Inflammatory pain-induced signaling events following a conditional deletion of the N-methyl-D-aspartate receptor in spinal cord dorsal horn.

The N-methyl-d-aspartate (NMDA) receptor in the spinal cord dorsal horn (SCDH) is one of the mechanisms involved in central sensitization during chronic pain. Previously, this laboratory created a spatio-temporal knockout (KO) of the N-methyl-d-aspartate receptor I (NR1) subunit in the mouse SCDH. The NR1 KO completely blocks NR1 gene and subsequent NMDA receptor expression and function in SCDH neurons. In the NR1 KO mice, the mechanical and cold allodynia induced at 24 h after complete Freund's adjuvant (CFA) was reduced. However, the protective effects of KO were transient and were not seen at 48 h after CFA. These observations suggest the presence of NMDA-independent pathways that contribute to CFA-induced pain. CFA induces the activation of several signaling cascades in the SCDH, including protein kinase C (PKC)gamma and extracellular signal-regulated kinases (ERK1/2). The phosphorylation of PKCgamma and ERK1/2 was inhibited in the SCDH of NR1 KO mice up to 48 h after CFA treatment, suggesting that these pathways are NMDA receptor-dependent. Interestingly, neuronal cyclooxygenase (COX) -2 expression and microglial p38 phosphorylation were induced in the SCDH of the NR1 KO at 48 h after CFA. Our findings provide evidence that inflammatory reactions are responsible for the recurrence of pain after NR1 KO in the SCDH.

Methadyl acetate (LAAM) in the treatment of heroin addicts. II. Double-blind comparison of gradual and abrupt detoxification.

One hundred nineteen patients were admitted to a six-month (26-week) prenaltrexone detoxification program comparing abrupt and gradual withdrawal from methadyl acetate (LAAM) therapy. All patients were brought to a maintenance level of 50, 50, and 65 mg (Monday, Wednesday, and Friday). Patients randomly assigned to the gradual group (group G) began 4-mg/wk reduction the Monday of week 9 and reached zero dosage (placebo) the Monday of week 23; patients in the abrupt group (group A) continued to receive 50, 50, and 65 mg until the Monday of week 23, when their dosage was dropped to zero (placebo). All patients were given placebo for four weeks. This study showed the superiority of abrupt withdrawal over this gradual-withdrawal schedule. Forty-six percent of group P compared with 28% of group G made the transition to naltrexone treatment. Severity of withdrawal problems was in no case significantly greater in group A.

Plasma immunoreactive beta-endorphin levels in depression. Effect of electroconvulsive therapy.

Immunoreactive (ir) plasma beta-endorphin level was assayed in ten symptomatic patients with a unipolar major depressive disorder and in 16 psychiatrically normal controls matched for age and sex. Plasma ir-beta-endorphin level in depressed patients was similar to that in controls. All depressed patients was similar to that in controls. All depressed patients had a transient, approximately threefold increase in ir-beta-endorphin after each use of electroconvulsive therapy (ECT). The increase of plasma ir-beta-endorphin level after ECT parallels the transient elevation of adrenocorticotropic hormone level reported by others and probably reflects a hypothalamic response to ECT.

Glucocorticoid regulation of enkephalins in cultured rat adrenal medulla.

The effect of dexamethasone on enkephalin-containing (EC) peptide levels and preproenkephalin mRNA levels was determined in adrenal medullary explants (glands) from sham and hypophysectomized (hypox) rats. Culture for 4 days in serum-free medium without dexamethasone resulted in a 13- and 4-fold increase in EC peptide levels in sham and hypox glands, respectively. The addition of dexamethasone (10(-5) M) produced a 20- to 26-fold increase in EC peptides in sham and hypox glands. In serum free medium, hypox glands showed a concentration dependent increase in EC peptides with the ED50 for dexamethasone equal to 5.7 x 10(-7) M. Since the glucocorticoid antagonist RU486 partially blocked the rise in EC peptides in sham glands, it appears that the increase in EC peptides in sham glands in the absence of dexamethasone is a result of a higher concentration of endogenous corticosterone in sham compared to hypox glands. Dexamethasone resulted in a 6-fold increase in preproenkephalin mRNA in hypox glands cultured for 2 days. This increase was approximately proportional to the increase in EC peptides seen at 4 days. In serum free medium progesterone, testosterone, and deoxycorticosterone failed to increase EC peptides in hypox glands. These results indicate that glucocorticoid treatment is required for maximal proenkephalin gene expression and EC peptide biosynthesis in cultured glands.

