GABA-immunoreactive Terminals Research Articles

The GABAergic-like system in the cyprid of Balanus amphitrite (=Amphibalanus amphitrite) (Cirripedia, Crustacea)

In the present study, biochemical and immunochemical methods were used to investigate the presence and distribution of GABA, glutamate decarboxylase (GAD), GABABR1 and GABAA γ2 subunit receptors and the vesicular GABA transporter (vGAT) in the cyprid of Balanus amphitrite (=Amphibalanus amphitrite). GAD65/67 immunoreactive neuron cell bodies and nerve fibers were detected in the central nervous system. Paired GAD65/67 immunoreactive nerves running from the posterior ganglion to the body and limb muscles were detected. Thin GABA-immunoreactive nerve terminals were present on striated muscular fibers and in the antennules. Furthermore, GABA, GAD65/67, GABABR1 and GABAAγ2 subunit receptors and vGAT were observed in the lateral compound eyes, and GABAAγ2 subunit receptor immunoreactivity was seen in the naupliar eye. These results suggest a neurotransmitter/neuromodulatory role for GABA in thoracic muscle contraction and regulatory functions in compound eyes and antennules of B. amphitrite cyprids.

Ultrastructural characterization of relationship between serotonergic and GABAergic structures in the ventral part of the oral pontine reticular nucleus

The ventral part of the oral pontine reticular nucleus (vRPO) is involved in the generation and maintenance of rapid eye movement (REM) sleep. Both GABAergic and serotonergic neurotransmission have been implicated in the control of the sleep–wakefulness cycle. Nevertheless, the synaptic organization of serotonergic terminals in the vRPO has not yet been characterized. We performed an electron microscope study of serotonin-immunoreactive (5-HT-IR) terminals using immunoperoxidase or immunogold–silver methods. In a second set of experiments, combining GABA immunoperoxidase and 5-HT immunogold–silver techniques, we examined inputs from GABA-immunoreactive (GABA-IR) terminals to serotonergic neurons. 5-HT-IR terminals were located primarily on dendrites and occasionally on somata of unlabeled and 5-HT-IR neurons. The majority of the synapses formed by 5-HT-IR terminals were of the symmetrical type, making contacts primarily with unlabeled dendritic profiles. Moreover, 5-HT-IR terminals contacted unlabeled axon terminals that formed asymmetric synapses on dendrites. Double immunolabeling experiments showed 5-HT-IR and GABA-IR afferents, in apposition to each other, making synapses with the same dendrites. Finally, GABA-IR terminals innervated 5-HT-IR and GABA-IR dendrites. Our findings indicate that serotonin would modulate the neuronal activity through inhibitory or excitatory influences, although the action of serotonin on the vRPO would predominantly be inhibitory. Moreover, the present results suggest that the serotonin modulation of vRPO neurons might involve indirect connections. In addition, GABA might contribute to the induction and maintenance of REM sleep by inhibiting serotonergic and GABAergic neurons in the vRPO.

Thalamic nucleus submedius receives GABAergic projection from thalamic reticular nucleus in the rat

GABAergic projection from thalamic reticular nucleus to thalamic nucleus submedius in the medial thalamus of the rat was studied by using immunohistochemistry for GABA, retrograde labeling with Fluoro-Gold combined with immunohistochemistry for GABA, and anterograde labeling with biotinylated dextranamine. Immunohistochemistry displayed that only GABA immunoreactive terminals were observed in the thalamic nucleus submedius, while GABA immunoreactive neuronal cell bodies were located in the thalamic reticular nucleus and lateral geniculate nucleus. Injection of Fluoro-Gold into the thalamic nucleus submedius resulted in massive retrogradely labeled neuronal cell bodies in the rostroventral portion of the ipsilateral thalamic reticular nucleus and a few in the contralateral thalamic reticular nucleus, and most of these cell bodies showed GABA immunopositive staining. Many biotinylated dextranamine anterogradely labeled fibers and terminals in the thalamic nucleus submedius were observed after injection of biotinylated dextranamine into the thalamic reticular nucleus. The present results provide a morphological evidence for a hypothesis that a disinhibitory effect on output neurons elicited by opioid or 5-hydroxytryptamine inhibiting a GABAergic terminal in the thalamic nucleus submedius may lead to activation of the descending inhibitory system and depression of the nociceptive inputs at the spinal cord level.

