GABA(A) and GABA(C) receptors have contrasting effects on excitability in superior colliculus

We have recently found that GABA(C) receptor subunit transcripts are expres sed in the superficial layers of rat superior colliculus (SC). In the prese nt study we used immunocytochemistry to demonstrate the presence of GABA(C) receptors in rat SC at protein level. We also investigated in acute rat br ain slices the effect of GABA(A) and GABA(C) receptor agonists and antagoni sts on stimulus-evoked extracellular field potentials in SC. Electrical sti mulation of the SC optic layer induced a biphasic, early and late, potentia l in the adjacent superficial layer. The late component was completely inhi bited by 6-cyano-7-nitroquinoxaline-2,3-dione or CoCl2, indicating that it was generated by postsynaptic activation. Muscimol, a potent GABA(A) and GA BA(C) receptor agonist, strongly attenuated this postsynaptic potential at concentrations >10 mu M. In contrast, the GABA(C) receptor agonist cis-amin ocrotonic acid, as well as muscimol at lower concentrations (0.1-1 mu M) in creased the postsynaptic potential. This in; crease was blocked by (1,2,5,6 -tetrahydropyridine-4-yI)methylphosphinic acid. a novel competitive antagon ist of GABA(C) receptors. Our findings demonstrate the presence of function al GABA(C) rereptors in SC and suggest a disinhibitory role of these recept ors in SC neuronal circuitry.