Organization by sensory modality in the reticular formation of the rat

The indirect corticopontine pathway relays perioral sensory signals to the cerebellum via the mesodiencephalic junction

In the cerebro-cerebellar loop, outputs from the cerebral cortex are thought to be transmitted via monosynaptic corticopontine gray (PG) pathways and subsequently relayed to the cerebellum. However, it is unclear whether this pathway is used constitutively for cerebro-cerebellar transduction. We examined perioral sensory pathways by unit recording from Purkinje cells in ketamine/xylazine-anesthetized mice. Infraorbital nerve stimulations enhanced simple spikes (SSs) with short and long latencies (first and second peaks), followed by SS inhibition. The second peak and SS inhibition were suppressed by muscimol (a GABAA agonist) injections into not only the PG but also the mesodiencephalic junction (MDJ). The pathway from the secondary somatosensory area (SII) to the MDJ, but not the cortico-PG pathway, transmitted the second peak signals. SS inhibition was processed in the SII and primary motor area. Thus, the indirect cortico-PG pathway, via the MDJ, is recruited for perioral sensory transduction.

Auditory and Lower Limb Tactile Prepulse Inhibition in Primary Restless Legs Syndrome: Clues to Its Pathophysiology

The resting sensory discomfort transiently relieved upon movement of the affected area in restless legs syndrome suggests that sensorimotor integration mechanisms, specifically gating, may be altered in the disease. The authors sought to determine the effects of prepulse auditory and tactile stimulation applied to lower limbs on the blink reflex of patients with restless legs syndrome and healthy subjects. Seventeen patients with restless legs syndrome and 17 age- and sex-matched healthy controls were investigated. Auditory stimuli and tactile lower limb stimulation were applied as prepulses. The R2 response of the blink reflex induced by electrical stimulation applied to the right supraorbital nerve was selected as the test stimulus. Time intervals between prepulses and response-eliciting stimuli were 40, 70, 90, 110, and 200 milliseconds. There were no differences in either the auditory or tactile prepulse conditions between patients and controls and no differences between these measures within subject groups. We concluded that the tactile lower limb and the auditory prepulse effects on the brainstem interneurons mediating the blink reflex share common neural pathways. Because forebrain interneurons mediate these prepulse effects, they are likely not involved in the disordered sensorimotor interaction of restless legs syndrome.

Neuroleptics and operant behavior: The anhedonia hypothesis

Neuroleptic drugs disrupt the learning and performance of operant habits motivated by a variety of positive reinforcers, including food, water, brain stimulation, intravenous opiates, stimulants, and barbiturates. This disruption has been demonstrated in several kinds of experiments with doses that do not significantly limit normal response capacity. With continuous reinforcement neuroleptics gradually cause responding to cease, as in extinction or satiation. This pattern is not due to satiation, however, because it also occurs with nonsatiating reinforcement (such as saccharin or brain stimulation). Repeated tests with neuroleptics result in earlier and earlier response cessation reminiscent of the kind of decreased resistance to extinction caused by repeated tests without the expected reward. Indeed, withholding reward can have the same effect on responding under later neuroleptic treatment as prior experience with neuroleptics themselves; this suggests that there is a transfer of learning (really unlearning) from nonreward to neuroleptic conditions. These tests under continuous reinforcement schedules suggest that neuroleptics blunt the ability of reinforcers to sustain responding at doses which largely spare the ability of the animal to initiate responding. Animals trained under partial reinforcement, however, do not respond as well during neuroleptic testing as animals trained under continuous reinforcement. Thus, neuroleptics can also impair responding (though not response capacity) that is normally sustained by environmental stimuli (and associated expectancies) in the absence of the primary reinforcer. Neuroleptics also blunt the euphoric impact of amphetamine in humans. These data suggest that the most subtle and interesting effect of neuroleptics is a selective attenuation of motivational arousal which is (a) critical for goal-directed behavior, (b) normally induced by reinforcers and associated environmental stimuli, and (c) normally accompanied by the subjective experience of pleasure. Because these drugs are used to treat schizophrenia and because they cause parkinsonian-like side effects, this action has implications for a better understanding of human pathology as well as normal motivational processes.

