大鼠延髓头端腹内侧区甩尾相关神经元在电针镇痛中的可能作用

在浅麻醉状态下的大鼠,用辐射热烫尾并同步记录延髓头端腹内侧区神经元单位放电,按紧随甩尾动作发生前细胞放电频率的变化而将其分为甩尾前放电骤停的撤型细胞,放电骤增的给型细胞和无变化的中性细胞。电针双侧“次髎”穴引起动物甩尾反射抑制时,撤型细胞自发放电增加,与电针前相比差异显著(P<0.001),给型细胞电针后自发放电频率的改变与电针前相比无显著差异(P>0.05),两类细胞的甩尾相关反应均被抑制。结果提示:撤型细胞可能是延髓头端腹内侧区参与针刺镇痛的主要传出神经元。     

电刺激大鼠外侧缰核对延髓甩尾相关细胞的活动和甩尾反射的影响

在浅麻醉大鼠上,在延髓腹内侧结构内观察到三种具有不同放电类型的细胞,即乃尾前放电骤停的撤反应细胞,甩尾前放电骤增的给反应细胞和甩尾无关的中性细胞。电刺激外侧缰核可抑制撤反应细胞的自发放电,加强给反应细胞自发放电,从而易化两类细胞的甩尾相关反应,同时易化伤害刺激引起的甩尾反射。实验结果说明,外侧缰核对节段性防御反射有易化作用,这种易化作用可能是通过延髓内撤反应和反应细胞的协同活动而实现的。     

Evidence for GABA-mediated control of putative nociceptive modulating neurons in the rostral ventromedial medulla: iontophoresis of bicuculline eliminates the off-cell pause.

This study was undertaken to test the hypothesis that gamma-aminobutyric acid (GABA) is an endogeneous neurotransmitter regulating the activity of a class of putative nociceptive modulatory neurons (termed "off-cells") in the rostral ventromedial medulla (RVM) of the barbiturate-anesthetized rat. Off-cells, which are believed to correspond to the RVM output neuron that inhibits nociceptive processing at the level of the spinal cord, exhibit an abrupt pause in firing that begins immediately prior to the occurrence of the tail flick response (TF), a nocifensive reflex evoked by application of noxious heat to the tail. Single-unit recording and iontophoretic techniques were used to examine the ability of the GABAA receptor antagonist bicuculline methiodide (BIC) to antagonize selectively the characteristic off-cell pause. Iontophoretic application of BIC (5-30 nA) blocked the TF-related pause in each of the off-cells tested. This effect of BIC was generally slow in onset, and outlasted the period of application by several minutes. BIC iontophoresis also eliminated the cyclic alternation between active and silent periods that is often displayed by off-cells in lightly anesthetized rats. BIC application did not have a consistent effect on the firing of two other classes of RVM neurons ("on-cells" and "neutral cells"). Iontophoretically applied BIC antagonized the inhibitory effect of iontophoretically applied GABA, but not that produced by glycine. The glycine receptor antagonist strychnine did not mimic the action of BIC on off-cell activity. These data demonstrate antagonism of a synaptically evoked response using iontophoretic application of BIC, and provide strong evidence that the inhibitory neurotransmitter GABA mediates the TF-related off-cell pause. Taken together with behavioral experiments demonstrating that a GABA-mediated inhibitory process within RVM is crucial in permitting execution of the TF response, the present observations point to the significant functional relevance of GABA transmission within RVM in modulation of nociception.     

Putative Nociceptive Modulating Neurons in the Rostral Ventromedial Medulla of the Rat: Firing of On- and Off-Cells Is Related to Nociceptive Responsiveness

In the unstimulated, lightly anesthetized rat, both on- and off-cells exhibit alternating periods of silence and activity lasting from several seconds to a few minutes. In the preceding paper, we showed that the active periods of all cells of the same class are always in phase, whereas the firing of cells of different classes is invariably out of phase. Thus, the pattern of firing of any single on- or off-cell provides a useful indication of the excitability of all on- and off-cells in the rostral ventromedial medulla (RVM).

