GABAergic synapses in the brain identified with antisera to GABA and its synthesizing enzyme, glutamate decarboxylase

The GABAB receptor agonist STX209 reverses the autism‑like behaviour in an animal model of autism induced by prenatal exposure to valproic acid

Autism spectrum disorder (ASD) is a lifelong neurodevelopmental condition characterized by impaired social interaction, compromised communication, and restrictive or stereotyped behaviours and interests. Due to the complex pathophysiology of ASD, there are currently no available medical therapies for improving the associated social deficits. Consequently, the present study investigated the effects of STX209, a selective γ‑aminobutyric acid type B receptor (GABABR2) agonist, on an environmental rodent model of autism. The mouse model of autism induced by prenatal exposure to valproic acid (VPA) was used to assess the therapeutic potential of STX209 on autism‑like behaviour in the present study. This study investigated the effects of STX209 on VPA model mice via behavioral testing and revealed a significant reversal of core/associated autism‑like behavior, including sociability and preference for social novelty, novelty recognition, locomotion and exploration activity and marble‑burying deficit. This may be associated with STX209 correcting dendritic arborization, spine density and GABABR2 expression in hippocampus of VPA model mice. However, expression of glutamic acid decarboxylase 65/67 in the hippocampus were not altered by STX209. The present results demonstrated that STX209 administration ameliorated autism‑like symptoms in mice exposed to VPA prenatally, suggesting that autism‑like symptoms in children with a history of prenatal VPA exposure may also benefit from treatment with the GABABR2 agonist STX209.

Ventral Tegmental Area Projection Regulates Glutamatergic Transmission in Nucleus Accumbens

The ventral tegmental area (VTA) projection to the nucleus accumbens shell (NAcSh) regulates NAcSh-mediated motivated behaviors in part by modulating the glutamatergic inputs. This modulation is likely to be mediated by multiple substances released from VTA axons, whose phenotypic diversity is illustrated here by ultrastructural examination. Furthermore, we show in mouse brain slices that a brief optogenetic stimulation of VTA-to-NAc projection induced a transient inhibition of excitatory postsynaptic currents (EPSCs) in NAcSh principal medium spiny neurons (MSNs). This inhibition was not accompanied by detectable alterations in presynaptic release properties of electrically-evoked EPSCs, suggesting a postsynaptic mechanism. The VTA projection to the NAcSh releases dopamine, GABA and glutamate, and induces the release of other neuronal substrates that are capable of regulating synaptic transmission. However, pharmacological inhibition of dopamine D1 or D2 receptors, GABAA or GABAB receptors, NMDA receptors, P2Y1 ATP receptors, metabotropic glutamate receptor 5, and TRP channels did not prevent this short-term inhibition. These results suggest that an unknown mechanism mediates this form of short-term plasticity induced by the VTA-to-NAc projection.

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Identification of oscillatory firing neurons associated with locomotion in the earthworm through synapse imaging

We used FM imaging to identify neurons that receive sensory feedback from the body wall in a circuit for octopamine (OA)-evoked rhythmic locomotion in the earthworm, Eisenia fetida. We visualized synapses in which postsynaptic neurons receive the sensory feedback, by using FM1-43 dye to label the synapses of both motor and sensory pathways that are associated with locomotion, then clearing the motor pathway synapse labeling, and finally identifying the target synapses by distinguishing physiologically functional synapses through destaining using a high-K(+) solution. A pair of synaptic regions associated with the sensory feedback was found to be located two or three cell body-widths away from the midline, between the anterior parts of the roots of the second lateral nerves (LNs) at the segmental ganglia (SGs). Using conventional intracellular recording and dye loading of the cell bodies surrounding these synaptic regions, we identified a pair of bilateral neurons with cell bodies larger than those of other cells in these regions, and named them "Oscillatory firing neurons Projecting to Peripheral nerves" (OPPs). These had a bipolar shape and projected neurites to the ipsilateral first and third LNs, fired rhythmically, and had a burst timing synchronized with the motor pattern bursts from the ipsilateral first LNs. Current injection into an OPP caused firing in the ipsilateral first LNs, supporting the hypothesis that OPPs functionally project to the peripheral nerves. OPPs also sent neurites to the adjacent anterior and posterior SGs, suggesting connections with the adjacent segments. We conclude that FM imaging can be used to identify neurons involved in specific functions, and that OPPs are the first neurons to be associated with OA-induced locomotion in the earthworm.

GABAergic inputs from direct and indirect striatal projection neurons onto cholinergic interneurons in the primate putamen

Striatal cholinergic interneurons (ChIs) are involved in reward-dependent learning and the regulation of attention. The activity of these neurons is modulated by intrinsic and extrinsic γ-aminobutyric acid (GABA)ergic and glutamatergic afferents, but the source and relative prevalence of these diverse regulatory inputs remain to be characterized. To address this issue, we performed a quantitative ultrastructural analysis of the GABAergic and glutamatergic innervation of ChIs in the postcommissural putamen of rhesus monkeys. Postembedding immunogold localization of GABA combined with peroxidase immunostaining for choline acetyltransferase showed that 60% of all synaptic inputs to ChIs originate from GABAergic terminals, whereas 21% are from putatively glutamatergic terminals that establish asymmetric synapses, and 19% from other (non-GABAergic) sources of symmetric synapses. Double pre-embedding immunoelectron microscopy using substance P and Met-/Leu-enkephalin antibodies to label GABAergic terminals from collaterals of "direct" and "indirect" striatal projection neurons, respectively, revealed that 47% of the indirect pathway terminals and 36% of the direct pathway terminals target ChIs. Together, substance P- and enkephalin-positive terminals represent 24% of all synapses onto ChIs in the monkey putamen. These findings show that ChIs receive prominent GABAergic inputs from multiple origins, including a significant contingent from axon collaterals of direct and indirect pathway projection neurons.

The gray area between synapse structure and function-Gray's synapse types I and II revisited

On the basis of ultrastructural parameters, the concept was formulated that asymmetric Type I and symmetric Type II synapses are excitatory and inhibitory, respectively. This "functional Gray synapses concept" received strong support from the demonstration of the excitatory neurotransmitter glutamate in Type I synapses and of the inhibitory neurotransmitter γ-aminobutyric acid in Type II synapses, and is still frequently used in modern literature. However, morphological and functional evidence has accumulated that the concept is less tenable. Typical features of synapses like shape and size of presynaptic vesicles and synaptic cleft and presence of a postsynaptic density (PsD) do not always fit the postulated (excitatory/inhibitory) function of Gray's synapses. Furthermore, synapse function depends on postsynaptic receptors and associated signal transduction mechanisms rather than on presynaptic morphology and neurotransmitter type. Moreover, the notion that many synapses are difficult to classify as either asymmetric or symmetric has cast doubt on the assumption that the presence of a PsD is a sign of excitatory synaptic transmission. In view of the morphological similarities of the PsD in asymmetric synapses with membrane junctional structures such as the zonula adherens and the desmosome, asymmetric synapses may play a role as links between the postsynaptic and presynaptic membrane, thus ensuring long-term maintenance of interneuronal communication. Symmetric synapses, on the other hand, might be sites of transient communication as takes place during development, learning, memory formation, and pathogenesis of brain disorders. Confirmation of this idea might help to return the functional Gray synapse concept its central place in neuroscience.

