Regional Differences in the Molecular Weight Profiles of Corticotropin and a-Melanotropin in the Hypothalamus

The molecular weight profiles of immunoreactive ACTH (ACTH_i) and immunoreactive α-MSH (α-MSH_i) were investigated in two regions of the hypothalamus: the region that is known to contain the perikarya of the ACTH/α-MSH neurons (medial basal hypothalamus) and a region that contains axons of these perikarya (preoptic anterior hypothalamus). Acetic acid extracts of the entire hypothalamus, medial basal hypothalamus, and preoptic anterior hypothalamus obtained from adult male rats were subjected to gel filtration under denaturing conditions. ACTH_i and α-MSH_i were quantified by RIA. The gel filtration profiles of ACTH in extracts of the entire hypothalamus were indicative of the presence of three molecular weight classes of ACTH_i: big ACTH (mol wt, ≥ 9000), 5.7K ACTH (mol wt, -5700), and 4.5K ACTH (mol wt, 4500). The molecular weight profiles of ACTH_i and the molar ratio of αMSH _i to ACTH_i in the medial basal hypothalamus and in the preoptic anterior hypothalamus differed markedly. Of the total ACTH_i in the medial basal hypothalamus, 64% was associated with big ACTH, 12% with 5.7K ACTH, and 24% with 4.5K ACTH; the molar ratio of α-MSH_i to total ACTH_i was 3. Of the total ACTH_i in the preoptic anterior hypothalamus, 38% was associated with big ACTH, 20% with 5.7K ACTH, and 42% with 4.5K ACTH; the molar ratio of α-MSHi to ACTH_i was 10. Infusion of vinblastine, an inhibitor of axonal transport, into a lateral ventricle of the brain resulted in a doubling of the concentrations in the medial basal hypothalamus of ACTH_i and αMSH_i as well as in a change in the molecular weight profile of ACTH_i. Of the total ACTH_i in this region, 40% was associated with big ACTH, 19% with 5.7K ACTH, and 41% with 4.5K ACTH. On the other hand, vinblastine did not affect the concentrations or the molecular weight profiles of ACTH; and α-MSH_i in the preoptic anterior hypothalamus. If big ACTH were the precursor to 5.7K ACTH and 4.5K ACTH as well as α-MSH, then these results would indicate that processing of big ACTH occurs along the axons of the ACTH/α-MSH neurons and that axonal transport is not a requirement for this processing. Thus, processing of ACTH and α-MSH occurs in two loci of the ACTH/α-MSH neuron: the perikaryon and the axon.