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PubMed1982

Resistance of a crayfish sensory interneurone to hyperinnervation by acceptable afferents.

1. Intact normal innervation of muscle fibres and other peripheral targets usually prevents regenerating nerves from forming synapses with the targets. Whether intact innervation similarly prevents synapse formation on central target neurones has rarely been tested. This question was examined here for interneurone A of the crayfish last abdominal ganglion. 2. Interneurone A normally receives synaptic input from mechanoreceptor neurones distributed over the side of the tailfan ipsilateral to interneurone A's axon and unilateral dendrites. When the five nerve roots carrying mechanoreceptor axons of one side are cut and central and peripheral ends of one or more are sutured together, regeneration and reinnervation of interneurone A occurs over some two to six weeks. If peripheral ends of roots from the 'wrong' (contralateral) side of the body are sutured to ipsilateral central stumps, they also form connexions with interneurone A. When roots from the two sides of the body are simultaneously tied to a central stump, functional connexion formation occurs equally well for afferents from both sides. Therefore, roots of the two sides seem to be equivalent in their ability to reinnervate interneurone A. 3. If peripheral ends of roots from one side of the tailfan are tied to roots on the intact opposite side of the body, the cut axons appear to grow into the last ganglion but usually do not form functional synapses there. The intact innervation therefore seems to exclude further innervation by other acceptable afferents. 4. It is known that mechanoreceptors are added to the tailfan at moult. Exclusion of extra innervation often broke down partially in animals that moulted during the present experiments. This suggests the possibility that synapse formation or exchange may be controlled by moult-inducing hormones.

Animals↗
PubMed1988

Respiratory interneurones in the thoracic spinal cord of the cat.

1. The discharges of spontaneously firing neurones, whose activity was modulated in phase with the central respiratory cycle, were recorded in the thoracic ventral horn (T3-T9) of anaesthetized, paralysed cats. 2. Out of 310 neurones, forty-six were positively identified as motoneurones by antidromic activation or spike-triggered averaging, fifty-four were positively identified as interneurones by antidromic activation from other spinal cord segments and ninety were indirectly identified as interneurones by virtue of their positions or firing rates as compared to the motoneurones. 3. Units were classified as inspiratory (64%), expiratory (25%) or post-inspiratory (7%) according to the times of their maximum firing rates. The remaining 4% consisted of units whose discharges were either strongly locked to the respiratory pump cycle or did not fit into the other categories. All but one of the motoneurones were classified as inspiratory or expiratory. 4. Inspiratory and expiratory units were further classified as early, late or tonic according to the starting times of their discharges in the respiratory cycle. The interneurones (both positively and indirectly identified) included more of the early and tonic categories and more fast-firing units than did the motoneurones in both the inspiratory and expiratory groups. 5. The locations of the motoneurones corresponded to the known positions of the intercostal and interchondral motor nuclei, including clear segregation of inspiratory and expiratory populations. The locations of neither the interneurones nor the unidentified units were segregated according to their firing patterns. These neurones were concentrated in the medial half of the ventral horn and were found generally more dorsally than the positions of the motoneurones, though their positions overlapped considerably with this group. 6. The axons of the positively identified interneurones were identified from one to five segments caudally and mostly contralaterally, but were not traced to their terminations. Some axons were located by microstimulation and found to run in the ventral or ventromedial white matter. Conduction velocities covered a wide range, 8 to around 100 m/s, mean 53 m/s. 7. Preliminary calculations indicate that there may be almost 10 times more respiratory thoracic interneurones as respiratory bulbospinal neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1995

Reciprocal inhibition between wrist flexors and extensors in man: a new set of interneurones?