Is there a brainstem substrate for action selection?

The search for the neural substrate of vertebrate action selection has focused on structures in the forebrain and midbrain, and particularly on the group of sub-cortical nuclei known as the basal ganglia. Yet, the behavioural repertoire of decerebrate and neonatal animals suggests the existence of a relatively self-contained neural substrate for action selection in the brainstem. We propose that the medial reticular formation (mRF) is the substrate's main component and review evidence showing that the mRF's inputs, outputs and intrinsic organization are consistent with the requirements of an action-selection system. The internal architecture of the mRF is composed of interconnected neuron clusters. We present an anatomical model which suggests that the mRF's intrinsic circuitry constitutes a small-world network and extend this result to show that it may have evolved to reduce axonal wiring. Potential configurations of action representation within the internal circuitry of the mRF are then assessed by computational modelling. We present new results demonstrating that each cluster's output is most likely to represent activation of a component action; thus, coactivation of a set of these clusters would lead to the coordinated behavioural response observed in the animal. Finally, we consider the potential integration of the basal ganglia and mRF substrates for selection and suggest that they may collectively form a layered/hierarchical control system.

The pedunculopontine nucleus--auditory input, arousal and pathophysiology

This review describes the role of the pedunculopontine nucleus (PPN) in various functions, including sleep-wake mechanisms, arousal, locomotion and in several pathological conditions. Special emphasis is placed on the auditory input to the PPN and the possible role of this nucleus in the manifestation of the P1 middle latency auditory evoked response. The importance of these considerations is evident because the PPN is part of the cholinergic arm of the reticular activating system. As such, the auditory input to this region may modulate the level of arousal of the CNS and, consequently, abnormalities in the processing of this input can be expected to have serious consequences on the level of excitability of the CNS. The involvement of the PPN in such disorders as schizophrenia, anxiety disorder and narcolepsy is discussed.

Auditory input to the pedunculopontine nucleus: II. Unit responses

The pedunculopontine nucleus (PPN) has been implicated in sleep-wake control, arousal responses, and motor functions. The PPN also has been implicated in the generation of the P1 middle-latency auditory-evoked potential. The present study was undertaken to determine the nature of the responsiveness of single neurons in and around the PPN following auditory stimulation. Somatosensory responsiveness also was tested in some cells. These results demonstrate a) the presence of a significant proportion of PPN neurons that respond to auditory click stimuli; b) two populations of neurons showing either low threshold/short latency/low habituation or high threshold/longer latency/high habituation; c) the responses of longer latency neurons precede the onset and peak of depth- and vertex-recorded middle-latency auditory-evoked potentials; d) thresholds of longer latency neurons similar to the threshold for wave A in the intact cat, the P13 potential in the intact rat, or the startle reflex; and e) convergent somatosensory and auditory responses at a similar latency in a number of PPN neurons. These findings suggest that neurons in and around the PPN may participate in auditory and somatosensory information processing related to arousal, and may contribute to the manifestation of the P1 auditory middle-latency evoked potential.

Visual and auditory cortical lesions following acquisition of an intensity discrimination in rats fail to disrupt cross-modal transfer

The effect of visual or auditory decortication on cross-modal transfer of an intensity discrimination was examined in rats. Twenty animals were first trained under either visual-auditory (V-A) or auditory-visual (A-V) cross-modal transfer (CMT) in a shuttlebox using a shock avoidance pardigm. Prior to the second training session, five of the A-V animals received auditory ablations and five V-A animals received visual ablations. The other 10 animals served as controls and received sham operations. The results reveal that CMT occurred in both experimental groups following cortical ablations. It is possible that information regarding stimulus intensity was transferred from a cortical region used during the original training session to the cortex used in the second or retraining session, prior to surgery. Alternatively, it may be that some subcortical structure (e.g. the amygdala, superior colliculus, or reticular formation) may be involved in CMT of intensity.

