Inhibitory action of the ventral noradrenergic bundle on the lateral hypothalamic neurons through alpha-noradrenergic mechanisms in the rat

Electrical stimulation of the ventral noradrenergic bundle (V-NA bundle) produced 3 types of responses in lateral hypothalamic neurons: IPSPs, a polysynaptic EPSP-IPSP sequence and antidromic spikes. The IPSPs were considered to be monosynaptic due to the fixed latencies seen at stimulus intensities. Iontophoretic application of an alpha-NA antagonist blocked only the presumed monosynaptic inhibition. Most of the glucose-sensitive neurons were inhibited by stimulation of the V-NA bundle. These results may account for the hyperphagia and obesity produced by selective lesions of the V-NA bundle.     

Electrophysiologic properties of red nucleus neurons in the rat brain slices]

Intracellular recordings from the red nucleus (RN) neurons were made in experiments on the rat brain slices. Passive membrane properties (input resistance and membrane time constant) of the RN neurons were evaluated. Phenomena of potential-dependent rebound depolarization and time-dependent inward rectification were revealed by means of passing hyperpolarizing current pulses through the recorded cells. Injections of depolarizing currents caused repetitive firing of neurons with frequencies directly depending on the intensity of injected currents. Repetitive firing was also characterized by a fast frequency adaptation during injections of currents of different intensities. Stimulation of a region of slices presumably corresponding to the decussation of brachium conjunctivum evoked mainly monosynaptic EPSPs with a "fast"-rise time in the RN neurons, which suggests activation of the synaptic input from the cerebellar nucleus interpositus. Stimulation of the same region sometimes evoked EPSP-IPSP mixtures or pure IPSPs in the RN neurons.     

Electrophysiology of rat thalamo-cortical relay neurons: an in vivo intracellular recordingf and labeling study

Membrane properties and responses to frontal cortical stimulation were studied on electrophysiologically and morphologically identified thalamo-cortical neurons in anesthetized rats. Those neurons generated membrane responses resembling the low-threshold Ca-spike, g_K(Ca) and I_A that have been previously demonstrated in in vitro studies of thalamic neurons. Stimulation of the frontal cortex evoked a sequence of responses; antidromic spike, initial depolarization, long duration hyperpolarization and a short period of depolarization. The initial depolarization was considered to be a monosynaptic excitatory postsynaptic potential (EPSP) which overlapped with an inhibitory postsynaptic potential (IPSP). Major constituents of the long duration hyperpolarization were considered to be short duration IPSPs and long duration disfacilitations of cortical inputs.     

Morphological analysis of cat masseteric motoneurons after intracellular staining with horseradish peroxidase

Intracellular injection of horseradish peroxidase (HRP) into 58 masseteric motoneurons identified by antidromic activation was performed in cats under pentobarbital anesthesia. Monosynaptic EPSPs were evoked by masseteric nerve stimuli in 52 cells, and were absent in the remaining six cells. The antidromic nature of the evoked spikes was confirmed by IS-SD separation observed at high frequency (50 Hz) stimulation. Motoneurons with monosynaptic excitation from masseter afferents showed IPSPs following stimulation of lingual and inferior alveolar nerves. Motoneurons which did not show monosynaptic excitation from masseter afferents showed no IPSPs from the above nerves. There were no differences in cell size or the number of stem dendrites between motoneurons with and without monosynaptic EPSPs. No recurrent collaterals were observed in any motor axons. Motoneurons with monosynaptic EPSPs were located at all rostrocaudal levels throughout the trigeminal motor nucleus, whereas motoneurons without such EPSPs were encountered only at the middle level. Dendrites of motoneurons with monosynaptic EPSPs did not extend into the medial portion of the nucleus where motoneurons innervating the anterior belly of the digastric muscle were located. In contrast, motoneurons without monosynaptic EPSPs had dendrite branches extending well into the medial part. The results show that there are two subpopulations of masseteric motoneurons that differ in peripheral inputs as well as dendritic morphology.     

Intracellular study of rat globus pallidus neurons: membrane properties and responses to neostriatal, subthalamic and nigral stimulation.

