Immunogold electron microscopic demonstration of glutamate and GABA in normal and deafferented cerebellar cortex: correlation between transmitter content and synaptic vesicle size

Synthesis and proton NMR spectroscopy of intra-vesicular gamma-aminobutyric acid (GABA)

We report the synthesis of vesicles containing gamma-aminobutyric acid (GABA), and their proton nuclear magnetic resonance ((1)H NMR) spectra. These vesicles were constructed to more closely mimic the intracellular environment wherein GABA exists. For this study, these GABA-containing vesicles were examined under (1)H NMR as a potential platform for future studies on the differences between aqueous phantoms, ex vivo brain extracts, and in vivo magnetic resonance spectroscopy results. We found that intra-vesicular GABA faithfully yielded the chemical shifts and J-coupling constants of free aqueous GABA, alongside the chemical shift signals of the vesicle wall.

Hermansky-Pudlak protein complexes, AP-3 and BLOC-1, differentially regulate presynaptic composition in the striatum and hippocampus

Endosomal sorting mechanisms mediated by AP-3 and BLOC-1 are perturbed in Hermansky-Pudlak Syndrome, a human genetic condition characterized by albinism and prolonged bleeding (OMIM #203300). Additionally, mouse models defective in either one of these complexes possess defective synaptic vesicle biogenesis (Newell-Litwa et al., 2009). These synaptic vesicle phenotypes were presumed uniform throughout the brain. However, here we report that AP-3 and BLOC-1 differentially regulate the composition of presynaptic terminals in the striatum and dentate gyrus of the hippocampus. Quantitative immunoelectron microscopy demonstrated that the majority of AP-3 immunoreactivity in both wild-type striatum and hippocampus localizes to presynaptic axonal compartments, where it regulates synaptic vesicle size. In the striatum, loss of AP-3 (Ap3d(mh/mh)) resulted in decreased synaptic vesicle size. In contrast, loss of AP-3 in the dentate gyrus increased synaptic vesicle size, thus suggesting anatomically specific AP-3-regulatory mechanisms. Loss-of-function alleles of BLOC-1, Pldn(pa/pa), and Muted(mu/mu) revealed that this complex acts as a brain-region-specific regulator of AP-3. In fact, BLOC-1 deficiencies selectively reduced AP-3 and AP-3 cargo immunoreactivity in presynaptic compartments within the dentate gyrus both at the light and/or electron microscopy level. However, the striatum did not exhibit these BLOC-1-null phenotypes. Our results demonstrate that distinct brain regions differentially regulate AP-3-dependent synaptic vesicle biogenesis. We propose that anatomically restricted mechanisms within the brain diversify the biogenesis and composition of synaptic vesicles.

The effects of vesicular volume on secretion through the fusion pore in exocytotic release from PC12 cells

Many spikes in amperometric records of exocytosis events initially exhibit a prespike feature, or foot, which represents a steady-state flux of neurotransmitter through a stable fusion pore spanning both the vesicle and plasma membranes and connecting the vesicle lumen to the extracellular fluid. Here, we present the first evidence indicating that vesicular volume before secretion is strongly correlated with the characteristics of amperometric foot events. L-3,4-dihydroxyphenylalanine and reserpine have been used to increase and decrease, respectively, the volume of single pheochromocytoma cell vesicles. Amperometry and transmission electron microscopy have been used to determine that as vesicle size is decreased the frequency with which foot events are observed increases, the amount and duration of neurotransmitter released in the foot portion of the event decreases, and vesicles release a greater percentage of their total contents in the foot portion of the event. This previously unidentified correlation provides new insight into how vesicle volume can modulate the activity of the exocytotic fusion pore.

Quantal size and variation determined by vesicle size in normal and mutant Drosophila glutamatergic synapses

Quantal size and variation at chemical synapses could be determined presynaptically by the amount of neurotransmitter released from synaptic vesicles or postsynaptically by the number of receptors available for activation. We investigated these possibilities at Drosophila glutamatergic neuromuscular synapses formed by two separate motor neurons innervating the same muscle cell. At wild-type synapses of the two neurons we found a difference in quantal size corresponding to a difference in mean synaptic vesicle volume. The same finding applied to two mutants (dlg and lap) in which synaptic vesicle size was altered. Quantal variances at wild-type and mutant synapses were similar and could be accounted for by variation in vesicular volume. The linear relationship between quantal size and vesicular volume for several different genotypes indicates that glutamate is regulated homeostatically to the same intravesicular concentration in all cases. Thus functional differences in synaptic strength among glutamatergic neurons of Drosophila result in part from intrinsic differences in vesicle size.

