80 VISION 



Pharmacology— The current hypothesis concerning the mechanism of 

 synaptic transmission between photoreceptors and second-order retinal 

 neurons is that a chemical transmitter is continuously released in the dark, 

 thus depolarizing the horizontal cells. Light causes a reduction in the release 

 of the transmitter, which in turn hyperpolarizes the membrane. Two impor- 

 tant questions are (1) What is the evidence supporting this hypothesis? and 

 (2) What chemical transmitters are involved? 



Dowling and Ripps (1973) tested this hypothesis by bathing the skate 

 retina in Ringer's solution containing magnesium. It is known that when Mg 

 is applied in high concentrations to chemically mediated synapses, trans- 

 mission is blocked (Katz and Miledi 1967). Fifteen to 25 s after placing a drop 

 of 100 mJH MgCl on the skate retina, the horizontal cell membrane hyper- 

 polarized while light-evoked ERGs and S-potentials decreased in amplitude. 

 After 3 min the membrane potential had dropped to —60 mV and neither 

 ERGs nor S-potentials could be evoked. The effect of Mg is reminiscent of 

 that of intense light stimulation and agrees with the supposition that a 

 transmitter is continuously released in the dark, thus maintaining the 

 horizontal cells in a partly depolarized state. The effects of Mg Ringer's 

 lasted for about 30 min, after which membrane potentials returned to their 

 normal levels. 



After application of Mg Ringer's, the b-wave of the ERG disappeared, 

 leaving the a- and c-waves. As stated, the leading edge of the a- wave orig- 

 inates at the photoreceptors while the c-wave derives from the pigment 

 epithelium. Thus the Mg Ringer's affected those synapses proximal to the 

 photoreceptors. 



Detailed examination of the waveform showed that the rise time of the 

 leading edge of the a- wave was reduced. Since the b-wave seems to originate 

 in glial cells (Miller and Dowling 1970), its disappearance suggests that the 

 aspartate affects retinal elements internal to the receptors. Intracellular 

 recordings from horizontal cells support this contention, since application of 

 aspartate to the retina depolarizes the membrane (horizontal cell). This is 

 accompanied by an increasing hyperpolarization to light, which diminishes 

 and eventually disappears. When both b-wave and S-potential were moni- 

 tored simultaneously, their decrease in amplitude followed the same time 

 course. 



Ripps et al. (1976), using horseradish peroxidase (HRP), have provided 

 further evidence that continuous release of transmitter in the dark is reduced 

 when the receptors are stimulated by light. The dark-adapted retina of the 

 skate takes up HRP in the cells and processes of the outer plexiform layer. 

 Upon light adaptation, there was a marked reduction in the amount of HRP 

 found in this layer. Adding Mg also reduces the concentration of HRP in the 

 outer plexiform layer. This is expected, since Dowling and Ripps (1973) 

 showed that Mg blocks transmission between chemically mediated retinal 

 synapses and would therefore also block the uptake of HRP. 



The amount of HRP reaction product was "somewhat greater" in eyecups 

 treated with Na aspartate than in the controls. This is what would be 

 expected of a putative transmitter that depolarizes the membrane. 



