Electrophysiological and anatomical studies on thalamic mediodorsal nucleus projections onto the prefrontal cortex in the cat

Electrical stimulation of the mediodorsal nucleus (MD) of the thalamus elicited field potentials in the gyrus proreus (PRO), frontalis (FR), rectus (RE) and cinguli anterior (CIant) of the ipsilateral prefrontal and adjacent cortical areas in cats. The results of a laminar field potential analysis indicate that the field potentials can be regarded as a combination of deep and superficial thalamocortical responses. By injecting horseradish peroxidase (HRP) into the MD, HRP-labeled terminals were distributed in the prefrontal and adjacent cortical areas where the field potentials were elicited. Densely labeled terminals in cortical layer I were distributed where the superficial thalamocortical responses were prominent, while those in layers III–V were distributed in the areas where the deep thalamocortical responses were prominent.     

The distribution of neurons in the substantia nigra pars reticulata with input from the motor, premotor and prefrontal areas of the cerebral cortex in monkeys

We investigated the distribution of neurons in the substantia nigra pars reticulata (SNr) which received cortical input. The activities of single SNr neurons were studied extracellulary in awake monkeys. SNr neurons showed excitatory and/or inhibitory responses to cortical stimulation. These responses were considered to be mediated by the subthalamic nucleus and striatum, respectively. The neurons receiving inhibitory input from the motor, premotor and supplementary motor areas (Motor-related cortical areas) were located in the lateral part of the SNr, whereas those with input from the medial, dorsal and orbital areas of the prefrontal cortex (PFmdo) were frequently found in the rostro–medial part of this nucleus. SNr neurons with inhibitory input from the ventral periprincipal area (PSv) were mainly distributed in the intermedio–lateral portion, with some degree of overlap with input from other cortical areas. The distribution of the excitatory input was almost similar to that of inhibitory one, but the excitatory input from the PSv was much stronger than that from the PFmdo. Some SNr neurons receiving cortical input were proved to project to the thalamus. Our results support the existence of several parallel organization of the cortico–basal ganglia loop circuits [G.E. Alexander, M.R. DeLong, P.L. Strick, Parallel organization of functionally segregated circuits linking basal ganglia and cortex, Ann. Rev. Neurosci., 9, 1986, pp. 357–381.], but interaction between the loops can not be ignored.     

Regulation of dopamine function in the prefrontal cortex of the rat by the thalamic mediodorsal nucleus

Dopamine (DA) utilisation has been assessed in the medial bank of the prefrontal cortex (FCx) and the agranular insular cortex (AgCx) of the rat in response to unilateral manipulations of the thalamic mediodorsal nucleus (MD). The ratios of 3,4-dihydroxyphenylacetic acid (DOPAC):DA and 4-hydroxy-3-methoxyphenylacetic acid (homovanillic acid, HVA):DA are used as indices of DA utilisation and were shown to increase in the ipsilateral FCx following electrical stimulation of lateral MD. A similar response was observed 1 hr after an infusion of the excitotoxin sodium ibotenate into lateral MD, although in this case the increase in DA utilisation in FCx was bilateral. Longer periods of recovery after ibotenate treatment (2 day and 1 week) produced DA utilisation ratios that had returned to near control values and by 1 week a significant decrease was detected in HVA:DA of the contralateral FCx. All treatments had little effect on DA utilisation in AgCx, although there was a tendency towards enhanced ratios after electrical stimulation and short-term ibotenate injection. These findings suggest that stimulation of MD neurones may tend to activate the DA system in their convergent terminal regions of cortex. It is argued that these influences result from interactions at the level of the DA terminal rather than at the cell bodies of mesocortical DA neurones.     

Thalamic paraventricular nucleus neurons collateralize to innervate the prefrontal cortex and nucleus accumbens

The prefrontal cortex and nucleus accumbens are primary recipients of medial thalamic inputs, prominently including projections from the thalamic paraventricular nucleus. It is not known if paraventricular neurons collateralize to innervate both the prefrontal cortex and nucleus accumbens. We used dual retrograde tract tracing methods to examine this question. A small population of paraventricular neurons was found to innervate the prefrontal cortex and medial nucleus accumbens. These data suggest that the thalamic paraventricular nucleus may coordinately influence activity in the prefrontal cortex and ventral striatum.     

