Dose-dependent effects of D-N-allylnormetazocine on regional cerebral metabolic rates for glucose

Effects of the intravenous administration of [Lys7]dermorphin on local cerebral glucose utilization in the rat

The use of analgesic opioids in the clinical setting is hampered by the reinforcing and addictive properties of these drugs. Moreover, chronic administration of conventional opioids is accompanied by progressive reduction of the analgesic effects, that often forces clinicians to increase dosages, exposing a subject to serious side-effects. Thus, interest is growing in the development and characterization of synthetic opioid agonists with lower reinforcing properties than conventional opioids. [Lys7]dermorphin is a mu1 receptor agonist with 20-30 times stronger analgesic properties than morphine. Previous data indicate that the drug causes fewer side-effects than conventional opioids, and is less likely to produce physical dependence than morphine. In this study we investigated the effects of the intravenous administration of a range of doses of [Lys7]dermorphin (0.002, 0.01 and 0.05 mg/kg) on local cerebral glucose utilization in the rat, by means of the quantitative [14C]2-deoxyglucose method. The results of the study showed dose-related reductions of cerebral metabolic rates for glucose in limbic, sensory-motor and autonomic regions following the intravenous administration of [Lys7]dermorphin. Such pattern of changes is similar to those measured earlier following the administration of analgesic doses of drugs stimulating mu-opioid receptors. Within the nucleus accumbens, and the shell portion in particular, we did not measure any increase of glucose utilization, rather a significant decrease following the administration of the higher dose of [Lys7]dermorphin. These findings contribute to the definition of the functional consequences of the administration of [Lys7]dermorphin, and indirectly suggest the lack of effect of the drug on mesolimbic dopamine neurotransmission.

Cerebral glucose metabolism during opioid withdrawal following methylnaloxonium injection into the locus coeruleus

Previous studies have demonstrated a widespread stimulation of regional cerebral metabolic rate(s) for glucose (rCMRglc) in morphine-dependent rats subjected to opioid withdrawal precipitated by systemic injection of naloxone. Nonetheless, many of the behavioral signs of opioid withdrawal are produced by intracerebral injections of an opioid antagonist, methylnaloxonium (MN), into the locus coeruleus (LC). The purpose of the present work was to determine the extent to which cerebral metabolic alterations in opioid withdrawal could be initiated by a local action in LC. Intracerebral injections of MN into LC increased rCMRglc in morphine-dependent rats, and the anatomical distribution of this effect was similar to that produced by systemic injections of naloxone. The present data support the view that LC is a major substrate of opioid withdrawal in the brain, and they suggest that LC plays an important role in changing rCMRglc during opioid withdrawal induced by systemic naloxone administration.