Auditory fear conditioning increases CS-elicited spike firing in lateral amygdala neurons even after extensive overtraining

We examined the influence of extensive overtraining (75 trials) of auditory fear conditioning on the expression of conditional stimulus (CS)-elicited spike firing in lateral amygdala (LA) neurons. Single units were recorded from chronic multichannel electrodes implanted in the LA of conscious and freely moving rats. In sequential training sessions, the rats received either five or 70 fear conditioning trials, which consisted of a white-noise CS and a coterminating footshock unconditional stimulus (US). Unpaired (sensitization) controls received the same number of trials, but the CS and US were explicitly unpaired. Paired CS-US presentations yielded robust increases in CS-elicited spike firing in LA neurons after both five and 70 conditioning trials, and the magnitude of the spike firing increases was correlated with the expression of conditional freezing to the CS. After 75 training trials, maximal conditioning-related increases in LA firing were exhibited within 20 ms of CS onset, indicating that this increase is mediated by direct thalamo-amygdala projections. There was no significant increase in CS-elicited spike firing or freezing behaviour in the unpaired group. These results complement amygdala lesion studies [e.g. Maren, S. (1999a) J. Neurosci., 19, 8696-8703] and support the view that the basolateral complex of the amygdala is involved in the encoding and storage of fear memories even after extensive overtraining.