Mesencephalic cuneiform nucleus and its ascending and descending projections serve stress-related cardiovascular responses in the rat

The aim of the present study was to explore the neuroanatomic network that underlies the cardiovascular responses of reticular formation origin in the region of the cuneiform nucleus (CNF). The study was performed in urethane anesthetized male Wistar rats. The left iliac artery was supplied with a catheter for the measurement of systemic blood pressure. Low intensity electrical stimulation of the mesencephalic reticular formation (MRF) in the vicinity of the CNF always resulted in pressor and bradycardiac responses, whereas stimulation in the parabrachial nucleus (PB) and Kölliker-Fuse nucleus (KF) led to a pressor response and a small tachycardiac response. The cuneiform area may be placed in the center of a circuit that serves a specific autonomic response pattern to stress: parallel activation of the sympathetic (pressor response) and parasympathetic limb (bradycardia). The efferent connections of the effective stimulation sites in the MRF and the CNF area, were investigated by anterograde tracing with the lectin Phaseolus vulgaris leucoagglutine (PHA-L). The CNF sends descending fibers to the gigantocellular reticular nuclei (GI), the motor nucleus of the vagus (DMNV) and nucleus tractus solitarius (NTS). These projections are probably involved in the bradycardiac response to stimulation. The descending pathway to the NTS/DMNV and GI may therefore be the parasympathetic limb of the circuit. Furthermore, the CNF sends ascending fibers to limbic forebrain areas and descending fibers to the PB-KF complex. The KF in its turn projects to the rostroventrolateral medullary nucleus (RVLM) and the intermediolateral cell column (IML). These latter projections are partly involved in producing the pressor response and thereby represent the sympathetic limb of the circuit. Accordingly, the transection of the descending fibers from the CNF to the PB-KF complex resulted in a decreased pressor and an increased bradycardiac response. This suggests that a baroreceptor reflex-induced bradycardia which results from blood pressure increase can be excluded as the origin of the stimulation-induced bradycardia, and that the pressor and bradycardiac responses are two independent moieties. It cannot be excluded that ascending fibers from the CNF are also involved in producing the pressor response. On the basis of the present physiological and neuroanatomical study, a brain circuit has been proposed in which the cuneiform nucleus has a central position. The described brain circuit may serve a passive coping strategy to novel, painful or threatening stimuli during which the animals show orientation/attention or freezing behavior accompanied by a bradycardiac and pressor response.

The mesencephalic locomotor region is activated during the auditory startle response of the unrestrained rat

We describe an acoustically evoked potential in the midbrain of the rat which occurred in conjunction with the auditory startle response, 'startle correlated potential'. This potential had a variable latency to the onset of the startle-eliciting acoustic stimuli, but was precisely coupled to the startle response in the electromyogram (EMG) of the temporal muscle which was simultaneously recorded. We tried to localize the source of this potential by recording evoked potentials at different recording sites in individual awake and unrestrained rats using a specially constructed microdrive. The potential may be generated in part by neurons in the region of the pedunculopontine tegmental nucleus, lying within the electrophysiologically defined mesencephalic locomotor region (MLR). We suggest, therefore, that the startle correlated potential reflects the activation of the MLR during the startle response. Timing calculations make it unlikely that the startle correlated potential is generated by a sensorimotor relay within the primary startle circuit which produced a fast startle twitch in the temporal muscle. Instead, the startle correlated potential probably reflects the involvement of the MLR in a later secondary startle response or in response modulation, e.g. habituation. In our opinion the most interesting possibility is that the MLR could be activated during a startle response to inhibit and reset the current motor program.

