The Ventral Photoreceptor Cells of Limulus

Abstract

The ventral photoreceptors of Limulus polyph,mus are unipolar cells with large, ellipsoidal somas located long both "lateral olfactory nerves." As a consequence of their size and location, the cells are easily impaled with microelectrodes. The cells have an average resting potential of --48 my. The resting potential is a function of the external concentration of K. When the cell is illuminated, it gives rise to the typical "receptor potential" seen in most invertebrate photoreceptors which consists of a transient phase followed by a maintained phase of depolarization. The amplitude of the transient phase depends on both the state of adaptation of the cell and the intensity of the illumination, while the amplitude of the maintained phase depends only on the intensity of the illumination. The over-all size of the receptor potential depends on the external concentration of Na, e.g. in sodium-free seawater the receptor potential is markedly reduced, but not abolished. On the other hand lowering the Ca concentration produces a marked enhancement of both components of the response, but predominantly of the steady-state component. Slow potential fluctuations are seen in the dark-adapted cell when it is illuminated with a low intensity light. A spike-like regenerative process can be evoked by either the receptor potential or a current applied via a microelectrode. No evidence of impulse activity has been found in the axons of these cells. The ventral photoreceptor cell has many properties in common with a variety of retinular cells and therefore should serve as a convenient model of the primary receptor cell in many invertebrate eyes. I N T R O D U C T I O NT h e ionic basis for receptor potentials in photoreceptors has been investigated in only a few preparations, namely, the cells in the o m m a t i d i a of the horseshoe c r a b (Kikuchi, Naito, and T a n a k a , 1962), the retinular cells of the crayfish (Eguchi, 1965), and the retinular cells of the h o n e y b e e d r o n e (Fulpius and B a u m a n n , 1969). T h e s e preparations all have the disadvantages of being quite small and e m b e d d e d in a meshwork of pigment-containing cells and glial cells. In the studies cited above it was shown that sodium is ira3Io

Electrophysiological measurement of the number of rhodopsin molecules in single Limulus photoreceptors.

“…1977 by this method). The average capacitance as determined from five cells was 4.2 +-1.4 x 10 -9 F, a value in reasonable agreement with the results of Millecchia and Mauro (1969a). This implies a total membrane area of 4.2 × 10 -3 cm ~.…”

“…This implies a total membrane area of 4.2 × 10 -3 cm ~. If the photoreceptor ~¢ere not invaginated, its surface area would be 5 × 10 -4 cm 2 (Millecchia and Mauro, 1969a). Thus most of the plasma membrane in the ventral photoreceptor is in the microvilli.…”

Protein Kinase C Activators Inhibit the Visual Cascade inLimulusVentral Photoreceptors at an Early Stage

“…4C, n ϭ 4). These responses are indistinguishable from control responses, as described by others (Millecchia and Mauro, 1969a).…”

Light-Evoked and Spontaneous Discrete Waves in the Ventral Nerve Photoreceptor of Limulus

“…The ease of penetrating these cells with intracellular microelectrodes and the simplicity of their structure have made them a good model system for studying the initial steps of the transduction of light by a receptor into an electric current. Millecchia and Mauro (1969) have shown that discrete waves can also be observed in the ventral nerve receptor. In this paper it will first be shown that for the ventral receptor Eq.…”