Opioid-activated postsynaptic, inward rectifying potassium currents in whole cell recordings in substantia gelatinosa neurons

The Paraventricular Thalamic Nucleus and Its Projections in Regulating Reward and Context Associations

The paraventricular thalamic nucleus (PVT) is a brain region that mediates aversive and reward-related behaviors as shown in animals exposed to fear conditioning, natural rewards, or drugs of abuse. However, it is unknown whether manipulations of the PVT, in the absence of external factors or stimuli (e.g., fear, natural rewards, or drugs of abuse), are sufficient to drive reward-related behaviors. Additionally, it is unknown whether drugs of abuse administered directly into the PVT are sufficient to drive reward-related behaviors. Here, using behavioral as well as pathway and cell-type specific approaches, we manipulate PVT activity as well as the PVT-to-nucleus accumbens shell (NAcSh) neurocircuit to explore reward phenotypes. First, we show that bath perfusion of morphine (10 µM) caused hyperpolarization of the resting membrane potential, increased rheobase, and decreased intrinsic membrane excitability in PVT neurons that project to the NAcSh. Additionally, we found that direct injections of morphine (50 ng) in the PVT of mice were sufficient to generate conditioned place preference (CPP) for the morphine-paired chamber. Mimicking the inhibitory effect of morphine, we employed a chemogenetic approach to inhibit PVT neurons that projected to the NAcSh and found that pairing the inhibition of these PVT neurons with a specific context evoked the acquisition of CPP. Lastly, using brain slice electrophysiology, we found that bath-perfused morphine (10 µM) significantly reduced PVT excitatory synaptic transmission on both dopamine D1 and D2 receptor-expressing medium spiny neurons in the NAcSh, but that inhibiting PVT afferents in the NAcSh was not sufficient to evoke CPP.

Contribution of µ Opioid Receptor-expressing Dorsal Horn Interneurons to Neuropathic Pain-like Behavior in Mice

BACKGROUND

Intersectional genetics have yielded tremendous advances in our understanding of molecularly identified subpopulations and circuits within the dorsal horn in neuropathic pain. The authors tested the hypothesis that spinal µ opioid receptor-expressing neurons (Oprm1-expressing neurons) contribute to behavioral hypersensitivity and neuronal sensitization in the spared nerve injury model in mice.

METHODS

The authors coupled the use of Oprm1Cre transgenic reporter mice with whole cell patch clamp electrophysiology in lumbar spinal cord slices to evaluate the neuronal activity of Oprm1-expressing neurons in the spared nerve injury model of neuropathic pain. The authors used a chemogenetic approach to activate or inhibit Oprm1-expressing neurons, followed by the assessment of behavioral signs of neuropathic pain.

RESULTS

The authors reveal that spared nerve injury yielded a robust neuroplasticity of Oprm1-expressing neurons. Spared nerve injury reduced Oprm1 gene expression in the dorsal horn as well as the responsiveness of Oprm1-expressing neurons to the selective µ agonist (D-Ala2, N-MePhe4, Gly-ol)-enkephalin (DAMGO). Spared nerve injury sensitized Oprm1-expressing neurons, as reflected by an increase in their intrinsic excitability (rheobase, sham 38.62 ± 25.87 pA [n = 29]; spared nerve injury, 18.33 ± 10.29 pA [n = 29], P = 0.0026) and spontaneous synaptic activity (spontaneous excitatory postsynaptic current frequency in delayed firing neurons: sham, 0.81 ± 0.67 Hz [n = 14]; spared nerve injury, 1.74 ± 1.68 Hz [n = 10], P = 0.0466), and light brush-induced coexpression of the immediate early gene product, Fos in laminae I to II (%Fos/tdTomato+: sham, 0.42 ± 0.57% [n = 3]; spared nerve injury, 28.26 ± 1.92% [n = 3], P = 0.0001). Chemogenetic activation of Oprm1-expressing neurons produced mechanical hypersensitivity in uninjured mice (saline, 2.91 ± 1.08 g [n = 6]; clozapine N-oxide, 0.65 ± 0.34 g [n = 6], P = 0.0006), while chemogenetic inhibition reduced behavioral signs of mechanical hypersensitivity (saline, 0.38 ± 0.37 g [n = 6]; clozapine N-oxide, 1.05 ± 0.42 g [n = 6], P = 0.0052) and cold hypersensitivity (saline, 6.89 ± 0.88 s [n = 5] vs. clozapine N-oxide, 2.31 ± 0.52 s [n = 5], P = 0.0017).

CONCLUSIONS

The authors conclude that nerve injury sensitizes pronociceptive µ opioid receptor-expressing neurons in mouse dorsal horn. Nonopioid strategies to inhibit these interneurons might yield new treatments for neuropathic pain.

EDITOR’S PERSPECTIVE

Critical Assessment of G Protein-Biased Agonism at the μ-Opioid Receptor

G protein-biased agonists of the μ-opioid receptor (MOPr) have been proposed as an improved class of opioid analgesics. Recent studies have been unable to reproduce the original experiments in the β-arrestin2-knockout mouse that led to this proposal, and alternative genetic models do not support the G protein-biased MOPr agonist hypothesis. Furthermore, assessment of putatively biased ligands has been confounded by several factors, including assay amplification. As such, the extent to which current lead compounds represent mechanistically novel, extremely G protein-biased agonists is in question, as is the underlying assumption that β-arrestin2 mediates deleterious opioid effects. Addressing these current challenges represents a pressing issue to successfully advance drug development at this receptor and improve upon current opioid analgesics.

Opioid Receptors in Immune and Glial Cells-Implications for Pain Control

Opioid receptors comprise μ (MOP), δ (DOP), κ (KOP), and nociceptin/orphanin FQ (NOP) receptors. Opioids are agonists of MOP, DOP, and KOP receptors, whereas nociceptin/orphanin FQ (N/OFQ) is an agonist of NOP receptors. Activation of all four opioid receptors in neurons can induce analgesia in animal models, but the most clinically relevant are MOP receptor agonists (e.g., morphine, fentanyl). Opioids can also affect the function of immune cells, and their actions in relation to immunosuppression and infections have been widely discussed. Here, we analyze the expression and the role of opioid receptors in peripheral immune cells and glia in the modulation of pain. All four opioid receptors have been identified at the mRNA and protein levels in immune cells (lymphocytes, granulocytes, monocytes, macrophages) in humans, rhesus monkeys, rats or mice. Activation of leukocyte MOP, DOP, and KOP receptors was recently reported to attenuate pain after nerve injury in mice. This involved intracellular Ca²⁺-regulated release of opioid peptides from immune cells, which subsequently activated MOP, DOP, and KOP receptors on peripheral neurons. There is no evidence of pain modulation by leukocyte NOP receptors. More good quality studies are needed to verify the presence of DOP, KOP, and NOP receptors in native glia. Although still questioned, MOP receptors might be expressed in brain or spinal cord microglia and astrocytes in humans, mice, and rats. Morphine acting at spinal cord microglia is often reported to induce hyperalgesia in rodents. However, most studies used animals without pathological pain and/or unconventional paradigms (e.g., high or ultra-low doses, pain assessment after abrupt discontinuation of chronic morphine treatment). Therefore, the opioid-induced hyperalgesia can be viewed in the context of dependence/withdrawal rather than pain management, in line with clinical reports. There is convincing evidence of analgesic effects mediated by immune cell-derived opioid peptides in animal models and in humans. Together, MOP, DOP, and KOP receptors, and opioid peptides in immune cells can ameliorate pathological pain. The relevance of NOP receptors and N/OFQ in leukocytes, and of all opioid receptors, opioid peptides and N/OFQ in native glia for pain control is yet to be clarified.

Electrophysiological Actions of N/OFQ

Whilst the nociceptin/orphanin FQ (N/OFQ) receptor (NOP) has similar intracellular coupling mechanisms to opioid receptors, it has distinct modulatory effects on physiological functions such as pain. These actions range from agonistic to antagonistic interactions with classical opioids within the spinal cord and brain, respectively. Understanding the electrophysiological actions of N/OFQ has been crucial in ascertaining the mechanisms by which these agonistic and antagonistic interactions occur. These similarities and differences between N/OFQ and opioids are due to the relative location of NOP versus opioid receptors on specific neuronal elements within these CNS regions. These mechanisms result in varied cellular actions including postsynaptic modulation of ion channels and presynaptic regulation of neurotransmitter release.

