The efferent connections of the head of the caudate nucleus in the cat: an experimental morphological study with special reference to a projection to the raphe nuclei

Multiple striatal representation in primate substantia nigra

The pattern of arborization of the striatonigral fibers in the squirrel monkey (Saimiri sciureus) was studied with Phaseolus vulgaris-leucoagglutinin (PHA-L) and biocytin as anterograde tracers. Single, small injections of PHA-L or biocytin in either the caudate nucleus or the putamen give rise to at least four distinct, nonoverlapping but interconnected fiber plexuses that are distributed throughout the rostrocaudal extent of the substantia nigra pars reticulata (SNr) according to a strikingly precise and constant sequence. These plexuses, which comprise numerous fibers that closely entwine unlabeled dendrites of SNr neurons (woolly fibers), often lie at the base of dopaminergic cell columns of the substantia nigra pars compacta (SNc). Long and varicose fibers emerge dorsally from SNr plexuses and climb along the ventrally oriented dendrites of dopaminergic SNc neurons, as visualized with tyrosine hydroxylase immunohistochemistry. These fibers appear to contact en passant both dendrites and cell bodies of dopaminergic neurons belonging to the ventral tier of SNc. Anterograde double-labeling experiments involving small deposits of PHA-L and biocytin in adjacent areas of the caudate nucleus and the putamen reveal that neighboring striatonigral cell populations form two distinct sets of terminal plexuses that remain well segregated throughout SNr. Plexuses from the two sources interdigitate in some parts of SNr, but never intermix. Furthermore, the woolly fibers in these plexuses are composed exclusively of either PHA-L- or biocytin-labeled elements; none of them display both types of labeling. These results reveal that the striatonigral projection in primates is highly divergent and that the striatum has multiple representations at nigral levels. They also indicate that striatal information is conveyed to the substantia nigra in a highly ordered fashion through multiple segregated channels.

Organization of the striatal projections from the rostral caudate nucleus to the globus pallidus, the entopeduncular nucleus, and the pars reticulata of the substantia nigra in the cat

This study explores the organization of the striatal projections from the rostral caudate nucleus to the output nuclei of the basal ganglia in the cat. Tracer deposits were stereotaxically injected in different dorsoventral, mediolateral, and rostrocaudal sectors of the head of the caudate nucleus using horseradish peroxidase (HRP) conjugated with wheat germ agglutinin (HRP-WGA) either alone or mixed with free HRP. After the injections, a detailed analysis of the terminal labeling was carried out within the globus pallidus (GP), the entopeduncular nucleus (Ep), and the substantia nigra (SN) pars reticulata (SNR). Our findings illustrate how different dorsoventral, mediolateral, and rostrocaudal parts of the rostral caudate nucleus project primarily to similarly positioned but spatially segregated parts of GP. The striatoentopeduncular pathway was also organized topographically, but there was overlapping by projections from different parts of the rostral caudate nucleus. Areas of topographical segregation and zones of overlap were detected in the organization of the striatal projections from the rostral caudate nucleus to SNR. These results raise the possibility of distinct striatal actions upon different sectors of the output nuclei of the basal ganglia and, indirectly, upon their targets in the thalamus and brainstem.

Regional differences in the cat caudate nucleus as to the effectiveness in inducing contraversive head-turning by electrical stimulation

An attempt was made to re-examine regional differences in the cat caudate nucleus as to the effectiveness in inducing contraversive head-turning by electrical stimulation and to analyze the time course of head-turning quantitatively. In 5 of the total 9 cats, the right sensorimotor cortex and its surrounding areas had been ablated chronically. While the awake, unrestrained cat maintained a stable standing posture facing forward, stimulation was applied systematically to various points in and around the caudate nucleus with a movable stimulating electrode. Trains of stimulating current pulses of less than 300 microA were given, mostly at a rate of 100 Hz for 5 s. In most experiments in which stimulation was given to the side of the intact cerebral cortex, stimulation of caudal portions of the head of the caudate nucleus was effective in inducing contraversive head-turning, but that of its rostral portions was ineffective. In experiments on the side of chronic cortical ablation, similar results were obtained. These results suggested that head-turning induced by stimulation of the caudate nucleus was brought about not by the activation of the corticofugal fibers from these cortical areas by a current spreading to the internal capsule, but by the activation of caudate neurons. Hence, it was demonstrated that there were regional differences in the cat caudate nucleus as to the effectiveness in inducing head-turning. The mean of the shortest latencies of the onset of head-turning for individual stimulation points was 396 ms (S.D., 210 ms) for the side of the intact cerebral cortex, and 454 ms (S.D., 289 ms) for the side of the cortical ablation. Statistically, there was no significant difference between them. Therefore, it was further revealed that the elimination of the sensorimotor cortex did not affect the caudate-induced head-turning in terms of the latency of its onset.

