Receptor reserve and turnover of alpha-2 adrenoceptors that mediate the clonidine-induced inhibition of rat locus coeruleus neurons in vivo.

The population of reserve alpha-2 adrenoceptors that mediate the inhibitory effect of clonidine on the activity of locus coeruleus neurons has been studied using extracellular recordings in anesthetized rats. Animals were pretreated with the irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1-2-dihydroquinoline (EEDQ). In rats pretreated with EEDQ (1, 2 and 6 mg/kg, i.p., 6 hr before experiment), there was an increase in firing rate, a reduction in firing regularity (i.e., increased variation coefficient) and an increase in burst firing of locus coeruleus neurons. Partial receptor inactivation with EEDQ (1 and 2 mg/kg, i.p.) caused a dose-dependent shift to the right of dose-effect curves for i.v. administered clonidine together with a reduction in its maximal effect. Higher doses of EEDQ (6 mg/kg, i.p.) completely abolished the effect induced by clonidine. This blockade was associated with a progressive decrease in the number of remaining receptors (noninactivated receptors). The pseudo-constant of dissociation for the drug-receptor complex was calculated to be approximately 70 micrograms/kg. The receptor occupancy-effect relationship was hyperbolic giving a value of only approximately 4% occupancy at 50% maximal effect. Estimates of noninactivated receptors and percentage of receptor occupancy at 50% of maximal effect were comparable when locally administered clonidine was used. After complete receptor inactivation with EEDQ (6 mg/kg), dose-effect curves for clonidine recovered gradually. The inhibitory effect of clonidine returned faster (half-life = 14 hr) than the receptor pool (half-life = 37 hr). These results indicate that locus coeruleus neurons have a large reserve of alpha 2 adrenoceptors that in addition, are rapidly turned over.