Striatal GPR88 expression is confined to the whole projection neuron population and is regulated by dopaminergic and glutamatergic afferents

GPR88, an orphan G protein-coupled receptor, was designated Strg/GPR88 for striatum-specific G protein-coupled receptor (K. Mizushima et al. (2000)Genomics, 69, 314-321). In this study, we focused on striatal GPR88 protein localization using a polyclonal antibody. We established that the distribution of immunoreactivity in rat brain matched that of GPR88 transcripts and provided evidence for its exclusive neuronal expression. GPR88 protein is abundant throughout the striatum of rat and primate, with expression limited to the two subsets of striatal projection medium spiny neurons (MSNs) expressing preprotachykinin-substance P or preproenkephalin mRNAs. Ultrastructural immunolabelling revealed the GPR88 concentration at post-synaptic sites along the somatodendritic compartments of MSNs, with pronounced preference for dendrites and dendritic spines. The GPR88-rich expression, in both striatal output pathways, designates this receptor as a potential therapeutic target for diseases involving dysfunction of the basal ganglia, such as Parkinson's disease. Hence, we investigated changes of GPR88 expression in a model of Parkinson's disease (unilateral 6-hydroxydopamine-lesioned rats) following repeated L-DOPA treatment. In dopamine-depleted striatum, GPR88 expression was differentially regulated, i.e. decreased in striatopallidal and increased in striatonigral MSNs. L-DOPA treatment led to a normalization of GPR88 levels through dopamine D1 and D2 receptor-mediated mechanisms in striatopallidal and striatonigral MSNs, respectively. Moreover, the removal of corticostriatal inputs, by ibotenate infusion, downregulated GPR88 in striatopallidal MSNs. These findings provide the first evidence that GPR88 is confined to striatal MSNs and indicate that L-DOPA-mediated behavioural effects in hemiparkinsonian rats may involve normalization of striatal GPR88 levels probably through dopamine receptor-mediated mechanisms and modulations of corticostriatal pathway activity.

The effect of prefrontal stimulation on the firing of basal forebrain neurons in urethane anesthetized rat

The basal forebrain (BF) contains a heterogeneous population of cholinergic and non-cholinergic corticopetal neurons and interneurons. Neurons firing at a higher rate during fast cortical EEG activity (f>16Hz) were called F cells, while neurons that increase their firing rate during high-amplitude slow-cortical waves (f<4Hz) were categorized as S-cells. The prefrontal cortex (PFC) projects heavily to the BF, although little is known how it affects the firing of BF units. In this study, we investigated the effect of stimulation of the medial PFC on the firing rate of BF neurons (n=57) that were subsequently labeled by biocytin using juxtacellular filling (n=22). BF units were categorized in relation to tail-pinch induced EEG changes. Electrical stimulation of the medial PFC led to responses in 28 out of 41 F cells and in 8 out of 9 S cells. Within the sample of responsive F cells, 57% showed excitation (n=8) or excitation followed by inhibitory period (n=8). The remaining F cells expressed a short (n=6) or long inhibitory (n=6) response. In contrast, 6 out of the 8 responsive S cells reduced their firing after prefrontal stimulation. Among the F cells, we recovered one cholinergic neuron and one parvalbumin-containing (PV) neuron using juxtacellular filling and subsequent immunocytochemistry. While the PV cell displayed short latency facilitation, the cholinergic cell showed significant inhibition with much longer latency in response to the prefrontal stimulus. This is in agreement with previous anatomical data showing that prefrontal projections directly target mostly non-cholinergic cells, including GABAergic neurons.

Effect of basal forebrain neuropeptide Y administration on sleep and spontaneous behavior in freely moving rats

Neuropeptide Y (NPY) is present both in local neurons as well as in fibers in the basal forebrain (BF), an area that plays an important role in the regulation of cortical activation. In our previous experiments in anaesthetized rats, significant EEG changes were found after NPY injections to BF. EEG delta power increased while power in theta, alpha, and beta range decreased. The aim of the present experiments was to determine whether NPY infusion to BF can modulate sleep and behavior in freely moving rats. In this study, microinjections were made into the BF. Saline was injected to the control side, while either saline or one of two doses of NPY (0.5 microl, 300-500 pmol) to the treated side. EEG as well as behavioral changes were recorded. Behavioral elements after the NPY injections changed in a characteristic fashion in time and three consecutive phases were defined. In phase I (half hour 2), activated behavioral items (moving, rearing, grooming) appeared frequently. In phase II (half hours 3 and 4) activity decreased, while motionless state increased. Reappearance of activity was seen in phase III (half hours 5 and 6). NPY injections caused sleep-wake changes. The three phases described for behavioral changes were also reflected in the sleep data. During phase I, lower NPY dose increased wakefulness and decreased deep sleep. Reduced behavioral activity seen in phase II was partially reflected in the sleep. In this phase, wakefulness tended to increase in the third half hour, while decreased in the 4th half hour. Deep sleep and total slow wave sleep non-significantly decreased in the third and increased in the 4th half hour. In most cases, wakefulness was elevated again during Phase III, while sleep decreased. Length of single sleep-wake epochs did not change after NPY injections. Our results suggest a role for NPY in the integration of sleep and behavioral stages via the BF.

Maintenance of inhibitory interneurons and boutons in sensorimotor cortex between middle and old age in Fischer 344 X Brown Norway rats

Ultrastructurally identified inhibitory synapses in layer II of rat sensorimotor cortex decline between middle and old age [Poe, B.H., Linville, C., Brunso-Bechtold, J., 2001. Age-related decline of presumptive inhibitory synapses in the sensorimotor cortex as revealed by the physical disector. J. Comp. Neurol. 439, 65-72]. The current study investigated whether a loss or shrinkage of gamma-aminobutyric acid (GABA)ergic interneurons contribute to that decline. Coronal sections from middle-aged (15-17 months) and old (25-29 months) Fischer 344 X Brown Norway male rats were immunoreacted with antibodies to the GABA synthesizing enzyme glutamic acid decarboxylase (GAD); the calcium-binding protein parvalbumin (PV), or the neuronal marker NeuN. The number of GAD-immunoreactive (IR), PV-IR, and NeuN-IR cells were determined stereologically using the optical disector technique and the cross-sectional areas of GAD-IR cells were measured in layers II/III, IV, V and VI of sensorimotor cortex. Neither the number of GAD-IR or NeuN-IR cells, nor the size of GAD-IR cells, declined significantly between middle and old age. A modest decline in the PV-IR subset of inhibitory interneurons was observed, predominantly due to changes in layers V and VI. Stereological analysis of layer II/III GAD-IR boutons revealed a stability of immunocytochemically identified inhibitory terminals. Taken together, these results indicate a general maintenance of overall GABAergic neurons in sensorimotor cortex between middle and old age and the loss of ultrastructurally identified inhibitory synapses may be due to the decline of a subset of GABAergic terminals.

EEG effect of basal forebrain neuropeptide Y administration in urethane anaesthetized rats

Neuropeptide Y (NPY) is present both in local neurons as well as in fibers in the basal forebrain (BF), an area that plays an important role in the regulation of cortical activation. In previous studies, NPY axons were found to innervate corticopetal cholinergic cells in this area. In addition, identified NPY positive neurons have been shown to be silent during cortical activation, but active during slow EEG waves. However, no in vivo studies have shown the effect of local NPY release in the BF on the EEG. In the present experiments, the EEG was examined following NPY injection (0.5 microl, 300-500 pmol) into the BF of urethane-anaesthetized rats. Fronto-parietal EEG was recorded on both sides and relative EEG power was calculated in the delta (0-3 Hz), theta (3-9 Hz), alpha (9-16 Hz) and beta (16-48 Hz) frequency bands. We found a significant increase in relative delta power and a decrease in the power of all higher frequency bands (theta, alpha, beta) after NPY injection. These results suggest that NPY can inhibit cortical activation via the BF.