GABA immunoreactivity in the human cerebellar cortex: a light and electron microscopical study

The distribution of gamma-aminobutyric acid (GABA) in surgical samples of human cerebellar cortex was studied by light and electron microscope immunocytochemistry using a polyclonal antibody generated in rabbit against GABA coupled to bovine serum albumin with glutaraldehyde. Observations by light microscopy revealed immunostained neuronal bodies and processes as well as axon terminals in all layers of the cerebellar cortex. Perikarya of stellate, basket and Golgi neurons showed evident GABA immunoreactivity. In contrast, perikarya of Purkinje neurons appeared to be negative or weakly positive. Immunoreactive tracts of longitudinally- or obliquely-sectioned neuronal processes and punctate elements, corresponding to axon terminals or cross-sectioned neuronal processes, showed a layer-specific pattern of distribution and were seen on the surface of neuronal bodies, in the neuropil and at microvessel walls. Electron microscope observations mainly focussed on the analysis of GABA-labelled axon terminals and of their relationships with neurons and microvessels. GABA-labelled terminals contained gold particles associated with pleomorphic vesicles and mitochondria and established symmetric synapses with neuronal bodies and dendrites in all cortex layers. GABA-labelled terminals associated with capillaries were seen to contact the perivascular glial processes, basal lamina and endothelial cells and to establish synapses with subendothelial unlabelled axons.

AMPA receptor subunit expression in trigeminal neurons during postnatal development

Trigeminal motoneurons (Mo5) and mesencephalic trigeminal neurons (Me5) are important constituents of the neural circuitry responsible for jaw movements. Non-N-methyl-D-aspartate (NMDA) glutamate receptors are a critical component of the brainstem circuitry responsible for reflex and centrally activated jaw movements; however, little is known about the expression of these receptors in neonatal oral-motor circuitry. Receptor immunohistochemistry using affinity-purified polyclonal antibodies directed against GluR1, GluR2/3/4c, and GluR4, respectively, and a monoclonal antibody directed against the GluR2 subunit, were used in rats at postnatal day (P)1, P3, P5, P10, P19-21, P32-35, and P60 to describe the expression of the alpha-amino-d-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor in Mo5 and Me5 neurons. In Mo5, immunoreactivity was noted for all antibodies throughout the time frame sampled. Neurons in caudal portions of Me5 displayed immunoreactivity to each antibody except at P60 when GluR2 immunoreactivity was absent. Neurons located in rostral Me5 displayed GluR2/3/4c and GluR4 immunoreactivity throughout the time frame, GluR1 immunoreactivity emerged at P3 and a transient expression of GluR2 expression was observed between P10 and P32-35. The lack of labeling of some neurons in both regions, coupled with differences in temporal expression, suggests that there are differences in the AMPA receptor phenotype within and between Mo5 and Me5 during postnatal development. Differences in AMPA subunit composition suggest a complex role for AMPA-mediated glutamatergic neurotransmission in brainstem circuits controlling jaw movements.

Analysis of multiquantal transmitter release from single cultured cortical neuron terminals

Application of single synapse recording methods indicates that the amplitude of postsynaptic responses of single CNS synapses can vary greatly among repeated stimuli. To determine whether this observation could be attributed to synapses releasing a variable number of transmitter quanta, we assessed the prevalence of multiquantal transmitter release in primary cultures of cortical neurons with the action potential (AP)-dependent presynaptic turnover of the styryl dye FM1-43 (,; ). It was assumed that if a high proportion of vesicles within a terminal were loaded with FM1-43 the amount of dye released per stimulus would be proportional to the number of quanta released and/or the probability of release at a terminal. To rule out differences in the amount of release (between terminals) caused by release probability or incomplete loading of terminals, conditions were chosen to maximize both release probability and terminal loading. Three-dimensional reconstruction of terminals was employed to ensure that bouton fluorescence was accurately measured. Analysis of the relationship between the loading of terminals and release indicated that presumed larger terminals (>FM1-43 uptake) release a greater amount of dye per stimulus than smaller terminals, suggesting multiquantal release. The distribution of release amounts across terminals was significantly skewed toward higher values, with 13-17% of synaptic terminals apparently releasing multiple quanta per AP. In conclusion, our data suggest that most synaptic terminals release a relatively constant amount of transmitter per stimulus; however, a subset of terminals releases amounts of FM1-43 that are greater than that expected from a unimodal release process.

