Morphine and substance P release in the spinal cord

Lack of inflammation or immune response in cyst tissue of patients with symptomatic non-hydrocephalic pineal cysts

Pineal cysts are frequently encountered as incidental findings in magnetic resonance imaging, usually devoid of symptoms, yet some patients exhibit symptomatic manifestations possibly associated with the cyst, even in the absence of hydrocephalus. The etiology of these symptoms remains contentious. This study aims to investigate the presence of lymphatic endothelial cell (LEC) markers and indications of inflammation or immune response within the pineal cysts of patients experiencing symptomatic non-hydrocephalic presentations. Eight patients who underwent surgical excision of their cysts were included in the study. Immunohistochemistry was utilized to assess the expression of LYVE-1, PDPN, and VEGFR3 as LEC markers, alongside IL-6 and CD3 for indications of inflammation or immune activity. Our analysis revealed an absence of inflammatory markers or immune response. However, a distinct expression of VEGFR3 was observed, likely localized to neurons within the pineal cyst tissue. We propose that these VEGFR3+ neurons within the pineal cyst may contribute to the headache symptoms reported by these patients. Further investigations are warranted to substantiate this hypothesis.

Neural mechanisms of spinal cord stimulation

Neuromodulation, specifically spinal cord stimulation (SCS), relieves pain and improves organ function. This chapter discusses the limited information presently available about the underlying mechanisms that explain the beneficial effects of treating patients with SCS. Where applicable, information is presented about translational research that illustrates the importance of collaboration between clinicians, basic scientists, and engineers. This chapter presents the infant stage of studies that attempt to explain the mechanisms which come into play for treating neuropathic pain, ischemic pain in peripheral vascular disease, and diseases of the visceral organs, specifically the gastrointestinal tract and the heart. The basic science studies will demonstrate how SCS acts on various pain syndromes and diseases via multiple pathways in the central nervous system as well as in somatic structures and visceral organs.

C2 spinal cord stimulation induces dynorphin release from rat T4 spinal cord: potential modulation of myocardial ischemia-sensitive neurons

During myocardial ischemia, the cranial cervical spinal cord (C1-C2) modulates the central processing of the cardiac nociceptive signal. This study was done to determine 1) whether C2 SCS-induced release of an analgesic neuropeptide in the dorsal horn of the thoracic (T4) spinal cord; 2) if one of the sources of this analgesic peptide was cervical propriospinal neurons, and 3) if chemical inactivation of C2 neurons altered local T4 substance P (SP) release during concurrent C2 SCS and cardiac ischemia. Ischemia was induced by intermittent occlusion of the left anterior descending coronary artery (CoAO) in urethane-anesthetized Sprague-Dawley rats. Release of dynorphin A (1-13), (DYN) and SP was determined using antibody-coated microprobes inserted into T4. SCS alone induced DYN release from laminae I-V in T4, and this release was maintained during CoAO. C2 injection of the excitotoxin, ibotenic acid, prior to SCS, inhibited T4 DYN release during SCS and ischemia; it also reversed the inhibition of SP release from T4 dorsal laminae during C2 SCS and CoAO. Injection of the kappa-opioid antagonist, nor-binaltorphimine, into T4 also allowed an increased SP release during SCS and CoAO. CoAO increased the number of Fos-positive neurons in T4 dorsal horns but not in the intermediolateral columns (IML), while SCS (either alone or during CoAO) minimized this dorsal horn response to CoAO alone, while inducing T4 IML neuronal recruitment. These results suggest that activation of cervical propriospinal pathways induces DYN release in the thoracic spinal cord, thereby modulating nociceptive signals from the ischemic heart.

Intradermal injection of capsaicin induces acute substance P release from rat spinal cord dorsal horn

Increased release of substance P (SP) from the dorsal horn following noxious stimuli, such as spinal administration of capsaicin, has been demonstrated in previous studies. However, changes in the release of SP in response to intradermal injection of capsaicin still remain unknown. This study was designed to demonstrate in vivo spinal SP release following intradermal injection of capsaicin (3%, 50 microl), using polyimide tubing with a single hole introduced into the rat dorsal horn. The changes in the content of SP in the rat dorsal horn tissues before and after capsaicin (3%, 50 microl) injection were also investigated. The SP concentration in the samples was analyzed using an enzyme-linked immunosorbent assay (ELISA). We found that intradermal injection of capsaicin induced a quick SP release within the dorsal horn. The peak of the release appeared around 10 min after the injection. In contrast, intradermal injection of capsaicin had no significant effect on the SP content in the dorsal horn. This study has provided direct evidence of the effect of intradermal injection of capsaicin on SP release within the dorsal horn, with the major source being from the central terminals of primary afferents.

