The effect of U-50,488H tolerance-dependence and abstinence on the levels of dynorphin (1-13) in brain regions, spinal cord, pituitary gland and peripheral tissues of the rat

Effect of chronic treatment with morphine, midazolam and both together on dynorphin(1-13) levels in the rat

We have recently reported that midazolam, a benzodiazepine receptor agonist that is also a short acting anesthetic and analgesic drug, can produce analgesia and decrease morphine tolerance and dependence in the rat by interacting with the opioidergic system. This study was designed to investigate the chronic effect of midazolam and/or morphine on the levels of dynorphin(1-13) in the pituitary gland, different brain regions, spinal cord and peripheral tissues of the rat. Four sets of animals were used: (I) saline-saline; (II) midazolam (0.03, 0.3 or 3.0 mg/kg, body wt., i.p.)-saline; (III) saline-morphine (10.0 mg/kg, body wt., s.c.); and (IV) midazolam-morphine (0.03, 0.3 or 3.0 mg/kg midazolam + 10.0 mg/kg morphine) groups. The first saline or midazolam injection was given i.p. and after 30 min, the second injection of saline or morphine was given s.c. daily for 11 days. Animals were sacrificed on the 11th day, 60 min after the last injection and dynorphin(1-13) was measured in indicated tissues by radioimmunoassay method. The midazolam treated animals showed a significant decrease in dynorphin(1-13) levels in the cortex, cerebellum, cervical region of spinal cord, heart and adrenals, and a significant increase in the hypothalamus, striatum and lumbar region of the spinal cord. The morphine treated animals showed a significant decrease in dynorphin(1-13) levels in the pituitary gland, hypothalamus, hippocampus, striatum, cerebellum, pons, medulla, kidneys, adrenals and spleen, and a significant increase only in the lumbar region of the spinal cord. When both drugs were injected together there was no effect on pituitary gland, kidneys and spleen. These drugs antagonize each other's effect on dynorphin(1-13) in the hypothalamus, striatum, cerebellum, pons, medulla and heart. However, the midazolam-morphine combination significantly increases dynorphin(1-13) levels in the hippocampus, cortex, midbrain, cervical and lumbar regions of the spinal cord, and adrenals. These results suggest the involvement of dynorphin(1-13) in the inhibition of morphine-induced tolerance and dependence by midazolam in the rat. These results may also help us in understanding the intrinsic mechanisms involved in narcotic tolerance and dependence.

Effect of chronic treatment with morphine, midazolam, and both together on beta-endorphin levels in the rat

We have recently reported that a short-acting anesthetic and analgesic drug midazolam can produce analgesia and decrease morphine tolerance and dependence in the rat by interacting with the opioid system. This study was designed to investigate the effect of midazolam, morphine, and both together on beta-endorphin levels in the rat. Male Sprague-Dawley rats were divided into four groups: (1) saline-saline; (2) saline-morphine; (3) midazolam-saline, and (4) midazolam-morphine groups. First, saline or midazolam injection was given IP and after 30 min a second injection of saline or morphine was given subcutaneously once daily for 11 days. Animals were sacrificed on 11th day 60 min after the last injection, to measure beta-endorphin by radioimmunoassay. Saline-morphine-treated animals showed a significant increase in beta-endorphin levels in the cortex, pons, medulla, lumbar spinal cord, adrenals, and spleen, and a decrease only in its level in pituitary. Midazolam-saline-treated animals showed a significant increase in beta-endorphin levels only in the medulla, and a decrease in its levels in hippocampus, striatum, and adrenals. Saline-morphine-treated animals did not show any changes in plasma beta-endorphin, but animals treated with midazolam-saline had a significant decrease in plasma beta-endorphin. In rats treated with morphine and midazolam together, beta-endorphin levels in cortex, lumbar spinal cord, and spleen decreased to the similar levels observed in rats treated with saline-saline; in pons and cervical spinal cord the levels were even lower than that found in saline-saline group. The decrease in pituitary beta-endorphin in morphine-midazolam-treated rats was due to morphine's own activity, whereas the decrease in plasma beta-endorphin in hippocampus in the morphine-midazolam group was a synergistic effect of morphine and midazolam. The beta-endorphin level in adrenal glands in the morphine-midazolam-treated animals was not different from that found in rats treated with morphine alone but was still higher than that in the saline-saline group. In general, it appears that chronic treatment with morphine stimulates the beta-endorphinergic system. A concomitant treatment with midazolam abolishes the stimulatory effect of morphine on the beta-endorphinergic system. These results may help us in understanding the intrinsic mechanisms involved in narcotic tolerance and dependence.

