J Neurophysiol (April 1, 2003). 10.1152/jn.00842.2002
Submitted on Submitted 18 September 2002; accepted in final form 30 November 2002

GABA_C Receptors in the Rat Superior Colliculus and Pretectum Participate in Synaptic Neurotransmission

Department of Zoology and Neurobiology, Ruhr-University, D-44780 Bochum, Germany

Boller, Mathias and Matthias Schmidt. GABA_C Receptors in the Rat Superior Colliculus and Pretectum Participate in Synaptic Neurotransmission. J. Neurophysiol. 89: 2035-2045, 2003. In mammals, GABA_C receptors seem to be specifically expressed in the retina and the subcortical visual system, with highest extraretinal expression levels in the superior colliculus (SC). Although its presence in the superficial SC has been demonstrated physiologically, a direct involvement of this receptor type in fast synaptic neurotransmission still awaits verification. We addressed the question of a possible synaptic localization of GABA_C receptors by performing in vitro whole-cell patch-clamp recordings of inhibitory postsynaptic currents (IPSCs) in single neurons of the rat SC and the neighboring pretectal nuclear complex, where GABA_C receptors are also expressed at significant levels. To increase the likelihood to record IPSCs we induced spontaneous activity by application of the potassium channel blocker 4-aminopyridine (4-AP) and blocked glutamate-mediated excitatory neurotransmission with kynurenic acid. All 4-AP-induced postsynaptic currents were of synaptic origin because they were completely suppressed by lidocaine or by substitution of extracellular calcium with cobalt. In 40% of the SC cells and in 60% of the pretectal neurons, IPSCs in the presence of 4-AP and kynurenic acid were only partly blocked by the selective GABA_A receptor antagonist bicuculline. Inhibitory currents that were insensitive to bicuculline, however, could be blocked by coapplication of either the specific GABA_C receptor antagonist 1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid or picrotoxin, an unselective GABA_A and GABA_C receptor antagonist. We conclude that GABA_C receptors are, at least partially, located synaptically in SC and pretectal neurons in the rat, which indicates a direct function of this receptor type for synaptic processing in both structures.