Excitatory acoustic responses in the inferior colliculus of the rat are increased by GABAB receptor blockade.

This study sought to investigate the influence of GABAB receptor activation on acoustically induced excitation within the rat inferior colliculus. To this end, the GABAB receptor antagonist, CGP 35348, was applied systemically and iontophoretically. Single and multibarrel electrodes were used for extracellular recordings within the central nucleus of the inferior colliculus. The experimental model, a paired-pulse stimulus paradigm, applied two identical acoustic stimuli, 200 msec apart, evoking corresponding responses characterized by the second being consistently weaker than the first. Abolishment of the acoustically evoked response, following iontophoretic application of the GABAB receptor agonist, L-baclofen, verified the existence of GABAB receptors in all inferior colliculus cells tested. Intravenous application of CGP 35348 (200 mg/kg) evoked a 24% overall increase in stimulus responses. Likewise, a 13% increase in total evoked excitation was observed, following iontophoretic application. There was no significant reduction of inhibition on the second evoked response in the paired-pulse model, following either systemic or iontophoretic application of CGP 35348. This result implies that the decreased magnitude of the second response, with an interpulse interval of 200 msec, is not influenced by GABAB receptor mediated inhibition. These findings do indicate, however, that GABAB receptors play a small, but significant role during the processing of acoustic information, within the inferior colliculus.