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

Existing data on the expression of H(4) 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 H(4) receptor expression and localization in the brain, spinal cord, and dorsal root ganglia (DRG). Here we show that transcripts of H(4) receptor are present in all analyzed regions of the human CNS, including spinal cord, hippocampus, cortex, thalamus and amygdala, with the highest levels of H(4) mRNA detected in the spinal cord. In rat, H(4) mRNA was detected in cortex, cerebellum, brainstem, amygdala, thalamus and striatum. Very low levels of H(4) mRNA were detected in hypothalamus, and no H(4) signal was detected in the rat hippocampus. Fairly low levels of H(4) mRNA were detected in examined peripheral tissues including spleen and liver. Interestingly, strong expression of H(4) mRNA was detected in the rat DRG and spinal cord. Immunohistochemical analysis revealed expression of H(4) receptors on neurons in the rat lumbar DRG and in the lumbar spinal cord. Our observations provide evidence of the H(4) presence in both human and rodent CNS and offer some insight into possible role of H(4) in itch and pain.