Substance P release in the cat spinal cord upon afferent C-fibre stimulation is not attenuated by clonidine at analgesic doses

In anaesthetized cats, antibody microprobes were used to measure the release of immunoreactive substance P (irSP) in the lumbar dorsal horn during electrical stimulation of primary afferent fibres at intensities suprathreshold for unmyelinated fibres. Release of irSP was detected in the region of the superficial dorsal horn. This evoked release was not reduced by clonidine hydrochloride, administered intravenously or by superfusion of the dorsal cord surface. Microprobes inserted during cord superfusion with lignocaine hydrochloride detected less irSP along their entire length, including in the region of evoked release. The results suggest that the analgesic action of clonidine does not involve reduced release of SP from the central terminals of nociceptors in the spinal cord.

Left vagal stimulation induces dynorphin release and suppresses substance P release from the rat thoracic spinal cord during cardiac ischemia

Electrostimulatory forms of therapy can reduce angina that arises from activation of cardiac nociceptive afferent fibers during transient ischemia. This study sought to determine the effects of electrical stimulation of left thoracic vagal afferents (C(8)-T(1) level) on the release of putative nociceptive [substance P (SP)] and analgesic [dynorphin (Dyn)] peptides in the dorsal horn at the T(4) spinal level during coronary artery occlusion in urethane-anesthetized Sprague-Dawley rats. Release of Dyn and SP was measured by using antibody-coated microprobes. While Dyn and SP had a basal release, occlusion of the left anterior descending coronary artery only affected SP release, causing an increase from lamina I-VII. Left vagal stimulation increased Dyn release, inhibited basal SP release, and blunted the coronary artery occlusion-induced release of SP. Dyn release reflected activation of descending pathways in the thoracic spinal cord, because vagal afferent stimulation still increased the release of Dyn after bilateral dorsal rhizotomy of T(2)-T(5). These results indicate that electrostimulatory therapy, using vagal afferent excitation, may induce analgesia, in part, via inhibition of the release of SP in the spinal cord, possibly through a Dyn-mediated neuronal interaction.

Baclofen and the Release of Neuropeptides in the Cat Spinal Cord

The antibody microprobe technique was used to study the effect of baclofen on the release of immunoreactive substance P and immunoreactive calcitonin gene-related peptide within the lower lumbar spinal cord of pentobarbitone-anaesthetized spinalized cats. Both peptides were released in the region of the substantia gelatinosa during ipsilateral noxious cutaneous stimulation or high-intensity electrical stimulation of a hind limb nerve. Intravenous administration of baclofen suppressed the excitation of lumbar dorsal horn neurons, but did not produce detectable alterations of the evoked release of immunoreactive substance P or immunoreactive calcitonin gene-related peptide in the superficial grey matter dorsal to these neurons. The results suggest that the antinociceptive action of baclofen does not involve a reduction of the intraspinal release of substance P or calcitonin gene-related peptide from the central terminals of nociceptive sensory fibres.

MOR-1-immunoreactive neurons in the dorsal horn of the rat spinal cord: evidence for nonsynaptic innervation by substance P-containing primary afferents and for selective activation by noxious thermal stimuli

