Evaluation of Postsurgical Hyperalgesia and Sensitization After Open Inguinal Hernia Repair: A Useful Model for Neuropathic Pain?

Cutaneous mechanical hyperalgesia can be induced in healthy volunteers in early phase analgesic studies to model central sensitization, a key mechanism of persistent pain. However, such hyperalgesia is short-lived (a matter of hours), and is used only for assessing only single drug doses. In contrast, postsurgical peri-incisional hyperalgesia may be more persistent and hence be a more useful model for the assessment of the efficacy of new analgesics. We undertook quantitative sensory testing in 18 patients at peri-incisional and nonoperated sites before open inguinal hernia repair and up to the 24th postsurgical week. The spatial extent of punctate hyperalgesia and brush allodynia at the peri-incisional site were greatest at weeks 2 and 4, but had resolved by week 24. Heat allodynia, suggestive of local inflammation or peripheral sensitization, was not observed; instead, there were deficits in cold and heat sensory detection that persisted until week 24. The findings suggest that central sensitization contributes significantly to mechanical hyperalgesia at the peri-incisional site. The prolonged duration of hyperalgesia would be advantageous as a pain model, but there was considerable variability of mechanical hyperalgesia in the cohort; the challenges of recruitment may limit its use to small, early phase analgesic studies. PERSPECTIVE: Peri-incisional mechanical hyperalgesia persists for ≥4 weeks after open inguinal hernia repair and reflects central sensitization; this may have usefulness as a model of chronic pain to assess the potential of antineuropathic analgesics.

Clinical trial of the p38 MAP kinase inhibitor dilmapimod in neuropathic pain following nerve injury

Current treatments of neuropathic pain arising from conditions such as nerve injury/compression are only partially effective, and limited in their use by side-effects. p38 mitogen-activated protein kinase (MAPK) is involved in the regulation and synthesis of inflammatory mediators, and is the target for a novel class of cytokine-suppressive anti-inflammatory drugs. p38 inhibitors may reduce neuronal sensitisation in preclinical models of neuropathic pain, particularly where there is a substantial inflammatory component. An exploratory, multicentre, double-blind, placebo-controlled, two-period, cross-over trial was undertaken to evaluate the effect of dilmapimod (SB-681323), a selective p38 MAPK inhibitor, on neuropathic pain symptoms and signs. Fifty patients with nerve trauma, radiculopathy or carpal tunnel syndrome were randomised; 43 patients completed the study. Eligible patients received oral dilmapimod and placebo twice daily for 2 weeks, with an intervening washout period of 2-4 weeks. Subjects attended weekly for efficacy and safety assessments, which included evaluation of daily and current pain intensity using an 11-point numerical rating scale (NRS), quantitative sensory testing, allodynia and global impression of change. There was a statistically significant reduction in the primary endpoint of average daily pain score during the second week of treatment among patients treated with dilmapimod (15 mg/day) compared to placebo using NRS [0.80; 95% CI (0.28, 1.33); p=0.0034]. A similar trend for effect was seen in some secondary endpoints. Dilmapimod was well tolerated, with no clinically relevant safety findings. p38 MAPK inhibitors merit further evaluation for neuropathic pain in larger clinical trials, particularly for clinically meaningful analgesic effect size.

Neurophysiological markers of central sensitisation in the trigeminal pathway and their modulation by the cyclo-oxygenase inhibitor ketorolac

Central sensitisation is a key mechanism of migraine; understanding its modulation by anti-migraine drugs is essential for rationalising treatment. We used an animal model of central trigeminal sensitisation to investigate neuronal responses to dural electrical stimulation as a putative electrophysiological marker of sensitisation and its modulation by ketorolac. In anaesthetised rats, responses of single convergent wide-dynamic range neurons of the spinal trigeminal nucleus to dural electrical simulation were recorded in parallel to their ongoing activity and responses to facial mechanical stimulation before and after a short-term dural application of an IS. Both ongoing activity and responses to dural electrical stimuli were enhanced by the inflammatory challenge, whereas neuronal thresholds to mechanical skin stimulation were reduced ( p < .05, N = 12). Intravenous ketorolac (2 mg/kg, N = 6) reduced ongoing activity and responses to dural electrical stimulation, and increased mechanical thresholds versus vehicle controls ( p < .05, N = 6). We conclude that neuronal responses to dural electrical stimulation can serve as a suitable marker which together with admitted electrophysiological signs can objectively detect central trigeminal sensitisation and its modulation by anti-migraine treatments in this preclinical model of migraine.

Use of sensory methods for detecting target engagement in clinical trials of new analgesics

The translation of analgesic efficacy seen in preclinical pain models into the clinic is problematic and is associated with a number of factors that may result in the failure of clinical trials to detect the effect of investigational therapeutic agents. The use of translational pain biomarkers in phase I trials can potentially reduce some of these risks by measuring the interaction between the drug and its target (termed target engagement) in humans. To serve this purpose, sensory tests and other measures of pharmacological activity in nociceptive pathways need to be identified, based on the preclinical profile of the drug being tested and the feasibility of human assessments. Here we discuss some examples to assess the utility of sensory and related pain biomarkers in the early phase of evaluation of novel analgesics for confirmation of target engagement in humans. The emphasis is on the TRPV1 antagonists, but some other target mechanisms are also discussed in examining the validity of this approach.

Clinical development of TRPV1 antagonists: targeting a pivotal point in the pain pathway

TRPV1 is a noxious heat, capsaicin (vanilloid) and acid receptor for which the development of antagonists represents a novel therapeutic approach for the treatment of pain. TRPV1 antagonists have entered early clinical development and initial reports indicate that they have demonstrated pharmacodynamic effects consistent with TRPV1 antagonist activity and anti-hyperalgesic action in humans. Should these effects extend to the relief of symptoms experienced by patients with chronic pain then this class of compounds may offer one of the first novel mechanisms of action for the treatment for pain for many years. In this article we will discuss recent progress and challenges in the field in this highly competitive area of drug discovery.

