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.     

The lateral vestibular nucleus of the toadfish Opsanus tau: Ultrastructural and electrophysiological observations with special reference to electrotonic transmission

The lateral vestibular nucleus of the toadfish Opsanus tau was localized by means of axonal iontophoresis of Procion Yellow. The ultrastructure of the lateral vestibular nucleus neurons was then correlated with their electrophysiological properties. The lateral vestibular nucleus consists of neurons of various sizes which are distributed in small clusters over a heavily myelinated neuropil. The perikarya and main dendrites of the large and the small neurons are surrounded by a synaptic bed, which is separated from the neighboring neuropil by a layer of thin astrocytic processes. The synaptic bed contains three main classes of axon terminals, club endings, large and small terminals, the first being quite infrequent. All the large terminals as well as the occasionally observed club endings contain a pure population of rounded synaptic vesicles. In some of the small axon terminals there are also rounded vesicles; however, the majority contain flattened vesicles or a pleomorphic population. These data indicate that the small terminals originate from different afferent sources. The synaptic interfaces of the large boutons and of the club endings bear three types of junctional complexes: attachment plates, gap junctions and active zones. Those showing both gap junctions and active zones were designated as morphologically ‘mixed synapses’. Gap junctions, although in large number, have only been observed at the synaptic interfaces between terminals with rounded vesicles and the perikarya or the dendrite of the lateral vestibular nucleus neurons. Therefore electrotonic coupling would only be possible by way of presynaptic fibers. Some axons observed in the neuropil were found to establish gap junctional complexes with two different dendritec profiles and this observation is in favour of electrotonic coupling by way of presynaptic terminals.Field and intracellular potentials were recorded in the lateral vestibular nucleus. The field potential evoked by stimulation of the vestibular nerve consisted of an early positive-negative wave followed by a slow negativity, and that evoked by spinal cord stimulation was composed of an antidromic potential followed by a slow negative wave. Vestibulo-spinal neurons were identified by their antidromic spikes. In these cells, stimulation of the ipsilateral vestibular nerve evoked an excitatory postsynaptic potential with two components. The short delay of the first component of this excitatory postsynaptic potential and its ability to follow paired stimulation at close intervals without reduction of the second response suggest that it is transmitted electrotonically from primary vestibular afferent fibers. By contrast the latency of the second peak of the vestibular evoked excitatory postsynaptic potential and its sensitivity to high stimulus frequencies are compatible with monosynaptic chemically mediated transmission from primary vestibular afferents. Spinal stimulation evoked graded antidromic depolarizations in vestibulo-spinal neurons. The latency of these potentials was too short to allow for chemical transmission through afferents or recurrent collaterals and suggests electrotonic spread of antidromic activity from neighboring neurons. An important finding is that the graded antidromic depolarizations can initiate spikes; thus coupling between neurons in the lateral vestibular nucleus is sufficiently close that a cell can be excited by activity spread from neighboring cells. Similar graded depolarizations were recorded in identified primary vestibular afferents; their latencies and time course indicate that they were brought about by electrotonic spread of postsynaptic potentials and spikes to the impaled presynaptic fibers; this confirms the morphological evidence that coupling between lateral vestibular nucleus neurons occurs, at least in part, by way of presynaptic vestibular axons. As the spinal stimulus strength was increased, these graded depolarizations became large enough to initiate spikes which presumably propagate to the vestibular receptors. Thus antidromic invasion of the presynaptic terminals may provide negative feedback by preventing their re-excitation at short intervals after a synchronous discharge of an adequate number of postsynaptic cells. Excitatory inputs to the neurons of the lateral vestibular nucleus were identified from the spinal cord and from the contralateral vestibular nerve. Long latency excitatory postsynaptic potentials large enough to excite the cells were recorded following spinal stimulation; the threshold intensity for evoking them was consistently higher than that adequate to generate the graded antidromic depolarizations. Field potentials recorded after stimulation of the contra lateral vestibular nerve consisted of an initial positive negative wave followed by a slow negative wave. the stimulus intensity for evoking these potentials was the same or slightly above the threshold for those evoked in the lateral vestibular nucleus on the stimulated side. Also lateral vestibular nucleus neurons exhibited excitatory postsynaptic potentials large enough to excite the cells following stimulation of the contralateral vestibular nerve. but no inhibitory postsynaptic potentials were detected. This lack of commissural inhibition indicates a qualitative difference between the central organization of these cells in the toadfish and in mammals.The presence of neurons in the lateral vestibular nucleus which send their axons to the labyrinth was confirmed by their heavy staining with Procion Yellow following axonal iontophoresis. In a number of vestibular neurons. abruptly rising spikes were evoked at short latencies after adequate stimulation of the ipsilateral vestibular nerve. Graded stimuli applied to the vestibular nerve evoked graded short latency depolarizations as well as long latency excitatory postsynaptic potentials in these presumed efferent neurons to the labyrinth; the former could indicate electrotonic coupling of the efferent cells or electrotonic transmission from primary afferents, resulting in a short latency feedback loop.From these studies, the synaptic organization of the lateral vestibular nucleus neurons is compared with that of the Mauthner cells of teleosts, and the possibility of a dual mode of transmission, electrical and chemical, by primary vestibular afferents is discussed.     

