Cutaneous stimuli releasing immunoreactive substance P in the dorsal horn of the cat

Spinal cord substance P receptor immunoreactivity increases in both inflammatory and nerve injury models of persistent pain

Numerous studies have implicated the primary afferent derived neuropeptide, substance P, which exerts its effects via the neurokinin-1/substance P receptor, in the transmission of nociceptive messages at the level of the spinal cord. Immunocytochemical studies demonstrate that the substance P receptor is concentrated in neurons of lamina I of the superficial dorsal horn. Since alterations in the number and distribution of the receptor may underlie persistent pain conditions, we have used immunocytochemistry to study the distribution of the receptor in two very different rat models of persistent pain: chronic inflammation, which is associated with increased levels of substance P, and sciatic nerve section, which is associated with decreased levels of substance P in the dorsal horn. Inflammation was produced by unilateral hindpaw injection of complete Freund's adjuvant. We report that there is an up-regulation of substance P receptor immunoreactivity in the superficial laminae of the dorsal horn in both injury models. The increase was found at all time points studied (up to one week after induction of inflammation and up to two weeks after sciatic nerve section). The increase in substance P receptor immunoreactivity was not only present in the medial part of the dorsal horn at segment L4, which is the region of input of the afferents from the hindpaw, but also in the lateral parts of the dorsal horn, and at segments rostral (L1) and caudal (S1) to the afferent input from the hindpaw. These results indicate that the up-regulation of the receptor is not predictable merely by the change in the concentration of substance P in the dorsal horn. Furthermore, the non-topographic up-regulation of substance P receptor in these different conditions may contribute to the central sensitization of dorsal horn nociceptors under conditions of persistent pain.

The effect of a unilateral inflammation at the rat's ankle joint on the expression of preprotachykinin-A mRNA and preprosomatostatin mRNA in dorsal root ganglion cells--a study using non-radioactive in situ hybridization

In rats with an acute (2 days) and chronic (20 days) unilateral ankle joint inflammation (induced by Freund's complete adjuvant), the proportion of dorsal root ganglion cells containing preprotachykinin-A mRNA or preprosomatostatin mRNA was determined using non-radioactive in situ hybridization. At the acute stage of inflammation, the proportion of neurons containing preprotachykinin-A mRNA was similar to that in control rats. At the chronic stage, the proportion of neurons expressing preprotachykinin-A mRNA was significantly higher on the inflamed side than on the contralateral side. The proportion of dorsal root ganglion cells containing preprosomatostatin mRNA did not change. These data suggest that inflammation influences the synthesis of substance P but not of somatostatin in afferent neurons.

Opioid antagonists naloxone, beta-funaltrexamine and naltrindole, but not nor-binaltorphimine, reverse the increased hindpaw withdrawal latency in rats induced by intrathecal administration of the calcitonin gene-related peptide antagonist CGRP8-37

We recently demonstrated that intrathecal administration of calcitonin gene-related peptide 8-37 (CGRP8-37), a selective antagonist of calcitonin gene-related peptide receptors, dose-dependently increased the latency to hindpaw withdrawal responses induced by both thermal and mechanical stimulation in intact rats, indicating a role for CGRP and its receptors in the transmission of presumed nociceptive information in the spinal cord. The present study was performed to explore the interaction between CGRP and opioids in the spinal cord of rats. The effects of naloxone, a non-selective opioid receptor antagonist, and three different selective opioid receptor antagonists on the increased latency to withdrawal response induced by intrathecal injection of CGRP8-37 were explored. Intrathecal administration of 10 nmol of CGRP8-37 induced a significant bilateral increase in hindpaw withdrawal latency to both thermal and mechanical stimulation. The effect was partly reversed by intrathecal injection of 4 or 8 micrograms of naloxone, 10 nmol of either the mu opioid receptor antagonist beta-funaltrexamine or the delta opioid receptor antagonist naltrindole, but not by 10 nmol of the kappa opioid receptor antagonist nor-binaltorphimine. These results indicate that mu and delta, but not kappa, opioid receptors are involved in the modulation of post-synaptic effects and/or release of CGRP and other neurotransmitters.

Tachykininergic tone in the spinal cord of the rabbit: dependence on nociceptive input arising from invasive surgery

In the decerebrated and spinalized rabbit, reflexes evoked in the gastrocnemius medialis muscle nerve by electrical stimulation of teh sural nerve are suppressed after blockade of NK-tachykinin receptors. This observation suggests that endogenous tachykinins tonically enhance transmission between sural nerve afferents and gastrocnemius motoneurons. In the present study we have investigated some possible sources of this tachykininergic tone. Electrical stimulation of the sural nerve at 1 Hz, as used in our previous investigation, leads to increased gastrocnemius reflex responses with successive stimuli. We examined reflexes evoked by pairs of sural stimuli separated by intervals of 10-1000 ms, and found that responses to the second stimuli of such pairs were significantly enhanced at intervals from 50 to 500 ms. Treatment with the NK receptor antagonist CP-99,994 [(+)-(2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine; 1 mg/kg, i.v.] reduced gastrocnemius reflex responses per se, but did not alter the facilitatory effects of pairing sural stimuli. Subsequent treatment with the glutamate N-methyl-D-aspartate receptor blocker dizocilpine (0.5 mg/kg, i.v.) further reduced reflex responses and abolished paired-pulse facilitation. In a second set of experiments, rabbits were prepared so that reflexes could be studied with minimal surgical preparation of the hindlimb. Reflex responses recorded in this way were enhanced by treatment with CP-99,994 (up to 1 mg/kg, i.v.). Subsequent administration of the opioid receptor antagonist naloxone (1-10 microg/kg, i.v.) increased reflexes, as seen previously in surgically-prepared animals. These data show that tachkininergic modulation of spinal reflexes in the rabbit results from the nociceptive input arising from surgical preparation of the leg. In contrast, tonic opioidergic inhibition of reflexes is not substantially dependent on such input.

