Melatonin signal transduction in hamster brain: inhibition of adenylyl cyclase by a pertussis toxin-sensitive G protein.
Melatonin signal transduction was examined in median eminence/pars tuberalis (ME/PT) explants from Djungarian hamsters. High affinity melatonin receptors in hamster ME/PT were first quantified by in vitro autoradiography using the potent melatonin agonist 125I-labeled melatonin ([125I]MEL). Scatchard analysis of [125I]MEL binding in ME/PT revealed high affinity receptors [dissociation constant (Kd) = 2.75 X 10(-11) M]. [125I]MEL binding was markedly reduced by guanine nucleotides; treatment with the nonhydrolyzable GTP analog guanosine 5'-O-(3-thiotriphosphate) caused a 10-fold decrease in receptor affinity. Melatonin (10 nM) significantly inhibited forskolin-stimulated cAMP accumulation in ME/PT, but not in pituitary or pineal glands. In ME/PT explants, melatonin and 6-chloromelatonin inhibited forskolin-stimulated cAMP accumulation in a dose-dependent manner with similar potency (significant inhibition for each at concentrations greater than or equal to 100 pM). Serotonin significantly inhibited forskolin-stimulated cAMP levels only at doses greater than or equal to 100 microM. Inhibition of [125I]MEL binding in ME/PT by these three indolamines paralleled that determined for inhibition of forskolin-stimulated cAMP accumulation. Pertussis toxin treatment (1 microgram/ml) blocked the ability of melatonin (10 nM) to inhibit forskolin-stimulated cAMP accumulation and significantly reduced [125I]MEL binding. Pertussis toxin ADP-ribosylated the alpha-subunits of at least two guanine nucleotide-binding proteins in ME/PT explants with molecular weights of approximately 40 K. Melatonin did not increase phosphodiesterase activity in ME/PT explants. The results strongly suggest that a signal transduction pathway for melatonin in mammals involves inhibition of adenylyl cyclase by a pertussis toxin-sensitive guanine nucleotide-binding protein.
Carlson
L L
LL
Laboratory of Developmental Chronobiology, Massachusetts General Hospital, Boston 02114.
Weaver
D R
DR
Reppert
S M
SM
D037361:Adenylate Cyclase Toxin; D000067956:Adenylyl Cyclase Inhibitors; D000818:Animals; D001921:Brain / Q000187:drug effects / Q000201:enzymology / Q000502:physiology*; D006224:Cricetinae; D000242:Cyclic AMP / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000502:physiology*; D007700:Kinetics; D008550:Melatonin / Q000502:physiology*; D009924:Organ Culture Techniques; D037342:Pertussis Toxin; D044094:Receptors, Melatonin; D017981:Receptors, Neurotransmitter / Q000502:physiology*; D015398:Signal Transduction / Q000187:drug effects; D010566:Virulence Factors, Bordetella / Q000494:pharmacology*
D037361:Adenylate Cyclase Toxin; D000067956:Adenylyl Cyclase Inhibitors; D044094:Receptors, Melatonin; D017981:Receptors, Neurotransmitter; D010566:Virulence Factors, Bordetella; D000242:Cyclic AMP; D037342:Pertussis Toxin; D019204:GTP-Binding Proteins; D008550:Melatonin