Comparison of [125I]-Bolton-Hunter substance P binding in young and aged rat spinal cord

Binding of [I-125]-labeled Bolton-Hunter substance P ([I-125]-BHSP) to NK1 receptors was investigated in the spinal cord of young (3-4 month) and aged (14-16 month) rats. In homogenates of whole spinal cord, the affinity (equilibrium dissociation constant, similar to 210 pM) and maximum density of [I-125]-BHSP binding sites (similar to 0.25 fmol/mg wet weight) were similar for young and aged rats. Autoradiographic studies revealed a similar distribution of [I-125]-BHSP sites in both young and old rats at all spinal levels. Intense binding was observed in the superficial dorsal horn (laminae I-III), grey commissure (lamina X) and thoracic intermediolateral cell column (IML) with lower levels of binding in the deeper dorsal horn (laminae IV-VI) and ventral horn (laminae VII-IX). However, the density of [I-125]-BHSP sites was significantly (P < 0.05) lower in lamina X of lumbar sections of aged rats compared with young controls. These studies suggest that ageing is associated with a selective loss of NK1 receptors in lamina X of the lumbar spinal cord, although the affinity of NK1 receptors in aged rats is unchanged. (C) 1998 Elsevier Science B.V.