Sex differences in the effect of prepubertal GALP infusion on growth, metabolism and LH secretion

Abstract

The hypothalamic neuropeptide, galanin-like peptide (GALP), is known to have an effect on energy expenditure and reproduction in adult male rats, but little work has been done on prepubertal rats. We hypothesized that hypothalamic GALP is involved in physiological changes associated with the onset of puberty. To test this hypothesis, we first determined the postnatal ontogeny of GALP gene expression via in situ hybridization of developing male and female rat pups through adulthood. GALP gene expression was not observed in either male or female rat pups until after postnatal day (PND) 10 and did not reach adult-like levels until after weaning (PND25). To determine if exogenous GALP could induce the onset of puberty, PND25 male and female rats were implanted with lateral ventricular cannulas connected to an osmotic minipump that delivered either GALP or vehicle. GALP infusion significantly (p<0.05) increased body weight, food intake, and metabolic rate in male but not female rats compared to control infusion. After 2 weeks, GALP infusion had no significant effect on the onset of puberty, percent body fat, nor plasma levels of insulin, FSH or gonadal steroids in either sex; however, GALP did significantly (p<0.05) increase plasma levels of LH and leptin in male but not female rats and increased plasma growth hormone (GH) in both sexes. Our observations further demonstrate a sex difference in GALP responsiveness in prepubertal rats. These data suggest that GALP may be involved with the prepubertal increase in circulating leptin, LH, and GH resulting in an increase in metabolic rate and lean growth associated with puberty.

Fig. 1

Postnatal ontogeny of GALP gene expression. The presence of hypothalamic GALP mRNA was determined via in situ hybridization. GALP gene expression was not observed until after PND10. Analysis by two way ANOVA (sex × age) shows a significant (p < 0.001) increase in GALP gene expression over time. No sex difference was observed in GALP gene expression at any postnatal age. * = p <0.05, *** = p < 0.001.

Fig. 2

Photomicrographs of representative GALP ISH. The right column (bar = 40 μm) is a higher power image of the panels in the left column (bar = 100 μm). All photomicrographs are from similar coronal sections of the Arc at the level of the median eminence and ventromedial hypothalamic nucleus. 3v = third ventricle, arrows indicate silver grain clusters denoting GALP-expressing cells.

Fig. 2

Photomicrographs of representative GALP ISH. The right column (bar = 40 μm) is a higher power image of the panels in the left column (bar = 100 μm). All photomicrographs are from similar coronal sections of the Arc at the level of the median eminence and ventromedial hypothalamic nucleus. 3v = third ventricle, arrows indicate silver grain clusters denoting GALP-expressing cells.

Fig. 3

Central GALP infusion significantly increased food intake in male (A) but not female (B) prepubescent rats compared to control infusion. Asterisk indicates statistical significance, p < 0.01.

Fig. 4

Central GALP infusion significantly increased body weight in male (A) but not female prepubescent rats compared to control infusions. By the end of the infusion period, GALP-treated male rats weighed significantly (p < 0.05) more than all other groups. Error bars are presented but are too small to resolve. Asterisk indicates p < 0.05.

Fig. 5

Rats were habituated to the metabolic chamber for 30 minutes, then oxygen consumption measured for one hour. Central GALP infusion significantly increase male (A) but not female (B) oxygen consumption suggesting an increase in metabolic rate. Asterisks indicate statistical significance, p < 0.001.