Enkephalin-induced stimulation of humoral and cellular immune reactions in aged rats

Twenty-month-old Wistar rats received intraperitoneal injections of the opioid pentapeptide, methionine-enkephalin (Met-Enk) in periods before and after immunization with cellular and soluble antigens. Animals were treated with 0.2 mg of Met-Enk/kg b.w., a dose previously found to increase immune capacity in young adult rats. Saline-treated 20-month-old, and Met-Enk-treated rats and saline-treated 8-week-old controls were set up for each experimental group. Immune performance was evaluated by plaque-forming cell response, antibody production and various immunoinflammatory reactions. At autopsy, thymus and spleen were weighed and processed for histological examination. The results showed that 0.2 mg dose of Met-Enk produced significant enhancement of both humoral and cellular immune responses in senescent rats. Methionine-enkephalin treatment also induced a significant increase in thymus and spleen weights in these animals. Analysis of the cellular make up of these organs revealed the enlargement of cortical and medullary areas, and pronounced pyroninophilia in the subcortical zone of the thymus and thymus-dependent areas of the spleen. The results suggest that Met-Enk exerts an immunorestorative activity in aged animals, and that changes in the opioid system may play an important role in the maintenance of immune functions during senescence.