Reduced development of tolerance to the analgesic effects of morphine and clonidine in PKC gamma mutant mice

Neurochemical characterization of neuronal populations expressing protein kinase C gamma isoform in the spinal cord and gracile nucleus of the rat

Protein kinase C gamma (PKCgamma) is widely distributed throughout the CNS and is thought to play a role in long term hyper-excitability in nociceptive neurones. Here, we provide the first report of PKCgamma cells in the dorsal column nuclei of the adult rat. Retrograde labeling of PKCgamma cells from the thalamus with choleragenoid revealed that 25% of the PKCgamma positive gracile cells projected to the thalamus. Further, we have characterized the distribution of PKCgamma within gracile nucleus in terms of colocalization with various neurotransmitter receptors or enzymes and calcium binding proteins, and compared this with PKCgamma colocalization in cells of laminae I-III of the spinal cord. We show that approximately 90% of the PKCgamma cells in the gracile nucleus and 60% in the dorsal horn were immuno-positive for the AMPA receptor subunit glutamate 2/3 (GluR2/3). Little coexpression was seen with neurokinin 1 receptor, nitric oxide synthase (NOS) and the AMPA receptor subunit GluR1, markers of distinct neuronal subpopulations. In the spinal cord, a quarter of PKCgamma cells expressed calbindin, but very few cells did so in the gracile nucleus. Electrical stimulation at c-fiber strength of the normal or injured sciatic nerve was used to induce c-fos as a marker of postsynaptic activation in the spinal cord and gracile nucleus. Quantitative analysis of the number of PKCgamma positive gracile cells that expressed also c-fos increased from none to 24% after injury, indicating an alteration in the sensory activation pattern in these neurones after injury. C-fos was not induced in inner lamina II following c-fiber electrical stimulation of the intact or axotomized sciatic nerve, indicating no such plasticity at the spinal cord level. As dorsal column nuclei cells may contribute to allodynia after peripheral nerve injury, pharmacological modulation of PKCgamma activity may therefore be a possible way to ameliorate neuropathic pain after peripheral nerve injury.

Plasticity of adenylyl cyclase-related signaling sequelae after long-term morphine treatment

Adaptations to long-term morphine treatment resulting in tolerance are protective by counteracting the consequences of sustained opioid receptor activation. Consequently, the manifestation of specific adenylyl cyclase (AC)-related neurochemical sequelae of long-term morphine treatment should depend on the consequences of short-term mu-opioid receptor (MOR) activation. We tested this by comparing complementary chemical sequelae of long-term morphine treatment among cells in which short-term MOR activation inhibited instead of stimulated AC activity. Short-term activation of MOR in Chinese hamster ovary (CHO) cells stably transfected with MOR (MOR-CHO) inhibits AC activity. Long-term morphine treatment of these cells increased AC and Gbeta phosphorylation, membrane protein kinase Cgamma (PKCgamma) translocation, and MOR G(s) association. All converge, shifting the consequences of short-term MOR activation from Galpha(i)/Galpha(o) inhibitory to AC stimulatory signaling. In contrast, overexpression of the Gbetagamma-stimulated AC isoform AC2 (which converted MOR-coupled inhibition to stimulation of AC) eliminated or reversed these adaptations to long-term morphine treatment; it negated the increase in Gbeta phosphorylation and PKCgamma translocation while reversing the increase in AC phosphorylation and MOR G(s) association. These adaptations greatly attenuated MOR-coupled stimulation of AC activity. Altered overexpression of AC protein per se was not a confounding factor because MOR-CHO overexpressing AC1, which is inhibited by short-term MOR activation, manifested adaptations to long-term morphine treatment qualitatively identical with those of MOR-CHO. These results reveal that adaptations elicited by long-term morphine treatment depend on the effects of short-term MOR activation. This dynamic and pliable nature of tolerance mechanisms could represent a new paradigm for pharmacotherapeutics.

Agonist-selective mechanisms of GPCR desensitization

The widely accepted model of G protein-coupled receptor (GPCR) regulation describes a system where the agonist-activated receptors couple to G proteins to induce a cellular response, and are subsequently phosphorylated by a family of kinases called the G protein-coupled receptor kinases (GRKs). The GRK-phosphorylated receptor then acts as a substrate for the binding of a family of proteins called arrestins, which uncouple the receptor and G protein so desensitizing the agonist-induced response. Other kinases, principally the second messenger-dependent protein kinases, are also known to play a role in the desensitization of many GPCR responses. It is now clear that there are subtle and complex interactions between GRKs and second messenger-dependent protein kinases in the regulation of GPCR function. Functional selectivity describes the ability of agonists to stabilize different active conformations of the same GPCR. With regard to desensitization, distinct agonist-activated conformations of a GPCR could undergo different molecular mechanisms of desensitization. An example of this is the mu opioid receptor (MOPr), where the agonists morphine and [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO) induce desensitization of the MOPr by different mechanisms, largely protein kinase C (PKC)- or GRK-dependent, respectively. This can be best explained by supposing that these two agonists stabilize distinct conformations of the MOPr, which are nevertheless able to couple to the relevant G-proteins and produce similar responses, yet are sufficiently different to trigger different regulatory processes. There is evidence that other GPCRs also undergo agonist-selective desensitization, but the full therapeutic consequences of this phenomenon await further detailed study.

Protein kinase C as a stress sensor

While there are many reviews which examine the group of proteins known as protein kinase C (PKC), the focus of this article is to examine the cellular roles of two PKCs that are important for stress responses in neurological tissues (PKC gamma and epsilon) and in cardiac tissues (PKC epsilon). These two kinases, in particular, seem to have overlapping functions and interact with an identical target, connexin 43 (Cx43), a gap junction protein which is central to proper control of signals in both tissues. While PKC gamma and PKC epsilon both help protect neural tissue from ischemia, PKC epsilon is the primary PKC isoform responsible for responding to decreased oxygen, or ischemia, in the heart. Both do this through Cx43. It is clear that both PKC gamma and PKC epsilon are necessary for protection from ischemia. However, the importance of these kinases has been inferred from preconditioning experiments which demonstrate that brief periods of hypoxia protect neurological and cardiac tissues from future insults, and that this depends on the activation, translocation, or ability for PKC gamma and/or PKC epsilon to interact with distinct cellular targets, especially Cx43. This review summarizes the recent findings which define the roles of PKC gamma and PKC epsilon in cardiac and neurological functions and their relationships to ischemia/reperfusion injury. In addition, a biochemical comparison of PKC gamma and PKC epsilon and a proposed argument for why both forms are present in neurological tissue while only PKC epsilon is present in heart, are discussed. Finally, the biochemistry of PKCs and future directions for the field are discussed, in light of this new information.

