DIFFERENTIAL-EFFECTS OF SOCIAL STRESS ON CENTRAL SEROTONERGIC ACTIVITY AND EMOTIONAL REACTIVITY IN LEWIS AND SPONTANEOUSLY HYPERTENSIVE RATS

Social stress by repeated defeat has been shown to be endowed with neu roendocrine and behavioural effects that render this stress model usef ul to identify adaptive mechanisms. Among these mechanisms, those rela ted to central serotonergic systems (e.g., hippocampal 5-HT1A receptor s, cortical 5-HT2A receptors) have been particularly underlined. Nonet heless, how (i) the neuroendocrine and behavioural effects of social s tress are affected by the genetic status of the animal, and (ii) this status affects the relationships between central serotonergic systems and adaptive processes has not been studied so far. The present study has thus analysed the effects of repealed defeat (once a day for seven days) by Long-Evans resident rats upon the psychoneuroendocrine profi le of Lewis rats and spontaneously hypertensive rats previously charac terized for their contrasting social and anxiety-related behaviours. R epeated defeat decreased in a time-dependent manner, body weight growt h and food intake in both strains, these decreases being, however, mor e severe and longer lasting in Lewis rats. This strain-dependent diffe rence could not be accounted for by differences in physical contacts w ith the resident rats as the number of attacks and their latency throu ghout the stress period were similar between spontaneously hypertensiv e and Lewis rats. When exposed to an elevated plus-maze test of anxiet y, the unstressed Lewis rats entered less the open arms than their spo ntaneously hypertensive counterparts, thus confirming that Lewis rats are more anxious than spontaneously hypertensive rats. This difference was amplified by stress as the latter increased anxiety-related behav iours in Lewis rats only. These strain-and stress-related differences were associated with differences in locomotor activity, this being inc reased in unstressed Lewis compared with spontaneously hypertensive ra ts; moreover, stress triggered hypolocomotion in the former but not th e latter strain. Lastly, in the forced swimming lest, Lewis rats,spent more time immobile than spontaneously hypertensive rats with stress i ncreasing immobility in a strain-independent manner. Beside the aforem entioned metabolic changes, the activity of the hypothalamo-pituitary- adrenal axis was slightly stimulated in a strain-independent manner by the stressor, as assessed by increased corticosterone levels and adre nal weights, and decreased thymus weights. In Lewis, but not in sponta neously hypertensive rats, midbrain serotonin metabolism was increased by stress, a difference associated with an increased Bmax value of co rtical [H-3]ketanserin binding at 5-HT2A receptors. On the other hand, the Bmax value of hippocampal [H-3]8-hydroxy-2-(di-n-propylamino)tetr alin binding at 5-HT1A receptors was decreased by stress, this reducti on being amplified in spontaneously hypertensive compared with Lewis r ats. This study shows that the psychoneuroendocrine responses to socia l stress may have a genetic origin, and that the use of socially stres sed Lewis and spontaneously hypertensive rats may provide an important paradigm to study adaptive processes. However, whether the aforementi oned strain-dependent differences in central serotonergic systems (par tly or totally) underlie the distinct profiles of emotivity measured i n spontaneously hypertensive and Lewis rats, is discussed in the conte xt of the relationships between serotonergic systems and behavioural r esponses to novel environments. (C) 1997 IBRO. Published by Elsevier S cience Ltd.