Melanotan II's thermogenic effects are mediated by CB1 receptors in the PVH of male rats

The melanocortin system is a critical regulator of energy balance, with the MC4R receptor playing a key role in appetite and energy expenditure. Agonists of this receptor, like Melanotan II, are known to increase thermogenesis, particularly in brown adipose tissue (BAT), which burns fat to produce heat. However, the precise neural circuits and interacting pathways are not fully understood. Separately, the endocannabinoid system, via the CB1 receptor, is known to influence energy metabolism, often promoting energy storage. Understanding how these two powerful systems interact, specifically within key brain regions like the paraventricular hypothalamic nucleus (PVH), could reveal novel mechanisms for controlling metabolic rate and body temperature, a crucial aspect of metabolic health and disease.

The study used male rats to investigate the interaction between MC4R and CB1 systems. In the first experiment, rats received infusions of the MC4R agonist Melanotan II (MTII) into the third brain ventricle, along with either the CB1 agonist δ(9)-THC or the CB1 antagonist AM251. Researchers measured whole-body thermogenesis via indirect calorimetry and brown adipose tissue (BAT) thermogenesis by tracking interscapular BAT (iBAT) temperature. They also assessed fatty acid and glucose uptake in iBAT using (14)C-bromopalmitate and (3)H-deoxyglucose tracers. A second experiment localized the effect by infusing MTII directly into the paraventricular hypothalamic nucleus (PVH) while infusing δ(9)-THC or AM251 into the fourth ventricle.

Activating MC4R with MTII significantly stimulated whole-body energy expenditure and increased iBAT temperature. This effect was directly opposed by the CB1 system. The CB1 agonist δ(9)-THC blunted MTII's effects on both energy expenditure and iBAT temperature, whereas the CB1 antagonist AM251 tended to potentiate them. Further analysis showed that δ(9)-THC also blocked the MTII-induced increase in fatty acid ((14)C-bromopalmitate) and glucose ((3)H-deoxyglucose) uptake into iBAT.

Critically, δ(9)-THC also attenuated the stimulating effect of MTII on the expression of key thermogenic genes in iBAT, including Pgc1α, Dio2, Cpt1b, and Ucp1. When MTII was infused directly into the PVH, subsequent administration of δ(9)-THC still blunted the rise in iBAT temperature and blocked the increase in (14)C-bromopalmitate uptake and Pgc1α and Dio2 gene expression. This demonstrates that the paraventricular nucleus of the hypothalamus (PVH) is a key site for this CB1-mediated modulation of melanocortin-driven thermogenesis.