Behavior of neurons in the abducens nucleus of the alert cat--II. Internuclear neurons

Subsets of extraocular motoneurons produce kinematically distinct saccades during hunting and exploration

Animals construct diverse behavioral repertoires by moving a limited number of body parts with varied kinematics and patterns of coordination. There is evidence that distinct movements can be generated by changes in activity dynamics within a common pool of motoneurons or by selectively engaging specific subsets of motoneurons in a task-dependent manner. However, in most cases, we have an incomplete understanding of the patterns of motoneuron activity that generate distinct actions and of how upstream premotor circuits select and assemble such motor programs. In this study, we used two closely related but kinematically distinct types of saccadic eye movement in larval zebrafish as a model to examine circuit control of movement diversity. In contrast to the prevailing view of a final common pathway, we found that in the oculomotor nucleus, distinct subsets of motoneurons were engaged for each saccade type. This type-specific recruitment was topographically organized and aligned with ultrastructural differences in motoneuron morphology and afferent synaptic innervation. Medially located motoneurons were active for both saccade types, and circuit tracing revealed a type-agnostic premotor pathway that appears to control their recruitment. By contrast, a laterally located subset of motoneurons was specifically active for hunting-associated saccades and received premotor input from pretectal hunting command neurons. Our data support a model in which generalist and action-specific premotor pathways engage distinct subsets of motoneurons to elicit varied movements of the same body part that subserve distinct behavioral functions.

Relative contribution of lateral vestibular neuron and abducens internuclear neuron inputs to the discharge activity of medial rectus motoneurons

Medial rectus motoneurons mediate nasally directed horizontal eye movements. These motoneurons receive two major excitatory inputs, from the abducens internuclear neurons (ABD Ints) and neurons of the lateral vestibular nucleus whose axons course through the ascending tract of Deiters (ATD). In the present work, we have recorded in the alert chronic cat preparation the discharge activity of these two premotor neurons simultaneously with eye movements, to discern their relative contribution to the firing pattern of medial rectus motoneurons. ABD Int discharge was accurately correlated with eye movements, displaying high sensitivities to eye position and eye velocity. ATD neurons also discharged in relation to spontaneous and vestibular eye movements but showed significantly lower eye position and eye velocity sensitivities. Outstandingly, ATD neurons presented a significantly lower eye position threshold for recruitment compared to both ABD Ints and medial rectus motoneurons. Therefore, ATD neurons exhibited eye position and velocity signals during spontaneous and vestibular eye movements, which were of lower magnitude than those of ABD Ints, but due to their low recruitment threshold, they could play a significant role in facilitating ABD Int signal transmission onto medial rectus motoneurons.

Locomotor efference copy signaling and gaze control: An evolutionary perspective

Neural replicas of the spinal motor commands that drive locomotion have become increasingly recognized as an intrinsic neural mechanism for producing gaze-stabilizing eye movements that counteract the perturbing effects of self-generated head/body motion. By pre-empting reactive signaling by motion-detecting vestibular sensors, such locomotor efference copies (ECs) provide estimates of the sensory consequences of behavioral action. Initially demonstrated in amphibian larvae during spontaneous fictive swimming in deafferented in vitro preparations, direct evidence for a contribution of locomotor ECs to gaze stabilization now extends to the ancestral lamprey and to tetrapod adult frogs and mice. Supporting behavioral evidence also exists for other mammals, including humans, therefore further indicating the mechanism's conservation during vertebrate evolution. The relationship between feedforward ECs and vestibular sensory feedback in ocular movement control is variable, ranging from additive to the former supplanting the latter, depending on vestibular sensing ability, and the intensity and regularity of rhythmic locomotor movements.

Preservation of KCC2 expression in axotomized abducens motoneurons and its enhancement by VEGF

The potassium chloride cotransporter 2 (KCC2) is the main Cl- extruder in neurons. Any alteration in KCC2 levels leads to changes in Cl- homeostasis and, consequently, in the polarity and amplitude of inhibitory synaptic potentials mediated by GABA or glycine. Axotomy downregulates KCC2 in many different motoneurons and it is suspected that interruption of muscle-derived factors maintaining motoneuron KCC2 expression is in part responsible. In here, we demonstrate that KCC2 is expressed in all oculomotor nuclei of cat and rat, but while trochlear and oculomotor motoneurons downregulate KCC2 after axotomy, expression is unaltered in abducens motoneurons. Exogenous application of vascular endothelial growth factor (VEGF), a neurotrophic factor expressed in muscle, upregulated KCC2 in axotomized abducens motoneurons above control levels. In parallel, a physiological study using cats chronically implanted with electrodes for recording abducens motoneurons in awake animals, demonstrated that inhibitory inputs related to off-fixations and off-directed saccades in VEGF-treated axotomized abducens motoneurons were significantly higher than in control, but eye-related excitatory signals in the on direction were unchanged. This is the first report of lack of KCC2 regulation in a motoneuron type after injury, proposing a role for VEGF in KCC2 regulation and demonstrating the link between KCC2 and synaptic inhibition in awake, behaving animals.

Role of locomotor efference copy in vertebrate gaze stabilization

Vertebrate locomotion presents a major challenge for maintaining visual acuity due to head movements resulting from the intimate biomechanical coupling with the propulsive musculoskeletal system. Retinal image stabilization has been traditionally ascribed to the transformation of motion-related sensory feedback into counteracting ocular motor commands. However, extensive exploration of spontaneously active semi-intact and isolated brain/spinal cord preparations of the amphibian Xenopus laevis, have revealed that efference copies (ECs) of the spinal motor program that generates axial- or limb-based propulsion directly drive compensatory eye movements. During fictive locomotion in larvae, ascending ECs from rostral spinal central pattern generating (CPG) circuitry are relayed through a defined ascending pathway to the mid- and hindbrain ocular motor nuclei to produce conjugate eye rotations during tail-based undulatory swimming in the intact animal. In post-metamorphic adult frogs, this spinal rhythmic command switches to a bilaterally-synchronous burst pattern that is appropriate for generating convergent eye movements required for maintaining image stability during limb kick-based rectilinear forward propulsion. The transition between these two fundamentally different coupling patterns is underpinned by the emergence of altered trajectories in spino-ocular motor coupling pathways that occur gradually during metamorphosis, providing a goal-specific, morpho-functional plasticity that ensures retinal image stability irrespective of locomotor mode. Although the functional impact of predictive ECs produced by the locomotory CPG matches the spatio-temporal specificity of reactive sensory-motor responses, rather than contributing additively to image stabilization, horizontal vestibulo-ocular reflexes (VORs) are selectively suppressed during intense locomotor CPG activity. This is achieved at least in part by an EC-mediated attenuation of mechano-electrical encoding at the vestibular sensory periphery. Thus, locomotor ECs and their potential suppressive impact on vestibular sensory-motor processing, both of which have now been reported in other vertebrates including humans, appear to play an important role in the maintenance of stable vision during active body displacements.

VEGF is an essential retrograde trophic factor for motoneurons

VEGF was initially discovered due to its angiogenic activity and therefore named "vascular endothelial growth factor." However, its more recently discovered neurotrophic activity may be evolutionarily more ancient. Our previous work showed that all the changes produced by axotomy on the firing activity and synaptic inputs of abducens motoneurons were completely restored after VEGF administration. Therefore, we hypothesized that the lack of VEGF delivered by retrograde transport from the periphery should also affect the physiology of otherwise intact abducens motoneurons. For VEGF retrograde blockade, we chronically applied a neutralizing VEGF antibody to the lateral rectus muscle. Recordings of extracellular single-unit activity and eye movements were made in alert cats before and after the application of the neutralizing antibody. Our data revealed that intact, noninjured abducens motoneurons retrogradely deprived of VEGF exhibited noticeable changes in their firing pattern. There is a general decrease in firing rate and a significant reduction in eye position and eye velocity sensitivity (i.e., a decrease in the tonic and phasic components of their discharge, respectively). Moreover, by means of confocal immunocytochemistry, motoneurons under VEGF blockade showed a marked reduction in the density of afferent synaptic terminals contacting with their cell bodies. Altogether, the present findings demonstrate that the lack of retrogradely delivered VEGF renders abducens motoneurons into an axotomy-like state. This indicates that VEGF is an essential retrograde factor for motoneuronal synaptic drive and discharge activity.

Transmitter and ion channel profiles of neurons in the primate abducens and trochlear nuclei

Extraocular motoneurons initiate dynamically different eye movements, including saccades, smooth pursuit and vestibulo-ocular reflexes. These motoneurons subdivide into two main types based on the structure of the neuro-muscular interface: motoneurons of singly-innervated (SIF), and motoneurons of multiply-innervated muscle fibers (MIF). SIF motoneurons are thought to provoke strong and brief/fast muscle contractions, whereas MIF motoneurons initiate prolonged, slow contractions. While relevant for adequate functionality, transmitter and ion channel profiles associated with the morpho-physiological differences between these motoneuron types, have not been elucidated so far. This prompted us to investigate the expression of voltage-gated potassium, sodium and calcium ion channels (Kv1.1, Kv3.1b, Nav1.6, Cav3.1-3.3, KCC2), the transmitter profiles of their presynaptic terminals (vGlut1 and 2, GlyT2 and GAD) and transmitter receptors (GluR2/3, NMDAR1, GlyR1α) using immunohistochemical analyses of abducens and trochlear motoneurons and of abducens internuclear neurons (INTs) in macaque monkeys. The main findings were: (1) MIF and SIF motoneurons express unique voltage-gated ion channel profiles, respectively, likely accounting for differences in intrinsic membrane properties. (2) Presynaptic glutamatergic synapses utilize vGlut2, but not vGlut1. (3) Trochlear motoneurons receive GABAergic inputs, abducens neurons receive both GABAergic and glycinergic inputs. (4) Synaptic densities differ between MIF and SIF motoneurons, with MIF motoneurons receiving fewer terminals. (5) Glutamatergic receptor subtypes differ between MIF and SIF motoneurons. While NMDAR1 is intensely expressed in INTs, MIF motoneurons lack this receptor subtype entirely. The obtained cell-type-specific transmitter and conductance profiles illuminate the structural substrates responsible for differential contributions of neurons in the abducens and trochlear nuclei to eye movements.

