Estrogen receptor-immunoreactive forebrain neurons project to the ventrolateral hypothalamus in female guinea pigs

In rodents, the facilitation of sexual receptivity by estradiol and progesterone is suspected to be mediated by a network of neurons containing estrogen and progestin receptors. In female guinea pigs, this network would include estrogen receptor-immunoreactive (ER-ir) neurons located within the rostro-ventral ventrolateral hypothalamus (r-vVLH). This hypothesis predicts that a proportion of the neurons projecting to the r-vVLH contains estrogen receptors. This prediction was tested through retrograde tracing combined with immunocytochemistry for estrogen receptors. Retrogradely labelled neurons were particularly abundant within the medial preoptic nucleus (MPN), bed nucleus of the stria terminalis (BST), anterior hypothalamus, amygdala, and lateral parabrachial nucleus. As predicted by the hypothesis, retrogradely labelled neurons were mostly observed in estrogen receptor-rich areas. Retrogradely labelled neurons also containing estrogen receptor-immunoreactivity (ER-IR) were primarily found within the MPN, BST, and amygdala. However, a majority of retrogradely labelled neurons did not contain ER-IR. As the preoptic area and the r-vVLH are both responsive to estradiol in the facilitation of sexual receptivity by progesterone, these data are consistent with the hypothesis tested. However, our data also suggest that the network of neurons controlling sexual receptivity may include elements not directly sensitive to estradiol. Finally, the location of retrogradely labelled neurons is discussed with respect to the stimuli provided to the r-vVLH in the context of sexual receptivity facilitated by estradiol and progesterone.