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.     

The effects of a hypothalamic peptide factor,MIF and its cyclic analog on tolerance to haloperidol in the rat

The effects of the hypothalamic peptide, ProLeuGlyNH₂ (MIF) and its analog, cyclo (LeuGly) (CLG) on the development of tolerance to haloperidol were investigated in male Sprague-Dawley rats. Chronic oral administration of haloperidol (1.5 mg/kg/day) for 21 days resulted in the development of tolerance to its pharmacological effects. A dose of haloperidol (3 mg/kg ip) exhibited an absence of cataleptic as well as hypothermic response in chronically haloperidol treated rats. Subcutaneous administration of MIF or CLG (2 mg/kg each) daily one hour prior to haloperidol injection blocked the haloperidol-induced tolerance as evidenced by the appearance of both the cataleptic and hypothermic responses. It is concluded that the hypothalamic peptide hormone, MIF may be important in regulating chronic effects of neuroleptic drugs.     

Interaction of MIF-I or alpha-MSH with D-amphetamine or chlorpromazine on thermoregulation and motor activity of rats maintained at different ambient temperatures

Administration of several doses of MIF-I or alpha-MSH did not modify colonic temperature or the level of motor activity of rats in ambient temperatures of 4 degree or 20 degrees C. However, the thermoregulatory but not motor effects of the interaction between MIF-I or alpha-MSH with d-amphetamine were dependent upon ambient temperature. At 4 degree C, 1.0 mg/kg of both peptides enhanced the d-amphetamine-induced hypothermia, but at 20 degrees C both peptides blocked the hyperthermic effects of d-amphetamine. The hypothermic effect of chlorpromazine (CPZ) at 4 degree C and 20 degrees C was blocked by 1.0 mg/kg MIF-I but not by 1.0 mg/kg alpha-MSH. No linear dose response relationships between various doses of MIF-I or alpha-MSH and thermal responses were found. Administration of melanin or the use of hypophysectomized rats did not alter the significant interactions observed after peripheral injections.     

Ethanol-induced hypothermia. Cross tolerance with morphine

The development of tolerance to ethanol-induced hypothermia and hypnosis, and cross-tolerance with morphine was studied in mice and rats. Ethanol significantly decreased the body temperature in rats (3.0 and 3.2 g/kg) and in mice (3.5 and 4.0 g/kg). Chronic administration of ethanol resulted in the tolerance not only to ethanol hypothermia but also to hypothermic effects of morphine in examined animals. Implantation of morphine pellets caused the development of cross tolerance to ethanol-induced hypothermia in rats but not in mice. The hypnotic effect of ethanol was significantly shorter in chronic alcoholized rats but not in morphine-implanted rats. Neither chronic ethanol administration nor implantation of morphine pellets changed the duration of ethanol-induced hypnosis in mice. These results seem to support the hypothesis on the opiate-like mechanism of ethanol action.     

The effect of single and chronic administration of imipramine on clonidine-induced hypothermia in the rat

The effect of imipramine (IMI) on the hypothermic action of clonidine, 50 μg/kg iv., was examined after a single dose and after 7, 14 and 21 days of IMI administration in doses of 2 and 10 mg/kg i.p. in rats. Single administration of IMI both in a dose of 2 and 10 mg/kg does not effect clonidine-induced hypothermia. IMI in a dose of 10 mg/kg given for one week significantly blocks the response to clonidine administration, but it has practically no effect in a dose of 2 mg/kg. After a three-week treatment also a dose of 2 mg/kg blocks clonidine-induced hypothermia. It has been demonstrated that the chronic administration of IMI in contrast to the single one significantly blocks clonidine hypothermia.     

