Inhibitory nigral influence on cerebellar evoked responses in the rat ventromedial thalamic nucleus

Summary The ventral medial nucleus of the thalamus (VM) has been shown in rats and cats to constitute a common target for nigro- and cerebellothalamic pathways. In the present study the responses of VM neurons to ipsilateral substantia nigra (SN) and contralateral cerebellar nuclei stimulation were analyzed in the rat. The typical response of VM neurons to SN stimulation consisted of a pure short-latency (1.1–3 ms), short-duration (7–17 ms) IPSP. Latencies of these responses were accorded well with the conduction velocity of the nigrothalamic fibers as measured on the basis of antidromic activation of this pathway. A high percentage (58%) of the thalamic neurons receiving the inhibitory nigral effect were also affected by cerebellar stimulation. The cerebellar effect consisted of a short-latency depolarizing potential which could trigger an action potential. These responses were invariably blocked during the course of SN evoked inhibition. Such convergence was found with fastigial as well as interpositus/dentatus evoked responses.The demonstration of an interaction between the processing of SN and cerebellar output at thalamic level provides an important clue for the understanding of the neurophysiologic mechanisms by which SN acts on motor behavior.     

The mode of pallido-thalamic transmission investigated with intracellular recording from cat thalamus

Summary Pallido-thalamic transmission was studied by intracellular recording from neurons in the ventrolateral (VL) and ventroanterior (VA) nuclei of the thalamus in cats anesthetized with pentobarbital. Stimulation of the entopeduncular nucleus (ENT) produced short latency, inhibitory postsynaptic potentials in the VL-VA neurons (1.60 ms on average). When stimuli were applied closer to the VL-VA region along the pallido-thalamic pathway, i.e., to the rostral Forel's field, the IPSP latency was significantly reduced. Linear regression analysis of the IPSP latency against conduction distance between different stimulating and recording positions indicated that the IPSP was produced through a monosynaptic pathway at a conduction velocity of 5 to 11 m/s.The neurons which received IPSPs from the ENT distributed in the rostromedial VL and in the rostral VA, whereas relay cells responding only to the contralateral brachium conjunctivum were found in the caudal VL and in the dorsolateral portion of the rostral VL-VA complex. Reciprocal convergence of pallidal and cerebellar impulses were observed in only a small number of cells, which were located in the border between the two neuron groups.Recording of extracellular field potentials and focal stimulation within and around the rostral VL also indicated that the fiber potentials arose from the ENT nucleus and propagated along a bundle of fibers which terminated within the rostromedial VL-VA complex. These results are all explicable by assuming that the entopeduncular neurons are inhibitory in nature and so inhibit thalamic neurons monosynaptically.     

Monosynaptic inhibition of pallidal neurons by axon collaterals of caudato-nigral fibers

Summary The effects produced in the entopeduncular nucleus (ENT) by stimulation of the substantia nigra (SN) were studied in cats anesthetized with pentobarbital. SN stimulation evoked positive field potentials localized in ENT and long latency IPSPs in ENT neurons. The IPSP seemed to be produced monosynaptically since 1. it was evoked by single shocks to SN and had a constant shape and latency, 2. there was no temporal facilitation of the second IPSP when two shocks were applied to SN at various intervals, and 3. an IPSP of similar shape, but shorter latency, was evoked from the diencephalic region between SN and ENT. Linear regression analysis of the latency vs. conduction distance for these two IPSPs, mediated by the same fibers, indicated a synaptic delay of less than 0.7 msec and a conduction velocity for these fibers of about 1 m/sec.Stimulation of the caudate nucleus evoked IPSPs with shapes similar to those evoked from SN; and there was a strong interaction between SN and caudate-evoked IPSPs, suggesting that they were mediated by the same fibers. Acute experiments on cats in which the caudate nucleus had been destroyed 3 to 7 weeks earlier showed that SN stimulation did not evoke positive field potentials in ENT or IPSPs in ENT cells. It was concluded that SN stimulation antidromically activated caudato-nigral fibers that monosynaptically inhibited ENT cells via axon collaterals.     

Convergence of afferent fibers from the entopeduncular nucleus and the substantia nigra pars reticulata onto single neurons in the ventromedial thalamic nucleus: an electron microscope study in the cat

Convergence of afferent fibers from the entopeduncular nucleus (EP) and the substantia nigra pars reticulata (SNr) onto single neurons in the ventromedial thalamic nucleus (VM) was shown electron microscopically in the cat. Axon terminals from the EP were anterogradely labeled with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injected into the EP. Axon terminals from the SNr were degenerated by injecting ibotenic acid into the SNr. Both WGA-HRP-labeled axon terminals and degenerating ones were found on single neuronal profiles in the VM.     

