ACTH neuromodulation of the developing motor system and neonatal learning in the rat.

ACTH peptides influence the developing nervous system during the first three weeks of life in the rat. ACTH 4-10 and Org 2766 (10.0 micrograms/kg) accelerate the expression of motor hyperactivity usually exhibited in 15-day old normal animals, with ACTH 4-10 increasing the force of extensor digitorum longus muscle contraction amplitude. Following cold stress and peptide treatment, rate changes in motor activity from one age to the next are dramatically enhanced, with vertical activity being exhibited at an earlier age than controls. Grasping ability is similarly enhanced in 13-day old ACTH 4-10-treated animals. The retention of a T-maze learning paradigm is significantly enhanced in 16-day old ACTH 4-10 (10.0 micrograms/kg)-treated and Org 2766 (0.01 micrograms/kg)-treated animals, with these animals running the maze significantly faster than controls. Peptide treatment appears to reverse the apparent turning preference in the maze during extinction. It is suggested that ACTH peptides modulate the organization of the nervous system and facilitate neurotransmission, and may act on dopaminergic and cholinergic neurotransmitter systems. Motor behavior seems to reflect underlying neural substrates that are integrated to produce the overt behavior of the organism.