Intragastric administration of cyclo(Leu-Gly) inhibits the development of tolerance to the analgesic effect of morphine in the rat

The effect of intragastric administration of cyclo(Leu-Gly), a cyclic dipeptide derived from melanotropin release inhibiting factor (Pro-Leu-Gly-NH2), on the development of tolerance to the analgesic effect of morphine in the rat was determined. The tolerance to morphine in the rat was induced by subcutaneous implantation of four morphine pellets during a 3-day period. The rats which served as controls were implanted with placebo pellets. The analgesic response to a challenge dose of morphine was determined by the tail-flick test. The tail-flick latencies were determined before and then every 30 min for 180 min. The analgesic response was computed by determining the area under the time-response curve. Implantation of morphine pellets resulted in the development of tolerance as evidenced by decreased analgesic response to morphine in morphine pellet implanted rats as compared to placebo pellet implanted rats. Chronic intragastric administration of cyclo(Leu-Gly) (4 to 16 mg/kg) inhibited the development of tolerance to morphine. A dose of 8 mg/kg of cyclo(Leu-Gly) completely blocked the tolerance to morphine. The study provides for the first time evidence that intragastric administration of a cyclic peptide can inhibit the development of tolerance to morphine, and that effective neuropeptides and their analogs can be developed as potential drugs to inhibit opiate-induced tolerance.     

Modification of d-amphetamine- or chlorpromazine-induced hypothermia by β-endorphin,MIF-I,and α-MSH: Mediation by the dopaminergic system

The effects of β-endorphin, MIF-I, and α-MSH on d-amphetamine- and CPZ-induced hypothermias in rats kept at 4°C were tested in three experimental groups: (a) intact; (b) rats with lesions of the olfactory tubercle; and (c) rats in which the link between the DA mesolimbic pathway and the striatum was disconnected. All drugs tested alone (except MIF-I) caused significant hypothermia. Pretreatment with CPZ, MIF-I, and α-MSH potentiated d-amphetamine-induced hypothermia in intact rats. Pretreatment with α-MSH potentiated CPZ-induced hypothermia. β-Endorphin partially blocked d-amphetamine-induced hypothermia, but did not interact with CPZ, MIF-I, or α-MSH. All potentiations were either reduced or disappeared in the incisioned rats. CPZ and α-MSH caused hypothermia in olfactory tubercle-lesioned rats. The results indicate that: (a) the DA mesolimbic pathway is involved in the hypothermic response of all drugs tested; (b) an intact feedback loop is required for the potentiation of the hypothermic response of CPZ on d-amphetamine, MIF-I on d-amphetamine, and α-MSH on d-amphetamine and CPZ; (c) β-endorphin acts as a partial blocker of d-amphetamine; MIF-I is a weak potentiator of d-amphetamine. α-MSH acts as a negative modulator of the DA system, most probably in the striatum.     

Effect of continuous infusions of dexfenfluramine on food intake, body weight and brain amines in rats

The present experiments describe the effects of continuous SC infusion, via osmotic minipump, of dexfenfluramine on food intake and body weight of male and female rats. It was found that the food intake of male rats was reduced by infusions of both 3 and 6 mg/kg/day although tolerance developed within 2-4 days at the lower dose. Further, these rats showed tolerance to an acute anorectic test dose of dexfenfluramine. Body weight loss was sustained by both groups. In older (6-8 mo old) female rats, some of which had previously nursed three litters, the anorectic effects of dexfenfluramine (3 and 6 mg/kg/day) were sustained throughout the 6 day infusion, and weight loss was substantial. The effects did not differ between bred and virgin rats of comparable age. The lower dose of dexfenfluramine produced no depletion of brain serotonin (5HT), although 5HIAA was reduced. Both compounds were depleted by the higher dose. The 3 mg/kg/day dose, in select rat populations, may be a close model for the mode of dexfenfluramine administration to humans.     

Microwave attenuation of ethanol-induced hypothermia: ethanol tolerance, time course, exposure duration, and dose response studies

