VAGAL ACTIVATION IN NOVELTY-INDUCED TACHYCARDIA DURING THE LIGHT PHASE IN THE RAT

The effects of repeated exposure to a novel test box on cardiac and be havioral activities (locomotion, rearing, grooming, scanning, and immo bility) were studied in rats tested during the dark phase (''dark'' ra ts) or the light phase (''light'' rats) of the lighting cycle, using a telemetry system for registration of ECGs during the first and fifth tests. Heart rare (HR) was used to monitor sympathetic and parasympath etic activity; the PQ interval was used to monitor parasympathetic act ivity. Behavior was videotaped simultaneously. In light rats, the firs t and fifth exposures to the test box resulted in higher increases of active behavior and HR than in dark rats, whereas the duration of the PQ interval of the ECG was increased in light rats only. This indicate s that in the light phase novelty induces active behavior associated w ith an increase in both sympathetic and vagal outflow, whereas in the dark phase behavioral activation is predominantly associated with incr eased sympathetic activity, without appreciable changes in vagal outfl ow. In addition, light rats showed less active behavior during the fif th than during the first exposure, indicating behavioral habituation. This behavioral habituation to the test box in the light phase coincid ed with vagal habituation (a diminution of the PQ interval). The incre ase of the tachycardiac response during the fifth exposure as compared to the first exposure suggests that it is not likely that sympathetic outflow was part of the habituation process. In dark rats no behavior al or cardiac habituation was found. (C) 1998 Elsevier Science Inc.