Diencephalic Noradrenaline Depletion Impairs the Corticosterone Response to Footshock but does not Affect Conditioned Fear

This study tested the hypothesis that hypothalamic noradrenaline (NA) depletion induced by 6‐hydroxydopamine alters neuroendocrine, but not behavioural, responses to aversive stimuli. Sham‐operated and NA depleted rats were exposed to pairings of an auditory (clicker) CS and (footshock) US in a distinctive environment. Subjects were tested for preference of a ‘safe’ environment over the one in which they were shocked, as a measure of effective conditioning to the contextual stimuli present in the distinctive environment. Subjects were also tested, in a separate operant chamber, for the suppression of drinking in the presence of the auditory stimulus, as a measure of effective conditioning to the explicit auditory CS. Blood samples were collected immediately following each phase of the behavioural experiment and were later analysed for plasma Corticosterone concentration. Behavioural and Corticosterone responses of individual control animals to the CS were positively correlated, consistent with previous results. This correlation was not present in the NA depleted group. The lesioned rats also showed a severely attenuated Corticosterone response to the footshock US. By contrast, NA depletion had no effect on any behavioural measure of CS or contextual conditioning. Together with previous experiments, these results suggest that diencephalic NA projections are more likely to mediate neuroendocrine, and coeruleo‐cortical NA projections are more likely to mediate behavioural responses to conditioned and unconditioned aversive stimuli.