Hippocampal 4-Hz oscillations emerge during stationary running in a wheel and are resistant to medial septum inactivation

Recent studies described 2-4 Hz oscillations in the hippocampus of rats performing stationary locomotion on treadmills and other apparatus. Since the 2-4 Hz rhythm shares common features with hippocampal theta (5-12 Hz) oscillations - such as a positive running speed-amplitude relationship and modulation of spiking activity - many have questioned whether these rhythms are related or independently generated. To answer this question, we analyzed local field potentials (LFP) and spiking activity from the dorsal CA1 of rats executing a spatial alternation task and running for [~]15 s in a wheel during the intertrial intervals both before and after muscimol injection into the medial septum. We observed 4-Hz oscillations during wheel runs, but not during maze runs. The 4-Hz amplitude positively correlated with wheel speed and negatively with maze speed. Also, the amplitude of 4-Hz and theta oscillations were inversely related. Medial septum inactivation abolished hippocampal theta but preserved 4-Hz oscillations. It also affected the entrainment of pyramidal cells and interneurons by 4-Hz rhythmic activity. In all, these results dissociate the underlying mechanism of 4-Hz and theta oscillations in the rat hippocampus. HighlightsO_LI4-Hz oscillations emerge in the rat hippocampus during wheel running and correlate with speed C_LIO_LI4-Hz and theta amplitude are inversely correlated C_LIO_LI4-Hz oscillations are resistant to medial septum inactivation C_LIO_LIRhythmic discharge of interneurons and pyramidal cells at 4-Hz are reduced by medial septum inactivation C_LI In briefLima and Belchior tested whether the pharmacological inactivation of the medial septum affects hippocampal 4-Hz oscillations associated with stationary running. They found that septal inactivation preserved 4-Hz oscillations while abolished theta, which dissociate the underlying mechanisms of these rhythms in the rat hippocampus.