Physiological properties of globus pallidus (GP) neurons were studied intracellularly in anesthetized rats. More than 70% of the neurons exhibited continuous repetitive firing of 2-40 Hz, while others exhibited periodic burst firing or no firing. The repetitively firing neurons exhibited the following properties: spike accommodation; spike frequency adaptation; continuous firing with a frequency of about 100 Hz generated by intracellular current injections; fast anomalous rectification; ramp-shaped depolarization upon injection of depolarizing current; and post-active hyperpolarization. The burst firing neurons evoked a large depolarization with multiple spikes in response to depolarizing current, and a similar response was observed after the termination of hyperpolarizing current. The few neurons which did not fire spontaneous spikes exhibited strong spike accommodation when they were stimulated by current injections. The continuously firing neurons were antidromically activated by stimulation of the neostriatum (Str) (23 of 68), the subthalamic nucleus (STh) (55 of 75), and the substantia nigra (SN) (25 of 46). The antidromic latencies of the 3 stimulus sites were very similar (about 1 ms). None of the burst firing neurons were antidromically activated. Three non-firing neurons evoked antidromic responses only after Str stimulation. Only repetitively firing neurons evoked postsynaptic responses following stimulation of the Str and the STh. Stimulation of the Str evoked initial small EPSPs with latencies of 2-4 ms and strong, short duration IPSPs with latencies of 2-12 ms. Stimulation of the STh evoked short latency EPSPs overlapped with IPSPs. Frequently, these responses induced by Str and STh stimulation were followed by other EPSPs lasting 50-100 ms. These results indicated: (1) that the GP contains at least 3 electrophysiologically different types of neurons; (2) that GP projections to the Str, the STh, and the SN are of short latency pathways; (3) that Str stimulation evokes short latency EPSPs followed by IPSPs and late EPSPs in GP neurons; and (4) that STh stimulation evokes short latency EPSPs overlapped with short latency IPSPs and late EPSPs in GP neurons.     

Responses of rat lateral hypothalamic neuron activity to vestibular nuclei stimulation

Effects of lateral vestibular nucleus (LVN) stimulation on neuronal activity in the rat lateral hypothalamic area (LHA), including specific glucose-sensitive neurons, were investigated by extracellular and intracellular recordings in vivo. Stimulation of the contralateral LVN evoked 3 types of response in 46% (111/240) of the neurons recorded extracellularly: long latency (38.1 +/- 23.6 ms) excitation (62/111, 56%), short latency (6.9 +/- 3.1 ms) excitation-inhibition (33/111, 30%), and inhibition with 20.1 +/- 11.1 ms latency (16/111, 14%). Glucose-sensitive neurons, which were identified by electrophoretic application of glucose, did not respond specifically to such stimulation. Neuronal activity was recorded intracellularly from 31 LHA neurons, of which 13 responded to LVN stimulation. Seven of the 13 neurons showed a long latency EPSP (10.4 +/- 5.5 ms) and the remaining 6 exhibited an EPSP-IPSP sequence with shorter latency (4.5 +/- 3.0 ms). The amplitude of these responses was graded with a change in stimulus intensity. The EPSPs of both types of response were considered to be polysynaptic because of shortening of latencies by higher current stimulation. Since the LHA is implicated in the regulation of autonomic nerve activity, the present results showing polysynaptic pathways from the LVN to the LHA suggest functional involvement of the LHA in vestibulo-autonomic responses.     

Convergence of projections from the rat hippocampal formation, medial geniculate and basal forebrain onto single amygdaloid neurons: an in vivo extra- and intracellular electrophysiological study.

We recorded extra- and intracellular responses from rat amygdaloid neurons in vivo after electrical stimulation of the hippocampal formation (dentate gyrus, hippocampal fields CA3 and CA4, entorhinal cortex, subicular complex); medial geniculate; and basal forebrain (diagonal band, ventral pallidum, olfactory tubercle, nucleus accumbens, bed nucleus of stria terminalis, lateral preoptic area, substantia innominata). Stimulation of hippocampal formation structures evoked IPSPs or EPSP-IPSP sequences in which the IPSP had a lower threshold than the EPSP. Recordings from candidate inhibitory neurons in the amygdala indicated that excitatory afferents from the hippocampal formation contact both amygdaloid inhibitory and principal neurons (feedforward inhibition), and that the inhibitory neurons have a lower threshold of activation. Medial geniculate stimulation also evoked EPSP-IPSP sequences. In marked contrast to these results, stimulation of basal forebrain structures evoked short latency IPSPs in amygdaloid neurons. This provides the first physiological evidence for direct inhibition of the amygdala by the basal forebrain. Basal forebrain stimulation also evoked EPSP-IPSP sequences in amygdaloid neurons. Individual amygdaloid neurons could show responses to stimulation of the hippocampal formation, basal forebrain, and medial geniculate, indicating that synaptic input from these areas converges onto single amygdaloid cells. The findings provide further information about the synaptic organization of afferents to the amygdala, and indicate that single amygdaloid neurons play a role in the synaptic integration of input from these diverse sources.     