Focal dorsal raphe stimulation and pinnal electrical stimulation modulate spontaneous and noxious evoked responses in thalamic neurons

This study investigated the nocieceptive responses of single neurons within the nucleus parafascicularis (PF) thalami of the rat following two modes of electrical stimulation known to induce analgesia. It was found that both focal electrical dorsal raphe stimulation (DRS) and bilateral pinnal (ear) electrical stimulation (PES) converge on the same PF neurons, affecting both the spontaneous discharges and the noxious evoked responses toward these neurons. The effects of different stimulus current intensity, frequency and pulse duration were also examined. It was found that for both DRS and PES at pulse frequency of 10 Hz and current amplitude of 10 microA are the optimal parameters to modulate both the spontaneous and the noxious evoked responses. These stimuli produced prolonged effects related to the duration of stimulation. The external (PES) low current stimulation which was delivered below the sensory threshold was as effective in modulating noxious responses as the invasive DRS in intact animals and in animals with bilateral dorsolateral-funiculus ablation. It was observed that dorsal lateral funiculus ablation (DLFx) did not modify the DRS and the PES effects. These observations further support the existence of an ascending pain modulation pathway.

A case of postanoxic encephalopathy with cortical action and brainstem reticular reflex myoclonus

A patient with postanoxic encephalopathy, with both action- and stimulus-sensitive reflex myoclonus, is described. The action myoclonus was multifocal and cortical in origin. In contrast, reflex myoclonus elicited by somaesthetic and auditory stimulation was generalised. The earliest reflex electromyograph activity was recorded in the sternocleidomastoid; myoclonic activity then spread up the brainstem and down the spinal cord, suggesting that this reflex myoclonus had its origin in the caudal brainstem. Stimulus sensitivity was greatest in the limbs. The bulbospinal motor pathways involved in the generalised reflex myoclonus were rapidly conducting, and this characteristic distinguishes this form of brainstem reflex myoclonus from that described in hyperekplexia.

How visual inputs to the ponto-bulbar reticular formation are used in the synthesis of premotor signals during orienting

The primate superior colliculus (SC) is known as a structure subserving the transformation of visual information into "commands" for orienting eye movements. Collicular burst neurons discharging with short lead times in relation to visually triggered or spontaneous saccades are supposed to be the output elements linking the SC to immediately premotor pattern generators. In this paper we summarize some data available for the cat's SC neurones, identified as tecto-reticulo-spinal projection cells (TRSN), and reticulospinal neurones (RSN), identified as receiving excitatory collicular input. Some TRSNs respond to visual stimuli in the absence of orienting movements and, hence, their signals cannot be regarded as motor "commands", in spite of their proven connections with premotor pools in the brain stem and with the spinal cord. Moreover, a small fraction of RSNs belonging to polysynaptic descending collicular pathways also displays visual responses dissociated from movement, in addition to discharges related to the performance of orienting eye-head synergies. The processes of visual to motor transformation, assumed by current models as being definitively accomplished in the SC, appear thus to be partially performed in the reticular network incorporating the overlapping collaterals of tectal projection cells and their target neurons in the reticular core. It is concluded that, at least as for visuomotor transformations underlying orienting movements in the cat, the deep division of the SC and the brain stem reticular formation represent an ensemble, rather than a sequence of hierarchically arranged levels of processing.

Turning responses evoked by stimulation of visuomotor pathways

Lateral eye, head, and body movements are produced by electrical stimulation of many brain regions from frontal cortex to pons. A new collision method shows that at least 5 separate axon bundles mediate stimulation-elicited lateral head and body movements in rats. One bundle passes between the rostromedial tegmentum and medial pons, with conduction velocities of 0.8-18 m/s. A second bundle passes between the superior colliculus and contralateral medial pons, with conduction velocities of 1.7-13 m/s. A third bundle passes between the superior colliculus and ventrolateral pons, with conduction velocities of 1.3-20 m/s. A fourth bundle passes between the internal capsule and medial substantia nigra, with conduction velocities of 0.9-4.4 m/s. A fifth bundle passes between the anteromedial cortex and rostral striatum, with conduction velocities of 2.4-36 m/s. Collision effects have not been observed between the anteromedial cortex and the internal capsule, medial substantia nigra, superior colliculus, rostromedial tegmentum, or medial pons, which suggests that these sites are not connected by axons mediating turning. Possible synaptic linkages between the 5 bundles and possible transmitters are discussed.

