GABAB receptors regulate chick retinal calcium waves

Correlated spiking activity and associated Ca2+ waves in the developing retina are important in determining the connectivity of the visual system. Here, we show that GABA, via GABA(B) receptors, regulates the temporal characteristics of Ca2+ waves occurring before synapse formation in the embryonic chick retina. Blocking ionotropic GABA receptors did no affect these Ca2+ transients. However, when these receptors were blocked, GABA abolished the transients, as did the GABA(B) agonist baclofen. The action of baclofen was prevented by the GABA(B) antagonist p-3-aminopropyl-p-diethoxymethyl phosphoric acid (CGP35348). CGP35348 alone increased the duration of the transients, showing that GABA(B) receptors are tonically activated by endogenous GABA. Blocking the GABA transporter GAT-1 with 1-(4,4-diphenyl-3-butenyl)-3-piperidine carboxylic acid (SKF89976A) reduced the frequency of the transients. This reduction was prevented by CGP35348 and thus resulted from activation of GABA(B) receptors by an increase in external [GABA]. The effect of GABA(B) receptor activation persisted in the presence of activators and blockers of the cAMP-PKA pathway. Immunocytochemistry showed GABA(B) receptors and GAT-1 transporters on ganglion and amacrine cells from the earliest times when Ca2+ waves occur (embryonic day 8). Patch-clamp recordings showed that K+ channels on ganglion cell layer neurons are not modulated by GABA(B) receptors, whereas Ca2+ channels are; however, Ca2+ channel blockade with omega -conotoxin-GVIA or nimodipine did not prevent Ca2+ waves. Thus, the regulation of Ca2+ waves by GABA(B) receptors occurs independently of N- and L-type Ca2+ channels and does not involve K+ channels of the ganglion cell layer. GABA(B) receptors are likely to be of key importance in regulating retinal development.