Functional synapses are established between ciliary ganglion neurones in dissociated cell culture

Abstract

The formation of specific synaptic connections is a central part of development in the nervous system. The parasympathetic ciliary ganglion of the chick is a useful population of neurones for studying synaptogenesis both because its development has been well characterized in vivo¹ and because its neurones can be maintained and examined in long-term dissociated cell culture^2,3. The ciliary ganglion contains two classes of neurones: choroid neurones that innervate smooth muscle in the choroid layer of the eye, and ciliary neurones that innervate striated muscle in the iris and ciliary body. Both classes of neurone are cholinergic and both receive excitatory cholinergic input from preganglionic neurones in the accessory oculomotor nucleus. Ciliary ganglion neurones do not seem to innervate each other in vivo^4,5, even though they have matching neurotransmitter and receptor types. In cell culture, the neurones acquire high levels of choline acetyltransferase activity, form cholinergic synapses on skeletal myotubes when present in the cultures, and have significant levels of acetylcholine (ACh) sensitivity^2,6, as they do in vivo. We report here that cholinergic synaptic transmission does occur between the neurones in cell culture. These results indicate that ciliary ganglion neurones can innervate each other, and suggest that additional constraints exist in vivo to prevent them from doing so.