Changes in Feeding Activity, Plasma Luteinizing Hormone, and Testes Weight in Ring Doves Following Hypothalamic Injections of Prolactin

Abstract

Microinjections of ovine prolactin were administered unilaterally to the ventromedial hypothalamic nucleus and the preopticsuprachiasmatic region in adult male ring doves in an attempt to determine the site(s) at which intracranial injections of prolactin act to alter feeding behaviour and gonadotropin secretion in this species. Food intake and body weight were measured daily during a 6-day pretreatment period and the 5-day treatment period that immediately followed. During the treatment period, birds received twice daily injections (0.5 PI) of either 2.5 ng ovine prolactin or saline vehicle. An additional group of birds with cannulae in the ventromedial nucleus were given twice daily injections of 25 ng ovine prolactin. Although food consumption was unaffected by low dose prolactin treatment, birds given 25 ng prolactin injections into the ventromedial nucleus showed a significant augmentation in food intake. Injections of 25 ng prolactin into the preoptic area also increased feeding; however, the magnitude of this hyperphagic response, as expressed relative to pretreatment levels, was less than that observed following prolactin injection into the ventromedial nucleus. No differences were observed between prolactin-treated and vehicle-treated birds in either cannulation group when testes weights and plasma luteinizing hormone concentrations were compared at the end of the treatment period. However, the possibility that prolactin influenced changes in luteinizing hormone and testes weight relative to baseline values could not be assessed due to constraints imposed by the experimental paradigm used. These results suggest that prolactin-sensitive neurons in the ventromedial hypothalamic region and the preoptic area are potential sites of prolactin action in promoting hyperphagia in ring doves. However, the role of these sites in mediating prolactin-induced suppression of gonadotropin secretion in this species remains to be clarified.

Inhibitory Effects of Anti-Prolactin Receptor Antibodies on Prolactin Binding in Brain and Prolactin-Induced Feeding Behavior in Ring Doves

“…The fact that both PRL-treated groups showed greater food intake responses than did their corresponding vehi cle control groups during the treatment period further confirmed previous findings [5,22] that PRL can act on the VMN to increase feeding activity in male doves. Although anti-PRL receptor IgG treatment appeared to suppress PRL-induced feeding, the fact that birds given AS IgG + vehicle ate significantly less during the treat ment period than during the pretreatment period raised questions about whether the blunted PRL-induced food intake response of the AS IgG-treated birds was due to competitive inhibition between AS IgG and PRL for binding to the PRL receptor in the VMN or to an anorexic effect of the antibodies themselves.…”

“…Nonbreeding adult male ring doves were first fitted unilaterally with 28 gauge stainless steel guide cannulae (Plastics One, Roanoke. Va., USA) targeted to the VMN using stereo taxic procedures as described previously [5]. After a recovery period of at least 7 days, baseline food consumption and body weight were measured in all subjects once daily at 09.40 h for 7 consecutive days.…”

“…The amount of IgG administered at each injection represented the equivalent of that present in 0.2 pi of undi luted NRS or antiserum. The 50-ng dose of oPRL employed in this experiment had been previously shown to induce a greater food intake response than VMN injections of 2.5 of 25 ng oPRL [5,22] but is still below the threshold dose required to induce a detectable increase in food intake following intracerebroventricular injection [21], Treatments were scheduled twice daily at approximately 10.30 h and 17.30 h and continued for 5 consecutive days. Injections were delivered through a 5-pl Hamilton syringe with an attached 33-gauge infusion cannula assembly (Plastics One), which was screwed on to the nylon threads of the guide cannula.…”

“…Although PRL stimulates robust feeding responses when administered systemically [21], the concentration of plasma PRL in non-breeding doves [36] is likely to be below that needed to induce hyperphagia when injected into the cerebral ventricles [4,21] or into discrete brain sites [5,22], This argues against an important role of prolactin in regulating feeding activi ty in non-breeding birds. The degree to which circulating PRL gains access to PRL receptors in the brains of non breeding birds is also unclear, as injection studies reveal detectable but limited CNS uptake of blood-borne l25I-ovine PRL in non-breeding doves when assessed at 2 h post-injection [18].…”

“…Among those effects elicited by PRL administra tion, the increase in feeding behavior is particularly dra matic, with male birds showing approximately 100% increase in daily food intake in response to twice-daily i.c.v. infusions of PRL at a dose of 2 pg/bird [4], The sites of this action in the brain were further examined [5,22] by administering PRL to a variety of diencephalic brain loci that had been previously shown to contain PRL binding sites [19]. Collectively, these two studies showed that PRL can induce food intake responses in male doves when it is administered into three regions of the diencephalon: the preoptic area (POA), ventromedial hypothalamic nucleus (VMN), and tuberal hypothalamus (TU).…”