Efferent connections of the caudate nucleus, including cortical projections of the striatum and other basal ganglia: an autoradiographic and horseradish peroxidase investigation in the cat

Tritiated tracer was injected into the head of the caudate nucleus in cats. Following such injections, labeling is present within extensive regions of both the globus pallidus and entopeduncular nucleus, where it presents a mottled or meshlike appearance. These projections are topographically organized in that there is simple correspondence between the mediolateral, dorsoventral, and rostrocaudal origin of the caudate projection and its input to the globus pallidus and entopeduncular nucleus. Transported tracer is also present within the substantia nigra, where it is most abundant within the pars reticularis. However, distinct labeling also overlies cells of the pars compacta, and lesser amounts of labeling are present within the pars lateralis and within the retrorubral area. Following injections of horseradish peroxidase into the caudate nucleus, and subsequent tissue processing by the tetramethylbenzidine (TMB) method of Mesulam ('78), labeled anterograde fibers are present in abundance within the globus pallidus, entopeduncular nucleus, and all subdivisions of the substantia nigra, thus confirming the autoradiographic findings. Also, it is especially obvious in this HRP material that, contrary to previous degeneration studies, both the rostromedial and caudolateral parts of the pars lateralis of the substantia nigra contain numerous anterogradely labeled fibers. Retrogradely labeled neurons are also present within the substantia nigra of these same tissue sections, where they are most abundant within the pars compacta, but lesser numbers of labeled neurons are also present within the pars reticularis, pars lateralis, retrorubral area, and ventral tegmental area on the ipsilateral side, and all of these same subdivisions of the substantia nigra on the contralateral side. Also, within the subthalamic nucleus in these experiments, there are anterogradely labeled fibers, as well as retrogradely labeled neurons, which are interpreted to represent a reciprocal connection between the subthalamic nucleus and the striatum. In a separate series of experiments, horseradish peroxidase was injected into the motor cortex-specifically into the anterior sigmoidal gyrus. Following such injections, labeled neurons representing afferents to the motor cortex are found in all subcortical nuclei commonly known as the "basal ganglia," including the caudate nucleus, putamen, globus pallidus, entopeduncular nucleus, substantia innominata, nucleus of the diagonal band of Broca, medial septal nucleus, claustrum, and basolateral amygdaloid nucleus.

Long-term local and distal increases in tryptophan hydroxylase activity following intracerebral kainic acid injections in the rat

The administration of kainic acid (1--2 micrograms) into the right striatum of adult rats resulted in a marked local increase in tryptophan hydroxylase activity (+ 54--106%). This change was significant as soon as on the second day after the treatment and persisted for at least 12 days. In addition, long-lasting elevations of tryptophan hydroxylase activity were also observed in the anterior raphe area, septum and ipsilateral hippocampus and cerebral cortex. In contrast, the intrahippocampal injection of kainic acid (1 microgram) induced a long-term increase in tryptophan hydroxylase activity only in the injected structure. In all cases, the changes in tryptophan hydroxylase activity were associated with significant increases in the Vmax of the enzyme with no alteration of its apparent affinities for tryptophan and the pterin cofactor. Studies of the sensitivity of tryptophan hydroxylase from control and from kainic acid-treated rats to in vitro activating conditions (Ca2+-dependent phosphorylation, partial trypsinization, exposure to sodium dodecyl sulfate) suggest that the intrastriatal injection of the neurotoxin induced a long-lasting activation of the enzyme. These findings indicate that intracerebral injections of kainic acid may be a valuable approach to explore further the mechanisms controlling tryptophan hydroxylase activity in vivo.

Ultrastructure and acetylcholinesterase content of neurones forming connections between the striatum and substantia nigra of rat

Neurones in the striatum and substantia nigra (zona compacta) of the rat were divided into several classes according to differences in morphological appearance and ultrastructural localization of acetylcholinesterase (AChE). To determine which of the neurone types form connections between the substantia nigra and striatum, the ultrastructural localization of AChE was combined with retrograde axonal transport of horseradish peroxidase (HRP). The products of the histochemical reactions for HRP and AChE are localized in separate intracellular compartments and so identification of the efferent cell types was possible. In the striatum there are two morphological classes of AChE-containing neurone and two or three varieties of cell which do not stain for the enzyme. Only one neurone type projects to the substantia nigra and this is a cell with no AChE which makes up 96% of the total neurone population. In the substantia nigra there are two types of neurone with AChE: a cell with rather variable amounts of AChE which projects to the striatum, and another rarer type, possibly an interneurone.

