Colocalization of neurotransmitters in presynaptic boutons of inhibitory synapses in the lamprey spinal cord

Electron-microscopic immunocytochemical studies were performed to detect GABA and glycine immunoreactivity in presynaptic axon terminals in the central gray matter of the spinal cord of the lampreyLampetra fluviatilis. The immunopositive presynaptic terminals contacting identified dendrites of motoneurons and unidentified postsynaptic profiles included terminals immunopositive for GABA only (44%) and glycine only (26%), as well as terminals containing GABA and glycine (30%). Glycine-immunopositive presynaptic terminals contained flattened synaptic vesicles. Large synaptic vesicles with dense cores were present along with classical synaptic vesicles in 74% of GABA-immunopositive boutons. Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 85, No. 4, pp. 515–522, April, 1999.     

GABA- and glycine-like immunoreactivity in axonal endings presynaptic to the vibrissa afferents in the cat trigeminal interpolar nucleus

The goal of this study was to analyze the synaptic interaction of primary afferents with GABA- and/or glycine-immunopositive presynaptic endings in the cat trigeminal interpolar nucleus (Vi). Fast adapting vibrissa afferents were labeled by intra-axonal injections of horseradish peroxidase. Postembedding immunogold labeling on serially cut ultrathin sections and quantitative ultrastructural analysis of the labeled boutons and their presynaptic endings (p-endings) in the Vi were performed. The majority of p-endings presynaptic to labeled boutons (83%) were immunopositive for both GABA and glycine and 8% were immunopositive for glycine alone. A small fraction of p-endings were immunopositive for GABA alone (4%) or immunonegative for both GABA and glycine (4%). Ultrastructural parameters related to synaptic release, i.e. bouton volume, mitochondrial volume, and active zone area, were significantly larger in the labeled boutons of primary afferents than in the p-endings. The volume of labeled boutons was positively correlated with the number of the postsynaptic dendrites and p-endings. In addition, fairly large-sized labeled boutons and p-endings were frequently observed in the Vi. These results reveal that large majority of vibrissa afferents in the Vi are presynaptically modulated by interneurons immunopositive for both GABA and glycine, and suggest that the Vi plays a distinct role in the processing of orofacial sensory information, different from that of other trigeminal sensory nuclei.     

Physiological and morphological correlates of presynaptic inhibition in primary afferents of the lamprey spinal cord.

Patch-clamp recordings in a whole-cell mode were performed on dorsal sensory cells enzymatically isolated from the spinal cord of two lamprey species, Ichthyomyzon unicuspis and Lampetra fluviatilis. The voltage-activated currents through calcium channels were analysed. GABA and the specific GABA(B) receptor agonist baclofen reduced the peak amplitude of inward Ba2+ current, as a robust alternate charge carrier through voltage-dependent Ca2+ channels. These effects were dose-dependent and reversible. GABA(B) receptor antagonists, 2-hydroxysaclofen and delta-amino-n-valeric acid, blocked the reduction of Ba2+ currents by GABA and baclofen, while bicuculline, a GABA(A) receptor antagonist, had no blocking action. GABA and baclofen did not modify the dorsal sensory cell membrane conductance, indicating that they did not activate ligand-gated channels. However, GABA, but not baclofen, considerably increased membrane conductance and induced Cl- currents in isolated multipolar neurons (presumably interneurons and/or motoneurons). These findings suggest that GABA and baclofen action on lamprey dorsal sensory cells is mediated by GABA(B) receptors. We concluded that GABA-mediated presynaptic inhibition of lamprey dorsal sensory cell fibers results from GABA(B) receptor activation followed by a decrease of inward voltage-activated calcium currents. Appositions of GABA-immunoreactive boutons to horseradish peroxidase-labeled fibers from the dorsal root were observed at the ultrastructural level in the dorsal column using postembedding immunogold cytochemistry. It seems likely that these appositions represent the morphological substrate of dorsal sensory cell fiber presynaptic inhibition. In very rare cases, ultrastructural features were observed which could be interpreted as synaptic specializations between the GABA-immunoreactive boutons and the primary afferent fibers. The extrasynaptic action of GABA as a basis of presynaptic inhibition of this population of primary afferent neurons is discussed.     

