title: Light and norepinephrine similarly prevent damping of the melatonin rhythm in cultured chick pineal cells: regulation of coupling between the pacemaker and overt rhythms?
abstract: The circadian rhythm of melatonin output displayed by chick pineal cells in static culture damps rapidly in constant red light (RR). This can be seen in the first cycle following a switch from a cycle of 12 hr white light (L) and 12 hr red light (R) to RR. Melatonin output is higher during the "day" in R than it is in L, but higher that next night (in R) after daytime L than after daytime R. This effect might be due entirely to the entraining effect of L. Alternatively, the higher nocturnal output after daytime L could be related to the acute suppression caused by L; it might be a "rebound" phenomenon. These alternative hypotheses differ in their predictions for the effects of norepinephrine (NE) and pertussis toxin (PT). Previous results dissociated the acute and entraining effects of L: PT blocks the acute effect but not the entraining effect of L. NE mimics the acute effect of L (and is blocked by PT), but not the entraining effect. If L prevents damping entirely by entrainment, then NE should not mimic and PT should not block this same-cycle effect of daytime L on nocturnal melatonin output. However, the present research found that NE did mimic and PT did block this effect, indicating that the ability of L to prevent damping is mediated by a same-cycle "rebound" following L's acute inhibition of melatonin production. Furthermore, NE enhanced the "rebound" effect of daytime L, and cycles substituting NE for L were effective in driving the melatonin rhythm. Lowering extracellular potassium did not induce a "rebound," and adding exogenous melatonin did not prevent one. The difference between nocturnal melatonin synthesis after daytime R and that after daytime L or NE implies regulation of coupling between the output of the circadian pacemaker and melatonin production. These results also suggest a role for NE in regulating and maintaining the expression of the melatonin rhythm.
authors.affiliation: Section on Biochemical Pharmacology, National Institute of Mental Health, Bethesda, Maryland 20892.
authors.forename: M
authors.initials: M
authors.lastname: Zatz
mesh_terms: D000818:Animals; D001683:Biological Clocks / Q000502:physiology*; D002478:Cells, Cultured; D002645:Chickens; D008027:Light; D008550:Melatonin / Q000378:metabolism*; D009568:Nitrendipine / Q000494:pharmacology; D009638:Norepinephrine / Q000494:pharmacology*; D010507:Periodicity; D037342:Pertussis Toxin; D010870:Pineal Gland / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D014660:Vasoactive Intestinal Peptide / Q000494:pharmacology; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
chemical_list: D010566:Virulence Factors, Bordetella; D014660:Vasoactive Intestinal Peptide; D009568:Nitrendipine; D037342:Pertussis Toxin; D008550:Melatonin; D011188:Potassium; D009638:Norepinephrine