Hyperprolactinemia decreases naloxone binding in the arcuate nucleus of ovariectomized rats

Hyperprolactinemia suppresses endogenous prolactin (PRL) secretion and inhibits LH release in ovariectomized rats. Opiate peptides appear to mediate the suppressive effects of hyperprolactinemia on both endogenous PRL and LH secretion. A mechanism by which hyperprolactinemia may change the ability of opiates to influence PRL and LH is by altering the density of opiate receptors. We, therefore, examined the effect of hyperprolactinemia on the density of naloxone binding sites in hypothalamic regions that are important in the regulation of PRL and LH secretion. Female rats, ovariectomized for 4 days, were treated with ovine prolactin (oPRL) every 8 h for 2 days, and naloxone binding sites were measured using autoradiographic procedures. oPRL treatment suppressed the concentration of naloxone binding sites throughout the arcuate nucleus but had no effect in the median eminence, suprachiasmatic nucleus, and medial preoptic nucleus. There is evidence that the tuberoinfundibular dopaminergic neurons of the arcuate nucleus are directly influenced through opiate receptors. We propose that the observed decrease in the density of opiate receptors may occur on dopaminergic neurons. This theory provides an explanation for a mechanism for the suppression of endogenous PRL and LH by hyperprolactinemia: a decrease in opiate receptors will decrease opiate suppression of dopamine neurons allowing dopamine activity to increase. Increase in dopamine release are known to decrease PRL and LH secretion in ovariectomized rats. Alternatively, decreased naloxone binding may result from homologous down-regulation of receptors due to increased opiate activity. If opiate activity increases, it may directly inhibit LHRH neurons and may suppress the activity of inhibitory neurons leading to increased dopamine activity.