A note on lordosis in male rats and the influence of the medial preoptic area and of the ventromedial nucleus of the hypothalamus

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Male rats do not only display copulatory behaviors typical of their own sex. In addition, they readily display lordosis if given adequate tactile stimulation of the per-ineal region and the flanks, either by a mounting male or by the experimenter's hand. Since male rats do not frequently mount other males and since most males do not like to be mounted by other males, the latter procedure is the one most commonly used. In some strains, about 50% of intact, untreated males display lordosis, while the proportion of males showing this behavior may be 0 in other strains (Sodersten et al., 1974). The lordosis behavior is reduced following castration and can be restored by treatment with estradiol or testosterone (Sodersten and Larsson, 1975). The effects of estradiol, as well as those of testosterone, are blocked by an estrogen receptor antagonist (Sodersten and Larsson, 1974), suggesting that aromatization of testosterone and subsequent stimulation of estrogen receptors is crucial. In intact rats, there is no relationship between blood androgen or estrogen concentrations and the propensity to display lordosis (Sodersten et al., 1974), showing that males displaying female behavior have no endocrine alteration. Thus, the display of lordosis in some strains of male rats seems to be a normal element of the behavioral repertoire. What is of interest in the present context is that lesions of the medial preoptic area enhance lordosis behavior in male rats independently of whether the lesion is electrolytic or produced by an axon-sparing neurotoxin (Hennessey et al, 1986; Olster, 1993). These data show that the preoptic area is not only essential for the display of male sexual behavior, but also that it inhibits the display of female sexual behavior in males. As we soon will see, the preoptic area also inhibits the display of lordosis in female rats.

The ventromedial nucleus of the hypothalamus is best known for its role in lordosis behavior in females of many rodent species as we will see in the next section. However, it also plays a role in lordosis behavior in the male rat. Indeed, lesion of the ventromedial nucleus completely eliminates this behavior (Chateau et al., 1987). It would appear, then, that the neural control of male rat lordosis is identical to that of female lordosis. A question that immediately comes to mind regards the possible effects of ventromedial nucleus lesions on male behavior in male rats. If preoptic lesions facilitate female behaviors, then it could be expected that ventromedial nucleus lesions would facilitate male behaviors. This is exactly what happens. Bilateral lesion of this nucleus stimulates most aspects of copulatory behavior. The latencies to mount and intromit were reduced while the number of mounts, intromissions and ejaculations displayed during tests was enhanced (Christensen et al., 1977). These observations confirm those of an earlier study (Dorner et al., 1969), but contradict those of a more recent one (Harding and McGinnis, 2005). However, the lesions in the latter study generally left parts of the ventromedial nucleus intact and the remaining parts could well be enough for maintained function. The importance of lesion size will be discussed shortly. For the moment, I conclude that lesions of the ventromedial nucleus of the hypothalamus in male rats facilitate male sexual behavior. There seems to exist, then, a reciprocal inhibition between male and female behavior in the way that the site responsible for male behavior patterns inhibits the site responsible for female behavior patterns and vice versa.

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