Synaptic organization of expansion motoneurons ofNavanax inermis

The opisthobranch mollusc, Navanax, feeds by rapid pharyngeal expansion that sucks in prey followed by peristaltic swallowing that moves prey into the esophagus. Several identifiable neurons on the ventral surface of the buccal ganglia control radial musculature within the pharyngeal wall, contraction of which leads to pharyngeal expansion. These are considered expansion motoneurons because their axons run into the muscle and twitches and EMGs occur one for one with action potentials. The motoneurons are electrotonically coupled. Electrotonic PSPs, the components of spread associated with impulses, can summate with subthreshold DC depolarizations to yield synchronous impulses in coupled cells. During a train of responses the later electrotonic PSPs can be facilitated because of increase in amplitude and duration of the presynaptic impulses. Expansion motoneurons are synaptically connected by two apparently interneuronal pathways: a low threshold pathway activated by subthreshold depolarization of the two largest expansion motoneurons (the G-cells) that inhibits the entire population, and a high threshold pathway that is activated by a train of G-cell impulses and produces largely excitatory PSPs in the smaller expansion motoneurons and an EPSP--IPSP sequence in the G-cells. Coupling among expansion motoneurons can be abolished by chemical inhibitory synaptic inputs that are activated by electrical stimulation of the pharyngeal nerve or tactile stimulation of the pharyngeal wall. This uncoupling phenomenon can be explained by a simple equivalent circuit in which inhibitory synapses along the coupling pathway short circuit electrotonic spread. Uncoupling can outlast the evoking stimulus by several seconds. During uncoupling the smaller expansion motoneurones can fire independently while the G-cell is inhibited, and impulses still propagate from somata to the periphery. The expansion motoneuron population receives excitatory input from the mechanoreceptors in protractor muscles. Mechanical stimulation of the pharyngeal wall activates primary sensory neurons in the buccal ganglia that fire during excitation and during inhibition and uncoupling of expansion motoneurons.     

An Organotypic Spinal Cord - Dorsal Root Ganglion - Skeletal Muscle Coculture of Embryonic Rat. II. Functional Evidence for the Formation of Spinal Reflex Arcs In Vitro

Electrical properties of motoneurons, muscle fibres and dorsal root ganglion (DRG) cells were studied in an organotypic coculture of embryonic rat spinal cord, dorsal root ganglia and skeletal muscle. The motoneurons were identified by their morphology and position in culture. Their size and input conductance were significantly larger than those of spinal interneurons. Intracellular current injection evoked action potentials in all motoneurons, but only evoked stable repetitive firing patterns in some. Excitability was correlated to somatic size and the rate of spontaneous excitatory input. It is suggested that the somatic growth and the increase in excitability is regulated by the excitatory afferents. The motoneurons showed spontaneous excitatory and inhibitory postsynaptic potentials and action potentials which disappeared with the application of various agents known to inhibit excitability or excitatory synaptic transmission. Excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs respectively) were distinguished by their shape, reversal potential and pharmacology. IPSPs could be depolarizing or hyperpolarizing in different cells. A higher percentage of cells with hyperpolarizing IPSPs was found in older cultures and in the presence of skeletal muscle, suggesting a reversal of the polarity of IPSPs with development. The spontaneous muscle contractions observed in the cultures could be due either to innervation, spontaneous oscillations of the membrane potential, or electrical coupling between neighbouring fibres. A small percentage of DRG cells showed spontaneous action potentials, all of which were found in cultures with spontaneous muscle contractions. The electrical stimulation of DRG afferents evoked mono- and polysynaptic EPSPs in motoneurons, endplate potentials and muscle contractions. The stimulation of the ventral horns evoked endplate potentials and muscle contractions via mono- or polysynaptic pathways. Together these results indicate that appropriate and functional contacts were established in the culture between myotubes and DRG cells, between DRG cells and motoneurons, and between motoneurons and muscle fibres.     

