The effects of the TRPV1 receptor antagonist SB-705498 on trigeminovascular sensitisation and neurotransmission.

This report examines the effect of the transient receptor potential vanilloid 1 receptor antagonist SB-705498 on neurotransmission and inflammation-induced sensitisation in the trigeminovascular sensory system. A single-neuron electrophysiological animal model for neurovascular head pain was used to evaluate dural and facial noxious inputs and the effects of SB-705498 administered by intravenous (i.v.) injection. Electrical and mechanical stimulation of the dura mater and the facial skin activated second-order neurons in the trigeminal nucleus caudalis of cats, with A-delta latencies. Intravenous injection of SB-705498 (2 mg kg(-1)) produced a slowly developing and long-lasting suppression of responses to dural and skin stimulation. Maximum suppression occurred by 1 h and reached 41% for dura and 24% for skin. Intravenous injection of drug vehicle did not produce significant suppression of responses to stimulation of either dura or skin. Intravenous injection of SB-705498 produced a brief and small rise in blood pressure and dural blood flow, which both returned to normal before suppression of the responses to stimulation became manifest. Application of "inflammatory soup" to the dura mater produced a pronounced increase in dural blood flow and induced a slowly developing increase in the responses of neurons to both electrical and mechanical stimulations of their facial and dural receptive fields. This sensitisation reached a maximum in 60-90 min, at which time responses had risen to approximately twice that of control levels seen before the application of inflammatory soup. Intravenous injection of SB-705498 subsequent to the development of sensitisation produced a slowly developing, prolonged and statistically significant reversal of the sensitisation induced by inflammatory soup. Maximum reversal of sensitisation to electrical stimulation occurred by 150-180 min, when responses had fallen to, or below, control levels. At 70-85 min following injection of SB-705498, the responses of previously sensitised neurons to mechanical stimulation of dura mater and facial receptive field had also returned to near control levels. SB-705498 was also able to prevent the development of sensitisation; application of inflammatory soup to the dura mater induced a slowly developing increase in the responses of neurons to electrical stimulation of the skin and dura mater in cats which had received an i.v. injection of vehicle for SB-705498 but not in cats which had received the active drug. Blood levels of SB-705498 were maximal immediately following i.v. injection and declined over the following 2 h. Significant brain levels of SB-705498 were maintained for up to 9 h. These results suggest that SB-705498 may be an effective suppressant and reversal agent of the sensitisation to sensory input which follows inflammation in the trigeminovascular sensory distribution but may not be particularly useful in blocking primary pain processes such as migraine headache. SB-705498 could thus potentially prevent, modify or reverse the cutaneous trigeminal allodynia seen in certain migraine conditions, especially "transformed" migraine.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Novel systemically active antagonists of the glycine site of the N-methyl-D-aspartate receptor: electrophysiological, biochemical and behavioral characterization.

A series of novel tricyclic pyrido-phthalazine-dione derivatives was tested for antagonistic effects at the strychnine-insensitive modulatory site of the N-methyl-D-aspartate (NMDA) receptor (glycineB). All compounds displaced [3H]MDL-105,519 binding to rat cortical membranes with IC50 values of between 90 nM and 3.6 microM. In patch-clamp experiments, steady-state inward current responses of cultured hippocampal neurons to NMDA (200 microM, glycine 1 microM) were antagonized by these same compounds with IC50 values of 0.14 to 13.8 microM. The antagonism observed was typical for glycineB antagonists, i.e., they induced desensitization and their effects were not use or voltage dependent. Moreover, increasing concentrations of glycine were able to decrease their apparent potency. Much higher concentrations (>100 microM) were required to antagonize alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced currents. They were potent, systemically active NMDA receptor antagonists in vivo against responses of single neurons in the rat spinal cord to microelectrophoretic application of NMDA with ID50 values in the low milligram per kilogram i.v. range. They also inhibited pentylenetetrazol-, NMDA- and maximal electroshock-induced convulsions in mice with ED50 values ranging from 8 to 100 mg/kg i.p. The duration of anticonvulsive action was rather short but was prolonged by the organic acid transport inhibitor probenecid (200 mg/kg). The agents tested represent a novel class of systemically active glycineB antagonists with greatly improved bioavailability.

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.