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I. Fahrenfort, J. Klooster, T. Sjoerdsma, M. Kamermans; A Role for Extracellular Carbonic Anhydrase in Synaptic Transmission Between Cones and Horizontal Cells . Invest. Ophthalmol. Vis. Sci. 2006;47(13):389.
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© ARVO (1962-2015); The Authors (2016-present)
Neuronal activity is known to induce changes in intracellular and extracellular pH, which are governed by the enzyme carbonic anhydrase (CA). In the retina inhibition of both extracellular and intracellular CA have been shown to enhance the slow light–induced changes in extracellular pH (Oakley and Wen, 1989; Borgula et al, 1989; Wolfensberger et al, 1999) . Furthermore, it has been suggested that extracellular pH changes are involved in synaptic transmission (DeVries, 2001; Hirasawa and Kaneko, 2003; Vessey et al, 2005). We studied the involvement of extracellular CA in synaptic transmission in the retina.
Goldfish retinas were fixed in 4% paraformaldehyde and incubated in a rabbit antibody against CA 14. The distribution of the immunolabel was studied using conventional LM immunohistofluorescence. Cone calcium currents and feedback responses were measured using the patch clamp technique. HC light–responses were measured using the current clamp technique.
Extracellular CA 14 immunolabel is abundantly present throughout the retina as has been reported for rat retina (Nagelhus et al, 2005). Its presence in the OPL suggesting a role for extracellular CA in synaptic transmission between cones and horizontal cells. Inhibiting extracellular CA with the relatively membrane–impermeant inhibitor, benzolamide reduced synaptic transmission between cones and horizontal cells. 500 uM benzolamide hyperpolarized horizontal cells with 6.7 +/– 1.3 mV and reduced the amplitude of their light responses with 34 +/– 7 % (n=4). The amplitude of the feedback–mediated responses were relatively more affected. Addition of benzolamide reduced the feedback–mediated secondary depolarization with 94 +/– 6% (n=4). In cones, benzolamide reduced feedback–mediated responses without significantly reducing the amplitude of Ica (n=6)
Our results show that blocking extracellular CA reduces synaptic transmission between cones and horizontal cells. The presence of extracellular CA in the extracellular space suggests that the pH is well buffered, arguing against a role for protons as the neurotransmitter in feedback from horizontal cells to cones.
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