The challenge of chronic pain

Mechanistic Insight into the Effects of Curcumin on Neuroinflammation-Driven Chronic Pain

Chronic pain is a persistent and unremitting condition that has immense effects on patients' quality of life. Studies have shown that neuroinflammation is associated with the induction and progression of chronic pain. The activation of microglia and astrocytes is the major hallmark of spinal neuroinflammation leading to neuronal excitability in the projection neurons. Excessive activation of microglia and astrocytes is one of the major contributing factors to the exacerbation of pain. However, the current chronic pain treatments, mainly by targeting the neuronal cells, remain ineffective and unable to meet the patients' needs. Curcumin, a natural plant product found in the Curcuma genus, improves chronic pain by diminishing the release of inflammatory mediators from the spinal glia. This review details the role of curcumin in microglia and astrocytes both in vitro and in vivo and how it improves pain. We also describe the mechanism of curcumin by highlighting the major glia-mediated cascades in pain. Moreover, the role of curcumin on inflammasome and epigenetic regulation is discussed. Furthermore, we discuss the strategies used to improve the efficacy of curcumin. This review illustrates that curcumin modulating microglia and astrocytes could assure the treatment of chronic pain by suppressing spinal neuroinflammation.

Multifunctional Opioid-Derived Hybrids in Neuropathic Pain: Preclinical Evidence, Ideas and Challenges

When the first- and second-line therapeutics used to treat neuropathic pain (NP) fail to induce efficient analgesia—which is estimated to relate to more than half of the patients—opioid drugs are prescribed. Still, the pathological changes following the nerve tissue injury, i.a. pronociceptive neuropeptide systems activation, oppose the analgesic effects of opiates, enforcing the use of relatively high therapeutic doses in order to obtain satisfying pain relief. In parallel, the repeated use of opioid agonists is associated with burdensome adverse effects due to compensatory mechanisms that arise thereafter. Rational design of hybrid drugs, in which opioid ligands are combined with other pharmacophores that block the antiopioid action of pronociceptive systems, delivers the opportunity to ameliorate the NP-oriented opioid treatment via addressing neuropathological mechanisms shared both by NP and repeated exposition to opioids. Therewith, the new dually acting drugs, tailored for the specificity of NP, can gain in efficacy under nerve injury conditions and have an improved safety profile as compared to selective opioid agonists. The current review presents the latest ideas on opioid-comprising hybrid drugs designed to treat painful neuropathy, with focus on their biological action, as well as limitations and challenges related to this therapeutic approach.

SIRT3 alleviates neuropathic pain by deacetylating FoxO3a in the spinal dorsal horn of diabetic model rats

BACKGROUND

The underlying mechanisms of neuropathic pain remain unclear. This work aimed to investigate the role of Sirtuin3 (SIRT3), an nicotinamide adenosine dinucleotide+-dependent histone deacetylase, in the development of neuropathic pain induced by type 2 diabetes mellitus (T2DM) and to explore the associated mechanisms.

METHODS

Diabetic neuropathic pain (DNP) in rats was induced by high-fat diet/low-dose streptozotocin. The pain behaviors were examined using the von Frey and Hargreaves tests. The levels of SIRT3, manganese superoxide dismutase (MnSOD) and catalase (CAT) were determined using Western blot and RT-qPCR. The acetylation, phosphorylation and ubiquitination of forkhead box class O3a (FoxO3a) were analyzed by immunoprecipitation and Western blot.

RESULTS

SIRT3 expression and activity were significantly reduced in the spinal dorsal horn of DNP model rats. Overexpression of spinal SIRT3 reversed the pain hypersensitivity in the DNP model rats, but knockdown of spinal SIRT3 mimicked the pain effect, eliciting pain hypersensitivity in normal rats. Moreover, overexpression of spinal SIRT3 in DNP model rats increased the FoxO3a level and upregulated the antioxidant genes MnSOD and CAT by deacetylating FoxO3a and inhibiting FoxO3a phosphorylation and ubiquitination. Knockdown of spinal SIRT3 in normal rats decreased the FoxO3a level and downregulated MnSOD and CAT by inhibiting the deacetylation of FoxO3a and further increasing FoxO3a phosphorylation and ubiquitination.

