The purpose of this study was to investigate the properties of cholinergic transmission at nascent synapses formed by neurons from the embryonic chick retina. By using a cell culture system in which striated muscle cells served as postsynaptic targets for dissociated retinal neurons, it was possible to record synaptic activity soon after the establishment of a functional cholinergic synapse. Postsynaptic potentials ranged from a few hundred microvolts to greater than 10 mV. The high temperature dependence (Q10 of 10) of this cholinergic transmission indicated that the release of acetylcholine from the embryonic retinal neurons was not injury-related. The wide range in event amplitudes did not appear to be due to electrotonic conduction from adjacent myotubes. Rather, a lack of correlation between event amplitude and rise time indicated that the release of acetylcholine occurred over a confined area of contact. Amplitude histograms of these events always showed a unimodal distribution, with small events being most common. No pattern to the timing of the events was evident. In addition, the inward current blockers, tetrodotoxin and cadmium did not affect this activity.
Taken together, our findings indicate that the embryonic retinal neurons studied in this culture system spontaneously release acetylcholine in a pulsatile manner by a mechanism that is stimulus-independent and highly temperature sensitive. A hypothesis is that the large variation in size of postsynaptic events at these newly formed synapses is caused by the spontaneous release of packets containing differing amounts of acetylcholine.