Anatomical distribution and response patterns of reticular neurons active in relation to acoustic startle

A population of reticulospinal neurons with short latency response to startle-inducing stimuli was identified in the nucleus reticularis pontis caudalis (NRPC) and nucleus gigantocellularis (NRGC) of the medial pontomedullary reticular formation. The threshold and magnitude of response to auditory stimuli was correlated in these cells and in the muscles mediating startle. Startle-related neurons were significantly more likely to have high conduction velocity spinal projections than adjacent cells not related to startle. Startle-related cells were not 'dedicated' to startle, but were active in relation to spontaneous movements. Both the unit response of the startle-related cells and the startle response recorded in muscles were suppressed by the prior presentation of a weak prepulse. Thus, prepulse inhibition of startle occurs at, or prior to, the medial pontomedullary reticular formation. We conclude that these reticulospinal cells convey the output of the brainstem system modulating and triggering startle.

Excitant amino acids and audiogenic seizures in the genetically epilepsy-prone rat. II. Efferent seizure propagating pathway

Previous studies indicate that the inferior colliculus is the brain stem auditory nucleus most sensitive to the chemical blockade of audiogenic seizures in the genetically epilepsy-prone rat. Other auditory structures do not appear to be as important. This study attempted to define the efferent pathways involved in propagation of the seizure from the colliculus to the spinal cord where the motor components of the convulsion are generated. This study also determined whether certain nuclei which have been implicated in the propagation of seizures in other epilepsy models are involved in audiogenic seizures. The excitant amino acid antagonist, 2-amino-7-phosphonoheptanoate, was infused bilaterally into several of those sites. The drug was effective in significantly reducing seizure severity with infusion of 5 nmol bilaterally into the midbrain and the pontine reticular formation or the substantia nigra. However, similar drug doses were not effective when infused into the entopeduncular nucleus even though prominent behavioral effects were observed with this infusion. Infusion of 2-amino-7-phosphonoheptanoate into the prepiriform cortex resulted in a small but significant reduction in seizure severity. These results suggest that inhibition of excitatory transmission within the substantia nigra and the reticular formation effectively blocks the output pathway for the audiogenic seizures, whereas the role of the prepiriform cortex in this process is relatively minor.

Chronic pain as a reticular formation syndrome

Evidence was previously presented to support the thesis that chronic pain is activated by neuronal elements that make up the multisynaptic short axon core of the reticular system (Andy and Peeler 1985). The present thesis, that chronic pain is a reticular formation syndrome, is based on a retrospective analysis of four patients with chronic pain who were successfully treated with a lesion in the anterior thalamus and stimulation electrode implants in the posterior thalamus and pontomesencephalic brain stem. The reticular formation was the common underlying anatomic substrate at those three sites. In addition to chronic pain, all the patients had other symptoms attributable to other body organs and systems. The number and type of symptoms that made up the syndrome differed between patients. Symptoms making up the core of the syndrome were pain, anxiety, nervousness, insomnia, and depression. Experimental and clinical findings are briefly presented to demonstrate the various reticular formation sites, pragmatically considered "reticular functional systems," from which symptoms may arise. It is hypothesized that the symptoms are recruited by a low threshold "pain oscillator" that is generated at one reticular site and subsequently permeates the rest of the reticular system. Therapeutic stimulation inactivates the low threshold system by "jamming" it.

Elicitation of intraspecific defensive behaviors in the rat by microinjection of picrotoxin, a gamma-aminobutyric acid antagonist, into the midbrain periaqueductal gray matter

Behavioral reactions induced in the rat by microinjections of a gamma-aminobutyric acid (GABA) antagonist (picrotoxin; 25 and 50 ng in 0.25 microliter) into the midbrain periaqueductal gray matter were measured in an open-field test and when the animal was confronted by a conspecific introduced into its cage (i.e. resident-intruder paradigm). In the open-field, microinjections of picrotoxin significantly increased backward locomotion while decreasing self-grooming. In the resident-intruder paradigm, microinjections of picrotoxin selectively increased defensive reactions (defensive uprights, defensive sideways, retreat) while offensive behaviors were rather reduced. In addition, the actual nature of the effects was found to depend upon the intruder's relative position. Defensive reactions were significantly increased when the partner was on the side contralateral to the injection site, whereas social approach behaviors (fur investigation, anogenital investigation) were decreased when the partner was located on the ipsilateral side. These data suggest the involvement of GABAergic synapses within the midbrain periaqueductal gray matter in the control of intraspecific defensive behaviors in the rat.