Putative inhibitory collicular boutons contact large neurons and their dendrites in the dorsal cochlear nucleus of the rat

Within the circuits of the acoustic nuclei, the inferior colliculus sends descending (collicular) terminals to control with a feedback mechanism, part of the activity of the dorsal cochlear nucleus (DCN). It is not known whether this descending projection is prevalently excitatory or inhibitory. Using the neuronal tracer Wheat Germ Agglutinin conjugated to Horse Radish Peroxidase (WGA-HRP) the connections between the inferior colliculus and the DCN of the rat have been investigated. By far most retrograde labelled large neurons were glycine and GABA negative (pyramidal and giant neurons) and rare medium-size cells were glycine positive. The ultrastructural immunocytochemical analysis for glycine and GABA shows that mainly large, excitatory, neurons innervate the inferior colliculus. Rare medium-size glycine-positive cells with intermediate characteristics between pyramidal and cartwheel cells, seem also to project to the colliculus. Few WGA-HRP labelled boutons contact the large cells or their dendrites, have symmetric pre- and post-synaptic thickenings, contain pleomorphic and/or flat vesicles, and are labelled for GABA or glycine. Since no GABA labelled cells in both the dorsal and ventral cochlear nucleus were retrograde labelled from the colliculus, the source of these intrinsic anterograde labelled boutons must be external to the cochlear nucleus. GABA positive neurons are both present in the inferior colliculus (injected with the tracer) and superior olivary complex (not injected with the tracer). This suggests that the double labelled boutons (WGA-HRP and GABA) are inhibitory GABA-ergic collicular terminals contacting the excitatory neurons of the DCN. Other few boutons or mossy fibers containing round vesicles and immunonegative for both glycine and GABA, were also seen contacting the large neurons and their dendrites in the DCN. As the round vesicles boutons may be derived from other retrograde cells of the cochlear nucleus (pyramidal and stellate cells) and those glycine positive from the glycinergic neurons in paraolivary nuclei, it is more likely that only the WGA-HRP and GABA labelled boutons are true collicular terminals.     

Measurement and analysis of postsynaptic potentials using a novel voltage-deconvolution method

This study investigated cellular and synaptic mechanisms of cholinergic neuromodulation in the in vitro lamprey spinal cord. Most spinal neurons tested responded to local application of acetylcholine (ACh) with depolarization and decreased input resistance. The depolarization persisted in the presence of either tetrodotoxin or muscarinic antagonist scopolamine and was abolished with nicotinic antagonist mecamylamine, indicating a direct depolarization through nicotinic ACh receptors. Local application of muscarinic ACh agonists modulated synaptic strength in the spinal cord by decreasing the amplitude of unitary excitatory and inhibitory postsynaptic potentials. The postsynaptic response to direct application of glutamate was unchanged by muscarinic agonists, suggesting a presynaptic mechanism. Cholinergic feedback from motoneurons was assessed using stimulation of a ventral root in the quiescent spinal cord while recording intracellularly from spinal motoneurons or interneurons. Mainly depolarizing potentials were observed, a portion of which was insensitive to removal of extracellular Ca2+, indicating electrotonic coupling. Hyperpolarizing potentials were also observed and were attenuated by the glycinergic antagonist strychnine, whereas depolarizing responses were potentiated by strychnine. Mecamylamine also reduced hyperpolarizing responses. The pharmacology of these responses suggests a Renshaw-like feedback pathway in lamprey. Immunohistochemistry for choline acetyltransferase, performed in combination with retrograde filling of motoneurons, demonstrated a population of nonmotoneuron cholinergic cells in the lamprey spinal cord. Thus endogenous cholinergic modulation of the lamprey spinal locomotor network is likely produced by both motoneurons and cholinergic interneurons acting via combined postsynaptic and presynaptic actions.     

