M3 but not M4 muscarinic receptors in the rostromedial tegmental nucleus are involved in the acquisition of morphine-induced conditioned place preference.

Opioids strongly inhibit GABAergic neurons in the rostromedial tegmental nucleus (RMTg) that expresses μ-opioid receptors to induce rewarding and psychomotor effects. M3 and M4 muscarinic receptors are co-localized with μ-opioid receptors at these GABAergic neurons. This study explored whether RMTg M3 and M4 muscarinic receptors are involved in regulating opioid-induced reward and locomotion via a conditioned place preference (CPP) paradigm. Selective muscarinic receptor agonists and antagonists were both singly and combinatorically injected into the RMTg to examine their effects on the acquisition of systemic morphine-induced CPP and locomotor activity. The M3 muscarinic receptor agonist, pilocarpine, inhibited the acquisition of morphine-induced CPP, whereas its antagonist, 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide (4-DAMP, 1 μg/side), reversed the inhibitory effect of pilocarpine (30 μg/side). Additionally, 4-DAMP increased locomotor activity while pilocarpine (30 μg/side) partially decreased locomotor activity when combined with morphine. In contrast, the M4 muscarinic receptor agonist, LY2033298 (0.1 and 0.2 μg/side), and antagonist, tropicamide (20 and 40 μM/side), did not affect the acquisition of morphine-induced CPP or locomotor activity. Taken together, our findings suggest that RMTg M3 muscarinic receptors are involved in opioid-induced rewarding and psychomotor effects. Therefore, RMTg M3 muscarinic receptors may represent a promising target for the treatment of opioid addiction.