An autoradiographic study of topographical relationships between pallidal and cerebellar projections to the cat thalamus

Summary Injections of ³H-leucine were made in the entopeduncular nucleus or dentate nucleus of the cerebellum in eight cats. The terminal projection zones of both pathways in the thalamus were studied using the sagittal plane and their relationships to one another as well as to cytoarchitectural boundaries of thalamic nuclei were compared. The data indicate that the territories controlled by the two projection systems are almost entirely segregated. The segregation is mainly along the antero-posterior axis as the main pallidal projection zone occupies the medio-ventral VA while the main dentate projection zone lies posterior to it in the VL. Furthermore, the dorsolateral part of the VA not occupied by pallidal projections receives dentate projections. In the VM, both afferent systems terminate in the lateral part of the nucleus with pallidal territory located anteriorly and dentate territory located posteriorly, again without overlap. As the delineations of nuclear subdivisions in the ventral thalamus of the cat have been a subject of some controversy, it is suggested that the boundaries of the VA, VL and VM in the cat thalamus be defined on the basis of basal ganglia and cerebellar projection zones.Abbreviations used in the Text and in Fig. 5 AM anterior medial nucleus - AV anterior ventral nucleus - BC brachium conjunctivum - CA anterior commissure - CC crus cerebri - CP posterior commissure - CD caudate nucleus - CE centrum medianum - CLN central lateral nucleus - DN dentate nucleus - EPN entopeduncular nucleus - FF Forel's field - FN fastigial nucleus - FR fasciculus retroflexus - HL lateral habenular nucleus - HM medial habenular nucleus - INA anterior interposite nucleus - INP posterior interposite nucleus - IC internal capsule - LD lateral dorsal nucleus - LG lateral geniculate body - MD medial dorsal nucleus - MTT mamillothalamic tract - NR red nucleus - OT optic tract - PAC paracentral nucleus - PF parafascicular nucleus - PV pulvinar - RT reticular thalamic nucleus - SM submedian nucleus - SN substantia nigra - SNr substantia nigra pars reticularis - STN subthalamic nucleus - VF ventral posterior nucleus - VA ventral anterior nucleus - VL ventral lateral nucleus - VM ventral medial nucleus - ZI zona incerta Supported in part by a grant from the American Parkinson Disease Association and NIH grant R01NS19280     

Electrophysiological study of spinothalamic inputs to ventrolateral and adjacent thalamic nuclei of the cat.

1. Extracellular and intracellular methods were used to record from fibers and neurons in the ventral lateral (VL) and adjacent nuclei of the cat thalamus. The receptive fields of the recorded units were analyzed and the units tested for inputs from the medial lemniscus (ML) and spinothalamic tract (STT) by electrical stimulation of the dorsal columns (DC) and ventrolateral funiculus (VLF) at the C2-3 spinal level. 2. Thirty-eight STT fibers were isolated in the thalamus. Their conduction velocities ranged from 15 to 75 m/s (mode 36 m/s). Adequate stimuli were found for 23 of these fibers. Seventeen were low-threshold (LT), 3 were wide-dynamic-range (WDR), and 3 were high-threshold (HT) units. 3. Five STT fibers were intra-axonally injected. Three were sufficiently well filled for analysis of their terminal fields. An intermediate-velocity STT fiber (conduction velocity 38 m/s) had a 4.3-microns axon and a single large terminal field in the central lateral nucleus (CL). The other two STT fibers were smaller, with diameters of 2.5 and 2.3 microns, conduction velocities of 15 and 19 m/s, and terminal fields made up of a few small boutons at the borders of the ventral posterior lateral nucleus (VPL). 4. Of 319 neurons isolated, 14 out of 129 (10.8%) in VL, 14 out of 76 (18.4%) in the VPL or ventral posterior medial (VPM) nucleus, 27 out of 64 (42.2%) in the CL nucleus, and 5 out of 50 (10%) in the reticular nucleus (R) responded at latencies less than 50 ms to VLF stimuli. A train of three pulses was more effective in driving VLF-responding neurons in all these nuclei than a single pulse. VLF-responding cells were widely dispersed in VL, concentrated in a focus in CL, and distributed around the borders of VPL. Most of those in VL and a small number in CL could be antidromically activated by stimulation of motor cortex. 5. Latencies of presynaptic responses (STT fibers) to VLF stimulation were short and varied from 0.8 to 3.9 ms (mode 1.6 ms). Despite this, very few fast-responding neurons were found. These were six VPL neurons (2.5 to 4 ms), one VL neuron (3 ms), and four CL neurons (3-4 ms). The initial spike latencies of the majority of thalamic neurons responding to VLF stimulation appeared in two peaks, one between 6 and 8 ms and the other at 10-15 ms.(ABSTRACT TRUNCATED AT 400 WORDS)     