Increased response to morphine in mice lacking protein kinase C epsilon

The protein kinase C (PKC) family of serine-threonine kinases has been implicated in behavioral responses to opiates, but little is known about the individual PKC isozymes involved. Here, we show that mice lacking PKCepsilon have increased sensitivity to the rewarding effects of morphine, revealed as the expression of place preference and intravenous self-administration at very low doses of morphine that do not evoke place preference or self-administration in wild-type mice. The PKCepsilon null mice also show prolonged maintenance of morphine place preference in response to repeated testing when compared with wild-type mice. The supraspinal analgesic effects of morphine are enhanced in PKCepsilon null mice, and the development of tolerance to the spinal analgesic effects of morphine is delayed. The density of mu-opioid receptors and their coupling to G-proteins are normal. These studies identify PKCepsilon as a key regulator of opiate sensitivity in mice.

Evidence for an important role of protein phosphatases in the mechanism of morphine tolerance

Acute morphine antinociception has been shown to be blocked by very low picogram doses of okadaic acid indicating that inhibition of protein phosphatase PP2A allows for increases in phosphorylation to inhibit antinociception. Comparative studies in morphine tolerant animals have not been reported. In the present study, we showed a significant increase in the total phosphatase activity in the periaqueductal gray matter (PAG) from morphine-pelleted versus placebo-pelleted mice, 72-h after pellet implantation. This supports our hypothesis that phosphatase activity is increased in tolerance as a compensatory mechanism for the increase in kinase activity during the development of tolerance. We also demonstrated that i.c.v. administration of the phosphatase inhibitor okadaic acid (3 pmol/mouse; a dose tested to be inert in placebo-pelleted mice) enhanced the level of morphine antinociceptive tolerance assessed by the tail immersion test, 72-h following pellet implantation. This was supported by the fact that the same treatment with okadaic acid blocked the increase in phosphatase activity in PAG of morphine tolerant mice indicating that selective inhibition of PP2A contributes to enhanced levels of morphine tolerance. We have previously reported that PKC or PKA inhibitors reversed morphine antinociceptive tolerance in mice. The current study shows that i.c.v. administration of the PKC inhibitors bisindolylmaleimide I or Go6976 reversed the enhanced level of morphine tolerance induced by okadaic acid treatment to the same level of tolerance observed in non-okadaic acid-treated tolerant mice. However, the PKA inhibitor PKI-(14-22)-amide only partially reversed the enhancement of morphine tolerance induced by okadaic acid. Our data suggest an important role for the balance between kinases and phosphatases in modulating tolerance levels. Further studies will be directed towards a better understanding of the role of different phosphatase isoforms in morphine tolerance.

Potentiation of spinal alpha(2)-adrenoceptor analgesia in rats deficient in TRPV1-expressing afferent neurons

The alpha(2)-adrenoceptors (alpha(2)-ARs) are located on primary afferent terminals and on neurons in the spinal cord dorsal horn. However, their relative contribution to the analgesic effect of the alpha(2)-AR agonists is not known. In this study, we determined the role of certain presynaptic alpha(2)-ARs in the antinociceptive effect produced by intrathecal administration of the alpha(2)-AR agonist clonidine. TRPV1-expressing sensory neurons were removed by resiniferatoxin (RTX). The effect of intrathecal injection of clonidine was measured by testing the paw withdrawal response to noxious mechanical or heat stimuli. In RTX-treated rats, the alpha(2A)-AR-immunoreactivity co-expressed with TRPV1-expressing terminals in the spinal cord was eliminated. However, the alpha(2C)-AR-immunoreactivity in the spinal cord was little changed. Surprisingly, intrathecal administration of clonidine produced a much greater increase in the mechanical withdrawal threshold in RTX- than in vehicle-treated rats. The duration of the clonidine effect was also significantly increased in RTX-treated rats. Furthermore, in the vehicle-treated group, although intrathecal injection of clonidine produced a large increase in the thermal withdrawal latency, it only had a small and short-lasting effect on the mechanical withdrawal threshold. This study provides new information that the antinociceptive effect of spinally administered alpha(2)-AR agonists is largely modality-specific. Loss of TRPV1-expressing sensory neurons leads to a reduction in presynaptic alpha(2A)-ARs but paradoxically potentiates the effect of clonidine on mechano-nociception.

Resistance to morphine analgesic tolerance in rats with deleted transient receptor potential vanilloid type 1-expressing sensory neurons

Deletion of transient receptor potential vanilloid type 1 (TRPV1)-expressing afferent neurons reduces presynaptic mu opioid receptors but paradoxically potentiates the analgesic efficacy of mu opioid agonists. In this study, we determined if removal of TRPV1-expressing afferent neurons by resiniferatoxin (RTX), an ultrapotent capsaicin analog, influences the development of opioid analgesic tolerance. Morphine tolerance was induced by daily intrathecal injections of 10 microg of morphine for 14 consecutive days or by daily i.p. injections of 10 mg/kg of morphine for 10 days. In vehicle-treated rats, the effect of intrathecal or systemic morphine on the mechanical withdrawal threshold was gradually diminished within 7 days. However, the analgesic effect of intrathecal and systemic morphine was sustained in RTX-treated rats at the time the morphine effect was lost in the vehicle group. Furthermore, the mu opioid receptor-G protein coupling in the spinal cord was significantly decreased ( approximately 22%) in vehicle-treated morphine tolerant rats, but was not significantly altered in RTX-treated rats receiving the same treatment with morphine. Additionally, there was a large reduction in protein kinase Cgamma-immunoreactive afferent terminals in the spinal dorsal horn of RTX-treated rats. These findings suggest that loss of TRPV1-expressing sensory neurons attenuates the development of morphine analgesic tolerance possibly by reducing mu opioid receptor desensitization through protein kinase Cgamma in the spinal cord. These data also suggest that the function of presynaptic mu opioid receptors on TRPV1-expressing sensory neurons is particularly sensitive to down-regulation by mu opioid agonists during opioid tolerance development.

How important is protein kinase C in μ-opioid receptor desensitization and morphine tolerance?

