Sex and strain differences in septal driving of the hippocampal theta rhythm as a function of frequency: Effects of gonadectomy and gonadal hormones

The function relating septal stimulation frequency to threshold current for driving a hippocampal theta rhythm has a minimum at 7.7 Hz in the unselected free-moving male rat. This minimum is absent in the female. The female function is unaffected by the oestrous cycle or ovariectomy: the male function changes to one characteristic of the female 7–11 weeks after castration. Treatment with testosterone restores the function in the castrated male rat to one typical of the intact male and also produces such a function in the ovariectomised female rat.We also investigated this function in rats selectively bred for high (the Maudsley Reactive or MR strain) and low (the Maudsley Non-reactive or MNR) fearfulness. MR males resembled unselected males in having a minimum threshold at 7.7 Hz. MNR males lack this minimum and thus resemble unselected males after administration of fear-reducing drugs. Neither MR nor MNR females display a minimum threshold at 7.7 Hz; MR females responded to exogenous testosterone by developing a 7.7 Hz minimum threshold, but MNR females were unresponsive to the hormone. These results suggest that selective breeding for low fearfulness in the MNR strain has produced permanent changes in the functioning of the septo-hippocampal system which resemble those produced temporarily in unselected rats by fear-reducing drugs.