In this study, we measured the latency of the tail flick response (TF) at set intervals while recording from TF-related neurons in RVM, and were able to demonstrate a significant relationship between the spontaneous firing of both on- and off-cells and the latency of the TF response. If noxious heat is applied at a time when an off-cell is spontaneously active (or an on-cell is silent), the TF latency is longer than if the TF trial falls during a period in which the off-cell is silent (or the on-cell is active). This correlation between on- and off-cell firing and changes in TF latency is consistent with a nociceptive modulatory role for either or both cell classes. These findings support the hypothesis that off-cells inhibit and on-cells facilitate spinal nociceptive transmission and reflexes.     

Putative nociceptive modulating neurons in the rostral ventromedial medulla of the rat: firing of on- and off-cells is related to nociceptive responsiveness

In the unstimulated, lightly anesthetized rat, both on- and off-cells exhibit alternating periods of silence and activity lasting from several seconds to a few minutes. In the preceding paper, we showed that the active periods of all cells of the same class are always in phase, whereas the firing of cells of different classes is invariably out of phase. Thus, the pattern of firing of any single on- or off-cell provides a useful indication of the excitability of all on- and off-cells in the rostral ventromedial medulla (RVM). In this study, we measured the latency of the tail flick response (TF) at set intervals while recording from TF-related neurons in RVM, and were able to demonstrate a significant relationship between the spontaneous firing of both on- and off-cells and the latency of the TF response. If noxious heat is applied at a time when an off-cell is spontaneously active (or an on-cell is silent), the TF latency is longer than if the TF trial falls during a period in which the off-cell is silent (or the on-cell is active). This correlation between on- and off-cell firing and changes in TF latency is consistent with a nociceptive modulatory role for either or both cell classes. These findings support the hypothesis that off-cells inhibit and on-cells facilitate spinal nociceptive transmission and reflexes.     

Putative nociceptive modulatory neurons in the rostral ventromedial medulla of the rat display highly correlated firing patterns

Recent work in this laboratory has identified two classes of putative nociceptive modulating neurons in the rostral ventromedial medulla (RVM) of the rat: "off-cells," which pause beginning just prior to the tail flick response (TF) evoked by noxious heat, and "on-cells," which accelerate shortly before the occurrence of the TF. In the unstimulated, lightly anesthetized rat, the spontaneous firing pattern of individual on- and off-cells consists of alternating periods of silence and activity lasting from several seconds to a few minutes. In the present study, simultaneous recordings were made from pairs of TF-related neurons, and the relationships among the firing patterns of cells within a class and between cells of different classes were determined. All cells of a given class showed fluctuations in spontaneous discharge that were in phase. On the other hand, there was a striking reciprocity of firing between the two cell classes, such that a decrease in activity of cells of one class was accompanied by an increase in activity of cells of the other class. These observations point to the existence of integrating mechanisms that coordinate the activity of all members of each class of TF-related neurons. Thus, the pattern of activity of any single on- or off-cell provides a useful index of the excitability of all cells of that class. Moreover, because of the highly reciprocal nature of the firing of the two classes, it is possible to infer the current state of both cell populations from the pattern of activity of any single TF-related neuron.     

Putative Nociceptive Modulatory Neurons in the Rostral Ventromedial Medulla of the Rat Display Highly Correlated Firing Patterns

Recent work in this laboratory has identified two classes of putative nociceptive modulating neurons in the rostral ventromedial medulla (RVM) of the rat: “off-cells,” which pause beginning just prior to the tail flick response (TF) evoked by noxious heat, and “on-cells,” which accelerate shortly before the occurrence of the TF. In the unstimulated, lightly anesthetized rat, the spontaneous firing pattern of individual on- and off-cells consists of alternating periods of silence and activity lasting from several seconds to a few minutes.