The effects of extrasynaptic substance P on nociceptive neurons in laminae I and II in rat lumbar spinal dorsal horn

Inflammation of the skin induces release and extrasynaptic spread of neuropeptides such as substance P mainly in spinal laminae I and II and causes changes in discharge properties of nociceptive neurons in spinal dorsal horn. To evaluate the role of extrasynaptic substance P we have superfused the spinal cord at recording segment with artificial cerebrospinal fluid or with substance P. A total of 102 multireceptive neurons responding to both noxious and innocuous skin stimulation were recorded in laminae I or II of lumbar spinal dorsal horn in pentobarbital anaesthetized rats. During superfusion with substance P (10 or 100 microM) significant increases of background activities (from 2.2 +/- 0.6 to 8.4 +/- 3.2 imp./s, mean +/- S.E.M.), enlargement of cutaneous receptive fields (from 359.9 +/- 60.4 to 465.5 +/- 77.3 mm2) and enhanced responses to mechanical (from 89.1 +/- 22.7 to 147.0 +/- 27.5 imp./5 s) but not thermal noxious skin stimuli were observed in the 22 neurons tested. Noxious heat-evoked responses and C-fibre-evoked responses were changed in both directions. In 50 other neurons, the coefficients of dispersion of interspike intervals, which is an indicator of burst-like discharges, were significantly reduced (from 60.4 +/- 5.5 to 52.7 +/- 5.3) after application of substance P. Substance P induced oscillations in background activities in 13 of 40 non-rhythmic neurons and depressed oscillations in 2 of 11 neurons. Cross-correlations of discharges of pairs simultaneously recorded neurons were flat (n = 4), or had a central peak (n = 19) or a central trough (n = 2) and were not changed qualitatively by extrasynaptic substance P. Thus, extrasynaptic substance P can modify not only discharge patterns in the spinal dorsal horn.

Substance P- and GABA-like immunoreactivities are co-localized in axonal varicosities in the superficial laminae of cat but not rat spinal cord

In the present study, we applied a combination of pre-embedding peroxidase-based immunocytochemistry and post-embedding immunogold staining to examine the synaptic interactions of substance P (SP) and gamma-aminobutyric acid (GABA) in the superficial laminae of the dorsal horn of cat and rat spinal cord. We demonstrate for the first time the co-existence of SP and GABA immunoreactivities in axonal boutons in laminae I-III of cat spinal dorsal horn. In cat, most SP + GABA immunoreactive (IR) axonal boutons established synapses with SP-IR or non-IR dendrites. These synapses were exclusively symmetric. Quantitative analysis showed that the percentage of SP/GABA double labelled bouton profiles was higher (7%) in lamina I but was considerably lower in laminae IIo, IIi and III. Similarly, the density (number of bouton profiles per 100 microns2) of SP + GABA-IR bouton profiles was highest in lamina I. However, in agreement with previous studies, the co-localization of SP and GABA immunoreactivities was never detected in the rat dorsal horn. In both species, SP + GABA-IR or GABA-IR axonal bouton profiles were never seen presynaptic to SP-IR boutons. These findings provide a morphological basis for the interaction of excitatory and inhibitory agents in the nociceptive circuits in the dorsal horn of the cat and rat spinal cord.

Implication of a nitric oxide synthase mechanism in the action of substance P: L-NAME blocks thermal hyperalgesia induced by endogenous and exogenous substance P in the rat

The effects of i.p. administration of the nitric oxide synthase inhibitor NG-nitro-L-arginine methylester (L-NAME) and its inactive isomer, D-NAME, were tested in two nociceptive paradigms in the rat. In the first paradigm, rats were lightly anaesthetized with a mixture of chloral hydrate (120 mg/kg, i.p.) and sodium pentobarbital (20 mg/kg, i.p.). Tail flick reaction times were monitored and thermal hyperalgesia was induced by immersion of the tail in hot water at 55 degrees C for 1.5 min. In the groups of rats pretreated with saline (n = 5), 100 mg/kg D-NAME (n = 6), 10 (n = 5) or 25 (n = 6) mg/kg L-NAME, this thermal injury induced a transient reduction in the reaction time that was 54-59% of the baseline value. However, in the groups of rats pretreated with 50 (n = 6) or 100 (n = 7) mg/kg L-NAME the reaction times were 73.9 +/- 2.7% (P < 0.05) and 102.3 +/- 0.9% (P < 0.001) of the baseline values respectively, indicating a block of the hyperalgesic responses seen in the other groups. As this hyperalgesia has been reported to be blocked by NK-1 receptor antagonists, it is suggested that it is due to the action of endogenous substance P. In the second paradigm, tail flick responses were monitored in the awake rat and thermal hyperalgesia was induced by intrathecal administration of substance P (6.5 nmol) via a chronically implanted catheter.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of neurokinin receptors in the induction but not the maintenance of mechanical allodynia in rat flexor motoneurones

1. Intrathecal (i.t.) injections of the (tachykinin) NK1 receptor agonist, substance P methyl ester (SPME; 20 pmol), or the NK2 receptor agonist, neurokinin A (NKA; 20 pmol), substantially decreased the cutaneous mechanical threshold and markedly enhanced the touch-evoked response of posterior biceps femoris-semitendinosus (PBF-ST) spinal flexor motoneurones in decerebrate-spinal rats. This cutaneous mechanical reflex allodynia was prevented by pretreatment with the NK1 antagonist RP 67580 (2.28 nmol, i.t.) and the NK2 antagonist MEN 10376 (0.7 nmol, i.t.), respectively. 2. Electrical stimulation of the sural nerve at C fibre strength or cutaneous application of the irritant, mustard oil, produced prolonged cutaneous mechanical allodynia in PBF-ST motoneurones (15 min and > 1 h, respectively). Pretreatment with RP 67580 but not MEN 10376 prevented this, but when RP 67580 was administered 25 min after the application of mustard oil, the established hypersensitivity of the flexor motor reflex was not reversed. The enantiomer of RP 67580, RP 68651 was without effect. 3. Injection of bradykinin (60 microM, 80 microliters) into the gastrocnemius muscle increased the cutaneous mechanical hypersensitivity of PBF-ST flexor motoneurones for 40-50 min. MEN 10376, but not RP 67580, prevented this, but only when administered prior to the bradykinin injection. 4. These results suggest that the induction, but not the maintenance, of cutaneous mechanical allodynia in flexor motoneurones is NK receptor dependent, with cutaneous C fibre conditioning inputs acting via NK1 and muscle C fibre conditioning inputs via NK2 receptor subtypes.