Excitatory inputs to cerebellar dentate nucleus neurons from the cerebral cortex in the cat

Summary 1. In anesthetized cats, we investigated excitatory and inhibitory inputs from the cerebral cortex to dentate nucleus neurons (DNNs) and determined the pathways responsible for mediating these inputs to DNNs. 2. Intracellular recordings were made from 201 DNNs whose locations were histologically determined. These neurons were identified as efferent DNNs by their antidromic responses to stimulation of the contralateral red nucleus (RN). Stimulation of the contralateral pericruciate cortex produced excitatory postsynaptic potentials (EPSPs) followed by long-lasting inhibitory postsynaptic potentials (IPSPs) in DNNs. The most effective stimulating sites for inducing these responses were observed in the medial portion (area 6) and its adjacent middle portion (area 4) of the precruciate gyrus. Convergence of cerebral inputs from area 4 and area 6 to single DNNs was rare. 3. To determine the precerebellar nuclei responsible for mediation of the cerebral inputs to the dentate nucleus (DN), we examined the effects of stimulation of the pontine nucleus (PN), the nucleus reticularis tegmenti pontis (NRTP) and the inferior olive (IO). Systematic mapping was made in the NRTP and the PN to find effective low-threshold stimulating sites for evoking monosynaptic EPSPs in DNNs. Stimulation of either the PN or the NRTP produced monosynaptic EPSPs and polysynaptic IPSPs in DNNs. Using a conditioning-testing paradigm (a conditioning stimulus to the cerebral peduncle (CP) and a test stimulus to the PN or the NRTP) and intracellular recordings from DNNs, we tested cerebral effects on neurons in the PN and the NRTP making a monosynaptic connection with DNNs. Conditioning stimulation of the CP facilitated PN- and NRTP-induced monosynaptic EPSPs in DNNs. This spatial facilitation indicated that the excitatory inputs from the cerebral cortex to DNNs are at least partly relayed via the PN and the NRTP. 4. Stimulation of the contralateral IO produced monosynaptic EPSPs and polysynaptic IPSPs in DNNs. These monosynaptic EPSPs were facilitated by conditioning stimulation of the CP, strongly suggesting that the IO is partly responsible for mediating excitatory inputs from the cerebral cortex to the DN. A comparison was made between the latencies of IO-evoked IPSPs in DNNs and the latencies of IO-evoked complex spikes in Purkinje cells. Such a comparison indicated that the shortest-latency IPSPs evoked from the IO were not mediated via the Purkinje cells and suggested the pathway mediated by inhibitory interneurons in the DN. 5. The functional significance of the excitatory inputs from the PN and the NRTP to the DN is discussed in relation to the motor control mechanisms of the cerebellum.     

Neuronal mechanisms of the interaction of Deiters' nucleus with some brainstem structures

The effects of stimulation of the interstitial nucleus of Ramón y Cajal, as well as the nucleus of Darkschewitsch, inferior olive and nucleus reticularis tegmenti pontis on the neuronal activity of the lateral vestibular nucleus of Deiters were studied by means of an intracellular recording technique. Stimulation of these structures is shown to lead to antidromic and orthodromic activation of Deiters' neurons. Collaterals of vestibulospinal neurons entering these structures are revealed electrophysiologically. It was shown that stimulation of the rostral part of the inferior olive evoked in Deiters' neurons predominantly antidromic responses, whereas stimulation of the caudal part of the inferior olive leads mostly to synaptic activation. Stimulation of Ramón y Cajal's and Darkschewitsch's nuclei evokes mono- and/or polysynaptic excitatory and inhibitory postsynaptic potentials in Deiters' neurons. Mono-, oligo- and/or polysynaptic inhibitory postsynaptic potentials were also evoked by stimulation of nucleus reticularis tegmenti pontis, as well as the rostral and particularly, caudal parts of the inferior olive. Stimulation of the caudal part of the inferior olive evoked mono-, oligo- and/or polysynaptic excitatory postsynaptic potentials in Deiters' neurons. Convergence of influences from the stimulated structures on the vestibular neurons under study was shown. A topical correlation between Deiters' nucleus and the brainstem nuclei mentioned above was found. The presence of inhibitory and excitatory interaction of these structures with Deiters' nucleus was established.     

