Purification of specific antibody against aspartate and immunocytochemical localization of aspartergic neurons in the rat brain

Amygdalofugal axon terminals immunoreactive for L-aspartate or L-glutamate in the nucleus accumbens of rats and domestic chickens: a comparative electron microscopic immunocytochemical study combined with anterograde pathway tracing

Several studies have shown that L-aspartate (Asp) is present in synaptic vesicles and released exocytotically from presynaptic terminals, possibly by Ca(2+)-dependent corelease of Asp and L-glutamate (Glu). It has been demonstrated that both excitatory amino acids (EAAs) are released from the rat striatum as part of corticostriatal neurotransmission. The single or colocalized occurrence of Asp and Glu in specific synaptic boutons of the chicken medial striatum/nucl. accumbens has been demonstrated by our group using ultrastructural immunocytochemistry. However, evidence for the presence of EAAs in any specific striatal pathway was only circumstantial. Here, we report on the distribution of Asp and Glu in specific synaptic terminals of the amygdalostriatal pathway, both in rat and chicken brains, combining anterograde tracing with postembedding immunogold labeling of Asp or Glu. Immunoreactivity for Asp and Glu was observed in amygdalofugal terminals with asymmetrical synaptic junctions (morphologically representing excitatory synapses) in both species. The postsynaptic targets were either dendritic spines or small dendrites, whereas axosomatic or axo-axonic connections were not observed. Ultrastructurally, the synaptic terminals immunoreactive for Asp were indistinguishable from those immunoreactive for Glu. The findigs are consistent with an Asp-Glu corelease mechanism, with a distinct synaptic contingent, evolutionarily conserved in the amygdalostriatal pathway.

Immunohistochemical demonstration of increased prostaglandin F₂α levels in the rat hippocampus following kainic acid-induced seizures

Prostaglandin (PG) F(2α) is one of the major prostanoids biosynthesized by cyclooxygenases (COXs) from arachidonic acid. Although it has been reported that there is a selective surge in PGF(2α) production in the hippocampus during kainic acid (KA)-induced seizure activity, the precise intra-hippocampal distribution of PGF(2α) has not been elucidated due to the paucity of effective histological techniques for detecting PGs in tissues. We investigated the tissue distribution of PGF(2α) in the rat hippocampus 30 min after KA injection by developing fixation and immunohistological-staining methods. To detect PGF(2α) directly on histological sections, we used systemic perfusion fixation with water-soluble carbodiimide fixative, followed by immersion of the brains in Zamboni's fixative. We then performed immunofluorescence staining with anti-PGF(2α) antibody, with negative control experiments used to confirm the staining specificity. Definitive immunolabeling for PGF(2α) was evident most markedly in pyramidal cells of the hippocampal cornu Ammonis (CA) 3 sector and neurons of the hilus in KA-treated rats. Immunolabeling for PGF(2α) was also evident in granule cells of the dentate gyrus. Double immunfluorescence staining revealed that PGF(2α)-immunopositive neurons expressed cytosolic phospholipases A(2), COX-2, and FP receptor. These results suggest that the major source of PGF(2α) production immediately after KA injection was neurons of the hippocampal CA3 sector, hilus and dentate gyrus. These neurons exert PGF(2α)-mediated functions via FP receptors in an autocrine/paracrine manner and may play pathophysiological roles in the acute phase (30 min) of excitotoxicity.

Changes in Localization of Amino Acids in the Detached Cat Retina

PURPOSE

To investigate the distribution of amino acids (glutamate, aspartate, glutamine, GABA, glycine) in detached retinas with minimum postmortem artifact and to clarify the relation between amino acid distribution and histopathological change in the outer portion of detached retinas.

METHODS

Unilateral retinal detachment was produced in cats by injecting 0.25% sodium hyaluronate into the subretinal space using a glass micropipet. The eyes were fixed by perfusion for 10 min, 1, 3, 6 and 24 h, 2, 3 and 7 days after detachment and then examined under conventional light- and electron-microscopic immunocytochemistry.

RESULTS

For glutamate, aspartate and glutamine, the inner segments and perikarya of the photoreceptor cells, which were not immunopositive in the normal retinas, showed various degrees of immunoreactivity immediately after retinal detachment. Photoreceptor cells with the strong immunoreactivity developed necrosis. The staining pattern of GABA and glycine scarcely changed during the course of retinal detachment.

CONCLUSIONS

Excess intracellular glutamate, aspartate and glutamine in photoreceptor cells may cause a part of neuronal death after retinal detachment.

Immunohistochemical evidence for the localization of neurons containing the putative transmitter L-proline in rat brain

We examined whether there are the neurotransmitter candidate amino acid L-proline containing neurons localized in the rat brain. Antibodies against L-proline conjugated with rabbit serum albumin were raised in a rabbit and purified with affinity chromatography. Strong L-proline-like immunoreactivity was confined to several groups of neurons in the arcuate nucleus (n) and supraoptic n in the hypothalamus and area postrema. The brainstem had markedly stained fibers in the medial longitudinal fasciculus and localized neuronal cell body labeling in the red n, mesencephalic trigeminal n, lateral reticular n, raphe obscurus n, solitary n, compact ambiguus n, motor trigeminal n and n of trapezoid body. Our findings are consistent with the hypothesis that L-proline may function as a neurotransmitter or neuromodulator in the brain.

Differential distribution of L-aspartate- and L-glutamate-immunoreactive structures in the arcopallium and medial striatum of the domestic chick (Gallus domesticus)

The role of amino acid neurotransmitters in learning and memory is well established. We investigated the putative role of L-aspartate as a neurotransmitter in the arcopallial-medial striatal pathway, which is known to be involved in passive avoidance learning in domestic chicks. Double immunocytochemistry against L-aspartate and L-glutamate was performed at both light and electron microscopic levels. L-aspartate- and L-glutamate-immunoreactive neurons in the arcopallium and posterior amygdaloid pallium were identified and counted by using fluorescence microscopy and confocal laser scanning microscopy. Most labeled neurons of arcopallium were enriched in glutamate as well as aspartate. However, the arcopallium and posterior amygdaloid pallium differed from a neighboring telencephalic region (nidopallium; formerly neostriatum) by containing a substantial proportion of cells singly labeled for L-aspartate (15%, vs. 5.3% in the nidopallium). Aspartate-labeled neurons constitute approximately 20%, 25%, 42%, and 28% of total in the posterior amygdaloid pallium and the medial, dorsal, and anterior arcopallia, respectively. Immunoelectron microscopy showed that L-aspartate was enriched in terminals of the medial striatum. The labeled terminals had clear and round vesicles and asymmetric junctions; similar to those immunoreactive to L-glutamate. Axon terminals singly labeled for L-aspartate made up 17% of the total. In addition, 7% of neuronal perikarya and 26% of all dendritic profiles appeared to be labeled specifically with L-aspartate but not L-glutamate. The results indicate that L-aspartate may play a specific role (as distinct from that of L-glutamate) in the intrinsic and extrinsic circuits instrumental in avian learning and memory.

L-aspartate-immunoreactive neurons in the rat enteric nervous system

L-aspartate (L-Asp) is an excitatory neurotransmitter in the central nervous system. In the present study, we demonstrate, for the first time, the presence of L-Asp in a particular neuronal cell class in the enteric nervous system (ENS). Scattered L-Asp-immunoreactive neuronal cell bodies and nerve fibers were found extensively in both the myenteric and submucosal plexus throughout the small and large intestines. Many L-Asp-immunoreactive nerve fibers, which originated from intrinsic nerve cell bodies, were found in the ganglia and interconnecting nerve bundles. Electron microscopy revealed that L-Asp-immunoreactive terminals frequently formed synaptic contacts with intrinsic nerve cells, suggesting that some L-Asp-immunoreactive neurons might function as interneurons. These results suggest that L-Asp-immunoreactive neurons play a significant role within the ENS to control intestinal functions. The presence of enteric L-Asp-immunoreactive neurons provides strong support for the proposal that L-Asp is a neuromodulator in the rat ENS.