Effects of eight-hour naloxone infusions on human subjects.

Twelve normal male volunteers received saline control, low-dose naloxone, and high-dose naloxone infusions during three weekly sessions. The sessions were 16 hr long: 1 hr for predrug assessments, 8 hr during which either naloxone or saline was infused in a double-blind procedure, and a 7-hr postdrug observation period. The 8-hr infusions of naloxone had no effect on experimental ischemic arm pain. In addition, the ischemic arm pain procedure did not significantly increase either plasma levels of cortisol or immunoreactive beta-endorphin, suggesting that the procedure was not stressful. The high-dose naloxone infusion resulted in a slightly aversive mood state and prevented the normal circadian decrease in cortisol levels. Both doses of naloxone increased systolic blood pressure and prevented the normal diurnal increase in temperature. The 8-hr infusions of naloxone did not result in changes in pain, mood, or physiological indices beyond what was present within a few hours after starting the infusion.

Disposition of acetylmethadol in relation to pharmacologic action.

The levels of acetylmethadol and its active biotransformation products were quantitated in the plasma and urine of subjects on acetylmethadol maintenance by the use of solvent extraction and gas-liquid chromatography. The time-course of plasma acetylmethadol, noracetylmethadol, and dinoracetylmethadol over 48 hr was determined concurrently with the time-action of pupillary constriction in 8 subjects receiving an average oral maintenance dose of 50 mg. The plasma level of acetylmethadol reached a peak at 4 hr and had nearly disappeared by 24 hr. The mean apparent half-life was 7 hr. The plasma level of noracetylmethadol peaked at 4 to 8 hr and slowly declined over the next 40 hr. The mean apparent half-life of noracetylmethadol was 48 hr. Dinoracetylmethadol plasma levels remained relatively constant throughout the dosing interval. The miotic effect reached a peak at 8 hr and decayed at a rate slower than that of plasma elimination of acetylmethadol but more rapidly than that of noracetylmethadol and dinoracetylmethadol. Substantial variation in the plasma levels of the compounds was observed in subjects after the same doses. The study indicated that the relatively long duration of opiate effects of acetylmethadol results from biotransformation to active and persistent metabolites.

Effects of acetylmethadol on plasma testosterone.

Plasma testosterone levels were measured in a group of 13 men maintained on acetylmethadol for treatment of opiate dependence. Prior to acetylmethadol administration, plasma testosterone levels were within normal adult ranges. Four hours following ingestion of acetylmethadol, plasma testosterone levels were significantly depressed and continued to fall 7-9 hr after drug administration. Plasma testosterone levels remained low 24-25 hr after the drug was taken, but, 48 hr following drug, plasma testosterone levels rose to values approximating those prior to drug. Depression of plasma testosterone levels following acetylmethadol ingestion was not only statistically but also biologically significant, since in many patients the levels were below the normal range for adult males.

Pharmacokinetics and pharmacodynamics of methadone in patients with chronic pain.

Concentrations of methadone in plasma, estimates of pain relief, and pupillary size were determined after a single intravenous dose (10 to 30 mg) of methadone hydrochloride to eight patients with chronic pain, five of whom had cancer. The pharmacokinetic parameter estimates reveal rapid and extensive distribution (Varea) and a slow apparent elimination half-life (t1/2) (mean Varea = 3.59 L/kg and harmonic mean t1/2 = 23 hours). The harmonic mean blood clearance is 106 ml/min, the harmonic mean renal clearance is 3.9 ml/min, the mean hepatic extraction ratio is 0.089, and plasma protein binding is 86% to 89%. These results suggest that only the free (unbound) fraction of methadone present in blood is extracted by the liver and that methadone can be classified as a low (hepatic)-extraction drug. The data were fit to a pharmacokinetic-pharmacodynamic model to obtain estimates of the steady-state plasma methadone concentration required to produce 50% of the maximum pain relief. This value varied from 0.04 to 1.13 micrograms/ml (mean = 0.29 micrograms/ml). These results indicate substantial interindividual variation in the relationship between changes in plasma methadone concentration and analgesia in patients with chronic pain receiving opioids. A pharmacokinetic-pharmacodynamic model may be useful for the individualization of analgesic dosage and therefore the optimization of pain management in patients with chronic pain.

