REDUCED LEVELS OF ACETYLCHOLINE-RECEPTOR EXPRESSION IN CHICK CILIARY GANGLION NEURONS DEVELOPING IN THE ABSENCE OF INNERVATION

Chick ciliary ganglion neurons receive innervation from a single sourc e, the accessory oculomotor nucleus (AON), and nicotinic ACh receptors (AChRs) mediate chemical synaptic transmission through the ganglion. Previous experiments examining the developmental expression of AChRs i n embryonic chick ciliary ganglion neurons in situ have shown that ACh R levels increase substantially in the neurons at the time of innervat ion. Prior to synapse formation, few AChRs are detected in the neurons . In the present experiments, the role of presynaptic inputs in induci ng an increase in AChRs was established by examining AChR levels in ci liary ganglion neurons that have been deprived of innervation by surgi cal ablation of the AON prior to synapse formation. AChR levels were d ramatically reduced in neurons of input-deprived ganglia as compared t o control innervated neurons at all developmental stages examined from embryonic day (ED) 5 to ED 12 as determined by indirect immunocytoche mical labeling of frozen ganglion sections with the anti-AChR monoclon al antibody mAb 35, and light microscopy. In contrast, neuronal somata of input-deprived and control ganglia had equivalent levels of immuno labeling f or three other components, a transmembrane glycoprotein of synaptic vesicles, SV2, and two microtubule-associated proteins, MAP 1 B and MAP 2, from ED 5 up to ED 10. The results demonstrate that presy naptic inputs specifically increase the levels of AChR expression in d eveloping neurons. In addition, changes in the levels of immunolabelin g for AChRs, SV2, MAP 1B, and MAP 2 in neuronal somata after ED 1 0 de monstrate that other major developmental events also influence the lev els of these components in neurons. Declines in the intensity of AChR, SV2, MAP 1 B, and MAP 2 immunolabeling within a subset of neuronal so mata in both operated and control ganglia at ED 10 and 12 coincide wit h the period of neuronal cell death. Increases in AChR labeling in the rest of the neuronal population of input-deprived ganglia at ED 1 2 s uggest that, in addition to innervation, synapse formation with the pe ripheral target tissue influences AChR levels in developing neurons in situ.