Cerebellar inhibitory control of the vestibulo-ocular reflex investigated in rabbit IIIrd nucleus

Summary In anaesthetized rabbits, the vestibulo-ocular reflex was evoked by electric stimulation of VIIIth nerve and was observed by recording postsynaptic potentials and relevant field potentials in Illrd nucleus. The electric stimulation of flocculus produced a prominent inhibition of the vestibulo-ocular reflex in both the inhibitory component relayed by the superior vestibular nucleus and the excitatory component mediated by the brachium conjunctivum. The excitatory component mediated by the medial vestibular nucleus appeared to be free of the flocculus inhibition. The flocculus inhibition was blocked very effectively by systemic injection of picrotoxin. That the flocculus inhibitory action is due to monosynaptic postsynaptic inhibition of secondary vestibular neurones was demonstrated by direct stimulation of, and also by recording from, the superior nucleus. Recording from the superior nucleus was also performed in anaesthetized cats. All of these above results indicate that Purkinje cells in flocculus projecting to vestibular and cerebellar nuclei cells have inhibitory synaptic action. Flocculus stimulation produced also an excitatory effect upon vestibular nuclei neurones. However, this effect could be attributed to intracerebellar activation of the primary vestibular fibers which pass into the flocculus.     

Dynamic properties of corticothalamic excitatory postsynaptic potentials and thalamic reticular inhibitory postsynaptic potentials in thalamocortical neurons of the guinea-pig dorsal lateral geniculate nucleus

The properties of postsynaptic potentials evoked by stimulation of cortical, retinal and GABAergic thalamic afferents were examined in vitro in thalamocortical neurons of the guinea-pig dorsal lateral geniculate nucleus. Brief trains of stimulation (2–10 stimuli) delivered to corticothalamic fibers led to a frequency-dependent increase in excitatory postsynaptic potential amplitude associated with an increase in activation of both N-methyl-d-aspartate and non-N-methyl-d-aspartate glutamate receptors. In addition, repetitive stimulation of corticothalamic fibers also gave rise to a slow excitatory postsynaptic potential that was blocked by local application of the glutamate metabotropic receptor antagonist α-methyl-4-carboxyphenylglycine. In contrast, repetitive stimulation of optic tract fibers resulted in monosynaptic excitatory postsynaptic potentials that did not potentiate and were not followed by the generation of a slow excitatory postsynaptic potential.Repetitive activation of the optic radiation also evoked both GABA_A and GABA_B receptor-mediated inhibitory postsynaptic potentials. These inhibitory postsynaptic potentials exhibited frequency-dependent depression during repetitive activation. The presence of frequency-dependent facilitation of corticothalamic excitatory postsynaptic potentials and frequency-dependent decrement of inhibitory postsynaptic potentials, as well as the ability of corticothalamic fibers to activate glutamate metabotropic receptors, suggests that sustained activation of corticothalamic afferents in vivo may result in postsynaptic responses in thalamocortical cells that are initially dominated by GABAergic inhibitory postsynaptic potentials followed by prominent monosynaptic excitatory postsynaptic potentials as well as a slow depolarization of the membrane potential.Therefore, the corticothalamic system may inhibit or enhance the excitability and responsiveness of thalamocortical neurons, based both on the spatial and temporal features of thalamocortical interactions.     

