Brain histamine: plasma corticosterone spontaneous locomotor activity and temperature.

Hypothalamic histamine exhibited circadian fluctuations in male Sprague-Dawley rats; low values were found during the dark period when spontaneous locomotor activity (S.L.A.) and temperature were elevated. A relatively high hypothalamic histamine level was observed during the early period of the light cycle and was associated with decreased S.L.A. and temperature. Histamine concentration was high when corticosterone levels were low at the end of the dark cycle and during the morning hours (4 a.m.-1 p.m.); but histamine levels were relatively constant while corticosterone concentration dropped during afternoon and early night hours (4 p.m.-10 p.m.). Furthermore, the lowest hypothalamic histamine level (at 1 a.m.) was associated with the average plasma corticosterone value, thus no consistent relationship between histamine and corticosterone levels could be observed. Circadian fluctuations in brain histamine may support its role in brain function.