GABAergic neurons comprise a major cell type in rodent visual relay nuclei: an immunocytochemical study of pretectal and accessory optic nuclei

Evolution of Local Circuit Neurons in Two Sensory Thalamic Nuclei in Amniotes

Local circuit neurons are present in the thalamus of all vertebrates where they are considered inhibitory. They play an important role in computation and influence the transmission of information from the thalamus to the telencephalon. In mammals, the percentage of local circuit neurons in the dorsal lateral geniculate nucleus remains relatively constant across a variety of species. In contrast, the numbers of local circuit neurons in the ventral division of the medial geniculate body in mammals vary significantly depending on the species examined. To explain these observations, the numbers of local circuit neurons were investigated by reviewing the literature on this subject in these two nuclei in mammals and their respective homologs in sauropsids and by providing additional data on a crocodilian. Local circuit neurons are present in the dorsal geniculate nucleus of sauropsids just as is the case for this nucleus in mammals. However, sauropsids lack local circuits neurons in the auditory thalamic nuclei homologous to the ventral division of the medial geniculate body. A cladistic analysis of these results suggests that differences in the numbers of local circuit neurons in the dorsal lateral geniculate nucleus of amniotes reflect an elaboration of these local circuit neurons as a result of evolution from a common ancestor. In contrast, the numbers of local circuit neurons in the ventral division of the medial geniculate body changed independently in several mammalian lineages.

Asymmetric retinal direction tuning predicts optokinetic eye movements across stimulus conditions

Across species, the optokinetic reflex (OKR) stabilizes vision during self-motion. OKR occurs when ON direction-selective retinal ganglion cells (oDSGCs) detect slow, global image motion on the retina. How oDSGC activity is integrated centrally to generate behavior remains unknown. Here, we discover mechanisms that contribute to motion encoding in vertically tuned oDSGCs and leverage these findings to empirically define signal transformation between retinal output and vertical OKR behavior. We demonstrate that motion encoding in vertically tuned oDSGCs is contrast-sensitive and asymmetric for oDSGC types that prefer opposite directions. These phenomena arise from the interplay between spike threshold nonlinearities and differences in synaptic input weights, including shifts in the balance of excitation and inhibition. In behaving mice, these neurophysiological observations, along with a central subtraction of oDSGC outputs, accurately predict the trajectories of vertical OKR across stimulus conditions. Thus, asymmetric tuning across competing sensory channels can critically shape behavior.

Brain-wide mapping of inputs to the mouse lateral posterior (LP/Pulvinar) thalamus-anterior cingulate cortex network

The rodent homolog of the primate pulvinar, the lateral posterior (LP) thalamus, is extensively interconnected with multiple cortical areas. While these cortical interactions can span the entire LP, subdivisions of the LP are characterized by differential connections with specific cortical regions. In particular, the medial LP has reciprocal connections with frontoparietal cortical areas, including the anterior cingulate cortex (ACC). The ACC plays an integral role in top‐down sensory processing and attentional regulation, likely exerting some of these functions via the LP. However, little is known about how ACC and LP interact, and about the information potentially integrated in this reciprocal network. Here, we address this gap by employing a projection‐specific monosynaptic rabies tracing strategy to delineate brain‐wide inputs to bottom‐up LP→ACC and top‐down ACC→LP neurons. We find that LP→ACC neurons receive inputs from widespread cortical regions, including primary and higher order sensory and motor cortical areas. LP→ACC neurons also receive extensive subcortical inputs, particularly from the intermediate and deep layers of the superior colliculus (SC). Sensory inputs to ACC→LP neurons largely arise from visual cortical areas. In addition, ACC→LP neurons integrate cross‐hemispheric prefrontal cortex inputs as well as inputs from higher order medial cortex. Our brain‐wide anatomical mapping of inputs to the reciprocal LP‐ACC pathways provides a roadmap for understanding how LP and ACC communicate different sources of information to mediate attentional control and visuomotor functions.

Distinct purinergic receptor-mediated currents of rat oculomotor integrator neurons characterized by different firing patterns

The prepositus hypoglossi nucleus (PHN) and the interstitial nucleus of Cajal (INC) are oculomotor neural integrators involved in the control of horizontal and vertical gaze, respectively. We previously reported that local application of adenosine 5'-trisphosphate (ATP) to PHN neurons induced P2X receptor-mediated fast inward currents, P2Y receptor-mediated slow inward currents, and/or adenosine P1 receptor-mediated slow outward currents. In contrast to the findings on PHN neurons, the expression of functional purinergic receptors in INC neurons has not been examined. In this study, we investigated ATP-induced current responses in INC neurons and the distributions of the three current types across distinct firing patterns in PHN and INC neurons using whole cell recordings of rat brainstem slices. The application of ATP induced all three current types in INC neurons. Pharmacological analyses indicated that the fast inward and slow outward currents were mainly mediated by the P2X and P1 subtypes, respectively, corresponding to the receptor subtypes in PHN neurons. However, agonists of the P2Y subtype did not induce the slow inward current in INC neurons, suggesting that other subtypes or mechanisms are responsible for this current. Analysis of the distribution of the three current types in PHN and INC neurons revealed that the proportions of the currents were distinctly dependent on the firing patterns of PHN neurons whereas the proportion of the fast inward current was higher during all firing patterns of INC neurons. The different distributions of ATP-induced currents suggest distinct modes of purinergic modulation specific to horizontal and vertical integrators.NEW & NOTEWORTHY The roles of purinergic signaling on vertical (mediated by the interstitial nucleus of Cajal; INC) and horizontal (prepositus hypoglossal nucleus; PHN) gaze control are not understood. Here, we report three current types induced by ATP in INC neurons; the distribution of these current types across different types of INC neurons is different from that in PHN neurons. These results suggest distinct modes of purinergic modulation in horizontal and vertical gaze control centers.

Global Neurotoxicity: Quantitative Analysis of Rat Brain Toxicity Following Exposure to Trimethyltin

The organotin, trimethyltin (TMT), is a highly toxic compound. In this study, silver-stained rat brain sections were qualitatively and quantitatively evaluated for degeneration after systemic treatment with TMT. Degenerated neurons were counted using image analysis methods available in the HALO image analysis software. Specific brain areas including the cortex, inferior and superior colliculus, and thalamus were quantitatively analyzed. Our results indicate extensive and widespread damage to the rat brain after systemic administration of TMT. Qualitative results suggest severe TMT-induced toxicity 3 and 7 days after the administration of TMT. Trimethyltin toxicity was greatest in the hippocampus, olfactory area, cerebellum, pons, mammillary nucleus, inferior and superior colliculus, hypoglossal nucleus, thalamus, and cerebellar Purkinje cells. Quantification showed that the optic layer of the superior colliculus exhibited significantly more degeneration compared to layers above and below. The inferior colliculus showed greater degeneration in the dorsal area relative to the central area. Similarly, in cortical layers, there was greater neurodegeneration in deeper layers compared to superficial layers. Quantification of damage in various thalamic nuclei showed that the greatest degeneration occurred in midline and intralaminar nuclei. These results suggest selective neuronal network vulnerability to TMT-related toxicity in the rat brain.

Comparisons of Neuronal and Excitatory Network Properties between the Rat Brainstem Nuclei that Participate in Vertical and Horizontal Gaze Holding

Gaze holding is primarily controlled by neural structures including the prepositus hypoglossi nucleus (PHN) for horizontal gaze and the interstitial nucleus of Cajal (INC) for vertical and torsional gaze. In contrast to the accumulating findings of the PHN, there is no report regarding the membrane properties of INC neurons or the local networks in the INC. In this study, to verify whether the neural structure of the INC is similar to that of the PHN, we investigated the neuronal and network properties of the INC using whole-cell recordings in rat brainstem slices. Three types of afterhyperpolarization (AHP) profiles and five firing patterns observed in PHN neurons were also observed in INC neurons. However, the overall distributions based on the AHP profile and the firing patterns of INC neurons were different from those of PHN neurons. The application of burst stimulation to a nearby site of a recorded INC neuron induced an increase in the frequency of spontaneous EPSCs. The duration of the increased EPSC frequency of INC neurons was not significantly different from that of PHN neurons. The percent of duration reduction induced by a Ca²⁺-permeable AMPA (CP-AMPA) receptor antagonist was significantly smaller in the INC than in the PHN. These findings suggest that local excitatory networks that activate sustained EPSC responses also exist in the INC, but their activation mechanisms including the contribution of CP-AMPA receptors differ between the INC and the PHN.

