Electroretinographic investigation of circadian efferent neuromodulation of photoreceptor timing in the lateral eye of Limulus polyphemus

The sensitivity of the lateral eye of the horseshoe crab Limulus polyphemus increases at night as part of a circadian rhythm mediated by efferent nerves that run from the brain to the eye. These efferents are known to release Octopamine, which implements physiological changes that contribute to this increase. In addition to increasing the amplitude of electroretinograms (ERGs) in the lateral eye, Octopamine has been demonstrated to increase the duration of photoreceptor responses in vitro. The hypothesis underlying the present dissertation predicted that comparable prolongations of photoreceptor responses would be evident in in vivo ERGs. Temporal differences between daytime and nighttime ERGs were evaluated. Nighttime traces were longer in duration, as had been predicted; but, surprisingly, their latencies were accelerated compared to daytime ERGs. The next question was whether a second neurotransmitter contributes to the sensitivity increase at night. The nighttime latency accelerations in nighttime ERGs implicated Substance P as another possible candidate neurotransmitter because a known lateral eye efferent system is immunoreactive for a Substance P-like neuropeptide. Octopamine and Substance P were therefore injected into lateral eyes in vivo. Each mimicked one of the two different aspects of the differences in timing between daytime and nighttime ERGs: Octopamine increased the daytime duration of ERGs and Substance P accelerated response latencies. Finally, an unexpected discovery was also made during these hypothesis-driven experiments: evidence indicated that two separate clocks may control circadian rhythms in the ERG in this animal. One clock has been demonstrated in the brain and is known to affect ERG peak amplitude. But the present results indicate another clock may affect peak latency. This unexpected discovery also indicated that these clocks can become uncoupled, resulting in internal desynchronization. These results then generate a new hypothesis that the second clock controls the circadian signals of the Substance P-like efferent system and that the balance between these two efferent systems regulates lateral eye ERG temporal properties. Although such systems are well known in mammalian systems, no invertebrate system controlling different aspects of the same physiological system seems to have been reported previously