Thalamic afferents of area 4 and 6 in the dog: a multiple retrograde fluorescent dye study

In the present study, we compared the distribution of thalamocortical afferents of cortical area 4 to that of cortical area 6 in the dog, using fluorescent tracers. Multiple injections of combinations of two dyes (diamidino yellow dihydrochloride, Evans blue, fast blue, granular blue) were made into either the anterior and posterior sigmoid gyri or into the medial and lateral regions of the anterior sigmoid gyrus in the anesthetized dog. We found that the thalamic afferents of areas 4 and 6 arise from topographically organized bands of cells that traverse several thalamic nuclei and extend throughout the rostrocaudal extent of the thalamus. The most medial band included area 6-projecting neurons in the anterior nuclei, the rhomboid nucleus, the ventral anterior nucleus (VA), ventromedial nucleus (VM) and mediodorsal nucleus (MD). Within this band, cells projecting to medial area 6a tended to be more numerous in the anterior nuclei, anterior parts of VA and VM and anterior and caudal parts of MD. Fewer cells in MD but more cells in caudal parts of VA and VM projected to lateral area 6 a. Lateral bands of cells in central through lateral parts of VA and VL projected topographically to lateral area 4 on the anterior sigmoid gyrus and lateral through medial parts of postcruciate area 4. The most lateral band of cells in VL continued ventrally into the zona incerta. Area 4 also received input from VM and the central lateral (CL) and centrum medianum (CM) nuclei. Within regions of VA, VL and VM, cells from one band interspersed with cells from another, but there were very few double-labeled cells projecting to two cortical sites. When the present results are compared with our previous findings on the distribution of subcortical afferents to the motor thalamus, it appears that separate motor cortical areas may receive predominantly separate but also partially over-lapping pathways in the dog.Abbreviations AV Anterior ventral nucleus - AM anterior medial nucleus - Cb cerebellar nuclei - CeM central medial nucleus - CL central lateral nucleus - CM centrum medianum nucleus - EN entopeduncular nucleus - Hb habenula - LD lateral dorsal nucleus - MD mediodorsal nucleus - mt mammillothalamic tract - MV medioventral nucleus - Pf parafascicular nucleus - R reticular thalamic nucleus - rf retroflex fasciculus - Rh rhomboid nucleus - SN substantia nigra - VA ventral anterior nucleus - VL ventral lateral nucleus, principal division - VLd ventral lateral nucleus, dorsal division - VM ventral medial nucleus - VPL ventral posterior lateral nucleus - ZI zona incerta     

Nigral modulation of cerebello-thalamo-cortical transmission in the ventral medial thalamic nucleus

Summary In the rat, the highly active GABAergic neurons of the substantia nigra pars reticulata (SNR) are known to exert a tonic inhibitory influence on cells in the ventral medial thalamic nucleus (VM). Considering that this nucleus is involved in the transfer of cerebellar signals towards motor cortex, we investigated the role played by SNR in that transmission. For this purpose we examined how changes in nigral background activity are reflected in the reactivity of VM cells to their cerebellar input. We report here that a GABA induced nigral pause increases the efficacy of cerebellar afferent volleys in VM, whereas an increase of nigral background by bicuculline, interrupts cerebello-thalamo-cortical transmission. It is concluded that nigrothalamic neurons subserve a permanent gating of cerebellothalamo-cortical transmission in VM.     

Entopeduncular projection to the thalamic ventrolateral nucleus of the cat

Summary The effect of stimulation of the Entopeduncular nucleus on the thalamic ventrolateral nucleus (VL) was studied by recording with macroelectrodes and microelectrodes in cats anesthetized with chloralose or under local analgesia. An asynaptic response (0.5 ms shortest latency) was evoked in the VL following the Entopeduncular stimulation. It is demonstrated that this response is attributable to the activation of the Entopeduncular-VL fibers described by anatomists. The postsynaptic events revealed, since only a small proportion of VL cells are affected by Entopeduncular stimulation, that the influence exerted by the Entopeduncular nucleus on the VL is much weaker than the cerebellar one.     

Spinal input to thalamic VL neurons: evidence for direct spinothalamic effects.

