Distribution and Modulation of Histamine H3 Receptors in Basal Ganglia and Frontal Cortex of Healthy Controls and Patients with Parkinson's Disease

Abstract

Parkinson's disease (PD) is a brain degenerative disorder with unknown etiology, and specific degeneration of mesencephalic dopaminergic cells is a morphological manifestation of the disease. The central histaminergic system appears to be activated in PD, since the histaminergic innervation is increased in the substantia nigra. The aim of the present study was to investigate the expression and function of histamine H3 receptors in PD, using receptor mRNA in situ hybridization with oligonucleotide probes, receptor binding assay with a specific radioactive agonist, and GTP-γ-[35S]-binding assay as a tool to study the activation of the receptor G-protein. H3 receptor binding sites were detected using N-α-methylhistamine autoradiography in the basal ganglia and cortex, being most abundant in the substantia nigra and striatum. In PD substantia nigra we detected an increase of the receptor binding density. In situ hybridization study of the receptor mRNA revealed prominent sites of H3 receptor synthesis in the putamen, cortex, and globus pallidus, whereas very low mRNA expression was seen in the substantia nigra. In the PD pallidum externum, H3 receptor mRNA expression was elevated as compared with the normal brains. GTP-γ-[35S]-binding assay did not reveal any significant difference between PD and normal brains, although the density values in PD substantia nigra tended to be lower than in the normal brain, and density values in PD striatum were higher. The dopaminergic neurons did not express significant amount of H3 receptor mRNA, suggesting that the effects of H3 receptor-mediated modulation of dopamine release are indirect. Our data indicates modulation of the histamine H3 receptor in PD at the level of the mRNA expression in the striatum and receptor density in the substantia nigra. The receptor activity seems to be unchanged or decreased, as revealed by GTP-γ-[35S]-binding assay. Modulation of the histamine H3 receptor may influence the activity of other neurotransmitter systems, e.g., the GABAergic one, in the substantia nigra.