Effect of antagonism of the NMDA receptor on tolerance to [D-Pen2,D-Pen5]enkephalin, a delta 1-opioid receptor agonist

Pharmacological Modulation of Endogenous Opioid Activity to Attenuate Neuropathic Pain in Rats

We showed previously that spinal metabotropic glutamate receptor 1 (mGluR₁) signaling suppresses or facilitates (depending on the stage of estrous cycle) analgesic responsiveness to intrathecal endomorphin 2, a highly mu-opioid receptor-selective endogenous opioid. Spinal endomorphin 2 antinociception is suppressed during diestrus by mGluR₁ when it is activated by membrane estrogen receptor alpha (mERα) and is facilitated during proestrus when mGluR₁ is activated by glutamate. In the current study, we tested the hypothesis that in female rats subjected to spinal nerve ligation (SNL), the inhibition of spinal estrogen synthesis or blockade of spinal mERα/mGluR₁ would be antiallodynic during diestrus, whereas during proestrus, mGluR₁ blockade would worsen the mechanical allodynia. As postulated, following SNL, aromatase inhibition or mERα/mGluR₁ blockade during diestrus markedly lessened the mechanical allodynia. This was observed only on the paw ipsilateral to SNL and was eliminated by naloxone, implicating endogenous opioid mediation. In contrast, during proestrus, mGluR₁ blockade worsened the SNL-induced mechanical allodynia of the ipsilateral paw. Findings suggest menstrual cycle stage-specific drug targets for and the putative clinical utility of harnessing endogenous opioids for chronic pain management in women, as well as the value of, if not the necessity for, considering menstrual cycle stage in clinical trials thereof. PERSPECTIVE: Intrathecal treatments that enhance spinal endomorphin 2 analgesic responsiveness under basal conditions lessen mechanical allodynia in a chronic pain model. Findings provide a foundation for developing drugs that harness endogenous opioid antinociception for chronic pain relief, lessening the need for exogenous opioids and thus prescription opioid abuse.

The NMDA antagonist MK-801 induces hyperalgesia and increases CSF excitatory amino acids in rats: reversal by guanosine

Excitatory amino acids (EAAs) and their receptors play a central role in the mechanisms underlying pain transmission. NMDA-receptor antagonists such as MK-801 produce antinociceptive effects against experimental models of chronic pain, but results in acute pain models are conflicting, perhaps due to increased glutamate availability induced by the NMDA-receptor antagonists. Since guanosine and riluzole have recently been shown to stimulate glutamate uptake, the aim of this study was to examine the effects of guanosine or riluzole on changes in nociceptive signaling induced by MK-801 in an acute pain model. Rats received an i.p. injection of vehicle, morphine, guanosine, riluzole or MK-801 or a combined treatment (vehicle, morphine, guanosine or riluzole+MK-801) and were evaluated in the tail flick test, or had a CSF sample drawn after 30 min. Riluzole, guanosine, and MK-801 (0.01 or 0.1 mg/kg) did not affect basal nociceptive responses or CSF EAAs levels. However, MK-801 (0.5 mg/kg) induced hyperalgesia and increased the CSF EAAs levels; both effects were prevented by guanosine, riluzole or morphine. Hyperalgesia was correlated with CSF aspartate and glutamate levels. This study provides additional evidence for the mechanism of action of MK-801, showing that MK-801 induces hyperalgesia with parallel increase in CSF EAAs levels.

Blockade of NMDA receptors prevents analgesic tolerance to repeated transcutaneous electrical nerve stimulation (TENS) in rats

Repeated daily application of transcutaneous electrical nerve stimulation (TENS) results in tolerance, at spinal opioid receptors, to the antihyperalgesia produced by TENS. Since N-methyl-D-aspartate (NMDA) receptor antagonists prevent analgesic tolerance to opioid agonists, we hypothesized that blockade of NMDA receptors will prevent tolerance to TENS. In rats with knee joint inflammation, TENS was applied for 20 minutes daily at high-frequency (100 Hz), low-frequency (4 Hz), or sham TENS. Rats were treated with the NMDA antagonist MK-801 (0.01 mg/kg to 0.1 mg/kg) or vehicle daily before TENS. Paw withdrawal thresholds were tested before and after inflammation and before and after TENS treatment for 4 days. On day 1, TENS reversed the decreased mechanical withdrawal threshold induced by joint inflammation. On day 4, TENS had no effect on the decreased withdrawal threshold in the group treated with vehicle, demonstrating development of tolerance. However, in the group treated with 0.1 mg/kg MK-801, TENS significantly reversed the mechanical withdrawal thresholds on day 4, demonstrating that tolerance did not develop. Vehicle-treated animals developed cross-tolerance at spinal opioid receptors. Treatment with MK-801 reversed this cross-tolerance at spinal opioid receptors. In summary, blockade of NMDA receptors prevents analgesic tolerance to daily TENS by preventing tolerance at spinal opioid receptors.

