Enhanced chemosensitivity of chick parasympathetic neurones in co‐culture with myotubes.

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Abstract
The influence of target interaction on the electrophysiological properties of dissociated ciliary ganglion cells was investigated by testing the sensitivity of the neuronal somal membrane to ionophoretically applied acetylcholine (ACh). Variations in the percentage of cells responsive to the transmitter were measured with time in culture. At 24 h after plating, all cells responded to the ionophoretic pulse of ACh with a depolarization. However, 1 wk after plating (from 7-14 days) most of the neurons were unresponsive, and highly responsive cells (> 100 mV peak depolarization/nC) were extremely rare. At even later times in culture, neurons sensitive to the transmitter were again more frequent. When neurons were plated onto preformed pectoral myotubes, however, ACh sensitivity was maintained throughout a 3 wk culture period. Neuromuscular junctions were formed by the neurons, and, when sufficient neurons were present, all the muscle fibers tested showed evidence of functional synaptic transmission. Chemosensitivity to ACh was not maintained by neurons in muscle-free microcultures, even when responsive neurons in muscle-containing microcultures were present on the same cover-slip. Interneuronal synaptic contacts, defined by ultrastructural criteria, were formed in cultures of neurons alone, but evidence of widespread functional synaptic interaction between cells was not found at 7-14 days in culture. The maintenance of ACh sensitivity of cultured ciliary ganglion cells was enhanced by the presence of muscle in co-culture. The interneuronal synaptic contacts observed were apparently not as potent a stimulus as co-culture with muscle for the full expression of the cholinergic phenotype under these culture conditions.