Our previous studies using turkey hens have demonstrated that c-fos mRNA (a marker of neuronal activation) is expressed in gonadotrophin-releasing hormone-I (GnRH-I), vasoactive intestinal peptide (VIP) and dopamine (DA) neurones following electrical stimulation in the preoptic area. DA has been shown to have both stimulatory and inhibitory effects on the GnRH-I/luteinising hormone (LH), follicle-stimulating hormone (FSH) and VIP/prolactin (PRL) systems. To identify the DA neurones that mediate the stimulatory influences of photoperiod on the reproductive system, we examined c-fos mRNA induction in DA, GnRH-I, and VIP neurones in the turkey hypothalamus using a dark-interruption experimental design. A 30-min light period was provided to short day (6L:18D) photosensitive turkeys at times when birds were responsive to light (14h after first light) and at times when birds were unresponsive to light (8h and 20h after first light). The only area where DA neurones were activated when the birds were provided with light was in the nucleus premammillaris (PMM). The number of activated DA neurones was significantly greater when light was provided at 14h (during the photoinducible phase) than at 8h or 20h. At 14h, there was also an increase in the number of GnRH-I neurones activated in the area of the nucleus commissura pallii (nCPa), as well as an up-regulation of GnRH-I mRNA expression. No expression of c-fos mRNA was observed in VIP neurones in the nucleus infundibularis or up-regulation of VIP mRNA expression in any of the experimental light treatments. These results are the first evidence to demonstrate a relationship between the dopaminergic system in the PMM and the GnRH-I system in the nCPa during the photoinduction of avian reproductive activity.
- Avian species
- Gonadotrophic-releasing hormone-I
- Rhythmic activation