Differential distribution of vesicular glutamate transporters in the rat cerebellar cortex

The chemical organization of excitatory axon terminals in the rat cerebellar cortex was examined by immunocytochemistry and in situ hybridization histochemistry of vesicular glutamate transporters 1 and 2 (VGluT1 and VGluT2). Chemical depletion of the inferior olivary complex neurons by 3-acetylpyridine treatment almost completely removed VGluT2 immunoreactivity from the molecular layer, leaving VGluT1 immunoreactivity apparently intact. On the other hand, neuronal deprivation of the cerebellar cortex by kainic acid injection induced a large loss of VGluT1 immunoreactivity in the molecular layer. In the cerebellar granular layer, both VGluT1 and VGluT2 immunoreactivities were found in mossy fiber terminals, and the two immunoreactivities were mostly colocalized in single-axon terminals. Signals for mRNA encoding VGluT2 were found in the inferior olivary complex, and those for VGluT1 and VGluT2 mRNAs were observed in most brainstem precerebellar nuclei sending mossy fibers, such as the pontine, pontine tegmental reticular, lateral reticular and external cuneate nuclei. These results indicate that climbing and parallel fibers selectively use VGluT2 and VGluT1, respectively, whereas mossy fibers apply both VGluT1 and VGluT2 together to accumulate glutamate into synaptic vesicles. Since climbing-fiber and parallel-fiber terminals are known to make depressing and facilitating synapses, respectively, VGluT1 and VGluT2 might have distinct properties associated with those synaptic characteristics. Thus, it would be the next interesting issue to determine whether mossy-fiber terminals co-expressing VGluT1 and VGluT2 show synaptic facilitation or depression.

Neutral amino acid transporter ASCT1 is preferentially expressed in L-Ser-synthetic/storing glial cells in the mouse brain with transient expression in developing capillaries

Nonessential amino acid L-Ser plays an essential role in neuronal survival and differentiation, through preferential expression of the L-Ser biosynthetic enzyme 3-phosphoglycerate dehydrogenase (3PGDH), in particular in glial cells but not in neurons. To seek the molecular candidates responsible for glia-borne L-Ser transport, we performed histochemical analyses on amino acid transporter ASCT1, which prefers small neutral amino acids, such as Ala, Ser, Cys, and Thr, and mediates their obligatory exchange. At early developmental stages, neuroepithelial cells constituting the ventricular zone expressed ASCT1 mRNA and protein ubiquitously. Thereafter, ASCT1 expression was gradually downregulated in neuronal populations during the late embryonic and neonatal periods, whereas its high expression was transmitted to radial glial cells and then to astrocytes. High levels of ASCT1 were also detected in the olfactory ensheathing glia. The preferential glial expression of ASCT1 was consistent with that of 3PGDH, and their extensive colocalization was demonstrated at the cellular level. Moreover, high cellular contents of L-Ser were revealed in these glial cells by using a specific antibody to L-Ser. These results strongly suggest that a large amount of L-Ser is synthesized and stored in these glial cells and is released through ASCT1 in exchange for other extracellular substrates. In addition, we observed prominent expression of ASCT1 in capillary endothelial cells of embryonic and neonatal brains. Therefore, ASCT1 appears to be regulated to meet metabolic demands by differentiating and mature neurons through the transport of glia- and blood-borne small neutral amino acids.

Immunohistochemical localization of amino acids in the diabetic retina of Goto-Kakizaki rats

The Goto-Kakizaki (GK) rat is a spontaneous model of non-insulin-dependent diabetes mellitus without obesity and diabetic retinopathy. We examined the retinal distribution of L-glutamate, gamma aminobutyric acid (GABA), glycine, and L-aspartate as neurotransmitters in the GK rat retina, using an immunohistochemical method with high-affinity antibodies. The retinal structures in the GK rats were the same as the controls. However, in the GK rats, immunoreactivity of L-glutamate and GABA was observed in the Müller and photoreceptor cells in addition to the immunoreactivity in normal rats. There was no change in glycine distribution between GK rats and controls. In the GK rats, L-aspartate accumulated in the inner segment of the photoreceptor cells in addition to the normal distribution. We consider that these immunoreactivity patterns in the GK rat retina might be induced by ischemia associated with diabetes mellitus.

Immunohistochemical demonstration of L-serine distribution in the rat brain

L-Serine has been suggested by in vitro studies to be an important neurotrophic factor which supports survival and neurite outgrowth of neurons. It is also a precursor of D-serine, a putative neurotransmitter. In the present study, we raised antibodies against L-serine in a rabbit and examined immunohistochemical distribution of the amino acid in the rat brain. In the hippocampus and the cerebellar cortex, where neurotrophic effects of L-serine have been indicated, L-serine immunoreactivity was found primarily in astrocytes. In the brain stem, where neuronal distribution of D-serine was reported, positive staining for L-serine was located primarily in neurons. Regional differences of cellular distribution of L-serine were indicated.

Synaptic excitation in the dorsal nucleus of the lateral lemniscus

The dorsal nucleus of the lateral lemniscus (DNLL) is a distinct auditory neuronal group located ventral to the inferior colliculus (IC). It receives excitatory and inhibitory afferent inputs from various structures of the auditory lower brainstem and sends GABAergic inhibitory efferents mainly to the contralateral DNLL and the bilateral IC. The synaptic excitation in DNLL neurons consists of two components, an early fast depolarization and a later long lasting one. Glutamate is the probable excitatory neurotransmitter for DNLL neurons. alpha-Amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors mediate the early part of the excitation while N-Methyl-D-aspartate (NMDA) receptors mediate the long lasting component. The long lasting NMDA receptor-mediated component in the DNLL may contribute to a prolonged inhibition in the IC. The DNLL is thought to be a structure for processing binaural information. Most DNLL neurons in rat and bat are sensitive to interaural intensity differences (IIDs). They are excited by stimulation of the contralateral ear and inhibited by stimulation of the ipsilateral ear, showing an excitatory/inhibitory (EI) binaural response pattern. The EI pattern can be attributed to synaptic inputs that originate from various structures in the lower auditory brainstem and impinge on the DNLL neurons. In cat some DNLL neurons are sensitive to IIDs and some are sensitive to interaural time differences. In addition, DNLL neurons exhibit different temporal response patterns to contralateral tonal stimulation and respond to amplitude modulated tones, implying that DNLL may contribute to processing temporally complex acoustic information. DNLL neurons shape binaural responses in the contralateral inferior colliculus and auditory cortex through their inhibitory brainstem projections and contribute to the accuracy with which animals localize sounds in space.

Effect of excitatory amino acid neurotransmitters on acid secretion in the rat stomach

Excitatory amino acids (EAAs), in particular, L-aspartate (L-Asp) neurons and their processes, were localized in the rat stomach using a immunohistochemical method with specific antibodies against either L-Asp or its synthesizing enzyme, aspartate aminotransferase (AAT). Myenteric ganglia and nerve bundles in the circular muscle and in the longitudinal muscle were found to be AAT- or L-Asp-positive. In addition, AAT- or L-Asp-positive cells were also found in the muscle layer and the deep mucosal layer. The distribution of AAT- or L-Asp-positive cells in both the mucosal and muscle layers was heterogeneous in the stomach. In addition, L-Asp at 10(-6) M negligibly influenced acid secretion in an everted preparation of isolated rat stomach. However, according to our results, L-Asp markedly inhibited the histamine-stimulated acid secretion, but not the oxotremorine- or the pentagastrin-stimulated acid secretion. Furthermore, L-Asp also inhibited histamine-induced elevation of cAMP. L- Asp itself did not affect the cAMP level although it elevated the cGMP level in the stomach. Moreover, either (+)2-amino-5-phosphonovaleric acid or (+/-)3-(2-carboxypiperazin-4-yl)prophyl-1-phosphonic acid, i.e. two specific antagonists for N-methyl-D-aspartic acid (NMDA) receptors, blocked the inhibitory effect of L-Asp on histamine-stimulated acid secretion or histamine-induced elevation of cAMP. Since cAMP has been strongly implicated as the second messenger involved in histamine-induced acid secretion, we believe that L-Asp regulates acid secretion in the stomach by inhibiting histamine release through the NMDA receptors, subsequently lowering the level of cAMP and ultimately reducing acid secretion.

