Rewarding effects of M4 but not M3 muscarinic cholinergic receptor antagonism in the rostromedial tegmental nucleus

The rostromedial tegmental nucleus (RMTg) receives inputs from the laterodorsal tegmental and pedunculopontine tegmental nuclei, the two principle brainstem cholinergic nuclei. We tested the effects of RMTg M3 and M4 muscarinic cholinergic receptor antagonism in a conditioned place preference (CPP) paradigm in mice. RMTg infusions of the M3 muscarinic cholinergic receptor antagonist 1,1-Dimethyl-4-diphenylacetoxypiperidinium iodide (4-DAMP) do not result in the acquisition of CPP but increase locomotor activation. By contrast, RMTg infusions of the M4 muscarinic cholinergic receptor antagonist Tropicamide result in the acquisition of CPP but do not increase locomotor activation. The rewarding effects of RMTg Tropicamide infusions are dopamine-dependent as systemic pre-treatment with the broad-spectrum dopamine receptor antagonist flupenthixol prevents the acquisition of CPP induced by RMTg Tropicamide infusions. Under conditions of systemic dopamine receptor blockade, RMTg Tropicamide infusions significantly increase locomotor activation. These data provide further support for an important role of endogenous cholinergic input to the RMTg in reward function and suggest that the contributions of RMTg cholinergic input to rewarding and locomotor-activating effects involve differential contributions of RMTg M4 and M3 muscarinic receptors, respectively.