Lesions of the entorhinal cortex disrupt behavioral and neuronal responses to context change during extinction of discriminative avoidance behavior
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Rabbits given either electrolytic lesions of the entorhinal cortex or sham-lesions were trained to prevent a foot-shock by stepping in an activity wheel after one tone, a positive conditioned stimulus (CS+), and to ignore a different tone, a negative conditioned stimulus (CS–). Neuronal activity was recorded simultaneously in the basolateral nucleus of the amygdala, the CA1 cell field of hippocampus, anterior cingulate cortical area 24b and posterior cingulate cortical area 29c/d. The activity of neurons in the entorhinal cortex was recorded in the controls. Acquisition of conditioned avoidance responses (CRs) was not affected by lesions of the entorhinal cortex. Discriminative neuronal activity (greater neuronal responses to the CS+ than to the CS–) during CR acquisition was significantly enhanced in hippocampal area CA1 and attenuated in the basolateral amygdala in rabbits with lesions. Following acquisition to a criterion, two counterbalanced extinction tests were administered, one in the original context and the other in the presence of novel contextual stimuli. CR frequency was significantly reduced in controls but not in rabbits with lesions, during extinction with novel contextual stimuli, relative to performance in the original context. The rabbits with lesions also showed fewer inter-trial responses than controls during extinction in the original context but inter-trial response frequency in rabbits with lesions did not differ from the frequency in controls during extinction in the novel context. Neurons in the basolateral amygdala in controls showed discriminative activity during extinction in the original context but not in the novel context. Amygdalar neurons in the rabbits with lesions did not show discriminative activity during extinction in either context. Posterior cingulate cortical neurons in control rabbits did not show discriminative activity during extinction in the original context but these neurons exhibited robust discriminative activity in the novel context. Posterior cingulate cortical neurons in rabbits with lesions showed discriminative activity in both extinction sessions. The results indicated that the entorhinal cortex does not play a significant role in the acquisition of discriminative avoidance behavior, under the employed conditions of training. However, the interactions of neurons in the entorhinal cortex, amygdala and cingulate cortex are essential for contextual modulation of CRs during extinction.
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