Effective electromagnetic noise cancellation with beamformers and synthetic gradiometry in shielded and partly shielded environments

The major challenge of MEG, the inverse problem, is to estimate the very weak primary neuronal currents from the measurements of extracranial magnetic fields. The non-uniqueness of this inverse solution is compounded by the fact that MEG signals contain large environmental and physiological noise that further complicates the problem. In this paper, we evaluate the effectiveness of magnetic noise cancellation by synthetic gradiometers and the beamformer analysis method of synthetic aperture magnetometry (SAM) for source localisation in the presence of large stimulus-generated noise. We demonstrate that activation of primary somatosensory cortex can be accurately identified using SAM despite the presence of significant stimulus-related magnetic interference. This interference was generated by a contact heat evoked potential stimulator (CHEPS), recently developed for thermal pain research, but which to date has not been used in a MEG environment. We also show that in a reduced shielding environment the use of higher order synthetic gradiometry is sufficient to obtain signal-to-noise ratios (SNRs) that allow for accurate localisation of cortical sensory function.

The effects of the TRPV1 antagonist SB-705498 on TRPV1 receptor-mediated activity and inflammatory hyperalgesia in humans

TRPV1 is a cation channel activated by a range of noxious stimuli and highly expressed in nociceptive fibres. TRPV1 receptors are involved in pain and sensitisation associated with tissue injury and inflammation; hence, TRPV1 antagonists are potentially useful for the treatment of such pain states. SB-705498 is a potent, selective and orally bioavailable TRPV1 antagonist with demonstrated efficacy in a number of preclinical pain models. In this first-time-into-human study, we have investigated the pharmacodynamic and antihyperalgesic activity of SB-705498. The compound was safe and well tolerated at single oral doses up to 400mg. In a cohort of 19 healthy volunteers, we used a randomised placebo-controlled single-blind cross-over design to assess the effects of SB-705498 (400mg) on heat-evoked pain and skin sensitisation induced by capsaicin or UVB irradiation. Compared with placebo, SB-705498 reduced the area of capsaicin-evoked flare (P=0.0047). The heat pain threshold on non-sensitised skin was elevated following SB-705498 (estimated difference from placebo [95% confidence intervals]: 1.3 degrees C [0.07,2.53], P=0.019). Following capsaicin sensitisation, the heat pain threshold and tolerance were similar between SB-705498 and placebo. However, SB-705498 increased heat pain tolerance at the site of UVB-evoked inflammation (estimated difference from placebo: 0.93 degrees C [0.25,1.6], P=0.0054). The magnitude of the pharmacodynamic effects of SB-705498 appeared to be related to plasma concentration. These results indicate that SB-705498, at a clinically safe and well-tolerated dose, has target-specific pharmacodynamic activity in humans. These data provide the first clinical evidence that a TRPV1 antagonist may alleviate pain and hyperalgesia associated with inflammation and tissue injury.

Fibromyalgia patients show an abnormal dopamine response to pain

Fibromyalgia is characterized by chronic widespread pain and bodily tenderness and is often accompanied by affective disturbances. Accumulating evidence indicates that fibromyalgia may involve a dysfunction of modulatory systems in the brain. While brain dopamine is best known for its role in pleasure, motivation and motor control, recent evidence suggests that it is also involved in pain modulation. Because dopamine is implicated in both pain modulation and affective processing, we hypothesized that fibromyalgia may involve a disturbance of dopaminergic neurotransmission. Fibromyalgia patients and matched healthy control subjects were subjected to deep muscle pain produced by injection of hypertonic saline into the anterior tibialis muscle. In order to determine the endogenous release of dopamine in response to painful stimulation, we used positron emission tomography to examine binding of [(11)C]-raclopride (D2/D3 ligand) in the brain during injection of painful hypertonic saline and nonpainful normal saline. Fibromyalgia patients experienced the hypertonic saline as more painful than healthy control subjects. Control subjects released dopamine in the basal ganglia during the painful stimulation, whereas fibromyalgia patients did not. In control subjects, the amount of dopamine release correlated with the amount of perceived pain but in fibromyalgia patients no such correlation was observed. These findings provide the first direct evidence that fibromyalgia patients have an abnormal dopamine response to pain. The disrupted dopaminergic reactivity in fibromyalgia patients could be a critical factor underlying the widespread pain and discomfort in fibromyalgia and suggests that the therapeutic effects of dopaminergic treatments for this intractable disorder should be explored.

Accelerated brain gray matter loss in fibromyalgia patients: premature aging of the brain?

Fibromyalgia is an intractable widespread pain disorder that is most frequently diagnosed in women. It has traditionally been classified as either a musculoskeletal disease or a psychological disorder. Accumulating evidence now suggests that fibromyalgia may be associated with CNS dysfunction. In this study, we investigate anatomical changes in the brain associated with fibromyalgia. Using voxel-based morphometric analysis of magnetic resonance brain images, we examined the brains of 10 female fibromyalgia patients and 10 healthy controls. We found that fibromyalgia patients had significantly less total gray matter volume and showed a 3.3 times greater age-associated decrease in gray matter than healthy controls. The longer the individuals had had fibromyalgia, the greater the gray matter loss, with each year of fibromyalgia being equivalent to 9.5 times the loss in normal aging. In addition, fibromyalgia patients demonstrated significantly less gray matter density than healthy controls in several brain regions, including the cingulate, insular and medial frontal cortices, and parahippocampal gyri. The neuroanatomical changes that we see in fibromyalgia patients contribute additional evidence of CNS involvement in fibromyalgia. In particular, fibromyalgia appears to be associated with an acceleration of age-related changes in the very substance of the brain. Moreover, the regions in which we demonstrate objective changes may be functionally linked to core features of the disorder including affective disturbances and chronic widespread pain.

Low dose ketamine: a therapeutic and research tool to explore N-methyl-D-aspartate (NMDA) receptor-mediated plasticity in pain pathways

Ketamine is a dissociative anaesthetic that has been used in the clinic for many years. At low, sub-anaesthetic doses, it is a relatively selective and potent antagonist of the N-methyl-D-aspartate (NMDA) receptor. It belongs to the class of uncompetitive antagonists and blocks the receptor by binding to a specific site within the NMDA receptor channel when it is open. Like other compounds of this class, ketamine can cause hallucinations or other untoward central effects which limit its use in the clinic. Nevertheless, because of the evidence on the importance of NMDA receptor-mediated pLasticity in chronic pain, low doses of ketamine have been explored in a wide range of pain conditions. The majority of studies with ketamine have shown efficacy; however, it has not been possible to separate safely the pain relief from the side effects of the drug. Hence, clinical use of ketamine as a pain treatment is very limited. Nevertheless, ketamine has served as a useful tool to provide a compelling rationale for developing other NMDA antagonists. Some of the new compounds of this class, particularly those acting at the NR2B subtype of the NMDA receptor, have shown promise in preclinical and clinical studies.