1. The postsynaptic actions of afferents ascending in the ventrolateral quadrant and dorsal columns of the spinal cord were studied in neurons in the ventrolateral nucleus of the thalamus (VL) (n = 138) by use of intracellular recording procedures. Neurons were identified by their monosynaptic input from the cerebellum and, when possible, their antidromic activation from the motor cortex. The possible occurrence of monosynaptic transmission along spinothalamic fibers was investigated by estimating the intrathalamic delay time of postsynaptic responses and by examining the occurrence of temporal facilitation to double-shock stimulation. The experiments were performed in cats anesthetized with alpha-chloralose. 2. The majority of neurons (86%) responded with excitatory or inhibitory postsynaptic potentials to stimulation of the ascending paths. The response latencies of excitation on stimulation of the ventral quadrants at C3 ranged from 2.9 to 18 ms. Evidence for monosynaptic excitation after stimulation of (spinothalamic) afferents ascending in the ventrolateral quadrants was obtained for a number of neurons (n = 30). For these neurons, estimated intrathalamic delays were less than 1 ms and/or the neurons did not display temporal facilitation to double shocks. All the shortest latency responses (from C3) showed evidence of monosynaptic transmission. It is estimated that approximately 19-39% of neurons sampled may receive monosynaptic input. The spinal conduction velocities of the direct projections ranged from 10 to 35 m/s (median 20 m/s). 3. Much of the ascending input was mediated polysynaptically. For afferents ascending in the ventrolateral quadrant, estimated intrathalamic delays were greater than 1.5 ms and/or the postsynaptic responses displayed temporal facilitation to double shocks. The shortest latency from C3 of a polysynaptic response was 5 ms. Spatial interactions were observed between polysynaptic inputs from the ventrolateral quadrants and the dorsal columns, indicating that at least some of the pathways to VL are shared. 4. The data show that many neurons in the VL receive input ascending from the spinal cord via direct and indirect routes. Somatosensory information reaching VL could serve to adjust, during the course of movement execution, the cerebellar commands relayed by VL to the motor cortex.     

5-HT-mediated synaptic potentials in the dorsal raphe nucleus: interactions with excitatory amino acid and GABA neurotransmission

1. Intracellular recordings from neurons within dorsal raphe nucleus in slices from rat brain were used to study an inhibitory postsynaptic potential (IPSP) evoked by electrical stimulation. 2. The IPSP was observed in approximately 70% of neurons, had a latency to onset of 40-65 ms, reached a peak in 350-400 ms, had a total duration of 1-2 s, and reversed polarity at the potassium equilibrium potential. 3. This IPSP was blocked by spiperone (1 microM) and prolonged by fluoxetine (300 nM-30 microM) suggesting that it was mediated by 5-hydroxytryptamine (5-HT). 4. Superfusion with gamma-aminobutyric acid (GABA) and excitatory amino acid receptor antagonists were used to block "fast" synaptic potentials that preceded the IPSP such that it could be studied in isolation. Blockade of the GABA-mediated synaptic potentials increased the amplitude of the IPSP by 1.3-fold. The amplitude of the IPSP was reduced by 30% after blockade of the excitatory amino acid-mediated synaptic potential. 5. The results indicate that the IPSP recorded in dorsal raphe neurons was caused by 5-HT released at least in part from indirect (synaptically induced) excitation of 5-HT-containing cells within the slice.     

An analysis of potentially converging inputs to the rostral ventral thalamic nuclei of the cat

