Expression of Vasoactive Intestinal Peptide/Peptide Histidine Isoleucine in Several Hypothalamic Areas during the Turkey Reproductive Cycle: Relationship to Prolactin Secretion

Abstract

The present study examined the changes in vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI) gene expression in the hypothalamus and compared their level of expression to prolactin (PRL) secretion throughout the turkey reproductive cycle. Both VIP and PHI expression were determined using in situ hybridization histochemistry and reverse transcription-polymerase chain reaction/Southern blot analysis. Plasma PRL levels were determined by radioimmunoassay. The changes in total hypothalamic VIP and PHI mRNA steady-state levels mirrored the changes seen in plasma PRL across the reproductive stages. In situ hybridization revealed that VIP mRNA was highly expressed throughout the hypothalamus and predominated within the ventromedial nucleus (VMN), inferior hypothalamus, and infundibular nuclear complex (INF). PHI mRNA was expressed in very low abundance within the same areas expressing VIP mRNA. VIP mRNA abundance within the VMN and INF was lowest in nonphotostimulated hens (VMN, 13.8 ± 1.7; INF, 17.0 ± 1.8 arbitrary densitometric units (ADU)), intermediate in laying hens (VMN, 29.6 ± 3.3; INF, 35.4 ± 4.3 ADU), and highest in incubating hens (VMN, 76.4 ± 10.2; INF, 119.2 ± 3.4 ADU). Levels decreased when birds shifted from incubation to photorefractoriness (VMN, 75%; INF, 82%). This relationship was not observed within other areas of the hypothalamus. The expression of PHI mRNA was also highest in the VMN and INF of incubating hens but no correlation between PHI mRNA and the other reproductive states was observed. This study provides additional evidence that VIP is the avian PRL-releasing factor, and suggests that the central site for avian PRL regulation lies within the INF of the hypothalamus.