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.

Morphological characterization of substance P receptor-immunoreactive neurons in the rat spinal cord and trigeminal nucleus caudalis

Although there is considerable evidence that primary afferent-derived substance P contributes to the transmission of nociceptive messages at the spinal cord level, the population of neurons that expresses the substance P receptor, and thus are likely to respond to substance P, has not been completely characterized. To address this question, we used an antibody directed against the C-terminal portion of the rat substance P receptor to examine the cellular distribution of the receptor in spinal cord neurons. In a previous study, we reported that the substance P receptor decorates almost the entire dendritic and somatic surface of a subpopulation of spinal cord neurons. In the present study we have taken advantage of this labeling pattern to identify morphologically distinct subpopulations of substance P receptor-immunoreactive neurons throughout the rostral-caudal extent of the spinal cord. We observed a dense population of fusiform substance P receptor-immunoreactive neurons in lamina I at all segmental levels. Despite having the highest concentration of substance P terminals, the substantia gelatinosa (lamina II) contained almost no substance P receptor-immunoreactive neurons. Several distinct populations of substance P receptor-immunoreactive neurons were located in laminae III-V; many of these had a large, dorsally directed dendritic arbor that traversed the substantia gelatinosa to reach the marginal layer. Extensive labeling was also found in neurons of the intermediolateral cell column. In the ventral horn, we found that labeling was associated with clusters of motoneurons, notably those in Onuf's nucleus in the sacral spinal cord. Finally, we found no evidence that primary afferent fibers express the substance P receptor. These results indicate that relatively few, but morphologically distinct, subclasses of spinal cord neurons express the substance P receptor. The majority, but not all, of these neurons are located in regions that contain neurons that respond to noxious stimulation.

Infusion of substance P or neurokinin A by microdialysis alters responses of primate spinothalamic tract neurons to cutaneous stimuli and to iontophoretically released excitatory amino acids

The responses of 25 spinothalamic tract (STT) neurons to mechanical and thermal stimulation of the skin, as well as to a battery of iontophoretically applied excitatory amino acids (EAAs), were tested before and then during microdialysis of substance P (SP) or neurokinin A (NKA) into the dorsal horn of anesthetized monkeys. Neither peptide had significant effects on the background activity or the responses to mechanical or thermal stimulation of the skin. However, each peptide produced significant increases in the responses to iontophoretic application of one or more EAAs. In addition, following combined application of the EAAs and either SP or NKA, the responses of the cells to mechanical stimulation of the skin increased. Combined application of SP and NKA failed to produce an increase in responses to either the EAAs or to cutaneous stimuli that was greater than that produced by either peptide alone. It is concluded that SP and NKA produce an increase in the responses of STT cells to iontophoretic applications of EAAs and the combined effects of these compounds produce sustained increases in responses to mechanical stimulation of the skin. These changes mimic those observed when STT cells are sensitized by peripheral noxious stimuli, suggesting that the mechanism of induction and expression of sensitization involves the facilitation of dorsal horn neuron responses to EAAs by tachykinins.

NK1-tachykinin receptors and prolonged, stimulus-evoked alterations in the excitability of withdrawal reflexes in the decerebrated and spinalized rabbit

Intense natural or electrical stimulation of afferents from the toes or the heel results in prolonged changes in the excitability of the heel withdrawal reflex pathway in the rabbit. This study has investigated the roles played by tachykinin NK1 receptors in mediating these effects. Reflexes were evoked by electrical stimulation of the sural nerve and recorded from the gastrocnemius medialis muscle nerve. High-intensity electrical stimulation of the common peroneal nerve, or application of a crush stimulus to the toes, resulted in suppression of gastrocnemius reflex responses to between 30 and 50% of controls, from which recovery was complete in 15-25 min. In contrast, intense electrical stimulation of the sural nerve, or application of mustard oil to the heel, facilitated the sural to gastrocnemius reflex to two to four times control values. Recovery was rarely complete within 30 min of these stimuli. Administration of the NK1 receptor antagonist CP-96,345, but not its enantiomer CP-96,344, reduced gastrocnemius reflex responses to sural nerve stimulation per se; significantly decreased the time to recovery after common peroneal nerve stimulation and toe crush (but did not affect maximum inhibition); and significantly reduced the facilitation of reflexes resulting from sural nerve stimulation or mustard oil applied to the heel in the first 3-5 min after the application of the stimuli. Both CP-96,345 and CP-96,344 reduced blood pressure and heart rate. These data show that: (i) blockade of NK1-receptors reduces excitatory drive from sural nerve afferents to GM motoneurones; (ii) NK1-receptors are involved in the generation of the early excitatory events which follow stimulation of nociceptive afferents from the heel; and (iii) have a role in the later stages of prolonged, opioid-mediated inhibition of reflexes resulting from activation of fine afferents from the toes. We believe that (ii) and (iii) reflect a role for tachykinins as transmitters from small diameter primary afferent fibres.