Regulation of glutamatergic and GABAergic neurotransmission in the chick nucleus laminaris: role of N-type calcium channels

Neurons in the chicken nucleus laminaris (NL), the third order auditory nucleus involved in azimuth sound localization, receive bilaterally segregated (ipsilateral vs contralateral) glutamatergic excitation from the cochlear nucleus magnocellularis and GABAergic inhibition from the ipsilateral superior olivary nucleus (SON). Here, I investigate the voltage-gated calcium channels (VGCCs) that trigger the excitatory and the inhibitory transmission in the NL. Whole-cell recordings were performed in acute brainstem slices. The excitatory transmission was predominantly mediated by N-type VGCCs, as the specific N-type blocker ω-Conotoxin-GVIA (ω-CTx-GVIA, 1–2.5 μM) inhibited excitatory postsynaptic currents (EPSCs) by ∼90%. Blockers for P/Q- and L-type VGCCs produced no inhibition, and blockade of R-type VGCCs produced a small inhibition. In individual cells, the effect of each VGCC blocker on the EPSC elicited by activation of the ipsilateral input was the same as that on the EPSC elicited by activation of the contralateral input, and the two EPSCs had similar kinetics, suggesting physiological symmetry between the two glutamatergic inputs to single NL neurons. The inhibitory transmission in NL neurons was almost exclusively mediated by N-type VGCCs, as ω-CTx-GVIA (1 μM) produced a ∼90% reduction of inhibitory postsynaptic currents, whereas blockers for other VGCCs produced no inhibition. In conclusion, N-type VGCCs play a dominant role in triggering both the excitatory and the inhibitory transmission in the NL, and the presynaptic VGCCs that mediate the two bilaterally segregated glutamatergic inputs to individual NL neurons are identical. These features may play a role in optimizing coincidence detection in NL neurons.     

Synaptic mechanisms of interaction between Deiters' nucleus and the nuclei of some cranial nerves

The effects of stimulation of the vestibular nerve, spinal trigeminal nucleus, facial and hypoglossal nuclei of the cranial nerves on the neuronal activity in the lateral vestibular nucleus of Deiters were studied in cats anaesthetized with pentobarbitone. Stimulation of these nuclei was found to produce antidromic and synaptic activation of Deiters' neurons. Descending axon collaterals of the vestibular neurons to these brainstem structures were revealed. Stimulation of the VIIIth nerve, spinal trigeminal and facial nuclei evoked mono- and polysynaptic excitatory postsynaptic potentials in Deiters' neurons. Stimulation of the spinal trigeminal nucleus evoked mono- and polysynaptic inhibitory postsynaptic potentials and disfacilitation in Deiters' neurons. In some vestibular neurons inhibitory postsynaptic potentials were also evoked by stimulation of the nucleus hypoglossus. Convergence of influences from these structures on Deiters' neurons was shown to exist. The peculiarities and functional significance of the effects mentioned are discussed.     

Electrophysiological and morphological characterization of cytochemically-defined neurons in the caudal nucleus of tractus solitarius of the rat.

Morphological and electrophysiological properties of calbindin D-28k-, GABA- and dopamine-beta-hydroxylase-immunopositive neurons were investigated in the caudal nucleus of tractus solitarius of rats, using a patch-clamp whole-cell recording combined with intracellular staining and immunocytochemistry. Calbindin D-28K- and GABA-positive neurons had a small cell body (10.9+/-0.3 microm in diameter) and were distributed throughout the caudal nucleus of tractus solitarius. Double fluorescence immunocytochemistry revealed that calbindin- and GABA-positive neurons formed distinct subpopulations. Calbindin- and GABA-positive neurons double stained for biocytin showed extensive axon collaterals within the nucleus of tractus solitarius and some calbindin-positive, but not GABA-positive neurons, had also projection axons leaving the nucleus of tractus solitarius. Dopamine-beta-hydroxylase-immunopositive neurons had a small (10.8+/-0.3 microm) or large (17.2+/-0.4 microm) cell body. Neurons with a small cell body were observed in the dorsomedial nucleus at the level of the area postrema, and in the area postrema, while neurons with a large cell body were observed in the medial nucleus throughout the caudal nucleus of tractus solitarius. Double fluorescence immunocytochemistry revealed that almost all small dopamine-beta-hydroxylase-positive neurons were also immunoreactive for calbindin, while large dopamine-beta-hydroxylase-positive neurons were not. Double staining for dopamine-beta-hydroxylase and biocytin showed that neurons with a small cell body had moderate axon collaterals. On the contrary, neurons with a large cell body had few, if any, axon collaterals and a projection axon which could leave the nucleus of tractus solitarius. Following stimulation of the tractus solitarius, all neurons with a small cell body exhibited a polysynaptic excitatory response (type I neurons), while dopamine-beta-hydroxylase-immunopositive neurons with a large cell body exhibited a monosynaptic excitatory response (type II neurons) or an excitatory followed by an inhibitory response (type III neurons). Spontaneous and evoked excitatory postsynaptic currents of (type I neurons) calbindin- or GABA-positive neurons were reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. Spontaneous and evoked inhibitory postsynaptic currents of type III neurons were reversibly blocked by bicuculline. Type II neurons showed no spontaneous excitatory nor inhibitory postsynaptic currents. It was concluded that the three kinds of chemically-defined neurons formed distinct neuronal subpopulations in the caudal nucleus of tractus solitarius in terms of synaptic responses and morphological characteristics such as cell size and axonal trajectory.     