Pallidosubthalamic projection in the cat. Electron microscopic study

The degenerative changes within the cat's subthalamic nucleus (Sth) following lesions of the external pallidum were studied by electron microscopy. Four to five days following pallidal lesions a great number of terminals undergoing degenerative changes were encountered in the ipsilateral Sth. The contralateral Sth was free of degeneration. The degenerating terminals show predominantly the light degenerative type, less frequently the dark degenerative pattern, and occasionally exhibit signs of filamentous hyperplasia. The degenerated boutons usually insert on perikarya of the large Sth neurons, on proximal dendrites, and more rarely contact dendritic spines. They were observed neither to perform synaptic contacts with the perikarya of the small Sth neurons nor with other vesicle-containing profiles. On the basis of the ultrastructural aspect of the degenerating terminals, they were identified as F1 terminals, discriminated in a previous study (Romansky et al., 1978). The normal appearance, the synaptic relationships, and the degenerative features of the F1 terminials in the Sth closely resemble the entopeduncular terminals in the thalamus described by Rinvik and Grofová (1974a), and Grofová and Rinvik (1974). The possible contribution of the interrupted passing fibers to the observed degeneration is discussed. The present findings corroborate the relevant morphological, physiological, neurochemical, and neuropharmacological data in the literature.

Aspartate, glutamate and GABA levels in pallidum, substantia nigra, center median and dorsal raphe nuclei after cylindric lesion of caudate nucleus in cat

Aspartate, glutamate and GABA levels were determined in afferent and efferent projection nuclei of the striatum after unilateral cylindric lesion in the head of the caudate nucleus in cats. Two and four weeks after operation, GABA content was significantly reduced in substantia nigra and pallidum ipsilateral to the lesion. Glutamate (GLU) level was decreased in substantia nigra and pallidum only 4 weeks after lesion, whereas aspartate content in substantia nigra decreased significantly already after 2 weeks. No changes in the contents of these amino acids were detected in the dorsal raphe nucleus, which receives a projection from the caudate, as well as in the center median nucleus, which projects to the striatum. These experiments using longer survival times substantiate the role of GABA in caudato-pallidal and caudatonigral projections. The possibility is discussed that aspartate (ASP) could function as transmitter of cortico-nigral fibers.

The raphe nuclei of the rabbit brain stem

The raphe nuclei of the rabbit brain stem were found in the midline and adjacent reticular formation of the medulla, pons, and mesencephalon. Nuclei raphe obscurus, pallidus, and magnus were located in the medulla. Nucleus raphe pontis and the caudal portion of nuclei raphe dorsalis and centralis superior were present in the pons. The rostral portion of nuclei raphe dorsalis and centralis superior, and nuclei linearis caudalis and intermedius were present in the msencephalon. Wings of neurons extended from the midline clusters of raphe neurons into the adjacent reticular formation. These wings of neurons contained serotonergic perikarya which were cytoarchitecturally indistinguishable from the midline neurons. A detailed localization of these nuclei is presented in atlas form. These raphe nuclei contained heterogeneous populations of neurons which varied in the size, shape and density of the cell bodies. In addition, the dendritic branching, specific orientation of dendrites, and appearance of spines were distinct for each of the raphe nuclei. Individual raphe nuclei often contained several subpopulations of neurons characterized by unique spatial configuration and orientation. The main morphological similarities of the raphe nuclei are location in or adjacent to the midline, the presence of serotonergic cell bodies in all raphe nuclei except the linear nuclei, and heterogeneous cell populations.

Changes in morphine self-administration after tel-diencephalic lesions in rats

Rats were trained to bar-press for intravenous infusions of morphine sulfate during 1-h daily test sessions. Rates of morphine self-administration were enhanced by lesions of the frontal cortex and hippocampus and transiently reduced by lesions of the medial forebrain bundle and medial thalamus. Dose-response studies indicated that sensitivity to morphine's rewarding property was decreased by frontal cortical and hippocampal lesions. Lesions of the posterior cortex, the tuberculum olfactorium, and the nucleus accumbens had no effect on self-administration behavior. The results are discussed in relation to previous findings with caudate and brainstem lesions. A neuroanatomical substrate for morphine reinforcement is suggested.