CONCLUSIONS

These results suggest that, by deacetylating FoxO3a and further reducing its phosphorylation, ubiquitination and degradation in the spinal dorsal horn, SIRT3 stabilizes FoxO3a protein and inhibits oxidative stress, resulting in pain alleviation in T2DM model rats.

Electroacupuncture Reduces the Effects of Acute Noxious Stimulation on the Electrical Activity of Pain-Related Neurons in the Hippocampus of Control and Neuropathic Pain Rats

To study the effects of acupuncture analgesia on the hippocampus, we observed the effects of electroacupuncture (EA) and mitogen-activated protein kinase (MEK) inhibitor on pain-excited neurons (PENs) and pain-inhibited neurons (PINs) in the hippocampal area CA1 of sham or chronic constrictive injury (CCI) rats. The animals were randomly divided into a control, a CCI, and a U0126 (MEK1/2 inhibitor) group. In all experiments, we briefly (10-second duration) stimulated the sciatic nerve electrically and recorded the firing rates of PENs and PINs. The results showed that in both sham and CCI rats brief sciatic nerve stimulation significantly increased the electrical activity of PENs and markedly decreased the electrical activity of PINs. These effects were significantly greater in CCI rats compared to sham rats. EA treatment reduced the effects of the noxious stimulus on PENs and PINs in both sham and CCI rats. The effects of EA treatment could be inhibited by U0126 in sham-operated rats. The results suggest that EA reduces effects of acute sciatic nerve stimulation on PENs and PINs in the CA1 region of the hippocampus of both sham and CCI rats and that the ERK (extracellular regulated kinase) signaling pathway is involved in the modulation of EA analgesia.

Morphological and physiological evidence of a synaptic connection between the lateral parabrachial nucleus and neurons in the A7 catecholamine cell group in rats

BACKGROUND

The descending noradrenergic (NAergic) system is one of the important endogenous analgesia systems. It has been suggested that noxious stimuli could activate descending NAergic system; nevertheless, the underlying neuronal circuit remains unclear. As NAergic neurons in the A7 catecholamine cell group (A7) are a part of the descending NAergic system and the lateral parabrachial nucleus (LPB) is an important brainstem structure that relays ascending nociceptive signal, we aimed to test whether LPB neurons have direct synaptic contact with NAergic A7 neurons.

RESULTS

Stereotaxic injections of an anterograde tracer, biotinylated dextran-amine (BDA), were administered to LPB in rats. The BDA-labeled axonal terminals that have physical contacts with tyrosine hydroxylase-positive (presumed noadrenergic) neurons were identified in A7. Consistent with these morphological observations, the excitatory synaptic currents (EPSCs) were readily evoked in NAergic A7 neurons by extracellular stimulation of LPB. The EPSCs evoked by LPB stimulation were blocked by CNQX, a non-NMDA receptor blocker, and AP5, a selective NMDA receptor blocker, showing that LPB-A7 synaptic transmission is glutamatergic. Moreover, the amplitude of LPB-A7 EPSCs was significantly attenuated by DAMGO, a selective μ-opioid receptor agonist, which was associated with an increase in paired-pulse ratio.

CONCLUSIONS

Taken together, the above results showed direct synaptic connections between LPB and A7 catecholamine cell group, the function of which is subject to presynaptic modulation by μ-opioid receptors.