The metabolite morphine-6-glucuronide contributes to the analgesia produced by morphine infusion in patients with pain and normal renal function.

Morphine-6-glucuronide is a metabolite of morphine that binds to the opioid receptor and is analgesic in animals and humans. Although accumulation of morphine-6-glucuronide in patients with renal insufficiency has been implicated in morphine toxicity, the contribution of the metabolite to morphine analgesia in patients with normal renal function has not been established. To evaluate this contribution, we repeatedly sampled blood and assessed effects during and after a loading infusion with morphine (mean duration, 168 minutes) in 14 patients with chronic pain, all of whom had normal serum creatinine levels. Plasma concentrations of morphine and morphine-6-glucuronide were assayed by use of high performance liquid chromatography with electrochemical detection. Patients were divided into three groups on the basis of the molar concentration ratio of morphine-6-glucuronide:morphine from the start of the infusion until 240 minutes later: Group 1 (n = 5) had a mean ratio greater than or equal to 0.7:1; group 2 (n = 4) had a mean ratio less than 0.7:1 but greater than or equal to 0.4:1; and group 3 (n = 5) had a mean ratio less than 0.4:1. Time-effect plots revealed that average and peak relief were greater in group 1 than group 2 and greater in group 2 than group 3. For all patients, mean morphine-6-glucuronide:morphine ratio throughout the study was significantly correlated with mean pain relief (r = 0.611, p less than 0.02). These data suggest that morphine-6-glucuronide contributes to morphine analgesia in patients with normal renal function. The role of the metabolite should be considered when morphine is used clinically.

Pharmacokinetic-pharmacodynamic relationships of methadone infusions in patients with cancer pain.

To determine the relationship between changes in plasma methadone concentration and pharmacodynamic effects, plasma methadone profiles and pharmacodynamics (analgesia and sedation) were measured during and after the continuous infusion of methadone for 180 to 270 minutes in 15 patients with pain caused by cancer. An increase in plasma methadone concentration resulted in a rapid increase in pain relief or sedation. The estimates of values of 50% of maximum effect (Css50) for pain relief and sedation obtained with a pharmacokinetic-pharmacodynamic model varied tenfold to twentyfold among patients; the mean Css50 value for pain relief (0.359 +/- 0.158 [SD] micrograms/ml) was virtually the same as the mean Css50 value for sedation (0.336 +/- 0.205 [SD] micrograms/ml). Similarly, the mean gamma (slope function) for pain relief (4.4 +/- 3.8 [SD]) and sedation (5.8 +/- 5.4 [SD]) did not differ. Examination of hysteresis plots of data obtained during the infusion and for 4 to 5 hours after cessation of the infusion revealed a very rapid equilibration between plasma methadone values and the sites mediating pain relief. There was no indication of the development of tolerance to the pharmacodynamic effects of methadone during the study. This report describes a method for quantitating the pharmacokinetic-pharmacodynamic relationships of the desirable and undesirable effects of opioid analgesics.

Treatment of opioid-induced constipation with oral naloxone: a pilot study.

Opioids cause constipation by binding to specific opioid receptors in the enteric and central nervous systems. First-pass glucuronidation limits systemic bioavailability of oral naloxone. This study was designed to determine if oral naloxone could reverse opioid-induced constipation without precipitating abstinence or recrudescence of pain in opioid-dependent individuals. Concentrations of unmetabolized and total naloxone, including naloxone glucuronide, were measured by radioimmunoassay. A dose-related increase in symptoms of laxation resulted in all three opioid-dependent patients studied that paralleled the increase in active and total naloxone plasma levels. Withdrawal symptoms occurred with plasma naloxone area under the plasma concentration-time curves above 550 ng.min/ml and with dosing intervals less than 3 hours. Peak plasma levels did not predict withdrawal. Oral naloxone ameliorates opioid-induced constipation in opioid-dependent persons. Titration of dose to a maximum of 12 mg at least 6 hours apart may be needed to avoid adverse reactions.

