Abstract
The primary structure of frog neurotensin (fNT) has recently been determined and it has been shown that fNT is a potent stimulator of alpha-MSH secretion by frog pituitary melanotropes. In the present study, we have investigated the effects of fNT on the electrical activity of cultured frog melanotropes by using the patch-clamp technique and we have determined the pharmacological profile of the receptors mediating the effect of fNT. In the cell-attached configuration, fNT (10(-7) M) provoked an increase in the action current discharge followed by an arrest of spike firing. In the gramicidin-perforated patch configuration, fNT (10(-7) M) induced a depolarization accompanied by an increase in action potential frequency and a decrease in membrane resistance. Administration of graded concentrations (10(-10) to 10(-6) M) of fNT or the C-terminal hexapeptide NT(8-13) caused a dose-dependent increase in the frequency of action potentials with EC(50) of 2 x 10(-8) and 5 x 10(-9) M, respectively. The stimulatory effect of fNT was mimicked by various pseudopeptide analogs, with the following order of potency: Boc-[Trp(11)]NT(8-13) > Boc-[D-Trp(11)]NT(8-13) > Boc-[Lys(8,9), Nal(11)]NT(8-13) > Boc-[Psi11,12]NT(8-13). In contrast, the cyclic pseudopeptide analogs of NT(8-13), Lys-Lys-Pro-D-Trp-Ile-Leu and Lys-Lys-Pro-D-Trp-Glu-Leu-OH, did not affect the electrical activity. The NTS1 receptor antagonist and nts2 receptor agonist SR 48692 (10(-5) M) stimulated the spike discharge but did not block the response to fNT. In contrast, SR 142948A (10(-5) M), another NTS1 receptor antagonist and nts2 receptor agonist, inhibited the excitatory effect of fNT. The specific nts2 receptor ligand levocabastine (10(-6) M) had no effect on the basal electrical activity and the response of melanotropes to fNT. In cells which were dialyzed with guanosine-5'-O-(3-thiotriphosphate) (10(-4) M), fNT caused an irreversible stimulation of the action potential discharge. Conversely, dialysis of melanotropes with guanosine-5'-O-(2-thiodiphosphate) (10(-4) M) completely blocked the effect of fNT. Pretreatment of cells with cholera toxin (1 microg/ml) or pertussis toxin (0.2 microg/ml) did not affect the electrical response to fNT. Intracellular application of the G(o/i/s) protein antagonist GPAnt-1 (3 x 10(-5) M) had no effect on the fNT-evoked stimulation. In contrast, dialysis of melanotropes with the G(q/11) protein antagonist GPAnt-2A (3 x 10(-5) M) abrogated the response to fNT. The present data demonstrate that fNT is a potent stimulator of the electrical activity of frog pituitary melanotropes. These results also reveal that the electrophysiological response evoked by fNT can be accounted for by activation of a G(q/11)-protein-coupled receptor subtype whose pharmacological profile shares similarities with those of mammalian NTS1 and nts2 receptors.