Effect of the tachykinin NK1 receptor antagonists, RP 67580 and SR 140333, on electrically-evoked substance P release from rat spinal cord

In memory of Norman Bowery (1944-2016)

This article is in memory of Professor Norman Bowery (1944-2016). Norman was a pharmacologist who spent most of his career researching the pharmacology of γ-aminobutyric acid (GABA). He discovered a novel metabotropic receptor subtype, GABA_B, that is pharmacologically, and structurally different from the original ionotropic receptor now designated as GABA_A. In his research he also studied the neurotransmitters glutamate and substance P, two molecules whose release in parts of the spinal cord is inhibited by baclofen a GABA_B receptor agonist. Norman was interested in the therapeutic potential of interacting with the GABA_B receptor, in particular spasticity, pain and absence epilepsy.

Receptor distribution studies

Receptor distribution studies have played a key role in the characterization of receptor systems (e.g. GABA_B, NMDA (GluNRs), and Neurokinin 1) and in generating hypotheses to exploit these systems as potential therapeutic targets. Distribution studies can provide important information on the potential role of candidate receptors in normal physiology/disease and alert for possible adverse effects of targeting the receptors. Moreover, they can provide valuable information relating to quantitative target engagement (e.g. % receptor occupancy) to drive mechanistic pharmacokinetic/pharmacodynamic (PK/PD) hypotheses for compounds in the Drug Discovery process. Finally, receptor distribution and quantitative target engagement studies can be used to validate truly translational technologies such as PET ligands and pharmacoEEG paradigms to facilitate bridging of the preclinical/clinical interface and thus increase probability of success.

NK1 receptor blockade is ineffective in improving outcome following a balloon compression model of spinal cord injury

The neuropeptide substance P (SP) is a well-known mediator of neurogenic inflammation following a variety of CNS disorders. Indeed, inhibition of SP through antagonism of its receptor, the tachykinin NK1 receptor, has been shown to be beneficial following both traumatic brain injury and stroke. Such studies demonstrated that administration of an NK1 receptor antagonist reduced blood-brain-barrier permeability, edema development and improved functional outcome. Furthermore, our recent studies have demonstrated a potential role for SP in mediating neurogenic inflammation following traumatic spinal cord injury (SCI). Accordingly, the present study investigates whether inhibition of SP may similarly play a neuroprotective role following traumatic SCI. A closed balloon compression injury was induced at T10 in New Zealand White rabbits. At 30 minutes post-injury an NK1 receptor antagonist was administered intravenously. Animals were thereafter assessed for blood spinal cord barrier (BSCB) permeability, spinal water content (edema), intrathecal pressure (ITP), and histological and functional outcome from 5 hours to 2 weeks post-SCI. Administration of an NK1 receptor antagonist was not effective in reducing BSCB permeability, edema, ITP, or functional deficits following SCI. We conclude that SP mediated neurogenic inflammation does not seem to play a major role in BSCB disruption, edema development and consequential tissue damage seen in acute traumatic SCI. Rather it is likely that the severe primary insult and subsequent hemorrhage may be the key contributing factors to ongoing SCI injury.

Botulinum toxin B in the sensory afferent: transmitter release, spinal activation, and pain behavior

