Figure 2.

Histamine and dopamine synthesis are under local circadian control. (A) During the less active period (the “lights on” period) of mice, the dopaminergic and histaminergic cells have lower levels of transmitter synthesis for dopamine and histamine because the levels of tyrosine hydroxylase (TH) in the ventral tegmental area (VTA) and histidine decarboxylase (HDC) in the TMN are lower. The gene expression levels are controlled by clock genes in local circadian processes coordinated by the suprachiasmatic nucleus (SCN). (B) During the more active period of mice, the dark period, in histamine and dopamine cells levels of the TH and HDC proteins are higher, giving the potential for more transmitter release. (C) The relative levels of HDC and TH enzymes, as detected by immunoreactivity (IR) are shown schematically at the different times of day. Adapted from Chung et al. (2014) and Yu et al. (2014).