Evidence for "short loop" feedback regulation of prolactin and growth hormone secretion in the pigeon

Intraventricular prolactin inhibits hypothalamic vasoactive-intestinal polypeptide-expression in doves

While the role of prolactin in promoting the development of the crop-sac in members of the pigeon family (Columbiformes) is well established, its action in the central nervous system is less well understood. In the present study, prolactin was administered intracerebroventricularily (i.c.v.) in ring doves, and central expression of vasoactive-intestinal polypeptide (VIP) and gonadotropin-releasing hormone (GnRH), and the display of sexual behavior was investigated. Ovine-prolactin (1 microgram in 2 microliters o-prl) was injected daily for six days through chronically implanted cannula either prior to a 2-h period of courtship, or late in incubation. Control subjects were given vehicle injections and were otherwise identical to experimental animals. Prolactin administered prior to courtship resulted in a reduction of sexual behavior, and in a decrease in testicular weight but had no detectable effect on the number of neurons expressing VIP or GnRH. In contrast, i.c.v. prolactin during incubation resulted in a reduced number of infundibular VIP-positive neurons and decreased crop weight. We conclude that during incubation prolactin regulates its own synthesis and/or release by modulating VIP expression in infundibular neurons.

Regional distribution and species variation of prolactin binding sites in the brain

The binding of 125I-labeled ovine prolactin (125I-oPRL) to membranes from different brain regions of pigeon, rabbit, rat, pig, calf, horse, and ewe was studied. The hypothalamus from rabbit, pig, horse, and pigeon showed a low but specific binding for 125I-oPRL clearly different from the other brain regions examined (cortex and cerebellum), whereas in the brain from rat, calf, and ewe the binding was very small and quite uniform in the various regions. Also the membranes from choroid plexus of rabbit, pig, calf, and horse showed an evident specific binding for prolactin. The binding of 125I-oPRL to hypothalamus and choroid plexus membranes from rabbit and horse was inhibited in a dose-dependent manner by unlabeled oPRL and hGH but not by many other polypeptide hormones. Scatchard analysis of the binding revealed the presence of binding sites with low capacity and high affinity for ovine ligand.

Effects of putative neurotransmitters on release of prolactin from pituitary glands of the domestic fowl co-incubated with hypothalamic tissue

Pituitary glands and hypothalami from broiler fowl heads were incubated alone or together with histamine, gamma-aminobutyric acid (GABA) or acetylcholine (ACh) as well as with catecholamines or neurotransmitter antagonists. Histamine and ACh stimulated, whereas GABA inhibited, the hypothalamus-induced release of prolactin, responses blocked by their specific antagonists. The dopamine antagonist pimozide, but not adrenergic (both alpha and beta), serotoninergic or cholinergic antagonists, prevented the actions of histamine and GABA. None of the antagonists except the cholinergic blocker, atropine, affected ACh-induced release of prolactin. Neither histamine nor ACh prevented inhibition of prolactin release by dopamine or stimulation of prolactin release by noradrenaline. GABA did not affect the response to noradrenaline. Furthermore, histamine, GABA and ACh had no effects on thyrotrophin releasing hormone-stimulated release of prolactin directly at the pituitary level. These results suggest that histamine and GABA affect prolactin release from chicken pituitaries in vivo by modifying the activity of the dopaminergic system. Acetylcholine may stimulate the secretion of prolactin releasing factor from the hypothalamus.