Monosynaptic and disynaptic projections from the substantia nigra pars reticulata to the parafascicular thalamic nucleus in the rat

We examined a direct pathway and an indirect pathway via the reticular thalamic nucleus (RT) from the substantia nigra pars reticulata (SNr) to the parafascicular thalamic nucleus (PF) by using anterograde and retrograde tract tracing methods. After biotinylated dextranamine (BDA) injection into the dorsolateral part of the SNr, many labeled fibers and axon terminals were distributed in the ventral part of the RT, as well as in the ventrolateral part of the PF, bilaterally with an ipsilateral dominance. After BDA injection into the ventral part of the RT, a plexus of labeled axons was found bilaterally with an ipsilateral dominance in the ventrolateral part of the PF. After combined injections of BDA into the dorsolateral part of the SNr and cholera toxin B subunit (CTb) into the ventrolateral part of the PF on the same side, overlapping distribution of BDA-labeled fibers and CTb-labeled neurons was observed in the ventral part of the RT ipsilateral to the injection sites, where the BDA-labeled axon terminals made symmetrical synaptic contacts with soma and dendrites of the CTb-labeled neurons.     

Subthalamic nucleus lesion regularizes firing patterns in globus pallidus and substantia nigra pars reticulata neurons in rats

Subthalamic nucleus lesion altered the statistical properties of the firing patterns of globus pallidus and substantia nigra pars reticulata neurons recorded in urethane anesthetized rats by increasing the proportion of cells in both structures that fired with a very highly regular pattern (from 25%to 50%). In all cases, the most regularly firing neurons fired at a higher mean rate than did more slowly firing neurons. In contrast, globus pallidus lesion shifted the pattern of substantia nigra neurons towards more irregular firing and induced a bursty pattern in two neurons.     

Synaptic organization of GABAergic projections from the substantia nigra pars reticulata and the reticular thalamic nucleus to the parafascicular thalamic nucleus in the rat

The ventrolateral part of the parafascicular thalamic nucleus (PF), which is considered to take part in the control mechanism of orofacial motor functions, receives projection fibers not only from the dorsolateral part of the substantia nigra pars reticulata (SNr) but also from the ventral part of the reticular thalamic nucleus (RT) [Tsumori et al., Brain Res. 858 (2000) 429]. In order to better understand the influence of these fibers upon the PF projection neurons, the morphology, synaptology and chemical nature of them were examined in the present study. After ipsilateral injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into the dorsolateral part of the SNr and biotinylated dextran amine (BDA) into the ventral part of the RT, overlapping distributions of PHA-L-labeled SNr fibers and BDA-labeled RT fibers were seen in the ventrolateral part of the PF. At the electron microscopic level, the SNr terminals made synapses predominantly with the medium to small dendrites and far less frequently with the somata and large dendrites, whereas approximately half of the RT terminals made synapses with the somata and large dendrites and the rest did with the medium to small dendrites of PF neurons. Some of single dendritic as well as single somatic profiles received convergent synaptic inputs from both sets of terminals. These terminals were packed with pleomorphic synaptic vesicles and formed symmetrical synapses. After combined injections of PHA-L into the dorsolateral part of the SNr, BDA into the ventral part of the RT and wheat germ agglutinin-horseradish peroxidase (WGA-HRP) into the ventrolateral part of the striatum or into the rostroventral part of the lateral agranular cortex, WGA-HRP-labeled neurons were embedded in the plexus of PHA-L- and BDA-labeled axon terminals within the ventrolateral part of the PF, where the PHA-L- and/or BDA-labeled terminals were in synaptic contact with single somatic and dendritic profiles of the WGA-HRP-labeled neurons. Furthermore, the SNr and RT axon terminals were revealed to be immunoreactive for gamma-aminobutyric acid (GABA), by using the anterograde BDA tracing technique combined with immunohistochemistry for GABA. The present data suggest that GABAergic SNr and RT fibers may exert different inhibitory influences on the PF neurons for regulating the thalamic outflow from the PF to the cerebral cortex and/or striatum in the control of orofacial movements.     