In the present study, simultaneous recordings were made from pairs of TF-related neurons, and the relationships among the firing patterns of cells within a class and between cells of different classes were determined. All cells of a given class showed fluctuations in spontaneous discharge that were in phase. On the other hand, there was a striking reciprocity of firing between the two cell classes, such that a decrease in activity of cells of one class was accompanied by an increase in activity of cells of the other class.

These observations point to the existence of integrating mechanisms that coordinate the activity of all members of each class of TF-related neurons. Thus, the pattern of activity of any single on- or off-cell provides a useful index of the excitability of all cells of that class. Moreover, because of the highly reciprocal nature of the firing of the two classes, it is possible to infer the current state of both cell populations from the pattern of activity of any single TF-related neuron.     

Hyperalgesia during naloxone-precipitated withdrawal from morphine is associated with increased on-cell activity in the rostral ventromedial medulla

Hyperresponsiveness to noxious stimulation (hyperalgesia) is observed with naloxone-precipitated morphine withdrawal in several experimental models, and may be due to changes in central nervous system neurons. Previous studies have demonstrated that certain neurons in the rostral ventromedial medulla (on-cells) discharge just prior to nocifensive withdrawal reflexes and are inhibited by morphine. Because the tail flick latency (TFL) is shorter when on-cells are active, it has been proposed that on-cells facilitate nocifensive reflexes. The present study examined the hypothesis that the hyperalgesia observed following naloxone-precipitated withdrawal from morphine is caused by increased on-cell discharge. Rats were maintained in a lightly anesthetized state with chloral hydrate. Administration of saline (1.25 cc, i.v.) or morphine sulfate (1.25 mg/kg, i.v.) was followed by naloxone (1.0 mg/kg, i.v.). On- and off-cell activity was continuously recorded and was correlated with TFL and paw withdrawal threshold (PWT). As previously reported, morphine increased off-cell activity, blocked on-cell activity, and suppressed the tail flick and paw withdrawal reflexes. When naloxone was given after morphine, TFL and PWT were reduced to values significantly below baseline (hyperalgesia). Both spontaneous and reflex-related on-cell activity increased to levels greater than the premorphine baseline. Spontaneous off-cell activity decreased abruptly to near zero when morphine was followed by naloxone. Linear regression analysis during the hyperresponsive state revealed a significant correlation between increased on-cell activity and reduced TFL, but not between decreased off-cell activity and TFL. These findings are consistent with the hypothesis that on-cells facilitate spinal nocifensive reflexes, and that the naloxone-precipitated hyperalgesia is at least in part accounted for by increased on-cell activity. A neural model of opiate dependence, tolerance, and withdrawal is proposed.     

Hyperalgesia during Naloxone-Precipitated Withdrawal from Morphine Is Associated with Increased On-Cell Activity in the Rostral Ventromedial Medulla

Hyperresponsiveness to noxious stimulation (hyperalgesia) is observed with naloxone-precipitated morphine withdrawal in several experimental models, and may be due to changes in central nervous system neurons. Previous studies have demonstrated that certain neurons in the rostral ventromedial medulla (on-cells) discharge just prior to nocifensive withdrawal reflexes and are inhibited by morphine. Because the tail flick latency (TFL) is shorter when on-cells are active, it has been proposed that on-cells facilitate nocifensive reflexes. The present study examined the hypothesis that the hyperalgesia observed following naloxone-precipitated withdrawal from morphine is caused by increased on-cell discharge.

