Role of GABAergic inhibition in shaping the spatial frequency tuning of neurons and its contrast dependency in the dorsal lateral geniculate nucleus of cat

To understand how GABAergic inhibition contributes to the elaboration of spatial frequency (SF) tuning of neurons in the lateral geniculate nucleus, we examined the effects of the microiontophoretic administration of bicuculline methiodide (BIC) and gabazine, both antagonists of GABAA receptors, on visual responses to grating stimuli with high or low contrasts. BIC administration changed the shape of the SF tuning curve of the spike response from band-pass to low-pass. We took the tuning curve obtained under the BIC condition as an estimated excitatory contribution to the control tuning curve and then estimated the difference between tuning curves recorded with and without BIC as the tuning curve of the estimated GABAergic inhibitory contribution. The SF tuning profile of estimated inhibition (Estimated-Inh) varied widely from cell to cell, as did estimated excitation (Estimated-Ex). Nonetheless, the relationship that Estimated-Inh exhibited more low-pass tuning than did Estimated-Ex was well conserved in the majority of cells, and the relationship refined the SF tuning of Estimated-Ex toward the band-pass tuning of the geniculate output. Lowering the stimulus contrast decreased the response magnitude, but did not change the degree of band-pass tuning. The GABAergic refinement of the SF tuning was also observed at low stimulus contrast, but was weaker than at high contrast, suggesting that GABAergic inhibition is regulated in coordination with excitatory inputs to keep the degree of the band-pass tuning constant. We therefore concluded that the degree of band-pass tuning is conserved contrast invariantly in the lateral geniculate nucleus on the basis of the dynamic regulatory action of GABAergic inhibition.