Multiple Apparent Alpha-Noradrenergic Receptor Binding Sites in Rat Brain: Effect of 6-Hydroxydopamine

[ 3 H]Clonidine (26.7 Ci/mmole) binds with high affinity to sites on rat brain membranes with properties of alpha -noradrenergic receptors. [ 3 H]Clonidine binding shows a biphasic pattern in kinetic and saturation experiments. Fifty percent of the specific binding of 0.4 nM [ 3 H]clonidine is dissociated rapidly in 2 min by excess norepinephrine or clonidine, and the remaining 50% dissociates at a 10-fold slower rate. When slowly-dissociating [ 3 H]clonidine binding is examined in isolation, saturation curves show a single population of high-affinity sites with a K D of 0.5 nM and B max of 1.3-1.6 pmoles/g tissue in cerebral cortex. Rapidly-dissociating [ 3 H]clonidine binding, estimated from the difference between total binding and high-affinity binding, involves a single population of lower affinity sites with a K D of 3.0 nM and B max of 9-10 pmoles/g cerebral cortex tissue. Alpha agonists are in general more potent at the high affinity [ 3 H]clonidine site, while alpha antagonists are more potent at the low affinity site. α-Methylnorepinephrine is less potent than norepinephrine at the high affinity site, but more potent at the low affinity site. Neither [ 3 H]-clonidine binding site resembles the alpha receptor site labeled by [ 3 H]WB-4101. The distribution of high and low affinity [ 3 H]clonidine binding throughout rat central nervous system is different. High affinity binding levels vary 14 fold between lowest values in corpus striatum and cerebellum, and highest values in cerebral cortex. Low affinity binding varies less regionally, with highest levels in hypothalamus. 6-Hydroxydopamine treatment doubles the number of high affinity [ 3 H]clonidine sites in the cortex, but does not alter the number of low affinity sites. 6-Hydroxydopamine increases high affinity binding much more than low affinity binding throughout the brain. 6-Hydroxydopamine also increases by 50% the number of [ 3 H]WB-4101 and [ 3 H]epinephrine alpha receptor sites in the cerebral cortex.