Afferents to brain stem nuclei (brain stem raphe, nucleus reticularis pontis caudalis and nucleus gigantocellularis) in the rat as demonstrated by microiontophoretically applied horseradish peroxidase

Using a retrograde tracer technique with microiontophoretically applied horseradish peroxidase (HRP), afferent projections to the brain stem raphe nuclei (BR, raphe magnus, pallidus and obscurus) and to two adjacent reticular nuclei, nucleus reticularis pontis caudalis (nRPC) and nucleus gigantocellularis (nGC) were identified. The most striking difference between the afferent projections to the BR and the adjacent nuclei as determined by this method is that afferents to the BR originate primarily from structures rostral to the pons, especially the mesencephalic central gray and the dorsal and ventral tegmentum. In contrast, the two reticular nuclei studied (nGC and nRPC) received afferent projections within or caudal to the pons-medulla. For example, the nGC receives prominent afferent projections from the gray matter of the spinal cord. In addition, evidence for interconnections between all of the adjacent nuclei (BR, nGC and nRPC) was found. Such afferent projections are compatible with the notion that the brain stem raphe nuclei may serve as connections within the brain stem for a descending system, while the nGC may be a relay in a feedback loop between the spinal cord and the reticular formation.

The cerebellar projection from the raphe nuclei in the cat as studied with the method of retrograde transport of horseradish peroxidase

Following injections of horseradish peroxidase (HRP) in the cerebellar cortex and nuclei of the cat, the distribution of labeled cells in the raphe nuclei was mapped. The findings confirm those made previously in studies of retrograde cell degeneration following cerebellar ablations (Brodal et al., 1960a), and in addition reveal new details in the projection of the raphe nuclei onto the cerebellar cortex and nuclei. All the raphe nuclei except nucleus linearis intermedius and nucleus linearis rostralis project onto the cerebellar cortex. The nuclei raphe obscurus and pontis contribute the greatest number of afferents to the cerebellum. With the exception of lobule VI which probably is the recipient of a weak projection, all parts of the cerebellar cortex receive afferents from the raphe nuclei. The heaviest projection is to the vermis of lobules VIIA and X, and to crus II. The afferents to the cerebellar nuclei are few in number (Tables 2-6). The observations indicate that each raphe neuron probably projects to more than one terminal site in the cerebellum. The findings are discussed with reference to other efferent and afferent studies of the raphe nuclei. All these studies indicate that the raphe nuclei have widespread efferent and afferent connections, making them capable to participate in a variety of regulatory functions.

The neurological basis of motor asymmetry following unilateral 6-hydroxydopamine brain lesions in the rat: the effect of motor decortication

The role of the motor cortex in the mediation of asymmetric movement after unilateral 6-hydroxydopamine (6-OHDA) brain lesions has been examined in the rat. The effect of unilateral and bilateral sensorimotor decortication upon spontaneous and drug (apomorphine and amphetamine)-induced motor asymmetry has been studied following unilateral injection of 6-OHDA into the lateral hypothalamus. Both bilateral decortication and unilateral decortication on the side of the 6-OHDA lesion caused a transient reversal of the spontaneous motor asymmetry normally seen in 6-OHDA lesioned animals. Neither unilateral nor bilateral sensorimotor decortication abolished drug-induced turning behaviour. It is postulated that subcortical basal ganglia efferent pathways may be involved in the mediation of motor asymmetry after unilateral 6-OHDA lesions of the nigrostriatal dopaminergic projection.

Character and origin of rotatory movements evoked by electrical stimulation of the caudate nucleus in cats

High-frequency stimulation of the caudate nucleus in cats induces three types of rotatory responses: turning the head or stepping movements in a circle toward the side opposite to that of stimulation, psychomotor excitation with contralateral rotation, and ipsilateral turning of the head and trunk. Movements of the last type developed mainly from the ventro-lateral zones of the head of the caudate nucleus, whereas psychomotor excitation, on the other hand, developed from the dorsal zones. Rotatory movements do not depend significantly on a contribution of the internal capsule or cortex. Slow synchronized activity on the EEG at the time of rotation was abolished after curarization of the animals. Caudate rotation is partly due to activation of the globus pallidus, for electrical stimulation of that structure raises the threshold of the responses although it does not block them completely.