A survey of spinal collateral actions of feline ventral spinocerebellar tract neurons

The aim of this study was to identify spinal target cells of spinocerebellar neurons, in particular the ventral spinocerebellar tract (VSCT) neurons, giving off axon collaterals terminating within the lumbosacral enlargement. Axons of spinocerebellar neurons were stimulated within the cerebellum while searching for most direct synaptic actions on intracellularly recorded hindlimb motoneurons and interneurons. In motoneurons the dominating effects were inhibitory [inhibitory postsynaptic potentials (IPSPs) in 67% and excitatory postsynaptic potentials (EPSPs) in 17% of motoneurons]. Latencies of most IPSPs indicated that they were evoked disynaptically and mutual facilitation between these IPSPs and disynaptic IPSPs evoked by group Ia afferents from antagonist muscles and group Ib and II afferents from synergists indicated that they were relayed by premotor interneurons in reflex pathways from muscle afferents. Monosynaptic EPSPs from the cerebellum were accordingly found in Ia inhibitory interneurons and intermediate zone interneurons with input from group I and II afferents but only oligosynaptic EPSPs in motoneurons. Monosynaptic EPSPs following cerebellar stimulation were also found in some VSCT neurons, indicating coupling between various spinocerebellar neurons. The results are in keeping with the previously demonstrated projections of VSCT neurons to the contralateral ventral horn, showing that VSCT neurons might contribute to motor control at a spinal level. They might thus play a role in modulating spinal activity in advance of any control exerted via the cerebellar loop.     

Quantitative ultrastructure of physiologically identified premotoneuron terminals in the trigeminal motor nucleus in the cat

Little is known about the ultrastructure of synaptic boutons contacting trigeminal motoneurons. To address this issue, physiologically identified premotor neurons (n = 5) in the rostrodorsomedial part of the oral nucleus (Vo.r) were labeled by intracellular injections of horseradish peroxidase (HRP) in cats. The ultrastructure of 182 serially sectioned axon terminals from the five neurons was both qualitatively and quantitatively analyzed. In addition, the effects of the glycine antagonist strychnine, GABA(A) antagonist bicuculline, NMDA antagonist 2-amino-5-phosphonovalerate (APV), and non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on Vo.r-induced postsynaptic potentials in trigeminal motoneurons (n = 11) were examined to evaluate potential signaling substances of the premotor neurons. Labeled boutons made synaptic contacts with either jaw-closing or -opening motoneurons. All the boutons contained pleomorphic vesicles, and most formed a single symmetric synapse either on the somata or on primary dendrites. Morphometric analyses indicated that bouton volume, bouton surface area, apposed surface area, total active zone area, and mitochondrial volume were not different between boutons on jaw-closing and -opening motoneurons. Vesicle number and density, however, were higher for boutons on jaw-closing motoneurons. The five morphological parameters were positively correlated with bouton volume. Vesicle density was the exception, which tending to be negatively correlated. Intravenous infusion of strychnine or bicuculline suppressed Vo.r-induced inhibitory postsynaptic potentials (IPSPs) in jaw-closing motoneurons. Abolition of Vo. r-induced excitatory postsynaptic potentials in jaw-opening motoneurons with APV and CNQX unmasked IPSPs. The present results suggest that premotor neurons in the Vo.r are inhibitory and that positive correlations between the ultrastructural parameters associated with synaptic release and bouton size are applicable to the interneurons, as they are in primary afferents.     

Synaptology of the direct projections from the nucleus of the solitary tract to pharyngeal motoneurons in the nucleus ambiguus of the rat