Commissural responses of rat retrohippocampal neurons

Synaptic responses of commissurally activated rat subicular and entorhinal neurons were studied intracellularly in vivo by stimulating the contralateral dentate gyrus. The most prominent synaptic responses in both subicular and entorhinal neurons were inhibitory postsynaptic potentials (IPSPs). IPSPs were generated in combination with antidromic spikes and/or excitatory postsynaptic potentials (EPSPs) and orthodromic spikes. No dependency between any two response types were found. Commissurally projecting subicular neurons (identified by the presence of antidromic spikes evoked by contralateral stimulation) were found, extending previous anatomical studies. Commissurally projecting entorhinal neurons were found in layer II, confirming previous anatomical studies. Positive correlations between antidromic spike latency and depth of recording sites supported the interpretation that axons projected along the fiber bundles of the hippocampal commissures and angular bundle to distribute to their targets. Possible circuits that could have mediated the excitatory and inhibitory responses of these retrohippocampal neurons are considered.     

An intracellular analysis of the entopeduncular inputs on the centrum medianum-parafascicular nuclear complex in cats

Intracellular recording from the CM-PF neurons was performed by stimulation of the EN, the caudate nucleus (Cd), the cerebellar nuclei (CN) and the motor cortex in the cat under Nembutal anesthesia. Twenty-seven neurons in the CM-PF nuclear complex and two neurons near the habenular nucleus received monosynaptic inhibitory postsynaptic potentials (IPSPs; latency of 1.0–4.0 ms, mean 2.3 ms) by EN stimulation. Cd stimulation evoked excitatory postsynaptic potentials (EPSPs) followed by long hyperpolarizations in most of the CM-PF neurons and produced antidromic activation in 7 neurons. Six neurons received EPSPs (latencies of 4–7 ms) by cortical stimulation. CN stimulation affected only two neurons in the present study. Intracellular HRP staining revealed that some CM-PF neurons have polygonal or spindle-shaped somata with fine, long and sparsely spinous dendrites.     

Long C3-C5 propriospinal neurones in the cat

Intracellular recording was made in the C3-C5 segments of cats from cells identified as long propriospinal neurones (PNs) by antidromic activation from the lower thoracic segments. The cell bodies were in laminae VII and VIII and their ventrally located axons were either uncrossed or crossed. Stimulation of higher motor centres revealed monosynaptic excitatory postsynaptic potentials (EPSPs) from cortico-, rubro-, tecto-, reticulo-, interstitio-, fastigio- and trigeminospinal fibres. Monosynaptic inhibitory postsynaptic potentials (IPSPs) were evoked from reticulospinal fibres. These PSPs were in addition to the separately described effects from the vestibular nuclei. Monosynaptic EPSPs were also evoked in some cells from neck or forelimb afferents and disynaptic EPSPs or IPSPs from forelimb afferents.     

Pallidal and entopeduncular intracellular responses to striatal, cortical, thalamic, and sensory inputs

Intracellular recordings were made from pallidal and entopeduncular neurons in cats. Responses were evoked by direct brain stimulation and auditory and somatosensory stimuli. Brain sites stimulated were caudate nucleus, the precruciate area of the cerebral cortex, and the central median-parafascicular region of the thalamus. The predominant synaptic response pattern for all types of stimulation was an EPSP-IPSP sequence. Thirty percent of the responses were IPSPs only. Relatively few “pure” EPSPs were recorded. These patterns of synaptic responses were compared with those evoked by comparable stimuli to caudate neurons. In particular, the relatively high percentage of “pure” IPSPs in pallidal and entopeduncular cells contrasted with the rate occurrence of “pure” IPSPs in caudate neurons. This difference in incidence of response types may be attributed to anatomical differences in the fine structure of these nuclei.     

