Chloride distribution in the CA1 region of newborn and adult hippocampus by light microscopic histochemistry

GABA, the main inhibitory neurotransmitter in the central nervous system, e xerts its effect by rendering the postsynaptic GABA, receptors permeable to chloride ions. Thus, depolarizing or excitatory effects of GABA, experienc ed in early postnatal life or in certain regions and/or conditions of the a dult brain, is thought to be associated with a reversed transmembrane chlor ide gradient. However, there is only limited direct information about the c orrelation of the actual excitatory versus inhibitory effects of GABA and t he local chloride distribution. Precipitation of chloride with silver is a potential way to immobilize and visualize chloride ions in biological tissu e. We examined the applicability of light microscopic histochemistry, based on trapping tissue chloride with silver ions during freeze-substitution or aldehyde fixation, to visualize the chloride distribution in hippocampal s lices. The freeze-substitution procedure yielded better chloride retention while with aldehyde fixation tissue preservation was more appropriate. Both methods were qualitative only, had limited applicability to the superficia l 20-30 mum of slices, but were able to demonstrate a reduced extracellular -to-intracellular chloride gradient in the CA1 pyramidal neurons of the new born hippocampus as compared to adult animals. In the 4-aminopyridine model of epilepsy, redistribution of chloride from extracellular to intracellula r space could also be demonstrated.