Immunological stress in rats induces bodily alterations in saline-treated conspecifics

This work was developed during an investigation on the neuroendocrine-immune interaction in rats immune challenged with sheep red blood cells (SRBC). The structures used for evaluating the immunological response was the direct plaque-forming cells (PFC). An inbred strain of rat was used to overcome the problem of different timings in the peak humoral immune response. Normal rats were injected intraperitoneally with saline or SRBC and were killed 0, 3, 4, 5, and 6 days later. Body and gland weights were recorded, and. serum levels of corticosterone and prolactin were quantified by radioimmunoassay. The hormone levels and gland weights of the saline conspecifics and SRBC-treated rats were found to be similar. When new rats were housed in a separate room and treated with physiological saline, there were again no differences in the body and gland weights or the serum hormone levels between the two home cage control (HCC) groups of animals. Compared with saline conspecifics and SRBC-treated groups, the HCC groups had higher body weights from the third to the sixth day of treatment and had lower gland weights in absolute and relative analysis (pituitary, thyroid, and adrenals) mainly on the fourth and fifth days; thymus weights were highest on the third day. Corticosterone and prolactin levels were significantly lower on the fifth and sixth days, respectively. Because SRBC-treated rats showed a peak direct immune response on the fourth and fifth days and showed peak corticosterone levels on the fifth day after treatment, we conclude that the former animals were under stress and influenced their saline conspecifics through sound or smell. This conclusion agrees with other studies, showing that physically or emotionally stressed rats can influence conspecifics through noise and body odors.