Sublingual absorption of selected opioid analgesics.

Ongoing interest in the improvement of pain management with opioid analgesics had led to the investigation of sublingual opioid absorption. The present report determined the percent absorption of selected opioid analgesics from the oral cavity of normal subjects under conditions of controlled pH and swallowing when a 1.0 ml aliquot of the test drug was placed under the tongue for a 10-minute period. Compared with morphine sulfate at pH 6.5 (18% absorption), buprenorphine (55%), fentanyl (51%), and methadone (34%) were absorbed to a significantly greater extent (p less than 0.05), whereas levorphanol, hydromorphone, oxycodone, heroin, and the opioid antagonist naloxone were not. Overall, lipophilic drugs were better absorbed than were hydrophilic drugs. Plasma morphine concentration-time profiles indicate that the apparent sublingual bioavailability of morphine is only 9.0% +/- 11.9% (SD) of that after intramuscular administration. In the same subjects the estimated sublingual absorption was 22.4% +/- 9.2% (SD), indicating that the sublingual absorption method may overestimate apparent bioavailability. When the oral cavity was buffered to pH 8.5, methadone absorption was increased to 75%. Thus, an alkaline pH microenvironment that favors the unionized fraction of opioids increased sublingual drug absorption. Although absorption was found to be independent of drug concentration, it was contact time dependent for methadone and fentanyl but not for buprenorphine. These results indicate that although the sublingual absorption and apparent sublingual bioavailability of morphine are poor, the sublingual absorption of methadone, fentanyl, and buprenorphine under controlled conditions is relatively high.

Epidural and intrathecal opiates: cerebrospinal fluid and plasma profiles in patients with chronic cancer pain.

We studied the cerebrospinal fluid (CSF) and plasma concentration-time profiles of morphine, methadone, and beta-endorphin after lumbar epidural or intrathecal injection in 17 patients with cancer. After epidural injection, all three drugs reached peak levels in lumbar CSF within 34 minutes that were 50 to 1300 times higher than free drug concentrations in plasma. The rate of decline of CSF levels correlated with drug lipid solubility (methadone [t1/2 = 73 minutes] greater than morphine [126 minutes] greater than beta-endorphin [317 minutes]). Plasma levels were comparable with those after intragluteal injection of the same dose. In four patients given intrathecal morphine or methadone, CSF at the C1-2 level contained high levels of morphine as early as 1 hour after injection, but levels of methadone were lower or undetectable. Three of 17 patients reported improved analgesia initially, but none were improved at 2 weeks after chronic therapy. We conclude that analgesia induced by intrathecal or epidural morphine injections is caused by drug acting at both spinal and supraspinal sites. The use of spinal opiates such as morphine is of limited value in patients whose pain is not adequately managed by high systemic doses of morphine-like drugs.

Propoxyphene and norpropoxyphene kinetics after single and repeated doses of propoxyphene.

Plasma concentrations of propoxyphene (P) and its pharmacologically active metabolite norpropoxyphene (NP) were determined in normal subjects after single 130-mg oral doses and during and after 13 consecutive oral doses of 130 mg P, and in former heroin addicts who were maintained on 900 to 1200 mg of P per day. The data were analyzed using a first-pass elimination pharmacokinetic model. Both P and NP cumulated during repeated dosing to levels 5 to 7 times those after the first dose. In contrast, "maintenance" patients exhibited steady-state trough plasma NP cumulation that exceeded that of P by a factor of 13. Several changes in P and NP kinetics occurred during repeated dosing with P to the normal subjects: P clearance decreased from 994 to 508 ml/min, NP clearance decreased from 454 to 2210 ml/min, P half-life (t 1/2) increased from 3.3 to 11.8 hr, NP t 1/2 increased from 6.1 to 39.2 hr, and area under the concentration time curves for P and NP were doubled. These changes in kinetics during repeated dosing resulted in more extensive cumulation of P and NP than would be predicted from the single-dose kinetic profile. Changes in the extent of first-pass elimination of P result in variability in plasma P and NP that may contribute to P-induced toxicity.

