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.     

Increases in protein kinase C gamma immunoreactivity in the spinal cord of rats associated with tolerance to the analgesic effects of morphine

Our previous studies have indicated a critical role of protein kinase C (PKC) in intracellular mechanisms of tolerance to morphine analgesia. In the present experiments, we examined (1) the cellular distribution of a PKC isoform (PKC gamma) in the spinal cord dorsal horn of rats associated with morphine tolerance by utilizing an immunocytochemical method and (2) the effects of the N-methyl-D-aspartate receptor antagonist MK-801 on tolerance-associated PKC gamma changes. In association with the development of tolerance to morphine analgesia induced by once daily intrathecal administration of 10 micrograms morphine for eight days, PKC gamma immunoreactivity was clearly increased in the spinal cord dorsal horn of these same rats. Within the spinal cord dorsal horn of morphine tolerant rats, there were significantly more PKC gamma immunostained neurons in laminae I-II than in laminae III-IV and V-VI. Such PKC gamma immunostaining was observed primarily in neuronal somata indicating a postsynaptic site of PKC gamma increases. Moreover, both the development of morphine tolerance and the increase in PKC gamma immunoreactivity were prevented by co-administration of morphine with 10 nmol MK-801 between Day 2 and Day 7 of the eight day treatment schedule. In contrast, PKC gamma immunoreactivity was not increased in rats receiving a single i.t. administration of 10 micrograms morphine on Day 8, nor did repeated treatment with 10 nmol MK-801 alone change baseline levels of PKC gamma immunoreactivity. These results provide further evidence for the involvement of PKC in NMDA receptor-mediated mechanisms of morphine tolerance.     

Dextromethorphan potentiates morphine antinociception, but does not reverse tolerance in rats

The N-methyl-D-aspartate (NMDA) and cholecystokinin (CCK)-B receptors may have a role in the development and reversal of tolerance to morphine. In morphine-tolerant rats, addition of the CCK-B receptors antagonist CI 988 or the NMDA receptor blocker dextromethorphan enhanced the antinociceptive effect of morphine on the hot plate test. However, combined administration of CI 988 and dextromethorphan did not further potentiate the antinociceptive effect of morphine in tolerant rats. Dextromethorphan by itself had no effect in tolerant rats. In drug-naive rats, dextromethorphan by itself had no antinociceptive effect, but when combined with morphine or morphine and CI 988, it significantly potentiated the magnitude and duration of the effect of morphine. Thus, unlike the reversal of tolerance with CI 988 at doses that did not potentiate the effect of morphine, the antinociception observed with the NMDA antagonist in the presence of morphine in tolerant rats may not represent the reversal of tolerance, but may instead reflect the potentiation of morphine's analgesic effect by dextromethorphan.     

Interaction of intrathecally infused morphine and lidocaine in rats (part II): effects on the development of tolerance to morphine

BACKGROUND: There has been little information regarding the effects of local anesthetics on tolerance to opioids, although chronic use of combination of opioids and local anesthetics is popular for pain control. This study was designed to examine the effects of lidocaine on morphine tolerance to somatic and visceral antinociception. METHODS: Rats received a continuous intrathecal infusion of morphine (0.3-10 microg x kg(-1) x h(-1)), lidocaine (30-1000 microg x kg(-1). h(-1)), a combination of those, or saline. After 6- day infusion, intrathecal morphine challenge test (5 microg/10 microl) was performed, and time-response curve was constructed to assess the magnitude of tolerance. The tail flick (TF) test and colorectal distension (CD) test were used to measure somatic and visceral antinociceptive effects, respectively. RESULTS: Antinociceptive effects in the TF and CD tests caused by morphine challenge were reduced (P < 0.01) in the morphine infused groups. The magnitude of the tolerance was inversely associated with the amount of morphine infused. Lidocaine infusion induced no different change in the morphine challenge test from that seen in the saline infusion group. Development of tolerance was greater in morphine 3 microg x kg(-1) h(-1) than in morphine 0.75 microg x kg(-1) x h(-1) + lidocaine 150 microg x kg(-1) x h(-1) despite their similar antinociceptive effects during intrathecal infusion. The infusion of a low dose of morphine (0.3 microg kg(-1) x h(-1)) did not reduce the antinociceptive effects in the challenge test. CONCLUSION: Lidocaine in combination with morphine does not reduce tolerance to morphine nor develop cross-tolerance. The intrathecal infusion of morphine induced tolerance to somatic and visceral antinociception in a dose-dependent fashion.     

