Expression of cholinergic markers in the developing chick embryo spinal cord.

The expression of cholinergic neurotransmission in the developing spinal cord was followed with pre- and postsynaptic cholinergic markers and histoautoradiographic determinations of cholinergic receptors and ultrastructural studies. Two distinct steps in the development of cholinergic markers were evident. The first step of cholinergic expression occurs early in development and its characterized by a sudden and large increase in choline acetyltransferase activity, whereas the high affinity choline uptake mechanism remains undetectable and few muscarinic receptors are detectable. This phenomenon possibly reflects the expression of high choline acetyltransferase gene activity in developing neurons which are still not synaptically connected. The second step is also characterized by a sharp rise of choline acetyltransferase activity. This rise parallels an active development of high affinity choline uptake and big increase in both number and density of muscarinic receptors. Also nicotine receptors increase, as revealed by histoautoradiography. Specific morphogenetic events, such as the increase of synaptic junctions with postsynaptic thickenings and numerous presynaptic clear vesicles, occur at the same time. Such interactions may contribute to the full maturity of cholinergic neurotransmission of the spinal cord.