George H. Renninger

Efferent neurotransmission of circadian rhythms inLimulus lateral eye

SummaryWe investigated efferent neurotransmission in theLimulus lateral eye by studying the action of pharmacological agents on responses of photoreceptor cells in vitro. We recorded transmembrane potentials from single cells in slices of retina that were excised during the day and maintained for several days in a culture medium. Potentials recorded in the absence of pharmacological agents resemble those recorded from cells in vivo during the day.Octopamine, a putative efferent neurotransmitter, induced changes in photoreceptor potentials that mimicked in part those generated at night by a circadian clock located in the brain. Specifically, octopamine (100 to 500 μM) decreased the frequency of occurrence of quantum bumps in the dark and increased the amplitude of photoreceptor responses to intermediate and high light intensities. Similar actions were produced by naphazoline (25 to 100 (μM, potent agonist of octopamine), forskolin (8 to 400 μM, activator of adenylate cyclase), IBMX (1 mM, inhibitor of phosphodiesterase), and 8-bromo-cAMP (500 μM, analogue of cAMP). 8-bromo-cGMP (500 μM, analogue of cGMP) decreased the rate of spontaneous quantum bumps only.Our results support the hypothesis that (1) octopamine is an efferent neurotransmitter of circadian rhythms in theLimulus eye and that (2) it activates adenylate cyclase to increase levels of the second messenger, cAMP, in photoreceptor cells. Circadian changes in photoreceptor responses to moderate intensities may be a specific action of cAMP, since cGMP has no effect. Circadian changes in the rate of spontaneous quantum bumps may involve a less specific intermediate, since both cAMP and cGMP reduce bump rate. Characteristics of the retinal slice preparation precluded a detailed study of the effects of pharmacological agents on retinal morphology.

The morphology of the dorsal eye of the hydrothermal vent shrimp, Rimicaris exoculata

The bresiliid shrimp, Rimicaris exoculata, lives in large masses on the sides of hydrothermal vent chimneys at two sites on the Mid-Atlantic Ridge. Although essentially no daylight penetrates to depths of 3500 m, very dim light is emitted from the hydrothermal vents themselves. To exploit this light, R. exoculata has evolved a modified compound eye on its dorsal surface that occupies about 0.5% of the animals body volume. The eyes morphology suggests that it is extremely sensitive to light. The cornea of the dorsal eye is smooth with no dioptric apparatus. The retina consists of two wing-shaped lobes that are fused across the midline anteriorly. The rhabdomeral segments of the 7000 ommatidia form a compact layer of photosensitive membrane with an entrance aperture of more than 26 mm2. Within this layer, the volume density of rhabdom is more than 70%. Below the rhabdomeral segments, a thick layer of white diffusing cells scatters light upward into the photoreceptors. The arhabdomeral segments of the five to seven photoreceptors of each ommatidium are mere strands of cytoplasm that expand to accommodate the photoreceptor nuclei. The rhabdom is comprised of well-organized arrays of microvilli, each with a cytoskeletal core. The rhabdomeral segment cytoplasm contains mitochondria, but little else. The perikaryon contains a band of mitochondria, but has only small amounts of endoplasmic reticulum. There is no ultrastructural indication of photosensitive membrane cycling in these photoreceptors. Vestigial screening pigment cells and screening pigment granules within the photoreceptors are both restricted to the inner surface of the layer of the white diffusing cells. Below the retina, photoreceptor axons converge in a fanshaped array to enter the dorsal surface of the brain. The eyes size and structure are consistent with a role for vision in shrimp living at abyssal hydrothermal vents.

