Variability in the Drug Response of M ₄ Muscarinic Receptor Knockout Mice During Day and Night Time

Mice are nocturnal animals. Surprisingly, the majority of physiological/pharmacological studies are performed in the morning, i.e., in the non-active phase of their diurnal cycle. We have shown recently that female (not male) mice lacking the M ₄ muscarinic receptors (MR, M ₄KO) did not differ substantially in locomotor activity from their wild-type counterparts (C57Bl/6Tac) during the inactive period. Increased locomotion has been shown in the active phase of their diurnal cycle. We compared the effects of scopolamine, oxotremorine, and cocaine on locomotor response, hypothermia and spontaneous behavior in the open field arena in the morning (9:00 AM) and in the evening (9:00 PM) in WT and in C57Bl/6NTac mice lacking the M ₄ MR. Furthermore, we also studied morning vs. evening densities of muscarinic, GABA _A, D ₁-like, D ₂-like, NMDA and kainate receptors using autoradiography in the motor, somatosensory and visual cortex and in the striatum, thalamus, hippocampus, pons, and medulla oblongata. At 9:00 AM, scopolamine induced an increase in motor activity in WT and in M ₄KO, yet no significant increase was observed at 9:00 PM. Oxotremorine induced hypothermic effects in both WT and M ₄KO. Hypothermic effects were more evident in WT than in M ₄KO. Hypothermia in both cases was more pronounced at 9:00 AM than at 9:00 PM. Cocaine increased motor activity when compared to saline. There was no difference in behavior in the open field between WT and M ₄KO when tested at 9:00 AM; however, at 9:00 PM, activity of M ₄KO was doubled in comparison to that of WT. Both WT and KO animals spent less time climbing in their active phase. Autoradiography revealed no significant morning vs. evening difference. Altogether, our results indicate the necessity of comparing morning vs. evening drug effects