The pedunculopontine tegmental nucleus and the role of cholinergic neurons in nicotine self-administration in the rat: a correlative neuroanatomical and behavioral study

The objective of this study was to determine whether the pedunculopontine tegmental nucleus plays a role in the maintenance of nicotine self-administration, and whether the ascending cholinergic projection from this nucleus to midbrain dopamine neurons in the ventral tegmental area might be involved. Studies were done with rats trained to self-administer nicotine intravenously. Self-administration was examined before and after the pedunculopontine tegmental nucleus was lesioned with the ethylcholine mustard aziridinium ion, a selective cholinergic toxin. Lesions were assessed qualitatively and quantitatively in histological sections stained for either nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry to identify cholinergic neurons, or for Nissl. Self-administration was also tested after an acute manipulation in which microinfusions of the nicotinic cholinergic antagonist dihydro-β-erythroidine were made into the pedunculopontine tegmentum. Infusions of neurotoxin into the pedunculopontine tegmentum reduced nicotine self-administration behaviour when tested weeks later. Toxin treatment reduced the number of cholinergic neurons in the tegmentum, while largely sparing the non-cholinergic population in this area. Lesions were limited to the pedunculopontine area and did not extend to the neighboring laterodorsal tegmental nucleus or to the substantia nigra. Acute manipulation of the pedunculopontine tegmental nucleus with microinfusions of dihydro-β-erythroidine also produced an attenuation of nicotine self-administration. Collectively these data show that the pedunculopontine tegmental nucleus is part of the neuronal circuitry mediating nicotine self-administration, and that the population of cholinergic neurons is likely a critical element.