Melanostatin (NPY) inhibited electrical activity in frog melanotrophs through modulation of K+, Na+ and Ca2+ currents.
1. Melanostatin, a thirty-six amino acid peptide recently isolated from the frog brain due to its ability to inhibit alpha-melanocyte-stimulating hormone (alpha-MSH) release, is the amphibian counterpart of mammalian neuropeptide Y (NPY). The effect of synthetic melanostatin on the bioelectrical activity of cultured frog melanotrophs was studied in 124 cells by using the whole-cell patch-clamp technique. 2. In current-clamp experiments, melanostatin (1 microM) provoked a reversible hyperpolarization and a suppression of spontaneous action potentials. In some cells the hyperpolarizing response was absent, but an arrest of spike firing still occurred. 3. Melanostatin-induced hyperpolarization was associated with a decrease in membrane resistance. In voltage-clamp experiments, melanostatin induced an outward current at a constant command potential. This hyperpolarizing outward current appeared to be carried by potassium ions. 4. Cell dialysis with the non-hydrolysable GTP analogue guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) sustained the outward current produced by melanostatin. Dopamine (1 microM), which generates a similar hyperpolarizing outward current in frog melanotrophs, was not capable of increasing the current provoked by melanostatin and sustained by GTP gamma S. 5. Melanostatin also modulated voltage-operated currents. The amplitude of voltage-activated potassium current was increased by 30%. 6. Melanostatin reduced the fast sodium current. This inhibitory effect was rather persistent compared to the other modulated currents. 7. Melanostatin markedly scaled down high voltage-activated N- and L-like calcium currents. The activation kinetics of these two calcium currents were not altered by the peptide. 8. Pretreatment of melanotrophs with pertussis toxin (1 microgram ml-1) blocked melanostatin-induced inhibition of N- and L-like calcium currents. 9. It is concluded that the NPY-related peptide melanostatin generates a very complex pattern of electrical responses in frog melanotrophs, including hyperpolarization and modulation of voltage-activated currents underlying action potentials. G proteins appear to mediate at least part of these effects.
Valentijn
J A
JA
European Institute for Peptide Research, Laboratory of Molecular Endocrinology, CNRS URA 650, UA INSERM, University of Rouen, Mont-Saint-Aignan, France.
Vaudry
H
H
Kloas
W
W
Cazin
L
L
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000378:metabolism; D066298:In Vitro Techniques; D009075:MSH Release-Inhibiting Hormone / Q000494:pharmacology*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D010902:Pituitary Gland / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D011895:Rana ridibunda; D012964:Sodium / Q000378:metabolism
D009075:MSH Release-Inhibiting Hormone; D012964:Sodium; D011188:Potassium; D002118:Calcium