1. Interneurones mediating reciprocal inhibition between wrist flexors and extensors in man are characterized using both Renshaw cells and transarticular group I afferent activation. 2. Renshaw cells were activated by reflex discharges evoked by a tendon tap. The tendon tap was applied to the tendon of the muscles from which the Ia fibres responsible for the reciprocal inhibition originated. Contrary to what was observed both in the cat hindlimb and in human elbow muscles, this Renshaw cell activation never resulted in a long depression of the reciprocal inhibition between wrist flexors and extensors. 3. Convergence from group I elbow muscle afferents and antagonistic group I afferents onto interneurones mediating reciprocal inhibition between wrist muscles was revealed in post-stimulus time histogram (PSTH) experiments using the technique of spatial facilitation. 4. The characteristics of the interneurones mediating reciprocal inhibition between wrist flexors and extensors could therefore be summarized as follows: (a) they are fed by antagonistic group I afferents and group I afferents originating from both flexor and extensor elbow muscles; (b) they are not inhibited by Renshaw cells; (c) they are not excited by low threshold cutaneous afferents; and (d) they are probably interposed in a disynaptic pathway. 5. It is therefore concluded that interneurones mediating reciprocal inhibition between wrist flexors and extensors in man differ both from Ia interneurones and from interneurones interposed in the Ib reflex pathways and these characteristics are related to the complex circumduction movements developed in the wrist.

Adult↗
PubMed2002

Gating, modulation and subunit composition of voltage-gated K(+) channels in dendritic inhibitory interneurones of rat hippocampus.

GABAergic interneurones are diverse in their morphological and functional properties. Perisomatic inhibitory cells show fast spiking during sustained current injection, whereas dendritic inhibitory cells fire action potentials with lower frequency. We examined functional and molecular properties of K(+) channels in interneurones with horizontal dendrites in stratum oriens-alveus (OA) of the hippocampal CA1 region, which mainly comprise somatostatin-positive dendritic inhibitory cells. Voltage-gated K(+) currents in nucleated patches isolated from OA interneurones consisted of three major components: a fast delayed rectifier K(+) current component that was highly sensitive to external 4-aminopyridine (4-AP) and tetraethylammonium (TEA) (half-maximal inhibitory concentrations < 0.1 mM for both blockers), a slow delayed rectifier K(+) current component that was sensitive to high concentrations of TEA, but insensitive to 4-AP, and a rapidly inactivating A-type K(+) current component that was blocked by high concentrations of 4-AP, but resistant to TEA. The relative contributions of these components to the macroscopic K(+) current were estimated as 57 +/- 5, 25 +/- 6, and 19 +/- 2 %, respectively. Dendrotoxin, a selective blocker of Kv1 channels had only minimal effects on K(+) currents in nucleated patches. Coapplication of the membrane-permeant cAMP analogue 8-(4-chlorophenylthio)-adenosine 3':5'-cyclic monophosphate (cpt-cAMP) and the phosphodiesterase blocker isobutyl-methylxanthine (IBMX) resulted in a selective inhibition of the fast delayed rectifier K(+) current component. This inhibition was absent in the presence of the protein kinase A (PKA) inhibitor H-89, implying the involvement of PKA-mediated phosphorylation. Single-cell reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed a high abundance of Kv3.2 mRNA in OA interneurones, whereas the expression level of Kv3.1 mRNA was markedly lower. Similarly, RT-PCR analysis showed a high abundance of Kv4.3 mRNA, whereas Kv4.2 mRNA was undetectable. This suggests that the fast delayed rectifier K(+) current and the A-type K(+) current component are mediated predominantly by homomeric Kv3.2 and Kv4.3 channels. Selective modulation of Kv3.2 channels in OA interneurones by cAMP is likely to be an important factor regulating the activity of dendritic inhibitory cells in principal neurone-interneurone microcircuits.

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PubMed2003

Active dendrites and spike propagation in multi-compartment models of oriens-lacunosum/moleculare hippocampal interneurons.