Short-term plasticity after partial deafferentation in the oculomotor system

Medial rectus motoneurons are innervated by two main pontine inputs. The specific function of each of these two inputs remains to be fully understood. Indeed, selective partial deafferentation of medial rectus motoneurons, performed by the lesion of either the vestibular or the abducens input, initially induces similar changes in motoneuronal discharge. However, at longer time periods, the responses to both lesions are dissimilar. Alterations on eye movements and motoneuronal discharge induced by vestibular input transection recover completely 2 months post-lesion, whereas changes induced by abducens internuclear lesion are more drastic and permanent. Functional recovery could be due to some kind of plastic process, such as reactive synaptogenesis, developed by the remaining intact input, which would occupy the vacant synaptic spaces left after lesion. Herein, by means of confocal microscopy, immunocytochemistry and retrograde labeling, we attempt to elucidate the possible plastic processes that take place after partial deafferentation of medial rectus motoneuron. 48 h post-injury, both vestibular and abducens internuclear lesions produced a reduced synaptic coverage on these motoneurons. However, 96 h after vestibular lesion, there was a partial recovery in the number of synaptic contacts. This suggests that there was reactive synaptogenesis. This recovery was preceded by an increase in somatic neurotrophin content, suggesting a role of these molecules in presynaptic axonal sprouting. The rise in synaptic coverage might be due to terminal sprouting performed by the remaining main input, i.e., abducens internuclear neurons. The present results may improve the understanding of this apparently redundant input system.

Synaptic loss and firing alterations in Axotomized Motoneurons are restored by vascular endothelial growth factor (VEGF) and VEGF-B

Vascular endothelial growth factor (VEGF), also known as VEGF-A, was discovered due to its vasculogenic and angiogenic activity, but a neuroprotective role for VEGF was later proven for lesions and disorders. In different models of motoneuronal degeneration, VEGF administration leads to a significant reduction of motoneuronal death. However, there is no information about the physiological state of spared motoneurons. We examined the trophic role of VEGF on axotomized motoneurons with recordings in alert animals using the oculomotor system as the experimental model, complemented with a synaptic study at the confocal microscopy level. Axotomy leads to drastic alterations in the discharge characteristics of abducens motoneurons, as well as to a substantial loss of their synaptic inputs. Retrograde delivery of VEGF completely restored the discharge activity and synaptically-driven signals in injured motoneurons, as demonstrated by correlating motoneuronal firing rate with motor performance. Moreover, VEGF-treated motoneurons recovered a normal density of synaptic boutons around motoneuronal somata and in the neuropil, in contrast to the low levels of synaptic terminals found after axotomy. VEGF also reduced the astrogliosis induced by axotomy in the abducens nucleus to control values. The administration of VEGF-B produced results similar to those of VEGF. This is the first work demonstrating that VEGF and VEGF-B restore the normal operating mode and synaptic inputs on injured motoneurons. Altogether these data indicate that these molecules are relevant synaptotrophic factors for motoneurons and support their clinical potential for the treatment of motoneuronal disorders.

Effects of Selective Deafferentation on the Discharge Characteristics of Medial Rectus Motoneurons

Medial rectus motoneurons receive two main pontine inputs: abducens internuclear neurons, whose axons course through the medial longitudinal fasciculus (MLF), and neurons in the lateral vestibular nucleus, whose axons project through the ascending tract of Deiters (ATD). Abducens internuclear neurons are responsible for conjugate gaze in the horizontal plane, whereas ATD neurons provide medial rectus motoneurons with a vestibular input comprising mainly head velocity. To reveal the relative contribution of each input to the oculomotor physiology, single-unit recordings from medial rectus motoneurons were obtained in the control situation and after selective deafferentation from cats with unilateral transection of either the MLF or the ATD. Both MLF and ATD transection produced similar short-term alterations in medial rectus motoneuron firing pattern, which were more drastic in MLF of animals. However, long-term recordings revealed important differences between the two types of lesion. Thus, while the effects of the MLF section were permanent, 2 months after ATD lesioning all motoneuronal firing parameters were similar to the control. These findings indicated a more relevant role of the MLF pathway in driving motoneuronal firing and evidenced compensatory mechanisms following the ATD lesion. Confocal immunocytochemistry revealed that MLF transection produced also a higher loss of synaptic boutons, mainly at the dendritic level. Moreover, 2 months after ATD transection, we observed an increase in synaptic coverage around motoneuron cell bodies compared with short-term data, which is indicative of a synaptogenic compensatory mechanism of the abducens internuclear pathway that could lead to the observed firing and morphological recovery.SIGNIFICANCE STATEMENT Eye movements rely on multiple neuronal circuits for appropriate performance. The abducens internuclear pathway through the medial longitudinal fascicle (MLF) and the vestibular neurons through the ascending tract of Deiters (ATD) are a dual system that supports the firing of medial rectus motoneurons. We report the effect of sectioning the MLF or the ATD pathway on the firing of medial rectus motoneurons, as well as the plastic mechanisms by which one input compensates for the lack of the other. This work shows that while the effects of MLF transection are permanent, the ATD section produces transitory effects. A mechanism based on axonal sprouting and occupancy of the vacant synaptic space due to deafferentation is the base for the mechanism of compensation on the medial rectus motoneuron.

Functional Diversity of Neurotrophin Actions on the Oculomotor System

Neurotrophins play a principal role in neuronal survival and differentiation during development, but also in the maintenance of appropriate adult neuronal circuits and phenotypes. In the oculomotor system, we have demonstrated that neurotrophins are key regulators of developing and adult neuronal properties, but with peculiarities depending on each neurotrophin. For instance, the administration of NGF (nerve growth factor), BDNF (brain-derived neurotrophic factor) or NT-3 (neurotrophin-3) protects neonatal extraocular motoneurons from cell death after axotomy, but only NGF and BDNF prevent the downregulation in ChAT (choline acetyltransferase). In the adult, in vivo recordings of axotomized extraocular motoneurons have demonstrated that the delivery of NGF, BDNF or NT-3 recovers different components of the firing discharge activity of these cells, with some particularities in the case of NGF. All neurotrophins have also synaptotrophic activity, although to different degrees. Accordingly, neurotrophins can restore the axotomy-induced alterations acting selectively on different properties of the motoneuron. In this review, we summarize these evidences and discuss them in the context of other motor systems.

Transmitter inputs to different motoneuron subgroups in the oculomotor and trochlear nucleus in monkey

In all vertebrates the eyes are moved by six pairs of extraocular muscles enabling horizontal, vertical and rotatory movements. Recent work showed that each extraocular muscle is controlled by two motoneuronal groups: (1) Motoneurons of singly-innervated muscle fibers (SIF) that lie within the boundaries of motonuclei mediating a fast muscle contraction; and (2) motoneurons of multiply-innervated muscle fibers (MIF) in the periphery of motonuclei mediating a tonic muscle contraction. Currently only limited data about the transmitter inputs to the SIF and MIF motoneurons are available. Here we performed a quantitative study on the transmitter inputs to SIF and MIF motoneurons of individual muscles in the oculomotor and trochlear nucleus in monkey. Pre-labeled motoneurons were immunostained for GABA, glutamate decarboxylase, GABA-A receptor, glycine transporter 2, glycine receptor 1, and vesicular glutamate transporters 1 and 2. The main findings were: (1) the inhibitory control of SIF motoneurons for horizontal and vertical eye movements differs. Unlike in previous primate studies a considerable GABAergic input was found to all SIF motoneuronal groups, whereas a glycinergic input was confined to motoneurons of the medial rectus (MR) muscle mediating horizontal eye movements and to those of the levator palpebrae (LP) muscle elevating the upper eyelid. Whereas SIF and MIF motoneurons of individual eye muscles do not differ numerically in their GABAergic, glycinergic and vGlut2 input, vGlut1 containing terminals densely covered the supraoculomotor area (SOA) targeting MR MIF motoneurons. It is reasonable to assume that the vGlut1 input affects the near response system in the SOA, which houses the preganglionic neurons mediating pupillary constriction and accommodation and the MR MIF motoneurones involved in vergence.