Changing synaptic connections on cell bodies of growing identified spinal motoneurons of the eel, Anguilla

As the target musculature they innervate grows throughout life, certain segmental motoneurons from the spinal cord of Anguilla, readily identified on the basis of their form and position, also increase in size. In doing so, they present a steadily increasing target to the spinal and supraspinal neurons that innervate them. How the afferent neurons respond was assessed by measuring features of their synaptic boutons contacting the motoneuronal perikarya, as seen with electron microscopy. About 60% of the perimeter of the perikaryal profile of each motoneuron was found to be covered with synaptic bouton profiles, a value that is independent of the size of the motoneuron. Furthermore, the distances between synaptic profiles, their contact sizes (measured as apposition length) and the number and size of the vesicles each profile contains were all found to be relatively constant and also independent of motoneuronal size. In contrast, the number of synaptic profiles contacting a motoneuron correlated well with its perikaryal size. Our findings indicate that the challenge of a growing neuronal target is met by a steady increase in the number of contacting boutons, the form and spacing of which are held relatively constant; this strategy will require continual synaptic realignment at the target. Accepted: 18 September 2000     

An ultrastructural study of 5-hydroxytryptamine-, thyrotropin-releasing hormone- and substance P-immunoreactive axonal boutons in the motor nucleus of spinal cord segments L7-S1 in the adult cat

The distribution and fine structure of 5-hydroxytryptamine-, thyrotropin-releasing hormone- and substance P-immunoreactive synaptic boutons and varicosities were studied in the motor nucleus of the spinal cord segments L7-S1 in the cat, using the peroxidase-antiperoxidase immunohistochemical technique and analysis of ultrathin serial sections. The 5-hydroxytryptamine-, thyrotropin-releasing hormone- and substance P-immunoreactive boutons had a similar ultrastructural appearance as judged from serial section analysis. The boutons could be classified into two types on the basis of their vesicular content, with one type containing a large number of small agranular vesicles together with only a few, if any large granular vesicles, while the other type contained a large number of large granular vesicles in addition to small agranular vesicles. The vesicles were spherical or spherical-to-pleomorphic. Postsynaptic dense bodies (Taxi bodies) were occasionally observed in relation to all three types of immunoreactive boutons, which almost invariably formed synaptic junctions with dendrites. Judged by the calibre of the postsynaptic dendrites, the boutons were preferentially distributed to the proximal dendritic domains of motoneurons. In one case, a substance P-immunoreactive bouton formed an axosomatic synaptic contact. In addition to synaptic boutons, 5-hydroxytryptamine-, thyrotropin-releasing hormone- and substance P-immunoreactive axonal varicosities containing a large number of large granular and small agranular vesicles but lacking any form of conventional synaptic contact were observed. Such varicosities were either directly apposing surrounding neuronal elements or separated from the neurons by thin glial processes. The origin of the immunoreactive boutons was not traced, but it was thought likely that the main source of the boutons was neurons with their cell bodies located in the medullary raphe nuclei.     

The distribution of inhibitory and excitatory synapses on single, reconstructed jaw-opening motoneurons in the cat

In a previous study, we reported that the distribution of inhibitory input, in contrast to excitatory input, decreased somatofugally along dendrites of cat jaw-closing alpha-motoneurons [J Comp Neurol 414 (1999) 454]. The present study examined the distribution of GABA, glycine, and glutamate immunopositive boutons covering horseradish peroxidase-labeled cat jaw-opening motoneurons. The motoneurons were divided into four compartments: the soma, and primary, intermediate, and distal dendrites. Ninety-seven percent of the total number of studied boutons had immunoreactivity for at least one of the three amino acids. The proportion of boutons immunoreactive for GABA and/or glycine was lower than the proportion of boutons immunoreactive for glutamate. Boutons immunoreactive to glycine alone were more numerous than boutons double-labeled for GABA and glycine, which, in turn, occurred more frequently than boutons immunoreactive to GABA alone. The percentage synaptic covering (proportion of membrane covered by synaptic boutons) of the putatively excitatory (glutamate containing) and putatively inhibitory (GABA and/or glycine containing) boutons decreased somatofugally along the dendrites. Such systematic variations were not seen in the packing density (number of boutons per 100 microm(2)); the packing density showed a distinct drop between the soma and primary dendrites but did not differ significantly among the three dendritic compartments. Overall, the packing density was slightly higher for the putatively excitatory boutons than for the inhibitory ones. When taken together with previous analyses of jaw-closing alpha-motoneurons the present data on jaw-opening alpha-motoneurons indicate that the two types of neuron differ in regard to the nature of synaptic integration in the dendritic tree.     

