Optical characterization of a novel GABA response in early embryonic chick brainstem.

To examine the functional expression of embryonic GABA receptors, the inhibitory effects were studied of GABA (GABA responses) on the excitatory postsynaptic potentials evoked by vagal stimulus in seven- to 10-day-old embryonic chick brainstem slice preparations. A multiple-site optical recording technique was used, with a multiple element photodiode array system and a fast voltage-sensitive merocyanine-rhodanine dye (NK2761). First, in the GABA response, three components were pharmacologically identified: component 1, related to GABA(A) receptors; component 2, related to GABA(B) receptors; and component 3 which is insensitive to GABA(A) and GABA(B) antagonists, but is stimulated by both GABA(A) and GABA(B) agonists. Subsequently. the embryogenesis and early development of the three components were investigated, and early developmental maps of regional distribution patterns of the three components were constructed. Components 1 and 3 have already emerged in the seven-day-old embryonic brainstem preparation; component 2 appeared in the eight-day-old preparations. No component related to GABA(C) receptors was observed in the seven- to 10-day-old embryonic stages. From the pharmacological properties of component 3, we suggest that it is related to a new subtype, the GABA(D) receptor.