24-hour pattern of circulating prolactin and growth hormone levels and submaxillary lymph node immune responses in growing male rats subjected to social isolation

To assess the effect of social isolation of growing rats on 24-h rhythmicity of circulating prolactin and growth hormone (GH) levels and submaxillary lymph node immune responses, male Wistar rats were either individually caged or kept in groups (4–5 animals per cage) for 30 d starting on d 35 of life. Plasma prolactin and GH levels, and submaxillary lymph node lymphocyte subset populations, interferon (IFN)-γ release and mitogenic responses to concanavalin A (Con A) and lipopolysaccharide (LPS) were determined at six time intervals during the 24 h span. Social isolation brought about changes in mean values and 24-h pattern of plasma prolactin and GH levels and lymph node immune responses. After isolation, prolactin and GH mean values decreased, and lymph node T, B, non T-non B, CD8⁺, and CD4⁺-CD8⁺ cells augmented, whereas lymph node CD4⁺/CD8⁺ ratio, IFN-γ release and mitogenic responses decreased. Social isolation resulted in disruption of 24 h rhythmicity of every immune parameter tested. CD4⁺/CD8⁺ ratio, IFN-γ release and Concanavalin A (Con A) and lipopolysaccharide (LPS) responses correlated significantly with plasma prolactin or GH levels while T/B ratio correlated with plasma prolactin levels only. B, non T-non B, and CD4⁺-CD8⁺ cells correlated negatively with plasma prolactin. Modifications in mean value and 24-h rhythmicity of plasma prolactin and GH levels are presumably involved in the effect of social isolation on immune responsiveness.