Rats were maintained in a lightly anesthetized state with chloral hydrate. Administration of saline (1.25 cc, i.v.) or morphine sulfate (1.25 mg/kg, i.v.) was followed by naloxone (1.0 mg/kg, i.v.). On- and off-cell activity was continuously recorded and was correlated with TFL and paw withdrawal threshold (PWT). As previously reported, morphine increased off-cell activity, blocked on-cell activity, and suppressed the tail flick and paw withdrawal reflexes. When naloxone was given after morphine, TFL and PWT were reduced to values significantly below baseline (hyperalgesia). Both spontaneous and reflex-related on-cell activity increased to levels greater than the premorphine baseline. Spontaneous off-cell activity decreased abruptly to near zero when morphine was followed by naloxone. Linear regression analysis during the hyperresponsive state revealed a significant correlation between increased on-cell activity and reduced TFL, but not between decreased off-cell activity and TFL.

These findings are consistent with the hypothesis that on-cells facilitate spinal nocifensive reflexes, and that the naloxone-precipitated hyperalgesia is at least in part accounted for by increased on-cell activity. A neural model of opiate dependence, tolerance, and withdrawal is proposed.     

Underlying mechanisms of pronociceptive consequences of prolonged morphine exposure

The opioid analgesics, commonly exemplified by morphine, represent the best option for the treatment of severe pain and for the management of chronic pain states, of both malignant and nonmalignant origin. It is well recognized that the prolonged use of opioids is associated with a requirement for ever-increasing doses in order to maintain pain relief at an acceptable and consistent level. This phenomenon is termed analgesic tolerance. While the concept that tolerance can develop as a result of cellular adaptations to the presence of the opioid has been proposed, it is now becoming abundantly clear that tolerance may also be related to a state of hyperalgesia that results from exposure to the opioid itself. Patients who receive long-term opioid therapy sometimes develop unexpected, abnormal pain. Similar paradoxical opioid-induced pain has been confirmed in a number of animal studies, even during the period of continuous opioid delivery. A number of recent studies have demonstrated that such pain may be secondary to neuroplastic changes that occur in the brain and spinal cord. One such change may be the activation of descending pain facilitation mechanisms arising from the rostral ventromedial medulla (RVM) elicited in part by increased activity of cholecystokinin (CCK) in the RVM. A cascade of pronociceptive events may follow, such as opioid-induced upregulation of spinal dynorphin levels that promotes enhanced input from primary afferent nociceptors. This mechanism appears to depend on intact descending pathways from the RVM, since interrupting this pathway abolishes enhanced abnormal pain. Furthermore, extended opioid exposure also can elicit increased calcitonin gene related peptide (CGRP) and substance P expression in the dorsal root ganglia. It is probable that increased pain elicited by opioids is a critical factor in the behavioral manifestation of opioid tolerance because the same manipulations that block abnormal pain also block antinociceptive tolerance. Taken together, such studies show that opioids elicit systems-level adaptations resulting in pain due to descending facilitation, upregulation of spinal dynorphin, and enhanced, evoked release of excitatory transmitters from primary afferents. These adaptive changes in response to sustained exposure to opioids indicate the need for the evaluation of the clinical consequences of long-term opioid administration. Additionally, these findings suggest a need for novel chemistry involving design of agents that may counteract opiate-induced neuroplastic adaptations resulting in pain relief without analgesic tolerance.     

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.     

Induction of pain facilitation by sustained opioid exposure: relationship to opioid antinociceptive tolerance