Summary Potentially convergent inputs to cerebellar-receiving and basal ganglia-receiving areas of the thalamus were identified using horseradish peroxidase (HRP) retrograde tracing techniques. HRP was deposited iontophoretically into the ventroanterior (VA), ventromedial (VM), and ventrolateral (VL) thalamic nuclei in the cat. The relative numbers of labeled neurons in the basal ganglia and the cerebellar nuclei were used to assess the extent to which the injection was in cerebellar-receiving or basal ganglia-receiving portions of thalamus. The rostral pole of VA showed reciprocal connections with prefrontal portions of the cerebral cortex. Only the basal ganglia and the hypothalamus provided non-thalamic input to modulate these cortico-thalamo-cortical loops. In VM, there were reciprocal connections with prefrontal, premotor, and insular areas of the cerebral cortex. The basal ganglia (especially the substantia nigra), and to a lesser extent, the posterior and ventral portions of the deep cerebellar nuclei, provided input to VM and may modulate these corticothalamo-cortical loops. The premotor cortical areas connected to VM include those associated with eye movements, and afferents from the superior colliculus, a region of documented importance in oculomotor control, also were labeled by injections into VM. The dorsolateral portion of the VA-VL complex primarily showed reciprocal connections with the medial premotor (area 6) cortex. Basal ganglia and cerebellar afferents both may modulate this cortico-thalamo-cortical loop, although they do not necessarily converge on the same thalamic neurons. The cerebellar input to dorsolateral VA-VL was from posterior and ventral portions of the cerebellar nuclei, and the major potential brainstem afferents to this region of thalamus were from the pretectum. Mid- and caudo-lateral portions of VL had reciprocal connections with primary motor cortex (area 4). The dorsal and anterior portions of the cerebellar nuclei had a dominant input to this corticothalamo-cortical loop. Potentially converging brainstem afferents to this portion of VL were from the pretectum, especially pretectal areas to which somatosensory afferents project.List of Abbreviations AC central amygdaloid nucleus - AL lateral amygdaloid nucleus - AM anteromedial thalamic nucleus - AV anteroventral thalamic nucleus - BC brachium conjunctivum - BIC brachium of the inferior colliculus - Cd caudate nucleus - CL centrolateral thalamic nucleus - CM centre median nucleus - CP cerebral peduncle - CUN cuneate nucleus - DBC decussation of the brachium conjunctivum - DR dorsal raphe nuclei - EC external cuneate nucleus - ENTO entopeduncular nucleus - FN fastigial nucleus - FX fornix - GP globus pallidus - GR gracile nucleus - IC internal capsule - ICP inferior cerebellar peduncle - IP interpeduncular nucleus - IVN inferior vestibular nucleus - LD lateral dorsal thalamic nucleus - LGN lateral geniculate nucleus - LH lateral hypothalamus - LP lateral posterior thalamic complex - LRN lateral reticular nucleus - LVN lateral vestibular nucleus - MB mammillary body - MD mediodorsal thalamic nucleus - MG medial geniculate nucleus - ML medial lemniscus - MLF medial lengitudinal fasciculus - MT mammillothalamic tract - MVN medial vestibular nucleus - NDBB nucleus of the diagonal band of Broca - NIA anterior nucleus interpositus - NIP posterior nucleus interpositus - OD optic decussation - OT optic tract - PAC paracentral thalamic nucleus - PPN pedunculopontine region - PRO gyrus proreus - PRT pretectal region - PT pyramidal tract - PTA anterior pretectal region - PTM medial pretectal region - PTO olivary pretectal nucleus - PTP poterior pretectal region - Pul pulvinar nucleus - Put putamen - RF reticular formation - RN red nucleus - Rt reticular complex of the thalamus - S solitary tract - SCi superior colliculus, intermediate gray - SN substantia nigra - ST subthalamic nucleus - VA ventroanterior thalamic nucleus - VB ventrobasal complex - VL ventrolateral thalamic nucleus - VM ventromedial thalamic nucleus - III oculomotor nucleus - IIIn oculomotor nerve - 5S spinal trigeminal nucleus - 5T spinal trigeminal tract - VII facial nucleus     

Subthalamic stimulation-induced synaptic responses in substantia nigra pars compacta dopaminergic neurons in vitro.

The subthalamic nucleus (STN) is one of the principal sources of excitatory glutamatergic input to dopaminergic neurons of the substantia nigra, yet stimulation of the STN produces both excitatory and inhibitory effects on nigral dopaminergic neurons recorded extracellularly in vivo. The present experiments were designed to determine the sources of the excitatory and inhibitory effects. Synaptic potentials were recorded intracellularly from substantia nigra pars compacta dopaminergic neurons in parasagittal slices in response to stimulation of the STN. Synaptic potentials were analyzed for onset latency, amplitude, duration, and reversal potential in the presence and absence of GABA and glutamate receptor antagonists. STN-evoked depolarizing synaptic responses in dopaminergic neurons reversed at approximately -31 mV, intermediate between the expected reversal potential for an excitatory and an inhibitory postsynaptic potential (EPSP and IPSP). Blockade of GABA(A) receptors with bicuculline caused a positive shift in the reversal potential to near 0 mV, suggesting that STN stimulation evoked a near simultaneous EPSP and IPSP. Both synaptic responses were blocked by application of the glutamate receptor antagonist, 6-cyano-7-nitroquinoxalene-2,3-dione. The confounding influence of inhibitory fibers of passage from globus pallidus and/or striatum by STN stimulation was eliminated by unilaterally transecting striatonigral and pallidonigral fibers 3 days before recording. The reversal potential of STN-evoked synaptic responses in dopaminergic neurons in slices from transected animals was approximately -30 mV. Bath application of bicuculline shifted the reversal potential to approximately 5 mV as it did in intact animals, suggesting that the source of the IPSP was within substantia nigra. These data indicate that electrical stimulation of the STN elicits a mixed EPSP-IPSP in nigral dopaminergic neurons due to the coactivation of an excitatory monosynaptic and an inhibitory polysynaptic connection between the STN and the dopaminergic neurons of substantia nigra pars compacta. The EPSP arises from a direct monosynaptic excitatory glutamatergic input from the STN. The IPSP arises polysynaptically, most likely through STN-evoked excitation of GABAergic neurons in substantia nigra pars reticulata, which produces feed-forward GABA(A)-mediated inhibition of dopaminergic neurons through inhibitory intranigral axon collaterals.     