Extrasynaptic localization of GABA in the developing mouse cerebellum

In the adult brain, gamma-amino butyric acid (GABA) is synaptically released and mediates inhibitory transmission. Recent studies have revealed that GABA is a trophic factor for brain development. To reveal the distribution of GABA and its secretion mechanisms during brain development, we investigated the immunohistochemical localization of two molecules, GABA and vesicular GABA transporter (VGAT), which is a GABAergic vesicle protein, in the developing mouse cerebellum by means of newly developed antibodies. Furthermore, we tested the relationship between developmental changes in distribution of above two molecules in the presynapses and ontogeny of GABAergic synapses. GABAergic synapses were detected by immunohistochemistry for the GABA(A) receptor alpha1 subunit, which is an essential subunit for inhibitory synaptic transmission in the mature cerebellar cortex. Until postnatal day 7 (P7), GABA was localized throughout the GABAergic neurons, and VGAT accumulated at axon varicosities and growth cones, where the alpha1 subunit did not accumulate. After P10, both GABA and VGAT became confined to the terminal sites where the alpha1 subunit was localized. These results suggested that GABA was extrasynaptically released from axon varicosities and growth cones by vesicular secretion 'exocytosis' and from all parts of GABAergic neurons during the cerebellar development by non-vesicular secretion 'diacrine'.

Morphological development and maturation of the GABAergic synapses in the mouse cerebellar granular layer

In the adult central nervous system (CNS), gamma-amino butyric acid (GABA) is a predominant inhibitory neurotransmitter, which regulates glutamatergic activity. Recent studies have revealed that GABA serves as an excitatory transmitter in the immature CNS, and is involved in brain morphogenesis. To elucidate how GABA exerts its effect on immature neurons and how GABAergic synapses are formed, we examined both development of pre- and post-synaptic elements of the GABAergic synapses formed between granule and Golgi cells in the mouse cerebellar granular layer. Immunohistochemistry for glutamic acid decarboxylase (GAD) demonstrated that GABA was localized throughout the Golgi cells before postnatal day 7 (P7), and became confined to the axon terminals during the second postnatal week. Electron microscopic analysis demonstrated that GABAergic synapses were clearly detected at P10. In situ hybridization and immunohistochemistry for the GABA(A) receptor alpha1 and alpha6 subunits, which are mainly involved in inhibitory synaptic transmission, demonstrated that both subunits appeared at P7. Distribution of both subunits expanded in the granular layer with special reference to the development of GABAergic synapses. Furthermore, the majority of the subunits accumulated adjacent to the GABAergic terminals. These results suggested that in the granular layer, GABA might be non-synaptically secreted from Golgi cell axons and dendrites during the first postnatal week. From the second postnatal week, GABA is synaptically released and begins to mediate inhibitory transmission. Furthermore, it was suggested that GABAergic innervation could initiate expression and trafficking of the GABA(A) receptors containing the alpha1 and alpha6 subunits.

Normal formation of the postsynaptic elements of GABAergic synapses in the reeler cerebellum

Synaptic transmission mediated by gamma-amino butyric acid (GABA) plays an important role in inhibition of glutamatergic excitatory transmission and expression of higher brain functions, such as memory, learning and anxiety. To elucidate mechanisms underlying formation of the postsynaptic elements for GABAergic transmission, we employed the reeler mutant mice in this study. In the reeler cerebellum, abnormal cytoarchitecture and an aberrant environment affect the formation of neural networks and maturation of neurons. We examined the expression and localization of GABA(A) receptor alpha subunits in the reeler cerebellum and determined whether various abnormalities in the reeler mice affected formation of the postsynaptic elements. In situ hybridization analysis revealed that the specific expression of alpha subunit mRNAs in each neuronal type was preserved. Abnormal expression of alpha subunits was not detected, although GABAergic networks were altered and neuronal maturation was severely disturbed. Immunohistochemistry for the alpha1 and alpha6 subunits, which were expressed abundantly in the reeler cerebellum, revealed that both subunit proteins accumulated at positions adjacent to GABAergic terminals. These results, taken together, suggested that expression of the GABA(A) receptor subunits in postsynaptic neurons might be genetically determined, but trafficking and accumulation of the subunit proteins at the GABAergic synapse may be induced by GABAergic innervation.

The light and electron microscopic characterisation of identified striato-ventrotegmental projection neurons in the domestic chick (Gallus domesticus)

A major projection of the medial striatum (lobus parolfactorius, LPO) of birds is the striato-ventrotegmental pathway projecting to the area ventralis tegmentalis. In the present study, we investigated the morphology and connectivity of striato-ventrotegmental neurons in the medial LPO. The neurons were identified by injecting the fluorescent retrograde tracer fast blue (FB) into the area ventralis tegmentalis. FB-labelled neurons in the LPO were targeted and iontophoretically injected with lucifer yellow (LY) in paraformaldehyde fixed slices. The fluorescent LY label in the filled neurons was then photoconverted, and the ultrastructure of cells was investigated. According to our results, the soma of striato-ventrotegmental neurons is rich in organelles, in particular rough and smooth endoplasmic reticula and they possess a large, unindented and slightly eccentric nucleus. The LY-labelled cells possess relatively few, sparsely spiny dendrites, and represent a type of medium-sized spiny projection neuron characteristic of the striata of birds. Axospinous synapses on the labelled cells are asymmetric and correspond morphologically to the glutamatergic excitatory type of terminals described previously in the LPO. Both symmetric and asymmetric axodendritic and axosomatic synapses were detected. Some symmetric synapses were GABA immunolabelled, whereas some asymmetric synapses were immunopositive to glutamate. Axon collaterals of labelled cells formed symmetric or asymmetric axodendritic synapses. Direct contact without interposing glial processes was observed between one of the FB-labelled neurons and an adjacent neuronal soma. There was also a microneuron attached to one of the labelled cells, which we identified as a neurogliaform 'dwarf' cell.

Neonatal monosodium glutamate treatment modifies glutamic acid decarboxylase activity during rat brain postnatal development

Monosodium glutamate (MSG) produces neurodegeneration in several brain regions when it is administered to neonatal rats. From an early embryonic age to adulthood, GABA neurons appear to have functional glutamatergic receptors, which could convert them in an important target for excitotoxic neurodegeneration. Changes in the activity of the GABA synthesizing enzyme, glutamic acid decarboxylase (GAD), have been shown after different neuronal insults. Therefore, this work evaluates the effect of neonatal MSG treatment on GAD activity and kinetics in the cerebral cortex, striatum, hippocampus and cerebellum of the rat brain during postnatal development. Neonatal MSG treatment decreased GAD activity in the cerebral cortex at 21 and 60 postnatal days (PD), mainly due to a reduction in the enzyme affinity (K(m)). In striatum, the GAD activity and the enzyme maximum velocity (V(max)) were increased at PD 60 after neonatal MSG treatment. Finally, in the hippocampus and cerebellum, the GAD activity and V(max) were increased, but the K(m) was found to be lower in the experimental group. The results could be related to compensatory mechanisms from the surviving GABAergic neurons, and suggest a putative adjustment in the GAD isoform expression throughout the development of the postnatal brain, since this enzyme is regulated by the synaptic activity under physiological and/or pathophysiological conditions.

Unique developmental patterns of GABAergic neurons in rat spinal cord

Gamma-aminobutyric acid (GABA)ergic neurons have been postulated to compose an important component of local circuits in the adult spinal cord, yet their identity and axonal projections have not been well defined. We have found that, during early embryonic ages (E12-E16), both glutamic acid decarboxylase 65 (GAD65) and GABA were expressed in cell bodies and growing axons, whereas at older ages (E17-P28), they were localized primarily in terminal-like structures. To determine whether these developmental changes in GAD65 and GABA were due to an intracellular shift in the distribution pattern of GAD proteins, we used a spinal cord slice model. Initial experiments demonstrated that the pattern of GABAergic neurons within organotypic cultures mimicked the expression pattern seen in embryos. Sixteen-day-old embryonic slices grown 1 day in vitro contained many GAD65- and GAD67-labeled somata, whereas those grown 4 days in vitro contained primarily terminal-like varicosities. When isolated E14-E16 slices were grown for 4 days in vitro, the width of the GAD65-labeled ventral marginal zone decreased by 40-50%, a finding that suggests these GABAergic axons originated from sources both intrinsic and extrinsic to the slices. Finally, when axonal transport was blocked in vitro, the developmental subcellular localization of GAD65 and GAD67 was reversed, so that GABAergic cell bodies were detected at all ages examined. These data indicate that an intracellular redistribution of both forms of GAD underlie the developmental changes observed in GABAergic spinal cord neurons. Taken together, our findings suggest a rapid translocation of GAD proteins from cell bodies to synaptic terminals following axonal outgrowth and synaptogenesis.

