Histamine type 3 receptor expression in mouse Scarpa's ganglia

OBJECTIVE: Histamine-related drugs such as betahistine are commonly used in the treatment of vertigo and related vestibular disorders. Their site and mechanism of action however deserve further investigation. We recently showed that the frog and mouse semicircular canal sensory epithelia, but not the vestibular (Scarpa’s) ganglia, express H1 histamine receptor subtype (Botta et al. 2008). Since it has been reported in the axolotl that H3 antagonists may act at the primary vestibular nerve afferents (Chavez et al. 2005), we have investigated the expression of H3 histamine receptor (H3R) subtype in the mouse vestibular ganglion. METHODS: H3 receptor mRNA expression was investigated by RT-PCR. For each of three total RNA extractions the vestibular ganglia were isolated from ~ 50 Swiss CD1 mice, and subsequent cDNA amplification was performed by using specific primers for H3 receptors designed in accordance with the published sequence. H3R location in the tissue was investigated by immunolabelling, using anti-H3R antibodies (Alpha Diagnostic International) on cryostat slices of the Scarpa’s ganglia and on paraffin embedded sections of the whole labyrinth (section thickness: 10 mm). RESULTS: RT-PCR experiments showed a band of 407 bp corresponding to H3R in the Scarpa’s ganglia. The immunolabelling study showed a clear signal for H3R in a subpopulation of Scarpa’s ganglia neurons, characterized by a large and roundish soma. Conversely, no clear staining for H3R was found in the vestibular sensory epithelia (cristae and maculae). CONCLUSIONS: This study shows that a subpopulation of mouse vestibular ganglia neurons express the histamine H3 receptor subtype, thus providing a molecular substrate for the postsynaptic action of histamine-related drugs reported in the literature.