Neural progenitor cell implants in the lesioned medial longitudinal fascicle of adult cats regulate synaptic composition and firing properties of abducens internuclear neurons

Transplants of neural progenitor cells (NPCs) into the injured CNS have been proposed as a powerful tool for brain repair, but, to date, few studies on the physiological response of host neurons have been reported. Therefore, we explored the effects of NPC implants on the discharge characteristics and synaptology of axotomized abducens internuclear neurons, which mediate gaze conjugacy for horizontal eye movements. NPCs were isolated from the subventricular zone of neonatal cats and implanted at the site of transection in the medial longitudinal fascicle of adult cats. Abducens internuclear neurons of host animals showed a complete restoration of axotomy-induced alterations in eye position sensitivity, but eye velocity sensitivity was only partially regained. Analysis of the inhibitory and excitatory components of the discharge revealed a normal re-establishment of inhibitory inputs, but only partial re-establishment of excitatory inputs. Moreover, their inhibitory terminal coverage was similar to that in controls, indicating that there was ultimately no loss of inhibitory synaptic inputs. Somatic coverage by synaptophysin-positive contacts, however, showed intermediate values between control animals and animals that had undergone axotomy, likely due to partial loss of excitatory inputs. We also demonstrated that severed axons synaptically contacted NPCs, most of which were VEGF immunopositive, and that abducens internuclear neurons expressed the VEGF receptor Flk1. Together, our results suggest that VEGF neurotrophic support might underlie the increased inhibitory-to-excitatory balance observed in the postimplant cells. The noteworthy improvement of firing properties of injured neurons following NPC implants indicates that these cells might provide a promising therapeutic strategy after neuronal lesions.

Dynamics of abducens nucleus neurons in the awake mouse

The mechanics of the eyeball and orbital tissues (the "ocular motor plant") are a fundamental determinant of ocular motor signal processing. The mouse is used increasingly in ocular motor physiology, but little is known about its plant mechanics. One way to characterize the mechanics is to determine relationships between extraocular motoneuron firing and eye movement. We recorded abducens nucleus neurons in mice executing compensatory eye movements during 0.1- to 1.6-Hz oscillation in the light. We analyzed firing rates to extract eye position and eye velocity sensitivities, from which we determined time constants of a viscoelastic model of the plant. The majority of abducens neurons were already active with the eye in its central rest position, with only 6% recruited at more abducted positions. Firing rates exhibited largely linear relationships to eye movement, although there was a nonlinearity consisting of increasing modulation in proportion to eye movement as eye amplitudes became small (due to reduced stimulus amplitude or reduced alertness). Eye position and velocity sensitivities changed with stimulus frequency as expected for an ocular motor plant dominated by cascaded viscoelasticities. Transfer function poles lay at approximately 0.1 and 0.9 s. Compared with previously studied animal species, the mouse plant is stiffer than the rabbit but laxer than cat and rhesus. Differences between mouse and rabbit can be explained by scaling for eye size (allometry). Differences between the mouse and cat or rhesus can be explained by differing ocular motor repertoires of animals with and without a fovea or area centralis.

Dual encoding of muscle tension and eye position by abducens motoneurons

Extraocular muscle tension associated with spontaneous eye movements has a pulse-slide-step profile similar to that of motoneuron firing rate. Existing models only relate motoneuron firing to eye position, velocity and acceleration. We measured and quantitatively compared lateral rectus muscle force and eye position with the firing of abducens motoneurons in the cat to determine fundamental encoding correlations. During fixations (step), muscle force increased exponentially with eccentric eye position, consistent with a model of estimate ensemble motor innervation based on neuronal sensitivities and recruitment order. Moreover, firing rate in all motoneurons tested was better related to eye position than to muscle tension during fixations. In contrast, during the postsaccadic slide phase, the time constant of firing rate decay was closely related to that of muscle force decay, suggesting that all motoneurons encode muscle tension as well. Discharge characteristics of abducens motoneurons formed overlapping clusters of phasic and tonic motoneurons, thus, tonic units recruited earlier and had a larger slide signal. We conclude that the slide signal is a discharge characteristic of the motoneuron that controls muscle tension during the postsaccadic phase and that motoneurons are specialized for both tension and position-related properties. The organization of signal content in the pool of abducens motoneurons from the very phasic to the very tonic units is possibly a result of the differential trophic background received from distinct types of muscle fibers.

GABAergic Projections to the Oculomotor Nucleus in the Goldfish (carassius Auratus)

The mammalian oculomotor nucleus receives a strong γ-aminobutyric acid (GABA)ergic synaptic input, whereas such projections have rarely been reported in fish. In order to determine whether this synaptic organization is preserved across vertebrates, we investigated the GABAergic projections to the oculomotor nucleus in the goldfish by combining retrograde transport of biotin dextran amine, injected into the antidromically identified oculomotor nucleus, and GABA immunohistochemistry. The main source of GABAergic afferents to the oculomotor nucleus was the ipsilateral anterior octaval nucleus, with only a few, if any, GABAergic neurons being located in the contralateral tangential and descending nuclei of the octaval column. In mammals there is a nearly GABAergic inhibitory inputs; thus, the vestibulooculomotor GABAergic circuitry follows a plan that appears to be shared throughout the vertebrate phylogeny. The second major source of GABAergic projections was the rhombencephalic reticular formation, primarily from the medial area but, to a lesser extent, from the inferior area. A few GABAergic oculomotor projecting neurons were also observed in the ipsilateral nucleus of the medial longitudinal fasciculus. The GABAergic projections from neurons located in both the reticular formation surrounding the abducens nucleus and the nucleus of the medial reticular formation have primarily been related to the control of saccadic eye movements. Finally, all retrogradely labeled internuclear neurons of the abducens nucleus, and neurons in the cerebellum (close to the caudal lobe), were negative for GABA. These data suggest that the vestibuloocular and saccadic inhibitory GABAergic systems appear early in vertebrate phylogeny to modulate the firing properties of the oculomotor nucleus motoneurons.

Encoding of eye position in the goldfish horizontal oculomotor neural integrator

Monocular organization of the goldfish horizontal neural integrator was studied during spontaneous scanning saccadic and fixation behaviors. Analysis of neuronal firing rates revealed a population of ipsilateral (37%), conjugate (59%), and contralateral (4%) eye position neurons. When monocular optokinetic stimuli were employed to maximize disjunctive horizontal eye movements, the sampled population changed to 57, 39, and 4%. Monocular eye tracking could be elicited at different gain and phase with the integrator time constant independently modified for each eye by either centripetal (leak) or centrifugal (instability) drifting visual stimuli. Acute midline separation between the hindbrain oculomotor integrators did not affect either monocularity or time constant tuning, corroborating that left and right eye positions are independently encoded within each integrator. Together these findings suggest that the "ipsilateral" and "conjugate/contralateral" integrator neurons primarily target abducens motoneurons and internuclear neurons, respectively. The commissural pathway is proposed to select the conjugate/contralateral eye position neurons and act as a feedforward inhibition affecting null eye position, oculomotor range, and saccade pattern.

Anatomical and pharmacological relationship between acetylcholine and nitric oxide in the prepositus hypoglossi nucleus of the cat: Functional implications for eye-movement control

The prepositus hypoglossi (PH) nucleus has been proposed as a pivotal structure for horizontal eye-position generation in the oculomotor system. Recent studies have revealed that acetylcholine (ACh) in the PH nucleus could mediate the persistent activity necessary for this process, although the origin of this ACh remains unknown. It is also known that nitric oxide (NO) in the PH nucleus plays an important role in the control of velocity balance, being involved in a negative feedback control of tonic signals arriving at the PH nucleus. As it could be expected that neurons taking part in eye-position generation must control their tonic background inputs, the existence of a relationship between nitrergic and cholinergic neurons is hypothesized. In the present study we analyzed the distribution, size, and morphology of choline acetyltransferase-positive neurons, and their relationship with neuronal nitric oxide synthase in the PH nucleus of the cat. As presumed, some 96% of cholinergic neurons were also nitrergic in the PH nucleus, suggesting that NO is regulating the level of ACh released by cholinergic PH neurons. Furthermore, we studied the alterations induced by muscarinic-receptor agonists and antagonists on spontaneous and vestibularly induced eye movements in the alert cat and compared them with those induced in previous studies by modification of NO levels in the same animal preparation. The results suggest that ACh is necessary for the generation of saccadic and vestibular eye-position signals, whereas the NO is stabilizing the eye-position generator by controlling background activity reaching cholinergic neurons in the PH nucleus.

The extraocular motor nuclei: organization and functional neuroanatomy

The organization of the motoneuron subgroups in the brainstem controlling each extraocular eye muscle is highly stable through the vertebrate species. The subgroups are topographically organized in the oculomotor nucleus (III) and are usually considered to form the final common pathway for eye muscle control. Eye muscles contain a unique type of slow non-twitch, fatigue-resistant muscle fiber, the multiply innervated muscle fibers (MIFs). The recent identification the MIF motoneurons shows that they too have topographic organization, but very different from the classical singly innervated muscle fiber (SIF) motoneurons. The MIF motoneurons lie around the periphery of the oculomotor nucleus (III), trochlear nucleus (IV), and abducens nucleus (VI), slightly separated from the SIF subgroups. The location of four different types of neurons in VI are described and illustrated: (1) SIF motoneurons, (2) MIF motoneurons, (3) internuclear neurons, and (4) the paramedian tract neurons which project to the flocculus. Afferents to the motoneurons arise from the vestibular nuclei, the oculomotor and abducens internuclear neurons, the mesencephalic and pontine burst neurons, the interstitial nucleus of Cajal, nucleus prepositus hypoglossi, the supraoculomotor area and the central mesencephalic reticular formation and the pretectum. The MIF and SIF motoneurons have different histochemical properties and different afferent inputs. The hypothesis that SIFs participate in moving the eye and MIFs determine the alignment seems possible but is not compatible with the concept of a final common pathway.