We addressed the hypothesis that intraplantar botulinum toxin B (rimabotulinumtoxin B: BoNT-B) has an early local effect upon peripheral afferent terminal releasing function and, over time, will be transported to the central terminals of the primary afferent. Once in the terminals it will cleave synaptic protein, block spinal afferent transmitter release, and thereby prevent spinal nociceptive excitation and behavior. In mice, C57Bl/6 males, intraplantar BoNT-B (1 U) given unilaterally into the hind paw had no effect upon survival or motor function, but ipsilaterally decreased: (1) intraplantar formalin-evoked flinching; (2) intraplantar capsaicin-evoked plasma extravasation in the hind paw measured by Evans blue in the paw; (3) intraplantar formalin-evoked dorsal horn substance P (SP) release (neurokinin 1 [NK1] receptor internalization); (4) intraplantar formalin-evoked dorsal horn neuronal activation (c-fos); (5) ipsilateral dorsal root ganglion (DRG) vesicle-associated membrane protein (VAMP); (6) ipsilateral SP release otherwise evoked bilaterally by intrathecal capsaicin; (7) ipsilateral activation of c-fos otherwise evoked bilaterally by intrathecal SP. These results indicate that BoNT-B, after unilateral intraplantar delivery, is taken up by the peripheral terminal, is locally active (blocking plasma extravasation), is transported to the ipsilateral DRG to cleave VAMP, and is acting presynaptically to block release from the spinal peptidergic terminal. The observations following intrathecal SP offer evidence for a possible transsynaptic effect of intraplantar BoNT. These results provide robust evidence that peripheral BoNT-B can alter peripheral and central terminal release from a nociceptor and attenuate downstream nociceptive processing via a presynaptic effect, with further evidence suggesting a possible postsynaptic effect.

A role for the sensory neuropeptide calcitonin gene-related peptide in endothelial cell proliferation in vivo

BACKGROUND AND PURPOSE

We have tested the hypothesis that calcitonin gene-related peptide (CGRP) is a mediator of capsaicin-induced angiogenesis in vivo.

EXPERIMENTAL APPROACH

In a series of experiments, the knee joints of rats were injected with CGRP, capsaicin or vehicle control. Groups of animals (n=6) were treated with the CGRP receptor antagonist BIBN4096BS and/or the NK₁ receptor antagonist SR140333. Endothelium, proliferating endothelial cell nuclei and macrophages were identified 24 h later in the synovium by immunohistochemistry and quantified by image analysis. mRNA for the receptors for CGRP and adrenomedullin were sought in normal and inflamed rat and human synovia using RT-PCR.

KEY RESULTS

Intra-articular CGRP injection increased the endothelial cell proliferation index, whereas macrophage infiltration and knee joint diameters were similar to saline-injected controls. CGRP-induced endothelial cell proliferation was dose-dependently inhibited by BIBN4096BS. mRNA for adrenomedullin and the CGRP receptor subunits were detected in normal and inflamed human and rat synovia. In capsaicin-induced synovitis, the increased endothelial cell proliferation index was partially blocked by administration of NK₁ or CGRP antagonists individually and was reduced to the level of saline controls by coadministration of both receptor antagonists.

CONCLUSIONS AND IMPLICATIONS

These data support the hypothesis that CGRP stimulates angiogenesis in vivo directly by activating CGRP receptors. Capsaicin-induced endothelial cell proliferation was completely blocked by coadministration of CGRP and NK₁ receptor antagonists, indicating that both CGRP and substance P may contribute to angiogenesis in this model of synovitis.

Enhanced nerve-stimulated muscarinic and neurokinin contractions of ileum from streptozotocin guinea-pigs

Diabetes mellitus can lead to neuropathy of enteric neurons, resulting in abnormal gut motility. These studies investigated voltage-dependent contributions of muscarinic M₃ receptor activation by acetylcholine and neurokinin NK₁ receptor activation by neurokinins to nerve-stimulated contractions of longitudinal ileal strips from STZ guinea-pigs, a type 1 diabetic model with insulin deficiency, but mild hyperglycaemia. Contractions to bethanechol, substance P methyl ester, and nerve stimulation were greater in diabetic as compared to control ileum. The muscarinic M₃ receptor antagonist 4-DAMP at lower voltages and the neurokinin NK₁ receptor antagonist SR140333 at higher voltages, but not the neurokinin NK₁ receptor antagonist CP-96,345, were more effective at inhibiting nerve-stimulated immediate peak contractions and total areas of contraction of ileum from diabetic as compared to control animals. For diabetic ileum, voltage-dependent increases in the areas of nerve-stimulated contraction were observed in the presence of 4-DAMP and CP-96,345 but not SR140333. At low voltages only, nerve-stimulated release of acetylcholine was greater from diabetic as compared to control ileum. Fluorescence intensity of tachykinin-like immunoreactivity was increased in ileal myenteric ganglia from diabetic as compared to control animals. In diabetic guinea-pigs, stronger ileal nerve-stimulated contractions reflected increased release of acetylcholine at lower voltages and tachykinins at higher voltages, as well as increased sensitivity of smooth muscle M₃ and NK₁ receptors to acetylcholine and tachykinins. Hypoinsulinaemia may be a primary contributor to intestinal motility dysfunction in type 1 diabetes mellitus.

