ULTRASTRUCTURAL EVIDENCE FOR INTRA-NUCLEAR AND EXTRANUCLEAR PROJECTIONS OF GABAERGIC NEURONS OF THE SUPRACHIASMATIC NUCLEUS

GABAergic projections of the suprachiasmatic nucleus (SCN) were demons trated in a double-labelling ultrastructural study which visualised th e efferents of the SCN by PHA-L tracing, diaminobenzidine (DAB) immuno cytochemistry, and GABA with immunogold postembedding staining. The re sults show a strong contralateral projection of the SCN that is partly GABA-containing. In addition, ipsilateral SCN projections to the dors omedial hypothalamus and periventricular part of the paraventricular n ucleus and subparaventricular nucleus were shown to contain GABA. The present results indicate that the SCN may utilize this inhibitory neur otransmitter to regulate and organize its own circadian rhythm as well as using GABA to transmit its diurnal information to other regions of the brain. (C) 1994 Wiley-Liss, Inc.