Intrathecal catheterization influences tolerance to chronic morphine in rats.

We evaluated the antinociceptive effects of acute and chronic morphine administered spinally via lumbar puncture in intrathecally catheterized and sham-surgery rats. The effects of acute morphine did not differ between groups. Catheterized rats developed tolerance to chronic morphine more rapidly, compared with sham and naive rats. Therefore, catheter presence facilitated development of opioid antinociceptive tolerance. Spinal astrogliosis, determined by measurement of 3-dimensional cell volumes, was observed in catheterized rats as indicated by significantly larger cell volumes compared with surgery-naive controls. Gliosis induced by chronic intrathecal morphine administered to surgery-naive animals was comparable to that observed in saline-treated catheterized rats.

Low dose alpha-2 antagonist paradoxically enhances rat norepinephrine and clonidine analgesia.

Ultralow-dose opioid antagonists prolong opioid antinociception and block tolerance. In this study we determined whether low doses of the α-2 adrenergic receptor (A2-R) antagonist, atipamezole, similarly influenced A2-R-induced antinociception and tolerance. In rats, intrathecal norepinephrine (NE) or clonidine in combination with atipamezole was tested using tail-flick and paw pressure tests. Acute tolerance to NE was induced by serial injections. Low-dose atipamezole significantly prolonged NE and clonidine-induced antinociception. Coadministration of atipamezole with A2-R agonists also prevented loss of agonist potency in the acute tolerance model. This study demonstrates paradoxical effects of low-dose A2-R antagonists augmenting A2-R agonist-induced analgesia.

Attenuation of opioid analgesic tolerance in p75 neurotrophin receptor null mutant mice.

Repeated exposure to opioid drugs can lead to the development of tolerance, which manifests as a reduction in analgesic potency, and physical dependence, a response indicated by a withdrawal syndrome. Accumulating evidence suggests that the nerve growth factor (NGF) family of neurotrophins may have an important modulatory role in the induction of opioid analgesia and opioid addiction. Because neurotrophins universally bind the p75 neurotrophin receptor (p75NTR), we investigated whether the activity of this receptor is involved in the development of opioid analgesic tolerance and physical dependence. We found that in both the wild-type and p75NTR-/- mice an acute systemic (i.p.) injection of morphine produced a maximal analgesic response as measured by the thermal tail-immersion test. Repeated injection of morphine over 5 days in wild-type mice resulted in a progressive decline of the analgesic effect and a concomitant loss of the agonist potency, reflecting development of morphine tolerance. However, the loss of morphine analgesia was not observed in p75NTR-/- mice. In the second part of this study, mice were given escalating doses of systemic (i.p.) morphine over 5 days and subsequently challenged with the opioid receptor antagonist naloxone. This challenge precipitated a robust withdrawal syndrome that was comparable in wild-type mice and p75NTR-/- mice. The findings suggest that p75NTR activity plays a critical role in the development of opioid analgesic tolerance but not in the induction or the expression of opioid physical dependence.

Subchronic MK-801 behavioural deficits in rats: partial reversal by the novel nitrate GT 1061.

Cognitive deficits are a core feature of schizophrenia that may be linked to abnormalities in GABA and nitric oxide (NO). Subchronic treatment with glutamate receptor antagonists produces similar deficits, providing a useful model to examine potential therapeutics. The present study investigated the effects of subchronic MK-801 (intraperitoneally; 0.5 mg/kg twice daily for 7 days) on amphetamine-induced locomotor activity and reversal learning in the water maze in rats, and the ability of the novel compound GT 1061 (4-methyl-5-(2-nitroxyethyl) thiazole HCl), containing dual pharmacophores producing NO- and GABA-mimetic activity, to ameliorate these effects. MK-801 enhanced locomotor responses to amphetamine. GT 1061 (0.1; not 0.0001, 0.001, 0.01, 1.0 mg/kg) further enhanced locomotion; the pro-GABA drug chlormethiazole (0.1, 1.0 mg/kg) had no significant effect. In saline-pretreated rats GT 1061 (0.1; not 0.0001, 0.001 mg/kg) increased amphetamine-induced locomotion; chlormethiazole (0.1, 1.0 mg/kg) had no effect. In the water maze, MK-801 impaired reversal learning after platform relocation. GT 1061 (0.001, 0.01, 0.1; not 0.0001 or 1.0 mg/kg) attenuated this impairment; chlormethiazole had no significant effect. These ameliorative effects of GT 1061 may be linked to the activation of NO- and GABA-dependent signaling and suggests a new direction for treating cognitive dysfunction in schizophrenia.

