Autocorrelogram sorting: A novel method for evaluating negative-feedback regulation of spike firing

Spontaneously firing units were recorded extracellularly from the hippocampus of anesthetized rats, and autocorrelograms were conventionally constructed. These conventional autocorrelograms were sorted into frequency-specific autocorrelograms according to the instantaneous firing frequency of the spike at Δt = 0, which was calculated based on the interval between the spike at Δt = 0 and the preceding spike. In this fashion, we found that autocorrelogram values were negatively correlated with instantaneous firing frequency during the 4–12 ms post-spike time period. The negative correlation during the 4–6 ms post-spike period could not have been due to the refractory period or GABAergic inhibition, and thus represented a third type of feedback regulation of spike firing completed within single neurons. Application of acetylcholine significantly enhanced this feedback regulation. Our ‘autocorrelogram sorting’ method thus proved to be successful in detecting cholinergically enhanced feedback regulation of spike firing intrinsic to single neurons.