Tailoring chronic pain treatments for the elderly: are we prepared for the challenge?

Chronic pain is increasingly recognized as a disease and accounts for substantial suffering and disability worldwide. The aging 'baby-boomer' generation is creating a tsunami of elderly patients (>65 years old) for global healthcare systems (between 2010 and 2030). The phenotypic expression of chronic pain in the elderly can be influenced by co-morbid diseases (e.g. diabetes, cancer, depression, Alzheimer's disease, etc.), changes in physiological competency (e.g. drug metabolism/elimination) or cognitive reserve. Will a shift in the drug discovery paradigm be required to improve efficacy, side-effects or positively impact quality of life (QoL) in the elderly with chronic pain? This review highlights a number of potential pitfalls that should be considered when delivering valued pain relief medicines tailored for the elderly.

Evaluation of an innovative population pharmacokinetic-based design for behavioral pharmacodynamic endpoints.

Pre-clinical behavioral pharmacology studies supporting indications like analgesia typically consist of at least three different studies; dose-finding, duration of effect, and tolerance-development studies. Pharmacokinetic (PK) plasma samples are generally taken from a parallel group of animals to avoid disruption of the behavioral pharmacodynamic (PD) endpoint. Our objective was to investigate if pre-clinical behavioral pharmacology studies in rats could be performed effectively by combining three studies into a single experimental design and using sparse PK sampling in the same animals as for PD. A refined dosing strategy was applied for a muscarinic agonist, AZD6088, using the rat spinal nerve ligation heat hyperalgesia model. PD measurements were performed on day 1, 3, 5 and 8. Two PK samples per day were taken day 2 and 4. In a separate control group, PD measurements were performed on rats without PK sampling. Data was analyzed using a population approach in NONMEM. The animals produced a consistent and reproducible response irrespective of day of testing suggesting that blood sampling on alternate days did not interfere with the PD responses. A direct concentration-effect relationship with good precision was established and no tolerance development was observed. The new design combining three studies into one and eliminating a satellite PK group realized substantial savings compared to the old design; animal use was reduced by 58% and time required to generate results was reduced by 55%. The design described here delivers substantial savings in animal lives, time, and money whilst still delivering a good quality and precise description of the PKPD relationship.

The M1/M4 preferring agonist xanomeline is analgesic in rodent models of chronic inflammatory and neuropathic pain via central site of action.

The role of muscarinic receptor subtype-1 (M1) in chronic pain is unclear. In an attempt to gain an understanding of its role, we have tested xanomeline, an M1/M4-preferring agonist, together with nonselective (scopolamine and pirenzepine), and selective (MT-7 and MT-3) muscarinic receptor (M1 and M4, respectively) antagonists in a number of inflammatory and neuropathic pain models. Xanomeline potently and effectively reversed tactile allodynia and heat hyperalgesia associated with established neuropathic and inflammatory pain in both rat and mouse models. Scopolamine and pirenzepine completely blocked the analgesic response to xanomeline, confirming that the analgesic effect is mediated by the muscarinic system. The highly selective M1 receptor toxin, MT-7, almost completely abolished the analgesic response to xanomeline when administered supraspinally. However, the highly selective M4 receptor toxin, MT-3, only marginally reversed the analgesia when given supraspinally, and had no effect when given spinally. In conclusion, the data presented show that the nonselective muscarinic agonist xanomeline is analgesic in models of persistent pain and suggest that the activation of supraspinal M1 receptors, and to a lesser extent supraspinal M4 receptors, contributes to that analgesia.

A peripherally restricted cannabinoid receptor agonist produces robust anti-nociceptive effects in rodent models of inflammatory and neuropathic pain.

