NK1 (substance P) receptor antagonists--why are they not analgesic in humans?

A novel approach to the use of animals in studies of pain: validation of the canine brief pain inventory in canine bone cancer

OBJECTIVE

To validate the Canine Brief Pain Inventory (CBPI), which is based on the human Brief Pain Inventory (BPI), in a canine model of spontaneous bone cancer.

DESIGN AND PARTICIPANTS

One hundred owners of dogs with bone cancer self-administered the CBPI on three occasions to test the reliability, validity, and responsiveness of the measure.

OUTCOME MEASURES

Factor analysis, internal consistency, convergent validity, and an extreme group validation assessment were completed using the responses from the first administration of the CBPI. Test-retest reliability was evaluated using two administrations of the instrument, 1 week apart. Responsiveness was tested by comparing responses 3 weeks apart.

RESULTS

The "severity" and "interference" factors hypothesized based on the BPI were demonstrated in the CBPI in dogs with bone cancer. Internal consistency was high (Cronbach's alpha, 0.95 and 0.93), as was test-retest reliability (kappa, 0.73 and 0.65). Convergent validity was demonstrated with respect to quality of life (r = 0.49 and 0.63). Extreme group validation against normal dogs showed significantly higher factor scores (P < 0.001 for both).

CONCLUSIONS

The CBPI reliably measures the same pain constructs in the companion canine model of spontaneous bone cancer as the BPI does in people with bone cancer. This innovative approach to preclinical outcomes development, validating a preclinical outcome measure that directly corresponds to an outcome measure routinely used in clinical research, applied to a readily available animal model of spontaneous disease could transform the predictive ability of preclinical pain studies.

The molecular epidemiology of pain: a new discipline for drug discovery

Recent candidate gene studies have identified and replicated the first associations between several common polymorphisms and pain severity in humans. Moreover, human studies in twins suggest high heritability for responses to experimental pain stimuli. Human genome-wide association studies of pain phenotypes might identify novel analgesic targets, help to prioritize research among current targets, and increase the likelihood of success for analgesic candidates emerging from animal studies. However, clinical research in pain has largely focused on small neurophysiology-based studies, so expansion of epidemiological understanding will be essential to the success of genetic or proteomic dissection of complex pain disorders. This Perspective outlines how methods of molecular epidemiology, proved effective in the study of other diseases, can enhance the returns from human genomic studies and expedite the development of new drugs to prevent or treat pain.

Predictive validity of animal pain models? A comparison of the pharmacokinetic-pharmacodynamic relationship for pain drugs in rats and humans

A number of previous reviews have very eloquently summarized pain models and endpoints in animals. Many of these reviews also discuss how animal models have enhanced our understanding of pain mechanisms and make forward-looking statements as to our proximity to the development of effective mechanism-based treatments. While a number of reports cite failures of animal pain models to predict efficacy in humans, few have actually analyzed where these models have been successful. This review gives a brief overview of those successes, both backward, providing validation of the models, and forward, predicting clinical efficacy. While the largest dataset is presented on treatments for neuropathic pain, this review also discusses acute and inflammatory pain models. Key to prediction of clinical efficacy is a lack of side effects, which may incorrectly suggest efficacy in animals and an understanding of how pharmacokinetic parameters translate from animals to man. As such, this review focuses on a description of the pharmacokinetic-pharmacodynamic relationship for a number of pain treatments that are effective in both animals and humans. Finally we discuss where and why animal pain models have failed and summarize improvements to pain models that should expand and improve their predictive power.

Role of tachykinin NK(1) and NK(2) receptors in colonic sensitivity and stress-induced defecation in gerbils

The pharmacology of tachykinin NK receptors varies greatly among species. The aim of the present study was to assess the role of NK(1) and NK(2) receptors in mediating colorectal distension-evoked nociception and psychological stress-induced defecation in gerbils, a species with human-like NK receptor pharmacology. The effects of the selective NK(1) and NK(2) receptor antagonists, aprepitant and saredutant, on acute (1 h) restraint stress-evoked defecation and plasma adenocorticotropin (ACTH) levels in gerbils were assessed. The effects of antagonists alone or in combination on colorectal distension-evoked visceral pain in conscious gerbils were evaluated using the visceromotor response as a surrogate marker of pain. Restraint stress increased fecal pellet output 2-3-fold and plasma ACTH levels 9-fold. Aprepitant inhibited the defecatory and endocrine responses to stress by 50%, while saredutant completely normalized the same parameters. Visceral pain responses during colorectal distension were attenuated by both compounds, but aprepitant (19+/-6% inhibition, P<0.01) was slightly more effective than saredutant (10+/-9% inhibition, P<0.05). A combination of both compounds resulted in an additive effect (30+/-10% inhibition, P<0.01). The results demonstrate that NK(1) and NK(2) receptors are involved in stress-related colonic motor alterations and visceral pain responses in gerbils and that combined antagonism provides enhanced inhibition of visceral pain responses. This suggests that for therapeutic use in for instance functional gastrointestinal disorders, dual NK(1)/NK(2) receptor antagonists may provide better clinical outcome than selective compounds.

Neuropathic pain: multiple mechanisms at multiple sites

Damage to nerves at various levels of the peripheral and central nervous systems will lead to sensory loss, but in a significant number of patients this is accompanied by a series of distressing painful signs and symptoms. Although animal models and clinical studies have shed much needed light on the underlying mechanisms that produce this maladaptive plasticity, the presently available drugs do not always fully control the pain. This review covers some of the important mechanisms that include ion channels, central processing through excitatory amino acid and neuropeptide receptors and, finally, the role of monoamine systems that originate in the brain and descend to alter spinal events. The targets for presently licensed and potential novel drugs are covered in this context, as are perspectives on future research priorities.

Pain mechanisms and their implication for the management of pain in farm and companion animals

Over the last two decades there has been a dramatic increase in the literature relating to the mechanisms and management of pain in domestic animals. Understanding the mechanisms of pain is crucial for its effective management. This review highlights the current understanding of the neurophysiology of nociception and the plastic changes involved in chronic pain states. Additionally, we describe a range of novel molecules and pathways that offer opportunities for the development of mechanism-based analgesic therapies. Pain management in animals is limited by pain assessment which remains highly subjective, with clinicians relying on indirect measures of pain, using rating scales and (less frequently) quantifiable physiological and behavioural parameters. The need for a systematic approach which would assess different pain components is well justified. Species-specific issues on pain assessment and management in mammalian companion and farm animals are addressed in the later part of this review.

