Byron C. Yoburn

Increased analgesic potency of morphine and increased brain opioid binding sites in the rat following chronic naltrexone treatment.

Implantation of rats with prolonged-release naltrexone pellets increased both morphines analgesic potency in the tailflick assay and radiolabeled opioid binding in the brain. The increases in both radiolabeled opioid binding and morphine potency were time-dependent. Implantation for 24 hours did not increase binding, whereas increases of approximately 45% were seen following 8 days of implantation. Similarly, morphines analgesic potency, measured as ED50 values, was increased by 50% following 8 days of exposure to naltrexone while a 24 hour exposure had no significant effect.

Constraints on the tailflick assay: Morphine analgesia and tolerance are dependent upon locus of tail stimulation

In three experiments, the locus of tail stimulation in the tailflick assay was found to be an important parameter in determining morphine action. Rats were intravenously infused (Experiment I), injected with morphine subcutaneously (Experiment II), or implanted subcutaneously with morphine pellets (Experiment III). Analgesia was evaluated periodically following drug administration using the tailflick test and 3 adjacent 1 in. tail areas. In all three experiments, the distal tail section was more sensitive to the analgesic effects of morphine than more proximal sections. In Experiments I and III, tolerance to the effects of morphine developed more slowly at the distal tail location. These results indicate that the locus of stimulation in the tailflick assay can profoundly affect the development of analgesia and tolerance to morphine.

Chronic vascular catheterization in the rat: Comparison of three techniques

Rats were implanted with an indwelling vascular cannula in the jugular vein, femoral artery or carotid artery, and evaluated for postsurgical weight changes and cannula patency. Complete details for surgical methods and materials are presented for each procedure. Over a 14 day period, the carotid artery procedure produced the most profound weight loss, while the jugular vein implantation was followed by minimal changes in body weight. Weight loss was intermediate for the femoral artery group. Body weight had returned to, or was above presurgical weight at 2, 4 and 6 days postsurgery for the jugular, femoral and carotid catheterizations, respectively. By 14 days following implantation 83%, 67% and 50% of the femoral, carotid and jugular cannulas, respectively, were patent. We conclude that for long-term sampling of blood in the rat, the femoral artery catheterization procedure is preferable in terms of patency and postsurgical weight loss.

Opioid receptor upregulation and supersensitivity in mice: Effect of morphine sensitivity

Mice of the Swiss-Webster strain obtained from two suppliers (Taconic, Charles River) were found to differ in their sensitivity to morphine. Mice from Taconic were approximately two-fold more sensitive to the analgesic and lethal effects of morphine compared to the Charles River mice. In a third strain, C3H/HEN, morphine was found to be more than 2.5 times more potent in producing analgesia than in the Charles River mice. Binding studies showed that the Taconic mice and C3H/HEN mice had approximately 40% and 60%, respectively, more specific [3H]naloxone binding sites in brain than did the less sensitive Charles River mice. When treated with chronic naltrexone for 8 days the analgesic potency of morphine was increased by approximately 90% for both Swiss-Webster mice and by 20% for the C3H/HENs. [3H]Naloxone binding was increased by 45-50% in the Swiss-Webster strains, but by only 33% in C3H/HEN mice. These data indicate that receptor upregulation is directly related to increases in morphine potency. Further, these findings suggest that initial sensitivity to morphine can determine the degree of functional supersensitivity and relative receptor upregulation produced by chronic opioid antagonist treatment.

Regulation of rat adrenal medullary enkephalins by glucocorticoids

Opioid peptides and their precursors of the proenkephalin family are found in the chromaffin cells of the rat adrenal medulla in low quantities. However, if the gland is denervated, there is a 10 to 20-fold increase in enkephalin-containing (EC) peptides consisting mostly of the precursor proenkephalin. The denervation-induced rise in medullary EC peptides is blocked by hypophysectomy, and partially reinstated by corticosterone, dexamethasone or ACTH treatment. In the intact rat, intermediate doses of corticosterone or dexamethasone reduce the denervation-induced increase in EC peptides, while a high dose of dexamethasone restores this response. These results indicate that glucocorticoids exert a permissive effect in vivo on the denervation-induced stimulation of EC peptide biosynthesis.

