Effects of dorsal rhizotomy on neurokinin receptor sub-types in the rat spinal cord: a quantitative autoradiographic study

The role of peptides in central sensitization

Peptides released in the spinal cord from the central terminals of nociceptors contribute to the persistent hyperalgesia that defines the clinical experience of chronic pain. Using substance P (SP) and calcitonin gene-related peptide (CGRP) as examples, this review addresses the multiple mechanisms through which peptidergic neurotransmission contributes to the development and maintenance of chronic pain. Activation of CGRP receptors on terminals of primary afferent neurons facilitates transmitter release and receptors on spinal neurons increases glutamate activation of AMPA receptors. Both effects are mediated by cAMP-dependent mechanisms. Substance P activates neurokinin receptors (3 subtypes) which couple to phospholipase C and the generation of the intracellular messengers whose downstream effects include depolarizing the membrane and facilitating the function of AMPA and NMDA receptors. Activation of neurokinin-1 receptors also increases the synthesis of prostaglandins whereas activation of neurokinin-3 receptors increases the synthesis of nitric oxide. Both products act as retrograde messengers across synapses and facilitate nociceptive signaling in the spinal cord. Whereas these cellular effects of CGRP and SP at the level of the spinal cord contribute to the development of increased synaptic strength between nociceptors and spinal neurons in the pathway for pain, the different intracellular signaling pathways also activate different transcription factors. The activated transcription factors initiate changes in the expression of genes that contribute to long-term changes in the excitability of spinal and maintain hyperalgesia.

Nociceptive spinal neurons expressing NK1 and GABAB receptors are located in lamina I

The nociceptive nature of spinal dorsal horn neurons expressing NK1 and gamma-aminobutyric acid (GABA)(B) receptors was evaluated in the rat. Immunodetection of the Fos protein, induced by noxious mechanical stimulation of the skin, was combined with immunocytochemistry for NK1 or GABA(B) receptors (double-immunostaining study) or both receptors (triple-immunostaining study). Neurons double-labeled for Fos and for each receptor largely prevailed in lamina I. The proportions of Fos-positive cells immunostained for NK1 or GABA(B) receptors were higher in lamina I than in the remaining spinal laminae. More Fos-positive cells were immunoreactive (IR) for GABA(B) receptors than for NK1 in all dorsal horn laminae. In the triple-immunostaining study, co-localization of NK1 and GABA(B) receptors occurred only in lamina I and was higher in neurons expressing Fos. As to the morphological lamina I cell class, NK1-positive cells belonged mainly to the fusiform type while similar proportions of fusiform, pyramidal and flattened NK1 neurons expressed GABA(B) receptors. No differences were found between those cell types as to the degree of nociceptive activation. The present results suggest that the co-localization of NK1 and GABA(B) receptors is a common feature of fusiform, pyramidal and flattened neurons in lamina I. Considering the participation of the three cell classes in various ascending systems, it is concluded that a simultaneous action of substance P (SP) and GABA may play an important role in the modulation of nociceptive input supraspinally transmitted from lamina I.

Tachykinin receptor (NK(1), NK(2), NK(3)) binding sites in the rat caudal brainstem following neonatal capsaicin administration

Binding of [125I]-Bolton-Hunter substance P ([125I]-BHSP), [125I]-neurokinin A and [3H]-senktide to tachykinin NK(1), NK(2) and NK(3) receptors, respectively, was examined in caudal brainstem sections of 10-week-old rats pretreated as neonates (P2) with capsaicin (50 mg/kg, s.c.) or vehicle. [125I]-BHSP binding was localised to the nucleus of the solitary tract (NTS), hypoglossal nucleus and inferior olivary complex, whereas [125I]-neurokinin A and [3H]-senktide binding were confined to the NTS. The distribution and density of binding sites were similar in vehicle- and capsaicin-pretreated rats, suggesting that sensory neuron ablation by neonatal capsaicin does not affect tachykinin receptor numbers in the rat caudal brainstem.

Prostaglandin E2 increases the expression of the neurokinin1 receptor in adult sensory neurones in culture: a novel role of prostaglandins

(1) Peripheral inflammation causes an increase in the proportion of primary afferent neurones that express neurokinin(1) (NK(1)) receptors for substance P (SP). This upregulation may contribute to the neuronal mechanisms of inflammatory pain. The aim of this study was to identify endogenous mediators that stimulate upregulation of NK(1) receptors in dorsal root ganglion (DRG) neurones. Cultured DRG neurones from the adult normal rat were exposed for 2 days to media that contained specific mediators, namely potassium in high concentration, prostaglandin E(2) (PGE(2)), somatostatin (SRIF), and compounds influencing second messenger cascades. After fixation neurones were labelled with an NK(1) receptor antibody. (2) Repetitive addition of the inflammatory mediator PGE(2) or dibutyryl-cyclic adenosine 3',5' monophophate (db-cAMP) to the culture medium enhanced the proportion of neurones with NK(1) receptor-like immunoreactivity from about 12% up to 40%. PGE(2)-induced upregulation was prevented by coadministration of PGE(2) and a protein kinase A inhibitor or SRIF to the medium. High potassium concentration, protein kinase C inhibitors and omission of nerve growth factor from the medium had no effect. (3) In calcium-imaging experiments, bath application of SP evoked increases of the intracellular calcium concentration in about 20% of the neurones. This proportion increased to about 40% after PGE(2)-pretreatment, but the increase was prevented when PGE(2) and SRIF were coadministered to the medium. (4) These data show that the expression of NK(1) receptor-like immunoreactivity in DRG neurones is regulated by the inflammatory mediator PGE(2). This upregulation depends on the intracellular adenylyl cyclase-protein kinase A pathway.

Localization of functional calcitonin gene-related peptide binding sites in a subpopulation of cultured dorsal root ganglion neurons

In this study we investigated whether cultured dorsal root ganglion (DRG) neurons from the adult rat express binding sites for calcitonin gene-related peptide (CGRP). These were identified on fixed cells by using CGRP labeled at the N-terminal site with 1.4-nm gold particles. After 1 day in culture, about 20% of small to medium-sized DRG neurons showed CGRP-gold binding. Binding of CGRP-gold was dose-dependently reduced by coadministration of CGRP. The calcium imaging technique in living cells revealed that the bath administration of CGRP evoked an increase of the intracellular calcium in up to 30% of the DRG neurons tested. Both depletion of intracellular calcium stores by thapsigargin or using a calcium-free medium blocked the CGRP-mediated increase of cytosolic calcium in most neurons. Thus intracellular and extracellular sources of calcium are relevant for the CGRP response. Using the whole-cell patch-clamp technique, about 30% of the neurons were found to exhibit an inward current and a depolarization upon administration of CGRP close to the neurons. Immunocytochemical double-labeling techniques showed that most of the CGRP-gold binding sites were expressed in unmyelinated (neurofilament 200-negative) DRG neurons. Most of the neurons with CGRP-gold binding sites also expressed the tyrosine kinase A receptor, and all of them showed CGRP-like immunoreactivity. This study shows, therefore, that a subpopulation of unmyelinated, peptidergic primary afferent neurons express CGRP binding sites that can be activated by CGRP in an excitatory direction. The binding sites may serve as autoreceptors because all of these neurons also synthesize CGRP. The activation of CGRP binding sites may sensitize primary afferent neurons and influence the release of mediators.