Effects of systemic, intracerebral, or intrathecal administration on an N-methyl-D-aspartate receptor antagonist on associative morphine analgesic tolerance and hyperalgesia in rats.

A flavor paired with morphine shifted to the right the function relating morphine dose to tail-flick latencies and provoked hyperalgesic responses when rats were tested in the absence of morphine. These learned increases in nociceptive sensitivity were not mediated by alterations in tail-skin temperature. Microinjection of the competitive N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonopentanoic acid (AP-5) into the lateral ventricle reversed the hyperalgesic responses but spared the tolerance to morphine analgesia. By contrast, systemic administration of the noncompetitive NMDA receptor antagonist MK-801 or intrathecal infusion of AP-5 reversed the hyperalgesic responses as well as the tolerance to morphine analgesia. The results demonstrate that associatively mediated tolerance to morphine analgesia can co-occur with hyperalgesic responses and are discussed relative to learned activation of endogenous pronociceptive mechanisms. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Absence of antinociceptive tolerance to improgan, a cimetidine analog, in rats

Improgan, an analog of the histamine receptor antagonist cimetidine, produces highly effective analgesia following intraventricular injection. The present study examined changes in the antinociceptive effects of improgan following once daily intraventricular injections. Improgan (100-150 microg) produced near maximal antinociception 10 and 30 min after daily administration on all 4 test days, whereas comparable morphine treatments (50 microg) induced considerable tolerance. Thus, improgan produced highly effective analgesia without the development of tolerance.     

Effects of intrathecal NMDA and non-NMDA antagonists on acute thermal nociception and their interaction with morphine

BACKGROUND: N-methyl-D-aspartate (NDMA) antagonists have minimal effects on acute nociception but block facilitated states of processing. In contrast, the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) antagonists decrease acute noxious responses. Morphine (a mu-opioid agonist) can also decrease acute nociceptive processing. The authors hypothesized that the interaction between morphine and AMPA receptor antagonists would be synergistic, whereas morphine and NMDA antagonists show no such interaction in acute nociception. METHODS: Sprague-Dawley rats (weight, 250-300 g) were implanted with chronic lumbar intrathecal catheters and were assigned to receive one of several doses of morphine--ACEA 1021 (NMDA glycine site antagonist), ACEA 2085 (AMPA antagonist), AP-5 (NMDA antagonist), saline or vehicle--and were tested for their effect on the response latency using a 52.5 degrees C hot plate. The combinations of morphine and other agents also were tested. RESULTS: Intrathecal morphine (ED50:2 microg/95% confidence interval, 1-4 microg) and ACEA 2085 (6 ng/2-15 ng), but not AP-5 or ACEA 1021, yielded a dose-dependent increase in the thermal escape latency. A systematic isobolographic analysis was carried out between intrathecal morphine and ACEA 2085 using the ED50 dose ratio of 357:1. A potent synergy was observed with decreased side effects. Morphine dose-response curves were carried out for morphine and fixed doses of ACEA 1021 (12 microg) or AP-5 (10 microg). No synergistic interactions were noted. CONCLUSIONS: Spinal mu-receptor activation and AMPA receptor antagonism showed a synergistic antinociception in response to an acute thermal stimulus. NMDA or NMDA glycine site antagonism had no effect alone nor did they display synergy with morphine. These results suggest an important direction for development of acute pain strategies may focus on the AMPA receptor.     

