Substance P in the chick retina: effects of light and dark

Monoclonal antibodies against substance P were used to study the distribution of the peptide in the retinae of chicks that had been kept in either total darkness or brightly lit conditions. In retinae from light-adapted chicks, substance P was found to be in normal sized amacrine cells and giant cells in the inner nuclear layer, in fibres in sublaminae 1,3-5 in the inner plexiform layer, in cells in the ganglion cell layer which may have been mostly displaced amacrine cells and in a few fibres in the optic nerve fibre layer. There was no apparent concentration of immunoreactive cell bodies in any part of the retina. After dark-adaption of the birds, the immunoreactivity in the retinae was much reduced; there were fewer visible cells in all parts of the retina and there was no reaction in the inner plexiform layer. When birds were taken from the dark and kept in the light for different periods, there was a gradual increase in the number of cells and in the overall immunoreaction. Substance P is thus like several other putative retinal neurotransmitters in that its concentration in the retina is regulated by light/dark. These experiments do not indicate how the changes occur, but they underline the need for a strict control of lighting conditions in experiments with these relatively slowly metabolized neurotransmitters.