Inhibition of tolerance to spinal morphine antinociception by low doses of opioid receptor antagonists.

Ultra-low doses of opioid receptor antagonists inhibit development of chronic spinal morphine tolerance. As this phenomenon mechanistically resembles acute tolerance, the present study examined actions of opioid receptor antagonists on acute spinal morphine tolerance. In adult rats, administration of three intrathecal injections of morphine (15 microg) at 90 min intervals produced a significant decline of the antinociceptive effect and loss of agonist potency in both the tail-flick and paw-pressure tests. These reduced responses, indicative of acute tolerance, were blocked by co-injection of morphine (15 microg) with naltrexone (NTX, 0.05 ng), D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTAP, 0.001 ng), naltrindole (0.06 ng), or nor-binaltorphimine (0.1 ng). Repeated injections of CTAP, naltrindole, or nor-binaltorphimine without morphine elicited a delayed weak antinociceptive response which was blocked by a high dose of naltrexone (2 microg). In another set of experiments, administration of low dose spinal (0.05 ng) or systemic (0.01 microg/kg) morphine produced a sustained thermal hyperalgesia. This response was blocked by opioid receptor antagonists at doses inhibiting development of acute morphine tolerance. Lastly, an acute spinal injection of morphine (15 microg) with naltrexone (0.05 ng) produced a sustained analgesic response; this was antagonized by adenosine receptor antagonist, 8-phenyltheophylline (3 microg). The results show that ultra-low doses of opioid receptor antagonists block acute tolerance to morphine. This effect may result from blockade of opioid excitatory effects that produce a latent hyperalgesia that then contributes to induction of tolerance. The sustained antinociception produced by combination of morphine with an opioid receptor antagonist shows dependency on the adenosine receptor activity.

The spinal basis of opioid tolerance and physical dependence: Involvement of calcitonin gene-related peptide, substance P, and arachidonic acid-derived metabolites.

Chronic opioid use in the management of pain is limited by development of analgesic tolerance and physical dependence. The mechanisms underlying tolerance-dependence are not entirely clear, however, recent evidence suggests that spinal adaptations leading to increased activity of sensory neuropeptides (calcitonin gene-related peptide (CGRP), substance P) and their downstream signaling messengers derived from metabolism of arachidonic acid: prostaglandins (PG), lipoxygenase (LOX) metabolites, and endocannabinoids, plays an important role in this phenomenon. In this communication we review the evidence implicating these factors in the induction and expression of opioid tolerance and physical dependence at the spinal level.

Analgesia and tolerance to intrathecal morphine and norepinephrine infusion via implanted mini-osmotic pumps in the rat.

The objectives of this study were to investigate the duration of analgesia and the development of tolerance following continuous intrathecal administration of morphine and norepinephrine alone, and morphine followed by norepinephrine via mini-osmotic pumps in the rat. Analgesia was assessed by the tail-flick test. In single pump experiments morphine 1 microliter (10 micrograms)/h (7 days) and 0.5 microliter (10 micrograms)/h (14 days) produced analgesia with tolerance by days 5-7. Norepinephrine 1 microliter (15 micrograms)/h (7 days) produced analgesia equivalent to that of morphine with tolerance developing by day 3. Following continuous intrathecal morphine 1 microliter (10 micrograms)/h for 5 days, norepinephrine 1 microliter (15 micrograms)/h for 7 days failed to produce a significant increase in analgesia. This was in contrast to the increase in analgesia seen when the norepinephrine infusion followed a saline infusion. Determination of the norepinephrine concentration in the solution from the osmotic pumps verified that the norepinephrine is stable for the treatment period.

The role of spinal neuropeptides and prostaglandins in opioid physical dependence.

