Expression of D₁ and D₂ dopamine receptors in the hypothalamus and pituitary during the turkey reproductive cycle: Colocalization with vasoactive intestinal peptide

The regulation of avian prolactin (PRL) secretion and PRL gene expression is influenced by hypothalamic vasoactive intestinal peptide (VIP), the PRL-releasing factor in avian species. Recent evidence indicates that D₁ and D₂ dopamine (DA) receptors play a pivotal role in VIP and PRL secretion. The differential expression of DA receptors located on hypothalamic VIP neurons and anterior pituitary cells may affect the degree of prolactinemia observed during the turkey reproductive cycle. The relative expression of D_1D and D₂ DA receptor subtype mRNA was quantitated using in situ hybridization histochemistry (ISH). D_1D and D₂ DA receptor mRNA was found expressed throughout the hypothalamus and pituitary. The expression of D_1D DA receptor mRNA in the hypothalamus was found to be 6.8-fold greater than that of D₂ DA receptor mRNA. Higher D_1D DA receptor mRNA content was found in the anterior hypothalamus (3.6-fold), the ventromedial nucleus (2.0-fold), the infundibular nuclear complex (INF; 1.9-fold), and the medial preoptic nucleus (1.5-fold) of laying hens as compared to that of reproductively quiescent non-photostimulated hens. The levels seen in incubating hyperprolactinemic hens were essentially the same as in laying hens, except for the INF where levels were 52% higher. During the photorefractory stage (hypoprolactinemia), the D_1D DA receptor mRNA was at its lowest level in all areas tested. No differences were observed in hypothalamic D₂ DA receptor mRNA abundance throughout the reproductive cycle, except for an increase in D₂ DA receptor mRNA within the INF of photorefractory hens. Also, a marked reduction in D₂ DA receptor mRNA was observed in the pituitary of incubating hens. Pituitary D_1D DA receptor levels did not change when birds entered the incubating phase. Double ISH revealed that D_1D and D₂ DA receptor mRNAs were co-expressed within neurons expressing VIP mRNA, predominantly within the lateral hypothalamus and INF. D_1D DA receptor mRNA was more highly expressed than D₂ DA receptor mRNA. The present findings clearly demonstrate that the expression of stimulatory D₁ DA receptor mRNA in the hypothalamus increases in hyperprolactinemic incubating hens, whereas inhibitory D₂ DA receptor mRNA increases in the pituitary of hypoprolactinemic photorefractory hens.