Cannabinoids are analgesic in man, but their use is limited by their psychoactive properties. One way to avoid cannabinoid receptor subtype 1 (CB1R)-mediated central side-effects is to develop CB1R agonists with limited CNS penetration. Activation of peripheral CB1Rs has been proposed to be analgesic, but the relative contribution of peripheral CB1Rs to the analgesic effects of systemic cannabinoids remains unclear. Here we addressed this by exploring the analgesic properties and site of action of AZ11713908, a peripherally restricted CB1R agonist, in rodent pain models. Systemic administration of AZ11713908 produced robust efficacy in rat pain models, comparable to that produced by WIN 55, 212-2, a CNS-penetrant, mixed CB1R and CB2R agonist, but AZ11713908 generated fewer CNS side-effects than WIN 55, 212-in a rat Irwin test. Since AZ11713908 is also a CB2R inverse agonist in rat and a partial CB2R agonist in mouse, we tested the specificity of the effects in CB1R and CB2R knock-out (KO) mice. Analgesic effects produced by AZ11713908 in wild-type mice with Freund's complete adjuvant-induced inflammation of the tail were completely absent in CB1R KO mice, but fully preserved in CB2R KO mice. An in vivo electrophysiological assay showed that the major site of action of AZ11713908 was peripheral. Similarly, intraplantar AZ11713908 was also sufficient to induce robust analgesia. These results demonstrate that systemic administration of AZ11713908, produced robust analgesia in rodent pain models via peripheral CB1R. Peripherally restricted CB1R agonists provide an interesting novel approach to analgesic therapy for chronic pain.

Modality of hyperalgesia tested, not type of nerve damage, predicts pharmacological sensitivity in rat models of neuropathic pain.

Although many types of nerve damage can cause neuropathic pain, there are substantial commonalities in neuropathic pain symptoms, and patients can be divided into sub-groups based on their symptom profile rather than etiology. Mechanism-based treatment suggests that pharmacotherapy should be chosen be based shared commonalities of symptoms rather than etiology. The aim of the present study was to determine whether type of injury (etiology) or behavioral endpoint (symptom) is a better predictor of pharmacological responsivity in the most commonly used rodent models of neuropathic pain. We used the chronic constriction injury (CCI) model to directly compare the temporal and pharmacological characteristics of four different types of evoked stimuli; heat, pressure, acetone cooling and punctate mechanical. We then compared heat hyperalgesia and mechanical allodynia endpoints across etiologies using the spinal nerve ligation (SNL) model. Evoked pain responses in both models had strikingly different temporal characteristics. We then tested three standard therapies for neuropathic pain from different drug classes, oxycodone, gabapentin, and amitriptyline. Notably, regardless of the model tested, or the time of onset, common endpoints showed near-identical pharmacological responses, and not all endpoints were equally sensitive to drug intervention within one model. Hypersensitivity to heat and pressure were highly responsive to oxycodone, gabapentin, and amitriptyline; whereas cold and mechanical allodynia were more difficult to reverse. Moreover, CCI- and SNL-induced mechanical allodynia was completely insensitive to amitriptyline treatment. We conclude that regardless of model and time course of presentation, different symptoms of peripheral neuropathy have unique pharmacological responses.

Discovery and pharmacological characterization of a small-molecule antagonist at neuromedin U receptor NMUR2.

Neuromedin U (NMU), through its cognate receptor NMUR2 in the central nervous system, regulates several important physiological functions, including energy balance, stress response, and nociception. By random screening of our corporate compound collection with a ligand binding assay, we discovered (R)-5'-(phenylaminocarbonylamino)spiro[1-azabicyclo[2.2.2]octane-3,2'(3'H)-furo[2,3-b]pyridine] (R-PSOP), a highly potent and selective NMUR2 antagonist. R-PSOP is a nonpeptidic small-molecule with the chemical composition C(20)N(4)O(2)H(22). In competition binding experiments, this compound was found to bind to NMUR2 with high affinity; the K(i) values were determined to be 52 and 32 nM for the human and rat NMUR2, respectively. Moreover, in functional assays measuring phosphoinositide turnover or intracellular calcium mobilization, R-PSOP strongly inhibited the responses stimulated by peptide agonists NMU-25, NMU-23, and NMU-8 in human embryonic kidney 293 cells expressing NMUR2. From Schild analyses, the functional K(b) values for R-PSOP were determined to be 92 and 155 nM at human and rat NMUR2, respectively. Highly selective for NMUR2, R-PSOP exhibited low affinity to the other subtype of NMU receptor, NMUR1, with a K(i) value >10 microM. R-PSOP in vivo attenuated NMU-23-evoked nociceptive responses in a rat spinal reflex preparation. To our knowledge, this is the first antagonist ever reported for NMU receptors. This compound could serve as a valuable tool for further understanding the physiological and pathophysiological roles of NMU system, while providing a chemical starting point that may lead to development of new therapeutics for treatment of eating disorders, obesity, pain, and stress-related disorders.

