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Department of Visual Science, University College London, UK. K.Binns@ucl.ac.uk

1. The superficial grey layer of the superior colliculus (SGS) contains a high proportion of GABAergic inhibitory neurones. We have investigated the role of GABA receptors in synaptic transmission of aspects of visual activity in the SGS that may be driven by inhibitory mechanisms, such as surround inhibition and response habituation. 2. Multi-barrel glass iontophoretic pipettes were used to record single neuronal activity in the SGS of urethane-anaesthetized rats. Visual stimulation was provided by the display of moving bars and stationary spots of light on a monitor placed in the receptive field. 3. Both ejection of GABA and the GABAB agonist baclofen reduced responses to moving bars (interstimulus intervals > or = 8 s). The effects of GABA were reversed by the GABAA antagonist bicuculline, and the effects of baclofen were antagonized by the GABAB antagonist CGP 35,348. 4. Surround inhibition was estimated by plotting the response to flashed spots of increasing diameter. In controls, expanding the spot diameter beyond the excitatory receptive field caused a decrease in the response. This inhibitory surround was reversibly reduced by bicuculline, but CGP 35,348 had no effect. 5. Response habituation is the progressive reduction in the visual response during repetitive stimulus presentation. In controls, the visual response was reduced to 44 +/- 3% of its initial level when a stimulus (moving bar) was presented 5 times with an interstimulus interval of 0.5 s. During CGP 35,348 ejection, response habituation was reversibly reduced. Bicuculline had no effect on response habituation. 6. The effects of bicuculline on surround inhibition in the superior colliculus are consistent with similar studies in the lateral geniculate nucleus which indicate that GABAA receptors mediate this effect. The function of GABAB receptors in the visual system is less well researched. The reduction of response habituation with CGP 35,348 demonstrates that, at least in the SGS, GABAB receptors have an important role in visual transmission which is distinct from that of GABAA receptors.

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