An iontophoretic study of single somatosensory neurons in rat granular cortex serving the limbs: a laminar analysis of glutamate and acetylcholine effects on receptive-field properties

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Abstract
Glutamate, acetylcholine (ACh), and bicuculline were delivered by ionotophoretic pipettes to the 545 neurons described in the preceding paper. Their response properties were examined to determine the effect of these compounds on the behavior of neurons in rat somatosensory cortex. The responses to glutamate covered a broad range. Some cells were completely depolarized by small amounts of this excitatory amino acid, whereas others were extremely insensitively requiring in excess of 100 nA to be excited. This range of sensitivities was seen throughout all cortical layers. Glutamate was most effective in uncovering new receptive fields or in enhancing preexisting somatic responses in the bottom of layer II/II and in layer IV. Receptive fields uncovered by glutamate had properties comparable to receptive fields observed without drugs. Overall, glutamate enhanced the ability of afferent inputs to drive 39% of the neurons tested. In 61% of the cells tested with glutamate there was no evidence of osmatic input even during excitation with glutamate. Of 50 cells displaying receptive fields, only two were enlarged by treatment with glutamate. For 36 other cells receptive fields of normal dimensions were uncovered during glutamate administration. Bicuculline uncovered most somatic inputs than either glutamate or ACh, leaving only 37% of 86 cells tested without of excitatory inputs from the skin. Bicuculline produced an average receptive-field enlargement of 87 times in 11 of 56 cells tested. This drug acted uniformly throughout the cortical layers. ACh excited 36.9% of the 360 cells tested. Those excited tended to be located in laminae Vb and VIb. The effects of ACh on afferent response properties could not be predicted from its ability to excite a cell. The magnitude of the response to 100 nA of ACh varied with the laminar position of the cell being tested, being weakest in layer II/III and greatest in layer Vb. Overall, 34.2% cells showed changes in afferent drive during ACh treatment. ACh enhanced the responses to somatic stimulation most frequently in laminae IV and V. Of the 90 neurons tested for long-term effects, 27% displayed effects of ACh that significantly outlasted the duration of ACh that significantly outlasted the duration of the ACh administration. In 18% of these, changes lasted for > 5 min, sometimes remaining altered for the duration of the time that the cell was studied. These long-term changes in excitability were generally produced by administration of ACh during the time that the cell was excited by glutamate or by somatic stimulation.