The Correlation Between Orientation Selectivity And Inhibition In Primary Visual Cortex Of Cat

The great majority of neurons in the primary visual cortex are tuned for local attributes of the image,such as stimulus orientation and spatial frequency.The discovery of orientation selectivity of neurons in primary visual cortex is recognized as a key development in visual neuroscience.The relative contributions of excitation and inhibition in generation orientation selectivity serve as a model problem for how synaptic circuitry of the cortex performs a complex computation.However,the role of inhibition in establishing sharp orientation selectivity is still debated,and the relationship between strength of inhibition and orientation selectivity is unclear.To investigate this relationship,we electrophoretically applied the inhibitory transmitter GABA and the GABA_A antagonist bicuculline on the same individual area 17 neurons in anesthetized cats.Neurons were classified as weakly orientation-selective,moderately orientation-selective,or strongly orientation-selective,according to the values of an orientation bias index.Orientation bias,half-width of the tuning curve at half-height and an orientation-specificity index were compared with or without GABA and bicuculline administration.GABA improved orientation selectivity with the greatest effects on weakly orientation-selective cells,smaller effects on moderately orientation-selective cells, and minimal effects on strongly orientation-selective cells;bicuculline diminished orientation selectivity the most on moderately and strongly orientation-selective cells, with minimal effects on weakly orientation-selective cells;the changes of orientation selectivity during GABA and bicuculline administration were due to changes of both bandwidth and orthogonal/optimal responses.We also found that orientation selectivity correlated with the level of neuron's spontaneous activity,cells with higher orientation selectivity had lower spontaneous rates.These results suggest that cells with weak orientation selectivity may be accounted for by insufficient inhibition,and those with strong selectivity tend to receive strong inhibition.Chronic morphine exposure could affect the visual response properties of area 17 neurons in cat,and lead to decreased orientation selectivity accompanied by the increase of spontaneous activity.Available evidences suggested that the morphine-derived degradation of GABAergic inhibition in visual system may be an important mechanism underlied the function degradation of V1 neurons.If GABA-mediated inhibition degrades during chronic morphine exposure,according our results,then the application of GABA on individual area 17 cells can improve visual function in chronic morphine exposure cats.To test this hypothesis,we have now studied the effects of electrophoretic application of GABA and bicuculline on the orientation selectivity and spontaneous activity in chronic morphine exposure cats. Bicuculline exerted a much weaker effect on neuronal responses in chronic morphine exposure than in control cats,confirming a degradation of GABA-mediated inhibition. On the other hand,the administration of GABA resulted in improved orientation selectivity,many treated cells in area 17 of chronic morphine exposure cats displayed responses typical of control cells.The body of evidence that is presented here significantly advances our knowledge of the relationship between inhibition strength and orientation selectivity. Cells with weak orientation selectivity may be accounted for by insufficient inhibition, and those with strong selectivity tend to received strong inhibition.Administration of GABA could result in improved orientation selectivity in chronic morphine exposure cats.The present results have important implications for the treatment of the function declines during chronic morphine exposure.