Medial Preoptic Area
Cross-section of rat midbrain, showing
the medial preoptic area
(Carlson, 1999)
The MPOA is located in the forebrain rostral to the hypothalamus. Lesions to this brain region severely impair the full expression of sexual and parenting behaviors. An important connection along the ventral amygdalofugal pathway is in the VTA, which is also a part of the mesolimbic pathway. The VTA is responsible for supplying DA to the MPOA, regulating the reward associated with sexual behavior, as well as food, water, and psychoactive drug consumption.
Neurotoxic lesions to both MPOAs upset normal parental behavior in males, even in males who have established these behaviors prior to the study (Sturgis & Bridges, 1997). Some male rats displayed various parenting behaviors, such as crouching over pups, retrieval of pups to nest, and physical contact. Males were given eight days to respond parentally to pups, latency scores were recorded. On the third consecutive day that males exhibited parental qualities, they were infused with either a saline vehicle or an NMA neurotoxin into both right and left MPOAs. Expression of full parenting behavior was recorded in both groups, showing that those who receive bilateral lesions of the MPOA were severely impaired in their ability to function on a parental level.
In males, the stimulation of androgen receptors elicits male-typical behaviors, including the sexual behaviors of anogenital sniffing, mounting, and ejaculation. McGinnis, et al. (1996), demonstrated that functional androgen receptors in the MPOA are necessary for the full compliment of mating behaviors to occur in male rats.
Concurrent androgen implants into both the VTA and the MPOA have a significantly higher effect on male-typical behaviors than implants in one of the two areas alone (Sipos & Nyby, 1996). This suggests that the androgen receptors in the MPOA are in charge of initiating sexual behaviors, while the VTA’s function is to reward the behavior.
Coolen & Wood (1999) discovered that Testosterone stimulation of the MPOA elicits male-typical sexual responses, even in those who have been castrated. The experiment began by introducing the male hamsters to females for thirty minutes, or until their first ejaculation. After castration, each male was implanted with a salastic capsule which contained either testosterone and cholesterol vehicle, and then were reintroduced to females. Sexual behavior of males was recorded to assess copulatory behavior on a long-term basis after their castration surgery. They used implants in both the MPOA and the AME, finding that dual stimulation did not significantly impact sexual behavior. They concluded that stimulating cortical areas that process different types of information relevant to sexual stimulation might elicit stronger sexual responses.
Fos is a chemical that is released during neural transmissions. Activity levels of Fos in the medial preoptic nucleus (MPN), a part of the MPOA, the bed nucleus of the stria terminalis (BNST), and the AME have indicated that these regions of the brain are vital to males’ sexual responsiveness (Greco, et al., 1998b). In conditions involving males who were allowed to mate, males who were allowed social but no sexual contact, and isolated males, results demonstrated that Fos levels were highest in mated males, followed by social controls, and there was no Fos present in the midbrains and spinal cords of the isolated group.
To study this Greco (1998a) and his colleagues set up three experimental conditions for the rats. The first involved males that were allowed to successfully copulate with females. Both rats were placed in the same cage, which allowed for all the important aspects of mating being present. The second condition allowed the male’s social, but no physical, contact with the females. This meant that the rats were placed the same cage, but in this cage there was a mesh barrier preventing physical contact of any sort. This still permitted the rat to be able to see, hear, and smell the female. The third condition used rats in isolation, allowed no contact with females. Greco and colleagues go on to explain that Fos-immunoreactivity in neurons in the MPN is localized to androgen receptors. Sexually relevant information gathered from olfactory and somatosensory stimuli is processed by androgen receptors that lead to the MPN.
Lorrain, Matuszewich, and Hull (1998) showed that the neurotransmitter 5-HT is also responsible for exciting the male sexual response. The study also displayed that through an agonist on the 5-HT1A receptor, ejaculation in males was facilitated.
Females also rely on the rewarding effects of the MPOA for proper reproductive behavior. The emotional changes necessary to exhibit maternal behaviors are facilitated here as well (Morgan, 1999). Estrogen, the hormone associated with the expression of maternal behaviors is in high concentrations in the MPOA. Lesions of the MPOA in females results in the halting of nesting, retrieving, and nursing behaviors (Numan, 1994).
The MPOA is rich in endogenous opioid receptors. The pathways that these receptors innervate are vital to the expression of maternal behavior by stimulating hormones like estrogen. More specifically, the MPN is very dense in opioid receptors, which mediates maternal behavior (Gulledge, 2000). The MPOA, and the androgen sensitive neurons that form the ventral amygdalofugal pathway leading to the MPN, specifically, relay information regarding sexually relevant stimuli through the brain, eliciting natural responses.