Neuronal and synaptic mechanisms of cortical inhibition

Data are presented concerning the latencies, amplitude and durations of IPSPs evoked in neurons of different cortical regions by peripheral and thalamocortical fibre stimulations and intracortical microstimulation in cat. The duration of IPSPs developing due to single afferent stimulus ranged between 20 and 250 ms (60-80 ms as a rule). In response to intracortical microstimulation monosynaptic IPSPs of 5-10 ms duration arose parallel with IPSPs of 20-100 ms duration. Barbiturates and chloralose increased the duration of IPSP up to 300-500 ms. The latencies of 73% of IPSPs evoked in the auditory cortex neurons by thalamocortical fibres stimulation exceed the latencies of monosynaptic EPSPs evoked by the same kind of stimuli not more than by 1.2 ms. Conclusion is made that inhibition developing in neurons of the cortical projection areas in response to afferent volley is a direct one and is produced by cortical inhibitory interneurons. Some cortical neurons were inhibited by a recurrent mechanism. Only 2% of IPSPs developed monosynaptically. The synaptic delay of IPSP evoked by intracortical microstimulation ranged between 0.3-0.4 ms. The length of the inhibitory neuron axons in layer IV of the auditory cortex reached 1.5 mm. The excitation conduction velocity in these axons was calculated to be 1.6-2.8 m/s (2.2 m/s on the average).     

Centrifugal innervation of the lamprey retina. Light- and electron microscopic and electrophysiological investigations

Centrifugal fibers and their synaptic connections were studied in retinas of the lamprey Lampetra fluviatilis. The morphological analysis of retinofugal and retinopetal elements was performed after their horseradish peroxidase (HRP) filling through either the cut optic nerve in isolated retina preparations or after intracerebral HRP injections. In flat-mounted retinas, labeled ganglion cell bodies with their dendritic arborizations as well as centrifugal axons were found. The topography of labeled ganglion cell bodies and fibers in semi-thin plastic sections is described. The electron microscopic analysis revealed that the centrifugal terminals synapse either upon unlabeled somata or profiles containing synaptic vesicles (PCSVs). In more rare cases these boutons seem to establish synaptic contacts on ganglion cell dendrites. The target cell bodies were located within the inner part of the inner nuclear layer, whereas postsynaptic dendrites and PCSVs were mainly observed in the outer portion of the internal synaptic layer. Stimulation of the optic nerve in isolated retinas produced antidromic responses in 23 neurons and in 9 of these cells, an antidromic spike was followed by a postsynaptic potential (PSP). Ten cells yielded no antidromic response, but showed PSPs sometimes associated with spikes. The morphological and physiological evidence obtained indicate that these PSP-generating cells were activated synaptically by centrifugal fibers and that in the lamprey retina, these fibers make contacts either with dendrites or somata of amacrine cells and probably with ganglion cell dendrites.     

Morphology and laminar distribution of electrophysiologically identified cells in the pigeon's optic tectum: an intracellular study

The responses of 65 cells to electrical stimulation of the contralateral optic nerve were intracellularly recorded in the pigeon optic tectum by using micropipettes filled with a solution of horseradish peroxidase. Nineteen of them were successfully labeled. Microscopic examination of the filled cells shows that our sample includes six pyramidal, ten ganglion, two stellate, and one bipolar horizontal cells. Thus, pyramidal and ganglion neurons constitute the most numerous types of cells in our sample. Pyramidal cells were located in layer II but mostly in its non-retinorecipient part, and they had restricted ascending dendritic trees oriented orthogonal to the tectal lamination. Ganglion cells were located in layer III with one exception, which was in sublayer IIi. These cells had non-oriented dendritic trees which ramify over considerable distances. Terminal dendritic branches from a number of pyramidal and ganglion cells extended superficially well within the region of optic fibers termination. In our study, ganglion cells constituted the efferent tectal elements. Pyramidal cells responded to optic nerve stimulation with a pure EPSP, with an EPSP-IPSP sequence, or with a pure IPSP. Ganglion cells always exhibited an IPSP either alone or preceded by an EPSP. Stellate and bipolar cells responded with a pure EPSP. The study of the laminar distribution of labeled and non-labeled cells shows from surface to depth, a gradual increase in the number of cells responding with an EPSP-IPSP or with a pure IPSP and a gradual decrease in the number of those exhibiting a pure EPSP. The analysis of the sensitivity of EPSPs and IPSPs to high frequency optic nerve stimulation shows that monosynaptic as well as polysynaptic EPSPs can be recorded from cells in the non-retinorecipient tectal region, a number of ganglion and pyramidal cells receive a direct retinal excitatory input as their dendrites pass through the region of optic endings, most IPSPs are polysynaptic, some cells located in the retinorecipient region may receive direct retinal inhibitory connections.     