A direct action of mu-opioid agonists on neurons in the spinal dorsal horn is thought to contribute to opiate-induced analgesia. In this study we have investigated neurons that express the mu-opioid receptor MOR-1 in rat spinal cord to provide further evidence about their role in nociceptive processing. MOR-1-immunoreactive cells were largely restricted to lamina II, where they comprised approximately 10% of the neuronal population. The cells received few contacts from nonpeptidergic unmyelinated afferents, but many from substance P-containing afferents. However, electron microscopy revealed that most of these contacts were not associated with synapses. None of the MOR-1 cells in lamina II expressed the neurokinin 1 receptor; however, the mu-selective opioid peptide endomorphin-2 was present in the majority (62-82%) of substance P axons that contacted them. Noxious thermal stimulation of the foot induced c-Fos expression in approximately 15% of MOR-1 cells in the medial third of the ipsilateral dorsal horn at mid-lumbar level. However, following pinching of the foot or intraplantar injection of formalin very few MOR-1 cells expressed c-Fos, and for intraplantar formalin injection this result was not altered significantly by pretreatment with systemic naloxone. Although these findings indicate that at least some of the neurons in lamina II with MOR-1 are activated by noxious thermal stimulation, the results do not support the hypothesis that the cells have a role in transmitting nociceptive information following acute mechanical or chemical noxious stimuli.

Presynaptic localization of the carboxy-terminus epitopes of the mu opioid receptor splice variants MOR-1C and MOR-1D in the superficial laminae of the rat spinal cord

Opioids inhibit nociceptive transmission at the level of the spinal cord, possibly through inhibition of neurotransmitter release by presynaptic mu opioid receptors (MORs) thus preventing the activation of ascending pathways and the perception of pain. Most nociceptive primary afferents are unmyelinated fibers containing peptides such as substance P and/or calcitonin gene-related peptide. However, few terminals contain both substance P and MOR. Recently, we identified new carboxy-terminal MOR splice variants that are localized in the superficial laminae of the dorsal horn. We now report the precise cellular distribution of two of these MOR-1 variants, MOR-1C (exon 7/8/9 epitope) and MOR-1D (exon 8/9 epitope), at the ultrastructural level. In the superficial laminae of the dorsal horn, the majority of the labeling of MOR-1C and MOR-1D was found in unmyelinated axons. This distribution contrasts with that of MOR-1 (exon 4 epitope), in which labeling is equally found in dendrites and soma, as well as in axons. The presence of dense core vesicles in many of the MOR-1C-like immunoreactive terminals implies that this splice variant might be involved in presynaptic inhibition of transmitter release from peptide-containing afferents to the dorsal horn. Consistent with this finding, confocal microscopy analyses showed that many MOR-1C profiles in laminae I-II also contained calcitonin gene-related peptide, whereas fewer MOR-1 profiles contained either substance P or calcitonin gene-related peptide in this same region. From these findings we suggest that there are differential distributions of MOR-1 splice variants as well as distinct peptide colocalizations in the dorsal horn.

Presynaptic regulation of spinal cord tachykinin signaling via GABA(B) but not GABA(A) receptor activation

Internalization of spinal cord neurokinin-1 receptors following noxious stimulation provides a reliable measure of tachykinin signaling. In the present study, we examined the contribution of GABAergic mechanisms to the control of nociceptor processing involving tachykinins. Spinal administration of the GABA(B) receptor agonist R(+)-baclofen in the rat, at antinociceptive doses, significantly reduced the magnitude of neurokinin-1 receptor internalization in neurons of lamina I in response to acute noxious mechanical or thermal stimulation. By contrast, administration of even high doses of the GABA(A) receptor agonists, muscimol or isoguvacine, were without effect. CGP55845, a selective GABA(B) receptor antagonist, completely blocked the effects of baclofen, but failed to increase the incidence of internalization when administered alone. These results provide evidence for a presynaptic control of nociceptive primary afferent neurons by GABA(B) but not GABA(A) receptors in the superficial laminae of the spinal cord, limiting tachykinin release. Because CGP5584 alone did not increase the magnitude of neurokinin-1 receptor internalization observed following noxious stimulation, there appears to be little endogenous activation of GABA(B) receptors on tachykinin-releasing nociceptors under acute stimulus conditions. The contribution of pre- and postsynaptic regulatory mechanisms to GABA(B) receptor-mediated antinociception was also investigated by comparing the effect of baclofen on Fos expression evoked by noxious stimulation to that induced by intrathecal injection of substance P. In both instances, baclofen reduced Fos expression not only in neurons that express the neurokinin-1 receptor, but also in neurons that do not. We conclude that baclofen acts at presynaptic sites to reduce transmitter release from small-diameter nociceptive afferents. Presynaptic actions on non-tachykinin-containing nociceptors could similarly account for the reduction by baclofen of noxious stimulus-induced Fos expression in neurokinin-1 receptor-negative neurons. However, the inhibition of Fos expression induced by exogenous substance P indicates that actions at sites postsynaptic to tachykinin- and/or non-tachykinin-containing primary afferent terminals must also contribute to the antinociceptive actions of GABA(B) receptor agonists.