The repeated administration of opiate drugs such as morphine results in the development of tolerance to their analgesic, rewarding (euphoric) and respiratory-depressant effects; thus, to obtain the same level of response with subsequent administrations, a greater dose must be used. Tolerance can limit the clinical efficacy of opiate drugs and enhance the social problems that are inherent in recreational opioid abuse. Surprisingly, the mechanism (or mechanisms) underlying the development of morphine tolerance remains controversial. Here, we propose that protein kinase C could have a crucial role in the desensitization of mu-opioid receptors by morphine and that this cellular process could contribute to the development and maintenance of morphine tolerance in vivo.

Determination of the role of conventional, novel and atypical PKC isoforms in the expression of morphine tolerance in mice

This study comprehensively determines the role of all the major PKC isoforms in the expression morphine tolerance. Pseudosubstrate and receptors for activated C-kinase (RACK) peptides inhibit only a single PKC isoform, while previously tested chemical PKC inhibitors simultaneously inhibit multiple isoforms making it impossible to determine which PKC isoform mediates morphine tolerance. Tolerance can result in a diminished effect during continued exposure to the same amount of substance. In rodents, morphine pellets provide sustained exposures to morphine leading to the development of tolerance by 72 h. We hypothesized that administration of the PKC isoform inhibitors i.c.v. would reverse tolerance and reinstate antinociception in the tail immersion and hot plate tests from the morphine released solely from the pellet. Inhibitors to PKC alpha, gamma and epsilon (100-625 pmol) dose-dependently reinstated antinociception in both tests. The PKC beta(I), beta(II), delta, theta, epsilon, eta and xi inhibitors were inactive (up to 2500 pmol). In other mice, the degree of morphine tolerance was determined by calculating ED50 and potency-ratio values following s.c. morphine administration. Morphine s.c. was 5.6-fold less potent in morphine-pelleted vs. placebo-pelleted mice. Co-administration of s.c. morphine with the inhibitors i.c.v. to either PKC alpha (625 pmol), gamma (100 pmol) or epsilon (400 pmol) completely reversed the tolerance so that s.c. morphine was equally potent in both placebo- and morphine-pelleted mice. The PKC beta(I), beta(II), delta, theta, epsilon, eta and xi inhibitors were inactive. Thus, PKC alpha, gamma and epsilon appear to contribute to the expression of morphine tolerance in mice.

Phosphorylation: A molecular switch in opioid tolerance

Protein phosphorylation is a key posttranslational modification mechanism controlling the conformation and activity of many proteins. Increasing evidence has implicated an essential role of phosphorylation by several major protein kinases in promoting and maintaining opioid tolerance. We review some of the most recent studies on protein kinase C (PKC), cyclic AMP dependent protein kinase A (PKA), calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase G (PKG), and G protein receptor kinase (GRK). These kinases act as the molecular switches to modulate opioid tolerance. Pharmacological interventions at one or more of the protein kinases and phosphatases may provide valuable strategies to improve opioid analgesia by attenuating tolerance to these drugs.

Evidence for a role of CaMKIV in the development of opioid analgesic tolerance

cAMP response-element binding protein (CREB), a transcription factor involved in learning, memory and drug addiction, is phosphorylated by calcium-calmodulin-dependent protein kinase IV (CaMKIV). Here, we show that CaMKIV-knockout (KO) mice developed less analgesic tolerance after chronic morphine administration with no alteration in physical dependence or acute morphine-induced analgesia. The increase in phosphorylated CREB expression observed in wild-type mice after chronic morphine was absent in CaMKIV-KO mice, while there was no difference in the expression or phosphorylation of the micro-opioid receptor between groups. Morphine-treated CaMKIV-KO mice showed less G-protein uncoupling from the micro-opioid receptor than did wild-type mice, while uncoupling was similar in control wild-type and KO mice. In addition, morphine reduced inhibitory transmission to a greater degree in CaMKIV-KO mice than in controls after chronic morphine exposure. Our results provide novel evidence for the role of CaMKIV in the development of opioid analgesic tolerance but not physical dependence.

Phosphorylation of neurogranin, protein kinase C, and Ca2+/calmodulin dependent protein kinase II in opioid tolerance and dependence

Activation of Ca2+/calmodulin dependent protein kinase II (CaMKII) and protein kinase C (PKC) are hallmarks of opioid tolerance and dependence. It is not known if the actions of these two kinases are synchronized by a common mechanism in opioid tolerance and dependence. Neurogranin (Ng), through mechanisms such as phosphorylation, has been previously proposed to regulate the activities of these protein kinases. We examined the phosphorylation status of neurogranin in mice that were made tolerant to opioids by morphine (100 mg/kg, s.c.). Increase in phosphorylation of neurogranin was found both in brains and spinal cords of morphine-treated mice, as compared to the untreated baseline or saline-treated mice. The effect appeared to correlate with the changes in the activities of PKC and CaMKII, and with the development of opioid tolerance and dependence. We have found that neurogranin activity is regulated in opioid tolerance and dependence. Neurogranin may, therefore, provide a potential mechanism interacting with both CaMKII and PKC in opioid tolerance and dependence.

Attenuation of opioid tolerance by antisense oligodeoxynucleotides targeting neurogranin

Neurogranin is capable of regulating protein kinase C and Ca2+/calmodulin-dependent protein kinase II. In this study, we examined the role of neurogranin in opioid tolerance. Increased phosphorylation of neurogranin was found in opioid tolerance. Opioid tolerance was absent in morphine (100 mg/kg)-treated mice that were also pretreated with neurogranin antisense oligodeoxynucleotides (2 microg/day, i.c.v. for 3 days). The behavioral effect correlated with the decreased expression of neurogranin. These data suggest that neurogranin may be critical in the development of opioid tolerance.

Signaling pathway of morphine induced acute thermal hyperalgesia in mice

Systemic administration of morphine induced a hyperalgesic response in the hot plate test, at an extremely low dose (1-10 microg/kg). We have examined in vivo whether morphine, at an extremely low dose, induces acute central hypernociception following activation of the opioid receptor-mediated PLC/PKC inositol-lipid signaling pathway. The PLC inhibitor U73122 and the PKC blocker, calphostin C, dose dependently prevented the thermal hypernociception induced by morphine. This effect was also prevented by pretreatment with aODN against PLCbeta3 at 2 nmol/mouse and PKCgamma at 2-3 nmol/mouse. Low dose morphine hyperalgesia was dose dependently reversed by selective NMDA antagonist MK801 and ketamine. This study demonstrates the presence of a nociceptive PLCbeta3/PKCgamma/NMDA pathway stimulated by low concentrations of morphine, through muOR1 receptor, in mouse brain. This signaling pathway appears to play an opposing role in morphine analgesia. When mice were treated with a morphine analgesic dose (7 mg/kg), the downregulation of PLCbeta3 or PKCgamma at the same aODN doses used for the prevention of the hyperalgesic effect induced, respectively, a 46% and 67% potentiation in analgesic response. Experimental and clinical studies suggest that opioid may activate pronociceptive systems, leading to pain hypersensitivity and short-term tolerance, a phenomenon encountered in postoperative pain management by acute opioid administration. The clinical management of pain by morphine may be revisited in light of the identification of the signaling molecules of the hyperalgesic pathway.

