Evidence for a nicotinic component to the actions of acetylcholine in cat visual cortex

Radioligand binding assays, receptor autoradiography and iontophoresis have been used to look for evidence of a nicotinic component to the actions of acetylcholine in cat visual cortex. [3H]Nicotine bound to a uniform population of high affinity binding sites in cat primary visual cortex. This binding was inhibited by nicotine agonists and antagonists but not muscarinic antagonists. The concentration of nicotinic binding sites was about 10% of that of muscarinic binding sites measured with [3H]N-methylscopolamine. The muscarinic sites were resolved into M1 and M2 subtypes. Quantitative receptor autoradiography showed that there were muscarinic sites in all layers, although they were least abundant in layer IV of area 17. In contrast, the nicotinic sites were most concentrated in layer IV in area 17. The concentration of this labelling was reduced at the 17/18 border and also at the 18/19 border. Layer I of the cingulate and suprasylvian gyri were also labelled. Electrolytic lesions of the lateral geniculate nucleus (LGN) led to a loss of nicotinic binding sites in layer IV of area 17, indicating that these sites are most likely located on the LGN terminals. Iontophoresis of mecamylamine, a nicotinic antagonist, decreased evoked responses in visual cortex, providing evidence that the [3H]nicotine binding sites are functional receptors and suggesting that the release of acetylcholine onto these receptors on the LGN terminals facilitates the input of visual information into visual cortex.