Molecular heterogeneity of central synapses: afferent and target regulation

Electrophysiological recordings show a functional spectrum even within a single class of synapse, with individual synapses ranging widely in fundamental properties, including release probability, unitary response and effects of previous stimulation on subsequent response. Molecular and cellular biological approaches have shown a corresponding diversity in the complement of ion channels, receptors, scaffolds and signal transducing proteins that make up individual synapses. Indeed, we believe that each individual synapse is unique, a function of presynaptic cell type, postsynaptic cell type, environment, developmental stage and history of activity. We review here the molecular diversity of glutamatergic and GABAergic synapses in the mammalian brain in the context of potential cell biological mechanisms that may explain how individual cells develop and maintain such a mosaic of synaptic connections.

Ultrastructural synaptic organization of axon terminals in the ventral part of the cat oral pontine reticular nucleus

In an attempt to contribute to the current knowledge of the brainstem reticular formation synaptic organization, the ultrastructure and distribution of synaptic terminal profiles on neurons in the ventral part of the oral pontine reticular nucleus (vRPO), the rapid eye movement (REM) sleep-induction site, were studied quantitatively. Terminals with asymmetric contacts and rounded vesicles were classified according to vesicle density as type I or II (high or low density, respectively). The area, apposed perimeter length, and mitochondrial area of type I terminals, on average, were significantly smaller than those of type II terminals. Type III and IV terminals had symmetric contacts and oval and/or flattened vesicles; type III terminals formed synapses between them and on initial axons. Type V and VI terminals showed characteristics intermediate to those of asymmetric and symmetric synapses. Interestingly, some terminal features were related to both terminal area and postsynaptic dendritic diameter. The percentages of different synapses sampled on somata were as follows: asymmetric synapses (usually formed by type II terminals; mean +/- S.D.), 26.4% +/- 3%; symmetric synapses, 46.7% +/- 5.2%; and intermediate synapses, 26.9% +/- 6.1%. The percentages of different synapses sampled on dendrites were asymmetric synapses, 62.1% +/- 9%; symmetric synapses, 25.6% +/- 8.1%; and intermediate synapses, 12.3% +/- 1.7%. Comparison between large- and small-diameter dendrites revealed that the percentages of symmetric synapses and type II terminals decreased, whereas the percentages of type I terminals increased as postsynaptic dendritic diameters became smaller. Synaptic density was approximately four times lower on somata than on dendrites. The vRPO synaptic organization reflects some patterns that are similar to those found in other regions of the central nervous system as well as specific synaptic patterns that are probably related to its functions: the generation and maintenance of REM sleep and the control of eye movement or limb muscle tone.

The medullary dorsal reticular nucleus facilitates pain behaviour induced by formalin in the rat

The influence of the dorsal reticular nucleus (DRt) on pain behaviour during the formalin test was studied in the rat by lesioning the nucleus through local application of electrical current or quinolinic acid. Animals in which the DRt was lesioned ipsilaterally to the paw injected with formalin spent less time in focused (licking, biting or scratching the injected paw) and total (focused pain behaviour plus protection of the injected paw during movements) pain behaviour, and showed paw-jerks less frequently than non-lesioned animals in both phases 1 and 2 of the test. Animals in which the DRt was lesioned contralaterally to the injected paw presented a decrease in total pain behaviour and number of paw-jerks only during phase 2. The number of superficial (laminae I-II) and deep (laminae III-VI) spinal dorsal horn cells expressing the c-fos proto-oncogene 2 h after subcutaneous injection of formalin was reduced by 34% and 50%, respectively, in animals with an ipsilateral DRt lesion as compared to non-lesioned rats. No differences in c-fos expression were observed after lesioning the DRt contralateral to the formalin injection. The results indicate that the DRt is involved in the facilitation of nociception during the formalin test by enhancing the response capacity of dorsal horn neurons to noxious stimulation. It is suggested that the pronociceptive action of the DRt is mediated by the reciprocal connections it establishes with the spinal dorsal horn.

Morphological substrates underlying opioid, epinephrine and gamma-aminobutyric acid inhibitory actions in the rat locus coeruleus

The locus coeruleus (LC) has been implicated in attentional processes related to orienting behaviors, learning and memory, anxiety, stress, the sleep-wake cycle, and autonomic control, as well as to contributing to the affective state. Direct activation of LC neurons causes desynchronization of the electroencephalogram, suggesting that the LC is an important modulator of the behavioral state. The LC has been an intensely studied neuronal system, as the physiology and pharmacology of this nucleus is well understood. This is mainly because of the similarity in neurochemical composition of LC cells which all contain norepinephrine in the rat. However, the homogeneity in neurotransmitter content in LC neurons is sharply contrasted by the heterogeneity of neurochemicals found in its afferent processes. Among these are axon terminals that contain inhibitory and excitatory amino acids, monoamines, and neuropeptides, many of which have been shown to exert differential physiological effects on LC discharge activity. Although much attention has focused on physiological activation of LC neurons, substantial evidence indicates that diverse afferents prominently inhibit noradrenergic cellular activity. Such inhibitory neurochemicals, which arise from local and extrinsic sources, include gamma-aminobutyric acid (GABA) and epinephrine as well as the neuropeptides methionine5-enkephalin and leucine5-enkephalin. Inhibitory transmission in the LC has widespread implications for norepinephrine release at diverse postsynaptic targets, and clinically useful pharmacological agents such as clonidine, an alpha2 adrenergic receptor agonist that potently inhibits the firing of LC neurons, alleviate some negative physical symptoms observed following withdrawal from opiates. In the present review, the synaptic and functional organization of selected inhibitory-type neurotransmitters in the LC obtained from immunoelectron microscopic data will be discussed.

Evidence for a GABAergic projection from the central nucleus of the amygdala to the brainstem of the macaque monkey: a combined retrograde tracing and in situ hybridization study

The central nucleus of the amygdala is interconnected with a variety of visceral and autonomic nuclei of the brainstem. These include the parabrachial nucleus, the nucleus of the solitary tract, the nucleus ambiguus and the dorsal motor nucleus of the vagus. Despite repeated attempts, neurochemical characterization of the major subcortical connections of the central nucleus has not yet been accomplished. Based on earlier immunohistochemical and in situ hybridization evidence indicating the presence of numerous GABAergic neurons in the macaque monkey central nucleus, we predicted that a sizeable portion of the descending projections may be GABAergic. We tested this hypothesis using a novel double labelling method with gold conjugated WGA-apoHRP as a retrograde tracer and in situ hybridization for detecting the mRNA that encodes the enzyme glutamic acid decarboxylase (GAD67) as a marker for GABAergic cells. Following WGA-apoHRP-gold injections into the brainstem, a large number of retrogradely labelled cells was observed in the medial and lateral divisions of the central nucleus. Of the retrogradely labelled cells observed in the medial division of the central nucleus, approximately half were double-labelled for GAD67 mRNA; about 30% double labelling was observed in the lateral division. These data support the view that a sizeable component of the central nucleus projection to the brainstem is GABAergic.

