Responsiveness to environmental stimuli after destruction of the locus coeruleus noradrenergic system: A review

The functional significance of the locus coeruleus noradrenergic system (LC NA) is still not fully understood. The aim of our work reviewed below, was to investigate the effects of the selective noradrenergic neurotoxin DSP-4 on behaviours displayed during inter- and intraspecies encounters and in response to stressogenic stimuli on cats and rats. It has been found that the destruction of LC NA did not affect prey killing in either cats or rats. Also, well established dominance-submissive order displayed during predatory competition remained unchanged, except for the situation when the stressful stimulus was presented to a pair of cats. In the presence of such a stimulus the DSP-4-treated cats attacked mice in front of their dominant partners, although they never did so before. In rats treated with DSP-4 a marked decrease in defensive episodes and an increase in offensive episodes was observed during social encounters in resident-intruder paradigm. It was found that these rats spent more time in the stressogenic highly illuminated centre of the open field than did the controls. Biochemical analysis showed a significant reduction of noradrenaline content in the forebrain structures and some changes in serotonin and dopamine levels; the latter are considered as an effect secondary to noradrenaline depletion. In light of other authors' data concerning the role of LC NA in brain and behavioural activity, three possible interpretations of our results are considered: (1) an increase in aggression; (2) fear reduction; (3) inadequate responsiveness to environmental cues.