Clearance from Cerebrospinal Fluid of Intrathecally Administered ??-Endorphin in Monkeys

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Abstract
Five adult male monkeys (Macaca mulatto) weighing 7.1–9.9 kg were given synthetic human β-endorphin (800 μg) and [14C]methoxy-inulin (50 μCi) in 400 μl of normal saline intrathecally. Serial samples of cerebrospinal fluid were drawn through a previously positioned indwelling spinal catheter and were assayed for concentrations of β-endorphin (determined by radioimmunoassay) and inulin (determined by liquid scintillation counter). Spinal fluid concentrations of β-endorphin and inulin peaked and declined in a parallel manner. The clearance ratio (calculated from the reciprocal of the ratio of the areas under the respective curves of elimination of the two species) remained remarkably similar from animal to animal, giving a mean value of 1.060 ± 0.090 (sem). This ratio, being near unity, suggests that β-endorphin is eliminated from spinal fluid in a fashion similar to that of inulin, which is removed exclusively by bulk absorption

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