The effect of µ-opioid receptor activation on GABAergic neurons in the spinal dorsal horn

The superficial dorsal horn of the spinal cord plays an important role in pain transmission and opioid activity. Several studies have demonstrated that opioids modulate pain transmission, and the activation of µ-opioid receptors (MORs) by opioids contributes to analgesic effects in the spinal cord. However, the effect of the activation of MORs on GABAergic interneurons and the contribution to the analgesic effect are much less clear. In this study, using transgenic mice, which allow the identification of GABAergic interneurons, we investigated how the activation of MORs affects the excitability of GABAergic interneurons and synaptic transmission between primary nociceptive afferent and GABAergic interneurons. We found that a selective µ-opioid agonist, [D-Ala², NMe-Phe⁴, Gly-ol]-enkephanlin (DAMGO), induced an outward current mediated by K⁺ channels in GABAergic interneurons. In addition, DAMGO reduced the amplitude of evoked excitatory postsynaptic currents (EPSCs) of GABAergic interneurons which receive monosynaptic inputs from primary nociceptive C fibers. Taken together, we found that DAMGO reduced the excitability of GABAergic interneurons and synaptic transmission between primary nociceptive C fibers and GABAergic interneurons. These results suggest one possibility that suppression of GABAergic interneurons by DMAGO may reduce the inhibition on secondary GABAergic interneurons, which increase the inhibition of the secondary GABAergic interneurons to excitatory neurons in the spinal dorsal horn. In this circumstance, the sum of excitation of the entire spinal network will control the pain transmission.

The Peptide PnPP-19, a Spider Toxin Derivative, Activates μ-Opioid Receptors and Modulates Calcium Channels

The synthetic peptide PnPP-19 comprehends 19 amino acid residues and it represents part of the primary structure of the toxin δ-CNTX-Pn1c (PnTx2-6), isolated from the venom of the spider Phoneutria nigriventer. Behavioural tests suggest that PnPP-19 induces antinociception by activation of CB1, μ and δ opioid receptors. Since the peripheral and central antinociception induced by PnPP-19 involves opioid activation, the aim of this work was to identify whether this synthetic peptide could directly activate opioid receptors and investigate the subtype selectivity for μ-, δ- and/or κ-opioid receptors. Furthermore, we also studied the modulation of calcium influx driven by PnPP-19 in dorsal root ganglion neurons, and analyzed whether this modulation was opioid-mediated. PnPP-19 selectively activates μ-opioid receptors inducing indirectly inhibition of calcium channels and hereby impairing calcium influx in dorsal root ganglion (DRG) neurons. Interestingly, notwithstanding the activation of opioid receptors, PnPP-19 does not induce β-arrestin2 recruitment. PnPP-19 is the first spider toxin derivative that, among opioid receptors, selectively activates μ-opioid receptors. The lack of β-arrestin2 recruitment highlights its potential for the design of new improved opioid agonists.

Mechanisms of μ-opioid receptor inhibition of NMDA receptor-induced substance P release in the rat spinal cord

The interaction between NMDA receptors and μ-opioid receptors in primary afferent terminals was studied by using NMDA to induce substance P release, measured as neurokinin 1 receptor internalization. In rat spinal cord slices, the μ-opioid receptor agonists morphine, DAMGO and endomorphin-2 inhibited NMDA-induced substance P release, whereas the antagonist CTAP right-shifted the concentration response of DAMGO. In vivo, substance P release induced by intrathecal NMDA after priming with BDNF was inhibited by DAMGO. ω-Conotoxins MVIIC and GVIA inhibited about half of the NMDA-induced substance P release, showing that it was partially mediated by the opening of voltage-gated calcium (Cav) channels. In contrast, DAMGO or ω-conotoxins did not inhibit capsaicin-induced substance P release. In cultured DRG neurons, DAMGO but not ω-conotoxin inhibited NMDA-induced increases in intracellular calcium, indicating that μ-opioid receptors can inhibit NMDA receptor function by mechanisms other than inactivation of Cav channels. Moreover, DAMGO decreased the ω-conotoxin-insensitive component of the substance P release. Potent inhibition by ifenprodil showed that these NMDA receptors have the NR2B subunit. Activators of adenylyl cyclase and protein kinase A (PKA) induced substance P release and this was decreased by the NMDA receptor blocker MK-801 and by DAMGO. Conversely, inhibitors of adenylyl cyclase and PKA, but not of protein kinase C, decreased NMDA-induced substance P release. Hence, these NMDA receptors are positively modulated by the adenylyl cyclase-PKA pathway, which is inhibited by μ-opioid receptors. In conclusion, μ-opioid receptors inhibit NMDA receptor-induced substance P release through Cav channel inactivation and adenylyl cyclase inhibition.

DAMGO modulates two-pore domain K(+) channels in the substantia gelatinosa neurons of rat spinal cord

The analgesic mechanism of opioids is known to decrease the excitability of substantia gelatinosa (SG) neurons receiving the synaptic inputs from primary nociceptive afferent fiber by increasing inwardly rectifying K⁺ current. In this study, we examined whether a µ-opioid agonist, [D-Ala2,N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO), affects the two-pore domain K⁺ channel (K2P) current in rat SG neurons using a slice whole-cell patch clamp technique. Also we confirmed which subtypes of K2P channels were associated with DAMGO-induced currents, measuring the expression of K2P channel in whole spinal cord and SG region. DAMGO caused a robust hyperpolarization and outward current in the SG neurons, which developed almost instantaneously and did not show any time-dependent inactivation. Half of the SG neurons exhibited a linear I~V relationship of the DAMGO-induced current, whereas rest of the neurons displayed inward rectification. In SG neurons with a linear I~V relationship of DAMGO-induced current, the reversal potential was close to the K⁺ equilibrium potentials. The mRNA expression of TWIK (tandem of pore domains in a weak inwardly rectifying K⁺ channel) related acid-sensitive K⁺ channel (TASK) 1 and 3 was found in the SG region and a low pH (6.4) significantly blocked the DAMGO-induced K⁺ current. Taken together, the DAMGO-induced hyperpolarization at resting membrane potential and subsequent decrease in excitability of SG neurons can be carried by the two-pore domain K⁺ channel (TASK1 and 3) in addition to inwardly rectifying K⁺ channel.

Distinct forms of synaptic inhibition and neuromodulation regulate calretinin-positive neuron excitability in the spinal cord dorsal horn

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CR+ spinal dorsal horn neurons form excitatory (Typical) and inhibitory (Atypical) subpopulations.

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Typical neurons received mixed (GABAergic and glycinergic) inhibition.

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Atypical neurons received inhibition dominated by glycine.

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Noradrenaline and serotonin evoke responses in Typical but not Atypical neurons.

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Enkephalins evoke responses in Atypical but not typical neurons.

The dorsal horn (DH) of the spinal cord contains a heterogenous population of neurons that process incoming sensory signals before information ascends to the brain. We have recently characterized calretinin-expressing (CR+) neurons in the DH and shown that they can be divided into excitatory and inhibitory subpopulations. The excitatory population receives high-frequency excitatory synaptic input and expresses delayed firing action potential discharge, whereas the inhibitory population receives weak excitatory drive and exhibits tonic or initial bursting discharge. Here, we characterize inhibitory synaptic input and neuromodulation in the two CR+ populations, in order to determine how each is regulated. We show that excitatory CR+ neurons receive mixed inhibition from GABAergic and glycinergic sources, whereas inhibitory CR+ neurons receive inhibition, which is dominated by glycine. Noradrenaline and serotonin produced robust outward currents in excitatory CR+ neurons, predicting an inhibitory action on these neurons, but neither neuromodulator produced a response in CR+ inhibitory neurons. In contrast, enkephalin (along with selective mu and delta opioid receptor agonists) produced outward currents in inhibitory CR+ neurons, consistent with an inhibitory action but did not affect the excitatory CR+ population. Our findings show that the pharmacology of inhibitory inputs and neuromodulator actions on CR+ cells, along with their excitatory inputs can define these two subpopulations further, and this could be exploited to modulate discrete aspects of sensory processing selectively in the DH.

Inhibitory effects of endomorphin-2 on excitatory synaptic transmission and the neuronal excitability of sacral parasympathetic preganglionic neurons in young rats

The function of the urinary bladder is partly controlled by parasympathetic preganglionic neurons (PPNs) of the sacral parasympathetic nucleus (SPN). Our recent work demonstrated that endomorphin-2 (EM-2)-immunoreactive (IR) terminals form synapses with μ-opioid receptor (MOR)-expressing PPNs in the rat SPN. Here, we examined the effects of EM-2 on excitatory synaptic transmission and the neuronal excitability of the PPNs in young rats (24–30 days old) using a whole-cell patch-clamp approach. PPNs were identified by retrograde labeling with the fluorescent tracer tetramethylrhodamine-dextran (TMR). EM-2 (3 μM) markedly decreased both the amplitude and the frequency of the spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs) of PPNs. EM-2 not only decreased the resting membrane potentials (RMPs) in 61.1% of the examined PPNs with half-maximal response at the concentration of 0.282 μM, but also increased the rheobase current and reduced the repetitive action potential firing of PPNs. Analysis of the current–voltage relationship revealed that the EM-2-induced current was reversed at −95 ± 2.5 mV and was suppressed by perfusion of the potassium channel blockers 4-aminopyridine (4-AP) or BaCl₂ or by the addition of guanosine 5′-[β-thio]diphosphate trilithium salt (GDP-β-S) to the pipette solution, suggesting the involvement of the G-protein-coupled inwardly rectifying potassium (GIRK) channel. The above EM-2-invoked inhibitory effects were abolished by the MOR selective antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), indicating that the effects of EM-2 on PPNs were mediated by MOR via pre- and/or post-synaptic mechanisms. EM-2 activated pre- and post-synaptic MORs, inhibiting excitatory neurotransmitter release from the presynaptic terminals and decreasing the excitability of PPNs due to hyperpolarization of their membrane potentials, respectively. These inhibitory effects of EM-2 on PPNs at the spinal cord level may explain the mechanism of action of morphine treatment and morphine-induced bladder dysfunction in the clinic.