Ultrastructural localization of neurotransmitter immunoreactivity in mossy cell axons and their synaptic targets in the rat dentate gyrus

Electrophysiologically identified and intracellularly biocytin-labeled mossy cells in the dentate hilus of the rat were studied using electron microscopy and postembedding immunogold techniques. Ultrathin sections containing a labeled mossy cell or its axon collaterals were reacted with antisera against the excitatory neurotransmitter glutamate and against the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). From single- and double-immunolabeled preparations, we found that 1) mossy cell axon terminals made asymmetric contacts onto postsynaptic targets in the hilus and stratum moleculare of the dentate gyrus and showed immunoreactivity primarily for glutamate, but never for GABA; 2) in the hilus, glutamate-positive mossy cell axon terminals targeted GABA-positive dendritic shafts of hilar interneurons and GABA-negative dendritic spines; and 3) in the inner molecular layer, the mossy cell axon formed asymmetric synapses with dendritic spines associated with GABA-negative (presumably granule cell) dendrites. The results of this study support the view that excitatory (glutamatergic) mossy cell terminals contact GABAergic interneurons and non-GABAergic neurons in the hilar region and GABA-negative granule cells in the stratum moleculare. This pattern of connectivity is consistent with the hypothesis that mossy cells provide excitatory feedback to granule cells in a dentate gyrus associational network and also activate local hilar inhibitory elements.

The pontine A5 noradrenergic cells which project to the spinal cord dorsal horn are reciprocally connected with the caudal ventrolateral medulla in the rat

A disynaptic pathway linking the caudal ventrolateral medulla (VLM) to the spinal cord via the A5 noradrenergic cell group of the pons has recently been described in the rat. In the present work, the projections of the A5 to the VLM and to the spinal dorsal horn were studied with double-tracing techniques combined with immunostaining of the noradrenaline-synthesizing enzyme dopamine-beta-hydroxylase. Cholera toxin subunit B (CTb) injected into the VLM and fluoro-gold injected into the spinal dorsal horn produced double retrograde labelling of A5 neurons immunoreactive for dopamine-beta-hydroxylase, which received appositions of fibre varicosities labelled anterogradely with CTb injected into the VLM. After injecting CTb into the A5, retrogradely labelled neurons occurred in the VLM. These neurons were contacted by anterogradely labelled fibres from the A5 group. These observations indicate that the VLM cells acting upon the A5 spinally projecting neurons, which are likely to exert an alpha2-adrenoreceptor-mediated inhibition on the spinal cord, are targeted by collaterals of the A5 spinal cord-bound axons. The A5-VLM pathway may be the anatomical substrate of a negative feedback circuit whereby the modulatory action of the VLM on the spinal cord is self-inhibited through activation of the A5.

Ultrastructural study of cholinergic and noncholinergic neurons in the pars compacta of the rat pedunculopontine tegmental nucleus

A group of medium-to-large cholinergic neurons situated in the dorsolateral mesopontine tegmentum comprises the pedunculopontine tegmental nucleus (PPT). The PPT pars compacta (PPT-pc), which occupies the lateral part of the caudal two-thirds of the nucleus, contains a dense aggregation of cholinergic neurons. In the present study, we have employed immunohistochemistry for choline acetyltransferase (ChAT) and electron microscopy to investigate the ultrastructure and synaptic organization of neuronal elements in the PPT-pc. Our results demonstrate that: (1) ChAT-immunoreactive (i.e., cholinergic) PPT-pc neurons are characterized by abundant cytoplasm and organelles, and have few axosomatic synapses (both asymmetric and symmetric); (2) ChAT-immunoreactive dendrites comprise 6-15% of total dendritic elements in the neuropil; the mean percentage of dendritic membrane covered by synaptic terminals is approximately 15%, and nearly all synapses with ChAT-immunoreactive dendrites are asymmetric; (3) within the boundaries described by cholinergic PPT-pc, there are noncholinergic neurons which, in contrast, exhibit a lucent cytoplasm and a higher frequency of axosomatic synapses (10.5% versus 3.7% for cholinergic neurons); and (4) noncholinergic neurons are morphologically heterogeneous with one subpopulation exhibiting a mean diameter that approximates that of cholinergic cells (i.e., > 15 microns and < 20 microns) and a very high frequency of axosomatic synapses (> 20%). Only 0.2-0.7% of terminal elements in the neuropil were ChAT-immunoreactive and these were not observed to synapse with cholinergic dendrites or somata. This relative paucity of terminal labeling and lack of cholinergic-cholinergic interactions seems inconsistent with the recognized and prominent physiological actions of acetylcholine on cholinergic PPT-pc neurons, and suggests a methodological limitation and/or a potential paracrine-like action of nonsynaptically released acetylcholine in the PPT region.

Electron microscopic evidence for coexistence of leucine5-enkephalin and gamma-aminobutyric acid in a subpopulation of axon terminals in the rat locus coeruleus region

We recently described ultrastructural evidence for morphologically heterogeneous axon terminals containing the endogenous opioid peptide, methionine5-enkephalin (ENK), that formed synapses with neurons containing the catecholamine synthesizing enzyme, tyrosine hydroxylase, in the locus coeruleus (LC) of the rat brain. The morphological characteristics of these terminals suggested that ENK may be co-localized with either an excitatory or inhibitory amino acid. To further test this hypothesis, we combined immunogold-silver localization of gamma-aminobutyric acid (GABA) and immunoperoxidase labeling for ENK in single sections through the LC, in the present study, to determine whether ENK and GABA were contained within single axon terminals. Light microscopic analysis of ENK and GABA immunoreactivities in the LC indicated that both transmitters were enriched in the dorsal pons. Although electron microscopy revealed that ENK and GABA were located primarily in axon terminals, some dendrites also contained immunolabeling for GABA. The dense core vesicles were consistently the most immunoreactive in ENK containing axon terminals and were identified toward the periphery of the axon terminal distal to the synaptic specialization. Axon terminals containing either ENK or GABA immunoreactivities contained pleomorphic vesicles as well as large dense core vesicles, varied in size and formed heterogeneous types of synaptic specializations (i.e. asymmetric vs. symmetric). Approximately 38% (n = 76) of the axon terminals containing ENK immunoreactivity (n = 200) also contained GABA. Some axon terminals containing peroxidase labeling for ENK (22%; n = 44) converged on common targets with GABA-labeled axon terminals. Finally, a few ENK-labeled axon terminals (14%; n = 28) formed asymmetric (excitatory-type) synapses with dendrites containing gold-silver labeling for GABA. The results, therefore, indicate that the opioid peptide, ENK, and the inhibitory amino acid, GABA, may influence LC neurons by concerted actions via (1) release from a common axon terminal, and (2) via separate sets of afferents converging on similar portions of the plasmalemma of target neurons. Furthermore, these studies also suggest a cellular substrate for opioid inhibition of LC neurons via activation (i.e. asymmetric synapses) of inhibitory GABAergic neurons. Future studies are required to determine whether the receptive sites for ENK and GABA are located at similar sites on the plasma membranes of LC neurons pre- or postsynaptically and whether there is differential release of either transmitter from single terminals in the LC.

Compound synapses within the GABAergic innervation of the auditory inner hair cells in the adolescent mouse

Ultrastructural investigation of the gamma-aminobutyric acid (GABA) component of the inner spiral bundle in adolescent mice revealed a pathway of glutamic acid decarboxylase (GAD)-positive and -negative fibers and vesiculated endings that contact inner hair cells and their afferents through a complex of axosomatic and axodendritic synapses. Ultrastructural details were investigated by using conventional electron microscopy. Several synaptic arrangements were observed: Main axosomatic synapses form between vesiculated endings and individual or adjoining inner hair cells (interreceptor synapses). Spinous synapses form on long, spinelike processes that protrude from inner hair cells to reach distant efferent endings. The efferent endings associate with inner hair cells and their synaptic afferents through compound synapses-serial, "converging," and triadic-otherwise characteristic of sensory relay nuclei. Serial synapses form by the sequential presynaptic alignment of the efferent-->receptor-->afferent components. Converging synapses result from the simultaneous apposition of a receptor ribbon synapse and a presynaptic efferent terminal on a recipient afferent dendrite. Triadic synapses comprise a vesiculated efferent ending in contact with an inner hair cell and with its synaptic afferent. Additionally, efferent endings may form simple axodendritic and axoaxonal synapses with GAD-negative vesiculated endings. The combination of different synaptic arrangements leads to short chains of compound synapses. It is assumed that these synaptic patterns seen in the adolescent mouse represent adult synaptology. The patterns of synaptic connectivity suggest an integrative role for the GABA/GAD lateral efferent system, and imply its involvement in the pre- and postsynaptic modulation of auditory signals.