This study examined the role of spinal calcitonin gene-related peptide (CGRP), substance P, and prostaglandins in the development and expression of opioid physical dependence. Administration of escalating doses (5 - 100 mg kg-1, i.p.) of morphine for 7 days markedly elevated CGRP and substance P- immunoreactivity in the dorsal horn of the rat spinal cord. Naloxone (2 mg kg-1, i.p.) challenge decreased both CGRP and substance P immunoreactivity and precipitated a robust withdrawal syndrome. Acute intrathecal pre-treatment with a CGRP receptor antagonist, CGRP(8 - 37) (4, 8 microg), a substance P receptor antagonist, SR 140333 (1.4, 2.8 microg), a cyclo-oxygenase (COX) inhibitor, ketorolac (30, 45 microg), and COX-2 selective inhibitors, DuP 697 (10, 30 microg) and nimesulide (30 microg), 30 min before naloxone challenge, partially attenuated the symptoms of morphine withdrawal. CGRP(8 - 37) (8 microg), but no other agents, inhibited the decrease in CGRP immunoreactivity. Chronic intrathecal treatment with CGRP(8 - 37) (4, 8 microg), SR 140333 (1.4 microg), ketorolac (15, 30 microg), DuP 697 (10, 30micro g), and nimesulide (30 microg), delivered with daily morphine injection significantly attenuated both the symptoms of withdrawal and the decrease in CGRP but not substance P immunoreactivity. The results of this study suggest that activation of CGRP and substance P receptors, at the spinal level, contributes to the induction and expression of opioid physical dependence and that this activity may be partially expressed through the intermediary actions of prostaglandins.

Spinal administration of lipoxygenase inhibitors suppresses behavioural and neurochemical manifestations of naloxone-precipitated opioid withdrawal.

1. This study investigated the role of spinal lipoxygenase (LOX) products in the induction and expression of opioid physical dependence using behavioural assessment of withdrawal and immunostaining for CGRP and Fos protein expression in the spinal cord. 2. Administration of escalating doses (5-50 mg kg-1; i.p.) of morphine for 5 days markedly elevated CGRP-like immunoreactivity in the dorsal horn of the rat spinal cord. Naloxone (2 mg kg-1; i.p.) challenge precipitated a robust withdrawal syndrome that depleted CGRP-like immunoreactivity and increased the number of Fos-like immunoreactive neurons in the dorsal horn. 3. Intrathecal administration of NDGA (10, 20 microg), a nonselective LOX inhibitor, AA-861 (1.5, 3 microg), a 5-LOX selective inhibitor, or baicalein (1.4, 2.8 microg), a 12-LOX selective inhibitor, concurrently with systemic morphine for 5 days or as a single injection immediately preceding naloxone challenge, blocked the depletion of CGRP-like immunoreactivity, prevented increase in the number of Fos-like immunoreactive neurons in the dorsal horn, and significantly attenuated the morphine withdrawal syndrome. 4. The results of this study suggest that activity of LOX products, at the spinal level, contributes to the expression of opioid physical dependence, and that this activity may be expressed through increased sensory neuropeptide release.

Involvement of spinal lipoxygenase metabolites in hyperalgesia and opioid tolerance.

This study investigated role of spinal lipoxygenase metabolites in induction of hyperalgesia and development of opioid analgesic tolerance. In the rat, nociception was measured using formalin and tail-flick tests. Intrathecal administration of leukotriene receptor agonist (LTB4) augmented the second phase of the formalin response and marginally increased sensitivity to acute thermal stimulation in the tail-flick test, responses suppressed by 6-(6-(3R-hydroxy-1E,5Z-undecadien-1-yl)-2-pyridinyl)-1,5S-hexanediol (U75302), a leukotriene BLT receptor antagonist. Treatment with 15-hydroxyperoxyeicosatetranoic acid (HPETE) increased phase II formalin activity, but had no effect on tail-flick responses. 12-HPETE failed to produce an effect in either nociceptive test. In the second part of this study, chronic spinal morphine for 5 days produced progressive decline in morphine antinociception and loss in analgesic potency. These effects were attenuated by co-administration of morphine with selective and nonselective lipoxygenase inhibitors. These results suggest involvement of lipoxygenase metabolites in both pain modulation and induction of opioid tolerance at the spinal level.

