THE CONTROL OF CIRCADIAN-RHYTHMS AND THE LEVELS OF VASOACTIVE-INTESTINAL-PEPTIDE MESSENGER-RNA IN THE SUPRACHIASMATIC NUCLEUS ARE ALTERED IN SPONTANEOUSLY HYPERTENSIVE RATS

Vasoactive intestinal peptide (VIP) has been localized within the supr achiasmatic nucleus of the hypothalamus (SCN) and appears to play an i mportant role in the entrainment of circadian rhythms with the light-d ark (LD) cycle. The spontaneously hypertensive rat (SHR), an inbred st rain used extensively in research on primary hypertension, has signifi cantly more VIP mRNA in its brain than normotensive Wistar-Kyoto contr ol (WKY) rats. Because VIP levels are abnormally high in SHR rats the present study examined whether the mechanisms controlling circadian rh ythms are also altered in SHR rats. When entrained to a 24 h LD cycle, SHR rats began their wheel-running rhythm approximately 1.5 h earlier than WKY controls. SHR rats re-entrained to a phase delay in the LD c ycle more slowly than did WKY rats, but tended to re-entrain to a phas e advance more rapidly. The free-running period of SHR rats in both co nstant light and constant dark was significantly shorter than that of WKY rats. In SHR rats, phase delays produced by 1-h pulses of light we re less than one-half the magnitude of the delays seen in WKY rats; ho wever, the phase advances were nearly twice that of WKY rats. Using in situ hybridization, the SCN levels of mRNA encoding VIP were found to be significantly greater in SHR rats, but the mRNA levels of another peptide important for entrainment, gastrin releasing peptide, did not differ between SHR and WKY rats. These data indicate that the mechanis ms controlling circadian rhythms in SHR rats differ significantly from those controlling rhythms in WKY rats and that VIP mRNA is significan tly elevated within the SCN of SHR rats.-The role of VIP in the entrai nment of circadian rhythms is discussed.