Evidence for a role of N-methyl-D-aspartate receptors in L-arginine-induced attenuation of morphine antinociception

Twice daily injections of L-arginine (50, 100 or 200 mg/kg, i.p.) for 4 days dose-dependently, decreased morphine antinociception in male Swiss-Webster mice as measured by the tail-flick test. To determine the possible role of N-methyl-D-aspartate (NMDA) receptor in the action of L-arginine, the effects of MK-801, a noncompetitive antagonist of the NMDA receptor and of LY 235959, a competitive antagonist of the NMDA receptor on L-arginine-induced attenuation of morphine antinociception were determined. MK-801 (0.01-0.10 mg/kg, i.p.) or LY 235959 (1.0-4.0 mg/kg, i.p.) given 10 min before each injection of L-arginine (200 mg/kg, i.p.) reversed the action of the letter in a dose-dependent manner on morphine antinociception. It is concluded that NMDA receptors are involved in the action of L-arginine in attenuating morphine antinociception.     

Possible mechanisms of salt-induced hypertension in Dahl salt-sensitive rats

The effects of acute and chronic administration of cocaine on the antinociception and tolerance to the antinociceptive actions of mu-(morphine), kappa-(U-50,488H), and delta-([D-Pen2,D-Pen5]enkephalin; DPDPE), opioid receptor agonists were determined in male Swiss-Webster mice. Intraperitoneal injection of 40 mg/kg of cocaine by itself produced weak antinociceptive response as measured by the tail-fick test but the lower doses were ineffective. Administration of morphine (10 mg/kg, SC), U-50,488H (25 mg/kg, IP) or DPDPE (10 microg/mouse, ICV) produced antinociception in mice. Cocaine (20 mg/kg) potentiated the antinociceptive action of morphine and DPDPE but had no effect on U-50,488H-induced antinociception. Administration of morphine (20 mg/kg, SC), U-50,488H (25 mg/kg, IP) or DPDPE (20 microg/mouse, ICV) twice a day for 4 days resulted in the development of tolerance to their antinociceptive actions. Tolerance to the antinociceptive actions of morphine and U-50,488H was inhibited by concurrent treatment with 20 or 40 mg/kg doses of cocaine; however, tolerance to the antinociceptive action of DPDPE was not modified by cocaine. It is concluded that cocaine selectively potentiates the antinociceptive action of mu- and delta- but not of the kappa-opioid receptor agonist. On the other hand, cocaine inhibits the development of tolerance to the antinociceptive actions of mu- and kappa- but not of delta-opioid receptor agonists in mice.     

Acute tolerance to spinally administered morphine compares mechanistically with chronically induced morphine tolerance

The mechanistic similarity between acutely and chronically induced morphine tolerance has been previously proposed but remains largely unexplored. Our experiments examined the modulation of acutely induced tolerance to spinally administered morphine by agonists that affect the N-methyl-D-aspartate receptor and nitric oxide synthase systems. Antinociception was detected via the hot water (52.5 degrees C) tail flick test in mice. Intrathecal pretreatment with morphine (40 nmol) produced a 9.6-fold rightward shift in the morphine dose-response curve. This shift confirmed the induction of acute spinal morphine tolerance. Intrathecal copretreatment with the receptor antagonists (competitive and noncompetitive, respectively) dizolcipine (MK801, 3 nmol) or LY235959 (4 pmol) and morphine [40 nmol, intrathecally (i.t.)] attenuated acute tolerance to morphine measured 8 hr later. A 60-min pretreatment of 7-nitroindazole (6 nmol, i.t.), a selective neuronal NOS inhibitor, followed by administration of morphine (40 nmol, i.t.) blocked the induction of morphine tolerance. Intrathecal copretreatment with morphine (40 nmol, i.t.) and agmatine (4 nmol, i.t.), an imidazoline, receptor agonist and putative nitric oxide synthase inhibitor, almost completely abolished acute spinal morphine tolerance. The results of these experiments agree with previous reports using models of chronically induced morphine tolerance. This evidence supports the proposal that the mechanisms responsible for acute morphine tolerance parallel those underlying chronic morphine tolerance. This study attests to the powerful predictive value of acute induction as a model for morphine tolerance.     

