Presynaptic modulation by GABAB receptors of glutamatergic excitation and GABAergic inhibition of neostriatal neurons.

1. The present experiments investigated the effects of gamma-amino-butyric acidB (GABAB) receptor stimulation on the excitatory and inhibitory responses of neostriatal neurons evoked by stimulation of the subcortical white matter in a rat neostriatal slice preparation. 2. Intracellular recordings showed that single-impulse stimulation of the corpus callosum evoked monosynaptic, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive excitatory postsynaptic potentials (EPSPs) that were attenuated by the GABAB receptor agonist, p-chlorophenyl-GABA (baclofen, 0.5-10 microM) in a concentration-dependent manner. Baclofen also blocked the GABAA-mediated inhibition of neostriatal cell responses, which were revealed by paired-impulse stimulation of the subcortical white matter. Both of these effects persisted in slices in which the anterior cortex was removed, indicating that the site of action for baclofen was intrinsic to the neostriatum. The GABAB antagonist 3-amino-2-hydroxy-2-(4-chlorophenyl)-propanesulfonic acid (saclofen, 250-500 microM) reversed the depressant actions of baclofen on both the excitatory and inhibitory responses of neostriatal cells. 3. Concentrations of baclofen as high as 100 microM, which markedly attenuated EPSP amplitude, did not exert direct effects on resting membrane potential, current-voltage relationship, input resistance, or spike threshold and thus appeared to have no postsynaptic effect on the population of neurons recorded. 4. These results indicate that, in contrast to other regions of the CNS, the depressant effects of baclofen on glutamate-dependent EPSPs are mediated exclusively through GABAB receptors located presynaptically on the terminals of glutamatergic afferents.(ABSTRACT TRUNCATED AT 250 WORDS)