Prolactin-releasing peptide, food intake, and hydromineral balance in goldfish

Abstract

A potential role for prolactin-releasing peptide (PrRP) in appetite regulation and hydromineral balance in goldfish was examined. PrRP was found to be expressed in discrete regions of the goldfish brain, in particular, the hypothalamus. Intraperitoneal (IP) or intracerebroventricular administration of PrRP had dose-dependent effects to suppress food intake in goldfish. Hypothalamic PrRP mRNA expression significantly increased after feeding, as well as after 7 days of food deprivation. Refeeding fish after 7 days food deprivation did not result in a postprandial increase in PrRP mRNA expression. These data suggest an anorexigenic role for PrRP in the short term around a scheduled meal time, but not over the longer term. IP injection of PrRP significantly increased pituitary prolactin (PRL) mRNA levels, suggesting involvement in the regulation of lactotroph activity. Acclimating goldfish to an ion-poor environment decreased serum osmolality and increased PrRP and PRL mRNA levels, providing evidence for PrRP involvement in hydromineral balance through its actions on lactotrophs. Acclimation to ion-poor water diminished the anorexigenic properties of PrRP in goldfish, indicating that a role for PrRP in goldfish satiation is counterbalanced by alternate systemic needs (i.e., osmoregulatory). This was further supported by an ability to reinstate the anorexigenic actions of PrRP in fish acclimated to ion-poor water by feeding a salt-rich diet. These studies provide evidence that PrRP is involved in regulating appetite and hydromineral balance in fish, and that the degree of involvement in either process varies according to overall systemic needs in response to environmental conditions.

The role of RFamide peptides in feeding

“…This contrast between chicks and mammals has also been reported for ghrelin and growth hormone-releasing hormone, both of which inhibit feeding in chicks, yet stimulate it in rats (Vaccarino et al 1985, Furuse et al 2001, Nakazato et al 2001. In goldfish, hypothalamic PrRP mRNA expression is modified by feeding status, and either intraperitoneal or ICV administration of fish PrRP peptide has been shown to suppress food intake (Kelly & Peter 2006). It is therefore important to keep in mind that while an evolutionarily conserved role of RFamides in the regulation of feeding behaviour seems clear, the specific actions can vary greatly between class and species.…”

Stretch-activated cation channel TRPV4 mediates hyposmotically induced prolactin release from prolactin cells of mozambique tilapia Oreochromis mossambicus

“…To coordinate hyperosmoregulatory functions in various tissues, osmosensitive mechanisms play an essential role in controlling PRL secretion. Although there are several reports that PRL release is under hypothalamic control, the relationship between these regulatory pathways and osmoregulation is still unclear (11,15,22,30,31,40).Previous studies have shown that PRL cells of tilapia are intrinsically responsive to changes in extracellular osmolality (12,23,32,37). The release of PRL from dispersed PRL cells is stimulated by physiologically relevant decreases in extracellular osmolality (36), a response that is closely related to cell volume changes (33,34,45), and is consistent with the hyperosmoregulatory role of PRL.…”

“…To coordinate hyperosmoregulatory functions in various tissues, osmosensitive mechanisms play an essential role in controlling PRL secretion. Although there are several reports that PRL release is under hypothalamic control, the relationship between these regulatory pathways and osmoregulation is still unclear (11,15,22,30,31,40).…”

Functions of Two Distinct “Prolactin-Releasing Peptides” Evolved from a Common Ancestral Gene

“… a Rat (41) . b Chicken (5) . c Mudskipper (12) . d Sea bream (14) . e Goldfish (45) . +: Definite expression; –: little or nothing .Blank columns indicate “not examined.” Distribution of PrRP has not yet been investigated in teleost . In rats, PrRP mRNA expression is also observed in thymus, trachea, submandibular gland, adrenal gland, and uterus (see PrRP and PrRP-R) .…”

“…PrRP2 mRNA is distributed in the central nervous system in non-mammalian vertebrates (5, 12, 14, 45, 46) (Table 1). Further, a histological survey has been performed only in teleosts using the antisera against teleost PrRP2 whose cross-reactions with PrRP are unclear.…”