It is well known that interneurons are heterogeneous in their morphologies, biophysical properties, pharmacological sensitivities and electrophysiological responses, but it is unknown how best to understand this diversity. Given their critical roles in shaping brain output, it is important to try to understand the functionality of their computational characteristics. To do this, we focus on specific interneuron subtypes. In particular, it has recently been shown that long-term potentiation is induced specifically on oriens-lacunosum/moleculare (O-LM) interneurons in hippocampus CA1 and that the same cells contain the highest density of dendritic sodium and potassium conductances measured to date. We have created multi-compartment models of an O-LM hippocampal interneuron using passive properties, channel kinetics, densities and distributions specific to this cell type, and explored its signalling characteristics. We found that spike initiation depends on both location and amount of input, as well as the intrinsic properties of the interneuron. Distal synaptic input always produces strong back-propagating spikes whereas proximal input could produce both forward- and back-propagating spikes depending on the input strength. We speculate that the highly active dendrites of these interneurons endow them with a specialized function within the hippocampal circuitry by allowing them to regulate direct and indirect signalling pathways within the hippocampus.

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PubMed2004

Global dendritic calcium spikes in mouse layer 5 low threshold spiking interneurones: implications for control of pyramidal cell bursting.

Interneuronal networks in neocortex underlie feedforward and feedback inhibition and control the temporal organization of pyramidal cell activity. We previously found that lower layer neocortical interneurones can reach action potential threshold in response to the stimulation of a single presynaptic cell. To better understand this phenomenon and the circuit roles of lower layer neocortical interneurones, we combined two-photon calcium imaging with whole cell recordings and anatomical reconstructions of low threshold spiking (LTS) interneurones from mouse neocortex. In both visual and somatosensory cortex, LTS interneurones are somatostatin-positive, concentrated in layer 5 and possess dense axonal innervation to layer 1. Due to the LTS properties, these neurones operate in burst and tonic modes. In burst mode, dendritic T-type calcium channels boosted small synaptic inputs and triggered low threshold calcium spikes, while in tonic mode, sodium-based APs evoked smaller calcium influxes. In both modes, the entire dendritic tree of LTS interneurones behaved as a 'global' single spiking unit. This, together with the fact that synaptic inputs to layer 5 LTS cells are facilitating, and that their axons target the dendritic region of the pyramidal neurones where bursts are generated, make these neurones ideally suited to detect and control burst generation of individual lower layer pyramidal neurones.

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PubMed2004

Hippocampal gamma-frequency oscillations: from interneurones to pyramidal cells, and back.

GABAergic interneurones are necessary for the emergence of hippocampal gamma-frequency network oscillations, during which they play a key role in the synchronization of pyramidal cell firing. However, it remains to be resolved how distinct interneurone subtypes contribute to gamma-frequency oscillations, in what way the spatiotemporal pattern of interneuronal input affects principal cell activity, and by which mechanisms the interneurones themselves are synchronized. Here we summarize recent evidence from cholinergically induced gamma-frequency network oscillations in vitro, showing that perisomatic-targeting GABAergic interneurones provide prominent rhythmic inhibition in pyramidal cells, and that these interneurones are synchronized by recurrent excitation. We conclude by presenting a minimal integrate-and-fire network model which demonstrates that this excitatory-inhibitory feedback loop is sufficient to explain the generation of intrahippocampal gamma-frequency oscillations.

Animals↗
PubMed2006

Synaptic kainate currents reset interneuron firing phase.

Hippocampal interneuron activity has been linked to epileptogenesis, seizures and the oscillatory synaptic activity detected in behaving rats. Interneurons fire at specific times in the rhythmic cycles that comprise these oscillations; however, the mechanisms controlling these firing patterns remain unclear. We have examined the role of synaptic input in modulating the firing of spontaneously active rat hippocampal interneurons. We find that synaptic glutamate receptor currents of 20-30 pA increase instantaneous firing frequency and reset the phase of spontaneously firing CA1 stratum oriens interneurons. Kainate receptor (KAR)-mediated currents are particularly effective at producing this phase reset, while AMPA receptor currents are relatively ineffective. The efficacy of KAR-mediated currents is probably due to their 3-fold longer decay. Given the small amplitude of the currents needed for this phase reset, coincident activation of only a few KAR-containing synapses could synchronize firing in groups of interneurons. These data suggest that KARs are potent modulators of circuit behaviour and their activation alters hippocampal interneuron output.

