Agrin mRNA expression in the developing chick Edinger-Westphal nucleus.

Agrin is a large extracellular matrix protein that directs the accumulation of acetylcholine receptors at the neuromuscular junction. Recent evidence suggests that agrin may be involved in organizing synapses in the visual system as well. Focussing on the pathway that controls accommodation and pupilloconstriction, this study examined the temporal pattern of agrin expression with reference to the organization of cholinergic synapses between embryonic chick Edinger-Westphal and ciliary ganglion neurons. In situ hybridization with an S35-labeled agrin cRNA probe was used to characterize agrin expression in the Edinger-Westphal nucleus during development. Agrin mRNA was detected in the Edinger-Westphal nucleus at all time points studied, from embryonic day 7 (E7, Hamburger and Hamilton stage 31) through newly hatched chicks. Throughout this period, agrin mRNA expression in Edinger-Westphal neurons was lower than in nearby oculomotor and trochlear neurons, suggesting that cells projecting to neuronal targets may require less agrin than those projecting to muscle. Agrin mRNA expression in the Edinger-Westphal nucleus at E7, E8, E9, and E10 was significantly higher than at E12. The early appearance of agrin mRNA coincides with the period during which acetylcholine receptors are being organized on ciliary ganglion neurons, consistent with the possibility that agrin contributes to neuron-neuron synapse formation in this pathway.