During the pharyngeal phase of the swallowing reflex, the nucleus of the solitary tract (NTS) receives peripheral inputs from the pharynx by means of the glossopharyngeal ganglion and is the location of premotor neurons for the pharyngeal (PH) motoneurons. The semicompact formation of the nucleus ambiguus (AmS) is composed of small and medium-sized neurons that do not project to the pharynx, and large PH motoneurons. We investigated whether the neurons in the NTS projected directly to the PH motoneurons or to the other kinds of neurons in the AmS by using the electron microscope. When wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was injected into the NTS after cholera toxin subunit B-conjugated HRP (CT-HRP) injections into the pharyngeal muscles of male Sprague-Dawley rats, many nerve terminals anterogradely labeled with WGA-HRP were found to contact PH motoneurons retrogradely labeled with CT-HRP. Most of the labeled axodendritic terminals (63%) contained pleomorphic vesicles with symmetric synaptic contacts (Gray's type II), and the remaining ones contained round vesicles with asymmetric synaptic contacts (Gray's type I). About 14% of the axosomatic terminals on PH motoneuron in a sectional plane were anterogradely labeled, and about 70% of the labeled axosomatic terminals were Gray's type II. Observations of serial ultrathin sections revealed that both the small and the medium-sized neurons received only a few labeled axosomatic terminals that were exclusively Gray's type I. These results indicate that the NTS neurons may send mainly inhibitory as well as a few excitatory inputs directly to the PH motoneurons in the AmS. J. Comp. Neurol. 393:391–401, 1998. © 1998 Wiley-Liss, Inc.     

The role of premotor interneurons in phase-dependent modulation of a cutaneous reflex during swimming in Xenopus laevis embryos.

Phase-dependent reflex modulation during fictive "swimming" in Xenopus laevis embryos has been examined with intracellular recordings from rhythmically active spinal neurons. (1) At rest, cutaneous trunk or tail skin stimulation evokes EPSPs in motoneurons and premotor excitatory and inhibitory interneurons of the opposite motor system. During swimming, these EPSPs can only be evoked during the depolarized phase of activity and can then produce extra action potentials that lead to phase-dependent reflexes in ventral roots. On the stimulated side, IPSPs are evoked in rhythmic neurons that can block centrally generated action potentials if the stimulus coincides with the inhibited phase of the swimming cycle. This inhibition suppresses ventral root discharge in a phase-dependent manner. (2) The presence of premotor interneurons in the crossed reflex pathway suggests two parallel routes for cutaneous excitation to reach the motoneurons, one direct and the other indirect through excitatory premotor interneurons. During swimming, the crossed excitation through both routes is gated by the rhythm-generating circuit to allow summation in motoneurons only during the depolarized phase of the swim cycle. (3) Following phase-dependent reflexes, the frequency of swimming is raised for several cycles, a phenomenon that requires sensory activation of premotor rhythm-generating interneurons. The results provide evidence on the role of identified premotor spinal interneurons in phase-dependent reflex modulation.     

Chronic selective activation of excitatory opioid receptor functions in sensory neurons results in opioid 'dependence' without tolerance.

We previously showed that mouse sensory dorsal root ganglion (DRG) neurons chronically exposed to 1 microM D-ala2-D-leu5-enkephalin (DADLE) or morphine for > 2-3 days in culture become tolerant to the usual opioid inhibitory receptor-mediated effects, i.e. shortening of the duration of the calcium-dependent component of the action potential (APD), and supersensitive to opioid excitatory APD-prolonging effects elicited by low opioid concentrations. Whereas nanomolar concentrations of dynorphin(1-13) or morphine are generally required to prolong the APD of naive DRG neurons (by activating excitatory opioid receptors), femtomolar levels become effective after chronic opioid treatment. Whereas 1-30 nM naloxone or diprenorphine prevent both excitatory and inhibitory opioid effects but do not alter the APD of native DRG neurons, both opioid antagonists unexpectedly prolong the APD of most of the chronic opioid-treated cells. In the present study, chronic exposure of DRG neurons to 1 microM DADLE together with cholera toxin-B subunit (which selectively blocks GM1 ganglioside-regulated opioid excitatory, but not inhibitory, receptor functions) prevented the development of opioid excitatory supersensitivity and markedly attenuated tolerance to opioid inhibitory effects. Conversely, sustained exposure of DRG neurons to 1 nM DADLE, which selectively activates excitatory opioid receptor functions, resulted in characteristic opioid excitatory supersensitivity but no tolerance. These results suggest that 'dependence'-like properties can be induced in chronic opioid-treated sensory neurons in the absence of tolerance. On the other hand, development of some components of tolerance in these cells may require sustained activation of both excitatory, as well as inhibitory, opioid receptor functions.     