Habenular and other midbrain raphe afferents demonstrated by a modified retrograde tracing technique

Afferents to th midbrain dorsal and median raphe nuclei in the rat were studied by means of the horseradish peroxidase (HRP) retrograde transport method. The HRP was given by means of a modified iontophoretic delivery technique. This technique permitted an efficient and localized deposition of a high concentration of HRP into the raphe nuclei. Afferents to the raphe as determined by this method could be categorized into 2 classes; those exclusively to the raphe and those also positive for adjacent reticular formation. The most striking afferent area to the raphe, both in terms of selectivity and density, was the lateral habenula. This result is in accord with previous studies using degeneration methods which indicate an habenular projection to the raphe area. There were afferents exclusively positive for the dorsal raphe nucleus emanating from the nucleus of the solitary tract. Most other raphe afferent areas were also positive for the reticular formation (e.g;, prefrontal cortex, medial forebrain bundle, preoptic nuclei, and reticular formation). The existence of a major afferent system from the lateral habenula to the midbrain raphe is consistent with the concept of a "dorsal pathway" which might be responsible for relaying information from forebrain limbic structures to the "midbrain limbic areas".

Serotonergic afferents to the dorsal raphe nucleus: evdience from HRP and synaptosomal uptake studies

Afferent connections of the serotonin (5-HT)-containing dorsal raphe nucleus were investigated in the rat utilizing the horseradish peroxidase (HRP) retrograde cell labeling technique. Small quantities (0.1-0.5 mul) of HRP solutions were infused into the dorsal raphe, and the brains were examined 19-72 h later for retrograde transport of the enzyme. Intrinsic connections within the dorsal raphe nucleus were revealed by this mapping technique, as was an input to the dorsal raphe from another serotonergic cell group, the median raphe nucleus. Little evidence was found for projections from other, more remote, brain sites. A serotonergic innervation of the dorsal raphe was also demonstrated by the presence of high affinity uptake of [3H]5-HT (Km=0.17 muM) into synaptosomal suspensions of the dorsal raphe nucleus. Synaptosomal uptake of [3H]5-HT was blocked by selective destruction of serotonergic axon terminals induced by the intraventricular injection of 200 mug of 5,7-dihydroxytryptamine following desipramine HCl pretreatment, but not by destruction of catecholaminergic axon terminals induced by intraventricularly injected 6-hydroxydopamine (2 X 250 mug). The uptake of [3H]-5-HT by synaptosomes of the dorsal raphe was comparable to that of striatal and hypothalamic synaptosomes, and markedly greater than that of synaptosomes from the cerebellum or nearby dorsal central gray or midbrain reticular formation, indicating the presence of a relatively dense serotonergic innervation. These data together indicate that neurons in the dorsal raphe nucleus receive a prominent serotonergic input that is derived, at least in part, from other neurons within the dorsal nucleus and from a neighboring raphe nucleus.

The precise localization of nigral afferents in the rat as determined by a retrograde tracing technique

Afferent innervation of the rat substantia nigra (SN) was studied by the retrograde horseradish peroxidase (HRP) method. High concentrations of HRP were deposited in discrete subregions of the SN by means of a microiontophoretic delivery system. Using this technique it was possible to demonstrate that the caudatonigral projection system is arranged topographically; All portions of the caudate-putamen except for a central medial core were found to contain HRP positive cells, indicative of retrograde transport. In the positive areas a much larger percentage of cells (30-50%) were found to participate in this projection than has previously been reported. Only medium size cells (12-20 mum) were found to contain the HRP reaction product. Other areas found to heavily innervate the SN were the globus pallidus, central nucleus of the amygdala and dorsal raphe nucleus. Areas containing fewer reactive cells but which also appear to innervate the SN included the prefrontal cortex and lateral habenula. These results emphasize the importance of striatonigral projections which recent studies have suggested contain a GABAergic link.

Schizophrenic psychology, associative learning and the role of forebrain dopamine

It is suggested that schizophrenic thinking can be explained as a lowering of levels of significance for acceptance of conclusions based on inductive logic. The formal similarity between inductive logic and operant, or classical conditioning is pointed out. It is thus possible to explain the therapeutic effects of neuroleptic drugs by referring to the effect of these, and related drugs, and of lesions of ascending dopamine pathways, on acquisition of conditioned responses. It is tentatively suggested that recognition of association of related features of the environment, whether in humans or animals occurs in the basal ganglia by a dopamine dependent process. A role is suggested for neocortical noradrenaline in consolidaton of newly acquired associations. Implications and tests of this hypothesis are discussed.