Morphological change of substance P receptor immunoreactive dendrites of preganglionic sympathetic neurons

Substance P (SP)-containing terminals and SP receptors (SPRs) are found on the dendrites of preganglionic sympathetic neurons (PSNs) in the intermedio-lateral nucleus (IML) of the spinal cord. The SP-containing fibers were thought to be of supraspinal origin. However, the primary sensory nerve fibers terminated around PSNs, and some of them directly on PSNs. We observed approximately 150 SPR-immunoreactive (ir) varicosities on the dendrites of PSNs in slices of the first thoracic segment (T1) in control rats. The number of varicosities decreased to 41% 14 days after hemisection of the spinal cord at the fourth cervical segment (C4), and to 55% 14 days after sectioning the spinal dorsal roots at the C8 and T1 levels. The number of varicosities decreased by 33% in 8-week-old rats which had been administered capsaicin subcutaneously within 24 hours after birth to eradicate unmyelinated sensory fibers. However, varicosities increased by 15% 15 minutes after injection of capsaicin into the plantar surface of the front paw to stimulate somatosensory nerve fibers in adult rats. The results demonstrate that SPR-ir varicosities on the dendrites of PSNs were modulated not only by the supraspinal nervous system but also directly by the primary sensory nerve terminals.

Neonatal capsaicin treatment decreased substance P receptor immunoreactivity in lamina III neurons of the dorsal horn

Using immunohistochemistry and in situ hybridization technique, the distribution of substance P (SP) and SP receptors was studied in the dorsal horn of the rat spinal cord after neonatal capsaicin treatment. Sprague-Dawley rats administered 100 mg/kg of capsaicin subcutaneously within 24 h after birth were examined at 8 weeks of age. In the capsaicin administered rats, slight reduction of SP immunoreactivities in lamina I, and severe decrease in lamina II were observed. In the control group, SP receptor-mRNA was observed in all laminae, and SP receptor-immunoreactivities were seen to be intense in laminae I and III. In contrast, in the capsaicin administered rats, the SP receptor-mRNA expression was low in laminae II-V, and SP receptor immunoreactivities decreased in laminae III-V. Furthermore, the density of the SP receptor immunoreactivities was considerably decreased in the nerve cells of lamina III. We concluded that elevation of the threshold to painful stimulation in rats was as a result of the decrease in SP immunoreactive afferent fibers in laminae I and II, decrease of the SP receptor-mRNA in the laminae II-V, or the decrease in SP receptor immunoreactive neurons in laminae III-V.

Respiratory actions of tachykinins in the nucleus of the solitary tract: effect of neonatal capsaicin pretreatment

1. The respiratory response to microinjection of capsaicin and tachykinin receptor agonists into the commissural nucleus of the solitary tract (cNTS) was investigated in adult, urethane-anaesthetized rats which had been pretreated with capsaicin (50 mg kg(-1) s.c.) or vehicle (10% Tween 80, 10% ethanol in saline) as day 2 neonates. 2. Microinjection of capsaicin (1 nmol) into the cNTS of vehicle-pretreated rats, significantly reduced respiratory frequency (59 breaths min(-1), preinjection control, 106 breaths min(-1)) without affecting tidal volume (VT). In capsaicin-pretreated rats, the capsaicin-induced bradypnoea was markedly attenuated (minimum frequency, 88 breaths min(-1); control, 106 breaths min(-1)). 3. In vehicle-pretreated rats, microinjection of substance P (SP, 33 pmol), neurokinin A (NKA, 33 pmol) and NKB (330 pmol), and the selective NK(1) tachykinin receptor agonists, [Sar(9), Met(O(2))(11)]-SP (33 pmol) and septide (10 pmol), increased VT (maxima, 3.60 - 3.93 ml kg(-1)) compared with preinjection control (2.82 ml kg(-1)), without affecting frequency. The selective NK(3) agonist senktide (10 pmol) also increased VT (3.93 ml kg(-1)) which was accompanied by a bradypnoea (-25 breaths min(-1)). The selective NK(2) agonist, [Nle(10)]-NKA(4-10) (330 pmol) increased VT slightly but significantly decreased frequency (-12 breaths min(-1)). In capsaicin-pretreated rats, VT responses to SP and [Sar(9), Met(O(2))(11)]-SP were increased whereas the response to septide was abolished. Both the VT and bradypnoeic responses to senktide and [Nle(10)]-NKA(4-10) were significantly enhanced. 4. These results show that neonatal capsaicin administration markedly reduces the respiratory response to microinjection of capsaicin into the cNTS. The destruction of capsaicin-sensitive afferents appears to sensitize the NTS to SP, NKB, [Sar(9),Met(O(2))(11)]-SP, senktide and [Nle(10)]-NKA(4-10). Moreover, the loss of septide responsiveness in capsaicin-pretreated rats, suggests that 'septide-sensitive' NK(1) receptors may be located on the central terminals of afferent neurons.

Monoarticular antigen-induced arthritis leads to pronounced bilateral upregulation of the expression of neurokinin 1 and bradykinin 2 receptors in dorsal root ganglion neurons of rats

STATEMENT OF FINDINGS: This study describes the upregulation of neurokinin 1 and bradykinin 2 receptors in dorsal root ganglion (DRG) neurons in the course of antigen-induced arthritis (AIA) in the rat knee. In the acute phase of AIA, which was characterized by pronounced hyperalgesia, there was a substantial bilateral increase in the proportion of lumbar DRG neurons that express neurokinin 1 receptors (activated by substance P) and bradykinin 2 receptors. In the chronic phase the upregulation of bradykinin 2 receptors persisted on the side of inflammation. The increase in the receptor expression is relevant for the generation of acute and chronic inflammatory pain.

Peptide autoreceptors: does an autoreceptor for substance P exist?

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

NK1, NMDA, 5HT1a, and 5HT2 receptor binding sites in the rat lumbar spinal cord: modulation following sciatic nerve crush

Quantitative receptor binding autoradiography was used to study the NK1, NMDA, 5HT1a, and 5HT2 receptor binding densities in the adult rat lumbar spinal cord from 3 days to 20 weeks following a unilateral crush lesion of the sciatic nerve. NK1 binding density increased unilaterally in the superficial dorsal horn on the side of the sciatic crush to reach levels 60% above controls by 4 weeks following the lesion and returned to control values by 12 weeks. NMDA binding density increased bilaterally and equally in both the dorsal and ventral horns to reach 300% of control values at 2 weeks following the crush and returned to near control values by 20 weeks following the lesion. Serotonergic receptor binding did not change. The changes in NK1 receptor binding density on postsynaptic dorsal horn cells are consistent with a response to the decrease and recovery in the synthesis and transport of tachykinins by the dorsal root ganglion cells following peripheral nerve injury. the bilateral changes in NMDA receptor binding are more likely mediated by polysynaptic pathways in the spinal cord that respond to the changes in metabolic events of the dorsal root ganglion cells evoked by axotomy and regeneration.

