HORIZONTAL CELL SPINULE DYNAMICS IN FISH ARE AFFECTED BY REARING IN MONOCHROMATIC LIGHT

Blue acaras (Aequidens pulcher, Cichlidae) were reared for 1 yr in whi te or monochromatic ''red'', ''green'' and ''blue'' lights to study th e function and control mechanisms of horizontal cell (HC) spinules in the synaptic pedicles of cones. Ratios of spinules per synaptic ribbon (S/R) were determined in tangential sections in both single and doubl e cones. We found that the S/R ratios in light adapted retinae decreas ed with decreasing wavelength of the rearing light in all cone types, Conversely, there was an increasing number of incompletely formed spin ules with the highest frequency in the blue light group. Dark adaptati on resulted in the complete degradation of mature spinules. However, s ignificant numbers of incompletely degraded spinules were observed in the group reared in blue light. Fish reared in blue light which were t ransferred to white light formed mature spinules when light adapted an d still had vestigial spinules when dark adapted. The mechanisms of sp inule formation and degradation and the control of spinule dynamics ap pear to be fully developed in fish reared in monochromatic light. Howe ver, long-term chromatic deprivation seems to induce a compensatory mo dulation of spinule dynamics. A working hypothesis is formulated that interprets the observed effects as manifestations of differences in th e activition of dopaminergic interplexiform cells (light adapted) and the sensitivity to glutamate of HCs (dark adapted). Our findings are c onsistent with the hypothesis that spinules are involved in sign-inver ting feedback transmission from HCs to cones. Copyright (C) 1996 Elsev ier Science Ltd