Tail-clamping stimulation increases phase-locking neurons in the hippocampus of anaesthetized rat

Phase-locking is an important encoding mode in the hippocampal region of brain where neuronal firing concentrates on a certain phase of the periodic field potential, especially θ rhythm. It can be observed during many behaviors while the locked-phase varies accordingly. In this study, the changes of neuronal phase-locking induced by somatosensory stimulation of tail-clamping were investigated. The local field potential and spike signals were recorded in hippocampal CA1 region of anesthetized rats with microelectrode array. The phase-locking relationship was analyzed with Raleigh test and spike-phase histogram. The results showed that during spontaneous activity, 3 of 20 examined neurons were phase-locking to the θ rhythm of the field potential. However, during the tail clamping periods, the number of phase-locking neurons increased from 3 to 8, and the mean locked-phase shifted towards the negative peak of the field potential cycle, which became θ rhythm dominated. These findings suggest that the hippocampus may encode somatosensory information with the phase-locking neurons and by shifting their locked-phases.