On the neurotrophic control of acetylcholine receptors at frog end-plates reinnervated by the vagus nerve.

To test whether the properties of subsynaptic acetylcholine (ACh) receptors in skeletal muscle fibre are influenced by the type of the innervating neurone some pharmacological properties of ACh receptor in normal end-plates and in denervated end-plates reinnervated by the vagus nerve in the frog were compared. Blockade of nerve-evoked synaptic currents by 200 microM-hexamethonium was stronger at vagus-reinnervated than at normal end-plates. Blockade at both types of junctions was voltage dependent. The effect of hexamethonium on equilibrium currents induced by bath-applied ACh and carbamylcholine was similar at the two types of junctions. At both normal and vagus-reinnervated junctions, decamethonium had similar partial agonist properties. Following a step in membrane potential, the relaxations of ACh-induced conductance changes at the two types of junctions were affected in a similar fashion by hexamethonium: hyperpolarization first produced a fast decrease and then a slow exponential increase in conductance. Upon depolarization, a fast increase was followed by an exponential decline to its original level. The time constant of the slow relaxation was slightly prolonged compared to control. These findings are consistent with a fast blocking action of open channels by hexamethonium. The effectiveness of hexamethonium in blocking end-plate currents was reduced in the presence of (+)-tubocurarine, indicating that hexamethonium has a competitive blocking action on the receptors. These results do not indicate that the pharmacological properties of the ACh receptors are changed after an end-plate is reinnervated by a preganglionic neurone. The differential effect of hexamethonium on transmission at normal and vagus-reinnervated end-plates is discussed as a consequence of different transmitter release characteristics at the two types of junctions.