Introduction

The somatomedins or insulin-like growth factors are a family of growth-promoting peptide hormones consisting of insulin-like growth factors I (IGF-I) and II (IGF-II) as well as their variant forms (Hall & Sara, 1983). The first members of this family to be characterized were isolated from human adult plasma by Rinderknecht and Humbel (1978a,b) and termed IGF-I and IGF-II due to their structural homology to insulin. IGF-I and IGF-II are homologous single chain peptides with intrachain disulphide bridges and consist of 70 and 67 amino acids respectively. Several variant forms of these peptides have been identified at both the cDNA and protein level (Jansen et al, 1985; Zumstein, Luthi & Humbel, 1985).

The somatomedins were first suggested to play a role in neural function prior to their characterization. Studies of hormonal influences on brain development led to the hypothesis that brain growth was regulated by a growth factor whose production could be stimulated by growth hormone (Sara & Lazarus, 1974; Sara et al, 1974). In a tempts to characterize this brain growth factor it became obvious that it was a member of the somatomedin family (Sara et al., 1976). It took many years however until this factor could be isolated and its amino acid sequence established (Sara et al, 1986). During this time it also became clear that the factor was not only involved in the regulation of brain growth but also was involved in the maintenance of the mature nervous system (Sara, Hall & Wetterberg, 1981).

In this review, the role of the somatomedins as neuropeptides will be discussed, with emphasis being placed upon their characterization and biosynthesis as well as their biological role in the nervous system.