Inhibition of the acquisition of conditioned place aversion by dopaminergic lesions of the central nucleus of the amygdala in morphine-treated rats

Opioid withdrawal: role in addiction and neural mechanisms

Withdrawal from opioids involves a negative affective state that promotes maintenance of drug-seeking behavior and relapse. As such, understanding the neurobiological mechanisms underlying withdrawal from opioid drugs is critical as scientists and clinicians seek to develop new treatments and therapies. In this review, we focus on the neural systems known to mediate the affective and somatic signs and symptoms of opioid withdrawal, including the mesolimbic dopaminergic system, basolateral amygdala, extended amygdala, and brain and hormonal stress systems. Evidence from preclinical studies suggests that these systems are altered following opioid exposure and that these changes mediate behavioral signs of negative affect such as aversion and anxiety during withdrawal. Adaptations in these systems also parallel the behavioral and psychological features of opioid use disorder (OUD), highlighting the important role of withdrawal in the development of addictive behavior. Implications for relapse and treatment are discussed as well as promising avenues for future research, with the hope of promoting continued progress toward characterizing neural contributors to opioid withdrawal and compulsive opioid use.

Double Dissociation of Monoacylglycerol Lipase Inhibition and CB1 Antagonism in the Central Amygdala, Basolateral Amygdala, and the Interoceptive Insular Cortex on the Affective Properties of Acute Naloxone-Precipitated Morphine Withdrawal in Rats

Both CB1 receptor antagonism and agonism, in particular by 2-arachidonyl glycerol (2-AG), have been shown to reduce somatic symptoms of morphine withdrawal (MWD). Here we evaluated the effects of both systemic pretreatment with the monoacylglycerol lipase (MAGL) inhibitor MJN110 (which selectively elevates 2-AG) and central administration of both MJN110 and the CB1 antagonist (AM251) on the affective properties of MWD. Acute MWD induced place aversion occurs when naloxone is administered 24 h following a single exposure to a high dose of morphine. Systemic pretreatment with the MAGL inhibitor, MJN110, prevented the aversive effects of acute MWD by a CB1 receptor-dependent mechanism. Furthermore, in a double dissociation, AM251 infusions into the central amygdala, but MJN110 infusions into the basolateral amygdala, interfered with the naloxone-precipitated MWD induced place aversion. As well, MJN110, but not AM251, infusions into the interoceptive insular cortex (a region known to be activated in acute MWD) also prevented the establishment of the place aversion by a CB1 mechanism of action. These findings reveal the respective sites of action of systemically administered MJN110 and AM251 in regulating the aversive effects of MWD.

Differential effects of naloxone on rewarding electrical stimulation of the central nucleus of the amygdala and parabrachial complex in a place preference study

The central nucleus of the amygdala (CeA) is considered to be involved in different affective, sensory, regulatory, and acquisition processes. This study analyzed whether electrical stimulation of the PB-CeA system induces preferences in a concurrent place preference (cPP) task, as observed after stimulation of the parabrachial-insular cortex (PB-IC) axis. It also examined whether the rewarding effects are naloxone-dependent. The results show that electrical stimulation of the CeA and external lateral parabrachial subnucleus (LPBe) induces consistent preference behaviors in a cPP task. However, subcutaneous administration of an opiate antagonist (naloxone; 4mg/ml/kg) blocked the rewarding effect of the parabrachial stimulation but not that of the amygdala stimulation. These results are interpreted in the context of multiple brain reward systems that appear to differ both anatomically and neurochemically, notably with respect to the opiate system.

CB1 antagonism: interference with affective properties of acute naloxone-precipitated morphine withdrawal in rats

RATIONALE

Modulation of the endocannabinoid system has been found to interfere with opiate withdrawal. The potential of activation and blockade of the endocannabinoid system to prevent the aversive-affective state of naloxone-precipitated morphine withdrawal (MWD) was investigated in a one-trial conditioned place aversion (CPA) paradigm.

OBJECTIVE

CPA provides a sensitive measure of the motivational effects of acute MWD. The potential of the fatty acid amide hydrolase (FAAH) inhibitors, URB597 and PF-3845, the CB1 antagonist/inverse agonist, AM251, and the neutral CB1 antagonists, AM4113 and AM6527 (oral), to interfere with establishment of a MWD-induced CPA was investigated. As well, the potential of AM251 and AM4113 to interfere with reinstatement of a previously established MWD-induced CPA was investigated.

MATERIALS AND METHODS

Using a one-trial place conditioning paradigm, rats were administered naloxone (1 mg/kg, subcutaneous (s.c.)) 24 h after receiving a high dose of morphine (20 mg/kg, s.c.) and were placed on the conditioning floor. To determine the effect of each pretreatment drug on the establishment of the MWD-induced CPA, URB597 (0.3 mg/kg, intraperitoneally (i.p.)), PF-3845 (10 mg/kg, i.p.), AM251 (1 or 2.5 mg/kg, i.p.), AM4113 (1 or 2.5 mg/kg, i.p.), and AM6527 (5 mg/kg, oral) were administered prior to conditioning.

RESULTS

AM251 (2.5, but not 1 mg/k), AM4113, and AM6527, but not URB597 or PF-3845, interfered with the establishment of the MWD-induced CPA. AM251 and AM4113 did not prevent reinstatement of the CPA.

CONCLUSIONS

Neutral antagonism of the CB1 receptor reduces the aversive affective properties of morphine withdrawal.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).