Potentiation of nociceptive responses to low pH injections in humans by prostaglandin E2

Inflammation and trauma lead to tissue acidification and release of inflammatory mediators, including prostaglandin E2 (PGE2). Protons can evoke pain through acid-sensing ion channels (ASICs) and TRPV1 receptors. In this study, we examined whether PGE2 can potentiate proton-induced nociception in humans on injection into skin and muscle. Psychophysical and vascular responses to microinjections of protons (pH 6.0 and 6.5), PGE2 (10-6 and 10-7 M) and their combinations into forearm skin (30 microL) or anterior tibial muscle (50 microL) were assessed in 16 male subjects. Pain intensity, axon reflex erythema, and heat pain thresholds were recorded after skin challenge; pain intensity and thresholds for pressure-evoked pain were recorded after intramuscular injections. Intradermal or intramuscular injections of PGE2 induced very low levels of pain similar to saline. Administration of low pH caused moderate pain within 5 seconds that declined rapidly over 15 to 20 seconds. In comparison, coinjection of low pH with PGE2 led to a biphasic profile of the pain response. Combined pH + PGE2 stimulation provoked significantly increased pain in the second phase after injections (20 to 100 seconds) both in skin and muscle, whereas the initial injection pain was not enhanced. Heat pain thresholds were reduced after PGE2 and combined pH + PGE2, whereas flare responses were rather attenuated on coadministration of low pH with PGE2. Intriguingly, when compared with skin, muscle pain was significantly lower in the initial phase (0 to 15 seconds) but significantly higher in the second phase (20 to 100 seconds after injection).

PERSPECTIVE

PGE2 can potentiate nociceptor activation by protons in human skin and muscle, indicated by increased sustained pain ratings. This can be best explained by TRPV1 sensitization in the presence of PGE2, a mechanism potentially relevant for inflammatory and injury-induced pain.

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.

The antiallodynic effect of NMDA antagonists in neuropathic pain outlasts the duration of the in vivo NMDA antagonism

Clinical reports have described a long-lasting relief in neuropathic pain patients treated with NMDA receptor antagonists; it is unclear, however, what mediates this effect. In this work, we have used two NMDA antagonists of different class to investigate if the antiallodynic effects in a rat neuropathy model can outlast their in vivo NMDA antagonism. Both the uncompetitive NMDA antagonist ketamine and the glycine(B) antagonist MRZ 2/576 inhibited neuronal responses to iontophoretic NMDA in anaesthetised rats with the time course consistent with their known pharmacokinetics (t(1/2) approximately 10-12min, similar in control and nerve-injured rats). Surprisingly, the antiallodynic effects of the same doses of the NMDA antagonists in the neuropathic pain model were long-lasting (>3h with ketamine, >24h with MRZ 2/576). The effect of ketamine was further prolonged (>24h) when combined with a short-acting opioid, fentanyl, which only produced a short effect ( approximately 40min) when given alone. The duration of centrally mediated side effects of ketamine and MRZ 2/576 was short, similar to the in vivo NMDA antagonism. We speculate that NMDA receptor blockade may down-regulate the central sensitisation triggered by nerve injury, resulting in a long-lasting antiallodynic effect. Development of short-acting NMDA antagonists could represent a strategy for improving their tolerability.

Antiallodynic effects of NMDA glycine(B) antagonists in neuropathic pain: possible peripheral mechanisms

NMDA receptors are implicated in central sensitisation underlying chronic pain, and NMDA antagonists have a potential for the treatment of neuropathic pain. Functional NMDA receptors are also present on primary afferents, where they may play a role in pro-nociceptive plasticity. The importance of this mechanism in neuropathic pain remains unclear. In the present work, we have compared in models of chronic pain the effects of NMDA antagonists at the glycine(B) site with different central access. L-701,324 (the centrally active antagonist) and 5,7-dichlorokynurenic acid (5,7-DCK, known to have limited central access) were tested after systemic administration in rats in the formalin test and in two models of neuropathic pain. The ability of these compounds to exert central actions (sedation, ataxia) was tested in the open field locomotion test; central NMDA antagonism in vivo was tested in anaesthetised rats on responses of spinal cord neurones to iontophoretic NMDA. Both L-701,324 (2.15-21.5 mg/kg i.p.) and 5,7-DCK (10-46.4 mg/kg i.v.) dose-dependently inhibited Phase II of formalin-evoked behaviour. Likewise, both compounds reversed cold allodynia in the chronic constriction injury model and tactile allodynia in animals with spinal nerve ligation. However, only L-701,324 was able to inhibit neuronal responses to NMDA in the antihyperalgesic dose range; 5,7-DCK was inactive on NMDA responses up to 46.4 mg/kg i.v. or 68.1 mg/kg i.p. Consistent with the lack of inhibition of central NMDA-evoked activity, 5,7-DCK did not alter spontaneous behaviour in the open field test, whereas it was significantly inhibited by L-701,324. Thus, peripheral NMDA receptors may substantially contribute to the efficacy of NMDA antagonists in neuropathic pain.

NMDA Antagonists and Neuropathic Pain - Multiple Drug Targets and Multiple Uses

NMDA (N-methyl-D-aspartate) receptors are one class of ionotropic receptor for the ubiquitous excitatory neurotransmitter L-glutamate. The receptor is made up of four protein subunits combined from a larger library of proteins, which gives this receptor a great deal of variability. This explains the large number of modulatory sites, a variety of sites at which antagonists can interact, and therefore a number of potential drug targets. Sensitivity of the NMDA ion channel to ambient levels of Mg++ gives it a voltage dependence that suits a function of responding to intense synaptic activation; the ability of the channel to admit Ca++ tends to trigger long-term processes. The receptor is thereby involved in long-term physiological processes such as learning and memory as well as in pathological processes such as neuropathic pain. Separating these functions therapeutically with NMDA antagonists has been a major difficulty, and has not yet been achieved with currently-available agents. This review summarises the preclinical rationale, based on animal models, and the clinical evidence on the use of NMDA antagonists in pain states. It also summarises the details of the receptor so as to explain the rationale for targeting either specific sites on the receptor, or exploiting anatomical differences in subtype expression, so as to provide the beneficial effects of NMDA receptor block with an improved side effect profile. In particular, agents that are selective for receptors that include the NR2B subunit preclinically have a substantially better profile for treating neuropathic pain than do current NMDA antagonists; some emerging clinical evidence supports this view.

