2005 Volume 42 Issue 2 Pages 87-100
A wealth of functional data confirms the involvement of hypothalamic vasoactive intestinal peptide (VIP) and gonadotropin releasing hormone (GnRH) in the regulation of the avian reproductive cycle. However, very little is known about the neurotransmitters or the anatomical locations of the hypophysiotropic neurons mediating the transition from one reproductive state to the next. Dopamine (DA) stimulates prolactin (PRL) and luteinizing hormone (LH) secretion by acting on VIP and GnRH neurons, respectively. DA may inhibit PRL secretion by antagonizing the action of VIP at the level of the pituitary, and limits LH secretion through presynaptic inhibition of GnRH release at the median eminence (ME) level. The stimulatory and inhibitory effects of DA are mediated via D1 and D2 DA receptors, respectively. However, the dopaminergic neuronal groups/subgroups which regulate the VIP/PRL and GnRH/LH systems remain to be clarified. Studies utilizing electro-pharmacological techniques in combination with radioimmunoassay, immunocytochemistry, and in situ hybridization histochemistry yield results suggesting the presence of a stimulatory dopaminergic pathway from the preoptic area (POA) to the infundibular nuclear complex (INF) area, where VIP neurons preside over the regulation of PRL secretion, as well as DA projections within the preoptic area-anterior hypothalamus (POA-AM) areas where the GnRH neurons reside that control LH secretion. DA neurons projecting to the ME which mediate the inhibition of the VIP/PRL and GnRH/LH systems remain to be identified.