A role for the endocannabinoid enzymes monoacylglycerol and diacylglycerol lipases in cue‐induced cocaine craving following prolonged abstinence

Abstract

Following exposure to drugs of abuse, long‐term neuroadaptations underlie persistent risk to relapse. Endocannabinoid signaling has been associated with drug‐induced neuroadaptations, but the role of lipases that mediate endocannabinoid biosynthesis and metabolism in regulating relapse behaviors following prolonged periods of drug abstinence has not been examined. Here, we investigated how pharmacological manipulation of lipases involved in regulating the expression of the endocannabinoid 2‐AG in the nucleus accumbens (NAc) influence cocaine relapse via discrete neuroadaptations. At prolonged abstinence (30 days) from cocaine self‐administration, there is an increase in the NAc levels of diacylglycerol lipase (DAGL), the enzyme responsible for the synthesis of the endocannabinoid 2‐AG, along with decreased levels of monoacylglycerol lipase (MAGL), which hydrolyzes 2‐AG. Since endocannabinoid‐mediated behavioral plasticity involves phosphatase dysregulation, we examined the phosphatase calcineurin after 30 days of abstinence and found decreased expression in the NAc, which we demonstrate is regulated through the transcription factor EGR1. Intra‐NAc pharmacological manipulation of DAGL and MAGL with inhibitors DO‐34 and URB‐602, respectively, bidirectionally regulated cue‐induced cocaine seeking and altered the phosphostatus of translational initiation factor, eIF2α. Finally, we found that cocaine seeking 30 days after abstinence leads to decreased phosphorylation of eIF2α and reduced expression of its downstream target NPAS4, a protein involved in experience‐dependent neuronal plasticity. Together, our findings demonstrate that lipases that regulate 2‐AG expression influence transcriptional and translational changes in the NAc related to drug relapse vulnerability.

CaMKII Modulates Diacylglycerol Lipase-α Activity in the Rat Nucleus Accumbens after Incubation of Cocaine Craving

“…We note that DGL levels were increased at WD30 in this prior study ( Mitra et al, 2021 ), in contrast to the lack of change reported here. This is most likely because of the fact that Mitra et al (2021) measured DGL in NAc shell, whereas we analyzed NAc core; there were also significant differences in the cocaine self-administration regimen. While the work cited above focused on synaptic depression expressed presynaptically via CB1R stimulation, other studies link mGlu5-dependent reinstatement of cocaine seeking to postsynaptically expressed LTD mediated via AMPAR internalization ( Schmidt et al, 2013 ; Benneyworth et al, 2019 ).…”

“…Overall, these results argue that synaptic depression can promote (Novak et al, 2010;Bilbao et al, 2020;Mitra et al, 2021) or be dissociated from (Murray et al, 2021) drug or natural 20 20 reward seeking. Thus, the functional significance of the observed reduction in DGL activity observed after prolonged cocaine withdrawal remains unclear.…”

“…These mice do not express mGlu5-dependent synaptic depression or demonstrate cue-induced reinstatement of cocaine or saccharin seeking; however, after 2-AG elevation with monoacylglycerol lipase (MAGL) inhibition, reinstatement of saccharin seeking is restored ( Novak et al, 2010 ; Bilbao et al, 2020 ). Furthermore, it was found that DGL and MAGL were dysregulated after prolonged abstinence (30 d) from cocaine self-administration, and that intra-NAc administration of a DGL inhibitor at this withdrawal time reduced cue-induced cocaine craving, while inhibition of MAGL (to increase 2-AG levels) had the opposite effect ( Mitra et al, 2021 ). We note that DGL levels were increased at WD30 in this prior study ( Mitra et al, 2021 ), in contrast to the lack of change reported here.…”

“…Furthermore, it was found that DGL and MAGL were dysregulated after prolonged abstinence (30 d) from cocaine self-administration, and that intra-NAc administration of a DGL inhibitor at this withdrawal time reduced cue-induced cocaine craving, while inhibition of MAGL (to increase 2-AG levels) had the opposite effect ( Mitra et al, 2021 ). We note that DGL levels were increased at WD30 in this prior study ( Mitra et al, 2021 ), in contrast to the lack of change reported here. This is most likely because of the fact that Mitra et al (2021) measured DGL in NAc shell, whereas we analyzed NAc core; there were also significant differences in the cocaine self-administration regimen.…”

“…However, our results fall short of demonstrating that loss of this synaptic depression contributes to incubated craving. In fact, other findings argue that this synaptic depression promotes reward seeking ( Novak et al, 2010 ; Bilbao et al, 2020 ; Mitra et al, 2021 ).…”