Acute restraint stress modifies the heart rate biorhythm in the poststress period

We studied the changes in the heart and the activity biorhythms in mice exposed to acute (one 120-minute session) and repeated (7 two-hour sessions) restraint stress in 129J1/CF1 mice (WT) and in mice without M(2) muscarinic receptors (M(2)KO) during the prestress period, during stress (STR) and for five days after the last stress session (POST). There were changes in the mesor (a midline based on the distribution of values across the circadian cycles; decreased in M(2)KO by 6% over all POST), day means (inactive period of diurnal rhythm in mice; higher in M(2)KO and further increased on STR and on the second to the fifth POST) and night means (active period; lower by 13% in M(2)KO and remained decreased in STR and in POST). The total area under the curve was decreased both in the WT and M(2)KO on STR and in all POST. Repeated stress caused changes over all days of STR, but the initial values were restored in POST. The average night values were decreased, and the day means were increased by 16% over all STR in M(2)KO. The day means decreased by 14% in the 4 POST in WT. The activity biorhythm parameters were almost unchanged. We show here that stress can specifically affect heart biorhythm in M(2)KO mice, especially when the stress is acute. This implies the role of M(2) muscarinic receptor in stress response.