PERSPECTIVE

Observed tolerance to the clinical treatment of TENS could be prevented by administration of pharmaceutical agents with NMDA receptors activity such as ketamine or dextromethorphan.

Effects of chronic dizocilpine on acute pain and on mRNA expression of neuropeptides and the dopamine and glutamate receptors

The mesocorticolimbic circuitry has been implicated in the pathophysiology of several neuropsychiatric syndromes like chronic pain and addiction. The aim of this study was to evaluate the effects of dizocilpine (MK-801), a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, on sensorimotor behaviors and the consequent changes in the dopamine, glutamate, and opiate systems in rats. Five groups of rats were subjected to acute tests for nociception (hot plate and paw pressure) before and after MK-801 (0.05, 0.1, 0.2 and 0.4 mg/kg, i.p.) or saline. Another two groups received daily i.p. saline or MK-801 (0.4 mg/kg) for 15 days. The nociceptive tests were performed on days 1, 7, and 14. On day 15 the rats received the last injection and were immediately sacrificed. We measured the mRNA expression, by in situ hybridization (ISH), of various dopamine and glutamate receptors, and enkephalin (Enk), dynorphin (Dyn), and substance P (SP) in the striatum, nucleus accumbens (NAC), piriform and cingulate cortex. Acute MK-801, dose-dependently, resulted in hyperalgesia. The chronic effects of 0.4 mg/kg MK-801 showed an extinction of the acute hyperalgesic effects especially with the hot plate test. The ISH studies revealed a decrease in mRNA expression of Enk and SP in the striatum and NAC. Our results indicate that the reversal of acute MK-801-induced hyperalgesia, with repeated exposure to systemic MK-801, is not directly related to changes in dopamine and glutamate receptors and might involve alteration of the striatal neuropeptide system.

Dextromethorphan and ketamine potentiate the antinociceptive effects of mu- but not delta- or kappa-opioid agonists in a mouse model of acute pain

Animal and clinical studies have reported potentiation of opioid antinociception by NMDA receptor antagonists such as ketamine and dextromethorphan. The aim of this study was to compare these clinically available NMDA antagonists in combination with classical morphine, mu-selective fentanyl-like opioids, the delta-opioid agonist SNC80 and the kappa-opioid agonist U50,488H. Using a mouse hot-plate test, dose-response relationships were first determined for all compounds individually and then for opioids co-administered with fixed doses of ketamine or dextromethorphan. All compounds were administered intraperitoneally ED(50) values were calculated from the proportion of animals failing to exhibit any response within a fixed cut-off criterion of 30 s. To varying degrees, all compounds produced increases in response latencies over time. Dextromethorphan produced lower ED(50) values for morphine, fentanyl and sufentanil but exerted no effect on the potency of SNC80 or U50,488H. Similarly, ketamine potentiated the antinociceptive potency of morphine, fentanyl and sufentanil but not SNC80 or U50,488H. In summary, these results support the use of mu-opioid agonists in combination with NMDA antagonists, but suggest that there may be no advantage in combining dextromethorphan or ketamine with delta- or kappa-opioids in the management of acute pain.

Dextromethorphan potentiates the antinociceptive effects of morphine and the delta-opioid agonist SNC80 in squirrel monkeys

Dextromethorphan (DXM) is a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist shown to prevent the development of tolerance to the antinociceptive effects of morphine in rodents. DXM also potentiates the antinociceptive effects of the mu-opioid receptor agonist morphine under some conditions; however, the effect of DXM in combination with opioids other than morphine has not been well characterized. This study determined the antinociceptive effects of DXM administered alone or in combination with morphine or the delta-opioid receptor (DOR) agonist SNC80 using a squirrel monkey titration procedure. In this procedure, shock (delivered to the tail) increases in intensity every 15 s (0.01-2.0 mA) in 30 increments. Five lever presses during any given 15-s shock period produces a 15-s shock-free period after which shock resumes at the next lower intensity. This assay provides a measure of antinociception that is separable from motor effects [response rate (RR)]. Morphine (0.3-3.0 mg/kg i.m.) and SNC80 (1.0-10 mg/kg i.m.), but not DXM (1.0-10 mg/kg i.m.) dose- and time-dependently increased the intensity below which monkeys (n = 4) maintained shock 50% of the time [median shock level (MSL)]. Doses of morphine and SNC80 that alone did not increase MSL were potentiated by DXM. Importantly, these combinations did not significantly alter RR. These data support previous findings with other NMDA receptor antagonists and morphine using this procedure and also extend those findings to a DOR agonist.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