Analgesia, enhancement of spinal morphine antinociception, and inhibition of tolerance by ultra-low dose of the α2A-adrenoceptor selective antagonist BRL44408.

Ultra-low doses of non-selective α2-adrenoceptor antagonists augment acute spinal morphine antinociception and block morphine tolerance; however, the receptor involved in mediating these effects is currently unknown. Here, we used tail flick and paw pressure tests on the rat to investigate the acute analgesic and tolerance-inducing effects of spinal morphine and norepinephrine alone or in combination with an ultra-low dose of the α2A-adrenoceptor antagonist, BRL44408. We also assessed the potential antinociceptive effects of BRL44408 alone following spinal administration. A spinal dose of BRL44408, over 1000-fold lower than that required to inhibit clonidine-induced antinociception (1.65ng/10µL), significantly prolonged morphine and norepinephrine action in both nociception tests. Following repeated morphine or norepinephrine injections, 1.65ng BRL44408 attenuated both the decline of antinociceptive effect and increase in morphine ED50 values, responses indicative of acute morphine tolerance. BRL44408 administered alone produced a delayed antinociceptive effect unrelated to repeated nociceptive testing. This response was partially reduced by the α2-adrenoceptor antagonist atipamezole (10µg). Ultra-low dose BRL44408 was able to inhibit the loss of morphine- and norepinephrine-induced antinociceptive response, and prevent the loss of drug potency due to repeated agonist exposure. This implicates the spinal α2A-adrenoceptor subtype in the action of ultra-low dose α2-adrenoceptor antagonists on morphine and norepinephrine tolerance. The BRL44408-induced analgesia is partially dependent on its interaction with the α2-adrenoceptors. Thus, this agent class may be useful in pain therapy.

Stereo-selective inhibition of spinal morphine tolerance and hyperalgesia by an ultra-low dose of the alpha-2-adrenoceptor antagonist efaroxan.

Ultra-low doses of alpha-2 (α2)-adrenoceptor antagonists augment spinal morphine antinociception and inhibit tolerance, but the role of receptor specificity in these actions is unknown. We used the stereo-isomers of the α2 adrenoceptor antagonist, efaroxan to evaluate the effect of receptor specificity on the induction of spinal morphine tolerance and hyperalgesia. Tail flick and paw pressure tests were first used to evaluate high dose efaroxan (12.6 µg) and its stereo-isomers on clonidine analgesia in intrathecally catheterized rats. Ultra-low doses of individual isomers (1.3 ng) were then co-administered with morphine (15 µg) to determine their effects on acute antinociceptive tolerance and hyperalgesia induced by low dose spinal morphine (0.05 ng). Results demonstrate that high dose (+) efaroxan antagonized clonidine-induced antinociception, while (-) efaroxan had minimal effect. In addition, an ultra-low dose of (+) efaroxan (1.3 ng), substantially lower than required for receptor blockade, inhibited the development of acute morphine tolerance, while (-) efaroxan was less effective. Racemic (±) efaroxan effects were similar to those of (+) efaroxan. Furthermore, low dose morphine (0.05 ng) produced sustained hyperalgesia in the tail flick test and this was blocked by co-injection of (+) but not (-) efaroxan (1.3 ng). Given the isomer-specific efaroxan effects and their different receptor potencies, we suggest that inhibition of opioid tolerance by ultra-low dose efaroxan involves a specific interaction with spinal α2-adrenoceptors in this model. Likewise, inhibitory effects of adrenoceptor antagonists on morphine tolerance may be due to blockade of opioid-induced hyperalgesia.

Neonatal ventral hippocampal lesions in male and female rats: effects on water maze, locomotor activity, plus-maze and prefrontal cortical GABA and glutamate release in adulthood.

