Different roles for GABA_A and GABA_B receptors in visual processing in the rat superior colliculus

Abstract

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. 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. Both ejection of GABA and the GABAB agonist baclofen reduced responses to moving bars (interstimulus intervals ≥ 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 35348. 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 35348 had no effect. 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 35348 ejection, response habituation was reversibly reduced. Bicuculline had no effect on response habituation. 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 35348 demonstrates that, at least in the SGS, GABAB receptors have an important role in visual transmission which is distinct from that of GABAA receptors.

Regulation of Burst Activity through Presynaptic and Postsynaptic GABABReceptors in Mouse Superior Colliculus

“…These GABA B R mechanisms have accounted for response habituation observed in the SC (Binns and Salt, 1997). In addition to this functional role, the electrophysiological characteristics together with dense expression of GABA B Rs in the SC (Bowery et al, 1987;Chu et al, 1990) suggest that such long-lasting, slow responses may contribute to the regulation of burst activity, particularly to the termination of bursts.…”

Investigating TMS–EEG Indices of Long-Interval Intracortical Inhibition at Different Interstimulus Intervals

“…Some important examples of this include the role of GABA in moderating synaptic plasticity [1], in mediating sensory acuity via surround inhibition [2][3][4] and in the generation of cortical oscillatory activity [5]. The functional importance of GABA is further illustrated by observations that GABAergic tone is modified in several different movement pathologies [see ; 6] and psychiatric conditions [7,8].…”

GABAergic mechanisms for shaping transient visual responses in the mouse superior colliculus

“…The mechanisms that make ON responses transient also remain largely unknown. A previous study demonstrated the different roles of GABA A and GABA B receptors on visual information processing, but did not precisely examine the effects of GABA receptor blockade on the nature of transient ON activity (Binns and Salt, 1997). Our previous study in slice preparations demonstrated that GABAergic sSC interneurons regulate the duration of discharges through a mechanism of feed-forward inhibition mediated by both GABA A and GABA B receptors in non-GABAergic and GABAergic sSC neurons (Kaneda et al, 2008), suggesting that GABAergic neurons contribute to transient activity.…”