NMDA and opioid receptors: their interactions in antinociception, tolerance and neuroplasticity

Over the last several years, significant progress has been made in our understanding of interactions between the N-methyl-D-aspartate (NMDA) and opioid receptors. Such interactions have been demonstrated at two distinct sites: (1) modulation of NMDA receptor-mediated electrophysiological events by opioids; and (2) intracellular events involving interactions between NMDA and opioid receptors. Furthermore, a considerable number of studies have shown the involvement of such interactions in neural mechanisms of nociceptive transmission, antinociception in acute and chronic pain states, opioid tolerance/dependence, and neuroplasticity. Importantly, emerging evidence indicates that activation of NMDA receptors may differentially modulate functions mediated by distinct opioid receptor subtypes, namely mu, delta, and kappa receptors. These studies have greatly enriched our knowledge regarding both NMDA and opioid receptor systems and have shed light on neurobiology of both acute and chronic pain. The advancement of such knowledge also promotes new strategies for better clinical management of pain patients.

Effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides from 1994 through 1998

Effects of peptides on the central nervous system are presented in two ways so as to provide a cross-listing. In the first table, the peptides are listed alphabetically. In the second table, the central nervous system effects are arranged alphabetically. No longer can there be any doubt that peptides affect the central nervous system, sometimes in several ways.

Antinociceptive tolerance to the mu-opioid agonist DAMGO is dose-dependently reduced by MK-801 in rats

Morphine has been used in previous studies that investigate interactions between the spinal cord mu-opioid and N-methyl-D-aspartate (NMDA) receptors in mechanisms of antinociceptive tolerance. Although morphine acts primarily on the mu-receptor, it also activates other subtypes of opioid receptors. In the present study, the selective mu-opioid agonist, D-Ala2-N-Me-Phe4,Gly-ol5-enkephalin (DAMGO), was used to further test the hypothesis. Repeated intrathecal (i.t.) administration of 6 microg DAMGO (twice daily) in rats for 7 days resulted in an approximately 17-fold rightward shift of the cumulative dose-response curve (the tail-flick test) on Day 8 compared to that on Day 1. This rightward shift of the dose-response curve was prevented by the i.t. co-administration with DAMGO of the NMDA receptor antagonist MK-801 (10 = 5 > 2.5 >> 1.25 nmol > saline). Further, a lower dose range of MK-801 (2.5 > 1.25 nmol > 0.625 > 0.313 = saline) was effective to prevent the antinociceptive tolerance to a lower dose (1.5 microg) of DAMGO using the same i.t. administration regimen. Thus, the present results provide further evidence supporting a cellular and intracellular model of opioid tolerance involving interactions between the mu-opioid and the NMDA receptors in the spinal cord.

Effects of NMDA receptor antagonists on delta1- and delta2-opioid receptor agonists-induced changes in the mouse brain [3H]DPDPE binding