Schizophrenia is characterized by diverse behavioural and neurochemical abnormalities that may be differentially expressed in males and females. Male rats with neonatal ventral hippocampal lesions (nVHL) have commonly demonstrated behavioural and neurochemical abnormalities similar to those in schizophrenia. Fewer studies have used female rats. We investigated the hypothesis that male and female nVHL rats will demonstrate behavioural abnormalities accompanied by decreased GABA and l-glutamate release in the prefrontal cortex (PFC). On postnatal day (P) 7 rats received VH injections of ibotenate (3.0 microg/0.3 microl/side; n=18) or saline (n=21) or no injections (n=22). On P56, rats began water-maze, locomotor activity and elevated plus maze testing, and were then sacrificed for potassium-evoked GABA and l-glutamate release from PFC slices. nVHL rats showed impaired performance in water maze acquisition and match-to-sample tasks, increased spontaneous and amphetamine-induced locomotor activity and increased percent open-arm time. These behavioural changes were similar in males and females. These effects were accompanied by significantly reduced potassium-evoked l-glutamate release in male and female nVHL rats relative to controls, and non-significantly lower GABA release. Findings support the notion that behavioural abnormalities in post-pubertal male and female nVHL rats are associated with decreases in PFC neurotransmitter release.

Spinal modulation of calcitonin gene-related peptide by endocannabinoids in the development of opioid physical dependence.

Studies implicate endocannabinoids in the acute and chronic actions of opioid drugs, including the genesis of physical dependence. Previous evidence suggests that spinal release of calcitonin gene-related peptide (CGRP) and activation of its receptors contribute to opioid physical dependence. The release of CGRP at the spinal level is modulated by cannabinoid (CB1)-receptors. Thus, this study examined whether CB1-receptor activity mediates changes in CGRP underlying development of opioid physical dependence. Systemic morphine administration for 5-days elevated CGRP-immunoreactivity in the rat spinal dorsal horn. In situ hybridization of dorsal root ganglion (DRG) neurons revealed an increase in CGRP mRNA during initial (day 1-3) but not later phase (day 4-5) of morphine treatment. CGRP-immunoreactivity in DRG neurons, however, was increased in the later phase of morphine treatment. Naloxone challenge to morphine-treated animals precipitated an intense withdrawal syndrome that depleted CGRP-immunoreactivity and increased Fos expression in the dorsal horn. The Fos-response primarily occurred in neurons that expressed CGRP receptor component protein (RCP) suggesting CGRP activity contributes to neuronal activation during precipitated withdrawal. Spinal slices obtained from morphine-treated animals showed higher levels of CGRP release than from saline controls. Intrathecal co-administration of CB1-receptor antagonists, AM-251 or SR141716A, with daily morphine attenuated the behavioral manifestations of withdrawal. Treatment with AM-251 also reduced the depletion of CGRP, suppressed Fos-induction, and prevented the increase in capsaicin-evoked spinal CGRP release. Altogether, this study suggests that endocannabinoid activity, expressed via CB1-receptors, contributes to the induction of opioid physical dependence through spinal modulation of CGRP.

Inhibition of neurokinin-1-substance P receptor and prostanoid activity prevents and reverses the development of morphine tolerance in vivo and the morphine-induced increase in CGRP expression in cultured dorsal root ganglion neurons.

Chronic treatment with opioid drugs such as morphine leads to the development of tolerance, which manifests as a loss of drug potency. The mechanisms underlying this phenomenon are poorly understood, but recent evidence suggests that increased activity of nociceptive sensory transmitters [calcitonin gene-related peptide (CGRP) and substance P] and other signalling messengers (prostaglandins) contribute to its development. Chronic intrathecal morphine administration to rats for 7 days produced analgesic tolerance. Co-administration of SR140333, a selective substance P receptor (neurokinin-1) antagonist, or nimesulide, a cyclooxygenase-2-selective inhibitor, augmented the acute effects of morphine, prevented morphine tolerance and reversed established tolerance. In cultured adult dorsal root ganglion neurons, exposure to morphine for 5 days increased the number of neurons expressing CGRP immunoreactivity. Co-exposure with the peptide CGRP receptor antagonist CGRP8-37, SR140333 or nimesulide prevented the morphine-induced increase in the expression of CGRP immunoreactivity. Additionally, BIBN4096BS, a nonpeptide CGRP receptor antagonist, stereoselectively produced similar effects. In summary, this investigation demonstrates that activity of CGRP and substance P contributes to both the induction and expression of opioid analgesic tolerance. Additionally, it highlights the involvement of prostaglandins generated by spinal cyclooxygenase-2 activity in the genesis of opioid tolerance. The neuropeptide and prostanoid activity contributing to tolerance is expressed at the level of the primary afferents terminating in the spinal cord. The combination of opioids with agents that block this activity may represent a useful strategy for the prevention as well as the reversal of clinical opioid tolerance.