Controlled superfusion of the rat spinal cord for studying non-synaptic transmission: an autoradiographic analysis

Recently, evidence has been raised that long-term changes in the central nervous system are mediated by extrasynaptic spread of neuropeptides ('volume transmission'). To study the effects of volume transmission in the spinal cord we developed the technique of controlled superfusion of the rat cord dorsum. This paper presents quantitative data about the spread, local spinal tissue concentration and redistribution of (2-[125I]iodohistidyl)neurokinin applied for 15, 30 or 60 min to the spinal cord dorsum in concentrations of 0.05 or 50 microM (10 microliters). Analysis of autoradiograms of sagittal and transverse spinal cord sections was done by computer-assisted densitometry. Under all experimental conditions, the spread of radiolabel into the superfused spinal cord segments reached Rexed's laminae V and VI; maximal spread (1.6 +/- 0.3 mm) was measured after superfusion for 30 min. The amount of radiolabel decreased in ventral direction as a function of distance. Highest tissue concentrations of neurokinin A (NKA) were obtained within the superficial spinal cord up to a depth of 0.5 mm and ranged from 700 to 2000 pmol/g following superfusions for 15 or 30 min with 50 microM NKA. Thus, these tissue concentrations were 25-70 times lower than the concentration of NKA in the superfusate. Since pool content was not exchanged, the radioactivity within the spinal cord was lower after superfusion periods of 60 min than after 15 or 30 min. Detection of radiolabel in blood and urine suggests that capillary clearance is relevant and limits the accumulation of the peptide within the spinal cord tissue and the spread into deeper laminae. The controlled superfusion of the rat cord dorsum is a useful method to mimick the spinal release of endogenous neuropeptides such as NKA during intense noxious stimulation, and it can be employed for versatile investigations of the effects of neuroactive molecules on the processing of sensory information in the intact spinal network.

Somatostatin-like immunoreactivity in primary afferents of the medial articular nerve and colocalization with substance P in the cat

The proportion of somatostatin-containing dorsal root ganglion cells innervating the knee joint of the cat via the medial articular nerve was determined by using retrograde labeling with fast blue and immunohistochemistry. Immunoreactivity was found in 8.6% of labeled cell bodies. In colchicine-treated ganglia, the proportion increased to 16.8%. Only small and intermediate-sized perikarya showed somatostatin-like immunoreactivity, indicating that this neuropeptide is synthesized predominantly in primary afferent units with unmyelinated sensory axons but may also be present in primary afferents with thinly myelinated sensory fibers. Colchicine treatment had no influence on the cell size distribution. Colocalization of somatostatin with substance P was determined by comparing the proportions of immunopositive dorsal root ganglion cells after incubation with antibodies against substance P or somatostatin or with a mixture of both. Substance P-like immunoreactivity was found in 18.1% (untreated ganglia) and 19.6% (colchicine treated ganglia) of the labeled neurons. After incubation with a mixed antibody solution, 18.2% of joint afferents in untreated and 19.9% of the cells in colchicine-treated ganglia were immunopositive. Comparing this result with the results obtained using somatostatin and substance P antibodies alone, one can calculate that both neuropeptides are colocalized in about 17% of the cat's knee joint afferents. About 3% of the neurons contain only substance P, whereas almost none of the neurons contain only somatostatin. Based on this fact, one can assume that both neuropeptides are coreleased in peripheral tissue as well as in the central nervous system.

Afferent volley patterns and the spinal release of immunoreactive substance P in the dorsal horn of the anaesthetized spinal cat

Microprobes bearing immobilized antibodies to the C-terminus of substance P were used to measure release of this neuropeptide in the spinal cord of the anaesthetized spinal cat in response to peripheral nerve stimulation. Release of substance P was just detectable in laminae I, II with 150 stimuli (0.5 Hz, 5 min) and was near maximal with 300 stimuli. Using two periods of stimulation of 10 min separated by 15 min, greater levels of substance P were detected during the second period. Fifteen to 25 min after two periods of peripheral nerve stimulation levels of substance P detected by microprobes were still elevated above those present prior to stimulation. Stimulation with bursts of three impulses when delivering a fixed number of stimuli resulted in detection of increased levels of substance P at sites adjacent to the areas of maximal release. The results suggest that maximal release of substance P from the central terminals of primary afferent fibres occurs with relatively few impulses and at low frequencies in agreement with what is known of release from the peripheral terminals of these fibres.

Central acute pain mechanisms

This account will consider the events occurring in the spinal cord that give rise to acute pain. The transmission and control of acute pain is not immutable but subject to plasticity so that the dividing line between acute and chronic pain is difficult to draw. Very brief acute pain is transmitted in a simple way and rarely produces difficulties in treatment. The situation within the spinal cord changes if the stimulus continues. After only seconds of C-fibre stimulation, additional peptides are released from C fibres, and spinal N-methyl-D-aspartate (NMDA) receptor activation and nitric oxide production occur. Soon after, genes are induced in central neurones, and increases and decreases in diverse pharmacological systems involved in pain transmission and modulation occur over periods of only a few hours. This rapid plasticity has important implications for the pharmacological treatment of acute because both the level of pain transmission and pain modulation will be altering over time.

Anticonvulsants suppress c-Fos protein expression in spinal cord neurons following noxious thermal stimulation

The expression of the nuclear immediate-early gene-encoded protein c-Fos in spinal cord dorsal horn neurons of the rat following noxious thermal stimulation was compared in carbamazepine-, valproate- and phenytoine-treated animals. Single intraperitoneal injection of carbamazepine (50 mg/kg), valproate (300 mg/kg) or intravenous injection of phenytoine (20 mg/kg) before noxious stimulation reduced the number of c-Fos immunoreactive neurons to 65-80% of control levels in superficial laminae and to 30-60% in deep laminae of the dorsal horn. Pretreatment with carbamazepine or valproate for 4 or 8 days combined with an injection immediately before noxious stimulation further significantly decreased the number of c-Fos neurons in the deep dorsal horn only in animals treated with valproate. The observation that activity-dependent gene expression in the spinal cord is effectively modulated by anticonvulsants discloses a novel therapeutic potential of these compounds. Presumably via an acute suppression of high-frequency repetitive firing and/or altered synaptic transmission of intraspinal or descending neurotransmitter systems these drugs gain access to neuroplastic mechanisms which might be relevant for the restoration of physiological levels of neuronal excitability in the central nervous system.