Intracellular electrophysiological and morphological study of the medullary inspiratory neurons of the decerebrate rat.

Intracellular recordings and labelings with horseradish peroxidase (HRP) of inspiratory neurons were performed in decerebrate, paralyzed and ventilated rats. A total of 58 neurons were located within the ventrolateral medulla. They were identified as bulbospinal neurons (n = 15), cranial motoneurons (n = 9) and not antidromically activated (NAA) neurons (n = 34) by antidromic stimulation or HRP labeling, or both. These inspiratory neurons had rhythmical changes in membrane potentials similar to those reported in cats, i.e. an abrupt depolarization at the onset of phrenic discharge followed by trajectories of depolarization which evolved into augmenting I, bell-shaped I or decrementing I patterns until a rapid repolarization at the start of expiration. All types were hyperpolarized during expiration by chloride-dependent inhibitory postsynaptic potentials (IPSPs) which were demonstrated in 13 neurons from which the reversal was obtained. Such IPSPs were apparent in two waves throughout expiration, an early one in post-inspiration (stage I of expiration) and a late one in late expiration (stage II of expiration). These properties are also similar to those of feline inspiratory medullary neurons. Four labeled bulbospinal neurons had axonal collaterals which were ipsi- and contralateral to the site of their somata. Two of 6 labeled NAA neurons exhibited profuse axonal arborizations within various medullary nuclei. No medullary axonal collateral was seen from 6 labeled motoneurons. These results indicate that even though in the rat a single concentration of inspiratory neurons within the ventrolateral medulla has been demonstrated, there is no fundamental difference in the organization of the inspiratory neuronal network compared to that of the cat.     

Correlation between intrinsic firing patterns and thalamocortical synaptic responses of neurons in mouse barrel cortex.

We used a thalamocortical slice preparation to record both spike trains and synaptically evoked responses from neurons of mouse barrel cortex. Cells were classified as regular spiking (RS), intrinsically bursting (IB), or fast spiking (FS) according to their temporal firing patterns when injected with current. RS cells were further separated into two subtypes, RS1 and RS2 cells, the latter encountered only in the infragranular layers. Synaptic responses were elicited by focal electrical stimuli in the ventrobasal nucleus of the thalamus (VB) while holding the cells at different membrane potentials. Postsynaptic potentials were classified as excitatory (EPSPs) or inhibitory (IPSPs), and their latencies were measured from the onset of the extracellularly recorded fiber volley in layer IV. EPSPs fell into three groups, according to latency. Those in the early cluster had latencies shorter than 1 msec and were coincident with the postsynaptic layer IV population response; they were considered monosynaptic. A second group, with latencies between 1.3 and 2.5 msec, were coincident with all IPSPs and were classified as disynaptic. The rest had latencies longer than 5 msec and were considered polysynaptic. The synaptic order of a cell was correlated with its laminar position and its electrophysiological class. Specifically, monosynaptic responses were restricted to infragranular RS cells and to FS cells, while disynaptic EPSPs were found in supragranular RS cells and in IB cells. Disynaptic IPSPs were found in both deep and superficial layers; in the deep layers they nearly always followed monosynaptic EPSPs, while in the superficial layers they were mostly found in isolation. We conclude that the intrinsic spiking characteristics of a neuron are an important determinant of its position in the cortical circuit and may have a substantial role in determining its response properties.     

Caudate evoked synaptic activities in nigral neurons

Intracellular recordings from neurons in the substantia nigra have revealed three varieties of monosynaptic PSPs in response to stimulation of the ipsilateral head of the caudate nucleus: short (3–5 msec) latency EPSPs; short (3–5 msec) latency IPSPs; and long (15–20 msec) latency IPSPs. The data indicate that at least three efferent fiber systems link the caudate to the nigra: two fast conducting axonal systems of comparable diameters mediate the short-latency PSPs whereas a slow conducting axonal system mediates the long-latency PSPs. The caudate evoked long-latency IPSPs in nigral neurons were preceded by antidromic and orthodromic potentials in the motor cortex; these cortical potentials are regarded as epiphenomena. The dual, facilitatory-inhibitory control of caudate on nigral neurons is consonant with the proposal that the caudate self-regulates its input from the nigra. The caudate-evoked EPSPs in nigral neurons are sine qua non for the operation of the caudato-nigrothalamic projection system.     

