Cooling of Medial Septum Reveals Theta Phase Lag Coordination of Hippocampal Cell Assemblies

The medial septum is critical in theta generation by two possible mechanisms: a unitary ‘pacemaker’ timing signal is imposed on the hippocampal system or it may assist in organizing target subcircuits within the phase space of theta oscillations. Therefore, this study used temperature manipulation of the medial septum to test these models. Long Evans rats were implanted with silicon probes and tungsten wires to record local field potential and spikes from the CA1 pyramidal layer. The tip of the cooling device was implanted at AP: +0.8mm, ML: 0.6mm, and lowered 6 mm below the brain surface, after which it was attached to the skull and base.

After recovery from surgery, the animals were water restricted to perform a spatial alternation task in a figure 8-shape maze. The behavior session typically lasted 40 min, consisting of 40 control trials, after which the cooling was applied by manually placing a small amount of dry ice in the cooling chamber. Cooling typically peaked after about 60 seconds after the cooling onset and lasted for about 10–12 minutes, corresponding to approximately 50 trials. The animal would continue the task for a total number of trials ranging from 80 to 200. This dataset includes electrophysiology and behavioral data.