Oestrogen‐Induced Vigorous Mounting in Female Rats Carrying Hypothalamic Knife Cuts

Abstract

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 µg 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.

Sex and Region‐Specific Regulation of Oestrogen Receptor β in the Rat Hypothalamus

“…In the knockout mice study, ERβ had little or no influence on lordosis behaviour in contrast to ERα (21). Therefore, sexually dimorphic expression of ERβ in the VMN may relate to the other behaviours, such as male sexual behaviour (22), food intake (23), etc.…”

Estrogen and the neural mediation of female–male mounting in the rat.

“…Both the mPOA and VMN have been implicated in the regulation of female and male mounting of sexually receptive females. However, lesions of the mPOA or VMN decrease or increase, respectively, the mounting of female rats by males and females treated with testosterone (Arendash & Gorski, 1983; Christensen, Nance, & Gorski, 1977; Dörner, Docke, & Hinz, 1969; Gray & Brooks, 1984; Heimer & Larsson, 1966, 1967; Matsumoto & Yamanouchi, 2000; McGinnis, Williams, & Lumia, 1996; Nance, Christensen, Shryne, & Gorski, 1977; Ohnishi, Kondo, & Sakuma, 2003). FMM serves a sexual function in rats (Afonso & Pfaus, 2006), whereas female mounting of other females (FFM) in this species appears to be an agonistic social behavior, for example, to maintain dominance hierarchies (Fang & Clemens, 1999a, 1999b).…”

Transient reversal of olfactory preference following castration in male rats: Implication for estrogen receptor involvement

“…In male rats, ventral de-efferentation of the lateral septum promotes lordosis behavior in castrated male rats primed with estradiol [21][22][23]. In contrast, vigorous mount activities toward receptive females are displayed by female rats with ibotenic acid lesions [24] or caudal transections [25] in the ventromedial nucleus of the hypothalamus. Although sex differentiation of brain occurs via perinatal sex steroids, the neural substrates for both masculine and feminine behaviors are still conserved in the brain, but normally one is activated and the other of opposite-sex is inhibited.…”