Effects of diphenhydramine iontophoretically applied onto neurons in the medial and lateral vestibular nuclei.

Electrophysiological studies were carried out to elucidate the effects of diphenhydramine, an antihistamine (H1-receptor blocking) drug, on neuron activities in the medial vestibular nucleus (MVN) and lateral vestibular nucleus (LVN) of cats using a microiontophoretic method. According to the firing pattern and latency of the first spike with vestibular nerve stimulation, neurons in the MVN and LVN were classified into two groups: monosynaptic and polysynaptic neurons. In the MVN, the spike generation of polysynaptic neurons was dose-dependently inhibited with the iontophoretic application of diphenhydramine up to 200 nA, and that of the monosynaptic neurons was also suppressed by the maximum dose of 200 nA. In contrast to the MVN neurons, the spike generation of LVN monosynaptic neurons remained unaffected with diphenhydramine up to 200 nA, although an inhibition of the LVN polysynaptic neurons was obtained with 200 nA of the drug. These results suggest that small doses of diphenhydramine more selectively interfere with synaptic transmission in the MVN neuron than that in the LVN neuron.