Effect of an Agonist of β2 Adrenoreceptors on Inhibition in the Rat Hippocampal CA1 Area Induced by Activation of GABAB Receptors

In experiments on rat brain slices using extracellular recording, we studied the effects of an agonist of beta(2) adrenoreceptors, metaproterenol (MPT), on reactions of pyramidal neurons of the hippocampal CA1 area induced by activation of GABA(B) receptors. Isolated application of an agonist of GABA(B) receptors, baclofen (10 mu M), resulted in intense inhibition (by 50% or more during the 1st min of action) of orthodromic field discharges (OFDs) in the pyramidal layer of the above-mentioned area of the hippocampus; the discharges were evoked by electrical stimulation of Schaffer collaterals. On the 3rd to 4th min, OFDs were suppressed nearly completely. After washing out from baclofen, the parameters of the evoked responses never completely restored to the initial level. In all cases, simultaneous application of 150 mu M MPT and 10 mu M baclofen prevented full manifestation of the inhibitory effect of the latter agent on OFDs of pyramidal neurons. The amplitude of evoked responses decreased, but the relative intensity of inhibition under these conditions during 2-min-long application was significantly lower than that upon isolated action of baclofen. The recovery of the amplitude of evoked responses in the course of washing out under conditions of parallel action of MPT was more rapid and, in some cases, complete. Therefore, our experiments showed that GABA(B)-ergic inhibitory transmission in the rat hippocampal CA1 area in vitro can be suppressed significantly by the beta(2) adrenoreceptor agonist.