Dorsal column nuclei neurons recorded in a brain stem-spinal cord preparation: characteristics and their responses to dorsal root stimulation

Recordings were obtained from dorsal column nucleus (DCN) neurons in a neonatal rat brain stem-spinal cord preparation to study their basic electrophysiological properties and responses to stimulation of a dorsal root. Whole-cell patch-clamp recordings were made from 21 neurons that responded to dorsal root stimulation with a fast excitatory postsynaptic potential (EPSP). These neurons were located lateral to, but at the level of, the area postrema at depths of 100-268 microm below the dorsal surface of the brain. The neurons could be divided into groups according to the shape of their action potentials or voltage responses to hyperpolarizing current steps; however, the response profiles of the groups of neurons to dorsal root stimulation were not significantly different and all neurons were considered together. Dorsal root stimulation elicited excitatory postsynaptic potentials (EPSPs) in all neurons with a very low variability in onset latency and an ability to follow 100-Hz stimulation, indicating that they were mediated by activation of a monosynaptic pathway. The peak amplitude of the EPSP increased with membrane hyperpolarization, and applications of the non-NMDA receptor antagonists 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2, 3-dione (NBQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) decreased the amplitude of the EPSP to 14.2% of the control response (n = 6). The descending phase of the EPSP decreased with membrane hyperpolarization and was reduced by the N-methyl-D-aspartate (NMDA) receptor antagonist AP-5 (n = 2). The EPSPs were also reduced in amplitude by applications of the gamma-aminobutyric acid-B (GABA(B)) receptor agonist baclofen, which had no effect on membrane potential or input resistance. These results show that fast EPSPs in DCN neurons elicited by dorsal root stimulation are mediated by an excitatory amino acid acting at both non-NMDA and, to a lesser extent, NMDA receptors. In addition, GABA acting at presynaptic GABA(B) receptors can inhibit these responses.     

Excitatory synaptic potentials in neurons of the deep nuclei in olivo-cerebellar slice cultures.

Excitatory postsynaptic potentials evoked in neurons of the deep cerebellar nuclei, either by electrical stimulation within the nuclei in cerebellar slice cultures or by electrical stimulation of olivary explants in olivo-cerebellar co-cultures, were investigated in the rat by means of intracellular recordings. In neurons of the deep cerebellar nuclei, stimulation of the nuclear tissue, as well as stimulation of the olivary tissue, induced a fast rising excitatory postsynaptic potential, followed by an inhibitory postsynaptic potential and a long-lasting excitation. The fast rising excitatory postsynaptic potential and the following inhibitory postsynaptic potential were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. The remaining depolarization was abolished by D-(-)-2-amino-5-phosphonovalerate, suggesting that this potential was mediated by N-methyl-D-aspartate receptors. With only D-(-)-2-amino-5-phosphonovalerate added to the bath, the slow excitation was depressed, whereas the fast excitatory and inhibitory postsynaptic potentials were not affected. In the presence of bicuculline, the 6-cyano-7-nitroquinoxaline-2,3-dione- and the D-(-)-2-amino-5-phosphonovalerate-sensitive excitatory postsynaptic potentials elicited by stimulation of the olivary tissue had the same latency, and were both graded with stimulation strength. The time-to-peak and the duration of the D-(-)-2-amino-5-phosphonovalerate-sensitive excitatory postsynaptic potentials were considerably longer than those of the 6-cyano-7-nitroquinoxaline-2,3-dione-sensitive excitatory postsynaptic potentials.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of prenatal auditory enrichment on developmental expression of synaptophysin and syntaxin 1 in chick brainstem auditory nuclei

