An investigation of the role of the locus coeruleus in anxiety and agonistic behaviour

The possible role of the locus coeruleus system in mediating anxiety responses and in controlling aggressive behaviours was investigated in rats in the following tests: (i) the social interaction anxiety test, (ii) the anxiosoif test and (iii) the home-cage intruder test. Rats with bilateral lesions of the system induced by 6-hydroxydopamine (OHDA) injections anterior and lateral to the locus coeruleus had mean cortical noradrenaline concentrations of 19.7 ng/g compared with a mean concentration of 173 ng/g in controls. Lesioned rats had similar responses to controls in the social interaction test over a range of test conditions, and responded to ACTH as did controls. The mean latency to drinking in the anxiosoif test was similar in lesioned rats and controls. These results confirm earlier findings that locus coeruleus lesioned rats are not impaired in their responses to anxiety provoking stimuli. When tested in pairs in the anxiosoif test lesioned animals showed more pushing, ☐ing, and wrestling than did controls. In the home-cage intruder test locus coeruleus lesioned rats ☐ed more with a heavier dominant male rat (the ‘intruder’), and were significantly less likely to turn on their backs to prevent the intruder from attacking, and more often jumped on the intruder's back. However lesioned rats did not show an increased incidence of severe attacks or biting.

The findings constitute further evidence that the locus coeruleus system is not involved in anxiety mechanisms but may be involved in the control of some components of agonistic behaviour.