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250 EFFECTS OF STRESSof the fibers that connect the prefrontal cortex with the amygdala (Correll et al.,2005). The lesion of these afferents resulted in a substantial decrease in centralmedial nucleus neuron activity; there was a significant decrease in both the baselinefiring rate and in the population of active neurons. The neurons in the central medialnucleus typically exhibited on average a small decrease in firing rate upon presentationof a footshock. Moreover, this response showed accommodation withrepeated footshocks, returning to baseline after four repetitions of the shock.However, after lesion of the prefrontal cortical afferents, there was a dramaticchange in response: The neurons showed a significant footshock-evoked increasein activity, and there was no accommodation to repeated footshock presentation.Therefore, in the control rat, the prefrontal cortex provides a potent suppressionof baseline activity, as well as a strong suppression in the response of this regionto acute noxious stimuli.Conditioned Responses Within the Amygdala-PrefrontalCortical CircuitIn addition to short-term responses evoked by noxious or threatening stimuli, theamygdala circuit also shows longer-term plasticity with repeated presentation of astimulus. Moreover, this plasticity is dependent on the nature of the stimulus presented.Thus, presentation of an odor to the nose of an anesthetized rat will cause anincrease in excitatory synaptic potentials (i.e., EPSPs) within the lateral amygdalanucleus. However, if the odor is presented repeatedly, the amplitude of the EPSPsoccurring during the odor presentation will progressively decrease, and after 5-8presentations actually fall below the spontaneous nonodor baseline (Rosenkranz &Grace, 2002b). This would be a type of habituation, in which the neuron has “learned”that the odor is benign and therefore the response amplitude decreases. A footshock,even in an anesthetized rat, typically evokes a much higher amplitude response,consisting of a membrane depolarization and spike discharge. If the footshock isdelivered at the same time as a novel odor, after several pairings the odor itself beginsto elicit a high-amplitude evoked response when presented alone. Therefore,this would be a neuronal equivalent of a “fear conditioning” paradigm within theamygdala, in which the odor has taken on the activational properties of the footshockwith which it was paired. Note that this conditioned response can take place evenwhen the entire procedure is done in the anesthetized rat. On the other hand, if thepairing is done while the animal is awake, leading to behavioral correlates of a fearresponse (i.e., freezing, decreased exploration) when the odor is presented alone,subsequent electrophysiological recordings from the anesthetized rat show that theamplitude of the response to the conditioned odor versus the nonpaired odor is significantlygreater as compared to when the entire procedure takes place in the anesthetizedrat (Rosenkranz & Grace, 2002b; Rosenkranz et al., 2003).