Effects of galanin-like peptide (GALP) on locomotion, reproduction, and body weight in female and male mice
Introduction
Galanin-like peptide (GALP) is a 60-amino acid neuropeptide of which amino acids 9–21 share 100% sequence identity with the first 13 amino acids of the neuropeptide galanin (Cunningham, 2004, Ohtaki et al., 1999). GALP was initially cloned from the pig, human, and rat, and cDNAs from non-human primates and mice have recently been characterized (Cunningham et al., 2002, Jureus et al., 2001, Ohtaki et al., 1999). In all species examined, GALP and galanin are encoded by distinct genes (Cunningham, 2004). In contrast to galanin, which is widely distributed throughout the brain, the expression of GALP within the mammalian central nervous system is exclusively localized to the hypothalamic arcuate nucleus, the median eminence, and the infundibular stalk (Jureus et al., 2001, Kerr et al., 2000, Larm and Gundlach, 2000, Takatsu et al., 2001). The highly localized distribution of hypothalamic cells expressing GALP mRNA is well conserved across mammalian species (Cunningham, 2004, Cunningham et al., 2002, Jureus et al., 2001). Fibers expressing GALP immunoreactivity are located in several forebrain regions implicated in the regulation of feeding, energy balance, and/or reproduction including the paraventricular nucleus (PVN), medial preoptic area (mPOA), bed nucleus of the stria terminalis (BNST), periventricular nucleus, and the arcuate nucleus (Takatsu et al., 2001).
The arcuate nucleus, where most GALP-containing neurons are located, plays a critical role in the regulation of mammalian energy balance and reproduction (Gottsch et al., 2004), and several discrete populations of neurons in this nucleus are targets for the regulatory hormone leptin (McMinn et al., 1998, McMinn et al., 2000). Furthermore, in both rodents and primates, the arcuate nucleus contains mRNA of numerous regulatory neuropeptides involved in feeding and/or reproduction including neuropeptide Y (NPY), α-melanocyte-stimulating hormone (α-MSH), agouti-related protein (AgRP), and galanin (Cone et al., 2001). Immunocytochemistry and in situ hybridization analyses have revealed that most GALP neurons express leptin receptors (Cunningham et al., 2002, Jureus et al., 2000, Takatsu et al., 2001) and as many as 40% express the NPY Y1 receptor (Cunningham et al., 2004). Collectively, these data suggest that GALP has a critical role in the regulation of energy balance and/or reproduction. Although central infusions of GALP stimulate acute feeding (1 h) in rats (Lawrence et al., 2002, Matsumoto et al., 2002, Seth et al., 2003), other studies in mice and rats have reported decreases in long-term (14 h, 24 h) food intake and a simultaneous reduction in body weight following GALP treatment (Hansen et al., 2003, Krasnow et al., 2003, Krasnow et al., 2004, Lawrence et al., 2002). Interestingly, male mice also display decreased locomotor activity for several hours after GALP infusion (Krasnow et al., 2003), an effect not observed in rats (Cunningham, 2004).
In addition to their effects on feeding, GALP infusions also significantly influence male reproductive physiology and behavior. GALP infusions stimulate luteinizing hormone (LH) release in a dose-dependent manner in male rats, mice, and macaques (Cunningham et al., 2004, Krasnow et al., 2003, Krasnow et al., 2004, Matsumoto et al., 2001). This GALP-induced LH release is likely mediated via GnRH I-dependent pathways, as GnRH I receptor antagonists prevent GALP from stimulating LH release in rats and primates (Cunningham et al., 2004, Matsumoto et al., 2001). Furthermore, GALP can cause in vitro release of GnRH from hypothalamic explants and GALP infusions also increase Fos immunoreactivity in GnRH neurons (Matsumoto et al., 2001, Seth et al., 2004). Recently, Fraley et al. (2004) reported increased male sexual behavior in gonad-intact and castrated rats infused with GALP, indicating that GALP's effects on reproduction also include the regulation of behavior.
Interestingly, while GALP's regulatory function in males has been well studied, no studies have addressed the physiological and behavioral roles of GALP in females of any mammalian species. We therefore conducted several studies in female mice to elucidate the role of GALP in reproduction, locomotion, and energy balance. Specifically, we addressed the following critical questions: (1) Does GALP effect LH release and body weight in female mice, and if so, are these effects mediated by GnRH-dependent pathways? (2) Are GALP's locomotor effects previously reported in male mice also present in females, and if so, what is the underlying basis for the reduced activity? (3) Does GALP stimulate sexual behavior in mice of either sex as it does in male rats?
Section snippets
Subjects
All experiments used adult (2–4 months of age), initially sexually-naïve male and female C57BL/6J mice purchased from the Jackson Laboratory (Bar Harbor, ME). Female mice were group housed (2–3 per cage) with food (Harlan Diet 8604; Harlan Teklad, Madison, WI) and water available ad libitum. After cannula implantation, female mice were individually housed. Male mice were singly housed for the entire duration of the study. The room was maintained on a 12-h light, 12-h dark photoperiod (lights
GALP increases LH release in female mice via a GnRH-dependent pathway
Central icv infusions of GALP stimulated the release of LH in female mice in a dose-dependent manner (F[3,30] = 4.79; P < 0.01; Fig. 1). All three GALP doses elicited significant increases in LH levels relative to control infusions of saline (P < 0.05 for each dose relative to saline; Fig. 1A). The highest dose of GALP (2.5 nmol) resulted in an approximate 2-fold increase in LH secretion relative to the lowest dose (0.5 nmol) tested (Fig. 1A).
Pre-treatment of female mice with Antide, a potent
Discussion
This is the first study to analyze the effects of GALP on female physiology and behavior in any mammalian species. We found that GALP has similar regulatory effects on LH release, energy balance, and locomotor activity in female mice as previously reported for male mice. In contrast, GALP had opposing effects on sexual behavior in both male and female mice as compared to its previously reported stimulatory actions on male rat sexual behavior.
Similar to its effects in male mice, rats, and
Acknowledgments
The authors thank Dr. Robert Steiner for generously providing the GALP. The authors also thank Aileen Wills for excellent animal care and technical support. This work is supported by NIMH grants F32 MH070084, K02 MH01349, and R01 MH57759. RIA assays were conducted by the University of Virginia Ligand Assay and Analysis Core Lab, which is supported by NICHD Grant U54 HD28934.
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2017, NeuropeptidesCitation Excerpt :However, our results show no differences in motor activity between GALP and control treatments; therefore, the effects of GALP on metabolic rate that we observed must be due to non-locomotive factors. Other reports have demonstrated an alteration in motor activity in mice following central GALP treatments (Kauffman et al., 2005b; Krasnow et al., 2002); however, our data clearly demonstrate that this is not the case in the rat. Our results show that GALP injection has no significant effects on activity levels in male rats.