Early endoscopic realignment as primary therapy for complete posterior urethral disruptions

Circadian changes in the interactions between L-NG-nitroarginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, and morphine-induced antinociception were investigated by the mouse hot-plate test. Both the basal pain sensitivity and morphine-induced analgesia undergo significant 24 h variations. L-NAME (40 mg/kg, i.p.) alone did not show any antinociceptive activity, but potentiated morphine-induced analgesia when combined with morphine at all injection times. In terms of percentage absolute potentiation (%AP), L-NAME dramatically augmented the analgesic effect of morphine in the late dark period at 19 hours after lights on (HALO). It is concluded that nitric oxide (NO) is involved in the modulation of the analgesic effect of morphine; thus, the L-NAME and morphine combination might be beneficial in alleviating pain.     

Spinal amino acid release and precipitated withdrawal in rats chronically infused with spinal morphine

Glutamate receptors are implicated in the genesis of opioid tolerance and dependence. Factors governing release of amino acids in systems chronically exposed to opiates, however, remain undefined. Using rats, each prepared with a spinal loop dialysis catheter and with a chronic lumbar intrathecal infusion catheter connected to a subcutaneous minipump, the release of amino acids before and during antagonist-precipitated withdrawal in unanesthetized rats was examined. Spinal infusion of morphine (20 nmol/micro l/hr) for 4 d had little effect on resting release of amino acids. In morphine-infused, but not saline-infused, rats naloxone (2 mg/kg, i.p.) evoked an immediate increase in the release of L-glutamate (299 +/- 143%) and taurine (306 +/- 113%) but not other amino acids. The magnitude and time course of the release of these amino acids significantly correlated with behavioral indices of withdrawal intensity. Acute intrathecal pretreatment immediately before naloxone with clonidine (20 microg; alpha2 agonist), MK-801 (3 microg; noncompetitive NMDA antagonist), or aminophosphonopentanoic acid (AP-5; 3 microg; competitive NMDA antagonist) suppressed naloxone-induced increases in spinal L-glutamate and taurine release and behavioral signs of withdrawal in spinal morphine-infused rats. Results point to a correlated increase in spinal L-glutamate release, which contributes to genesis of the opioid withdrawal syndrome. Agents such as clonidine that suppress opioid withdrawal may owe their action to an inhibition of excitatory amino acid release. The effects of MK-801 and AP-5 suggest a glutamate-evoked glutamate release.     

Differential antagonism by MK-801 against antinociception induced by opioid receptor agonists administered supraspinally in mice

Various doses of MK-801 ((+/-)-5-methyl-10,11-dihydro-5H-dibenzo(a,d) cyclohepten-5, 10-imine maleate), a non-competitive N-methyl-D-aspartic acid (NMDA) receptor antagonist (0.001-1 microgram) injected intracerebroventricularly (i.c.v.) alone did not show any antinociceptive effect. MK-801 (0.001-1 microgram i.c.v.) dose dependently attenuated the inhibition of the tail-flick and hot plate responses induced by i.c.v. administered morphine (1 microgram), [D-Pen2, D-Pen5]enkephalin (DPDPE; 10 micrograms), and U50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeoce tamide ) 60 micrograms). However, the inhibition of the tail-flick and hot plate responses induced by i.c.v. administered beta-endorphin (1 microgram) was not changed by i.c.v. administered MK-801. Our results indicate that, at the supraspinal level, NMDA receptors are involved in the production of antinociception induced by supraspinally administered morphine, DPDPE, and U50,488H but not beta-endorphin.     