Neural activity plays an important role in shaping the developing brain. We have determined the consequence of increased auditory stimulation on the developmental profile of synaptic proteins, synaptophysin and syntaxin 1, in the chick brainstem auditory nuclei, nucleus magnocellularis and nucleus laminaris, by immunohistochemistry and western blotting techniques. The chick embryos were provided with patterned sounds of species-specific calls or musical notes of a sitar, a stringed instrument, in a graded manner from embryonic day 10 (E10) through hatching, for 15 min every hour. During normal synaptogenesis of nucleus magnocellularis and nucleus laminaris, synaptophysin immunoreactivity increased significantly from E8 to E20, in parallel with synapse formation, and reduced at hatching. The embryos receiving species-specific sound stimuli exhibited a similar pattern with higher levels of immunoreactivity, though the difference between the study groups was not statistically significant. The music stimulated embryos showed an earlier peak at E16, followed by a gradual decline until hatching. In all three groups studied, syntaxin immunoreactivity showed a surge at E12, followed by a decline at E16 and subsequent stabilization. The stimulated groups continually expressed higher amounts of syntaxin immunoreactivity. The results suggest that prenatal sound stimulation enhances the normal pattern of synaptic protein expression in these auditory nuclei.     

Ascending projections of posterior canal-activated excitatory and inhibitory secondary vestibular neurons to the mesodiencephalon in cats

The axonal projections of 62 posterior canal (PC)-activated excitatory and inhibitory secondary vestibular neurons were studied electrophysiologically in cats. PC-related neurons were identified by monosynaptic activation elicited by electrical stimulation of the vestibular nerve and activation following nose-up rotation of the animal's head. Single excitatory and inhibitory neurons were identified by antidromic activation following electrical stimulation of the contralateral and ipsilateral medial longitudinal fasciculus, respectively. The oculomotor projections of identified neurons were confirmed with a spike-triggered averaging technique. The axonal projections of the identified neurons were then studied by systematic, antidromic stimulation of the mesodiencephalon. Excitatory neurons showed two main types of axonal projections. In one type, axonal branches were issued to the interstitial nucleus of Cajal, central gray, and thalamus including the ventral posterolateral, ventral posteromedial, ventral lateral, ventral medial, centromedian, central lateral, lateral posterior, and ventral lateral geniculate nuclei. The other type was more frequently observed, giving off axon collaterals to the above-mentioned regions and to Forel's field H as well. Inhibitory neurons issued axonal branches to limited areas which included the central gray, interstitial nucleus of Cajal, its adjacent reticular formation and caudalmost part of Forel's field H, but not the rostral part of the Forel's field H and the thalamus. These results suggest that PC-related excitatory neurons participate in the genesis of vertical eye movements and in the perception of the vestibular sensation, and that PC-related inhibitory neurons seem to take part only in the genesis of vertical eye movements.Deceased     

Excitatory connections between neurons of the central cervical nucleus and vestibular neurons in the cat

 The central cervical nucleus (CCN) of the cat receives input from upper cervical muscle afferents, particularly primary spindle afferents. Its axons cross in the spinal cord, and while in the contralateral restiform body give off collaterals to the vestibular nuclei. In order to study the connections between CCN axons and vestibular neurons, we stimulated the area of the CCN in decerebrate cats while recording intra- or extracellularly from neurons in the contralateral vestibular nuclei. CCN stimulation evoked excitatory postsynaptic potentials (EPSPs) or extracellularly recorded firing in the lateral, medial and descending vestibular nuclei. The latency of EPSPs (mean 1.6 ms) was on average 0.4 ms longer than the latency of antidromic spikes evoked in the CCN by stimulation of the contralateral vestibular nuclei (mean 1.2 ms), demonstrating that the excitation was typically monosynaptic. The results provide further evidence that the CCN is an important excitatory relay between upper cervical muscle afferents and neurons in the contralateral vestibular nuclei. Received: 1 August 1996 / Accepted: 16 December 1996     

Electrophysiological properties of cortical synaptic inputs of rubro-spinal neurons

Excitatory postsynaptic potentials (EPSPs) of the red nucleus neurons evoked by stimulation of the cerebellar nucleus interpositus as well as the sensorimotor and association parietal regions of the cerebral cortex were studies in acute cats. As for the first two structures, a monosynaptic connection of the association cortex with rubro-spinal neurons was shown to exist. The analyses of the time characteristics of the unitary EPSPs suggested a localization of synapses of fibers from the association cortex closer to the soma when compared with those which originated from axons of the sensorimotor cortical cells.     

