The endocannabinoid system as possible target for the treatment of obesity related disorders: beyond cannabinoid receptors

Worldwide, obesity is one of the most diffused pathologies. Several lines of recent evidence link obesity with a higher risk of neurological pathologies such as anxiety and depression; this consideration stems from the observation that palatable food consumption becomes sometimes necessary to overcome negative mental states, thus sustaining the vicious circle of “food addiction”. Endogenous acylethanolamides (such as anandamide, oleoylethanolamide, palmitoylethanolamide) might play a key role in this scenario since they participate to the mechanisms regulating both reward and mood tone. The present study aims to explore whether the abstinence from a palatable diet, after a long-term ad libitum consumption of it, might produce alterations of the emotional reactivity and mood tone and whether the pharmacological inhibition of fatty acid amide hydrolase (FAAH) by PF-3845 treatment (which causes an increase of acylethanolamide tone) might ameliorate such alterations. We used a rat model of diet-induced obesity based on a cafeteria-style diet. After the first 40 days of extended exposure to cafeteria diet, rats underwent an abstinence period of 28 days. During this period, animals were separated in two different groups: one group was chronically treated with PF-3845 (10 mg/kg, intraperitoneally) and the other group was treated with vehicle (ethanol/tween80/saline 5/5/90). At the end of the treatment rats were subjected to behavioral tests such as the open field test, the elevated plus maze and the forced swimming test and then were sacrificed. Brains were collected to investigate markers of monoaminergic transmission, endocannabinoid tone and inflammation in different brain areas implicated in reward, anxiety and depression. To meet this aim we used different experimental approaches including immunofluorescence, HPLC and western blot analysis. Pearson correlations tests were also conducted to correlate behavioural parameters to monoamine tissue levels in the different brain areas, as well as to investigate monoaminergic and endocannabinoid protein correlations throughout the different brain areas analysed. Our results demonstrate that long term abstinence from a palatable diet might impact both mood and reward processes: in particular, animals subjected to cafeteria diet withdrawal show an anxiety-like behavior and a depressive-like phenotype. Moreover, several neurochemical alterations are also detected in key brain areas involved in the control of mood and reward processes. These include alterations of the expression of proteins partaking to the endocannabinoid system and to the inflammatory-response, alterations in the monoaminergic tone. The repeated administration of the FAAH inhibitor PF-3845 during the abstinence period exerts an anxiolytic and antidepressant effect in abstinent rats and is effective in normalizing protein expression variations and monoamine content alterations in almost all brain areas analyzed. Pearson correlation tests display pattern variations both consequently to diet abstinence and pharmacological treatment. The results obtained from the present study are important in evidencing that palatable food exposure and subsequent abstinence can cause neuroadaptive changes that turn into the development of depression and anxiety, raising the importance of these comorbidities in obesity and palatable food consumption. Most importantly, we observe that enhancing the endogenous tone of acylethanolamides and endocannabinoids by blocking their enzymatic degradation might represent a novel protective strategy for psychiatric comorbidities in obese subjects.