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

New topics in vasopressin receptors and approach to novel drugs: involvement of vasopressin V1a and V1b receptors in nociceptive responses and morphine-induced effects

Arginine vasopressin (AVP) receptors have been classified into V1a, V1b, and V2 subtypes. Recent studies have demonstrated the involvement of AVP in anti-nociception and in morphine-induced anti-nociception. However, the roles of individual AVP-receptor subtypes have not been fully elucidated. Here, we have summarized the role of V1-receptor subtypes in behavioral responses to noxious stimuli and to morphine. In this review, we focus on studies using mice lacking the V1a receptor (V1a(-/-) mice) and the V1b receptor (V1b(-/-) mice).

Influence of stress in the effects of morphine on pituitary-adrenocortical activity in the guinea-pig

1. Modifications by stress of the effects of morphine on pituitary-adrenocortical activity were examined in the guinea-pig at different lengths of time. 2. An increase in plasma cortisol but not in beta-endorphin levels was obtained in guinea-pigs stressed for 60 min. 3. Morphine (50 mg kg i.p.) enhanced beta-endorphin and cortisol levels 5 min after injection and decreased beta-endorphin concentration 30 and 90 min after its administration. 4. In stressed guinea-pigs plasma beta-endorphin levels were reduced 5 min after morphine injection. 5. These results indicate that stress can alter the effects of morphine on pituitary-adrenal activity in the guinea-pig and that the type of modification may be dependent on the time of the sampling.

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.