mu-Opioid receptors often colocalize with the substance P receptor (NK1) in the trigeminal dorsal horn

Substance P (SP) is a peptide that is present in unmyelinated primary afferents to the dorsal horn and is released in response to painful or noxious stimuli. Opiates active at the mu-opiate receptor (MOR) produce antinociception, in part, through modulation of responses to SP. MOR ligands may either inhibit the release of SP or reduce the excitatory responses of second-order neurons to SP. We examined potential functional sites for interactions between SP and MOR with dual electron microscopic immmunocytochemical localization of the SP receptor (NK1) and MOR in rat trigeminal dorsal horn. We also examined the relationship between SP-containing profiles and NK1-bearing profiles. We found that 56% of SP-immunoreactive terminals contact NK1 dendrites, whereas 34% of NK1-immunoreactive dendrites receive SP afferents. This result indicates that there is not a significant mismatch between sites of SP release and available NK1 receptors, although receptive neurons may contain receptors at sites distant from the peptide release site. With regard to opioid receptors, we found that many MOR-immunoreactive dendrites also contain NK1 (32%), whereas a smaller proportion of NK1-immunoreactive dendrites contain MOR (17%). Few NK1 dendrites (2%) were contacted by MOR-immunoreactive afferents. These results provide the first direct evidence that MORs are on the same neurons as NK1 receptors, suggesting that MOR ligands directly modulate SP-induced nociceptive responses primarily at postsynaptic sites, rather than through inhibition of SP release from primary afferents. This colocalization of NK1 and MORs has significant implications for the development of pain therapies targeted at these nociceptive neurons.

Dual ultrastructural localization of mu-opiate receptors and substance p in the dorsal horn

Opiates active at the mu-opiate receptor (MOR) produce antinociception, in part, through actions involving substance P (SP), a peptide present in both unmyelinated primary afferents and interneurons within the dorsal horn. We examined potential functional sites for interactions between SP and MOR by using dual electron microscopic immunocytochemical localization of antisera against SP and a sequence-specific antipeptide antibody against MOR in rat cervical spinal dorsal horn. The distribution was compared with that of the functionally analogous dorsal horn of the trigeminal nucleus caudalis. Many of the SP-immunoreactive terminals in the dorsal horn contacted dendrites that contain MOR (53% in trigeminal; 70% in cervical spinal cord). Conversely, within the cervical spinal dorsal horn 79% of the MOR-labeled dendrites that received any afferent input were contacted by at least one SP-containing axon or terminal. Although SP-immunoreactive dendrites were rare, many of these (48%) contained MOR, suggesting that the activity of SP-containing spinal interneurons may be regulated by MOR ligands. A few SP-labeled terminals also contained MOR (12% in trigeminal; 6% in cervical spinal cord). These data support the idea that MOR ligands produce antinociception primarily through modulation of postsynaptic second-order nociceptive neurons in the dorsal horns of spinal cord and spinal trigeminal nuclei, some of which contain SP. They also suggest, however, that in each region, MOR agonists can act presynaptically to control the release of SP and/or glutamate from afferent terminals. The post- and presynaptic MOR sites are likely to account for the potency of MOR agonists as analgesics.

The mu-opioid receptor (MOR1) is mainly restricted to neurons that do not contain GABA or glycine in the superficial dorsal horn of the rat spinal cord

