Hippocampal type 1 (movement-related) theta rhythm positively correlates with serotonergic activity

To investigate the relationship between the hippocampal υ activity (or Rhythmical Slow Activity, RSA) and the hippocampal serotonergic activity during spontaneous behavior, simultaneous recordings of i) hippocampal EEG, ii) sleepwake activity, and iii) hippocampal levels of the serotonin (5-HT) metabolite 5-hydroxyndolacetic acid (5-HIAA - measured by in vivo voltammetry and infrared telemetry) were performed. The results show that hippocampal type 1 RSA recorded during wakefulness and voluntary movements (such as walking), is positively correlated to hippocampal 5-HIAA levels. Since in the experimental conditions used in the study, 5-HIAA levels are a reliable index of 5-HT release, the results support the hypothesis that hippocampal type 1 RSA is generated by a serotonergic mechanism. In contrast, hippocampal type 2 RSA recorded during desynchronized sleep is negatively correlated with 5-HT release, suggesting a different neurochemical mechanism for its production. These results also show that, in the experimental condition of this study, hippocampal RSA power spectrum has a main peak frequency of 3.5 during wakefulness, and of 6.5 Hz during desynchronized sleep.