Innate differences in medial septal area burst firing neurons and the hippocampal theta rhythm during ambulation in selectively bred rat lines

Previous studies have demonstrated a higher mean peak frequency of the hippocampal (HPC) theta rhythm during REM sleep in alcohol-nonpreferring (NP) rats when compared with alcohol-preferring (P) rats. Burst firing neurons of the medial septal area (MS/VDB) are thought to pace the HPC theta rhythm during REM sleep and ambulation. Therefore, extracellular action potentials of MS/VDB burst firing neurons and HPC-CA1 field potentials were recorded simultaneously in ambulating P and NP rats. These recordings revealed that the mean peak frequency of the HPC theta rhythm during ambulation was higher in NP rats (7.62 +/- 0.12 Hz) as compared with P rats (7.21 +/- 0.14 Hz) (P < 0.05). Consistent with the difference in the HPC theta rhythm, the burst pattern of MS/VDB neurons exhibited a shorter inter-burst interval in the NP rats (NP 82.4 +/- 5.4 ms, P 97.4 +/- 8.1 ms, P < 0.05). The difference in the inter-burst interval was confirmed by the distribution of inter-spike intervals in cumulative inter-spike interval histograms and the frequency of peaks in the mean cumulative autocorrelation histograms for P and NP rats. The mean cumulative autocorrelation histograms for P and NP rats also revealed that the regularity of the burst pattern in NP rats was sustained over a longer time period as determined by the decay constant. The cross-correlation of MS/VDB burst activity and the HPC theta rhythm showed a strong relationship between the two signals in P and NP rats. In both P and NP rats, two similar phase relationships were observed between MS/VDB bursting neurons and the HPC theta rhythm. These findings are in agreement with the hypothesis that MS/VDB burst firing neurons are responsible for the variation in the HPC theta rhythm between the two lines. Other mechanisms consistent with these findings are also discussed.