EFFECTS OF HISTAMINE AND ALPHA-FLUOROMETHYLHISTIDINE INJECTIONS ON CIRCADIAN PHASE OF FREE-RUNNING RHYTHMS

Sequential IP injections of α-fluoromethylhistidine at CT-6 and CT-18 elicited transient delay during the injection period in the onset of activity in the free-running circadian rhythms of locomotor and drinking activities. Histamine injection ICV resulted in various changes, permanent or transient, in the circadian phase of the free-running rhythms depending on the subjective time of the injection: it caused transient advance in onset of activity when injected at CT-0, 3, 6, 9, 18 or 21, whereas it caused permanent phase-delay when injected at CT-12 or 15. These permanent effects were plotted in a phase-response curve. Histamine administration at CT-6 and CT-18 resulted in transient, reciprocal changes of almost equivalent amplitude changes to those of histamine depletion at the corresponding subjective times. Although this transient shift may represent a masking effect of the drug, HA may modulate the circadian synchronization mechanism(s) during the advance sensitive phase. Moreover, HA seems to elicit phase-delay by affecting the light-relaying tract. © 1990.