Time dependent loss of tissue GABA content and immunoreactivity in hippocampal slices

Glutamate dependence of GABA levels in neurons of hypoxic and hypoglycemic rat hippocampal slices

Hypoxia may increase GABA levels in neurons by ATP depletion-induced activation of glutamate decarboxylase and by inhibiting GABA transaminase. Hypoglycemia, which also depletes ATP, reduces neuronal levels of GABA and its precursor glutamate. We examined whether differences in glutamate levels may contribute to these altered GABA levels in hippocampal slices. GABA levels were highly correlated with endogenous glutamate levels during both hypoxia and hypoglycemia (R=0.93 for combined data). Hypoxia maximally increased GABA levels (146+/-6.3% of control, S.E.M.) when glutamate remained above 90% of control levels and ATP was at 30% of control levels. Hypoglycemia with similar ATP levels and glutamate levels at 40% of control decreased GABA levels to 55% of control. Effects of inhibitors of glutamate decarboxylase and GABA transaminase suggested that increased synthesis and decreased catabolism may both contribute to increased hypoxic GABA levels. Immunocytochemical studies suggested that hypoxia increased GABA concentrations primarily in neurons and their processes, but not in glial cells. Severe hypoxic ATP depletion increased the release of both GABA and glutamate. Hypoxia increased GABA levels in neurons, while hypoglycemia with a similar severity of ATP depletion decreased GABA levels. Much of the difference may be related to lower levels of precursor glutamate during hypoglycemia. The twofold higher levels of neuroprotective GABA available for release during hypoxia may contribute to differences in the pathophysiology of these metabolic insults.

Metabolism of the dorsal cochlear nucleus in rat brain slices

In vitro brain slices of the cochlear nucleus have been used for electrophysiological and pharmacological studies. More information is needed about the extent to which the slice resembles in vivo tissue, since this affects the interpretation of results obtained from slices. In this study, some chemical parameters of the dorsal cochlear nucleus (DCN) in rat brain slices were measured and compared to the in vivo state. The activities of malate dehydrogenase and lactate dehydrogenase were reduced in some DCN layers of incubated slices compared to in vivo brain tissue. The activities of choline acetyltransferase and acetylcholinesterase were increased or unchanged in DCN layers of slices. Adenosine triphosphate (ATP) concentrations for in vivo rat DCN were similar to those of cerebellar cortex. Compared with in vivo values, ATP concentrations were decreased in the DCN of brain slices, especially in the deep layer. Vibratome-cut slices had lower ATP levels than chopper-cut slices. Compared with the in vivo data, there were large losses of aspartate, glutamate, glutamine, gamma-aminobutyrate and taurine from incubated slices. These amino acid changes within the slices correlated with the patterns of release from the slices.

Neurons in the rat occipital cortex co-expressing the substance P-receptor and GABA: a comparison between in vivo and organotypic cultures

The morphology and the distribution of neurons expressing the NK1-receptor (NK1R) and the co-expression of gamma-aminobutyric acid (GABA) in these neurons were studied in the rat occipital cortex and in organotypic cultures (OTCs) derived from this structure. By employing immunohistochemistry, we demonstrate that the NK1R-expressing neurons are non-pyramidal neurons and co-express GABA. Some differences were noted between in vivo and OTCs. NK1R-expressing neurons in OTCs had larger somata and longer dendrites and the proportion stained with an anti-GABA-antibody (approximately 50%) was smaller than in vivo (90%). The preferential location of NK1R-expressing neurons in layers II/III and VI, seen in vivo is not present in OTCs where these neurons distribute rather homogeneously through layers II-VI. Our findings imply that in contrast to the cat and monkey, in the rat occipital cortex the effects of substance P are almost exclusively mediated via inhibitory interneurons.

