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

The present experiments compared the noradrenaline and behavioural responses of inbred Maudsley reactive (MR) and non-reactive (MNRA) rats when they are exposed to the light or dark arena of a light/dark shuttle-box. Behavioural 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 efflux 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 rats was maintained throughout exposure to the test zone. Strain differences in activity/visit and time/visit were evident but it was not possible to discern whether this could be attributed to the strain difference in the hypothalamic noradrenaline response. Nevertheless, it remains possible that, by comparison with MR rats, the prolonged noradrenaline response in the hypothalamus of MNRA rats could contribute to their well-documented, greater resistance to aversive environmental stimuli.     

Effects of stress and chronic propranolol infusion on cardiac beta-receptor function in the Maudsley Reactive and Non-Reactive rat strains

The effects of beta-adrenoceptor blockade on left ventricular beta-receptor characteristics were evaluated in the Maudsley Reactive (MR/Har) and Non-Reactive (MNRA/Har) rat strains. After propranolol infusion for one week administered by subcutaneously implanted osmotic minipumps, differences in beta-adrenoceptor-agonist interactions between strains were assessed by comparison of isoproterenol competition for [125I]iodocyanopindolol (ICYP) binding sites in crude left ventricular homogenate preparations from both strains. Propranolol exposure had no significant effect on the binding parameters investigated (Bmax, Kd, % receptors in high affinity state, IC50). In contrast to previous observations in naive animals, these experiments revealed no strain differences in the sham-implanted Maudsleys in any of the left ventricular beta-adrenoceptor parameters investigated. We hypothesize that the surgical procedures and/or presence of the plastic inserts acted as stressors in the Maudsley strains and thus altered beta-adrenoceptor function. This hypothesis is supported by the observation that exposure to another stress (footshock) also eliminated strain differences in ventricular beta-receptor binding variables.