Enkephalinas activity in various brain structures of naloxone-treated and morphine-insensitive rats

Abstract

Morphine injected subcutaneously in a dose of 2 mg/kg body weight exerted an analgesic effect in some Wistar rats (morphine-sensitive animals), as was indicated by a significantly prolonged latency of the tail-flick response, but failed to produce analgesia in others (morphine-insensitive animals). In morphine-sensitive rats, the striatum had the highest enkephalinase A activity, followed in decreasing order by the mesencephalon, hippocampus, pons, cortex, medulla oblongata, and hypothalamus. Thirty minutes after intraperitoneal administration of naloxone (0.3 mg/kg body weight) to morphine-sensitive rats, enkephalinase activity fell significantly in the hippocampus, striatum, and cortex, remained unchanged in the pons and medulla oblongata, and rose significantly in the meseneephalon and insignificantly in the hypothalamus; generally similar differences in enkephalinase activity from naloxone-untreated morphine-sensitive rats were recorded in the brain structures of morphine-insensitive rats given saline instead of naloxone.