Cholinergic transmission by embryonic retinal neurons in culture: inhibition by dopamine

The function of neurotransmitters in ontogeny remains unclear, although it is well known that both pre- and postsynaptic components of certain neurotransmitter systems are present from early in morphogenesis. The objective of this study was to establish a culture system that would permit an analysis of the physiological effects of dopamine on immature neurons. Specifically, dopamine-mediated effects on synaptic transmission by cholinergic neurons of the embryonic chick retina were explored. To do this, a retina-muscle culture system was used. In previous physiological studies, striated muscle cells in culture have proved useful as postsynaptic targets for cholinergic neurons of the immature retina. It is reported here that dopamine can inhibit synaptic responses of cultured muscle cells that are innervated by neurons of the embryonic chick retina. This inhibitory effect is blocked reversibly by the dopamine antagonists, haloperidol and fluphenazine. With the culture system used in this developmental study, dopamine-mediated inhibition can be examined with either explants of retina or with dissociated retinal neurons. When a low density of dissociated cells is plated, it is possible to examine relatively isolated, visually identified, presynaptic, cholinergic neurons. The results show that an inhibitory response to dopamine is expressed by neurons derived from retinas which are at an early stage of ontogeny. The finding that inhibition by dopamine could be demonstrated to develop at 90% of the retina-muscle synapses indicates that the cholinergic neurons studied in this experimental system are a relatively homogenous population with respect to their responsiveness to dopamine.