Blockade of cue- and drug-induced reinstatement of morphine-induced conditioned place preference with intermittent sucrose intake

Inhibitory effects of processed Aconiti tuber on morphine-induced conditioned place preference in rats

AIM OF THE STUDY

Our previous studies indicated that processed Aconiti tuber (PAT), a traditional Chinese herbal medicine, had antinociceptive effects and inhibitory effects on morphine tolerance by activation of kappa-opioid receptor (KOR). Preclinical studies also demonstrated that KOR agonists functionally attenuate addictive behaviors of morphine, such as conditioned place preference (CPP). Therefore, we hypothesize that PAT may inhibit morphine-induced CPP in rats.

MATERIALS AND METHODS

(1) Five groups of rats (n=8 for each group) were alternately subcutaneous (s.c.) injected with morphine 10mg/kg (one group receive normal saline as a control) and normal saline for 8 days and oral co-administrated with distilled water or PAT 0.3, 1.0, or 3.0 g/kg daily on days 2-9 during CPP training, respectively. (2) Other four groups of rats were randomly s.c. injected with nor-binaltorphimine (nor-BNI; 5mg/kg) or normal saline (as a control) 120 min before alternately s.c. with morphine and normal saline and oral co-administrated with distilled water or PAT 3.0 g/kg daily. Each rat was acquired pre-conditioning and post-conditioning CPP data and assayed dynorphin concentrations by radioimmunoassay in rat's nucleus accumbens (NAc) after CPP training.

RESULTS

(1) PAT 1.0 or 3.0 g/kg dose-dependently decreased the morphine-induced increase of CPP scores. (2) Nor-BNI completely antagonized the inhibition of PAT on morphine-induced CPP. (3) PAT dose-dependently increased dynorphin content in rats' NAc after CPP training.

CONCLUSIONS

(1) PAT dose-dependently inhibited morphine-induced CPP. (2) The inhibition of PAT on morphine-induced CPP was probably due to activation of KOR by increasing dynorphin release in rats' NAc.

Implication of dopaminergic projection from the ventral tegmental area to the anterior cingulate cortex in μ-opioid-induced place preference

Despite the importance of prefrontal cortical dopamine in modulating reward, little is known about the implication of the specific subregion of prefrontal cortex in opioid reward. We investigated the role of neurons projecting from the ventral tegmental area (VTA) to the anterior cingulate cortex (ACG) in opioid reward. Microinjection of the retrograde tracer fluorogold (FG) into the ACG revealed several retrogradely labelled cells in the VTA. The FG-positive reactions were noted in both tyrosine hydroxylase (TH)-positive and -negative VTA neurons. The released levels of dopamine and its major metabolites in the ACG were increased by either the electrical stimulation of VTA neurons or microinjection of a selective μ-opioid receptor (MOR) agonist, (D-Ala²,N-MePhe⁴,Gly-ol⁵) enkephalin (DAMGO), into the VTA. MOR-like immunoreactivity was seen in both TH-positive and -negative VTA neurons projecting to the ACG. The conditioned place preference induced by intra-VTA injection of DAMGO was significantly attenuated by chemical lesion of dopaminergic terminals in the ACG. The depletion of dopamine in the ACG induced early extinction of μ-opioid-induced place preference. The levels of phosphorylated DARPP32 (Thr34) and phosphorylated CREB (Ser133) were increased in the ACG of rats that had maintained the morphine-induced place preference, whereas the increases of these levels induced by morphine were blocked by pre-treatment of a selective dopamine D1 receptor antagonist SCH23390. These findings suggest that VTA-ACG transmission may play a crucial role in the acquisition and maintenance of μ-opioid-induced place preference. The activation of DARPP32 and CREB through dopamine D1 receptors in the ACG could be implicated in the maintenance of μ-opioid-induced place preference.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 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).

Aripiprazole blocks reinstatement but not expression of morphine conditioned place preference in rats

Aripiprazole is an atypical antipsychotic drug primarily characterized by partial agonist activity at dopamine(DA) D2 receptors and serotonin-1A (5-hydroxytryptamine, 5-HT1A) receptors and minimal side effects.Based on its pharmacological profile, including stabilization of mesocorticolimbic DA activity (a pathway implicated in drug addiction), we investigated the effects of aripiprazole on relapse to morphine seeking in rats. In experiment 1, rats underwent morphine-induced conditioned place preference (CPP) training with alternate injections of morphine (5 mg/kg, s.c.) and saline (1 ml/kg, s.c.) for 8 consecutive days. To examine the effect of aripiprazole on the expression of morphine-induced CPP, rats received aripiprazole (0, 0.03, 0.1,and 0.3 mg/kg, i.p.) 30 min before testing for the expression of CPP. In experiment 2, rats underwent the same CPP training as in experiment 1 and subsequent extinction training. To examine the effect of aripiprazole on reinstatement of morphine-induced CPP, rats received aripiprazole 30 min before testing for reinstatement of CPP. In experiment 3, to assess the effects of aripiprazole on locomotor activity, aripiprazole was administered 30 min before testing for locomotor activity. Aripiprazole significantly decreased the reinstatement of CPP induced by a priming injection of morphine but had no effect on the expression of morphine-induced CPP or locomotor activity. The D2 and 5-HT1A partial agonist and 5-HT2A antagonist properties of aripiprazole likely account for the blockade of relapse to drug seeking. These findings suggest that aripiprazole may have therapeutic value for reducing craving and preventing relapse to drug seeking.