Participation of substance P in spinal physiological responses to peripheral aversive stimulation

Analgesia by Deletion of Spinal Neurokinin 1 Receptor Expressing Neurons Using a Bioengineered Substance P-Pseudomonas Exotoxin Conjugate

Cell deletion approaches to pain directed at either the primary nociceptive afferents or second-order neurons are highly effective analgesic manipulations. Second-order spinal neurons expressing the neurokinin 1 (NK1) receptor are required for the perception of many types of pain. To delete NK1+ neurons for the purpose of pain control, we generated a toxin–peptide conjugate using DTNB-derivatized (Cys₀) substance P (SP) and a N-terminally truncated Pseudomonas exotoxin (PE35) that retains the endosome-release and ADP-ribosylation enzymatic domains but with only one free sulfhydryl side chain for conjugation. This allowed generation of a one-to-one product linked by a disulfide bond (SP-PE35). In vitro, Chinese hamster ovary cells stably transfected with the NK1 receptor exhibited specific cytotoxicity when exposed to SP-PE35 (IC₅₀ = 5 × 10⁻¹¹M), whereas the conjugate was nontoxic to NK2 and NK3 receptor-bearing cell lines. In vivo studies showed that, after infusion into the spinal subarachnoid space, the toxin was extremely effective in deleting NK1 receptor-expressing cells from the dorsal horn of the spinal cord. The specific cell deletion robustly attenuated thermal and mechanical pain sensations and inflammatory hyperalgesia but did not affect motoric capabilities. NK1 receptor cell deletion and antinociception occurred without obvious lesion of non–receptor-expressing cells or apparent reorganization of primary afferent innervation. These data demonstrate the extraordinary selectivity and broad-spectrum antinociceptive efficacy of this ligand-directed protein therapeutic acting via receptor-mediated endocytosis. The loss of multiple pain modalities including heat and mechanical pinch, transduced by different populations of primary afferents, shows that spinal NK1 receptor-expressing neurons are critical points of convergence in the nociceptive transmission circuit. We further suggest that therapeutic end points can be effectively and safely achieved when SP-PE35 is locally infused, thereby producing a regionally defined analgesia.

Xenon suppresses nociceptive reflex in newborn rat spinal cord in vitro; comparison with nitrous oxide

Although analgesic action of xenon has been reported, little is known about the effect of xenon at the spinal cord, which plays a crucial role in nociceptive transmission. We studied the effect of xenon on nociceptive reflex (the slow ventral root potential) and the monosynaptic reflex in neonatal rat spinal cord in vitro in comparison with nitrous oxide. Xenon (30%) and nitrous oxide (30%) were applied for 17 min through superfusing artificial cerebrospinal fluid. Xenon and nitrous oxide significantly reduced the amplitude of nociceptive reflex by approximately 70% and approximately 25%, respectively. Xenon and nitrous oxide also significantly reduced the amplitude of the monosynaptic reflex by approximately 35% and approximately 15%, respectively. These results indicate that xenon suppressed the synaptic transmission at the spinal cord, especially those of the slow ventral root potential, which reflect nociceptive transmission.

Abnormalities of somatosensory perception in patients with painful osteoarthritis normalize following successful treatment

To investigate the effect of chronic nociceptive pain on somatosensory perception, quantitative sensibility testing was performed in the most painful area and the homologous contralateral side in 14 patients with painful osteoarthritis of the hip. Twelve patients were reassessed in a painfree state 6-14 months following surgery. Von Frey filaments were used to test low-threshold mechanoreceptive function. Pressure pain sensitivity was assessed with a pressure algometer and thermal sensitivity with a Thermotest. Sex- and age-matched controls were examined in the corresponding areas at similar time intervals. There was no statistically significant difference between groups in the sensitivity to light touch and innocuous cold in either session. Compared to controls, patients had increased sensitivity to pressure pain in the most painful area (p < 0.002), bilaterally increased sensitivity to innocuous warmth (p < 0.03), cold pain (p< 0.05) and a tendency toward bilaterally increased sensitivity to heat pain (p = 0.054) before surgery. In the painful area, patients' sensitivity to pressure pain decreased (p < 0.04) and, remaining within normal limits, sensitivity to light touch increased (p < 0.006) compared to values prior to surgery. No statistically significant differences between the groups were seen following surgery, indicating that the sensibility changes had been maintained by chronic nociceptive pain.