Colocalization of γ-aminobutyric acid and acetylcholine in neurons in the laterodorsal and pedunculopontine tegmental nuclei in the cat: a light and electron microscopic study

Cholinergic and γ-aminobutyric acid (GABA) mechanisms in the dorsolateral pontomesencephalic tegmentum have been implicated in the control of active (REM) sleep and wakefulness. To determine the relationships between neurons that contain these neurotransmitters in this region of the brainstem in adult cats, combined light and electron microscopic immunocytochemical procedures were employed. Light microscopic analyses revealed that choline acetyltransferase (ChAT) and GABA immunoreactive neurons were distributed throughout the laterodorsal and pedunculopontine tegmental nuclei (LDT and PPT). Surprisingly, approximately 50% of the ChAT immunoreactive neurons in these nuclei also contained GABA. Using electron microscopic pre-embedding immunocytochemistry, GABA immunoreactivity was observed in somas, dendrites and axon terminals in both the LDT and PPT. Most of the GABA immunoreactive terminals formed symmetrical synapses with non-immunolabeled dendrites. Electron microscopic double-immunolabeling techniques revealed that ChAT and GABA were colocalized in axon terminals in the LDT/PPT. Approximately 30% of the ChAT immunoreactive terminals were also GABA immunoreactive, whereas only 6–8% of the GABA immunoreactive terminals were ChAT immunoreactive. Most of the ChAT/GABA immunoreactive terminals formed symmetrical synapses with non-immunolabeled dendrites; however, ChAT/GABA immunoreactive terminals were also observed that contacted ChAT immunoreactive dendrites. With respect to ChAT immunoreactive postsynaptic profiles, approximately 40% of the somas and 50% of the dendrites received synaptic contact from GABA immunoreactive terminals in both the LDT and PPT. These findings (a) indicate that there are fundamental interactions between cholinergic and GABAergic neurons within the LDT/PPT that play an important role in the control of active sleep and wakefulness and (b) provide an anatomical basis for the intriguing possibility that a mechanism of acetylcholine and GABA co-release from the terminals of LDT/PPT neurons is involved in the regulation of behavioral states.

Ultrastructural study of glutamate- and GABA-immunoreactive terminals contacting the primary afferent fibers in frog spinal cord: A double postembedding immunocytochemical study

The in vitro HRP application to the dorsal root of the frog spinal cord produced an intensive staining of primary afferent fibers. A double postembedding GABA and glutamate immunocytochemical study revealed GABA- or glutamate-immunopositive presynaptic boutons establishing axo-axonic synapses onto HRP-stained primary afferent fibers in the spinal cord intermedial zone.

Localization of dopamine D1 and D2 receptors in the rat neostriatum: Synaptic interaction with glutamate‐ and GABA‐containing axonal terminals

In order to determine the synaptic interactions between the glutamate- and GABA-containing axonal terminals and the two subpopulations of medium spiny neurons in the rat neostriatum, double immunocytochemistry was performed. Sections of perfuse-fixed rats were used. Immunoreactivity for dopamine D1 and D2 receptors was used as a marker for the two subpopulations of spiny neurons that give rise to the direct and indirect pathways, respectively. Receptor immunoreactivity was first revealed by preembedding immunostaining. Postembedding colloidal gold labeling was then performed to reveal immunoreactivity for glutamate or GABA. The results were analyzed at the electron microscopic level. Both the D1-immunoreactive, presumed striatonigral/entopeduncular neurons, and the D2-immunoreactive, presumed striatopallidal neurons, were found to receive qualitatively similar synaptic inputs from glutamate-immunoreactive terminals and GABA-immunoreactive terminals. The present results indicate that the different classes of spiny neurons are thus likely to be under a similar regime of excitatory and inhibitory control. Synapse 38:413–420, 2000. © 2000 Wiley-Liss, Inc.

