Distribution and possible origin of β-endorphin and ACTH in discrete brainstem nuclei of rats

Immunoreactive ACTH and β-endorphin in the lower brainstem nuclei of intact and brainstem-hemisected rats were quantified by radioimmunoassay. The distribution of these peptides was almost identical throughout the lower brainstem. The concentrations of ACTH and β-endorphin were essentially equal when expressed on a molar basis. Both peptides were distributed unevenly in the lower brainstem. High concentrations were found in the periaqueductal central gray matter and the dorsal raphe nucleus, and moderate levels were present in the locus coeruleus, the parabrachial nuclei, the nucleus raphe magnus and in the nucleus of the solitary tract. β-Endorphin was measurable in all 43 brainstem nuclei investigated; ACTH was non-detectable in the red and lateral cuneate nuclei. Except in certain areas in the medulla oblongata (nucleus of the solitary tract, lateral reticular nucleus, reticular formation) ACTH and β-endorphin declined in brainstem nuclei 10 days after midbrain hemisections. Retrograde accumulation of ACTH and β-endorphin was found in the arcuate nucleus 3 days after midbrain hemisection, which was mainly ipsilateral to the lesion. Data from brainstem-hemisected rats also indicated that ACTH and β-endorphin in the nucleus of the solitary tract are primarily of local origin, whereas the lateral reticular nucleus (A1 and A5 catecholaminergic cell groups) and medullary reticular formation may receive ACTH and β-endorphin innervation from both hypothalamic and medullary neurons.