Differential effect of procaine injection into the rostral and caudal part of the nucleus pontis oralis on hippocampal theta rhythm in urethane-anesthetized rats

The nucleus reticularis pontis oralis (RPO) is a reticular structure important for the regulation of paradoxical sleep (PS). However, the data concerning the relation between the RPO and the main tonic indicator of PS, hippocampal theta rhythm, are contradictory: although electrical or cholinergic stimulation of the RPO evoked well-synchronized theta activity, the electrolytic lesion of the structure had no effect on theta. In our experiment, the effect of procaine injections into different parts of the RPO on the electrical activity of the hippocampus, as well as on tail pinch-elicited hippocampal theta rhythm was assessed in urethanized rats. Power spectral analysis was performed using a Fast Fourier Transform routine in 1-Hz and 3-Hz bands between 0.6 and 12 Hz frequency. We have found that unilateral procaine inactivation of neurons in the caudal part of the RPO blocked the sensory-elicited theta rhythm. The same injection into the rostral RPO either had no effect or evoked long-lasting episodes of theta rhythm without sensory stimulation. These results suggest functional diversity of the parts of the RPO in mechanisms underlying production of hippocampal theta.