Diurnal rhythm of vasopressin mRNA species in the rat suprachiasmatic nucleus: independence of neuroendocrine modulation and maintenance in explant culture

Abstract

Vasopressin mRNA in the suprachiasmatic nucleus (SCN) of the rat brain exhibits diurnal variation in poly(A) tail length; a single species of mRNA identical in size to that in other hypothalamic nuclei is expressed in the light phase of the daily cycle whereas a second, smaller species is expressed in the dark phase. We have investigated the neuroendocrine factors which may regulate this rhythm by comparing vasopressin mRNA size with Northern analysis of RNA extracted from SCN tissue taken at 09.00 h (light phase) and 21.00 h (dark phase). The consistent rhythmic variation observed in normal male rats was not modified in either adrenalectomized or castrated animals or in ovariectomized female rats. The rhythm was also not disrupted following treatment with the serotonin-depleting agent parachlorophenylalanine, or following treatment with either melatonin or the benzodiazepine, triazolam. We next investigated whether the daily pattern of expression was maintained in isolated SCN. Microdissected blocks containing the paired SCN were explanted into culture at various times of the day for a period of 4 h. Vasopressin mRNA extracted from light phase cultures (11.00-15.00 h) exhibited no size change from control SCN mRNA taken at either 11.00 h or 15.00 h. In contrast, mRNA from cultures maintained over the period 16.00-20.00 h (lights off: 18.00 h) exhibited a marked size difference from 16.00 h controls, being similar to the smaller species observed in 20.00 h controls. Similarly, vasopressin mRNA from cultures maintained over the period 05.00-09.00 h (lights on 06.00 h) was similar in size to 09.00 h controls and contrasted to the pattern of expression observed in 05.00 h controls. We conclude that the diurnal rhythm of vasopressin mRNA poly(A) tail length in the SCN does not appear to be coordinated by extrinsic neuroendocrine signals, rather it is controlled directly by mechanisms endogenous to the SCN. © 1989.