Effect of Ω‐conotoxin, a calcium channel blocker, on the circadian rhythm in rats

Using an Alzet osmotic minipump, we examined the effect of continuous infusion of O‐conotoxin, a N‐type voltage‐sensitive calcium channel blocker, into the third cerebral ventricle above the suprachiasmatic nucleus (SCN) of the hypothalamus on the circadian drinking rhythm of rats maintained under a 12‐h light and 12‐h dark cycle and constant darkness. O‐Conotoxin (10‐6 mol/h) infusion disrupted the rhythm in both conditions. In contrast, infusions of nifedipine (104 mol/h), a dihydropyridine sensitive L‐type calcium channel blocker, did not eliminate the rhythm. These findings suggest that N‐type voltage‐sensitive calcium channels are involved in the mechanism of generation of the circadian rhythm driven by the circadian oscillator in the SCN.