Inhibitory substantia nigra inputs to the pedunculopontine neurons

Summary Responses of 43 pedunculopontine area (PPN area) neurons to electrical stimulation of the substantia nigra (SN) were studied in anesthetized rats. An intracellular recording technique was used to demonstrate that SN stimulation evoked hyperpolarizing potentials, which were identified by intracellular injections as inhibitory postsynaptic potentials (IPSPs). These IPSPs were often followed by a rebound depolarization that originates several spike potentials. These IPSPs were characterized as monosynaptic, with latencies varying from 1.0 to 8.5 ms. Similar results were observed in some animals with chronic unilateral coronal lesion just rostral to subthalamic nucleus (STH), which severed the rostral afferents. PPN area neurons were also antidromically activated by SN stimulation. Two PPN area projection neurons were clearly identified. Mean latency of one group was 0.71 ms; mean latency of the second group was 5.16 ms. The morphological analysis of a neuron inhibited by SN stimulation and labeled with horseradish peroxidase (HRP) demonstrated that the soma was fusiform in shape, with the axon originating in the soma and collaterals and a large dendritic field extending in the ventrodorsalis direction. The results indicate that the PPN area is reciprocally connected with the SN, which elicits an inhibitory effect on PPN area neurons.     

A study of neuronal circuitry mediating the saccadic suppression in the rabbit

Summary Stimulation of the deep layers of the superior colliculus (SC) evoked an IPSP in the relay cells of the lateral geniculate nucleus (LGN). The latency of the JPSP ranged from 3.3 to 4.7 ms with an average of 3.87±0.56 ms (S.D.). The IPSP from SC stimulation was proposed to be mediated by the recurrent inhibitory circuit to LGN, since the recurrent inhibitory interneurones in the thalamic reticular nucleus (R) responded to the same stimulation with a latency of 2.14±0.43 ms, which was 1.73 ms shorter than the latency of the IPSP in LGN relay cells. This was in good agreement with our previous observation that the recurrent interneurones always fired about 1.8 ms prior to the onset of the recurrent IPSP in LGN (Lo and Xie 1987b). The recurrent inhibitory interneurones could also be excited by stimulation of the central lateral nucleus (CL) with a very short latency (0.57±0.15 ms), suggesting a monosynaptic connection between the central lateral nucleus and the reticular recurrent interneurones. This suggestion was supported by the fact that CL neurones, which projected to the striate cortex (Cx), were antidromically excited by stimulation of the caudal part of R where the recurrent inhibitory interneurones were situated. CL neurone's response to stimulation of the deep layers of SC (SC-CL response) has a latency of 1.68±0.56 ms, which was comparable with the difference between the latency of SC-R response and that of CL-R response, just as expected from the notion that the saccadic suppression is mediated by a circuit of SC (deep layers) -CL-R-LGN.     

Topographical projections from the posterior thalamic regions to the striatum in the cat,with reference to possible tecto-thalamo-striatal connections