Male Swiss-Webster mice were injected intracerebroventricularly (i.c.v.) with [D-Pen2,D-Pen5]enkephalin (20 microg/mouse) twice a day for 2 days. This procedure resulted in down-regulation of binding sites for [3H][D-Pen2,D-Pen5]enkephalin as evidenced by a 52% decrease in the Bmax value. Twice daily injections of (+)-5-methyl-10,11-dihydro-5H-dibenzo-[a,d]-cyclohepten-5,10-imine (MK-801) (0.1 mg/kg, i.p.) or [(-)3-SR,4a-RS,8a-SR-6-(phosphonomethyl)-1,2,3,4,4a,5,6,7,8,8a-decahy droisoquinoline-3-carboxylic acid] (LY 235959) (2 mg/kg, i.p.), the noncompetitive and competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, respectively, for 2 days did not alter the Bmax or Kd value of [3H][D-Pen2,D-Pen5]enkephalin binding to the mouse brain. Concurrent treatment of MK-801, but not of LY 235959 with [D-Pen2,D-Pen5]enkephalin, reversed the decreases in Bmax value of [3H][D-Pen2,D-Pen5]enkephalin. Twice daily injections of [D-Ala2,Glu4] deltorphin II (20 microg/mouse) for 2 days caused an increase in the Kd value, but not the Bmax value of [3H][D-Pen2,D-Pen5]enkephalin to bind to brain membranes. Concurrent treatment of [D-Ala2,Glu4]deltorphin II with LY 235959 reversed the increase in Kd value of [3H][D-Pen2,D-Pen5]enkephalin binding induced by multiple injections of [D-Ala2,Glu4]deltorphin II, but MK-801 had no effect. The results suggest that multiple injections of delta1- and delta2-opioid receptor agonists down-regulate delta1-opioid receptors of the brain by modifying Bmax and Kd values of [3H][D-Pen2,D-Pen5]enkephalin binding, respectively. MK-801 and LY 235959 reverse delta1- and delta2-opioid receptor agonists-induced down-regulation of brain delta1-opioid receptor, respectively, apparently by different mechanisms. It is concluded that short term treatment of mice with delta1-opioid receptor agonist down-regulates brain delta1-opioid receptors by decreasing Bmax of the ligand which is partially reversed by concurrent treatment with MK-801 but not by LY 235959. On the other hand, short term treatment of mice with delta2-opioid receptor agonist down-regulates brain delta1-opioid receptors by increasing Kd of the ligand which is partially reversed by concurrent treatment with LY 235959 but not by MK-801.

Effect of multiple injections of U-50, 488H, a kappa-opioid receptor agonist, on the activity of nitric oxide synthase in brain regions and spinal cord of mice

1. The time course of the effects of multiple injections of U-50,488H, a kappa-opioid receptor agonist, and its subsequent termination on its analgesic action and nitric oxide synthase (NOS) activity was determined in the brain regions and spinal cord of the mouse. 2. Male Swiss-Webster mice were rendered tolerant to U-50,488H by twice-daily injections of the drug (25 mg/kg, IP) for 4 days. Vehicle-injected mice served as controls. 3. In tolerant mice, NOS activity was unchanged in brain regions and the spinal cord after treatment with U-50,488H. During abstinence from U-50,488H, NOS activity was found to be increased in the cortex and remainder of the brain, but no change was noted in the cerebellum, midbrain and spinal cord. 4. These studies demonstrate that withdrawal from the short-term treatment with U-50,488H in mice causes induction of NOS in certain brain regions. However, long-term treatment and withdrawal from U-50,488H are not associated with changes in the central NOS activity and indicate a possible adaptation in the NOS activity.

Time course of the changes in central nitric oxide synthase activity following chronic treatment with morphine in the mouse: reversal by naltrexone

1. The time course of the effect of chronic administration of morphine on the activity of nitric oxide synthase (NOS) in the brain regions and spinal cord of the mouse was determined. The effect of naltrexone by itself on the NOS activity and that induced by morphine also were determined. 2. Male Swiss Webster mice were implanted subcutaneously with a pellet containing 25 mg of morphine free base for 4 days. Placebo pellet implanted mice served as controls. 3. Twenty-four hours after treatment with morphine, NOS activity decreased in the cerebellum, midbrain, cortex and remainder of the brain as well as in the spinal cord. Forty-eight and 72 hr after the treatment with morphine, NOS activity increased in the cerebellum and cortex, but no change was observed in the other brain regions and spinal cord. Twenty-four hours after morphine pellet removal (withdrawal), NOS activity in all brain regions and the spinal cord has returned to normal. 4. Implantation of a pellet containing 10 mg of naltrexone did not alter NOS activity in any brain region or spinal cord for 24, 48 and 72 hr or 24 hr after removal of the pellet. 5. Implantation of a naltrexone pellet in conjunction with a morphine pellet blocked the changes in NOS activity in the brain region and spinal cord induced by morphine. 6. It is concluded that the initial decrease in NOS activity by morphine may be related to enhanced motor activity, whereas the increase in NOS activity in certain brain regions may be associated with tolerance-physical dependence development. Additionally, the changes in central NOS activity by morphine appear to be mediated by opioid receptors because they were blocked by concurrent treatment with naltrexone.