Glutamate receptors underlying excitatory synaptic transmission in the rat's lateral superior olive studied in vitro

Glutamate receptors underlying synaptic excitation in the rat's lateral superior olive were studied by whole-cell patch clamp recordings in a brain slice preparation. Recordings from two morphological types of cells, bipolar and multipolar, identified by intracellular labeling with biocytin, showed that there were no obvious differences in responses mediated or modulated by ionotropic and metabotropic receptors between these two types of neurons. The excitatory postsynaptic potentials (EPSPs) elicited by ipsilateral stimulation of the trapezoid body consisted of two components. An earlier component, which had faster rise time constant and decay time constant, was mediated by non-NMDA receptors. A later component, which had slower rise time and decay time constants, was mediated by NMDA receptors. Suprathreshold responses (action potentials), which arose from the early component, were always abolished by the non-NMDA antagonist, CNQX, but not by the NMDA antagonist, APV. These results suggest that both non-NMDA and NMDA receptors are present in LSO neurons, and that fast excitatory transmission in LSO is primarily mediated by non-NMDA receptors. The metabotropic glutamate receptor agonists, t-ACPD and L-AP4, reduced the size of EPSPs evoked by stimulation of the ipsilateral trapezoid body in LSO neurons; the reductive action of t-ACPD was reversed by the antagonist, MCPG, indicating that metabotropic glutamate receptors, probably group II and III subtypes, can modulate excitatory synaptic transmission in LSO.

Immunohistochemical localization of arginine and citrulline in rat renal tissue

Using antibodies highly specific for L-arginine and L-citrulline, we localized these amino acids in rat kidney with immunohistochemical methods. Highest levels of arginine immunoreactivity were observed in epithelial cells of proximal tubules in the outer stripe of the outer medulla and the collecting ducts in the cortex. Staining intensity of proximal convoluted tubules in the outer stripe decreased from the inner side to the outer side. In the inner medulla, collecting ducts were labeled with moderate intensity. Staining within the cortex was apparent only with collecting ducts. Citrulline immunoreactivity was localized in the epithelial cells of collecting ducts both in the cortex and medulla. Immunoreactivity was also found in glomerular podocytes and in the epithelial cells of proximal convoluted tubules in the outer medulla. These localizations were different from those of other amino acids previously reported. The precise cellular distribution of arginine and citrulline in rat kidney was determined for the first time by an immunohistochemical method in the present study.

D-aspartate localizations imply neuronal and neuroendocrine roles

Though L-amino acids predominate in living organisms, substantial levels of free D-serine and D-aspartate occur in mammals, especially in nervous and endocrine tissues. Using an antibody specific for glutaraldehyde-fixed D-aspartate, we have localized D-aspartate in rat tissues. In the brain we observe discrete neuronal localizations of D-aspartate, especially in the external plexiform layer of the olfactory bulb, hypothalamic supraoptic and paraventricular nuclei, the medial habenula, and certain brainstem nuclei. In rats 3-4 weeks old, we observe D-aspartate in septal nuclei and in a subset of stellate and basket cells of the cerebellum. D-aspartate is also concentrated in glands, including the epinephrine cells of the adrenal medulla, the posterior pituitary, and the pineal gland. Levels in the pineal gland are the highest of any mammalian tissue. D-aspartate oxidase, visualized by enzyme histochemistry, is concentrated in neurons of the hippocampus, cerebral cortex, and olfactory epithelium, as well as choroid plexus and ependyma. Localizations of D-aspartate oxidase are reciprocal to D-aspartate, suggesting that the enzyme depletes endogenous stores of the amino acid and might inactivate synaptically released D-aspartate.

Distribution of calretinin-immunoreactive septal axons in the normal and deafferented medial habenula of adult rats

To characterize the neural circuitry and plasticity of the septohabenular pathway, the present study analyzes the distribution of calretinin-immunoreactive fibers within the normal and deafferented medial habenula (MHb) at the light and ultrastructural levels. In the adult rat, a dense plexus of calretinin-positive fibers was found throughout the entire MHb neuropil; these immunoreactive terminals formed asymmetric synaptic contacts with unstained dendritic profiles. Calretinin-positive axons that innervate the MHb originated from neurons of the ipsilateral posterior septum, specifically those of the nucleus septofimbrialis and the nucleus triangularis. Unilateral deafferentation of the MHb resulted in the complete loss of calretinin-immunostained fibers within the ipsilateral MHb after 7 days; no reduction was apparent on the contralateral side. Four weeks after unilateral MHb deafferentation, new calretinin-immunoreactive fibers were found confined to the caudal regions of the MHb, these axons again formed asymmetrical contacts with unstained dendritic profiles. No calretinin-positive axons, however, were found within the MHb at 4 weeks following bilateral deafferentation, thus suggesting that the source of these new fibers within the long-term deafferented MHb arises from the contralateral septal neurons. Supporting this idea, injections of biotinylated dextran amine into the 4-week deafferented MHb resulted in retrogradely labeled somata observed in the contralateral posterior septum. These data reveal that septal projections to the MHb, which are normally ipsilateral, respond to a unilateral deafferentation by extending contralateral fibers that cross the midline at the habenular commissure and reinnervate the caudal regions of the nucleus.

Evidence for aspartate-immunoreactive neurons in the neostriatum of the rat: modulation by the mesencephalic dopamine pathway via D1-subtype of receptor

Aspartate-like immunoreactivity was visualized in the neostriatum of rats using indirect immunofluorescence techniques and antibodies raised against aspartate conjugated to keyhole limpet hemocyanine. In normal rats only a few aspartate-positive cell bodies with limited processes were observed. A moderate increase was seen after treatment with (+)methamphetamine and haloperidol. A dramatic increase in the number and fluorescence intensity was observed in the unilaterally 6-hydroxy-dopamine lesioned rats after multiple injections of the D1-dopamine receptor agonist SKF 38393. In these rats strongly fluorescent processes as well as extensive terminal varicose fibre networks were observed. This increase could partly be blocked by the D1-dopamine receptor antagonist SCH 23390. Using a modified technique the aspartate-positive cell bodies and processes were observed even when the antiserum was diluted 1:80,000. Positive cell bodies and fibres were also seen on the ipsilateral side outside the neostriatum, for example in the islet of Calleja and in the piriform cortex. The aspartate-positive cells were negative for dopamine- and cyclic AMP-regulated phosphoprotein-32, a marker for neurons bearing dopamine D1-receptor subtype. A proportion of the aspartate-positive neurons (20%) contained neuropeptide tyrosine-like immunoreactivity. On adjacent sections there was a marked up-regulation of preprodynorphin-like immunoreactivity. The up-regulation of dynorphin and aspartate was only observed when there was an almost complete denervation of the neostriatum as visualized with antiserum to tyrosine hydroxylase, a marker for dopamine fibres. The present results raise the possibility that aspartate may act as a neurotransmitter released from interneurons in the neostriatum.

Glutamate-like immunoreactivity in synaptic terminals of the posterior cingulopontine pathway: a light and electron microscopic study in the rabbit

A postembedding immunoperoxidase method for light microscopy was used to localize glutamate-like immunoreactivity in the rabbit basilar pontine nuclei. Labelled fibre bundles, neuronal cell bodies and numerous puncta of diverse size were heavily glutamate immunoreactive throughout all subdivisions of the pontine nuclei. To determine whether some of the glutamate-immunoreactive puncta were synaptic terminals of posterior cingulate cortical neurons, a double-labelling technique involving an anterograde tract-tracing method and a postembedding immunogold procedure for electron microscopy was used. A quantitative evaluation of gold particle densities revealed that anterogradely labelled cingulopontine synaptic terminals were about twice as immunoreactive as their postsynaptic dendrites, perikaryal and glial profiles and about three times more than symmetric synaptic terminals. The present results indicate that the posterior cingulopontine projection contains high levels of glutamate at its synaptic terminals. This observation provides further support to the role for glutamate as a neurotransmitter in the corticopontine pathway.

