THE RAPID ASSEMBLY OF SYNAPTIC SITES IN PHOTORECEPTOR TERMINALS OF THE FLYS OPTIC LOBE RECOVERING FROM COLD SHOCK

When a housefly, Musca domestica, is subject to cold exposure (0 degre es C for 24 hr), a number of obvious changes are seen in the first opt ic neuropil, or lamina, beneath the compound eye. In particular, the n umber of afferent photoreceptor synapses declines by about 30%. This l oss is dramatically restored after warm recovery at 23 degrees C for 2 4 hr. Synapses disappear at an average rate of 2-3/hr during cold expo sure and reappear at a maximal rate of more than 20/hr during the firs t 2 hr of warm recovery. Thereafter their number temporarily overshoot s control values, to increase at 6 hr of warm recovery to 60% above th eir cold-exposed minimum. The number subsequently returns more or less to normal. These changes demonstrate the lability of synaptic sites u nder these conditions, with individual sites forming and disappearing rapidly. The changes also interrupt the close correlation between syna ptic number and the surface area of the receptor terminal, a correlati on that normally conserves synaptic spacing density. The density is pr eserved during cold exposure but increases during warm recovery at a t ime when the addition of newly formed synapses exceeds the slower incr ease in receptor terminal size.