Neurophysiological mechanisms involved in photo-entrainment of the circadian rhythm from the Aplysia eye.

Abstract

AbstractAn attempt was made to identify the neurophysiological processes involved in entrainment of the circadian rhythm of spontaneous optic nerve potentials from the Aplysia eye by determining whether pharmacological agents or ion substitutions could block phase shifts produced by single light pulses. Knowing which physiological processes are involved in entrainment should help define the morphological pathway traveled by entrainment information.A secretory step does not appear to be involved in the flow of entrainment information from the environment to the circadian oscillator. A treatment (HiMg LoCa) capable of inhibiting secretion did not interfere with phase shifting by light. Furthermore, treating eyes with putative transmitters or extracts of eyes did not phase shift the free running rhythm. Also, the phase shifting information is not translated into action potentials before reaching the oscillator since TTX–HiMg LoCa solutions did not block the light‐induced phase shift. The photoreceptor potential does seem to be important for light‐induced phase shifts. A correlation was found between the effects of treatments on the ERG and their effects on the light‐induced phase shift. Solutions which decreased the ERG by 90% or more blocked phase shifting whereas solutions which decreased the ERG by less than 74% had no effect on phase shifting by light.The results from these studies are consistent with two pathways for the flow of phase shifting information to the circadian oscillator. The circadian oscillator may be associated with receptor cells and the entrainment pathway would include a step involving the photoreceptor potential. Alternatively, the circadian oscillator may be associated with secondary cells and receive entrainment information via the photoreceptor potential and passive spread of current through a gap junction. Higher order cells than second‐order ones are probably not involved in the entrainment pathway.