Effects of ibogaine on the development of tolerance to antinociceptive action of mu-, delta- and kappa-opioid receptor agonists in mice

The effects of ibogaine, an alkaloid isolated from the bark of the African shrub, Tabernanthe iboga, on the development of tolerance to the antinociception action of morphine, U-50,488H and [D-Pen2,D-Pen5]enkephalin (DPDPE), which are mu-, kappa- and delta-opioid receptor agonists, respectively, were determined in male Swiss-Webster mice. Mice were rendered tolerant to opioid receptor agonists by injecting morphine (20 mg/kg, s.c.), U-50,488H (25 mg/kg, i.p.) or DPDPE (20 microg/mouse, i.c.v.) twice a day for 4 days. Ibogaine (20, 40 or 80 mg/kg, i.p.) given twice a day for 4 days did not alter the tail-flick latency. Ibogaine (40 or 80 mg/kg, i.p.) injected 10 min before each injection of morphine inhibited the development of tolerance to the antinociceptive action of morphine, however, the lower dose of ibogaine (20 mg/kg, i.p.) was ineffective. Ibogaine (20, 40 or 80 mg/kg, i.p.) given prior to the injection of U-50,488H or DPDPE did not modify the development of tolerance to their antinociceptive action. It is concluded that ibogaine inhibits selectively the development of tolerance to the antinociceptive action of mu- but not kappa- or delta-opioid receptor agonists in mice.

Modification of the binding of [3H]MK-801 to brain regions and spinal cord of rats treated chronically with U-50,488H, a kappa-opioid receptor agonist

Male Sprague-Dawley rats were rendered tolerant to U-50,488H by twice-daily injections of the drug (25 mg/kg, i.p.) for 4 days. In tolerant rats, the binding of [3H]MK-801 was increased in pons and medulla and corpus striatum but decreased in midbrain and hippocampus and was due to changes in Bmax values. In U-50,488H-abstinent rats, the binding of [3H]MK-801 was increased in pons and medulla and hippocampus, and decreased in midbrain and amygdala. In hippocampus, the Bmax of [3H]MK-801 was increased but the Kd was decreased whereas in amygdala and pons and medulla, the changes were due to alterations in the Bmax values. Previous studies have shown that NMDA receptor antagonists block the tolerance to the analgesic action of U-50,488H in rodents. The present studies demonstrate differential changes in the NMDA receptors of brain regions of U-50,488H-tolerant and -abstinent rats.

Effects of multiple intracerebroventricular injections of [D-Pen2,D-Pen5]enkephalin and [D-Ala2,Glu4]deltorphin II on tolerance to their analgesic action and on brain delta-opioid receptors

Male Swiss-Webster mice were injected intracerebroventricularly (i.c.v.) with [D-Pen2,D-Pen5]enkephalin (DPDPE), a delta 1-opioid receptor agonist (20 micrograms per mouse) twice a day for either 2 or 4 days. Vehicle injected mice served as controls. Treatment of mice with DPDPE for 2 or 4 days decreased its analgesic response by 44 and 76%, respectively in comparison to vehicle injected mice. Treatment of mice with DPDPE for 2 or 4 days decreased density (Bmax) of [3H]DPDPE to bind to brain homogenates by 77 and 76%, respectively, in comparison to vehicle injected controls but the apparent dissociation constant (kd) values were not altered. The effects of i.c.v. injections of [D-Ala2,Glu4]deltorphin II (deltorphin II), a delta 2-opioid receptor agonist (20 micrograms per mouse) twice a day for either 2 or 4 days on its analgesic response as well as on brain receptors for [3H]DPDPE were also determined. The analgesic response to deltorphin II decreased by 51 and 78%, respectively in mice treated with deltorphin II for 2 or 4 days, respectively. Two or four days treatment with deltorphin II decreased the Bmax of [3H]DPDPE by 76 and 87%, respectively. The 2-day treatment also increased the Kd value by 58%, but the 4-day treatment with deltorphin II had no effect on the Kd of [3H]DPDPE to bind to brain membranes. Thus, multiple injections of delta 1- or delta 2-opioid receptor agonists results in the development of tolerance to their analgesic action and the intensity of tolerance increases with the duration of treatment. Both delta 1- and delta 2-opioid receptor agonist, on chronic administration, result in the down-regulation of delta 1-opioid receptors labeled with [3H]DPDPE.