Opioid analgesics are frequently used for the long-term management of chronic pain states, including cancer pain. The prolonged use of opioids is associated with a requirement for increasing doses to manage pain at a consistent level, reflecting the phenomenon of analgesic tolerance. It is now becoming clearer that patients receiving long-term opioid therapy can develop unexpected abnormal pain. Such paradoxical opioid-induced pain, as well as tolerance to the antinociceptive actions of opioids, has been reliably measured in animals during the period of continuous opioid delivery. Several recent studies have demonstrated that such pain may be secondary to neuroplastic changes that result, in part, from an activation of descending pain facilitation mechanisms arising from the rostral ventromedial medulla (RVM). One mechanism which may mediate such pain facilitation is through the increased activity of CCK in the RVM. Secondary consequences from descending facilitation may be produced. For example, opioid-induced upregulation of spinal dynorphin levels seem to depend on intact descending pathways from the RVM reflecting spinal neuroplasticity secondary to changes at supraspinal levels. Increased expression of spinal dynorphin reflects a trophic action of sustained opioid exposure which promotes an increased pain state. Spinal dynorphin may promote pain, in part, by enhancing the evoked release of excitatory transmitters from primary afferents. In this regard, opioids also produce trophic actions by increasing CGRP expression in the dorsal root ganglia. Increased pain elicited by opioids is a critical factor in the behavioral manifestation of opioid tolerance as manipulations which block abnormal pain also block antinociceptive tolerance. Manipulations that have blocked enhanced pain and antinociceptive tolerance include reversible and permanent ablation of descending facilitation from the RVM. Thus, opioids elicit systems-level adaptations resulting in pain due to descending facilitation, upregulation of spinal dynorphin and enhanced release of excitatory transmitters from primary afferents. Adaptive changes produced by sustained opioid exposure including trophic effects to enhance pain transmitters suggest the need for careful evaluation of the consequences of long-term opioid administration to patients.     

Development of tolerance to the excitatory effect of morphine and cross-tolerance to the inhibitory action of β-endorphin in the isolated rat vas deferens

In the isolated rat vas deferens, morphine caused an increase in the neuromuscular twitch evoked by electrical stimulation. In rats chronically treated with morphine, the concentration of the opiate required to cause a 50% increase in the twitch was about 3 times larger than needed to elicit the same response in vasa from rats treated with a placebo. The simultaneous administration of naloxone plus morphine partially antagonized tolerance development by about 22%. Morphine tolerance was extended to other opiate- like alkaloids such as etonitazene and a derivative of azidomorphine (CAM). In contrast to the effects of morphine, β-endorphin inhibited neuromuscular transmission. Vasa from rats chronically treated with morphine were about 10 times less sensitive to the inhibitory effect of β-endorphin as compared to paired placebo treated controls. Chronic naloxone treatment in conjuction with morphine significantly reduced the cross-opiate tolerance by 66%. Present results suggest that morphine may interact at two different sites in the nerve terminals of the rat vas deferens.     

Microiontophoretically applied THIP effects upon nociceptive responses of neurons in medial thalamus

Sprague-Dawley rats anesthetized with urethane were used to study the single cell responses of medial thalamic neurons following noxious input and their interactions with gamma-aminobutyric acid (GABA) agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) and morphine sulfate applied microintophoretically . The majority of the medial thalamic neurons responded to noxious stimulation by an increase in their firing rate. Local application of both THIP and morphine attenuated the spontaneous and the noxious evoked responses of these neurons. The possibility of a role for GABA in mediating nonopiate pain suppression is discussed.     

Involvement of ERK Phosphorylation of Trigeminal Spinal Subnucleus Caudalis Neurons in Thermal Hypersensitivity in Rats with Infraorbital Nerve Injury

To evaluate the involvement of the mitogen-activated protein kinase (MAPK) cascade in orofacial neuropathic pain mechanisms, this study assessed nocifensive behavior evoked by mechanical or thermal stimulation of the whisker pad skin, phosphorylation of extracellular signal-regulated kinase (ERK) in trigeminal spinal subnucleus caudalis (Vc) neurons, and Vc neuronal responses to mechanical or thermal stimulation of the whisker pad skin in rats with the chronic constriction nerve injury of the infraorbital nerve (ION-CCI). The mechanical and thermal nocifensive behavior was significantly enhanced on the side ipsilateral to the ION-CCI compared to the contralateral whisker pad or sham rats. ION-CCI rats had an increased number of phosphorylated ERK immunoreactive (pERK-IR) cells which also manifested NeuN-IR but not GFAP-IR and Iba1-IR, and were significantly more in ION-CCI rats compared with sham rats following noxious but not non-noxious mechanical stimulation. After intrathecal administration of the MEK1 inhibitor PD98059 in ION-CCI rats, the number of pERK-IR cells after noxious stimulation and the enhanced thermal nocifensive behavior but not the mechanical nocifensive behavior were significantly reduced in ION-CCI rats. The enhanced background activities, afterdischarges and responses of wide dynamic range neurons to noxious mechanical and thermal stimulation in ION-CCI rats were significantly depressed following i.t. administration of PD98059, whereas responses to non-noxious mechanical and thermal stimulation were not altered. The present findings suggest that pERK-IR neurons in the Vc play a pivotal role in the development of thermal hypersensitivity in the face following trigeminal nerve injury.     