Expression of tachykinin receptors inXenopus oocytes injected with poly (A)(+) RNA from cat dorsal root ganglion

The expression of the types of tachykinin receptors in the dorsal root ganglion (DRG) neurons by means ofXenopus oocyte expressing system was studied. Poly(A)(+) RNAs were extracted from cat cervical and lumbar DRG. Two days after injection of Poly (A)(+) RNAs, the oocytes were recorded with the two-electrode voltage clamp technique. In the oocytes injected with DRG poly(A)(+) RNA, [Sar(9), Met(O(2))(11)]-substance P(Sar -SP, 1 mumol/L), neurokinin A (NKA, 1 mumol/L) or [beta-Ala(8)]-neurokinin A((4-10)) (Ala-NKA, 1 mumol/L) produced an inward current comprising a rapid spike and a long sustained oscillatory component for several minutes. Sar-SP induced response was blocked by NK-1 antagonist L-668, 169 (1 mumol/L), but not by NK-2 antagonist L-659, 877(1mumol/L). In contrast, Ala-NKA and NKA responses were only blocked by L-659, 877. The oocytes injected with DH Poly(A)(+)RNA also responded to Sar-SP and NKA with similar inward currents, which were selectively blocked by L-668, 169 and L-659, 877, respectively. These tachykinins-induced responses had a potent desensitization. The present data indicate expression of NK-1 and NK-2 receptors in DRG neurons, suggesting that there may be tachykinin autoreceptors on the nociceptive primary afferent terminals.

Modulation of basal and stress-induced amygdaloid substance P release by the potent and selective NK1 receptor antagonist L-822429

It has been shown that anxiety and stress responses are modulated by substance P (SP) released within the amygdala. However, there is an important gap in our knowledge concerning the mechanisms regulating extracellular SP in this brain region. To study a possible self-regulating role of SP, we used a selective neurokinin-1 (NK1) receptor antagonist to investigate whether blockade of NK1 receptors results in altered basal and/or stress-evoked SP release in the medial amygdala (MeA), a critical brain area for a functional involvement of SP transmission in enhanced anxiety responses induced by stressor exposure. In vitro binding and functional receptor assays revealed that L-822429 represents a potent and selective rat NK1 receptor antagonist. Intra-amygdaloid administration of L-822429 via inverse microdialysis enhanced basal, but attenuated swim stress-induced SP release, while the low-affinity enantiomer of L-822429 had no effect. Using light and electron microscopy, synaptic contacts between SP-containing fibres and dendrites expressing NK1 receptors was demonstrated in the medial amygdala. Our findings suggest self-regulatory capacity of SP-mediated neurotransmission that differs in the effect on basal and stress-induced release of SP. Under basal conditions endogenous SP can serve as a signal that tonically inhibits its own release via a NK1 receptor-mediated negative feedback action, while under stress conditions SP release is further facilitated by activation of NK1 receptors, likely leading to high local levels of SP and activation of receptors to which SP binds with lower affinity.