Chronic opioid antagonist treatment facilatates nonopioid, stress-induced analgesia

Chronic exposure to opioid antagonists produces increases brain opioid receptors and enhances morphine analgesia. Since opioid antagonists could affect both opioid and nonopioid analgesic systems, the present study evaluated whether chronic opioid antagonist treatment with naltrexone alters the nonopioid analgesia produced by cold-water swims (CWS). Rats were implanted (SC) with two, 30 mg naltrexone pellets. The pellets were removed 8 days later or left in place and rats tested 24 hr later for analgesia (tail-flick) following a 3.5 min CWS or morphine (3 mg/kg, SC). As expected, morphine analgesia was potentiated in rats with naltrexone pellets removed, but was blocked in rats tested with the naltrexone still implanted. In contrast, naltrexone pretreatment potentiated CWS analgesia, irrespective of whether the pellets were removed or left in place. These findings confirm the nonopioid nature of CWS analgesia and indicate that chronic treatment with an opioid antagonist can affect both opioid and nonopioid analgesic mechanisms.

Characterization of enkephalins in rat adrenal medullary explants

In the rat, removal of depolarizing stimuli to the adrenal medulla by surgical denervation in vivo or by explanting adrenal medullae has been shown to dramatically increase preproenkephalin mRNA, and enkephalin-containing (EC) peptides. To further elucidate the cellular basis of these effects and the role of transsynaptic influences on post-translational processing, we have defined the time course, and characterized EC peptides in rat adrenal medullary explants in control and depolarized states. The rise in EC peptides begins after 1 day in culture and reaches a peak at 4-7 days. Although the onset of the increase in EC peptides in culture is delayed by 12-24 h compared to the changes seen in vivo, following surgical denervation, the time course of peak and duration is remarkably similar. Size exclusion chromatography (SEC) revealed that the major species of newly appearing EC peptides in explanted glands is a high molecular weight peptide of approximately 18,000 with a Met-/Leu-enkephalin ratio of approximately 6. These results suggest that proenkephalin, the initial precursor of the EC peptide family, is the major EC peptide that accumulates in rat adrenal medullary explants. A low-molecular weight EC peptide, found by high-performance liquid chromatography to be free Met-enkephalin, is a minor component of the culture induced increase in EC peptides. Culturing of medullae in the presence of depolarizing concentrations of K+ prevents the accumulation of the proenkephalin-like EC peptides and free enkephalins.(ABSTRACT TRUNCATED AT 250 WORDS)

The graded and quantal nature of opioid analgesia in the rat tailflick assay.

Opioid agonists routinely increase the latency to respond in the rodent tailflick assay. The nature of this effect was investigated in 5 experiments using several parametric variations and routes of administration. Morphine and methadone were found to produce both quantal and graded effects in all experiments. In cases where quantal effects were observed, the majority of animals also responded in a graded manner during subsequent testing. The increase in latency to respond in the tailflick assay produced by opioid agonists is not accurately characterized as predominantly quantal.

Naltrexone Maintenance: Effect on Morphine Sensitivity in Normal Volunteers

Naltrexone has been tested in numerous clinical trials for both safety and efficacy in the treatment of relapse to opioid dependence. In the doses used for this purpose, it has been found to be safe and pharmacologically effective. Recently it has been found that chronic treatment with naloxone or naltrexone produces supersensitivity to opioid agonists in rodents. Seven normal human volunteers were tested for opiate sensitivity, using morphine 8 mg iv before and after 2 weeks of naltrexone treatment at 50 mgpo qd. There were no significant differences in morphine-induced respiratory depression before or after naltrexone treatment. These results indicate that at the dose of morphine used in this study, it is unlikely that naltrexone maintenance would increase the risk of opiate toxicity.