Neurokinin A and substance P vary independently in different regions of rat sensory neurons

Neurokinin A (NKA) and substance P (SP) are often co-localized in small sensory neurons and have been suggested to subserve similar roles. We have now compared tissue levels of NKA and SP in rat cervical dorsal root ganglia with those in the central and peripheral terminations of the same neurons. We found that NKA content was less than that of SP in dorsal root neuron perikarya and in two peripheral tissues (superior cervical ganglion and ear skin) containing SP axon terminals from cervical spinal ganglia; in a third peripheral tissue, trachea, equal amounts of NKA and SP were present. By contrast, in the spinal cord containing the central terminals of these sensory neurons there was almost twice as much NKA as SP. Our results indicate that, although NKA and SP are co-localized in sensory neurons, their levels vary independently, suggesting distinct functional roles.

The effects of pretreatment with tachykinin antagonists and galanin on the development of spinal cord hyperexcitability following sciatic nerve section in the rat

The effects of acute section of the sciatic nerve on the excitability of the flexor reflex was examined in decerebrate, spinalized, unanaesthetized rats. In control experiments without drugs, the excitability of the flexor reflex was dramatically increased in two phases following axotomy. An early intense, brief reflex hyperexcitability was followed by a less intense, prolonged period of facilitation. The selective NK1 tachykinin receptor antagonist CP-96,345 injected intrathecally at lower (1.2-2.4 nmol) and higher (12 nmol) doses blocked both components of spinal sensitization. The selective NK2 tachykinin receptor antagonist Men 10376 at a dose of 2.4 nmol also reduced both response components, as did the same dose of the inhibitory neuropeptide galanin. Thus, antagonists of excitatory neuropeptides released during and after nerve section, such as substance P and neurokinin A, can block the spinal response to peripheral nerve injury. Furthermore, the inhibitory neuropeptide galanin also reduced spinal cord sensitization.

Neurokinin actions on substantia gelatinosa neurones in an adult longitudinal spinal cord preparation

We have used an adult longitudinal spinal cord preparation to study the effects of a range of selective neurokinin analogues on single neurones located exclusively within the substantia gelatinosa. Since the preparation retained attached dorsal roots it was possible synaptically to activate the substantia gelatinosa neurones by electrical stimulation of their afferent fibres, thus providing a means of studying directly the role of neurokinins in mechanisms of primary afferent transmission. The actions of three agonists selective for the three NK receptor subtypes (NK1, GR73632; NK2, GR64349; NK3, senktide), and a highly selective antagonist at NK1 receptors (GR82334) were investigated. Experiments were performed on a total of 274 substantia gelatinosa neurones, estimates of conduction velocity for evoked responses suggested that the majority of these neurones were innervated by unmyelinated afferents. A large proportion responded to iontophoretically applied neurokinin agonists. The majority responded to NK1, fewer responded to NK2; some, although not all, of the neurones tested responded to both NK1 and NK2 agonists. In most cases the responses were excitatory, although inhibitory effects were observed in some neurones. None of the neurones tested responded to NK3 agonist. Excitatory and inhibitory actions could be demonstrated following abolition of synaptic transmission by removal of calcium, suggesting direct mechanisms for both effects. The antagonist alone failed to modify either spontaneous firing or firing in response to afferent stimulation in any of the neurones studied, even though the doses used were shown to be effective in selectively antagonising responses to the NK1 agonist, suggesting that neither relied on the endogenous release of neurokinins.

Non-specific effects of the tachykinin NK1 receptor antagonist, CP-99,994, in antinociceptive tests in rat, mouse and gerbil

We have examined the antinociceptive activity of the potent and selective tachykinin NK1 receptor antagonist, CP-99,994, and its less active enantiomer, CP-100,263, in a variety of models in rat, mouse and gerbil. Administered systemically to gerbil or mouse CP-99,994 but not CP-100,263 stereo selectively inhibited a caudally directed biting and scratching elicited by intrathecal administration of the tachykinin NK1 receptor agonist, GR73632. In contrast, both CP-99,994 (ED50 = 3 (1-6) mumol.kg-1 s.c.) and CP-100,263 (4 (2-10)), were equipotent at inhibiting acetylcholine-induced abdominal constrictions in mice. Similarly, both enantiomers were also equipotent in reducing formalin-induced licking in gerbil (CP-99,994 (10.1 (5.7-18.6)), CP-100,263 (13.8 (7.8-27.1)) and rat (100 mumol.kg-1 s.c.). Finally, in the spinalised, anaesthetised rat, CP-99,994 dose-dependently and significantly inhibited the flexion reflex evoked by noxious pinch (5.0 (3.3-7.5) mumol.kg-1 i.v.), whereas the less active enantiomer, CP-100,263, was without significant effect when tested up to 30 mumol.kg-1. Our results demonstrate that in the spinal cord, CP-99,994 exhibits a tachykinin NK1 receptor mediated antinociceptive action.