Chronic Oral Gabapentin Reduces Elements of Central Sensitization in Human Experimental Hyperalgesia

Background

In chronic pain, increased activity from intact or damaged peripheral nerve endings results in an enhanced response in central pain transmission systems, a mechanism known as central sensitization. Central sensitization can also be invoked in human experimental models. Therefore, these models may be useful to characterize novel analgesics in humans. The anticonvulsant gabapentin has demonstrated efficacy in patients with neuropathic pain, but its mode of action remains unclear. This study examined the effects of gabapentin on signs of central sensitization (brush and pinprick hyperalgesia) in a human model of capsaicin-evoked pain, using a gabapentin dosing regimen similar to that used in the clinic. The aims were to determine whether gabapentin, dosed in a manner similar to that used in the clinic, affected the various components of central sensitization and to assess the utility of this model for characterizing novel analgesics.

Methods

Intradermal capsaicin (100 microg/20 microl) was administered in the volar forearm of 41 male human volunteers to induce pain and clinical signs of central sensitization. Gabapentin (titrated to 2,400 mg daily) or placebo was given orally for 15 days in a randomized, double-blind, parallel-group design. The capsaicin test was conducted at baseline and after gabapentin or placebo. Endpoints were the size of areas of brush-evoked allodynia (with cotton gauze) and pinprick hyperalgesia (with von Frey filament), and the intensity of ongoing brush- and pinprick-evoked pain.

Results

Gabapentin significantly reduced the area of brush allodynia compared with placebo (P &lt;/= 0.05) and insignificantly attenuated the area of pinprick hyperalgesia. Gabapentin had no significant effect on spontaneous and evoked pain intensity.

Conclusion

Oral gabapentin, administered to healthy volunteers in a regimen similar to that used in treating chronic neuropathic pain, reduces measures of central sensitization evoked by intradermal capsaicin. This suggests that the pain-relieving effect in chronic neuropathic pain condition is linked to the effect of gabapentin on central sensitization. The ability of the capsaicin model to detect the efficacy of this standard treatment of neuropathic pain suggests that it may have a predictive value for detection of efficacy in human subjects.

The Effect of Intravenous Infusion of Adenosine on Electrically Evoked Hyperalgesia in a Healthy Volunteer Model of Central Sensitization

Human pain models invoking central sensitization, one of the key mechanisms of chronic pain, may be useful for characterizing new analgesics. A new model of electrical hyperalgesia can detect the efficacy of several analgesic mechanisms. Because IV adenosine can alleviate neuropathic pain, we investigated its effect on experimental sensitization. This was a double-blinded, randomized, two-period crossover study in 20 healthy volunteers. Current pulses (0.5 ms; 1 Hz) were applied intracutaneously to achieve pain rating of approximately 5 on a 0-10 numeric rating scale. Pain, areas of pinprick hyperalgesia, and tactile allodynia were assessed during the 2.5-h stimulation period. Adenosine (50 microg. kg(-1). min(-1)) and placebo were infused IV over 60 min. Additional testing was performed 24 h after each treatment. Adenosine reduced the area of pinprick hyperalgesia during the infusion compared with placebo; there was no significant effect on tactile allodynia or pain rating. The effect on hyperalgesia developed over 15 min and was significant (P < or = 0.05) for the rest of the infusion period. There was no difference between treatments at 24 h. Thus, in accordance with reports on neuropathic pain, adenosine reduced central sensitization in the human model of electrical hyperalgesia. However, adenosine did not have the long-term effects seen in patients. The model can investigate mechanisms of drugs for the treatment of chronic pain.

Novel approaches to targeting glutamate receptors for the treatment of chronic pain: review article

Glutamatergic mechanisms are implicated in acute and chronic pain, and there is a great diversity of glutamate receptors that can be used as targets for novel analgesics. Some approaches, e.g. NMDA receptor antagonism, have been validated clinically, however, the central side-effects have remained the main problem with most compounds. Recently, some novel approaches have been explored as new compounds targeting some modulatory sites at the NMDA receptor (glycine(B) and NR2B-subtype selective antagonists), as well as kainate and metabotropic glutamate receptors, have been discovered. Many of these compounds have demonstrated efficacy in animal models of chronic pain, and some of them appear to have a reduced side-effect liability compared to clinically tested NMDA antagonists. These recent advances are reviewed in the present work.

P2X receptors and nociception

The potential importance for nociception of P2X receptors, the ionotropic receptors activated by ATP, is underscored by the variety of pain states in which this endogenous ligand can be released. Several important findings have been made recently indicating that P2X receptors can be involved in pain mechanisms both centrally and in the periphery. The roles of ATP at these two sites and the P2X receptor subtypes involved appear to be different. In the periphery, ATP can be released as a result of tissue injury, visceral distension, or sympathetic activation and can excite nociceptive primary afferents by acting at homomeric P2X(3) or heteromeric P2X(2/3) receptors. Centrally, ATP released from central afferent terminals or second order neurons can modulate neurotransmitter release or postsynaptically activate neurons involved in central nociceptive transmission, with P2X(2), P2X(4), P2X(6), and some other receptors being potentially involved. Evidence from in vivo studies suggests that peripheral ATPergic mechanisms are most important under conditions of acute tissue injury and inflammation whereas the relevance of central mechanisms appears to be more limited. Furthermore, the release of ATP and P2X receptor-mediated afferent activation appear to have been implicated in visceral and neuropathic pain; the importance of the ATPergic component in these states needs to be investigated further. Thus, peripheral P2X receptors, and homomeric P2X(3) and/or heteromeric P2X(2/3) receptors in particular, constitute attractive targets for analgesic drugs. The development of selective antagonists of these receptors, suitable for a systemic in vivo use although apparently difficult, may prove a useful strategy to generate analgesics with a novel mechanism of action.

NMDA receptor antagonists as analgesics: focus on the NR2B subtype

Ifenprodil and a group of related compounds are selective antagonists of NR2B-containing NMDA receptors. These compounds are antinociceptive in a variety of preclinical pain models and have a much lower side-effect profile compared with other NMDA receptor antagonists. It remains unclear whether the improved safety of these compounds is due to their subtype selectivity or to a unique mode of inhibition of the receptor. Human trials have so far confirmed the good tolerability of these subtype-selective NMDA receptor antagonists; however, whether they are as effective as other NMDA receptor antagonists in pain patients remains to be demonstrated.