Design, synthesis, and biological evaluation of novel bifunctional C-terminal-modified peptides for delta/mu opioid receptor agonists and neurokinin-1 receptor antagonists

A series of bifunctional peptides that act as agonists for delta and mu opioid receptors with delta selectivity and as antagonist for neurokinin-1 (NK1) receptors were designed and synthesized for potential application as analgesics in various pain states. The peptides were characterized using radioligand binding assays and functional assays using cell membrane and animal tissue. Optimization was performed on the fifth residue which serves as an address moiety for both receptor recognitions. It had critical effects on both activities at delta/mu opioid receptors and NK1 receptors. Among the synthesized peptides, H-Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-O-3,5-Bzl(CF3) 2 (5) and H-Tyr-D-Ala-Gly-Phe-Nle-Pro-Leu-Trp-O-3,5-Bzl(CF3)2 (7) had excellent agonist activity for both delta opioid and mu opioid receptors and excellent antagonist activity for NK1 receptors. These results indicate that the rational design of multifunctional ligands with opioid agonist and neurokinin-1 antagonist activities can be accomplished and may provide a new tool for treatment of chronic and several pain states.

Pharmacological, Pharmacokinetic, and Primate Analgesic Efficacy Profile of the Novel Bradykinin B1 Receptor Antagonist ELN441958

The bradykinin B(1) receptor plays a critical role in chronic pain and inflammation, although efforts to demonstrate efficacy of receptor antagonists have been hampered by species-dependent potency differences, metabolic instability, and low oral exposure of current agents. The pharmacology, pharmacokinetics, and analgesic efficacy of the novel benzamide B(1) receptor antagonist 7-chloro-2-[3-(9-pyridin-4-yl-3,9-diazaspiro[5.5]undecanecarbonyl)phenyl]-2,3-dihydro-isoindol-1-one (ELN441958) is described. ELN441958 competitively inhibited the binding of the B(1) agonist ligand [(3)H]desArg(10)-kallidin ([(3)H]DAKD) to IMR-90 human fibroblast membranes with high affinity (K(i) = 0.26 +/- 0.02 nM). ELN441958 potently antagonized DAKD (but not bradykinin)-induced calcium mobilization in IMR-90 cells, indicating that it is highly selective for B(1) over B(2) receptors. Antagonism of agonist-induced calcium responses at B(1) receptors from different species indicated that ELN441958 is selective for primate over rodent B(1) receptors with a rank order potency (K(B), nanomolar) of human (0.12 +/- 0.02) approximately rhesus monkey (0.24 +/- 0.01) > rat (1.5 +/- 0.4) > mouse (14 +/- 4). ELN441958 had good permeability and metabolic stability in vitro consistent with high oral exposure and moderate plasma half-lives in rats and rhesus monkeys. Because ELN441958 is up to 120-fold more potent at primate than at rodent B(1) receptors, it was evaluated in a primate pain model. ELN441958 dose-dependently reduced carrageenan-induced thermal hyperalgesia in a rhesus monkey tail-withdrawal model, with an ED(50) approximately 3 mg/kg s.c. Naltrexone had no effect on the antihyperalgesia produced by ELN441958, indicating a lack of involvement of opioid receptors. ELN441958 is a novel small molecule bradykinin B(1) receptor antagonist exhibiting high oral bioavailability and potent systemic efficacy in rhesus monkey inflammatory pain.

Protection of endogenous enkephalin catabolism as natural approach to novel analgesic and antidepressant drugs

The most efficient drugs to alleviate severe pain are opioid compounds. However, their chronic use could be associated with serious drawbacks, such as tolerance, respiratory depression and constipation. Therefore, there is a need for compounds able to efficiently alleviate inflammatory and neurogenic pain following chronic treatment. The discovery that the endogenous opioid peptides, enkephalins, are inactivated by two metallopeptidases, neutral endopeptidase and aminopeptidase N, which can be blocked by synthetic dual inhibitors, represents a promising way to develop 'physiological' analgesics devoid of morphine side effects. These dual inhibitors also have antidepressant-like properties through enkephalin-related activation of delta-opioid receptors. This is expected to reduce the emotional component of pain in humans. This article reviews the promising data obtained for future development of a new class of analgesic that could be of major interest in a number of severe and chronic pain syndromes.

Contribution of substance P and neurokinin A to the differential injury-induced thermal and mechanical responsiveness of lamina I and V neurons

In a previous report, we compared the properties of lamina V neurons of the spinal cord dorsal horn in wild-type mice and in mice with a deletion of the preprotachykinin-A (PPT-A) gene, which encodes substance P (SP) and neurokinin A (NKA). The mutant mice had pronounced deficits in the response to thermal stimulation, both before and after mustard oil induced sensitization. Here, we extended our analysis to the properties of lamina I neurons and also examined responsiveness to mechanical stimulation. Consistent with the properties of lamina V neurons, in the PPT-A mutant mice we found significantly reduced responses of lamina I neurons to noxious thermal stimulation, and mustard oil sensitization of these neurons to heat was lost. In contrast, not only were the responses of lamina I neurons to noxious mechanical stimulation unchanged in the mutant mice, but in neither the wild-type nor the mutant mice could sensitization be induced. However, mustard oil profoundly sensitized lamina V neurons to mechanical stimulation in both wild-type and mutant mice. We conclude that SP and/or NKA are required for the transmission of noxious thermal stimulation by lamina I and V neurons, both before and after tissue injury. The persistence of mechanical sensitization of lamina V neurons in the mutant mice further shows that mustard oil induces mechanical and thermal sensitization through different mechanisms. Finally, we conclude that lamina I sensitization to mechanical stimulation is not required for this form of injury-increased responsiveness of lamina V neurons.

