| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
1 The Graduate Center, CUNY, New York, NY
2 Hunter College, CUNY, New York, NY
* Corresponding author: rchappell{at}gc.cuny.edu
A decade ago, Wu et al. reported evidence of a dense band of ionic zinc in the region of the photoreceptor terminals of the salamander retina (1). They speculated that zinc may play a neuromodulatory role in the outer retina, including possible feedback onto photoreceptors to down-regulate transmitter release, as well as feed-forward onto second order cells. Subsequently, a high ionic zinc concentration was identified in a similar region near the base of the photoreceptors in the all-rod retina of the skate (2). In addition, Ugarte and Osborne (3) have reported recently that a dense band of ionic zinc, located in the photoreceptor region of the light-adapted rat retina, is redistributed under dark-adapted conditions.
Zinc has been known to affect the response of receptors on a number of retinal cell types to various neurotransmitters (4). In the skate, zinc regulates both GABA (2) and glutamate (5) receptors of isolated cells. Furthermore, the zinc chelator histidine (an amino acid endogenous in the retina, where it may play various roles in cell metabolism and disease (6,7,8,9,10)) has been shown to enhance the size of the b-wave of the electroretinogram (ERG) of the skate (5) and zebrafish (11). Histidine can also increase the membrane currents recorded postsynaptically from horizontal cells during voltage-clamp in the skate retinal slice preparation (12). The effects of histidine support the notion that endogenous zinc may be playing a role in the physiological response of the retina to light.
Recent studies of the effects of histidine on the zebrafish ERG have demonstrated an increase in sensitivity of its mixed rod-cone retina in the presence of a zinc chelator (11). Here, we report studies of the effect of histidine on the retina of the skate, Raja erinacea, indicating that application of this zinc chelator enhances the sensitivity of the b-wave of the ERG in an all-rod retina.
Skates were obtained through the Marine Resources Center of the Marine Biological Laboratory (Woods Hole, MA). The animals were allowed to dark-adapt for at least one hour prior to an experiment. After approved euthanasia, the eyes were enucleated and dissected under dim red light. The anterior portion of the eye, including cornea and lens, was removed; and the remaining retinal eyecup preparation was used for ERG recordings. Eyecups were placed into a chamber over a silver chloride reference electrode within a Faraday cage. The active silver chloride electrode was connected to superfusion solutions in the eyecup via a glass capillary containing Ringer/agar. ERG responses to increasing intensities of illumination were recorded while the preparation was superfused (
0.5 ml/min) with skate-modified Ringer’s solution (2) alone, or to which 200 µM picrotoxin (to block GABAergic receptors in the retina known to be zinc-sensitive (2)), and then 100 µM histidine plus 200 µM picrotoxin had been added. We had found that responses in picrotoxin did not increase after the first 10 min, but that ERG responses in histidine could continue to increase for up to 30 min. Therefore, the preparation was kept in the dark for 30 min in Ringer, at least 10 min in picrotoxin, and at least 30 min in histidine plus picrotoxin prior to recording an intensity-response series. One-second flashes were used to elicit ERG responses from which b-wave amplitudes were measured. The intensity of the unattenuated beam (Log I = 0) was 260 µW/cm2.
Individual ERG responses recorded from one skate eyecup preparation in response to stimulation at an intensity of Log I = -3.0 are shown in Figure 1A. Note that in addition to increased b-wave amplitude observed when picrotoxin, or histidine plus picrotoxin, were applied, an increased a-wave as well as a more prominent OFF component were often observed, as described previously by Chappell and Rosenstein (13) using picrotoxin alone.
|
) for the intensity-response curve obtained in Ringer. The curves plotted with the intensity-response data in Figure 1D are the best fit of the data in Ringer, picrotoxin, and histidine plus picrotoxin, respectively, to the Naka-Rushton relation, using the least-squares approximation in Origin. Using this approach, we could obtain the value of the intensity corresponding to the half amplitude () of the Naka-Rushton curves, as well as the Vmax for each of these curves. The values of Vmax obtained increased from 0.95 in Ringer, to 1.09 in 200 µM picrotoxin, and 1.4 in 100 µM histidine plus 200 µM picrotoxin, while the values of shifted from a Log I of -4.3 to -4.52 to -5.13, respectively.
The shift of toward dimmer intensities represents an increase in sensitivity of 0.8 log units (roughly a factor of 6) in the presence of histidine. Such an increase in sensitivity is consistent with the feedback model proposed by Wu et al. (1), in which they suggest that Zn2+ may feed back onto photoreceptor terminals to reduce Ca2+ entry and thereby reduce vesicular transmitter release from the photoreceptor terminals. It would also be consistent with a reduced inhibition of glutamate receptors on second-order retinal neurons (5), or possibly even an action of zinc on hemichannels (15), since hemichannels have been suggested as a means of feedback from horizontal cells onto cones in the carp retina, where the hemichannel blocker cambenoxolone has been shown to alter feedback-mediated responses (16). Whatever mechanism is involved, the evidence that removal of extracellular zinc by chelation alters the sensitivity of the physiological response to light in an all-rod retina suggests that zinc may be playing an important role as a neuromodulator of rod afferent pathways in the vertebrate retina. Supported by PSC/CUNY grant 65711-0034.
Literature Cited
This article has been cited by other articles:
|
|
 |
|
  R. L. Chappell, H. Qian, J. Zakevicius, and H. Ripps Histidine Suppresses Zinc Modulation of Connexin Hemichannels Biol. Bull., December 1, 2004; 207(3): 188 - 190. [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
) of these curves determined in Ringer, 200 µM picrotoxin, and in 100 µM histidine plus 200 µM picrotoxin were -4.3, -4.5, and -5.1, respectively. Thus, in addition to a 50% increase in Vmax,