Alpha2 adrenergic receptors are located prejunctionally in the Auerbach's plexus of the guinea pig small intestine: Direct demonstration by radioligand binding

We provide direct evidence that alpha₂-receptors in the guinea pig small intestine are localized prejunctionally in neurons of the Auerbach's plexus. The alpha₂-agonist ligand ³H]clonidine bound to a single saturable class of sites with a K_d of 1–2 nM and a capacity of approximately 70 fmol/mg protein in membranes from the innervated longitudinal and circular muscle layers of the intestine. By a special dissection technique the Auerbach's plexus could be completely removed from the longitudinal muscle. In these denervated preparations the clonidine binding sites were virtually completely removed whereas the expected binding was observed in innervated controls. The innervated preparations also contained a small number of alpha₁-receptors as revealed by binding with ³H]prazosin (capacity approximately 18 fmol/mg protein with a K_d of 0.4-0₃7 nM). Thus, the present study suggests that alpha₂-receptors (³H]clonidine binding sites) are localized in neurons (i.e., prejunctionally) in the Auerbach's plexus of the guinea pig small intestine.