Brain-derived neurotrophic factor induces NMDA receptor subunit one phosphorylation via ERK and PKC in the rat spinal cord

Brain-derived neurotrophic factor (BDNF) is involved in the modulation of synaptic transmission in the spinal cord, and several circumstantial lines of evidence suggest that it has the ability to modulate the activity of the NMDA receptor. Here we dissect the signalling mechanisms by which BDNF exerts its neuromodulatory role on the NMDA receptor subunit 1 (NR1). Using a preparation of adult isolated dorsal horn with dorsal roots attached, we found that electrical stimulation of roots induced a concomitant release of BDNF and an increased phosphorylation of NR1, which was partly prevented by the BDNF sequestering molecule, TrkB-IgG. Using a second approach in vitro, we confirmed that both exogenous glutamate and BDNF (but not other neurotrophins) were able to induce NR1 phosphorylation, in particular at residue Ser-897. NR1 phosphorylation induced by BDNF was blocked by a TrkB inhibitor, an ERK inhibitor and a PKC inhibitor but not a PKA inhibitor. Activation of PKC using exogenous PMA also led to NR1 phosphorylation. Together these data suggest that BDNF modulates the activity of the receptor by phosphorylation via the kinases ERK and PKC.

(S)-AMPA inhibits electrically evoked calcitonin gene-related peptide (CGRP) release from the rat dorsal horn: reversal by cannabinoid receptor antagonist SR141716A

Previous studies in the hippocampus and cerebellum demonstrate that depolarisation of postsynaptic neurones stimulates the rapid synthesis and release of an endocannabinoid that retrogradely interacts with pre-synaptic CB(1) to modulate neurotransmitter release. This study evaluated whether depolarisation of second order neurones in the dorsal horn of the spinal cord by the AMPA receptor agonist, (S)-AMPA, would modulate sensory neurotransmission via release of endocannabinoids. Using an isolated rat dorsal horn with dorsal root attached in vitro preparation the release of calcitonin gene-related peptide (CGRP) after electrical stimulation of the dorsal roots was measured. Superfusion of either WIN55,212-2 (1 microM) or (S)-AMPA (1 microM) significantly attenuated CGRP release in a CB(1)-dependent manner (SR141716A, 5 microM). This provides indirect pharmacological evidence for an AMPA-evoked release of endogenous cannabinoids inhibiting peptide release from primary afferent neurons. This study confirms that CGRP release from the dorsal horn is modulated via CB(1) activation. Furthermore a depolarising stimulus also modulates CGRP release, potentially via the release of endogenous cannabinoids.

Basal and activity-induced release of substance P from primary afferent fibres in NK1 receptor knockout mice: evidence for negative feedback

The concept that NK1 receptors are located pre-junctionally on substance P (SP)-containing nerves, acting as autoreceptors to inhibit SP release, has been suggested, but remains a controversial issue. To further investigate the existence of this receptor on central and peripheral terminals of primary afferent fibres, NK1 receptor knockout mice and an NK1 receptor antagonist were used in nerve-attached tissue preparations. These were the isolated dorsal horn of the spinal cord with dorsal roots attached, and the hairy skin of the hind paw with attached saphenous nerve. The results reveal that in the dorsal horn preparation, basal release of SP is significantly higher in NK1(-/-) mice than NK1(+/+) mice (P<0.05, n=7 mice/strain). However, a difference in SP release evoked in the dorsal horn by electrical stimulation of the dorsal roots or capsaicin application was not observed. In contrast, antidromic electrical stimulation of the saphenous nerve caused a substantially greater release of SP in the skin of NK1(-/-) mice than in NK1(+/+) mice (P<0.05, n=5 to 6 mice/strain). These results provide evidence for the existence of NK1 autoreceptors on sensory nerves in skin, which may be relevant to the modulation of their peripheral pathophysiological effector functions.

Effect of peptidases on the ability of exogenous and endogenous neurokinins to produce neurokinin 1 receptor internalization in the rat spinal cord