Parallel information processing channels created in the retina

In the retina, several parallel channels originate that extract different attributes from the visual scene. This review describes how these channels arise and what their functions are. Following the introduction four sections deal with these channels. The first discusses the "ON" and "OFF" channels that have arisen for the purpose of rapidly processing images in the visual scene that become visible by virtue of either light increment or light decrement; the ON channel processes images that become visible by virtue of light increment and the OFF channel processes images that become visible by virtue of light decrement. The second section examines the midget and parasol channels. The midget channel processes fine detail, wavelength information, and stereoscopic depth cues; the parasol channel plays a central role in processing motion and flicker as well as motion parallax cues for depth perception. Both these channels have ON and OFF subdivisions. The third section describes the accessory optic system that receives input from the retinal ganglion cells of Dogiel; these cells play a central role, in concert with the vestibular system, in stabilizing images on the retina to prevent the blurring of images that would otherwise occur when an organism is in motion. The last section provides a brief overview of several additional channels that originate in the retina.

Contribution of GABA(C) receptors to inhibition in the rodent accessory optic system

The medial terminal nucleus (MTN) of the mammalian accessory optic system controls vertical compensatory eye movements. It consists of two neuronal populations which respond best either to upward or to downward visual image shifts. The two cell classes are located spatially separate in the dorsal or in the ventral subdivision of the MTN, respectively. Pronounced GABAergic pathways have been described to exist between neurons in the two MTN subdivisions indicating that inhibitory interactions play a significant role for the generation of MTN cell response properties. Yet, the types of GABA receptors which mediate these inhibitory interactions are unknown. Functionally, it is of particular interest to know whether GABA(C) receptors, as in other subcortical visual centers, participate in inhibitory mechanisms in MTN neurons. We therefore performed whole-cell patch clamp recordings from MTN neurons in acute mouse midbrain slices. We monitored excitatory and inhibitory postsynaptic responses to afferent stimulation and applied specific GABA receptor agonists and antagonists to identify the GABA receptor types present in MTN neurons. We found that more than 80% of the neurons in both MTN subdivisions express functional GABA(C) receptors that can be activated by specific receptor agonists. A blockade of GABA(C) receptors, on the other hand, either reduced or enhanced postsynaptic inhibition, indicating that both postsynaptic and presynaptic functions are served by this receptor type. This, together with earlier results, suggests that GABA(C) receptors play a general role for the control of neuronal excitability in subcortical visual pathways.

The commissural transfer of the horizontal optokinetic signal in the rat: a c-Fos study

We applied the Fos method in rats subjected to horizontal optokinetic stimulation (OKS) to study whether optokinetic information is transferred through the commissural pretectal fibres from one optic tract nucleus (NOT) to the opposite. In binocular as well as in monocular nasalward OKS, the highest Fos immunoreactivity was found in the NOT contralateral to the nasalward stimulation, as expression of the activation either of direction-selective cells and of commissural neurons. Even the opposite NOT showed many Fos-positive cells activated by the opposite nucleus throughout the commissural pretectal pathway. They might be the GABA positive cells, which are thought to allow the activation in one nucleus to be transformed into inhibition of the opposite side. In monocular temporalward OKS, the inhibition on direction-selective cells and the consequent silencing of commissural neurons caused the faint immunoreactivity in the NOT contralateral to eye stimulated. In the opposite nucleus the few Fos-positive cells emerged as a consequence of the lack of the normal tonic commissurally mediated inhibition.

Inferior olive and oculomotor system

Three subnuclei within the inferior olive are implicated in the control of eye movement; the dorsal cap (DC), the beta-nucleus and the dorsomedial cell column (DMCC). Each of these subnuclei can be further divided into clusters of cells that encode specific parameters of optokinetic and vestibular stimulation. DC neurons respond to optokinetic stimulation in one of three planes, corresponding to the anatomical planes of the semicircular canals. Neurons in the beta-nucleus and DMCC respond to vestibular stimulation in the planes of the vertical semicircular canals and otoliths. Each these olivary nuclei receives excitatory and inhibitory signals from pre-olivary structures. The DC receives excitatory signals from the ipsilateral nucleus of the optic tract (NOT) and inhibitory signals from the contralateral nucleus prepositus hypoglossi (NPH). The beta-nucleus and DMCC receive inhibitory signals from the ipsilateral nucleus parasolitarius (Psol) and excitatory signals from the contralateral dorsal Y group. Consequently, the olivary projection to the cerebellum, although totally crossed, still represents bilateral sensory stimulation. Inputs to the inferior olive from the NOT, NPH, Psol or Y-group discharge at frequencies of 10-100 imp/s. CFRs discharge at 1-5 imp/s; a frequency reduction of an order of magnitude. Inferior olivary projections to the contralateral cerebellum are sagittally arrayed onto multiple cerebellar folia. These arrays establish coordinate systems in the flocculus and nodulus, representing head-body movement. These climbing fiber-defined spatial coordinate systems align Purkinje cell discharge onto subjacent cerebellar and vestibular nuclei. In the oculomotor system, olivo-cerebellar circuitry enhances and modifies eye movements based on movement of the head-body in space.

The accessory optic system: basic organization with an update on connectivity, neurochemistry, and function

The accessory optic system (AOS) is formed by a series of terminal nuclei receiving direct visual information from the retina via one or more accessory optic tracts. In addition to the retinal input, derived from ganglion cells that characteristically have large receptive fields, are direction-selective, and have a preference for slow moving stimuli, there are now well-characterized afferent connections with a key pretectal nucleus (nucleus of the optic tract) and the ventral lateral geniculate nucleus. The efferent connections of the AOS are robust, targeting brainstem and other structures in support of visual-oculomotor events such as optokinetic nystagmus and visual-vestibular interaction. This chapter reviews the newer experimental findings while including older data concerning the structural and functional organization of the AOS. We then consider the ontogeny and phylogeny of the AOS and include a discussion of similarities and differences in the anatomical organization of the AOS in nonmammalian and mammalian species. This is followed by sections dealing with retinal and cerebral cortical afferents to the AOS nuclei, interneuronal connections of AOS neurons, and the efferents of the AOS nuclei. We conclude with a section on Functional Considerations dealing with the issues of the response properties of AOS neurons, lesion and metabolic studies, and the AOS and spatial cognition.

The mammalian superior colliculus: laminar structure and connections

The superior colliculus is a laminated midbrain structure that acts as one of the centers organizing gaze movements. This review will concentrate on sensory and motor inputs to the superior colliculus, on its internal circuitry, and on its connections with other brainstem gaze centers, as well as its extensive outputs to those structures with which it is reciprocally connected. This will be done in the context of its laminar arrangement. Specifically, the superficial layers receive direct retinal input, and are primarily visual sensory in nature. They project upon the visual thalamus and pretectum to influence visual perception. These visual layers also project upon the deeper layers, which are both multimodal, and premotor in nature. Thus, the deep layers receive input from both somatosensory and auditory sources, as well as from the basal ganglia and cerebellum. Sensory, association, and motor areas of cerebral cortex provide another major source of collicular input, particularly in more encephalized species. For example, visual sensory cortex terminates superficially, while the eye fields target the deeper layers. The deeper layers are themselves the source of a major projection by way of the predorsal bundle which contributes collicular target information to the brainstem structures containing gaze-related burst neurons, and the spinal cord and medullary reticular formation regions that produce head turning.

The pretectum: connections and oculomotor-related roles

Research over the past two decades in mammals, especially primates, has greatly improved our understanding of the afferent and efferent connections of two retinorecipient pretectal nuclei, the nucleus of the optic tract (NOT) and the pretectal olivary nucleus (PON). Functional studies of these two nuclei have further elucidated some of the roles that they play both in oculomotor control and in relaying oculomotor-related signals to visual relay nuclei. Therefore, following a brief overview of the anatomy and retinal projections to the entire mammalian pretectum, the connections and potential roles of the NOT and the PON are considered in detail. Data on the specific connections of the NOT are combined with data from single-unit recording, microstimulation, and lesion studies to show that this nucleus plays critical roles in optokinetic nystagmus, short-latency ocular following, smooth pursuit eye movements, and adaptation of the gain of the horizontal vestibulo-ocular reflex. Comparable data for the PON show that this nucleus plays critical roles in the pupillary light reflex, light-evoked blinks, rapid eye movement sleep triggering, and modulating subcortical nuclei involved in circadian rhythms.

