A novel phenylaminotetralin (PAT) recognizes histamine H1 receptors and stimulates dopamine synthesis in vivo in rat brain.

A series of novel phenylaminotetralins (PATs) previously was shown to recognize discrete binding sites that are stereoselectively labeled by [3H]-(-)-trans-1-phenyl-3-N,N-dimethylamino-1,2,3,4-tetrahydronaphthalen e (H2-PAT) and highly localized in catecholaminergic nerve terminal regions in guinea pig forebrain. Furthermore, certain PATs stimulate tyrosine hydroxylase and dopamine synthesis in guinea pig and rat brain in vitro. In the current studies, we characterized sites labeled by [3H]-(-)-trans-H2-PAT and measured effects of PATs on dopamine synthesis in vivo in rat brain. [3H]-(-)-Trans-H2-PAT binds saturably (Bmax approximately 13 fmol/mg protein) and with high affinity (K(D) approximately 0.5 nM) to a single population of sites in rat brain. The ligand binding profile of [3H]-(-)-trans-H2-PAT labeled sites is very similar to histamine H1 receptors labeled with [3H]-mepyramine. After i.c.v. injection to rats, (+/-)-trans H2-PAT (4-40 nmoles/kg) stimulates dopamine synthesis (to about 180% of control levels) selectively in the limbic brain region nucleus accumbens vs. the extrapyramidal region striatum; this effect is fully blocked by (+/-)-cis-H2-PAT and the H1 antagonist triprolidine. At higher doses (> 40 nmoles/kg), the observed stimulation of dopamine synthesis is attenuated to control levels, likely due to activation of feedback mechanisms resulting from non-receptor mediated displacement of intraneuronal dopamine. We propose that PATs represent a novel class of ligands for H1 receptors that can modulate tyrosine hydroxylase activity and dopamine synthesis in the limbic region of mammalian forebrain.