Effect of chronic administration of morphine, U-50, 488H and [D-Pen2, D-Pen5]enkephalin on the concentration of cGMP in brain regions and spinal cord of the mouse

The effects of chronic administration and subsequent withdrawal of mu-, kappa- and delta-opioid receptor agonists on the levels of cyclic GMP in several brain regions and spinal cord of mice were determined in an attempt to further study the role of NO cascade in opioid actions. The agonists at mu-, kappa- and delta-opioid receptor included morphine, U-50,488H and DPDPE, respectively. Tolerance to morphine was associated with highly significant increases in cGMP levels in corpus striatum (41%), cortex (36%), midbrain (73%) and cerebellum (51%) relative to controls. Abstinence caused increases in cGMP levels in corpus striatum (61%) and pons and medulla (45%). Tolerance to U-50,488H resulted in increases in cGMP levels in midbrain (52%) whereas abstinence from U-50,488H increased the cGMP levels in pons and medulla (76%). Tolerance to DPDPE was associated with increases in cGMP levels in hypothalamus (12%) and pons and medulla (33%) but decreases in cerebellum (66%) and spinal cord (58%). Abstinence from DPDPE produced increases in cGMP levels in pons and medulla (14%) but decreases in cerebellum (67%) and spinal cord (50%). Overall treatment with morphine and U-50,488H produced increases in cGMP levels in brain regions whereas DPDPE produced decreases in brain regions and spinal cord. Previous studies have shown that chronic administration of mu- and kappa-opioid receptor agonists induce NO synthase (NOS) in certain brain regions and that the inhibitors of NO synthase attenuate tolerance to mu- and kappa- but not to delta-opioid receptors agonists. Since activation of NO increases the production of cGMP, the present results demonstrating alterations of cGMP levels by mu-, kappa- and delta-opioid receptor agonists are consistent with the behavioral results with NOS inhibitors on tolerance to mu-, kappa- and delta-opioid receptor agonists.

Endogenous opiates: 1996

This paper is the nineteenth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1996 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress, tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Effects of some 7-arylidene and 7-heteroarylidene morphinan-6-ones on the antinociceptive activity of [D-Pen2, D-Pen5]enkephalin and [D-Ala2, Glu4]deltorphin II and on multiple opioid receptors

The in vivo and functional effects of several 7-arylidene and 7-heteroarylidene morphinan-6-ones were determined at the mu-, delta-, and kappa-opioid receptors using the guinea pig brain membranes, guinea pig ileum (GPI), and mouse vas deferens (MVD). In vivo effects included the antagonism by these compounds given subcutaneously on the antinociceptive actions of intracerebroventricularly injected [D-Pen2, D-Pen5]enkephalin (DPDPE) and [D-Ala2, Glu4]deltorphin II (deltorphin II), the highly selective putative delta 1- and delta 2-opioid receptor agonists. Finally, the partition coefficients of these compounds were estimated (CLOGP) and determined experimentally at pH 7.4 in the 1-octanol/water system. Compared with E-7-benzylidenenaltrexone (BNTX), most compounds except for E-7-(4-chlorobenzylidene)naltrexone, were more potent at delta-opioid receptors than at the mu-opioid receptor, whereas, in comparison to the kappa-opioid receptor, the activities of the E-7-arylidene or E-7-heteroarylidene naltrexone derivatives at the delta-receptor were in the following order, where the 7-substituents were: 4-fluorobenzylidene- > benzylidene > 3-pyridylmethylene- > 4-pyridylmethylene- > 1-methyl-2-imidazolylmethylene > 4-chlorobenzylidene. In the MVD preparation, the potencies at the delta-opioid receptor, in comparison to BNTX, were in the following order, where the 7-substituents were: benzylidene = 1-methyl-2-imidazolylmethylene- > 4-fluorobenzylidene- = 3-pyridylmethylene- = 4-pyridylmethylene-. All compounds antagonized delta 1, and delta 2-opioid receptor agonist-induced analgesia. The antagonist potencies at the delta 1-opioid receptor were in the following order, where the 7-substituents were: benzylidene- > 4-chlorobenzylidene- > 4-fluorobenzylidene- > 3-pyridylmethylene- > 1-methyl-2-imidazolymethylene- approximately 4-pyridylmethylene-, whereas at the delta 2-opioid receptor, the order was benzylidene- > 4-chlorobenzylidene- > 4-fluorobenzylidene- > 3-pyridylmethylene- > 1-methyl-2-imidazolymethylene- > 4-pyridylmethylene. In general, all compounds exhibited greater potency at the delta 2- than delta 1-opioid receptor. The computed partition coefficients were, as expected, greater than the apparent log P values, which were determined experimentally. Generally, the lipophilicity values in decreasing order were: 4-chlorobenzylidene- > 4-fluorobenzylidene- > benzylidene > 3-pyridylmethylene- = 4-pyridylmethylene- > 1-methyl-2-imidazolylmethylene-. In general, the benzylidene and 4-pyridylmethylene derivatives, which have medium lipophilicities, were equally effective at the delta 1- and delta 2-receptors; the 3-pyridylmethylene and 1-methyl-2-imidazolylmethylene derivatives had lower lipophilicities and were more selective for the delta 2- than delta 1-receptor; the 4-chlorobenzylidene and 4-fluorobenzylidene derivatives were more lipophilic and had intermediate activity. The plot of pED50 values for the in vivo tests for the delta 1- and delta 2-receptors showed that the two receptors are not independent with respect to this series of compounds.