Localization of GABA (gamma-aminobutyric acid) markers in the turtle's basal optic nucleus

Recent physiological data have demonstrated that retinal slip, the sensory code of global visual pattern motion, results from complex interactions of excitatory and inhibitory visual inputs to neurons in the turtle's accessory optic system (the basal optic nucleus, BON). In the present study, the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), its synthetic enzyme, glutamic acid decarboxylase (GAD-67) and its receptor subtypes GABA(A) and GABA(B) receptors were localized within the BON. GABA antibodies revealed cell bodies and processes, whereas antibodies against GAD revealed a moderate density of immunoreactive puncta throughout the BON. GAD in situ hybridization labeled BON cell bodies, indicating a possible source of inhibition intrinsic to the nucleus. Ultrastructural analysis revealed terminals positive for GAD that exhibit symmetric synaptic specializations, mainly at neuronal processes having small diameters. Neurons exhibiting immunoreactivity for GABA(A) receptors were diffusely labeled throughout the BON, with neuronal processes exhibiting more labeling than cell bodies. In contrast, GABA(B)-receptor-immunoreactive neurons exhibited strong labeling at the cell body and proximal neuronal processes. Both these receptor subtypes are functional, as evidenced by changes of visual responses of BON neurons during application to the brainstem of selective receptor agonists and antagonists. Therefore, GABA may be synthesized by BON neurons, released by terminals within its neuropil and stimulate both receptor subtypes, supporting its role in mediating visually evoked inhibition contributing to modulation of the retinal slip signals in the turtle accessory optic system.

GABAC receptors in the rat superior colliculus and pretectum participate in synaptic neurotransmission

In mammals, GABA(C) receptors seem to be specifically expressed in the retina and the subcortical visual system, with highest extraretinal expression levels in the superior colliculus (SC). Although its presence in the superficial SC has been demonstrated physiologically, a direct involvement of this receptor type in fast synaptic neurotransmission still awaits verification. We addressed the question of a possible synaptic localization of GABA(C) receptors by performing in vitro whole-cell patch-clamp recordings of inhibitory postsynaptic currents (IPSCs) in single neurons of the rat SC and the neighboring pretectal nuclear complex, where GABA(C) receptors are also expressed at significant levels. To increase the likelihood to record IPSCs we induced spontaneous activity by application of the potassium channel blocker 4-aminopyridine (4-AP) and blocked glutamate-mediated excitatory neurotransmission with kynurenic acid. All 4-AP-induced postsynaptic currents were of synaptic origin because they were completely suppressed by lidocaine or by substitution of extracellular calcium with cobalt. In 40% of the SC cells and in 60% of the pretectal neurons, IPSCs in the presence of 4-AP and kynurenic acid were only partly blocked by the selective GABA(A) receptor antagonist bicuculline. Inhibitory currents that were insensitive to bicuculline, however, could be blocked by coapplication of either the specific GABA(C) receptor antagonist 1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid or picrotoxin, an unselective GABA(A) and GABA(C) receptor antagonist. We conclude that GABA(C) receptors are, at least partially, located synaptically in SC and pretectal neurons in the rat, which indicates a direct function of this receptor type for synaptic processing in both structures.

Mesencephalic and diencephalic afferent connections to the thalamic nucleus rotundus in the lizard, Psammodromus algirus

The present work is an analysis of the afferent projections to the thalamic nucleus rotundus in a lizard, both at the light- and electron-microscopic level, using biotinylated dextran amine (BDA) as a neuroanatomical tracer. This study has confirmed previously reported afferent projections to nucleus rotundus in reptiles and has also identified a number of new cellular aggregates projecting to this dorsal thalamic nucleus. After BDA injections into nucleus rotundus, retrogradely labelled neurons were observed consistently within the following neuronal groups in the midbrain and the diencephalon: (i) the stratum griseum centrale of the optic tectum; (ii) the nucleus subpretectalis in the pretectum; (iii) the nucleus ansa lenticularis posterior, the posterior nucleus of the ventral supraoptic commissure, and the posteroventral nucleus, in the dorsal thalamus and (iv) the lateral suprachiasmatic nucleus and part of the reticular complex in the ventral thalamus. Tectal axons entering nucleus rotundus were fine and varicose and formed exclusively asymmetric synaptic contacts, mainly on small dendritic profiles. Rotundal neurons had symmetric synapses made by large boutons probably of nontectal origin. After comparing our results with those in other reptiles, birds and mammals, we propose that the sauropsidian nucleus rotundus forms part of a visual tectofugal pathway that conveys mesencephalic visual information to the striatum and dorsal ventricular ridge, and is similar to the mammalian colliculo-posterior/intralaminar-striatoamygdaloid pathway, the function of which may be to participate in visually guided behaviour.

Postnatal development of the retinal projection to the nucleus of the optic tract and accessory optic nuclei in the hooded rat

Retinal projections to the nucleus of the optic tract (NOT) and accessory optic nuclei (AON) were studied in the postnatal hooded rat after monocular injection of cholera toxin B subunit (CTB) into the vitreous chamber of the eye. At all postnatal ages, retinal axons were labeled sensitively; they revealed dense projections to the contralateral, and sparse but distinct projections to the ipsilateral, NOT and AON. The CTB labeling enabled the first delineation of the complete morphology of developing retinal axons in the ipsilateral NOT and AON. From postnatal day (P) 1 to P3, axons with complex growth cones were seen, and unbranched collaterals with simple growth cones increased and extended gradually. At P6, complex growth cones disappeared while branched collaterals with simple growth cones as well as small-sized varicosities increased. By P12 (two days before eye-opening) the adult-like pattern of terminal arbors appeared. The branched collaterals with tiny, small-sized varicosities present probably represented developing synaptic boutons. At P16 (after eye opening), the pattern of terminal arbors was well developed, almost to the same extent as in the adult. By contrast, a broadly distributed, transient retinal projection around NOT and AON was gradually eliminated; it started to disappear during the first few postnatal days, and was fully retracted by the time of eye-opening time to a pattern normal for the adult.

Pretectal connections in turtles with special reference to the visual thalamic centers: a hodological and gamma-aminobutyric acid-immunohistochemical study

Projections of the pretectal region to forebrain and midbrain structures were examined in two species of turtles (Testudo horsfieldi and Emys orbicularis) by axonal tracing and immunocytochemical methods. Two ascending gamma-aminobutyric acid (GABA)ergic pathways to thalamic visual centers were revealed: a weak projection from the retinorecipient nucleus lentiformis mesencephali to the ipsilateral nucleus geniculatus lateralis pars dorsalis and a considerably stronger projection from the nonretinorecipient nucleus pretectalis ventralis to the nucleus rotundus. The latter is primarily ipsilateral, with a weak contralateral component. The interstitial nucleus of the tectothalamic tract is also involved in reciprocal projections of the pretectum and nucleus rotundus. In addition, the pretectal nuclei project reciprocally to the optic tectum and possibly to the telencephalic isocortical homologues. Comparison of these findings with previous work on other species reveals striking similarities between the pretectorotundal pathway in turtles and birds and in the pretectogeniculate pathway in turtles, birds, and mammals.

Synaptic and neurochemical features of calcitonin gene-related peptide containing neurons in the rat accessory optic nuclei

Within the rodent visual system, calcitonin gene-related peptide (CGRP) is selectively expressed in neurons in the accessory optic nuclei (AON), including the dorsal terminal nucleus (DTN), lateral terminal nucleus (LTN) and medial terminal nucleus (MTN). To determine whether CGRP-immunoreactive neurons are involved in visual circuitry, electron microscopic preparations were analyzed from normal rats and rats with optic nerve transections. A co-localization analysis was also made because CGRP-labeled neurons had features of GABAergic neurons. Thus, sections were prepared for light microscopy to determine whether CGRP-containing neurons also had glutamate decarboxylase (GAD) and other markers for GABAergic neurons, such as calcium binding proteins: calbindin (CB), calretinin (CR) and parvalbumin (PV). Electron microscopy of the DTN and LTN showed CGRP-labeled somata and dendrites that were postsynaptic to axon terminals forming asymmetric synapses. Many of these axon terminals degenerated following optic nerve transection indicating that retinal ganglion cells form synapses with CGRP-labeled neurons in the AON. In the DTN, LTN and MTN, CGRP-labeled axon terminals formed symmetric synapses with unlabeled somata as well as dendritic shafts and spines. Consistent with this type of synapse being GABAergic were the co-localization data showing that about 90% of the CGRP-labeled neurons co-localized GAD in the AON. Many CGRP-labeled neurons showed immunostaining for CR (40%) whereas only a few had labeling for CB (5%). No CGRP-labeled neurons had PV. These data show that CGRP-containing neurons receive direct retinal input and represent a subpopulation of GABAergic neurons which differentially co-express calcium-binding proteins.

On the functional anatomy of the nucleus of the optic tract-dorsal terminal nucleus commissural connection in the opossum (Didelphis marsupialis aurita)

Immunocytochemical methods revealed the presence of GABA in cell bodies and terminals in the nucleus of the optic tract-dorsal terminal nucleus, the medial terminal nucleus, the lateral terminal nucleus and the interstitial nucleus of the superior fasciculus of the opossum (Didelphis marsupialis aurita). Moreover, after unilateral injections of rhodamine beads in the nucleus of the optic tract-dorsal terminal nucleus complex and processing for GABA, double-labelled cells were detected in the ipsilateral complex, up to 400 microns from the injected site, but not in the opposite. Analysis of the distributions of GABAergic and retrogradely-labelled cells throughout the contralateral nucleus of the optic tract-dorsal terminal nucleus showed that the highest density of GABAergic and rhodamine-labelled cells overlapped at the middle third of the complex. Previous electrophysiological data obtained in the opossum had suggested the existence, under certain conditions, of an inhibitory action between the nucleus of the optic tract-dorsal terminal nucleus of one side over the other. The absence of GABAergic commissural neurons may imply that this inhibition is mediated by an excitatory commissural pathway that activates GABAergic interneurons.

