Comodulation of Limulus lateral eye photoreceptors by efferent neuromodulators

To survive, adaption is an essential response of all animals confronted by changed environments. This is often accomplished by neuromodulation which is widely present in neural systems. A novel form of visual efferent neuromodulation has recently been discovered in the lateral eye of Limulus, an ideal model visual system. This work demonstrated that individual efferent neuromodulators impressively modulated the temporal properties of this eye’s visual responses to light. Octopamine particularly prolongs the repolarization of RPs (RP-) and the corresponding part of ERGs, while substance P specifically accelerates the depolarization of RPs (RP+) and the corresponding part of ERGs. These opposite effects therefore raise the question of whether an antagonistic modulatory system exists in the lateral eye of Limulus. The present research thereby used physical, chemical, and biological methods to determine whether and how these two neuromodulators interact when they are applied to Limulus lateral eye photoreceptors; this is called comodulation. Dramatic temporal changes in the responses to visual stimuli as well as other biological characteristics were revealed under comodulation that were not due to the effects of any physical or chemical factors. The half-peak latency, an index of temporal properties, was temperature dependent, speeding up at cold while it slowing down at warm. The largest effect observed was a 20 ms acceleration at cold while 15 ms delay occurred at warm. And at cold higher concentrations, novel biological characteristics were discovered, such as bi-directionally augmented RP durations, hyperpolarized membrane potentials, and an unexpected gain in response. These results clearly demonstrated that octopamine and substance P do biologically interact in those photoreceptors. Moreover, these interactions are not simply linear in nature, but rather depend on a complex nonlinearity. The present findings suggest a new way of changing the temporal bandpass of photoreceptors so that the brain can adjust the limited capacity of its distal visual system to respond effectively to temporal variations in its environment. These impressive temporal modifications produced by efferent neuromodulation not only functionally benefit Limulus natural behaviors (such as mating) but may also be helpful in understanding human psychological problems, such as attention deficit disorders