NK-3 receptors mediate enhancement of substance P release from capsaicin-sensitive spinal cord afferent terminals

1. The effects of NK-3 receptor agonists on the release of substance P-immunoreactivity (SP-LI) have been investigated using superfused rat spinal cord synaptosomes. 2. The Ca2+-dependent overflow of SP-LI evoked by 35 mM KCl was concentration-dependently enhanced by senktide (EC50 = 52 nM; maximal effect = 70%) or [MePhe7]NKB (EC50 = 5.5 nM; maximal effect 125%), both selective agonists at receptors of the NK-3 type. 3. The potentiation of the SP-LI overflow elicited by 100 nM senktide or [MePhe7]NKB was prevented by the NK-3 receptor antagonist (+)-SR142801. The antagonist halved, at 10 nM, and almost abolished, at 100 nM, the effect of both agonists. The effect of senktide or [MePhe7]NKB was insensitive to antagonists at NK-1 or NK-2 receptors. 4. Capsaicin (0.1-1 microM) stimulated SP-LI release in a concentration-dependent manner from spinal cord synaptosomes. The SP-LI overflow elicited by 1 microM capsaicin was completely dependent on external Ca2+. Senktide could not affect the capsaicin-evoked release of SP-LI. 5. Senktide failed to potentiate the K+-evoked overflow of SP-LI from synaptosomes previously exposed for 15 min in superfusion to capsaicin. 6. The results show that release-enhancing NK-3 receptors are located on axon terminals of capsaicin-sensitive primary afferent neurones in the spinal cord. Antagonists at NK-3 receptors might help controlling pain transmission.

Astroglial distribution of neurokinin-2 receptor immunoreactivity in the rat spinal cord

Two mouse monoclonal antibodies, 11H9.1 and 1G7.10, raised against the COOH-terminus peptide (359-390) of the rat neurokinin-2 receptor, were used to visualize by light and electron microscope immunocytochemistry the distribution of this receptor in adult rat spinal cord. At all spinal levels, immunoreactivity was mainly observed in two narrow crescentic zones bordering the gray matter of the dorsal and ventral horns, and around the central canal. In the light microscope, this labelling was the densest within the outer part of lamina I facing the dorsal column, where it took the form of minute dots and streaks scattered in the neuropil. In the electron microscope, such a localization was exclusively astrocytic and essentially involved astrocytic leaflets, as indicated by the size and irregular shape of the immunostained processes, their location between and around neuronal profiles, and their occasional display of glial filaments. The diaminobenzidine reaction product showed some predilection for the plasma membrane and was occasionally seen at gap junctions of these labelled processes. Many labelled astrocytic leaflets were observed in the immediate vicinity of axon terminals containing large dense-cored vesicles, and around fibres morphologically identifiable as primary afferent, unmyelinated C-fibres. These observations suggest that astrocytic neurokinin-2 receptors could define the effective sphere of neurokinin A neuromodulation in rat spinal cord, via alterations in the regulation of the extracellular environment and glutamate uptake by astrocytes and/or the release of putative astroglial mediators. The astrocyte neurokinin-2 receptors, activated by extrasynaptic neurokinin A, might thus co-operate with neurokinin-1 and neurokinin-3 neuronal receptors in the modulation of nociceptive information.

Distribution of substance P receptor binding in dorsal column nuclei of rat, cat, monkey and human

In the present study, substance P receptor binding was localized in the dorsal column nuclei (DCN) of the rat, cat, monkey, and human. Bolton-Hunter-labeled [125I]substance P binding was most concentrated in the cell nests of the core region, but was present throughout the DCN of each species. The distribution of substance P receptors may reconcile apparent mismatches between the widespread responsiveness of DCN neurons to substance P and the restricted distribution of substance P containing afferents.

Comparison of [125I]-bolton-hunter substance P binding in young and aged rat spinal cord

Binding of [125I]-labeled Bolton-Hunter substance P ([125I]-BHSP) to NK1 receptors was investigated in the spinal cord of young (3-4 month) and aged (14-16 month) rats. In homogenates of whole spinal cord, the affinity (equilibrium dissociation constant, approximately 210 pM) and maximum density of [125I]-BHSP binding sites ( approximately 0.25 fmol/mg wet weight) were similar for young and aged rats. Autoradiographic studies revealed a similar distribution of [125I]-BHSP sites in both young and old rats at all spinal levels. Intense binding was observed in the superficial dorsal horn (laminae I-III), grey commissure (lamina X) and thoracic intermediolateral cell column (IML) with lower levels of binding in the deeper dorsal horn (laminae IV-VI) and ventral horn (laminae VII-IX). However, the density of [125I]-BHSP sites was significantly (P<0.05) lower in lamina X of lumbar sections of aged rats compared with young controls. These studies suggest that ageing is associated with a selective loss of NK1 receptors in lamina X of the lumbar spinal cord, although the affinity of NK1 receptors in aged rats is unchanged.

The tachykinin NK1 receptor. Part II: Distribution and pathophysiological roles

The tachykinin NK1 receptor is widely distributed in both the central and peripheral nervous system. In the CNS, NK1 receptors have been implicated in various behavioural responses and in regulating neuronal survival and degeneration. Moreover, central NK1 receptors regulate cardiovascular and respiratory function and are involved in activating the emetic reflex. At the spinal cord level, NK1 receptors are activated during the synaptic transmission, especially in response to noxious stimuli applied at the receptive field of primary afferent neurons. Both neurophysiological and behavioural evidences support a role of spinal NK1 receptors in pain transmission. Spinal NK1 receptors also modulate autonomic reflexes, including the micturition reflex. In the peripheral nervous system, tachykinin NK1 receptors are widely expressed in the respiratory, genitourinary and gastrointestinal tracts and are also expressed by several types of inflammatory and immune cells. In the cardiovascular system, NK1 receptors mediate endothelium-dependent vasodilation and plasma protein extravasation. At respiratory level, NK1 receptors mediate neurogenic inflammation which is especially evident upon exposure of the airways to irritants. In the carotid body, NK1 receptors mediate the ventilatory response to hypoxia. In the gastrointestinal system, NK1 receptors mediate smooth muscle contraction, regulate water and ion secretion and mediate neuro-neuronal communication. In the genitourinary tract, NK1 receptors are widely distributed in the renal pelvis, ureter, urinary bladder and urethra and mediate smooth muscle contraction and inflammation in response to noxious stimuli. Based on the knowledge of distribution and pathophysiological roles of NK1 receptors, it has been anticipated that NK1 receptor antagonists may have several therapeutic applications at central and peripheral level. At central level, it is speculated that NK1 receptor antagonists could be used to produce analgesia, as antiemetics and for treatment of certain forms of urinary incontinence due to detrusor hyperreflexia. In the peripheral nervous system, tachykinin NK1 receptor antagonists could be used in several inflammatory diseases including arthritis, inflammatory bowel diseases and cystitis. Several potent tachykinin NK1 receptor antagonists are now under evaluation in the clinical setting, and more information on their usefulness in treatment of human diseases will be available in the next few years.

