Evaluation of the Role of Melanocortin 3 and 4 Receptors in Leptin-Stimulated and Spontaneous Growth Hormone Secretion in Rats

Abstract

It has been reported that the melanocortin 4-receptor (MC4-R) may act downstream of leptin to mediate its effects on food intake and several neuroendocrine functions (the reproductive system, the hypothalamo-pituitary-thyroid axis, and prolactin secretion). However, no previous study examined whether MC4-R mediates leptin stimulatory actions on growth hormone (GH) secretion, or whether MC4-R signaling is involved in spontaneous pulsatile GH release in fed rats. Therefore in this study we examined the involvement of both MC3-R and MC4-R (the predominant MC-R subtypes expressed in the brain) in these two aspects of GH secretion in freely-moving male rats. In both fed and 3-day fasted rats, plasma GH levels were determined every 15 min over 5 h after single intracerebroventricular injections of the following substances or vehicle. Fasting diminished and leptin (0.3 nmol) reinstated the GH pulse amplitude without affecting the pulse frequency. Neither HS014 (1.0 nmol, a selective MC4-R antagonist) nor agouti-related peptide (1.0 nmol, a non-selective MC3/4-R antagonist) was effective in altering leptin-stimulated or spontaneous GH secretion. In addition, neither melanotan-II (1.0 nmol, a non-selective MC3/4-R agonist) nor γ₁-melanocyte-stimulating hormone (10 nmol, a selective MC3-R agonist) affected significantly GH release in fasted rats. We have previously demonstrated that stimulation or blockade of MC4-R, achieved by the same drug dosage as in this study, significantly affect luteinizing hormone and prolactin secretion in rats. The present results thus suggest that neither MC4-R nor MC3-R is involved in leptin-stimulated or spontaneous GH secretion, or at least that the level of MC4-R involvement in GH secretion is much lower than that in luteinizing hormone and prolactin release regulation.