Microinjections of quaternary scopolamine into the pedunculopontine tegmental nucleus induce a conditioned place aversion

The pedunculopontine tegmental nucleus (PPTg) has been proposed to be critical in drug and food reward. It is a major source of cholinergic inputs to the substantia nigra and ventral tegmental area, areas important in reward, and is believed to modulate activity of dopamine neurons through a cholinergic mechanism. The firing rate of cholinergic PPTg cells is regulated by muscarinic autoreceptors that can be blocked to increase cell firing. If the PPTg modulates dopamine neurons involved in reward, then a muscarinic antagonist microinjected into the PPTg should be rewarding. To test this hypothesis, bilateral microinjections of scopolamine methyl bromide (5 $ mu$g or 20 $ mu$g) or 0.9% saline were used as reward treatments in the conditioned place preference test. On the test day, rats from both doses of drug avoided the drug-paired chamber, which suggests that cholinergic PPTg cells are not involved in reward