Histamine H1 receptors are expressed in mouse and frog semicircular canal sensory epithelia.

Histamine-related drugs are commonly used in the treatment of vertigo and related vestibular disorders. Their site and mechanism of action, however, are still poorly understood. To increase our knowledge of the histaminergic system in the vestibular organs, we have investigated the expression of H1 and H3 histamine receptors in the frog and mouse semicircular canal sensory epithelia. Analysis was performed by mRNA reverse transcriptase-PCR, immunoblotting and immunocytochemistry experiments. Our data show that both frog and mouse vestibular epithelia express H1 receptors. Conversely no clear evidence for H3 receptors expression was found.

Simulation of cupulolithiasis and canalolithiasis by an animal model.

The physical mechanisms responsible for cupulolithiasis and canalolithiasis have been investigated by two groups of experiments in isolated posterior semicircular canal (SCC) of frog (Rana esculenta L.). First, clouds of 10-30 isolated otoconia were let to fall (snowfall of otoconia) either through the ampulla onto the cupula, or inside the long arm of the canal, opposite to the cupula. Second, microspheres ranging 30 to 350 microm in diameter were gently moved to and fro inside the long arm of the canal by a micromanipulator. The effects were evaluated by recording the firing rate (Nfr) of the ampullary nerve. Snowfall of otoconia produced detectable changes of Nfr only when otoconia got in contact with the cupula, but not when falling through the endolymph. Movement of the microspheres in the canal long arm induced Nfr changes only if the microsphere diameter exceeded about 50 microm. Although the exact microsphere size needed for receptor stimulation may depend on the experimental conditions, these results strongly suggest that debris moving inside a SCC (canalolithiasis) can produce transcupular pressures able to stimulate ampullar receptors only if they have suitable size, whereas isolated otoconia cannot, except when lying on the cupula (cupulolithiasis).

Convection, buoyancy or endolymph expansion: what is the actual mechanism responsible for the caloric response of semicircular canals?

The mechanisms underlying caloric nystagmus are still matter of debate. The original theory proposed by Barany and more recently by Pau and Limberg suggested that convective endolymphatic currents were involved. In contrast Gentine et al. suggested that the main mechanism responsible for caloric nystagmus is buoyancy due to calorization of the endolymph, without the need of continuous convective currents. Finally, other authors (Scherer and Clarke, Arai et al.) proposed that thermal expansion or contraction of the endolymph were involved. In the present study experimental conditions have been considered able to discriminate between these different models. The experiments, were carried out on isolated labyrinth preparations of the frog. Only the predictions of the model based on buoyancy were fully consistent with the experimental results whereas those provided by the other models were not.