Up-regulation of brain N-methyl-D-aspartate receptors following multiple intracerebroventricular injections of [D-Pen2, D-Pen5] enkephalin and [D-Ala2, Glu4]deltorphin II in mice

The effects of chronic administration of [D-Pen2, D-Pen5]enkephalin and [D-Ala2, Glu4]deltorphin II, the selective agonists of the delta 1- and delta 2-opioid receptors, on the binding of [3H]MK-801, a noncompetitive antagonist of the N-methyl-D-aspartate receptor, were determined in several brain regions of the mouse. Male Swiss-Webster mice were injected intracerebroventricularly (i.c.v.) with [D-Pen2, D-Pen5]enkephalin or [D-Ala2, Glu4]deltorphin II (20 micrograms/mouse) twice a day for 4 days. Vehicle injected mice served as controls. Previously we have shown that the above treatment results in the development of tolerance to their analgesic activity. The binding of [3H]MK-801 was determined in brain regions (cortex, midbrain, pons and medulla, hippocampus, striatum, hypothalamus and amygdala). At 5 nM-concentration, the binding of [3H]MK-801 was increased in cerebral cortex, hippocampus, and pons and medulla of [D-Pen2, D-Pen5]enkephalin treated mice. In [D-Ala2, Glu4]deltorphin II treated mice, the binding of [3H]MK-801 was increased in cerebral cortex and hippocampus. The changes in the binding were due to increases in the Bmax value of [3H]MK-801. It is concluded that tolerance to delta 1- and delta 2-opioid receptor agonists is associated with up-regulation of brain N-methyl-D-aspartate receptors, however, some brain areas affected differ with the two treatments. The results are consistent with the recent observation from this laboratory that N-methyl-D-aspartate receptors antagonists block tolerance to the analgesic action of delta 1- and delta 2-opioid receptor agonists.

Effects of chronic administration of 7-benzylidene-7-dehydronaltrexone and naltriben on the antinociceptive actions of delta 1- and delta 2-opioid receptor agonists

The effects of chronic administration of 7-benzylidene-7-dehydronaltrexone, a delta 1-opioid receptor antagonist and naltriben, a delta 2-opioid receptor antagonist, on the antinociceptive responses to [D-Pen2, D-Pen5] enkephalin and [D-Ala2, Glu4]deltorphin II, delta 1- and delta 2-opioid receptor agonists, respectively, were determined in the mouse. Female B6C3F1 mice were given 7-benzylidene-7-dehydronaltrexone (3 mg/kg/day), naltriben (1 mg/kg/day) or the vehicle by subcutaneously implanted Alzet osmotic minipumps for 7 days. Both [D-Pen2, D-Pen5]enkephalin and [D-Ala2, Glu4]deltorphin II administered intracerebroventricularly (i.c.v.) produced antinociceptive as measured by the tail-flick test with ED50 values of 6.76 and 6.68 micrograms/mouse, respectively. Chronic administration of 7-benzylidene-7-dehydronaltrexone lowered the ED50 of [D-Pen2, D-Pen5]enkephalin but not of [D-Ala2, Glu4]deltorphin II. Chronic administration of naltriben lowered the ED50 of [D-Ala2, Glu4]deltorphin II but had no effect on the ED50 of [D-Pen2, D-Pen5]enkephalin. The binding of [3H][D-Pen2, D-Pen5]enkephalin to whole brain membranes of chronic 7-benzylidene-7-dehydronaltrexone-treated mice did not differ from chronic vehicle-treated mice. On the other hand, chronic administration of naltriben resulted in slight but reproducible elevation in the Bmax value of [3H][D-Pen2, D-Pen5]enkephalin to bind to whole brain membranes in comparison to vehicle-injected controls. The results suggest that chronic treatment with delta 1- and delta 2-opioid receptor antagonist cause behavioral supersensitivity to their agonists, respectively, and provides further evidence for the existence of delta-opioid receptor subtypes.