Paradoxical effects of the opioid antagonist naltrexone on morphine analgesia, tolerance, and reward in rats.

Opioid agonists such as morphine have been found to exert excitatory and inhibitory receptor-mediated effects at low and high doses, respectively. Ultra-low doses of opioid antagonists (naloxone and naltrexone), which selectively inhibit the excitatory effects, have been reported to augment systemic morphine analgesia and inhibit the development of tolerance/physical dependence. This study investigated the site of action of the paradoxical effects of naltrexone and the generality of this effect. The potential of ultra-low doses of naltrexone to influence morphine-induced analgesia was investigated in tests of nociception. Administration of intrathecal (0.05 and 0.1 ng) or systemic (10 ng/kg i.p.) naltrexone augmented the antinociception produced by an acute submaximal dose of intrathecal (5 microg) or systemic (7.5 mg/kg i.p.) morphine in the tail-flick test. Chronic intrathecal (0.005 and 0.05 ng) or systemic (10 ng/kg) naltrexone combined with morphine (15 microg i.t.; 15 mg/kg i.p.) over a 7-day period inhibited the decline in morphine antinociception and prevented the loss of morphine potency. In animals rendered tolerant to intrathecal (15 microg) or systemic (15 mg/kg) morphine, administration of naltrexone (0.05 ng i.t.; 10 and 50 ng/kg i.p.) significantly restored the antinociceptive effect and potency of morphine. Thus, in ultra-low doses, naltrexone paradoxically enhances morphine analgesia and inhibits or reverses tolerance through a spinal action. The potential of naltrexone to influence morphine-induced reward was also investigated using a place preference paradigm. Systemic administration of ultra-low doses of naltrexone (16.7, 20.0, and 25.0 ng/kg) with morphine (1.0 mg/kg) extended the duration of the morphine-induced conditioned place preference. These effects of naltrexone on morphine-induced reward may have implications for chronic treatment with agonist-antagonist combinations.

Prior ibuprofen exposure does not augment opioid drug potency or modify opioid requirements for pain inhibition in total hip surgery.

PURPOSE: In previous animal studies, a prior exposure to non-steroidal anti-inflammatory drugs (NSAID) augmented opioid drug potency. This study was designed to answer the question whether a similar effect can be attained in man. The objective was to use NSAID for preoperative pain reduction and at the same time use the NSAID exposure to reduce opioid requirements for pain inhibition in major orthopedic surgery. METHODS: In this double-blind, randomized study, 50 patients scheduled for total hip surgery were included. Patients of Group I received a placebo drug three times a day two weeks before surgery, and those allocated to Group II received ibuprofen (600 mg) three times a day. For surgical anesthesia, all patients received intrathecal bupivacaine 20 mg plus 0.1 mg morphine in a total volume of 4 mL. RESULTS: The preoperative or postoperative visual analogue scale pain scores or the amount of iv morphine showed no differences between the two groups in the first 24 hr after surgery. The median total blood loss in the ibuprofen group was 1161 mL vs 796 mL in the placebo group (P < 0.01). CONCLUSION: Pretreatment with ibuprofen before major hip surgery does not improve the pain scores or reduce morphine requirement but significantly increases blood loss. Considering the presence of relevant adverse effects, pretreatment with a non-selective NSAID is not recommended.

Neuroprotection against ischemic brain injury conferred by a novel nitrate ester.

Nitrates exhibit a selectivity of action in different tissue types not fully recognized: in particular, the neuromodulatory and cardiovascular properties can be dissociated. A novel nitrate showed relatively weak systemic effects, but in the middle cerebral artery occlusion rat model of focal ischemia, reduced the cerebral infarct by 60-70% when administered 4 h after the onset of ischemia.