Substance P releases and augments the morphine-evoked release of adenosine from spinal cord

The effects of substance P on the morphine-evoked release of adenosine were examined. Substance P alone produced a multiphasic effect on release of adenosine, with release occurring at low nanomolar concentrations and at a micromolar concentration, but not at intermediate concentrations. An inactive dose of substance P augmented the morphine-evoked release of adenosine at a nanomolar concentration of morphine. Release of adenosine by substance P alone (1 nM) or substance P/morphine (100 nM/10 nM) was Ca2(+)-dependent and originated from capsaicin-sensitive nerve terminals.

Receptor localization in the mammalian dorsal horn and primary afferent neurons

The dorsal horn of the spinal cord is a primary receiving area for somatosensory input and contains high concentrations of a large variety of receptors. These receptors tend to congregate in lamina II, which is a major receiving center for fine, presumably nociceptive, somatosensory input. There are rapid reorganizations of many of these receptors in response to various stimuli or pathological situations. These receptor localizations in the normal and their changes after various pertubations modify present concepts about the wiring diagram of the nervous system. Accordingly, the present work reviews the receptor localizations and relates them to classic organizational patterns in the mammalian dorsal horn.

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.

Modulation of neurotransmitter receptors following unilateral L1-S2 deafferentation: NK1, NK3, NMDA, and 5HT1a receptor binding autoradiography

Following surgical deafferentation of the spinal cord, cut dorsal roots degenerate, and spared projections compensate for this loss by collateral sprouting (reactive reinnervation). Light microscopic immunocytochemistry has shown sprouting by selected undamaged intraspinal projections, including those that express the transmitters substance P and serotonin. Quantitative immunoelectron microscopy supports these results by demonstrating loss and subsequent recovery of substance P-containing terminals and an increase in serotonin-containing terminals. To test the hypothesis that changes in afferent innervation modulate neurotransmitter receptors on second-order neurons, we used receptor binding autoradiography in this model. Adult rats were subjected to L1-S2 unilateral dorsal rhizotomy and killed at 1, 2, 6, or > 20 weeks after surgery. Receptor binding densities of tachykinin (neurokinins-1 and -3), glutamate (N-methyl-D-aspartate), and serotonin (serotonin-1a) receptors were assayed in the lumbar dorsal horn. Neurokinin-1 binding density was increased in lamina II of the deafferented side by 1 week after surgery, remained elevated at 2 weeks, and returned to control values by 6 weeks. Neurokinin 3 binding density was elevated at 2 weeks and then returned to control levels. N-methyl-D-aspartate receptor binding showed slight but not statistically significant increased binding density at 6 and at > 20 weeks. No significant changes were found in serotonin-1a receptor binding density. The elevations in tachykinin receptor binding density occur when afferents in the dorsal horn are degenerating and suggest reactive up-regulation of the receptor. The return to normal levels coincides with reactive reinnervation in the spinal cord, which restores synaptic numbers. Changes in N-methyl-D-aspartate binding occur much later than the restitution of synaptic numbers but may indicate a role for this receptor in synaptic stabilization following reactive reinnervation.

Distribution of neuropeptide FF (FLFQPQRFamide) receptors in the adult rat spinal cord: effects of dorsal rhizotomy and neonatal capsaicin

By using quantitative autoradiography and highly selective iodinated ligands, we quantified modifications in neuropeptide FF binding sites in the superficial layers (laminae I and II) of the cervical (C6-C8 segments) and lumbar (L3-L5 segments) enlargements in two models: (i) rats neonatally treated with capsaicin; (ii) rat submitted 15 days before to unilateral dorsal rhizotomies. We comparatively analysed the distribution of mu-opioid binding sites in the same animals. We have shown that the [125I]YLFQPQRFamide (neuropeptide FF sites) labelling is not significantly modified following selective damage of fine afferent fibres by neonatal capsaicin treatment. In the cervical and lumbar enlargements, capsaicin-treated/control binding ratios for [125I]YLFQPQRFamide were 0.90 and 0.86, respectively. While unilateral dorsal rhizotomy induced a drastic decrease in [125I]FK-33-824 labelling in the side ipsilateral to the lesion as compared to the intact side of (yielding ratios of 0.29 and 0.31 for cervical and lumbar levels, respectively), [125I]YLFQPQRFamide labelling was not significantly modified, yielding ratios of 0.98 and 0.91 for cervical and lumbar levels, respectively. These data suggest that, in contrast with a majority of mu-opioid receptors, neuropeptide FF receptors are not located on fine primary afferent fibers carrying nociceptive information from the fore- or hindlimb in the rat. This preferential postsynaptic localization, together with the reported "morphine modulating" action of this peptide, support the proposal of a role for neuropeptide FF in intraspinal modulation of nociceptive input.

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.

The types of neuron in spinal dorsal horn which possess neurokinin-1 receptors

In order to provide further information about the types of spinal neuron which possess neurokinin-1 receptors, we have carried out pre-embedding immunocytochemistry on sections of rat lumbar spinal cord with an antiserum raised against a synthetic peptide corresponding to part of the sequence of the receptor, and combined this with post-embedding immunocytochemistry to detect GABA and glycine. Numerous neuronal cell bodies showing neurokinin-1 receptor-immunoreactivity were seen in lamina I, laminae III-VI, the lateral spinal nucleus and the area around the central canal. Most of the cells observed in lamina III were small and had relatively restricted dendritic trees which could often not be followed into lamina II, however some larger cells in laminae III and IV had dendrites which extended through lamina II and into lamina I. Cells of the latter type are likely to represent a major target of substance P released from small-diameter primary afferents in the superficial dorsal horn. The great majority (255 out of 283) of spinal neurons which possessed neurokinin-1 receptor-immunoreactivity, including all of those in lamina I, were not GABA- or glycine-immunoreactive, however a few cells in the deep part of the dorsal horn and the lateral spinal nucleus and several cells near the central canal were GABA-immunoreactive, and some of these were also glycine-immunoreactive. These results suggest that substance P acts through neurokinin-1 receptors mainly on excitatory neurons within the spinal cord.

