Properties of GABA-activated whole-cell currents in bipolar cells of the rat retina

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Abstract
This paper describes experiments on GABA-activated whole-cell membrane currents in bipolar cells freshly isolated from the adult rat retina. The main goal was to determine whether bipolar cell responses to GABA could be resolved in terms of mediation by the GABAA receptor, the GABAB receptor, or both. Bipolar cells were isolated by gentle enzymatic dissociation and identified by their distinct morphology. GABA agonists and antagonists were applied focally by pressure and the resultant currents were recorded under whole-cell voltage clamp. In all bipolar cells tested, GABA (0.1–100 μM) induced a monophasic response associated with a conductance increase (IGABA). The shift in reversal potential for IGABA as a function of pipet [CI] paralleled that predicted based on the Nernst equation for Cl−. IGABA was mimicked by muscimol (5–20 μM) and antagonized by bicuculline (20–100 μM). Baclofen (0.1–1.0 mM) produced no apparent conductance change. “Hot spots” of sensitivity to GABA which might be associated with regions of synaptic contact were not found; both the soma and processes of all bipolar cells were responsive to focally applied GABA. Furthermore, all bipolar cells tested responded to glycine.In conclusion, we have established the presence of GABAA receptors on rat retinal bipolar cells. Our data suggest further that these cells lack GABAB receptors. Finally, our observation that bipolar cells in the rat retina are relatively homogeneous in terms of their sensitivity to GABA and glycine lead us to postulate that the functional significance of the presence of receptors and their distribution on a neuron may be dictated more by the topography of the presynaptic inputs than by its inherent chemosensitivity.