Dextromethorphan shows efficacy in experimental pain (nociception) and opioid tolerance.

The oral antitussive dextromethorphan is a clinically available N-methyl-D-aspartate receptor antagonist. Dextromethorphan has analgesic efficacy in the experimental formalin test, blocks the nociceptive activation of the immediate-early gene, c-fos proto-oncogene, and prevents and reverses the development of opiate analgesic tolerance in experimental models. These data suggest that dextromethorphan should be evaluated in a controlled clinical trial for analgesic efficacy in zoster-associated neuralgia.

Chronic morphine therapy for cancer pain: plasma and cerebrospinal fluid morphine and morphine-6-glucuronide concentrations.

Morphine-6-glucuronide (M-6-G) is an active metabolite that may contribute to the clinical effects produced by systemic administration of morphine. To help clarify the extent to which M-6-G may cross the blood-brain barrier and exert effects, we employed high-performance liquid chromatography with electrochemical detection to measure the concentrations of M-6-G and morphine in the plasma and either ventricular (three patients) or lumbar (eight patients) CSF of cancer patients receiving chronic morphine therapy. The mean ratio of morphine in ventricular CSF:morphine in plasma was 0.71; the same ratio for M-6-G was only 0.077. The average molar ratio of M-6-G: morphine in ventricular CSF was 0.207, and the average molar ratio in plasma was 1.89. Although sampling problems render the lumbar CSF results less reliable, they were very similar. Thus, plasma contained approximately twice as much M-6-G as morphine, whereas CSF contained only one-fifth to one-third as much. These data confirm that M-6-G in plasma is distributed into CSF, but to a far lesser extent than morphine. They help explain animal data demonstrating much higher potency of M-6-G on administration into CSF than systemic administration and indicate that the degree to which M-6-G contributes to morphine effects in humans remains an unresolved question.

Gabapentin enhances the antinociceptive effects of spinal morphine in the rat tail-flick test.

The antinociceptive effects of the combination of spinal morphine and gabapentin were evaluated in the tail-flick test in rats. The intrathecal coadministration of a subantinociceptive dose of morphine at 0.2 microgram and gabapentin at 300 micrograms produced significant antinociception. Pretreatment with spinal gabapentin at 300 micrograms shifted the dose-response curve of spinal morphine to the left with a decrease in morphine ED50 value from 1.06 micrograms to 0.34 microgram. The antinociceptive effects produced by the combination of a subantinociceptive dose of morphine and gabapentin were reversed by spinal naloxone at 30 micrograms but were not reversed by spinal bicuculline at 0.3 microgram. Furthermore, the concurrent administration of spinal naloxone at 30 micrograms with the combination of morphine and gabapentin blocked antinociception, while the concurrent administration of spinal bicuculline at 0.3 microgram failed to prevent antinociception. These results indicate that the combination of spinal gabapentin and morphine produces an enhancement of antinociception that appears to involve the spinal mu opioid receptors. Furthermore, repeated administration of gabapentin for 3 days did not affect the enhancing effect of gabapentin on the antinociceptive effect of morphine, indicating that tolerance did not develop to gabapentin's ability to enhance morphine antinociception.

Oral ketamine is antinociceptive in the rat formalin test: role of the metabolite, norketamine.