Positive and negative cis-acting elements are required for hematopoietic expression of zebrafish GATA-1

BACKGROUND: The Na+,K+-adenosine triphosphatase is a ubiquitous enzyme system that maintains the ion gradient across the plasma membrane of a variety of cell types, including cells in the central nervous system. We investigated the antinociceptive effect of intrathecally administered ouabain and examined its potential interaction with spinal morphine and lidocaine. METHODS: Using rats chronically implanted with lumbar intrathecal catheters, the ability of intrathecally administered ouabain, morphine, and lidocaine and of mixtures of ouabain-morphine and ouabain-lidocaine to alter tail-flick latency was examined. To characterize any interactions, isobolographic analysis was performed. The effects of pretreatment with intrathecally administered atropine or naloxone also were tested. RESULTS: Intrathecally administered ouabain (0.1-5.0 microg), morphine (0.2-10.0 microg), and lidocaine (25-300 microg) given alone produced significant dose- and time-dependent antinociception, but systemic administration of ouabain did not produce such an effect. The median effective dose (ED50) values for intrathecally administered ouabain, morphine, and lidocaine were 2.3, 5.0, and 227.0 microg, respectively. Isobolographic analysis exhibited a synergistic interaction after the coadministration of ouabain and morphine. With ouabain and lidocaine, there was no such evidence of synergism. Intrathecally administered atropine, but not naloxone, completely blocked the antinociceptive effect of ouabain and attenuated its interaction with spinally administered morphine. CONCLUSIONS: Intrathecally administered ouabain produces antinociception, at least in part, via an enhancement of cholinergic transmission in the spinal nociceptive processing system. The results of the interaction of ouabain with morphine and lidocaine suggest that modulation of Na+-,K+-electrochemical gradients and thus subsequent release of neurotransmitters in the spinal cord are likely to play important roles in the spinal antinociceptive effect of intrathecally administered ouabain.     

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.     

Effect of morphine-induced catalepsy, lethality, and analgesia by a benzodiazepine receptor agonist midazolam in the rat

Previously we have shown that intrathecal administration of midazolam can increase or decrease morphine-induced antinociception, depending upon relative concentration of these drugs by modulating spinal opioid receptors, and it also can inhibit morphine-induced tolerance and dependence in the rat. Now we report that midazolam also influences catalepsy, lethality, and analgesia induced by morphine in the rat. In the acute treatment, animals were first treated with saline or midazolam (0.03 to 30.0 mg/kg, b.wt., IP), and 30 min later with a second injection of saline or morphine (1.0 to 100.0 mg/kg, b.wt., SC). The catalepsy was measured 60 min after the second injection and lethality was checked after 24 h. Midazolam injection increased the morphine-induced catalepsy and lethality. In the chronic treatment, animals were injected with two injections daily for 11 days. The first injection consisted of saline or midazolam (0.03 to 3.0 mg/kg, b.wt., IP), and 30 min later with a second injection of saline or morphine (10.0 mg/kg, b.wt., IP) was given. Lethality, antinociception, and body weight were measured. Chronic morphine treatment also increased lethality in a dose-dependent manner. Chronic treatment with midazolam and morphine increased the antinociception on day 11, as measured in the tail-flick and hot-plate tests. Midazolam administration also prevented the morphine-induced weight loss. These results suggest a strong interaction between midazolam and morphine in altering catalepsy, lethality, and analgesia in rat.     

Postoperative analgesia after co-administration of clonidine and morphine by the intrathecal route in patients undergoing hip replacement

Postoperative analgesia after intrathecal co-administration of clonidine hydrochloride (75 micrograms) and morphine sulfate (0.5 mg) was compared with analgesia produced after either intrathecal morphine (0.5 mg) or 0.9% sodium chloride in 90 patients undergoing total hip replacement under bupivacaine spinal anesthesia. Patient-controlled morphine requirements were significantly reduced (P < 0.001) postoperation by both clonidine/morphine (median 5 mg/24 h) and morphine (median 7 mg/24 h) compared with control (saline) (median 28 mg/24 h). However, no significant additional reduction in postoperative analgesic requirements was shown with the clonidine/morphine combination compared with morphine alone. Visual analog pain scores, although good in all groups at all times, were significantly poorer in the control group at 2 h (P < 0.04) and 4 h (P < 0.001) after operation compared with both treatment groups, and significantly poorer than the clonidine/morphine group at 6 h (P < 0.002) and 24 h (P < 0.009) postoperation. Mean arterial blood pressure was significantly lower in the clonidine/morphine group than in the two other groups (P < 0.001) between 2 and 5 h after operation. The incidence of emesis was similar in the clonidine/morphine and morphine groups and was significantly more than in the control group.     

