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

Field population spikes, orthodromically elicited (PS2), upon commissural stimulation, in the hippocampal, CA3 region of the anaesthetized rat, were determined at stimulation intensities (I-stim) varying from 0.25 to 6 mA. A sharp, stimulation-dependent increase in the PS2 amplitude was observed with increasing I-stim up to 0.5 mA, followed by an exponential decrease in PS2 at higher I-stim reaching a steady value with I-stim above 3 mA. Intravenous (iv) infusion of bicuculline (BIC) increased the PS2, when elicited by I-stim between 0.25 and 6 mA, though the stimulus - response (S-R) curve consistently presented a local minimum in between 0.5 to 1 mA. Baclofen (10 and 30 mg/kg iv) dose-dependently reduced PS2 at the lowest I-stim (0.25 mA), both in control and in BIC-treated animals. At higher stimulation intensities, however, baclofen (30 mg/kg iv) increased PS2, while barely affecting the population spike in BIC-treated animals. The input-output dependence of this CA3 response appeared able to provide a convenient in vivo model revealing the components of GABAergic modulation.