Melanotropins as growth factors.

Peptides that regulate the growth of tissues, whether in a positive or negative manner, are termed growth factors. The melanocortins, neurotrophic sequences that correspond to peptide fragments contained within ACTH-(1-13), beneficially affect neural growth during development and regeneration. Analogues of ACTH-(4-9) (Org 2766) and ACTH-(4-10) (BIM 22015) are capable of sustaining neurite outgrowth from cultured dorsal root ganglion and spinal cord cells in the absence of nerve growth factor. The development of sexually dimorphic behavior in both male and female rats is influenced by perinatal administration of ACTH. This change appears to be correlated with changes in the growth and metabolism of developing serotonergic and dopaminergic systems in the hypothalamic nuclei associated with male and female sexual behavior. Similar melanotropic influences are found in the developing neuromuscular system. Neuromuscular development is accelerated by perinatal administration of melanocortins, provoking both nerve and muscle to attain early maturation. However, the responding tissue varies pivotally with age: early in gestation, embryonic muscle is acutely sensitive to peptide exposure; but once innervation has occurred, only the developing nerve reacts to melanocortin treatment. Melanocortins have little if any effect on the normal, adult neuromuscular system. Following peripheral nerve injury or pathology, melanotropins once again become effective growth factors, accelerating and enhancing nerve regeneration and muscle reinnervation. Electrophysiological, morphological, biochemical, and functional tests all indicate that ACTH-(4-10), Org 2766, BIM 22015, and alpha-MSH improve various facets of nerve regeneration, the degree to which the specific parameter is improved being dependent on the peptide fragment, its dosage, and pattern of administration. BIM 22015, while less effective as a neurotrophic factor, has potent myotrophic effects that the other peptides lack. Org 2766 may provide some protective action to the injured CNS as demonstrated by tests of cognitive function following brain lesions, although evaluation of recovery is sometimes enigmatic. Recovery from destruction of the nigrostriatal system is more easily measured through tests of motor function and open field behavior, both of which support a protective role for Org 2766. Compensatory mechanisms, including the presence of increased tyrosine hydroxylase and greater density of dopaminergic fibers, may be involved. Melanocortins are effective growth factors in sciatic nerve regeneration in neonatal rats. Both alpha-MSH and ACTH-(4-10) favor the formation of morphologically normal end plates despite the trauma following nerve crush at postnatal day 2.(ABSTRACT TRUNCATED AT 400 WORDS)