Organization of the zona incerta in the macaque: an electron microscopic study

BACKGROUND

The zona incerta (ZI) receives projections from many telencephalic and brainstem structures. On the basis of its connectivity and physiology, this nucleus has been implicated in the control of saccadic eye movements. Because of the complexity of its afferent signals and its simple efferent signal, there must be much local interaction within the ZI to integrate these various afferents. The purpose of this study was to investigate, at the ultrastructural level, whether the ZI contains the anatomical substrata which could subserve the control of eye movements.

METHODS

Blocks of tissue from the ZI of macaque monkeys were prepared for electron microscopy using standard techniques. Some of these animals were taken specifically for electron microscopy. Others had received injections of tracer substances and were prepared for electron microscopy subsequent to tracer visualization.

RESULTS

Cell bodies of medium-large neurons were found in our preparations. They have large nucleoli and relatively small volumes of karyoplasm. Cell bodies and dendrites of all sizes have many synaptic contacts. Three types of synaptic profiles were found, designated Types 1, 2, and 3. Type 1 profiles are symmetrical and contact cell bodies and small dendrites. Type 2 profiles are thought to be presynaptic dendrites and may have symmetrical or asymmetrical contacts. Type 3 profiles are asymmetrical and primarily contact small dendrites. Many synapses contacted vesicle-containing profiles. In some cases, it was clear that these profiles participated in serial synapses on presumptive presynaptic dendrites. Other profiles appeared to be axoaxonic contacts.

CONCLUSIONS

Afferent and efferent signals are likely to be modulated extensively within the ZI. Therefore, there needs to be complex interactions between neuronal elements of the ZI and its afferents. This study demonstrates that this nucleus possesses the structural substrata to subserve diverse roles, such as the gating of saccadic eye movements.

Ionotropic and metabotropic glutamate receptors show unique postsynaptic, presynaptic, and glial localizations in the dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) is a major brain center for integration of auditory information, and excitatory amino acid neurotransmission plays a central role in the processing of this information. In this study, the distribution of glutamate receptors was examined with preembedding immunocytochemistry, using 14 antibodies to ionotropic (GluR1, GluR2/3, GluR4, GluR5-7, GluR6/7, KA2, NR1, NR2A/B, delta 1/2) and metabotropic (mGluR1 alpha, mGluR2/3, mGluR5) glutamate receptor subtypes. Each of these antibodies produced a specific immunolabeling pattern, including a variety of postsynaptic, presynaptic, and glial localizations. Some antibodies showed widespread distribution patterns, notably the antibodies to the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits, GluR2 and GluR3, and the N-methyl-D-aspartate (NMDA) receptor subunit, NR1. In contrast, antibodies to other glutamate receptor subunits produced more restricted distribution patterns, especially that to GluR1, which stained the outer neuropil of the DCN, cartwheel cells, and a small population of presumptive interneurons associated with the dorsal acoustic stria, but produced little or no staining in fusiform cells or deep DCN neurons. Staining of the postsynaptic density and membrane of the granule cell-parallel fiber/cartwheel cell spins synapse was most prevalent with delta 1/2 and mGluR1 alpha antibodies. A unique pattern of staining was found with mGluR2/3 antibody--with staining concentrated in Golgi cells and unipolar brush cells of the middle to deep DCN. Distribution of some glutamate receptors in the DCN shows similarities to that of the cerebellum, where delta 2 and mGluR1 alpha may modulate neurotransmission at parallel fiber synapses, while mGluR2 and/or mGluR3 may modulate mossy terminal function.

