Enkephalin is essential for the molecular and behavioral expression of cocaine sensitization

Abstract

Behavioral sensitization to cocaine is associated to neuroadaptations that contribute to addiction. Enkephalin is highly expressed in mesocorticolimbic areas associated with cocaine-induced sensitization; however, their influence on cocaine-dependent behavioral and neuronal plasticity has not been explained. In this study, we employed a knockout (KO) model to investigate the contribution of enkephalin in cocaine-induced behavioral sensitization. Wild-type (WT) and proenkephalin KO mice were treated with cocaine once daily for 9 days to induce sensitization. Additionally, to clarify the observations in KO mice, the same procedure was applied in C57BL/6 mice, except that naloxone was administered before each cocaine injection. All animals received a cocaine challenge on days 15 and 21 of the treatment to evaluate the expression of locomotor sensitization. On day 21, microdialysis measures of accumbal extracellular dopamine, Western blotting for GluR1 AMPA receptor (AMPAR), phosphorylated ERK2 (pERK2), CREB (pCREB), TrKB (pTrkB) were performed in brain areas relevant for sensitization from KO and WT and/or naloxone- and vehicle pre-treated animals. We found that KO mice do not develop sensitization to the stimulating properties of cocaine on locomotor activity and on dopamine release in the nucleus accumbens (NAc). Furthermore, pivotal neuroadaptations such as the increase in pTrkB receptor, pERK/CREB and AMPAR related to sensitized responses were absent in the NAc from KO mice. Consistently, full abrogation of cocaine-induced behavioral and neuronal plasticity after naloxone pre-treatment was observed. We show for first time that the proenkephalin system is essential in regulating long-lasting pivotal neuroadaptations in the NAc underlying behavioral sensitization to cocaine.

Enkephalin as a Pivotal Player in Neuroadaptations Related to Psychostimulant Addiction

“…after a long-term abstinence from the drug (12 days after last injection). Met-enkephalin immunoreactivity elevations induced by chronic cocaine is not dependent of cocaine challenge administration (7.5 mg/kg i.p., day 21), as this effect on met-enkephalin immunoreactivity was also observed after saline challenge injection ( 19 ).…”

“…Although several pharmacological and genetic approaches demonstrate a role of both MOPr and DOPr in psychostimulant-induced behavioral effects, the role of the endogenous opioid peptides in this process has not been fully examined. Previous studies from our lab have demonstrated a long-lasting increase in enkephalin levels within the mesocorticolimbic circuit after psychostimulant administration ( 18 , 19 ). Enkephalin has also been shown to positively modulate dopamine and glutamate neurotransmission within this circuit ( 20 – 25 ).…”

“…Psychostimulant-induced PENK mRNA expression at striatal level may be the result of multiple neurotransmitter interactions ( 31 , 51 , 52 ). Cocaine and amphetamine stimulate the PENK mRNA expression in striatal neurons ( 19 , 28 , 31 , 35 , 49 ), which mostly express D2 receptors ( 53 , 54 ), and also induce prodynorphin and substance P in striatal neurons ( 31 ), which mainly express D1 receptors ( 53 , 55 ). Similarly, the full D1 receptor agonist SKF-82958 induced PENK, prodynorphin and substance P gene expression in both the dorsal and ventral striatum ( 33 ).…”

“…The role of MOPr and DOPr in the development and expression of psychostimulant sensitization has been shown pharmacologically. It has been reported that naloxone and naltrexone, non-selective opioid receptor antagonists, attenuate the development of sensitization to cocaine in rats ( 103 ) and mice ( 19 , 88 , 104 ). Naltrindole, a DOPr antagonist ( 87 ) and CTAP (D-Phe-cyc(Cys-Tyr-D-Trp-Arg-Thr-Pen)-Thr-NH2), a selective MOPr antagonist( 105 ), also reduce cocaine-induced sensitization in rats.…”

“…Despite all these studies, and reports demonstrating that met-enkephalin and MOPr have a prominent role in the ventral tegmental area at the initial step of sensitization ( 101 , 117 , 118 ), there is still no explanation in the literature of the influence of enkephalin on the psychostimulant-induced neuronal plasticity underpinning long-term sensitization. Data from our lab demonstrate an essential role of enkephalin in the development of neuroadaptations in the nucleus accumbens leading to cocaine-induced psychomotor sensitization ( 19 ). PENK knockout mice treated chronically with cocaine (9 days x 15 mg/kg) do not become sensitized to cocaine's properties stimulating locomotor activity and dopamine release in the nucleus accumbens 21 days after starting drug treatment.…”