The involvement of substance P and neurokinin-1 receptors in the responses of rat dorsal horn neurons to noxious but not to innocuous mechanical stimuli applied to the knee joint

In 29 anesthetized rats, the involvement of substance P and neurokinin-1 receptors in the spinal processing of mechanosensory innocuous and noxious information from the knee and ankle joint was investigated. In 21 rats, multibarrel electrodes were used to record from 46 spinal cord neurons with afferent input from the knee joint and to administer agonists and antagonists by microinophoresis. In 35 of 46 nociceptive neurons, substance P (ejected at 20-120 nA) caused an excitation and/or an increase in responses to innocuous and noxious pressure applied to the knee and ankle. These effects were reduced by ionophoretic application of the specific neurokinin-1 receptor antagonist CP96,345 (ejected at 25-80 nA) but not by CP96,344, its inactive enantiomer. CP96,345 dose-dependently reduced the responses to noxious pressure applied to the knee joint in 28/28 substance P-sensitive neurons but not those to innocuous pressure in 23/23 substance P-sensitive wide dynamic range neurons. CP96,345 did not affect responses to pressure in substance P-insensitive neurons and the inactive enantiomer CP96,344 had no effect in any of the neurons tested. Using microprobes coated with antibody to substance P, intraspinal release of immunoreactive substance P was found to be evoked by noxious pressure applied to the knee but not by innocuous pressure in 8 rats. Both sets of data suggest a role for substance P and neurokinin-1 receptors in the neuronal mechanisms in the spinal cord related to nociception and pain in the normal joint.

Intraspinal release of immunoreactive calcitonin gene-related peptide during development of inflammation in the joint in vivo--a study with antibody microprobes in cat and rat

This study addressed the intraspinal release of immunoreactive calcitonin gene-related peptide in vivo during mechanical stimulation of the normal joint and during the development of an acute experimental inflammation in the knee joint in the anaesthetized cat (spinalized) and rat (not spinalized). Release was assessed using microprobes coated with antibody to calcitonin gene-related peptide; inhibition of binding of [125I]calcitonin gene-related peptide to these probes following insertion into the spinal cord is equated with intraspinal release of the endogenous (unlabelled) peptide. Probes inserted prior to inflammation showed marked basal release of immunoreactive calcitonin gene-related peptide in the dorsal horn with a maximum in the superficial dorsal horn in the absence of intentional stimulation. The pattern of binding of [125I]calcitonin gene-related peptide was not or only minimally changed by innocuous mechanical stimuli (flexion of and innocuous pressure to the knee in the cat and innocuous pressure to the knee of the rat) but was significantly altered by electrical stimulation of the tibial nerve in the cat (sufficient to excite unmyelinated afferent fibres), indicating release of the peptide by the latter stimulus. During the first hours of the development of an experimental inflammation in the knee joint induced by intra-articular injections of kaolin and carrageenan, the pattern of binding of [125I]calcitonin gene-related peptide changed. In the cat, the level of immunoreactive calcitonin gene-related peptide showed a persistent increase in the gray matter and up to the surface of the cord and release was slightly increased by innocuous stimuli. In the rat, increased levels of immunoreactive calcitonin gene-related peptide were mainly seen in the superficial and deep dorsal horn during innocuous pressure (this stimulus did not evoke release of the peptide prior to inflammation) and noxious pressure applied to the injected knee, whereas increased basal levels were only observed at later stages. These data show that the development of an acute experimental inflammation in the joint is associated with an enhancement of the intraspinal release of immunoreactive calcitonin gene-related peptide. Since the changes in the release were noted at an early stage, within the first hours, they could contribute to the generation of inflammation-evoked changes of the responsiveness of spinal cord neurons and hence to the mechanisms inducing inflammatory pain.

Modulation of spinal excitability: co-operation between neurokinin and excitatory amino acid neurotransmitters

Activation of C fibres with strong 'potentially tissue damaging' chemical, mechanical or thermal stimuli produces painful sensations that are significantly enhanced during pathological conditions, such as neuropathy and inflammation. The pronounced painful symptoms of hyperalgesia and allodynia are induced, in part, by the development of spinal hyperexcitability. This involves plastic changes in synaptic transmission between primary afferents and dorsal horn neurones induced by sustained activity of peripheral nociceptors. L. Urban, S. W. N. Thompson and A. Dray describe some of the central mechanisms that account for central hyperexcitability occurring in hyperalgesia and allodynia based on evidence from experiments both in vivo and in vitro with neurokinin and N-methyl-D-aspartate receptor antagonists.

Substance P, neurofilament, peripherin and SSEA4 immunocytochemistry of human dorsal root ganglion neurons obtained from post-mortem tissue: a quantitative morphometric analysis

Immunocytochemical studies on lumbar dorsal root ganglia obtained at routine postmortem 24-36 h after death were carried out, and neuronal cross-sectional areas measured. The subjects were elderly (76-81 years), of both sexes, had died from heart attack or haemorrhage, and had no clinical evidence of clinical neuropathy or of disease known to be associated with neuropathy. The data were consistent between ganglia from the three subjects. There were striking similarities with data from other species. Two populations of cell profiles with overlapping size distributions were distinguished with an anti-neurofilament antibody, neurofilament-rich (45% of cell profiles) with a large mean area and neurofilament-poor with a smaller mean area. Anti-substance P and anti-peripherin antibodies both labelled a population with a small mean area, with extensive co-localization between them. There were also some differences between these human dorsal root ganglia and dorsal root ganglia from some other species. More neuronal profiles were labelled for substance P in humans (44%) than in rat (20%). More neuronal profiles were labelled for SSEA4 (stage specific embryonic antigen 4) in human (40.5%) than in rat dorsal root ganglia (10%), and the SSEA4-positive profiles were relatively smaller in human than in rat. No selective accumulation of lipofusin in profiles of large cells was apparent. This study also shows that quantitative morphometric analysis of immunocytochemically labelled dorsal root ganglion neuronal profiles can be carried out successfully on human sensory ganglia obtained at post-mortem. This is the first demonstration of the two main subgroups of dorsal root ganglia neurones with neurofilament-rich and poor somata in human tissue. The size distributions of neurons with neurofilament, substance P and peripherin are consistent with these neuronal populations having similar functional properties to those described in other species. From the known sensory and fibre loss with aging, it is speculated that the loss of some large diameter neurones with myelinated fibres and low mechanical thresholds, might account for the high percentage of neurones expressing substance P.

