A stain for ischaemic or excessively stimulated neurons

Loyez' myelin stain sometimes impregnates the perikarya and processes of neurons. This is rare in normal adult mice, but can be provoked by various means including electrical stimulation and mild ischaemia shortly before fixation. Passing a direct current or a train of pulses across the dura of the cerebral cortex produces a patch at the site of stimulation in which the neurons, but not the glia, are Loyez-impregnated; pulse-trains evoke such impregnation even at remote sites, presumably through orthodromic and/or antidromic activation. The strength and duration of stimulation necessary to provoke impregnation are below the threshold for causing an overt lesion. Local ischaemia produced by irreversibly occluding a small subarachnoid artery for as little as 7 min evokes a small patch of impregnated neurons in the superficial layers adjacent to the occluded artery. After slightly longer-lasting (e.g. 17 min) ischaemia, the impregnated zone spans most of the cortical depth. Mild global ischaemia produced by irreversibly occluding one or both common carotid arteries 30 min before fixation causes large numbers of neurons to be impregnated in many parts of the brain. That interventions as different as electrical stimulation and arterial occlusion both lead to neuronal impregnation may most plausibly be explained by the fact that they both cause cells to be depolarized. The method will be useful for visualizing overstimulated or ischaemic neurons, and may be applicable to the tracing of neural pathways.