Glial uptake of intracerebroventricularly injected D-serine in the rat brain: an immunocytochemical study

Recent studies have shown that free D-serine, an agonist for the N-methyl-D-aspartate receptor, is present in the forebrain of rats. In present study, we raised antibodies against D-serine and examined the distribution of both endogenous and intracerebroventricularly administered D-serine in the rat forebrain by an immunocytochemical method. D-Serine-like immunoreactive cells were found in glial cells in the brains of the rats injected with D-serine into the lateral ventricle. No immunoreactive cells were seen in the brains of untreated rats. The results suggest that glial cells may accumulate and catabolize D-serine to regulate the concentration of this neuroactive amino acid in the forebrain.

Morphine amplifies norepinephrine (NE)-induced LH release but blocks NE-stimulated increases in LHRH mRNA levels: comparison of responses obtained in ovariectomized, estrogen-treated normal and androgen-sterilized rats

In these studies we examined the temporal effects of intracerebroventricular (i.c.v.) infusions of norepinephrine (NE) on plasma LH and on LHRH mRNA levels in the organum vasculosum of the lamina terminalis (OVLT) and in neurons located in the rostral (r), middle (m) and caudal (c) preoptic areas (POA) of ovariectomized, estrogen-treated rats. Thereafter, we compared these responses to those which occur in androgen-sterilized rats (ASR). NE infusions not only increased plasma LH concentrations but within 1 h after NE, LHRH mRNA levels also were increased significantly in the OVLT and rPOA but not in the mPOA or cPOA. By 4 h, these message levels still were elevated in the OVLT and rPOA and they now also were significantly higher than control values in the mPOA and cPOA. While NE also increased LH secretion in ASR, the plasma LH concentrations obtained were markedly blunted compared to control values. Moreover, NE infusions did not alter single cell levels of LHRH mRNA in any region of the rostral hypothalamus. Previously, we have reported that morphine (s.c.) markedly amplifies NE-induced LH release and questioned whether these responses are accompanied by concomitant augmented increases in LHRH mRNA levels. Morphine alone did not affect basal LHRH mRNA or plasma LH levels. However, when rats were pretreated with morphine (-15 min) and NE was infused i.c.v. at 0 time, significant amplification of LH release occurred but, unexpectedly, morphine completely blocked NE-induced increases in LHRH mRNA levels in all of the neurons we examined. Morphine also amplified LH release in ASR but these responses were significantly less than those obtained in control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of periaqueductally injected transmitter antagonists on forebrain-elicited vocalization in the squirrel monkey

In 15 squirrel monkeys, vocalization-eliciting electrodes were implanted into the following forebrain structures: anterior cingulate cortex, genu of the internal capsule, amygdala, bed nucleus of the stria terminalis, hypothalamus, midline thalamus, inferior thalamic peduncle and periventricular grey. Then, injections of 29 transmitter antagonists were made into the midbrain periaqueductal grey (PAG) and their effects tested on the elicitability of vocalization from the forebrain. Vocalization could be blocked completely with glutamate antagonists. NMDA receptor antagonists as well as kainate/quisqualate receptor antagonists were effective. Facilitatory effects, i.e. a decrease in threshold of forebrain-elicited vocalization, was obtained with GABA-A receptor, glycine and opioid antagonists. The facilitatory effect of the opioid antagonist naloxone was limited to vocalizations expressing aversive emotional states. GABA-A receptor antagonists not only facilitated forebrain-induced vocalization but also produced vocalization themselves, i.e. without concomitant forebrain stimulation. No effects were obtained with antagonists of muscarinic and nicotinic receptors, with the GABA-B receptor antagonist phaclofen and antagonists of the monoamines dopamine, noradrenaline, adrenaline, serotonin and histamine. It is concluded that the PAG represents a crucial relay station of the vocalization-controlling system. In this station, transmission of vocalization-relevant information depends upon the activation of glutamatergic synapses. Inhibitory control is exerted by GABA, glycine and endogenous opioids. Acetylcholine, dopamine, noradrenaline, adrenaline, serotonin and histamine may play a transient modulatory role; forebrain-induced vocalization, however, does not depend upon the cholinergic or monoaminergic activation of PAG neurons.

Ankyrin immunoreactivity in adult rat cerebellum

An antibody recognizing all ankyrin isoforms was employed to localize ankyrin in the cerebellum by light and electron microscopy. White matter (myelinated fiber tracts) did not contain ankyrin. Granule cell bodies and axons contained ankyrin which extended into the parallel fibers and their synapses. Purkinje cells contained a nearly uniform plasma membrane undercoating of ankyrin in the cell body. However, in both of these cell types ankyrin was not detected in dendrites. Basket and stellate cells did not contain detectable ankyrin. Golgi neuroepithelial cell bodies and processes contained ankyrin while myelin and myelinated axons did not. These results indicate that the membrane skeletal protein ankyrin occurs in some, but not all, neuronal and glial cell types in the cerebellum. For the neurons in which it does occur its distribution is polarized, being limited to cell bodies and axons while not occurring in dendrites.

Conceptual history of the nigrostriatal dopamine system

The history of the nigrostriatal dopamine system may provide a prime example of the two faces of scientific development. First, a given concept is replaced by another simply as a result of methodologies being improved, and second, successive technical improvements make seemingly settled controversies even more complicated and disputable. The nigrostriatal pathway, which had been unrecognizable with Nauta's silver impregnation method, became apparent by use of the more sensitive silver impregnation method of Fink-Heimer. The sensitivity of the latter method, however, was still insufficient to reveal the whole extent of another ascending dopamine system, the mesocortical dopamine system, until its existence was established through the application of glyoxylic acid fluorescent histochemistry. Electron microscopic analysis of nigrostriatal dopamine synapses in properly fixed tissue was initiated by the demonstration of dark type terminal degeneration, which was induced by either electrolytic lesions or chemical destruction with a specific toxin (6-hydroxydopamine) of the substantia nigra and medial forebrain bundle. The degenerating terminal boutons, thus produced, invariably formed postsynaptic membrane specializations of asymmetric type. However, the asymmetric nature of the synaptic morphology, although later confirmed by the combined study of chemical lesions and autoradiographic anterograde tracing, was seriously challenged with the introduction of electron microscopic immunohistochemistry. The latter method has consistently revealed that symmetric en passant synapses or axonal varicosities with no synaptic membrane specializations are the only tissue compartments immunoreactive to antibodies against dopamine and its synthetic enzyme tyrosine hydroxylase. In view of the fact that more than 95% of the nigrostriatal projection neurons are dopaminergic, it is difficult to satisfactorily interpret all the available and seemingly paradoxical fine structural data. In this context, a novel concept has emerged in the process of eliminating all the possible alternative interpretations. The concept is that single nigrostriatal neurons form two chemically distinct types of synapses, one dopaminergic symmetric en passant bouton and another non-dopaminergic (still chemically unclassified) asymmetric terminal bouton. If the concept is a valid one, it contradicts Dale's long standing principle, as defined by Eccles: at all the axonal branches of a neuron there is liberation of the same transmitter substance or substances. Furthermore, a certain population of substantia nigra pars reticulata neurons has recently been recognized to be immunoreactive to both dopamine synthetic tyrosine hydroxylase and GABA synthetic glutamate decarboxylase. These single neurons send projections to both the striatum and superior colliculus by way of axon collaterals.(ABSTRACT TRUNCATED AT 400 WORDS)

N-methyl-D,L-aspartic acid differentially affects LH release and LHRH mRNA levels in estrogen-treated ovariectomized control and androgen-sterilized rats

