The Role of Melatonin and Bromocriptine in the Regulation of Prolactin Secretion in Animals – A Review

Changes in the concentration of melatonin and prolactin are associated with response to day length. The factors that stimulate the release of PRL include, among others, TRH, VIP, endorphins, oestrogen, and adrenaline. PRL secretion inhibitors include DA, GABA, progesterone and bromocriptine (exogenous compound), used in the treatment of hyperprolactinemia D2 receptor agonist. The biological activity of this compound is to stimulate dopamine D2 receptors in the pituitary, which inhibits PRL secretion via the tuberoinfundibular pathway (Fitzgerald and Dinan, 2008). In sheep treated with bromocriptine there is a decrease in PRL concentration and an increase in the sensitivity of cells producing LH to GnRH, but there is no disturbance in the course of the estrous cycle. Rams show decreased libido and changes in sexual behaviour. The administration of bromocriptine throughout the period of sexual activity and the period of rearing offspring affects maternal instinct, which involves the inhibition of PRL secretion. It is not clarified whether the effect of bromocriptine on sexual behaviour is associated with its direct impact on behavioural centres in the brain, or indirectly through the regulation of PRL secretion. However, the seasonal variations in the effects of bromocriptine on sexual behaviour can strengthen the hypothesis that the effect of bromocriptine on sexual development behaviour is associated with changes in the intensity of PRL secretion rather than by the inhibition of behavioural bromocriptine brain centres. The process of regulation of prolactin secretion by bromocriptine requires further examination.