Role of the inferior colliculus in the inhibition of acoustic startle in the rat

Fifteen rats were tested for amplitude reduction of the acoustic startle response using auditory and visual prestimuli. Eight subjects then received large lesions of the inferior colliculus, and the remaining subjects served as normal controls. All animals were reassessed on a post-test identical to the pre-test. In addition, all subjects were tested for latency reduction of startle using auditory prestimuli. There were no significant differences between groups on the pre-test for startle amplitude, visual amplitude reduction, or auditory amplitude reduction, nor did the control group differ significantly on these measures from pre-test to post-test. After surgery, the lesion group displayed a large, significant increase in startle amplitude. Auditory prestimuli were no longer effective in reducing startle amplitude in this group, but visual prestimuli still produced reliable amplitude reduction. Both groups displayed reliable latency reduction to auditory prestimuli; the groups were not significantly different from each other on this measure. These data support the proposition that the inferior colliculus is part of a neural circuit for startle amplitude reduction by auditory prestimuli.

Cortical and tectal control of visual orientation in the gerbil: evidence for parallel channels

Two experiments were carried out with Mongolian gerbils to determine the roles of optic tectum and visual cortex in the mediation of visually guided head turns and locomotion elicited and controlled by discrete visual targets. In Experiment 1, the behavior of animals with either a sham operation, a bilateral lesion of optic tectum, or a bilateral ablation of areas 17, 18a, and 18b was recorded on videotape as they ran from the center of a circular arena toward a small visual target projected in different locations around the perimeter of the arena. The amplitude and direction of the head turns and the accuracy of their locomotor responses were reconstructed from a frame by frame analysis of the videotapes. Sham-operate gerbils made a series of head turns before running accurately and efficiently toward the target. The gerbils with lesions of areas 17, 18a, and 18b rarely made more than one head turn before running toward the perimeter of the arena. Although the single head turn they did make was often well-correlated with the position of the target in their visual field, the direction of their locomotor response was largely determined by the direction and amplitude of that head turn. As a consequence, these animals undershot the target more often than did the sham-operate animals, and even ran into the visual half field opposite the target if their head turn had also been made into that half field. Unlike the sham operates, these animals were unable to make further adjustments in their orientation toward the stimulus after their initial head turn. The head turns and locomotor behavior of the gerbils with lesions of optic tectum were even more disorganized and inaccurate than those of the posterior decorticates. Nevertheless, when the target was presented within 45 degrees from their visual midline, their head turns and locomotor responses showed a systematic relationship with the eccentricity of the target. Their behavior to stimuli outside this central wedge of their visual field was completely disorganized and showed no relationship to the location of the target. In Experiment 2, unilateral lesions of area 17 were performed in the gerbils that had already received bilateral tectal lesions to determine whether such lesions would affect the "residual" ability of these animals to orient toward stimuli located within the central portion of their visual field. During retesting, these animals were able to respond to targets only if they were located in the central portion of the field ipsilateral to the cortical lesion.(ABSTRACT TRUNCATED AT 400 WORDS)

Upper visual space neglect and motor deficits after section of the midbrain commissures in the cat

The neurological deficits following section of the midbrain commissures were studied in the cat. After a lesion of the commissures between the superior and inferior colliculi, with or without involvement of the posterior commissure, the animals showed a long lasting inattention for stimuli in the upper visual space, lack of exploratory head movements towards the neglected space, head ventroflexion and vertical paralysis of gaze. After a lesion of the commissure between the superior colliculi or of its rostral part only, the same symptomatology appeared, but it was short lasting. After a lesion of the posterior commissure, the head was kept dorsiflexed, the exploratory head movements towards the lower visual space were reduced and the stimuli presented in this space were often neglected. There was a paralysis of vertical eye movements. The findings are discussed in the frame of a premotor theory of neglect.

