Figure 4.

Signaling pathways triggered by histamine receptor activation. Activation of the H1R-H4R receptors, figures (a–d). The H1R receptor is widely expressed in the hippocampus, cholinergic and aminergic brain stem nuclei, thalamus, and cortex. H2R is located in the basal ganglia, amygdala, hippocampus, and brain cortex. High H3R densities have been demonstrated in the anterior cerebral cortex, hippocampus, amygdala, striatum, olfactory tubercle, cerebellum, substantia nigra, and brain stem. Finally, expression of H4R mRNA has been reported in the amygdala, cerebellum, corpus callosum, frontal cortex, hippocampus, and thalamus.