ABSTRACT
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.
Subject(s)
Conditioning, Operant/drug effects , Dopamine/metabolism , Enkephalin, Ala(2)-MePhe(4)-Gly(5)-/pharmacology , Gyrus Cinguli/drug effects , Motivation/drug effects , Nerve Net/drug effects , Prefrontal Cortex/drug effects , Receptors, Opioid, mu/agonists , Ventral Tegmental Area/drug effects , Animals , Association Learning/drug effects , Benzazepines/pharmacology , Cyclic AMP Response Element-Binding Protein/metabolism , Dopamine and cAMP-Regulated Phosphoprotein 32/metabolism , Male , Mental Recall/drug effects , Neurons/drug effects , Rats , Rats, Sprague-Dawley , Receptors, Dopamine D1/antagonists & inhibitors , Receptors, Dopamine D1/drug effects , Synaptic Transmission/drug effectsABSTRACT
The intermittent administration of methamphetamine produces behavioral sensitization to methamphetamine. In the limbic forebrain, mainly including the nucleus accumbens, of mice that had been intermittently treated with methamphetamine, we found a significant increase in mRNA of a chemokine, CCR2. This increase was accompanied by a significant increase in histone H3 lysine 4 (H3K4) trimethylation at its promoter. Interestingly, the maintenance of sensitization to methamphetamine-induced hyperlocomotion was significantly decreased in CCR2 knockout mice. These findings suggest that increased CCR2 associated with epigenetic modification after the intermittent administration of methamphetamine may be associated with the maintenance of sensitization to methamphetamine-induced hyperlocomotion.
Subject(s)
Central Nervous System Stimulants/pharmacology , Epigenesis, Genetic/genetics , Methamphetamine/pharmacology , Motor Activity/drug effects , Motor Activity/genetics , RNA, Messenger/genetics , Receptors, CCR2/genetics , Animals , Limbic System/drug effects , Male , Mice , Mice, Inbred C57BL , Mice, Knockout , Nucleus Accumbens/drug effects , Promoter Regions, Genetic/geneticsABSTRACT
Dopamine receptor antagonists are commonly used to counter the adverse effects of opioids such as hallucinations, delusions and emesis. However, most of these agents themselves have side effects, including extrapyramidal symptoms. Here, we investigated the effect of the dopamine system stabilizer aripiprazole on morphine-induced dopamine-related actions in mice. Morphine-induced hyperlocomotion and reward were significantly suppressed by either the dopamine receptor antagonist prochlorperazine or aripiprazole. Catalepsy was observed with a high dose of prochlorperazine, but not with an even higher dose of aripiprazole. The increased level of dialysate dopamine in the nucleus accumbens stimulated by morphine was significantly decreased by pretreatment with aripiprazole. These results suggest that the co-administration of aripiprazole may be useful for reducing the severity of morphine-induced dopamine-related side effects.