Supraspinal injection of Substance P attenuates allodynia and hyperalgesia in a rat model of inflammatory pain

The neuropeptide Substance P (SP), that has a high affinity for the neurokinin 1 (NK1) receptor, is involved in modulation of pain transmission. Although SP is thought to have excitatory actions and promote nociception in the spinal cord, the peptide induces analgesia at the supraspinal level. The aim of this study was to evaluate the role of supraspinal SP and the NK1 receptor in inflammatory pain induced by injection of carrageenan in the hind paw of the rat. There are two nociceptive behavioral responses associated with this pain state: mechanical allodynia and heat hyperalgesia. Because the NK1 receptor colocalizes with the MOP receptor in supraspinal sites involved in pain modulation, we also decided to study the possible involvement of the opioid system on SP-induced analgesia. We found that treatment with SP, at doses of 3.5, 5 and 7 μg/5 μl/rat i.c.v., clearly showed inhibition of allodynia and hyperalgesia. Pretreatment with the selective NK1 antagonist L-733,060 (10mg/kg i.p.) blocked the SP-induced analgesia, suggesting the involvement of the NK1 receptor. This SP-induced analgesia was significantly reduced by administration of the opioid antagonist naloxone (3mg/kg s.c.). This reduction occurred when SP was administered either before or after the carrageenan injection. These results suggest a significant antinociceptive role for SP and the NK1 receptor in inflammatory pain at the supraspinal level, possibly through the release of endogenous opioids.

TRPA1 antagonists as potential analgesic drugs

The necessity of safe and effective treatments for chronic pain has intensified the search for new analgesic drugs. In the last few years, members of a closely-related family of ion channels, called transient receptor potential (TRP) have been identified in different cell types and their functions in physiological and pathological conditions have been characterized. The transient receptor potential ankyrin 1 (TRPA1), originally called ANKTM1 (ankyrin-like with transmembrane domains protein 1), is a molecule that has been conserved in different species during evolution; TRPA1 is a cation channel that functions as a cellular sensor, detecting mechanical, chemical and thermal stimuli, being a component of neuronal, epithelial, blood and smooth muscle tissues. In mammals, TRPA1 is largely expressed in primary sensory neurons that mediate somatosensory processes and nociceptive transmission. Recent studies have described the role of TRPA1 in inflammatory and neuropathic pain. However, its participation in cold sensation has not been agreed in different studies. In this review, we focus on data that support the relevance of the activation and blockade of TRPA1 in pain transmission, as well as the mechanisms underlying its activation and modulation by exogenous and endogenous stimuli. We also discuss recent advances in the search for new analgesic medicines targeting the TRPA1 channel.

Roles of A-type potassium currents in tuning spike frequency and integrating synaptic transmission in noradrenergic neurons of the A7 catecholamine cell group in rats

We investigated voltage-dependent K(+) currents (I(K)) in noradrenergic (NAergic) A7 neurons. The I(K) evoked consisted of A-type I(K) (I(A)), which had the characteristics of a low threshold for activation (approximately -50 mV), fast activation/inactivation, and rapid recovery from inactivation. Since the I(A) were blocked by heteropodatoxin-2 (Hptx-2), a specific Kv4 channel blocker, and the NAergic A7 neurons were shown to be reactive with antibodies against Kv4.1/Kv4.3 channel proteins, we conclude that the I(A) evoked in NAergic neurons are mediated by Kv4.1/Kv4.3 channels. I(A) were also evoked using voltage commands of a single action potential (AP), a subthreshold voltage change between two consecutive APs, or excitatory postsynaptic potential (EPSP) activity recorded in current-clamp mode (CCM). Blockade of the I(A) by 4-AP, a broad spectrum I(A) blocker, or by Hptx-2 increased the half-width and spontaneous firing of APs and reduced the amount of synaptic drive needed to elicit APs in CCM, showing that the I(A) play important roles in regulating the shape and firing frequency of APs and in synaptic integration in NAergic A7 neurons. Since these neurons are the principal projection neurons to the dorsal horn of the spinal cord, these results also suggest roles for Kv4.1/4.3 channels in descending NAergic pain regulation.