Lack of pressure pain modulation by heterotopic noxious conditioning stimulation in patients with painful osteoarthritis before, but not following, surgical pain relief

To investigate the influence of chronic nociceptive pain on endogenous pain modulation, the effect of heterotopic noxious conditioning stimulation (HNCS) on perception of various somatosensory modalities was assessed in 15 patients with painful osteoarthritis of the hip. Thirteen patients were re-assessed when pain-free 6-14 months following surgery. Sex- and age matched healthy subjects assessed at similar time intervals served as controls. The effects of HNCS were tested using the upper extremity submaximal effort tourniquet test. Subjects rated tourniquet-induced pain intensity on a visual analogue scale (VAS). Quantitative sensory testing (QST) was performed contralaterally to the maximally painful area in 13 patients and contralaterally to the second most painful area in two patients (i.e. lateral thigh n = 12, frontal thigh n = 1, lateral calf n = 2). Sensibility was assessed before, during and 45 min following the tourniquet test. Perception thresholds to light touch were assessed using von Frey filaments and pressure pain thresholds by pressure algometry. Perception thresholds to non-painful and painful warmth and cold were determined using a Thermotest. In both sessions, patients rated the tourniquet-induced pain higher than controls at the start (P < 0.003 and P < 0.006, respectively), but not at the end of the tourniquet test. Decreased sensitivity to light touch (P < 0.001) and innocuous cold (P < 0.002) was seen during the tourniquet in patients and controls alike, on both occasions, while perception thresholds to innocuous warmth and heat pain remained unaffected. In the first session, pressure pain thresholds increased during the tourniquet test in controls (P < 0.002), but not in patients. In the second session, pressure pain thresholds increased during the tourniquet test in controls (P < 0.001) and in patients (P < 0.02). In conclusion, no pressure pain modulation was induced by HNCS in patients before surgery, as opposed to controls, suggesting a dysfunction in systems subserving 'diffuse noxious inhibitory controls' (DNIC). Normal pressure pain modulation induced by HNCS was seen when patients were re-assessed in a pain-free state following surgery, indicating that the dysfunction of DNIC had been maintained by chronic nociceptive pain.

In vitro prostanoid release from spinal cord following peripheral inflammation: effects of substance P, NMDA and capsaicin

1. Spinal prostanoids are implicated in the development of thermal hyperalgesia after peripheral injury, but the specific prostanoid species that are involved are presently unknown. The current study used an in vitro spinal superfusion model to investigate the effect of substance P (SP), N-methyl-d-aspartate (NMDA), and capsaicin on multiple prostanoid release from dorsal spinal cord of naive rats as well as rats that underwent peripheral injury and inflammation (knee joint kaolin/carrageenan). 2. In naive rat spinal cords, PGE2 and 6-keto-PGF1alpha, but not TxB2, levels were increased after inclusion of SP, NMDA, or capsaicin in the perfusion medium. 3. Basal PGE2 levels from spinal cords of animals that underwent 5-72 h of peripheral inflammation were elevated relative to age-matched naive cohorts. The time course of this increase in basal PGE2 levels coincided with peripheral inflammation, as assessed by knee joint circumference. Basal 6-keto-PGF1alpha levels were not elevated after injury. 4. From this inflammation-evoked increase in basal PGE2 levels, SP and capsaicin significantly increased spinal PGE2 release in a dose-dependent fashion. Capsaicin-evoked increases were blocked dose-dependently by inclusion of S(+) ibuprofen in the capsaicin-containing perfusate. 5. These data suggest a role for spinal PGE2 and NK-1 receptor activation in the development of hyperalgesia after injury and demonstrate that this relationship is upregulated in response to peripheral tissue injury and inflammation.