Vanilloid receptor-1 (TRPV1)-dependent activation of inhibitory neurotransmission in spinal substantia gelatinosa neurons of mouse

Inhibitory neurotransmission in spinal cord dorsal horn is mainly mediated by gamma-amino butyric acid (GABA) and glycine. By patch clamp recordings and correlative immunocytochemistry, we studied here the effect of 2 microM capsaicin-induced vanilloid receptor-1 (TRPV1) activation on IPSCs in spinal lamina II neurons from post-natal mice. Specificity was confirmed after pre-incubation with the competitive antagonist SB366791 (10 microM). After a single capsaicin pulse, an intense increase of spontaneous IPSC (sIPSC) frequency was observed in the presence of NBQX 10 microM (62/81 neurons; approximately 76%) or NBQX 10 microM + AP-5 20-100 microM (27/42 neurons; approximately 64%). Only a subpopulation (approximately 40%) of responsive neurons showed a significant amplitude increase. Seventy-two percent of the neurons displayed pure GABA(A) receptor-mediated sIPSCs, whereas the remaining ones showed mixed GABAergic/glycinergic events. After two consecutive capsaicin pulses, frequency rises were very similar, and both significantly higher than controls. When the second pulse was given in the presence of 4 microM L732,138, a selective antagonist of the substance P (SP) preferred receptor NK1, we observed a significant loss in frequency increase (63.90% with NBQX and 52.35% with NBQX + AP-5). TTX (1 microM) largely (approximately 81.5%) blocked the effect of capsaicin. These results show that TRPV1 activation on primary afferent fibers releases SP. The peptide then excites inhibitory neurons in laminae I, III and IV, leading to an increased release of GABA/glycine in lamina II via a parallel alternative pathway to glutamate.

Blocking sodium channels to treat neuropathic pain

Neuropathic pain remains a large unmet medical need. A number of therapeutic options exist, but efficacy and tolerability are less than satisfactory. Based on animal models and limited data from human patients, the pain and hypersensitivity that characterize neuropathic pain are associated with spontaneous discharges of normally quiescent nociceptors. Sodium channel blockers inhibit this spontaneous activity, reverse nerve injury-induced pain behavior in animals and alleviate neuropathic pain in humans. Several sodium channel subtypes are expressed primarily in sensory neurons and may contribute to the efficacy of sodium channel blockers. In this report, the authors review the current understanding of the role of sodium channels and of specific sodium channel subtypes in neuropathic pain signaling.

Neurokinin-1 receptor antagonism in a human model of visceral hypersensitivity

BACKGROUND

Substance P acting via the neurokinin-1 receptor is involved in the development of hyperalgesia, although studies using neurokinin-1 receptor antagonists (NK-1RA) in human somatic pain have been disappointing.

AIM

To evaluate whether Substance P is involved in the development of human visceral pain/hyperalgesia using a selective NK-1RA.

METHODS

Using a validated human model of acid-induced oesophageal allodynia, pain thresholds to electrical stimulation (mA) were measured in the proximal oesophagus and the foot (somatic control), pre- and for 4 h postdistal oesophageal acid in 14 healthy subjects, using a double-blind, randomized, two-period, crossover study. Measurements were taken on the third day of dosing with either an oral NK-1RA or matching placebo, with 2 weeks washout between periods.

RESULTS

Baseline pain threshold did not differ between treatments (proximal oesophagus 37 +/- 7.4 mA NK-1RA vs. 38 +/- 10.1 placebo P = 0.81, foot 40 +/- 15 mA NK-1RA vs. 38 +/- 14 placebo P = 0.68). NK-1RA did not attenuate the reduction in pain threshold in the proximal oesophagus postacid infusion (AUC-394 +/- 279 NK-1RA vs. -262 +/- 397 placebo P = 0.54).

CONCLUSIONS

The lack of effect of NK-1RA on oesophageal pain threshold in our model does not support a role for Substance P in the development of acid-induced oesophageal allodynia.

Effects of oral pregabalin and aprepitant on pain and central sensitization in the electrical hyperalgesia model in human volunteers †

BACKGROUND

Central sensitization is an important mechanism of neuropathic pain; its human models could be useful for early detection of efficacy of novel treatments. The electrical hyperalgesia model invokes central sensitization by repetitive stimulation of the skin. To assess its predictive value, we have investigated pregabalin, a standard neuropathic pain treatment, and aprepitant, an NK(1) antagonist, as an example of a drug class active in animal models but not in neuropathic pain patients. Furthermore, we explored if combinations of either of these drugs with the COX-2 inhibitor parecoxib could improve its efficacy.

METHODS

This was a double-blind, two-period, placebo-controlled study using incomplete block design. Thirty-two healthy volunteers received either oral pregabalin (titrated to 300 mg) or aprepitant (titrated to 320 mg), or matching placebo over 6 days before testing. Sensitization was assessed over 3 h; at 2 h, subjects received either parecoxib (40 mg) or saline i.v.

RESULTS

Pregabalin significantly reduced the areas of punctate mechanical hyperalgesia and dynamic touch allodynia vs placebo (both P < 0.0001); no significant reduction in the area of hyperalgesia or allodynia vs placebo was observed with aprepitant. In the pregabalin + parecoxib treated group, the area of allodynia was significantly reduced (P < 0.0001) and the area of hyperalgesia insignificantly attenuated (P = 0.09) vs placebo + parecoxib; no efficacy improvement was observed with aprepitant + parecoxib.

CONCLUSIONS

The model can serve to predict analgesic efficacy in early human development and investigate the mechanism of action. The model could also be used to explore efficacy of analgesic combinations to provide a rationale for patient studies.

