In the male, the medial preoptic area is essential for male sexual behavior. This has been shown in several vertebrate species (Hillarp et al., 1954; Heimer and Larsson, 1966; Slimp et al., 1978; Koyama et al., 1984; Lloyd and Dixson, 1988) and it is most likely that equivalent structures in men are involved (see Paredes, 2003 and Paredes and Baum, 1997 for excellent reviews). Lesion of the medial preoptic area eliminates all sexual behavior irreversibly, provided the lesion includes most of the area. The dramatic deleterious effects are not due to damage to passing axons. Axon-sparing lesions of the medial preoptic area have the same effects as electrolytic or radiofrequency lesions (Hansen et al., 1982). Thus, cell bodies located within the medial preoptic area are necessary. Electrical stimulation of this structure produces intense sexual behavior that lasts as long as stimulation continues (Schmidt, 1968; Merari and Ginton, 1975; Satou, 1984). This effect has consistently been observed in several vertebrate classes. Other means for stimulating neuronal firing within the preoptic area are also effective for enhancing the intensity of sexual behavior. Blockade of GABAa (^-amino butyric acid) receptors with bicuculline leads to a dramatic reduction of the post-ejaculatory interval and an increase in the number of ejaculations per test (Fernandez-Guasti et al., 1986). Similar effects are seen when neurons are excited by depositing ferrous ions (Paredes and Agmo, 1992). Furthermore, electrophysiological studies have shown that neurons within the medial preoptic area enhance firing rate during copula-tory behavior. Sixteen per cent of the neurons did so during the male's pursuit of the female, another 35% during pelvic thrusting and 43% enhanced firing during the rapid withdrawal immediately following intromission. After an ejaculation, most neurons reduced firing (Shimura et al., 1994). This study suggests that preop-tic neurons are involved in the entire sequence of events forming male copulatory behavior. Finally, sexual behavior in inactive, castrated animals can be activated by implants of minute amounts of testosterone into the preoptic area (Davidson, 1966; Morgantaler and Crews, 1978; Balthazart and Surlemont, 1990), as always in several classes of vertebrates. The evidence for a role of the medial preoptic area in male sexual behavior throughout the vertebrates is, as we must accept, overwhelming. The conservation of function throughout a long evolutionary history from fish to primates strongly suggests that an equivalent area is also crucial in the human. We have, understandably, no experimental data confirming this, but I can see no good reason for doubts.
Over the years, there has been some discussion with regard to the exact role of the medial preoptic area. Some have proposed that it is involved in the executional but not the motivational aspects of sexual behavior. The main argument for that point of view comes from studies in which rats had been trained to perform operant responses for access to a receptive female, or where they had been trained to find a female in a specific location. After preoptic lesion, the males soon stopped copulating with the females but they still performed operant responses to get access to them (Everitt and Stacey, 1987; Everitt, 1990). Other studies have found that unconditioned responses, like pursuing a receptive female or approaching an inaccessible female, are abolished by preoptic lesions (Paredes et al., 1993; Hurtazo et al., 2003). Similarly, interest in odors from receptive females disappears after preoptic lesions in rats and ferrets (Paredes and Baum, 1995; Paredes et al., 1998). These data show that the reactions to sexual incentives are reduced or abolished by preoptic lesions and such an effect is normally interpreted as motivational. It is possible that responses that have been conditioned with sexual reward are more resistant to disappearance following castration than copulatory behavior is. Some experimental data support this notion (Edwards and Einhorn, 1986), and I and others (Hull et al., 1999; Paredes, 2003) are convinced that the medial preoptic area is crucial for sexual motivation. Whether it is crucial also for the execution of cop-ulatory reflexes is not known. If an animal is not motivated to copulate, it has no reason to display any copulatory behavior. A Solomonic hypothesis, which I favor with insistence, is that the current excitability of the preoptic area determines whether remote sexual incentives will activate approach behaviors or not. It also determines whether other stimuli, probably tactile, will activate the copulatory reflexes of mount and contraction of the striated penile muscles or not. In other words, both motivation and likelihood of execution of copulatory reflexes are determined by the excitability of the medial preoptic area. The observation that long-term enhancement of neuronal excitability within that area by kindling can activate sexual behavior in otherwise inactive rats (Paredes et al., 1990) may offer some support for the notion, at least concerning a role for the preoptic area in the control of sexual motivation.
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