Comparative, general pharmacology of SDZ NKT 343, a novel, selective NK1 receptor antagonist

1. The in vitro and in vivo pharmacology of SDZ NKT 343 (2-nitrophenyl-carbamoyl-(S)-prolyl-(S)-3-(2-naphthyl)alanyl-N-benzyl-N- methylamide), a novel tachykinin NK1 receptor antagonist was investigated. 2. SDZ NKT 343 inhibited [3H]-substance P binding to the human NK1 receptor in transfected Cos-7 cell membranes (IC50 = 0.62+/-0.11 nM). In comparison, in the same assay Ki values for FK888, CP 99,994, SR 140,333 and RPR 100,893 were 2.13+/-0.04 nM, 0.96+/-0.20 nM, 0.15+/-0.06 nM and 1.77+/-0.41 nM, respectively. SDZ NKT 343 showed a markedly lower affinity at rat NK1 receptors in whole forebrain membranes (IC50 = 451+/-139 nM). 3. SDZ NKT 343 caused an increase in EC50 as well as reduction in the number of binding sites (Bmax) determined for [3H]-substance P, suggesting a non-competitive interaction at the human NK1 receptor. SDZ NKT 343 also caused a reduction in the maximum elevation of [Ca2+]i evoked by substance P (SP) in human U373MG cells and depressed the maximum [Sar9]SP sulphone-induced contraction of the guinea-pig isolated ileum. The antagonism of SP effects on U373MG cells by SDZ NKT 343 was reversible. 4. SDZ NKT 343 showed weak affinity to human NK2 and NK3 receptors in transfected Cos-7 cells (Ki of 0.52+/-0.04 microM and 3.4+/-1.2 microM, respectively). SDZ NKT 343 was inactive in a broad array of binding assays including the bradykinin B2 receptor the histamine H1 receptor, opiate receptors and adrenoceptors. SDZ NKT 343 only weakly inhibited the voltage-activated Ca2+ and Na+ currents in guinea-pig dorsal root ganglion neurones. The enantiomer of SDZ NKT 343, (R,R)-SDZ NKT 343 was about 1000 times less active at human NK1 receptors expressed in Cos-7 cell membranes. 5. Contractions of the guinea-pig ileum by [Sar9]SP sulphone were inhibited by SDZ NKT 343 in a concentration-dependent manner, with an IC50 = 1.60+/-0.94 nM, while the enantiomer (R,R)-SDZ NKT 343 was 100 times less active (IC50 = 162+/-26 nM). In comparison, in the same assay IC50 values for other NK1 receptor antagonists CP 99,994, SR 140,333, RPR 100,893 and FK 888 were 2.90+/-07 nM, 0.14+/-0.02 nM, 11.4+/-2.9 nM and 2.4+/-0.83 nM, respectively. 6. In anaesthetized guinea-pigs i.v. administered SDZ NKT 343 antagonized [Sar9]SP sulphone-evoked bronchoconstriction (70% reduction at 0.4 mg kg(-1), i.v.). Basal airway resistance, mean arterial blood pressure and heart rate were not affected. 7. In conclusion, SDZ NKT 343 is a highly selective NK1 receptor antagonist with high potency at the human and guinea-pig receptors. SDZ NKT 343 may be used as a potential novel therapeutic agent in human diseases where NK1 receptor hyperfunction is involved.

Large dorsal horn neurons which receive inputs from numerous substance P-like immunoreactive axon terminals in the laminae I and II of the chicken spinal cord

Large neurons outlined with numerous substance P (SP)-like immunoreactive (LI) boutons were detected immunocytochemically in the dorsal horn of the chicken spinal cord at the light microscopic level. The cervical enlargement was mainly used for observations. By electron microscopy, asymmetrical synapses were observed between the SP-LI axon terminals and the soma and dendrites of the large neurons. Cell bodies of the large neurons were mostly localized in the lamina I and the region lateral to the lamina I. Some of the cell bodies were also located in the lamina II. Their dendrites extended in the lamina I, in the region lateral to the lamina I, and deeply in the lamina II. In the lamina II, dendrites of these neurons formed synapses with SP-containing central terminals in synaptic glomeruli known to originate from primary afferents. The findings suggest that these large neurons receive nociceptive information directly from primary afferents. In the light of previous investigations, these neurons are considered to be pain-transmitting long ascending tract neurons.

