Effects of cholinergic depletion on neuron activities in the cat visual cortex

1. Unilateral lesions of the nucleus basalis magnocellularis (nBM), a source of cholinergic projection to the cerebral cortex, were produced by injection of kainic acid in the cat. The lesions caused a significant reduction in density of choline acetyltransferase-immunoreactive terminals in the visual cortex ipsilateral to the lesions. 2. In the primary visual cortex ipsilateral to the lesions [acetylcholine (ACh)-depleted cortex], about half of the cells had weak or undetectable visual responses, whereas in the contralateral visual cortex almost all the cells had normal responsivity. The response selectivity, such as orientation and direction selectivities, of cortical cells was not affected by the depletion of ACh. 3. The microionophoretic application of ACh to cells under observation facilitated visual responses in 83% of the cells recorded from the ACh-depleted cortex, whereas it suppressed the responses in only 9%. The application of a muscarinic antagonist, atropine, to cells in the ACh-depleted cortex was ineffective, suggesting no residual ACh activity. 4. The mean current required to induce facilitation in the cortex ipsilateral to the lesion was significantly smaller than that required in the contralateral cortex and the visual cortex of the normal cat, suggesting a supersensitivity of receptors mediating the effect or a reduction in catabolism of exogenous ACh in the ACh-depleted cortex. 5. More than half of the cells that had been unresponsive to visual stimuli became clearly responsive during the ACh application. The response magnitude of cortical cells, as a whole, increased to the same degree as that observed during the ACh application in the normal cat. 6. In addition to the decrease in the average response magnitude, there was a remarkable variability in responses of cells to motion of the slit from sweep to sweep in the ACh-depleted cortex. The application of ACh to cortical cells decreased the variability of responses and consequently made the responses much more consistent. 7. These results suggest that without ACh supplied from the nBM, most of the cortical neurons could not respond briskly and consistently to excitatory inputs and that exogenously applied ACh could reverse such an impairment of cortical neurons through intact or even supersensitive postsynaptic receptors.