Acidic saline-induced primary and secondary mechanical hyperalgesia in mice

Most of our knowledge about chronic musculoskeletal pain is based on cutaneous pain models. To test the hypothesis that animals develop chronic muscular hyperalgesia following intramuscular acidic saline injections, primary hyperalgesia within the gastrocnemius muscle was analyzed compared to secondary cutaneous hyperalgesia in the hind paw that develops following intramuscular acid saline injection. Two acidic saline (pH 4) injections were administrated into the gastrocnemius of female CF-1 mice. The results indicate that mice developed a robust hypersensitivity bilaterally in primary (gastrocnemius muscle) secondary (cutaneous hind paw) sites that lasted up to 2 weeks. In addition, primary hyperalgesia correlated well with levels of Fos expression. Fos expression patterns in the spinal cord were different for primary secondary site stimulation. Hind-paw palpation stimulated ipsilateral Fos expression in the superficial spinal laminae at L4/L5 levels, bilaterally in deep laminae at L2-L5 spinal levels. In contrast, gastrocnemius compression stimulated widespread Fos expression in all regions of the ipsilateral dorsal horn within L2-L6 spinal segments. These findings indicate that acidic saline injection induces primary hyperalgesia in muscle that the patterns of Fos expression in response to primary vs secondary stimulation are strikingly different.

PERSPECTIVE

This study assesses primary site muscular pain, which is the main complaint of people with musculoskeletal conditions, and identifies spinal patterns activated by noxious mechanical stimuli to the gastrocnemius. This study demonstrates approaches to test nociception arising from muscle aids in our understanding of spinal processing of primary secondary site hyperalgesia.

Involvement of the Nociceptin/Orphanin FQ-NOP receptor system in the ventrolateral periaqueductal gray following mechanical allodynia in chronic pain

AIMS

It has been well documented that ventrolateral periaqueductal gray (vlPAG) matter is a crucial component of the descending pain modulatory system in the chronic pain condition. The aim of the present study was to identify the role of the vlPAG Nociceptin/Orphanin FQ/NOP receptor system in allodynia, a nociceptive behavioral response associated with chronic pain.

MAIN METHODS

We used two animal models of persistent pain: chronic constriction injury (CCI) and inflammation induced by carrageenan. In each, Nociceptin/Orphanin FQ transmission was abolished using UFP-101, a selective NOP receptor antagonist, which was injected into the vlPAG at a dose of 18 microg/1 microl/rat.

KEY FINDINGS

We found that treatment with the NOP antagonist reversed the decrease in allodynic threshold in CCI rats fourteen days after the ligature, which was the timepoint of the greatest reduction in threshold. Moreover, UFP-101 administered immediately prior to or 2 h after intra plantar (i.pl.) carrageenan injection prevented or reversed, respectively, allodynic behavior in rats with inflammation.

SIGNIFICANCE

Our findings support the hypothesis that the endogenous Nociceptin/Orphanin FQ/NOP receptor system is tonically active at the vlPAG level during neuropathic states or carrageenan inflammation.

Neurokinin 1 receptor activates transient receptor potential-like currents in noradrenergic A7 neurons in rats

Noradrenergic (NAergic) A7 neurons are involved in modulating nociception by releasing noradrenaline in the dorsal spinal cord. Since NAergic A7 neurons receive dense Substance P (Sub-P) releasing terminals from ventromedial medulla, here we tested the effect of Sub-P on them. Bath application of Sub-P induced an inward current (I(Sub-P)) in NAergic neurons, which was significantly blocked by Neurokinin 1 (NK1) receptor antagonist. The I(Sub-P) was reversed at approximately -20 mV, blocked by several TRP channel blockers, enhanced by OAG and negatively regulated by PKC. Immunohistochemistry staining showed that NAergic A7 neurons express high level of TRPC6 channel proteins, which is consistent with pharmacological properties of I(Sub-P) shown above, as TRPC6 channel is shown to be augmented by OAG and inhibited by PKC. In conclusion, the above results provide mechanism underlying postsynaptic action of Sub-P on NAergic A7 neurons and a role for TRPC6 channel in NAergic pain modulation.

