Detection of low level histamine H3 receptor occupancy by autoradiography

Histamine H3 receptor antagonists have been proposed as a novel approach to the treatment of cognitive, attentional, and sleep disorders. It is apparent that H3 receptor antagonists produce in vivo effects in preclinical animal models of central diseases across a wide dose range. In order to characterize the relationship between efficacy in the preclinical models and H3 receptor occupancy, a brain slice receptor autoradiography method was used. Brain slice receptor autoradiography requires less in vitro tissue processing, preserves brain structure, and provides anatomical localization of compound in the brain. Consistent with H3 receptor distribution, in vitro autoradiography experiments demonstrated specific binding of [3H]NAMH (N-α-methylhistamine) in rat cortex, and other brain regions, but not in cerebellum. Ex vivo H3R brain slice autoradiography was able to detect H3 receptor occupancy by reference antagonists at doses lower than previously found using a homogenate assay format. The method is relatively quick with image acquisition on a β-imager™ and is capable of detecting receptor occupancy in different brain regions simultaneously. Furthermore, the increased sensitivity should be useful in providing dosing guidelines for H3 antagonists in both preclinical and clinical settings.