The pineal eye in Xenopus laevis embryos and larvae: A photoreceptor with a direct excitatory effect on behaviour

Behavioural experiments demonstrate that embryos and young larval stages of Xenopus laevis when exposed to a sudden drop in light intensity may show locomotor activity. Recordings from motoneurone axons demonstrate that the integrity of the pineal eye photoreceptors are essential for this response to occur. Thus the pineal has a direct excitatory effect on behaviour. The pineal eye arises embryologically as a single dorsal vesicular evagination of the diencephalon and anatomically it is very similar to that of other developing amphibians. Recording from the pineal eye using a suction electrode demonstrates that suddenly lowering the light intensity evokes a burst of impulses followed by a raised firing frequency. Conversely, increasing the light intensity leads to a lowered firing frequency. With prolonged exposure to white light at a range of intensities, the frequency of spike discharge is dependent upon the light intensity. The pineal eye can therefore act as a luminance detector. The pineal photoreceptors are most sensitive to light of a wavelength near 520 nm, this probably enables maximum sensitivity to the wavelengths of light that penetrate the freshwater environment. The possible role of the pineal eye in controlling locomotor activity is discussed. © 1982 Springer-Verlag.