Trifluoperazine, an orally available clinically used drug, disrupts opioid antinociceptive tolerance

Calcium/calmodulin dependent protein kinase II (CaMKII) has been shown to play an important role in the generation and maintenance of opioid tolerance. In this study, trifluoperazine was studied for its effect on morphine tolerance in mice. Acute treatment with trifluoperazine (0.5 mg/kg, i.p.) completely reversed the established antinociceptive tolerance to morphine. Pretreatment with trifluoperazine also significantly attenuated the development of antinociceptive tolerance (p<0.01). Morphine induced a significant up-regulation of supraspinal and spinal CaMKII activity in tolerant mice, which was abolished after the pretreatment or acute treatment with trifluoperazine. These data suggested that trifluoperazine was capable of suppressing opioid tolerance, possibly by the mechanism of inhibiting CaMKII. Since trifluoperazine has been safely used as an antipsychotic drug, we propose that the drug should be studied in humans for the prevention and treatment of opioid tolerance and addiction.

Expression of spinal NMDA receptor and PKCgamma after chronic morphine is regulated by spinal glucocorticoid receptor

Spinal NMDA receptor (NMDAR), protein kinase C (PKC), and glucocorticoid receptor (GR) have all been implicated in the mechanisms of morphine tolerance; however, how these cellular elements interact after chronic morphine exposure remains unclear. Here we show that the expression of spinal NMDAR and PKCgamma after chronic morphine is regulated by spinal GR through a cAMP response element-binding protein (CREB)-dependent pathway. Chronic morphine (10 microg, i.t.; twice daily for 6 d) induced a time-dependent upregulation of GR, the NR1 subunit of NMDAR, and PKCgamma within the rat's spinal cord dorsal horn. This NR1 and PKCgamma upregulation was significantly diminished by intrathecal coadministration of morphine with the GR antagonist RU38486 or a GR antisense oligodeoxynucleotide. Intrathecal coadministration of morphine with an adenylyl cyclase inhibitor (2',5'-dideoxyadenosine) or a protein kinase A inhibitor (H89) also significantly attenuated morphine-induced NR1 and PKCgamma expression, whereas intrathecal treatment with an adenylyl cyclase activator (forskolin) alone mimicked morphine-induced expression of GR, NR1, and PKCgamma. Moreover, the expression of phosphorylated CREB was upregulated within the spinal cord dorsal horn after chronic morphine, and a CREB antisense oligodeoxynucleotide coadministered intrathecally with morphine prevented the upregulation of GR, NR1, and PKCgamma. These results indicate that spinal GR through the cAMP-CREB pathway played a significant role in NMDAR and PKCgamma expression after chronic morphine exposure. The data suggest that genomic interaction among spinal GR, NMDAR, and PKCgamma may be an important mechanism that contributes to the development of morphine tolerance.

Chronic morphine acts via a protein kinase Cgamma-G(beta)-adenylyl cyclase complex to augment phosphorylation of G(beta) and G(betagamma) stimulatory adenylyl cyclase signaling

Chronic morphine augments protein kinase C (PKC) phosphorylation of G(beta), which enhances the potency of G(betagamma) to stimulate adenylyl cyclase II (ACII) activity. The present study demonstrates an in vivo association between phosphorylated G(beta) and a specific PKC isoform, PKCgamma. We investigated the association of G(beta) and PKCgamma by assessing the ability of anti-PKCgamma antibodies to co-immunoprecipitate G(beta) from (32)P-radiolabeled Chinese Hamster Ovary cells stably transfected with a mu-opioid receptor (MOR-CHO). PKCgamma immunoprecipitate (IP) obtained from MOR-CHO membranes contained radiolabeled signals of approximately equals 33 and 36--38 kDa that were subsequently identified as G(beta)(s). Chronic morphine significantly increased ( approximately equals 75%) the magnitude of (32)P incorporated into G(beta) present in PKCgamma IP. This suggests that G(beta) is an in vivo substrate for PKCgamma, which mediates the chronic morphine-induced increment in G(beta) phosphorylation. In order to evaluate AC as a putative effector for phosphorylated G(betagamma), its presence in IP obtained using anti-AC antibodies was evaluated. Autoradiographic analyses of AC IP also revealed the presence of phosphorylated G(beta)(s), the magnitude of which was significantly enhanced ( approximately equals 60%) following chronic morphine treatment. This indicates that phosphorylated G(betagamma) associates and presumably interacts in vivo with AC, indicating that it is a target for the enhanced phosphorylated G(betagamma) that is generated following chronic morphine treatment. This would contribute to the previously observed shift from predominantly G(ialpha) inhibitory to G(betagamma) stimulatory AC signaling following chronic morphine. The PKCgamma-G(beta)-AC complex identified in this study provides an organizational framework for understanding the well-documented participation of PKCgamma in opioid tolerance-producing mechanisms.

The involvement of alpha 2A-adrenoceptors in morphine analgesia, tolerance and withdrawal in mice

Alpha(2)-adrenoceptor agonists potentiate opioid analgesia and alleviate opioid withdrawal. The effects of two alpha(2)-adrenoceptor agonists, clonidine (2 mg/kg) and dexmedetomidine (20 and 100 microg/kg), and the alpha(1)-adrenoceptor antagonist prazosin (0.5 mg/kg) were tested on morphine analgesia, tolerance, and withdrawal in wild-type and alpha(2A)-adrenoceptor knock-out (KO) mice. Analgesia and tolerance were assessed with the tail-flick test. Withdrawal was precipitated with naloxone. Prazosin potentiated morphine analgesia equally in both genotypes. Clonidine and dexmedetomidine had no analgesic effects in alpha(2A)-adrenoceptor KO mice, but morphine analgesia and tolerance were similar in both genotypes. Alpha(2A)-Adrenoceptor KO mice exhibited 70% fewer naloxone-precipitated jumps than wild-type mice; weight loss was similar in both genotypes. The alpha(2)-adrenoceptor agonists reduced opioid withdrawal signs only in wild-type mice. We conclude that alpha(2A)-adrenoceptors are not directly involved in morphine analgesia and tolerance, and not critical for potentiation of morphine analgesia by prazosin, but that alpha(2A)-adrenoceptors modulate the expression of opioid withdrawal signs in mice.

