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Anatomical and behavioral studies indicate that the dorsal Midbrain Central Grey (dMCG) and the Ventromedial Nucleus of the hypothalamus (VMH) are the principle brain structures that function in mediating sexual behavior in the female rat. Recently, the medial portion of the Zona Incerta (mZI) has been implicated as an additional brain structure that regulates sexual receptivity (lordosis) in the female rat. When the mZI of the female was destroyed via intracerebral injections with the neurotoxin, ibotenic acid, by Dornan et al. (1991), they reported that lordosis behavior was shown to be attenuated. Presently, little is known about the role of the mZI in the expression of male sexual behavior, but several studies indicate that it plays no role. Anatomical studies, however, have revealed that neural connections exist between the mZI and the dMCG in both male and female rats. Collectively this information suggests that the neuronal pathways between the mZI and the dMCG would differ in strength between male and female rats. We addressed this possibility by using the retrograde tract tracing approach with horseradish peroxidase (HRP). A 12% solution of HRP was injected bilaterally (volume, O.5~I/per side) into the dorsal portion of the Midbrain Central Gray in three animals (2 males and 1 female). After a 3-day survival period, animals were sacrificed and their brains processed for HRP histochemistry using a modified Mesualm method. The results of this analysis revealed more retrogradely labeled cells in the female brain than the male brain with a similiar injection site. These results suggest that in the rat, the neural pathway between the mZI and the dMCG may be a stronger in the female than in the male.



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