Abstract
Histamine is thought to deliver itch sensation via H1 receptors on primary sensory neurons. Our previous studies showed that H1 receptor-expressing dorsal root ganglion neurons (about 15-20%) are unmyelinated in the guinea pig. However, these may not be peptidergic and capsaicin-sensitive neurons. The profiles of H1 receptor-expressing neurons in the guinea pig are partly incompatible for the physiological data of histamine-sensitive neurons in the rat. In this study, we examined the expression of H1 receptor in the rat primary sensory neurons by immunohistochemistry and transcriptase-polymerase chain reaction (RT-PCR). The immunoreactivity for H1 receptor was seen in 20-30% of the dorsal root ganglia. These were also labeled by isolectin-B4, which is a marker of unmyelinated sensory neurons. Furthermore, half of these neurons were positive to substance P and TRPV1, a capsaicin receptor. A positive signals of H1 receptor mRNA in rat dorsal root and trigeminal ganglia were clearly detected by RT-PCR, but were weaker than those of GAPDH mRNA. On the other hand, H1 receptor mRNA signals in the guinea pig sensory ganglia were more intense than those of GAPDH mRNA. A few of vagal afferent neurons (about 5% of nodose ganglion neurons) were also immunoreactive to H1 receptors in the rat. No expression of H1 receptor mRNA was seen in the guinea pig nodose ganglion in our previous study. These results suggest that the expression pattern of histamine H1 receptor in primary sensory neurons of the rat is partially different from that of the guinea pig. [Jpn J Physiol 55 Suppl:S162 (2005)]
