Journal of Neurophysiology, Vol 66, Issue 6 1993-2001, Copyright © 1991 by APS
Dual effect of glycine on horizontal cells of the tiger salamander retina
S. Borges and M. Wilson
Department of Zoology, University of California, Davis 95616.
1. The effects of glycine on horizontal cells have been examined by
microelectrode recording from superfused retinas isolated from the
salamander. 2. Low concentrations of glycine (less than 50 microM)
hyperpolarized horizontal cells and increased the magnitude of their light
responses. Millimolar concentrations produced the opposite effect of
depolarizing these cells and reducing their light response amplitudes. 3.
In the presence of Co2+ and Mg2+ at concentrations sufficient to suppress
the light response, millimolar glycine still exerted a depolarizing effect
on horizontal cells, implying that this effect was largely a direct one on
horizontal cell membranes. 4. Although both the rod and the cone
contributions to horizontal cell light responses were reduced by millimolar
glycine, rod input was reduced more, suggesting that millimolar glycine may
also exert a presynaptic effect. 5. Strychnine (10 microns) antagonized the
effects of millimolar glycine and, in the absence of exogenously applied
glycine, caused horizontal cells to hyperpolarize and their light responses
to increase in amplitude. This result implies that, in darkness, glycine is
tonically released onto horizontal cells and maintains them in a state of
partial depolarization. 6. The low-concentration effect of glycine was
accompanied by an increased membrane resistance and receptive field size
but no change in the balance of rod and cone input. 7. Low concentrations
of glycine were often seen to cause a speeding of light responses, whereas
high concentrations sometimes caused a slowing of response kinetics.
Response kinetics were found to correlate with horizontal cell dark
membrane potential so that, positive to -30 mV, depolarization slowed
responses whereas kinetics at more negative values were largely independent
of voltage.