Reduction of substance P binding sites in the spinal dorsal horn after perineural capsaicin treatment in the rat

The long-term effect of perineural capsaicin treatment on the distribution of substance P (SP) binding sites was studied in the rat spinal dorsal horn using 125I-labelled Bolton-Hunter-SP. Three months after local application of capsaicin onto the sciatic nerve quantitative evaluation of the autoradiograms revealed a significant decrease in the density of SP binding sites of up to 48% in regions of laminae I and II of the spinal dorsal horn somatotopically related to the capsaicin treated sciatic nerve. It is suggested that reduction in SP binding may result from transganglionic and transsynaptic degenerative changes affecting postsynaptic structures. Changes in the distribution of SP binding sites may significantly contribute to functional alterations observed after perineural treatment with capsaicin.

Antinociceptive profiles of non-peptidergic neurokinin1 and neurokinin2 receptor antagonists: a comparison to other classes of antinociceptive agent

This study compared the antinociceptive properties of systemic administration of selective, non-peptidergic antagonists at neurokinin (NK1 and NK2) receptors to those of other classes of antinociceptive agent. (All doses are in mg/kg.) In mice, the NK1 antagonist, CP 99,994, preferentially (inhibitory dose50 (ID50) = 4.4) inhibited the late phase (LP) as compared to the early phase (EP) (16.1) of formalin-induced licking (FIL). A high dose (17.6) elicited ataxia in the rotarod test. Acetic acid-induced writhing was reduced at intermediate doses (10.0) whereas the tail-flick (TF) response to thermal and mechanical stimuli was inhibited only at high doses (22.7 and 17.7, respectively). Modulation of stimulus intensity did not modify the influence of CP 99,994 upon the response to heat. A similar pattern of data was acquired with RP 67,580, although this NK1 antagonist more potently inhibited writhing (2.8). In contrast, RP 68,651, the inactive isomer of RP 67,580, neither reduced the LP of FIL nor modified writhing indicating that these actions of RP 67,580 were stereospecific. Three further NK1 antagonists, SR 140,333, WIN 51,708 and WIN 62,577, likewise inhibited the LP of FIL and failed to modify the TF response at non-ataxic doses. Further, SR 140,333 (0.5) and WIN 51,708 (1.4) were potent ligands in the writhing procedure. The NK2 antagonist, SR 48,966, mimicked NK1 antagonists in preferentially inhibiting the LP (7.7) as compared to the EP (26.9) of FIL. Further, only at doses higher than those evoking ataxia (20.9) did SR 48,968 modify the TF response (36.5 and 32.0 for heat and pressure, respectively). However, it differed to NK1 antagonists in being inactive in the writhing test (> 40.0). In comparison to these NK1 and NK2 antagonists, the mu-opioid agonists (morphine and fentanyl) and kappa-opioid agonists (enadoline and U 69,593) equipotently inhibited all nociceptive responses at doses not provoking ataxia. While the glycine B receptor partial agonist, (+)-HA 966, selectively blocked the LP of FIL and did not evoke ataxia, the NMDA receptor channel blocker, (+)-MK 801, elicited antinociception only at doses close to those provoking ataxia. Finally, the NSAIDs, indomethacin and ibuprofen, the BK2 antagonist, Hoe 140 and the nitric oxide synthase (NOS) inhibitors, L-NAME and 7 nitroindazole, inhibited the LP (but not the EP) of FIL and (except for L-NAME) also reduced writhing: in contrast, they did not evoke ataxia and were inactive in the TF procedures.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropeptide receptors in developing and adult rat spinal cord: an in vitro quantitative autoradiography study of calcitonin gene-related peptide, neurokinins, mu-opioid, galanin, somatostatin, neurotensin and vasoactive intestinal polypeptide receptors

A number of neuroactive peptides including calcitonin gene-related peptide (CGRP), substance P, neurokinin B, opioids, somatostatin (SRIF), galanin, neurotensin and vasoactive intestinal polypeptide (VIP) have been localized in adult rat spinal cord and are considered to participate either directly and/or indirectly in the processing of sensory, motor and autonomic functions. Most of these peptides appear early during development, leading to the suggestion that peptides, in addition to their neurotransmitter/neuromodulator roles, may possibly be involved in the normal growth and maturation of the spinal cord. To provide an anatomical substrate for a better understanding of the possible roles of peptides in the ontogenic development of the cord, we investigated the topographical profile as well as variation in densities of [125I]hCGRP alpha, [125I]substance P/neurokinin-1 (NK-1), [125I]eledoisin/neurokinin-3 (NK-3), [125I]FK 33-824 ([D-Ala2, Me-Phe4, Met(O)ol5]enkephalin)/mu-opioid, [125I]galanin, [125I]T0D8-SRIF14 (an analog of somatostatin); [125I]neurotensin and [125I]VIP binding sites in postnatal and adult rat spinal cord using in vitro quantitative receptor autoradiography. Receptor binding sites recognized by each radioligand are found to be distributed widely during early stages of postnatal development and then to undergo selective modification to attain their adult profile of distribution during the third week of postnatal development. The apparent density of various receptor sites, however, are differently regulated depending on the lamina and the stage of development studied. For example, the density of mu-opioid binding sites, following a peak at postnatal day 4 (P4), declines gradually in almost all regions of the spinal cord with the increasing age of the animal. [125I]substance P/NK-1 binding sites, on the other hand, show very little variation until P14 and then subsequently decrease as the development proceeds. In the adult rat, most of these peptide receptor binding sites are localized in relatively high amounts in the superficial laminae of the dorsal horn. To varying extents, moderate to low density of various peptide receptor binding sites are also found to be present in the ventral horn, intermediolateral cell column and around the central canal. Taken together, these results suggest that each receptor-ligand system is regulated differently during development and may each uniquely be involved in cellular growth, differentiation and in maturation of the normal neural circuits of the spinal cord. Furthermore, the selective localization of various receptor binding sites in adult rat spinal cord over a wide variety of functionally distinct regions reinforces the neurotransmitter/modulator roles of these peptides in sensory, motor and autonomic functions associated with the spinal cord.

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.

Antagonism of nociceptive responses of cat spinal dorsal horn neurons in vivo by the NK-1 receptor antagonists CP-96,345 and CP-99,994, but not by CP-96,344

