Differences in central noradrenergic and behavioural responses of Maudsleynon-reactive and Maudsley reactive inbred rats on exposure to an aversive novel environment

The present experiments compared the noradrenaline and behavioural response s of inbred Maudsley reactive (MR) and non-reactive (MNRA) rats when they a re exposed to the light or dark arena of a light/dark shuttle-box. Behaviou ral scores confirmed that both strains of rats perceived the light arena to be more aversive than the dark one. Using in vivo microdialysis, exposure to the light, but not the dark, arena was found to increase noradrenaline e fflux in both the frontal cortex and the hypothalamus of MNRA and MR rats. However, whereas the increase in the frontal cortex of both strains and the hypothalamus of MR rats was transient, the hypothalamic response in MNRA r ats was maintained throughout exposure to the test zone. Strain differences in activity/visit and time/visit were evident but it was not possible to d iscern whether this could be attributed to the strain difference in the hyp othalamic noradrenaline response. Nevertheless, it remains possible that, b y comparison with MR rats, the prolonged noradrenaline response in the hypo thalamus of MNRA rats could contribute to their well-documented, greater re sistance to aversive environmental stimuli.