Distinct profiles of refilling of inhibitory neurotransmitters into presynaptic terminals projecting to spinal neurones in immature rats

Corelease of glycine and GABA from the single synaptic terminal (synaptic bouton) is well accepted in immature rat spinal cord and brainstem. However, it raises the question of how glycine and GABA are accumulated in the same synaptic vesicles and coreleased. To address this issue, spontaneous miniature inhibitory postsynaptic currents (mIPSCs) and focally evoked IPSCs (eIPSCs) mediated via a single synapse were recorded from synaptic bouton preparations of the rat immature sacral dorsal commissural nucleus (SDCN) neurones by whole-cell patch recording. Focal stimulation of a single synaptic bouton revealed that three different quantal releases occur from a single synaptic bouton: i.e. pure glycine, pure GABA, and mixed. Prolonged treatment with bafilomycin A1, a vacuolar-type H⁺/ATPase inhibitor, to the SDCN neurone greatly suppressed frequency and amplitude of the mIPSCs. During washing out of bafilomycin A1, complete recovery in the amplitude of glycinergic mIPSCs was observed, while that of GABAergic and mixed mIPSCs was incomplete. These observations indicate that three types of vesicles coexist in single synaptic terminals, and that refilling of glycine into the synaptic vesicle predominantes over GABA after pretreatment with bafilomycin A1 in immature rats. This could be explained by the decrease in the cytosolic concentration of GABA, or by the presence of subtypes of vesicular inhibitory amino acid transporter in the synaptic vesicle membrane.     

Recovery of synapses in axotomized adult cat spinal motoneurons after reinnervation into muscle

 Peripheral axotomy of adult cat spinal motoneurons induces a marked loss of synaptic boutons from the cell bodies and dendritic trees. The aim of the present study was to analyze the recovery of synaptic contacts in axotomized motoneurons following reinnervation into muscle. Adult cat spinal motoneurons were first deprived of their muscular contacts for 12 weeks and, then, allowed to reinnervate their target muscle. Two years later, regenerated motoneurons were labeled with horseradish peroxidase to allow quantitative ultrastructural analyses of the synaptic covering of the cell bodies and dendrites. Presynaptic boutons were classified according to their size and the shape of their synaptic vesicles. Results show that a recovery of synaptic covering occurs in the axotomized neurons after muscle reinnervation, but it affects various bouton types to different degrees. The number of S-type boutons synapsing with the soma was 70% higher after reinnervation than at 12 weeks after axotomy, while the number of F-type boutons had increased by only 13%. Compared with the normal situation, the number of S-type boutons synapsing with the proximal dendrites increased from 82% at 12 weeks after axotomy to 180% in the reinnervated state. In conclusion, in adult cat spinal motoneurons, the reestablishment of muscular contact is followed by a normalization of some of the synaptological changes induced by a prolonged state of axotomy. In certain respects restitution is incomplete, but in others it results in overcompensation. Received: 10 December 1997 / Accepted: 30 July 1998     

Ultrastructure of supraspinal dorsal root projections in the toad. II. The cerebellar granular layer

Summary Following section of the left dorsal roots, degenerating fibres and boutons were observed in the granular layer of the ipsilateral cerebellum. The degenerating terminals were identified as largeen passant varicosities of mossy fibres contacting the dendrites of presumptive granule cells. They contained round synaptic vesicles and neurofilaments and established Gray type I contacts. The terminals initially underwent filamentous degeneration with neurofilamentous hypertrophy, swollen mitochondria and loss of synaptic vesicles. At later survival times (6–30 days) they acquired an electron-dense appearance due to an increase and clumping of the filamentous component.After injection of horseradish peroxidase into the left cerebellum, all ipsilateral spinal ganglia showed a few (2–3%) labelled cells, indicating that a primary afferent contribution to this pathway originated from each segment of the spinal cord.     

