Exposure to constant darkness enhances the thermic response of the rat to a muscarinic agonist.
Bright artificial light is used to treat patients with major depression with a seasonal component ("Winter Depression"). Hyperactivity of muscarinic cholinergic systems is implicated in the pathophysiology of depression. Continual exposure to bright light for 7 days or during discrete portions of the photoperiod blunts the thermic response to a muscarinic agonist (oxotremorine) in the rat. Exposure to either 24 hours per day of bright light (in contrast to periods of circumscribed exposure) or darkness would tend to produce free-running. Observers have suggested that the reduced responsiveness to oxotremorine may result from the induction of free-running (the "free-running hypothesis"). The "free-running hypothesis" leads to the prediction that rats exposed to constant darkness will exhibit a reduction in thermic responsiveness to oxotremorine. The authors hypothesized that constant exposure to darkness would, contrary to the "free-running hypothesis," enhance the thermic response to oxotremorine. Rats (n = 12) exposed to constant darkness for 7 days exhibited super-sensitivity to oxotremorine 5 days after return to standard light/dark cycle in the vivarium. This argues against the hypothesis that the induction of free-running enhances sensitivity to the thermic effects of oxotremorine.
Flemmer
D D
DD
Department of Psychiatry, University of Texas School of Medicine, Houston 77225.
Dilsaver
S C
SC
Peck
J A
JA
D000818:Animals; D001831:Body Temperature / Q000187:drug effects*; D003624:Darkness; D004305:Dose-Response Relationship, Drug; D008297:Male; D019832:N-Methylscopolamine; D010095:Oxotremorine / Q000494:pharmacology; D010276:Parasympatholytics / Q000494:pharmacology; D010277:Parasympathomimetics / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D012602:Scopolamine Derivatives / Q000494:pharmacology
D010276:Parasympatholytics; D010277:Parasympathomimetics; D012602:Scopolamine Derivatives; D010095:Oxotremorine; D019832:N-Methylscopolamine