Joel E. Brown

Synapses of Horizontal Cells in Rabbit and Cat Retinas

Horizontal cells in the retinas of cats and rabbits are morphologically similar; in both species, two types can be distinguished in Golgistained material. Horizontal cells and their processes are readily recognized in electron micrographs, and many of the horizontal cell processes appear to make synaptic contacts with dendrites and somata of bipolar cells, and probably with other horizontal cells. The synapses of the horizontal cell appear similar to chemical synaptic contacts described throughout the nervous system. With the finding of synaptic contacts, it seems clear that retinal horizontal cells should be classified as neurons.

Cat retinal ganglion cell dendritic fields.

Abstract Retinal ganglion cells of the cat were stained by the techniques of Golgi (silver) and Ehrlich (in vivo methylene blue). These neurohistological techniques presumably stain a few nerve cells completely, and hence allowed the study of the form of the dendritic expansion of the cat ganglion cells. The dendritic expansions of all the cells studied ended in one plane within the inner plexiform layer; no multistratified types of cells were seen. The dendritic fields were more or less circular with the cell bodies placed centrally. The size distribution of the dendritic fields was bimodal. Sizes between 70 and 200 μ and from 400 to 700 μ were found. It is suggested that for the cat retinal ganglion cells the size of the dendritic field is related to the size of the center region of the corresponding (physiologically determined) concentric receptive field.

A Role for the Sodium Pump in Photoreception in Limulus

The membranes of photoreceptor cells in Limulus have an electrogenic sodium pump which contributes directly to membrane potential and whose activity is changed by light. These light-induced changes in pump activity underlie the receptor potential.

Conductance Changes Associated with Receptor Potential in Limulus Photoreceptors

The receptor potential in Limulus photoreceptors appears to be a consequence not of permeability changes in the cell membrane but of alterations in a light-sensitive constant-current generator.

A Hyperpolarizing Component of the Receptor Potential in the Median Ocelius of Limulus

There are two classes of photoreceptor cells in the median ocellus of Limulus. One class of cells respond to long wavelength (visible) stimuli with a depolarizing receptor potential and to near ultraviolet light with a biphasic, initially hyperpolarizing, receptor potential. The other class of receptors respond with a depolarization to near ultraviolet and with a biphasic response to visible light. In the latter type of cell, visible light can counteract the depolarization elicited by near ultraviolet light. The evidence suggests that there are two photopigments in each cell and that both are involved in the generation of receptor potential.

Receptive fields of rat retinal ganglion cells.

Abstract The receptive fields of rat retinal ganglion cells were studied quantitatively by the “area-intensity” method. By this means the antagonistic interaction of the center and the surround was quantitatively determined. For the units studied, the range of surround efficacy included units having (1) no demonstrable surround, (2) center versus surround antagonism and (3) surrounds which dominated the response of the cell to stimuli covering the entire receptive field.

Revisiting the Role of H+ in Chemotactic Signaling of Sperm

Chemotaxis of sperm is an important step toward fertilization. During chemotaxis, sperm change their swimming behavior in a gradient of the chemoattractant that is released by the eggs, and finally sperm accumulate near the eggs. A well established model to study chemotaxis is the sea urchin Arbacia punctulata. Resact, the chemoattractant of Arbacia, is a peptide that binds to a receptor guanylyl cyclase. The signaling pathway underlying chemotaxis is still poorly understood. Stimulation of sperm with resact induces a variety of cellular events, including a rise in intracellular pH (pHi) and an influx of Ca2+; the Ca2+ entry is essential for the chemotactic behavior. Previous studies proposed that the influx of Ca2+ is initiated by the rise in pHi. According to this proposal, a cGMP-induced hyperpolarization activates a voltage-dependent Na+/H+ exchanger that expels H+ from the cell. Because some aspects of the proposed signaling pathway are inconsistent with recent results (Kaupp, U.B., J. Solzin, J.E. Brown, A. Helbig, V. Hagen, M. Beyermann, E. Hildebrand, and I. Weyand. 2003. Nat. Cell Biol. 5:109–117), we reexamined the role of protons in chemotaxis of sperm using kinetic measurements of the changes in pHi and intracellular Ca2+ concentration. We show that for physiological concentrations of resact (<25 pM), the influx of Ca2+ precedes the rise in pHi. Moreover, buffering of pHi completely abolishes the resact-induced pHi signal, but leaves the Ca2+ signal and the chemotactic motor response unaffected. We conclude that an elevation of pHi is required neither to open Ca2+-permeable channels nor to control the chemotactic behavior. Intracellular release of cGMP from a caged compound does not cause an increase in pHi, indicating that the rise in pHi is induced by cellular events unrelated to cGMP itself, but probably triggered by the consumption and subsequent replenishment of GTP. These results show that the resact-induced rise in pHi is not an obligatory step in sperm chemotactic signaling. A rise in pHi is also not required for peptide-induced Ca2+ entry into sperm of the sea urchin Strongylocentrotus purpuratus. Speract, a peptide of S. purpuratus may act as a chemoattractant as well or may serve functions other than chemotaxis.

Photoelectric Potential from Photoreceptor Cells in Ventral Eye of Limulus

Intense colored light from a gas laser evokes a photoelectric potential in the photoreceptor cells of the ventral eye of Limulus. This potential has two components, both of which have the action spectrum of a rhodopsin with an absorbancy maximum at 530 nm. The evidence is consistent with the hypothesis that the photoelectric potential arises directly from the orderly array of rhodopsin molecules which are an integral part of the photoreceptor cell membrane.

Ion fluxes in photoreception in Limulus polyphemus ventral eye. I. The response of potassium efflux to light.

Abstract 1. 1. Radioactive tracers were used to study the effect of light on the efflux of K + from Limulus polyphemus ventral eye. Eyes were labeled in artificial sea water containing 42 KCl and then perfused with non-radioactive artificial sea water. 2. 2. After the first 2 min of perfusion, 42 K in the eye decreased with time as the sum of two exponential terms of half-times 0.3 and 2 h. 3. 3. Illumination of an eye labeled with 42 K caused an abrupt increase in the rate of release of 42 K. The additional 42 K released in response to light comes from the photoreceptor cell bodies rather than from the axons. 4. 4. The proportional increase in efflux rate upon illumination did not vary with the length of time that an eye was perfused. This suggests that the two exponential terms above reflect two classes of photoreceptor cells rather than photoreceptor cells and glial cells. 5. 5. By collection of perfusate at intervals of less than one second, it was shown that the instantaneous rate of 42 K release increases up to 20-fold at the onset of illumination. With continuous illumination, the rate remains above the dark rate, but by less than a factor of 2. Thus, the response of potassium efflux to light has transient and steady phases like the receptor potential. The increased driving force on internal potassium resulting from the receptor potential is not sufficient to explain the flux changes, if one makes the constant field assumption for the photoreceptor membrane. 6. 6. A double label method with 42 K and 42 K was used to show that the average rate of influx over a 10-min period is not affected by illumination. 7. 7. In eyes cooled in the dark to 2–3 °C, light still elicited an increase in the rate of 42 K release and large, prolonged receptor potentials. Thus, the photoreception mechanism of the ventral eye is not, as has been previously suggested, inactive at low temperatures.