The influence of environmental lighting on the amphibian pars intermedia: Receptors and possible pathways

The well-known "background adaptation" response of the anuran amphibians constitutes a clearly defined example of an effect of environmental illumination on pituitary secretion. Morphological studies strongly suggest that the secretion of MSH, the hormonal agent of background adaptation, is controlled by neurons which terminate in the intermediate lobe of the pituitary (pars intermedia). We have investigated the possible role of this innervation in the photic control of the pars intermedia of Rana pipiens by recording the electrical activity of single, intermediate lobe neurons with gold-plated, platinum-tipped micro-electrodes. These studies have shown that changes in illumination affect the activity of some neurons in the intermediate lobe, ant that the photoreceptors which mediate this response are clearly the lateral eyes. Light responsive neurons in the pars intermedia do not exhibit any detectable response to illumination of the pineal region. The latency of the response to light is consistent with the existence of a neural circuit linking the eyes with the pars intermedia, and studies of retinal projections in amphibians suggest several possible routes for a retino-pars intermedia pathway. The unique availability of the input and output stages of this retino-hypophyseal pathway may make it a useful model for the study of photo-neuroendocrine mechanisms.