Changes in GABA-immunoreactivity during development of the rostral subdivision of the nucleus of the solitary tract

GABA plays an important role in the processing of gustatory information in the rostral nucleus of the solitary tract. The following study used post-embedment immunohistochemistry in the rat brainstem to localize GABA at both the light and electron microscopic levels to characterize the developmental distribution of GABA and synaptogenesis of GABA-immunoreactive terminals in the rostral nucleus of the solitary tract. During the first postnatal week, GABA is present in the rostral nucleus of the solitary tract, but less of it is synaptic than any time later in development. Of the few synaptic terminals present at postnatal day 1, less than 20% are GABA-immunoreactive. This proportion more than doubles to reach adult levels by postnatal day 10. By weaning (postnatal day 20), GABA-immunoreactive cells are found in nearly the same density as in the adult. Development continues after weaning and is characterized by a disproportionate loss of non-GABA-containing cells. Finally, one previously identified subtype of GABA-immunoreactive terminal matures very late during the postweaning phase of development. The study provides the first analysis of the development of GABA-related circuitry in the rostral nucleus of the solitary tract using anatomical methods. These data provide the background with which to view the emerging physiology of developing taste neurons.

Localization of dopamine D1 and D2 receptors in the rat neostriatum: synaptic interaction with glutamate- and GABA-containing axonal terminals.

In order to determine the synaptic interactions between the glutamate- and GABA-containing axonal terminals and the two subpopulations of medium spiny neurons in the rat neostriatum, double immunocytochemistry was performed. Sections of perfuse-fixed rats were used. Immunoreactivity for dopamine D1 and D2 receptors was used as a marker for the two subpopulations of spiny neurons that give rise to the direct and indirect pathways, respectively. Receptor immunoreactivity was first revealed by preembedding immunostaining. Postembedding colloidal gold labeling was then performed to reveal immunoreactivity for glutamate or GABA. The results were analyzed at the electron microscopic level. Both the D1-immunoreactive, presumed striatonigral/entopeduncular neurons, and the D2-immunoreactive, presumed striatopallidal neurons, were found to receive qualitatively similar synaptic inputs from glutamate-immunoreactive terminals and GABA-immunoreactive terminals. The present results indicate that the different classes of spiny neurons are thus likely to be under a similar regime of excitatory and inhibitory control.

Glycine- and GABA-immunoreactive nerve terminals apposed to alpha-motoneurons during postnatal development in kittens: a quantitative study using the disector.

The distribution of gamma-aminobutyric acid (GABA) and/or glycine-immunoreactive (IR) terminal-like structures apposed to somatic and dendritic membrane of lumbar alpha-motoneurons in column 2 was examined in 1- and 3-week-old kittens and in the adult cat. This quantitative study was carried out using a postembedding technique on semithin sections and a stereological method, the disector. Analysis of immunoreactive terminals showed that the percentages of GABA-IR and glycine-IR terminals (these populations include terminals containing both GABA and glycine) apposed to somatic and proximal dendritic compartments of alpha-motoneurons are almost the same in kittens, while in the adult glycinergic innervation becomes predominant. This change results from: (1) the decrease in numbers of GABA-IR terminals contacting the somatic compartment between 3 weeks and adult stage, while the numbers of glycine-IR terminals show no significant changes after birth and the numbers of terminals containing both neurotransmitters (GABA-IR+glycine-IR) present transient changes and (2) the postnatal increase in the dendritic compartment, in numbers of GABA-IR, glycine-IR and GABA-IR+glycine-IR terminals; the increase being larger for glycine-IR terminals. Furthermore, using a postembedding immunogold technique, observations by electron microscopy showed that GABA-IR P boutons apposed to M boutons can already be identified at 1 week after birth.

Glycine-immunoreactive synaptic terminals in the nucleus tractus solitarii of the cat: ultrastructure and relationship to GABA-immunoreactive terminals.

