β‐Endorphin Cells in the Arcuate Nucleus: Projections to the Supraoptic Nucleus and Changes in Expression During Pregnancy and Parturition

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Abstract
Supraoptic nucleus oxytocin neurone activity and secretion are inhibited in late pregnancy and parturition by endogenous opioids. Here, we investigated alterations in the projections and gene expression of β-endorphin/pro-opiomelanocortin neurones in the arcuate nucleus in the pregnant rat. All regions of the arcuate nucleus were found to contain cells immunoreactive for β-endorphin fluorescent microbeads retrogradely transported from the supraoptic nucleus, and double-labelled neurones (β-endorphin plus microbeads), showing that β-endorphin neurones throughout the arcuate nucleus project to the supraoptic nucleus. There was an increase in the number of β-endorphin-immunoreactive cells in the arcuate nucleus and an increase in the density of β-endorphin fibres within the supraoptic nucleus and peri-supraoptic region in late pregnancy and parturition, suggesting enhanced expression of β-endorphin and increased β-endorphin innervation of the supraoptic nucleus. Pro-opiomelanocortin mRNA expression in the arcuate nucleus increased in late compared to early pregnancy: the number of positive neurones significantly increased in the caudal region. Fos expression (an indicator of neuronal activation) in the arcuate nucleus was colocalized in β-endorphin neurones in both proestrus and parturient rats, but the number of positive cells did not increase during parturition, suggesting lack of activation of β-endorphin neurones at birth. Thus, β-endorphin cells in the arcuate nucleus project to the supraoptic nucleus and increased innervation during pregnancy may explain the enhanced endogenous opioid inhibition of oxytocin neurones.

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