Neuron-restrictive silencer factor in periaqueductal gray contributes to remifentanil-induced postoperative hyperalgesia via repression of the mu-opioid receptor

BACKGROUND

The ultra-short-acting mu-opioid receptor (MOR) agonist remifentanil induces postoperative hyperalgesia both in preclinical and clinical research studies. However, the precise mechanisms remain unclear, although changes in opioid receptor expression might be a correlative feature. Neuron-restrictive silencer factor (NRSF) functions as a crucial regulator of MOR expression in specific neuronal cells. Using a mouse model of incisional postoperative pain, we assessed the expression of MOR and NRSF and investigated whether disruption of NRSF expression could prevent the postoperative nociceptive sensitization induced by surgical incision and subcutaneous infusion of remifentanil.

METHODS

Paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL) were independently used to assess mechanical allodynia and thermal hyperalgesia after surgery and cerebral ventricle injection of NRSF antisense oligonucleotide. Western blotting analyses were preformed to assess the expression levels of MOR and NRSF.

RESULTS

NRSF expression levels were enhanced after intraoperative infusion of remifentanil, resulting in repression of MOR expression in the periaqueductal gray (PAG). NRSF blockade with an NRSF antisense oligonucleotide significantly enhanced the expression levels of MOR and alleviated mechanical allodynia and thermal hyperalgesia induced by intraoperative infusion of remifentanil.

CONCLUSION

NRSF functions as a negative regulator of MOR in PAG and contributes to remifentanil-induced postoperative hyperalgesia. NRSF in PAG may be a potential target for this pain therapy.

Endogenous analgesia, dependence, and latent pain sensitization

Endogenous activation of µ-opioid receptors (MORs) provides relief from acute pain. Recent studies have established that tissue inflammation produces latent pain sensitization (LS) that is masked by spinal MOR signaling for months, even after complete recovery from injury and re-establishment of normal pain thresholds. Disruption with MOR inverse agonists reinstates pain and precipitates cellular, somatic, and aversive signs of physical withdrawal; this phenomenon requires N-methyl-D-aspartate receptor-mediated activation of calcium-sensitive adenylyl cyclase type 1 (AC1). In this review, we present a new conceptual model of the transition from acute to chronic pain, based on the delicate balance between LS and endogenous analgesia that develops after painful tissue injury. First, injury activates pain pathways. Second, the spinal cord establishes MOR constitutive activity (MORCA) as it attempts to control pain. Third, over time, the body becomes dependent on MORCA, which paradoxically sensitizes pain pathways. Stress or injury escalates opposing inhibitory and excitatory influences on nociceptive processing as a pathological consequence of increased endogenous opioid tone. Pain begets MORCA begets pain vulnerability in a vicious cycle. The final result is a silent insidious state characterized by the escalation of two opposing excitatory and inhibitory influences on pain transmission: LS mediated by AC1 (which maintains the accelerator) and pain inhibition mediated by MORCA (which maintains the brake). This raises the prospect that opposing homeostatic interactions between MORCA analgesia and latent NMDAR-AC1-mediated pain sensitization creates a lasting vulnerability to develop chronic pain. Thus, chronic pain syndromes may result from a failure in constitutive signaling of spinal MORs and a loss of endogenous analgesic control. An overarching long-term therapeutic goal of future research is to alleviate chronic pain by either (a) facilitating endogenous opioid analgesia, thus restricting LS within a state of remission, or (b) extinguishing LS altogether.

Neurochemical characterisation of lamina II inhibitory interneurons that express GFP in the PrP-GFP mouse

Background

Inhibitory interneurons in the superficial dorsal horn play important roles in modulating sensory transmission, and these roles are thought to be performed by distinct functional populations. We have identified 4 non-overlapping classes among the inhibitory interneurons in the rat, defined by the presence of galanin, neuropeptide Y, neuronal nitric oxide synthase (nNOS) and parvalbumin. The somatostatin receptor sst_2A is expressed by ~50% of the inhibitory interneurons in this region, and is particularly associated with nNOS- and galanin-expressing cells. The main aim of the present study was to test whether a genetically-defined population of inhibitory interneurons, those expressing green fluorescent protein (GFP) in the PrP-GFP mouse, belonged to one or more of the neurochemical classes identified in the rat.

Results

The expression of sst_2A and its relation to other neurochemical markers in the mouse was similar to that in the rat, except that a significant number of cells co-expressed nNOS and galanin. The PrP-GFP cells were entirely contained within the set of inhibitory interneurons that possessed sst_2A receptors, and virtually all expressed nNOS and/or galanin. GFP was present in ~3-4% of neurons in the superficial dorsal horn, corresponding to ~16% of the inhibitory interneurons in this region. Consistent with their sst_2A-immunoreactivity, all of the GFP cells were hyperpolarised by somatostatin, and this was prevented by administration of a selective sst₂ receptor antagonist or a blocker of G-protein-coupled inwardly rectifying K⁺ channels.

Conclusions

These findings support the view that neurochemistry provides a valuable way of classifying inhibitory interneurons in the superficial laminae. Together with previous evidence that the PrP-GFP cells form a relatively homogeneous population in terms of their physiological properties, they suggest that these neurons have specific roles in processing sensory information in the dorsal horn.

Relative contributions of peripheral versus supraspinal or spinal opioid receptors to the antinociception of systemic opioids

The contribution of supraspinal, spinal or peripheral mu-opioid receptors (MORs) to the overall antinociception of systemic centrally penetrating versus peripherally restricted opioids has not been thoroughly investigated. Therefore, we examined paw pressure thresholds in Wistar rats with complete Freund's adjuvant hindpaw inflammation following different doses of intraplantar (i.pl.) as well as intravenous (i.v.) fentanyl (6.25-50 μg/kg), morphine (1-7.5 mg/kg) or loperamide (1-7.5 mg/kg). Antagonism of the i.v. mu-opioid agonists by intracerebroventricular (i.c.v.), intrathecal (i.t.) or i.pl. naloxone-methiodide (NLXM) revealed the relative contributions of supraspinal, spinal and peripheral MOR to the overall antinociceptive effects. In parallel, the MOR density at these three levels of pain transmission was assessed by radioligand binding. Antinociceptive effects of i.v. fentanyl and morphine, but not of the peripherally restricted loperamide were two- to threefold greater and longer lasting compared with their i.pl. administration. I.c.v. but not i.pl. NLXM significantly antagonized fentanyl's and morphine's antinociception by 70-80%, whereas i.t. NLXM reduced it by 20-30%. In contrast, antinociception of i.v. loperamide was abolished by i.pl. but not by i.c.v. or i.t. NLXM. In parallel, a respective 32- and sixfold higher MOR density in supraspinal and spinal versus peripheral sensory neurons was detected. In conclusion, in comparison with supraspinal and spinal opioid receptors, peripheral opioid receptors do not significantly contribute to the antinociception of systemic fentanyl and morphine during inflammatory pain. Antinociception of their i.v. administration was superior over both i.v and i.pl. loperamide, acting exclusively via peripheral MOR. These findings may guide the future development of novel peripherally restricted opioids.

Preclinical and early clinical investigations related to monoaminergic pain modulation

The balance between descending controls, both excitatory and inhibitory, can be altered in various pain states. There is good evidence for a prominent alpha(2)-adrenoceptor-mediated inhibitory system and 5-HT(3) (and likely also 5-HT(2)) serotonin receptor-mediated excitatory controls originating from brainstem and midbrain areas. The ability of cortical controls to influence spinal function allows for top-down processing through these monoamines. The links between pain and the comorbidities of sleep problems, anxiety, and depression may be due to the dual roles of noradrenaline and of 5-HT in these functions and also in pain. These controls appear, in the cases of peripheral neuropathy, spinal injury, and cancer-induced bone pain to be driven by altered peripheral and spinal neuronal processes; in opioid-induced hyperalgesia, however, the same changes occur without any pathophysiological peripheral process. Thus, in generalized pain states in which fatigue, mood changes, and diffuse pain occur, such as fibromyalgia and irritable bowel syndrome, one could suggest an abnormal engagement of descending facilitations with or without reduced inhibitions but with central origins. This would be an endogenous central malfunction of top-down processing, with the altered monoamine systems underlying the observed symptoms. A number of analgesic drugs can either interact with or have their actions modulated by these descending systems, reinforcing their importance in the establishment of pain but also in its control.