Postembedding immunogold labeling methods applied to ultrathin and semithin sections of cat dorsomedial medulla showed that neuronal perikarya, dendrites, myelinated and nonmyelinated axons, and axon terminals in the nucleus tractus solitarii contain glycine immunoreactivity. Light microscopic observations on semithin sections revealed that these immunoreactive structures were unevenly distributed throughout the entire nucleus. At the electron microscopic level, synaptic terminals with high levels of glycine-immunoreactivity, assumed to represent those releasing glycine as a neurotransmitter, were discriminated from terminals containing low, probably metabolic levels of glycine-immunoreactivity, by a quantitative analysis method. This compared the immunolabeling of randomly sampled terminals with a reference level of labeling derived from sampling the perikarya of dorsal vagal neurones. The vast majority of these "glycinergic" terminals contained pleomorphic vesicles, formed symmetrical synaptic active zones, and targeted dendrites. They appeared to be more numerous in areas of the nucleus tractus solitarii adjoining the tractus solitarius, but rather scarce caudally, medially, ventrally, and in the dorsal motor vagal nucleus. In a random analysis of the entire nucleus tractus solitarii, 26.2% of sampled terminals were found to qualify as glycine-immunoreactive. In contrast, boutons immunoreactive for gamma-aminobutyric acid (GABA) were more evenly distributed throughout the dorsal vagal complex and accounted for 33.7% of the synaptic terminals sampled. A comparison of serial ultrathin sections suggested three subpopulations of synaptic terminals: one containing high levels of both GABA- and glycine-immunoreactivities (21% of all terminals sampled), one containing only GABA-immunoreactivity (12.7%), and relatively few terminals (5.2%) that were immunoreactive for glycine alone. These results were confirmed by dual labeling of sections using gold particles of different sizes. This study reports the first analysis of the ultrastructure of glycinergic nerve terminals in the cat dorsal vagal complex, and the pattern of coexistence of glycine and GABA observed provides an anatomical explanation for our previously reported inhibitory effects of glycine and GABA on neurones with cardiovascular and respiratory functions in the nucleus tractus solitarii.

Structure and function of gustatory neurons in the nucleus of the solitary tract. IV. The morphology and synaptology of GABA-immunoreactive terminals

In the visual, auditory and somatosensory systems, insight into the synaptic arrangements of specific types of neurons has proven useful in understanding how sensory processing within that system occurs. The neurotransmitter GABA is present in the nucleus of the solitary tract and based on the fact that the vast majority of cells respond to GABA, its agonists and antagonists, and that over 45% of synaptic terminals in the rostral subdivision of the nucleus of the solitary tract are GABA-immunoreactive, GABA is thought to play an important role in gustatory processing. The following study was carried out to establish the distribution of GABA-immunoreactive terminals within the nucleus of the solitary tract. Specifically, the distribution on to physiologically-identified gustatory neurons was determined using post-embedding electron immuno-histochemistry. GABA-immunoreactive terminals synapse with gustatory neuronal somata and all portions of their dendrites, but non-GABAergic terminals synapse only with distal dendrites of the gustatory cells and on to correspondingly small unidentified dendritic profiles in the neuropil. There is a differential distribution of two subtypes of GABA-immunoreactive terminals on to proximal and distal portions of the gustatory neurons as well. Finally, a model for the synaptic arrangements involving gustatory and GABAergic neurons is proposed.

Localization and source of γ aminobutyric acid immunoreactivity in the isthmic nucleus of the frog Rana esculenta

The distribution of γ-aminobutyric acid (GABA)-containing neurons and nerve fibers was studied in the isthmic nucleus of the frog Rana esculenta using light and electron microscopical immunohistochemical techniques. Approximately 0.5% of isthmic cells showed GABA immunopositivity, and the majority of these cells was found in the anterior one-third of the nucleus. A meshwork of GABA-immunostained fine beaded axons filled the entire isthmic nucleus. The GABA-immunoreactive terminals formed pericellular basket-like structures around a few cells both in the medulla and the cortex of the isthmic nucleus. To determine the source of GABA-positive fibers in the isthmic nucleus lesion experiments were carried out. After unilateral tectal ablation no change was observed in GABA immunoreactivity. Hemisectioning the tegmentum close to the anterior border of the isthmic nucleus, transection of the caudal tectal commissure and decussatio veli, or electrical lesioning of the anterodorsal tegmental nucleus all resulted in a moderate decrease in the density of GABA-positive fibers. Our results suggest that the majority of GABA-positive fibers derives from local GABA-positive cells, but some GABAergic afferents seem to arise in the tegmentum.

Pre-embedding staining for GAD67 versus postembedding staining for GABA as markers for central GABAergic terminals.