An electrophysiological study of a transient ipsilateral interpositorubral projection in neonatal cats

Summary We examined whether transient projections in the developing central nervous system of Mammalia form functional synapses on their target neurons, using transient ipsilateral interpositorubral (iIR) projection in kittens as a model system. Intracellular recordings were made from red nucleus (RN) neurons in 26 kittens aged 6–26 postnatal days (PD6-26). RN neurons were identified by monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by stimulation of contralateral nucleus interpositus (IN), and additionally by intracellular staining in a few cells. Sixty-nine out of 362 RN neurons responded to stimulation of the ipsilateral IN. Of the 69 cells, 25 showed depolarizing responses with relatively short latency (2.1–6.7 ms) in kittens up to PD20. Such responses were not observed in older animals. Varying stimulus strength revealed that the potentials were unitary. Paired-pulse facilitation of the potential was observed, suggesting that the depolarizations are EPSPs. Several lines of evidence were obtained suggesting that the EPSPs are evoked monosynaptically. They followed high-frequency stimulation up to 50 Hz, and their latencies remained constant with varying stimulus strength. The latencies of ipsilaterally induced EPSPs were always longer than those of contralateral ones, evidence consistent with the longer course of ipsilaterally projecting axons than that of contralateral ones (Song and Murakami 1990). The age of disappearance of the monosynaptic EPSPs, i.e., PD20, also corresponds roughly with that of the anatomically demonstrable iIR fibers (PD15–PD25; Song and Murakami 1990). It is thus concluded that the transient iIR fibers in kittens form functional synapses on RN neurons.     

Local synaptic circuits and epileptiform activity in slices of neocortex from children with intractable epilepsy.

1. Single and dual intracellular recordings were performed in neocortical slices obtained from tissue samples surgically removed from children (8 mo to 15 yr) for the treatment of intractable epilepsy. Electrical stimulation and glutamate microapplication were used to study local synaptic inputs to pyramidal cells. 2. In recordings with potassium-acetate electrodes, activation of presynaptic neocortical neurons with glutamate microdrops did not elicit a clear increase in postsynaptic potentials (PSPs) but did suppress current-evoked repetitive spike firing in recorded neurons. Bicuculline (10 microM) blocked this effect, suggesting it was caused by the activation of presynaptic gamma-aminobutyric acid (GABA) cells. In recordings with KCl electrodes, glutamate microdrops elicited an increase in the frequency and amplitude of depolarizing PSPs. Bicuculline (5-10 microM) blocked the glutamate-evoked PSPs, suggesting they were reversed GABAA-receptor-mediated inhibitory postsynaptic potentials (IPSPs). In one cell recorded with a KCl electrode (total n = 8), current-evoked spike trains elicited afterdischarges of reversed IPSPs, thus revealing a recurrent inhibitory circuit. Therefore local inhibitory synaptic circuits were robust and could be observed in tissue from patients as young as 11 mo. 3. In addition to short-latency (10-25 ms), monosynaptic excitatory postsynaptic potentials (EPSPs), electrical stimulation at low intensities sometimes elicited delayed EPSPs (20-60 ms). When GABAA-receptor-mediated synaptic inhibition was partially reduced in bicuculline (5-10 microM), electrical stimulation evoked large EPSPs at long and variable latencies (100-300 ms). Glutamate microapplication caused an increase in the frequency and amplitude of EPSPs; preliminary results suggest that glutamate microdrops were less likely to evoke EPSPs in tissue from younger patients (8-12 mo) than in slices from patients greater than 4 yr. Evidence for local excitatory synaptic circuits was thus found when synaptic inhibition was partially reduced. 4. After further reduction of inhibition in bicuculline (5-50 microM), electrical stimulation elicited epileptiform bursts. In pairs of simultaneously recorded neurons, bursts were generated synchronously from long-latency EPSPs (100-300 ms) in slices from patients as young as 8 mo. Reflected EPSPs at very long and variable latencies (500-1,100 ms) and repetitive epileptiform bursts could be evoked synchronously in pairs of cells. Glutamate activation of local presynaptic neurons elicited robust epileptiform events in recorded cells. This was seen in slices from patients as young as 16 mo. 5. These data provide physiological evidence for the presence of local inhibitory and excitatory synaptic circuits in human neocortex at least as early as 11 and 8 mo, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)     

Neuronal architecture in nucleus magnocellularis of the chicken auditory system with observations on nucleus laminaris: A light and electron microscope study