Role of rat sensory neuron-specific receptor (rSNSR1) in inflammatory pain: contribution of TRPV1 to SNSR signaling in the pain pathway.

Sensory neuron-specific receptors (SNSRs) belong to a large family of GPCRs, known as Mrgs (Mas-related genes), many of which are preferentially expressed in primary afferent nociceptors. Selective SNSR agonists produce pain-like behaviors in rats, showing that SNSR activation is sufficient to produce pain. However, it is unknown whether SNSR activation is necessary for pain either in the normal condition or in pathological pain states. Here we used small interfering RNA (siRNA) to acutely knockdown rat SNSR1 and test the hypothesis that this receptor mediates pain responses. Administration of siRNA to the lumbar spinal cord in rats dose-dependently knocked down rSNSR1 mRNA and protein and abolished heat hyperalgesia evoked by intradermal administration of specific rSNSR1 agonists. In rats with levels of rSNSR1 knockdown sufficient to block responses to the SNSR1 agonists, there was no effect on normal pain responses, but there was a significant reduction of heat hyperalgesia in an inflammatory pain model (Complete Freund's Adjuvant), supporting a role for rSNSR1 in inflammatory pain. Further in vivo studies revealed that SNSR1 knockdown had no effect on responses to intradermal capsaicin, a selective TRPV1 agonist. In contrast, a selective TRPV1 antagonist abolished heat hyperalgesia produced by an SNSR agonist, suggesting that TRPV1 receptors mediate rSNSR1-evoked responses. We also found that rSNSR1-like immunoreactivity, like TRPV1, is localized in the superficial dorsal horn of the spinal cord. We propose that rSNSR1 represents a new member of the receptors expressed on chemosensitive nociceptors responsible for detecting the "inflammatory soup" of mediators generated by tissue damage.

GABAA-Receptor blockade reverses the injury-induced sensitization of nociceptor-specific (NS) neurons in the spinal dorsal horn of the rat.

Single-unit electrical activity was recorded from 80 nociceptor-specific (NS) neurons in the dorsal horn of the lumbar spinal cord of pentobarbital anesthetized rats. Their responses to low- and high-intensity mechanical stimulation of their receptive fields (RFs) were recorded before and after the application of irritant agents [capsaicin (CAP) or mustard oil (MO)] to the RF. Before the applications of the irritants the neurons responded only to high-intensity stimuli, but after this procedure 20 of 28 neurons tested were sensitized, i.e., gave increased responses to high-intensity stimuli and showed novel responses to low-intensity mechanical stimulation as well as an Abeta-fiber afferent drive. CAP was more likely to induce sensitization than MO and the majority of sensitized neurons were located in the superficial dorsal horn. No relationship was found between the magnitude of the response to the sensitizing agent and the presence or absence of sensitization. Cumulative doses of two gamma-aminobutyric acid type A (GABA(A))-receptor antagonists, picrotoxin and bicuculline, were administered systemically or applied directly over the spinal cord. The GABA(A) antagonists reversed the sensitization of the neurons by reducing the novel low-threshold responses. These results show that NS neurons in the spinal dorsal horn can be sensitized by a sustained afferent discharge in peripheral nociceptors and that this sensitization can be reduced or reversed by low doses of GABA(A)-receptor antagonists. This provides evidence for a mechanism in which an enhanced GABAergic transmission can lead to hyperexcitability and sensitization of NS neurons in the dorsal horn.

In vivo recruitment by painful stimuli of AMPA receptor subunits to the plasma membrane of spinal cord neurons.

