Functional Supersensitivity of Antinociceptive Spinal Cord α2‐Adrenoceptors, Induced by Depletion of Endogenous Noradrenaline, is Associated with an Enhanced Sensitivity for Guanine Nucleotide Regulation of3H‐Clonidine Binding

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Abstract
In spinal cords from normal mice,3H‐clonidine bound to α2‐adrenoceptors with an apparant Kdof 4.6 nM and capacity (Bmax) of 430 fmol/mg protein. The non‐hydrolyzable GTP analogue Gpp(NH)p was found to dose‐dependently reduce the binding of3H‐clonidine the effect of Gpp(NH)p being essentially maximal at 10‐4M. Gpp(NH)p was found to reduce the apparent affinity as well as the apparent number of binding sites for3H‐clonidine; the Kdof3H‐clonidine being 16 nM and the Bmax, 300 fmol/mg protein when 10‐4M of the nucleotide was present. In mice pretreated with the noradrenaline neurotoxin DSP4 for 14 days prior to assay, the specific binding of nearly saturating concentrations (10–30 nM) of3H‐clonidine was virtually the same as for control mice. However, for the DSP4 treated animals the down‐regulation of3H‐clonidine binding induced by 10‐4M Gpp(NH)p was significantly higher than for control mice. Thus, at a3H‐clonidine concentration of 10 nM the down‐regulation induced by Gpp(NH)p was 64.1±2.6% for the DSP4 versus 58.9±2.3% for the control mice (P3H‐clonidine concentration of 30 nM the down‐regulation was 52.4±1.4% for DSP4 versus 41.8±4.1% for control mice (P2‐adrenoceptors.
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