[³H]8-OH-DPAT labels a 5-HT site coupled to inhibition of phosphoinositide hydrolysis in the dorsal raphe

The present study was undertaken to compare the properties of the [³H]8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) binding site in the dorsal raphe nucleus with the hippocampal 5-HT(1A) receptor. In both tissues inclusion of 1 mM Mg²⁺ enhanced specific [³H]8-OH-DPAT binding, while 1 mM GTP decreased radioligand binding. [³H]8-OH-DPAT appears to bind to a single population of binding sites in both the hippocampus and the dorsal raphe nucleus, although the K(d) for the radioligand at the dorsal raphe site was five times that observed at the hippocampal 5-HT(1A) receptor. Similarly, although 5-HT and selective 5-HT(1A) receptor ligands displayed high affinity for the [³H]8-OH-DPAT binding site in the dorsal raphe nucleus, the affinity at the dorsal raphe site was less than that observed at the hippocampal 5-HT(1A) receptor. 8-OH-DPAT inhibited forskolin-stimulated adenylyl cyclase activity in the hippocampus, but did not alter enzyme activity in the dorsal raphe nucleus. Conversely, 8-OH-DPAT inhibited the accumulation of [³H]inositol phosphates in the dorsal raphe nucleus, but not in the hippocampus. An inhibition of phosphoinositide hydrolysis in the dorsal raphe nucleus also was found with the putative 5-HT(1A) receptor selective ligands, flesinoxan and gepirone. However, addition of another putative 5HT(1A) receptor selective ligand, buspirone, did not alter the generation of [[³H]inositol phosphates, but blocked the inhibitory effect of 8-OH-DPAT on phosphoinositide hydrolysis. These studies demonstrate that the 8-OH-DPAT binding site in the dorsal raphe nucleus displays a binding profile which is similar to the hippocampal 5-HT(1A) receptor, but unlike this 5-HT(1A) receptor the binding site in the dorsal raphe nucleus is negatively coupled to phosphoinositide turnover.