Substance P and enkephalin immunoreactivities in axonal boutons presynaptic to physiologically identified dorsal horn neurons. An ultrastructural multiple-labelling study in the cat

A combination of intracellular electrophysiological recording and injection of horseradish peroxidase with ultrastructural immunocytochemistry was used to investigate the synaptic interplay between substance P- and enkephalin-immunoreactive axonal boutons and three types of functionally characterized dorsal horn neurons in the cat spinal cord. The dorsal horn neurons were classified as nociceptive specific, wide dynamic range and non-nociceptive based on their responses to innocuous and noxious stimuli. Most of the nociceptive neurons (either nociceptive specific or wide dynamic range) contained enkephalin immunoreactivity, but none of the non-nociceptive neurons were positive for enkephalin. Three types of immunoreactive boutons were found in contact with the functionally characterized dorsal horn neurons. These boutons were positive for either substance P, enkephalin, or substance P+enkephalin. Quantitative analysis revealed that the percentages of substance P-immunoreactive boutons apposed to the cell bodies, proximal dendrites and distal dendrites of nociceptive neurons were significantly higher than those of non-nociceptive neurons. Furthermore, the percentages of substance P+enkephalin-immunoreactive axonal boutons apposed to the distal dendrites of nociceptive neurons were significantly higher than those of non-nociceptive neurons and the percentages of enkephalin-immunoreactive boutons apposed to the cell bodies and proximal dendrites of nociceptive neurons were significantly higher than in non-nociceptive neurons. Finally, neither enkephalin-immunoreactive nor substance P+enkephalin-immunoreactive boutons were ever seen presynaptic to substance P-immunoreactive boutons. These results provide evidence of an anatomical substrate within the dorsal horn for the interaction of substance P-mediated with enkephalin-mediated mechanisms. The data support the idea that the modulation of nociceptive input in the dorsal horn by enkephalinergic neurons occurs mainly via a postsynaptic mechanism, and thus suggest that dorsal horn enkephalinergic neurons participate in a local inhibitory feedback loop in a distinct pathway from the previously postulated opioid-mediated depression of substance P release from primary afferent terminals.

Characteristics of nerve growth factor induced hyperalgesia in adult rats: dependence on enhanced bradykinin-1 receptor activity but not neurokinin-1 receptor activation

Treatment of adult rats with a single dose of nerve growth factor (NGF, 1 mg/kg, i.p.) results in a prolonged hypersensitivity to noxious thermal stimulation which becomes noticeable within 30 min of administration and lasts for several days. A significant mechanical hyperalgesia develops within 7 h following injection of NGF and persists for up to 7 days. In the present set of experiments we describe certain quantitative features of this hyperalgesia. The initial thermal hyperalgesia can be highly variable and is associated to some degree with the presence of an overt immunologic reaction. The mechanical hyperalgesia is reproducible enough to reveal a clear dependency on the dose of NGF. We also examined the pharmacological properties of the NGF-induced hyperalgesia. The bradykinin BK1 receptor antagonist des-Arg9[Leu8]BK transiently blocked the thermal hyperalgesia when injected 1 day after NGF administration whereas mechanical thresholds were further reduced under this protocol. The BK2 antagonist HOE 140 had no effect on this late NGF-induced hyperalgesia. Injection of the neurokinin NK1 receptor antagonist CP-96345 or its inactive enantiomer CP-96344 one day after NGF both induced a transient block of NGF-induced thermal hyperalgesia indicating a non-specific effect rather than an action at NK1 receptors. This was confirmed by finding no reversal of NGF-induced hyperalgesia by RP67580, another NK1 receptor blocker. These results suggest upregulation and activation of BK1 but not NK1 receptors as an additional, probably peripheral, mechanism for the late phase of NGF-induced thermal hyperalgesia.

Functional reorganization in the rat dorsal horn during an experimental myositis

In anaesthetized rats, the influence of an acute inflammation (2-8 h duration) of the gastrocnemius-soleus (GS) muscle on the excitability of dorsal horn neurones was studied using a mapping procedure. One of the main effects of the myositis was that the neurone population responding to GS A-fibre input increased in size. The increase was most marked in the lateral segments L6-L3 which received little input from the GS muscle in control animals. Excitability testing showed a myositis-induced lowering in threshold, combined with an increase in latency, jitter and input convergence. This suggests that new oligo- or polysynaptic connections become functional under the influence of a myositis. Neuronal effects induced by C fibres in the GS nerves were not significantly altered by a myositis, but C fibre-induced activations from the peroneal and sural nerves increased in the lateral dorsal horn. The results show that an acute myositis leads to marked changes in the functional connectivity of the dorsal horn within a few hours. The main increase in excitability took place in the lateral dorsal horn, where many neurones acquired a new input from the GS muscle. This mechanism may be involved in the spread or referral of muscle pain.