1. Intracellular recordings were made from cones and horizontal cells of the isolated carp retina and the mechanisms of synaptic transmission from cones of horizontal cells were studied by changing the ionic composition of the external medium. 2. Cones were depolarized and their light responses enhanced in low Ca high‐Mg medium. In the same medium, in which chemical transmission is suposed to be blocked, horizontal cells were hyperpolarized and their light responses disappeared. 3. When the synaptic input was removed, the membrane potential of horizontal cells agreed well with Ek. 4. In Na‐free medium both cones and horizontal cells were hyperpolarized and response disappeared. On reaplication of normal Ringer, horizontal cells showed a trenaient membrane potential reversal (inside positive), indicating that the horizontal cell has a high Na permeability under the influence of the endogenous transmitter. 5. Application of La produced little change in cones, while it strongly depolarized horizontal cells...
Recordings of light-evoked changes in extracellular K+ concentration (A[K+]o) were obtained in the retinas of frog and mudpuppy . In eyecup preparations, various recording approaches were used and provided evidence for a K increase near the outer plexiform layer (distal K increase). This distal K increase could be pharmacologically dissociated from the well-known, large K increase in the proximal retina by the application of ethanol and -y-aminobutyric acid . The distal K increase also often showed surround antagonism . A retinal slice preparation was used to permit electrode placement into the desired retinal layers under direct visual control and without the risk of electrode damage to adjacent layers . In the slice, a distinct distal K increase was found in the outer plexiform layer, in addition to the prominent K increase in the inner plexiform layer. Compared with eyecups, only weak K increases were found in the nuclear layers of the slice. This suggests that the K responses observed in the nuclear layers of eyecups may be generated by K' diffusing along the electrode track from the plexiform layers. In the context of current models of ERG b-wave generation, the magnitude of the recorded distal K increase, compared with the proximal K increase, seems too small to give rise to the bwave . However, the distal K increase may be differentially depressed by electrode dead space. It is also possible that if certain aspects of the models of bwave generation were modified, then the observed distal K increase could give rise to the b-wave .
The most important observations in the present study can be summarized as follows: 1. In horizontal and bipolar cells, blocking chemical transmission caused membrane potential changes of the same polarity as the responses to illumination in each cell type. 2. Acceleration of transmitter release from the receptor terminals depolarized the horizontal cells, which is the opposite polarity to the light response. 3. Stimulation by transretinal current flowing from receptor side to vitreous side depolarized the receptor terminals and triggered transmitter release, which in turn evoked transient postsynaptic potentials in horizontal and bipolar cells. The polarity of the postsynaptic responses was the opposite to that of the light-evoked responses in each type of cell. 4. Hyperpolarization of the receptor terminals by the current of relatively long duration flowing from vitreous side to receptor side reduced the transmitter release, as demonstrated by the horizontal cell hyperpolarization. 5. From these observations, it is inferred that the receptors release the transmitter in the dark and that the transmission depolarizes horizontal and off-center bipolar cells by increasing membrane permeability chiefly to Na+ and hyperpolarizes the on-center bipolar cell by decreasing membrane permeability to Na+.
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