A reciprocal inhibitory relationship between the ventromedial nucleus of the hypothalamus and the preoptic area neural circuits forming the basis for the bisexual potential

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We have now arrived at a point where it is most useful to refresh our recollections of male sexual behavior. It is probably not an abuse to suggest that any reader who has forgotten that the medial preoptic area is crucial for male sexual behavior should return to the section on males in this chapter and read it with some attention. The rather exciting point here is that the structure that stimulates male sexual behavior at the same time inhibits female sexual behavior, at least in rodents. In other words, the brain site in which nervous activity enhances the likelihood that male sexual behavior will be displayed reduces the likelihood that female sexual behavior will be displayed. We have already learned that lesion of the ventrome-dial nucleus of the hypothalamus reduces female sexual behavior and we learned in the previous section that lesion of this nucleus facilitates male sexual behavior. Again, the brain site making female sexual behavior more likely at the same time makes male behavior less likely. There seems to be a kind of reciprocal relationship between male and female sexual behavior with regard to its neural control. Low activity in the preoptic area combined with high activity in the ventromedial nucleus is associated with high likelihood for the display of female sexual behavior. This fact applies to males as well as to females. The attentive reader may remember that preoptic lesion also facilitates lordosis in males and that ventrome-dial nucleus lesion reduces it in males as much as it does in females.

If we now turn to the opposite situation, low activity in the ventromedial nucleus and high activity in the preoptic area, we find that it is associated with a high likelihood for the display of male sexual behavior. The interesting questions that appear immediately are whether preoptic lesion abolishes mounting in females in the same way as it does in males and whether ventromedial lesion facilitates mounting in females just as it does in males. The answer to these questions is unavailable at present. An abstract presented at the Society for Neuroscience meeting in Orlando, Florida, in 2002 contains the only existing data concerning the effects of lesions of the preoptic area and ventromedial nucleus of the hypothalamus on mounting behavior in female rats. Most unfortunately, the abstract itself only states that, 'The effects of these treatments are currently being evaluated and will be presented' (Afonso et al., 2002). My memory of the poster is somewhat vague after 4 long years, but I recollect that the data shown were most preliminary. However, it appears that a complete report will be submitted for publication before the writing of this book is finished. I can only advise any interested reader to look for it. Nevertheless, despite the uncertainty about the central nervous control of mounting in females, I dare to suggest that its control is similar to that in males. The difference between males and females is, then, extremely simple: in the male, high activity in the preoptic area and low activity in the ventromedial nucleus make the display of male copulatory behaviors more likely than that of female copulatory behaviors. In the female, high activity in the ventromedial nucleus and low activity in the preoptic area make the display of female copulatory behaviors more likely than that of male copulatory behaviors. However, both males and females have the potential for expressing behaviors typical of the opposite sex in addition to the potential of expressing behaviors typical of their own sex. The only difference between males and females is the likelihood of expression of a behavior, a quantitative difference. There is no qualitative sex difference in behavior, as far as I can see. This important issue will be addressed in more depth in Chapter 8.

A caveat needs to be added here. The reasoning in the preceding paragraph is based on rat data and it is limited to the probability of expressing the copulatory reflexes, basically mount and lordosis. Since we do not have the necessary data from other species, we cannot determine whether the principle of reciprocal inhibitory control of male and female copulatory motor patterns is valid in any species other than the rat. Furthermore, the activation of sexual incentive motivation by distant stimuli seems to depend on an intact preoptic area in both male and female rats. If the observations justifying this statement were confirmed in more experimental studies, then we could conclude that the preoptic area has a double function. It would then control the propensity to express male copulatory behaviors and the reactivity to distant sexual incentives in both sexes. On the contrary, the only function of the ventromedial nucleus would be the control of female cop-ulatory behaviors in both sexes.

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