Increased glutamate synaptic transmission in the nucleus raphe magnus neurons from morphine-tolerant rats

Currently, opioid-based drugs are the most effective pain relievers that are widely used in the treatment of pain. However, the analgesic efficacy of opioids is significantly limited by the development of tolerance after repeated opioid administration. Glutamate receptors have been reported to critically participate in the development and maintenance of opioid tolerance, but the underlying mechanisms remain unclear. Using whole-cell voltage-clamp recordings in brainstem slices, the present study investigated chronic morphine-induced adaptations in glutamatergic synaptic transmission in neurons of the nucleus raphe magnus (NRM), a key supraspinal relay for pain modulation and opioid analgesia. Chronic morphine significantly increased glutamate synaptic transmission exclusively in one class of NRM cells that contains μ-opioid receptors in a morphine-tolerant state. The adenylyl cyclase activator forskolin and the cAMP analog 8-bromo-cAMP mimicked the chronic morphine effect in control neurons and their potency in enhancing the glutamate synaptic current was significantly increased in neurons from morphine-tolerant rats. MDL12330a, an adenylyl cyclase inhibitor, and H89, a protein kinase A (PKA) inhibitor, reversed the increase in glutamate synaptic transmission induced by chronic morphine. In addition, PMA, a phorbol ester activator of protein kinase C (PKC), also showed an increased potency in enhancing the glutamate synaptic current in these morphine-tolerant cells. The PKC inhibitor GF109203X attenuated the chronic morphine effect. Taken together, these results suggest that chronic morphine increases presynaptic glutamate release in μ receptor-containing NRM neurons in a morphine-tolerant state, and that the increased glutamate synaptic transmission appears to involve an upregulation of both the cAMP/PKA pathway and the PKC pathway. This glutamate-mediated activation of these NRM neurons that are thought to facilitate spinal pain transmission may contribute to the reduced opioid analgesia during opioid tolerance.

Evidence for a long-term influence on morphine tolerance after previous morphine exposure: role of neuronal glucocorticoid receptors

Opioid analgesic tolerance is a pharmacological phenomenon that overtime diminishes the opioid analgesic effect. However, it remains unknown as to whether a previous opioid exposure would have a long-term influence on opioid tolerance upon subsequent opioid administration. Here, we show that the onset and degree of antinociceptive tolerance to a subsequent cycle of morphine exposure were substantially exacerbated in rats made tolerant to and then recovered from previous morphine administration, indicating a long-term influence from a previous morphine exposure on the development of morphine tolerance. Mechanistically, morphine exposure induced a cyclic AMP and protein kinase A-dependent upregulation of neuronal glucocorticoid receptors (GR) within the spinal cord dorsal horn, which was maintained after discontinuation of morphine administration and significantly enhanced upon a second cycle of morphine exposure. Prevention of the GR upregulation with GR antisense oligonucleotides as well as inhibition of GR activation with the GR antagonist RU38486 effectively prevented the exacerbated morphine tolerance after subsequent cycles of morphine exposure. The results indicate that a previous morphine exposure could induce lasting cellular changes mediated through neuronal GR and influence morphine analgesia upon a subsequent exposure. These findings may have significant implications in clinical opioid therapy and substance abuse.

Morphine-promoted survival of CEMx174 cells in early stages of SIV infection in vitro: involvement of the multiple molecular mechanisms

Progression of HIV infections to AIDS is a complex process and it differs considerably among individuals infected with HIV, influenced by both genetic and environmental factors. Opiates have been implicated to be a cofactor in HIV infections leading to AIDS. However, little is known about the molecular mechanisms involved in the effects of opioids on HIV infected immune cells. Cell cycle analysis was carried out by flow cytometry, the phosphorylation of mitogen-activated protein kinases ERK1 and ERK2 was detected by Western blotting assay, and changes of calcium concentration were monitored by scanning intracellular fluorescence intensity. In response to the treatment with morphine, SIV-infected cells were accumulated in G1 phase. Morphine increased the content of intracellular calcium in a time-dependent manner. In addition, morphine also elevated the levels of PKC activity and phosphorylated ERK1/2. Therefore, it is implicated that the calcium-PKC-MAPK cascade is involved in morphine-prolonged survival of SIV-infected cells in the early stages of virus infection.

Morphine aggravates the apoptosis of simian immunodeficiency virus infected CEM x174 cells in the prolonged culture in vitro

This study was designed to assess the in vitro effects of morphine on the lymphocytes infected with SIV. CEM x174 cells were cotreated with morphine and simian immunodeficiency virus (SIVmac239). Cells were cultured for 96 h and the effects of morphine on the viability of infected cells were determined. At the concentration of 1 micromol/l, morphine could inhibit the proliferation of CEM x174 cells at the culture of 72 h. The stronger effect was observed in the case of viral infection. During 72 h SIV loading, the cells were accumulated in S phase in all SIV infected groups. The S arrest was observed in every experimental group and statistically different from normal groups (P<0.05). The results from annexin V binding assay showed that SIV infection resulted in a lower proportion of vital cells and higher mortality compared with corresponding control (P<0.01). Morphine failed to induce detectable alteration in the cell cycle profile of viral infected cells. Western blotting showed that the synthesis of intracellular p53 and bax protein was gradually up-regulated in the virus-loading period of 72 h. Naloxone had an apparent additive rather than antagonistic effect on the morphine-associated enhancement of bax expression. The ratio of bax/bcl-2 proteins appeared to tilt the balance toward apoptosis. At 72 h of infection, 1 micromol/l of morphine significantly elevated the level of caspase-3. These results indicated that the alteration in the balance of intracellular apoptotic and anti-apoptotic elements is one of the reasons of accelerated progression of acquired immunodeficiency syndrome (AIDS) by opioids abuse.