The mu-opioid receptor MOR1 is present on primary afferent axons and a population of neurons in the superficial dorsal horn of the rat spinal cord. In order to determine which types of neuron possess the receptor we carried out pre-embedding immunocytochemistry with antibody to MOR1 and combined this with a post-embedding method to detect GABA and glycine in the rat. MOR1 immunoreactivity was seen on many small neurons in lamina II and a few in the dorsal part of lamina III. Although immunostaining was mainly restricted to the cell bodies and dendrites of these neurons, in some cases it was possible to see their axons, and a few of these entered lamina III. One hundred and thirty-nine MOR1-immunoreactive cells were tested with GABA and glycine antibodies, and the great majority of these (131 of 139; 94%) were not GABA or glycine immunoreactive, while the remainder showed GABA but not glycine immunoreactivity. These results suggest that most of the cells in the superficial dorsal horn which possess MOR1 are excitatory interneurons. They support the hypothesis that part of the action of mu-opioid agonists, such as morphine, involves the inhibition of excitatory interneurons which convey input from nociceptors to neurons in the deep dorsal horn, thus interrupting the flow of nociceptive information through polysynaptic pathways in the spinal cord.

Release of beta-endorphin immunoreactivity into ventriculo-cisternal perfusate by lumbar intrathecal capsaicin in the rat

A model employing perfusion of artificial cerebrospinal fluid from the lateral ventricle to the cisterna magna in the halothane anesthetized rat was used to study beta-endorphin release in the brain. Injection of 75 micrograms capsaicin into the lumbar intrathecal space released beta-endorphin immunoreactivity into perfusate. The release was blocked by intrathecal pretreatment with 1.25 mg lidocaine and the capsaicin receptor antagonist capsazepine (92 micrograms), showing that the release is caused by binding of capsaicin to a spinal receptor. The release was also blocked by intrathecal pretreatment with the NMDA antagonist MK-801 (3 micrograms) and the NK-1 receptor antagonist CP96,345 (200 micrograms), whereas the AMPA receptor antagonist NBQX (6 micrograms) yielded no significant inhibition. Surprisingly, morphine (30 micrograms) and sufentanil (1.5 micrograms) did not prevent release of beta-endorphin immunoreactivity, although blocking the cardiovascular responses to a noxious heat stimulus. High performance liquid chromatography characterization of perfusates collected after capsaicin injection showed that all beta-endorphin immunoreactivity coeluted with authentic beta-endorphin1-31. beta-Endorphin immunoreactivity in plasma was increased 10 min, but not 25 min, after capsaicin injection. Capsaicin injection abolished the motor and cardiovascular responses to tail immersion in 52.5 degrees C water. Addition of MK-801 (10(-4) mol/l) to the lateral ventricle-cisterna magna perfusate blocked the capsaicin-induced beta-endorphin release, showing that our previous demonstration of an NMDA receptor regulating arcuate nucleus beta-endorphin neuron activity has functional significance. We conclude that in this in vivo, anesthetized preparation including three hot water tail immersions, beta-endorphin can be released into a ventriculo-cisternal perfusate, by activation of the central axons of small primary afferent neurons by capsaicin. These data support the idea that central beta-endorphin may be released in response to prolonged, intense noxious stimulation.

Co-localization of mu-opioid receptor-like and substance P-like immunoreactivities in axon terminals within the superficial layers of the medullary and spinal dorsal horns of the rat

The presence of mu-opioid receptor-like immunoreactivity (MOR-LI) on axon terminals was confirmed by light and electron microscopy within the superficial layers of the medullary and spinal dorsal horns of the rat. By means of double-immunofluorescence histochemistry, co-localization of MOR-LI and substance P (SP)-LI was occasionally observed in axon terminals within the superficial layers of the dorsal horns.

Morphine produces a biphasic modulation of substance P release from cultured dorsal root ganglion neurons

We have previously reported that morphine produces a concentration-dependent multiphasic modulation (inhibitions and facilitations) of substance P (SP) release from trigeminal nucleus caudalis slices by activation of distinct populations of mu-, delta- and kappa-opioid receptors. In the present study, we have examined a wide range of morphine concentrations on K(+)-evoked SP release from dissociated rat dorsal root ganglion (DRG) neurons in culture. SP immunoreactivity was measured in the release buffer. Morphine produced a biphasic effect on K(+)-evoked SP release without affecting basal release. A concentration of 30 nM morphine facilitated SP release while a concentration of 1 microM suppressed release. Higher concentrations of morphine (10-30 microM) did not alter SP release. The facilitatory effect evoked by 30 nM morphine was abolished by opioid-receptor blockade with naloxone (30 nM) and the inhibitory effect produced by 1 microM morphine tended to be reversed. We conclude that an intact neuronal circuitry is not required for morphine to produce an opioid receptor mediated biphasic modulation of SP released from unmyelinated primary afferents. It is plausible that the dose-dependent biphasic effects of opioid agonists may also produce biphasic effects on nociception.

