Contribution of GABA(C) receptors to inhibition in the rodent accessory optic system

The medial terminal nucleus (MTN) of the mammalian accessory optic system controls vertical compensatory eye movements. It consists of two neuronal populations which respond best either to upward or to downward visual image shifts. The two cell classes are located spatially separate in the dorsal or in the ventral subdivision of the MTN, respectively. Pronounced GABAergic pathways have been described to exist between neurons in the two MTN subdivisions indicating that inhibitory interactions play a significant role for the generation of MTN cell response properties. Yet, the types of GABA receptors which mediate these inhibitory interactions are unknown. Functionally, it is of particular interest to know whether GABA(C) receptors, as in other subcortical visual centers, participate in inhibitory mechanisms in MTN neurons. We therefore performed whole-cell patch clamp recordings from MTN neurons in acute mouse midbrain slices. We monitored excitatory and inhibitory postsynaptic responses to afferent stimulation and applied specific GABA receptor agonists and antagonists to identify the GABA receptor types present in MTN neurons. We found that more than 80% of the neurons in both MTN subdivisions express functional GABA(C) receptors that can be activated by specific receptor agonists. A blockade of GABA(C) receptors, on the other hand, either reduced or enhanced postsynaptic inhibition, indicating that both postsynaptic and presynaptic functions are served by this receptor type. This, together with earlier results, suggests that GABA(C) receptors play a general role for the control of neuronal excitability in subcortical visual pathways.