Supraspinal vs spinal sites of the antinociceptive action of the subtype-selective NMDA antagonist ifenprodil

The N-methyl-D-aspartate (NMDA) antagonist ifenprodil and several structurally related compounds are highly selective for the NR2B-containing receptor subtype. This selectivity could provide an explanation for the reported difference of the analgesic and side-effect profile of ifenprodil-like compounds from other NMDA antagonists. In this work, we have queried if the ifenprodil-induced antinociception can be attributed to the block of NMDA receptors in the spinal cord. Ifenprodil and some other NMDA antagonists (MK-801, memantine) were tested in a model of inflammatory pain (Randall-Selitto) in rats. The in vivo NMDA antagonism was assessed in anaesthetised rats on responses of spinal dorsal horn (DH) neurones to iontophoretic NMDA and in the model of single motor unit (SMU) wind-up. Ifenprodil, MK-801 and memantine dose-dependently increased nociceptive thresholds in the Randall-Selitto model. Antinociceptive doses of the channel blockers selectively antagonised NMDA responses of DH neurones and inhibited wind-up. In contrast, antinociceptive doses of ifenprodil did not show any NMDA antagonism in electrophysiological tests. Although ifenprodil did not inhibit the SMU responses to noxious stimuli in spinalised rats, it markedly and dose-dependently inhibited nociceptive SMU responses in sham-spinalised rats. These results argue against the spinal cord being the principal site of antinociceptive action of ifenprodil; supraspinal structures seem to be involved in this effect.

The N-methyl-D-aspartate antagonistic and opioid components of d-methadone antinociception in the rat spinal cord

The d-enantiomer of the opioid methadone is a weak opioid with low micromolar affinity to the N-methyl-D-aspartate (NMDA) receptor. We have investigated the antinociception and in vivo NMDA antagonism after systemic administration of d-methadone in the rat spinal cord. d-Methadone caused antinociception in the Randall-Selitto model of inflammatory pain and inhibited the responses of hindlimb single motor units to noxious electrical and mechanical stimulation (ED(50) 6.6, 6.8 and 7.2 mg/kg intravenous (i.v.), respectively); the wind-up of these responses was only inhibited at the dose almost completely abolishing the baseline responses. d-Methadone inhibited the activity of spinal dorsal horn neurones evoked by both iontophoretic NMDA and (R, S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA, ED(50) 5.7 and 8.2 mg/kg i.v., respectively). After pre-treatment with naloxone, d-methadone was unable to inhibit nociception in the Randall-Selitto model, the NMDA- or AMPA-evoked neuronal activity or the motoneurone wind-up. Thus, in the antinociceptive dose range, the NMDA antagonism does not appear to contribute to the mechanism of d-methadone antinociception.

Antinociception and (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid antagonism by gabapentin in the rat spinal cord in vivo

Gabapentin, a novel anticonvulsant and analgesic with an unknown mechanism of action, was tested on spinal dorsal horn neurone activity evoked by iontophoretically applied N-methyl-D-aspartic acid (NMDA) and (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and on nociceptive responses of single motor units (SMU) in anaesthetised rats. Gabapentin (10-215 mg/kg, i.v.) dose-dependently and selectively inhibited AMPA-evoked neuronal responses (ED50 106+/-24 mg/kg); no effect on NMDA-evoked activity was observed. In the same dose-range, gabapentin (10-215 mg/ kg, i.v.) dose-dependently reduced SMU responses to noxious electrical and mechanical stimulation. We conclude that gabapentin acts as an AMPA antagonist in the rat spinal cord, and that this mechanism is likely to substantially contribute to the antinociceptive effect of the drug.

Effects of NMDA receptor antagonists on nociceptive responses in vivo: comparison of antagonists acting at the glycine site with uncompetitive antagonists

We have shown that members of a new series of tricyclic pyridophthalazine diones, defined as glycineB site NMDA antagonists in vitro, are selective and systemically active NMDA antagonists in vivo. In electrophysiological tests in alpha-chloralose anaesthetised rats, these compounds reduced nociceptive reflex responses. In conscious rats they displayed analgesic properties. These glycineB antagonists were compared electrophysiologically with several uncompetitive NMDA channel blockers. The degree of voltage dependence previously reported in vitro related to the effectiveness of the agents against different amplitude nociceptive responses of spinal cord neurones in vivo.

Actions of kainate and AMPA selective glutamate receptor ligands on nociceptive processing in the spinal cord

Kainate receptors expressing the GluR5 subunit of glutamate receptor are present at high levels on small diameter primary afferent neurones that are considered to mediate nociceptive inputs. This suggests that GluR5 selective ligands could be novel analgesic agents. The role of kainate receptors on C fibre primary afferents has therefore been probed using three compounds that are selective for homomeric GluR5 receptors. The agonist, ATPA, and the antagonists, LY294486 and LY382884, have been tested in four models of nociception: responses evoked by noxious stimulation of the periphery have been recorded electrophysiologically (1) from hemisected spinal cords from neonatal rats in vitro, (2) from single motor units in adult rats in vivo, (3) from dorsal horn neurones in adult rats in vivo, and (4) in hotplate tests with conscious mice. In some protocols comparisons were made with the AMPA selective antagonist GYKI 53655. The agonist ATPA reduced nociceptive reflexes in vitro, but failed to have effects in vivo. In all tests, the GluR5 antagonists reduced nociceptive responses but only at doses that also affected responses to exogenous AMPA. The AMPA antagonist reduced nociceptive responses at doses causing relatively greater reductions of responses to exogenous AMPA. The results indicate that GluR5 selective ligands do reduce spinal nociceptive responses, but they are not strongly analgesic under these conditions of acute nociception.

Coupling of sympathetic and somatic motor outflows from the spinal cord in a perfused preparation of adult mouse in vitro

1. The relationship between sympathetic and somatic motor outflows from thoraco-lumbar spinal cord was investigated in a novel arterially perfused trunk-hindquarters preparation of adult mouse. 2. Ongoing activity was present in both somatic motor (obturator, sciatic or femoral nerves) and sympathetic outflows (either renal nerve or abdominal sympathetic chain). Sympathetic activity was rhythmic with bursts frequencies of 0.6-2.2 Hz. No obvious rhythmic activity was found in the somatic motor outflow. There were periods during which sympathetic and somatic motor activity were correlated. 3. Addition of NMDA (20-80 microM) to the perfusate elicited repetitive burst discharges in the somatic motor outflow which were sometimes rhythmic. The frequency of these burst discharges/rhythmic activity varied between preparations but in all cases increased with increasing NMDA concentration. 4. NMDA induced burst discharges in the sympathetic outflow. This bursting activity was of the same frequency as the somatic motor outflow and the two were coupled as revealed by correlation analysis. Periods of coupling persisted for up to 3 min. 5. Administration of hexamethonium (300 microM), to block sympathetic ganglionic transmission, had no effect on the somatic motor activity but severely attenuated sympathetic nerve discharge. 6. The thoraco-sacral cord therefore has the neuronal machinery necessary for generating and coupling activity in somatic motor and sympathetic outflows. Our findings indicate a dynamic control over the degree of coupling. We discuss that the synchronization of these neural outflows reflects either coupling between two independent mechanisms or the presence of a common synaptic driver impinging on both somatic motor and sympathetic neurones.