The ventrolateral medulla of the rat is connected with the spinal cord dorsal horn by an indirect descending pathway relayed in the A5 noradrenergic cell group

The pathway conveying the descending inhibitory noradrenergic input elicited from the caudal ventrolateral medulla (VLM) onto the spinal cord dorsal horn was studied in the rat. Retrograde labeling with cholera toxin subunit B (CTb) injected into the dorsal horn was combined with immunostaining for dopamine-beta-hydroxylase (DBH) in the VLM and other brainstem nuclei containing noradrenergic cells. CTb-labeled neurons occurred in the lateral part of the VLM (VLMlat), located ventrolaterally to the DBH-immunoreactive cells of the A1 noradrenergic cell group. Neuronal profiles stained for CTb and DBH (double labeled) occurred in the A5 (31%), A6 (57%), and A7 (12%) noradrenergic cell groups. To ascertain whether noradrenergic cells targeting the spinal cord in those groups received projections from the VLMlat, this area was injected with the anterograde tracer biotinylated dextran amine (BDA). Labeled terminal fibers with boutons en passant were apposed to numerous double-stained neurons in the A5 cell group. Similar appositions occurred in small amounts in the ventral subcoerulear component of the A6. Correlated light and electron microscopic analyses of the labeled appositions revealed that the BDA-labeled axonal boutons contained spherical vesicles and were presynaptic at asymmetrical contacts to somata and dendritic profiles of the double-stained A5 neurons. These data indicate the occurrence of an indirect dysynaptic pathway connecting the VLM to the spinal cord, with a relay in the A5 cells. This pathway may convey the antinociceptive effects mediated by alpha 2-adrenoreceptors, which have been previously observed in the spinal cord following VLM stimulation.

The GABA/GAD innervation within the inner spiral bundle in the mouse cochlea

Stains with antibodies to glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) in the cochlea of postnatal and adult mice reveal within the inner spiral bundle a distinctive neuronal plexus intimately associated with the inner hair cells. This innervation provides endings that cradle the receptor poles of the sensory cells and lateral end collaterals that wind between the cells, distributing endings alongside and around them. Some GAD-positive fibers enter the inner pillar bundle, from where they distribute tunnel fibers to the outer hair cells and recurrent collaterals to the inner hair cells. The GABAergic innervation within the inner spiral bundle is present along the entire cochlear axis, with the highest density in its basal half. Stainings against calcitonin gene-related peptide (CGRP), which localizes in the cholinergic counterpart of the inner spiral bundle, reveal that some of these fibers parallel the GABAergic circuit. The present data, together with our previous demonstration of compound (serial, converging, triadic) efferent synapses within this pathway (Sobkowicz et al. (1995) Abst. ARO 18, 171) evidences the presence of a distinctive innervation to the inner hair cells, hitherto unrecognized. The expression of growth-associated protein-43 (GAP-43) within the inner hair cell innervation in the adult cochlea provides evidence for a continuous synaptic turnover and plasticity, thus emphasizing its functional importance.

Norepinephrine modulates excitability of neonatal rat optic nerves through calcium-mediated mechanisms

We report that norepinephrine markedly increases excitability of neonatal rat optic nerves. To investigate the mechanisms of the norepinephrine-induced excitability increase, we studied isolated optic nerves from 42 neonatal (< three days old) and five adult (> three months old) Long-Evan's hooded rats. Norepinephrine (10(-6), 10(-5) and 10(-4) M) rapidly and reversibly increased the amplitude (mean +/- S.D.: 3.5 +/- 1.7%, 12.1 +/- 2.8% and 35.6 +/- 8.4%) of compound action potentials elicited by submaximal stimulation of neonatal optic nerves. The beta-1 adrenoceptor antagonist atenolol (10(-5) M) blocked the norepinephrine-induced increase in excitability but the alpha antagonist phentolamine (10(-5) M) did not. The beta agonist isoproterenol (10(-5) and 10(-4) M) increased response amplitudes (8.7 +/- 4.1% and 25.8 +/- 4.6%) but the alpha-1 agonist methoxamine and alpha-2 agonist clonidine did not. The beta antagonist propranolol blocked the isoproterenol effect. Replacing Ca2+ with Mg2+ or adding 0.8 mM of Cd2+ reversibly blocked the norepinephrine effects. Extracellular K+ concentrations did not change in optic nerves during norepinephrine application. Blockade of K+ channels with apamin (10(-6) M) or tetraethylammonium (10(-3) M) did not prevent the excitatory effects of norepinephrine. Adult rat optic nerves were insensitive to both norepinephrine (10(-4) M) and isoproterenol (10(-4) M). Our results indicate that norepinephrine increases neonatal optic axonal excitability through Ca(2+)-dependent mechanisms. The data suggest that the adrenoceptors are situated on the axons, that the excitability changes are not due to changes in extracellular K+ concentration or K+ channels sensitive to apamin or tetraethylammonium. The sensitivity of rat optic nerves to norepinephrine declined with age. Axonal adrenoceptors may play a role in optic axonal development and injury.

Relational features of acetylcholine, noradrenaline, serotonin and GABA axon terminals in the stratum radiatum of adult rat hippocampus (CA1)

In a well-defined sector of adult rat hippocampus (CA1, stratum radiatum), the ultrastructural features of acetylcholine (ACh), noradrenaline (NA), serotonin (5-HT) and GABA axon terminals (varicosities) were compared by electron microscopy after immunostaining with antibodies against choline acetyltransferase, NA, 5-HT and glutamic acid decarboxylase. Approximately 100 sectional profiles of each type were analyzed for size, presence of a synaptic membrane specialization (synaptic incidence) and composition of the microenvironment. An equivalent number of immunonegative varicosity profiles selected at random from the same micrographs were similarly examined. ACh, NA and 5-HT varicosity profiles were of comparable size, and significantly smaller than GABA profiles. They exhibited a low frequency of junctional specialization, amounting to 7%, 15% and 21%, respectively, when extrapolated to the whole volume of these terminals. In contrast, GABA varicosities appeared entirely synaptic. The ACh, NA and 5-HT varicosities also differed from their GABA counterparts in being juxtaposed to a greater number of unlabeled axonal varicosities and a lower number of dendritic branches. In addition, the microenvironment of immunostained terminals showed a much lower number of dendritic spines than that of immunonegative varicosities. This latter finding was viewed as another indication that predominantly asynaptic varicosities do not maintain particular relationships with their immediate surround. It was also concluded that volume transmission represents a major mode of transmission for ACh, NA and 5-HT in adult rat hippocampus, thus contributing to the properties and functions assigned to these transmitters in this part of brain.

