Enhanced retinal and optic nerve excitability associated with demyelination in mice infected with Semliki Forest virus.

The physiological effects of Semliki Forest virus (SFV) induced demyelination on the visual system of mice have been examined by recording electroretinograms (ERGs) and the spike activity in the retinal ganglion cell axons in control and SFV-infected mice. The amplitudes of ERGs evoked by a single flash in the dark-adapted eyes of the SFV infected mice were abnormally enhanced, whereas flicker ERGs were slightly depressed. This hyperactivity was also seen in the ganglion cell axonal discharge. Both visually evoked and spontaneous activity recorded from the optic tract fibres of the infected mice were characterized by rhythmic oscillatory bursts of firing. Similar patterns were seen only very rarely in the evoked activity recorded from control mice. We suggest that this oscillatory firing might be a functional correlate of the types of 'positive symptoms' reported by multiple sclerosis patients with optic neuritis.