Time-related loss of glutamine from hippocampal slices and concomitant changes in neurotransmitter amino acids

A dramatic, time-dependent loss of L-glutamine was observed in mouse and rat hippocampal slices equilibrated in normal artificial CSF under static (no-flow) and superfused (constant-flow) conditions. Concomitant with the decline in L-glutamine, there was a significant, but less pronounced, decrease in levels of the neurotransmitter amino acids, gamma-aminobutyric acid, L-aspartate, and L-glutamate. The disappearance of L-glutamine was a result of diffusion from the tissue to the artificial CSF rather than chemical or biochemical transformation. The loss of amino acids from the hippocampal slices was prevented to different degrees by the addition of 0.5 mM exogenous L-glutamine to the artificial CSF. The levels of newly synthesized amino acids were also determined, because they may be more indicative of the neuronal activity than the total tissue levels of amino acids. The effects of perturbations in glutamine (length of the equilibration time and addition of exogenous glutamine) on newly synthesized glutamate were more pronounced under 4-aminopyridine-stimulated than control (unstimulated) conditions. Therefore, a loss of L-glutamine from the hippocampal slices may have neurophysiological effects and warrants further investigation.

Subdivisions in the multiple GABAergic innervation of granule cells in the dentate gyrus of the rat hippocampus

The sources of GABAergic innervation to granule cells were studied to establish how the basic cortical circuit is implemented in the dentate gyrus. Five types of neuron having extensive local axons were recorded electrophysiologically in vitro and filled intracellularly with biocytin (Han et al., 1993). They were processed for electron microscopy in order to reveal their synaptic organization and postsynaptic targets, and to test whether their terminals contained GABA. (1) The hilar cell, with axon terminals in the commissural and association pathway termination field (HICAP cell), formed Gray's type 2 (symmetrical) synapses with large proximal dendritic shafts (n = 18), two-thirds of which could be shown to emit spines, and with small dendritic branches (n = 6). Other boutons of the HICAP neuron were found to make either Gray's type 1 (asymmetrical) synapses (n = 4) or type 2 synapses (n = 6) with dendritic spines. Using a highly sensitive silver-intensified immunogold method for the postembedding visualization of GABA immunoreactivity, both the terminals and the dendrites of the HICAP cell were found to be immunopositive, whereas its postsynaptic targets were GABA-immunonegative. The dendritic shafts of the HICAP cell received synapses from both GABA-negative and GABA-positive boutons; the dendritic spines which densely covered the main apical dendrite in the medial one-third of the molecular layer received synapses from GABA-negative boutons. (2) The hilar cell, with axon terminals distributed in conjunction with the perforant path termination field (HIPP cell), established type 2 synapses with distal dendritic shafts (n = 17), most of which could be shown to emit spines, small-calibre dendritic profiles (n = 2) and dendritic spines (n = 6), all showing characteristics of granule cell dendrites. The sparsely spiny dendrites of the HIPP cell were covered with many synaptic boutons on both their shafts and their spines. (3) The cell with soma in the molecular layer had an axon associated with the perforant path termination field (MOPP cell). This GABA-immunoreactive cell made type 2 synapses exclusively on dendritic shafts (n = 20), 60% of which could be shown to emit spines. The smooth dendrites of the MOPP cell were also restricted to the outer two-thirds of the molecular layer, where they received both GABA-negative and GABA-positive synaptic inputs.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of raf-1 protein immunoreactivity in guinea pig hippocampal slices during the in vitro maintenance

The cellular raf-1 protooncogene encodes a cytoplasmic serine/threonine protein kinase; raf-1 is expressed not only in tumour cells, but also in normal brain tissue. However, the substrate of raf-1 kinase is unknown and its function is still hypothetical. In cell cultures, mitogen stimulation may alter its properties, including its kinase activity and subcellular distribution. The aims of the present study were: (1) light- and electron microscopic immunohistochemical localization of raf-1 kinase in hippocampal slices; (2) the follow-up of the possible changes in localization and detectability during the in vitro maintenance and low-frequency stimulation. Our results suggested the induction of neuronal raf-1 kinase during the in vitro maintenance. The possibilities of increased transcription and enzyme activation are discussed.