Sustained inhibition of neurotransmitter release from nontransient receptor potential vanilloid type 1-expressing primary afferents by mu-opioid receptor activation-enkephalin in the spinal cord

Removing transient receptor potential vanilloid type 1 (TRPV1)-expressing primary afferent neurons reduces presynaptic mu-opioid receptors but potentiates opioid analgesia. However, the sites and underlying cellular mechanisms for this paradoxical effect remain uncertain. In this study, we determined the presynaptic and postsynaptic effects of the mu-opioid receptor agonist [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO) using whole-cell patch-clamp recordings of lamina II neurons in rat spinal cord slices. Treatment with the ultrapotent TRPV1 agonist resiniferotoxin (RTX) eliminated TRPV1-expressing dorsal root ganglion neurons and their central terminals in the spinal dorsal horn and significantly reduced the basal amplitude of glutamatergic excitatory postsynaptic currents (EPSCs) evoked from primary afferents. Although RTX treatment did not significantly alter the concentration-response effect of DAMGO on evoked monosynaptic and polysynaptic EPSCs, it causes a profound long-lasting inhibitory effect of DAMGO on evoked EPSCs. Subsequent naloxone treatment did not reverse the prolonged inhibitory effect of DAMGO on evoked EPSCs. Furthermore, brief application of DAMGO produced a sustained inhibition of miniature EPSCs in RTX-treated rats. However, the concentration response and the duration of the effects of DAMGO on G protein-coupled inwardly rectifying K+ currents in lamina II neurons were not significantly different between vehicle- and RTX-treated groups. These data suggest that stimulation of mu-opioid receptors on non-TRPV1 afferent terminals causes extended inhibition of neurotransmitter release to spinal dorsal horn neurons. The differential effect of mu-opioid receptor agonists on different phenotypes of primary afferents provides a cellular basis to explain why the analgesic action of opioids on mechanonociception is prolonged when TRPV1-expressing primary afferents are removed.

Activation of GIRK channels in substantia gelatinosa neurones of the adult rat spinal cord: a possible involvement of somatostatin

Recent studies have suggested that spinal G-protein-coupled, inwardly rectifying K(+) (GIRK) channels play an important role in thermal nociception and the analgesic actions of morphine and other agents. In this study, we show that spinal GIRK channels are activated by an endogenous neurotransmitter using whole-cell patch-clamp recordings from substantia gelatinosa (SG) neurones in adult rat spinal cord slices. Although repetitive stimuli applied to the dorsal root did not induce any slow responses, ones focally applied to the spinal dorsal horn produced slow inhibitory postsynaptic currents (IPSCs) at a holding potential of -50 mV in about 30% of the SG neurones recorded. The amplitude and duration of slow IPSCs increased with the number of stimuli and decreased with removal of Ca(2+) from the external Krebs solution. Slow IPSCs were associated with an increase in membrane conductance; their polarity was reversed at a potential close to the equilibrium potential for K(+), calculated from the Nernst equation. Slow IPSCs were blocked by addition of GDP-beta-S into the patch-pipette solution, reduced in amplitude in the presence of Ba(2+), and significantly suppressed in the presence of an antagonist of GIRK channels, tertiapin-Q. Somatostatin produced an outward current in a subpopulation of SG neurones and the slow IPSC was occluded during the somatostatin-induced outward current. Moreover, slow IPSCs were significantly inhibited by the somatostatin receptor antagonist cyclo-somatostatin. These results suggest that endogenously released somatostatin may induce slow IPSCs through the activation of GIRK channels in SG neurones; this slow synaptic transmission might play an important role in spinal antinociception.

Compartment-specific modulation of GABAergic synaptic transmission by mu-opioid receptor in the mouse striatum with green fluorescent protein-expressing dopamine islands

The striatum is a heterogeneous mosaic of two neurochemically, developmentally, and functionally distinct compartments: the mu-opioid receptor (MOR)-enriched striosomes and the matrix. Preferential activation of the striosomes and persistent suppression of the matrix have recently been suggested to represent neural correlates of motor stereotypy. However, little is known concerning the physiological properties of the striosomes. We made patch-clamp recordings from medium spiny neurons in identified MOR-immunoreactive "dopamine islands" as striosomes in a slice preparation taken from transgenic mice expressing green fluorescent protein in tyrosine hydroxylase mRNA-containing neurons. Striosomal neurons differed electrophysiologically from cells in the matrix in having significantly less hyperpolarized resting membrane potentials and larger input resistances, suggesting developmental differences between the two types of cells. Moreover, corticostriatal EPSCs were inhibited by MOR activation to similar extents in the two compartments, although inhibition of IPSCs was observed only in the striosomes. This MOR-induced inhibition of IPSCs was presynaptically mediated, because MOR agonist invariably decreased IPSC amplitudes when postsynaptic G-protein was inactivated, significantly increased the paired-pulse ratio of the IPSCs, and decreased the frequency but not the amplitude of miniature IPSCs. These effects of MOR were mediated principally by 4-aminopyridine-sensitive K+ conductance via a cAMP-dependent pathway, which was further augmented by previous blockade of the protein kinase C cascade. These findings suggest that MOR activation by endogenous and/or exogenous MOR-selective opioid substances differentially regulates the activities of the striosome and matrix compartments and thus plays an important role in motivated behavior and learning.

Modulation of pain transmission by G-protein-coupled receptors

The heterotrimeric G-protein-coupled receptors (GPCR) represent the largest and most diverse family of cell surface receptors and proteins. GPCR are widely distributed in the peripheral and central nervous systems and are one of the most important therapeutic targets in pain medicine. GPCR are present on the plasma membrane of neurons and their terminals along the nociceptive pathways and are closely associated with the modulation of pain transmission. GPCR that can produce analgesia upon activation include opioid, cannabinoid, alpha2-adrenergic, muscarinic acetylcholine, gamma-aminobutyric acidB (GABAB), groups II and III metabotropic glutamate, and somatostatin receptors. Recent studies have led to a better understanding of the role of these GPCR in the regulation of pain transmission. Here, we review the current knowledge about the cellular and molecular mechanisms that underlie the analgesic actions of GPCR agonists, with a focus on their effects on ion channels expressed on nociceptive sensory neurons and on synaptic transmission at the spinal cord level.

Activation of group I metabotropic glutamate receptors on main olfactory bulb granule cells and periglomerular cells enhances synaptic inhibition of mitral cells

Granule and periglomerular cells in the main olfactory bulb express group I metabotropic glutamate receptors (mGluRs). The group I mGluR agonist 3,4-dihydroxyphenylglycine (DHPG) increases GABAergic spontaneous IPSCs (sIPSCs) in mitral cells, yet the presynaptic mechanism(s) involved and source(s) of the IPSCs are unknown. We investigated the actions of DHPG on sIPSCs and TTX-insensitive miniature IPSCs (mIPSCs) recorded in mitral and external tufted cells in rat olfactory bulb slices. DHPG, acting at mGluR1 and mGluR5, increased the rate but not amplitude of sIPSCs and mIPSCs in both cell types. The increase in mIPSCs depended on voltage-gated Ca2+ channels but persisted when ionotropic glutamate receptors and sodium spikes were blocked. Focal DHPG puffs onto granule cells or bath application after glomerular layer (GL) excision failed to increase mIPSCs in mitral cells. Additionally, GL excision reduced sIPSCs in mitral cells by 50%, suggesting that periglomerular cells exert strong tonic GABAergic inhibition of mitral cells. In contrast, GL DHPG puffs readily increased mIPSCs. These findings indicate that DHPG-evoked GABA release from granule cells requires spikes, whereas in the GL, DHPG facilitates periglomerular cell GABA release via both spike-dependent and spike-independent presynaptic mechanisms. We speculate that mGluRs amplify spike-driven lateral inhibition through the mitral-to-granule cell circuit, whereas GL mGluRs may play a more important role in amplifying intraglomerular inhibition after subthreshold input.

