HRH3 and Parkinson disease: When gene expression was examined in mice with the MPTP-induced model of the ESS of PD, changes in the expression of the Hrh1, Hrh3, and Hnmt genes associated with HAergic signal transduction, as well as the Chrm3 gene encoding the cholinergic muscarinic receptor, were found in the SN of adult mice with the ESS model of PD.