Supravital recordings of spectral transmission in the brains of two species of teleosts (Anguilla anguilla, Ictalurus nebulosus), an amphibian (Rana temporaria), a reptile (Lacerta muralis), two species of birds (Passer domesticus, Columba livia), and a mammal (Phodopus sungorus) indicate that photons of longer wavelengths (700-750 nm) penetrate approximately 1,000 times more effectively into the hypothalamus than photons of shorter wavelengths (400-450 nm). The decrease in transmission from 750 to 400 nm is slightly interrupted by a plateau around 500 to 540 nm because of the transmission characteristics of hemoglobin. There is a small, ill-defined transmission minimum around 430 nm corresponding to the transmission minimum of melanin and hemoglobin (soret band). The high light sensitivity of deep diencephalic photoreceptors involved in the control of photoneuroendocrine events characteristic of some non-mammalian vertebrates suggests the occurrence of photopigment-containing receptors and nerve cells summating the input of several photoreceptors. However, in addition to photopigments, there may also exist other photosensitive compounds that mediate non-visual photoneuroendocrine responses. © 1979 Springer-Verlag.