An arterially-perfused trunk-hindquarters preparation of adult mouse in vitro

We describe a preparation of arterially-perfused spinal cord with attached hindquarters, taken from adult mouse. This is the first preparation of adult mammalian spinal cord tissue to have the advantages of an in vitro approach whilst retaining intact intraspinal circuitry, sensory inputs, and somatic and sympathetic segmental outputs. The functional integrity of the preparation has been demonstrated by the motor and sympathetic reflexes that can readily be evoked by peripheral noxious thermal, mechanical and electrical stimuli, and also by bladder distension. The mechanical stability of the preparation allows intracellular recordings to be made from spinal dorsal or ventral horn neurones. The intact connectivity permits synaptic responses to be evoked by stimulation of functionally-defined peripheral sensory receptors. The preparation is relatively quick to set up and remains viable for more than 6 h. This model offers the opportunity to perform complex electrophysiological and pharmacological studies on functionally characterised synaptic responses of mature spinal neurones. The choice of the mouse will furthermore permit studies to be performed on genetically mutant strains.

Stimulus intensity, cell excitation and the N-methyl-D-aspartate receptor component of sensory responses in the rat spinal cord in vivo

The importance of receptors for N-methyl-D-aspartate in synaptic plasticity and in triggering long-term pronociceptive changes is explained by their voltage-dependence. This suggests that their contribution to acute nociceptive responses would be determined both by the magnitude of synaptic input and by the level of background excitation. We have now examined the role of N-methyl-D-aspartate receptors in acute nociceptive transmission in the spinal cord. Drugs selectively affecting activity mediated by these receptors were tested on responses of dorsal horn neurons to noxious stimuli of different intensities and at different levels of ongoing spike discharge. The drugs used were the N-methyl-D-aspartate receptor channel blocker ketamine; the competitive antagonists, 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (D-CPP) and D-2-amino-5-phosphonopentanoic acid (D-AP5), and the positive modulator thyrotropin-releasing hormone. The activity of dorsal horn wide dynamic range neurons was recorded extracellularly in alpha-chloralose-anaesthetized spinalized rats. Their responses to noxious stimuli (pinch, heat and electrical) were monitored in parallel with responses to iontophoretic N-methyl-D-aspartate and (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA). Drugs were given i.v. or (D-AP5) iontophoretically. At doses that selectively inhibited responses to exogenous N-methyl-D-aspartate, ketamine (4 or 8, mean 5 mg/kg i.v.) reduced the nociceptive responses of the majority of the cells in deep dorsal horn. Ketamine also reduced wind-up of the responses to repetitive electrical stimulation. Ketamine (4 or 8 mg/kg). D-CPP (2 mg/kg), D-AP5 (iontophoretically) and thyrotrophin-releasing hormone (1 mg/kg) were tested on different magnitude nociceptive responses evoked by alternating intensities of noxious heat or pinch. In percentage terms, the less vigorous responses were affected by all four drugs as much as or more than the more vigorous responses. When background activity of neurones was enhanced by continuous activation of C-fibres with cutaneous application of mustard oil, ketamine was less effective against superimposed noxious pinch responses. Ongoing background activity was affected in parallel with evoked responses. When background discharge of the cells was maintained at a stable level with continuous ejection of kainate, neither the N-methyl-D-aspartate antagonists nor thyrotrophin-relasing hormone affected the responses to noxious pinch or heat, although responses to exogenous N-methyl-D-aspartate were still blocked. The wind-up of the electrical responses was, however, reduced by ketamine irrespective of the level of background activity. The results indicate that under these conditions in vivo, N-methyl-D-aspartate receptors mediate ongoing low-frequency background activity rather than phasic high-frequency nociceptive responses. The effects of N-methyl-D-aspartate antagonists and positive modulators on nociceptive responses are evidently indirect, being secondary to changes in background synaptic excitation. These results cannot be explained simply in relation to the voltage-dependence of N-methyl-D-aspartate receptor-mediated activity; other factors, such as modulation by neuropeptides, must be involved.

Thyrotropin-releasing hormone facilitates spinal nociceptive responses by potentiating NMDA receptor-mediated transmission

The interaction of thyrotropin-releasing hormone (TRH) with NMDA receptor-mediated responses has been investigated in alpha-chloralose-anaesthetized spinalized rats with respect to its relevance to spinal nociceptive transmission. The effects of TRH and of the uncompetitive NMDA antagonist ketamine were tested on responses of dorsal horn wide dynamic range neurones to noxious pinch, heat and electrical stimuli in parallel with those to iontophoretically applied N-methyl-D-aspartate (NMDA) and AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid). Tests with NMDA blocking doses of ketamine (4 mg/kg i.v.) demonstrated a variable NMDA receptor-mediated component of all synaptic responses. TRH (0.5-1 mg/kg i.v.) enhanced the responses to NMDA (but not AMPA) in parallel with an increase of responses to all noxious stimuli and the 'wind-up' component of the responses to repeated electrical stimulation. This potentiation was completely reversed by a subsequent administration of ketamine (4 mg/kg i.v.). The results indicate that TRH facilitates nociceptive transmission in the spinal dorsal horn via a selective positive modulation of NMDA receptor-mediated transmission.