The ability of peptidases to restrict neurokinin 1 receptor (NK1R) activation by exogenously applied or endogenously released neurokinins was investigated by measuring NK1R internalization in rat spinal cord slices. Concentration-response curves for substance P and neurokinin A were obtained in the presence and absence of 10 microm thiorphan, an inhibitor of neutral endopeptidase (EC 3.4.24.11), plus 10 microm captopril, an inhibitor of dipeptidyl carboxypeptidase (EC 3.4.15.1). These inhibitors significantly decreased the EC50 of substance P to produce NK1R internalization from 32 to 9 nm, and the EC50 of neurokinin A from 170 to 60 nm. Substance P was significantly more potent than neurokinin A, both with and without these peptidase inhibitors. In the presence of peptidase inhibitors, neurokinin B was 10 times less potent than neurokinin A and 64 times less potent than substance P (EC50=573 nm). Several aminopeptidase inhibitors (actinonin, amastatin, bacitracin, bestatin and puromycin) failed to further increase the effect of thiorphan plus captopril on the NK1R internalization produced by 10 nm substance P. Electrical stimulation of the dorsal root produced NK1R internalization by releasing endogenous neurokinins. Thiorphan plus captopril increased NK1R internalization produced by 1 Hz stimulation, but not by 30 Hz stimulation. Therefore, NEN and DCP restrict NK1R activation by endogenous neurokinins when they are gradually released by low-frequency firing of primary afferents, but become saturated or inhibited when primary afferents fire at a high frequency.

Inhibition of neurokinin-1-substance P receptor and prostanoid activity prevents and reverses the development of morphine tolerance in vivo and the morphine-induced increase in CGRP expression in cultured dorsal root ganglion neurons

Chronic treatment with opioid drugs such as morphine leads to the development of tolerance, which manifests as a loss of drug potency. The mechanisms underlying this phenomenon are poorly understood, but recent evidence suggests that increased activity of nociceptive sensory transmitters [calcitonin gene-related peptide (CGRP) and substance P] and other signalling messengers (prostaglandins) contribute to its development. Chronic intrathecal morphine administration to rats for 7 days produced analgesic tolerance. Co-administration of SR140333, a selective substance P receptor (neurokinin-1) antagonist, or nimesulide, a cyclooxygenase-2-selective inhibitor, augmented the acute effects of morphine, prevented morphine tolerance and reversed established tolerance. In cultured adult dorsal root ganglion neurons, exposure to morphine for 5 days increased the number of neurons expressing CGRP immunoreactivity. Co-exposure with the peptide CGRP receptor antagonist CGRP8-37, SR140333 or nimesulide prevented the morphine-induced increase in the expression of CGRP immunoreactivity. Additionally, BIBN4096BS, a nonpeptide CGRP receptor antagonist, stereoselectively produced similar effects. In summary, this investigation demonstrates that activity of CGRP and substance P contributes to both the induction and expression of opioid analgesic tolerance. Additionally, it highlights the involvement of prostaglandins generated by spinal cyclooxygenase-2 activity in the genesis of opioid tolerance. The neuropeptide and prostanoid activity contributing to tolerance is expressed at the level of the primary afferents terminating in the spinal cord. The combination of opioids with agents that block this activity may represent a useful strategy for the prevention as well as the reversal of clinical opioid tolerance.

Substance P content in the cerebrospinal fluid and fluid perfusing cerebral ventricles during elicitation and inhibition of trigemino-hypoglossal reflex in rats

The aim of this study was to establish whether tooth pulp and periaqueductal central gray (PAG) stimulation affects the release of substance P (SP) into the fluid perfusing the cerebral ventricles in rats. The content of substance P in the cerebrospinal fluid and fluid perfusing cerebral ventricles was determined during incisor pulp stimulation with electrical impulses inducing nociceptive trigemino-hypoglossal reflex and then during inhibition of the reflex by stimulation of PAG. Perfusion of the cerebral ventricles was carried out using artificial cerebrospinal fluid (aCSF). SP-like immunoreactivity (SP-LI) was determined in the samples by radioimmunoassay. Samples were collected in four groups: first group-cerebrospinal fluid (CSF); second group-aCSF perfusates without stimulation; third group-aCSF perfusates during incisor pulp stimulation; fourth group-aCSF perfusates during incisor pulp stimulation and simultaneous inhibition of trigemino-hypoglossal reflex by PAG stimulation. It was shown that incisor pulp stimulation led to the increased release of SP-LI into the fluid perfusing cerebral ventricles. Stimulation of PAG reduced the release of SP-LI into the cerebro-ventricular system to the values obtained before the tooth pulp stimulation. The results indicate that PAG significantly inhibits the release of SP-LI into the rat cerebral ventricular system and may be involved in the inhibition of trigemino-hypoglossal reflex.

