Sequence of changes in neurofibrils (neurofilaments) induced in synaptic regions of bullfrogs by environmental temperature changes

Prolonged exposure of bullfrogs ( R. catesbeiana ) to a cold (6°C) environment causes a progressive increase in numbers of neurofibrillar boutons in synaptic regions of the midbrain and induces thickening of perikaryal neurofibrils. Quantitative estimates of the numbers of boutons in lamina 5 of the optic tectum with controlled conditions for reduced silver staining, show the effects are graded with respect to both temperature and duration of exposure. Initial acclimation to 18°C results in few neurofibrillar rings but upon changing to 6°C the counts increase linearly with time to reach a 20 fold increase at three weeks. Rewarming to 18°C causes a return to the original numbers in less than one day. In electron micrographs ring‐shaped bundles of 100 Å diameter neurofilaments are coextensive with the cold induced neurofibrillar boutons of light microscopy; both are absent following acclimation to a warm environment. The cold exposure also causes large accumulations of β glycogen particles to appear in axons, dendrites, and synaptic knobs.