Unaltered or decreased. Despite the significant loss of histaminergic neurons in AD, expression of HDC-mRNA in the TMN remains unaltered. The decline in H1R binding may have a role in the cognitive deficits of patients with AD.Many studies have also demonstrated the involvement of H3R, whose loss of integrity and/or increased expression may locally reduce HA concentration in the CNS. The gene discussed is HRH1; the disease is Alzheimer disease.