A cholinergic mechanism underlies persistent neural activity necessary for eye fixation

It is generally accepted that the prepositus hypoglossi (PH) nucleus is the site where horizontal eye-velocity signals are integrated into eye-position ones. However, how does this neural structure produce the sustained activity necessary for eye fixation? The generation of the neural activity responsible for eye-position signals has been studied here using both in vivo and in vitro preparations. Rat sagittal brainstem slices including the PH nucleus and the paramedian pontine reticular formation (PPRF) rostral to the abducens nucleus were used for recording intracellularly the synaptic activation of PH neurons from the PPRF. Single electrical pulses applied to the PPRF showed a monosynaptic projection on PH neurons. This synapse was found to be glutamatergic in nature, acting on alpha-amino-3-hydroxy-5-methylisoxazole propionate (AMPA)/kainate receptors. Train stimulation (100 ms, 50-200 Hz) of the PPRF evoked a depolarization of PH neurons, exceeding (by hundreds of ms) the duration of the stimulus. Both duration and amplitude of this long-lasting depolarization were linearly related to train frequency. The train-evoked sustained depolarization was demonstrated to be the result of the additional activation of cholinergic fibers projecting onto PH neurons, because it was prevented by slice superfusion with atropine sulfate and pirenzepine (two cholinergic antagonists), and mimicked by carbachol and McN-A-343 (two cholinergic agonists). These results were confirmed in alert behaving cats. Microinjections of atropine and pirenzepine evoked an ipsilateral gaze-holding deficit consisting of an exponential-like, centripetal eye movement following saccades directed toward the injected site. These findings suggest that the sustained activity present in PH neurons carrying eye-position signals is the result of the combined action of PPRF neurons and the facilitative role of cholinergic terminals, both impinging on PH neurons. The present results are discussed in relation to other proposals regarding integrative properties of PH neurons and/or related neural circuits.

Histochemical differences between motoneurons supplying multiply and singly innervated extraocular muscle fibers

The extraocular muscle fibers of vertebrates can be classified into two categories: singly innervated fibers (SIFs) and multiply innervated fibers (MIFs). In monkeys, the motoneurons of SIFs lie within the oculomotor, trochlear, and abducens nucleus, whereas the motoneurons of MIFs appear in separate subgroups in the periphery of the classical nuclei borders. In the present study, we investigated the histochemical properties of SIF and MIF motoneurons by using combined tract-tracing and immunofluorescence techniques. In monkeys, SIF and MIF motoneurons of extraocular muscles were identified by tracer injections into the belly or the distal myotendinous junction of the medial or lateral rectus muscle. Alternatively, the motoneurons were identified by choline acetyltransferase immunostaining. These techniques were combined with the detection of histochemical markers for perineuronal nets, nonphosphorylated neurofilaments, parvalbumin, or cytochrome oxidase. The experiments revealed that the MIF motoneurons in the periphery of the motonuclei do not contain nonphosphorylated neurofilaments or parvalbumin and lack perineuronal nets. In contrast, SIF motoneurons express all markers at high intensity. Cytochrome oxidase immunostaining was found in both motoneuron populations. An additional population of motoneurons with "MIF properties" was identified within the boundaries of the abducens nucleus, which could represent the motoneurons innervating MIFs in the orbital layer of lateral rectus muscle. Our data provide evidence that SIF and MIF motoneurons, which can be correlated with twitch motoneurons and presumed non-twitch motoneurons, differ in their histochemical properties. The absence of perineuronal nets, nonphosphorylated neurofilaments, and parvalbumin may help to identify the homologous MIF motoneurons in other species, including humans.

Effects of electrode penetrations into the abducens nucleus of the monkey: eye movement recordings and histopathological evaluation of the nuclei and lateral rectus muscles

Two adult rhesus monkeys that had undergone 2 years of electrode penetrations into their abducens and vestibular nuclei, for chronic eye movement studies, were examined histologically. An analysis of their VIth nucleus neurons and lateral rectus muscles revealed the following. Twenty-two percent of the large neurons (approximately 30 microm in diameter), on average, were missing and extensive neuropil disruption and gliosis was evident in the experimental side abducens nuclei as compared with the control side in each animal. While the lateral rectus muscles showed small, but inconsistent, changes in total fiber number, the muscle fiber diameters were altered, leading to a more homogenous muscle and making the typical orbital and global subdivisions of the muscle less distinct. Eye movement records from before and after the electrophysiological studies were comparable. We discuss how the complex architecture of the extraocular muscles as well as the possibility of polyneuronal innervation of single muscle fibers could explain our results.

Expression of Trk receptors in the oculomotor system of the adult cat

We examined the expression of the three Trk receptors for neurotrophins (TrkA, TrkB, and TrkC) in the extraocular motor nuclei of the adult cat by using antibodies directed against the full-Trk proteins in combination with horseradish peroxidase retrograde tracing. The three receptors were present in all neuronal populations investigated, including abducens motoneurons and internuclear neurons, medial rectus motoneurons of the oculomotor nucleus, and trochlear motoneurons. They were also present in the vestibular and prepositus hypoglossi nuclei. TrkA, TrkB, and TrkC immunopositive cells were found in similar percentages in the oculomotor and in the trochlear nuclei. In the abducens nucleus, however, a significantly higher percentage of cells expressed TrkB than the other two receptors, among both motoneurons (81.8%) and internuclear neurons (88.4%). The percentages obtained for the three Trk receptors in identified neuronal populations pointed to the colocalization of two or three receptors in a large number of cells. We used confocal microscopy to elucidate the subcellular location of Trk receptors. In this case, abducens motoneurons and internuclear neurons were identified with antibodies against choline acetyltransferase and calretinin, respectively. We found a different pattern of staining for each neurotrophin receptor, suggesting the possibility that each receptor and its cognate ligand may use a different route for cellular signaling. Therefore, the expression of Trk receptors in oculomotor, trochlear, and abducens motoneurons, as well as abducens internuclear neurons, suggests that their associated neurotrophins may exert an influence on the normal operation of the oculomotor circuitry. The presence of multiple Trk receptors on individual cells indicates that they likely act in concert with each other to regulate distinct functions.

Central nervous system lesions that can and those that cannot be repaired with the help of olfactory bulb ensheathing cell transplants

Growth-promoting macroglia (aldynoglia) with growth properties and immunological markers similar to Schwann cells, are found in loci of the mammalian CNS where axon regeneration occurs throughout life, like the olfactory sytem, hypothalamus-hypophysis and the pineal gland. Contrary to Schwann cells, aldynoglia mingle freely with astrocytes and can migrate in brain and spinal cord. Transplantation of cultured and immunopurified olfactory ensheathing cells (OECs) in the spinal cord after multiple central rhizotomy, promoted sensory and central axon growth and partial functional restoration, judging by anatomical, electrophysiological and behavioural criteria. OEC transplants suppressed astrocyte reactivity, thus generally favouring axon growth after a lesion. However, the functional repair promoted by OEC transplants was partial in the best cases, depending on lesion type and location. Cyst formation after photochemical cord lesion was partially prevented but neither the corticospinal tract, interrupted by a mild contusion, nor the sectioned medial longitudinal fascicle, did regrow after OEC transplantation in the injured area.

Recruitment order of cat abducens motoneurons and internuclear neurons

Abducens neurons undergo a dose-dependent synaptic blockade (either disinhibition or complete blockade) when tetanus neurotoxin (TeNT) is injected into the lateral rectus muscle at either a low (0.5) or a high dose (5 ng/kg). We studied the firing pattern and recruitment order in abducens neurons both in control and after TeNT injection. The eye position threshold for recruitment of control abducens neurons was exponentially related to the eye position and velocity sensitivities. We also found a constancy of recruitment threshold for different eye movement modalities (spontaneous, optokinetic, and vestibular). Exponential relationships were found, as well, for eye velocity sensitivity during saccades and for position and velocity sensitivities during the vestibulo-ocular reflex. Likewise, inverse relationships were found between recruitment threshold or position sensitivity with the antidromic latency in control abducens neurons. These relationships, however, did not apply following TeNT treatment. Neuronal firing after TeNT appeared either disinhibited (low dose) or depressed (high dose), but the relationships between neuronal sensitivities and recruitment still applied. However, the pattern of recruitment shifted toward the treated side as more inputs were blocked by the low- and high-dose treatments, respectively. Nonetheless, although the recruitment-to-sensitivity relationships persisted under the TeNT synaptic blockade, we conclude that synaptic inputs are determinant for establishing the recruitment threshold and recruitment spacing of abducens motoneurons and internuclear neurons.

A-, T-, and H-type currents shape intrinsic firing of developing rat abducens motoneurons

During postnatal development, profound changes take place in the excitability of nerve cells, including modification in the distribution and properties of receptor-operated channels and changes in the density and nature of voltage-gated channels. We studied here the firing properties of abducens motoneurons (aMns) in transverse brainstem slices from postnatal day (P) 1-13 rats. Recordings were made from aMNs in the whole-cell configuration of the patch-clamp technique. Two main types of aMn could be distinguished according to their firing profile during prolonged depolarizations. Both types were identified as aMns by their fluorescence following retrograde labelling with the lipophilic carbocyanine DiI in the rectus lateralis muscle. The first type (BaMns) exhibited a burst of action potentials (APs) followed by an adaptation of discharge and were encountered in approximately 70 % of aMns. Their discharge profile resembled that of adult aMns and was encountered in all aMns after P9. BaMns exhibited a hyperpolarization-induced rebound potential that was blocked by low concentrations of Ni2+ or by Ca2+-free external solution. This current had the properties of the T-type current. Action potentials of BaMns showed a complex afterhyperpolarization (AHP). An inward rectification was evidenced following hyperpolarization and was blocked by external application of caesium or ZD7288, indicating the presence of the hyperpolarization-activated cationic current (IH). Blocking the IH current almost doubled the input resistance of BaMns. The second class of aMns (DaMns) displayed a delayed excitation that was mediated by A-type K+ currents and was observed only between P4 and P9. DaMns exhibited immature characteristics: an action potential with a simple AHP, a linear current-voltage relation and a large input resistance. The number of aMns remained unchanged when both types were present (P5-P6) and later in development when only BaMns were encountered (P19), suggesting that DaMns mature into BaMns during postnatal development. We conclude that aMns display profound reorganization in their intrinsic excitability during postnatal development.