Pre-embedding immunocytochemistry for the active form of glutamate decarboxylase (GAD67) and postembedding staining for gamma-aminobutyric acid (GABA) were compared as markers for central GABAergic terminals in the phrenic motor nucleus, in which phrenic motor neurons had been retrogradely labeled with cholera toxin B-horseradish peroxidase. Nerve terminals with or without GAD67 immunoreactivity were identified in one ultrathin section. GABA was localized with immunogold in an adjacent section after etching and bleaching. GABA labeling density was assessed over 519 GAD67-positive and GAD67-negative nerve terminals in the phrenic motor nucleus. Frequency histograms showed that statistically higher densities of gold particles occurred over most GAD67-positive terminals. However, some GAD67-negative terminals also showed high densities of gold particles, and some GAD67-positive terminals showed low densities. Preabsorption of the anti-GABA antibody with a GABA-protein conjugate, but not with other amino acid-protein conjugates, significantly reduced gold labeling over both GAD67-positive and GAD67-negative terminals. These results show that the presence of GAD67 immunoreactivity correlates strongly with high densities of immunogold labeling for GABA in nerve terminals in the phrenic motor nucleus. Preabsorption controls indicate that authentic GABA was localized in the postembedding labeling procedure. Only a small proportion of intensely GABA-immunoreactive terminals lack GAD67, suggesting that both GAD67 and GABA are reliable markers of GABAergic nerve terminals.

Ultrastructural subtypes of glutamate-immunoreactive terminals on rat trigeminal motoneurones and their relationships with GABA-immunoreactive terminals.

Electron-microscopic immunolabelling methods were used to study the relationships between glutamate-immunoreactive and gamma-aminobutyric acid (GABA)-immunoreactive synapses on trigeminal motoneurones labelled by the retrograde transport of horseradish peroxidase. Serial sections were cut through the motor nucleus, alternate sections were incubated with antibodies to glutamate and GABA, and the immunopositive nerve terminal profiles were recognized using a quantitative, post-embedding immunogold method. Boutons exhibiting high levels of glutamate immunoreactivity and GABA-immunoreactive boutons both formed axo-dendritic and axo-somatic synaptic contacts on labelled motoneurones. Boutons strongly immunopositive for glutamate were not immunopositive for GABA, and vice versa. Strongly glutamate immunoreactive boutons received axo-axonic synaptic contacts but did not form such contacts, while GABA-immunoreactive boutons formed axo-axonic synapses but did not receive them. The presynaptic elements at all axo-axonic synapses on to glutamate-immunoreactive boutons sampled were GABA-immunopositive. These data provide ultrastructural evidence in support of the roles of glutamate and GABA as transmitters at synapses on trigeminal motoneurones, and for presynaptic control of transmission at glutamatergic synapses by GABA acting at receptors at axo-axonic synapses. The vast majority (more than 90%) of strongly glutamate immunoreactive boutons contained spherical synaptic vesicles, in contrast to GABA-immunoreactive boutons, which contained pleomorphic vesicles. Most of the glutamate-immunoreactive boutons (67%) formed asymmetrical synaptic active zones, many of which (47% of total) were associated with subsynaptic dense "Taxi" bodies (T-terminals), while a smaller population of boutons (21%) formed symmetrical synapses, and a few (11%) made synapses associated with subsynaptic cisternae (C-terminals). The heterogeneity of active zone ultrastructure of boutons identified as being glutamatergic on the basis of their high levels of immunolabelling is discussed in relation to possible differences in co-transmitters released, origins of the synaptic input or post-synaptic receptor subtypes activated.

Multiple inputs of GABA-immunoreactive neurons in the cuneate nucleus of the rat

Using anterograde transport of WGA-HRP and the experimental degeneration method for identification of corticocuneate (CCT) and primary afferent (PAT) terminals in conjunction with γ-amino butyric acid (GABA) and glutamate immunocytochemistry, this study has demonstrated that the GABA-immunoreactive (GABA-IR) neurons in the rat cuneate nucleus were post-synaptic to PATs (some of them being glutamate-IR), GABA-IR and GABA-negative terminals. The HRP-labelled CCTs did not make any synaptic contacts with GABA-IR neurons but with some GABA-negative dendrites. PATs labelled by HRP or showing degenerating features made direct synaptic contacts with the dendrites of GABA-IR neurons. Beside the above GABA-IR boutons also showed axosomatic and axodendritic synapses with the GABA-IR neurons. In `triple labeling' method for GABA, PAT and glutamate, it was found that the PATs which were usually glutamate-positive were presynaptic to the dendrites of GABA-IR neurons. Furthermore, some glutamate-IR terminals which were of non-PAT's origin also synapsed with the dendrites and somata of GABA-IR neurons. It is concluded from this study that the major inputs of GABA-IR neurons were from glutamate immunopositive PATs and glutamate terminals of non-PATs origin; other GABA-IR terminals either intrinsic or extrinsic also contributed to the afferent sources of GABA-IR neurons. The CCTs contributed very little, if any, to this input. It is suggested that the PATs and glutamate-IR terminals on GABA-IR neurons may be involved in lateral inhibition for increase of spatial precision. The synaptic contacts between GABA-IR boutons and dendrites or somata of GABA-IR neurons may provide a possible means for disinhibition.

