It was our aim to test if melatonin deficiency (due to surgical pinealectomy) decreased BAT activation and if melatonin replacement could revert this pattern to normal.
This is the first investigation, to our knowledge, to study BAT activation by 18F-FDG PET scan before and after melatonin treatment in animals. Although data of increased BAT mass and thermogenesis after melatonin replacement in many animal models and species have already been demonstrated (3,6, 14, 15, 18), we found relevant to investigate if 18F-FDG PET was able to detect those differences, in order to facilitate future research in animals as well as humans, in which indirect measures of BAT are more feasible to be detected than direct samples tissues. Indeed, the positive results of this actual study permitted our group to conduct an already published human study that demonstrated an increase in BAT volume and activity after melatonin replacement in a group of pinealectomized individuals (19).
In both isolated models, there were no differences between the groups, although some trends could be observed, including an unexpected higher maximal SUV in pinealectomized animals in room temperature. However, the melatonin deficient group had a clear, reduced acute thermogenic response, suggesting that melatonin proficiency is critical for thermoregulation in settings of acute challenging as temperature reduction. However, maybe in 23o C room temperature chronic exposure, which is not thermoneutral for rats, but clearly not as challenging as acute 4 o C exposure, other mechanisms of BAT activation independent of melatonin could be enough to maintain the normal thermoregulatory responses of the animal (3). In our recent human study that demonstrated an increase in BAT volume and activity after melatonin replacement in a group of pinealectomized individuals (19), some individuals have reasonable high baseline BAT activity.
The UCP-1 RNA data in the present study was performed as an additional tool to analyze and interpret our image data. Importantly, other published manuscripts, some of them by our own group, have already shown a decrease in UCP-1 RNA and UCP-1 protein expression after pinealectomy and a reversal of this reduction by melatonin replacement is slightly different experimental models (5, 13, 14, 18). Therefore, although our UCP-1 results are not original, they reinforce and validate those previous results.
The observed pattern of UCP-1 expression in our model is in line with 18F-FDG PETresults. The groups were statistically different; P group shas the lowest UCP-1 expression, which in some ways could explain the lower thermogenic capacity.
However, as already pointed out, an increase in UCP-1 RNA not necessarily means more heat production, as this recruited tissue can be inactive if there is no need to increase thermogenesis (3, 16, 17). Even differences in RNA and protein expression could arise, as post-transcriptional factors may influence protein synthesis. This could explain why even with lower UCP-1 expression, the pinealectomized group showed a normal BAT response in 18F-FDG PETin room temperature.
Our main issue in this particular study was to evaluate BAT responses in 18F-FDG PET. As many other different studies have demonstrated the physiological role of melatonin in body weight regulation, food intake, energy expenditure, metabolic risk factors, as well as many other parameters, we did not include those data as they would be redundant. Our group have previously shown that pinealectomy leads to a metabolic syndrome phenotype in rats and melatonin replacement reverts it (1). Melatonin have been shown to decrease body weight with a minimal decrease in food intake, suggesting an effect in energy expenditure, proposed to be mediated by BAT (2,3). For a more comprehensive review of metabolic consequences of melatonin deficiency and the physiological role of melatonin in several animal models, see references 1 and 2.
BAT physiology is very complex, and potential compounds aiming BAT increase can act in recruitment, activation, or both (3, 6, 11). Experimental models can help us to distinguish between the action of potential BAT recruiters, such as melatonin and the detection of BAT in vivo, by imaging techniques. Different models have the potential to investigate the role of several recruiters in different laboratory conditions, paving the way for human studies with potential compounds capable of activating or recruiting BAT, besides melatonin (11, 20). Our finding that melatonin seems critical for the acute BAT activation induced by cold measured by 18F-FDG PET is novel and can lead to future imaging studies in both animals and humans that will help to understand the physiological role of melatonin in relation to BAT and if melatonin could be a potential weapon for increasing melatonin recruitment and activation in humans, with potential therapeutic use in metabolic diseases. In the same way, the finding of reduced melatonin production leading to reduced BAT thermogenic responses can help the understanding of increased light-at-night exposure as a potential risk factor for obesity and metabolic diseases, as already suggested (19, 21–22).