Oestrogen-induced vigorous mounting in female rats carrying hypothalamic knife cuts

In ovariectomized Wistar rats, 1- or 2-mm wide knife cuts were placed in a coronal plane from the surface of the cortex to the floor of the cranial cavity to interrupt posterior efferents of the ventromedial nucleus of the hypothalamus (VMN). Sham-operated females had the same knife lowered to a depth of 7 mm. Ovariectomized nonoperated females were also used. After recovery, all rats received a single injection of 2 microg oestradiol benzoate, and were tested 48 h later for male and female sexual behaviour and partner preference. When placed with highly receptive stimulus females, the rats with the 2-mm cut showed a significantly higher incidence of mounting with ejaculatory thrusts than any other groups. When placed with stud males, 1-mm cut, as well as sham-operated females, had increased lordosis quotients. Similarly, both 1-mm and 2-mm cuts and sham operation enhanced the incidence of ear wiggling. Despite the display of a transsexual behaviour (i.e. vigorous mounting), all females with the cut showed heterosexual partner preference. Thus, the cut in the present study removed the inhibitory neural effect on mounting, which presumably descends from the VMN. In the absence of this inhibition, minute amounts of oestrogen sufficed to induce vigorous mounting. Sham operation in the present study appeared to interfere with certain inhibitory neural circuitry for lordosis.