The distribution of the a-MSH precursor proopiomelanocortin (POMC) has been investigated in more detail than that of a-MSH.
POMC is not only the precursor of MSH, but is also the source of some other neuroactive substances. Because of its importance in neuropeptide generation, the action of POMC and its function as a neuropeptide precursor are considered separately (Section 4.28). POMC is expressed in several tissues, including the pituitary, brain, skin, pancreas and testis; and its posttranslational processing varies in a tissue- and species-specific manner. a-MSH-containing neurons are found in different areas of the hypothalamus, such as in the preoptic area, the ventromedial nucleus of the hypothalamus and in the arcuate nucleus. In addition, MSH immunoreactivity occurs in the amygdala, the septum, the hippocampus, the caudate-putamen and the cortex.
The arcuate a-MSH immunoreactive axons project to the paraventricular nucleus of the hypothalamus, the dorsomedial hypothalamic nucleus and the lateral hypothalamic area. Interestingly, all these hypothalamic nuclei appear to be important in appetite regulation and energy homeostasis.
Neurons in the lateral arcuate nucleus exhibit immunoreactivity for a-MSH projections to pre-ganglionic sympathetic neurons in the spinal cord. Aside from this, a-MSH occurs in within the anterior and intermediate lobe of the pituitary.
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