Effect of an Antagonist of Vasoactive Intestinal Polypeptide on Biological Rhythm of Rest Activity in the Rat

Abstract Vasoactive Intestinal Polypeptide (VIP), has been found in different neurotransmitter systems and exists in various nerve tracts in the brain. Potential role of this peptide in physiological processes such as regulation of sleep and wakefulness, and biological rhythms has been confirmed in several reports. In the present research effects of intracerebroventricular (ICV) injection of a VIP antagonist, 4-CID- Phe6/leu 17, at different circadian time on rest-activity cycle of rats during free- run, has been investigated. Results of this study showed that brain clock in definite time of its endogenous rhythmic activity is sensitive to the antagonist and presents this sensivity through phase advance following ICV injection at CT 6. While injection of same drug at other circadian time and also injection of artificial cerebrospinal fluid has no effect on this rhythm. According to our experiment and considering the results of others, the sensitivity of locomotor, rhythms to VIP ergic system is revealed. In previous studies, effect of this antagonist on sleep homeostasis has also been demonstrated. Regarding difference in type and distribution of known VIP receptors in sleep homeostasis and biological rhythm of locomotor, activity, it is clear that above mentioned synthetic antagonist has no specificity for known VIP receptors.