To characterize further vasoactive intestinal peptide (VIP) as the prolactin-releasing factor in avian species, the present study examined hypothalamic VIP transcription and plasma prolactin (PRL) levels during the turkey reproductive cycle. The contribution of transcription to hypothalamic VIP mRNA steady-state levels and VIP content in response to gonadal stimulating photoperiod was also investigated. Nuclear run-on transcription assays were performed using nuclei isolated from hypothalami. Cytoplasmic VIP mRNA levels, and VIP content in the median eminence and plasma PRL levels were determined by Northern blot analysis and radioimmunoassays respectively. The alterations in VIP transcription mirrored the changes in cytoplasmic VIP mRNA and VIP content during the reproductive stages. VIP transcription, cytoplasmic VIP mRNA level and VIP content were lowest in non-photostimulated birds, higher (P<0.05) in laying hens, and greatest (P<0.05) in incubating birds. These increases paralleled the changes in circulating plasma PRL levels. Changes in VIP transcription (P>0.05) were not observed during the transition from incubation to photorefractoriness, even though there was a sharp decline in circulating plasma PRL levels (P<0.05). Following photostimulation, VIP transcription, cytoplasmic VIP mRNA levels, and VIP content increased as the hens progressed towards sexual maturity (P<0.05), and these increases were correlated with an increased plasma PRL level. These results suggest that VIP is regulated in large part at the transcriptional level during the turkey reproductive cycle and that this transcriptional regulation is coupled to the photo-induced increase in PRL secretion.

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