Spontaneous Ca2+ transients and their transmission in the developing chick retina.
The development of the central nervous system is dependent on spontaneous action potentials and changes in [Ca2+]i occurring in neurons [1-4]. In the mammalian retina, waves of spontaneous electrical activity spread between retinal neurons, raising [Ca2+]i as they pass [5-7]. In the ferret retina, the first spontaneous Ca2+ waves have been reported at postnatal day 2 and are thought to result from the Ca2+ influx associated with bursts of action potentials seen in ganglion cells at this time [5-7]. These waves depend on depolarisation produced by voltage-gated sodium channels, but their initiation and/or propagation also depends upon nicotinic cholinergic synaptic transmission between amacrine cells and ganglion cells [8]. Here, we report contrasting results for the chick retina where Ca2+ transients are seen at times before retinal synapse formation but when there are extensive networks of gap junctions. These Ca2+ transients do not require nicotinic cholinergic transmission but are modulated by acetylcholine (ACh), dopamine and glycine. Furthermore, they propagate into the depth of the retina, suggesting that they are not restricted to ganglion and amacrine cells. The transients are abolished by the gap-junctional blocker octanol. Thus, the Ca2+ transients seen early in chick retinal development are triggered and propagate in the absence of synapses by a mechanism that involves several neurotransmitters and gap junctions.