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.

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

1. The effects of the non-peptide tachykinin NK1 receptor antagonists, RP 67580, SR 140333, CP-96,345 and CP-99,994 have been investigated on electrically-evoked release of substance P-like immunoreactivity (SP-LI) from rat spinal cord slices. 2. RP 67580 (10 nM) and SR 140333 (1 nM), perfused 5 min prior to and during 8 min stimulation of the dorsal roots (20 V, 0.5 ms, 1 Hz), significantly enhanced SP-LI release by 213 +/- 43 (n = 8) and 203 +/- 31 (n = 5) % of control evoked release (187 +/- 16% of basal outflow, n = 22) respectively. Neither compound modified basal outflow of SP-LI (15.3 +/- 2.5 fmol/8 ml, n = 10). 3. RP 67580 (10 nM) did not modify electrically-evoked release of calcitonin gene-related peptide-LI from rat spinal cord slices. 4. CP-96,345 (10 nM) and CP-99,994 (1 and 10 nM) did not alter electrically-evoked SP-LI release; however, they both inhibited release at 1 microM. Inhibition was also induced by 1 microM RP 67580 but not 1 microM SR 140333. 5. The effect of the NK1 receptor agonists, [Sar9 Met (O2)11]SP and [Sar9]SP, could not be tested on SP-LI release due to interference with the substance P radioimmunoassay (RIA). The other NK1 receptor agonists used, GR 73632, [Pro9]SP and septide, which did not interfere with the RIA, increased SP-LI basal outflow by 1807 +/- 713% (n = 3), 1259 +/- 160% (n = 3) and 620 +/- 69% (n = 3) at 10 nM, 1 nM and 1 microM, respectively. At the same concentrations, the three agonists also enhanced electrically evoked SP-LI release by 204 +/- 38% (n = 6), 753 +/- 40% (n = 3) and 504 +/- 97% (n = 3), respectively. The GR 73632 (10 nM)-induced increase in electrically-evoked SP-LI release, was not prevented by SR140333 (100 nM). None of the agonists inhibited SP-LI release at lower concentrations (0.1 nM GR73632; 0.01 and 0.1 nM [Pro9]SP and 1-100 nM septide).6 NKA and NKB, at concentrations up to 10 nM which did not interfere with the RIA, did not modify electrically-evoked release of SP-LI.7 The ability of NKI receptor antagonists to enhance electrically-evoked SP-LI release supports the concept of an NK1 autoreceptor control mechanism at substance P nerve terminals within the dorsal horn of the rat spinal cord.

Opioidergic control of the spinal release of neuropeptides. Possible significance for the analgesic effects of opioids

Several neuropeptides play a key role in the transfer (substance P, calcitonin gene-related peptide, etc) and control (enkephalins, cholecystokinin, etc) of nociceptive messages from primary afferent fibres to spino-thalamic neurones in the dorsal horn of the spinal cord. This first relay in nociceptive pathways has been shown to be a major target for opioids such as analgesic drugs, and the effects of exogenous (mainly morphine) and endogenous opioids on the release of neuropeptides within the dorsal horn are reviewed here for a better understanding of the cellular mechanisms responsible for their antinociceptive action. Complex modulations of the in vitro (from tissue slices) and in vivo (in halothane-anaesthetized rats whose intrathecal space was perfused with an artificial cerebrospinal fluid) release of substance P and calcitonin gene-related peptide by opioids have been reported, depending on the opioid receptor (mu, delta, kappa, and their subtypes) stimulated by these compounds. In particular, the inhibition by delta agonists of substance P release from primary afferent fibres, and that by the concomitant stimulation of mu and kappa receptors of the release of calcitonin gene-related peptide are very probably involved in the analgesic action of specific opioids and morphine at the level of the spinal cord. Furthermore, the negative modulation (through presynaptic opioid autoreceptors) by delta and mu agonists of the spinal release of met-enkephalin, and the complex inhibitory/excitatory influence of delta, mu and kappa receptor ligands on the release of cholecystokinin within the dorsal horn very likely also contribute to the antinociceptive action of these drugs and morphine. The reviewed data strongly support the existence of functional interactions between mu and kappa receptors within the spinal cord, and their key role in the analgesic action of non specific opiates (acting on mu, delta and kappa receptors) such as morphine.

