Diurnal variations in brain and pituitary histamine and histamine-N-methyltransferase in the rat and guinea pig.

The circadian variation of histamine concentrations and histamine-N-methyltransferase (E.C. 2.1.1.8., HNMT) activities were measured in different parts of rat and guinea pig brain and pituitary glands after an adaptation to a daily 12-hour light and 12-hour dark schedule. A significantly higher histamine content was found in rat hypothalamus at 19.00 hours, at the end of the light period, then a decline during the dark hours followed by an increase during the daytime. No similar changes were seen either in the other parts of the brain or in the pituitary gland. In the guinea pig hypothalamus a somewhat reverse pattern of histamine content was seen as compared to that in the rat, the peak occurred at 8.00 hours and the lowest level at 14.00 hours. In other parts of the brain the changes were not significant. In rat hypothalamus there were also significant changes in HNMT activities, with a higher level during darkness. In the guinea pig, the HNMT activities were much higher than in the rat, and in hypothalamus, cortex and striatum there was a modest increase at 2.00 hours, with another upward trend at 14.00 hours. It will be of interest to see whether or not these species differences reflect different circadian activity rhythms and/or different living habits of these two species, and with which physiological functions they may be connected.