An opioid pathway in the hypothalamus of the turkey that stimulates prolactin secretion

Circulating prolactin (PRL) levels increase when dynorphin is infused into the turkey brain. This study tested the hypothesis that centrally infused dynorphin requires an intact vasoactive intestinal peptide (VIP) system in order to stimulate turkey PRL secretion. It also investigated the roles of the dopaminergic and serotonergic systems in dynorphin-induced PRL release. Drugs were infused into the third ventricle of anesthetized laying turkeys via stereotaxically guided cannulae and circulating blood was assayed for changes in PRL levels. When a highly selective κ opioid receptor antagonist was given prior to dynorphin injection, the PRL response to dynorphin was almost totally blocked. The coinfusion of either a serotonin (5-HT) or a D₁ dopamine (DA) receptor antagonist with dynorphin prevented the increase in PRL observed in birds when dynorphin was infused alone. On the other hand, the κ opioid receptor antagonist failed to prevent the 5-HT-induced release of PRL. In hens actively immunized against VIP, infused dynorphin was unable to increase plasma PRL levels and infused VIP gave a muted PRL rise, while large increases in PRL were seen in nonimmunized birds receiving the same infusions. These data show that: (1) dynorphin stimulates PRL secretion by activating κ opioid receptors in the avian hypothalamus, and (2) dynorphin, 5-HT, DA, and VIP stimulate avian PRL secretion via a common pathway expressing κ opioid, serotonergic, dopaminergic, and VIPergic receptors at synapses arranged serially in that functional order, with the VIPergic system as the final mediator (releasing factor).