Morphology and electrophysiological properties of hamster spinal dorsal horn neurons that express VGLUT2 and enkephalin

The excitatory amino acid glutamate mediates transmission at spinal synapses, including those formed by sensory afferent fibers and by intrinsic interneurons. The identity and physiological properties of glutamatergic dorsal horn neurons are poorly characterized despite their importance in spinal sensory circuits. Moreover, many intrinsic spinal glutamatergic synapses colocalize the opioid peptide enkephalin (ENK), but the neurons to which they belong are yet to be identified. Therefore, we used immunohistochemistry and confocal microscopy to investigate expression of the VGLUT2 vesicular glutamate transporter, an isoform reported in nonprimary afferent spinal synapses, and ENK in electrophysiologically identified neurons of hamster spinal dorsal horn. VGLUT2 immunoreactivity was localized in restricted fashion to axon varicosities of neurons recorded from laminae II-V, although the occurrence of immunolabeling in individual varicosities varied widely between cells (39 +/- 36%, n = 31 neurons). ENK colocalized with VGLUT2 in up to 77% of varicosities (17 +/- 21%, n = 21 neurons). The majority of neurons expressing VGLUT2 and/or ENK had axons with dense local terminations or projections consistent with propriospinal functions. VGLUT2 and ENK labeling were not correlated with cellular morphology, intrinsic membrane properties, firing patterns, or synaptic responses to sensory afferent stimulation. However, VGLUT2 expression was significantly higher in neurons with depolarized resting membrane potential. The results are new evidence for a population of dual-function dorsal horn interneurons that might provide another mechanism for limiting excitation within dorsal horn circuits during periods of strong sensory activation.

Differential Effects of Opioids on Sacrocaudal Afferent Pathways and Central Pattern Generators in the Neonatal Rat Spinal Cord

The effects of opioids on sacrocaudal afferent (SCA) pathways and the pattern-generating circuitry of the thoracolumbar and sacrocaudal segments of the spinal cord were studied in isolated spinal cord and brain stem-spinal cord preparations of the neonatal rat. The locomotor and tail moving rhythm produced by activation of nociceptive and nonnociceptive sacrocaudal afferents was completely blocked by specific application of the μ-opioid receptor agonist [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin acetate salt (DAMGO) to the sacrocaudal but not the thoracolumbar segments of the spinal cord. The rhythmic activity could be restored after addition of the opioid receptor antagonist naloxone to the experimental chamber. The opioid block of the SCA-induced rhythm is not due to impaired rhythmogenic capacity of the spinal cord because a robust rhythmic activity could be initiated in the thoracolumbar and sacrocaudal segments in the presence of DAMGO, either by stimulation of the ventromedial medulla or by bath application of N-methyl-d-aspartate/serotonin. We suggest that the opioid block of the SCA-induced rhythm involves suppression of synaptic transmission through sacrocaudal interneurons interposed between SCA and the pattern-generating circuitry. The expression of μ opioid receptors in several groups of dorsal, intermediate and ventral horn interneurons in the sacrocaudal segments of the cord, documented in this study, provides an anatomical basis for this suggestion.

Distinct populations of spinal cord lamina II interneurons expressing G-protein-gated potassium channels

Noxious stimuli are sensed and carried to the spinal cord dorsal horn by A delta and C primary afferent fibers. Some of this input is relayed directly to supraspinal sites by projection neurons, whereas much of the input impinges on a heterogeneous population of interneurons in lamina II. Previously, we demonstrated that G-protein-gated inwardly rectifying potassium (GIRK) channels are expressed in lamina II of the mouse spinal cord and that pharmacologic ablation of spinal GIRK channels selectively blunts the analgesic effect of high but not lower doses of intrathecal mu-opioid receptor (MOR) agonists. Here, we report that GIRK channels formed by GIRK1 and GIRK2 subunits are found in two large populations of lamina II excitatory interneurons. One population displays relatively large apparent whole-cell capacitances and prominent GIRK-dependent current responses to the MOR agonist [D-Ala2,N-MePhe4,Gly-ol5] -enkephalin (DAMGO). A second population shows smaller apparent capacitance values and a GIRK-dependent response to the GABA(B) receptor agonist baclofen, but not DAMGO. Ultrastructural analysis revealed that GIRK subunits preferentially label type I synaptic glomeruli, suggesting that GIRK-containing lamina II interneurons receive prominent input from C fibers, while receiving little input from A delta fibers. Thus, excitatory interneurons in lamina II of the mouse spinal cord can be subdivided into different populations based on the neurotransmitter system coupled to GIRK channels. This important distinction will afford a unique opportunity to characterize spinal nociceptive circuitry with defined physiological significance.

Spinal G-protein-gated potassium channels contribute in a dose-dependent manner to the analgesic effect of mu- and delta- but not kappa-opioids

Opioids can evoke analgesia by inhibiting neuronal targets in either the brain or spinal cord, and multiple presynaptic and postsynaptic inhibitory mechanisms have been implicated. The relative significance of presynaptic and postsynaptic inhibition to opioid analgesia is essentially unknown, as are the identities and relevant locations of effectors mediating opioid actions. Here, we examined the distribution of G-protein-gated potassium (GIRK) channels in the mouse spinal cord and measured their contribution to the analgesia evoked by spinal administration of opioid receptor-selective agonists. We found that the GIRK channel subunits GIRK1 and GIRK2 were concentrated in the outer layer of the substantia gelatinosa of the dorsal horn. GIRK1 and GIRK2 were found almost exclusively in postsynaptic membranes of putative excitatory synapses, and a significant degree of overlap with the mu-opioid receptor was observed. Although most GIRK subunit labeling was perisynaptic or extrasynaptic, GIRK2 was found occasionally within the synaptic specialization. Genetic ablation or pharmacologic inhibition of spinal GIRK channels selectively blunted the analgesic effect of high but not lower doses of the mu-opioid receptor-selective agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin. Dose-dependent contributions of GIRK channels to the analgesic effects of the -opioid receptor-selective agonists Tyr-D-Ala-Phe-Glu-Val-Val-Gly amide and [D-Pen(2,5)]-enkephalin were also observed. In contrast, the analgesic effect of the agonist (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl] benzeneacetamide methanesulfonate hydrate was preserved despite the absence of GIRK channels. We conclude that the activation of postsynaptic GIRK1 and/or GIRK2-containing channels in the spinal cord dorsal horn represents a powerful, albeit relatively insensitive, means by which intrathecal mu- and -selective opioid agonists evoke analgesia.

Blocking mu opioid receptors in the spinal cord prevents the analgesic action by subsequent systemic opioids

Systemically administered mu opioids may produce analgesia through inhibition of the ascending nociceptive transmission and activation of descending pathways. However, the relative importance of the spinal and supraspinal sites in the analgesic action of systemic opioids remains uncertain. It has been shown that systemic morphine can inhibit dorsal horn neurons independent of the descending system. In this study, we determined the extent to which spinal mu opioid receptors mediate the analgesic effect of systemic mu opioids. Rats were instrumented with an intrathecal catheter with the tip placed in the lumbar spinal cord. Nociception was measured by testing the paw withdrawal threshold in response to a noxious radiant heat or pressure stimulus. Surprisingly, intrathecal pretreatment with naloxone or H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP, a specific mu opioid receptor antagonist) completely blocked the inhibitory effect of intravenous morphine on mechanical nociception. Intrathecal naloxone or CTAP also abolished the effect of intravenous morphine on the withdrawal latency of the hindpaw, but not the forepaw, measured with a radiant heat stimulus. Furthermore, the inhibitory effect of subcutaneous fentanyl on mechanical nociception was eliminated by CTAP injected intrathecally. Intrathecal CTAP similarly abolished the effect of subcutaneous fentanyl on thermal nociception of the hindpaw but not the forepaw. Therefore, this study provides new information that when spinal mu opioid receptors are blocked, subsequent systemic administration of mu opioids fails to produce an analgesic effect. This finding highlights the important role of mu opioid receptors in the spinal cord in the antinociceptive action of opioids.

Loss of TRPV1-expressing sensory neurons reduces spinal mu opioid receptors but paradoxically potentiates opioid analgesia

Systemic administration of resiniferatoxin (RTX), an ultrapotent capsaicin analogue, removes transient receptor potential vanilloid type 1 (TRPV1)-expressing afferent neurons and impairs thermal but not mechanical nociception in adult animals. In this study, we determined how loss of TRPV1-expressing sensory neurons alters the antinociceptive effect of mu opioids and mu opioid receptors in the spinal cord. The effect of morphine and (D-Ala2,N-Me-Phe4,Gly-ol5)-enkephalin (DAMGO) was measured by testing the paw mechanical withdrawal threshold in rats treated with RTX or vehicle. RTX treatment deleted TRPV1-immunoreactive dorsal root ganglion neurons and nerve terminals in the spinal dorsal horn. Also the mu opioid receptor immunoreactivity was markedly reduced in the superficial dorsal horn of RTX-treated rats. However, RTX treatment did not affect the dorsal horn neurons labeled with both TRPV1- and mu opioid receptor-immunoreactivity. Surprisingly, intrathecal morphine or DAMGO produced a greater increase in the withdrawal threshold in RTX- than in vehicle-treated rats. The duration of the effect of intrathecal morphine and DAMGO in RTX-treated rats was also profoundly increased. Furthermore, the antinociceptive effect of systemic morphine was significantly potentiated in RTX-treated rats. The B(MAX) (but not K(D)) of [3H]-DAMGO binding and DAMGO-stimulated [35S]GTPgammaS activity in the dorsal spinal cord were significantly reduced in the RTX group. This study provides novel information that loss of TRPV1 afferent neurons eliminates presynaptic mu opioid receptors present on TRPV1-expressing afferent neurons but paradoxically potentiates the analgesic effect of mu opioid agonists. Mechano-nociception, transmitted through non-TRPV1 sensory neurons, is subject to potent modulation by mu opioid agonists.