Distribution and pharmacological characterization of primate NK-1 and NK-3 tachykinin receptors in the central nervous system of the rhesus monkey

Much attention has focused on tachykinin receptors as therapeutic targets for neuropsychiatric disorders, although their expressional distributions in the primate central nervous system (CNS) remain unclear. We cloned the genes encoding the NK-1 and NK-3 tachykinin receptors (referred to as rmNK-1 and rmNK-3) from the rhesus monkey (Macaca mulatta) brain and examined their pharmacological profiles and regional distributions in the CNS. The deduced rmNK-1 amino-acid sequence differed by only two amino acids from the human NK-1 (hNK-1). The deduced rmNK-3 amino-acid sequence was two amino acids shorter than human NK-3 (hNK-3), with a seven-amino-acid difference in sequence. Ligand binding studies revealed that the affinity of rmNK-1 to substance P (SP) was comparable to that of hNK-1 in cell lines that expressed individual receptors stably. Nonpeptide antagonists had similar effects on the binding of rmNK-1 and hNK-1. Affinity of rmNK-3 for NKB was stronger than for SP and the IC50 value was comparable with that of hNK-3. Ca2+ imaging showed that activations of both rmNK-1 and rmNK-3 by specific ligands, SP and senktide, induced increased intracellular Ca2+ in cell lines that stably expressed individual primate tachykinin receptors. The amounts of rmNK-1 and rmNK-3 mRNAs were quantitatively determined in the monkey CNS. The expression of rmNK-1 was observed in all of the cortical and subcortical regions, including the hippocampus and the amygdala. The putamen contained the most NK-1 mRNA in the brain, with less rmNK-3 mRNA found in the cortex compared to rmNK-1 mRNA. In the monkey hippocampus and amygdala, rmNK-1 mRNA was present at markedly higher concentrations than rmNK-3 mRNA. The present results provide an insight into the distinct physiological nature and significance of the NK-1 and NK-3 tachykinin systems in the primate CNS. These findings are indispensable for establishing model systems in the search for a subtype-specific tachykinin receptor agonist and antagonist for the treatment of neuropsychiatric disorders.

Pharmacological FMRI in the development of new analgesic compounds

Chronic pain is a major problem for the individual and for society. Despite a range of drugs being available to treat chronic pain, only inadequate pain relief can be achieved for many patients. There is therefore a need for the development of new analgesic compounds. The assessment of pain depends to date entirely on the subjective report of the patient, in contrast to many other clinical conditions where biomarkers that help determine the severity and stage of the disease enable the physician to monitor the course of the disease and treatment effects longitudinally. In this article, we illustrate that magnetic resonance-based imaging techniques have the potential to provide sensitive and specific biomarkers of the pain experience, as well as clarifying disease mechanisms. Functional magnetic resonance imaging (FMRI) is particularly suited to investigating the effects of pharmacological agents on pain processing within the human central nervous system. Combination of FMRI and drug administration is termed pharmacological FMRI (phFMRI). In addition to outlining several methodological considerations that have to be taken into account when performing phFMRI, we discuss phFMRI studies that have already used this technique to study the effects of analgesic compounds. These studies provide promising data for the use of phFMRI as sensitive tool in assessing a potential drug effect. Such pharmacodynamic readouts obtained early in the process of drug development would not only save the pharmaceutical industry substantial amounts of money, but would also avoid the unnecessary exposure of patients to molecules with limited or no therapeutic value. We are therefore optimistic that phFMRI will be used as a tool with high sensitivity and specificity for evaluating analgesic agents in early drug development and clinical studies.

Review article: modulation of the brain-gut axis as a therapeutic approach in gastrointestinal disease

BACKGROUND

The importance of bi-directional brain-gut interactions in gastrointestinal illness is increasingly being recognized, most prominently in the area of functional gastrointestinal disorders. Numerous current and emerging therapies aimed at normalizing brain-gut interactions are a focus of interest, particularly for irritable bowel syndrome and functional dyspepsia.

METHODS

A literature search was completed for preclinical and clinical studies related to central modulation of gastrointestinal functions and published in English between 1980 and 2006.

RESULTS

Existing data, while sparse, support the use of different classes of antidepressant drugs, including tricyclics, and selective and non-selective serotonin reuptake inhibitors in irritable bowel syndrome. Serotonin receptor agonists and antagonists with peripheral and possibly central effects are effective in treating specific subtypes of irritable bowel syndrome. Based largely on theoretical and preclinical evidence, several novel compounds that selectively target receptors at multiple levels within the brain-gut axis such as neurokinin, somatostatin and corticotropin-releasing factor receptor antagonists are promising.

CONCLUSIONS

This review discusses the rationale for modulation of the brain-gut axis in the treatment of functional gastrointestinal disorders and highlights the most promising current and future therapeutic strategies.

Emerging drugs for irritable bowel syndrome

Irritable bowel syndrome (IBS) is one of the most common chronic gastrointestinal disorders, yet its pathophysiology is incompletely understood and pharmacological treatments remain unsatisfactory. Current therapeutic choices include a range of drugs aimed at normalising bowel habits, reducing pain or treating comorbid psychological symptoms. However, this individual symptom-targeted approach remains unsatisfactory in terms of global symptom relief and patient satisfaction. In the last decade, further characterisation of IBS pathophysiology has provided new and exciting targets at different levels of the brain-gut axis for the development of several candidate drugs. Advances in clinical trial design will help to evaluate these compounds in different IBS patient populations.

Review article: current and emerging therapies for functional dyspepsia

Functional dyspepsia represents a heterogeneous group of gastrointestinal disorders marked by the presence of upper abdominal pain or discomfort. Although its precise definition has evolved over the last several decades, this disorder remains shrouded in controversy. The symptoms of functional dyspepsia may overlap with those of other functional bowel disorders including irritable bowel syndrome and non-erosive reflux disease. There may be coexistent psychological distress or disease complicating its presentation and response to therapy. Given the prevalence and chronicity of functional dyspepsia, it remains a great burden to society. Suspected physiological mechanisms underlying functional dyspepsia include altered motility, altered visceral sensation, inflammation, nervous system dysregulation and psychological distress. Yet the exact pathophysiological mechanisms that cause symptoms in an individual patient remain difficult to delineate. Numerous treatment modalities have been employed including dietary modifications, pharmacological agents directed at various targets within the gastrointestinal tract and central nervous system, psychological therapies and more recently, complementary and alternative treatments. Unfortunately, to date, all of these therapies have yielded only marginal results. A variety of emerging therapies are being developed for functional dyspepsia. Most of these therapies are intended to normalize pain perception and gastrointestinal motor and reflex function in this group of patients.

