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

Abstract

Binding of [ 125I]-labeled Bolton–Hunter substance P ([ 125I]-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, ∼210 pM) and maximum density of [ 125I]-BHSP binding sites (∼0.25 fmol/mg wet weight) were similar for young and aged rats. Autoradiographic studies revealed a similar distribution of [ 125I]-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 [ 125I]-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.