Neonatal Administration of Met-Enkephalin Facilitates Maze Performance of Adult Rats

Newborn rats were injected SC during the first week of life with 80 μ/kg Met-enkephalin, DSIP, MIF-I, or diluent. When tested 3 months later in a 12-choice maze for a reward of food, hungry rats injected neonatally with Met-enkephalin were found to run the maze faster and with fewer errors than the controls. DSIP and MIF-I did not improve performance in the maze, indicating some specificity to the findings. Tyrosine hydroxylase and choline acetyltransferase activity in several parts of the brain were not significantly different among the groups. Radioimmunoassay of brain parts from a small number of adult rats indicated slightly more DSIP-like material in the thalamus and striatum of females injected neonatally with DSIP as compared with those injected with diluent. The results extend our previous observations of the persistence of central effects of peripherally injected Met-enkephalin from several minutes to three months when administration occurs early in life. The findings further suggest an effect of peptides on the organization of the developing brain.