Effects of peripheral axotomy on presynaptic axon terminals with GABA-like immunoreactivity

The facial nerve was unilaterally crushed at its exit from the stylomastoid foramen in three 3-month old male rats. After 10 days survival, before the regenerating axons had reinnervated their target muscles, the facial nucleus was examined to determine central patterns of response in material prepared to demonstrate the presence of GABA-like immunoreactivity with postembedding procedures using gold-labeled secondary antibody. The uninjured nucleus served as a control. In both control and injured nuclei, the GABAergic terminals synapse with all parts of the motor neurons, except the axon, and exhibit diverse morphologies. GABAergic axon terminals vary in their size and in the electron density of their axoplasm and the majority of the terminals contain pleomorphic vesicle profiles that display a range in their packing density and size. In both control and injured facial nuclei, only approximately 40% of the axon terminal profiles with pleomorphic vesicles exhibit GABA immunoreactivity. A morphometric analysis of the synaptic vesicle profiles in the GABA-positive terminals reveals that following axotomy there is no change in the mean number of synaptic vesicle profiles per GABAergic terminal profile. However, the mean size of the synaptic vesicle profiles in these terminals shows an axotomy-induced 50% increase, without change in the shapes of the enlarged vesicle profiles. Also, the numerical density of gold particles associated with the GABA-positive terminals is consistently greater in the injured than the control axon terminals. In the control animals quantitative analysis of the relative distribution of all axon terminal profiles in the neuropil categorized by the shape of their vesicle profiles as round, pleomorphic, or flat is 57:37:6. Ten days after axotomy the ratio of these categories in the injured nucleus has shifted to 35:60:5. This study demonstrates that the functional state of a postsynaptic target can influence the morphology of vesicle profiles in presynaptic elements as well as patterns of its afferent input.

GABAergic innervation of the Mauthner cell and other reticulospinal neurons in the goldfish

The Mauthner cells are pair of identifiable hindbrain neurons that participate in the escape response of fishes. Membrane excitability in these cells is regulated by inhibitory neurons that use glycine as a transmitter. We examined the possibility that the inhibitory transmitter gamma-amino butyric acid (GABA) may also act on the Mauthner cells. We used immunocytochemical methods involving an antibody against glutamic acid decarboxylase (GAD), the synthesizing enzyme for GABA. Our study revealed dense GAD immunoreactive terminals surrounding the Mauthner cells. Puncta counts showed that the distribution of GAD immunoreactivity was densest at the distal lateral dendrite of the Mauthner cells; the distribution of puncta tapers gradually in regions closer to the soma. The axon cap was devoid of GABAergic immunoreactivity. We also performed unilateral lesions of the octaval nuclei to evaluate the origin of the GAD immunoreactive terminals. Following the lesions, we found marked decreases in GAD immunoreactive terminals on the proximal lateral dendrite, soma, and proximal ventral dendrite of both Mauthner cells. These results suggest that the octaval region contributes to bilateral inhibition of the Mauthner cells. The distal lateral dendrite of the ipsilateral Mauthner cell also showed a reduction in GAD immunoreactive terminals. This suggests that GABA mediates remote dendritic inhibition of this cell. GAD immunoreactive puncta also surrounded other large reticulospinal neurons, some of which are serially reiterated along the anterior-posterior axis of the hindbrain. Thus, GABA may also exert an influence not only on the Mauthner cells, but also on other reticulospinal neurons.

Descending projections from the medullary dorsal reticular nucleus make synaptic contacts with spinal cord lamina I cells projecting to that nucleus: an electron microscopic tracer study in the rat

An ultrastructural study is made of the synaptic contacts occurring between structures labelled anterogradely and retrogradely in the superficial dorsal horn following injections of cholera toxin subunit B or horseradish peroxidase in the dorsal reticular nucleus of the medulla oblongata of the rat. Both tracers revealed labelled axonal boutons in lamina I with round synaptic vesicles and a few large granular vesicles making asymmetrical synaptic contacts upon labelled somata and dendrites. After injections of Phaseolus vulgaris leucoagglutinin in the dorsal reticular nucleus, labelled boutons identical to those revealed by the two other tracers were presynaptic to unlabelled somata and dendrites. In addition, dorsoreticular neurons were labelled retrogradely following injections of cholera toxin subunit B into the superficial dorsal horn of the cervical enlargement. These observations show the occurrence of a reciprocal connection between dorsal reticular and lamina I neurons. Considering the putative excitatory nature of the axodendritic contacts in lamina I, a positive feedback circuit is suggested, whereby the nociceptive signals transmitted to the dorsal medullary reticular formation by marginal neurons are intensified.

Patterns of glutamate, glycine, and GABA immunolabeling in four synaptic terminal classes in the lateral superior olive of the guinea pig

The goal of this study was to correlate synaptic ultrastructure with transmitter specificity and function in the lateral superior olive (LSO), a nucleus that is thought to play a major role in sound localization. This was accomplished by means of postembedding immunogold immunocytochemistry. Four classes of synaptic terminals were identified in the LSO. They were distinguishable from one another both morphologically and on the basis of their different patterns of immunolabeling for glutamate, glycine, and gamma-aminobutyric acid (GABA). The highest level of glutamate immunoreactivity was found in terminals that contained round vesicles (R) and formed synaptic contacts with asymmetric synaptic junctions. Round-vesicle terminals predominated on small caliber dendrites by a ratio of at least 2:1 over the other classes combined. The thinnest dendrites were typically contacted by R terminals only. The ratio of R terminals to the other types decreased as the caliber of the dendritic profiles they apposed increased so that on the soma, R terminals were outnumbered by at least 2:1 by the other types. Terminals containing flattened vesicles (F) exhibited intense immunoreactivity for both glycine and glutamate, although the glutamate immunolabeling was not as high as that in the R terminals. Flattened-vesicle terminals formed symmetric synaptic contacts with their targets and their distribution was the reverse of that described for R terminals; i.e., they were most abundant on LSO perikarya and fewest on small caliber dendrites. Two terminal types, both containing pleomorphic vesicles and forming symmetric synaptic junctions, were found in far fewer numbers. One group contained large pleomorphic vesicles (LP) and was immunoreactive for both glycine and GABA. The other group contained small pleomorphic vesicles (SP) along with a few dense-core vesicles and labeled for GABA only. The LP terminals were preferentially distributed on somata and large-caliber dendrites, while the SP terminals most often contacted smaller dendrites. Previous work suggests that a large percentage of the R terminals arise from spherical cells in the ipsilateral cochlear nucleus and are excitatory in action. This pathway may use glutamate as a transmitter. Many of the F terminals are thought to originate from the ipsilateral medial nucleus of the trapezoid body and appear to be the inhibitory (glycinergic) terminals from a pathway that originates from the contralateral ear. The origins and functions of LP and SP terminals are unknown, but a few possibilities are discussed along with the significance of cocontainment of neuroactive substances in specific terminal types.

Glutamate immunoreactive terminals in the lateral amygdaloid nucleus: a possible substrate for emotional memory

The ultrastructure and synaptic associations of terminals immunoreactive for L-glutamate (Glu) were examined in the lateral nucleus of the amygdala (AL). All results reported here involved tissue fixed only with paraformaldehyde. The specificity of the antiserum with paraformaldehyde fixation conditions was assessed and confirmed by immuno-dot blot analysis: the reactivity of anti-Glu to glutamic acid was at least 1,000 times greater than the reactivity to other amino acids. At the light microscopic level, Glu-immunoreactive punctate processes and somata were present in AL. At the electron microscopic level, many Glu-immunoreactive terminals were identified. Data analysis was performed on 365 of these labeled terminals. Glu-immunoreactive terminals were 0.3-1.5 microns in diameter and contained numerous small, clear vesicles as well as mitochondria. Many (77%) of the terminals analyzed had morphologically identifiable synaptic specializations. Most (90%) of the Glu-immunoreactive terminals with synaptic specializations formed asymmetric synapses on spines or small dendrites; synaptic specializations on soma or proximal dendrites were rarely seen (< 1%). Glu-immunoreactive terminals were qualitatively compared to terminals in AL labeled with two other antisera: anti-glutaminase, a marker for the enzyme that catalyzes the conversion of glutamine to the releasable or transmitter form of Glu, and anti-gamma-aminobutyric acid (anti-GABA), a marker for the major inhibitory amino acid transmitter in the brain. Terminals immunoreactive for glutaminase, like those immunoreactive for Glu, formed mostly asymmetric synaptic specializations on spines or small dendrites. In contrast, GABA-immunoreactive terminals usually formed symmetric synapses on soma or proximal dendrites and were never observed to form asymmetric axo-spinous contacts. Although Glu is a metabolic precursor to GABA, these data indicate that the majority of Glu-immunoreactive terminals reflect the site of synthesis and release of Glu and not of GABA. In addition, these results provide morphological evidence that Glu plays a role in excitatory neurotransmission at synapses in AL and support the growing body of data implicating excitatory amino acid-mediated synaptic plasticity in-emotional learning and memory processes in AL.