Excitatory amino acids such as N-methyl-D,L-aspartic acid (NMDA) are thought to play an important role in the regulation of gonadotropin secretion. NMDA induces significant increases in plasma LH in a variety of animal models and these effects occur by activation of neural processes involved in excitation of LHRH neurons rather than by a direct action on the pituitary gland. We have taken advantage of this information to study the effects of NMDA on LH release and on changes in levels of LHRH mRNA in single neurons of adult rats treated neonatally with a high dosage of androgen. While iv NMDA evoked an increase in plasma LH in estrogen-treated ovariectomized control and androgen-sterilized rats (ASR), significantly less LH was released in ASR. LHRH mRNA levels in the organum vasculosum of the lamina terminalis (OVLT), the rostral (r), media (m) and caudal (c) preoptic (POA) regions were quantitated using in situ hybridization histochemistry and quantitative image analysis methods. LHRH mRNA levels in untreated controls and ASR did not differ in any of the brain regions examined. Within 1 h after NMDA, LHRH mRNA had increased significantly in OVLT and rPOA but not in mPOA and cPOA neurons of control rats and these mRNA levels remained elevated for 4 h. In contrast, NMDA treatment of ASR did not affect basal levels of LHRH mRNA in any region of the rostral hypothalamus. These observations suggest that neonatal androgen treatment of female rats either directly or indirectly affects the responsiveness of LHRH neurons to NMDA.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory amino acids as neurotransmitters of cortical and cerebellar projections to the red nucleus: an immunocytochemical study in the guinea pig

We combined a retrograde labeling technique with peroxidase immunocytochemistry to verify whether cortical and cerebellar neurons projecting to the red nucleus (RN) contain high concentrations of glutamate and aspartate as possible neurotransmitters. Injections of a tracer, colloidal gold-labeled enzymatically inactive horseradish peroxidase conjugated to wheatgerm agglutinin, into the RN of adult guinea pigs produced retrograde labeling of layer V cortical neurons, with a large predominance in the ipsilateral hemisphere. Corticorubral neurons were located in the granular parietal cortex (Gr), agranular frontal cortex (Ag), agranular cingulate cortex (Cg), and retrobulbar cortex (Rb). Large numbers of retrogradely labeled neurons were concentrated in contralateral interpositus and dentate cerebellar nuclei. We found the majority of corticorubral neurons to be immunostained by antibodies raised in rabbits against glutamate or aspartate conjugated to invertebrate hemocyanin by glutaraldehyde, supporting the hypothesis that excitatory amino acids are neurotransmitters of corticorubral projections. With either antiserum, immunostaining was found in 58-72% of corticorubral neurons in Ag and Gr; higher percentages were observed in Rb (80-85%) and Cg (up to 96%). Cross-sectional area measurements indicated that the perikarya of corticorubral neurons were larger in Ag and Gr than in Rb and Cg; in each area, soma size values of immunopositive corticorubral neurons tended to be larger than those of immunonegative ones. In the cerebellar nuclei, virtually all retrogradely labeled neurons were immunostained by glutamate and aspartate antisera, suggesting that excitatory amino acids might also be considered as possible neurotransmitters for cerebellorubral projections.

Effects of chemical stimulation in the periaqueductal gray on vocalization in the squirrel monkey

Twenty-nine agonists and 32 antagonists of more than 10 transmitters known to be present in the periaqueductal gray (PAG) have been injected into the squirrel monkey's PAG in order to test their effects on spontaneous vocalization at sites yielding vocalization with electrical stimulation. Vocalization could be elicited with the glutamate agonists sodium-L-glutamate, L-aspartic acid, L-homocysteic acid, N-methyl-D-aspartic acid, quisqualic acid, and kainic acid, the cholinergic agonists acetylcholine, carbachol, and muscarine, the monoaminergic agonist histamine, and the GABA antagonists bicuculline and picrotoxin. No vocalizations could be obtained with agonists of dopamine, noradrenaline, adrenaline, serotonin, GABA, glycine, nicotinic receptors, and endogenous opioids, as well as with antagonists of glutamate, acetylcholine, dopamine, noradrenaline, adrenaline, serotonin, histamine, glycine, GABA-B, delta- and mu-receptors. Blocking of spontaneous vocalization was obtained with the nonspecific glutamate antagonist kynurenic acid and the GABA-A receptor agonist muscimol. The results indicate that the production of vocalization depends upon the activation of glutamatergic synapses in the PAG. GABAergic afferents seem to have a tonic inhibitory control on the periaqueductal vocalization mechanism, while acetylcholine and histamine seem to exert only a transient modulatory control.

Biochemical and immunohistochemical demonstration of a tightly bound form of prostaglandin E2 in the rat brain

Basal levels of prostaglandin E2 in the rat brain were determined by radioimmunoassay to be 0.68-0.79 pmol/g brain. About one-third of the prostaglandin E2 (0.23-0.28 pmol/g) was resistant to extraction with ethanol, but could be recovered with a mixture of ethanol and 1 N HCl (9:1, v/v), indicating that a tightly bound form of prostaglandin E2 exists in the brain. The amount of the bound form of prostaglandin E2 was almost unchanged by pentylenetetrazole-induced convulsion or by transcardial perfusion with a formaldehyde solution, although these treatments resulted in 40- to 80-fold increases in prostaglandin E2 content extracted with ethanol at neutral pH. A polyclonal antibody against prostaglandin E2-albumin conjugates recognized the bound form of prostaglandin E2, giving a punctate appearance in many neuronal cell bodies in the brain. Although almost all of the neuronal perikarya were immunoreactive for prostaglandin E2, intense immunoreactivity was observed in the mitral cell layer of the olfactory bulb, layer V of the cerebral neocortex, anterodorsal and reticular nuclei of the thalamus, supraoptic, paraventricular, accessory neurosecretory and lateral mammaillary nuclei of the hypothalamus, mesencephalic trigeminal nucleus, nucleus of the trapezoid body and deep cerebellar nuclei. When the cerebral neocortical regions were observed electron microscopically, immunoreaction products were seen as fine granules which were clustered into small patches in the cytoplasm of neuronal cell bodies and proximal dendrites. No immunoreaction products were seen in glial cells or endothelial cells. These results suggest that prostaglandin E2 is involved in fundamental processes of neurons.

Cerebellar nuclei and the nucleocortical projections in the rat: Retrograde tracing coupled to GABA and glutamate immunohistochemistry

The amino acids GABA and glutamate (Glu) are thought to be the principal substances in the central nervous system responsible for neuronal inhibition and excitation. Their distributions among the different neurons in a defined pathway may thus be indicative of the contributions of the cells to pathway function. Examples of such neurons are those of the cerebellar nuclei which, while regulating output from the Purkinje cells of the cerebellar cortex, are also found to project back to the cerebellar cortex. Immunohistochemical experiments were done to identify GABA and glutamate (Glu) containing cells in the adult rat cerebellar nuclei. Consecutive semithin and serial vibratome sections were incubated with antisera raised in rabbit against GABA and Glu. In semithin sections, only small neurons were intensely GABA immunoreactive (GABA-IR) (31.7%), and the majority (80.5%) were Glu immunoreactive (Glu-IR) of different sizes. Consistent with Glu being a metabolic precursor for GABA, 75.4% of the GABA-IR population colocalized Glu. In vibratome sections GABA-IR neurons showed some local differences in number, whereas the Glu-IR were uniformly distributed in the three nuclei studied. Measured mean diameters for these neurons showed a distinct size difference for the GABA- and Glu-IR with little overlap. Cerebellar nuclei neurons projecting to the cortex (nucleocortical neurons, NCN) were identified by locally preinjecting the retrograde transported WGA-apoHRP-colloidal gold complex in the cerebellar cortex. Vibratome sections of these cerebellar were silver intensified for the retrograde tracer and double labeled for GABA and Glu. Of the total number of identified NCN, 8.7% were GABA-IR (10 animals) and 47.7% Glu-IR (5 animals). Many retrograde labeled NCN in the core of the thick sections were immunonegative for both amino acids due to poor antibody penetration, thus underestimating the proportions of cells containing GABA and Glu. The size distributions for the GABA-IR and Glu-IR NCN were similar to those measured in non-retrograde labeled nuclei in thick sections. The conclusions reached are that GABA-IR neurons of the cerebellar nuclei, including the NCN, use GABA as the presumed inhibitory neurotransmitter and that Glu-IR neurons may use Glu or another excitatory neurotransmitter.