The persistent increase in pain sensitivity observed after injury, known as hyperalgesia, depends on synaptic plasticity in the pain pathway, particularly in the spinal cord. Several potential mechanisms have been proposed, including post-synaptic exocytosis of the AMPA subclass of glutamate receptors (AMPA-R), which is known to play a critical role in synaptic plasticity in the hippocampus. AMPA-R trafficking has been described in spinal neurons in culture but it is unknown if it can also occur in spinal neurons in vivo, or if it can be induced by natural painful stimulation. Here we have induced referred mechanical hyperalgesia in vivo by intracolonic instillation of capsaicin in mice and have observed a recruitment of GluR1 AMPA-R subunits to neuronal plasma membranes in the lumbar spinal cord. Intracolonic capsaicin induced a rapid (10 min) increase in GluR1, but not GluR2/3 in the synaptosomal membrane fraction which lasted at least 3 h and a decrease in GluR1 subunit in the cytosolic fraction. Capsaicin treatment also provoked CaMKII activation and pre-treatment with a specific CaMKII inhibitor prevented the GluR1 trafficking. Brefeldin-A, an antibiotic that inhibits exocytosis of proteins, not only prevented GluR1 trafficking to the membrane but also inhibited referred hyperalgesia in capsaicin-treated mice. Our results show that delivery of GluR1 AMPA receptor subunits to the cell membrane through a CaMKII activity-dependent exocytotic regulated pathway contributes to the development of hyperalgesia after a painful stimulus. We conclude that AMPA-R trafficking contributes to the synaptic strengthening induced in the pain pathway by natural stimulation.

Understanding the signaling and transmission of visceral nociceptive events.

Visceral pain can be considered as part of the defense reactions of the body against harmful stimuli, particularly of those that impinge on the mucosal lining of hollow organs. It is a problem of considerable clinical relevance, and its neurobiological mechanisms differ from those of somatic nociceptive or neuropathic pain. Much progress had been made in recent years in the understanding of the functional properties of the visceral nociceptors that trigger pain states, their molecular mechanisms of activation and sensitization and on their central actions. Some molecular targets have been identified as key players in the activation and sensitization of visceral nociceptors, notably ASICs, TTX-resistant Na channels and the TRPV1 receptor. Some nonneural elements of visceral organs, such as the urothelium have been shown to play active roles in the transduction of visceral sensory events by mechanisms involving ATP release by the urothelial cells. Certain well-known neurotransmitters, such as the tachykinin family of neuropeptides, likely play an important role in the peripheral and central activation of visceral nociceptive afferents and in the generation of visceral hyperalgesia. This article reviews current evidence on the mechanisms of activation and sensitization of visceral nociceptive afferents and on their role in the triggering and maintenance of clinically relevant visceral pain states.

Presynaptic inhibition and spinal pain processing in mice: a possible role of the NKCC1 cation-chloride co-transporter in hyperalgesia.

We have examined the role of the NKCC1 sodium-potassium-chloride-cotransporter in the generation of touch-evoked pain. The pain behavior of NKCC1 knockout mice (KO) was studied and compared to that of heterozygous (HE) and wild-type (WT) littermates. NKCC1 KO mice showed an increase in tail flick latencies and a reduction of the duration of pain behavior induced by intradermal capsaicin compared to HE and WT mice. All three groups of animals expressed a normal level of plasma extravasation following capsaicin applications. NKCC1 KO mice showed a reduction in stroking hyperalgesia (touch-evoked pain) compared to WT and HE mice but no differences were detected between the three groups in the expression of punctate hyperalgesia. As the NKCC1 co-transporter is responsible for the generation of presynaptic inhibition between afferent terminals in the spinal cord, these results support the notion that presynaptic interactions between low and high threshold afferents can underlie touch-evoked pain.

Sensory neuron-specific receptor activation elicits central and peripheral nociceptive effects in rats.