Noradrenergic neurons in the locus coeruleus contribute to neuropathic pain

Current theories of neuropathic hypersensitivity include an imbalance of supraspinal inhibition and facilitation. Our overall hypothesis is that the locus coeruleus (LC), classically interpreted as a source of pain inhibition, may paradoxically result in facilitation after tibial and common peroneal nerve transection (spared sural nerve injury--SNI). We first tested the hypothesis that non-noxious tactile hind paw stimulation of the spared sural innervation territory increases neuronal activity in the LC in male rats. We observed a bilateral increase in the stimulus-evoked expression of transcription factors Fos and phosphorylated CREB (pCREB) in LC after SNI but not sham surgery; these markers of neuronal activity correlated with the intensity of tactile allodynia. We next tested the hypothesis that noradrenergic neurons contribute to the development of neuropathic pain. To selectively destroy these neurons, we delivered antidopamine-beta-hydroxylase saporin (anti-DbetaH-saporin) into the i.c.v. space 2 weeks before SNI. We found that anti-DbetaH-saporin, but not an IgG-saporin control, reduced behavioral signs of tactile allodynia, mechanical hyperalgesia, and cold allodynia from 3 to 28 days. after SNI. Our final experiment tested the hypothesis that the LC contributes to the maintenance of neuropathic pain. We performed SNI, waited 2 weeks for maximal allodynia and hyperalgesia to develop, and then administered the local anesthetic lidocaine (4%) directly into the LC parenchyma. Lidocaine reduced all behavioral signs of neuropathic pain in a reversible manner, suggesting that the LC contributes to pain facilitation. We conclude that, in addition to its well-known inhibition of acute and inflammatory pain, the LC facilitates the development and maintenance of neuropathic pain in the SNI model. Further studies are needed to determine the facilitatory pathways emanating from the LC.

Altered hippocampal long-term potentiation after peripheral nerve injury in mice

It has been clinically reported that patients with chronic pain often have accompanying cognitive deficiency, which hampers efficient medical treatment. In the present study, we investigated whether hippocampal synaptic plasticity, which has been considered to be a cellular model of learning and memory, could be influenced by chronic pain conditions using a murine model of neuropathic pain prepared by partial ligation of the sciatic nerve (the Seltzer model). In slices obtained from neuropathic animals, tetanus-induced long-term potentiation of CA1 hippocampal synaptic transmission was impaired, whereas long-term depression induced by low-frequency stimulation was similar in neuropathic and sham-treated (control) animals. Bath application of the beta-adrenoceptor agonist isoproterenol or the beta-adrenoceptor antagonist propranolol diminished the difference of synaptic plasticity between neuropathic and control mice. In the presence of isoproterenol, long-term potentiation was successfully induced in neuropathic mice. By contrast, long-term potentiation in sham-treated mice was impaired by propranolol which did not alter the already impaired long-term potentiation after peripheral nerve injury. These results suggest that beta-adrenergic functions are changed in chronic pain conditions, which may underlie the deficiency of long-term potentiation.

Supraspinal modulation of pain by cannabinoids: the role of GABA and glutamate

Recent physiological, pharmacological and anatomical studies provide evidence that one of the main roles of the endocannabinoid system in the brain is the regulation of gamma-aminobutyric acid (GABA) and glutamate release. This article aims to review this evidence in the context of its implications for pain. We first provide a brief overview of supraspinal regulation of nociception, followed by a review of the evidence that the brain's endocannabinoid system modulates nociception. We look in detail at regulation of supraspinal GABAergic and glutamatergic neurons by the endocannabinoid system and by exogenously administered cannabinoids. Finally, we review the evidence that cannabinoid-mediated modulation of pain involves modulation of GABAergic and glutamatergic neurotransmission in key brain regions.

