The Development of ACh- and GABA-Activated Currents in Embryonic Chick Ciliary Ganglion Neurons in the Absence of Innervation in vivo

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Chemical synaptic transmission in the chick ciliary ganglion is mediated by nicotinic ACh receptors. Ciliary ganglion neurons also express GABAA receptors, although there is no known source of GABAergic innervation of the ganglion, and the function of GABA receptors on these neurons is not known. We examined whether ACh and GABA receptors on embryonic chick ciliary ganglion neurons are regulated by presynaptic inputs. Whole-cell currents evoked by ACh or GABA in neurons soon after dissociation were taken to estimate the level of functional receptors in intact ganglia. We destroyed the accessory oculomotor nucleus (AON), the only source of synaptic input to the ganglion, on embryonic day (E) 4. We determined that 80% of the operations resulted in the virtual elimination of synaptic contacts in the ganglion, using P65 immunohistochemistry (a synaptic vesicle antigen) and direct ultrastructural examination. Previous experiments have shown that during normal development, ACh-activated currents increase over sevenfold between E6 and E18; GABA-activated currents increase only twofold, in proportion to cell size. We found that ACh-activated currents of uninnervated neurons at E14 and E18 were as large as control responses. Furthermore, ACh receptor-like molecules, visualized with monoclonal antibody 35 immunofluorescence, were concentrated in high density clusters on the surface of E18 neurons from AON-ablated embryos. GABA-activated currents were also not affected by AON destruction. We conclude that ACh and GABA receptors are not induced in embryonic chick ciliary ganglion neurons during development by contact with or soluble factors released from AON synaptic terminals.