Extracellular and intracellular studies were undertaken to test the effects of the non-peptide, substance P (NK-1) receptor antagonists CP-96,345 and CP-99,994, and of CP-96,344, the inactive enantiomer of CP-96,345, on the responses of spinal dorsal horn neurons to peripheral noxious and non-noxious cutaneous stimuli in spinalized cats anesthetized with alpha-chloralose. The effect of these agents on the response of dorsal horn neurons to iontophoretic application of substance P was also tested in extracellular studies. The substance P-induced slow, prolonged discharge of dorsal horn neurons was blocked by administration (0.5 mg/kg, i.v.) of CP-96,345 (n = 10) or CP-99,994 (n = 9), but was unaffected by CP-96,344 (n = 9). The response of substance P-sensitive neurons to noxious thermal stimulation of the cutaneous receptive field, especially the late afterdischarge phase, was also significantly inhibited by CP-96,345 (n = 10) and by CP-99,994 (n = 7). The response of such neurons to noxious pinch stimulation of the receptive field was also significantly inhibited by CP-96,345 (n = 7) and CP-99,994 (n = 8), but the response of three other substance P-sensitive neurons to pinch was unaffected by CP-96,345. CP-96,344 did not affect the response of any neuron tested to either of these noxious stimuli (noxious thermal, n = 7; pinch, n = 6). The response to hair afferent stimulation was unaffected by any of these compounds (CP-96,345, n = 16; CP-96,344, n = 5; CP-99,994, n = 6). In intracellular studies, the effect of these antagonists was tested on responses of dorsal horn neurons to noxious pinch stimulation or to a train of high intensity electrical stimulation of the superficial peroneal nerve. Both stimuli produced an initial fast depolarization followed by a slow and prolonged depolarization with corresponding discharge patterns. CP-96,345 (n = 3) and CP-99,994 (n = 6) selectively blocked the late, slow components of the stimulus-evoked response without affecting the early components. Responses to single electrical pulses of the same intensity and duration were not affected. CP-96,344 did not affect any of the responses tested (n = 5). The data indicate that nociceptive responses of a subset of spinal dorsal horn cells are selectively blocked by the NK-1 receptor antagonists, CP-96,345 and CP-99,994, thus confirming the involvement of NK-1 receptors in these responses.(ABSTRACT TRUNCATED AT 400 WORDS)

Tachykinin receptors in the spinal cord

In summary, all three tachykinin receptors appear to be important modulators of physiological systems in the spinal cord. However, although there is a good deal of data concerning binding characteristics in peripheral tissues, work done in the spinal cord is scanty, leading to a number of unanswered questions. Firstly, Lui et al. (1993) have suggested a discrepancy between the location of SP binding sites and SP containing terminals. This might explain the conflicting evidence on the role of NK1 receptors in the dorsal horn. Furthermore, evidence that NK2 receptors are involved in nociception is increasing, however binding sites for these receptors in the spinal cord have not been demonstrated. This appears to be due to the difficulty in locating an ideal receptor specific ligand. The role of NK2 receptors in autonomic function is also unclear, perhaps for the same reason. Finally, there is evidence indicating that NK3 binding sites are increased following transection of the LIV-VI dorsal roots, however, studies on the effects of inflammation have not been done, as they have with the NK1 and NK2 receptors. All of these and many more unanswered questions require further investigation.

Altered calcitonin gene-related peptide, substance P and enkephalin immunoreactivities and receptor binding sites in the dorsal spinal cord of the polyarthritic rat

The dorsal horn of the spinal cord, which forms the locus of first synapses in pain pathways, is an important site of interaction between calcitonin gene-related peptide (CGRP), substance P and enkephalin--the neuropeptides considered to be especially involved in the regulation of pain perception. Since adjuvant-induced arthritic rats provide a suitable model for peripheral inflammation and hyperalgesia, the possible alterations of immunoreactive CGRP, substance P and enkephalin as well as the binding sites for [125I]hCGRP alpha, [125I]substance P/neurokinin-1, (NK1) and [125I]FK-33-824/mu-opioid receptors were studied in the dorsal horn of the spinal cord receiving projections from the inflamed limbs. In arthritic rats compared to control animals, a bilateral increase in CGRP- and substance P-immunoreactive fibres and the presence of enkephalin-immunoreactive cell bodies were noted in the dorsal horn of the spinal cord. As for receptors, while a significant decrease in [125I]hCGRP alpha and [125I]substance P/NK1 binding sites was observed in selective layers, no measurable alteration in [125I]FK-33-824/mu-opioid binding sites was noted in any regions of the arthritic rat dorsal horn compared to the unaffected control rats. Following unilateral section of the peripheral nerve prior to induction of arthritis, CGRP- and substance P-immunoreactive fibres were markedly depleted and no enkephalin-positive neurons were observed in the ipsilateral dorsal horn. Analysis of receptor binding sites in denervated arthritic rats, however, exhibited differential responses, i.e. a significant increase in [125I]hCGRP alpha, a marked decrease in [125I]FK-33-824/mu-opioid and apparently no alteration in [125I]substance P/NK1 receptor binding sites were observed in the ipsilateral dorsal horn compared to the intact contralateral side. These results taken together provide anatomical evidence for a concerted role of these peptides in the regulation of adjuvant-induced hyperalgesia accompanying peripheral inflammation.

Spinal cord amino acid release and content in an arthritis model: the effects of pretreatment with non-NMDA, NMDA, and NK1 receptor antagonists

An experimental arthritis, induced by injection of the knee joint with kaolin and carrageenan, results in guarding of and decreased weight bearing on the limb. At the time of injection, a transient increased release of all amino acids examined is measurable in samples collected by microdialysis. A second and prolonged increase of aspartate (ASP), glutamate (GLU), and glutamine (GLN) concentrations follows after 3 h. The increased release at time of injection is blocked by microdialysis application of a non-N-methyl-D-aspartate (non-NMDA) or an NMDA receptor antagonist, and the release of ASP, GLU, and GLN in the late phase is blocked by pretreatment with a non-NMDA (CNQX), an NMDA (AP7) or a neurokinin 1 (NK1; CP-96,345) antagonist. Dorsal horn immunoreactive staining of GLU, substance P (SP), and calcitonin gene-related peptide (CGRP) is reflective of the events occurring in the late phase of amino acid release since GLU release is positively correlated with GLU staining density. Increased immunoreactivity for GLU, SP, and CGRP at 8 hr in the arthritic animals is differentially altered by pretreatment of the spinal cord dorsal horn with non-NMDA, NMDA, or NK1 receptor antagonists. The differential staining pattern for GLU, SP, and CGRP, the differential release of ASP and GLU, and the differential activation of the EAA and NK1 receptors implies that ASP, GLU, SP, and CGRP are each involved in the processing of sensory information and that their roles in the central sensitization occurring with the inflammatory process, are unique.

Time-dependent changes in Bolton-Hunter-labeled 125I-substance P binding in rat spinal cord following unilateral adjuvant-induced peripheral inflammation

Time-dependent changes in Bolton-Hunter-labeled 125I-substance P binding occurred in the dorsal horn of the spinal cord following unilateral adjuvant-induced inflammation in the hindpaw of the rat. Inflammation was characterized by measures of edema and hyperalgesia. Edema and hyperalgesia were both present 6 h after induction of inflammation. However, by eight days, hyperalgesia had dissipated while edema persisted. Six hours after the induction of inflammation, widespread decreases in Bolton-Hunter-labeled 125I-substance P binding occurred on both sides of the dorsal horn of spinal level L4 in comparison to the control group. However, by two days, widespread increases in Bolton-Hunter-labeled 125I-substance P binding occurred on both sides of the spinal cord at level L4 compared to the control group. The increase in radioligand binding was primarily due to a 10-fold increase in affinity of neurokinin-1 receptors for substance P. At later time-points of four and eight days, Bolton-Hunter-labeled 125I-substance P binding remained increased only in laminae I/II on the side of the spinal cord ipsilateral to inflammation. The changes in Bolton-Hunter-labeled 125I-substance P binding suggest that alterations in substance P synaptic transmission in the spinal cord may contribute to the increased excitability of spinal neurons that accompanies adjuvant-induced peripheral inflammation.

