YOUNG AND OLD RATS SHOW A FULL CYCLE OF THETA PHASE PRECESSION DURING THE FIRST PASS THROUGH A PLACE FIELD IN A FAMILIAR ENVIRONMENT.
1. ARL NSMA,
2. Evelyn F. McKnight Brain Inst., Univ. Arizona, Tucson, AZ
In all subregions of the hippocampus the timing of spikes shows a dynamic relationship to the hippocampal theta rhythm, a 5-10 Hz oscillation that is prominent in the rat hippocampus during active exploration (Vanderwolf, 1969). As a rat passes through a principal cell's place field, the timing of spikes shift relative to the local theta rhythm such that the spike timing occurs at earlier phases of the theta cycle. In both young and old rats, for the majority of CA1 pyramidal cells, this "theta phase precession" covers approximately 360 degrees or a complete theta cycle (O'Keefe and Recce, 1993; Skaggs et al., 1996; Shen et al., 1997; Ekstrom et al., 2001; Maurer et al., 2006). According to one model of CA1 experience-dependent place field expansion plasticity and phase precession, however, a monotonic phase advance should not be present during the first pass through an unexpanded place field (Mehta et al., 2002). Contrary to this idea, other empirical data show that in young rats phase precession occurs during conditions in which place fields do not show experience-dependent place field expansion (Skaggs et al., 1996; Ekstrom et al., 2001; Harris et al., 2002) and in aged rats with spatial memory and plasticity impairments (Shen et al., 1997; Ekstrom et al., 2002). This suggests that previous experience may not be necessary for 360 degrees of phase precession to occur. In the current experiment, we examined theta phase precession during the first traversal of a place field in young rats and in old rats with and without restored experience-dependent plasticity. Neurons from CA1 were recorded from young (9-12 months) and old rats (25-30 months) as they traversed a circular track in a familiar environment. In the old rats the NMDA receptor was modulated by the activity-dependent non-competitive antagonist memantine, which has been shown to restore experience-dependent plasticity (Burke et al., 2005). Neurons showing place-specific firing during the first pass through a place field exhibited a full 360 degrees of theta phase precession. This was consistent between young and old rats administered saline or memantine. These data contradict the hypothesis that phase precession in CA1 pyramidal cells is a result of experience dependent place field expansion.
Supported by McKnight Brain Research Foundation; AG012609; NS054465; NS020331