Peculiarities of receptor-channel complexes for inhibitory mediators in the membranes of lamprey spinal cord neurones

Effects of inhibitory mediators on the membranes of isolated lamprey spinal cord neurones were investigated by means of whole-cell recording and concentration clamp techniques. Glycine and gamma-aminobutyric acid (GABA) applications evoked desensitizing chloride currents. The concentrations at half-maximum effects were 16 microM for glycine- and 1.5 mM for GABA-activated currents. Ionic responses to applications of both amino acids demonstrated full cross-desensitization. Strychnine and bicuculline suppressed both glycine- and GABA-activated conductances to the same degree. We suggest that in the membranes of lamprey spinal cord neurones glycine and GABA act on the same receptor-channel complexes.     

Origin of phasic synaptic inhibition in myotomal motoneurons during fictive locomotion in the lamprey

The periodic membrane potential fluctuations in motoneurons during fictive locomotion in the lamprey, a primitive vertebrate, involve phasic synaptic excitation and inhibition. This paper investigates the origin of the phasic synaptic input to lamprey myotomal motoneurons in the in vitro spinal cord preparation with regard to the relative contribution of descending propriospinal input from interneurons in the local segment. The synaptic drive to myotomal motoneurons in the most rostral and the most caudal part of the spinal cord preparation are compared before and after selective spinal cord lesions. Current clamp recordings of the same cell before and after lesion showed that neither the excitatory phase nor the inhibitory phase was abolished after interruption of the descending or the ascending ipsilateral input, or after interrupting crossing segmental input by a local longitudinal midline incision. None of these sources thus appears to be alone responsible for the phasic synaptic drive. To quantitatively evaluate these effects, and in particular the contribution from the descending propriospinal fibres to the inhibitory phase, voltage clamp recordings were made in combination with a spinal cord hemisection just rostral to the motoneuron. The input from propriospinal interneurons in approximately 15 rostral segments may be responsible for as much as 70% of the phase of inhibitory current during the locomotor cycle. In accordance with these findings, a similar voltage clamp analysis of rostrally and caudally located motoneurons showed that the average peak-to-peak amplitude of the current fluctuations in rostral cells was approximately 50% of that in caudal cells.     

Involvement of GABA and glycine in recurrent inhibition of spinal motoneurons.

1. Recurrent inhibitory postsynaptic potentials (IPSPs) were recorded intracellularly from chloride-loaded motoneurons in the isolated lumbar spinal cord of neonatal rats (day 5-day 12). This in vitro preparation exhibited an intact and functional recurrent inhibitory pathway that displayed characteristics previously described for this pathway in other species. 2. Although strychnine (1-5 microM) depressed the chloride-dependent recurrent synaptic potentials evoked by ventral root stimulation by 48.2 +/- 2.7% (mean +/- SE, n = 13), confirming that part of the recurrent IPSP is mediated by a glycinergic mechanism, in every case a residual strychnine-resistant synaptic potential was observed. 3. The gamma-aminobutyric acid (GABA) antagonist bicuculline, in low concentrations (2-10 microM), depressed the recurrent synaptic potentials in a dose-dependent manner by 27.0 +/- 4.3% (range 0-49%, n = 19). Application of bicuculline almost eliminated the strychnine-resistant component of the IPSP. However, in some motoneurons, a small synaptic potential remained after combined application of strychnine and bicuculline. 4. The selective antagonists of GABA uptake, (+/-)-nipecotic acid (1 mM) and guvacine (1 mM), increased the amplitude of recurrent synaptic potentials in 12 of 16 motoneurons by 37.2 +/- 7.2% (range 12.6-84.2%). 5. The excitatory amino acid antagonists kynurenic acid (1 mM), 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX (10 microM)] and 6,7-dinitroquinoxaline-2,3-dione (10 microM) potentiated recurrent synaptic potentials in 5 of 7 motoneurons. However, CNQX (10-15 microM) in the presence of strychnine and bicuculline virtually abolished the synaptic potential remaining after application of the inhibitory amino acid antagonists. It is concluded that ventral root stimulation evokes a small excitatory amino acid-mediated synaptic potential in neonatal rat motoneurons. 6. An antidromic synaptic potential due to electrotonic coupling between motoneurons was unaffected by changes in membrane potential, chloride loading, or antagonists of glycine, GABA, excitatory amino acid, and acetylcholine receptors. 7. The results suggest that a major portion of the strychnine-resistant component of the IPSP is mediated by a GABAergic mechanism. It is concluded that both glycinergic and GABAergic mechanisms play a role in recurrent inhibition of motoneurons in the mammalian spinal cord. It is unknown whether these inhibitory amino acids are released by a single pool of Renshaw cells or by neurochemically distinct populations.     

