Binding of [³H]prazosin and [³H]p-aminoclonidine to α-adrenoceptors in rat spinal cord

α-Adrenoceptors in spinal cord appear to play a role in a number of physiologic processes including the control of blood pressure, pain and motor function. In order to evaluate more clearly these potential roles, the characteristics of binding of [³H]prazosin ([³H]PRZ) to spinal α₁ adrenoceptors and [³H]p-aminoclonidine ([³H]PAC) to spinal α₂ adrenoceptors were determined. Binding of each ligand to their respective adrenoceptors was saturable and Scatchard analysis revealed binding of each to a single class of adrenoceptors with characteristics of [³H]PRZ binding of Bmax = 78 fmol/mg protein and K_d = 0.75 nM and [³H]PAC binding B_max = 70 fmol/mg protein and K_d = 1.39 nM. Whereas [³H]PRZ specific binding (B_max) was unaltered by guanine nucleotides, [³H]PAC binding was increased with addition of 10 μM guanosine triphosphate (GTP) (P < 0.05) and decreased with either 50 μM GTP or guanyl-5′-yl-imidodiphosphate [Gpp(NH)p] (P < 0.01). Competition for specific [³H]PRZ and [³H]PAZ binding by various α₁and α₂ adrenoceptor vs 4300 nM for yohimbine) and [³H]PAC defines α₂ adrenoceptors (K_i = 1.06 nM for yohimbine vs 15480 nM for prazosin). Regional spinal cord studies demonstrated that dorsal spinal cord in the lumbar region contains the highest density of both [³H]PRZ (B_max = 93 ± 14 fmol/mg protein) and [³H]PAC (Bmax = 101 ± 6 fmol/mg protein). In contrast, lowest binding was evident in thoracic cord with equal levels in both dorsal and ventral regions (B_max = 44-48 fmol/mg protein). The regional distribution of both α₁and α₂ adrenoreceptors in spinal cord compares to the localization previously classified functional utilizing various pharmacological agonists and antagonists at norepinephrine receptors.