This report presents the major structural features of neurons and their afferent input in nucleus magnocellularis, the avian homologue of the mammalian anteroventral cochlear nucleus. Results of light-microscope observations, as seen in Golgi, Nissl, and normal fiber preparations, as well as ultrastructural morphology are reported. In addition, cells and axons in nucleus laminaris, the presumed homologue of the mammalian medial superior olivary nucleus, are also described.In Golgi-impregnated material, the mature principal cell in nucleus magnocellularis has an ovoid soma encrusted with somatic spines. A dendrite, when present, emerges from the cell soma, travels for a short distance and breaks into a tuft of stubby terminal branches. Foremost among the afferents to nucleus magnocellularis are auditory nerve axons that terminate in large, axosomatic endings, or endbulbs, covering a large portion of the somatic surface. Other afferents, which also end in relation to the perikaryon, are of undetermined and perhaps multiple origins. The neurons resemble the bushy cells of the mammalian anteroventral cochlear nucleus. Evidence is presented that individual axons from the nucleus magnocellularis bifurcate and send branches to the nucleus laminaris bilaterally, thus placing constraints on the binaural interactions possibly involved in lateralization functions.In electron micrographs, the end-bulbs appear as large, elongate structures which can cover a third of the cell soma. Multiple sites of synaptic specialization occur along these terminals. The synaptic membrane complexes may form directly on the cell body or on the sides or crests of somatic spines. These complexes are characterized by asymmetric membrane densities with a cluster of clear, spherical vesicles on the axonal side. Other small terminal profiles are also present on the somata receiving the end-bulbs. Dendritic profiles are scarce, in agreement with observations in Golgi impregnations.The structural findings indicate that the medial part of the nucleus magnocellularis is homologous to the anterior part of the mammalian anteroventral cochlear nucleus in that the neurons of nucleus magnocellularis are homologous to the bushy cells of the cat. On this basis, the cells in nucleus magnocellularis could faithfully preserve the acoustic response patterns generated in the auditory nerve. This should, in turn, allow a secure relay of bilateral latency differences essential for binaural interactions in the nucleus laminaris.     

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.     

Fast excitatory postsynaptic potentials and the responses to excitant amino acids of sympathetic preganglionic neurons in the slice of the cat spinal cord.

The properties of the excitatory postsynaptic potential evoked by focal stimulation and of the responses to excitatory amino acids were examined by intracellular recording from sympathetic preganglionic neurons in upper thoracic spinal cord slices of the adult cat. Single stimuli to the region dorsal to the intermedio-lateral nucleus evoked short-latency, presumably monosynaptic, excitatory postsynaptic potentials. The reversal potential of this response was -2.2 mV and became more negative when external Na+ or K+ concentration was decreased. The excitatory postsynaptic potential was depressed by the non-selective excitatory amino acid receptor antagonist cis-2,3-piperidine dicarboxylic acid and enhanced by a glutamate uptake inhibitor. The non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2.3-dione abolished the excitatory postsynaptic potential in 72% of neurons. In the remaining neurons, this antagonist only depressed the potential and unmasked a slower component which was abolished by the N-methyl-D-aspartate receptor antagonist D,L-2-amino-5-phosphonovaleric acid. In the presence of tetrodotoxin all neurons tested were depolarized by glutamate or aspartate, as well as by the selective agonists quisqualate, alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate. The glutamate-evoked depolarization reversed at a membrane potential of -2.0 mV and at a more negative value when external Na+ or K+ concentration was decreased. The response to alpha-amino-3-hydroxy-5-methylisoxazole propionic acid was abolished by 6-cyano-7-nitroquinoxaline-2,3-dione in all neurons tested and that to kainate in only one-third of the cells. In the remainder the response to kainate was only slightly depressed by this antagonist. The responses to glutamate and aspartate were only slightly depressed by the combined action of the various amino acid receptor antagonists used. The responses to N-methyl-D-aspartate were abolished by D,L-2-amino-5-phosphonovaleric acid. The punched-out region of the intermedio-lateral nucleus, maintained in vitro, released glutamate and aspartate in the absence of stimulation. Field stimulation (20 Hz) enhanced release by between 40 and 100%. The increase was prevented by superfusion with calcium-free Krebs. It is concluded that excitatory amino acids, acting on both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors, but mainly on the latter, are likely mediators of the monosynaptic excitatory postsynaptic potential evoked in sympathetic preganglionic neurons by the stimulated region. The efflux data suggest that glutamate and aspartate are among the mediators.