Telencephalic but not diencephalic noradrenaline depletion enhances behavioural but not endocrine measures of fear conditioning to contextual stimuli

Three experiments investigated the effects of primarily cortical or hypothalamic noradrenaline depletion on aversive conditioning of explicit and contextual stimuli in rats. In Expt. 1, two groups of rats were trained to respond under a variable interval schedule for food reward. One group of rats subsequently received injections of 6-hydroxydopamine into the dorsal noradrenergic bundle, resulting in profound depletion of cortical noradrenaline; the second group received vehicle injections. All rats were exposed to 5 pairings of an auditory stimulus (CS) and footshock (UCS) in a distinctive environment (the dark chamber of a place preference apparatus). During testing in a separate, neutral environment, DNAB-lesioned rats suppressed responding for food reward, in the presence of the aversive CS, to a greater degree than controls. Lesioned rats also showed a greater aversion to the distinctive environment in which they were shocked. In contrast, plasma corticosterone concentrations, measured immediately following each of these behavioural tests, revealed no differences between DNAB-lesioned and control rats. Expt. 2 showed that the DNAB lesion did not affect habituation to the light chamber of the place preference apparatus used in Expt. 1. Expt. 3 showed that 6-OHDA injection into the ventral noradrenergic bundle component of the central tegmental tract, which damages primarily the noradrenergic innervation of hypothalamus, had no effect on either behavioural or endocrine responses to conditioned aversive, explicit or contextual cues. The results are discussed in relation to other reports of the effects of DNAB lesions on simple associative learning in an aversive context.