Electron microscopic serial analysis of GABA presynaptic terminals on the axon hillock and initial segment of labeled abducens motoneurons in the rat

The aim of the present study was to provide a quantitative analysis of the synapses made onto the axon hillock and initial segment of rat abducens motoneurons retrogradely or intracellularly stained with HRP. GABA-immunoreactive terminals contacting these axons were visualized using a postembedding procedure. The presynaptic terminals contained either spherical or pleomorphic vesicles. γ-Aminobutyric acid (GABA)-immunoreactive axon terminals, which belonged to this last category, were distributed both onto axon hillocks and the proximal part of initial segments. The percentage of axonal membrane covered by synapses ranged from 44.1 to 68.2%. A quantitative analysis performed on a series of ultrathin sectioned terminals contacting the axon of an intracellularly labeled motoneuron revealed a significant correlation between the length of membrane apposition of the terminals and their perimeter or surface area, and also between the area of membrane apposition and terminal volume. GABA-immunoreactive terminals had a mean perimeter and volume that were larger than those of unlabeled axon terminals. The number of active zones was correlated with the area of apposition. Some hypotheses concerning the functional role of the GABAergic innervation of this particular part of the neuron are discussed.

Glycine-immunoreactive terminals in the rat trigeminal motor nucleus: light- and electron-microscopic analysis of their relationships with motoneurones and with GABA-immunoreactive terminals

Post-embedding immunolabelling methods were applied to semi-thin and ultrathin resin sections to examine the relationships between glycine- and γ-aminobutyric acid (GABA)-immunoreactive terminals on trigeminal motoneurones, which were identified by the retrograde transport of horseradish peroxidase injected into the jaw-closer muscles. Serial sections were cut through boutons and alternate sections were incubated with antibodies to glycine and GABA. Light-microscopic analysis of semi-thin sections revealed a similar pattern of glycine and GABA-immunoreactive boutons along the motoneurone soma and proximal dendrites, and of immunoreactive cell bodies in the parvocellular reticular and peritrigeminal areas surrounding the motor nucleus. Immunoreactive synaptic terminals on motoneurones were identified on serial ultrathin sections at electron-microscopic level using a quantitative immunogold method. Three populations of immunolabelled boutons were recognized: boutons immunoreactive for glycine alone (32%), boutons immunoreactive for GABA alone (22%), and boutons showing co-existence of glycine and GABA immunoreactivities (46%). Terminals which were immunoreactive for glycine only contained a higher proportion of flattened synaptic vesicles than those which were immunoreactive for GABA only, which contained predominantly spherical vesicles. Terminals which exhibited both immunoreactivities contained a mixture of vesicle types. All three classes of terminal formed axo-dendritic and axo-somatic contacts onto retrogradely labelled motoneurones. A relatively high proportion (25%)of boutons that were immunoreactive for both transmitters formed synapses on somatic spines. However, only GABA-immunoreactive boutons formed the presynaptic elements at axo-axonic contacts: none of these were found to contain glycine immunoreactivity. These data provide ultrastructural evidence for the role of glycine and GABA as inhibitory neurotransmitters at synapses onto jaw-closer motoneurones, but suggest that presynaptic control of transmission at excitatory (glutamatergic) synapses on motoneurones involves GABAergic, but not glycinergic inhibition.