Topical clonidine antinociception

Clonidine, an alpha-2 adrenergic agonist, is an extremely potent antinociceptive agent. However, the therapeutic utility of systemic clonidine for the treatment of pain is limited by centrally mediated side effects including sedation, hypotension and rebound hypertension. Given that alpha-2 adrenoceptors are expressed on the peripheral and central terminals of nociceptive fibers, we administered clonidine topically in order to avoid central effects. Here, we demonstrate that topical administration of clonidine to mice (via tail immersion) elicited antinociception in the radiant heat tail-flick test. The magnitude of antinociception was dependent upon the duration of exposure to the clonidine solution. Further, the antinociceptive activity of clonidine was limited to the portion of the tail exposed to drug solution suggesting that the actions of clonidine were locally mediated. Systemic pretreatment with the alpha-2 receptor antagonist, yohimbine, blocked the antinociceptive activity of topical clonidine. Concentrations of clonidine administered locally that were antinociceptive did not impair motor coordination as measured by the rota-rod test. However, doses of clonidine administered systemically that produced antinociception significantly impaired motor coordination. Repeated daily topical administration of clonidine resulted in antinociceptive tolerance. Tolerance to the antinociceptive actions of clonidine was not blocked by topical administration of the NMDA antagonist, ketamine. In conclusion, topical administration of clonidine elicits antinociception by blocking the emerging pain signals at peripheral terminals via alpha-2 adrenoceptors without producing the undesirable central side effects observed following the systemic administration. The ineffectiveness of topical ketamine to block topical clonidine antinociceptive tolerance suggests that peripheral NMDA receptors do not mediate local clonidine antinociceptive tolerance.

Neuronal protein kinase C gamma-dependent proliferation and hypertrophy of spinal cord astrocytes following repeated in vivo administration of morphine

Repeated administration of morphine induced a time-dependent inhibition of the morphine-induced antinociceptive action, indicating the development of tolerance to morphine. We demonstrated that mice tolerant to morphine exhibited a significant increase in the level of protein kinase Cgamma-like immunoreactivity (PKCgamma-IR) in the dorsal horn of the spinal cord. The PKCgamma-IR was exclusively colocalized with the neuron-specific markers neuronal nuclei (NeuN) and microtubule associated protein 2ab (MAP2ab). Here we found a dramatic increase in reactive astrocytes in the dorsal horn of the spinal cord following repeated treatment with morphine, as characterized by the increase and morphological changes in glial fibrillary acidic protein (GFAP)-positive cells. Furthermore, transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of the mouse GFAP promoter displayed enhanced levels of EGFP expression after repeated treatment with morphine. Under these conditions, mice lacking the PKCgamma gene failed to show any changes in astroglial hypertrophy or proliferation after repeated treatment with morphine. These findings strongly support the idea that the sustained activation of neuronal PKCgamma is implicated in the increased levels of reactive astrocytes in the dorsal horn of the spinal cord following repeated treatment with morphine. This neuron-glia communication may lead to the development of tolerance to morphine-induced antinociception.

Prevention of fentanyl-induced delayed pronociceptive effects in mice lacking the protein kinase Cγ gene

It has recently been reported in several nociceptive models of rats that the antinociceptive effect of fentanyl, an opioid analgesic widely used in the management of per-operative pain, was followed by paradoxical delayed hyperalgesia dependent on N-methyl-D-aspartate (NMDA) mechanisms. Events upstream of the NMDA receptor, especially the activation of the protein kinase Cgamma (PKCgamma), have been involved in the persistence of pain states associated with central sensitisation. In order to evaluate the contribution of the PKCgamma in early and delayed fentanyl nociceptive responses, we studied these effects in knock-out mice deficient in such a protein. We found that fentanyl antinociception was followed by the spontaneous appearance of prolonged hyperalgesia in the paw pressure and formalin tests, and allodynia in the Von Frey paradigm. In PKCgamma deficient mice, an enhancement of the early fentanyl antinociceptive effects was observed, as well as a complete prevention of the fentanyl delayed hyperalgesic/allodynic effects. Finally, naloxone administration in mice that had recovered their pre-fentanyl nociceptive threshold, precipitated hyperalgesia/allodynia in wild-type but not in mutant mice. This study identifies the PKCgamma as a key element that links opioid receptor activation with the recruitment of opposite systems to opioid analgesia involved in a physiological compensatory pain enhancement.

Dual effects of intrathecal BAM22 on nociceptive responses in acute and persistent pain--potential function of a novel receptor

Bovine adrenal medulla 22 (BAM22) peptide is one of the cleavage products of proenkephalin A. It binds with high affinity to both opioid receptors and a newly discovered receptor in vitro. This latter receptor was first named sensory neuron-specific receptor and is here named BAM peptide-activated receptor with non-opioid activity (BPAR). BPAR is uniquely distributed in small-diameter DRG neurons, most of which are associated with the IB4 class of nociceptor afferent. The present study examined the effects of intrathecal administration of BAM22 on formalin-induced nocifensive behaviors and tail-withdrawal latency in the rat. Intrathecal (i.t.) administration of BAM22 decreased nocifensive behavior scores, measured as the sum of flinching and lifting/licking, in the first and second phases of the formalin test. This decrease was partially attenuated by systemic injection of naloxone. In the presence of naloxone, i.t. BAM22 produced a dose-dependent suppression of the nocifensive behaviors observed during the formalin test. The ratio of the efficacy of BAM22 (5 nmol) in the presence of naloxone over that in the absence of naloxone was 0.65 for flinching and 0.74 for lifting/licking in the second phase. BAM22 at a dose of 5 nmol increased the tail-withdrawal latency by 193 and 119% of baseline in the absence and presence of naloxone, respectively. Systemic administration of naloxone alone enhanced the nocifensive behaviors in the second, but not in the first phase of the formalin test. Naloxone treatment did not alter the tail-withdrawal latency. These data confirm earlier in vitro data showing that BAM22 has both opioid and non-opioid biological actions. The non-opioid action of BAM22 involves inhibition of acute and persistent nociceptive behaviors at the spinal level, presumably mediated via BPAR. The name suggested for this novel receptor, its potential physiological function and its ligand are discussed. British Journal of Pharmacology (2004) 141, 423-430. doi:10.1038/sj.bjp.0705637

Signaling pathway involved in methionine enkephalin-promoted survival of lymphocytes infected by simian immunodeficiency virus in the early stage in vitro

