Abstract:There appear to be two anatomically distinct β‐endorphin (βE) pathways in the brain, the major one originating in the arcuate nucleus of the hypothalamus and a smaller one in the area of the nucleus tractus solitarius (NTS) of the caudal medulla. Previous studies have shown that these two proopiomelanocortin (POMC) systems may be differentially regulated by chronic morphine treatment, with arcuate cells down‐regulated and NTS cells unaffected. In the present experiments, we examined the effects of chronic opiate antagonist treatment on βE biosynthesis across different CNS regions to assess whether the arcuate POMC system would be regulated in the opposite direction to that seen after opiate agonist treatment and to determine whether different βE‐containing areas might be differentially regulated. Male adult rats were administered naltrexone (NTX) by various routes for 8 days (subcutaneous pellets, osmotic minipumps, or repeated intraperitoneal injections). Brain and spinal cord regions were assayed for total βE‐ir, different molecular weight immunoreactive β‐endorphin (βE‐ir) peptides, and POMC mRNA. Chronic NTX treatment, regardless of the route of administration, reduced total βE‐ir concentrations by 30–40% in diencephalic areas (the arcuate nucleus, the remaining hypothalamus, and the thalamus) and the midbrain, but had no effect on βE‐ir in the NTS or any region of the spinal cord. At the same time, NTX pelleting increased POMC mRNA levels in the arcuate to ∼ 140% of control values. These data suggest that arcuate POMC neurons are up‐regulated after chronic NTX treatment (whereas NTS and spinal cord systems remain unaffected) and that they appear to be under tonic inhibition by endogenous opioids. Chromatographic analyses demonstrated that, after chronic NTX pelleting, the ratio of full length βE1–31to more processed βE‐ir peptides (i.e., βE1–27and βE1–26) tended to increase in a dose‐dependent manner in diencephalic areas. Because βE1–31is the only POMC product that possesses opioid agonist properties, and βE1–27has been posited to function as an endogenous anatgonist of βE1–31, the NTX‐induced changes in the relative concentrations of βE1–31and βE1–27/βE1–26may represent a novel regulatory m