Evidence of a role for NK1 and CGRP receptors in mediating neurogenic vasodilatation in the mouse ear

The aims of this study were to develop a technique to measure blood flow in the mouse ear and to investigate the nature of the vasodilator mediator(s) involved in the response to capsaicin. The response to capsaicin, applied topically, was investigated in anaesthetized CD1 or Sv129+C57BL/6 wild-type (+/+) or NK(1) receptor knockout mice (-/-). Blood flow was assessed by laser Doppler flowmetry and oedema formation by (125)I-albumin accumulation. Capsaicin induced significant increases in blood flow (0.2 - 200 microg in 20 microl) and oedema (2 - 200 microg in 20 microl). The oedema response was absent in NK(1)-/- mice and NK(1)+/+mice treated with the selective NK(1) receptor antagonist SR140333 (480 nmol kg(-1)) as expected. Furthermore, the capsaicin-evoked increase in blood flow was significantly potentiated in the knockout mice (203% of wild-type response, P<0.05) and wild-type mice treated with SR140333 (201%, P<0.05). The CGRP receptor antagonist CGRP(8 - 37) (400 nmol kg(-1)) had no effect on capsaicin-induced blood flow in NK(1)+/+mice but abolished the increased blood flow to capsaicin in NK(1)-/-, and NK(1)+/+wild-type mice pre-treated with SR140333. The results indicate that neurogenic vasodilatation can be measured in the mouse ear. The capsaicin-induced increased blood flow involves activation of, and possible interactions between, both NK(1) and CGRP(1) receptors.

Denervation and NKI receptor block modulate stimulated CGRP and PGE2 release from rat skin

We investigated the possible neurogenic origin of prostaglandin E2 (PGE2) in the rat skin, in vitro. The hairy skin of one hindpaw was denervated and one week later the dorsal hindpaws were skinned to study the release of calcitonin gene-related peptide (CGRP) and PGE2 using the EIA technique. Stimulation with bradykinin (BK) caused a significant release of CGRP (1.4-fold increase) and PGE2 (3-fold) which was massively augmented under neurokinin I (NKI) receptor antagonist treatment (CGRP: 4-fold, PGE2: 5-fold). In denervated skin the BK-evoked CGRP release was lost whereas the PGE2 release was unchanged. Thus, neither nerve endings nor neuropeptides contribute essentially to BK-induced PGE2 release in the skin. However, excessive neuropeptide levels, as under NKI blockade facilitate PGE2 formation, which may play a role in sustained inflammation.

Peptide autoreceptors: does an autoreceptor for substance P exist?

The presence of autoreceptors for simple neurotransmitters at synapses in the mammalian nervous system is well established. By contrast, the evidence for such receptors modifying neuropeptide transmission is less obvious. Probably the most well characterized of the neuropeptides is substance P (SP), which appears to play a major role as a primary afferent modulator. This article highlights evidence to support the existence of autoreceptors that might modulate the release of this neuropeptide and which, therefore, could be important in the design of drugs affecting SP function, not only in sensory processing, but also elsewhere in the brain.

