CaMKII modulates diacylglycerol lipase-α activity in the rat nucleus accumbens after incubation of cocaine craving

Abstract

Relapse is a major challenge to the treatment of substance use disorders. A progressive increase in cue-induced drug craving, termed incubation of craving, is observed after withdrawal from multiple drugs of abuse in humans and rodents. Incubation of cocaine craving involves strengthening of excitatory synapses onto nucleus accumbens (NAc) medium spiny neurons via postsynaptic accumulation of high-conductance Ca²⁺-permeable AMPA receptors. This enhances reactivity to drug-associated cues and is required for the expression of incubation. Additionally, incubation of cocaine craving is associated with loss of the synaptic depression normally triggered by stimulation of metabotropic glutamate receptor 5 (mGlu5), leading to endocannabinoid production, and expressed presynaptically via cannabinoid receptor 1 activation. Previous studies have found alterations in mGlu5 and Homer proteins associated with the loss of this synaptic depression. Here we conducted co-immunoprecipitation studies to investigate associations of diacylglycerol lipase-α (DGL), which catalyzes formation of the endocannabinoid 2-arachidonylglycerol (2-AG), with mGlu5 and Homer proteins. Although these interactions were unchanged in the NAc core at incubation-relevant withdrawal times, the association of DGL with total and phosphorylated Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα) and CaMKIIβ was increased. This would be predicted, based on other studies, to inhibit DGL activity and therefore 2-AG production. This was confirmed by measuring DGL enzymatic activity. However, the magnitude of DGL inhibition did not correlate with the magnitude of incubation of craving for individual rats. These results suggest that CaMKII contributes to the loss of mGlu5-dependent synaptic depression after incubation, but the functional significance of this loss remains unclear.

Significance Statement

Cue-induced drug craving progressively intensifies or incubates after withdrawal from extended-access drug self-administration, augmenting relapse vulnerability. Incubation of cocaine craving in rats is accompanied by robust plasticity in the NAc including strengthening of excitatory synapses via postsynaptic AMPAR plasticity and the loss of endocannabinoid-dependent synaptic depression. Our results identify a novel mechanism that may account for this loss of synaptic depression, namely reduced activity of DGL, the enzyme that produces the endocannabinoid 2-AG, along with increased physical association of this enzyme with CaMKII, an association predicted to reduce DGL’s enzymatic activity. These findings expand our understanding of mechanisms underlying cocaine-induced alterations in endocannabinoid-dependent synaptic depression.