Hypothalamic, metabolic, and behavioral responses to pharmacological inhibition of CNS melanocortin signaling in rats

Abstract

The CNS melanocortin (MC) system is implicated as a mediator of the central effects of leptin, and reduced activity of the CNS MC system promotes obesity in both rodents and humans. Because activation of CNS MC receptors has direct effects on autonomic outflow and metabolism, we hypothesized that food intake-independent mechanisms contribute to development of obesity induced by pharmacological blockade of MC receptors in the brain and that changes in hypothalamic neuropeptidergic systems known to regulate weight gain [i.e., corticotropin-releasing hormone (CRH), cocaine-amphetamine-related transcript (CART), proopiomelanocortin (POMC), and neuropeptide Y (NPY)] would trigger this effect. Relative to vehicle-treated controls, third intracerebroventricular (i3vt) administration of the MC receptor antagonist SHU9119 to rats for 11 d doubled food and water intake (toward the end of treatment) and increased body weight ( approximately 14%) and fat content ( approximately 90%), hepatic glycogen content ( approximately 40%), and plasma levels of cholesterol ( approximately 48%), insulin ( approximately 259%), glucagon ( approximately 80%), and leptin ( approximately 490%), whereas spontaneous locomotor activity and body temperature were reduced. Pair-feeding of i3vt SHU9119-treated animals to i3vt vehicle-treated controls normalized plasma levels of insulin, glucagon, and hepatic glycogen content, but only partially reversed the elevations of plasma cholesterol ( approximately 31%) and leptin ( approximately 104%) and body fat content ( approximately 27%). Reductions in body temperature and locomotor activity induced by i3vt SHU9119 were not reversed by pair feeding, but rather were more pronounced. None of the effects found can be explained by peripheral action of the compound. The obesity effects occurred despite a lack in neuropeptide expression responses in the neuroanatomical range selected across the arcuate (i.e., CART, POMC, and NPY) and paraventricular (i.e., CRH) hypothalamus. The results indicate that reduced activity of the CNS MC pathway promotes fat deposition via both food intake-dependent and -independent mechanisms.

Fig. 1.

Effect of third cerebroventricular (i3vt) treatment for 11 d with vehicle (saline; n = 7), SHU9119 (0.5 nmol/d; n = 7), or SHU9119 (0.5 nmol/d) pair-fed to the vehicle-treated group (SHU9119/pair-fed;n = 7) on 24 hr food intake (A) and body weight (B). Both food intake (p < 0.0001 from day 2 till the end of treatment) and body weight (p < 0.05, days 2–7; p< 0.01, days 8–10) were significantly increased in i3vt SHU9119-treated animals compared with the i3vt vehicle-treated group.

Fig. 2.

Five day averages (days 5–10 after minipumps implantation) of body temperature (A) and locomotor activity (B) during the dark and light phase of rats treated third cerebroventricularly (i3vt) with vehicle (saline; n = 7), SHU9119 (0.5 nmol/d;n = 7), or SHU9119 (0.5 nmol/d) pair-fed with the vehicle-treated group (SHU9119/pair-fed, n = 7). *p < 0.05); **p < 0.01; ***p < 0.001 denote statistical difference with the i3vt vehicle-treated group. ⁺⁺p < 0.01 denotes statistical difference with the i3vt SHU9119-treated group.

Fig. 3.

Expression of mRNA for NPY, CART, and POMC in the arcuate hypothalamic nucleus, and CRH in the paraventricular nucleus of rats treated third cerebroventricularly (i3vt) for 11 d with vehicle (saline; n = 7; open bars), SHU9119 (0.5 nmol/d; n = 7; black bars), or SHU9119 (0.5 nmol/d) pair-fed with the vehicle-treated group (SHU9119/pair-fed; n = 7;hatched bars). Levels of mRNA were expressed as percentage of expression of mean value of the vehicle-treated group.