Electrical stimulation of nucleus raphe dorsalis changes morphine self-administration and withdrawal symptoms in rats

Habenular Neurons Expressing Mu Opioid Receptors Promote Negative Affect in a Projection-Specific Manner

BACKGROUND

The mu opioid receptor (MOR) is central to hedonic balance and produces euphoria by engaging reward circuits. MOR signaling may also influence aversion centers, notably the habenula (Hb), where the receptor is highly dense. Our previous data suggest that the inhibitory activity of MOR in the Hb may limit aversive states. To investigate this hypothesis, we tested whether neurons expressing MOR in the Hb (Hb-MOR neurons) promote negative affect.

METHODS

Using Oprm1-Cre knockin mice, we combined tracing and optogenetics with behavioral testing to investigate consequences of Hb-MOR neuron stimulation for approach/avoidance (real-time place preference), anxiety-related responses (open field, elevated plus maze, and marble burying), and despair-like behavior (tail suspension).

RESULTS

Optostimulation of Hb-MOR neurons elicited avoidance behavior, demonstrating that these neurons promote aversive states. Anterograde tracing showed that, in addition to the interpeduncular nucleus, Hb-MOR neurons project to the dorsal raphe nucleus. Optostimulation of Hb-MOR/interpeduncular nucleus terminals triggered avoidance and despair-like responses with no anxiety-related effect, whereas light-activation of Hb-MOR/dorsal raphe nucleus terminals increased levels of anxiety with no effect on other behaviors, revealing 2 dissociable pathways controlling negative affect.

CONCLUSIONS

Together, the data demonstrate that Hb neurons expressing MOR facilitate aversive states via 2 distinct Hb circuits, contributing to despair-like behavior (Hb-MOR/interpeduncular nucleus) and anxiety (Hb-MOR/dorsal raphe nucleus). The findings support the notion that inhibition of these neurons by either endogenous or exogenous opioids may relieve negative affect, a mechanism that would have implications for hedonic homeostasis and addiction.

Blockade of Glutamate Receptors within the Prelimbic Cortex Attenuate Concentration of Excitatory Amino Acids in the Morphine Self-administration in Rats

Background

The attitude of research on addiction has been done on the key role of glutamate. As a regard, the prelimbic cortex (PrL) has an important role in addiction, learning, and memory. We tried to investigate the level of glutamate and aspartate concentration after glutamate receptors blockade in this region in the morphine-addicted rats.

Materials and Methods

In this study, we examined the effects of local infusion of the N-methyl-D-aspartate receptor and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonists, 2-amino-5-phosphonovaleric acid (AP5), and 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX), into the PrL cortex on the level of excitatory amino acids (EAAs) and glycine. After 11 days of self-administration, the prelimbic area of the brain was taken out, and the EAAs and glycine concentration was measured by high-performance liquid chromatography.

Results

Morphine resulted in the significant increase in the EAAs concentration within this area (P ≤ 0.001). Microinjection of AP5 into this region before using of morphine significantly decreased the morphine-induced glutamate and aspartate concentration (P ≤ 0.001). CNQX had the same effect and significantly reduced the EAAs concentration compared to the morphine group (P ≤ 0.001). In addition, microinjection of AP5 and CNQX simultaneously increased glycine concentration (P ≤ 0.001).

Conclusions

These results show that morphine stimulates the EAAs release in the prelimbic area. It seems that microinjection of AP5 or CNQX in this region is effective in reducing morphine-induced EAA. It is suggested that EAA transmission in the PrL cortex may be a possible target for treatment of morphine addiction.

Blockade of prelimbic glutamate receptor reduces the reinforcing effect of morphine

The prelimbic cortex (PrL) as a part of the medial prefrontal cortex (mPFC) plays a crucial role in drug addiction. Previous studies have shown that glutamatergic transmission through the NMDA and AMPA receptors plays an important role in morphine rewarding properties. In this study, we evaluated the effect of glutamate receptors blockade within the PrL on morphine self-administration. Male Wistar rats were randomly selected and divided into 7 groups. Trained rats were placed in self-administration apparatus, where they pressed an active lever for receiving morphine (5 mg/mL) in test groups and saline in saline group during 11 consecutive days for 2 h per session. The effects of intra-prelimbic AMPA receptor antagonist (CNQX; 0.5 and 2.5 μg/0.5 μL) and the NMDA antagonist (AP5; 0.1 and 1 μg/0.5 μL) on self-administration were tested. Our results demonstrated that intra-prelimbic injection of different doses of CNQX and AP5, and co-administration of these 2 drugs before self-administration significantly decreased active lever pressing compared with morphine group (p < 0.001). Also, the number of self-infusion significantly decreased in test groups compared with morphine group (p < 0.001). These findings suggest that a reduction in PrL glutamatergic output can modulate morphine reinforcement.

