The Journal of Neurophysiology Vol. 82 No. 4 October 1999, pp. 2020-2023
Copyright ©1999 by the American Physiological Society

RAPID COMMUNICATION

GABA_A and GABA_C Receptors Have Contrasting Effects on Excitability in Superior Colliculus

 ¹Institute of Biotechnology, University of Helsinki, FIN-00014 Helsinki, Finland; and  ²General Zoology and Neurobiology, Ruhr-University Bochum, D-44780 Bochum, Germany

Pasternack, Michael, Mathias Boller, Belinda Pau, and Matthias Schmidt. GABA_A and GABA_C Receptors Have Contrasting Effects on Excitability in Superior Colliculus. J. Neurophysiol. 82: 2020-2023, 1999. We have recently found that GABA_C receptor subunit transcripts are expressed in the superficial layers of rat superior colliculus (SC). In the present study we used immunocytochemistry to demonstrate the presence of GABA_C receptors in rat SC at protein level. We also investigated in acute rat brain slices the effect of GABA_A and GABA_C receptor agonists and antagonists on stimulus-evoked extracellular field potentials in SC. Electrical stimulation of the SC optic layer induced a biphasic, early and late, potential in the adjacent superficial layer. The late component was completely inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione or CoCl₂, indicating that it was generated by postsynaptic activation. Muscimol, a potent GABA_A and GABA_C receptor agonist, strongly attenuated this postsynaptic potential at concentrations >10 µM. In contrast, the GABA_C receptor agonist cis-aminocrotonic acid, as well as muscimol at lower concentrations (0.1-1 µM) increased the postsynaptic potential. This increase was blocked by (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid, a novel competitive antagonist of GABA_C receptors. Our findings demonstrate the presence of functional GABA_C receptors in SC and suggest a disinhibitory role of these receptors in SC neuronal circuitry.