The tachykinin NK1 receptor in the brain: pharmacology and putative functions

After its discovery in 1931, substance P (SP) remained the only mammalian member of the family of tachykinin peptides for several decades. Tachykinins thus refer to peptides sharing the common C-terminal amino acid sequence Phe-X-Gly-Leu-Met x NH2. In recent years the family of mammalian tachykinins has grown with the isolation of two novel peptides from bovine and porcine central nervous system (CNS), neurokinin A and neurokinin B. In parallel with the identification of multiple endogenous tachykinins several classes of tachykinin receptors were discovered. The receptors described so far are named tachykinin NK1 receptor, tachykinin NK2 receptor and tachykinin NK1 receptor, respectively. The present review focuses on the pharmacology and putative function of tachykinin NK1 receptors in brain. The natural ligand with the highest affinity for the tachykinin NK1 receptor is SP itself. The C-terminal sequence is essential for activity, the minimum length of a fragment with reasonable affinity for the tachykinin NK1 receptor is the C-terminal hexapeptide. A rapid advance of knowledge was caused by development of non-peptidic tachykinin NK1 receptor antagonists. This area is under rapid development and a variety of different chemical classes of compounds are involved. Species-dependent affinities of tachykinin NK1 receptor antagonists reveal two clusters of compounds, targeting the tachykinin NK1 receptor subtype found in guinea pig, human or ferret or the one in rat or mouse, respectively. The most recently developed compounds are highly selective, enter the brain and are orally bioavailable. Distinct behavioural effects in experimental animals suggest the involvement of tachykinin NK1 receptors in nociceptive transmission, basal ganglia function or anxiety and depression. Recent clinical trials in man showed that tachykinin NK1 receptor antagonists are effective in treating depression and chemotherapy-induced emesis. Therefore, it is well possible that tachykinin NK1 receptor antagonists will be clinically used for treatment of specific CNS disorders within a short period of time.

NMDA receptor activation modulates evoked release of substance P from rat spinal cord

The possible modulation exerted by glutamate on substance P (SP) release from the rat spinal cord has been investigated. The N-methyl-D-aspartate (NMDA) receptor agonist, NMDA (1 microM), increased SP basal outflow by 46.5+/-10.9% (n = 3, P<0.01) without changing the evoked release of the peptide. Conversely, NMDA antagonists but not 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) inhibited both electrically-evoked and capsaicin-induced release of SP. In particular, D-2-amino-5-phosphonopentanoate (D-AP5; 50 microM) inhibited electrically-evoked and capsaicin-induced release of SP by 93+/-2.4% and 93.2+/-3.8% (n = 12, P<0.01), respectively. Functional pharmacological evidence is provided for glutamate exerting a positive feedback on SP release evoked by C fibre stimulation via NMDA receptor activation.

alpha-Lipoic acid corrects neuropeptide deficits in diabetic rats via induction of trophic support

This study compared the effects of treatment of diabetic rats with either alpha-lipoic acid (100 mg/kg/day i.p. 5 days/week) or with recombinant human nerve growth factor (rhNGF; 0.2 mg/kg s.c. 3 days/week) on NGF-like immunoreactivity (NGFLI) and neuropeptide Y-like immunoreactivity (NPYLI) levels in the sciatic nerve and on the release of substance P-like immunoreactivity (SPLI) from the spinal cord in response to electrical stimulation of the dorsal roots in vitro. Diabetic rats showed depletion of NGFLI and NPYLI, together with reduced release of SPLI. Treatment with NGF increased the sciatic nerve NGFLI (to four times that seen in untreated diabetic rats) and normalised stimulus-evoked release of SPLI, but did not affect the sciatic nerve NPYLI. Treatment with alpha-lipoic acid caused a small non-significant increase in sciatic nerve NGFLI, but normalised both NPYLI levels and stimulus-evoked release of SPLI. These findings indicate that alpha-lipoic acid can boost neurotrophic support in diabetic rats, with effects beyond those related to NGF.

L-733,060, a novel tachykinin NK1 receptor antagonist; effects in [Ca2+]i mobilisation, cardiovascular and dural extravasation assays