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PubMed1976

Identification and discharge patterns of spinal sympathetic interneurons.

Sympathetic interneurons, in the vicinity of the intermediolateral cell column of the cat thoracic spinal cord, were identified by determining whether the probability of spontaneously occurring unitary discharge was correlated in time with the R wave of the ECG. Fifteen units which could not be antidromically activated by stimulation of the cervical sympathetic nerve exhivited a positive post-R wave relationship. The discharge patterns of these cells were distinctly different from those of antidromically identified preganglionic neurons. The interneurons discharged spontaneously in bursts with short interspike intervals (less than 20 ms). Preganglionic neurons rarely discharged more than once during a cardiac cycle. Single shocks applied to medullary pressor sites evoked a train of spikes in the interneurons. Preganglionic units usually discharged only once to medullary pressor stimulation. Electrial activation of medullospinal inhibitory tracts suppressed the discharges of both interneurons and preganclionic units. This observation indicates that spinal inhibition is exerted at an interneuronal level within sympathoexictatory pathways. No evidence was found for the existence of inhibitory interneurons in the intermediolateral cell column. This study demonstrated that the post-R wave time interval histogram in combination with a test for antidromic activation can be used to differentiate between spinal sympathetic internuerons and preganglionic cells.

Animals↗
PubMed2004

Dorsal spinal interneurons forming a primitive, cutaneous sensory pathway.

In mammals, sensory projection pathways are provided by just three classes of spinal interneuron that develop from the roof-plate. We asked whether similar sensory projection interneurons are present primitively in a developing lower vertebrate where function can be more readily studied. Using an immobilized Xenopus tadpole spinal cord preparation, we define the properties and connections of spinal sensory projection interneurons using whole cell patch recordings from single neurons or pairs, identified by dye filling. Dorsolateral interneurons lie in the tadpole equivalent of the spinal dorsal horn, and have dorsally located dendrites and an ipsilateral ascending axon that projects into the midbrain. These neurons receive direct, mainly AMPA receptor (AMPAR)-mediated, excitation from skin touch sensory neurons. They in turn produce AMPAR and N-methyl-d-aspartate receptor (NMDAR)-mediated excitation of spinal locomotor pattern generator neurons rostrally on the same side of the cord. During swimming, they receive glycinergic modulatory inhibition from ascending interneurons in the spinal locomotor central pattern generator. We conclude that spinal dorsolateral interneurons are a primitive class of excitatory sensory projection neurons activated by ipsilateral cutaneous afferents and carrying excitation ipsilaterally and rostrally as far as the midbrain to initiate or accelerate swimming.

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PubMed2006

Identification of basolateral amygdala projection cells and interneurons using extracellular recordings.

This study tested whether firing rate and spike shape could be used to distinguish projection cells from interneurons in extracellular recordings of basolateral amygdala (BLA) neurons. To this end, we recorded BLA neurons in isoflurane-anesthetized animals with tungsten microelectrodes. Projection cells were identified by antidromic activation from cortical projection sites of the BLA. Although most projection cells fired spontaneously at low rates (<1 Hz), an important subset fired at higher rates (up to 6.8 Hz). In fact, the distribution of firing rates in projection cells and unidentified BLA neurons overlapped extensively, even though the latter cell group presumably contains a higher proportion of interneurons. The only difference between the two distributions was a small subset (5.1%) of unidentified neurons with unusually high firing rates (9-16 Hz). Similarly, distributions of spike durations in both cell groups were indistinguishable, although most of the fast-firing neurons had spike durations at the low end of the distribution. However, we observed that spike durations depended on the exact position of the electrode with respect to the recorded cell, varying by as much as 0.7 ms. Thus neither firing rate nor spike waveform allowed for unequivocal separation of projection cells from interneurons. Nevertheless, we propose the use of two firing rate cutoffs to obtain relatively pure samples of projection cells and interneurons: < or =1 Hz for projection cells and > or =7 Hz for fast-spiking interneurons. Supplemented with spike-duration cutoffs of > or =0.7 ms for projection cells and < or =0.5 ms for interneurons, this approach should keep instances of misclassifications to a minimum.