Circadian change in function of Limulus ventral photoreceptors

Efferent fibers from a central circadian clock innervate photoreceptors along the ventral nerve of Limulus and release octopamine when active. We have recorded ERG-like responses from the ventral eye in vivo over several day periods. We have also used intracellular microelectrodes to study changes in ventral photoreceptor function during exogenous applications of octopamine (the putative efferent neurotransmitter), IBMX (a phosphodiesterase inhibitor), and forskolin (an adenylate cyclase activator): (1) Responses to light measured at night from ventral photoreceptors in vivo are greater in amplitude than those recorded during the day; (2) Octopamine and agents that increase intracellular levels of cAMP in ventral photoreceptors decrease the rate of spontaneous (dark) bumps, increase photoreceptor response to light without changing threshold, and often increase the bump duration; and (3) These changes in function of ventral photoreceptors are similar to those that have been observed in the photoreceptor of the lateral eye during circadian clock activity at night, and in vitro in the presence of those same pharmacological agents.

Octopamine modulates photoreceptor function in the Limulus lateral eye.

Activity at night in efferent nerve fibers from a central circadian clock produces changes in photoreceptor function in the lateral compound eye of Limulus: the response to light is increased; membrane potential fluctuations (bumps) occurring in the dark are suppressed; and the duration of bumps occurring both in the dark and under dim illumination is increased (Barlow et al., 1977; Kaplan & Barlow, 1980; Barlow, 1983; Barlow et al., 1985). Efferent nerve terminals release octopamine when activated (Battelle et al., 1982; Battelle & Evans, 1984, 1986); exogenous octopamine in vitro produces some of the changes resulting from efferent nerve activity in vivo (Kass et al., 1988). We report here that the increase in both on-transient and steady-state response to light induced by octopamine in the lateral eye in vitro are concentration dependent with threshold at or below 100 nM, saturation at or above 100 microM, and half-maximal increase in the range 1-10 microM. Octopamine also reduces bump activity in the dark in a concentration-dependent way. Unlike the increase in light response, the dependence of this effect on octopamine concentration is extremely variable from specimen to specimen. The effects of exogenous octopamine on light response and bump activity can sometimes be reversed by removing octopamine from the medium bathing the in vitro preparation. Octopamine also increases bump duration, apparently in a concentration-dependent manner. We have not succeeded in reversing this increase in bump duration. The concentration dependence of changes in photoreceptor response described here agrees qualitatively with the dependence of cAMP levels on octopamine in Limulus photoreceptors (Kaupp et al., 1982), lending further support to the idea that cAMP acts as a second messenger in the circadian control of photoreceptor function. Our results also suggest that the changes induced in the transient and steady-state response to light by both efferent nerve activity and exogenous octopamine have a common origin, which may differ from that responsible for the modulation of bump activity.

Circadian photoreceptor organs inLimulus

SummaryThe circadian rhythm in the ERG amplitude of the lateral compound eye ofLimulus can be phase shifted either by general illumination or by illuminating combinations of the photoreceptor organs.1.For 15-min exposures, light confined to one lateral eye, or to the median ocelli, or to the ventral photoreceptor region resulted in the smallest phase shifts.2.Illuminating combinations of these organs produced larger shifts. The most effective combination tested included the median ocelli, the ventral photoreceptors, and one lateral eye. The phase shift resulting from illumination of this combination was only about one-half of the shift produced by general illumination.3.These results suggest that the circadian clock also receives light information from other, unidentified, photoreceptors located outside the prosoma.

Periodic Solutions of Certain Nonlinear Integral Equations with a Time Lag

Solutions y of equations of the type \[ ( 1 )\qquad y( t ) = m\left( {1 - \int_0^\infty {e^{ - s} g( {y( {t - \gamma - s} )} )ds} } \right) \] with

Theory of the response of the limulus retina to periodic excitation.

\gamma

The eccentric cell of theLimulus lateral eye: encoder of circadian changes in visual responses

a given positive number, with g a given function obeying

Retinal anatomy of Chorocaris chacei, a deep-sea hydrothermal vent shrimp from the mid-Atlantic ridge

g( 0 ) = 0

Consequences of delayed lateral inhibition in the retina of Limulus I. elementary theory of spatially uniform fields

and