Two mechanisms of photoendocrine transduction in cultured chick pineal cells: pertussis toxin blocks the acute but not the phase-shifting effects of light on the melatonin rhythm.

We have recently described a system, using dispersed chick pineal cells in static culture, which displays a persistent, photosensitive, circadian rhythm of melatonin release. Light has two apparent effects on this melatonin rhythm: the first is an acute inhibition of melatonin output, the second is entrainment of the underlying pacemaker. These two effects could be mediated by the same or different mechanisms. Pertussis toxin, which acts to block the function of transducin and certain other G-proteins, blocked the acute effects of light on chick pineal cells, but not the ability of light pulses to induce phase-dependent phase shifts of the rhythm. There must, therefore, be at least two mechanistic pathways by which light affects chick pineal melatonin production. Transducin or other pertussis toxin-sensitive G-proteins would appear to be involved in the acute effects of light on the melatonin-synthesizing apparatus, but not in the effects of light on the circadian pacemaker which generates the melatonin rhythm. Some plausible pertussis toxin-sensitive mechanisms are discussed.