Neurotensin Modulates the Electrical Activity of Frog Pituitary Melanotropes via Activation of a G-Protein-Coupled Receptor Pharmacologically Related to Both the NTS1 and nts2 Receptors of Mammals
Résumé
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.