Effect of dizocilpine (MK-801) on analgesia and tolerance induced by U-50,488H, a kappa-opioid receptor agonist, in the mouse

The effect of dizocilpine (MK-801), an N-methyl-D-aspartate (NMDA) receptor antagonist, on the analgesic response to U-50,488H, a kappa-opioid receptor agonist, and tolerance to the analgesic effect of U-50,488H was determined in mice. The doses of MK-801 used were 0.03-0.30 mg/kg, whereas U-50,488H was administered at a dose of 25 mg/kg. Intraperitoneal (i.p.) administration of U-50,488H (25 mg/kg) produced analgesia as evidenced by the delay in the tail-flick latency in the mouse and lasted for a period of 240 min. MK-801 (0.03-0.30 mg/kg, i.p.) given 30 min prior to the injection of U-50,488H did not modify U-50,488H-induced analgesia. Twice daily administration of U-50,488H (25 mg/kg) for 9 days produced tolerance to its analgesic action. Administration of MK-801 (0.03 and 0.10 mg/kg) injected 30 min before each injection of U-50,488H prevented the development of tolerance to its analgesic effect. The higher dose, 0.3 mg/kg, of MK-801 had a minimal effect on U-50,488H tolerance. It is concluded that MK-801 in doses which do not affect U-50,488H-induced analgesia blocks the development of tolerance to its analgesic action in mice. These studies suggest that NMDA receptors play a crucial role in the development of tolerance to kappa-opioid agonist in mice.

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.

Effects of naltrexone on the binding of [3H]D-Ala2, MePhe4, Gly-ol5-enkephalin to brain regions and spinal cord and pharmacological responses to morphine in the rat

1. The effects of naltrexone pellet implantation and removal on the analgesic and hypothermic effects of morphine and the binding of 3H-D-Ala2, MePhe4, Gly-ol5-enkephalin (DAMGO) to mu-opiate receptors in rat brain regions and spinal cord were determined. 2. Male Sprague-Dawley rats were implanted subcutaneously with a pellet containing 10 mg of naltrexone for 7 days. Placebo pellet implanted rats served as controls. The pellets were removed on day 8, and the analgesic and hyperthermic effects were determined in the rat 24 hr later. Morphine produced a dose-dependent analgesic and hyperthermic responses in rats implanted with placebo pellets. Enhanced analgesic and hyperthermic responses to morphine were produced in rats implanted with naltrexone pellets. 3. The binding constants (Bmax and Kd values) of [3H]DAMGO in regions of the brain (amygdala, hypothalamus, striatum, midbrain, hippocampus, pons + medulla and cortex), and spinal cord of rats with naltrexone pellet left intact or removed were determined. The Bmax values of [3H]DAMGO were increased in all brain regions and spinal cord of rats in which the naltrexone pellets were left in place or removed prior to sacrificing. However, the Kd values of [3H]DAMGO were unaffected by naltrexone treatment. 4. It is concluded that enhanced analgesic and hyperthermic response to morphine is produced in rats implanted with naltrexone pellets and such alterations in the pharmacological responses are due to up-regulation of mu-opiate receptors in all the brain regions and spinal cord. Additionally whether the pellets were left intact (receptors blocked) or removed (receptors not blocked), the mu-opiate receptors were up-regulated in spinal cord and all the regions of the brain.