Stable bellows cup electrode demonstrates low-frequency properties of long-term electroretinographic recordings in the Limulus lateral eye

Visual efference neuromodulates retinal timing: in vivo roles of octopamine, substance P, circadian phase, and efferent activation in Limulus

Efferent nerves coursing from the brain to the lateral eye of the horseshoe crab, Limulus polyphemus, increase its nighttime sensitivity to light. They release octopamine, which produces a categorical increase of photoreceptor response duration in vitro. Analogous in vivo timing effects on the electroretinogram (ERG) were demonstrated when octopamine was infiltrated into the eye of an otherwise intact animal; nighttime ERGs were longer than daytime ERGs. Related effects on the ERG were produced by daytime electrical stimulation of efferent fibers. Surprisingly, in a departure from effects predicted solely from in vitro octopamine data, nighttime ERG onsets were also accelerated relative to daytime ERG onsets. Drawing on earlier reports, these remarkable accelerations led to an examination of substance P as another candidate neuromodulator. It demonstrated that infiltrations of either modulator into the lateral eyes of otherwise intact crabs increased the amplitude of ERG responses but that each candidate modulator induced daytime responses that specifically mimicked one of the two particular aspects of the timing differences between day- and nighttime ERGs: octopamine increased the duration of daytime ERGs and substance P infiltrated during the day accelerated response onset. These results indicate that, in addition to octopamine's known role as an efferent neuromodulator that increases nighttime ERG amplitudes, octopamine clearly also affects the timing of photoreceptor responses. But these infiltration data go further and strongly suggest that substance P may also be released into the lateral eye at night, thereby accelerating the ERG's onset in addition to increasing its amplitude.

Visual efference in Limulus: in vitro temperature-dependent neuromodulation of photoreceptor potential timing by octopamine and substance P

Efferents from the brain of Limulus course toward its lateral eye and release octopamine and substance P into it. These neurotransmitters have previously been found to act as neuromodulators in this visual system by altering the size of its responses to light. We report here that both also modulate the timing of the receptor potentials (RPs) evoked by brief light flashes and that these timing effects are temperature dependent. Specifically: We extend our previous report that octopamine prolongs ambient RPs in a categorical fashion and here demonstrate that it does the same at colder temperatures. Categorical means that a given RP is either clearly prolonged in a dramatic fashion or its duration is otherwise unremarkable. Octopamine also accelerates the onsets of RPs when they are evoked by weak flashes under cold temperatures. Contrariwise, substance P accelerates RPs at all temperatures and this acceleration dramatically reduces the sluggishness that is otherwise typically present at low temperatures. Quantitative analysis of intensity-response functions also demonstrated that light sensitivity under substance P is significantly augmented. The plain temporal antagonism between these two modulators demonstrates that the visual system of Limulus possesses a well-poised mechanism which could be used to adjust the timing of its neural processing to interface well with the temporal characteristics of those visual stimuli that are currently present.