Corelease of two functionally opposite neurotransmitters by retinal amacrine cells: Experimental evidence and functional significance

Abstract

The Dale's law postulates that a neuron releases the same neurotransmitter from all its branches. In the case of mulitple neurotransmitters it would require all transmitters to be released from all branches. The retinal cholinergic amacrine cells contain and release &ggr;-aminobutyric (GABA) and, therefore, if GABA and acetylcholine (ACh) are released at the same sites, this could mean that amacrine cells simultaneously excite and inhibit postsynaptic cells. Conversely, if the two neurotransmitters are released at different synapses, or if their release is regulated in a distinct manner, they may play different physiological roles. Recent studies carried out in cultured cholinergic amacrine-like neurons showed that Ca2+-dependent release of ACh and GABA have a different sensitivity to membrane depolarization, to the effect of blockers of voltage gated Ca2+ channels (VGCC) and to the effect of presynaptic A1 adenosine receptors. Therefore, it is proposed that in retinal amacrine cells the Ca2+-dependent release of ACh and GABA occurs at distinct cellular locations. The possible nature of these release sites and the physiological significance of this model are discussed in this review.J. Neurosci. Res. 58: 475-479, 1999. © 1999 Wiley-Liss, Inc.