Tramadol produces outward currents by activating mu-opioid receptors in adult rat substantia gelatinosa neurones

1. An action of a tramadol metabolite, mono-O-dimethyl-tramadol (M1), on substantia gelatinosa (SG) neurones in adult rat spinal cord slices was examined by using the whole-cell patch-clamp technique. 2. In 41% of the neurones examined, superfusing M1 produced an outward current at -70 mV; this response reversed at a potential close to the equilibrium potential for K(+). M1 current hardly declined and persisted for >30 min after its washout. 3. M1 current correlated in amplitude with current produced by mu-opioid receptor agonist DAMGO in the same neurone, and largely reduced in amplitude in the presence of mu-opioid receptor antagonist CTAP but not alpha2-adrenoceptor antagonist yohimbine. In a neurone where M1 had no effect on holding currents, noradrenaline produced an outward current at -70 mV. 4. The amplitude of the M1 response, relative to that of the DAMGO response, exhibited an EC(50) value of 300 microM. 5. We conclude that M1 produces a persistent hyperpolarization by activating mu-opioid receptors in adult rat SG neurones. This could contribute to at least a part of pain alleviation produced by tramadol.

Effect of morphine on deep dorsal horn projection neurons depends on spinal GABAergic and glycinergic tone: implications for reduced opioid effect in neuropathic pain

The mu opioid agonist morphine has distinct effects on spinal dorsal horn neurons in the superficial and deep laminae. However, it is not clear if the inhibitory effect of morphine on dorsal horn projection neurons is secondary to its potentiating effect on inhibitory interneurons. In this study, we tested the hypothesis that removal of GABAergic and glycinergic inhibitory inputs attenuates the effect of morphine on dorsal horn projection neurons and the reduced spinal GABAergic tone contributes to attenuated morphine effect in neuropathic pain. Single-unit activity of deep dorsal horn projection neurons was recorded in anesthetized normal/sham controls and L(5) and L(6) spinal nerve-ligated rats. Spinal application of 10 microM morphine significantly inhibited the evoked responses of dorsal horn neurons in both normal/sham controls, and this effect was abolished by the specific mu opioid antagonist. However, the effect of morphine on dorsal horn projection neurons was significantly reduced in nerve-injured rats. Furthermore, topical application of the GABA(A) receptor antagonist bicuculline (20 microM) almost abolished the effect of morphine in normal/sham control rats but did not significantly attenuate the morphine effect in nerve-injured rats. On the other hand, the glycine receptor antagonist strychnine (4 microM) significantly decreased the effect of morphine in both nerve-injured and control animals. These data suggest that the inhibitory effect of opioids on dorsal horn projection neurons depends on GABAergic and glycinergic inputs. Furthermore, reduced GABAergic tone probably contributes to diminished analgesic effect of opioids in neuropathic pain.

Systemic Morphine Inhibits Dorsal Horn Projection Neurons through Spinal Cholinergic System Independent of Descending Pathways

Cholinergic circuitry and muscarinic receptors within the spinal cord have been proposed to contribute to the analgesic effects of systemic morphine. In this study, we determined whether the descending pathways are involved in the inhibitory effect of systemic morphine on dorsal horn projection neurons mediated by activation of the spinal cholinergic system. Single-unit activity of dorsal horn projection neurons was recorded in anesthetized rats. The neuronal responses to mechanical stimuli applied to the receptive field were determined before and after intravenous injection of morphine. The inhibitory effect of intravenous morphine on dorsal horn neurons was also tested before and after topical spinal application of the muscarinic antagonist atropine in both intact and spinally transected rats. Intravenous injection of 2.5 mg/kg morphine significantly inhibited the evoked response of dorsal horn neurons in both intact and spinally transected rats. Spinal topical application of the mu opioid antagonist H-d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP) completely blocked the effect of morphine on dorsal horn neurons. In addition, spinal application of 10 microM atropine significantly attenuated the effect of systemic morphine. In rats subjected to cervical spinal transection, atropine produced a similar attenuation of the inhibitory effect of systemic morphine on dorsal horn neurons. Data from this electrophysiological study suggest that systemic morphine inhibits ascending dorsal horn neurons through stimulation of spinal mu opioid receptors. Furthermore, activation of the local spinal cholinergic circuitry and muscarinic receptors is involved in the inhibitory effect of systemic morphine on dorsal horn projection neurons independent of descending pathways.

Synaptic modulation in pain pathways

All higher organisms possess a sensory system that allows them to detect potentially tissue-damaging (or noxious) stimuli. The proper functioning of this system is essential to protect their bodies from tissue damage. However, under pathological conditions after severe tissue injury and in inflammatory or neuropathic diseases, this system can become sensitized, and pain can then turn into a disease. Such exaggerated pain sensation (or hyperalgesia) can arise at different levels of integration. It can originate from an increased responsiveness of primary nociceptors, specialized nerve cells, which sense noxious stimuli, or from changes in the central processing of nociceptive input. Like other sensory input, nociceptive signals are relayed in the central nervous system by neurons, which communicate with each other mainly through chemical synapses. Changes in the excitability of these neurons or in the strength of their synaptic coupling provide the cellular basis for many forms of pathological pain. This review focuses on the synaptic processing of pain-related signals in the spinal cord dorsal horn, the first site of synaptic integration in the pain pathway. Particular emphasis is paid to synaptic processes underlying the generation of pathological pain evoked by inflammation or neuropathic diseases.

Opioid-Like Actions of Neuropeptide Y in Rat Substantia Gelatinosa: Y1 Suppression of Inhibition and Y2 Suppression of Excitation

Neuropathic pain that results from injury to the peripheral or CNS responds poorly to opioid analgesics. Y1 and Y2 receptors for neuropeptide Y (NPY) may, however, serve as targets for analgesics that retain their effectiveness in neuropathic pain states. In substantia gelatinosa neurons in spinal cord slices from adult rats, we find that NPY acts via presynaptic Y2 receptors to attenuate excitatory postsynaptic currents (EPSCs) and predominantly on presynaptic Y1 receptors to attenuate glycinergic and GABAergic inhibitory postsynaptic currents (IPSCs). Because NPY attenuates the frequency of TTX-resistant miniature EPSCs and IPSCs, perturbation of the neurotransmitter release process contributes to its actions at both excitatory and inhibitory synapses. These effects, which are reminiscent of those produced by analgesic opioids, provide a cellular basis for previously documented spinal analgesic actions mediated via Y1 and Y2 receptors in neuropathic pain paradigms. They also underline the importance of suppression of inhibition in spinal analgesic mechanisms.

Hyperpolarization of substantia gelatinosa neurons evoked by mu-, kappa-, delta 1-, and delta 2-selective opioids

With whole-cell recordings of substantia gelatinosa (SG) neurons from rat spinal cord slices, we investigated the effects of bath application of highly selective delta(1), delta(2), kappa and mu opioid agonists on membrane potential and conductance. Each agonist was applied at 0.5 to 1 micromol/L and evoked robust hyperpolarizations and conductance increases in a subset of neurons. The response magnitude means were similar across agonists at several concentrations; no excitatory effects were observed. Nine of 55 (16%) were hyperpolarized by delta(1) opioids, 2 of 45 (4%) by delta(2), 8 of 59 (14%) by kappa, and 35 of 67 (52%) by mu opioids. To test the hypothesis that SG neurons may be hyperpolarized by multiple opioid subtype agonists, we applied 2, 3, or 4 selective agonists to individual neurons. Most neurons were hyperpolarized only by mu opioids; however, a minority were hyperpolarized by multiple subtype-selective agonists. These results indicate that delta(1)- and delta(2)-selective opioids can also evoke robust hyperpolarizations in spinal SG neurons, that the relative abundance of hyperpolarizing responses was mu > > delta (1) approximately equal kappa > delta(2), and that some SG neurons can be hyperpolarized by more than 1 opioid subtype-selective agonist. These powerful inhibitory postsynaptic responses likely contribute to analgesia evoked by spinally and systemically administered opioid subtype-selective agonists.

