α-Bungarotoxin binding sites in the rat superior cervical ganglion are influenced by postganglionic axotomy

Abstract

α-Bungarotoxin binds in a saturable manner to membrane-associated receptors in rat superior cervical ganglia. Toxin binding is inhibited by some nicotinic ligands. The rate constant of association ( k on ) and dissociation ( k off ) of the toxin-receptor reaction has been estimated to be k on = 1.58 × 10 5M −1 s −1 and k off = 9.24 × 10 −6 s −1, respectively, at 24°C. The binding constants were not modified by decentralization, postganglionic axotomy or by drugs acting at the sympathetic postganglionic nerve terminals (reserpine and 6-hydroxydopamine). The number of the α-bungarotoxin bindings sites decreased slightly after preganglionic denervation (10% at 7 days). In contrast, postganglionic axotomy causes almost complete disappearance of the binding sites (90%) when the toxin binding assay was performed on whole ganglia ( in situ experiments). Since experiments on homogenates (binding in vitro) revealed a smaller (35%) loss of sites, it is suggested that the disappearance of the α-bungarotoxin binding sites in situ is a reflection of a partial loss of the original sites and the masking of the residual ones. The combination of pre- and postganglionic axotomy resulted in changes in the number of binding sites not significantly different from those occurring with postganglionic axotomy alone. It is concluded that the α-bungarotoxin binding sites in the rat superior cervical ganglion are influenced by postganglionic rather than preganglionic signals, but that chronic treatments with drugs acting at the postganglionic (adrenergic) nerve terminal (6-hydroxydopamine and reserpine) do not mimic the effect of the axotomy on the toxin binding sites.