Effects of morphine and their antagonism by dexamethasone on body temperature in restrained and unrestrained guinea-pigs

Adaptation of the pituitary-adrenal axis to daily repeated forced swim exposure in rats is dependent on the temperature of water

Comparison of exposure to certain predominantly emotional stressors reveals a qualitatively similar neuroendocrine response profile as well as a reduction of physiological responses after daily repeated exposure (adaptation). However, particular physical components of the stressor may interfere with adaptation. As defective adaptation to stress can enhance the probability to develop pathologies, we studied in adult male rats (n = 10/group) swimming behavior (struggling, immobility and mild swim) and physiological responses (ACTH, corticosterone and rectal temperature) to daily repeated exposure to forced swim (20 min, 13 d) at 25 or 36 °C (swim25 or swim36). Rats were repeatedly blood-sampled by tail-nick and hormones measured by radioimmunoassay. Some differences were observed between the two swim temperature groups after the first exposure to forced swim: (a) active behaviors were greater in swim25 than swim36 groups; (b) swim25 but not swim36 caused hypothermia; and (c) swim36 elicited the same ACTH response as swim25, but plasma corticosterone concentration was lower for swim36 at 30 min post-swim. After daily repeated exposure, adaptation in ACTH secretion was observed with swim36 already on day 4, whereas with swim25 adaptation was not observed until day 13 and was of lower magnitude. Nevertheless, after repeated exposure to swim25 a partial protection from hypothermia was observed and the two swim conditions resulted in progressive reduction of active behaviors. Thus, daily repeated swim at 25 °C impairs adaptation of the hypothalamic-pituitary-adrenal axis as compared to swim at 36 °C, supporting the hypothesis that certain physical components of predominantly emotional stressors can interfere with the process of adaptation.

Functional effects of systemically administered agonists and antagonists of mu, delta, and kappa opioid receptor subtypes on body temperature in mice

We have investigated the roles of peripheral and central mu, delta, and kappa opioid receptors and their subtypes in opioid-induced hypothermia in mice. Measuring rectal temperature after i.p. injection, opioid agonists [morphine, fentanyl, SNC80 ((+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)3-methoybenzyl]-N,N-diethylbenzamide), U50,488H ((trans-(dl)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide), and loperamide)] were tested alone or with opioid antagonists [naloxone, beta-funaltrexamine, naloxonazine, naltrindole, 7-benzylidenenaltrexone (BNTX), naltriben, nor-binaltorphimine, 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-(3-isothiocyanatophenyl)-2-(1-pyrrolidinyl)ethyl]acetamide (DIPPA), and methyl-naltrexone] given 15 min after the agonist. All agonists produced dose-related hypothermia, although at low doses, morphine and U50,488H produced hyperthermia. The effects of morphine and fentanyl were antagonized by naloxone and by the mu(1) antagonist naloxonazine. The delta(2) antagonist naltriben potentiated the hypothermic effect of mu agonists. SNC80-induced hypothermia was blocked by the delta antagonist naltrindole but not by the delta(1) antagonist BNTX. Depending on the dose, the delta(2) antagonist naltriben produced either a potentiation or an attenuation of the effect of SNC80. U50,488H-induced hypothermia was antagonized by the kappa antagonist nor-binaltorphimine but not by acute treatment with the irreversible kappa antagonist DIPPA. The peripherally acting opioid loperamide produced hypothermia that could be blocked by several mu-, delta-, or kappa-selective antagonists as well as the peripherally acting antagonist methyl-naltrexone. Methyl-naltrexone produced a weak potentiation of morphine-, fentanyl-, and U50,488H-induced hypothermia, whereas a significant attenuation of SNC80-induced hypothermia was observed. In conclusion, at high doses, morphine- and fentanyl-induced hypothermia may involve composite action on mu, kappa, and possibly delta opioid receptors after initial activation. In the mediation of delta opioid-induced hypothermia, no clear selectivity between the delta(1) and delta(2) subtypes was defined. The studies provide new evidence that maintenance of the initial effects of agonist/receptor activation vary with the agonist and the receptor. The existence of both central and peripheral components of opioid-induced hypothermia is also emphasized.

GABAergic mechanisms in morphine-induced hypothermia

Morphine produced a hypothermic effect in restrained rats which was antagonized by naloxone. This effect was completely inhibited by gamma-acetylenic-GABA, an inhibitor of GABA transaminase and by baclofen, a specific GABAB agonist. Pretreatment with diazepam, an agonist of the benzodiazepine receptor, partially inhibited morphine-induced hypothermia. Flumazenil, a benzodiazepine receptor blocker, potentiated the action of morphine on body temperature. A role of brain GABA in the thermoregulatory effects of morphine is proposed on the basis of these results.

Effects of morphine on plasma B-endorphin and cortisol levels and on body temperature in guinea-pigs pretreated with 6-hydroxydopamine

1. i.c.v. morphine administration (50 micrograms/10 microliters) to sham-lesioned guinea-pigs produces a decrease of body temperature 90 min after injections. 2. This effect was blocked in animals that received i.c.v. 6-OHDA injections 20 days before. 3. On the other hand, morphine injection to sham-lesioned guinea-pigs resulted 30 and 90 min later in suppressed plasma B-END and cortisol concentration. 4. This suppressive effect was increased 20 days after 6-OHDA injections. 5. These results indicate that in the guinea-pig the effects of morphine on both body temperature and plasma levels of B-END and cortisol are altered by hypothalamic DA lesions, and suggest that hypothalamic DA neurons would be one of the sites participating in morphine-induced delayed decreased in plasma B-END and cortisol.

Effect of selective monoamine oxidase inhibitors on the morphine-induced hypothermia in restrained rats

Morphine (30 mg/kg i.p.) produced a hypothermic effect in restrained rats which was antagonized by naloxone pretreatment (10 mg/kg s.c.). This hypothermia was inhibited by deprenyl pretreatment (5 mg/kg i.p.) and by beta-phenylethylamine treatment (25 mg/kg i.p.). However, the effect of morphine was partially potentiated when a higher dose of deprenyl (10 mg/kg i.p.) was administered. Pretreatment with clorgyline (1 mg/kg i.p.) potentiated the morphine-induced hypothermia. In contrast, the effect of morphine was antagonized when a higher dose of clorgyline was used (5 mg/kg i.p.). Based on these results, a possible role of brain serotonin and dopamine in the thermoregulatory effects of morphine is proposed in this paper.

Effect of ACTH-like peptides on morphine-induced hypothermia in unrestrained guinea pigs

Peripheral treatment with adrenocorticotropin (1-24) (ACTH1-24), at different doses and sequences, consistently antagonized the decrease in body temperature produced by morphine in the freely moving guinea pig, whereas adrenocorticotropin (4-10) (ACTH4-10), which lacks corticotrophic activity, was partially effective only when it was administered in a high dose 24 h prior to morphine. Centrally administered ACTH1-24 completely prevented the hypothermic effect of intracerebroventricularly (i.c.v.)-injected morphine. Likewise, the i.c.v. administration of ACTH4-10 was equally effective in blocking the i.c.v. morphine-induced hypothermia. Neither ACTH1-24 nor ACTH4-10 did produce changes in body temperature. These results suggest that peripherally administered ACTH1-24 antagonizes indirectly the actions of morphine through the release of adrenal corticosteroids, whereas centrally injected ACTH1-24 or ACTH4-10 act as direct antagonists of morphine effects through opioid receptors.