Modulation of histamine H3 receptors in the brain of 6-hydroxydopamine-lesioned rats.

Parkinson's disease is a major neurological disorder that primarily affects the nigral dopaminergic cells. Nigral histamine innervation is altered in human postmortem Parkinson's disease brains. However, it is not known if the altered innervation is a consequence of dopamine deficiency. The aim of the present study was to investigate possible changes in the H3 receptor system in a well-characterized model of Parkinson's disease--the 6-hydroxydopamine (6-OHDA) lesioned rats. Histamine immunohistochemistry showed a minor increase of the fibre density index but we did not find any robust increase of histaminergic innervation in the ipsilateral substantia nigra on the lesioned side. In situ hybridization showed equal histidine decarboxylase mRNA expression on both sides in the posterior hypothalamus. H3 receptors were labelled with N-alpha-[3H]-methyl histamine dihydrochloride ([3H] NAMH). Upregulation of binding to H3 receptors was found in the substantia nigra and ventral aspects of striatum on the ipsilateral side. An increase of GTP-gamma-[35S] binding after H3 agonist activation was found in the striatum and substantia nigra on the lesioned side. In situ hybridization of H3 receptor mRNA demonstrated region-specific mRNA expression and an increase of H3 receptor mRNA in ipsilateral striatum. Thus, the histaminergic system is involved in the pathological process after 6-OHDA lesion of the rat brain at least through H3 receptor. On the later stages of the neurotoxic damage, less H3 receptors became functionally active. Increased H3 receptor mRNA expression and binding may, for example, modulate GABAergic neuronal activity in dopamine-depleted striatum.