Pulsatile release of histamine in the hypothalamus of conscious rats

The pattern of histamine release was investigated in the hypothalamus of the conscious, freely moving rat over 20 h. Under anaesthesia, a guide cannula was stereotaxically inserted into the posterior hypothalamus. In the conscious animal, the stylet of the guide cannula was replaced by a push-pull cannula, and the hypothalamus was superfused with artificial cerebrospinal fluid. Histamine was determined radioenzymatically in the superfusate which was continuously collected in time periods of 20 min. The release rate of histamine fluctuated according to an ultradian rhythm (frequency: 1 cycle per 83 min) and a circadian rhythm with the highest release rate of histamine between 11:00 p.m. and 1:00 a.m. The release rate of histamine during darkness was higher than that during the light period. The results demonstrate that, in the brain, neuronal histamine is released according to rhythms with various frequencies.