Tectal afferents monosynaptically activate neurons in the pigeon isthmo-optic nucleus

Postsynaptic responses of 105 neurons in brain slices were intracellularly recorded from the isthmo-optic nucleus (ION) in pigeons, and 18 of these neurons were labeled with Lucifer yellow. Excitatory postsynaptic potentials (EPSPs) or spikes were produced in 93 cells, inhibitory postsynaptic potentials (IPSPs) in 10 cells, and EPSPs followed by IPSPs in two cells following electrical stimulation of the tecto-isthmooptic tract. The EPSPs occurred in an all-or-none fashion, with short latencies (1.3 +/- 0.6 ms). Repetitive stimulation increased their amplitude and duration, demonstrating that temporal summation was involved. Neurons producing excitatory responses were distributed throughout cellular layers of the nucleus. Pure IPSPs had a latency of 3.9 +/- 2.3 ms, and cells that responded in this manner were only distributed in the rostral portion of the nucleus. In the remaining two cells with EPSP-IPSP responses, the latency of excitatory responses was 1.5 ms in one cell and 1.4 ms in the other, and that of inhibitory responses was, respectively, 5.1 and 4.1 ms. Thus, it appeared that excitation was monosynaptic, whereas inhibition may be polysynaptic. Four single injections resulted in dye-coupled labeling, and two pairs of closely apposed cells fired spikes, probably resulting from spatial summation of their excitatory responses. The present study suggests that tectal cells directly activate ION neurons and that tectal fibers contact isthmo-optic neurons in a one-to-one fashion. Taken together with previous studies, it appears that the entire tecto-ION-retinal pathway is excitatory.     

Excitatory projection of the rat subicular complex to the cingulate cortex and synaptic integration with thalamic afferents

We studied the responses of rat cingulate cortex neurons to electrical stimulation of the subicular complex. Intracellular and 'quasi-intracellular' recordings from layer V posterior cingulate neurons showed that stimulation of the presubiculum or postsubiculum evoked EPSPs and action potentials. These were usually followed by shallow IPSPs averaging 122 ms in duration. Frequency potentiation of an EPSP was demonstrated in one case. Laminar analysis of field potentials provided evidence for a source of excitatory synaptic drive in layer II-III of the posterior cingulate cortex, where the subicular projections terminate, presumably on apical dendrites of layer V pyramids. Intracellular HRP injection of neurons showing EPSPs after subicular complex stimulation established that these responsive neurons were layer V pyramids. One cell with physiological properties characteristic of inhibitory interneurons was recorded in layer V. Stimulation of the thalamic nuclei lateralis and anterior ventralis also evoked EPSPs and action potentials in layer V cingulate neurons. In one cell it was possible to show that EPSPs evoked by presubicular stimulation and by nucleus anterior ventralis summed. These results indicate that subicular and thalamic afferents make excitatory synaptic contact onto dendrites of the same layer V cingulate neurons; that spatial summation can integrate the input from these two sources; and that inhibition from local interneurons limits the duration of this excitatory influence.     

Reactions of medial geniculate body neurons to stimulation of the auditory cortex]

Extra- and intracellular responses of pars principalis neurons in the medial geniculate body to stimulation of the first (AI), second (AII) and third (AIII) auditory cortex were studied in experiments on cats immobilized with d-tubocurarine. In geniculate neurons both antidromic (45-50%) and orthodromic (50-55%) reactions occurred in response to the auditory cortex stimulation. The latencies for antidromic and orthodromic responses were 0.3-2.5 ms and 2.0-ms, respectively. Late responses appeared with a latency of 30-200 ms. 63% of neurons responded antidromically to both AII and AI stimulation, that confirms the suggestion on the projection of a considerable number of the geniculate neurons to both auditory zones. Orthodromic responses of geniculate neurons consisted either of 1-2 spikes or a burst of 8-12 spikes with a frequency of 300-600/sec. The bursts are supposed to be the responses of inhibitory geniculate neurons. Intracellular recording showed the following responses: antidromic spikes, EPSP, EPSP-spike, EPSP-spike-IPSP, EPSP-IPSP and initial IPSP. Above 50% of initial IPSPs had the latency of 2.0-4.0 ms. They are supposed to be produced with the participation of intermediate inhibitory neurons located in the medial geniculate body.