Vasopressin-deficient suprachiasmatic nucleus grafts re-instate circadian rhythmicity in suprachiasmatic nucleus-lesioned arrhythmic rats.

It was investigated whether grafts of the suprachiasmatic nucleus could re-instate circadian rhythmicity in the absence of its endogenous vasopressin production and whether the restored rhythm would have the long period length of the donor. Grafts of 17-days-old vasopressin-deficient homozygous Brattleboro rat fetuses, homotopically placed in arrhythmic suprachiasmatic nucleus-lesioned Wistar rats, re-instated circadian drinking rhythm within 20-50 days similar as seen for grafts of heterozygous control fetuses. Period length of the recovered rhythm revealed a similar difference (average 24.3 vs. 23.8 h) as reported for the rhythm between the adult Brattleboro genotypes. In all transplants, also those of the two-third non-recovery rats, a surviving suprachiasmatic nucleus was visible as a vasoactive intestinal polypeptide-positive neuronal cell cluster, whereas heterozygous transplants also revealed the complementary vasopressinergic cell part. Explanation of the absence of recovery failed since no undisputable correlation emerged between recovery of rhythm and vasoactive intestinal polypeptide, vasopressin and/or somatostatin immunocytochemical characteristics of the suprachiasmatic nucleus of the transplant. Special focus on the somatostatinergic neurons revealed their presence only occasionally near or in between the vasoactive intestinal polypeptidergic and (in the case of heterozygous grafts) vasopressinergic cell cluster. However their aberrant cytoarchitectural position appeared not to have affected the possibility to restore drinking rhythm of the suprachiasmatic nucleus-lesioned arrhythmic rat. It was concluded that grafted Brattleboro fetal suprachiasmatic nucleus develop their intrinsic rhythm conform their genotype and that vasopressin is not a crucial component in the maintenance nor in the transfer of circadian activity of the biological clock for drinking activity. Vasopressin of the suprachiasmatic nucleus may instead serve modulation within the circadian system.