Morphological subclasses of lateral olivocochlear terminals? Ultrastructural analysis of inner spiral bundle in cat and guinea pig

The lateral olivocochlear efferent pathway terminates in vesicle-filled swellings in the inner spiral bundles under inner hair cells (IHCs) and has been suggested to include at least two chemically distinct subclasses (see, e.g., Vetter et al. [1991] Synapse 7:21-43). In the present study, the ultrastructure and peripheral targets of vesicle-filled swellings in the IHC area of the cat and guinea pig cochleas were quantitatively analyzed to determine 1) whether morphological subclasses could be defined based on swelling size or on the density, size or shape of clear and dense-cored vesicles and 2) whether swellings with different postsynaptic targets differed morphologically. In both cat and guinea pig, all swellings contained large, round, clear vesicles and a variable number of dense-core vesicles. Although evidence of clear-cut subclasses was not compelling, the smallest swellings tended to be rich in dense-core and poor in clear vesicles and rarely formed synaptic contacts. Most of the larger swellings, which tended to contain few dense-core vesicles and a rich complement of clear round vesicles, formed synapses with radial afferent fibers. However, there were no morphological differences between swellings contacting afferents originating on the modiolar vs. pillar sides of the IHC (the source of afferents with low and high spontaneous discharge rates, respectively). We conclude that 1) if distinct gamma-amino butyric acid (GABA)ergic and cholinergic subclasses of lateral olivocochlear (LOC) fibers exist, then the vesicle morphology of their terminals does not differ as it does in the central nervous system and that 2) if peptide neurotransmitters, such as calcitonin gene-related peptide and enkephalins, are packaged in dense-core vesicles, then the LOC terminals synapsing with IHC afferent fibers are not particularly rich in these peptides.

An antibody to agmatine localizes the amine in bovine adrenal chromaffin cells

Agmatine, a newly identified amine in mammalian brain, is an endogenous ligand for imidazoline and alpha 2-adrenergic receptors. We sought to develop a polyclonal antibody to agmatine suitable for immunocytochemistry. Agmatine was conjugated to keyhole limpet hemocyanin and injected into rabbits. The polyclonal antiserum so generated dose-dependently recognized the agmatine conjugate but not carrier protein by dot blot. Its reaction with the conjugate was selectively antagonized by agmatine but not related compounds. The antiserum, but not pre-immune or pre-adsorbed antiserum, selectively stained cultured adrenal chromaffin cells. Our results indicate that agmatine immunoreactivity is contained in a sub-population of adrenal chromaffin cells and, thus, these antibodies are useful for immunocytochemical localization of the amine in mammalian tissues.

Glutamate, gamma-aminobutyric acid and tachykinin-immunoreactive synapses in the cat nucleus tractus solitarii

Neurophysiological and pharmacological evidence suggests that glutamate, gamma-aminobutyric acid and tachykinins (substance P and neurokinin A) each have a role in cardiovascular regulation in the nucleus tractus solitarii. This study describes the ultrastructural relationships between nerve terminals immunoreactive for these substances in the nucleus tractus solitarii of the cat using post-embedding immunogold (single and double) labelling techniques on sections of tissue embedded in LR White resin. The technique combines a high specificity of labelling with good ultrastructural and antigenic preservation. Glutamate-immunoreactive terminals, recognized by their high density of gold particle labelling compared to the mean tissue level of labelling, accounted for about 40% of all synaptic terminals in the region of the nucleus tractus solitarii analysed (medial, dorsal, interstitial, gelatinosus and dorsolateral subnuclei). They appeared to comprise several morphological types, but formed mainly asymmetrical synapses, most often with dendrites of varying size, and contained spherical clear vesicles together with fewer dense-cored vesicles. Substance P- and neurokinin A-immunoreactive terminals were fewer in number (9% of all terminals) but similar in appearance, with the immunoreaction restricted to the dense-cored vesicles. Analysis of serial- and double-labelled sections showed a co-existence of substance P and neurokinin A-immunoreactivity in 21% of glutamate-immunoreactive terminals. Immunoreactivity for gamma-aminobutyric acid was found in 33% of all terminals in the nucleus tractus solitarii. These predominantly contained pleomorphic vesicles and formed symmetrical synapses on dendrites and somata. Possible sites of axo-axonic contact by gamma-aminobutyric acid-immunoreactive terminals onto glutamate-or tachykinin-immunoreactive terminals were rare, but examples of adjacent glutamate and gamma-aminobutyric acid-immunoreactive terminals synapsing on the same dendritic profile were frequent. These results provide an anatomical basis for a gamma-aminobutyric acid mediated inhibition of glutamatergic excitatory inputs to the nucleus tractus solitarii at a post-synaptic level.