Distribution and binding sites of substance P and calcitonin gene-related peptide and their capsaicin-sensitivity in the spinal cord of rats and chicken: a comparative study

In a comparative study, the distribution and binding sites of substance P (SP) and calcitonin gene-related peptide (CGRP) in the spinal cord, and their susceptibility towards capsaicin pretreatment were studied in rats and chicken. Rats: In accordance with the SP immunohistochemistry, specific binding sites for 125I-Bolton-Hunter-SP were highest in laminae I-III. Binding sites for 125I-0Tyr-rat-CGRP were found to be dense around the central canal, moderate in the dorsal and weak in the ventral horn. Neonatal capsaicin pretreatment, that reduced SP and CGRP immunoreactivities, increased SP specific binding sites in laminae I-III and X by 20 and 100%, respectively. An increase in CGRP binding density was detected in laminae IV, V and in the lumbar ventral horn. Displacement studies revealed a significant decrease of EC50-values for SP. Chicken: SP and CGRP immunoreactivities and SP specific binding sites were distributed similarly as in rats. Binding sites for radiolabelled CGRP, however, were highest in lamina X and in the ventral horn. Capsaicin (800 mg/kg) injected into the eggs 9 days before hatching had no influence on growth rate, nociception, peptide immunoreactivities and binding of the respective radioligands. The data demonstrated a different action of capsaicin on SP and CGRP and their specific binding sites in the spinal cord of rats and chicken and were discussed with regard to functional differences between these two animal species.

Inhibition by NK2 but not NK1 antagonists of carrageenan-induced preprodynorphin mRNA expression in rat dorsal horn lamina I neurons

Previous evidence indicated that NK2 rather than NK1 receptors play a central role in mediating the electrophysiological responses of dorsal horn neurons to brief cutaneous stimuli such as noxious heat (but not noxious pinch) and moderately sustained stimuli such as mustard oil, topically applied over 10-20 min. The present experiments were designed to investigate, by in situ hybridisation histochemistry, a delayed genomic response in dorsal horn neurons (the expression of preprodynorphin mRNA induced by intraplantar carrageenan injection) and explore the role of NK1 and NK2 receptors in mediating this response. In anaesthetised rats with bilateral intraplantar injections of carrageenan, neurokinin receptor antagonists were administered unilaterally by prolonged ionophoresis into the superficial dorsal horn. The marked increase in preprodynorphin mRNA expression elicited by carrageenan was inhibited (both in terms of number of expressing cells and their level of expression) by NK2 but not NK1 antagonists.

The effect of intrathecal administration of RP67580, a potent neurokinin 1 antagonist on nociceptive transmission in the rat spinal cord

The effect of intrathecal application of the selective neurokinin 1 (NK1) receptor antagonist RP67580 and its enantiomer RP68651 was studied on the responses of dorsal horn nociceptive neurones to formalin in the rat. The first and second phases of the formalin response were inhibited by RP67580 in a dose-related manner (1-10 micrograms), whereas RP68651 (5 micrograms) facilitated the second phase of the response. The same doses of RP67580 had minimal effects on the acute C-fibre responses. The NK1 receptor appears to play a role in prolonged nociceptive transmission in the spinal cord.

Changes in substance P and 5-HT binding in the spinal cord dorsal horn and lamina 10 after dorsolateral funiculus lesions

The present study was undertaken to determine whether changes in receptor binding of substance P (SP) and 5-hydroxytryptamine (5-HT) occur in lumbar spinal cord laminae 2, 3, 4 and 10 following interruption of descending SP and 5-HT input. These transmitters and spinal cord regions have been implicated in nociceptive and visceral functions. Quantitative receptor binding autoradiography was used to assess the binding of 2 nM [3H]SP and 2 nM [3H]5-HT to lumbar spinal cord sections taken from normal rats and rats with unilateral thoracic dorsolateral funiculus (DLF) lesions. Postoperative survival times ranged from 1 to 28 days. Substance P binding was above normal in laminae 2 and 3 ipsilateral to the lesion and in contralateral lamina 2 at 1 day postoperatively (DPO), and declined thereafter, reaching below normal levels by 28 DPO. Substance P binding in lamina 10 was significantly above normal at 7 and 14 DPO, but not at 1 or 28 DPO. Binding of 5-HT was above normal at 7 DPO in lamina 2 ipsilateral to the lesion, lamina 3 contralaterally, and lamina 10 bilaterally. These increases were not sustained, however, and at 28 DPO 5-HT binding was significantly below normal in laminae 2-4 bilaterally. The bilateral effects seen in the present study are consistent with the bilaterality of descending thoracic DLF projections demonstrated by the Fink-Heimer method.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioid and substance P receptor adaptations in the rat spinal cord following sub-chronic intrathecal treatment with morphine and naloxone

The effect of continuous intrathecal infusion with morphine (5 mu/h) or naloxone (2 micrograms/h) was investigated with regard to analgesia and the apparent density of mu- and delta-opioid and neurokinin-I/substance P receptors in the rat spinal cord. Morphine infusion increased tail-flick and paw-pressure responses until day 4 after the mini-osmotic pump implant. A decline in antinociception, reflecting tolerance to morphine, was then apparent in both tests. Quantitative in vitro receptor autoradiography of [125I]FK-33824, [125I][D.Ala2]deltorphin-I and [125I] Bolton-Hunter substance P binding sites, as ligands of mu, delta and neurokinin-I/substance P receptors, respectively, was performed on lumbosacral spinal cord sections of seven-days tolerant animals. Treatments with morphine and naloxone induced a similar increase (37%) in the number of delta binding sites in the superficial laminae of the dorsal horn. In contrast, the density of mu-opioid receptors was only affected by naloxone (50% increase). Neurokinin-I/substance P binding parameters were not altered by these treatments. Thus, it appears that delta-opioid binding sites may be of special relevance with regard to the development of tolerance to opiates in the spinal cord.

Capsaicin inhibits whereas rhizotomy potentiates substance P-induced release of excitatory amino acids in the rat spinal cord in vivo

The origin of the excitatory amino acids (EAA) aspartate (Asp) and glutamate (Glu) released into the dorsal spinal cord extracellular fluid of rats following intradialysate infusion of substance P (SP) was studied using neonatal capsaicin, dorsal rhizotomy and proximal spinal cord transection. Neonatal capsaicin (50 mg/kg i.p.) had no effect on basal EAA release, but significantly inhibited SP-induced release of both Asp (86%) and Glu (70%). Bilateral dorsal rhizotomy enhanced SP-induced release of Asp (152%) and had no effect on Glu release compared to sham-operated controls. Proximal spinal transection (T8-9) had no effect on basal or SP-induced release of EAAs compared to sham-operated controls. The ability of neonatal capsaicin to inhibit, and dorsal rhizotomy to potentiate Asp release correlates well with their distinct effects on hyperalgesia and suggests that these manipulations do not produce identical lesions. Neonatal capsaicin likely interferes with the normal development of EAA interneurons innervated by SP primary afferent C-fibers. Rhizotomy may result in a compensatory up-regulation of SP receptors on EAA interneurons.

