This study tested the hypothesis that cortical noradrenaline (NA) depletion induced by 6-hydroxydopamine (6-OHDA) widens attentional span, impairing the acquisition of conditioning to an explicit stimulus while enhancing conditioning to contextual stimuli. Sham-operated and NA-depleted rats were exposed to pairings of an auditory (clicker) CS and (footshock) US in a distinctive environment. Half of the lesioned and half of the control animals were trained with a short trace interval between presentations of clicker and shock, and half with a long trace. Associative learning theory predicts that a long trace interval should bias intact animals towards stronger contextual conditioning and, in contrast, a short trace interval should bias controls towards stronger CS conditioning. During testing, NA-depleted animals showed impaired fear conditioning to explicit cues, compared with controls, indicated by their reduced suppression of drinking when the CS was introduced into a separate, lick-operant chamber. In contrast, the same animals exhibited enhanced fear of contextual cues, as shown by their greater preference for a "safe" environment over the one in which they were shocked. The behavioral and plasma corticosterone responses of individual animals to the CS were positively correlated in both the lesion and sham groups. Corticosterone levels corroborated the impairment in CS conditioning caused by the lesion. In contrast, behavioral and corticosterone responses to contextual stimuli were not significantly correlated in either group, and there was no enhancement of the corticosterone response to contextual stimuli in the lesioned animals.
Mendeley saves you time finding and organizing research
Choose a citation style from the tabs below