Efferent pathways of the nucleus of the optic tract in monkey and their role in eye movements

To clarify the role of the pretectal nucleus of the optic tract (NOT) in ocular following, we traced NOT efferents with tritiated leucine in the monkey and identified the cell groups they targeted. Strong local projections from the NOT were demonstrated to the superior colliculus and the dorsal terminal nucleus bilaterally and to the contralateral NOT. The contralateral oculomotor complex, including motoneurons (C-group) and subdivisions of the Edinger-Westphal complex, including motoneurons (C-group) and subdivisions of the Edinger-Westphal complex, also received inputs. NOT efferents terminated in all accessory optic nuclei (AON) ipsilaterally; contralateral AON projections arose from the pretectal olivary nucleus embedded in the NOT. Descending pathways contacted precerebellar nuclei: the dorsolateral and dorsomedial pontine nuclei, the nucleus reticularis tegmenti pontis, and the inferior olive. Direct projections from NOT to the ipsilateral nucleus prepositus hypoglossi (ppH) appeared to be weak, but retrograde tracer injections into rostral ppH verified this projection; furthermore, the injections demonstrated that AON efferents also enter this area. Efferents from the NOT also targeted ascending reticular networks from the pedunculopontine tegmental nucleus and the locus coeruleus. Rostrally, NOT projections included the magnocellular layers of the lateral geniculate nucleus (lgn); the pregeniculate, peripeduncular, and thalamic reticular nuclei; and the pulvinar, the zona incerta, the mesencephalic reticular formation, the intralaminar thalamic nuclei, and the hypothalamus. The NOT could generate optokinetic nystagmus through projections to the AON, the ppH, and the precerebellar nuclei. However, NOT also projects to structures controlling saccades, ocular pursuit, the near response, lgn motion sensitivity, visual attention, vigilance, and gain modification of the vestibulo-ocular reflex. Any hypothesis on the function of NOT must take into account its connectivity to all of these visuomotor structures.

Dendritic morphology of projection neurons in the cat pretectum

The distribution and dendritic morphology of neurons in the cat pretectal nuclear complex were analyzed with respect to their projection to the ipsilateral dorsal lateral geniculate nucleus (LGNd) and the ipsilateral inferior olive (IO). Single and double retrograde tracing techniques were combined with intracellular injections of either horseradish peroxidase into electrophysiologically identified pretectal neurons or Lucifer Yellow into retrogradely labeled somata. Pretectal cells afferent to the LGNd were located in the nucleus of the optic tract (NOT), adjacent dorsal terminal nucleus of the accessory optic system (DTN), and posterior pretectal nucleus (NPP). Cells projecting to the IO were also distributed throughout the NOT-DTN and dorsal part of the NPP. Separate tracer injections (fluorogold and horseradish peroxidase [HRP] or granular blue) into the LGNd and the IO showed considerable overlap of labeled neurons in the NOT and dorsal NPP. Double-labeled neurons, however, were not observed after double tracer injections into LGNd and IO. Partial topographical segregation of the two populations was observed along the dorsoventral axis because LGNd-projecting neurons exhibited maximum density ventral to that of IO neurons. Pretectal cells to the LGNd had cell body diameters between 16 and 48 microns. Somatic shapes varied between fusiform and multipolar with considerable overlap between these two morphological appearances. Neurons projecting to the IO exhibited similar cell body sizes and their morphology also varied from fusiform to multipolar. Quantitative analysis of dendritic field size and orientation, number and order of dendritic arborizations, and symmetry of the dendritic tree revealed no statistically significant difference between the two neuronal populations. Hence, neurons of the two populations cannot be unequivocally identified just from the dendritic morphology. By contrast, dendritic morphology was correlated with the topographical location of either cell type within the pretectal nuclei rather than projection. Thus, the morphological appearance of neurons located dorsally predominantly was fusiform while neurons located ventrally mostly were multipolar.

Projection from the accommodation-related area in the superior colliculus of the cat

Our previous study has indicated that accommodative responses can be evoked with weak currents applied to a circumscribed area of the superior colliculus in the cat. We investigated efferent projections from this area with biocytin in the present study. The accommodation area in the superior colliculus was identified by systematic microstimulation in each of five anesthetized cats. Accommodative responses were detected by an infrared optometer. After mapping the superior colliculus, biocytin was injected through a glass micropipette into the accommodation area, where accommodative responses were elicited with low-intensity microstimulation. In addition, accommodative responses to stimulation of the superior colliculus were compared before and after an injection of muscimol, an agonist of inhibitory neurotransmitter, into the pretectum. Following the injection of biocytin, in the ascending projections, labeled terminals were seen mainly in the caudal portion of the nucleus of the optic tract, the nucleus of the posterior commissure, the posterior pretectal nucleus, the olivary pretectal nucleus, the mesencephalic reticular formation at the level of the oculomotor nucleus, and the lateral posterior nucleus of the thalamus on the ipsilateral side. Less dense terminals were seen in the anterior pretectal nucleus, the zona incerta, and the centromedian nucleus of the thalamus. In the descending projections, labeled terminals were observed mainly in the paramedian pontine reticular formation, the nucleus raphe interpositus, and the dorsomedial portion of the nucleus reticularis tegmenti pontis on the contralateral side. Less dense terminals were also seen in the nucleus of the brachium of the inferior colliculus, the cuneiform nucleus, the medial part of the paralemniscal tegmental field, and the dorsolateral division of the pontine nuclei on the ipsilateral side. Following the injection of muscimol into the pretectum, including the nucleus of the optic tract, the posterior pretectal nucleus, and the nucleus of the posterior commissure, accommodative responses evoked by microstimulation of the superior colliculus were reduced to 33-55% of the value before the injections. These findings suggest that the accommodation area in the superior colliculus projects to the oculomotor nucleus through the ipsilateral pretectal area, especially the nucleus of the optic tract, the nucleus of posterior commissure, and the posterior pretectal nucleus, and also projects to the pupilloconstriction area (the olivary pretectal nucleus), the vergence-related area (the mesencephalic reticular formation), and the active visual fixation-related area (the nucleus raphe interpositus).

Patterns of transmitter labelling and connectivity of the cat's nucleus of Darkschewitsch: a wheat germ agglutinin-horseradish peroxidase and immunocytochemical study at light and electron microscopical levels

Immunocytochemical studies using antibodies raised against a number of probable synaptic transmitters of the mesodiencephalic area, and fibre-tracing studies using wheat germ agglutinin-horseradish peroxidase (WGA-HRP), have been performed in adult cats. Glutamate and aspartate immunoreactivity produced a strong labelling of many cell bodies and terminals in the nucleus of Darkschewitsch (ND). gamma-Aminobutyrate (GABA) immunoreactivity in the ND appeared as a moderate label in some small neurones, and as a strong label in a few glial-like cells, in addition to being present in high levels to produce strong labelling in many GABA-immunopositive terminals that possessed pleomorphic vesicles. Some choline acetyltransferase-positive terminals and dendrites and a few substance P-positive fine fibres possessing varicosities also were observed in the ND. Following WGA-HRP injection in the ND, dense terminal labelling was seen ipsilaterally in the rostral half of the medial accessory olive, suggesting that there may be a certain degree of mediolateral and dorsoventral topographic correspondance within the ND-olive projection. In the same cases, many cell bodies containing HRP reaction product also were found 1) ipsilaterally in the motor cortex, anterior pretectal nucleus, and a restricted area of the caudal part of the substantia nigra pars reticulata; 2) contralaterally in the anterior and posterior interposed cerebellar nuclei as well as in a portion of the lateral cerebellar nucleus; and 3) bilaterally in the zona incerta, the posterior pretectal nucleus, the pedunculopontine tegmental nuclei, the spinal trigeminal nucleus, the dorsal column nuclei, and the spinal cord. Details of the interrelationships and functional considerations amongst the ND, adjacent nuclei, and longitudinal zones of the cerebellum are discussed.

Presynaptic localization of mu-opioid receptor-like immunoreactivity in retinal axon terminals within the terminal nuclei of the accessory optic tract: a light and electron microscope study in the rat

Neuropil within the terminal nuclei of the accessory optic tract of the rat showed intense to moderate mu-opioid receptor-like immunoreactivity (MOR-LI). After unilateral enucleation, MOR-LI within the terminal nuclei almost disappeared or was markedly reduced on the side contralateral to the operation. Electron microscopy revealed that MOR-LI axon terminals within the terminal nuclei were filled with round synaptic vesicles and in asymmetric synaptic contact mainly with dendritic profiles, and occasionally with somatic profiles.

