There are several brain structures in addition to the ventromedial hypothalamus and the preoptic area that might be of importance for female sexual behaviors.
In the beginning of this section I mentioned that the overlapping distribution of estrogen and progesterone receptors was too vast to offer specific cues as to crucial structures. Rather than using this distribution as a basis for speculations, some scientists have reasoned that brain regions activated by the execution of sexual behaviors or by sexually relevant stimuli somehow must be involved in the control of these behaviors or responses to these stimuli. As we know, there are reasonably accurate, non-invasive procedures for determining brain activity. We may recall that the favorite one among those working with rats is the post-mortem determination of the number of neurons expressing the immediate early gene c-fos. This has obviously been done in a large number of experiments. Typically, sexually receptive females are allowed different amounts of sexual interaction with males, or they are allowed to smell male odors. Sometimes, the male has been replaced by a syringe plunger. This object can easily be introduced into the vagina and the cervix can be stimulated in a controlled way. I will not review the large number of studies on the subject. A more interesting point to discuss is whether the activation of a particular brain structure during the execution of a particular behavior really means that the structure is important for that behavior. A priori there is no reason to expect that. Many brain structures may be active without being specifically involved in the behavior being executed. Thus, fos activation by itself does not necessarily mean anything. Additional data need to be presented before a convincing case can be made for the importance of any particular brain structure in any particular behavior.
A beautiful illustration to the point of view presented in the preceding paragraph can be found in a splendid little paper by Fay Guarraci and colleagues (Guarraci et al., 2004). To appreciate fully their ingenuity some background information is required. Vaginocervical stimulation occurs every time the male performs an intromission. In the female rat, this sensory stimulation has many consequences, as was mentioned in Chapter 3. It is, for example, important for the pacing of sexual interactions. It is also important for the physiological consequences of copulatory behavior, like the induction of pseudopregnancy. This is a state where the corpora lutea do not regress as fast as usual, but remain active for about 14 days. If the copulation had been fertile, the placenta would have started to produce large amounts of estrogens and progesterone long before that time, making the corpora lutea unnecessary for the maintenance of pregnancy. Anyway, vaginocervical stimulation is an important event associated with female copula-tory behavior and the brain structures showing enhanced fos expression as a consequence of it have been well described. They include, in addition to the obvious sites of the ventromedial nucleus and the medial preoptic area, the amygdala and the bed nucleus of the stria terminalis (Pfaus et al., 1993).
Ibotenic acid lesion of the medial amygdala failed to modify female rat sexual behavior in paced mating tests. Receptivity and proceptivity as well as all aspects of pacing were unaltered by the lesion. Similar results were obtained following lesion of the bed nucleus of the stria terminalis. Lesion of the medial preoptic area, in contrast, enhanced the duration and probability of escapes from the male following intromission and ejaculation (Guarraci et al., 2004). This was interpreted as suggestive of an increased impact of vaginocervical stimulation in lesioned animals, a conclusion already reached in a similar study from another laboratory a few years earlier (Yang and Clemens, 2000). The important point here is that the destruction of structures showing intense fos expression following copulation does not alter copulatory behavior. It seems quite reasonable to suggest, then, that these structures are not of any particular importance for that behavior. Furthermore, it should also be evident that, exactly as I stated many lines ago, activation of a brain structure during a behavior does not by itself imply that the structure is crucial or even important at all for the behavior. This annoying conclusion should be remembered when interpreting the very popular imaging studies in human subjects.
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