Physiological and anatomical evidence for an inhibitory trigemino-oculomotor pathway in the cat

During blink down‐phase, the levator palpebrae superioris (levator) muscle is inactivated, allowing the orbicularis oculi muscle to act. For trigeminal reflex blinks, the excitatory connections from trigeminal sensory nuclei to the facial nucleus have been described, but the pathway whereby the levator is turned off have not. We examined this question by use of both physiological and anatomical approaches in the cat. Intracellular records from antidromically activated levator motoneurons revealed that periorbital electrical stimulation produced bilateral, long latency inhibitory postsynaptic potentials (IPSPs). Central electrical stimulation of the principal trigeminal nucleus produced shorter latency IPSPs. Intracellular staining revealed that these motoneurons reside in the caudal central subdivision and have 10 or more poorly branched dendrites, which extend bilaterally into the surrounding supraoculomotor area. Axons penetrated in this region could be activated from periorbital and central electrodes. Neurons labeled from tracer injections into the caudal oculomotor complex were distributed in a crescent‐shaped band that lined the ventral and rostral aspects of the pontine trigeminal sensory nucleus. Double‐label immunohistochemical procedures demonstrated that these cells were not tyrosine hydroxylase‐positive cells in the Kölliker‐Fuse area. Instead, supraorbital nerve afferents displayed a similar crescent‐shaped distribution, suggesting they drive these trigemino‐oculomotor neurons. Anterograde labeling of the trigemino‐oculomotor projection indicates that it terminates bilaterally, in and above the caudal central subdivision. These results characterize a trigemino‐oculomotor pathway that inhibits levator palpebrae motoneurons in response to blink‐producing periorbital stimuli. The bilateral distributions of trigemino‐oculomotor afferents, levator motoneurons, and their dendrites supply a morphological basis for conjugate lid movements. J. Comp. Neurol. 520:2218–2240, 2012. © 2012 Wiley Periodicals, Inc.     

Identification of motoneurons and interneurons in the spinal network for escapes initiated by the mauthner cell in goldfish

We used intracellular recording and staining techniques to study the spinal circuitry of the escape behavior (C-start) initiated by the Mauthner axon (M-axon) in goldfish. Simultaneous intracellular recordings from one or both M-axons and a spinal neuron, followed by HRP labeling of the spinal cell, show that each M-axon makes monosynaptic, chemical excitatory synapses onto 2 populations of ipsilateral spinal neurons. The first consists of the large primary motoneurons that, based on earlier work (Fetcho, 1986), innervate exclusively the faster, white muscle fiber types in the myomeres. The second group of cells is formed by previously undescribed descending interneurons with ipsilateral axonal branches that have contacts with primary and secondary motoneurons spread over 2 or more body segments. Indirect evidence suggests that these descending interneurons are excitatory, and they may explain the polysynaptic activation of motoneurons observed in earlier studies of the spinal circuitry (Diamond, 1971). Both classes of neurons excited by the ipsilateral M-axon are disynaptically inhibited by the contralateral one. The morphology and physiology indicate that this inhibition is mediated by interneurons that are electrotonically coupled to one M-axon and have processes that cross the cord to inhibit contralateral neurons in the region where these postsynaptic cells receive excitatory input from the other M-axon. We have identified interneurons with the physiological and morphological features of these predicted crossed inhibitory interneurons. These cells are electrotonically coupled to the ipsilateral M-axon and receive a chloride-dependent disynaptic inhibitory input from the contralateral M-axon. Their very simple somata give rise to a process that crosses the spinal cord between the 2 M-axons. Once on the opposite side of the cord, the crossing process sends myelinated branches that run rostrally and caudally, roughly parallel to the contralateral M-axon. Processes that arise from these longitudinal branches terminate in a striking association with collaterals of the M-axon; nearly every M-axon collateral along the longitudinal course of an interneuron is met by a branch or branches of the interneuron whose terminals are apposed to neurons postsynaptic to the collateral.(ABSTRACT TRUNCATED AT 400 WORDS)     

