Analysis of the site of CN action.A, Plots of afferent volley (peak-to-peak amplitude) as function of stimulus duration show that neither CN alone (top) nor CN in presence of 8-SPT (bottom) affects the afferent volley. B, The efficacy of synaptic transmission was examined by plotting the initial rate of rise of EPSPs as function of the size of the afferent volley. Top, CN reduces the slope of the plot and therefore the efficacy of synaptic transmission. Bottom, This effect was nearly abolished by 8-SPT. C, Plots of population spike as function of initial slope of EPSP (E–S coupling).Top, For a given EPSP, CN decreases corresponding population spike; reduced coupling indicates that CN also affects postsynaptic mechanisms. Bottom, 8-SPT also attenuated this CN action. A and B were recorded from CA1 stratum radiatum of same slice. Data in C were recorded from CA1 pyramidal layer of another slice.