>I agree with your conclusion that the system is more complicated than the simple neural pathway depicted in the literature. The manipulations used in your studies may act directly on the retina-pineal neural pathway or indirectly on other neural tissues that affect components in the retina-pineal neural pathway. In one of our earlier studies [Pang, S.F., Li, Y, D.H. Jiang, M.L. Wang, and D.R. Jiao: Acute cerebral hemorrhage changes the nocturnal surge of plasma melatonin in humans. J. Pineal Res. 9:193-208 (1990)] we found that lesions in sites outside the retina-pineal neural pathway disrupted the melatonin rhythms in humans. One of the possible explanations is that these lesions (neural substrates) may affect the SCN signals to the pineal gland.
I agree with you up to a point. The acute effects on melatonin production that we see with serotonergic and nicotinic agonists could be acting through direct and indirect pathways. However, the phase shifting responses we see must involve the SCN at some level. The simple act of decreasing melatonin at night for a brief period does not in itself result in phase shifting.