A clinical study of multistage isoproterenol tilt table test for patients with unexplained syncope]

The present study examined whether eliprodil (SL 82.0715), an N-methyl-D-aspartate (NMDA) receptor antagonist acting on the polyamine sites induced expression of the 70 kDa heat shock protein (HSP70) in the rat brain. Whereas the NMDA channel blocker MK801 consistently induced HSP70 in posterior cingulate and retrosplenial cortices, eliprodil had no such effects even at the highest dose (50 mg/kg, intraperitoneally), supporting the idea that injury to the cerebrocortical neurones by NMDA receptor antagonists is probably related to specific sites of the receptor. Furthermore, eliprodil, given immediately after injection of MK801, blocked the effects of MK801 on HSP70. The result is discussed in terms of high affinity of eliprodil for the sigma receptor.     

Nociceptive and antinociceptive responses to intrathecally administered nicotinic agonists

Activation of spinal nicotinic receptors evokes a prominent algogenic response. Recently, epibatidine, a potent nicotinic agonist, was found to display an antinociceptive response after systemic administration. To examine the spinal component of this action, effects of three nicotinic agonists epibatidine, cytisine and nicotine--were given intrathecally (IT) and their antinociceptive activity was subsequently assessed. All agents elicited dose-dependent algogenic activity, characterized at lower doses by touch-evoked hyperactivity and at higher doses by intermittent vocalization and marked behavioral activity, with the rank order of potency being epibatidine > cytisine > nicotine. In addition, intrathecal epibatidine elicited a short-lasting, dose-dependent thermal antinociception. In contrast, the other nicotinic agonists at the highest usable dose failed to produce a significant antinociception. Mecamylamine, a nicotinic channel blocker, completely abolished the antinociceptive and algogenic responses of epibatidine. The competitive antagonist, alpha-lobeline, blocked both the analgesic and algogenic responses, but methyllycaconitine inhibited only the algogenic effects of epibatidine. Dihydro-beta-erythroidine, also a competitive antagonist, had no effect on the initial intense algogenic responses. The analgesic response to epibatidine was neither inhibited by naloxone nor by atropine. 2-Amino-5-phosphopentanoic acid, a competitive N-methyl-D-aspartate receptor antagonist, did not affect the analgesic response to intrathecal epibatidine or the intense initial algogenic response. Upon repeated administration (30-min interval), epibatidine (1 microg, IT) exhibited marked and rapid desensitization to both the analgesic and algogenic responses which recovered within 8 h. Pretreatment with two consecutive doses of cytisine (5 microg, IT, 30-min apart) inhibited the agitation and analgesic actions of intrathecal epibatidine. Thus, we contend that in addition to the typical nociceptive response elicited by spinal nicotinic agonists, intrathecal epibatidine also exhibits a pronounced but short-lasting antinociception. The analgesic and algogenic responses to intrathecal epibatidine may be mediated by distinct subtypes of spinal nicotinic receptors as suggested by the antagonist studies.     

Isolation, characterization and synthesis of a novel paradaxin isoform

Recently our laboratory found that tolerance to morphine-induced antinociception could be completely reversed with intracerebroventricular (i.c.v.) administration of a protein kinase A inhibitor, whereas intrathecal (i.t.) administration of the inhibitor produced hyperalgesia in morphine-tolerant mice. In the experiments described here, we sought to characterize further the role of phosphorylation events in supraspinal versus spinal opioid-mediated pain pathways and how such events might be involved in the development of antinociceptive tolerance. Two phosphatase inhibitors were administered centrally to determine whether they affected morphine-induced antinociception in naive or chronically morphine-treated mice. By the i.c.v. route, okadaic acid enhanced morphine-induced antinociception in tolerant mice and produced toxicity by the i.t. route. The calcineurin inhibitor ascomycin had no effect on antinociception following acute or chronic morphine treatment. These results suggest that increased activity of protein phosphatase types 1 and/or 2A in the brain may contribute to the development of morphine tolerance.