LAUSR

Neural activity and bodily functions are inherently rhythmic and related to each other. The occurrence of hippocampal sharp‐wave ripples (SPW‐Rs) and dentate spikes (DSs) supporting memory consolidation is regulated by the overall state of the brain, and they also seem to aggregate to a certain phase of the breathing cycle in naturally sleeping mice. Further, SPW‐Rs and DSs synchronize to a variable degree between hemispheres, but how this is affected by the neural and bodily state is unclear. Here, we recorded dorsal hippocampal local‐field potentials, electrocardiogram, and respiration for several hours under urethane anesthesia in adult male Sprague–Dawley rats. For a subset of rats, we injected atropine (50 mg/kg, i.p.) halfway into the recording to decrease cholinergic and parasympathetic tone. We found variable relation of hippocampal oscillations to breathing phase and none to the cardiac cycle phase. A decrease in breathing rate implying increased parasympathetic tone preceded the start of SPW‐R bouts. Roughly 90% of DSs and half of SPW‐Rs were unilateral. In most rats, SPW‐Rs were more often bilateral during slow breathing compared to faster breathing. Atropine reduced the proportion of bilateral SPW‐Rs. Both nonrapid eye movement sleep‐like state and atropine increased the proportion of bilateral DSs under urethane anesthesia. Finally, in naturally sleeping rats, both DSs and SPW‐Rs were bilateral ~60% of the time. In sum, urethane seems to desynchronize DSs but not SPW‐Rs, and a low cholinergic and/or parasympathetic tone seems to dissociate SPW‐Rs and to synchronize DSs in the two hippocampi. Whether these findings have relevance in terms of memory consolidation and behavior should be investigated in the future.