Putative commissural and collicular axo-somatic terminals on neurons of the rat ventral cochlear nucleus

The type of synaptic terminals from the cochlear nucleus and inferior colliculus that terminate in the contralateral ventral cochlear nucleus are not known. These terminals were studied with the electron microscope and immunogold after injection of wheat germ agglutinin conjugated to horseradish peroxidase into the inferior colliculus or into the cochlear nucleus. The tracer anterogradely labelled boutons onto the main neurons of the contralateral ventral cochlear nucleus. Most of these cells (95%) were glycine immuno-negative and represent excitatory neurons. After injection of the tracer into the contralateral inferior colliculus few anterogradely labelled boutons were seen on spherical and multipolar cells of type II in the anteroventral cochlear nucleus. Rare labelled boutons were present on multipolar cells of type I and II, globular neurons and octopus cells in the posteroventral cochlear nucleus. After injection into the contralateral dorsal and ventral cochlear nucleus labelled boutons were seen more frequently than after injection into the inferior colliculus. These terminals contacted most of large neurons, especially multipolar cells of type II and less frequently of type I. Also globular and spherical cells were contacted by commissural terminals. Octopus cells received less frequently putative commissural terminals. Most boutons contained pleomorphic vesicles and stored GABA. A lower number of boutons with pleomorphic and flat vesicles contained glycine and sometimes GABA, both inhibitory neurotransmitters. Few boutons containing round vesicles were immuno-negative for both glycine and GABA, and were considered putative commissural excitatory terminals. The latter often contacted glycinergic neurons of type II so that also these terminals might elicit an inhibition with at least a disynaptic mechanism after contralateral stimulation.     

Transition from GABAergic to glycinergic synaptic transmission in newly formed spinal networks.

The role of glycinergic and GABAergic systems in mediating spontaneous synaptic transmission in newly formed neural networks was examined in motoneurons in the developing rat spinal cord. Properties of action potential-independent miniature inhibitory postsynaptic currents (mIPSCs) mediated by glycine and GABA(A) receptors (GlyR and GABA(A)R) were studied in spinal cord slices of 17- to 18-day-old embryos (E17-18) and 1- to 3-day-old postnatal rats (P1-3). mIPSC frequency and amplitude significantly increased after birth, while their decay time decreased. To determine the contribution of glycinergic and GABAergic synapses to those changes, GlyR- and GABA(A)R-mediated mIPSCs were isolated based on their pharmacological properties. Two populations of pharmacologically distinct mIPSCs were recorded in the presence of glycine or GABA(A) receptors antagonists: bicuculline-resistant, fast-decaying GlyR-mediated mIPSCs, and strychnine-resistant, slow-decaying GABA(A)R-mediated mIPSCs. The frequency of GABA(A)R-mediated mIPSCs was fourfold higher than that of GlyR-mediated mIPSCs at E17-18, indicating that GABAergic synaptic sites were functionally dominant at early stages of neural network formation. Properties of GABA(A)R-mediated mIPSC amplitude fluctuations changed from primarily unimodal skewed distribution at E17-18 to Gaussian mixtures with two to three discrete components at P1-3. A developmental shift from primarily long-duration GABAergic mIPSCs to short-duration glycinergic mIPSCs was evident after birth, when the frequency of GlyR-mediated mIPSCs increased 10-fold. This finding suggested that either the number of glycinergic synapses or the probability of vesicular glycine release increased during the period studied. The increased frequency of GlyR-mediated mIPSCs was associated with more than a twofold increase in their mean amplitude, and in the number of motoneurons in which mIPSC amplitude fluctuations were best fitted by multi-component Gaussian curves. A third subpopulation of mIPSCs was apparent in the absence of glycine and GABA(A) receptor antagonists: mIPSCs with both fast and slow decaying components. Based on their dual-component decay time and their suppression by either strychnine or bicuculline, we assumed that these were generated by the activation of co-localized postsynaptic glycine and GABA(A) receptors. The contribution of mixed glycine-GABA synaptic sites to the generation of mIPSCs did not change after birth. The developmental switch from predominantly long-duration GABAergic inhibitory synaptic currents to short-duration glycinergic currents might serve as a mechanism regulating neuronal excitation in the developing spinal networks.     