The nigrostriatal projection in the cat. Part 1. Silver impregnation study

The course and destination of the degenerating nigrostriatal fibers were studied by selective silver impregnation methods in 37 cats with unilateral lesions in the substantia nigra. The nigrostriatal fibers ascend along the dorsomedial border of the substantia nigra to the prerubral area; they proceed for a short distance through the lateral hypothalamus, enter the medial part of the internal capsule and run in a dorsorostral direction to reach the head of the caudate nucleus and the rostral portion of the putamen. A smaller number of degenerating fibers obliquely cross the peduncular part of the internal capsule and traverse the entopeduncular nucleus and the pallidum to terminate in the central and caudal portions of the putamen. Some features of the topical distribution of the nigrostriatal tract are described. Apparently, the more anterior part of the pars compacta sends axons primarily to the head of the caudate nucleus and to the most rostral putamen. The most medially situated nigral neurons project to the fundus striati. The posteromedial cell groups of the pars compacta innervate primarily the central putamen and the caudal part of the caudate nucleus. The projection of the lateral cell group of the posterior zona compacta to the caudal putamen is sparser than from the other nigral groups, suggesting that a part of them has another destination, possibly lower in the neuraxis. The contribution of the pars of reticulata to the nigrostriatal connections seems to be modest, according to the small number of neurons; they project to the lateral caudate and putamen. Thus, the ascending nigrostriatal fibers mirror the distribution of the descending striatonigral fibers. No convincing evidence for the existence of a nigroentopeduncular and nigropallidal projection was found.

The discrimination of nine different types of synaptic boutons in the fundus striati (Nucleus accumbens septi)

An attempt has been made to discriminate additional types of synapses than have been previously described in the nucleus accumbens septi of the cat, which can, according to Brockhaus (1942), justifiably be termed the fundus striati due to the fact that it possesses all of the morphological and some of the neurochemical features of the striatum. This was undertaken in order to correlate at least one type of synapse with each different afferent pathway. Nine distinct types of synapses could be differentiated electron microscopically: Type I: axo-spinous synapses with sparse, small, round vesicles which seemed to be the nigro-striatal endings (35%). Type II: axo-somatic or axo-dendritic en passant synapses containing small, round vesicles (3%). Type III: axo-spinous synapses filled with densely-packed, small, round vesicles displaying strong postsynaptic thickenings which seem to be cortico-striatal (17%). Type IV: large axo-spinous synapses with densely-arranged, small, round vesicles contacting larger spines branching off a pedicle (9%). Type V: axo-somatic or axo-dendritic synapses containing large pleomorphic vesicles, probably axon collaterals (1%). Type VI: axo-somatic or axo-dendritic synapses with elongated small vesicles (20 X 45 nm) (3%). Type VII: large axo-somatic or axo-dendritic synapses filled by densely-packed, small, round vesicles (11%). Type VIII: large axo-somatic or axo-dendritic synapses containing loosely-arranged, small, round vesicles (8%). Type IX: axo-somatic or axo-dendritic synapses containing large, round vesicles in a translucent axoplasm (13%).

Raphe neurons and barbiturate induced rhythmic activity in the subthalamic nucleus and ventral tegmental area

A continuous rhythmic discharge in the subthalamic nucleus (STN), lateral habenular nucleus, and ventral tegmental area (VTA) is associated with tremor in cats lightly anesthetized with barbiturate. The pontine raphe nuclei centralis superior and inferior also show barbiturate dependent slow wave and unitary activity. Stimulation of the raphe region produces evoked potentials and synchronization of the oscillatory activity in STN and VTA. The latency of the evoked response is shortest in STN. Extensive raphe lesions abolish the rhythmic activity in STN and VTA, but lesions which do not completely destroy nuclei centralis superior and inferior are not effective. In single cell recordings 81% of 142 cells in the pontine raphe region and medial reticular formation are inhibited following VTA stimulation. Fifty-four percent of 129 cells in the same region were inhibited when tested with STN stimulation. Forty-seven raphe paramedian reticular formation cells showed convergence of inhibitory input from STN, VTA, and contralateral sciatic nerve (Group II and III fibers). Depletion of serotonin with rho-chlorophenylalanine did not affect the appearance of barbiturate rhythmic activity. It is concluded that the pontine raphe region is not essential for maintenance of the oscillatory activity in STN and VTA, and that the raphe region is on the output side of the system.