INPUT-OUTPUT DEPENDENCE REVEALS THE COMPONENTS OF GABAERGIC MODULATION IN RAT HIPPOCAMPAL CA3 IN-VIVO

Field population spikes, orthodromically elicited (PS2), upon commissu ral stimulation, in the hippocampal, CA3 region of the anaesthetized r at, were determined at stimulation intensities (I-stim) varying from 0 .25 to 6 mA. A sharp, stimulation-dependent increase in the PS2 amplit ude was observed with increasing I-stim up to 0.5 mA, followed by an e xponential decrease in PS2 at higher I-stim reaching a steady value wi th I-stim above 3 mA. Intravenous (iv) infusion of bicuculline (BIC) i ncreased the PS2, when elicited by I-stim between 0.25 and 6 mA, thoug h the stimulus - response (S-R) curve consistently presented a local m inimum in between 0.5 to 1 mA. Baclofen (10 and 30 mg/kg iv) dose-depe ndently reduced PS2 at the lowest I-stim (0.25 mA), both in control an d in BIC-treated animals. At higher stimulation intensities, however, baclofen (30 mg/kg iv) increased PS2, while barely affecting the popul ation spike in BIC-treated animals. The input-output dependence of thi s CA3 response appeared able to provide a convenient in vivo model rev ealing the components of GABAergic modulation.