Early functional organization of spinal neurons in developing lower vertebrates

The spinal neurons in the embryos and young larvae of two amphibians (Xenopus and Triturus) and two fish (Oryzias and Brachydanio) are described and compared. They can be placed into a limited number of common neuron classes: Rohon-Beard sensory, dorsolateral and dorsolateral commissural sensory interneurons, inhibitory ascending interneurons, two classes of inhibitory commissural interneuron, excitatory descending interneurons, motoneurons and possible sensory Kolmer-Agdhur neurons. In Triturus and other urodeles, there are also giant dorsolateral commissural sensory interneurons. The functions of the spinal neurons in simple flexion responses and swimming are considered in relation to evidence mainly from the Xenopus tadpole.     

Control of the central swallowing program by inputs from the peripheral receptors. A review

Swallowing is a complex motor sequence, usually divided into a buccopharyngeal stage (coordinated contractions of several muscles of the mouth, pharynx and larynx) and an esophageal stage, called primary peristalsis. This motor sequence depends on the activity of medullary interneurons belonging to the swallowing center which program through excitatory and inhibitory connections the sequential excitation of motoneurons and vagal preganglionic neurons responsible for the whole motor sequence. The activity of the medullary swallowing neurons can occur without feedback phenomena: it is truly a central activity indicating that swallowing depends on a central network which may function without afferent support. However, the swallowing neurons receive a strong afferent input suggesting the involvement of sensory feedbacks during swallowing. The swallowing neurons present a short latency activation on electrical stimulation of the peripheral afferent fibers supplying the region of the tract which is under their control. In addition, the neurons are activated by localized distensions of the swallowing tract, this distension having to be done more and more distally when the neuronal discharge occurs later and later during swallowing. Furthermore the swallowing discharge of the central neurons is increased either when a bolus is swallowed or during a slight distension of the corresponding region of the tract. Thus, under physiological conditions, swallowing neurons receive sensory information from pharyngeal and esophageal receptors and the central program may be modified by peripheral afferents that adjust the motor sequence to the size of the swallowed bolus. The inputs from the peripheral receptors can also exert inhibitory effects depending on the central connections between the swallowing neurons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Opposing parallel connections through crayfish local nonspiking interneurons

Unilateral local nonspiking interneurons in the terminal (sixth) abdominal ganglion of crayfish (Procambarus clarkii Girard) can be classified into two major groups of PL and AL types by their gross morphology and somatic position. These premotor interneurons are the neural components of uropod motor pattern formation. They receive sensory input from the exopodite of the contralateral side as well as that of the ipsilateral side. Small fluctuations in their membrane potentials cause sustained change in activity of the motoneurons innervating the uropod muscles. PL interneurons, which make noninverting connections to an identified closer, the reductor motoneuron No. 1, mainly receive excitatory input from the afferents of the contralateral exopodite, whereas inverting PL interneurons receive inhibitory input. AL interneurons receive distinctly different input from the afferents. Noninverting AL interneurons mainly receive inhibitory input, whereas inverting AL interneurons receive excitatory input. The rate of discharge of the reductor motoneurons is increased by sensory stimulation. The PL interneurons form either excitatory or disinhibitory pathways, which are relevant in function to the observed increase of the motoneuron. Conversely, the AL interneurons form either inhibitory or disfacilitatory pathways. Thus, the PL and the AL interneurons are fractionated in function and distinguishable in terms of their physiology by their input and output correlations. Functional meaning of the presence of these two types of unilateral local nonspiking interneurons of opposing connections in the uropod motor control system is discussed.     