The sensory neuron-specific G protein coupled receptors (SNSRs) have been described as a family of receptors whose expression in small diameter sensory neurons in the trigeminal and dorsal root ganglia suggests an implication in nociception. To date, the physiological function(s) of SNSRs remain unknown. Hence, the aim of the present study was to determine the effects of rat SNSR1 activation on nociception in rats. The pharmacological characterization of rat SNSR1 was initially performed in vitro to identify a specific ligand, which could be used subsequently in the rat for physiological testing. Among all ligands tested, gamma2-MSH was the most potent at activating rat SNSR1. Structure-activity relationship studies revealed that the active moiety recognized by rat SNSR1 was the C-terminal part of gamma2-MSH. The radiolabeled C-terminal part of gamma2-MSH, gamma2-MSH-6-12, bound with high affinity to membranes derived from rat skin and spinal cord, demonstrating the presence of receptor protein at both the proximal and distal terminals of dorsal root ganglia. To investigate the physiological role of SNSR, specific ligands to rat SNSR1 were tested in behavioral assays of pain sensitivity in rats. Selective rat SNSR1 agonists produced spontaneous pain behavior, enhanced heat and mechanical sensitivity when injected intradermally, and heat hypersensitivity when injected centrally, consistent with the localization of rat SNSR1 protein at central and peripheral sites. Together, these results clearly indicate that the SNSR1 plays a role in nociception and may provide novel therapeutic opportunities for analgesia.

Role of ion channels in mechanisms controlling gastrointestinal pain pathways.

Hypersensitivity or sensitization of nociceptive primary afferents in the gastrointestinal tract has been proposed as a mechanism for organic and functional gastrointestinal pain. This hypersensitivity can be the result of alterations, either induced by a sensitizing agent or without a peripheral cause, in the functional properties of ion channels located in primary afferents. The tetrodotoxin-resistent sodium channel, known as Na(v)1.8, is present in nociceptive primary afferents, including those from the gut, and it has been implicated as being the main candidate for the enhanced activity that characterizes nociceptor sensitization. Other voltage-gated channels, such as calcium and potassium channels, can also contribute to the sensitization of primary afferents observed in gastrointestinal pain states.

Extracellular signaling-regulated kinase-1 and -2 (ERK 1/2) mediate referred hyperalgesia in a murine model of visceral pain.

We have investigated the role of spinal extracellular signaling-regulated kinase-1 and -2 (ERK1/2) in a model of visceral pain and hyperalgesia induced by intracolonic instillation of irritants in adult mice. Instillation of either capsaicin or mustard oil induced a significant activation of lumbosacral spinal ERK1/2, measured by immunoblot, with a peak 2.4-fold increase over control levels between 45 and 90 min post-treatment. Intracolonic saline did not produce significant activation of lumbosacral spinal ERK1/2, and none of the treatments evoked ERK1/2 activation in thoracic or cervical spinal cord. These studies suggested a preferential nuclear localization, which was explored by subcellular fractionation. Both mustard oil and capsaicin produced a redistribution of phosphorylated ERK1/2 from cytosol into the nucleus that was statistically significant at 45 min after treatment. Spinal ERK1/2 activation with capsaicin treatment correlated with the development of prolonged referred hyperalgesia. The upstream inhibitor of ERK phosphorylation, U0126 (100-400 microg/kg, i.v., 10 min pre-capsaicin), dose-dependently inhibited referred hyperalgesia 3-6 h after capsaicin. Treatment with U0126 did not affect spontaneous pain behavior or colon inflammation. Our data show that ERK activation plays a specific role in maintaining prolonged referred (secondary) hyperalgesia in visceral pain. The time course and subcellular localization of the effects observed suggest that ERK is involved in transcriptional events underlying the maintenance of secondary hyperalgesia.

Secondary hyperalgesia and presynaptic inhibition: an update.

One of the most prominent features of secondary hyperalgesia is touch-evoked pain, i.e., pain evoked by dynamic tactile stimuli applied to areas adjacent or remote from the originating injury. It is generally accepted that the neurobiological mechanism of this sensory alteration involves the central nervous system (CNS) so that incoming impulses in low-threshold mechanoreceptors from the area of secondary hyperalgesia can evoke painful sensations instead of touch. Some years ago we proposed a mechanistic model for this form of pain based on presynaptic interactions in the spinal dorsal horn between the terminals of low-threshold mechanoreceptors and of nociceptors. Here we review the evidence gathered in support of this model in the intervening years with special reference to experimental studies of antidromic activity (Dorsal Root Reflexes--DRRs) in nociceptive afferents and on the acquisition of low-threshold inputs by nociceptor-specific neurons in the spinal dorsal horn. We also discuss and identify potential molecular mechanisms that may underlie the presynaptic interaction model and therefore that could be responsible for the development of secondary hyperalgesia.