脊髓运动神经元胞体上的突触超微结构

本文报道了大白鼠脊髓运动神经元轴-体突触的超微结构特点。根据电镜观察的结果说明脊髓运动神经元胞体表面的突触前囊内有五种形态的突触小泡,它们包括S形小泡(圆形小泡)、E形小泡(椭圆形小泡)、F形小泡(扁平形小泡)、G形小泡(颗粒小泡)和Co形小泡(有被小泡)等。作者认为脊髓灰质前角内运动神经元的轴-体突触可以区分为四种类型、S型突触(含圆形小泡)、F形突触(含椭圆形小泡或扁平形小泡)、S-F型突触(以圆形小泡占优势)和F-S型突触(以扁平形小泡或椭圆形小泡占优势)。后二种类型的突触为运动神经元上的混合小泡型(Mv型)轴-体突触。有关突触连接部位的形态特征、突触小泡的形态、突触的分型、突触膜和突触裂的特点,以及突触与神经胶质间的关系等问题在论文中作了讨论。     

Direct and indirect actions of thyrotropin-releasing hormone on neonatal rat motoneurons in vitro

In addition to causing a slow depolarization, thyrotropin-releasing hormone (TRH, 5-10 microM) evoked synaptic activity in antidromically identified motoneurons in thin transverse neonatal rat spinal cord slices. The synaptic activity but not the slow depolarization was reversibly abolished by TTX or low Ca2+/high Mg2+ solution. Furthermore, the TRH-induced synaptic activity was eliminated by the glutamate receptor antagonist kynurenic acid (0.5 or 1 mM) and/or the glycine receptor antagonist strychnine (1 microM). Our results indicate that in addition to depolarizing motoneurons, TRH modifies the activity of spinal interneurons which may secondarily alter the activity of motoneurons.     

Monoaminergic and GABAergic terminations in phrenic nucleus of rat identified by immunohistochemical labeling

The termination patterns of axons in the phrenic nucleus immunoreactive to synthetic enzymes for catecholamines and for serotonin and GABA were studied in rats. Spinal cord tissue in which phrenic motoneurons were retrogradely labeled with horseradish peroxidase was incubated with antisera against dopamine beta-hydroxylase, phenylethanolamine-N-methyltransferase, 5-hydroxytryptamine, and GABA to identify presumptive terminations of monoaminergic and GABAergic neurons onto identified phrenic motoneurons. In the C3 to C5 spinal cord, 5-hydroxytryptamine-, dopamine beta-hydroxylase- and GABA-like positive terminals with varicosities formed a dense network, with presumptive synaptic contacts on dendrites and somas of phrenic motoneurons. A similar pattern of terminations was also observed in adjacent (non-respiratory muscle) motoneuron pools. There were fewer phenylethanolamine-N-methyltransferase-positive terminal arborizations in the cervical spinal cord compared to thoracic spinal cord; phenylethanolamine-N-methyltransferase terminals were not seen in the vicinity of phrenic motoneurons. These results suggest that phrenic motoneuronal activity is influenced by multiple supraspinal inputs utilizing different neurotransmitters. These transmitters also mediate inputs to other (nearby) spinal motoneurons and thus are not unique for signal transmission to phrenic motoneurons.