Functional alterations of cat abducens neurons after peripheral tetanus neurotoxin injection

Tetanus neurotoxin (TeNT) cleaves synaptobrevin, a protein involved in synaptic vesicle docking and fusion, thereby preventing neurotransmitter release and causing a functional deafferentation. We injected TeNT into the lateral rectus muscle of adult cats at 0.5 or 5 ng/kg (low and high dose, respectively). In the periphery, TeNT slightly slowed motor axon conduction velocity, and at high doses, partially blocked neuromuscular transmission. TeNT peripheral actions displayed time courses different to the more profound and longer-lasting central actions. Central effects were first observed 2 days postinjection and reversed after 1 mo. The low dose induce depression of inhibitory inputs, whereas the high dose produce depression of both inhibitory and excitatory inputs. Simultaneous recordings of eye movement and neuronal firing revealed that low-dose injections specifically reduced inhibition of firing during off-directed saccadic movements, while high-dose injections of TeNT affected both inhibitory and excitatory driven firing patterns. Motoneurons and abducens interneurons were both affected in a similar way. These alterations resulted in modifications in all discharge characteristic analyzed such as background firing, threshold for recruitment, and firing sensitivities to both eye position and velocity during spontaneous movements or vestibulo-ocular reflexes. Removal of inhibition after low-dose injections also altered firing patterns, and although firing activity increased, it did not result in muscle tetanic contractions. Removal of inhibition and excitation by high-dose injections resulted in a decrease in firing modulation with eye movements. Our findings suggest that the distinct behavior of oculomotor and spinal motor output following TeNT intoxication could be explained by their different interneuronal and proprioceptive control.

Dynamics of abducens nucleus neuron discharges during disjunctive saccades

In this report, we provide the first characterization of abducens nucleus neuron (ABN) discharge dynamics during horizontal disjunctive saccades. These movements function to rapidly transfer the visual axes between targets located at different eccentricities and depths. Our primary objective was to determine whether the signals carried by ABNs during these movements are appropriate to drive the motion of the eye to which they project. We also asked whether ABNs encode eye movements similarly during disjunctive saccades and disjunctive fixation. To address the first objective we 1) assessed whether we could predict the discharge dynamics of individual neurons during disjunctive saccades based on their discharge properties during conjugate saccades and 2) directly estimated the sensitivity of individual neurons to either the ipsilateral/contralateral eye or the conjugate/vergence position and velocity using bootstrap statistics. Our main finding was that during disjunctive saccades in the direction ipsilateral to the recording site (ON-direction), the majority of ABNs preferentially encoded the velocity and the position of the ipsilateral eye. The remaining neurons predominantly encoded the conjugate motion of the eyes (i.e., were equally sensitive to the motion of both eyes). Generally, ipsilateral/contralateral eye based models better described neuronal discharges than conjugate/vergence based models, yet both model structures yielded similar conclusions. Moreover, the preferred eye of individual neurons based on their position and velocity sensitivities were generally well matched. We also found that for saccades in the OFF-direction, the pausing behavior of ABNs was similar during conjugate and disjunctive saccades, with the exception that for movements of small amplitudes, more ABNs paused during conjugate saccades. Finally, we found that putative motoneurons and internuclear neurons encoded ON- and OFF-direction disjunctive saccades in a similar manner. To address our second objective, we compared the discharge properties of individual ABNs during disjunctive saccades and disjunctive fixation. Good coherence was observed between the preferred eye of individual ABNs during the two behaviors. Taken together, our results indicate that although individual ABNs can encode the motion of both eyes to various degrees, the population drive of ABNs accounts for most of the movement of the ipsilateral eye during disjunctive saccades and disjunctive fixation.

Firing properties of axotomized central nervous system neurons recover after graft reinnervation

Axotomy produces changes in the electrical properties of neurons and in their synaptic inputs, leading to alterations in firing pattern. We have considered the possibility that these changes occur as a result of the target deprivation induced by the lesion. Thus, we have provided a novel target to axotomized central neurons by grafting embryonic tissue at the lesion site to study the target dependence of discharge characteristics. The extracellular single-unit electrical activity of abducens internuclear neurons was recorded in the alert behaving cat in control, after axotomy, and after axotomy plus the implantation of cerebellar primordium. As recently characterized (de la Cruz et al. [2000] J. Comp. Neurol. 427:391-404), firing alterations induced by axotomy included an overall decrease in firing rate and a loss of eye-related signals, i.e., eye position and velocity neuronal sensitivities, that do not resume to normality with time. The grafting of a novel target to the injured abducens internuclear neurons restored the normal firing and sensitivities as recorded in the majority of units. To study the reinnervation of the implant, we performed anterograde labeling with biocytin combined with electron microscopy visualization. Axons of abducens internuclear neurons grew into the transplant sprouting into granule cell and molecular layers, as characterized by the immunostaining for gamma-aminobutyric acid and calbindin D-28k. Ultrastructural examination of labeled axons and boutons revealed the establishment of synaptic contacts, mainly axodendritic, with different cell types of the grafted cerebellar cortex. Therefore, these data indicate that axotomized central neurons resume to normal firing after the reinnervation of a novel target.

Hypoglossal and reticular interneurons involved in oro-facial coordination in the rat

Chewing, swallowing, breathing, and vocalization in mammals require precise coordination of tongue movements with concomitant activities of the mimetic muscles. The neuroanatomic basis for this oro-facial coordination is not yet fully understood. After the stereotaxic microinjection of retrograde and anterograde neuronal tracers (biotin-dextran, Fluoro-Ruby, Fluoro-Emerald, and Fluoro-Gold) into the facial and hypoglossal nuclei of the rat, we report here a direct bilateral projection of hypoglossal internuclear interneurons onto facial motoneurons. We also confirm the existence of a small pool of neurons in the dorsal part of the brainstem reticular formation that project ipsilaterally to both facial and hypoglossal nuclei. For precise tracer injections, both motor nuclei were located and identified by the electrical antidromic activation of their constituent motoneurons. Injections of retrograde tracers into the facial nucleus consistently labeled neurons in the hypoglossal nucleus. These neurons prevalently lay in the ipsilateral side, were small in size, and, like classic intrinsic hypoglossal local-circuit interneurons, had several thin dendrites. Reverse experiments - injections of anterograde tracers into the hypoglossal nucleus - labeled fine varicose nerve fiber terminals in the facial nucleus. These fiber terminals were concentrated in the intermediate subdivision of the facial nucleus, with a strong ipsilateral prevalence. Double injections of different tracers into the facial and the hypoglossal nuclei revealed a small, but constant, number of double-labeled neurons located predominantly ipsilateral in the caudal brainstem reticular formation. Hypoglossal internuclear interneurons projecting to the facial nucleus, as well as those neurons of the parvocellular reticular formation that project to both facial and hypoglossal nuclei, could be involved in oro-facial coordination.

Response of abducens internuclear neurons to axotomy in the adult cat

The highly specific projection of abducens internuclear neurons on the medial rectus motoneurons of the oculomotor nucleus constitutes an optimal model for investigating the effects of axotomy in the central nervous system. We have analyzed the morphological changes induced by this lesion on both the cell bodies and the transected axons of abducens internuclear neurons in the adult cat. Axotomy was performed by the transection of the medial longitudinal fascicle. Cell counts of Nissl-stained material and calretinin-immunostained abducens internuclear neurons revealed no cell death by 3 months postaxotomy. Ultrastructural examination of these cells at 6, 14, 24, and 90 days postaxotomy showed normal cytological features. However, the surface membrane of axotomized neurons appeared contacted by very few synaptic boutons compared to controls. This change was quantified by measuring the percentage of synaptic coverage of the cell bodies and the linear density of boutons. Both parameters decreased significantly after axotomy, with the lowest values at 90 days postlesion ( approximately 70% reduction). We also explored axonal regrowth and the possibility of reinnervation of a new target by means of anterograde labeling with biocytin. At all time intervals analyzed, labeled axons were observed to be interrupted at the caudal limit of the lesion; in no case did they cross the scar tissue to reach the distal part of the tract. Nonetheless, a conspicuous axonal sprouting was present at the caudal aspect of the lesion site. Structures suggestive of axonal growth were found, such as large terminal clubs, from which short filopodium-like branches frequently emerged. Similar findings were obtained after parvalbumin and calretinin immunostaining. At the electron microscopy level, biocytin-labeled boutons originating from the sprouts appeared surrounded by either extracellular space, which was extremely dilated at the lesion site, or by glial processes. The great majority of labeled boutons examined were, thus, devoid of neuronal contact, indicating absence of reinnervation of a new target. Altogether, these data indicate that abducens internuclear neurons survive axotomy in the adult cat and show some form of axonal regrowth, even in the absence of target connection.