Anatomy and physiology of chronic pain

Although much has been accomplished in the past several decades, treatment of chronic pain remains imperfect. This article presents the anatomy and physiology of the pain system along with the neurobiologic changes that occur in the establishment and maintenance of chronic pain states.

Effects of endomorphin on substantia gelatinosa neurons in rat spinal cord slices

1. Whole-cell patch recordings were made from substantia gelatinosa (SG) neurons in transverse lumbar spinal cord slices of 15- to 30-day-old rats. 2. Endomorphin 1 (EM-1) or EM-2 (<or=10 microM) hyperpolarized or induced an outward current in 26 of the 66 SG neurons. The I-V relationship showed that the peptide activates an inwardly rectifying K+ current. 3. EM-1 or EM-2 (0.3-10 microM) suppressed short-latency excitatory postsynaptic currents (EPSCs) and long-latency inhibitory postsynaptic currents (IPSCs) in nearly all SG neurons tested or short-latency IPSCs in six of the 10 SG neurons. [Met5] enkephalin or [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) (1-10 microM) depressed EPSCs and IPSCs. EM-1 or EM-2 depressed synaptic responses without causing a significant change in holding currents or inward currents induced by glutamate. 4. Glutamate also evoked a short-latency outward current in five SG neurons or a biphasic current in two neurons; the outward current was blocked by tetrodotoxin (TTX, 0.3 microM) or bicuculline (10 microM). 5. EM-1 or DAMGO (1 or 5 microM) attenuated the glutamate-evoked outward or biphasic currents in four of the seven SG neurons. EM-1 (1 microm) reduced the frequency, but not the amplitude of miniature EPSCs or miniature IPSCs. 6.. Naloxone (1 microM) or the selective micro-opioid receptor antagonist beta-funaltrexamine (beta-FNA, 25 microM) antagonized the action of EM; EM-induced hyperpolarizations persisted in the presence of the kappa-opioid receptor antagonist (nor-binaltorphimine dihydrochloride, 1 microM) and/or sigma-opioid receptor antagonist (naltrindole hydrochloride, 1 microM). 7. It may be concluded that EM acting on micro-opioid receptors hyperpolarizes a population of SG neurons by activating an inwardly rectifying K+ current, and attenuates excitatory and inhibitory synaptic currents evoked in a population of SG neurons, probably by a presynaptic site of action.

Morphology and axonal arborization of rat spinal inner lamina II neurons hyperpolarized by mu-opioid-selective agonists

The ventral or inner region of spinal substantia gelatinosa (SG; lamina II(i)) is a heterogeneous sublamina important for the generation and maintenance of hyperalgesia and neuropathic pain. To test whether II(i) neurons can be hyperpolarized by the mu-opioid agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO; 500 nM) and to address possible downstream consequences of mu-opioid-evoked inhibition of II(i) neurons, we combined in vitro whole-cell, tight-seal recording methods with fluorescent labeling of the intracellular tracer biocytin and confocal microscopy. Twenty-one of 23 neurons studied had identifiable axons. Nine possessed axons that projected ventrally into laminae III-V; six of these were hyperpolarized by DAMGO. Three of four neurons with identifiable axons that projected to lamina I were hyperpolarized by DAMGO. Most neurons could be classified as either islet cells or stalked cells. Five of nine labeled islet cells and only two of seven stalked cells were hyperpolarized by DAMGO. Three were stellate cells: one resembled a spiny cell and three could not be classified. DAMGO hyperpolarized each of the stellate cells, the spiny cell, and 1 of the unclassified cells. Our data support the hypothesis that part of the action of mu-opioid agonists involves the inhibition of interneurons that are part of a polysynaptic excitatory pathway from primary afferents to neurons in the deep and/or superficial dorsal horn.

Dynamic balance of metabotropic inputs causes dorsal horn neurons to switch functional states

Sensory relay structures in the spinal cord dorsal horn are now thought to be active processing structures that function before supraspinal sensory integration. Dorsal horn neurons directly receive nociceptive (pain) signals from the periphery, express a high degree of functional plasticity and are involved in long-term sensitization and chronic pain. We show here that deep dorsal horn neurons (DHNs) in Wistar rats can switch their intrinsic firing properties from tonic to plateau or endogenous bursting patterns, depending upon the balance of control by metabotropic glutamate (mGlu) and GABA(B) receptors. We further show that this modulation acts on at least one common target, the inwardly rectifying potassium channel (Kir3). Finally, we found that these firing modes correspond to specific functional states of information transfer in which dorsal horn neurons can faithfully transmit, greatly enhance or block the transfer of nociceptive information.

Postsynaptic action mechanism of somatostatin on the membrane excitability in spinal substantia gelatinosa neurons of juvenile rats

We used tight-seal, whole-cell recording in juvenile rat spinal slices to investigate the action of somatostatin on substantia gelatinosa neurons. Bath application of somatostatin caused a robust and repeatable hyperpolarization or outward current in substantia gelatinosa neurons. Somatostatin inhibited spontaneous action potentials in subpopulation of substantia gelatinosa neurons. The amplitude of dorsal root-evoked excitatory postsynaptic currents and the frequency of spontaneous excitatory postsynaptic currents were not affected by somatostatin. The current induced by somatostatin developed almost instantaneously and did not show any time-dependent inactivation. The current-voltage relationship exhibited inward rectification. The conductance of somatostatin-sensitive current increased with the concentration of external K(+). The reversal potentials in different external K(+) concentrations were close to the K(+) equilibrium potentials. The effect of somatostatin was dose-dependent, with an EC(50) of 113 nM. The somatostatin-sensitive current was blocked by low concentration of extracellular Ba(2+) but not by glibenclamide, an inhibitor of ATP-sensitive K(+) channels. Hyperpolarization-activated cation current in a subpopulation of substantia gelatinosa neurons was not affected by somatostatin. In neurons recorded with an internal solution containing GTPgammaS, somatostatin induced outward current and hyperpolarization that did not reverse on washing. When the spontaneous induction of outward current with GTPgammaS was greatest, somatostatin did not induce any outward currents. Furthermore, intracellular dialysis of GDPbetaS, a G-protein antagonist, abolished the effect of somatostatin. In addition, SST-sensitive neurons were fewer in slices incubated with pertussis toxin than in adjacent control slices incubated without pertussis toxin. These results suggest that somatostatin decreases the postsynaptic membrane excitability of substantia gelatinosa neurons by a pertussis toxin-sensitive G-protein-mediated activation of an inwardly rectifying K(+) conductance.

Morphine-3beta-D-glucuronide suppresses inhibitory synaptic transmission in rat substantia gelatinosa

High doses of intrathecally applied morphine or morphine-3beta-D-glucuronide (M3G) produce allodynia and hyperalgesia. Whole-cell patch-clamp recordings were made from substantia gelatinosa neurons in transverse slices of adult rat lumbar spinal cord to compare the actions of M3G with those of the mu-opioid agonist, DAMGO ([D-Ala(2),N-Met-Phe(4),Gly-ol(5)]-enkephalin), and the ORL(1) agonist, nociceptin/orphanin FQ (N/OFQ). M3G (1-100 microM) had little or no effect on evoked excitatory postsynaptic currents (EPSC) and no effect on postsynaptic membrane conductance. In contrast, 1 microM DAMGO and 1 microM N/OFQ reduced the amplitude of evoked EPSCs and activated an inwardly rectifying K(+) conductance. M3G did not attenuate the effect of DAMGO or N/OFQ on evoked EPSC amplitude. However, 1 to 100 microM M3G reduced the amplitude of evoked GABAergic and glycinergic inhibitory postsynaptic current (IPSC) by up to 48%. This effect was naloxone-insensitive. The evoked IPSC was also attenuated by DAMGO, but not by N/OFQ. Because M3G reduced the frequency of tetrodotoxin-insensitive miniature IPSCs and increased paired-pulse facilitation, it appeared to act presynaptically to disinhibit substantia gelatinosa neurons. This effect, which does not appear to involve mu-opioid or ORL(1) receptors, may contribute to the allodynia and hyperalgesia observed after intrathecal application of high doses of morphine.