Neuronal activation at the spinal cord and medullary pain control centers after joint stimulation: a c-fos study in acute and chronic articular inflammation

Chronic inflammatory pain induces short- and long-term central changes, which have been mainly studied at the spinal cord level. Supraspinal pain control centers intrinsically connected with the dorsal horn are also prone to be affected by chronic inflammatory pain. C-fos expression was used as a neuronal activation marker at spinal and supraspinal levels to i) compare acute and chronic articular inflammation, and ii) analyze the effects of brief innocuous or noxious stimulation of a chronically inflamed joint. Acute articular inflammation was induced by an inflammatory soup with prostaglandin E(2) and bradykinin, both at 10(-5) M. Chronic articular inflammation consisted of 14 days of monoarthritis. Early c-fos expression was studied 4 min after inflammatory soup injection or stimulation of the arthritic joint whereas late c-fos expression was evaluated 2 h after those stimuli. At the spinal cord, the analysis was focused on the dorsal horn (laminae I-V) and supraspinally, five major regions of the endogenous pain control system were considered: the caudal ventrolateral medulla (VLM), the dorsal reticular nucleus (DRt), the ventral reticular nucleus (VRt), the nucleus of the solitary tract (Sol) and the rostroventromedial medulla (RVM). Acute articular inflammation induced early and late increases in c-fos expression at the spinal level and late increases supraspinally whereas the effects of monoarthritis were more moderate and restricted to the spinal cord. When monoarthritic animals were subjected to gentle touch or bending of the joint, early increases in c-fos expression were detected supraspinally, but not at the spinal level. In this region, noxious mechanical stimulation induced late increases in non-inflamed animals and both early and late increases in monoarthritic rats. Supraspinally, noxious stimulation induced only late increases in c-fos expression. The present results show complex differences in the patterns of c-fos expression between the spinal cord and medullary areas of the pain control system during articular inflammation, which indicate that the somatosensory system is differentially affected by the installation of chronic pain.

The role of P2X₇ in pain and inflammation

The P2X₇ purinoceptor is unique amongst the P2X receptor family in that its activation is able to stimulate the release of mature, biologically active interleukin-1β (IL-1β), as well as a variety of other proinflammatory cytokines. Coupled with the predominate localisation of this receptor to immunocytes of haemopoetic origin, this receptor is an obvious candidate to play a major and pivotal role in processes of pain and inflammation. Using genetically modified animals that lack the P2X₇ receptor, several investigators have shown that these mice do indeed demonstrate a blunted inflammatory response, and fail to develop pain following both inflammatory and neuropathic insult. These animals also show altered cytokine production in response to inflammatory stimulus, which is far broader than merely modulation of IL-1β release. In this short article, we review the role of the P2X₇ receptor in modulating the release of cytokines and other mediators, and discuss the findings made from P2X₇ receptor-deficient animals. As well as highlighting outstanding questions regarding this intriguing receptor, we also speculate as to the potential therapeutic benefit of P2X₇ receptor modulation.

The therapeutic potential of Na+and Ca2+channel blockers in pain management

Chronic pain affects a large percentage of the population, representing a socio-economic burden. Current treatments are characterised by suboptimal efficacy and/or side effects that limit their use. Among several approaches to treating chronic pain, voltage-sensitive Ca(2+) and Na(+) channels are promising targets. This review evaluates the preclinical evidence that supports the involvement of these targets, with specific attention to those subtypes that appear more strictly correlated with pain generation and sustainment, as well as those compounds that modulate the activity of Ca(2+) and/or Na(+) channels that are currently in clinical development for chronic pain conditions.

