Failure of prolyl-leucyl-glycinamide to alter analgesia measured by the Takemori test in morphine-pretreated rats

Comparative effects of Pro-Leu-Gly-NH2 and cyclo(Leu-Gly) administered orally on the development of tolerance to the analgesic effect of morphine in the rat

Comparative effects of Pro-Leu-Gly-NH2 (MIF) and cyclo(Leu-Gly) (CLG) administered orally at different stages of chronic morphine treatment on the development of tolerance to the analgesic effect of morphine in the rat were determined. Male Sprague-Dawley rats were implanted with either 6 placebo or morphine pellets during a 7-day period. Implantation of morphine pellets resulted in the development of a high degree of tolerance as evidenced by a decrease in the analgesic response to morphine. Administration of CLG (8 and 16 mg/kg/day) on day 5, 6 and 7 of implantation inhibited the development of tolerance to morphine but 4 and 32 mg/kg doses had no effect. Further, CLG (2 mg/kg/day for 7 days) inhibited the development of tolerance but higher doses (4 and 8 mg/kg) had no effect. MIF (26 and 52 mg/kg) administered orally on the last three days of the implantation schedule inhibited the development of tolerance to morphine. MIF (6.5 mg/kg/day for 7 days) inhibited the development of tolerance but the higher doses had no effect. Concurrent administration of MIF (6.5 mg/kg) and CLG (2 mg/kg) for seven days failed to inhibit the development of tolerance. A single dose of MIF or CLG administered a day before the assessment of tolerance did not affect the morphine tolerance. Thus, even after a significant degree of tolerance to morphine had developed, neuropeptides like MIF and CLG given orally, in appropriate doses, can inhibit development of tolerance to morphine and restore the analgesic effect of morphine.

Structure activity relationship studies with hypothalamic peptide hormones III. Effect of melanotropin-release inhibiting factor and analogs on tolerance to morphine in the rat

The effects of several analogs of melanotropin-release inhibiting factor (MIF, Pro-Leu-Gly-NH2) on the development of tolerance to the hyperthermic, hypothermic and cataleptic actions of morphine were investigated in male Sprague-Dawley rats. The analogs that were examined included. Pro-Gly-Gly-NH2 (I), Pro-VAl-Gly-NH2 (II), Pro-Leu-beta-Ala-NH2 (III), Pro-Leu-Gly-NHCH3 (IV), Pro-Leu-NH2 (V) and cyclo (Pro-Gly) (VI). Subcutaneous implantation of four morphine pellets (each containing 75 mg of morphine free base) during a 3-day period was used to develop tolerance to the pharmacological effects of morphine. Concurrent daily subcutaneous administration of any of the above peptides (I through VI) at a 10 mu mol/kg dose did not modify the development of tolerance to morphine-induced hyperthermia, hypothermia or catalepsy. The development of tolerance to morphine was, however, inhibited by equivalent doses of MIF. Treatment with these peptides did not alter the distribution of morphine in brain and plasma. It is concluded that the structural requirements for the inhibitory effect of MIF on the development of tolerance to morphine are very strict and that the following modifications in the structure of MIF result in the loss of activity (a) substitution of Gly or Val in place of Leu (b) replacement of Gly-NH2 with Gly-NHCH3 or beta-Ala-NH2 (c) removal of Gly, and (d) removal of Leu followed by cyclization of Pro-Gly.

Facilitation of morphine-induced tolerance and physical dependence by prolyl-leucyl-glycinamide

The pretreatment of mice with 30 mg/kg morphine s.c. did not alter the analgesic effect of morphine in mice pretreated with saline but decreased the analgesic effect of morphine in mice pretreated with prolyl-leucyl-glycinamide (PLG). Tolerance was evaluated by the effect of PLG on morphine-induced enhancement of naloxone potency which is a measure of the capacity of naloxone to antagonize morphine-induced analgesia and is postulated to be an indicator of tolerance development. The naloxone potency of PLG-treated mice was 2-fold greater than that of control mice. PLG did not alter the whole brain levels of morphine, nor did it alter the naloxone potency in mice which were not pretreated with 30 mg/kg morphine. In mice treated with 100 mg/kg morphine or implanted with 50 mg morphine pellets for 24 or 72 h, the amount of naloxone required to induce jumping was not altered by PLG. However, PLG treatment did increase the hypothermia and body weight loss seen after naloxone-induced withdrawal. Administration of PLG to morphine-dependent mice 1 h prior to naloxone did not modify the resultant hypothermia or body weight loss. These results indicate that PLG facilitated the development of morphine tolerance and dependence.

Effects of peptides related to neurohypophyseal hormones on ethanol tolerance

Mice were rendered tolerant to the hypothermic effect of ethanol by forcing them to inhale ethanol vapor for 3 days. One day after withdrawal, tolerance was assessed by determining the response of the mice to an acute 3 g/kg IP challenge dose of ethanol. Thirty minutes before the injection of ethanol, saline or peptide solution was SC injected. The peptides studied were des-Gly9-Arg8-vasopressin (a peptide with reduced peripheral endocrine activities), oxytocin, and analogs and fragments of these peptides. None of the peptides, with the possible exception of oxytocin, affected body temperature in naive animals or the acute hypothermic response to ethanol in non-tolerant mice. Des-Gly9-Arg8-vasopressin enhanced the expression of tolerance to ethanol hypothermia; shorter fragments of vasopressin did not share this effect. Oxytocin attenuated the expression of tolerance but this may been due to an interaction with the acute effects of ethanol.