Is a retrosplenial (cingulate) pathway involved in the mediation of high frequency hippocampal rhythmical slow...

In previous experiments we demonstrated that in rats there are two kinds of hippocampal rhythmical slow activity patterns (RSA or theta) as defined by the dominating EEG frequencies. RSA with a frequency of 6-8 Hz appeared during exploratory behavior (locomotion), whereas stimulation of the dorsomedial hypothalamus (DMH) elicited RSA with frequencies of 8-12 Hz. To determine the neural pathways involved in the mediation of these two types of RSA, local injections of tetracaine were made either in the medial septum or in the cingulate cortex in order to reversibly interrupt the functional activity of these loci. Blockade of the medial septum suppressed the 6-8 Hz 'walking-associated' RSA in the hippocampal EEG, but had no effect on the 8-12 Hz DMH-driven RSA. On the other hand, a tetracaine injection into the cingulate cortex selectively blocked the high-frequency RSA elicited by DMH stimulation, but had no effect on the 6-8 Hz 'walking-associated' RSA. Both effects disappeared between 30 and 90 min after tetracaine injection. We conclude that the DMH-driven RSA is mediated by the cingulum and/or fibers traveling through the cingulate cortex (retrosplenial region) and thus, that this type of RSA operates without septal involvement.