This study investigated the properties of a novel piperidine ether-based tachykinin NK1 receptor antagonist L-733,060, ((2S,3S)-3-((3,5-bis(trifluoromethyl)phenyl)methyloxy)-2-phenyl piperidine and its 2R,3R-enantiomer L-733,061 on [Ca2+]i mobilisation in Chinese hamster ovary cells transfected with human tachykinin NK1 receptors, compared to their effects in rodent cardiovascular and neurogenic plasma extravasation assays. Using FURA-2-imaging techniques, L-733,060 inhibited substance P-induced [Ca2+]i mobilisation with an estimated affinity of 0.8 nM whereas L-733,061 (30-300 nM) did not. No significant effects of L-733,060 were observed on mean arterial blood pressure or heart rate in conscious or anaesthetised rats at doses of < 3000 micrograms kg-1 i.v. L-733,060 also stereoselectively inhibited neurogenic plasma extravasation in rat dura produced by electrical stimulation of trigeminal nerves with an ID50 of 212 +/- 19 micrograms kg-1 i.v. Thus, L-733,060 is a novel antagonist of human tachykinin NK1 receptors which stereoselectively inhibits neurogenic plasma extravasation at doses that do not cause adverse cardiovascular effects.

Effect of interleukin-1 beta on the release of substance P from rat isolated spinal cord

Superfusion of rat spinal cord slices with rat interleukin-1 beta resulted in a significant enhancement of electrically evoked substance P-like immunoreactivity with a maximal effect (> 2-fold increase) at 0.1 ng/ml, whereas higher concentration (10-50 ng/ml) of the cytokine inhibited (approximately 50%) the release of the neuropeptide. Interleukin-1 beta (0.1 ng/ml) potentiation of substance P-like immunoreactivity release was abrogated by co-perfusion with interleukin-1 receptor antagonist (10-100 ng/ml) or with indomethacin (1 microM). Superfusion of spinal cord with interleukin-1 beta inhibited electrically evoked calcitonin gene-related peptide-like immunoreactivity release. Modulation of substance P-like immunoreactivity release from the spinal cord by interleukin-1 beta may represent a mechanism responsible for the hyperalgesic action of the cytokine characteristic of the inflammatory response.

Spinal cord SP release and hyperalgesia in monoarthritic rats: involvement of the GABAB receptor system

1. Monoarthritis was induced in Lewis rats by interdermal injection in the left hind paw of a suspension of Mycobacterium tubercolusis in mineral oil (500 micrograms 100 microliters-1). Controls were injected with 100 microliters mineral oil. 2. Withdrawal latencies to thermal stimuli of the inflamed paw, the contralateral and both paws of control rats were measured at daily intervals after injection by the plantar test. 3. After detection of the pain threshold, rat spinal cords were removed and horizontal dorsal slices were mounted in a 3-compartment bath to measure electrically-evoked release of substance P-like immunoreactivity (SP-LI). 4. The inflamed paw of monoarthritic rats exhibited a lower pain threshold to thermal stimuli than the contralateral paw of the same animals and both paws of control rats. Inflamed paw hyperalgesia was maximal two days after injection, and declined gradually between 7 to 21 days with no evidence of excitability of withdrawal reflexes after 28 days. 5. During the 28 days study, monoarthritic rats gained less weight than control rats. 6. Electrical stimulation of the dorsal roots attached to rat isolated spinal cord slices induced a significant increase (174 +/- 18% of basal outflow which was 30.3 fmol 8 ml-1, n = 5) in SP-LI release. 7. One-week after induction of inflammation no differences in the amount of SP-LI released from the spinal cord of incomplete Freund's adjuvant-treated rats (IFA) and Freund's adjuvant-treated rats (CFA) were detected. Two weeks after, CFA spinal cord tended to release more SP-LI than IFA cords and, 21 days after injection, the spinal cord of CFA rats released significantly more peptide than IFA rats (17.8 +/- 2.8 fmol ml-1, n = 12 and 6.9 +/- 3.2 fmol ml-1, n = 9, respectively).8. Twenty-one days after treatment, the evoked release from monoarthritic rat spinal cords was increased by 263 + 42% (n = 3) in the presence of the GABAB receptor antagonist, CGP 36742 (100 micro M)which also significantly potentiated monoarthritis-induced hyperalgesia up to 45 min after injection(100 mgkg-1, i.p.).9. These findings may provide a basis for a novel approach to chronic pain therapy but also an explanation for the lack of analgesia produced by the GABAB agonist, baclofen, in chronic as compared to acute pain.