Increased numbers of substance P-containing sensory neurons in a rat strain with a genetic neurotrophic defect

The GH inbred Wistar rat possesses reduced numbers of sympathetic motor neurons. In the present study, we report that substance P (SP) concentrations in superior cervical ganglion, spinal cord, iris and trachea of GH rats are about two-fold those in normal rats, and that SP-containing sensory neuron numbers are elevated in GH rats. These data suggest increased perinatal survival of SP neurons in the GH strain, due to reduced competition by sympathetic neurons for limited amounts of nerve growth factor. By contrast with the situation in iris and trachea, we found no difference between GH and normal rats in SP content of ear skin, atrium or stomach. This accords with previous findings that only some SP sensory neurons are responsive to nerve growth factor.

The calcitonin gene-related peptide antagonist CGRP8-37 increases the latency to withdrawal responses in rats

The present study explored the effects of calcitonin gene-related peptide (CGRP) and its antagonist CGRP8-37 on the latency to hindpaw withdrawal responses induced by both thermal and mechanical stimulation in rats. (1) Intrathecal injection of 10 nmol of CGRP had no effects on the latency to hindpaw withdrawal; intrathecal injection of 5 nmol of substance P (SP) decreased the latency to both withdrawal responses. (2) Intrathecal administration of 5 nmol or 10 nmol of CGRP8-37, but not 1 nmol, induced a significant increase in hindpaw withdrawal latency. (3) Intrathecal administration of CGRP8-37 not only reversed the SP-induced decrease in latency to both withdrawal responses but also mediated a significant increase in response latency compared to basal levels. The demonstrated results suggest that intrathecal administration of CGRP8-37 has a possible antinociceptive effect, and CGRP receptors in the spinal cord may be involved.

Altered tachykinin expression by dorsal root ganglion neurons in a rat model of neuropathic pain

The experiments described in the present study approached nerve injury from both a biochemical and anatomical perspective by monitoring changes in expression of preprotachykinin (PPT) mRNA encoding the prototypic tachykinin substance P and related peptide species in neurons of the rat dorsal root ganglia (DRG) following unilateral chronic constriction injury of the sciatic nerve. In situ hybridization histochemistry (ISHH) analyses in conjunction with computer-assisted image processing were employed to quantify levels of PPT mRNA distributed in DRG neurons. Injury-induced changes in PPT mRNA expression by affected DRG neurons included: (1) at early postoperative times, generally increased levels of PPT mRNA associated with small and intermediate-size B cells exhibiting normal morphology, (2) at late postoperative times, markedly decreased levels of PPT mRNA associated with degenerating B cells, and (3) induction of PPT gene expression by large A cells which is highly correlated with degenerative morphological changes. The significant aspects of these changes are discussed with special emphasis on the contribution of altered transmitter expression by DRG neurons to the pathophysiology of causalgia. In particular, the induction of PPT gene expression by many of the large neurons undergoing degenerative changes may represent an important biochemical parameter which is associated with the development and persistence of experimental allodynia.

Substance P released endogenously by high-intensity sensory stimulation potentiates purinergic inhibition of nociceptive dorsal horn neurons induced by peripheral vibration

To investigate the interaction at the spinal level of endogenously released substance P with the effects of endogenously released adenosine, extracellular single-unit activity was recorded from dorsal horn neurons in the anesthetized cat. Vibration to the skin inhibited on-going activity of nociceptive neurons; 20 mg/kg caffeine reversibly blocked this inhibition, indicating mediation via adenosine receptors. In half of the cases, this inhibition was potentiated by iontophoretic application of substance P. High-intensity electrical stimulation to a sensory nerve produced excitation which was blocked by an NK-1 (substance P) receptor antagonist, implicating an endogenous neurokinin. When electrical stimulation preceded the vibrational stimulus, the inhibitory effect of vibration was potentiated. Thus, we suggest that endogenous substance P may potentiate the inhibitory response to endogenous adenosine. The results have important implications for integration of inputs from different sensory modalities, especially as they relate to nociception and pain.

Lack of effect of microinjection of noradrenaline or medetomidine on stimulus-evoked release of substance P in the spinal cord of the cat: a study with antibody microprobes

1. Experiments were performed on barbiturate anaesthetized, spinalized cats to investigate the effect of microinjected noradrenaline or medetomidine on the release of immunoreactive substance P in the dorsal spinal cord following peripheral nerve stimulation. The presence of immunoreactive substance P was assessed with microprobes bearing C-terminus-directed antibodies to substance P. 2. Noradrenaline or medetomidine were microinjected into the grey matter of the spinal cord, near microprobe insertion sites, at depths of 2.5, 2.0, 1.5 and 1.0 mm below the spinal cord surface with volumes of approximately 0.125 microliters and a concentration of 10(-3) M. 3. In the untreated spinal cord, electrical stimulation of the ipsilateral tibial nerve (suprathreshold for C-fibres) elicited release of immunoreactive substance P which was centred in and around lamina II. Neither noradrenaline nor medetomidine administration in the manner described produced significant alterations in this pattern of nerve stimulus-evoked release. 4. In agreement with recent ultrastructural studies these results do not support a control of substance P release by catecholamines released from sites near to the central terminals of small diameter primary afferent fibres.