Neuropeptides as synaptic transmitters

Neuropeptides are small protein molecules (composed of 3-100 amino-acid residues) that have been localized to discrete cell populations of central and peripheral neurons. In most instances, they coexist with low-molecular-weight neurotransmitters within the same neurons. At the subcellular level, neuropeptides are selectively stored, singularly or more frequently in combinations, within large granular vesicles. Release occurs through mechanisms different from classical calcium-dependent exocytosis at the synaptic cleft, and thus they account for slow synaptic and/or non-synaptic communication in neurons. Neuropeptide co-storage and coexistence can be observed throughout the central nervous system and are responsible for a series of functional interactions that occur at both pre- and post-synaptic levels. Thus, the subcellular site(s) of storage and sorting mechanisms into different neuronal compartments are crucial to the mode of release and the function of neuropeptides as neuronal messengers.

A role for fMRI in optimizing CNS drug development

Drug development today needs to balance agility, speed and risk in defining the probability of success for molecules, mechanisms and therapeutic concepts. New techniques in functional magnetic resonance imaging (fMRI) promise to be part of a sequence that could transform drug development for disorders of the central nervous system (CNS) by examining brain systems and their functional activation dynamically. The brain is complex and multiple transmitters and intersecting brain circuits are implicated in many CNS disorders. CNS therapeutics are designed against specific CNS targets, many of which are unprecedented. The challenge is to reveal the functional consequences of these interactions to assess therapeutic potential. fMRI can help optimize CNS drug discovery by providing a key metric that can increase confidence in early decision-making, thereby improving success rates and reducing risk, development times and costs of drug development.

The neurobiology of itch

The neurobiology of itch, which is formally known as pruritus, and its interaction with pain have been illustrated by the complexity of specific mediators, itch-related neuronal pathways and the central processing of itch. Scratch-induced pain can abolish itch, and analgesic opioids can generate itch, which indicates an antagonistic interaction. However, recent data suggest that there is a broad overlap between pain- and itch-related peripheral mediators and/or receptors, and there are astonishingly similar mechanisms of neuronal sensitization in the PNS and the CNS. The antagonistic interaction between pain and itch is already exploited in pruritus therapy, and current research concentrates on the identification of common targets for future analgesic and antipruritic therapy.

Targeting Pain-Suppressed Behaviors in Preclinical Assays of Pain and Analgesia: Effects of Morphine on Acetic Acid-Suppressed Feeding in C57BL/6J Mice

Pain increases the rate, frequency, or intensity of some behaviors (eg, withdrawal responses) and suppresses other behaviors (eg, feeding). Our laboratories are developing assays to test analgesic drug candidates using measurements of pain-suppressed rather than pain-elicited behaviors. Such assays may model important aspects of clinical pain and provide a means for distinguishing true analgesics from drugs that produce motor impairment. The present study compared effects of the mu opioid analgesic morphine and the nonanalgesic neuroleptic haloperidol on intraperitoneal acetic acid-induced writhing (a pain-elicited behavior) and suppression of feeding behavior (a pain-suppressed behavior). In feeding studies, C57BL/6J mice were given access to a dish containing 8 mL Ensure(trade mark) liquid food (0-100% in water) during daily sessions (7.5-120 min). Levels of consumption were dependent on both Ensure concentration and session duration. Intraperitoneal injection of acetic acid (0.10-0.56%) produced a time- and concentration-dependent decrease in Ensure consumption. Morphine (1 mg/kg) prevented both acid-induced writhing and acid-induced suppression of feeding, whereas the dopamine antagonist haloperidol inhibited writhing without preventing acid-induced suppression of feeding. The effects of morphine were time-dependent, selective for acid-suppressed feeding, and naltrexone-reversible. These results suggest that assays of pain-suppressed behaviors may complement assays of pain-elicited behaviors in preclinical studies of candidate analgesics.

PERSPECTIVE

This paper presents a new preclinical strategy for assessing pain and analgesia in mice that is congruent with current methods of pain assessment in the clinic. This strategy may therefore be a useful complement to more traditional procedures for assessing pain and analgesia.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

A Lack of Antinociceptive or Antiinflammatory Effect of Botulinum Toxin A in an Inflammatory Human Pain Model

Several in vitro and in vivo investigations have shown that botulinum toxin A (BoNT/A) can inhibit the release of substance P and excitatory amino acids. Recently, a marked antinociceptive effect of BoNT/A and inhibition of glutamate release was observed in an animal pain model with inflammatory sensitization. In the present study, we tested the antiinflammatory and antihyperalgetic effect of BoNT/A in a well-characterized human inflammatory pain model. Using a randomized, double-blind, paired study design, we compared the effects of 100 mouse units of BoNT/A versus pure saline. Thermal and mechanical pain testings and superficial skin blood flow measurements were performed at baseline, at 48 h (in normal skin), and at 72 h (in inflamed skin) thereafter. Ultraviolet B irradiation resulted in a local inflammation with significant primary and secondary hyperalgesia. However, despite the evidence of efficacy on sudomotor function, BoNT/A had no effect on pain measures in either normal or inflamed skin. Signs of inflammation and primary and secondary hyperalgesia were found to be unaffected by BoNT. We have confirmed that BoNT/A has no direct effect on acute, noninflammatory pain. Furthermore, despite highly promising data from animal research, we have not observed antiinflammatory or antinociceptive effects of BoNT/A in human inflammatory pain.

Mechanisms of Disease: mechanism-based classification of neuropathic pain—a critical analysis

Classification of neuropathic pain according to etiology or localization has clear limitations. The discovery of specific molecular and cellular events following experimental nerve injury has raised the possibility of classifying neuropathic pain on the basis of the underlying neurobiological mechanisms. Application of this approach in the clinic is problematic, however, owing to a lack of precise tools to assess symptoms and signs, and difficulties in correlating symptoms and signs with mechanisms. Development and validation of diagnostic methods to identify mechanisms, together with pharmacological agents that specifically target these mechanisms, seems to be the most logical and rational way of improving neuropathic pain treatment.