Reticular formation of the lower brainstem. A common system for cardiorespiratory and somatomotor functions: discharge patterns of neighboring neurons influenced by somatosensory afferents

Extracellular recordings were made from 103 neurons located in the medial parts of the reticular formation of the lower brainstem of chloralose-urethane anesthetized dogs. Activities of 2 or 3 neighbouring neurons under identical conditions could be recorded with one electrode. In 9 recordings it was possible to register simultaneously up to 5 neurons with two electrodes placed in both halves of the medulla. Action potentials of individual neighbouring neurons were identified by amplitude discrimination. The influences of somatosensory afferents from skin, joints and muscles on neuronal discharge patterns were tested. Responses of single neurons were characterized by multisensory afferent spectra including afferents from various parts of the body. The combinations of afferents converging onto neighbouring neurons were similar, whereas neurons in more distant parts of the medulla revealed different combinations of converging afferents. In long-lasting recordings the influence of somatosensory afferents on the discharge behaviour changed from time to time. When the discharge behaviour was mainly determined by somatosensory afferents, neighbouring neurons were shown to be organized in sub-populations. The results led to the conclusion that in this part of the reticular formation different types of functional organization of the neuronal network are possible. The type of functional organization depends on the actual preponderances of different inputs to the neurons.

Afferent connections of the mesencephalic reticular formation: a horseradish peroxidase study in the rat

The afferent connections of the mesencephalic reticular formation were studied experimentally in the rat by the aid of the retrograde horseradish peroxidase tracer technique. The results suggest that the rostral portion of the mesencephalic reticular formation receives its main input from the cerebral cortex, the zona incerta and the fields of Forel, the central gray substance, the nuclei reticularis pontis oralis and caudalis, and the deep cerebellar nuclei. Substantial input to the same territory of the mesencephalic reticular formation appears to come from the superior colliculus, the substantia nigra, the parabrachial area, the spinal trigeminal nucleus, and the nucleus reticularis gigantocellularis, whereas several other brain structures, among which the locus coeruleus and the raphe complex, seem to represent modest but consistent additional input sources. The afferentation of more caudal portions of the mesencephalic reticular formation appears to conform to the general pattern outlined above with only three exceptions; the cerebral cortex, the deep cerebellar nuclei and the spinal trigeminal nucleus seem to be relatively modest sources of projections to these levels. Considering that the mesencephalic reticular formation is a critical structure in the "ascending activating systems" the present results, confirming and extending those of many other investigators, characterize a set of pathways that seem to be an important part of the anatomical substrate of the sleep-walking cycle.

Acute effects of alcohol on photic evoked potentials of rats: lateral geniculate nucleus and reticular formation

This study examined the effects of ethanol on photic evoked potentials recorded from the dorsal lateral geniculate nucleus (LGN) and midbrain reticular formation (MRF) of chronically implanted albino rats. Animals were given intraperitoneal injections of saline, or of 0.5, 1.0, 1.5 or 2.5 g ethanol/kg body weight on separate days. Evoked potentials were recorded at 5, 20, 40 and 60 min following injection. An early positive component recorded from each structure was depressed in amplitude by only the 2.5 g/kg alcohol dose, while the succeeding negative component was depressed by both the 1.5 and 2.5 g/kg doses. Latencies of both early components in each structure were increased by the 1.5 and 2.5 g/kg alcohol doses. Alcohol doses of 1.0-2.5 g/kg depressed the amplitude of a later positive component in the LGN (latency of 78 msec), but latency was not altered. In contrast, a late positive component in the MRF (latency of 150 msec) was both decreased in amplitude and increased in latency by only the 2.5 g/kg dose. These results on subcortical structures are discussed in relation to alcohol's effects on cortical evoked potentials.