CP-96,345, but not its stereoisomer, CP-96,344, blocks the nociceptive responses to intrathecally administered substance P and to noxious thermal and chemical stimuli in the rat

The effects of subcutaneous administration of the non-peptide NK-1 (substance P) receptor antagonist, CP-96,345, and its stereoisomer, CP-96,344, were tested in three nociceptive paradigms in the rat. In the first paradigm, tail flick responses were monitored before and after intrathecal administration of substance P (6.5 nmol) in rats pretreated subcutaneously with saline, CP-96,344 (5 mg/kg) or CP-96,345 (5 mg/kg). In the control groups, pretreated with saline (n = 6) or with CP-96,344 (n = 5), substance P reduced the tail flick reaction time at 1 min after administration to 38.3 +/- 5.1 (mean +/- S.E.M.) and 32.1 +/- 7.7% of the mean baseline value, respectively. In contrast, in the group pretreated with CP-96,345 (n = 6) the reaction time following administration of substance P was 98.8 +/- 3.3% of the baseline reaction time; this value was not significantly different from the baseline value of this group, indicating a block (P < 0.01) of the substance P-induced facilitation of the tail flick response. In the second paradigm, rats were anesthetized with a mixture of chloral hydrate (120 mg/kg, i.p.) and sodium pentobarbital (20 mg/kg, i.p.), and the effects were determined on tail flick reaction time of a sustained noxious cutaneous stimulation, immersing the tip of the tail in hot water at 55 degrees C. In the groups of rats pretreated with saline (n = 4) or with CP-96,344 (n = 7), this noxious stimulus produced a transient decrease in reaction time to 62-74% of the baseline value.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative autoradiographic distribution of calcitonin gene-related peptide (hCGRP alpha) binding sites in the rat and monkey spinal cord

Calcitonin gene-related peptide (CGRP) has been implicated in various spinal functions on the basis of its presence in the substantia gelatinosa and motoneurons and the biological effects induced by intrathecal CGRP injections. We investigated here the comparative distribution of [125I]hCGRP alpha binding sites in various segments of the rat and monkey spinal cord. The immunocytochemical localization of CGRP-like material in rat spinal cord was also evaluated for comparison. In the rat spinal cord, high densities of [125I]hCGRP alpha binding sites were observed in lamina I, in a U-shaped band that included lamina X and the medial parts of laminae III-IV and in the intermediolateral and intermediomedial nuclei. The substantia gelatinosa (lamina II) contained relatively lower, but still significant, densities of [125I]hCGRP alpha binding sites, while the ventral horn showed low amounts of specific labeling. CGRP-like immunoreactive fibers, on the other hand, were heavily concentrated in laminae I-II and in the reticulated portion of lamina V of the dorsal horn. Immunoreactivity to CGRP antiserum was also noted in fibers around the central canal and in a number of motoneurons of the ventral horn. In the monkey spinal cord, [125I]hCGRP alpha binding sites were present in lamina I in a U-shaped band that included lamina X and the medial portions of laminae V-VI. Relatively low levels of [125I]hCGRP alpha binding were detected in laminae II to IV of the dorsal horn, while the ventral horn was more enriched with specific [125I]hCGRP alpha binding sites. Thus, it appears that the autoradiographic distribution of [125I]hCGRP alpha sites is species dependent in the spinal cord. Additionally, some differences are observed between the localization of [125I]hCGRP alpha binding sites and immunoreactive material in the rat spinal cord. These differences may be relevant to the purported roles of CGRP-like peptides in spinal functions such as nociception, control of sympathetic output, and motor control.

Experimental peripheral neuropathy decreases the dose of substance P required to increase excitatory amino acid release in the CSF of the rat spinal cord

Partial ligation of the sciatic nerve of rats produces hyperalgesia similar to that seen in humans following nerve injury. In this study, we used microdialysis of the spinal cord cerebral spinal fluid (CSF) to test the hypothesis that hyperalgesia is due to an enhanced release of excitatory amino acids (EAA) in response to substance P (SP). Intrathecal SP caused release of aspartate and glutamate in the CSF of rats with partial sciatic ligation at a dose of SP that did not cause release in sham operated animals. Neonatal capsaicin pretreatment blocked SP-induced EAA release in both sham and sciatic ligated animals. Release of EAAs in ligated animals was not significantly different from release in sham-operated animals following higher doses of SP or chemical nociceptive stimulation. These results demonstrate a partial sciatic ligation-induced decrease in the dose of SP required to initiate EAA release in the CSF of the spinal cord, a change which could play an important role in hyperalgesia.

[125I]vasoactive intestinal polypeptide binding sites: quantitative autoradiographic distribution in the rat spinal cord

The quantitative autoradiographic distribution of [125I]vasoactive intestinal polypeptide (VIP) receptor binding sites was investigated in the rat spinal cord. [125I]VIP binding sites are discretely distributed, with a rostro-caudal gradient, along the longitudinal length of the cord; highest densities of sites being observed in its lumbar and sacral segments. In transverse sections, highest levels of [125I]VIP sites are present in laminae I and II, around the central canal, and in the parasympathetic lateral horn of the sacral segment. Moderate densities are seen along the medial border of the dorsal horn and the sympathetic lateral horn of the thoracic cord. Low amounts of labeling are observed in most structures of the ventral horn while white matter areas are apparently devoid of specific [125I]VIP binding. Thus, the distribution of spinal [125I]VIP receptor sites correlates well with that of VIP-like immunoreactive materials and support possible roles for this peptide in sensory neurotransmission and in the control of autonomic functions.

Novel substance P antagonist, CP-96,345, blocks responses of cat spinal dorsal horn neurons to noxious cutaneous stimulation and to substance P

Responses of dorsal horn neurons to iontophoretic application of substance P (80-120 nA), and to noxious thermal and noxious mechanical stimulations of the receptive field in the hind limb were tested in adult cats before and after the administration of the specific, non-peptide, NK-1 receptor antagonist CP-96,345 (0.5 mg/kg, i.v.). CP-96,345 inhibited the response of the neurones to substance P and also the response of these substance P-sensitive neurones to noxious thermal stimulation. The response of the substance P-insensitive neurones to noxious heat stimulations were, however, unaffected by CP-96,345. The effect of CP-96,345 on the response of neurones to noxious mechanical stimulation was variable. The results confirm the role of substance P in thermal nociception.