Projections of the lateral terminal accessory optic nucleus of the common marmoset (Callithrix jacchus)

The connections of the lateral terminal nucleus (LTN) of the accessory optic system (AOS) of the marmoset monkey were studied with anterograde 3H-amino acid light autoradiography and horseradish peroxidase retrograde labeling techniques. Results show a first and largest LTN projection to the pretectal and AOS nuclei including the ipsilateral nucleus of the optic tract, dorsal terminal nucleus, and interstitial nucleus of the superior fasciculus (posterior fibers); smaller contralateral projections are to the olivary pretectal nucleus, dorsal terminal nucleus, and LTN. A second, major bundle produces moderate-to-heavy labeling in all ipsilateral, accessory oculomotor nuclei (nucleus of posterior commissure, interstitial nucleus of Cajal, nucleus of Darkschewitsch) and nucleus of Bechterew; some of the fibers are distributed above the caudal oculomotor complex within the supraoculomotor periaqueductal gray. A third projection is ipsilateral to the pontine and mesencephalic reticular formations, nucleus reticularis tegmenti pontis and basilar pontine complex (dorsolateral nucleus only), dorsal parts of the medial terminal accessory optic nucleus, ventral tegmental area of Tsai, and rostral interstitial nucleus of the medial longitudinal fasciculus. Lastly, there are two long descending bundles: (1) one travels within the medial longitudinal fasciculus to terminate in the dorsal cap (ipsilateral >> contralateral) and medial accessory olive (ipsilateral only) of the inferior olivary complex. (2) The second soon splits, sending axons within the ipsilateral and contralateral brachium conjunctivum and is distributed to the superior and medial vestibular nuclei. The present findings are in general agreement with the documented connections of LTN with brainstem oculomotor centers in other species. In addition, there are unique connections in marmoset monkey that may have developed to serve the more complex oculomotor behavior of nonhuman primates.

Distribution of GAD-like immunoreactivity in the retina and central visual system of Rana pipiens

Immunohistochemistry was used to survey the distribution of glutamic acid decarboxylase (GAD), the synthesizing enzyme for gamma-aminobutyric acid (GABA), throughout the visual system of the frog Rana pipiens. GAD-like immunoreactivity (GAD-LI) was assessed in the retina, in primary retinorecipient targets, and in thalamic nuclear groups postsynaptic to primary retinal terminal zones. Within the retina, the inner plexiform layer displayed intense GAD-LI, but immunoreactivity was absent in the ganglion cell layer (GCL). Putative amacrine, bipolar, and horizontal cell somata were also labeled. Centrally, GAD-LI was observed in all primary visual nuclei. Dense, GAD-like immunoreactive punctate structures (puncta), presumably terminals, were observed in the pretectal nucleus lentiformis mesencephali, posterior thalamic neuropil, and uncinate neuropil. GAD-like immunoreactive puncta were noted in several laminae of the optic tectum, with the highest concentrations located within the 9th and 8th laminae. Moderate numbers of GAD-like immunoreactive puncta were found in the mesencephalic nucleus of the basal optic root, and two thalamic neuropils--corpus geniculatum and neuropil of Bellonci. The ventrolateral area, posterocentral nucleus, and posterolateral nucleus all contained sparse amounts of GAD-LI. These observations suggest that GABA plays an important physiological role in all central visual areas of Rana pipiens.

Immunocytochemical localization of the GABAA/benzodiazepine receptor beta2/beta3 subunits in the optic tectum of the salmon

The optic tectum of the salmon is a primary visual center with direct input from the retina via the optic tract. The structure is homologous with the superior colliculus of the mammalian brain. We have studied the distribution of immunoreactivity against the GABAA/benzodiazepine receptor beta2/beta3 subunits with a monoclonal antibody (BD-17) in the optic tectum of the salmon brain. A weak immunoreactivity is found in the rostral stratum marginale (SM), strong labelling of the neuropil is shown in a thin band in stratum opticum (SO), two bands in stratum fibrosum et griseum superficiale (SFGS) and two bands in stratum griseum centrale (SGC). Immunoreactive perikarya with neurites that extend radially through the stratum album centrale (SAC) are located in the stratum periventriculare. BD-17 immunoreactivity is to a great extent located in tectal layers that receive direct retinal input, i.e. the SO, SFGS and SGC. These layers are known to receive input also from other visual centers, such as the pretectum (SO, SFGS), the nucleus isthmi (SO, SFGS, SGC), as well as non-visual regions as the telencephalon (SGC). High levels of 2-[125I]-iodomelatonin binding sites have previously been demonstrated in all layers of the salmon optic tectum except the SM and SPV. Thus it appears likely that GABA and/or benzodiazepines and melatonin play a role in visual processing in the optic tectum of teleost fish.

GABAergic and non-GABAergic neurons in the nucleus of the optic tract project to the superior colliculus: an ultrastructural retrograde tracer and immunocytochemical study in the rabbit

Both the nucleus of the optic tract (NOT) and the superior colliculus (SC) are thought to play important roles in the regulation of eye movements. The superior colliculus contributes to visual orientation and saccades, and the nucleus of the optic tract contributes to the detection of slow movements of the visual surround. Recently, a GABAergic projection has been described between these two nuclei in the cat, a species with frontal vision. The present study aimed at determining whether a similar GABAergic pathway exists in the rabbit, a species with lateral vision. To study this pathway we used the retrograde tracer cholera-toxin (CTB) to identify NOT neurons projecting to the SC and GABA-antibody immunostaining to identify GABA-containing neurons and processes. CTB injections into the superficial laminae of the SC showed that GABAergic and non-GABAergic neurons in the NOT project to the SC. Both types of neurons have structural characteristics similar to other projection neurons in the NOT. In contrast to the NOT neurons projecting to the inferior olive (IO) which are mainly located in the rostral NOT, the GABAergic and non-GABAergic NOT-SC neurons are situated throughout the nucleus. The somata and principal dendrites of both neuron types receive numerous synaptic contacts from GABAergic terminals and only a few from retinals. The NOT projection neurons to the SC thus establish prominent excitatory and inhibitory links between the two structures, suggesting the existence of separate circuits that could interact through a GABAergic and non-GABAergic NOT-SC projection. It is further suggested that these circuits may be involved in the regulation of saccades in the SC during optokinetic nystagmus.

Anatomical connections of the primate pretectal nucleus of the optic tract

The pretectal nucleus of the optic tract (NOT) plays an essential role in optokinetic nystagmus, the reflexive movements of the eyes to motion of the entire visual scene. To determine how the NOT can influence structures that move the eyes, we injected it with lectin-conjugated horseradish peroxidase and characterized its afferent and efferent connections. The NOT sent its heaviest projection to the caudal half of the ipsilateral dorsal cap of Kooy in the inferior olive. The rostral dorsal cap was free of labeling. The NOT sent lighter, but consistent, projections to other visual and oculomotor-related areas including, from rostral to caudal, the ipsilateral pregeniculate nucleus, the contralateral NOT, the lateral and medial terminal nuclei of the accessory optic system bilaterally, the ipsilateral dorsolateral pontine nucleus, the ipsilateral nucleus prepositus hypoglossi, and the ipsilateral medial vestibular nucleus. The NOT received input from the contralateral NOT, the lateral terminal nuclei bilaterally, and the ipsilateral pregeniculate nucleus. Although our injections involved the pretectal olivary nucleus (PON), there was neither orthograde nor retrograde labeling in the contralateral PON. Our results indicate that the NOT can influence brainstem preoculomotor pathways both directly through the medial vestibular nucleus and nucleus prepositus hypoglossi and indirectly through both climbing and mossy fiber pathways to the cerebellar flocculus. In addition, the NOT communicates strongly with other retino-recipient zones, whose neurons are driven by either horizontal (contralateral NOT) or vertical (medial and lateral terminal nuclei) fullfield image motion.

The contribution of GABA-mediated inhibition to response properties of neurons in the nucleus of the optic tract in the rat

The contribution of GABA-mediated inhibition to the generation of directional selectivity of neurons in the nucleus of the optic tract (NOT) and the dorsal terminal nucleus of the accessory optic system (DTN) was examined in anaesthetized rats by iontophoretic application of the GABAA receptor antagonist bicuculline methiodide. Spontaneous and visually evoked NOT-DTN cell activities were always increased by bicuculline application. The directional selectivity of NOT-DTN cells to slowly moving whole-field stimuli, expressed as the direction index, was reduced for most neurons. However, the difference between firing rates during stimulus movements in the preferred and in the non-preferred direction did not change systematically. On average, this difference was not significantly affected in the majority of the neurons, although bicuculline more strongly increased the activity during movement in the preferred or non-preferred direction in some of the neurons. These results indicate that directionally selective neurons in the rat NOT-DTN receive GABAergic inhibition which is most likely tonic and independent of the stimulus direction.