Discharge characteristics of axotomized abducens internuclear neurons in the adult cat

The aim of the present work was to characterize the axotomy-induced changes in the discharge properties of central nervous system neurons recorded in the alert behaving animal. The abducens internuclear neurons of the adult cat were the chosen model. The axons of these neurons course through the contralateral medial longitudinal fascicle and contact the medial rectus motoneurons of the oculomotor nucleus. Axotomy was carried out by the unilateral transection of this fascicle (right side) and produced immediate oculomotor deficits, mainly the incapacity of the right eye to adduct across the midline. Extracellular single-unit recording of abducens neurons was carried out simultaneously with eye movements. The main alteration observed in the firing of these axotomized neurons was the overall decrease in firing rate. During eye fixations, the tonic signal was reduced, and, on occasion, a progressive decay in firing rate was observed. On-directed saccades were not accompanied by the high-frequency spike burst typical of controls; instead, there was a moderate increase in firing. Similarly, during the vestibular nystagmus, neurons hardly modulated during both the slow and the fast phases. Linear regression analysis between firing rate and eye movement parameters showed a significant reduction in eye position and velocity sensitivities with respect to controls, during both spontaneous and vestibularly induced eye movements. These firing alterations were observed during the 3 month period of study after lesion, with no sign of recovery. Conversely, abducens motoneurons showed no significant alteration in their firing pattern. Therefore, axotomy produced long-lasting changes in the discharge characteristics of abducens internuclear neurons that presumably reflected the loss of afferent oculomotor signals. These alterations might be due to the absence of trophic influences derived from the target.

Why move the eyes if we can move the head?

To see while moving is a very basic and integrative sensorimotor function in vertebrates. To maintain visual acuity, the oculomotor system provides efficient compensatory eye movements for head and visual field displacements. Other types of eye movement allow the selection of new visual targets and binocular vision and stereopsis. Motor and premotor neuronal circuits involved in the genesis and control of eye movements are briefly described. The peculiar properties and robust biomechanics of the oculomotor system have allowed it to survive almost unchanged through vertebrate evolution.

Limits to the capacity of transplants of olfactory glia to promote axonal regrowth in the CNS

Olfactory bulb ensheathing cell (OBEC) transplants promoted axonal regeneration in the spinal cord dorsal root entry zone and in the corticospinal tract. However, OBECs failed to promote abducens internuclear neuron axon regeneration when transplanted at the site of nerve fibre transection. In experiments performed in both cats and rats, OBECs survived for up to 2 months, lining themselves up along the portion of the regrowing axons proximal to the interneuron cell body. However, OBECs migrated preferentially towards abducens somata, in the direction opposite to the oculomotor nucleus target. OBECs seem to promote nerve fibre regeneration only where preferred direction of glial migration coincides with the direction of axonal growth towards its target.

GABA-immunoreactive internuclear neurons in the ocular motor system of lampreys

The presence of internuclear neurons in the abducens and oculomotor nuclei of lampreys [González, M.J., Pombal, M.A., Rodicio, M.C. and Anadón, R., Internuclear neurons of the ocular motor system of the larval sea lamprey, J. Comp. Neurol. 401 (1998) 1-15] indicates that coordination of eye movements by internuclear neurons appeared early during the evolution of vertebrates. In order to investigate the possible involvement of inhibitory neurotransmitters in internuclear circuits, the distribution of gamma-aminobutyric acid (GABA) in the extraocular motor nuclei of the lamprey was studied using immunocytochemical techniques. Small GABA-immunoreactive (GABAir) neurons were observed in the three ocular motor nuclei. Numerous GABAir neurons were observed in the group of internuclear neurons of the dorsal rectus oculomotor subnucleus. A second group of GABAir neurons was observed among and below the trochlear motoneurons. Two further groups of GABAir interneurons, periventricular and lateral, were located in the abducens nucleus among the cells of the caudal rectus and the ventral rectus motor subnuclei, respectively. In addition to the presence of GABAir neurons, in all the ocular motor nuclei the motoneurons were contacted by numerous GABAir boutons. Taken together, these results suggest that GABA is involved as a neurotransmitter in internuclear pathways of the ocular motor system of lampreys.

Comparing extraocular motoneuron discharges during head-restrained saccades and head-unrestrained gaze shifts

Burst neurons (BNs) in the paramedian pontine reticular formation provide the primary input to the extraocular motoneurons (MNs) during head-restrained saccades and combined eye-head gaze shifts. Prior studies have shown that BNs carry eye movement-related signals during saccades and carry head as well as eye movement-related signals during gaze shifts. Therefore MNs receive signals related to head motion during gaze shifts, yet they solely drive eye motion. Here we addressed whether the relationship between MN firing rates and eye movements is influenced by the additional premotor signals present during gaze shifts. Neurons in the abducens nucleus of monkeys were first studied during saccades made with the head stationary. We then recorded from the same neurons during voluntary combined eye-head gaze shifts. We conclude that the activity of MNs, in contrast to that of BNs, is related to eye motion by the same dynamic relationship during head-restrained saccades and head-unrestrained gaze shifts. In addition, we show that a standard metric-based analysis [i.e., counting the number of spikes (NOS) in a burst] yields misleading results when applied to the same data set. We argue that this latter approach fails because it does not properly consider the system's dynamics or the strong interactions between eye and head motion.

Quantitative analysis of abducens neuron discharge dynamics during saccadic and slow eye movements

The mechanics of the eyeball and its surrounding tissues, which together form the oculomotor plant, have been shown to be the same for smooth pursuit and saccadic eye movements. Hence it was postulated that similar signals would be carried by motoneurons during slow and rapid eye movements. In the present study, we directly addressed this proposal by determining which eye movement-based models best describe the discharge dynamics of primate abducens neurons during a variety of eye movement behaviors. We first characterized abducens neuron spike trains, as has been classically done, during fixation and sinusoidal smooth pursuit. We then systematically analyzed the discharge dynamics of abducens neurons during and following saccades, during step-ramp pursuit and during high velocity slow-phase vestibular nystagmus. We found that the commonly utilized first-order description of abducens neuron firing rates (FR = b + kE + r, where FR is firing rate, E and are eye position and velocity, respectively, and b, k, and r are constants) provided an adequate model of neuronal activity during saccades, smooth pursuit, and slow phase vestibular nystagmus. However, the use of a second-order model, which included an exponentially decaying term or "slide" (FR = b + kE + r + uE - c), notably improved our ability to describe neuronal activity when the eye was moving and also enabled us to model abducens neuron discharges during the postsaccadic interval. We also found that, for a given model, a single set of parameters could not be used to describe neuronal firing rates during both slow and rapid eye movements. Specifically, the eye velocity and position coefficients (r and k in the above models, respectively) consistently decreased as a function of the mean (and peak) eye velocity that was generated. In contrast, the bias (b, firing rate when looking straight ahead) invariably increased with eye velocity. Although these trends are likely to reflect, in part, nonlinearities that are intrinsic to the extraocular muscles, we propose that these results can also be explained by considering the time-varying resistance to movement that is generated by the antagonist muscle. We conclude that to create realistic and meaningful models of the neural control of horizontal eye movements, it is essential to consider the activation of the antagonist, as well as agonist motoneuron pools.

Neural organization from the superior colliculus to motoneurons in the horizontal oculomotor system of the cat

The neural organization of the superior colliculus (SC) projection to horizontal ocular motoneurons was analyzed in anesthetized cats using intracellular recording and transneuronal labeling. Intracellular responses to SC stimulation were analyzed in lateral rectus (LR) and medial rectus (MR) motoneurons and internuclear neurons in the abducens nucleus (AINs). LR motoneurons and AINs received excitation from the contralateral SC and inhibition from the ipsilateral SC. The shortest excitation (0.9-1.9 ms) and inhibition (1.4-2.4 ms) were mainly disynaptic from the SC and were followed by tri- and polysynaptic responses evoked with increasing stimuli or intensity. All MR motoneurons received excitation from the ipsilateral SC, whereas none of them received any short-latency inhibition from the contralateral SC, but some received excitation. The latency of the ipsilateral excitation in MR motoneurons (1.7-2.8 ms) suggested that this excitation was trisynaptic via contralateral AINs, because conditioning SC stimulation spatially facilitated trisynaptic excitation from the ipsilateral vestibular nerve. To locate interneurons mediating the disynaptic SC inputs to LR motoneurons, last-order premotor neurons were labeled transneuronally after injecting wheat germ agglutinin-conjugated horseradish peroxidase into the abducens nerve, and tectoreticular axon terminals were labeled after injecting dextran-biotin into the ipsilateral or contralateral SC in the same preparations. Transneuronally labeled neurons were mainly distributed ipsilaterally in the paramedian pontine reticular formation (PPRF) rostral to retrogradely labeled LR motoneurons and the vestibular nuclei, and contralaterally in the paramedian pontomedullary reticular formation (PPMRF) caudomedial to the abducens nucleus and the vestibular nuclei. Among the last-order premotor neuron areas, orthogradely labeled tectoreticular axon terminals were observed only in the PPRF and the PPMRF contralateral to the injected SC and seemed to make direct contacts with many of the labeled last-order premotor neurons in the PPRF and the PPMRF. These morphological results confirmed that the main excitatory and inhibitory connections from the SC to LR motoneurons are disynaptic and that the PPRF neurons that receive tectoreticular axon terminals from the contralateral SC terminate on ipsilateral LR motoneurons, whereas the PPMRF neurons that receive tectoreticular axon terminals from the contralateral SC terminate on contralateral LR motoneurons.

