A comprehensive ultrastructural examination of one cross-sectional level (middle 1/3) of the arcuate nucleus of the hypothalamus (AH) of male rats several weeks after castration or after two weeks of morphine treatment confirmed a marked increase in lamellar whorls of endoplasmic reticulum in AH neurons in each group. A comparable incidence of AH whorls was not detected in rats treated with lactose, those treated for only 1–3 days with morphine, or in those given testosterone plus morphine for 2 weeks. It is postulated that the testosterone deficiency following either castration or chronic morphine treatment stimulated the observed increase in AH whorls. A close correspondence was noted between the distribution within the AH of neurons containing whorls and those reported by others to contain luteinizing hormone releasing hormone (LH-RH). The possibility that whorls may be a marker for hypothalamic neurons which play a role in the LH-RH regulatory system warrants further consideratio
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