gamma-Aminobutyric acid immunoreactive structures in the nucleus tractus solitarius: a light and electron microscopic study

gamma-Aminobutyric acid immunoreactive perikarya and boutons in the nucleus tractus solitarius of the cat were examined at both the light and electron microscopic level. Immunoreactive neurones were found predominantly in the parvocellular subdivision of the nucleus tractus solitarius and to a lesser degree in all the other subdivisions of the nucleus tractus solitarius and the dorsal vagal motonucleus. All the immunoreactive perikarya observed were similar in size and morphology. gamma-Aminobutyric acid immunoreactive boutons were observed throughout the nucleus tractus solitarius. However, in contrast to its high content of immunoreactive perikarya the parvocellular subdivision contained the lowest density of immunoreactive boutons. Ultrastructural examination of immunoreactive boutons in the different regions of the nucleus tractus solitarius revealed that they formed synaptic specializations, predominantly with dendritic shafts, all of which were of the symmetric type. This pattern of innervation was observed throughout the medial, commissural, ventrolateral and parvocellular subdivisions of the nucleus tractus solitarius.

Evidence for GABAergic interneurons in the red nucleus of the painted turtle

Immunocytochemical and electrophysiological evidence supporting the presence of GABAergic interneurons in the turtle red nucleus is presented. Injections of HRP into the spinal cord produced labeling of large neurons in the contralateral red nucleus. The peroxidase-antiperoxidase (PAP) method revealed smaller cells immunoreactive to an antibody against glutamate decarboxylase (GAD), the synthetic enzyme for the inhibitory neurotransmitter GABA, that were interspersed among larger immunonegative neurons. Similar small neurons were densely immunostained by antibodies to GABA-glutaraldehyde conjugates obtained from different sources and applied according to pre-embedding and postembedding protocols. Rubrospinal neurons retrogradely labeled with HRP measured 16 and 27 microns in mean minor and major cell body diameters, while GABA-like immunopositive neurons situated within the red nucleus measured 7 and 13 microns. There was very little overlap in soma size between the two cell populations. Therefore, we suggest that the GAD- and GABA-positive neurons may be local inhibitory interneurons. This notion is further supported by observations of pre-embedding immunostaining for GAD and postembedding immunostaining for GABA showing that the turtle red nucleus is amply innervated by immunoreactive axon terminals. These puncta are closely apposed to cell bodies and dendrites of both immunonegative large neurons and immunopositive small neurons. Moreover, immunogold staining at the electron microscopic level demonstrated that GABA-like immunoreactive axon terminals with pleomorphic synaptic vesicles formed symmetric synapses with cell bodies and dendrites of the two types of red nucleus cells. These ultrastructural features are commonly assumed to indicate inhibitory synapses. A moderately labeled bouton with round vesicles and asymmetric synapses was also observed. In addition, the two types of red nucleus neurons received asymmetric axosomatic and axodendritic synapses with GABA-negative boutons provided with round vesicles, features usually associated with excitatory functions. To obtain electrophysiological evidence for inhibition, intracellular recordings from red nucleus neurons were conducted using an in vitro brainstem-cerebellum preparation from the turtle. Small, spontaneous IPSPs were recorded from 7 out of 14 red nucleus cells studied. These morphological and physiological results provide strong support for concluding that the turtle red nucleus, like its mammalian counterpart, contains GABAergic inhibitory interneurons. While we have not identified the main source of input to these interneurons, in view of the scarce development of the reptilian cerebral cortex, this input is unlikely to come from the motor cortex as it does in mammals.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural description of glutamate-, aspartate-, taurine-, and glycine-like immunoreactive terminals from five rat brain regions

The ultrastructural localization of putative excitatory (glutamate, aspartate) and inhibitory (taurine, glycine) amino acid neurotransmitters is described in several selected rat brain regions. In general, axon terminal profiles immunoreactive for excitatory amino acids formed asymmetric synapses with non-immunoreactive small diameter dendritic profiles or dendritic spines. In the cerebellum, both mossy fiber terminals and parallel fiber terminals were immunoreactive for glutamate and aspartate. In the hippocampus, mossy fiber terminals within the stratum lucidum of the CA3 region were immunoreactive for glutamate. Localization of glutamate and aspartate to cerebellar parallel and mossy fibers, as well as the identification of glutamate in hippocampal mossy fibers, is consistent with the excitatory nature of these fibers as described in previous physiological studies. Glutamate-like immunoreactive terminals were also identified in subnucleus caudalis of the spinal trigeminal nucleus and in the dorsal horn of the spinal cord. Immunoreactive axon terminals for two putative inhibitory neurotransmitters, glycine and taurine, displayed a greater number of morphological variations in synaptic structure. In the cerebellum, taurine-like immunoreactivity was present in both basket cell axon terminals which formed symmetric synapses with Purkinje cell neurons, and in a few mossy fiber terminals which formed asymmetric synapses with dendritic spines. In the area dentata of the hippocampus, taurine-like immunoreactive profiles formed asymmetric synapses with dendritic elements. Glycine-like immunoreactive terminals formed symmetric synapses with cell perikarya in both the ventral horn of the spinal cord and in the cochlear nuclei, and on axon terminals in the spinal trigeminal and cochlear nuclei. In contrast, some glycine-like immunoreactive terminals formed asymmetric synapses with distal dendritic profiles in the spinal cord and spinal trigeminal nucleus. The localization of taurine to cerebellar basket cell axons and glycine to axon terminals that synapse on ventral horn motor neuron perikarya is consistent with the hypothesis that these amino acids are functioning as inhibitory neurotransmitters at these synapses. Taurine localization to cerebellar mossy fibers and to fibers in the molecular layer of the dentate gyrus may be more consistent with a proposed neuromodulator role of taurine.

The development of GABAergic neurons in the rat hippocampal formation. An immunocytochemical study

Recent studies have indicated that hippocampal GABAergic neurons in both the dentate gyrus and Ammon's horn are generated prenatally. Although the adult distribution of GABAergic neurons has been previously described by numerous investigators, the early postnatal appearance of these neurons has not been described. In the present study, immunocytochemical methods were used to localize GABAergic neurons with antisera to both GABA and its synthesizing enzyme, glutamate decarboxylase (GAD). The GABA-positive neurons appeared at the earliest postnatal day (PND) examined, 4 PND. In contrast, GAD-positive cells were not observed until 6 PND, and the number of these neurons remained less than that of the GABA-positive neurons until 14 PND. These findings indicated that immunocytochemically detectable amounts of GAD were not present in many young GABAergic neurons. Both GABA- and GAD-positive hippocampal neurons showed two large increases in number during the 4-8 PND and 12-16 PND time periods, and they reached about 90% of adult levels before 18 PND. The regional distribution of GABA- and GAD-positive neurons throughout the hippocampal formation was homogeneous for all ages examined except 4 PND. At this age, the GABA-positive cells appeared in clusters in the proximal CA3 and the distal CA1 relative to the dentate gyrus. In addition, the number of hippocampal neurons immunostained in adult preparations for both antisera to GABA and GAD showed a similar number and distribution. The data on the developmental appearance of GABA and GAD immunoreactivities are consistent with biochemical data for the development of GABA concentration and GAD activity in the hippocampal formation. Together, these data provide important information about the functional maturation of the hippocampal GABAergic system in the first 3 weeks of rat brain development.