Summary Projections from the posterior thalamic regions to the striatum were studied in the cat by the anterograde tracing method after injecting wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) into the caudalmost regions of the lateroposterior thalamic nucleus (caudal LP), suprageniculate nucleus (Sg) and magnocellular division of the medial geniculate nucleus (MGm). The results were further confirmed by the retrograde tracing method after injecting WGA-HRP into the regions of the caudate nucleus (Cd) and putamen (Put) where afferent fibers from the caudal LP, Sg and MGm were distributed. Fibers from the MGm, Sg or caudal LP were distributed mainly in the medial, middle or lateral part of the caudal half of the putamen (caudal Put), respectively. Although there was a considerable overlap, thalamostriatal fibers from the caudal LP terminated more caudally than those from the MGm. On the other hand, thalamocaudate fibers from the MGm, Sg and lateral part of the caudal LP overlapped with each other in the ventrolateral part of the caudal half of the caudate nucleus (caudal Cd). Fibers from the medial part of the caudal LP were distributed in the ventral part of the caudal Cd. In the superior colliculus (SC) of the cats with WGA-HRP injections in the caudal LP, retrogradely labeled neuronal cell bodies were mainly seen ipsilaterally in the superficial SC layer, and simultaneously, anterogradely labeled axon terminals were observed in the striatum. On the other hand, when WGA-HRP was injected into the Sg or MGm, labeled SC neurons were mainly located in the intermediate and deep SC layers. Thus, ascending impulses from the superficial SC layer may possibly be conveyed ipsilaterally via the caudal LP to the ventral and ventrolateral parts of the caudal Cd and the lateral part of the caudal Put, whereas those from the intermediate and deep SC layers may be relayed via the Sg and/or MGm to the ventrolateral part of the caudal Cd and the middle and medial parts of the caudal Put.Abbreviations AC anterior commissure - Am amygdaloid nucleus - Cd caudate nucleus - Ce centromedial nucleus - CL centrolateral nucleus - Cl claustrum - CM-Pf centre médian-parafascicular complex - CP cerebral peduncle - d deep SC layer - EC external capsule - Ep entopeduncular nucleus - GP globus pallidus - i intermediate SC layer - IC internal capsule - Ip interpeduncular nucleus - LG lateral geniculate nucleus - LP lateroposterior nucleus - MD mediodorsal nucleus - MG medial geniculate nucleus - MGm magnocellular division of MG - MGp principal division of MG - NBIC nucleus of brachium of inferior colliculus - O oculomotor nucleus - OT optic tract - Pom medial division of posterior group of thalamus - Pt pretectum - Pul pulvinar nucleus - Put putamen - Pv paraventricular nucleus of thalamus - R reticular nucleus of thalamus - Rh rhomboid nucleus - RN red nucleus - s superficial SC layer - SC superior colliculus - Sg suprageniculate nucleus - SN substantia nigra - SNpc pars compacta of SN - SNpr pars reticulata of SN - V lateral ventricle - VA ventroanterior nucleus - VL ventrolateral nucleus - VM ventromedial nucleus - WGA-HRP wheat germ agglutinin-HRP conjugate     

Suprachiasmatic nucleus communicates with anterior thalamic paraventricular nucleus neurons via rapid glutamatergic and gabaergic neurotransmission: state-dependent response patterns observed in vitro

The hypothalamic suprachiasmatic nucleus uniquely projects to the midline thalamic paraventricular nucleus. To characterize this projection, patch clamp techniques applied in acute rat brain slice preparations examined responses of anterior thalamic paraventricular nucleus neurons to focal suprachiasmatic nucleus stimulation. Whole cell recordings from slices obtained during daytime (n=40) revealed neurons with a mean membrane potential of -66+/-1.2 mV, input conductance of 1.5+/-0.1 nS and state-dependent tonic or burst firing patterns. Electrical stimulation (one or four pulses) in suprachiasmatic nucleus elicited monosynaptic excitatory postsynaptic potentials (mean latency of 12.6+/-0.6 ms; n=12), featuring both AMPA and N-methyl-D-aspartate-glutamate receptor-mediated components, and monosynaptic bicuculline-sensitive inhibitory postsynaptic potentials (mean latency of 16.6+/-0.6 ms; n=7) reversing polarity at -72+/-2.6 mV, close to the chloride equilibrium potential. Glutamate microstimulation of suprachiasmatic nucleus also elicited transient increases in spontaneous excitatory or inhibitory postsynaptic currents in anterior thalamic paraventricular neurons. Recordings from rats under reverse light/dark conditions (n=22) yielded essentially similar responses to electrical stimulation. At depolarized membrane potentials, suprachiasmatic nucleus-evoked excitatory postsynaptic potentials triggered single action potentials, while evoked inhibitory postsynaptic potentials elicited a silent period in ongoing tonic firing. By contrast, after manual adjustment of membrane potentials to hyperpolarized levels, neuronal response to the same "excitatory" stimulus was a low threshold spike and superimposed burst firing, while responses to "inhibitory" stimuli paradoxically elicited excitatory rebound low threshold spikes and burst firing. These data support the existence of glutamatergic and GABAergic efferents from the suprachiasmatic nucleus to its target neurons. Additionally, in thalamic paraventricular nucleus neurons, responses to activation of their suprachiasmatic afferents may vary in accordance with their membrane potential-dependent intrinsic properties, a characteristic typical of thalamocortical neurons.     