Electrophysiological studies on the postnatal development of intracerebellar nuclei neurons in rat cerebellar slices maintained in vitro. I. Postsynaptic potentials

The development of the synaptic responses of intracerebellar nuclei neurons was studied in the rat by the use of thick sagittal cerebellar slices maintained in vitro. It has been shown that functional excitatory synapses are present on these neurons from birth, probably due to climbing and/or mossy fiber collaterals; functional inhibitory synapses, due to monosynaptic projections of Purkinje cell axons onto intracerebellar nuclei, are present as early as postnatal day 2; and a more complex pattern of synaptic responses, including short latency excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs), longer latency IPSPs, and late depolarizing responses, can be elicited in nuclear neurons as early as postnatal day 3, indicating an early development of some complete functional cerebellar circuits involving the intracerebellar nuclei.     

An intracellular recording study of stimulus-specific response properties in cat area 17

Stimulus-specific response properties, such as direction or orientation selectivity, were studied intracellularly in cells recorded from area 17 of the cat. In all 5 direction selective complex cells and one orientation selective simple cell successfully studied, visually evoked excitatory postsynaptic potentials (EPSPs) were tuned to the preferred direction or orientation. Visually evoked inhibitory postsynaptic potentials (IPSPs) were also tuned to the preferred direction/orientation of stimulus. IPSPs evoked by the non-preferred stimulus when present were smaller than those evoked by the preferred stimulus. IPSPs were undetected in two of the 5 cells tested. These results suggest that directionally/orientationally tuned EPSPs make a major contribution to stimulus specificity in visual cortical neurons but IPSPs evoked by a stimulus with null-direction/orientation may sharpen the stimulus specificity.     

Effects of glutamate agonists on the isolated neurons from the locomotor network of the mollusc Clione limacina

In the pteropod mollusc, locomotor rhythm is produced by the central pattern generator (CPG), mainly by the reciprocal activity of interneurons, groups 7 and 8, which are active in the phases of the dorsal and ventral flexion of the wings, respectively. Both groups produce excitation in the CPG neurons controlling the same phase of the locomotor cycle, and inhibition in the neurons of the opposite phase. As previously suggested, the connections of the group 7 interneurons to the follower CPG neurons are glutamatergic. However, the properties of the glutamatergic receptors in the postsynaptic cells are unknown. In this work, the identified CPG motoneurons of the antagonistic groups 1 and 2 were isolated from the CNS, and their responses to the local application of glutamate agonists were examined. Glutamate elicited opposite effects in the neurons of these two groups, reproducing excitatory and inhibitory influence of the group 7 interneurons. Interestingly, micromolar concentrations of glutamate agonists kainate, domoate and AMPA elicited excitation in both types of the motoneurons. To test for the possible of the NMDA receptors, NMDA was applied, as well as an equimolar mixture of NMDA and glycine. The isolated motoneurons showed excitatory response to these applications only at high (millimolar) concentrations. These results suggest that the motoneurons of group 1 possess excitatory glutamate receptors and that motoneurons of group 2 may posses a composite population of receptors, which are responsible for the inhibitory action of glutamate and excitatory action of its AMPA/kainate agonists.     

Evidence that glycine mediates the postsynaptic potentials that inhibit lumbar motoneurons during the atonia of active sleep

Postsynaptic inhibition of somatic motoneurons underlies the atonia of active sleep. This inhibitory control depends, in large measure, on the bombardment of motoneurons during active sleep by a unique class of large-amplitude inhibitory postsynaptic potentials (IPSPs). These potentials are present only during this behavioral state and have therefore been designated as active sleep-specific IPSPs (AS-IPSPs). The present study was concerned with determining the neurotransmitter that mediates these AS-IPSPs. Lumbar motoneurons were recorded intracellularly during quiet and active sleep in intact, undrugged, normally respiring cats. The frequency and waveform parameters of the inhibitory postsynaptic potentials recorded from these motoneurons were examined following the microiontophoretic juxta-cellular administration of strychnine (a glycine receptor antagonist) and picrotoxin and bicuculline (GABA receptor antagonists). Microiontophoretically applied strychnine abolished the AS-IPSPs and a majority of smaller-amplitude IPSPs. Neither picrotoxin nor bicuculline modified the frequency, amplitude, or rising phase of the AS-IPSPs or the smaller-amplitude IPSPs. We conclude that the postsynaptic inhibitory drive that impinges on motoneurons during active sleep is principally mediated by glycine or a glycinergic substance.     

