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

The Dale's law postulates that a neuron releases the same neurotransmitter from all its branches. In the case of multiple neurotransmitters it would r equire all transmitters to be released from all branches. The retinal choli nergic amacrine cells contain and release gamma-aminobutyric (GABA) and, th erefore, if GABA and acetylcholine (ACh) are released at the same sites, th is could mean that amacrine cells simultaneously excite and inhibit postsyn aptic cells. Conversely, if the two neurotransmitters are released at diffe rent synapses, or if their release is regulated in a distinct manner, they may play different physiological roles. Recent studies carried out in cultu red cholinergic amacrine-like neurons showed that Ca2+-dependent release of ACh and GABA have a different sensitivity to membrane depolarization, to t he effect of blockers of voltage gated Ca2+ channels (VGCC) and to the effe ct of presynaptic Al adenosine receptors. Therefore, it is proposed that in retinal amacrine cells the Ca2+-dependent release of ACh and GABA occurs a t distinct cellular locations. The possible nature of these release sites a nd the physiological significance of this model are discussed in this revie w. J. Neurosci. Res. 58: 475-479, 1999. (C) 1999 Wiley-Liss, Inc.