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 (Istim) varying from 0.25 to 6 mA. A sharp, stimulation-dependent increase in the PS2 amplitude was observed with increasing Istim up to 0.5 mA, followed by an exponential decrease in PS2 at higher Istim, reaching a steady value with Istim above 3 mA. Intravenous (iv) infusion of bicuculline (BIC) increased the PS2, when elicited by Istim 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 Istim (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.