Methionine enkephalin, the endogenous opioid peptide, has a diversity of effects on the immune system. Although the biological effects of the pentapeptide have been well documented, little is known about the intracellular events involved in the effects of opioids on human immunodeficiency virus (HIV) infected immune cells. In the present investigation, the possible mechanism of apoptosis alleviated by exposure of methionine enkephalin at 1 micromol/l to CEM x 174 cells, the hybrid lymphocytes, infected with simian immunodeficiency virus (SIV) in vitro is elucidated. Apoptosis and cell cycle analysis is carried out by flow cytometry, the phosphorylation of mitogen-activated protein kinases (MAPK) ERK1 and ERK2 is detected by Western blotting assay, and changes of calcium concentration were analyzed using the calcium-sensitive dye Fluo-3 AM. The results exhibit that methionine enkephalin at the concentrations of 1 micromol/l increase remarkably the proportion of vital cells and decrease the apoptotic cells based on annexin V binding assay. In response to the treatment with methionine enkephalin, SIV-infected cells display a prolonged survival and are accumulated in G1 phase. Methionine enkephalin increase obviously the content of intracellular calcium in normal cells within 1-2 min and maintains a high level within monitoring time. However, the intracellular calcium reaches the highest level at 1 min and subsequently decline to background in SIV infected group. In addition, methionine enkephalin also elevates the levels of protein kinase C (PKC) activity and phosphorylated extracellular signal-regulated kinase (ERK) 1/2. It is proposed that calcium-PKC-MAPK cascade is involved in methionine enkephalin-prolonged survival of SIV-infected cells in the early stages of virus infection. The results provide a further evidence for potential use of methionine enkephalin on the therapy of Acquired Immunodeficiency Syndrome (AIDS).

Opioid hyperalgesia and tolerance versus 5-HT1A receptor-mediated inverse tolerance

In addition to analgesia, opioids also produce paradoxical hyperalgesic effects following acute and chronic treatment. In this article, we review the occurrence of this hyperalgesia under several conditions, and discuss the potential mechanisms and clinical implications. We also review recent evidence that paradoxical analgesia and inverse tolerance induced by stimulation of 5-HT(1A) receptors, which is a mirror image of opioid-induced hyperalgesia and tolerance, might achieve clinically significant analgesia in chronic pain.

The expression of a high level of morphine antinociceptive tolerance in mice involves both PKC and PKA

We have previously reported that intracerebroventricular (i.c.v.) injection of either a PKC or PKA inhibitor completely reversed the expression of 5- to 8-fold morphine antinociceptive tolerance. We developed a model of 45-fold morphine tolerance that included a 75-mg morphine pellet and twice daily morphine injections. PKC inhibitor doses of bisindolylmaleimide I and Gö-7874 that completely reversed 8-fold tolerance only partly reversed the 45-fold level of antinociceptive tolerance. A component of tolerance was resistant to PKC inhibition, since even higher inhibitor doses failed to further reverse the high level of morphine tolerance. In addition, the 45-fold tolerance was only partly reversed by the PKA inhibitor KT-5720 at a dose previously cited by others to reverse 5-fold tolerance. Another PKA inhibitor 4-cyano-3-methylisoquinoline only partly reversed the morphine tolerance as well. In other experiments PKC and PKA inhibitors were co-administered together to determine their effectiveness for completely reversing the 45-fold level of morphine tolerance. Co-administering either bisindolylmaleimide I with KT-5720, or Gö-7874 with KT-5720, completely reversed the high level of tolerance. The high level of morphine tolerance was also completely reversed by co-administering Gö-7874 with 4-cyano-3-methylisoquinoline. Thus, high levels of morphine tolerance may reflect increases in protein phosphorylation by the terminal kinases of both the adenylyl cyclase and phosphatidylinositol cascades in brain and spinal cord areas critical to the expression of antinociception.

Region-specific changes in NMDA receptor mRNA induced by chronic morphine treatment are prevented by the co-administration of the competitive NMDA receptor antagonist LY274614

The steady-state mRNA levels of the NMDA receptor NR1 subunit were determined by a quantitative solution hybridization assay in selected CNS regions associated with antinociception in the rat. Tissues were obtained by microdissection from rats treated chronically with morphine alone or in combination with LY274614, a competitive NMDA receptor antagonist. Morphine treatment for 7 days resulted in the development of tolerance to morphine's analgesic effect and produced a significant decrease in the steady-state NR1 mRNA levels in the spinal cord dorsal horn (by 16%), and an elevation in nucleus raphe magnus and medial thalamus (by 26 and 38%, respectively). The NR1 mRNA levels were unchanged in the lateral paragigantocellular nucleus, locus coeruleus, periaqueductal grey, and sensorimotor cortex. NMDA receptor binding in the spinal cord measured with [3H]MK-801 was reduced approximately 50% by chronic morphine treatment. Co-administration of LY274614 (s.c. at 24 mg/kg/24 h via an osmotic pump) not only attenuated the development of morphine tolerance but also prevented the changes in the NR1 mRNA levels induced by chronic morphine administration. Neither a 7-day infusion of LY274614 nor an acute injection of morphine (10 mg/kg, s.c.) changed the NR1 mRNA levels. These results suggest that changes in the expression of the NR1 mRNA induced by chronic morphine in three CNS regions involved in antinociception are associated with the development of morphine tolerance and in the spinal cord, morphine tolerance is associated with the downregulation of NMDA receptors.

Opioids: a review

Recent discoveries in opioid pharmacology help explain the enormous variability in clinical responses to these powerful analgesics. Although there is only one m opioid receptor gene, splice variants of that gene's expression result in a panoply of different functioning receptors. Other sources of variable response include polymorphisms in the m opioid receptor regulatory region, and pharmacokinetic differences because of cytochrome P-450 mono-oxygenase heterogeneity. Analgesic tolerance is likely the key phenomenon limiting the benefit of opioids. A plethora of intracellular pathways affects this. Among them are the N-methyl-D-aspartate receptor, protein kinase C gamma activity, nitric oxide synthase, and GM1 ganglioside content of the neuronal membrane. Clinical studies undercut the routine use of meperidine in most settings. Other studies have shown better ways to diminish opioid side effects.