Four experiments were conducted to quantify the reported attenuation by microwave (MW) irradiation of ethanol-induced hypothermia. In one experiment rats were irradiated (continuous wave 2.45 GHz, specific absorption rate = 0.3 W/kg) or sham irradiated for 45 min, injected with 3.6 g/kg, 20% (v/v) ethanol (EtOH) or saline (NaCl) i.p.. Colonic temperature was monitored at 20-min intervals for 2 h. This procedure was repeated for 8 days to determine the rate of tolerance development to the hypothermic effect of ethanol. While MW irradiation did significantly attenuate EtOH-induced hypothermia, it did not enhance or retard the rate of tolerance development. To determine the duration of irradiation necessary to attenuate EtOH-induced hypothermia, groups of rats were irradiated or sham irradiated for 5, 15, 30, or 60 min prior to EtOH injection and subsequent temperature measurements. The attenuation was apparent only after 60 min of irradiation. To determine the duration of the attenuation effect after irradiation, rats were injected with EtOH or NaCl at 0, 30, 60, 120, or 480 min after 45 min of irradiation or sham irradiation. The attenuation effect was apparent among rats injected 0 to 30 min after irradiation and for the first 40 min for groups injected at 120 min. Additional rats were injected with NaCl or 0.9, 1.8, or 2.7 g/kg of EtOH i.p. following 45 min of irradiation or sham irradiation to determine if the attenuation effect depends on the dose of EtOH administered. Attenuation of EtOH-induced hypothermia was more apparent at lower doses of EtOH than at higher doses. These results indicate that the effect is an acute response to irradiation, and rule out several other potential explanations.     

Attenuation of morphine tolerance and dependence by alpha-melanocyte stimulating hormone(alpha-MSH).

Repeated administration of morphine resulted in significant reduction of its analgesic potency. If 0.1 mg/kg α-MSH was coadministered, the tolerance development was attenuated, 1 mg/kg MIF (MSH release inhibiting factor), given simultaneously with morphine, did not affect tolerance. Injecting, however, MIF 1 hour prior to the daily opiate treatment resulted in accelerated development of tolerance supposedly by lowering the plasma α-MSH level at the time of morphine administration. Of the morphine abstinence symptoms the naloxone-induced jumping in morphine pretreated mice could not be modified either by α-MSH coadministration or by MIF pretreament, but the withdrawal body weight loss was found to be diminished by the former and increased by the latter peptide. The possible role of α-MSH in preventing the development of tolerance to the analgesic effect of endogenous opioid peptides is discussed.     

Pharmacological activities of the MIF-1 analogues Pro-Leu-Gly, Tyr-Pro-Leu-Gly and pareptide

The pharmacological activities of the related free acid analogues of MIF-1, Pro-Leu-Gly (PLG) and Tyr-Pro-Leu-Gly (YPLG), were investigated because of the possibility that they may be formed during the digestion of milk and wheat proteins in vivo. The amino acid sequences-Tyr-Pro-Leu-Gly- and -Pro-Leu-Gly- are present in proteins from these foods. Chronic administration of either PLG (0.25 mg/kg, SC, BID) or the control substance, pareptide (0.25 mg/kg, SC, BID), antagonized the development of tolerance to the cataleptic effects of haloperidol in mice. The effect of YPLG (0.25 mg/kg, SC, BID) on the development of this tolerance was borderline and not statistically significant. Nanomolar concentrations of PLG, YPLG, and pareptide each increased the in vitro binding of ³H-apomorphine to rat striatal receptors. In this in vitro system, bell-shaped dose response curves were observed for each peptide. The effects of these peptides on tolerance development and apomorphine binding are similar to those previously reported for MIF-1 and demonstrate that amidation at the carboxyl terminus is not required for biological activity.     

Pharmacological responses to acute morphine administration in normotensive Wistar-Kyoto and spontaneously hypertensive rats

The effect of acute administration of morphine on analgesia, hyperthermia, hypothermia and catalepsy was determined in spontaneously hypertensive (SH) rats and normotensive Wistar-Kyoto (WKY) rats. A greater analgesic and hyperthermic response to morphine was observed in SH rats than in WKY rats. A dose of morphine (50 mg/kg ip) which produced hypothermia in WKY rats produced pronounced hyperthermia in SH rats. The cataleptic response to morphine was lower in SH rats. The cataleptic response to morphine was lower in SH rats than in WKY rats. The brain and plasma levels of morphine in SH rats were significantly lower as compared to the WKY rats at any dose of morphine used but the ratio of brain to plasma did not differ. It is concluded that SH rats exhibit altered sensitivity to morphine in comparison with their normotensive counterparts.     

