<Emphasis Type="SmallCaps">l</Emphasis>-5-hydroxytryptophan resets the circadian locomotor activity rhythm of the nocturnal Indian pygmy field mouse, <Emphasis Type="Italic">Mus terricolor</Emphasis> |
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Authors: | Priyoneel?Basu Email author" target="_blank">Muniyandi?SingaravelEmail author Chandana?Haldar |
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Institution: | (1) Chronobiology Laboratory, Department of Zoology, Banaras Hindu University, Varanasi, India;(2) Pineal Research Laboratory, Department of Zoology, Banaras Hindu University, Varanasi, India |
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Abstract: | We report that l-5-hydroxytryptophan (5-HTP), a serotonin precursor, resets the overt circadian rhythm in the Indian pygmy field mouse, Mus terricolor, in a phase- and dose-dependent manner. We used wheel running to assess phase shifts in the free-running locomotor activity
rhythm. Following entrainment to a 12:12 h light–dark cycle, 5-HTP (100 mg/kg in saline) was intraperitoneally administered
in complete darkness at circadian time (CT)s 0, 3, 6, 9, 12, 15, 18, and 21, and the ensuing phase shifts in the locomotor
activity rhythm were calculated. The results show that 5-HTP differentially shifts the phase of the rhythm, causing phase
advances from CT 0 to CT 12 and phase delays from CT 12 to CT 21. Maximum advance phase shift was at CT 6 (1.18 ± 0.37 h)
and maximum delay was at CT 18 (−2.36 ± 0.56 h). No extended dead zone is apparent. Vehicle (saline) at any CT did not evoke
a significant phase shift. Investigations with different doses (10, 50, 100, and 200 mg/kg) of 5-HTP revealed that the phase
resetting effect is dose-dependent. The shape of the phase–response curve (PRC) has a strong similarity to PRCs obtained using
some serotonergic agents. There was no significant increase in wheel-running activity after 5-HTP injection, ruling out behavioral
arousal-dependent shifts. This suggests that this phase resetting does not completely depend on feedback of the overt rhythmic
behavior on the circadian clock. A mechanistic explanation of these shifts is currently lacking. |
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