Tactile allodynia in patients with postherpetic neuralgia: Lack of change in skin blood flow upon dynamic stimulation

Tactile allodynia is a common, troublesome feature of neuropathic pain. Allodynia has been proposed to involve abnormal Abeta-afferent coupling in the dorsal horn resulting in C-fibre activation and increased skin blood flow (SBF). Thus, changes in SBF could provide an objective measure of allodynia. We searched for this mechanism in patients with postherpetic neuralgia (PHN) with varying degrees of cutaneous sensory loss. We mapped the allodynic area in PHN patients using cotton buds and von Frey hairs. Quantitative thermal testing was performed to assess small fibre function in the affected and mirror-image areas. At a subsequent visit the area of allodynia was remapped. Then the SBF in the affected and control areas was quantified before and after allodynic stimulation using laser Doppler imaging and subsequent single point continuous monitoring to detect rapid changes. We enrolled 10 PHN patients (medians: age 77 yrs, duration 20 months, ongoing pain 5). The allodynic area (range 11-546 cm2) was stable across the sessions. Thermal testing showed similar (n=5) or reduced (n=5) warmth and pain sensation in the affected versus control area. Following allodynic stimulation (median evoked pain-5) we saw no changes in SBF using either imaging (repeated measures ANOVA, P=0.73) or single point monitoring. This was the case for all patients regardless of the degree of sensory impairment in the affected dermatome. In conclusion, in a representative population of PHN patients we found no evidence of changes in SBF in response to allodynic stimulation. Hence, SBF measurements are not suitable for assessing allodynia.

Substance P receptor antagonists in psychiatry: rationale for development and therapeutic potential

Increasing evidence suggests that substance P (SP) and its receptor (neurokinin [NK]-1 receptor [NK1R]) might play an important role in the modulation of stress-related, affective and/or anxious behaviour. First, SP and NK1R are expressed in brain regions that are involved in stress, fear and affective response (e.g. amygdala, hippocampus, hypothalamus and frontal cortex). Second, the SP content in these areas changes upon application of stressful stimuli. Third, the central administration of SP produces a range of fear-related behaviours. In addition, the SP/NK1R system shows significant spatial overlap with neurotransmitters such as serotonin and noradrenaline (norepinephrine), which are known to be involved in the regulation of stress, mood and anxiety. Therefore, it was hypothesised that blockade of the NK1R might have anxiolytic as well as antidepressant effects. Preclinical studies investigating the effects of genetic or pharmacological NK1R inactivation on animal behaviour in assays relevant to depression and anxiety revealed that the behavioural changes resemble those seen with reference antidepressant or anxiolytic drugs. Furthermore, antagonism or genetic inactivation of the NK1R causes alterations in serotonin and norepinephrine neuronal transmission that are likely to contribute to the antidepressant/anxiolytic activity of NK1R antagonists but that are--at least partially--distinct from those produced by established antidepressant drugs. This underlines the conceivable unique mechanism of action of this new class of compounds. In three independent clinical trials with three different compounds (aprepitant [MK-869], L-759274 and CP-122721), an antidepressant effect of NK1R antagonists could be demonstrated. These results, however, have been challenged by recent failed studies with aprepitant. There are numerous indications from preclinical studies that, in addition to SP and NK1R, other neurokinins and/or neurokinin receptors might also be involved in the modulation of stress-related behaviour and that exclusive blockade of the NK1R might not be sufficient to produce consistent anxiolytic and antidepressant effects. One such candidate is the neurokinin-2 receptor (NK2R), and clinical trials to assess the antidepressant effects of NK2R antagonists are currently underway. Of special interest might also be substances that block more than one receptor type such as NK1/2R antagonists or NK1/2/3R antagonists. These compounds may be more efficacious in antagonising the effects of SP than compounds that only block the NK1R.

Effects of capsaicin on visceral smooth muscle: a valuable tool for sensory neurotransmitter identification

Studying the visceral effects of the sensory stimulant capsaicin is a useful and relatively simple tool of neurotransmitter identification and has been used for this purpose for approximately 25 years in the authors' and other laboratories. We believe that conclusions drawn from experiments on visceral preparations may have an impact on studies dealing with the central endings of primary afferent neurons, i.e. research on nociception at the spinal level. The present review concentrates on the effects of capsaicin--through the transient receptor potential vanilloid receptor type 1 (TRPV1) receptor--on innervated gastrointestinal, respiratory and genitourinary smooth muscle preparations. Tachykinins and calcitonin gene-related peptide (CGRP) are the most widely accepted transmitters to mediate "local efferent" effects of capsaicin-sensitive nerves in tissues taken from animals. Studies more and more frequently indicate a supra-additive interaction of various types of tachykinin receptors (tachykinin NK(1), NK(2), NK(3) receptors) in the excitatory effects of capsaicin. There is also evidence for a mediating role of ATP, acting on P(2) purinoceptors. Non-specific inhibitory actions of capsaicin-like drugs have to be taken into consideration while designing experiments with these drugs. Results obtained on human tissues may be sharply different from those of animal preparations. Capsaicin potently inhibits tone and movements of human intestinal preparations, an effect mediated by nitric oxide (NO) and/or vasoactive intestinal polypeptide.

Galanin and its receptors: introduction to the Third International Symposium, San Diego, California, USA, 21-22 October 2004

The Third Galanin Symposium presented many different and exciting results on galanin research reflecting a major progress since the previous symposium in 1998. A major impression was the many possible relationships of galaninergic mechanisms to important brain functions such as development, cognition and ageing as well as many aspects related to a wide spectrum of diseases, including Alzheimer's disease, anxiety/depression, addiction, obesity, pain and tumour growth. These studies were based on an extensive armament of methodologies including various strains of transgenic mice. Unfortunately, the pharmaceutical industry had only a minor participation. Nevertheless, exciting developments in the generation of agonists and antagonists are emerging, providing hope that we at the next symposium will be able to validitate many of the challenging hypotheses concerning galanin and disease with the help of pharmacological tools.

Tools for target identification and validation

Reliable technologies for addressing target identification and validation are the foundation of successful drug development. Microarrays have been well utilized in genomics/proteomics approaches for gene/protein expression profiling and tissue/cell-scale target validation. Besides being used as an essential step in analyzing high-throughput experiments such as those involving microarrays, bioinformatics can also contribute to the processes of target identification and validation by providing functional information about target candidates and positioning information to biological networks. Antisense technologies (including RNA interference technology, which is recently very 'hot') enable sequence-based gene knockdown at the RNA level. Zinc finger proteins are a DNA transcription-targeting version of knockdown. Chemical genomics and proteomics are emerging tools for generating phenotype changes, thus leading to target and hit identifications. NMR-based screening, as well as activity-based protein profiling, are trying to meet the requirement of high-throughput target identification.

