Pharmacological characterization and regional distribution of alpha-noradrenergic binding sites of rat spinal cord

In order better to interpret their physiological role in rat spinal cord, we characterized binding sites of [3H]WB-4101 and [3H]p-aminoclonidine ( [3H] PAC), and determined their regional distribution. These binding sites have characteristics required for, respectively, alpha 1 and alpha 2 receptors of norepinephrine. Binding to these sites is saturable, with Kd values of 0.38 nM and 35 nM for high and low affinity binding sites respectively of [3H]WB-4101; and 1.7 nM, for a single binding site of [3H]PAC. For whole cord, Bmax values are 52 and 320 (high and low affinity sites respectively); and 21 fmol/mg protein. Catecholamines compete stereoselectively for these sites, while selected noradrenergic agents compete with an order of potency corresponding to their relative activity at the alpha 1 and alpha 2 receptors. We conclude that spinal alpha 1 and alpha 2 binding sites have the same pharmacologic properties as corresponding peripheral sites. The alpha 2 and, to a lesser degree, the alpha 1 binding sites vary in concentration with region. Our results support the contention that alpha 2 binding sites subserve neuronal function in the spinal cord.