Inhibitory nigral influence on tectospinal neurons,a possible implication of basal ganglia in orienting behavior

Summary We have established in previous electrophysiological studies that the substantia nigra pars reticulata (SNr) exerts a potent inhibitory influence on cells located in the intermediate and deep tectal strata. The present study demonstrates that, in the rat, the tectospinal neurons constitute one of the cellular populations of the tectum on which the SN exerts its influence.Tectospinal neurons were identified using the antidromic activation method. Following SNr stimulation 17/37 (45%) of these cells showed a shortlatency (1.5–2 ms) short-duration (7–15 ms) inhibition. This effect was revealed by a blockade of spontaneous and peripherally evoked discharges. Moreover, in some cases the nigral inhibition delayed the antidromic invasion of the somato-dendritic portion of the neuron. Tectospinal neurons have been considered as one of the neuronal substrate by which the superior colliculus (SC) can promote head orienting movements. The evidence that the SNr influences the responsiveness of these cells to their peripheral sensory inputs suggests that the basal ganglia and in particular the SNr are involved in the integrative sensorimotor processes underlying head orienting movements.This work was supported by DGRST Grant 83 C 0336     

Responses of rat lateral hypothalamic neuronal activity to fastigial nucleus stimulation

1. The aim of this study was investigation of neuronal mechanisms underlying inputs from the fastigial nucleus (FN) to the lateral hypothalamic area (LHA). 2. In male anesthetized rats, 295 extracellular and 82 intracellular recordings of LHA responses to electrical stimulation of the FN, which elicited stimulus-locked pressor responses, were examined. 3. Contralateral FN stimulation evoked three types of responses in 48% of spontaneously firing LHA neurons: inhibition with 11 +/- 6 (SD) ms latency followed by excitation (30%), excitation with 15 +/- 12.5 ms latency (14%), and excitation followed by inhibition with 6 +/- 4 ms latency (4%). 4. Contralateral FN stimulation after transection of the inferior cerebellar peduncle (ICP), which resulted in a substantial fall of the fastigial pressor response, also evoked the three types of responses. These responses were unaffected by transection of the ICP. 5. Neuronal activity was recorded intracellularly from 82 LHA neurons, of which 36 (44%) responded to FN stimulation. Of the 36 neurons, 24 showed inhibitory postsynaptic potentials (IPSPs) with a mean latency of 7.5 +/- 2 ms. Of the 24 neurons, 16 were checked for change in IPSP latency with stimulus intensity, and 11 were considered to be monosynaptically connected since their latencies were constant when FN stimulation intensity was changed. The remaining 12 exhibited excitatory postsynaptic potentials (EPSPs) with a longer latency of 10.5 +/- 3 ms, which indicated polysynaptic conduction. The reversal potentials of the IPSP and EPSP were estimated to be about -77 mV and -13 mV, respectively. 6. Most glucose-sensitive neurons (78%), which were identified by their inhibition in response to electrophoretically applied glucose, were inhibited by FN stimulation, whereas only 7% of the glucose-insensitive neurons responded to such stimulation. 7. From the results, it was concluded that LHA neurons receive inhibitory monosynaptic and excitatory polysynaptic inputs from the FN via the superior cerebellar peduncle. These connections may contribute to hypothalamic modulation of feeding behavior.     

Dorsal column input to thalamic VL neurons: an intracellular study in the cat

Summary This study investigated the role of the ventral lateral (VL) nucleus of the thalamus as a lemniscal relay to motor cortex. Intracellular recordings were obtained from thalamic VL relay neurons in cats anesthetized with chloralose, following stimulation of the dorsal column nuclei. VL neurons were identified by their short-latency input from the cerebellar nuclei, their antidromic activation from motor cortex and their anatomical location. A total of 105 neurons was studied. The occurence of temporal facilitation to double volleys was also examined. It was found that 80/105 (75%) neurons responded with excitation and/or inhibition to stimulation of the dorsal column nuclei. The latencies of the postsynaptic responses ranged from 2.0 to 20 ms (median 10.0 ms). The latencies of nearly all responses (79/80) were > 3 ms and nearly all responses (45/47) which were tested for it, displayed temporal facilitation to double shock stimulation, consistent with polysynaptic transmission. Effective stimulation sites were found in the gracile and cuneate nuclei. Recording sites were located throughout VL, including the border region with the ventral posterior lateral nucleus (VPL). There was no obvious topographic relationship between location of recording site and latency or polarity (excitation versus inhibition) of the synaptic responses. This is consistent with dorsal column input diffusely distributed over VL. When the recording electrodes penetrated VPL, characteristics of the EPSPs were indicative of monosynaptic transmission (short latency, no temporal facilitation). This clear transition from VL to VPL suggests that it is not necessary to define, on physiological grounds, a separate border region between these two nuclei. The data provide evidence that dorsal column information reaches VL neurons polysynaptically, not monosynaptically. This indicates that VL is part of a long-latency, not short-latency path through the dorsal column nuclei to motor cortex.