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.     

The effect of pyramidal stimulation upon tail muscle motoneurons in the decerebrate cat

This study aimed to determine the effects of the corticospinal tract (CST) on the motoneurons innervating the tail muscles in cats. The stimulation of the pyramidal tract predominantly evoked excitatory postsynaptic potentials (EPSPs; 48/90 motoneurons: 53%). Single-pulse stimulation produced EPSPs in 18 of 48 motoneurons, but double shocks evoked postsynaptic potentials in most of the remaining cells (26/48). Monosynaptic excitatory connections between pyramidal tract fibers and tail motoneurons were confirmed in 4 motoneurons. Inhibitory postsynaptic potentials (IPSPs) were recorded from motoneurons innervating long tendinous tail muscles (7/90: 8%) and the shortest neuronal pathways of IPSPs were shown to be disynaptic pathways. Interactions between the CST and reflex pathways from low-threshold muscle and cutaneous afferents innervating the tail and hindlimbs were observed.     

Naloxone reduces the amplitude of IPSPs evoked in lumbar motoneurons by reticular stimulation during carbachol-induced motor inhibition

During active sleep or carbachol-induced motor inhibition, electrical stimulation of the medullary nucleus reticularis gigantocellularis (NRGc) evoked large amplitude, glycinergic inhibitory postsynaptic potentials (IPSPs) in cat motoneurons. The present study was directed to determine whether these IPSPs, that are specific to the state of active sleep, are modulated by opioid peptides. Accordingly, intracellular recordings were obtained from lumbar motoneurons of acute decerebrate cats during carbachol-induced motor inhibition while an opiate receptor antagonist, naloxone, was microiontophoretically released next to the recorded cells. Naloxone reversibly reduced by 26% the mean amplitude of NRGc-evoked IPSPs (1.9+/-0.2 mV (S.E.M.) vs. 1.4+/-0.2 mV; n=11, control and naloxone, respectively, p<0.05), but had no effect on the other waveform parameters of these IPSPs (e.g., latency-to-onset, latency-to-peak, duration, etc.). The mean resting membrane potential, input resistance and membrane time constant of motoneurons following naloxone ejection were not statistically different from those of the control. These data indicate that opioid peptides have a modulatory effect on NRGc-evoked IPSPs during carbachol-induced motor inhibition. We therefore suggest that endogenous opioid peptides may act as neuromodulators to regulate inhibitory glycinergic synaptic transmission at motoneurons during active sleep.     

Local disinhibition of neocortical neuronal circuits causes augmentation of glutamatergic and GABAergic synaptic transmission in the rat neostriatum in vitro

Intra- and extracellular recordings were performed to investigate the influence of local disinhibition of neocortical circuits on corticostriatal synaptic transmission. In rat brain slices with preserved corticostriatal connections, electrical stimulation of the neocortex elicited composed postsynaptic responses in neostriatal neurons consisting of glutamatergic excitatory postsynaptic potentials (EPSPs) and weakly expressed GABAA receptor-mediated inhibitory postsynaptic potentials (IPSPs). Following local application of the GABAA receptor antagonist bicuculline to the neocortex, neocortical neurons responded to intracortical stimulation with transient paroxysmal depolarizations. Simultaneously, the amplitude of neocortically evoked EPSPs recorded from neostriatal neurons was found to be enhanced without changes in duration. Similarly, the amplitude of IPSPs increased following disinhibition of neocortical circuits. In addition and in contrast to EPSPs, the duration of the IPSPs was found to be markedly prolonged. The results demonstrate that local disinhibition of neocortical neuronal circuits potentiates both excitatory and inhibitory synaptic transmission in striatal neurons. However, compared to AMPA receptor-mediated excitation, GABAA receptor-mediated inhibition becomes more efficient due to a marked prolongation of IPSPs. The pronounced augmentation of inhibition can be attributed to a strong activation of inhibitory interneurons within the striatum.