NAN-190 - AGONIST AND ANTAGONIST INTERACTIONS WITH BRAIN 5-HT1A RECEPTORS

NAN-190 has been reported to be a 5-HT1A antagonist in drug discrimination studies. In order to determine if the effect of NAN-190 was directly due to competitive inhibition at 5-HT1A receptors, 5-HT1A-mediated inhibition of adenylyl cyclase in hippocampal membranes was investigated. NAN-190 (10-10-10-5 M), by itself, was found to have no effect on forskolin-stimulated adenylyl cyclase. NAN-190, however, did shift the 5-carboxamidotryptamine (a 5-HT1A agonist) log-concentration inhibition curve to the right in a concentration-dependent manner, typical of competitive antagonism. Schild analysis revealed a KB 5-HT1A antagonist using the in vitro adenylyl cyclase system. [3H]NAN-190 was synthesized and its 5-HT1A receptor binding properties were characterized and compared with the 5-HT1A agonist radioligand, [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT). The 5-HT1A agonists, serotonin (5-HT) and 8-hydroxy-2-(di-n-propylamine(tetralin (8-OH-DPAT) competed with equal affinities regardless of the radiogland used to label the 5-HT1A receptors. [3H]NAN-190 and [3H]8-OH-DPAT labeled the same number of sites in rat hippocampus, striatum and frontal cortex. Guanosine-5′-O-(3-thio)triphosphate (GTPγS) and 5-guanyly;-imidodiphosphate (GppNHp)m non-hydrolyzable analogs of GTP, inhibited specific [3H]NAN-190 binding. Adenosine-5′-O-(3-thio)triphosphate (ATPγS) and 5-adenylyl-imidodiphosphate (AppNHp) were ineffective. This guanylyl nucleotide-specific effect is generally associated with agonist radioligand binding to a GTP-binding protein coupled receptor. However, [3H]8-OH-DPAT was far more sensitive than [3H]NAN-190 to the Bmax reducing effects of GTP and GTPγS. We propose that the test for a reduction in Bmax by non-hydrolyzable guanylyl nucleotides may be more sensitive than other tests for quantifying agonist activity and may demonstrate that NAN-190 has low intrinsic activity. In summary, NAN-190 displayed antagonist-like properties in functional models of 5-HT1A receptor activity and possibly partial agonist-like properties in radioligand binding experiments. © 1990.