Photoreceptor-specific proteins in the mammalian pineal organ: immunocytochemical data and functional considerations.

The mammalian pineal organ contains photoreceptor-specific proteins, whose distribution shows conspicuous variation among different species of mammals. Nevertheless, the following general conclusions can be drawn: immunoreactions for S-antigen and recoverin labeled more pinealocytes than the rod-opsin immunoreaction. The intensity of the recoverin- and S-antigen immunoreactions varied from cell to cell. alpha-Transducin immunoreaction was absent from the pineal organ of all mammals investigated with the exception of the blind mole rat. Immunoreaction for the cyclic GMP-gated cation channel was undetectable in the pineal organ of all mammals investigated. The functional significance of photoreceptor-specific proteins in the mammalian pineal organ remains unknown. It has been speculated that the S-antigen might be involved in adrenergic transduction mechanisms. To test this assumption, we have started to analyze calcium responses of single rat pinealocytes to norepinephrine stimulation using the Fura-2 technique. The cells were subsequently labeled by means of S-antigen immunocytochemistry. These combined investigations showed that variation in S-antigen immunoreactivity is not correlated with differences in the rapid calcium response to stimulation with norepinephrine. It remains to be determined whether cells displaying different intensities of the S-antigen immunoreaction show different cyclic AMP responses to noradrenergic stimulation. Investigations along this line should help to clarify further whether there is indeed a relation between the expression of S-antigen and noradrenergic transduction mechanisms in the mammalian pineal organ.