Specific binding of alpha-bungarotoxin to synaptic membranes in rat sympathetic ganglion: computer best-fit analysis of electron microscope radioautographs.

In the rat superior cervical sympathetic ganglion (SCG), alpha-bungarotoxin (alpha BT) demonstrates binding that is saturable and inhibited by nicotinic ligands. However, alpha BT does not inhibit the physiological response of ganglionic neurons to preganglionic stimulation or to exogenously applied acetylcholine. Thus the specificity of alpha BT for ganglionic nicotinic cholinergic receptors has been questioned. The present study provides a morphological localization of the binding sites of 125I-labelled alpha BT in the rat SCG using the method of Blackett and Parry on electron microscopic radioautographs. The distribution of grains resulting from specific binding was calculated by subtracting the nonspecific distribution (alpha BT in the presence of D-tubocurarine, a known nicotinic ligand) from the total grain distribution (alpha BT alone). A hypothetical grain distribution was obtained based on the geometrical properties of the tissue sections. A computer minimizing routine was employed to adjust the relative weights of each of the potential sources of hypothetical grains until a 'best-fit' with the real grain distributions occurred. The nonspecific binding of alpha BT was uniform across all tissue components, with the exception of a significant concentration on the membrane of the ganglion cell body. By contrast, the specific binding of alpha BT was highly localized to synaptic membranes, and to a lesser extent, to dendritic membranes.