Modifying effects of histamine on circadian rhythms and neuronal excitability

Histaminergic activity shows a clear circadian rhythm: high levels during t he active period (in rodents at night, in monkeys and humans during the day ), and low levels during the sleep period. Histamine appears to be necessar y for the maintenance of the cireadian rhythmicity of the adrenocortical ho rmone release, locomotor activity and food intake, and the sleep-wakefulnes s cycle. In addition, a role for histaminergic neurons in the light entrain ment is implicated. In phase shift studies, histamine given centrally seems to entrain the activity rhythm in the same way as light impulses and inhib ition of histamine synthesis seems to block the entrainment by light. Impor tantly, histamine participates in the control of arousal and may be implica ted in the sleep disturbances in hepatic encephalopathy. Furthermore, evide nce suggests a role for histamine in overall neuronal excitability and seiz ure susceptibility both in animals and humans. Thus, we conclude that hista mine may exert modifying effects on cireadian rhythmicity and neuronal exci tability. (C) 2001 Elsevier Science B.V. All rights reserved.