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PubMed2003

Differential modulation of CA1 and dentate gyrus interneurons during exploration of novel environments.

Parallel recordings of hippocampal principal cells and interneurons were obtained as rats foraged in familiar and adjacent, novel environments. Firing rates of each cell type were assessed as a function of spatial location. Many CA1 interneurons exhibited large decreases in activity in the novel compared with the familiar environment. Dentate gyrus interneurons, however, were much more likely to exhibit large increases in firing in the novel environment. Neither effect was correlated with basic interneuron discharge properties such as degree of theta modulation, baseline firing rate or degree of spatially modulated discharge. Both CA1 and dentate gyrus interneuron rate changes extended into regions of the familiar environment bordering the novel environment. Principal cells in CA1 and dentate gyrus exhibited similar patterns of place specific activity each being indicative of incorporation of novel spatial information into the spatial representation of the familiar environment. The data indicate that inhibitory networks in the CA1 and dentate gyrus areas are modulated in a divergent fashion during the acquisition of novel spatial information and that interneuron activities can be used to detect those regions of an environment subject to redistribution of principal cell spatial activity patterns.

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PubMed2004

Functional properties of fast spiking interneurons and their synaptic connections with pyramidal cells in primate dorsolateral prefrontal cortex.

Recent studies suggest that fast-spiking (FS) interneurons of the monkey dorsolateral prefrontal cortex (DLPFC) exhibit task-related firing during working-memory tasks. To gain further understanding of the functional role of FS neurons in monkey DLPFC, we described the in vitro electrophysiological properties of FS interneurons and their synaptic connections with pyramidal cells in layers 2/3 of areas 9 and 46. Extracellular spike duration was found to distinguish FS cells from non-FS interneuron subtypes. However, a substantial overlap in extracellular spike duration between these populations would make classification of individual interneurons difficult. FS neurons could be divided into two main morphological groups, chandelier and basket neurons, with very similar electrophysiological properties but significantly different horizontal spread of the axonal arborization. In paired cell recordings, unitary inhibitory postsynaptic potentials (IPSPs) elicited by FS neurons in pyramidal cells had rapid time course, small amplitude at resting membrane potential, and were mediated by GABA(A) receptors. Repetitive FS neuron stimulation, partially mimicking the sustained firing of interneurons in vivo, produced short-term depression of the unitary IPSPs, present at connections made by both basket and chandelier neurons and due at least in part to presynaptic mechanisms. These results suggest that FS neurons and their synaptic connections with pyramidal cells have homogeneous physiological properties. Thus different functional roles of basket and chandelier neurons in the DLPFC in vivo must arise from the distinct properties of the interneuronal axonal arborization or from a different functional pattern of excitatory and inhibitory connections with other components of the DLPFC neuronal network.

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PubMed2003

Excitatory inputs to CA1 interneurons show selective synaptic dynamics.

The dynamic properties of synapses between neurons in the hippocampal CA1 area are important for the frequency-dependent signal transfer of the network. We have examined the synaptic dynamics of excitatory inputs to CA1 interneurons and pyramidal cells using whole cell voltage-clamp recordings. The CA1 network was activated using extracellular stimulation electrodes at the Schaffer collaterals (feedforward activation) or at the Alveus (activation of the feedback loop). The dynamic properties of input from the Schaffer collaterals to CA1 interneurons (basket and bistratified cells) were different from the synaptic dynamics of input from the Alveus. Synaptic input from the Schaffer collaterals to CA1 interneurons showed facilitation for most frequencies. After 10 stimuli the synaptic response reached a plateau level that was approximately 150% of the first response in the train. In contrast, the plateau levels of Alveus inputs to interneurons were not different from the first responses for frequencies <or=40 Hz. Paired-pulse facilitation of Schaffer input was stronger than for Alveus input. Cells in stratum oriens with horizontal dendritic trees appeared to be a special group of interneurons because Alveus input to these cells showed strong facilitation with plateau levels of 200% of the first responses. Schaffer input to CA1 basket and bistratified cells showed similar synaptic dynamics compared with Schaffer input to pyramidal cells for frequencies <or=80 Hz. The synaptic dynamics of Schaffer and Alveus input depended only weakly on the stimulus intensity. The difference between the dynamics of Alveus and Schaffer input to CA1 interneurons implies that the relative contribution of feedforward and -back inhibition to network activity depends on the frequency of the input signal at the afferent fibers, adding a level of complexity to transient responses.