Antinociceptive effect of systemic PC 12, a prodrug mixed inhibitor of enkephalin-degrading enzymes, in normal and arthritic rats

The antinociceptive effects of various i.v. doses (2.5, 3.75, 5, 10, 15 and 20 mg/kg) of a prodrug mixed inhibitor of enkephalin degrading enzymes, PC 12, were tested in normal and Freund's adjuvant-induced arthritic rats, using vocalization thresholds to paw pressure as a nociceptive test. In normal animals, PC 12 produced a dose-dependent antinociceptive effect over the 2.5-15 mg/kg range, but the time-course of the response was shorter with the dose of 20 mg/kg. In arthritic rats, PC 12 had no significant effect at 2.5 mg/kg, but induced a highly significant dose-dependent antinociceptive action with 3.5 and 5 mg/kg, which decreased with the highest doses. The antinociceptive effect of PC 12 (10 mg/kg i.v.) was prevented by naloxone (0.5 mg/kg i.v.) in the two categories of rats, providing evidence for the involvement of opioid receptors. These results are discussed in relation with the increased level of endogenous opioid substances in the spinal dorsal horn of arthritic rats and the nociceptive test used.

Opioid control of the release of calcitonin gene-related peptide-like material from the rat spinal cord in vivo

The possible control by opioids of the spinal release of calcitonin gene-related peptide-like material (CGRPLM) was investigated in halothane-anaesthetized rats whose intrathecal space was perfused with an artificial cerebrospinal fluid. Morphine (20 mg/kg i.v.; or at 10-100 microM added to the perfusing fluid), the mu selective agonist DAGO (10 microM) and the kappa selective agonist U 50488 H (10 microM) did not affect the spontaneous outflow of the CGRPLM. In contrast, the selective delta agonist DTLET (10 microM) significantly increased CGRPLM release. The latter effect could be prevented by the selective delta antagonist naltrindole (10 microM) as expected from the involvement of this class of opioid receptors. However, the addition of naltrindole alone to the perfusing fluid did not modify CGRPLM outflow, indicating that endogenous opioids do not exert a tonic control of CGRP-containing fibers through the stimulation of delta receptors. In contrast, intrathecal perfusion with naloxone (10 microM) or nor-binaltorphimine (10 microM), a selective antagonist of kappa receptors, produced a marked increase in spinal CGRPLM release, suggesting that endogenous opioids acting at mu and kappa receptors, respectively, exert a tonic inhibitory control of CGRP-containing fibers. Indeed, a significant decrease in the spinal release of CGRPLM release could be evoked by the combined addition of U 50488 H (10 microM) plus DAGO (10 microM) to the perfusing medium, indicating that the simultaneous stimulation of both kappa and mu receptors is required for this negative control to occur. This could notably be achieved with morphine (10 microM) in the presence of naltrindole (10 microM) which also produced a significant reduction in the spinal release of CGRPLM. In conclusion, morphine per se did not change CGRPLM release because this drug triggers opposite positive (through the stimulation of delta receptors) and negative (through the concomitant stimulation of both kappa and mu receptors) control mechanisms within the rat spinal cord.

Excitation of cutaneous afferent nerve endings in vitro by a combination of inflammatory mediators and conditioning effect of substance P