Differential mechanisms of morphine antinociceptive tolerance revealed in (beta)arrestin-2 knock-out mice

Morphine induces antinociception by activating mu opioid receptors (muORs) in spinal and supraspinal regions of the CNS. (Beta)arrestin-2 (beta)arr2), a G-protein-coupled receptor-regulating protein, regulates the muOR in vivo. We have shown previously that mice lacking (beta)arr2 experience enhanced morphine-induced analgesia and do not become tolerant to morphine as determined in the hot-plate test, a paradigm that primarily assesses supraspinal pain responsiveness. To determine the general applicability of the (beta)arr2-muOR interaction in other neuronal systems, we have, in the present study, tested (beta)arr2 knock-out ((beta)arr2-KO) mice using the warm water tail-immersion paradigm, which primarily assesses spinal reflexes to painful thermal stimuli. In this test, the (beta)arr2-KO mice have greater basal nociceptive thresholds and markedly enhanced sensitivity to morphine. Interestingly, however, after a delayed onset, they do ultimately develop morphine tolerance, although to a lesser degree than the wild-type (WT) controls. In the (beta)arr2-KO but not WT mice, morphine tolerance can be completely reversed with a low dose of the classical protein kinase C (PKC) inhibitor chelerythrine. These findings provide in vivo evidence that the muOR is differentially regulated in diverse regions of the CNS. Furthermore, although (beta)arr2 appears to be the most prominent and proximal determinant of muOR desensitization and morphine tolerance, in the absence of this mechanism, the contributions of a PKC-dependent regulatory system become readily apparent.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Inhibition of spinal protein kinase Calpha expression by an antisense oligonucleotide attenuates morphine infusion-induced tolerance

Protein kinase C isoforms including the alpha isozyme have been implicated in morphine tolerance. In the present study, we examined the effect of intrathecal delivery of an antisense oligonucleotide targeting rat protein kinase Calpha mRNA on the expression of spinal protein kinase Calpha isozyme and spinal morphine tolerance. Continuous intrathecal infusion of rats with morphine produced an increase in paw withdrawal threshold to thermal stimulation on day 1, which disappeared by day 5. On day 6, a bolus intrathecal injection of morphine (a probe dose) produced significantly less analgesia in morphine-infused rats than in saline-infused rats, suggesting tolerance. Intrathecal treatment with the protein kinase Calpha antisense concurrent with spinal morphine infusion not only maintained the analgesic effect of morphine during the 5-day infusion, it also significantly increased responsiveness to the probe morphine dose on day 6. In comparison, the missense used in the same treatment paradigm had no effect. The inhibitory effect of protein kinase Calpha antisense on spinal morphine tolerance was dose-dependent, and reversible. Intrathecal treatment with the antisense, but not the missense, in rats decreased expression of spinal protein kinase Calpha mRNA and protein, as revealed by real-time quantitative reverse transcription-polymerase chain reaction and western blots. Expression of the gamma isozyme was not affected by the oligonucleotides. The antisense also attenuated protein kinase C-mediated phosphorylation in spinal cord. These results demonstrate that selective reduction in the expression of the spinal protein kinase Calpha isozyme followed by a decrease of local protein kinase C-mediated phosphorylation will reverse spinal morphine infusion-induced tolerance. This finding is consistent with the view that tolerance produced by morphine infusion is dependent upon an increase in phosphorylation by protein kinase C, and also it emphasizes that the protein kinase Calpha isozyme and its activation in spinal cord may specifically participate in the phenomenon of opiate tolerance.

Chronic morphine induces downregulation of spinal glutamate transporters: implications in morphine tolerance and abnormal pain sensitivity

Tolerance to the analgesic effects of an opioid occurs after its chronic administration, a pharmacological phenomenon that has been associated with the development of abnormal pain sensitivity such as hyperalgesia. In the present study, we examined the role of spinal glutamate transporters (GTs) in the development of both morphine tolerance and associated thermal hyperalgesia. Chronic morphine administered through either intrathecal boluses or continuous infusion induced a dose-dependent downregulation of GTs (EAAC1 and GLAST) in the rat's superficial spinal cord dorsal horn. This GT downregulation was mediated through opioid receptors because naloxone blocked such GT changes. Morphine-induced GT downregulation reduced the ability to maintain in vivo glutamate homeostasis at the spinal level, because the hyperalgesic response to exogenous glutamate was enhanced, including an increased magnitude and a prolonged time course, in morphine-treated rats with reduced spinal GTs. Moreover, the downregulation of spinal GTs exhibited a temporal correlation with the development of morphine tolerance and thermal hyperalgesia. Consistently, the GT inhibitor l-trans-pyrrolidine-2-4-dicarboxylate (PDC) potentiated, whereas the positive GT regulator riluzole reduced, the development of both morphine tolerance and thermal hyperalgesia. The effects from regulating spinal GT activity by PDC were at least in part mediated through activation of the NMDA receptor (NMDAR), because the noncompetitive NMDAR antagonist MK-801 blocked both morphine tolerance and thermal hyperalgesia that were potentiated by PDC. These results indicate that spinal GTs may contribute to the neural mechanisms of morphine tolerance and associated abnormal pain sensitivity by means of regulating regional glutamate homeostasis.

Neuronal apoptosis associated with morphine tolerance: evidence for an opioid-induced neurotoxic mechanism

Tolerance to the analgesic effect of an opioid is a pharmacological phenomenon that occurs after its prolonged administration. Activation of the NMDA receptor (NMDAR) has been implicated in the cellular mechanisms of opioid tolerance. However, activation of NMDARs can lead to neurotoxicity under many circumstances. Here we demonstrate that spinal neuronal apoptosis was induced in rats made tolerant to morphine administered through intrathecal boluses or continuous infusion. The apoptotic cells were predominantly located in the superficial spinal cord dorsal horn, and most apoptotic cells also expressed glutamic acid decarboxylase, a key enzyme for the synthesis of the inhibitory neurotransmitter GABA. Consistently, increased nociceptive sensitivity to heat stimulation was observed in these same rats. Mechanistically, the spinal glutamatergic activity modulated morphine-induced neuronal apoptosis, because pharmacological perturbation of the spinal glutamate transporter activity or coadministration of morphine with the NMDAR antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate affected both morphine tolerance and neuronal apoptosis. At the intracellular level, prolonged morphine administration resulted in an upregulation of the proapoptotic caspase-3 and Bax proteins but a downregulation of the antiapoptotic Bcl-2 protein in the spinal cord dorsal horn. Furthermore, coadministration with morphine of N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (a pan-caspase inhibitor) or acetyl-aspartyl-glutamyl-valyl-aspart-1-aldehyde (a relatively selective caspase-3 inhibitor) blocked morphine-induced neuronal apoptosis. Blockade of the spinal caspase-like activity also partially prevented morphine tolerance and the associated increase in nociceptive sensitivity. These results indicate an opioid-induced neurotoxic consequence regulated by the NMDAR-caspase pathway, a mechanism that may have clinical implications in opioid therapy and substance abuse.