Involvement of endothelin in morphine tolerance in neuroblastoma (SH-SY5Y) cells

Long-term use of morphine in pain management leads to adverse effects, such as development of antinociceptive tolerance. We have previously shown the involvement of central endothelin (ET) mechanisms in morphine analgesia and development of tolerance in vivo. The present study was conducted to investigate the in vitro mechanism of interaction of the ET(A) receptor antagonist, BMS182874, and morphine during acute and chronic morphine tolerance in SH-SY5Y cells. SH-SY5Y cells were exposed to acute and chronic treatment with vehicle, morphine, ET-1, BMS182874, or morphine plus BMS182874. Activation of G-protein-coupled receptors in SH-SY5Y cells was determined using [35S]GTPgammaS binding assays. Acute morphine treatment produced a concentration-dependent increase in GTP binding. Median effective concentration (EC50) values were significantly decreased after acute morphine treament, suggesting sensitization of opioid receptors. Chronic morphine treatment produced a lower maximal response of GTP binding compared with both control (vehicle treated) and acute morphine treatment, indicating uncoupling of G-proteins. Acute and chronic exposure of cells to ET-1 did not affect changes in ET-1-induced GTP binding. BMS182874 treatment alone (acute or chronic) did not produce G-protein activation. However, in cells chronically cotreated with 10 microM morphine and 1 microM BMS182874, morphine-induced GTP stimulation was significantly higher than control (vehicle treated). The EC50 value after control treatment was 414 nM, and was significantly increased in chronically morphine-treated cells (>1000 nM ). However, the EC50 value in cells receiving a chronic treatment of BMS182874 and 63 nM morphine was significantly reduced compared with control (vehicle treated) and chronic morphine treatment. ET(A) antagonists significantly enhance the coupling of G-protein to opioid receptors. Therefore, we propose that restoration of morphine antinociception by ET(A) antagonists in morphine-tolerant animals is likely via a G-protein mediated mechanism.     

Involvement of central endothelin receptors in neonatal morphine withdrawal

The involvement of central endothelin (ET) receptors in neonatal morphine tolerance has been demonstrated. The present study investigates the role of central ET receptors in morphine withdrawal in neonatal rats. The aim was to determine whether activation of G-proteins coupled to opioid and ET receptors by morphine and various ET receptor modulators is affected during morphine withdrawal in neonatal rats. Pregnant female rats were rendered tolerant to morphine by chronic exposure to morphine pellets during 7 days. On Day 8, pellets were removed and rats were allowed to undergo withdrawal for 24 hrs. Rat pups were delivered by cesarean section. G-protein stimulation induced by morphine; ET-1; the ET(A) receptor antagonist, BMS182874; and the ET(B) receptor agonist, IRL1620, were determined in the brain of neonatal rats undergoing morphine withdrawal by [35S]GTPgammaS binding assay. Morphine produced higher (P < 0.05) maximal stimulation of G-protein in the morphine-withdrawal group (83.60%) compared with the placebo group (66.81%). ET-1-induced G-protein stimulation was also altered, and the median effective concentration (EC50) during morphine withdrawal (170.60 nM) was significantly higher than placebo (62.5 nM; P< 0.05). The maximal stimulation induced by the ET(A) receptor antagonist, BMS182874, in the morphine-withdrawal group (86.07%; EC50 = 31.25 nM) was significantly higher than in the placebo group (EC50 > 1000 nM). The ET(B) agonist, IRL1620, induced G-protein stimulation was similar in placebo (73.43%, EC50 = 13.26 nM) and morphine-withdrawal groups (75.08%, EC(50) = 11.70 nM), respectively. To our knowledge, this is the first report indicating involvement of central ET(A) receptors in neonatal morphine withdrawal.     