Internuclear neurons of the ocular motor system of the larval sea lamprey

The internuclear neurons of the ocular motor system of lampreys are characterized here for the first time. Horseradish peroxidase (HRP), fluorescein-, or Texas red-(TRDA) coupled dextran-amine applied into the oculomotor nucleus of larval lamprey (Petromyzon marinus) retrogradely labeled two populations of contralateral abducens interneurons, one lateral and the other periventricular. Tracer application to the abducens nucleus anterogradely labeled thick contralateral fibers that specifically contact the medial rectus motor subnucleus by means of large boutons. Local application of TRDA to this subnucleus allowed identification of the lateral abducens interneurons as the origin of this projection. Electron microscopy of the medial rectus motor subnucleus showed large boutons bearing round synaptic vesicles that contact on the perikarya, as well as small boutons with pleomorphic vesicles. This lateral rectus (abducens) -- medial rectus (oculomotor) internuclear projection of lampreys appears to be similar to those involved in the coordination of horizontal eye movements in mammals. The periventricular abducens interneurons projected bilaterally to other oculomotor subnuclei. Tracer application to the abducens nucleus labeled a group of small interneurons in the ipsilateral dorsal rectus motor subnucleus. Anterograde labeling indicates that oculomotor interneurons project ipsilaterally to the ventral rectus abducens subnucleus, thus, corresponding to oculomotor interneurons found in mammals and frogs. The interneurons of the dorsal rectus and ventral rectus motor subnuclei are probably involved in the control of conjugate vertical eye movements. The present results strongly suggest that the internuclear coordination of conjugate eye movements appeared in the earliest vertebrates. The homologies of extraocular muscles of lampreys and gnathostomes were reexamined.

Localization of parvalbumin, calretinin, and calbindin D-28k in identified extraocular motoneurons and internuclear neurons of the cat

Calcium-binding proteins have been shown to be excellent markers of specific neuronal populations. We aimed to characterize the expression of calcium-binding proteins in identified populations of the cat extraocular motor nuclei by means of immunohistochemistry against parvalbumin, calretinin, and calbindin D-28k. Abducens, medial rectus, and trochlear motoneurons were retrogradely labeled with horseradish peroxidase from their corresponding muscles. Oculomotor and abducens internuclear neurons were retrogradely labeled after horseradish peroxidase injection into either the abducens or the oculomotor nucleus, respectively. Parvalbumin staining produced the highest density of immunoreactive terminals in all extraocular motor nuclei and was distributed uniformly. Around 15-20% of the motoneurons were moderately stained with antibody against parvalbumin, but their axons were heavily stained, indicating an intracellular segregation of parvalbumin. Colchicine administration increased the number of parvalbumin-immunoreactive motoneurons to approximately 85%. Except for a few calbindin-immunoreactive trochlear motoneurons (1%), parvalbumin was the only marker of extraocular motoneurons. Oculomotor internuclear neurons identified from the abducens nucleus constituted a nonuniform population, because low percentages of the three types of immunostaining were observed, calbindin being the most abundant (28.5%). Other interneurons located within the boundaries of the oculomotor nucleus were mainly calbindin-immunoreactive. The medial longitudinal fascicle contained numerous parvalbumin- and calretinin-immunoreactive but few calbindin-immunoreactive axons. The majority of abducens internuclear neurons projecting to the oculomotor nucleus (80.7%) contained calretinin. Moreover, the distribution of calretinin-immunoreactive terminals in the oculomotor nucleus overlapped that of the medial rectus motoneurons and matched the anterogradely labeled terminal field of the abducens internuclear neurons. Parvalbumin immunostained 42% of the abducens internuclear neurons. Colocalization of parvalbumin and calretinin was demonstrated in adjacent semithin sections, although single-labeled neurons were also observed. Therefore, calretinin is proven to be a good marker of abducens internuclear neurons. From all of these data, it is concluded that parvalbumin, calretinin, and calbindin D-28k selectively delineate certain neuronal populations in the oculomotor system and constitute valuable tools for further analysis of oculomotor function under normal and experimental conditions.

Localization of parvalbumin, calretinin, and calbindin D-28k in identified extraocular motoneurons and internuclear neurons of the cat

Calcium-binding proteins have been shown to be excellent markers of specific neuronal populations. We aimed to characterize the expression of calcium-binding proteins in identified populations of the cat extraocular motor nuclei by means of immunohistochemistry against parvalbumin, calretinin, and calbindin D-28k. Abducens, medial rectus, and trochlear motoneurons were retrogradely labeled with horseradish peroxidase from their corresponding muscles. Oculomotor and abducens internuclear neurons were retrogradely labeled after horseradish peroxidase injection into either the abducens or the oculomotor nucleus, respectively. Parvalbumin staining produced the highest density of immunoreactive terminals in all extraocular motor nuclei and was distributed uniformly. Around 15-20% of the motoneurons were moderately stained with antibody against parvalbumin, but their axons were heavily stained, indicating an intracellular segregation of parvalbumin. Colchicine administration increased the number of parvalbumin-immunoreactive motoneurons to approximately 85%. Except for a few calbindin-immunoreactive trochlear motoneurons (1%), parvalbumin was the only marker of extraocular motoneurons. Oculomotor internuclear neurons identified from the abducens nucleus constituted a nonuniform population, because low percentages of the three types of immunostaining were observed, calbindin being the most abundant (28.5%). Other interneurons located within the boundaries of the oculomotor nucleus were mainly calbindin-immunoreactive. The medial longitudinal fascicle contained numerous parvalbumin- and calretinin-immunoreactive but few calbindin-immunoreactive axons. The majority of abducens internuclear neurons projecting to the oculomotor nucleus (80.7%) contained calretinin. Moreover, the distribution of calretinin-immunoreactive terminals in the oculomotor nucleus overlapped that of the medial rectus motoneurons and matched the anterogradely labeled terminal field of the abducens internuclear neurons. Parvalbumin immunostained 42% of the abducens internuclear neurons. Colocalization of parvalbumin and calretinin was demonstrated in adjacent semithin sections, although single-labeled neurons were also observed. Therefore, calretinin is proven to be a good marker of abducens internuclear neurons. From all of these data, it is concluded that parvalbumin, calretinin, and calbindin D-28k selectively delineate certain neuronal populations in the oculomotor system and constitute valuable tools for further analysis of oculomotor function under normal and experimental conditions.

Effects of botulinum neurotoxin type A on abducens motoneurons in the cat: alterations of the discharge pattern

The discharge characteristics that abducens motoneurons exhibit after paralysis of the lateral rectus muscle with botulinum neurotoxin type A were studied in the alert cat. Antidromically identified motoneurons were recorded during both spontaneous and vestibularly induced eye movements. A single injection of 0.3 ng/kg produced a complete paralysis of the lateral rectus muscle lasting for about 12-15 days, whereas after 3 ng/kg the paralysis was still complete at the longest time checked, three months. Motoneurons recorded under the effect of the low dose showed differences in their sensitivities to both eye position and velocity according to the direction of the previous and ongoing movements, respectively. These directional differences could be explained by post-saccadic adaptation of the non-injected eye in the appropriate direction for reducing ocular misalignment. Thus, backward and forward post-saccadic drifts accompanied on- and off-directed saccades, respectively. The magnitude of the drift was similar to the magnitude of changes in eye position sensitivity. The discharge of the high-dose-treated motoneurons could be described in a three-stage sequence. During the initial 10-12 days, motoneuronal discharge resembled the effects of axotomy, particularly in the loss of tonic signals and the presence of exponential-like decay of firing after saccades. In this stage, the conduction velocity of abducens motoneurons was reduced by 21.4%. The second stage was characterized by an overall reduction in firing rate towards a tonic firing at 15-70 spikes/s. Motoneurons remained almost unmodulated for all types of eye movement and thus eye position and velocity sensitivities were significantly reduced. Tonic firing ceased only when the animal became drowsy, but was restored by alerting stimuli. In addition, the inhibition of firing for off-directed saccades was more affected than the burst excitation during on-directed saccades, since in many cells pauses were almost negligible. These alterations could not be explained by adaptational changes in the movement of the non-injected eye. Finally, after 60 days the initial stages of recovery were observed. The present results indicate that the high dose of botulinum neurotoxin produces effects on the motoneuron not attributable to the functional disconnection alone, but to a direct effect of the neurotoxin in the motoneuron and/or its synaptic inputs.

The vestibular system as a model of sensorimotor transformations. A combined in vivo and in vitro approach to study the cellular mechanisms of gaze and posture stabilization in mammals

To understand the cellular mechanisms underlying behaviours in mammals, the respective contributions of the individual properties characterizing each neuron, as opposed to the properties emerging from the organization of these neurons in functional networks, have to be evaluated. This requires the use, in the same species, of various in vivo and in vitro experimental preparations. The present review is meant to illustrate how such a combined in vivo in vitro approach can be used to investigate the vestibular-related neuronal networks involved in gaze and posture stabilization, together with their plasticity, in the adult guinea-pig. Following first a general introduction on the vestibular system, the second section describes various in vivo experiments aimed at characterizing gaze and posture stabilization in that species. The third and fourth parts of the review deal with the combined in vivo-in vitro investigations undertaken to unravel the physiological and pharmacological properties of vestibulo-ocular and vestibulo-spinal networks, together with their functional implications. In particular, we have tried to use the central vestibular neurons as examples to illustrate how the preparation of isolated whole brain can be used to bridge the gap between the results obtained through in vitro, intracellular recordings on slices and those collected in vivo, in the behaving animal.