Morphine history sensitizes postsynaptic GABA receptors on dorsal raphe serotonin neurons in a stress-induced relapse model in rats

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Previous work has shown that the dorsal raphe nucleus (DR)-5-HT system is inhibited by swim stress via stimulation of GABA synaptic activity by the stress neurohormone corticotropin-releasing factor (CRF). Additionally, the DR 5-HT system is regulated by opioids. The present study tests the hypothesis that the DR 5-HT system regulates stress-induced opioid relapse. In the first experiment, electrophysiological recordings of GABA synaptic activity in 5-HT DR neurons were conducted in brain slices from Sprague-Dawley rats that were exposed to swim stress-induced reinstatement of previously extinguished morphine conditioned place preference (CPP). Behavioral data indicate that swim stress triggers reinstatement of morphine CPP. Electrophysiology data indicate that 5-HT neurons in the morphine-conditioned group exposed to stress had increased amplitude of inhibitory postsynaptic currents (IPSCs), which would indicate greater postsynaptic GABA receptor density and/or sensitivity, compared to saline controls exposed to stress. In the second experiment, rats were exposed to either morphine or saline CPP and extinction, and then 5-HT DR neurons from both groups were examined for sensitivity to CRF in vitro. CRF induced a greater inward current in 5-HT neurons from morphine-conditioned subjects compared to saline-conditioned subjects. These data indicate that morphine history sensitizes 5-HT DR neurons to the GABAergic inhibitory effects of stress as well as to some of the effects of CRF. These mechanisms may sensitize subjects with a morphine history to the dysphoric effects of stressors and ultimately confer an enhanced vulnerability to stress-induced opioid relapse.

Impaired emotional-like behavior and serotonergic function during protracted abstinence from chronic morphine

BACKGROUND

Opiate abuse is a chronic relapsing disorder, and maintaining prolonged abstinence remains a major challenge. Protracted abstinence is characterized by lowered mood, and clinical studies show elevated comorbidity between addiction and depressive disorders. At present, their relationship remains unclear and has been little studied in animal models. Here we investigated emotional alterations during protracted abstinence, in mice with a history of chronic morphine exposure.

METHODS

C57BL6J mice were exposed to a chronic intermittent escalating morphine regimen (20-100 mg/kg). Physical dependence (naloxone-precipitated withdrawal), despair-related behaviors (tail suspension test), and social behaviors were examined after 1 or 4 weeks of abstinence. Stress hormones and forebrain bioamine levels were analyzed at the end of morphine regimen and after 4 weeks of abstinence. Finally, we examined the effects of chronic fluoxetine during abstinence on morphine-induced behavioral deficits.

RESULTS

Acute naloxone-induced withdrawal was clearly measurable after 1 week, and became undetectable after 4 weeks. In contrast, social and despair-related behaviors were unchanged after 1 week, but low sociability and despair-like behavior became significant after 4 weeks. Chronic morphine regimen increased both corticosterone levels and forebrain serotonin turnover, but only serotonergic activity in the dorsal raphe remained impaired after 4 weeks. Remarkably, chronic fluoxetine prevented depressive-like behavioral deficits in 4-week abstinent mice.

CONCLUSIONS

During protracted abstinence, the immediate consequences of morphine exposure attenuate, whereas fluoxetine-sensitive emotional alterations strengthen with time. Our study establishes a direct link between morphine abstinence and depressive-like symptoms and strongly suggests that serotonin dysfunction represents a main mechanism contributing to mood disorders in opiate abstinence.

Treadmill exercise reduces self-administration of morphine in male rats

Exercise can activate the same pathways as morphine. The aim of the present study was to clarify the effect of short-term and mid-term exercises on the self-administration of morphine in rats. Male Wistar rats were initially trained to receive small pellets of food by pressing the active lever in self-administration apparatus. Rats were divided into 4 groups: Saline, Morphine, Exercise 1 (11 days) and Exercise 2 (30 days). Their jugular vein was cannulated. The animals were placed in self-administration apparatus and allowed to self-administer morphine (0.5mg per infusion all test groups) or saline (Saline group) during consecutive days, for 2h/sessions. In the group 1 the rats were running before each session of self-administration and of group Exercise 2, 30 days before surgery as well as before each session. The pressing numbers of active and passive levers in each group and among different groups were compared. The number of active lever pressing of Morphine group was significantly higher than Saline group (p<0.001). In Exercise 1 and Exercise 2 groups, the number of active lever pressing was significantly lower than Morphine group (p<0.001). As exercise can activate many neurotransmitter systems involved in the addiction process and increase the release of endorphins, it is likely that could decrease the morphine self-administration in this experimental setup.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 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).