Selective lesions of the fimbria and the fornix in the rat: differential effects on CA1 and dentate theta.

The effects of electrolytic lesions of the dorsal fornix and the dorsomedial fimbria on the CA1 and the dentate theta rhythms (theta s) recorded from the dorsal hippocampal formation were investigated in the ether-anesthetized rat. The results showed that (i) fornix lesions mainly affected CA1 theta, (ii) fimbrial lesions mainly affected dentate theta, and (iii) combined fornix-fimbria lesions suppressed both CA1 and dentate theta s. When considered in connection with other observations, these data suggest that the septal projections pacing the CA1 theta may course essentially within the dorsomedial fornix whereas those pacing the dentate theta may pass essentially within the dorsomedial fimbria. Moreover, our data provide new support for the hypothesis that at least two septohippocampal neural systems are anatomically and functionally independent and capable of controlling the theta activity of the dorsal hippocampal formation of the anesthetized rat.