D1 and D2 dopamine receptor messenger ribonucleic acid in brain and pituitary during the reproductive cycle of the turkey hen

The regulation of prolactin secretion during the reproductive cycle of seasonal breeding birds appears to be largely under the stimulatory influence of hypothalamic vasoactive intestinal peptide (VIP). However, the factors influencing VIP secretion, and hence prolactin release, in birds remain largely unexplored. Recent evidence has demonstrated that dopamine and dopamine receptors may affect VIP and prolactin release in birds. The differential expression of dopamine receptors on hypothalamic VIP-releasing neurons may affect the degree of prolactinemia observed during the reproductive cycle of birds. In order to examine this hypothesis, we used reverse transcription-polymerase chain reaction to quantitate the levels of D1 and D2 dopamine receptor subtype mRNAs in the brain of the domestic turkey hen during the reproductive cycle. No significant difference in hypothalamic expression of D1 or D2 dopamine receptor subtypes during the reproductive cycle was observed. However, pronounced differences in D1(D) and D2 mRNAs were detected in cortex and cerebellum. Interestingly, there was a dramatic increase in pituitary D1(D) receptor mRNA during the reproductive stages of laying and incubation of eggs, which paralleled the hyperprolactinemic state of the turkey reproductive cycle. In addition, pituitary D2 receptor mRNA steadily increased throughout the reproductive cycle. In light of these observations, a modified hypothesis regarding the effects of dopamine on prolactin secretion is discussed.