The role of the pituitary-adrenal axis in the hyperthermia induced by acute peripheral or central (preoptic anterior hypothalamus) administration of morphine to unrestrained rats

Entanglement between thermoregulation and nociception in the rat: the case of morphine

In thermoneutral conditions, rats display cyclic variations of the vasomotion of the tail and paws, the most widely used target organs in current acute or chronic animal models of pain. Systemic morphine elicits their vasoconstriction followed by hyperthermia in a naloxone-reversible and dose-dependent fashion. The dose-response curves were steep with ED50 in the 0.5-1 mg/kg range. Given the pivotal functional role of the rostral ventromedial medulla (RVM) in nociception and the rostral medullary raphe (rMR) in thermoregulation, two largely overlapping brain regions, the RVM/rMR was blocked by muscimol: it suppressed the effects of morphine. "On-" and "off-" neurons recorded in the RVM/rMR are activated and inhibited by thermal nociceptive stimuli, respectively. They are also implicated in regulating the cyclic variations of the vasomotion of the tail and paws seen in thermoneutral conditions. Morphine elicited abrupt inhibition and activation of the firing of on- and off-cells recorded in the RVM/rMR. By using a model that takes into account the power of the radiant heat source, initial skin temperature, core body temperature, and peripheral nerve conduction distance, one can argue that the morphine-induced increase of reaction time is mainly related to the morphine-induced vasoconstriction. This statement was confirmed by analyzing in psychophysical terms the tail-flick response to random variations of noxious radiant heat. Although the increase of a reaction time to radiant heat is generally interpreted in terms of analgesia, the present data question the validity of using such an approach to build a pain index.

Increases of CCK mRNA and peptide in different brain areas following acute and chronic administration of morphine

The present study examined whether either acute or chronic administration of morphine resulted in changes in the content of CCK mRNA and CCK immunoactive peptide in selective areas of the rat brain and spinal cord. Two hours after a single injection of morphine (10 mg/kg, s.c.), CCK mRNA significantly increased in the hypothalamus (0.8-fold) and spinal cord (2-fold) relative to the CCK mRNA content in saline-injected controls. No significant differences in CCK mRNA were observed in the frontal cortex, hippocampus, midbrain or brainstem. There were no significant alterations in CCK immunoreactivity in any brain regions and spinal cord after the acute treatment with morphine. Upon repeated morphine administration, the content of CCK mRNA in both the hypothalamus and the spinal cord was further elevated by at least 3-fold. A significant increase of CCK mRNA content in brain stem (2.8-fold) was also observed following chronic morphine administration. In contrast to the acute exposure to morphine, chronic administration resulted in significant increases in CCK immunoactive peptide in hypothalamus (2.6-fold), spinal cord (2.1-fold) and brainstem (1.6-fold), but not in the other brain areas. These results demonstrate that morphine, especially following repeated administrations, stimulates endogenous CCK biosynthesis in selective brain regions.

A kappa opiate agonist, U50,488H, enhances energy expenditure in rats

The effect of U50,488H (a selective kappa opiate agonist) on oxygen consumption was measured in either resting and free-moving rats. In both states, U50,488H provokes an increase in oxygen consumption. In resting rats, the increase occurs at lower doses than in free-moving rats. The explanation could be that in the free-moving rats the drug results in an increase in energy expenditure, partially compensated by a decrease in activity.