[Effects of prolonged darkness and background illumination on cone horizontal cells in carp retina: a correlative study of morphology and physiology].

Using intact, immobilized carp preparations, changes in light responsiveness of cone horizontal cells and ultrastructures of their terminals in cone pedicles (HCTs) were correlatively examined in prolonged (greater than 2 h) darkness and after the presentation of a dim background light. Following background illumination cone horizontal cells exhibit high light responsiveness and HCTs give rise to a lot of long, fingerlike or ball-like extensions, called slender or round spinules. When the retina is left in the dark for more than 2 h, the light responsiveness of these cells is depressed, which is accompanied by a dramatic decrease of spinules. Thus light responsiveness of the cone horizontal cells seems to be well correlated with the number of spinules. The results suggest that spinules may play an important role in regulating light responsiveness of cone horizontal cells following background illumination by altering the efficacy of signal transfer across the synapses between cone photoreceptors and cone horizontal cells.