An efficient enzyme immunoassay for glutamate using glutaraldehyde coupling of the hapten to microtiter plates

In order to coat microtiter plates for enzyme immunoassays (EIAs), amino acids and other haptens are usually coupled to larger protein molecules. The formation of such conjugates is not always reproducible. This may lead to inconsistent hapten-protein stoichiometries, unfavorable orientation of the hapten on the protein and/or well-to-well variation in the concentration of the available hapten. In the assay described here the excitatory amino acid (EAA) Glu is coupled directly to polystyrene microtiter wells with GA. Each step of the assay was tested for maximum efficiency. The resulting EIA with Glu as a competitor gave excellent reproducibility (coefficient of variation = 5.87%), an EC50 of 2.02 X 10(-5) M and a detection limit of 1.26 X 10(-6) M. This EIA method is generally useful for a variety of antisera to amino acids and small peptides and a wide range of competing substances. It can be used to characterize the conformational requirements for antigen binding, to assay for glutamate or to identify compounds with glutamate-like structure in unknown solutions.

Predominant localization in glial cells of free L-arginine. Immunocytochemical evidence

Nitric oxide has been recently identified as an endogenous activator of the soluble guanylate cyclase in the brain as well as in vascular endothelial cells and macrophages. In the present study, we determined the localization of free arginine in the brain because nitric oxide was formed from the terminal guanido group of L-arginine. Anti-arginine antiserum was raised in guinea pigs by repeated injection of L-arginine covalently conjugated to guinea pig serum albumin via glutaraldehyde. Specific anti-arginine antibody was purified from the antiserum by using an affinity gel coupled with L-arginine. Arginine-like immunoreactivity in the rat brain and spinal cord was found concentrated mainly in astrocytes including Bergmann glial cells in the cerebellum and processes of astrocytes around blood vessels. The present results suggest that glial cells, particularly astrocytes, are the main locus of L-arginine, a nitric oxide precursor, in the brain.

Aspartate-like immunoreactivity in vestibulospinal neurons in gerbils

In an effort to further characterize vestibulospinal pathways in the gerbil, immunocytochemistry was combined with retrograde identification of neurons. Vestibulospinal neurons were retrogradely labeled following injections of horseradish peroxidase into the cervical cord of anesthetized gerbils. Sections were reacted with nickel acetate-diaminobenzidine for horseradish peroxidase, giving a black reaction product. Sections were incubated in polyclonal antisera to aspartate, incubated in an avidin-biotin-peroxidase procedure, and reacted to give a brown reaction product. Alternatively, fluoro-gold was used as a retrograde tracer and aspartate-like immunoreactivity was demonstrated with avidin conjugated to Texas red. Cells stained with aspartate-like immunoreactivity, were located in all vestibular nuclei. Double-labeled cells were located in the medial nucleus and in the lateral vestibular nucleus where many of the large cells were double labeled.

Immunocytochemical localization of N-acetyl-aspartate with monoclonal antibodies

N-Acetyl-aspartate is found in high concentrations in all areas of the brain, but is undetectable in non-neuronal tissue. In order to characterize the cellular localization of N-acetyl-aspartate in brain, highly specific monoclonal antibodies against N-acetyl-aspartate were produced by fusing spleen lymphocytes obtained from mice immunized with N-acetyl-aspartate conjugated to thyroglobulin by carbodiimide with P3/x63-Ag8.653 mouse myeloma cells. Clones were selected which secrete IgG2a(k) antibodies highly specific for conjugated N-acetyl-aspartate. Only 3-6% cross-reactivity with conjugated N-acetyl-aspartate-glutamate was observed at high antibody concentrations, whereas no cross-reactivity (less than 1%) was observed with conjugated N-acetyl-glutamate or aspartate. Preincubation of the antibodies with 0.5 mg/ml conjugated N-acetyl-aspartate blocked immunoreactivity more than 90%, while preincubation with conjugated N-acetyl-aspartate-glutamate and free N-acetyl-aspartate had no effect. Immunocytochemical staining has shown that N-acetyl-aspartate-like immunoreactivity is localized in neurons, which are widely distributed throughout the brain. The immunoreactive neurons exhibited intense staining of the perikarya, proximal dendrites and axons. No consistent pattern of distribution of immunoreactivity was observed with regard to primary neurotransmitter characteristics of stained neurons although neurons with long projections or extensive arbors, such as pyramidal cells in cortex, locus coeruleus, motor neurons and Purkinje cells, stained much more intensively than local circuit neurons.

Excitatory amino acid transmitters and their receptors in neural circuits of the cerebral neocortex

In 1954, L-glutamate (Glu) and L-aspartate (Asp) were first suggested as being excitatory synaptic transmitters in the cerebral cortex. Since then, evidence has mounted steadily in favor of the view that Glu and Asp are major excitatory transmitters in the neocortex. Many of the experimental studies which reported how Glu/Asp came to satisfy the criteria for transmitters in the neocortex are reviewed here, according to the methods employed. Since the question of which particular synaptic sites in cortical neural circuits Glu/Asp operate as excitatory transmitters has not previously been reviewed, particular attention is given to efferent, afferent and intrinsic neural circuits of the visual and somatosensory cortices, where circuitry is relatively clearly delineated. Recent studies using chemical assays of released amino acids, high-affinity uptake mechanisms of Glu/Asp from nerve terminals, the direct micro-iontophoretic administration of Glu/Asp antagonists, and immunocytochemical techniques have demonstrated that almost all corticofugal efferent projections employ Glu/Asp as excitatory synaptic transmitters. Evidence indicating that thalamocortical afferent projections, including geniculocortical projections and some intrinsic connections are glutamatergic, is also reviewed. Thus, the results highlighted here indicate that the main framework of neocortical circuitry is operated by Glu/Asp. Pharmacological studies indicate that synaptic receptors for Glu/Asp can be classified into a few subtypes, including N-methyl-D-aspartate (NMDA) and quisqualate/kainate (non-NMDA) types. Some evidence indicating the sites of operation of NMDA and non-NMDA receptors in neocortical circuitry is reviewed, and the distinct, functional significance of these two types of Glu/Asp receptors in information processing in the neocortex is proposed.

Immunolocalization of cytosolic aspartate aminotransferase (cAAT) in axon terminals that form synapses in the rat cerebellar cortex. A study at the electron microscopic level

This study was conducted to determine the ultrastructural localization of cytosolic aspartate aminotransferase (cAAT)-like immunoreactivity in the cerebellar cortex in the rat. The isoenzyme was found both in excitatory and inhibitory axon terminals, but not in the climbing fibers of the molecular layer. These findings suggest that cAAT may have a different role in the excitatory and inhibitory synapses, and that climbing fibers of the molecular layer do not appear to use aspartate as neurotransmitter.

Effect of climbing fiber deprivation on release of endogenous aspartate, glutamate, and homocysteate in slices of rat cerebellar hemispheres and vermis

Aspartate (Asp) and/or glutamate (Glu) have been proposed as putative excitatory transmitters released from synaptic terminals of the olivo-cerebellar climbing fiber afferents to the Purkinje cells. Investigations of the climbing fiber transmitter(s) separately for hemispheres and vermis were performed to examine whether the current controversy over the role of Asp as a neurotransmitter in the climbing fibers may be due to topographic differences. K(+)-induced Ca2(+)-dependent release of endogenous substances was investigated in slices of cerebellar hemisphere and vermis of control rats and those deprived of climbing fibers by 3-acetylpyridine (3-AP) treatment. A release of Asp and Glu, as well as a small but significant release of homocysteic acid (HCA) was confirmed in control rats. Climbing fiber deprivation by 3-AP treatment reduced the stimulated release of Asp by 48% in slices of cerebellar hemispheres, but not in vermis. Climbing fiber deprivation completely abolished the release of HCA in both hemispheres and vermis. The release of HCA, Asp, and Glu from slices of control and climbing fiber-deprived rats evoked by 50 mM K+ was greater than 90% Ca2(+)-dependent. These results support the hypothesis that Asp is a transmitter candidate of the climbing fibers projecting to the cerebellar hemispheres, but not to the vermis, and provide the first evidence that HCA can be linked to a specific pathway.

