Histamine acts as a neurotransmitter in the brain. Histamine is synthesized from histidine by catalyzing histidine decarboxylase (HDC). In the central nervous system, HDC-positive neurons are in tuberomammillary nucleus of posterior hypothalamus and project their axons to entire brain. However, the roles of HDC in brain functions are not fully elucidated. In the present study, we generated mice with brain-specific deletion of HDC to investigate the importance of histamine for adult brain.

We stereotaxically microinjected adeno-associated virus (AAV) expressing

Cre-recombinase into tuberomammillary nucleus of adult HDC flox mice (HDC cKO mice). Immunohistochemical analysis showed Cre expression in tuberomammillary nucleus in HDC cKO mice. We confirmed the reduced HDC mRNA expression and the decreased histamine content in HDC cKO brain. Light/dark box tests showed that HDC cKO mice spent a shorter amount of time in the light room. In the tail suspension tests, immobility time was prolonged in HDC cKO mice. These results indicated that the inhibition of HDC activity in adult brain reduced histamine content and induced anxiety- and depression-like behaviors in mice.