The molecular weight profiles of immunoreactive ACTH (ACTHi) and immunoreactive α-MSH (α-MSHi) 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. ACTHi and α-MSHi 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 ACTHi: big ACTH (mol wt, ≥ 9000), 5.7K ACTH (mol wt, -5700), and 4.5K ACTH (mol wt, 4500). The molecular weight profiles of ACTHi and the molar ratio of αMSH i to ACTHi in the medial basal hypothalamus and in the preoptic anterior hypothalamus differed markedly. Of the total ACTHi 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-MSHi to total ACTHi was 3. Of the total ACTHi 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 ACTHi 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 ACTHi and αMSHi as well as in a change in the molecular weight profile of ACTHi. Of the total ACTHi 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-MSHi 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.
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