Distinct adenosine deaminase-containing inputs to the substantia nigra from the striatum and tuberomammillary nucleus

Immunohistochemical, neuroanatomical and lesion methods were used to investigate the projections of adenosine deaminase immunoreactive (ADA-IR) neurons in the striatum (caudate/putamen) and hypothalamus to the substantia nigra (SN). Striatal ADA-IR neurons were distributed within two zones; anteriorly in the medial and ventromedial extreme of the head and body of the striatum, and posteriorly in the tail of the striatum. The posterior hypothalamus contained ADA-positive neurons which were confined to the tuberomammillary nucleus (TM). The SN was devoid of ADA-positive neurons, but contained two distinct types of ADA-IR fiber terminations. One type was confined to bands located at the ventrolateral and dorsomedial borders of the pars reticulata and consisted of fine puncta. The other type was distributed throughout the SN and consisted of long, beaded fibers. Injections of the retrograde tracer Fluoro-gold (FG) into the SN gave rise to FG-labelling of significant numbers of ADA-IR neurons in both the striatum and TM. Medial SN injections preferentially labelled ADA-IR neurons in the anterior striatum and lateral SN injections labelled posterior ADA-IR striatal neurons. Kainic acid lesions of the anterior medial striatum selectively abolished the punctate ADA-IR band in the medial SN and left the long, ADA-IR nigral fibers in an apparently hypertrophied state. Despite depletion of ADA-IR neurons in the striatum by kainic acid, ADA activity increased significantly at striatal lesion sites. The results suggest that the SN receives two topographically segregated fine terminal fields from striatal ADA-IR neurons, and a substantial innervation from ADA-IR neurons in the TM as well. These findings add to the heterogeneous chemical composition of nigral afferents and are discussed in the context of adenosine neuromodulatory mechanisms in the striatonigral system.     

Temporal profile of apoptosis at ipsilateral ventroposterior thalamic nucleus and substantia nigra in adult hypertensive rats after cortex infarction

研究背景：已有研究发现,在正常血压的脑梗死模型中,凋亡可发生在远离梗死灶的同侧丘脑和黑质等部位。但是,在高血压脑梗死模型中,这种远隔部位的继发性损害发生情况和相关机制仍不清楚。 研究目标：目前研究验证如下假说：在成年高血压大鼠,神经元凋亡与皮层梗死后丘脑和黑质的神经元丢失和继发性改变有关。 设计、时间和设施：本研究所应用的动物模型、免疫组化和凋亡检测等实验手段均在中山大学附属第一医院神经科实验室完成,完成时间在2006年10月至2008年7月之间。 材料：单克隆小鼠抗大鼠微管相关蛋白（MAP2）和胶质原纤维酸性蛋白（GFAP）分别购自美国Sigma公司和BD Pharmingen公司;生物素结合的山羊抗小鼠IgG和Cy3结合的驴抗小鼠IgG分别购自丹麦DAKO公司和美国Jackson免疫研究实验室;凋亡检测试剂盒则购自瑞士Roche公司和美国Upstate公司。 方法：32只肾血管性高血压大鼠通过电凝法制作右侧大脑中动脉闭塞（MCAO）模型。MCAO后1,2,3和4周,使用免疫组化检测梗死灶同侧丘脑和黑质的MAP2（神经元特异性标记物）和GFAP（神经胶质细胞特异性标记物）表达,同时使用凋亡检测试剂盒检测凋亡细胞。 主要结果分析方法：通过使用Kontron IBAS 2.5自动图像分析系统（德国）分析梗死灶同侧丘脑和黑质5个随机视野中（10×40）MAP2、GFAP阳性细胞,以及TUNEL染色细胞的阳性单位信号值并进行定量分析。 结果：肾血管性高血压大鼠MCAO后4周内,梗死灶同侧丘脑和黑质内MAP2表达逐渐降低而GFAP表达逐渐增加,均以4周为明显;而神经元凋亡明显增加,以2周为明显。 结果：肾血管性高血压大鼠MCAO后梗死灶同侧丘脑和黑质的神经元丢失可能为凋亡的继发性结果。目前关于凋亡动态变化的研究为抗凋亡治疗提供了可能的时间窗     