Glutamate immunoreactivity is enriched over pinealocytes of the gerbil pineal gland

Mammalian pinealocytes have been shown to contain synaptic-like microvesicles with putative secretory functions. As a first step to elucidate the possibility that pinealocyte microvesicles store messenger molecules, such as neuroactive amino acids, we have studied the distributional pattern of glutamate immunoreactivity in the pineal gland of the Mongolian gerbil (Meriones unguiculatus) at both light- and electron-microscopic levels. In semithin sections of plastic-embedded pineals, strong glutamate immunoreactivity could be detected in pinealocytes throughout the pineal gland. The density of glutamate immunolabeling in pinealocytes varied among individual cells and was mostly paralleled by the density of immunostaining for synaptophysin, a major integral membrane protein of synaptic and synaptic-like vesicles. Postembedding immunogold staining of ultrathin pineal sections revealed that gold particles were enriched over pinealocytes. In particular, a high degree of immunoreactivity was associated with accumulations of microvesicles that filled dilated process terminals of pinealocytes. A positive correlation between the number of gold particles and the packing density of microvesicles was found in three out of four process terminals analyzed. However, the level of glutamate immunoreactivity in pinealocyte process endings was lower than in presumed glutamatergic nerve terminals of the cerebellum and posterior pituitary. The present results provide some evidence for a microvesicular compartmentation of glutamate in pinealocytes. Our findings thus lend support to the hypothesis that glutamate serves as an intrapineal signal molecule of physiological relevance to the neuroendocrine functions of the gland.

Amino acid immunocytochemistry of primary afferent terminals in the rat dorsal horn

We combined transganglionic tracing methods with postembedding electron microscopic immunocytochemistry to determine whether identified primary afferent fibers terminating in spinal laminae I-IV may use glutamate and aspartate as neurotransmitters. Sciatic injections of wheat-germ agglutinin conjugated to horseradish peroxidase labeled fine afferent fibers with terminals in laminae I-II of the lumbar spinal cord, whereas injections of the B subunit of cholera toxin conjugated to horseradish peroxidase labeled primary afferent terminals in deeper laminae. Many labeled primary afferent terminals in superficial laminae were involved in glomerular synaptic arrangements; others established nonglomerular contacts. Most glomerular arrangements were clearly immunopositive for glutamate, compared with dendrites, astrocytes, or terminals immunopositive for gamma-aminobutyric acid (GABA). The degree of enrichment varied in labeled terminals of different morphological types. Aspartate was enriched, though to a lesser degree than glutamate, in labeled central terminals of glomeruli in superficial laminae. Labeled primary afferent terminals in laminae III-IV were immunopositive for glutamate, though at lower levels than glomerular terminals in superficial laminae. Aspartate was not enriched in these terminals compared with dendrites, glia, and GABA-positive terminals. These results support a neurotransmitter role for glutamate in primary afferents to the dorsal horn. Quantitative differences in the content of glutamate in identified primary afferent terminals may be related to functional differences. Enrichment of aspartate in terminals in superficial but not deep laminae is compatible with a role for this amino acid in sustained, NMDA-mediated phenomena characteristic of activity in fine caliber afferents.

