Localization of histamine H4 receptors in the central nervous system of human and rat

Existing data on the expression of H4 histamine receptor in the CNS are conflicting and inconclusive. In this report, we present the results of experiments that were conducted in order to elucidate H4 receptor expression and localization in the brain, spinal cord, and dorsal root ganglia (DRG). Here we show that transcripts of H4 receptor are present in all analyzed regions of the human CNS, including spinal cord, hippocampus, cortex, thalamus and amygdala, with the highest levels of H4 mRNA detected in the spinal cord. In rat, H4 mRNA was detected in cortex, cerebellum, brainstem, amygdala, thalamus and striatum. Very low levels of H4 mRNA were detected in hypothalamus, and no H4 signal was detected in the rat hippocampus. Fairly low levels of H4 mRNA were detected in examined peripheral tissues including spleen and liver. Interestingly, strong expression of H4 mRNA was detected in the rat DRG and spinal cord. Immunohistochemical analysis revealed expression of H4 receptors on neurons in the rat lumbar DRG and in the lumbar spinal cord. Our observations provide evidence of the H4 presence in both human and rodent CNS and offer some insight into possible role of H4 in itch and pain.