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 D(1) and D(2) 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 D(2) DA receptor subtype mRNA was quantitated using in situ hybridization histochemistry (ISH). D(1D) and D(2) 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 D(2) 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 D(2) DA receptor mRNA abundance throughout the reproductive cycle, except for an increase in D(2) DA receptor mRNA within the INF of photorefractory hens. Also, a marked reduction in D(2) 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 D(2) 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 D(2) DA receptor mRNA. The present findings clearly demonstrate that the expression of stimulatory D(1) DA receptor mRNA in the hypothalamus increases in hyperprolactinemic incubating hens, whereas inhibitory D(2) DA receptor mRNA increases in the pituitary of hypoprolactinemic photorefractory hens. Copyright 2003 S. Karger AG, Basel