Light and electron microscopic immunocytochemical localization of AMPA-selective glutamate receptors in the rat spinal cord

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors are probably the most widespread excitatory neurotransmitter receptors of the central nervous system, and they play a role in most normal and pathological neural activities. However, previous detailed studies of AMPA subunit distribution have been limited mainly to the brain. Thus, a comprehensive study of AMPA receptor subunit distribution was carried out on sections of rat spinal cord and dorsal root ganglia, which were immunolabeled with antibodies made against peptides corresponding to C-terminal portions of the AMPA receptor subunits: GluR1, GluR2/3, and GluR4. In the spinal cord, labeling was most prominent in the superficial dorsal horn, motoneurons, and nuclei containing preganglionic autonomic neurons. Immunostaining also was observed in neurons in other regions including those known to contain Renshaw cells and Ia inhibitory cells. Although overall immunostaining was lighter with antibody to GluR1 than with GluR2/3 and 4, there were neurons that preferentially stained with antibody to GluR1. These "GluR1 intense" neurons were usually fusiform and most concentrated in lamina X. In dorsal root ganglia, immunostaining of ganglion cell bodies was moderate to dense with antibody to GluR2/3 and light to moderate with antibody to GluR4. Possible neuroglia in the spinal cord (mainly GluR2/3 and 4) and satellite cells in dorsal root ganglia (GluR4) were immunostained. Electron microscopic studies of the superficial dorsal horn and lateral motor column showed staining that was restricted mainly to postsynaptic densities and associated dendritic and cell body cytoplasm. In dorsal horn, colocalization of dense-cored vesicles with clear, round synaptic vesicles was observed in unstained presynaptic terminals apposed to stained postsynaptic densities. Subsynaptic dense bodies (Taxi-bodies) were associated with some stained postsynaptic densities in both the superficial dorsal horn and lateral motor column. Based on several morphological features including vesicle structure and presence of Taxi-bodies, it is likely that at least some of the postsynaptic staining seen in this study is apposed to glutamatergic input from primary sensory afferent terminals.

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.

Ultrastructural localization of calretinin immunoreactivity in lobule V of the rat cerebellum

Ultrastructural localization of a calcium-binding protein, calretinin, in the cerebellar cortex of the rat was examined with an immunoperoxidase method. The cerebellar lobule V was investigated in detail. In the molecular layer, calretinin immunoreactivity was found in parallel fibers and in their varicosities that formed synapses with unlabeled Purkinje cell dendritic thorns and Golgi cell dendrites. In the granular layer, most granule cells were immunostained for calretinin. Within immunostained granule cells, the labels were found in the cytoplasmic matrix of both the perikaryon and primary dendrite, and in euchromatin patches of the nucleus. Some mossy fiber terminals also stained for calretinin. In the white matter, a few axons were labeled for calretinin and surrounded by unstained myelin sheath. It is suggested that calretinin may play a role at the presynaptic sites of the parallel and mossy fiber terminals.

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.

Light and electron immunocytochemical localization of AMPA-selective glutamate receptors in the rat brain

Since four AMPA-type excitatory amino acid receptor subunits have been cloned recently, it is now possible to localize these important molecules in the nervous system. A comprehensive study of AMPA receptor immunocytochemistry was carried out on vibratome sections of rat brain, which were immunolabeled with antibodies made against peptides corresponding to the C-terminal portions of AMPA-receptor subunits: GluR1, GluR2/3, and GluR4. Labeling was most prominent in forebrain structures such as the olfactory bulb and tubercle, septal nuclei, amygdaloid complex, hippocampus, induseum griseum, habenula, and interpeduncular nucleus, and in the cerebellum. Different patterns of immunolabeling were evident with the antibodies to the four subunits, with marked contrast between densely and lightly stained structures with antibody to GluR1, widespread dense staining with antibody to GluR2/3, and moderate staining with antibody to GluR4. In the parietal cortex, some non-pyramidal neurons were more densely stained than pyramidal cells with antibodies to GluR1. Neurons of the main olfactory bulb, other than granule cells, were most densely stained with antibody to GluR1. In the cerebellum, Bergmann glia were densely stained with antibodies to GluR1 and 4, while neurons, other than granule cells, were most densely stained with antibody to GluR2/3. Immunolabeling patterns of all antibodies were consistent with that of previous in situ hybridization histochemistry studies and with the overall pattern of 3H-AMPA binding. Electron microscopy of thin sections taken from immunolabeled vibratome sections of hippocampus and cerebral cortex showed staining which was restricted mainly to postsynaptic densities and adjacent dendritoplasm, and to neuron cell body cytoplasm. We saw no convincing examples of stained presynaptic terminals, and only limited evidence of glial staining, excepting Bergmann glia.