Effect of nitric oxide synthase inhibition on tolerance to the analgesic action of D-Pen2, D-Pen5 enkephalin and morphine in the mouse

The effects of NG-nitro-L-arginine (NNA), an inhibitor of nitric oxide synthase (NOS) on the development of tolerance to the analgesic action of D-Pen2, D-Pen5 enkephalin (DPDPE), a delta 1 opioid receptor agonist, and morphine were determined in the mouse. Tolerance to DPDPE was induced in male Swiss-Webster mice by twice daily intracerebroventricular (i.c.v.) injections of the drug (20 micrograms/mouse) for 4 days. NNA was injected intraperitoneally (i.p.) 10 min before each injection of DPDPE. Chronic injections of DPDPE resulted in development of tolerance to its analgesic action. Multiple injections of NNA by itself did not modify the analgesic response to DPDPE. Concurrent injections of NNA did not affect the development of tolerance to the analgesic action of DPDPE. Twice daily injections of morphine (15 mg/kg s.c.) for 4 days resulted in the tolerance to its analgesic action. Concurrent administration of NNA with morphine attenuated the development of tolerance to morphine. It is concluded that NOS inhibition attenuates morphine, but not delta, opioid agonist-induced tolerance in the mouse.

Effects of morphine-3-glucuronide on the antinociceptive activity of peptide and nonpeptide opioid receptor agonists in mice

The effects of morphine-3-glucuronide (M3G), a metabolite of morphine, were determined on the antinociceptive actions, as measured by the tail flick test, of morphine, a mu-opioid receptor agonist, of U-50,488H, a kappa-opioid receptor agonist of [D-Pen2, D-Pen3]enkephalin (DPDPE), a delta 1-opioid receptor agonist, and of [D-Ala2,Glu4]deltorphin II (deltorphin II), a delta 2-opioid receptor agonist in mice. Morphine administered ICV (2.5 micrograms/ mouse) or SC (10 mg/kg), U-50,488H (25 mg/kg, IP), DPDPE (15 micrograms/mouse; ICV), and deltorphin II (15 micrograms/mouse, ICV) produced antinociception in mice. Intraperitoneal or ICV injections of M3G did not produce any effect on the tail flick latency nor did it affect the antinociception-induced by morphine, U-50,488H, DPDPE, or deltorphin II. Previously M3G has been shown to antagonize the antinociceptive effects of morphine in the rat. It is concluded that in the mouse, M3G neither produces hyperalgesia nor modifies the actions of mu-, kappa-, delta 1-, or delta 2-opioid receptor agonists.

Nitric oxide synthase inhibition attenuates tolerance to morphine but not to [D-Ala2, Glu4] deltorphin II, a delta 2-opioid receptor agonist in mice

The effects of NG-nitro-L-arginine (L-NNA) and NG-monomethyl-L-arginine (L-NMMA), two potent inhibitors of nitric oxide synthase (NOS) on the development of tolerance to the analgesic action of [D-Ala2, Glu4] deltorphin II (deltorphin II), a delta 2-opioid receptor agonist, and morphine, a mu-opioid receptor agonist, were determined in mice. Male Swiss-Webster mice were rendered tolerant to deltorphin II by twice daily ICV injections of the drug for 4 days. Tolerance to morphine was induced by twice daily s.c. injections of the drug for 4 days. Multiple injections of deltorphin II (20 micrograms/mouse) or morphine (15 mg/kg) resulted in the development of tolerance to their analgesic action as evidenced by decreases in the response in comparison to mice injected with vehicle. Concurrent administration of L-NNA or L-NMMA (2,4, or 8 mg/kg, i.p.) had no effect on the development of tolerance to the analgesic action of deltorphin II. However, the same doses of L-NNA or L-NMMA inhibited the development of tolerance to the analgesic action of morphine. Acute treatment with L-NNA or L-NMMA did not modify deltorphin II- or morphine-induced analgesia in mice. It is concluded that NOS inhibition attenuates tolerance to the analgesic action of morphine but not to that of deltorphin II, a delta 2-opioid receptor agonist, in the mouse.