The effect of thyrotropin releasing hormone and morphine on gastrointestinal transit

The effect of thyrotropin releasing hormone (TRH) alone and in combination with morphine on the gastrointestinal transit was investigated by using the charcoal meal test in mice. The intraperitoneal (IP) administration of TRH decreased the transit when given in a dose of 1.0 mg/kg 10 min prior to the meal. The intracerebroventricular (ICV) administration of TRH (10 μg/mouse) also inhibited the transit when given just prior to the charcoal meal. Subcutaneous (SC) administration of morphine (5, 10 and 20 mg/kg) inhibited gastrointestinal transit in a dose dependent manner. When TRH (1, 3 and 10 mg/kg, IP as well as 0.3 μg, ICV) which had no effect on the transit by itself was combined with morphine (10 mg/kg, SC), an enhancement in the inhibition of the transit was observed. TRH-induced inhibition of the transit was antagonized by naloxone (0.1 mg/kg, SC). It is concluded that TRH inhibits gastrointestinal transit in the mouse possibly via the opiate receptor system.     

The effects of cyclic 3'5' adenosine monophosphate on the acute tolerance induced by ethanol in male rats.

The effect of cyclic 3′5′ adenosine monophosphate (cAMP) on the acute tolerance induced by ethanol was studied in male rats. The acute tolerance was measured with a hexobarbital anesthesia method, where the dose of hexobarbital needed to obtain a burst suppression of 1 second or more in EEG is determined. Ethanol 2.0 g/kg was given ip 0.25 or 3 h prior to the threshold determination. cAMP 10 mg/kg or saline was given iv 6 h prior to the threshold determination.After saline pre-treatment less hexobarbital was needed 0.25 h after ethanol administration compared to 3 h after ethanol administration, although the blood levels were similar. An acute tolerance had developed. Pre-treatment with cAMP had no effect on the dose of hexobarbital needed without ethanol nor on the dose needed 3.0 h after ethanol administration. 0.25 h after ethanol more hexobarbital was needed in the animals pre-treated with cAMP compared with the corresponding saline treated animals. The dose of hexobarbital was as large as the one needed 3.0 h after ethanol. Thus cAMP seems to facilitate the induction of acute tolerance to ethanol while the hexobarbital threshold as such is uninfluenced.     

Interactive effects of intracisternal oxytocin and other centrally active substances on colonic temperatures of mice

Oxytocin (OXY) administered intracisternally to adult male mice produced a significant dose-related (1-4 micrograms) increase in colonic temperatures at an ambient temperature of 25 degrees C. The maximal rise in temperature occurred 30 min after administration of the peptide. The interactive effects on colonic temperature of central OXY with equimolar amounts of neurotensin, bombesin or beta-endorphin or of 2 2 mg/kg of chlorpromazine were investigated. OXY significantly antagonized the hypothermia produced by all of these substances. Pretreatment of mice with haloperidol or naloxone failed to prevent OXY-induced hyperthermia. The hyperthermic action of OXY and the interactive effects of OXY with other peptides on thermoregulation may be physiologically significant during parturition and lactation.     

A comparison of grooming behavior potencies of neurohypophyseal nonapeptides

We have previously demonstrated that intracerebroventricular (ICV) administration of oxytocin (OXY) enhanced grooming behaviors in male and female rats at a 1 microgram dose. In the present study female rats were injected ICV with 1 microgram OXY or equimolar doses of other peptides. At this dose arginine-vasopressin (AVP), arginine-vasotocin (AVT) and lysine-vasopressin (LVP), as well as alpha-MSH, were as effective as OXY in increasing grooming behavior. At equimolar doses, ACTH1-10, tocinoic acid (the ring structure of OXY) and Pro-Leu-Gly-NH2 (the tail structure of OXY) had no significant effect on grooming behavior. The potency of AVP and AVT was determined across a 0.05-5 microgram dose range. Grooming scores increased in an apparent linear manner across a similar OXY dose range. Both AVP and AVT, however, manifested an inverted U grooming response curve. Maximum grooming scores resulted from a 0.1 microgram dose of AVT or a 0.5 microgram AVP dose. Analyses of the aspects of grooming separately found that nonapeptides OXY, AVP and AVT all elevated body grooming, washing, and scratching. Because AVT and AVP administration resulted in grooming scores significantly higher than OXY at lower doses, we concluded that the CNS is more sensitive to the effects of AVT and AVP on grooming behavior than OXY.     

