CHLORIDE IS PREFERENTIALLY ACCUMULATED IN A SUBPOPULATION OF DENDRITES AND PERIGLOMERULAR CELLS OF THE MAIN OLFACTORY-BULB IN ADULT-RATS

GABA is predominantly an inhibitory transmitter. Mediated by GABA(A) r eceptors, GABA opens chloride channels, induces a passive flux of chlo ride ions, which is usually directed from extracellular to intracellul ar space, and hyperpolarizes postsynaptic neurons. Recent electrophysi ological data suggested that GABA may also depolarize neurons and exer t excitatory actions. However, it remained unclear whether excitatory GABA effects are based on reversed transmembrane chloride gradient due to modifications in extracellular or intracellular chloride concentra tions. Here, the first histochemical evidence is provided for local re distribution of chloride in the CNS of healthy adult rats. Olfactory b ulbs were examined using freeze substitution, silver trapping of chlor ide and intensification techniques at light and electron microscopic l evel. The chloride content of precipitates was evidenced by electron s pectroscopic imaging using a CEM 902 (Zeiss) electron microscope. Chlo ride concentration was high in a subpopulation of some periglomerular cell bodies and isolated dendritic profiles, while it seemed to be ver y low in certain parts of the glomerular neuropil including intercellu lar clefts. Data suggest that reversed chloride gradients can be demon strated by cytochemical methods, and may be responsible for excitatory GABA effects on selected periglomerular neurons and dendrites in the olfactory glomeruli. Conditions leading to chloride redistribution in the CNS of normal adult rats remain to be determined.