Aspartate-like and glutamate-like immunoreactivities in the inferior olive and climbing fibre system: a light microscopic and semiquantitative electron microscopic study in rat and baboon (Papio anubis)

A post-embedding immunogold procedure was used to analyse, in a semiquantitative manner, the distributions of aspartate-like and glutamate-like immunoreactivities in the inferior olive and climbing fibre system in rats and baboons. The neurons in the inferior olive were uniformly labelled for aspartate as well as glutamate, indicating a 100% co-localization of these two amino acids in the cell bodies. The level of glutamate-like immunoreactivity in the climbing fibre terminals was similar to that in the parent cell bodies, as judged by a computer-assisted calculation of gold particle densities. In contrast, the level of aspartate-like immunoreactivity in the climbing fibre terminals was only one-seventh of that of the olivary neurons. No differences were found between the hemispheres and vermis. Nerve terminals in the inferior olive were generally moderately labelled with the aspartate antiserum, as were cell bodies of astrocytes. With a few exceptions, the results obtained in baboons were similar to those in rats. Notably, no evidence was found of an enrichment of aspartate-like immunoreactivity in climbing fibres. The present results do not support previous data suggesting that aspartate is the transmitter of the climbing fibres but indicate that glutamate or another excitatory compound should be considered as candidate for this role. Our findings show that the presence of aspartate-like immunoreactivity in cell bodies is an unreliable indicator of transmitter identity.

Synaptic organization of septal projections in the rat medial habenula: a wheat germ agglutinin-horseradish peroxidase and immunohistochemical study

The synaptic organization of septal inputs to the rat habenular complex of the dorsal diencephalon was examined employing the anterograde tracer wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). The cellular distribution of substance P (SP) and choline acetyltransferase (ChAT) immunoreactivity was also studied at the light and electron microscopic level. Following placements of tracer within the entire septum, labeled axons were observed in the stria medullaris and in the medial and lateral subnuclei of the habenula. Following injections of tracer in the nuclei triangularis and septofimbrialis of the posterior septum, the medial subnucleus was heavily labeled, whereas the lateral subnucleus was devoid of peroxidase activity. The medial subnucleus possessed labeled myelinated axons and terminals that contained clear, spherical vesicles and formed asymmetric contacts with dendritic spines and shafts. Terminals possessing WGA-HRP activity also formed non-synaptic junctions with other labeled or unlabeled terminals. SP and ChAT immunoreactivity in normal and colchicine-treated animals was confined to dendrites and somata within the medial habenula. Terminals containing clear spherical vesicles formed asymmetric synaptic contacts with these immunoreactive somatic and dendritic profiles. Based on the combined anterograde tracing and immunohistochemical data, it is proposed that septal projections provide a direct innervation to habenular neurons that contain ChAT or SP activity. These septal inputs may play an important role in the facilitation of the ChAT- and SP-positive habenular neurons, both of which provide prominent afferent inputs to the interpeduncular nucleus. Thus, neurons of the habenula and interpeduncular nucleus are under the direct and indirect influence of septal neurons within the limbic forebrain circuit.

Excitatory transmitter amino acid-containing neurons in the rat visual cortex: a light and electron microscopic immunocytochemical study

The distribution and morphology of neurons labelled with antisera to glutamate or aspartate were examined, at the light and electron microscope levels, in the rat visual cortex. Using widely accepted light microscopic features as well as well-established nuclear, cytoplasmic, and synaptic criteria, we noted that glutamate-immunoreactive neurons were pyramidal cells distributed in layers II-VI, with an increased concentration in layers II and III. Aspartate immunoreactivity was localized chiefly to pyramidal neurons in layers II-VI. However, approximately 10% of immunolabeled cells were nonpyramidal neurons scattered throughout the cortex. Cell-body measurements revealed that, for both groups of neurons, layer V contained the largest labelled neurons, whereas layers IV and VI contained the smallest. Furthermore, in every layer, aspartate-stained neurons were larger than glutamate-positive cells. Finally, glutamate- and aspartate-labelled axon terminals formed asymmetrical synapses, which are presumably excitatory in nature, primarily with dendritic spines. These findings, together with recent detailed studies of the projections of glutamate- and aspartate-labelled cortical neurons, may provide essential background information for studies aimed to elucidate the function(s) of excitatory amino acids in the cortex and their role in pathological conditions.

Glutamate-positive neurons and axon terminals in cat sensory cortex: a correlative light and electron microscopic study

Immunocytochemical methods were used to perform a correlative light and electron microscopic study of neurons and axon terminals immunoreactive to the antiglutamate (Glu) serum of Hepler et al. ('88) in the visual and somatic sensory areas of cats. At the light microscopic level, numerous Glu-positive neurons were found in all layers except layer I of both cortical areas. On the basis of the dendritic staining of Glu-positive cells, two major morphological categories were found: pyramidal cells, which were the most frequent type of immunostained neuron, and multipolar neurons, which were more numerous in layer IV of area 17 than in any other layer. A large number of Glu-positive neurons, however, did not display dendritic labelling and were considered unidentified neurons. Counts of labelled neurons were performed in the striate cortex; approximately 40% were Glu-positive. Numerous lightly stained punctate structures were observed in all cortical layers: the majority of these Glu-positive puncta were in the neuropil. After resectioning the plastic sections for electron microscopy it was observed that: 1) the majority of neurons unidentifiable at light microscopic level were indeed pyramidal neurons except in layer IV of area 17, where many stained cells were probably spiny stellate neurons. Some Glu-positive neurons, however, exhibited clear ultrastructural features of nonspiny nonpyramidal cells; 2) all synaptic contacts made by Glu-positive axon terminals were of the asymmetric type, but not all asymmetric synaptic contacts were labelled. The vast majority of postsynaptic targets of Glu-positive axons were unlabelled dendritic spines and shafts. The present results provide further evidence that Glu (or a closely related compound) is probably the neurotransmitter of numerous excitatory neurons in the neocortex.

Non-N-methyl-D-aspartate receptors may mediate ipsilateral excitation at lateral superior olivary synapses

Principal cells of the lateral superior olivary nucleus (LSO) are thought to receive a direct excitatory input from spherical bushy cells located in the ipsilateral ventral cochlear nucleus (VCN) and an indirect input from the contralateral VCN globular bushy cells via a secure synapse in the medial nucleus of the trapezoid body (MNTB). MNTB bushy cells project to the somata and proximal dendrites of LSO principal cells. LSO neurons display phasic 'chopper' temporal response patterns to ipsilateral tone-burst stimuli at characteristic frequency (CF), while binaural stimuli suppress this ipsilaterally evoked activity. This suppression is sensitive to interaural differences in intensity, phase and time, suggesting a role for these neurons in the localization of sound in space. In the present study, the nature of the neurotransmitter mediating fast ipsilateral excitation of LSO neurons was examined using iontophoretic application of excitant amino acid (EAA) agonists and antagonists. N-methyl-D-aspartate (NMDA) and quisqualate (QUIS) were used as agonists, while the selective NMDA receptor antagonist D. L-2-amino-5-phosphonovaleric acid (APV), and the non-selective receptor EAA antagonist cis-2,3-piperidine-dicarboxylic acid (PDA) were used to study ipsilaterally evoked neuronal responses. In 3 additional experiments the selective non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) replaced PDA. Ipsilateral, tone-evoked and spontaneous activities were generally enhanced by EAA agonists while partial blockade of tone-evoked, ipsilateral excitation was observed with EAA antagonists. Both PDA and DNQX more effectively blocked ipsilateral tone-evoked excitations and spontaneous activity than did the NMDA-receptor antagonist, APV.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA and glycine immunoreactivity in the guinea pig superior olivary complex