Imbalance between the expression of NK1 and GABAB receptors in nociceptive spinal neurons during secondary hyperalgesia: A c-fos study in the monoarthritic rat

The neurochemical changes that operate in nociceptive spinal cord circuits during secondary hyperalgesia are largely unknown, in particular with respect to the balance between excitatory and inhibitory neurotransmission. In this study we evaluated the expression of NK1 and GABA(B) receptors in nociceptive spinal neurons in a model of secondary hyperalgesia consisting of noxious mechanical stimulation of the hindlimb skin close to a joint chronically inflamed by complete Freund's adjuvant. In spinal segments receiving input from that skin area, Fos-immunodetection was combined with immunocytochemistry for NK1 receptors, GABA(B) receptors or both receptors. In control and monoarthritic animals, neurons double-labeled for Fos and each receptor occurred mainly in laminae I and IV-V. In lamina I, the percentage of NK1 neurons expressing Fos was higher in monoarthritics while lower percentages of GABA(B) neurons expressed Fos. The percentage of Fos-positive cells expressing NK1 immunoreaction did not change in monoarthritics but that of Fos cells with GABA(B) immunoreaction was lower in these animals. In laminae IV-V, a large increase in Fos expression was detected in monoarthritic rats but the relative proportions of Fos-positive neurons expressing each receptor were similar in the two groups. Co-localization of NK1 and GABA(B) receptors occurred only in lamina I neurons in both experimental groups with no differences between control and monoarthritic animals in the percentages of Fos-positive neurons that expressed the receptors. Considering the participation of lamina I neurons bearing NK1 and GABA(B) receptors in several spinofugal systems, it is possible that the imbalance between excitatory and inhibitory actions exerted, respectively, by substance P and GABA may subserve secondary hyperalgesia by increasing ascending transmission of nociceptive input.

Elimination of neurokinin-1 receptor neurons in caudal nucleus reverses the effects of systemic bicuculline on c-Fos expression in rat trigeminal sensory nucleus: I. High intensity electrical stimulation of the trigeminal ganglion

Although neurokinin-1 receptor (NK-1)-bearing neurons are distributed in lamina I of the trigeminal caudal nucleus (Vc) and constitute major projection neurons, little is known about their fundamental role(s) in nociceptive processing. This study examines the effect of intra cisterna magna injection of substance P (SP) conjugated to saporin (SP-Sap; 5 microM, 5 microl) [with/without systemic administration of bicuculline] on c-Fos expression in the trigeminal sensory nucleus (TSN) induced 2 h after 10 min repetitive electrical stimulation of the trigeminal ganglion (TG) at high intensity (1.0 mA, 5 Hz, 5 ms) in the urethane-anesthetized rat. In the SP-Sap-treated rats, the numbers of NK-1-immunopositive neurons in laminae I and III of the Vc decreased compared with rats similarly pretreated with saline (Sal; 5 microl) or blank-saporin (Bl-Sap; 5 microM, 5 microl). In Sal- or Bl-Sap-treated controls, high intensity stimulation induced c-Fos expression in neurons throughout the full extent of ipsilateral superficial layers of the Vc (VcI/II), magnocellular zone of the Vc (VcIII/IV) and the dorsal or dorsomedial subdivisions of the rostral TSN above the obex (trigeminal principal, oral (Vo) and interpolar nuclei). Preadministration of bicuculline (2 mg/kg, i.p.) decreased the numbers of c-Fos-immunopositive neurons in the VcI/II, VcIII/IV and Vo in Sal- or Bl-Sap-treated controls. In contrast, high intensity stimulation induced less c-Fos-immunopositive neurons in the VcI/II and Vo of rats treated with SP-Sap compared with those in Sal- or Bl-Sap-treated controls. In SP-Sap-treated rats preadministered with bicuculline, the numbers of c-Fos-immunopositive neurons in the VcI/II and Vo were increased compared with the SP-Sap-treated rats preadministered with Sal. These results suggest that NK-1-immunopositive neurons in laminae I and III of Vc play a pivotal role in the nociceptive specific processing in the TSN through GABA(A) receptors.

Glutamate and tachykinin receptors in central sensitization of withdrawal reflexes in the decerebrated rabbit

This study assessed the involvement of NMDA and group I metabotropic glutamate receptors, and tachykinin NK1 and NK3 receptors, in central sensitization of withdrawal reflexes in the decerebrated rabbit. Reflexes evoked in the ankle flexor tibialis anterior and the knee flexor semitendinosus by electrical stimulation at the base of the toes were enhanced for 29-63 min after application of 20% mustard oil to the tips of the toes. Selective antagonists of mGlu1, mGlu5, NMDA and NR2B-subunit-containing NMDA glutamate receptors, as well as NK1, and NK3 receptors, and a non-selective blocker of all tachykinin receptors, were assessed for their effects on the magnitude and duration of the increase in reflexes induced by mustard oil. Dizocilpine, an antagonist of all NMDA receptors (1 mg intrathecal) abolished facilitation of tibialis anterior reflexes and significantly reduced the magnitude and duration of increase of the semitendinosus response. The NR2B-subtype selective antagonist CP-101,606 decreased the magnitude of facilitation of both reflexes but had no effect on duration of enhancement. Selective antagonists for the mGlu1 (CPCCOEt, 1-3 mg intrathecal), mGlu5 (MPEP, 0.2-1 mg intrathecal), NK1 (L-733,060, 0.3 mg intrathecal) or NK3 (SR 142,801, 1 mg kg(-1) i.v.) receptors had no effect on the amplitude or duration of sensitization. However, the non-selective tachykinin receptor blocker ZD-6021 (0.3 mg intrathecal) reduced the amplitude but not the duration of sensitization in the flexor reflexes. Combination of ZD-6021 with CP-101,606 (doses as above) decreased both aspects of the sensitization response. Dizocilpine reduced reflexes evoked from the heel per se, and dizocilpine, CP-101,606 and ZD-6021 reduced arterial blood pressure. Otherwise the drugs used had no effects on baseline variables. The present data confirm the importance of NMDA receptors as a critical part of the process of central sensitization, provide no evidence for a role of metabotropic glutamate receptors, and show that simultaneous blockade of all tachykinin receptors is required to reveal their role in hyperalgesia. The data further indicate that a combined pharmacological approach offers a potential way forward for the development of new antihyperalgesic agents.

