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Free keywords:
Morphine; NMDA receptor; noradrenaline; reinforcement; sensitization; withdrawal
Abstract:
The brain's noradrenergic system is involved in the development of behaviours induced by drugs of abuse, e.g. dependence and withdrawal, and also reward or psychomotor effects. To investigate how noradrenergic system activity is controlled in the context associated with drug−induced behaviours, we generated a Cre/loxP mouse model in which the essential glutamate NMDA receptor subunit NR1 is ablated in cells expressing dopamine β−hydroxylase (Dbh). As a result, the noradrenergic cells in NR1DbhCre mice lack the NMDA receptor−dependent component of excitatory post−synaptic currents. The mutant mice displayed no obvious behavioural alterations, had unchanged noradrenaline content and mild increase in dopamine levels in the nucleus accumbens. Interestingly, NR1DbhCre animals did not develop morphine−induced psychomotor sensitization. However, when the morphine injections were preceded by treatment with RX821002, an antagonist of α2−adrenergic receptors, the development of sensitization was restored. Conversely, pretreatment with clonidine, an agonist of α2−adrenergic receptors, blocked development of sensitization in wild−type mice. We also found that while the development of tolerance to morphine was normal in mutant mice, withdrawal symptoms were attenuated. These data reveal that NMDA receptors on noradrenergic neurons regulate development of opiate dependence and psychomotor sensitization, by controlling drug−induced noradrenaline signalling