The tetrodotoxin-resistant Na+ channel Nav1.8 is essential for the expression of spontaneous activity in damaged sensory axons of mice.

The tetrodotoxin-resistant sodium channel alpha subunit, Nav1.8, is exclusively expressed in primary sensory neurons and is suggested to play a role in the generation of ectopic action potentials after axonal injury and thereby contribute to neuropathic pain. Here we investigated the involvement of Nav1.8 in ectopic impulse generation in damaged axons by examining spontaneous activity and mechanosensitivity in neuromas formed by section of the saphenous nerve in Nav1.8 null mice and in their wild-type littermates. We recorded 522 identified units from 24 neuromas in vitro at two time points, 8-11 days (median 10 days) and 19-29 days (median 22 days) post-operatively. At approximately 10 days, neither genotype showed spontaneous activity, but a significantly higher proportion of fibres were mechanosensitive in wild-type (54%) compared to Nav1.8 null neuromas (18%). At approximately 22 days, 19% of fibres recorded in wild-type neuromas showed spontaneous activity, whereas only one fibre of the 238 (0. %) recorded in neuromas taken from null mice showed ongoing activity. In recordings at approximately 22 days, a similar proportion of fibres were mechanosensitive in wild-type and Nav1.8 null neuromas (51 and 46%, respectively). We conclude that Nav1.8 is essential for the expression of spontaneous activity in damaged sensory axons, and may also contribute to the development of ectopic mechanosensitivity.

Deficits in visceral pain and referred hyperalgesia in Nav1.8 (SNS/PN3)-null mice.

The tetrodotoxin-resistant sodium channel alpha subunit Nav1.8 is expressed exclusively in primary sensory neurons and is proposed to play an important role in sensitization of nociceptors. Here we compared visceral pain and referred hyperalgesia in Nav1.8-null mice and their wild-type littermates in five tests that differ in the degree to which behavior depends on spontaneous, ongoing firing in sensitized nociceptors. Nav1.8-null mice showed normal nociceptive behavior provoked by acute noxious stimulation of abdominal viscera (intracolonic saline or intraperitoneal acetylcholine). However, Nav1.8-null mutants showed weak pain and no referred hyperalgesia to intracolonic capsaicin, a model in which behavior is sustained by ongoing activity in nociceptors sensitized by the initial application. Nav1.8-null mice also showed blunted pain and hyperalgesia to intracolonic mustard oil, which sensitizes nociceptors but also provokes tissue damage. To distinguish between a possible role for Nav1.8 in ongoing activity per se and ongoing activity after sensitization in the absence of additional stimuli, we tried a visceral model of tonic noxious chemical stimulation, cyclophosphamide cystitis. Cyclophosphamide produces cystitis by gradual accumulation of toxic metabolites in the bladder. In this model, Nav1.8-null mice showed normal responses. There were no differences between null mutants and their normal littermates in tissue damage and inflammation evoked by any of the stimuli tested, suggesting that the behavioral differences are not secondary to impairment of inflammatory responses. We conclude that there is an essential role for Nav1.8 in mediating spontaneous activity in sensitized nociceptors.

Activation of spinal extracellular signaling-regulated kinase-1 and -2 by intraplantar carrageenan in rodents.

We have examined the activation of spinal extracellular signaling-regulated kinase (ERK) in juvenile rats and adult mice after intraplantar carrageenan or saline and its relationship to pain behavior. In rats, intraplantar carrageenan evoked a peak five-fold activation of spinal ERK at 30 min measured by immunoblot. Saline injection resulted in a two-fold activation. This differential ERK activation correlated with a 2.5-fold greater pain response and the development of secondary hyperalgesia in carrageenan-injected rats, whereas both saline and carrageenan produced similar primary hyperalgesia. In mice, carrageenan injection produced a peak 3.5-fold activation of ERK, but saline was ineffective. We conclude that ERK activation may underlie spinal nociceptive processing and secondary hyperalgesia after carrageenan inflammation.