Avian prolactin (PRL) secretion is regulated by vasoactive intestinal peptide (VIP) neurons residing in the infundibular nuclear complex (INF) of the hypothalamus. This VIPergic activity is modulated by stimulatory dopaminergic inputs. Dynorphin, serotonin (5-HT), dopamine (DA) and VIP all appear to stimulate PRL secretion along a hypothalamic pathway, expressing ĸ opioid, serotonergic, dopaminergic and VIPergic receptors in succession, with the VIPergic system as the final mediator. Electrical stimulation (ES) within the turkey hypothalamus at the level of the medial preoptic area (POA), the ventromedial hypothalamic nucleus (VMN), the INF or the median eminence (ME) results in the release of PRL. When the selective D₁ DA receptor antagonist SCH-23390 HCl was infused intraventricularly at the rate of 10 nmol/min, ES in the POA or VMN was unable to increase PRL levels, while ES in the INF and ME did increase PRL to the same level as that of controls. These results were interpreted to suggest that the D₁ DA receptors involved in PRL release lie caudally to the VMN and dorsally to the INF. Bilateral microinjections (50 ng) of the D₁ DA receptor agonist SKF-38393 HCl into the POA or VMN failed to produce any increase in PRL, while similar microinjections in the INF increased PRL significantly within 15 min. Bilateral microinjections of the D₁ DA antagonist (50 ng) into the INF blocked the rise in PRL associated with ES in the POA. Bilateral microinjections of a D₂ DA antagonist (50 ng) into the INF failed to block PRL secretion induced by ES in the POA. Tract tracing, using double-label immunocytochemistry, revealed the presence of a monosynaptic dopaminergic pathway projecting from the POA to the INF. These data imply that the only hypothalamic D₁ DA receptors involved in the regulation of avian PRL secretion are those residing within the INF in the same region as the VIP neurons known to be involved in PRL secretion.

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 D₁ and D₂ 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 D₂ DA receptor subtype mRNA was quantitated using in situ hybridization histochemistry (ISH). D_1D and D₂ 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 D₂ 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 D₂ DA receptor mRNA abundance throughout the reproductive cycle, except for an increase in D₂ DA receptor mRNA within the INF of photorefractory hens. Also, a marked reduction in D₂ 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 D₂ 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 D₂ DA receptor mRNA. The present findings clearly demonstrate that the expression of stimulatory D₁ DA receptor mRNA in the hypothalamus increases in hyperprolactinemic incubating hens, whereas inhibitory D₂ DA receptor mRNA increases in the pituitary of hypoprolactinemic photorefractory hens.

The neural and neurochemical substrates regulating reproduction in birds remain vaguely defined. The findings that electrical stimulation in the medial preoptic area (ES/MPOA) or intracerebroventricular infusion of dopamine (DA) stimulated luteinising hormone (LH) and prolactin (PRL) release in female turkeys, led to the suggestion that ES/MPOA might help to clarify the DA circuitry regulating LH and PRL. We used c-fos mRNA and tyrosine hydroxylase immunoreactivity as measured by double in situ hybridisation/immunocytochemistry (ISH/ICC) to determine which group/subgroup of DA neurones was activated following unilateral ES/MPOA. To establish that the reproductive neuroendocrine system was activated, double ISH/ICC was also conducted on c-fos/gonadotrophin-releasing hormone-I (GnRH-I) and c-fos/vasoactive intestinal peptide (VIP). Changes in circulating LH and PRL were determined by radioimmunoassay. Unilateral ES/MPOA (100 microA, right side) of anaesthetised laying turkeys for 30 min increased circulating LH and PRL levels. It also induced c-fos mRNA expression on the ipsilateral side by all GnRH-I neurones within the septopreoptic region, implying that GnRH-I neurones in this region share similar circuitry. VIP neurones within the nucleus infundibularis were the only VIP group to show c-fos mRNA expression, suggesting their involvement in ES/MPOA induced PRL release. c-fos mRNA expression was also observed in a subgroup of DA neurones in the nucleus mamillaris lateralis (ML). To our knowledge, the present study is the first to show that activation of DAergic cells in the ML is associated with the activation of GnRH-I and VIP neurones and the release of LH and PRL. It is likely that ES/MPOA activated VIP/GnRH-I neurones via activation of DA neurones in the ML, as this was the only DA subgroup that showed c-fos mRNA expression.