Inhibition of neuronal activity in the nucleus of the optic tract due to electrical stimulation of the medial terminal nucleus in the rat

Morphologically, a GABAergic connection between the medial terminal nucleus of the accessory optic system and the nucleus of the optic tract, two primary visual nuclei involved in the optokinetic reflex, has been demonstrated. In this study it was investigated if the medial terminal nucleus forms an inhibitory input to movement direction selective units in the nucleus of the optic tract. Neurons in the nucleus of the optic tract were visually stimulated with moving large random square patterns in their preferred and non-preferred direction, and their activity was recorded extracellularly. Concomitantly, bipolar electrical stimulation was applied to the medial terminal nucleus and its effect was studied on the visual responses of units in the nucleus of the optic tract. Units in the nucleus of the optic tract were strongly inhibited during electrical stimulation of the medial terminal nucleus. The role of GABA in mediating this inhibition was investigated by applying bicuculline, a GABAA receptor antagonist, iontophoretically to the recorded units in the nucleus of the optic tract. However, although average spike rate levels of units in the nucleus of the optic tract increased with bicuculline, bicuculline did not reduce inhibition invoked by electrical stimulation of the medial terminal nucleus. A possible explanation for this observation is that this inhibition is GABAB receptor mediated.

Segregation of direction selective neurons and synaptic organization of inhibitory intranuclear connections in the medial terminal nucleus of the rat: an electrophysiological and immunoelectron microscopical study

A combined electrophysiological and morphological investigation of the medial terminal nucleus (MTN) in the rat was undertaken, aimed at a better understanding of the relationship between structure and function in this nucleus. The locations of upward and downward direction selective units in the MTN were documented with extracellular electrophysiological recording. By means of tracer experiments, with Phaseolus vulgaris-leucoagglutinin, biocytin, and cholera toxin subunit B-horseradish peroxidase, the internal connections of the MTN, its retinal afferents, and the projection neurons to the inferior olive were visualized. Terminals originating from the retina and from internal connections were characterized at the ultrastructural level. Their termination pattern on cells in the MTN, including identified inferior olive projection neurons, were determined. Additionally, postembedding GABA immunocytochemistry was performed to identify GABAergic elements. From reconstructions of the positions of electrophysiologically recorded units in the MTN, a local segregation between upward and downward direction selective units was revealed. Upward direction selective units were found in the dorsal part and ventromedially, whereas downward direction selective units were found ventral and laterally in the MTN. The MTN receives optic fibers via two separate routes which, based on their trajectory, presumably terminate in different parts of the MTN: the inferior fascicle of the accessory optic tract in the dorsal part, and the posterior fiber bundle of the superior fascicle in the ventral part of the MTN. A correspondence has been found between the segregation of direction selective units and the areas in the MTN where the retinal fibers from the two pathways distribute. It is, therefore, proposed that the inferior fasciculus conveys upward direction selectivity and the posterior fiber bundle downward direction selectivity, and that the two fiber bundles terminate segregated in the MTN. After anterograde tracing from the eye, retinal terminals were found evenly distributed throughout the MTN. They are characterized as GABA negative R-type terminals. After retrograde tracing from the inferior olive, identified MTN-inferior olive projection neurons were found in the dorsal MTN and medially in the ventral MTN. Their location in the MTN suggests that MTN-inferior olive projection neurons are upward direction selective. MTN-inferior olive projection neurons are large non-GABAergic cells, with a variable form. A majority of both F- and R-type terminals were found to make synaptic contacts on the dendrites of MTN cells. MTN-inferior olive projection neurons did not differ from other neurons in this respect.

Immunohistochemical study and an electron microscopic observation of the nucleus of the optic tract in the rat

In the present study, gamma aminobutyric acid (GABA)-positive cells and terminals were detected in the nucleus of the optic tract (NOT) in the rat. However, GABA-positive cells in the NOT receiving projections directly from retinal ganglion cells could not be identified. To determine the role of GABA in the projection from the retinal ganglion cells to the NOT, we used electron microscopy to observe this nucleus of a rat in which one eyeball had been removed. In the nerve synapse binding region of the NOT on the side opposite to the enucleated eyeball, the symmetrical and asymmetrical types were recognized; the ratio between the two was the same as before the eyeball was enucleated. From these results, we conclude that the inhibitory axon terminals in the NOT do not have GABA activity. The GABA-positive cells seem to act indirectly as intrinsic interneurons, when processing or transmitting the information from the retina to the inferior olivary nucleus.

GABAergic and non-GABAergic projections of accessory optic nuclei, including the visual tegmental relay zone, to the nucleus of the optic tract and dorsal terminal accessory optic nucleus in rat

This study examines the non-gamma-amino butyric acid (GABA)ergic (group I neurons) and GABAergic neurons (group II neurons) of the accessory optic system projecting to the nucleus of the optic tract (NOT)/dorsal terminal nucleus (DTN) of the accessory optic system in rat. These nuclei include the dorsal (MTNd) and ventral (MTNv) divisions of the medial terminal nucleus, the lateral terminal nucleus, the interstitial nucleus of the superior fasciculus, the posterior fibers, and the visual tegmental relay zone. GABAergic neurons of these nuclei that do not target the NOT/DTN (group III neurons) have also been observed. The fluorescent retrograde tracer fluoro-gold was injected into the pretectum, targeting the NOT/DTN and the tissue prepared immunocytochemically to reveal neurons containing the neurotransmitter GABA. Three groups of neurons (groups I, II, and III neurons) were examined in terms of their distribution, density, and percentage present. Group I neurons are single-labeled with fluoro-gold and represent non-GABAergic neurons projecting to the NOT/DTN. These neurons are of the highest density in the lateral terminal nucleus (204 neurons/mm2). Their densities are also substantial in the MTNv (120 neurons/mm2), interstitial nucleus of the superior fasciculus, posterior fibers (96 neurons/mm2), and visual tegmental relay zone (93 neurons/mm2). Group II neurons are double-labeled with fluoro-gold and GABA. They form a system of GABAergic neurons projecting to the NOT/DTN, which are exceedingly dense in the MTNd (78 neurons/mm2) but are also dense in both the visual tegmental relay zone (49 neurons/mm2) and MTNv (33 neurons/mm2). Group III neurons are GABAergic neurons that do not target the NOT/DTN but must project to other brain nuclei and/or be interneurons. These are of extremely high concentration in the visual tegmental relay zone (316 neurons/mm2) and are also of substantial densities in the MTNd (77 neurons/mm2), lateral terminal nucleus (72 neurons/mm2), and MTNv (44 neurons/mm2). The MTNd has the highest percentage of GABAergic neurons projecting to the NOT/DTN (72%). GABAergic neurons also form significant percentages of the projections to the NOT/DTN from the visual tegmental relay zone (34%) and MTNv (21%). The percentage of the total GABAergic neurons that project to the NOT/DTN is the highest in the MTNd (50%) and MTNv (42%). The described GABAergic afferents to the NOT/DTN may function to process information concerned with the compensation for retinal slip.

Directional asymmetry of the frog monocular optokinetic nystagmus: cholinergic modulation

The frog monocular optokinetic gaze nystagmus (OKN) was studied by coil recordings after intravitreal administration of cholinergic drugs into the closed eye. Before injection, the frog displayed OKN for stimulations in the temporo-nasal (T-N) direction only. The injection of muscarinic agonists, as well as that of nicotinic antagonists, provoked the appearance of a naso-temporal (N-T) component, the slow phase velocity gain then being strongly and significantly increased. The abolition of the OKN directional asymmetry indicates that acetylcholine seems to act in opposite ways through muscarinic and nicotinic binding sites. The GABAergic and cholinergic systems may interact to generate and modulate OKN in the frog.