MOR-1-immunoreactive neurons in the dorsal horn of the rat spinal cord: evidence for nonsynaptic innervation by substance P-containing primary afferents and for selective activation by noxious thermal stimuli

A direct action of mu-opioid agonists on neurons in the spinal dorsal horn is thought to contribute to opiate-induced analgesia. In this study we have investigated neurons that express the mu-opioid receptor MOR-1 in rat spinal cord to provide further evidence about their role in nociceptive processing. MOR-1-immunoreactive cells were largely restricted to lamina II, where they comprised approximately 10% of the neuronal population. The cells received few contacts from nonpeptidergic unmyelinated afferents, but many from substance P-containing afferents. However, electron microscopy revealed that most of these contacts were not associated with synapses. None of the MOR-1 cells in lamina II expressed the neurokinin 1 receptor; however, the mu-selective opioid peptide endomorphin-2 was present in the majority (62-82%) of substance P axons that contacted them. Noxious thermal stimulation of the foot induced c-Fos expression in approximately 15% of MOR-1 cells in the medial third of the ipsilateral dorsal horn at mid-lumbar level. However, following pinching of the foot or intraplantar injection of formalin very few MOR-1 cells expressed c-Fos, and for intraplantar formalin injection this result was not altered significantly by pretreatment with systemic naloxone. Although these findings indicate that at least some of the neurons in lamina II with MOR-1 are activated by noxious thermal stimulation, the results do not support the hypothesis that the cells have a role in transmitting nociceptive information following acute mechanical or chemical noxious stimuli.

Direct excitation of mitral cells via activation of alpha1-noradrenergic receptors in rat olfactory bulb slices

The main olfactory bulb receives a significant modulatory noradrenergic input from the locus coeruleus. Previous in vivo and in vitro studies showed that norepinephrine (NE) inputs increase the sensitivity of mitral cells to weak olfactory inputs. The cellular basis for this action of NE is not understood. The goal of this study was to investigate the effect of NE and noradrenergic agonists on the excitability of mitral cells, the main output cells of the olfactory bulb, using whole cell patch-clamp recording in vitro. The noradrenergic agonists, phenylephrine (PE, 10 microM), isoproterenol (Isop, 10 microM), and clonidine (3 microM), were used to test for the functional presence of alpha1-, beta-, and alpha2-receptors, respectively, on mitral cells. None of these agonists affected olfactory nerve (ON)-evoked field potentials recorded in the glomerular layer, or ON-evoked postsynaptic currents recorded in mitral cells. In whole cell voltage-clamp recordings, NE (30 microM) induced an inward current (54 +/- 7 pA, n = 16) with an EC(50) of 4.7 microM. Both PE and Isop also produced inward currents (22 +/- 4 pA, n = 19, and 29 +/- 9 pA, n = 8, respectively), while clonidine produced no effect (n = 6). In the presence of TTX (1 microM), and blockers of excitatory and inhibitory fast synaptic transmission [gabazine 5 microM, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) 10 microM, and (+/-)-2-amino-5-phosphonopentanoic acid (APV) 50 microM], the inward current induced by PE persisted (EC(50) = 9 microM), whereas that of Isop was absent. The effect of PE was also observed in the presence of the Ca(2+) channel blockers, cadmium (100 microM) and nickel (100 microM). The inward current caused by PE was blocked when the interior of the cell was perfused with the nonhydrolyzable GDP analogue, GDPbetaS, indicating that the alpha1 effect is mediated by G-protein coupling. The current-voltage relationship in the absence and presence of PE indicated that the current induced by PE decreased near the equilibrium potential for potassium ions. In current-clamp recordings from bistable mitral cells, PE shifted the membrane potential from the downstate (-52 mV) toward the upstate (-40 mV), and significantly increased spike generation in response to perithreshold ON input. These findings indicate that NE excites mitral cells directly via alpha1 receptors, an effect that may underlie, at least in part, increased mitral cell responses to weak ON input during locus coeruleus activation in vivo.

Mu opiates inhibit long-term potentiation induction in the spinal cord slice

Long-term potentiation (LTP) involves a prolonged increase in neuronal excitability following repeated afferent input. This phenomenon has been extensively studied in the hippocampus as a model of learning and memory. Similar long-term increases in neuronal responses have been reported in the dorsal horn of the spinal cord following intense primary afferent stimulation. In these studies, we utilized the spinal cord slice preparation to examine effects of the potently antinociceptive mu opioids in modulating primary afferent/dorsal horn neurotransmission as well as LTP of such transmission. Transverse slices were made from the lumbar spinal cord of 10- to 17-day-old rats, placed in a recording chamber, and perfused with artificial cerebrospinal fluid also containing bicuculline (10 microM) and strychnine (1 microM). Primary afferent activation was achieved in the spinal slice by electrical stimulation of the dorsal root (DR) or the tract of Lissauer (LT) which is known to contain a high percentage of small diameter fibers likely to transmit nociception. Consistent with this anatomy, response latencies of LT-evoked field potentials in the dorsal horn were considerably slower than the response latencies of DR-evoked potentials. Only LT-evoked field potentials were found to be reliably inhibited by the mu opioid receptor agonist [D-Ala(2), N-Me-Phe(4), Gly(5)] enkephalin-ol (DAMGO, 1 microM), although evoked potentials from both DR and LT were blocked by the AMPA/kainate glutamate receptor antagonist 6-cyano-7-nitroquinoxalene-2,3-dione. Moreover repeated stimulation of LT produced LTP of LT- but not DR-evoked potentials. In contrast, repeated stimulation of DR showed no reliable LTP. LTP of LT-evoked potentials depended on N-methyl-D-aspartate (NMDA) receptor activity, in that it was attenuated by the NMDA antagonist APV. Moreover, such LTP was inhibited by DAMGO interfering with LTP induction mechanisms. Finally, in whole cell voltage-clamp studies of Lamina I neurons, DAMGO inhibited excitatory postsynaptic current (EPSC) response amplitudes from LT stimulation-evoked excitatory amino acid release but not from glutamate puffed onto the cell and increased paired-pulse facilitation of EPSCs evoked by LT stimulation. These studies suggest that mu opioids exert their inhibitory effects presynaptically, likely through the inhibition of glutamate release from primary afferent terminals, and thereby inhibit the induction of LTP in the spinal dorsal horn.

Interference of biocytin with opioid-evoked hyperpolarization and membrane properties of rat spinal substantia gelatinosa neurons

In our laboratory, preliminary whole-cell, tight seal recordings of rat spinal substantia gelatinosa neurons including biocytin in the patch pipette yielded a significantly smaller proportion of neurons hyperpolarized by selective opioid agonists compared with recordings without biocytin. Therefore, we investigated the effects of biocytin inclusion on opioid responses and other membrane properties during whole-cell, tight seal recordings of these neurons. The percentage of neurons hyperpolarized by mu-, delta(1)-, and kappa-selective opioids was significantly reduced when 1% but not < or =0.2% biocytin was included in the recording pipette, compared with neurons recorded without biocytin. However, a significantly higher proportion of neurons fired spontaneous action potentials with either 0.05-0.2 or 1% biocytin compared to no biocytin. Resting membrane potential, input impedance and the proportion of neurons displaying transient outward rectification were each significantly altered for neurons recorded with 1% but not 0.05-0.2% biocytin. These effects may be due to a relatively specific blockade of diverse potassium channel types. Because efficient labeling can be achieved with 0.1% biocytin with whole-cell recording, higher concentrations are contraindicated.

Cellular and synaptic adaptations mediating opioid dependence

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

Differential actions of spinal analgesics on mono-versus polysynaptic Adelta-fibre-evoked field potentials in superficial spinal dorsal horn in vitro

Processing of nociceptive information can be modulated at various levels in spinal cord that may range from changes of neurotransmitter release from primary afferent Adelta- or C-fibres to excitability changes of spinal interneurones or motoneurones. The site and mechanism of action of spinal analgesics has been assessed with a number of in vivo and in vitro methods with sometimes conflicting results. Here, we have used transverse spinal cord slices with attached dorsal roots to simultaneously record mono- and polysynaptic Adelta-fibre-evoked field potentials in superficial spinal dorsal horn. Two classical spinal analgesics, morphine and clonidine, and the metabotropic glutamate receptor agonist (IS,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD) differentially affected mono- and polysynaptic Adelta-fibre-evoked transmission in spinal dorsal horn. Polysynaptic responses were dose-dependently inhibited while the monosynaptic response remained unaffected. These results suggest that spinal analgesics may preferentially affect polysynaptic but not monosynaptic Adelta-fibre-evoked responses in superficial spinal dorsal horn.

Molecular mechanisms and regulation of opioid receptor signaling

Cloning of multiple opioid receptors has presented opportunities to investigate the mechanisms of multiple opioid receptor signaling and the regulation of these signals. The subsequent identification of receptor gene structures has also provided opportunities to study the regulation of receptor gene expression and to manipulate the concentration of the gene products in vivo. Thus, in the current review, we examine recent advances in the delineation basis for the multiple opioid receptor signaling, and their regulation at multiple levels. We discuss the use of receptor knockout animals to investigate the function and the pharmacology of these multiple opioid receptors. The reasons and basis for the multiple opioid receptor are addressed.