丘脑底核毁损术对帕金森病大鼠脚内核和黑质网状部的影响

目的 :观察毁损丘脑底核 (STN)对帕金森病 (PD)大鼠脚内核 (EP)及黑质网状部 (SNr)γ -氨基丁酸(GABA)能系统的影响。方法 :将 6 0只Wistar大鼠随机分为 6组 ,每组 10只。对照组采用 6 -OHDA立体定向注入大鼠右侧前脑内侧束 (MFB)和中脑被盖腹侧区 (VTA) ,制成偏侧PD模型。实验组分为第Ⅰ、Ⅱ、Ⅲ、Ⅳ和Ⅴ组 ,分别于 6 -OHDA注射前 7d、注射后 1h、2h、3d、7d5个不同时间点 ,局部注射海人藻酸 (KA)破坏STN。 4周后处死大鼠 ,采用免疫组化染色方法 ,定量测量各组大鼠SNr区和EP区的GABA免疫反应阳性区面积和免疫反应强度。实验数据采用方差分析和t检验统计学处理。结果 :GABA免疫组化显示Ⅰ、Ⅱ、Ⅲ、Ⅳ、Ⅴ实验组的注射侧EP面积分别为正常侧的 72 9%、83 7%、79 7%、88 1%、90 1%。对照组注射侧为正常侧面积的 139 1% (P <0 0 5、0 0 1)。各实验组注射侧EP的GABA免疫反应强度 (积分光密度 )均较正常侧减少 ,对照组注射侧较正常侧积分光密度增加 (P <0 0 5 ,P <0 0 1)。Ⅰ、Ⅱ、Ⅲ、Ⅳ、Ⅴ实验组的注射侧SNr面积分别为正常侧的 90 6 %、86 9%、87 3%、80 5 %、80 4%。对照组注射侧面积为正常侧的 10 8 1% (P <0 0 5、0 0 1) ,各实验组注射侧SNr的GABA免疫反应强度均较正常侧减少 ,对照组注射     

Axosomatic input to subpopulations of cortically projecting pyramidal neurons in primate prefrontal cortex

Pyramidal cells, the major class of cortical excitatory neurons, can be divided into different subpopulations based upon the target region of their principal axon projection. The activity of pyramidal neurons is regulated in part through inhibitory synaptic inputs to the soma from local circuit neurons. However, little is known about how the density of these axosomatic inputs differs among subpopulations of pyramidal neurons in the prefrontal cortex of primates. In this study, retrograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was used to identify pyramidal neurons in monkey prefrontal cortex (areas 9 and 46), which were labeled via either associational (ipsilateral hemisphere) or callosal (contralateral hemisphere) principal axon projections. Ultrastructural analysis revealed that the relative number of terminals apposed to the somatic membrane did not differ between associational and callosal neurons. However, neurons in the supragranular layers were apposed by a significantly greater number of axon terminals than were neurons in the infragranular layers. These findings suggest that the laminar environment of a neuron may play a more important role than principal axon projection in determining the amount of axosomatic inhibitory input it receives. Synapse 25:326–334, 1997. © 1997 Wiley-Liss, Inc.     

Visual pretectal neurons projecting to the dorsal lateral geniculate nucleus and pulvinar nucleus in the cat

We observed morphological subtypes of visual pretectal neurons ascending to the dorsal thalamus, following injections of wheat germ agglutinin conjugated to horseradish peroxidase into the dorsal lateral geniculate nucleus (LGNd) or the pulvinar nucleus. These neurons are composed of fusiform cells and small-sized multipolar cells in the olivary pretectal nucleus, superficial horizontal cells, fusiform cells, small-, medium- and large-sized multipolar cells in the optic tract nucleus, and small- and medium-sized multipolar cells in the posterior pretectal nucleus. When somal size of the neurons projecting to the LGNd was compared to the size of neurons projecting to the pulvinar, the neuronal groups were not identical.     

Simultaneous recording of substantia nigra neurons and voltammetric release of dopamine in the caudate of behaving cats

Simultaneous electrophysiological recordings of single dopamine-containing neurons in the pars compacta of the substantia nigra and the voltammetric release of dopamine in the caudate were made in the behaving cat. Unit activity showed no significant changes during sleep and small changes during active waking, while the release of dopamine in post-synaptic target regions of the caudate nucleus decreased by approximately 35% during sleep and increased approximately 50% during movement. These data demonstrate that recording the electrophysiological activity of single dopamine-containing neurons alone does not accurately reflect the functional state of the central dopamine system. The present study is the first report on the simultaneous measurement of the post-synaptic release of a neurotransmitter and the electrophysiological recording of neurons identified to contain that transmitter substance.     

