The autoradiographic distribution of substance P binding sites in guinea-pig vas deferens is altered by capsaicin pretreatment

Abstract

The distribution of binding sites for [¹²⁵I]Bolton-Hunter substance P (BHSP) was investigated in vasa deferentia from normal, capsaicin-pretreated and vehicle-pretreated guinea-pigs, using qualitative and quantitative autoradiography. Dense binding of BHSP was seen over the outer longitudinal muscle with less over the inner longitudinal muscle. Very low specific binding occurred to the circular muscle and was absent in the mucosa. Characterization in slide-mounted sections showed that binding was saturable and of high affinity, with equilibrium dissociation constant (K_D) 91 ± 15 pM. BHSP was displaced by substance P > neurokinin A > neurokinin B, suggesting binding to NK-1 receptors. Capsaicin pretreatment had no effect on the lengthwise distribution of binding sites but significantly altered their relative distribution between the different smooth muscle layers. There was a very marked increase in BHSP binding over the inner longitudinal muscle and the inner part of the circular muscle layer, whereas binding was virtually abolished over the outer longitudinal muscle, compared with vehicle control. Capsaicin-sensitive binding sites over the outer longitudinal muscle may be located presynaptically on capsaicin-susceptible primary afferent sensory neurons. In contrast, the increase in number of binding sites associated with the inner longitudinal muscle may be due to receptor upregulation resulting from loss of sensory innervation, and suggests that these binding sites are postsynaptic.