Medial terminal nucleus terminals in the nucleus of the optic tract contain GABA: an electron microscopical study with immunocytochemical double labeling of GABA and PHA-L

In this study the medial terminal nucleus (MTN) projection to the nucleus of the optic tract (NOT) was investigated in pigmented rats at the light and electron microscopical levels with a new combination of techniques. MTN terminals were anterogradely labeled with Phaseolus vulgaris-leucoagglutinin (PHA-L). Preembedding immunocytochemistry, followed by gold intensification, was used to visualize PHA-L. Postembedding immunocytochemistry with 15 nm immunogold particles was carried out to demonstrate the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Both PHA-L and GABA labeling can be easily discriminated at the electron microscopical level even when present in the same neuronal profiles. Light microscopically MTN-NOT fibers proved to have several branches with many varicosities. MTN terminals were found concentrated in terminal fields. Electron microscopically, it was shown that MTN boutons display characteristics resembling F-type terminals, i.e., terminals with dark mitochondria, pleomorphic vesicles, and symmetrical synapses. All NOT afferents originating from the MTN contained GABA and made multiple contacts exclusively with GABA negative NOT somata and dendrites. These results indicate the existence of a strong and direct inhibitory input onto GABA negative projection neurons in the NOT. This substantiates earlier physiological and morphological reports. It was further demonstrated that the location and organization of MTN terminals in the neuropil differ from that of the retinal input: MTN terminals are largely separated from retinal terminals. MTN terminal fields contain large amounts of GABA positive F terminals in contrast to retinal terminal areas. MTN terminals take part in irregularly shaped agglomerations of terminals, which contain many F terminals and dendritic processes and are surrounded by a glial sheet. Retinal terminals are found grouped together in small circular arrangements contacting a central dendrite.

Distribution of GABA immunoreactive systems in the forebrain and midbrain of the chameleon

An immunocytochemical method, using glutaraldehyde fixation and an antiserum developed against a GABA-glutaraldehyde protein conjugate, permitted direct visualization of GABAergic structures in the brain of a reptile (chameleon). GABA immunoreactive cell bodies and nerve terminals were observed to be evenly distributed throughout the forebrain and midbrain. In the forebrain, GABA-positive perikarya were shown in all cortical areas, the septal area, the striatum, the dorsal ventricular ridge, and in the nucleus accumbens. In the midbrain, the optic tectum contained a dense and laminar distribution of GABA neurons. These neurons were also observed in the lateral geniculate nucleus, nucleus profundus mesencephali, nucleus opticus tegmenti and substantia nigra. Immunoreactive nerve fibers and terminals were observed in the same structures and, additionally, in the tractus septo-hypothalamicus, habenula complex, median eminence, intermediate lobe of the pituitary, basal part of the subcommissural organ, torus semicircularis and nucleus reticularis isthmi. These results provide a framework for a further electron microscopic analysis of the GABAergic innervation of some encephalic areas involved in physiological regulations particular to this species especially the visual system.

Cerebellar projections to the somatic pretectum in the cat

Neurons in the somatic pretectum receive input from the dorsal column nuclei (DCN) and project to a comparable "somatic" portion of the dorsal accessory nucleus of the inferior olive (DAO). This somatic DAO is reciprocally connected with the anterior interpositus nucleus of the cerebellum. One question that arises is whether this circuitry is further controlled by an output specifically from the anterior interpositus nucleus to the somatic pretectum. Wheatgerm agglutinin conjugated to horseradish peroxidase was injected into various parts of the cat pretectum. Injection sites were interpreted as including the somatic pretectum if neurons in the DCN were retrogradely labeled and if anterograde terminal labeling occurred in somatic DAO. The locations of retrogradely labeled neurons within the deep cerebellar nuclei were then compared in cases in which the injection sites included or excluded the somatic pretectum. In all cases in which the injection site included the somatic pretectum, retrogradely labeled neurons were observed in the anterior interpositus nucleus as well as in the lateral cerebellar nuclei. In some of these cases, neurons in the posterior interpositus and medial nuclei were also labeled. In contrast, in cases in which the pretectal injection site was located outside or at the border of the somatic pretectum, retrogradely labeled neurons were observed only in the lateral, posterior interpositus, and medial nuclei. Thus, the somatic pretectum appears to receive input primarily from neurons in the anterior interpositus nucleus, along with some input from neurons in the lateral nucleus. These results provide additional evidence for a pathway through the DCN in which sequentially processed somatic information has access to and is modulated by cerebellar circuitry. The existence of such a pathway supports the conclusion that neurons in the DCN convey somatic information important not only for cutaneous, kinesthestic, and other bodily sensations, but also for the control of movement.

A GABAergic projection from the pretectum to the dorsal lateral geniculate nucleus in the cat

To study the projection from the pretectum to the lateral geniculate nucleus, we placed wheat-germ agglutinin conjugated to horseradish peroxidase into the lateral geniculate nuclei of six cats, allowed this marker to be retrogradely transported by afferent axons to their parent somata in the pretectum, and revealed the label in these cells with stabilized tetramethylbenzidine histochemistry. In three cases we made large pressure injections that completely infiltrated the lateral geniculate nucleus and extended into neighboring thalamic nuclei; in the other three we made smaller iontophoretic injections largely confined to the A- and C-laminae of the lateral geniculate nucleus. In both types of injection we found labeled pretectal cells mainly in the nucleus of the optic tract but also found some cells labeled in the olivary pretectal nucleus and the posterior pretectal nucleus. After one of the larger injections we analysed both sides of the pretectum and found that 11% of the labeled cells were located contralaterally and were distributed in the same three nuclei. We analysed only the ipsilateral side in the remaining five cats. In those five experiments we also immunohistochemically stained the pretectal sections with an antibody directed against the neurotransmitter, GABA. Of the retrogradely labeled pretectal cells, 40% were also labeled for GABA, and those were similar in soma size (350 microns 2 in cross-sectional area) to those labeled only with the retrograde marker (331 microns 2). GABA-positive cells not labeled by retrograde transport were smaller (246 microns 2) than either of these other cells populations. These results indicate that at least 40% of the cells involved in the projection from the pretectum to the lateral geniculate nucleus are GABAergic. We suggest that this extrathalamic projection may serve to inhibit thalamic GABAergic cells. This, in turn, would disinhibit geniculate relay cells, thereby facilitating the geniculate relay of retinal information to cortex.

Opioid receptors in the accessory optic system of the rat: effects of monocular enucleation

The presence and concentrations of each of the three subtypes of opioid receptors (mu, kappa, and delta) has been studied in the accessory optic nuclei (dorsal, lateral, and medial terminal nuclei and the interstitial nucleus of the superior fasciculus, posterior fibers: DTN, LTN, MTN, and inSFp) in normal young rats with radioligands directed towards each opioid receptor subtype. The changes in mu opioid receptors have also been investigated in monocularly enucleated rats in which one eye was removed and the rats sacrificed at postoperative day (PO) 2, 3, 5, 7, 14, and 30. As the MTN is the only accessory optic nucleus of the rat large enough for semiquantitative evaluation, the mu receptor population of the MTN has been subjected to optical microdensitometric analysis. All four of the accessory optic nuclei (AOS nuclei) are found to contain exceedingly high levels of mu opioid receptor binding with the selective radioligand [3H]-[D-Ala,MePhe4, Gly-ol5] (DAGO), low levels of kappa opioid receptor binding using the radioligand [3H]-[ethylketocyclazocine] (EKC) together with the competing agents [D-Pro4]-morphiceptin and [D-Ser2,Thr6]-Leu-enkephalin, and an absence of delta opioid receptor binding with the radioligand [3H]-[D-Ala2,D-Leu5]-enkephalin (DADLE) combined with the competing agent [D-Pro4]-morphiceptin. Monocular enucleation, as studied on the mu opioid receptor population with this experimental approach, results in virtually a complete loss of mu opioid receptors throughout all four of the contralaterally located AOS nuclei, including both dorsal and ventral subdivisions of the medial terminal nucleus (MTNd,v). Kappa and delta receptors are very few (kappa receptors) or are lacking (delta receptors) in the AOS nuclei, and for this reason, the effects of monocular enucleation on these two opioid receptor subtypes have not been investigated. Monocular enucleation also produces a significant lowering in mu receptor binding in other primary optic nuclei (the lateral geniculate nuclei, nucleus of the optic tract, and superficial layers of the superior colliculus) and in the pars principalis of the medial geniculate nucleus (description of changes in mu receptors in non-accessory optic primary optic nuclei will be considered elsewhere). Microdensitometric study of the MTNd,v shows that the decreased binding of mu receptors in this nucleus is barely detectable (about 6%) at PO2 and rises to 6-15% at PO3. At PO5 receptor loss reaches approximately 62%, whereas at PO7 it is about 81% complete. At PO14 and PO30, the mu receptor loss is nearly complete at around 93%.(ABSTRACT TRUNCATED AT 400 WORDS)

Thalamic origin of neuropeptide Y innervation of the accessory optic nucleus of the pigeon (Columba livia)

Immunohistochemical and tracing techniques were used in combination to reveal the source of a neuropeptide Y-like immunoreactive (NPY-LI) plexus in the nucleus of the basal optic root (nBOR) of the pigeon accessory optic system. Injections of rhodamine-labeled latex microspheres into nBOR produced retrograde labeling of a population of neurons interposed between the principal optic nucleus of the dorsolateral thalamus (equivalent to the mammalian dorsal lateral geniculate nucleus) and the ventral lateral geniculate nucleus. The retrogradely labeled neurons were distributed mainly in the immediate vicinity of the lateral, dorsal, and ventral aspects of the nucleus rotundus. Immunohistochemical methods revealed many NPY-containing somata within the same intergeniculate thalamic area. Double-labeling immunohistochemical and retrograde tracing experiments evidenced that many NPY-LI neurons in the intergeniculate area contained rhodamine microspheres that had been previously injected into the ipsilateral nBOR. The projection of that general thalamic area to the nBOR was then confirmed by means of anterograde transport of Phaseolus vulgaris leucoagglutinin. In these experiments, the intergeniculate region was demonstrated to project to all divisions of the nBOR and to every other retino-recipient structure, including the suprachiasmatic nucleus. Finally, electrolytic lesions of the intergeniculate area produced a dramatic reduction in the number of NPY-LI axons and terminals within the ipsilateral nBOR and also within other retino-recipient structures. These data indicate the existence of a thalamic NPY-LI projection to the pigeon nBOR of the accessory optic system. This chemically specific projection originates from the intergeniculate area, which was shown in this study to project to all other retino-recipient structures. Thus, NPY may have a role in the functional organization of the accessory optic system and also of the avian visual system as a whole.

