Normal to increased. TMN volume and number of TMN neurons remain within the normal range, whereas the levels of HDC mRNA, HMT activity within the inferior frontal gyrus (IFG), H1R, and H3R mRNA levels are increased. These findings indicate a functional increase in brain histaminergic signaling in HD that is linked to aberrant striatal function. This evidence concerns the gene HRH3 and Huntington disease.