Modulation of excitatory amino acid responses by tachykinins and selective tachykinin receptor agonists in the rat spinal cord

1. The effects of tachykinins and agonists selective for the three subtypes of neurokinin (NK) receptor have been tested on spinal neuronal responses both to the excitatory amino acids (EAAs) NMDA, AMPA and kainate, and to noxious heat stimuli. The agonists were applied by microiontophoresis in in vivo experiments in alpha-chloralose-anaesthetized, spinalized rats. 2. The NK1-selective agonist, GR 73632, enhanced responses to all three EAAs similarly, whilst the NK2-selective agonist, GR64349, reduced responses to AMPA and kainate without affecting those to NMDA, and the NK3 selective agonist, senktide, enhanced responses to AMPA and kainate. 3. The endogenous ligands substance P (SP) and neurokinin A (NKA) both enhanced responses to NMDA with little effect on responses to kainate, whereas neurokinin B (NKB) selectively enhanced responses to kainate without affecting those to NMDA. 4. The effects of GR73632 on EAA responses showed some differences between the dorsal and ventral horn, with more selectivity towards enhancement of NMDA responses in the ventral horn, but a smaller maximum effect. 5. Background activity was significantly enhanced by GR73632, GR64349, SP and NKA but not by senktide or NKB. GR73632 had the greatest effect on background firing, but this action was variable between cells and was related both to the location within the spinal cord and to the degree of spontaneous activity prior to GR73632 administration. 6. Responses to noxious heat were enhanced consistently only by NKA. 7. These data show that selective agonists for the tachykinin receptors are capable of modulating EAA responses differentially. SP, NKA and NKB appear to act via more than one receptor type when modulating EAA responses in vivo. This indicates that NK-EAA interactions can be more specific than suggested hitherto, with the combined actions at NKI and NK2 receptors biasing EAA responsiveness towards NMDA receptor mediated functions, whereas NK3 receptor activation would have the opposite effect. The physiological role of such interactions is likely to be complex.

Endogenous modulation of excitatory amino acid responsiveness by tachykinin NK1 and NK2 receptors in the rat spinal cord

1. The effects of selective tachykinin (neurokinin, NK) NK1 and NK2 receptor antagonists have been examined on spinal neurones in alpha-chloralose anaesthetized, spinalized rats. They were tested for effects on responses both to excitatory amino acids (EAA) and to noxious heat stimuli. They were also tested for their ability to reverse the actions of selective NK agonists. 2. The NK1-selective antagonists GR82334 (peptide) and CP-99,994 (non-peptide), when applied by microiontophoresis, both reduced responses to kainate > AMPA > NMDA. Intravenous CP-99,994 (3 mg kg-1) also reduced responses to kainate but had inconsistent effects on nociceptive responses. 3. GR82334, applied microiontophoretically, reduced the enhancement by the selective NK1 agonist, GR73632 of both responses to EAAs and background activity. Systemic CP-99,994 (< or = 10 mg kg-1) failed to reverse the effects of GR73632. 4. The selective peptide NK2 antagonist, GR103537, had no consistent effects on responses to EAAs when applied by iontophoresis. In contrast, the non-peptide NK2 antagonist, GR159897, administered systemically (0.5-2 mg kg-1, i.v.) enhanced responses to kainate (but not NMDA); responses to noxious heat were enhanced only weakly. 5. Iontophoretically-administered GR103537 attenuated the effects of the NK2 agonist GR64349, which selectively reduced responses to kainate compared to those to NMDA. Systemically administered GR159897 (< or = 2 mg kg-1, i.v.) caused little antagonism of the effects of GR64349. 6. The data indicate that under these conditions the non-peptide antagonists are not reliable at reversing the actions of selective NK agonists. 7. These results suggest that there is a tonic release of endogenous tachykinins that can modulate glutamatergic neurotransmission in the spinal cord. They provide further support for the hypothesis that release of endogenous NKs acting on NK1 and NK2 receptors can promote NMDA receptor mediated glutamatergic transmission.

A comparison of intravenous NBQX and GYKI 53655 as AMPA antagonists in the rat spinal cord

1. The effects of intravenous administration of two alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) antagonists were studied on responses of single neurones to iontophoretically applied excitatory amino acids. The tests were performed on spinal neurones in alpha-chloralose anaesthetized, spinalized rats. 2. Both the quinoxaline, NBQX (2-16 mg kg-1) and the 2,3-benzodiazepine, GYKI 53655 (2-8 mg kg-1) dose-dependently decreased responses to AMPA. 3. Both compounds were short acting, with half-recovery times of 15 min for NBQX and 7 min for GYKI 53655. 4. The selectivity for responses to AMPA over those to N-methyl-D-aspartate (NMDA) was significantly poorer for systemic NBQX than for either systemic GYKI 53655 or iontophoretic NBQX, suggesting that systemic NBQX may be converted to a less selective metabolite. 5. GYKI 53655 is therefore likely to be a more valuable tool than NBQX for the study of AMPA receptor-mediated processes in vivo.

AMPA receptors have an equal role in spinal nociceptive and non-nociceptive transmission

The relative roles of receptors for AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) in spinal nociceptive and non-nociceptive transmission were studied on dorsal horn wide dynamic range neurones in alpha-chloralose-anaesthetized spinalized rats. The effects of systemically administered competitive and non-competitive AMPA antagonists (the quinoxalinedione NBQX and the 2,3-benzodiazepine GYKI 53655) were examined on responses to peripheral noxious heat and non-noxious tap stimuli as well as to iontophoretic AMPA and N-methyl-D-aspartate (NMDA). Both NBQX and GYKI 53655 dose-dependently reduced responses to peripheral stimuli and to AMPA. GYKI 53655, the more selective antagonist of AMPA vs NMDA, decreased heat and tap responses to the same extent. The results indicate that AMPA receptors play a significant and equal, but not exclusive, role in mediating nociceptive and non-nociceptive spinal transmission.

Thyrotropin-releasing hormone (TRH)-induced facilitation of spinal neurotransmission: a role for NMDA receptors

Thyrotropin-releasing hormone (TRH) is known to enhance spinal reflexes and modulate NMDA receptors in supraspinal areas. We have investigated the relationship between TRH and NMDA receptors in the spinal cord of alpha-chloralose-anaesthetized spinalized rats. TRH was tested (a) on dorsal horn neurone responses to iontophoretic NMDA, AMPA and kainate and (b) on spinal reflexes evoked by noxious pinch and electrical stimulation (2 Hz) shown to involve NMDA receptor activation. TRH given i.v. (0.5 mg/kg) or iontophoretically selectively potentiated neuronal responses to NMDA. TRH (0.5-2 mg/kg) also dose-dependently increased single motor unit reflex responses. The NMDA antagonist ketamine (2 mg/kg i.v.) abolished these TRH effects; ketamine reduced single motor unit (SMU) reflex responses more effectively when administered after TRH than in pre-TRH control tests. These results indicate that TRH-induced facilitation of spinal sensory transmission involves NMDA receptor activation.