Influence of the postsynaptic target on the functional properties of neurons in the adult mammalian central nervous system

In this review we have attempted to summarize present knowledge concerning the regulatory role of target cells on the expression and maintenance of the neuronal phenotype during adulthood. It is well known that in early developmental stages the survival of neurons is maintained by specific neurotrophic factors derived from their target tissues. Neuronal survival is not the only phenotype that is regulated by target-derived neurotrophic factors since the expression of electrophysiological and cytochemical properties of neurons is also affected. However, a good deal of evidence indicates that the survival of neurons becomes less dependent on their targets in the adult stage. The question is to what extent are target cells still required for the maintenance of the pre-existing or programmed state of the neuron; i.e., what is the functional significance of target-derived factors during maturity? Studies addressing this question comprise a variety of neuronal systems and technical approaches and they indicate that trophic interactions, although less apparent, persist in maturity and are most easily revealed by experimental manipulation. In this respect, research has been directed to analyzing the consequences of disconnecting a group of neurons from their target-by either axotomy or selective target removal using different neurotoxins-and followed (or not) by the implant of a novel target, usually a piece of embryonic tissue. Numerous alterations have been described as taking place in neurons following axotomy, affecting their morphology, physiology and metabolism. All these neuronal properties return to normal values when regeneration is successful and reinnervation of the target is achieved. Nevertheless, most of the changes persist if reinnervation is prevented by any procedure. Although axotomy may represent, besides target disconnection, a cellular lesion, alternative approaches (e.g., blockade of either the axoplasmic transport or the conduction of action potentials) have been used yielding similar results. Moreover, in the adult mammalian central nervous system, neurotoxins have been used to eliminate a particular target selectively and to study the consequences on the intact but target-deprived presynaptic neurons. Target depletion performed by excitotoxic lesions is not followed by retrograde cell death, but targetless neurons exhibit several modifications such as reduction in soma size and in the staining intensity for neurotransmitter-synthesizing enzymes. Recently, the oculomotor system has been used as an experimental model for evaluating the functional effects of target removal on the premotor abducens internuclear neurons whose motoneuronal target is destroyed following the injection of toxic ricin into the extraocular medial rectus muscle. The functional characteristics of these abducens neurons recorded under alert conditions simultaneously with eye movements show noticeable changes after target loss, such as a general reduction in firing frequency and a loss of the discharge signals related to eye position and velocity. Nevertheless, the firing pattern of these targetless abducens internuclear neurons recovers in parallel with the establishment of synaptic contacts on a presumptive new target: the small oculomotor internuclear neurons located in proximity to the disappeared target motoneurons. The possibility that a new target may restore neuronal properties towards a normal state has been observed in other systems after axotomy and is also evident from experiments of transplantation of immature neurons into the lesioned central nervous system of adult mammals. It can be concluded that although target-derived factors may not control neuronal survival in the adult nervous system, they are required for the maintenance of the functional state of neurons, regulating numerous aspects of neuronal structure, chemistry and electro-physiology.(ABSTRUCT TRUNCATED)

A quantitative study of electroencephalography, eye movements and neck electromyography characterizing the sleep-wake cycle of the guinea-pig

The qualitative and quantitative characteristics of cerebral cortex electrical activity, ocular motility and muscular activity were studied in six head-restrained guinea-pigs during wakefulness, slow-wave and paradoxical sleep. Animals were chronically implanted with bipolar electrodes in the obliquus capitis muscle for electromyographic recordings and epidurally through the parietal bones for electroencephalographic (EEG) recordings. Eye movements were recorded using the scleral search-coil technique. After postoperative recovery and a short period of habituation to immobilization, head-restrained animals exhibited a polyphasic sleep-wake cycle similar to what has already been described in the unrestrained guinea-pig. Paradoxical sleep periods of mean duration 110 +/- 42 s occurred at a mean interval of 32.2 +/- 7.2 min. Amplitude and frequency components of EEG activity were different for each state of vigilance. EEG amplitude was highest and frequency range lowest-with two well-defined peaks at 4 and 10 Hz-during slow-wave sleep. During paradoxical sleep, frequencies were higher and amplitudes lower than during wakefulness. Three types of eye movement intermingled with periods of ocular fixation were recorded: saccadic movements during wakefulness and paradoxical sleep, slow drifts during slow-wave sleep and paradoxical sleep, and a new type of eye movement-bursts of high-velocity eye oscillations during paradoxical sleep. Saccadic eye movements during paradoxical sleep were more frequent and showed higher velocities and amplitudes than during wakefulness. During paradoxical sleep the episodes of eye oscillation (8-14 Hz) occurred quite regularly every 1.6 s and had a mean duration of 1.4 s. During wakefulness, the obliquus muscle activity displayed a burst-tonic pattern. Bursting components were closely related to saccadic eye movements directed to the side of the recorded muscle. The muscle activity was predominantly tonic during slow-wave sleep and was completely absent during paradoxical sleep except for small bursts or twitches. These twitches were tightly synchronized with the occurrence of the rapid eye movements oriented towards the side of the recorded obliquus muscle, as during wakefulness. These results strongly suggest that paradoxical sleep is characterized by the oscillatory discharge of at least two neuronal populations: the brainstem saccadic generators and the tecto-reticular spinal network which underlies gaze-orienting behaviour during wakefulness. The occurrence of rhythmic discharges at approximately 11 Hz may explain the spinal motoneurons' inhibition during paradoxical sleep in order to avoid anarchic motor behavior. Whether these neuronal oscillations are simply an epiphenomenon or have functional implications remains to be determined.

Are laws that govern behavior embedded in the structure of the CNS? The case of Hering's law

The pattern of axonal terminations of individual premotoneuronal medium lead burst neurons (MLBs) has been elucidated with the help of the intraaxonal recording and horseradish peroxidase injection technique in alert behaving monkeys. These findings indicate that individual MLBs do not influence individual muscles; instead they influence groups of muscles. Horizontal medium lead burst neurons (EBNs) project to ipsilateral lateral rectus motoneurons directly, and could contact contralateral medial rectus motoneurons indirectly, through the internuclear neurons of the ipsilateral abducens nucleus. Also, upward MLBs project to motoneurons innervating both the superior rectus and the inferior oblique muscles of both eyes. Finally, downward MLBs project to ipsilateral motoneurons innervating the ipsilateral inferior rectus muscle and the contralateral superior oblique muscle. All in all, the pattern of oculomotor terminations of MLBs provides experimental support for Hering's law of equal innervation. EBNs also project to the nucleus prepositus hypoglossi while vertical MLBs also project to the interstitial nucleus of Cajal (NIC). These structures are thought to participate in the process of "velocity to position integration", in the horizontal and vertical planes respectively. Intracellular recording from individual neurons of the NIC in alert behaving monkeys followed by biocytin or horseradish peroxidase injections demonstrates that their axons pass through the posterior commissure on their way to vertical extraocular motoneurons. It also demonstrates that these fibers carry a phasic signal related to saccades and a tonic signal related to eye position in a precise quantitative manner. Work is in progress to determine whether their pattern of oculomotor terminations is also appropriate for Hering's law.

Effects of target depletion on adult mammalian central neurons: functional correlates

The physiological signals and patterns of synaptic connectivity that CNS neurons display after the loss of their target cells were evaluated in adult cats for one year. Abducens internuclear neurons were chosen as the experimental model because of their highly specific projection onto the medial rectus motoneurons of the oculomotor nucleus. Selective death of medial rectus motoneurons was induced by the injection into the medial rectus muscle of ricin, a potent cytotoxic lectin that leaves the presynaptic axons intact. The electrical activity of antidromically identified abducens internuclear neurons was recorded in chronic alert animals, during both spontaneous and vestibularly induced eye movements, before and after target removal. During the three weeks that followed ricin injection, abducens internuclear neurons exhibited several firing-related abnormal properties. There was an overall reduction in firing rate with a corresponding increase in the eye position threshold for recruitment. In addition, neuronal sensitivities to eye position and velocity were significantly decreased with respect to control data. Bursting activity was also altered since low-frequency delayed burst accompanied the saccades in the on-direction and, occasionally, internuclear neurons exhibited low-frequency discharges associated with off-directed saccades. Intracellular recordings carried out seven and 15 days after ricin injection demonstrated no significant changes in their electrical properties, although a marked depression of synaptic transmission was evident. The amplitude of both excitatory and inhibitory postsynaptic potentials of vestibular origin was reduced by 60-85% with respect to controls. However, postsynaptic potentials recorded one month after ricin injection showed normal amplitude values which persisted unaltered one year after target loss. Recovery of synaptic transmission occurred at the same time as the re-establishment of normal eye-related signals in the discharge pattern of abducens internuclear neurons recorded in alert cats from days 25-30 post lesion. The functional restoration of firing properties was maintained in the long term (one year). Conversely, abducens motoneurons showed normal firing and synaptic patterns at all time intervals analysed. These results demonstrate that, after an initial period of altered physiological properties, abducens internuclear neurons survive the loss of their target motoneurons and regain a normal discharge pattern and afferent synaptic connections.