DTG and (+)-3-PPP inhibit a ligand-activated hyperpolarization in mammalian neurons

The effects of three compounds with high affinity for the haloperidol-sensitive alpha-binding site were studied with intracellular recordings in the vitro neuronal preparations of the rat locus ceruleus, rat dorsal raphe and the guinea pig submucous plexus. Both (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)-3-PPP] and 1,3-di-o-tolylguanidine (DTG) inhibited the hyperpolarization induced by a ligand-activated potassium conductance. In the locus ceruleus, (+)-3-PPP and DTG produced a maximal 40 to 45% inhibition of the [Met5]enkephalin hyperpolarization, and had EC50 values of 6.6 and 2.2 microM, respectively. In the submucous plexus, the two compounds had a similar action on the alpha-2 adrenoceptor agonist UK14304 hyperpolarization, producing a maximal 50% inhibition with EC50 values of 140 and 32 nM, respectively. In addition, DTG inhibited the alpha-2-mediated inhibitory postsynaptic potential in both preparations. In contrast, (+)-3-PPP increased and prolonged the inhibitory postsynaptic potential. This action is qualitatively similar to the actions of cocaine on locus ceruleus and submucous plexus neurons. Haloperidol (1-10 microM) shared none of these actions. It is concluded that DTG and (+)-3-PPP are inhibitors of the opiate and alpha-2-mediated hyperpolarization at a postreceptor site, possibly the potassium channel. In addition, (+)-3-PPP, but not DTG, inhibits norepinephrine reuptake. None of these effects appear to be related to the sigma -binding site, because haloperidol acted as neither an agonist nor an antagonist.