A broad mixture of inflammatory mediators ("inflammatory soup") was used to investigate the responsiveness of primary afferents from rat hairy skin in an in vitro skin-saphenous nerve preparation. In addition, a conditioning effect of the tachykinin substance P on chemosensitivity of nociceptors was examined. Inflammatory soup (IS) was made up in synthetic interstitial fluid from bradykinin, serotonin, histamin and prostaglandin E2 (all 10(-5) M). In addition, the potassium and the hydrogen ion concentration (7 mM, pH 7.0) and the temperature (39.5 degrees C) were elevated. The latter agents, in a control solution, did not excite nociceptors (n = 5). IS was repeatedly superfused over the receptive fields for 5 min at 10 min intervals; substance P (SP 10(-6) and 10(-5) M) was applied during the last 5 min of the interval and during the subsequent IS stimulation. IS excited more than 80% of the mechano-heat sensitive ("polymodal") afferents with slowly conducting nerve fibres (n = 72), but none of the low-threshold mechanoreceptive slow and fast conducting units (n = 17). Slow conducting afferents with high mechanical threshold (n = 35) were weakly, and less frequently (< 20%), driven by IS. A majority, but not all, of the responsive units showed tachyphylaxis upon repeated IS application. None, however, lost its responsiveness completely. Conditioning heat stimulation (32-46.5 degrees C in 20 s) did not enhance the subsequent IS response, which may indicate that sensitizing substances normally released by a noxious heat stimulus were already contained in IS. No sensitization to mechanical (von Frey) or heat stimulation could be established in the period after the IS response had subsided and after the washout was completed, respectively. A short-lived sensitization may have been overlooked under these temporal restrictions. Conditioning SP in 10(-5) M but not in 10(-6) M concentration significantly increased the IS response of polymodal C fibres, by 58% on average (n = 14). SP did not excite the units. Comparing with previous data, we conclude that there is a significant synergism between inflammatory mediators, acting to induce more intense and more sustained discharge via many nociceptors than single mediators alone could achieve. Conditioning substance P can further enhance this algogenic action. Mechanisms of interaction and relative contributions of single substances remain to be elucidated.

Analgesic doses of morphine do not reduce noxious stimulus-evoked release of immunoreactive neurokinins in the dorsal horn of the spinal cat

1. Antibody microprobes were used to detect immunoreactive neurokinin A release in the dorsal spinal cord of barbiturate-anaesthetized spinal cats. 2. Noxious mechanical stimulation of the ipsilateral hind paw and electrical stimulation (suprathreshold for unmyelinated primary afferent fibres) of the ipsilateral tibial nerve evoked immunoreactive neurokinin A release. 3. Systemic morphine, 5 mg kg-1, i.v., did not block immunoreactive neurokinin A release in response to these stimuli. 4. Subsequent naloxone administration, 0.5 mg kg-1, i.v., did not alter this stimulus-evoked release. 5. Basal levels of immunoreactive neurokinin A were unaltered by morphine or naloxone. 6. These results suggest that the analgesic effects of morphine at the spinal cord level are not brought about by activation of presynaptic opiate receptors on neurokinin A containing afferent terminals.

Intraspinal release of substance P and calcitonin gene-related peptide during opiate dependence and withdrawal

The antibody microprobe technique was used to study the release of immunoreactive substance P and immunoreactive calcitonin gene-related peptide within the lower lumbar spinal cord of anaesthetized spinalized cats pretreated twice daily for 3.5 days with increasing doses of morphine hydrochloride (2-20 mg/kg, i.p.). Both peptides were released in the region of the substantia gelatinosa during noxious cutaneous thermal stimulation or high-intensity electrical stimulation of a hind limb nerve. Intravenous administration of naloxone increased the nociceptive excitation of lumbar dorsal horn neurons, but did not alter the evoked release of immunoreactive substance P or immunoreactive calcitonin gene-related peptide in the superficial gray matter dorsal to these neurons. In addition, the release of both peptides was not significantly different to that detected under similar experimental conditions in opioid-naive cats. The results suggest that alterations in neuropeptide release from the central terminals of nociceptive primary afferent neurons do not occur during states of opiate dependence and withdrawal, and thus do not contribute to the characteristic signs of these phenomena in dependent animals.

Morphine does not reduce the intraspinal release of calcitonin gene-related peptide in the cat

In anaesthetised cats, antibody microprobes were used to measure the release of immunoreactive calcitonin gene-related peptide (irCGRP) in the lumbar dorsal horn during stimulation of non-nociceptive or nociceptive primary afferent fibres. Release of irCGRP was detected in the substantia gelatinosa, where immunostaining for CGRP was subsequently observed. Intravenous administration of morphine did not affect release of irCGRP detected during either non-nociceptive or nociceptive afferent stimulation. The results suggest that the analgesic action of morphine does not involve reduced release of CGRP from the central terminals of nociceptive primary afferents.