The neurobiology of thalamic amnesia: Contributions of medial thalamus and prefrontal cortex to delayed conditional discrimination

Although medial thalamus is well established as a site of pathology associated with global amnesia, there is uncertainty about which structures are critical and how they affect memory function. Evidence from human and animal research suggests that damage to the mammillothalamic tract and the anterior, mediodorsal (MD), midline (M), and intralaminar (IL) nuclei contribute to different signs of thalamic amnesia. Here we focus on MD and the adjacent M and IL nuclei, structures identified in animal studies as critical nodes in prefrontal cortex (PFC)-related pathways that are necessary for delayed conditional discrimination. Recordings of PFC neurons in rats performing a dynamic delayed non-matching-to position (DNMTP) task revealed discrete populations encoding information related to planning, execution, and outcome of DNMTP-related actions and delay-related activity signaling previous reinforcement. Parallel studies recording the activity of MD and IL neurons and examining the effects of unilateral thalamic inactivation on the responses of PFC neurons demonstrated a close coupling of central thalamic and PFC neurons responding to diverse aspects of DNMTP and provide evidence that thalamus interacts with PFC neurons to give rise to complex goal-directed behavior exemplified by the DNMTP task.     

Selective responsiveness of medial prefrontal cortex neurons to the meaningful stimulus with a low probability of occurrence in rats

Multi-unit neuronal activity was recorded in the medial prefrontal cortex (mPFC) of 13 chronically prepared male rats while they performed a two-tone discrimination task. Tones at 1000 and 2000 Hz were sequentially presented at intervals of 3-6 s. The duration of each tone was 0.8 s. Rats were trained to press a bar within 1.2 s after the cessation of the 1000 Hz tone (target), and not to press the bar when the other tone (non-target) was presented. Intracranial electrical stimulation (ICS) of the medial forebrain bundle was given as a reward immediately after the rats had correctly responded to the target tone. Probability of the target occurrence was either 30% or 70% in different sessions. When the target tone was presented on only 30% of the trials, the mPFC neurons in the majority of rats tested (10/13) exhibited phasic excitation about 100 ms after the onset of the target tone. However, when the target tone occurred on 70% of the trials, mPFC neurons in most of rats (11/13) did not show excitatory responses, and in some of them (5/13) were inhibited. No mPFC neurons exhibited significant responses to the non-target tone, regardless of its probability. These results suggest that the mPFC neurons selectively respond to meaningful events with a low probability of occurrence.     

Changes in midline thalamic recruiting responses in the prefrontal cortex of the rat during the development of chronic limbic seizures

Purpose:   Mesial temporal lobe epilepsy (MTLE) is a common form of epilepsy that affects the limbic system and is associated with decreases in memory and cognitive performance. The medial prefrontal cortex (PC) in rats, which has a role in memory, is associated with and linked anatomically to the limbic system, but it is unknown if and how MTLE affects the PC.
Methods:   We evoked responses in vivo in the PC by electrical stimulation of the mediodorsal (MD) and reuniens (RE) nuclei of the thalamus at several time points following status epilepticus, before and after onset of spontaneous seizures. Kindled animals were used as additional controls for the effect of seizures that were independent of epilepsy.
Results:   Epileptic animals had decreased response amplitudes and significantly reduced recruiting compared to controls, whereas kindled animals showed an increase in both measures. These changes were not associated with neuronal loss in the PC, although there was significant loss in both the MD and RE in the epileptic animals.
Conclusions:   There is a significant reduction in the thalamically induced evoked responses in the PCs of epileptic animals. This finding suggests that physiologic dysfunction in MTLE extends beyond primary limbic circuits into areas without overt neuronal injury.     

Effects of electrical stimulation on acetylcholine synthesis in cat caudate nucleus

The concentration of endogenous- and deuterium-labeled acetylcholine (ACh) in the cat caudate nucleus was determined after stimulation of either the substantia nigra or the precruciate cortex. In this procedure the caudate nucleus is exposed surgically, and a coring device is used to obtain biopsy specimens which are immediately frozen in liquid nitrogen. Samples are collected at rest, 5 min after stimulation, and again 5 min after a resting period. An infusion of 2H9-choline is maintained during these manipulations to provide a label for ACh synthesis. Electrical stimulation of the substantia nigra, which increases the release of dopamine, produced a decrease in endogenous ACh and the newly synthesized deuterium-labeled ACh. Stimulation of the precruciate cortex produced no significant effect on the levels or synthesis of ACh, but attenuated the effect of subsequent nigral stimulation. These preliminary results indicate that stimulation of the substantia nigra has a net excitatory effect on ACh synthesis in the caudate. This stimulation apparently is modulated by input from the cortex.     