Spinal dorsal horn neurone targets for nociceptive primary afferents: do single neurone morphological characteristics suggest how nociceptive information is processed at the spinal level

It has become increasingly clear that nociceptive information is signalled by several anatomically distinct populations of primary afferents that target different populations of neurones in the spinal cord. It is probable that these different systems all give rise to the sensation pain and hence, an understanding of their separate roles and the processes that they employ, may offer ways of selectively targeting pain arising from different causes. The review focuses on what is known of the anatomy of neurones in LI-III of the spinal dorsal horn that are implicated in nociception. The dendritic geometry and synaptic input of the large LI neurones that receive input from primary afferents containing substance P that express neurokinin 1 (NK(1)) receptors suggests that these neurones may monitor the extent of injury rather than the specific localisation of a discrete noxious stimulus. This population of neurones is also critically involved in hyperalgesia. In contrast neurones in LII with the morphology of stalked cells that receive primary afferent input from glomerular synapses may be more suitable for fine discrimination of the exact location of a noxious event such as a sting or parasite attack. The review focuses as far as possible on precisely defined anatomy in the belief that only by understanding these anatomical relationships will we eventually be able to interpret the complex processes occurring in the dorsal horn. The review attempts to be an accessible guide to a sometimes complex and highly specialised literature in this field.

Cell signaling and the genesis of neuropathic pain

Damage to the nervous system can cause neuropathic pain, which is in general poorly treated and involves mechanisms that are incompletely known. Currently available animal models for neuropathic pain mainly involve partial injury of peripheral nerves. Multiple inflammatory mediators released from damaged tissue not only acutely excite primary sensory neurons in the peripheral nervous system, producing ectopic discharge, but also lead to a sustained increase in their excitability. Hyperexcitability also develops in the central nervous system (for instance, in dorsal horn neurons), and both peripheral and spinal elements contribute to neuropathic pain, so that spontaneous pain may occur or normally innocuous stimuli may produce pain. Inflammatory mediators and aberrant neuronal activity activate several signaling pathways [including protein kinases A and C, calcium/calmodulin-dependent protein kinase, and mitogen-activated protein kinases (MAPKs)] in primary sensory and dorsal horn neurons that mediate the induction and maintenance of neuropathic pain through both posttranslational and transcriptional mechanisms. In particular, peripheral nerve lesions result in activation of MAPKs (p38, extracellular signal-regulated kinase, and c-Jun N-terminal kinase) in microglia or astrocytes in the spinal cord, or both, leading to the production of inflammatory mediators that sensitize dorsal horn neurons. Activity of dorsal horn neurons, in turn, enhances activation of spinal glia. This neuron-glia interaction involves positive feedback mechanisms and is likely to enhance and prolong neuropathic pain even in the absence of ongoing peripheral external stimulation or injury. The goal of this review is to present evidence for signaling cascades in these cell types that not only will deepen our understanding of the genesis of neuropathic pain but also may help to identify new targets for pharmacological intervention.

Serotoninergic-mediated inhibition of substance P sensitive deep dorsal horn neurons: a combined electrophysiological and morphological study in vitro

Dorsal horn neurons that express the neurokinin 1 receptor (NK-1R) play an important role in nociceptive processing. The targetting of NK-1R neurons by serotoninergic (5-hydroxytryptamine, 5-HT) axons would provide a straightforward means to exert an inhibitory analgesic effect at spinal level. This study used single cell electrophysiology to analyse and correlate the responses of rat deep DH neurons in vitro to both 5-HT and the NK-1R agonist [Sar9,Met(O2)11]-substance P (SP). Subsequently a combination of immunocytochemistry and confocal imaging was applied to biocytin-filled laminae III-VI neurons to reveal putative 5-HT innervation in this neuronal sample. A population of neurons was identified in which 5-HT (50 microM) significantly attenuated the dorsal root-evoked excitatory postsynaptic potential and [Sar9,Met(O2)11]-SP (2 microM) induced a direct tetrodotoxin-resistant depolarisation. Immunolabelling revealed that all of these neurons were inhibited by 5-HT, including those that were excited by [Sar9,Met(O2)11]-SP, were overlaid by a plexus of 5-HT immunoreactive fibres and in some instances, closely apposed putative contacts with somata and proximal dendrites identified although their incidence was low. Inhibition by 5-HT of deep DH neurons directly responsive to SP may account at least in part for monoamine-induced modulation of nociceptive processing in the spinal cord.

Pain-like behaviours in animals – how human are they?

The use of genetically manipulated animals in conjunction with classical physiological and biochemical measurement has unravelled many pathological changes in animal models of chronic pain that bear some striking similarities to those described in several chronic pain conditions in humans. In this article, I highlight several limitations in the validation of animal models of chronic pain and the methods that are used for assessing pain-like behaviours in these models. Alternative methods for assessing pain and stress in animals, which might better reflect the diverse symptomotology of chronic pain in humans, are proposed.

Substance P microinjected into the periaqueductal gray matter induces antinociception and is released following morphine administration

The aims of the present study were to investigate, in rats, the behavioral effects of substance P (SP) microinjected into the ventrolateral periaqueductal gray (PAG) and the effects of the neurokinin 1 (NK-1) receptor antagonist [d-Arg1, d-Trp7, 9, Leu11]-substance P (Spantide). The effect of morphine administration on the release of SP in the ventrolateral PAG was also investigated using microdialysis in awake rats. SP microinjected into the ventrolateral part of the PAG induced significant increases in the hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulation as an antinociceptive response. The NK-1 receptor antagonist blocked these effects but exhibited no antinociceptive effect alone. Subcutaneous administration of morphine increased basal SP-like immunoreactivity (SP-LI) release in the microdialysate obtained from the ventrolateral PAG of freely moving rats. Our results demonstrate that SP injected into the ventrolateral PAG induces an antinociceptive effect via activation of NK-1 receptors. Morphine administered systemically induces the release of SP in the ventrolateral PAG. We suggest that an increased release of SP in the PAG may contribute to opioid antinociception.