Fate of septohippocampal neurons following fimbria-fornix transection: a time course analysis

Neurons in the medial septum (MS) and vertical limb of the diagonal band (vDB) undergo degenerative changes following transection of their axons. These changes have been well studied by histological techniques such as Nissl stains and immunocytochemistry. A dramatic loss of stained neurons occurs following axotomy and this has been interpreted as indicative of neuronal death. However, since the staining intensity and the size of affected neurons may be reduced by axotomy, it is possible that the apparent neuronal death may actually be due to a decrease in somal size or the ability to detect neurons by routine histological methods. The present study describes the effects of axotomy on MS and vDB neurons which have been labeled by hippocampal injections of the retrograde tracer, Fluoro-Gold (FG), prior to transection of the fimbria-fornix and supracallosal stria. The number of FG-labeled neurons in the MS decreased by 21% at three weeks, 36% at six weeks, and 31% at ten weeks after fimbria-fornix transection. The reduction was statistically significant at 6 and 10 weeks. The number of FG-labeled neurons in the vDB showed no reduction at three weeks but was decreased by 31% and 37% at six and ten weeks, respectively. This was statistically significant only at 10 weeks. By comparison, the number of neurons immunoreactive for choline acetyltransferase (ChAT) was reduced by 75-80% at these time points. The size (area and diameter) of FG-labeled somata decreased in both the MS and vDB within three weeks following fimbria-fornix transection and remained relatively constant at the six- and ten-week time points.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural organization of the retino-pretecto-olivary pathway in the rabbit: a combined WGA-HRP tracing and GABA immunocytochemical study

The ultrastructural organization of the pretecto-olivary projection neurons within the nucleus of the optic tract and dorsal terminal accessory optic nucleus of rabbits was studied by using anti-GABA immunolabelling and retrograde transport of WGA-HRP. GABA-like immunoreactivity was determined with a postembedding colloidal gold technique. WGA-HRP was injected in the dorsal cap of the inferior olive. The WGA-HRP-labelled neurons were incubated with gold-substituted silver peroxidase. Neurons projecting to the inferior olive had large to medium-sized cell bodies and were GABA negative. In the nucleus of the optic tract, projection neurons are found in the rostral parts, while the majority of the local GABAergic interneurons are mainly found in the caudal parts. In the dorsal terminal nucleus both types of neurons are intermingled. The projection neurons were frequently in synaptic contact by GABAergic terminals. These neurons also receive retinal afferents indicating the existence of a two-step synaptic connection from the retina to the inferior olive. It is suggested that this class of projection neurons forms the "direction-selective" neurons that can be antidromically stimulated from the inferior olive. The GABAergic terminals on the identified projection neurons are of axonal origin (F-terminals), whereas presynaptic dendrites of interneurons (P-terminals) were seldom observed to be in synaptic contact with retrogradely labelled profiles. The strong input of GABA on direction-selective neurons indicates that GABA is directly involved in modulating retinal signals to the inferior olive.

Visual telencephalon modulates directional selectivity of accessory optic neurons in pigeons

The directional selectivity of units within the nucleus of the basal optic root (nBOR) of the accessory optic system (AOS) was studied before and after lesions of the visual telencephalon (visual Wulst) in urethane-anesthetized pigeons. In intact pigeons, most nBOR units preferred upward motion with a temporal component or downward motion with a nasal component. The ipsilateral and bilateral telencephalic lesions generated a dramatic reduction in the number of cells with optimal responses to upward motion. The overall distribution of preferred directions was still bimodal following ipsilateral or bilateral Wulst lesions, with most units showing best responses to a straight temporal or to downward-nasal directions. The contralateral Wulst lesions produced, instead, a marked reduction in downward preferences. The nBOR units which were studied in these cases showed mainly upward-temporal and upward-nasal responses. These data suggest an involvement of the visual Wulst in the determination of the directional selectivity of nBOR neurons in the pigeon. Specifically, the responses of nBOR units to upward motion appeared to depend on the integrity of the telencephalic descending systems which impinge, in both direct and indirect ways, upon that AOS nucleus. Taken together with data for the mammalian AOS, the present results indicate that nonretinal afferents to AOS nuclei have an important role in the functional organization of that subcortical visual pathway.

Neurotransmitters, receptors, and neuropeptides in the accessory optic system: an immunohistochemical survey in the pigeon (Columba livia)

Immunohistochemical techniques were used to survey the distribution of several conventional transmitters, receptors, and neuropeptides in the pigeon nucleus of the basal optic root (nBOR), a component of the accessory optic system. Amongst the conventional neurotransmitters/modulators, the most intense labeling of fibers/terminals within the nBOR was obtained with antisera directed against glutamic acid decarboxylase (GAD) and serotonin (5-HT). Moderately dense fiber plexuses were seen to label with antibodies directed against tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT). GAD-like immunoreactivity (GAD-LI) was found in many small and medium-sized perikarya within the nBOR. Some of the medium-sized cells were occasionally positive for ChAT-LI. Cell body and dendritic staining was also commonly seen with the two tested antisera against receptors-anti-GABA-A receptor and anti-nicotinic acetylcholine receptor. The antisera directed against various neuropeptides produced only fiber labeling within the nBOR. The densest fiber plexus staining was observed with antiserum against neuropeptide Y (NPY-LI), while intermediate fiber densities were seen for substance P (SP-LI) and cholecystokinin (CCK-LI). A few varicose fibers were labeled with antisera against neurotensin (NT), leucine-enkephalin (L-ENK), and the vasoactive intestinal polypeptide (VIP). Unilateral enucleation produced an almost complete elimination of TH-LI in the contralateral nBOR. SP-LI and CCK-LI were also decreased after enucleation. No apparent changes were seen for all other substances. These results indicate that a wide variety of chemically-specific systems arborize within the nBOR. Three of the immunohistochemically defined fiber systems (TH-LI, SP-LI, and CCK-LI fibers) were reduced after removal of the retina, which may indicate the presence of these substances in retinal ganglion cells. In contrast, the fibers exhibiting ChAT-LI, GAD-LI, 5-HT-LI, NPY-LI, NT-LI, L-ENK-LI, and VIP-LI appear to be of nonretinal origin. Two different populations of nBOR neurons exhibited GAD-LI and ChAT-LI. However, these two populations together constituted only about 20% of the nBOR neurons.

GABA-like immunoreactivity of neurons in the chicken diencephalon and mesencephalon

The chick brain is a useful model system for studying the ontogeny and phylogeny of neural circuitry, especially that of the visual system. In this study the distribution of cells and processes showing GABA-like immunoreactivity (GABA+) in the diencephalon and mesencephalon of the posthatch chick was determined immunohistochemically with a polyclonal antibody to GABA and compared with the results of similar studies in mammals. Most of the small GABA+ cells were found in the chick visual centers such as the nucleus lateralis anterior, suprachiasmatic nucleus, ventral lateral geniculate, optic tract, dorsolateralis anterior pars lateralis, lentiformis mesencephali, ectomammillary nucleus, area pretectalis, and the optic tectum. Large GABA+ cells were found in the following nuclei: reticularis superior, posteroventralis thalami, subpretectalis, isthmi pars magnocellularis, interstitio-pretectosubpretectalis, mesencephalicus lateralis pars dorsalis. These large cell-containing nuclei receive projections from visual or auditory centers. GABA+ axons were found throughout the diencephalon and mesencephalon but were especially prominent in the ansa lenticularis, fasciculus medialis longitudinalis, and optic tract. The distribution of GABA+ cells in the chick is more widespread than in rodents and exhibits an increased association with the visual centers suggesting a correlation with the specialized visual requirements of the bird.