Control of substantia nigra pars reticulata neurons by the nucleus accumbens

In alpha-chloralose anesthetized, immobilized and ventilated cats stimulation of the nucleus accumbens (NA) evokes either inhibition or brief excitation followed by inhibition of extracellularly recorded spontaneously active or sural driven units in substantia nigra pars reticulata (SNpr). Inhibition peaked 50-80 msec following the onset of NA stimulation and persisted for about 300 msec. Pharmacologic interventions designed to characterize this inhibition were performed. Bicuculline, 0.01 to 0.1 mg/kg IV, consistently antagonized NA elicited suppression of SNpr cells. Diazepam 0.5 mg/kg effectively reversed bicuculline actions. Inhibition of spontaneously active units as well as sural excited units was blocked by bicuculline. Strychnine 0.1 to 0.5 mg/kg IV failed to affect inhibition arising from NA. These data suggest that, analogous to the GABAergic striatonigral pathway, inhibition of SNpr cells arising from NA utilizes GABA as a transmitter.     

The insular region: Part of the prefrontal cortex?

Prefrontal and insular regions in different mammalian species are described on the basis of Brodmann's cytoarchitectonic findings. It is shown that the prefrontal cortex, defined cytoarchitectonically, and defined as the projection area of the mediodorsal thalamic nucleus, is often a considerably discordant area. An inverse relationship is assumed between the extent of insular cortex, which on the basis of mediodorsal thalamic afferents has to be termed prefrontal, and frontal cortex, which on the basis of the existence of a granular cell layer four has to be termed prefrontal. It is hypothesized that, because of a lower concordance of additional criteria, on which cortex may be termed prefrontal, and because of evidence, which makes the role of the mediodorsal nucleus for a definition of the prefrontal cortex questionable, nonprimates in comparison to primates in general will possess only “weaker copies” of the primate prefrontal cortex. Tentative support for this view is indicated.     

Pedunculopontine-evoked excitation of substantia nigra neurons in the rat

The effects of electrical stimulation of the nucleus tegmenti pedunculopontinus on the unitary activity of identified neurons of the rat substantia nigra were studied. The experiments were carried out in intact rats as well as in animals bearing either chronic bilateral electrolytic lesions of the deep cerebellar nuclei or an acute lesion of the ipsilateral subthalamic nucleus. Excitation of both compacta and reticulata cells of the substantia nigra (many of the latter being output neurons since they are antidromically activated from the superior colliculus) was the predominant response recorded. Two types of excitations could be distinguished. The first was a direct orthodromic excitation (latency 2.9 ± 1.6ms; duration3.7 ± 1.9ms). The second was a sparse and less pronounced activation (latency 5.2 ± 1.8ms; duration13.0 ± 3.0ms). These two types of excitation were the only responses recorded in intact rats (10/51, 19.6%, orthodromic and 10/51, 19.6%, diffuse activation). When the cerebellar nuclei were destroyed 7–21 days prior to the recording, both excitations were still found (10/59, 16.9% and 15/59, 25.4%, respectively), whereas a minority (3/59, 5.0%) of neurons were inhibited. Conversely, when the subthalamic nucleus was lesioned the orthodromic response was still present (9/42, 21.4%) whereas the occurrence of the diffuse excitation greatly decreased (3/42, 7.1%) and a greater number of inhibitions (6/42, 14.2%) appeared. A small population of cells (12/85, 14.1%) were excited from the contralateral pedunculopontine nucleus either by the orthodromic or by the diffuse excitation. The total number of nigral neurons antidromically activated from the ipsilateral pedunculopontine nucleus was 9/152 (5.9%). The results provide evidence that the nucleus tegmenti pedunculopontinus gives a dual excitatory input to the substantia nigra either through a probable direct connection or through a polysynaptic pathway via the subthalamic nucleus. A few cells from both parts of the substantia nigra, in turn, project back to the nucleus tegmenti pedunculopontinus. In addition, our data give further support to the view that output fibers from the deep cerebellar nuclei do not synapse in the substantia nigra in the rat.