Hierarchical glucocorticoid-endocannabinoid interplay regulates the activation of the nucleus accumbens by insulin

- CB1 and insulin receptors form physical complex in the rat nucleus accumbens.
- Insulin receptor activation stimulates accumbal glucose uptake.
- Accumbal glucocorticoid receptor activation stimulates endocannabinoid 2-AG release.
- 2-AG by activating CB1 receptors, prevents insulin from stimulating glucose uptake.
- This model may explain how dexamethasone may cause cerebral insulin resistance.

Here we asked if insulin activation of the nucleus accumbens in vitro is reflected by an increase in 3H-deoxyglucose ([3H]DG) uptake, thus subserving a new model to study molecular mechanisms of central insulin actions. Additionally, we investigated the dependence of this insulin effect on endocannabinoids and corticosteroids, two major culprits in insulin resistance. We found that in acute accumbal slices, insulin (3 and 300 nM but not at 0.3 nM) produced an increase in [3H]DG uptake. The synthetic cannabinoid agonist, WIN55212-2 (500 nM) and the glucocorticoid dexamethasone (10 μM), impaired insulin (300 nM) action on [3H]DG uptake. The glucocorticoid receptor (GcR) antagonist, mifepristone (10 μM) prevented dexamethasone from inhibiting insulin's action. Strikingly, this anti-insulin action of dexamethasone was also blocked by two CB1 cannabinoid receptor (CB1R) antagonists, O-2050 (500 nM) and SR141716A (500 nM), as well as by tetrahydrolipstatin (10 μM), an inhibitor of diacylglycerol lipases-the enzymes responsible for the synthesis of the endocannabinoid, 2-arachidonoyl-glycerol (2-AG). On the other hand, the blockade of the post-synaptic 2-AG metabolizing enzymes, α,β-serine hydrolase domain 6/12 by WWL70 (1 μM) also prevented the action of insulin, probably via increasing endogenous 2-AG tone. Additionally, an anti-insulin receptor (InsR) antibody immunoprecipitated CB1Rs from accumbal homogenates, indicating a physical complexing of CB1Rs with InsRs that supports their functional interaction. Altogether, insulin stimulates glucose uptake in the nucleus accumbens. Accumbal GcR activation triggers the synthesis of 2-AG that in turn binds to the known CB1R-InsR heteromer, thus impeding insulin signaling.