DIFFERENT ROLES FOR GABA(A) AND GABA(B) RECEPTORS IN VISUAL PROCESSING IN THE RAT SUPERIOR COLLICULUS

1. The superficial grey layer of the superior colliculus (SGS) contain s a high proportion of GABAergic inhibitory neurones. We have investig ated the role of GABA receptors in synaptic transmission of aspects of visual activity in the SGS that may be driven by inhibitory mechanism s, such as surround inhibition and response habituation. 2. Multi-barr el glass iontophoretic pipettes were used to record single neuronal ac tivity in the SGS of urethane-anaesthetized rats. Visual stimulation w as provided by the display of moving bars and stationary spots of ligh t on a monitor placed in the receptive field. 3. Both ejection of GABA and the GABA(B) agonist baclofen reduced responses to moving bars (in terstimulus intervals greater than or equal to 8 s). The effects of GA BA were reversed by the GABA(A) antagonist bicuculline, and the effect s of baclofen were antagonized by the GABA(B) antagonist CGP 35348. 4. Surround inhibition was estimated by plotting the response to flashed spots of increasing diameter. In controls, expanding the spot diamete r beyond the er;citatory receptive field caused a decrease in the resp onse. This inhibitory surround was reversibly reduced by bicuculline, but CGP 35348 had no effect. 5. Response habituation is tile progressi ve reduction in the visual response during repetitive stimulus present ation. In controls, the visual response was reduced to 44 +/- 3 % of i ts initial level when a stimulus (moving bar) was presented 5 times wi th an interstimulus interval of 0.5 s. During CGP 35348 ejection, resp onse habituation was reversibly reduced. Bicuculline had no effect on response habituation. 6. The effects of bicuculline on surround inhibi tion in the superior colliculus are consistent with similar studies in the lateral geniculate nucleus which indicate that GABA(A) receptors mediate this effect. The function of GABA(B) receptors in the visual s ystem is less well researched. The reduction of response habituation w ith CGP 35348 demonstrates that, at least in the SGS, GABA(B) receptor s have an important role in visual transmission which is distinct from that of GABA(A) receptors.