Involvement of the intralaminar parafascicular nucleus in muscarinic-induced antinociception in rats

The thalamic contribution to cholinergic-induced antinociception was examined by microinjecting the acetylcholine (ACh) agonist carbachol into the intralaminar nucleus parafascicularis (nPf) of rats. Pain behaviors organized at spinal (spinal motor reflexes), medullary (vocalizations during shock), and forebrain (vocalization afterdischarges, VADs) levels of the neuraxis were elicited by noxious tailshock. Carbachol (0.5, 1, and 2 microg/side) administered into nPf produced dose-dependent elevations of vocalization thresholds, but failed to elevate spinal motor reflex threshold. Injections of carbachol into adjacent sites dorsal or ventral to nPf failed to alter vocalization thresholds. Elevations in vocalization thresholds produced by intra-nPf carbachol were reversed in a dose-dependent manner by local administration of the muscarinic receptor antagonist atropine (30 and 60 microg/side). These results provide the first direct evidence supporting the involvement of the intralaminar thalamus in muscarinic-induced antinociception. Results are discussed in terms of the contribution of nPf to the processing of the affective dimension of pain.

Serotonin and noradrenaline reuptake inhibitors in animal models of pain

Animal models of chronic pain serve as an experimental basis for testing new therapeutic interventions and for mechanistic investigations. In an animal model of chronic pain, based on the injection of formalin into the paw of a rodent, inhibitors of noradrenaline reuptake such as nisoxetine, nortriptyline and maprotiline and dual inhibitors of the noradrenaline and serotonin reuptake such as imipramine and milnacipran produce potent anti-nociceptive effects, whereas selective serotonin reuptake inhibitors, such as fluvoxamine, are much less potent. In another model, neuropathic pain resulting from the chronic constriction injury of the sciatic nerve was prevented by the dual uptake inhibitor, venlafaxine. The experimental model involving ligation of the 5th spinal nerve induces behavioural signs in rats and mice that are similar to the symptoms of human neuropathic pain. In this model amitriptyline, a non-selective serotonin and noradrenaline reuptake blocker, the preferential noradrenaline reuptake inhibitor, desipramine and the selective serotonin and noradrenaline reuptake inhibitors, milnacipran and duloxetine, produce a decrease in pain sensitivity whereas the selective serotonin reuptake inhibitor, fluoxetine, is ineffective. Antidepressants acting on the noradrenergic or both the noradrenergic and serotonergic systems thus appear to be more effective than those working on the serotonin system alone.

Pathways that elicit long-term changes in gene expression in nociceptive neurons following nerve injury: contributions to neuropathic pain

Chronic neuropathic pain following nerve injury or inflammation is mediated by transcription-dependent changes in neurons that comprise the nociceptive pathway. Among these changes is often a long-term hyperexcitability (LTH) in primary nociceptors that persists long after the lesion has healed. LTH is manifest by a reduction in threshold and an increased tendency to fire action potentials. This increased excitability activates higher order neurons in the pathway, leading to the perception of pain. Efforts to ameliorate chronic pain would therefore benefit if we understood how LTH is induced, but studies toward this goal are impeded by the complexity and heterogeneity of vertebrate nervous systems. Fortunately, LTH is an evolutionarily conserved mechanism that underlies defensive behaviors across phyla, including invertebrates. Thus, the same electrophysiological changes that underlie LTH in vertebrate nociceptive neurons are seen in their counterparts in the experimentally favorable mollusk Aplysia californica. Nociceptive neurons of Aplysia are readily accessible and large enough to approach using a variety of cell and molecular approaches not possible in higher organisms. Studies of the molecular cascades activated by injury to Aplysia peripheral nerves has focused on a group of positive injury signals that are retrogradely transported from the injury site in the axon to the cell nucleus where they regulate gene transcription. One of these, protein kinase G, is activated by nitric oxide synthetase and its activation in axons is required for the induction of LTH after injury. This pathway, and the transcriptional events that it activates, are targets for therapeutic intervention for chronic pain.