Antinociceptive and nociceptive actions of opioids

Although the opioids are the principal treatment options for moderate to severe pain, their use is also associated with the development of tolerance, defined as the progressive need for higher doses to achieve a constant analgesic effect. The mechanisms which underlie this phenomenon remain unclear. Recent studies revealed that cholecystokinin (CCK) is upregulated in the rostral ventromedial medulla (RVM) during persistent opioid exposure. CCK is both antiopioid and pronociceptive, and activates descending pain facilitation mechanisms from the RVM enhancing nociceptive transmission at the spinal cord and promoting hyperalgesia. The neuroplastic changes elicited by opioid exposure reflect adaptive changes to promote increased pain transmission and consequent diminished antinociception (i.e., tolerance).     

Increased brain P-glycoprotein in morphine tolerant rats

The objective of this study was to determine whether chronic morphine exposure increased P-glycoprotein in rat brain. Male Sprague-Dawley rats were treated with morphine, saline, or dexamethasone for 5 days. On day 6, antinociceptive effect was measured to evaluate the extent of functional tolerance to morphine. Brain P-glycoprotein was detected by Western blot analysis of whole brain homogenate. Morphine- and dexamethasone-treated rats exhibited decreased antinociceptive response when compared to saline-treated controls. Brain P-glycoprotein was approximately 2-fold higher in morphine-treated rats compared to saline controls based on Western blot analysis. Chronic morphine exposure appears to increase P-glycoprotein in rat brain. P-glycoprotein induction may enhance morphine efflux from the brain, thus reducing morphine's pharmacologic activity. Induction of P-glycoprotein may be one mechanism involved in the development of morphine tolerance.     

Regulation by chronic clonidine of adenylate cyclase and cyclic AMP-dependent protein kinase in the rat locus coeruleus

Clonidine and morphine are known to produce tolerance and dependence in rat locus coeruleus (LC) neurons after chronic administration based on electrophysiological criteria. Previous studies have shown that morphine tolerance and dependence is associated with increases in levels of adenylate cyclase, pertussis toxin-mediated ADP-ribosylation of G-proteins, and cyclic AMP-dependent protein kinase in this brain region. The present study was aimed at investigating whether clonidine tolerance and dependence is also associated with alterations in these intracellular messengers. It was found that, similar to chronic morphine, chronic (2 weeks) clonidine administration, under conditions that produce electrophysiological evidence of tolerance and dependence in LC neurons, increased levels of adenylate cyclase activity and cyclic AMP-dependent protein kinase activity in this brain region, but not in several other regions studied, which included the frontal cortex, neostriatum, and dorsal raphe. However, the changes induced by chronic clonidine in the LC, at maximal doses and duration of treatment, were only approximately 50% in magnitude of those observed in response to morphine. Unlike chronic morphine, chronic clonidine produced no change in G-protein ADP-ribosylation levels in the LC. Chronic administration of a number of other drugs, namely diazepam, chloral hydrate, and dextromethorphan, which produce electrophysiological actions distinct from those of clonidine and morphine in the LC, failed to alter adenylate cyclase and cyclic AMP-dependent protein kinase in this brain region. The results indicate that increased levels of adenylate cyclase and cyclic AMP-dependent protein kinase represent common adaptations by LC neurons to chronic clonidine and morphine, and raise the possibility that such changes contribute to the development of clonidine and morphine tolerance and dependence in these neurons.