Light Sensitivity of the Pineal Organ in Poikilothermic and Homeothermic Vertebrates

Abstract

During the past 25 years extraocular photoreceptive systems have received considerable interest after it was shown that they coordinate processes within an organism with the photoperiod of the environment. Although it seems firmly established that there are deep photoreceptors scattered throughout the diencephalon in mammalian species, the pineal organs of poikilotherms stand out as relatively highly developed photosensory structures (Groos 1982). Whereas earlier studies indicating the presence of extraocular photoreceptors were based mainly on indirect evidence such as the effect of removal or localized stimulation of putative photoreceptors (v. Frisch 1911) direct evidence for the presence of functioning extraretinal light receptors was obtained by applying neurophysiological methods after most results pointed to neural structures as the sites of extraocular photoreception (Dodt and Heerd 1962). Although electrical recording from pineal photosensitive structures does not provide information about the possible effector systems, this technique helps the investigator determine effective parameters of stimulation so that subsequent experiments can be designed in which the appropriate stimuli are applied to the pineal system of the otherwise undisturbed animal. In general, the mode of action of pineal and parietal photoreception resembles that of ocular photoreception in that onset and offset of light are followed by inhibitory and excitatory changes in neuronal activity. The similarities in the effects of light reflect the common origin of ocular and extraocular photoreceptors from di-encephalic structures (Studnička 1898).