Effects of intraseptally injected dopamine and noradrenaline on hippocampal synchronized theta wave activity in rats

To clarify the functional role of catecholamine in the septal area, we investigated the effects of dopamine (DA) and noradrenaline (NA) injected into the medial septum (MSN) on hippocampal synchronized theta wave activity (TWA) in immobilized rats. The injection of DA (1-4 micrograms) into the MSN dose-relatedly enhanced hippocampal TWA, i.e., an increase of the total power in 3-7 Hz bands and little modification of the peak frequency. An enhancing effect of DA on TWA was also observed by the application into the diagonal band, but not into the lateral septum, nucleus accumbens, and lateral ventricle, indicating a selective effect of DA on the MSN and diagonal band. Furthermore, the enhancing effect of DA was blocked with the systemic treatment of haloperidol. Apomorphine (2 micrograms) injected into the MSN also increased the total power. NA (1-4 micrograms) enhanced TWA; however, NA, unlike DA, increased the peak frequency without modifying the total power, and the effect was not blocked by haloperidol, suggesting that the effect of NA on the septo-hippocampal neurons is different from that of DA. Muscimol (0.1 microgram) and baclofen (0.05 microgram) injected into the MSN depressed TWA, as indicated by a decrease of the total power. These results suggest that DA and NA, when injected into the MSN, heighten the functional level of the hippocampus.