Abstract: Previous studies have shown that emotional stress induced by randomly giving an empty water bottle to rats during watering periods for 14 consecutive days activated both the hypothalamic-pituitary-adrenal axis and the sympathetic nervous system, leading to the increased blood levels of corticosterone and norepinephrine. It also elicited attacking and exploring behaviors, suppressed the level of specific anti-ovalbumin IgG antibody production which was negatively related to norepinephrine level. However, three day empty bottle presentation had no significant effect on anti-OVA antibody production. These results suggest that the duration of stressors determines the consequences of body reactions induced by stress, at least, humoral immune responses. In order to examine whether the duration of stress exerts an accumulative effect or an adaptive effect on body reactions, the present study was designed to investigate the dynamic changes in behavioral, immune and neuroendocrine responses of rats that were subjected to chronic emotional stress with different durations.
Male Wistar rats were divided into four groups: (1) emotional stress (induced by randomly giving an empty water bottle to rats at set watering time) (n=19); (2) physiological stress (given neither empty water bottles nor water during the same watering periods as for emotional stress group) (n=19); (3) control 1 (allowed free access to water during watering periods)(n=15); and (4) control 2 (given water ad libitum during the whole experimental periods)(n=15). Behavioral changes, humoral immune responses and the reaction of sympathetic nervous system were measured and compared during different days(14，21 and 28 days) of stress. The behaviors observed included attacking, exploring and grooming. Norepinephrin levels in plasma were determined using high-performance liquid chromatography. Serum levels of IgG antibodies to Ovalbumin were determined by enzyme linked immunosorbent assay.
It was found that emotional stress stably induced aggressive behavior of rats over the whole experimental period. Compared with controls, rats subjected to 14，21 or 28 days of stress showed decrease in the level of specific anti-OVA antibody to a similar degree. Both 14-day and 28-day stress activated the sympathetic nervous system, leading to the increased blood levels of norepinephrine. However, a decline of norepinephrine level was also obvious from 14-day to 28-day stress. The body weight showed an early decrease during the first 14-day stress period but it was gradually recovered after prolonged stress exposure.
Taken together, these results demonstrated different adaptive processes of behavior, immune, neuroendocrine and physical responses to chronic stress, and may be helpful for further understanding the complex relationships among various stress responses

Key words: emotional stress, humoral immunity, norepinephrine, behavior