Cholinergic and glutamatergic transmission in medial vestibular nucleus neurons responding to lateral roll tilt in rats

Abstract

The responses of the medial vestibular nucleus (MVN) neurons to lateral tilt and the neurotransmitters mediating otolith information to MVN neurons were investigated using rats. A computer-operated goniometer was tilted 20° clockwise and counterclockwise at an angular speed of 5°/s and paused in the inclined positions for 10 s to record neuronal responses in the static phase. The 185 MVN neurons recorded were classified into eight types according to their responses to tilt (α, β, γ, δ, ε, ζ, η and θ). A majority showed increased firing in response to ipsilateral tilting and decreased firing in response to contralateral tilting (α type: 31.4%) or exhibited the reverse pattern (β type: 36.8%). Further, other groups of neurons increased (γ type) or decreased (δ type) firing rates to either side tilting and increased (ε and ζ type) or decreased (η and θ type) firing only on one side. Atropine or l-glutamic acid diethyl ester hydrochloride (GDEE) applied microiontophoretically antagonized tilt-induced firing of α type neurons in 58.8% or 60.0%, respectively, and of β type neurons in 66.7% or 58.3%, respectively. When the effects of atropine and GDEE were examined in the same neurons, antagonizing effects of both drugs on tilt-induced firing were obtained in 28.6% and 40.0% of α and β type neurons, respectively. These results suggest that both acetylcholine and glutamate act as neurotransmitters in the transmission of otolith information to most MVN neurons.