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[3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) is thought to label a single population of serotonin (5-HT)1A receptor, but some reports implicate multiple binding sites exist. In addition, while 5-HT1A receptor activates or inhibits adenylate cyclase, 5-HT1A receptor high and low affinity states are not reported. In this experiment, we found that [3H]8-OH-DPAT had multiple binding sites, which contained 5-HT1A receptor high and low affinity states, in rat brain membranes. [3H]8-OH-DPAT saturation binding experiment revealed high and low affinity binding sites existed. High affinity binding site was dense in hippocampus and sparse in striatum. 5-HT agonist and antagonist biphasically displaced [3H]8-OH-DPAT binding in frontocortical, hippocampal, and striatal membranes. These drugs potently displaced high affinity [3H]8-OH-DPAT binding, but clomipramine (5-HT reuptake inhibitor) potently displaced low affinity binding. High affinity [3H]8-OH-DPAT binding site was decreased by guanosine triphosphate and Na+, but increased by divalent cations, implicating coupling with G protein(s). Low affinity [3H]8-OH-DPAT binding site was decreased by cations, especially by monovalent cations and Ca2+. After the destruction of 5-HT neuron by parachloroamphetamine, only the low affinity binding site decreased. These results indicate that [3H]8-OH-DPAT not only labels the 5-HT1A receptor high and low affinity states but has presynaptic binding site relating to 5-HT reuptake site.
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