[The mechanisms of the participation of the peptidergic systems in regulating respiratory center function]

In experiments on different species of animals respiratory stimulating effects of naloxone, TRH and its analogue RGH 2202 during respiratory rhythmogenesis disturbances, evoked by hyperventilation of lungs, bleeding and intoxication with cyanides or opiates, were investigated. The role of opiate and NMDA receptors in respiratory centre function is discussed.

The NMDA-receptor antagonist dizocilpine (MK-801) suppresses the memory facilitatory action of thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) has been shown to improve memory both in animal amnesia models and in humans. In an earlier study we have found that respiratory stimulant action of TRH is mediated through the N-methyl-D-aspartate (NMDA) receptors. Since brain NMDA receptors are implicated in neuronal plasticity, we have investigated whether the non-competitive NMDA antagonist dizocilpine antagonizes the memory-facilitatory action of TRH. TRH (5 mg/kg i.p.) increased passive avoidance scores in the rats in both memory retention and retrieval tests. Dizocilpine (0.2 mg/kg i.p.) caused no significant performance changes. However, it completely blocked the improvement of retention caused by TRH and reversed its effect on retrieval. We conclude that the facilitatory effect of TRH on avoidance retention and retrieval may be mediated by NMDA receptors.

[Central nervous system stimulants as nonspecific antagonists of morphine-induced respiratory depression]

The respiratory effects of pymadin, amiridine, thyrotropin-releasing hormone (TRH), its analog RGH 2202, and pentetrazol and their interaction with morphine were studied in anesthesized rats. During intravenous injection or local application to the medulla oblongata, pymadin, amiridine, TRH and RGH 2202 were shown to enhance the respiratory activity of the diaphragm and to abolish its morphine-induced inhibition. TRH and RGH 2202 proved to be the most potent and safe antagonists of morphine-induced respiratory depression. These agents given in the doses sufficient to completely abolish morphine-induced respiratory depression unchanged its antinociceptive activity. Pentetrazol in the tested dose range failed to increase diaphragmatic respiratory activity or to eliminate its depression induced by morphine.

Stimulant effect of thyrotropin-releasing hormone and its analog, RGH 2202, on the diaphragm respiratory activity, and their antagonism with morphine: possible involvement of the N-methyl-D-aspartate receptors

Thyrotropin-releasing hormone (TRH) was reported to stimulate respiration and abolish the respiratory depressant effect of morphine-like analgesics. Some TRH analogs which have a diminished hormonal activity may be of interest as potential non-specific opioid antagonists. The mechanism of this effect of TRH and its analogs is still unclear. Thus, in the present work the respiratory stimulant effect of TRH and its analog RGH 2202 was studied in the urethane-anesthetized vagotomized artificially-ventilated rats. The integrated diaphragmatic electromyogram was used to evaluate the effects of the drugs. TRH and RGH 2202 administered either i.v. or directly onto the dorsal medullary surface significantly increased the respiratory activity of the diaphragm. TRH and RGH 2202 also effectively antagonized the diaphragm activity depression caused by morphine. The latency, time course and activity of RGH 2202 turned out to be close to those of TRH. The possible involvement of N-methyl-D-aspartate (NMDA) receptors in the mechanism of action of TRH and RGH 2202 was also investigated. It was shown that the non-competitive NMDA antagonists ketamine and MK-801 and the competitive antagonist D-amino-5-phosphonovalerate after local or i.v. administration prevented or discontinued the diaphragm activity stimulation by TRH and RGH 2202. Moreover, they blocked the antagonistic action of TRH and RGH 2202 on the morphine-induced diaphragm activity depression. Thus, we conclude, that TRH and RGH 2202 cause similar stimulant effects on the respiratory activity of the diaphragm and effectively antagonize its depression by morphine. These effects are likely to be mediated by the NMDA receptors located in the central respiratory structures.

[The characteristics of the interaction of the thyroliberin analog RGH 2202 and narcotic analgesics]

The effect of an thyroliberin analogue RGH 2202 on respiration inhibition induced by narcotic analgetics and on their antinociceptive action was studied in experiments on mice, rabbits and rats. RGH 2202 (10 mg/kg, intravenously) was found to exert no effect on the antinociceptive effect of morphine in mice and rabbits. Meanwhile, RGH 2202 in doses of 1 to 5 mg/kg intravenously abolished the respiration inhibition induced by fentanyl (0.02-0.07 mg/kg, intravenously) or morphine (10 mg/kg, intravenously) in rabbits and rats. After administration of RGH 2202 in doses of 10-40 mg/kg intravenously no significant changes of arterial blood pressure or heart rate were observed. The results obtained suggest that RGH 2202 may be used for relieving the inhibitory effects of narcotic analgetics on the respiratory centre during intoxication with these agents and in anesthesiological practice.

[The possible participation of N-methyl-D-aspartate receptors in the stimulating action on respiration of thyroliberin and its analog RGH 2202]

The effects of ketamine and an antagonist of N-methyl-D-aspartate receptors dibenzocyclohepteniline maleate (MK-801) on the changes in the bioelectrical activity of the diaphragm induced by thyroliberine and its analogue L-pyro-2-aminoadipil-L-leucyl-proline amide (RGH 2202) were studied in experiments on vagotomized urethane-anesthetized rats. It was shown that ketamine (5 mg/kg, intravenously) and MK-801 (1 mg/kg, intravenously) eliminate and prevent an increase of the index of pulmonary ventilation caused by thyroliberine (5 mg/kg, intravenously) and RGH 2202 (5 mg/kg, intravenously). The data obtained suggest the involvement of N-methyl-D-aspartate receptors in the mechanism of the stimulating effects of thyroliberine and its analogue on respiration.

[Neurotropic effects of tranquilizers administered intrathecally]

The effects of intrathecally administered benzodiazepines diazepam, flurazepam and flunitrazepam on the pain thresholds in rats were studied by using chronically implanted catheters. Despite benzodiazepine-induced marked muscle relaxation and sedative effect, benzodiazepines produced no significant changes in the thresholds during tests of tail flick and paw compression whereas morphine was effective in both tests. Experiments with 14C-diazepam and 3H-flunitrazepam showed that after intrathecal administration these substances rapidly enter the brain. It was found that 10-15 minutes after intrathecal administration 3H-flunitrazepam concentration in the frontal parts of the brain is 1/4 of its concentration in the lumbar spinal cord. The data suggest that these benzodiazepines possess no antinociceptive activity at intrathecal administration to rats and that way of administration fails to prevent their action on the brain.