The present study was designed to evaluate the oral efficacy and bioavailability of ketamine. Antinociceptive efficacy was determined with the rat formalin test and oral bioavailability by the measurement of plasma and brain concentrations of ketamine and its major metabolite, norketamine. Oral ketamine in a dose range from 30 to 180 mg/kg or saline was given prior to intraplantar formalin and the flinching behavior was measured. Oral ketamine dose-dependently reduced the flinching during phase 2, while flinching during phase 1 was reduced only with the highest dose given. Following oral ketamine at 100 mg/kg, blood and brain samples were obtained and plasma and brain ketamine and norketamine levels were measured using high-performance liquid chromatography (HPLC). The average concentration ratio of norketamine/ketamine, as expressed by the area under the curve (AUC) value, was 6.4 for plasma and 2.9 for brain. These results demonstrate that a significant amount of norketamine is formed by first pass biotransformation of ketamine and is distributed to the brain. Competition binding assays for the [3H]MK-801-labeled non-competitive site of the N-methyl-D-aspartate receptor (NMDA) receptor revealed that both norketamine and ketamine displaced [3H]MK-801 at low micromolar concentrations with Ki values of 2.5 and 0.3 mM in the forebrain, and 4.2 and 1.0 mM in the spinal cord, respectively. Spinal norketamine was approximately equipotent to ketamine in producing antinociceptive effects during phase 2 of the formalin test. Thus, norketamine appears to contribute to the antinociceptive effects of oral ketamine through its NMDA receptor antagonist activity.

Competitive and non-competitive NMDA antagonists block the development of antinociceptive tolerance to morphine, but not to selective mu or delta opioid agonists in mice.

N-Methyl-D-aspartate (NMDA) receptor antagonists have been shown to block the development of antinociceptive tolerance to morphine. Assessment of the effects of NMDA antagonists on development of antinociceptive tolerance to selective opioid mu (mu) and delta (delta) agonists, however, has not been reported. In these experiments, selective mu and delta receptor agonists, and morphine, were repeatedly administered to mice either supraspinally (i.c.v.) or systemically (s.c.), alone or after pretreatment with systemic NMDA antagonists. Antinociception was evaluated using a warm-water tail-flick test. Repeated i.c.v. injections of mu agonists including morphine, fentanyl, [D-Ala2, NMePhe4, Gly-ol]enkephalin (DAMGO) and Tyr-Pro-NMePhe-D-Pro-NH2 (PL017) or [D-Ala2, Glu4]deltorphin, a delta agonist, or s.c. injections of morphine or fentanyl, produced antinociceptive tolerance as shown by a significant rightward displacement of the agonist dose-response curves compared to controls. Single injections or repeated administration of MK801 (a non-competitive NMDA antagonist) or LY235959 (a competitive NMDA antagonist) at the doses employed in this study did not produce behavioral toxicity, antinociception or alter the acute antinociceptive effects of the tested opioid agonists. Consistent with previous reports, pretreatment with MK801 or LY235959 (30 min prior to agonist administration throughout the tolerance regimen) prevented the development of antinociceptive tolerance to i.c.v. or s.c. morphine. Neither NMDA antagonist, however, affected the development of antinociceptive tolerance to i.c.v. fentanyl, DAMGO, or [D-Ala2, Glu4]deltorphin. Additionally, MK801 pretreatment did not affect the development of antinociceptive tolerance to i.c.v. PL017 or to s.c. fentanyl. Further, MK801 pretreatment also did not affect the development of tolerance to the antinociception resulting from a cold-water swim-stress episode, previously shown to be a delta-opioid mediated effect. These data lead to the suggestion that the mechanisms of tolerance to receptor selective mu and delta opioids may be regulated differently from those associated with morphine. Additionally, these findings emphasize that conclusions reached with studies employing morphine cannot always be extended to 'opiates' in general.

N-methyl-D-aspartate (NMDA) receptors, mu and kappa opioid tolerance, and perspectives on new analgesic drug development.

This laboratory perspective reviews the pharmagologic approaches that have been used in preclinical animal models to demonstrate the ability of competitive (LY274614) and noncompetitive (MK801 and dextromethorphan) N-methyl-D-aspartate (NMDA) receptor antagonists to attenuate or reverse the development of morphine tolerance. We provide additional data to support previous observations that these NMDA antagonists modulate morphine (mu) opioid tolerance but do not affect U50488H (kappa 1) opioid tolerance. A strategy, which utilizes efficacy as an NMDA receptor antagonist and clinical safety, provides the basis for a discussion of the clinical potential of dextromethorphan, ketamine, and felbamate as modulators of opioid tolerance in pain patients or opioid addicts. The potential use of NMDA receptor antagonists and nitric oxide synthase (NOS) inhibitors in neuropathic pain is also discussed.