Transient and prolonged effects of acetylcholine administered iontophoretically to neurons in cat primary somatosensory cortex

This thesis describes the modulatory effects of acetylcholine (ACh) on neuronal responsiveness in the primary somatosensory cortex of cats. In 52 anesthetized animals, the responses of 440 cells were studied in three series of experiments. In series 1, iontophoretic administration of various doses (7-200 nA) of ACh produced increased firing rates in 16% (33/203) of the neurons tested. Responses to somatic stimuli were modulated by ACh in 29% of the cases. These modulatory effects were predominantly excitatory, with only 5% of the cells being depressed by ACh. In series 2 and 3, 50 nA of ACh applied in the absence of other overt stimula altered the responsiveness of a cell to subsequent tactile stimulation or iontophoretically-applied glutamate. This occurred in 21% (13/61) of the cells. In contrast, when 50 nA of ACh was administered simultaneously with glutamate and/or receptive field stimulation, altered responsiveness occurred in 77% (76/99) of the cases. One-third (32/99) of these cells displayed prolonged changes in responsiveness, lasting from 7 min to over 1 h.Cells modulated by ACh were found in all cortical layers. The percentage of cholinoceptive cells was highest in layer VI (78%), and was not less than 50% in any layer. Both long and short-term potentiating effects of ACh on responses to somatic stimulation and responses to glutamate administration could be reversibly blocked by low iontophoretic doses of atropine