Ultrastructural Characteristics of Glutamatergic and GABAergic Terminals in Cat Lamina IX Before and After Spinal Cord Injury

The present study was designed to: 1) morphologically characterize cat glutamate and GABAergic synaptic terminals in lamina IX in the intact spinal cord at the electron microscopic level using postembedding immunochemical techniques and 2), begin an analysis of how the synaptic architecture of glutamate and GABAergic terminals changes elfter an ipsilateral spinal cord hemisection. The present study shows that glutamate immunoreactive terminals are characterized by a wide synaptic cleft, asymmetric synaptic membrane densities and spherical synaptic vesicles. Most of the glutamatergic terminals are presynaptic to small or medium size dendrites. In contrast, GABAergic terminals display typical pleomorphic synaptic vesicles, a narrow synaptic cleft and a symmetrical membrane density. Qualitative analysis indicated that 13-17 months after hemisection, the length of the synaptic active zones in both glutamatergic and GABAergic terminals ipsilateral to hemisection is longer than those observed in the terminals contralateral to hemisection or in normal control cats. Furthermore, the perimeters of both dendrites and either glutamate or GABA immunoreactive terminals are longer on the hemisected side compared with those observed in the nonhemisected side of the spinal cord. These results are important for complete understanding of the mechanisms which underlie locomotor recovery in mammals following spinal cord injury. (J Spinal Cord Med 1997;20:311-318)

Postnatal development of GABA-immunoreactive terminals in the reticular and ventrobasal nuclei of the rat thalamus: a light and electron microscopic study.

The postnatal development of inhibitory GABAergic circuits in the thalamic reticular and ventrobasal nuclei was studied in rats ranging from the day of birth to the end of the third postnatal week by means of a postembedding immunogold staining procedure to visualize GABA. In the reticular nucleus, GABA labeling was present from birth in cell bodies, dendrites, growth cones and a few synaptic terminals, whereas in the ventrobasal nucleus it was exclusively in axonal processes identifiable as growth cones, vesicle-rich profiles and synaptic terminals. In both nuclei, GABA-labeled synaptic terminals were, however, very scarce and immature in neonatal animals and they became numerous and morphologically mature only after the end of the second postnatal week. These findings suggest that inhibitory synaptic responses in the somatosensory thalamus are not yet fully mature throughout the first two postnatal weeks and support the hypothesis that GABA may initially play trophic roles. The relatively late maturation of the thalamic GABAergic system may have important functional consequences, as the reticulothalamic circuits are responsible for the generation of spindle wave oscillations whose cellular mechanisms are also involved in the generation of spike-and-wave (absence) seizures in humans and in animal models.

Direct evidence of an extensive GABAergic innervation of the spinal dorsal horn by fibres descending from the rostral ventromedial medulla.

A long line of studies emphasizes the contribution of serotonergic fibres descending from the rostral ventromedial medulla in the control of spinal nociceptive information processing. A growing body of evidence, however, suggests that the relative contribution of serotonin to the mediation of spinal neuronal activity from the rostral ventromedial medulla may require re-evaluation. It has recently been substantiated that, in addition to the serotonergic fibres, the spinal dorsal horn receives an abundant non-serotonergic projection from the rostral ventromedial medulla. Furthermore, stimulation in the rostral ventromedial medulla could result in a powerful inhibition of nociceptive spinothalamic tract cells without any detectable serotonin release in the dorsal horn. After labelling raphe-spinal axons and axon terminals in the rat by iontophoretic injections of the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin into the central region of the rostral ventromedial medulla (nucleus raphe magnus) and revealing GABA and glycine immunoreactivities of the labelled raphe-spinal terminals and their postsynaptic targets by postembedding immunocytochemical methods, here we demonstrate an extensive GABAergic projection from the rostral ventromedial medulla to the spinal dorsal horn. We show that the majority of the labelled raphe-spinal terminals in laminae I-IIo and IV-V contain GABA and some of the GABA-immunoreactive terminals are also immunoreactive for glycine. We also disclose that GABA-immunoreactive raphe-spinal terminals establish synaptic contacts primarily with GABA- and glycine-negative, presumably excitatory, spinal neurons, including Calbindin-D28k- as well as parvalbumin-immunoreactive cells in both laminae I-IIo and IV-V. The results suggest that volleys in fibres descending from the rostral ventromedial medulla may evoke GABA release from raphe-spinal terminals, and the released GABA, in some cases probably acting together with glycine, might play a crucial, as yet mostly unidentified, role in the inhibition of nociceptive information processing in the dorsal horn of the spinal cord.