Animals↗
PubMed1980

Triggering of locust jump by multimodal inhibitory interneurons.

1. The locust jump is triggered by a sudden inhibition of activity in hindleg flexor tibiae motoneurons following cocontraction of the hindleg flexor and extensor tibiae muscles. The main result of this investigation was the identification of two interneurons (one for each hindleg) that monosynaptically inhibit flexor tibiae motoneurons and whose properties are all consistent with them being the trigger interneurons for initiating a jump. 2. These interneurons receive strong excitatory input from many sensory modalities (visual, auditory, tactile, and proprioceptive). Because of their multimodal response characteristics, we designated them M-neurons. A particularly strong excitatory input to each M-neuron is from both descending contralateral movement detector (DCMD) interneurons. 3. The threshold for spike initiation in the M-neurons is high (approximately 14 mV). As a consequence, input from any one sensory modality alone rarely initiates action potentials. 4. Each M-neuron is depolarized by sensory input from leg proprioceptors. We propose that proprioceptive feedback during the cocontraction phase depolarizes the M-neurons to decrease their threshold, thus enabling extrinsic sensory stimuli to generate action potentials in both M-neurons and in so doing trigger a jump. The function of the proprioceptive gating of inhibitory transmission from the various sensory systems to the flexor motoneurons (via the M-neurons) is to ensure the development of a strong isometric contraction of the extensor tibiae muscle, and thus a powerful jump in response to external stimuli. 5. Insofar as the initiation of the locust jump depends on sensory convergence onto large identified interneurons, this behavior is similar to ballistic movements in some other animals such as the crayfish tail flip and the startle response in fish. The unique feature of the locust jump is that the trigger interneurons initiate the jump only after a preceding phase (cocontraction) has been accomplished.

Animals↗
PubMed1980

Distributed processing by visual interneurons of crayfish brain. II. Network organization and stimulus modulation of synaptic efficacy.

1. Multiple interactions were examined between five or six visual neurons simultaneously monitored in the circumesophageal connective. 2. A single neuron can make divergent connections to at least five other visual interneurons. 3. Conversely, a single cell may receive convergent inputs from up to four visual interneurons. 4. The convergent interactions are sufficiently intense so that 80--90% of a postsynaptic cell's visual activity can be attributed to observed network interactions. 5. Connectivity diagrams suggest that the descending interneurons, which arise in the visual neuropil of the brain, are organized into three interconnected layers: a) neurons that receive input from the optic nerve and project to other visual interneurons, b) neurons that both receive input and project to other descending interneurons in the brain--these cells exhibit a preponderance of reciprocal interactions, c) neurons that receive input from both the first and second network layers and project exclusively to the more caudal ganglia of the ventral nerve cord. 6. The network is systematically organized with respect to visual and nonvisual responsiveness. The cells of the first layer exhibit the strongest visual responses. The cells of the third layer exhibit spontaneous activity and the strongest tactile and/or proprioceptive responses. 7. The intensity of the network interactions is under stimulus control. The synaptic efficacy of a presynaptic spike can vary by over 100-fold as a consequence of stimulus presentation and/or location. The expressed organization of the network thus exhibits a dynamic, stimulus-dependent, plasticity. 8. The results indicate that the descending visual interneurons of the brain rather than forming a parallel tract actually constitute a complex distributed network. Furthermore, the results indicate the feasibility of population neural coding based on stimulus-dependent inpulse coordination in an array of neurons.

Animals↗
PubMed1984

Activity of commissural interneurons in spinal cord of Xenopus embryos.