Stimulation-produced analgesia: evidence for somatotopic organization in the midbrain

The somatotopic organization of structures in the midbrain which mediate stimulation-produced analgesia (SPA) was studied by stimulating a series of neural loci using a movable electrode. The strength and distribution of analgesia at 7 fields on the body surface, as measured by the application of noxious pinch, was clearly related to the dorsoventral stimulation locus. SPA at dorsal sites was distributed at the ears and, to a lesser extent, at the forepaws. At progressively more ventral loci, maximal analgesia appeared successively at the forepaws, hindpaws and tail. This organization was maintained throughout the rostrocaudal extent of the midbrain. However, rostral stimulation sites tended to produce analgesia in smaller, more discrete fields than those produced by caudal stimulation sites. Further studies showed that the relationship between SPA and stimulation site is influenced by the electrical stimulation current level. Each site had an optimum current level, so that current intensities higher or lower than the optimum produced decreased analgesia. These results reveal a possible neural mechanism for somatotopic organization in hyperstimulation analgesia in which an intense somatic input at a body site produces analgesia in the same or adjacent segments.

Cross-correlation analysis of midbrain reticular neuron pairs during sleep-waking cycle of the cat

By simultaneously inserting three extracellular microelectrodes, separated by 1.0 mm from each other, into the cat's midbrain reticular formation (MRF), temporal cross-correlation of firing of neuron pairs was measured to investigate the mode of interaction among the MRF neurons during different states of sleep and wakefulness. None of 97 neurons pairs studied gave clear-cut cross-correlograms suggesting cascade connection between two neurons. However, 29 neuron pairs showed weakly synchronized firing which occurred periodically with a mean interval of 1.23 s. This interval was different from the respiratory cycle. The rhythmic synchronization appeared most obviously during slow wave sleep. The synchronized firing was encountered more often in the pairs of adjacent neurons picked up with a single electrode and in the pairs picked up with separate electrodes positioned along the mediolateral axis, when compared with the neuron pairs located along the rostrocaudal axis. About 80% of the neurons (37 our of 46) which showed periodically synchronized discharge had no corresponding periodicity in their auto-correlograms. Simple synaptic linkage conceivable for the observed periodic synchronization of a neuron pair would be shared inhibition with the common inhibitory source activated periodically. Some neurons in the dorsal raphe nucleus and in the solitary tract nucleus were also examined for the cross-correlation with the MRF neurons. However, no positive relationship was found.

The origin of the spinomesencephalic tract in the rat: an anatomical study using the retrograde transport of horseradish peroxidase

An anatomical technique based on the retrograde transport of horseradish peroxidase (HRP) was used to investigate the projections of spinal cord neurons to the mesencephalic tegmentum in the rat. Restricted unilateral injections were confined to central grey, cuneiformis areas, and superior colliculus. Injections into all these loci produced labeling in similar spinal areas. Only quantitative differences were noted. In the spinal grey matter, numerous labeled cells were regularly encountered in the marginal zone, the lateral part of the neck of the dorsal horn, and the dorsal grey commissure. Projections from the marginal zone and neck of the dorsal horn were predominantly contralateral. In the white matter, a pronounced bilateral labeling was observed in the nucleus of the dorsolateral funiculus, thus confirming our previous electrophysiological findings (Menétrey et al., '80). This distribution of labeled cells was commonly observed throughout the whole length of the cord. Additional sites of projecting cells have also been identified at the most rostral levels (obex, C1, C2). They mostly derived from spinal extensions of the dorsal column nuclei and lateral cervical nucleus contralaterally; from the lateral ventral horns bilaterally and from the nucleus commissuralis ipsilaterally. This study is thus a clear confirmation that the mesencephalic tegmentum constitutes a target for various somatosensory inputs originating from spinal cord, dorsal column nuclei, and lateral cervical nucleus. Moreover, from these results together with those obtained for the spinothalamic tract in the rat, it appears that marginal and dorsolateral funiculus neurons preferentially project to the mesencephalic tegmentum. The importance of marginal zone projections underlines the involvement of the spinomesencephalic tract in pain mechanisms.