Levonorgestrel and norethindrone alter insulin action on skeletal muscle of the female rat

Prospective studies of women receiving oral contraceptives suggest that the progestin component may induce insulin resistance and variable deterioration of glucose tolerance. Because the tissue sites and nature of this insulin antagonism are not well-defined, we studied the effects of two parenterally administered progestins, levonorgestrel (NG) and norethindrone (NE), on insulin-regulated glucose uptake and phenylalanine release by the perfused rat hindquarter. Female rats were injected sc for 14 days with NG or NE (10 or 30 micrograms/kg/day). Low-dose NG and high-dose NE approximate the per kg dose received by women taking a high-dose progestin oral contraceptive. Phenylalanine release and glucose uptake (nmole/min/g) by the perfused hindquarters were calculated from the A-V difference for each. Progestin treatment (30 micrograms/kg/d) significantly reduced phenylalanine release from hindquarters perfused without exogenous insulin. Hindquarters from the high dose NG and low and high dose NE rats perfused with insulin (100 microU/ml) released 22% less phenylalanine than control rats perfused with the same insulin concentration (P less than 0.01) but the net suppression below baseline was similar in the control and steroid-treated groups. High-dose progestin treatment did not alter glucose uptake by hindquarters perfused without exogenous insulin. Insulin (100 microU/ml) increased glucose uptake by hindquarters of control and progestin-treated rats as compared to animals in the same treatment group perfused without exogenous insulin (P less than 0.01). High dose NE impaired insulin-stimulated glucose uptake 24% below values of the control group (P less than 0.01). The other NE and NG doses had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pro-Leu-Gly-NH2 and pareptide inhibit development of tolerance to haloperidol catalepsy in the mouse

Single doses of MIF-1 (0.03-2.0 mg/kg, SC) and chronic pretreatments with MIF-1 (0.03-2.0 mg/kg, SC, BID, 3 1/2 days) or pareptide (0.25 mg/kg, SC, BID, 3 1/2 days) did not affect the acute cataleptic response to haloperidol in the mouse. Chronic pretreatment with haloperidol (8.0 mg/kg, IP, BID, 3 days) decreased the duration of catalepsy in mice given smaller challenge dose of haloperidol (2.0 or 3.0 mg/kg, IP) 15 hours after the last pretreatment injections. Administration of either MIF-1 or pareptide to mice also chronically pretreated with haloperidol antagonized the development of tolerance.     

Effects of neuropeptide Y on rat body temperature in normal conditions and after ethanol administration]

It was shown that intracerebroventricular (icv) administration of 2 micrograms neuropeptide Y (NPY) increased the rectal temperature in rats 2.5 hours postinjection. During 5 days we analysed dynamics of the effect of NPY on alcohol-induced hypothermia in this particular interval. 2 micrograms of NPY were given daily 30 min prior to 25% solution of ethanol (3 g/kg weight rat) intraperitoneal injection. It was found that NPY can prevent the attenuation of alcohol hypothermia on the 3-d and 4-th injection day. It was supposed that the inhibitory effect of NPY on the development of alcohol tolerance may be due to the capacity of NPY to increase food behavior. So it's known that activation of other competitor motivation may inhibit the development of alcoholism.     

Opiate receptor antagonism by l-prolyl-l-leucyl-glycinamide, MIF-I

Chronic administration of l-prolyl-l-leucyl-glycinamide (MIF-I) to mice slightly reduced morphine's antinociceptive activity in the hot-plate test and modified the biphasic motor activity response to morphine. MIF-I antagonized the initial depression of activity and potentiated the increased motor activity phase. Chronic treatment of rats with MIF-I prevented morphine's antinociceptive activity in the tail flick test. MIF-I partly antagonized the inhibition by morphine of the coaxially stimulated guinea-pig ileum preparation. The inhibition of the ileum produced by ethylketocyclazocine was weakly antagonized by MIF-I. In contrast, MIF-I had no effect on the inhibition of the stimulated mouse vas deferens produced by Leu-enkephalin. The findings show that MIF-I weakly and selectively inhibits μ-type opiate receptors which suggests that MIF-I could be an endogenous inhibitor of opiate receptors.