Horseradish peroxidase- (HRP) filled microelectrodes have been used to examine the anatomy and physiology of "commissural interneurons," a morphologically defined class of spinal cord interneuron in Xenopus laevis embryos. Commissural interneurons have unipolar cell bodies in the dorsal half of the spinal cord. Their dendrites lie in the mid to ventral parts of the lateral tracts and their axons cross the cord ventrally, T branch, and ascend and descend on the opposite side of the cord. Recordings were made from animals immobilized in tubocurarine and responding to natural stimulation with three patterns of fictive motor activity. During episodes of fictive swimming, commissural interneurons are phasically excited to fire 1 spike/cycle in phase with motor discharge on the same side and receive a midcycle inhibitory postsynaptic potential (IPSP) in phase with motor discharge on the opposite side. Rhythmic activity is superimposed on a background depolarization. During periods of synchrony, phasic excitatory input doubles in frequency so that cells fire with half the swimming cycle period. The background depolarization is generally stronger than during swimming. During periods of fictive struggling, evoked by electrical stimulation of the skin, commissural interneurons fire a burst of spikes per cycle, cells being relatively hyperpolarized when motoneurons on the opposite side are active. In response to ipsilateral skin stimulation, some cells receive an IPSP at a latency of 12-20 ms. This precedes the onset of fictive locomotion. We discuss how anatomy and activity of commissural interneurons is suitable for a reciprocal inhibitory role.

Animals↗
PubMed1991

Three types of inhibitory postsynaptic potentials generated by interneurons in the anterior thalamic complex of cat.

1. These experiments were carried out to study how thalamic interneurons generate inhibitory postsynaptic potentials (IPSPs) in relay cells. Intracellular recordings were performed in the anterior thalamic (AT) nuclei, a nuclear group in which interneurons constitute the only intrathalamic source of gamma-aminobutyric acid (GABA). 2. In the AT complex, as in most dorsal thalamic nuclei, interneurons can influence relay cells through their presynaptic dendrites (PSDs) and their axons. This dual mode of action is paralleled by a different termination pattern of prethalamic fibers and cortical axons on interneurons. Prethalamic fibers, which in the AT nuclei arise in the mammillary bodies (MBs), end mostly on PSDs, whereas cortical terminals usually synapse on the parent dendrites of PSDs. We therefore took advantage of the differential mode of termination of cortical and MB afferents on interneurons to infer the respective roles of the axons and PSDs of interneurons in the genesis of the IPSPs recorded from relay cells. 3. In all responsive AT cells, cortical stimuli delivered at low frequency (less than or equal to 0.5 Hz) evoked a biphasic IPSP, with an early and a late phase, having a total duration of 221.96 +/- 8.18 ms (mean +/- SE). The early part of the IPSP (termed A) had a reversal potential (ER) close to the equilibrium potential for Cl- ions: -79.25 +/- 2.14 mV. Furthermore, it reversed in polarity after impalement of the cells with KCl-filled pipettes. The late IPSP (termed B) always began before the end of the early IPSP, 45.93 +/- 2.50 ms after the onset of the A-IPSP. The B-IPSP had an ER of -109 +/- 2.4 mV and was not affected by Cl- injection. 4. By contrast, MB stimuli delivered at low frequency (less than or equal to 0.5 Hz) evoked a triphasic IPSP having a total duration of 220.5 +/- 9.42 ms in most (61.2%) AT cells. The IPSP with the shortest latency (termed a) was evoked only by MB stimuli. Before the return of the membrane potential to the resting level, a second hyperpolarizing potential began (7.41 +/- 0.46 ms after the onset of the a-IPSP). This second inhibitory phase was biphasic and had electrophysiological characteristics similar to the biphasic A- and B-IPSP evoked by cortical stimulation. Both the MB-evoked a- and A-IPSPs had an ER close to the equilibrium potential for Cl- ions (-72.22 +/- 0.68 and -72 +/- 0.82 mV, respectively) and reversed in polarity after impalement of the cells with KCl-filled pipettes.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