Release of immunoreactive galanin in the spinal cord of rats with ankle inflammation: studies with antibody microprobes

Antibody microprobes bearing antibodies to the carboxy-terminus of rat galanin were inserted into the spinal cords of anaesthetized normal rats and those in which ankle inflammation had been induced by the unilateral subcutaneous injection of Freund's adjuvant four to six days previously. In normal rats, a basal presence of immunoreactive galanin was detected in the dorsal horn. Similar levels of immunoreactive galanin were found in the dorsal horn of both sides of the spinal cord in animals with unilateral ankle inflammation. Flexing the ankle or compressing the foot in normal rats failed to alter levels of immunoreactive galanin detected by microprobes. In animals with ankle inflammation, prolonged periods of ankle flexion did release immunoreactive galanin in the ipsilateral dorsal horn. Subsequent noxious ankle compression in these animals did not increase but rather decreased immunoreactive galanin in the dorsal horn to below basal levels. The reason for this decrease is unknown but it may represent an inhibition of release or a depletion of spinal stores of galanin.

Evidence for localized release of substance P within rat spinal cord evoked by physiological and electrical stimuli

Antibodies immobilized onto the outer surface of glass microelectrodes were used to measure and localize substance P (SP) release in the spinal cords of anaesthetized rats. Utilizing a C-terminally directed antibody, significant levels of SP were not found in the lumbar spinal cord in the absence of peripheral noxious stimulation. Following noxious heating or pinch of the ipsilateral hind paw or electrical stimulation of the ipsilateral tibial nerve at C-fibre strength, significant amounts of released SP were detected. This noxious stimulus-evoked release of SP was primarily in the region of the substantia gelatinosa. In conclusion, the antibody microprobe technique can be employed to focally detect the release of neuropeptide in vivo, even in structures as small as rat spinal cord. The technique reveals that SP release in the rat follows broadly the same pattern as that previously reported in the cat.

Detection of immunoreactive substance P-like substances from cat brainstem sites during fatiguing isometric contractions

Isometric contractions were generated on the left hindlimb muscles from adult cats (n = 11) anesthetized with alpha-chloralose (75 mg/kg) to determine whether immunoreactive substance P (irSP) was released from either the right periaqueductal grey (PAG) or ventrolateral medulla (VLM), sites shown to be involved with the integration of the muscle pressor response. The release of immunoreactive SP was measured using SP antibody-coated microelectrodes that were inserted into the PAG or the VLM during periods of rest, fatiguing isometric contractions and post-contraction. Mean arterial pressure increased by 78 +/- 11 mmHg during the contractions. There was a release of irSP from sites in the medulla during the contractions compared to the non-contraction periods but none was detected from the PAG in response to muscle stimulation. These results provide further evidence that SP-like substances may be involved with the central integration of the muscle pressor response.

Involvement of substance P and excitatory amino acids in aversive behavior elicited by intrathecal capsaicin

The rat given an intrathecal injection of capsaicin (0.3-10 nmol/rat) through a lumbar puncture showed biting or licking the tail and hind paws. The substance P antagonist, CP-96,345 (3 nmol/rat), co-administered intrathecally with capsaicin (10 nmol/rat), caused a significant inhibition of the behavioral responses to capsaicin (10 nmol/rat). When co-administered intrathecally with the NMDA antagonist, 2-amino-5-phosphonovaleric acid (APV, 10 nmol/rat), the capsaicin (10 nmol/rat) -induced behavioral responses were significantly inhibited. A co-administration of the non-NMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 nmol/rat), resulted in a significant reduction of the behavioral responses produced by capsaicin (10 nmol/rat). Administration of the combination of two antagonists (CP-96,345 and either APV or CNQX, or APV and CNQX) more markedly inhibited the behavioral responses to capsaicin (10 nmol/rat) than when either antagonist was co-administered with capsaicin. The results suggest that aversive behaviors induced by intrathecal capsaicin are mediated not only by the activation of NK-1 receptors but also by that of NMDA and non-NMDA receptors.

Interaction between substance P and excitatory amino acid receptors in modulation of nociceptive responses of cat spinal dorsal horn neurons

Co-effects of microelectrophoretic application of 2-amino-5-phosphonovalerate (APV), ketamine, 6,7-dinitroqinoxaline-2,3-dione (DNQX), kynurenate (Kyn) and spantide on 34 spinal dorsal horn neurons were studied. Co-application of spantide and APV or DNQX produced a synergetic inhibition of responses by tibial stimulation in 10/23 and 3/8 neurons, respectively, and also in 3/8 neurons by administration of DL-sodium homocysteate. Spantide enhanced APV- or ketamine-induced inhibition of C responses of 8/19 neurons to sural stimulation, DNQX induced that of 2/4 neurons to gastrocnemius-soleous stimulation (GS), and Kyn induced that to both sural (8/19) and GS (2/4). The results suggest an interaction of SP and NMDA, non-NMDA receptors in processing spinal nociception.

Inhibition in the release of immunoreactive beta-endorphin from the periaqueductal grey during isometric contractions of cat hind-limb muscles: the effects of clonidine

Glass microelectrodes, coated with antibodies specific for beta-endorphin, were inserted into the right periaqueductal grey (PAG) (at PO.5-1.0 mm, LR 2.0 mm and 6.0 mm below the dorsal surface of the colliculi) of cats anesthetized with alpha-chloralose to determine whether immunoreactive beta-endorphins (ir-END) were released in response to fatiguing isometric contractions of the hind-limb muscles. Probes were inserted into the PAG prior to, during and following muscle contraction in the absence or presence of clonidine. ir-END was released from the PAG up to 3 h after surgery was completed while cats remained at rest. In the absence of clonidine, mean arterial pressure increased by 65 +/- 12 mmHg during contractions and the release of ir-END was inhibited during the contraction periods compared to resting periods. Clonidine abolished the pressor response to muscular contraction when injected into the PAG, but did not cause the release of ir-END during fatiguing isometric contractions. These data suggest that isometric contractions of skeletal muscle do not induce the release of ir-END-like substances from the PAG and clonidine does not attenuate the muscle pressor response by causing the release of ir-END from this level in the PAG.