Immunoperoxidase immunocytochemistry was employed to examine the distribution of gamma-aminobutyric acid (GABA)-and glycine (GLY)-immunoreactive cells, fibers, and terminals in the guinea pig superior olivary complex. The nuclei studied were the lateral superior olive (LSO), medial superior olive (MSO), superior paraolivary nucleus (SPN), and the medial, ventral, and lateral nuclei of the trapezoid body (MNTB, VNTB, and LNTB, respectively). The majority of LSO neurons exhibited GABA-immunoreactive (+) labeling. These same neurons were also lightly GLY+. Extensive perisomatic punctate GLY + labeling was observed on most LSO neurons; these puncta most likely correspond to synaptic terminals. A very small number of MSO fusiform neurons were GABA +, and none were GLY +. The GLY positive perisomatic punctate labeling around most MSO neurons, although abundant, was not as profuse as that observed in the LSO. The MNTB neurons corresponding to the principal and elongate types were intensely GLY + and were contacted by small numbers of GLY + puncta. There was extensive GLY + punctate labeling in the SPN that surrounded the cell bodies of most of its large, radiate neurons and many of the smaller, fusiform neurons. The few large, radiate neurons that were lightly GLY + possessed far fewer GLY + puncta on their perikarya. The distribution of GABA + puncta was generally diffuse and scattered throughout the nuclei described above. In the VNTB and LNTB, several large neurons of various shapes were GLY + as were the small, oval neurons. The extent of GLY + punctate labeling was quite variable in both nuclei. The majority of perikarya in the VNTB and LNTB were GABA +. A light distribution of GABA + puncta was observed on most cell bodies in both nuclei. Peridendritic GABA + punctate labeling was dense in the VNTB neuropil. Two small populations of GLY + neurons were observed outside of the named nuclei of the SOC; one was located dorsal to the LSO, near its dorsal hilus, and the other was identified near the medial pole of the LSO. The somata of both populations possessed extremely sparse GLY + punctate labeling. In general, these results agree with and expand on findings in rodents from previous studies. There appears, however, to be differences between the guinea pig and cat with regard to the proportions of GABA + neurons in the LSO and GLY + punctate labeling in the MSO.

GABA, glycine, glutamate, aspartate and taurine in the perihypoglossal nuclei: an immunocytochemical investigation in the cat with particular reference to the issue of amino acid colocalization

The differential distribution of glutamate (Glu), aspartate (Asp), glycine (Gly), gamma-aminobutyric acid (GABA) and taurine (Tau) was investigated in the cat's perihypoglossal nuclei. Serial semi-thin (0.5 micron) sections through the perihypoglossal nuclei were incubated with antisera raised against the mentioned amino acids with the aim of studying possible co-localization. In each experiment different measures were undertaken in order to screen for possible cross-reactivities, and all sections were processed together with test conjugates in order to ascertain the specificity of the antisera used. A very high proportion of the neurons in the perihypoglossal nuclei (about 90%) shows strong immunostaining for Asp and also displays distinct immunoreactivity for Glu in neighbouring sections. About 25% of the cells in the perihypoglossal nuclei are intensely immunostained for Gly, but very few cells show immunoreactivity for GABA. Only glial cells appear to be immunostained for Tau. Neurons that are Gly(+) also display Glu and Asp immunoreactivities. The neuropil of the perihypoglossal nuclei shows a high density of GABA(+), Gly(+) and Glu(+) puncta mainly representing stained axons and terminals. Fewer Asp(+) puncta and very few Tau(+) nerve terminal-like puncta are seen. Details of the regional distribution of immunopositive neurons and puncta within the perihypoglossal nuclei are described. The findings are discussed with particular reference to the possible role of the mentioned amino acids as transmitter substances in the known synaptic circuitry of the perihypoglossal nuclei.

Glutamate and aspartate immunoreactivity in corticospinal neurons of rats

A combination of retrograde tracers and immunostaining was employed to test whether corticospinal tract (CST) neurons in rats may use amino acid excitatory neurotransmitters. CST neurons were identified following injections of either Diamidino Yellow (DY) or colloidal gold-labeled enzymatically inactive horseradish peroxidase conjugated to wheat germ agglutinin (WGAapoHRP-Au) in the spinal cord. As retrograde tracers, the two substances seemed to be equally effective, but WGAapoHRP-Au was better suited than DY as a tracer to use in combination with immunocytochemistry. Sections through the primary sensorimotor cortex, which contained the bulk of identified CST neurons, and the secondary somatosensory cortex were processed with antisera raised in rabbits against glutamate (Glu) or aspartate (Asp) conjugated by glutaraldehyde to hemocyanin. In rats with DY injections, about 60-75% of the CST neurons were Glu-immunopositive, with higher ratios in SI and MI than in SII. Similar results were obtained in all areas examined from the rats with injections of WGAapoHRP-Au. Only sections from rats with injections of WGAapoHRP-Au were processed for Asp immunostaining. In this material, between 65 and 75% of the CST neurons were Asp-immunopositive, with a slightly higher ratio in SI and MI than in SII. The possibility that these results might reflect limited penetration of the antiserum and/or staining of the same population of CST neurons by either antiserum was addressed in sections processed with both the Glu and Asp antisera. In sections incubated in a mixture of the two antisera, the percentage of immunostained CST neurons was higher, about 90%, than in sections processed for only one of the two antisera. Furthermore, in rats in which Glu and Asp antibodies were visualized by two distinguishable immunostainings, four populations of CST neurons were identifiable: 1) neurons only immunopositive for Glu, 2) neurons only immunopositive for Asp, 3) neurons likely to be stained by both, and 4) neurons immunonegative for both antisera. Twenty-five to 30% of CST neurons were positive for only one antiserum, and about 50% were positive for both. No preferential distribution was evident for any one of these populations of neurons. However, perikaryal cross-sectional areas were larger for the double-stained than for the single-stained CST neurons. Glutamergic and aspartergic transmission in CST neurons has been proposed in several publications in which methods other than immunocytochemistry were employed.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurochemical heterogeneity among corticofugal and callosal projections

Biochemical, physiological, and anatomical studies over the past 30 years have firmly established glutamate (Glu) as the major neurotransmitter of those cortical neurons which give rise to corticofugal pathways. In the present study we utilized immunohistochemistry, with an antibody directed against Glu, in conjunction with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) histochemistry to examine the Glu-containing neurons which give rise to corticofugal and callosal projections of the rat. Injections of WGA-HRP into the pons labeled cells in layer V of both visual and somatosensory cortices. WGA-HRP-labeled cells which also stained for Glu were large pyramids and in the visual cortex constituted approximately 42% of the total number of neurons which had effectively transported WGA-HRP, while the percentage was 56% in the somatosensory cortex. Following caudate/putamen injections, WGA-HRP-labeled cells were confined to layer V of the somatosensory and motor cortices. Of these cells, 40% in the somatosensory cortex and 53% in the motor cortex were also stained for Glu. Finally, after WGA-HRP injections in the visual cortex numerous WGA-HRP-positive neurons were found throughout layers II-VI around the boundaries between area 17 and areas 18 and 18a of the contralateral hemisphere. Here, 38% of these cells were also labeled for Glu, but this percentage was higher (49%) when layers II-III were considered alone. These findings show that Glu is not the neurotransmitter used overwhelmingly in corticofugal and callosal projections and that different proportions of neurons are Glu-immunoreactive in the systems examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Demonstration of GABA immunoreactive cells in the inferior olive of baboons (Papio papio and Papio anubis)

The distribution of gamma-aminobutyric acid (GABA)-like immunoreactivity was studied in semithin sections through the inferior olivary complex in two baboon species. About 5% of the olivary neurons were GABA-immunoreactive. The GABA-immunoreactive neurons differed from the large majority of olivary neurons by their smaller size and their lower contents of aspartate, as judged by analysis of alternate sections labelled with an aspartate antiserum. The present observations raise the possibility that in primates the GABAergic modulation of the activity of the climbing fibre system is effected not only by the previously described input from the cerebellar nuclei and other extrinsic sources, but that intrinsic neurons also participate.

Quantitative electron microscopic immunocytochemistry of neuroactive amino acids

Amino acids are of crucial importance in brain function, not only as metabolic intermediates and building blocks of proteins, but also as mediators of interneuronal communication. This dual role of the amino acids distinguishes them from other neurotransmitter candidates, and implies that they are unlikely to be restricted to neurons using them as transmitters. This calls for a quantitative approach when attempts are made to analyse the distribution of transmitter amino acids by means of immunocytochemistry. The present review deals with recent methodological developments that have made it possible to utilize specific antisera to explore the cellular and subcellular distribution of neuroactive amino acids in a quantitative manner.