Neurokinin-1 receptors on lumbar spinothalamic neurons in the rat

Neurokinin-1 projection cells in the rat dorsal horn receive synaptic contacts from axons that possess alpha2C-adrenergic receptors

Thealpha2C subclass of adrenergic receptor (alpha2C-AR) mediates some of the antinociceptive actions of norepinephrine in the spinal cord. Axon terminals, which possess this receptor, are concentrated in the superficial dorsal horn and originate from spinal interneurons. We performed a series of combined tract-tracing and immunocytochemical studies to determine whether alpha2C-AR-immunoreactive axons target projection neurons that possess the neurokinin-1 (NK-1) receptor because such cells are likely to transmit nociceptive information to the brain. Spinomedullary neurons were labeled by stereotaxic injection of the B-subunit of cholera toxin (CTb) into the caudal ventrolateral medulla of three anesthetized adult rats. After 3 d, the animals were anesthetized again and fixed by perfusion. Sections were cut from midlumbar segments and reacted with antibodies to reveal alpha2C-ARs, CTb, and NK-1 receptors. Retrogradely labeled neurons possessing the NK-1 receptor (n = 45) were examined with confocal microscopy to investigate their relationship with alpha2C-AR-immunoreactive axons. Numerous alpha2C-AR axons were apposed to cell bodies and proximal dendrites of cells in lamina I and also to distal dendrites that originate from labeled cell bodies in lamina III/IV. A combined confocal and electron microscopic method confirmed that these appositions were synaptic. Additional experiments showed that virtually all alpha2C-AR terminals in contact with labeled cells are also immunoreactive for the vesicular glutamate transporter 2 and therefore are glutamatergic. These data suggest that norepinephrine can modulate excitatory synaptic transmission from spinal interneurons to projection cells by acting at alpha2C-ARs. This could be one of the mechanisms that underlie the antinociceptive actions of norepinephrine.

The challenge of chronic pain

Chronic pain is a complex problem with staggering negative health and economic consequences. The complexity of chronic pain is presented within Cervero and Laird's model that describes three phases of pain, including pain without tissue damage, pain with tissue damage and inflammation, and neuropathic pain. The increased afferent input in phases 2 and 3 of chronic pain produces marked changes in primary afferents, dorsal root ganglia, and spinal cord dorsal horn. These changes promote the symptoms of chronic pain, including spontaneous pain, hyperalgesia, and allodynia. Increased afferent input also evokes supraspinal input to the dorsal horn, including biphasic innervation from the ventromedial medulla and A7 catecholamine cell group, that promotes hyperalgesia and allodynia. More rostral brain structures, such as the lateral hypothalamus, amygdala, and hippocampus, may also play a role in chronic pain. Although much has been discovered about the multiple pathological mechanisms involved in chronic pain, further research is needed to fully comprehend these mechanisms.

Intrathecal substance p-saporin attenuates operant escape from nociceptive thermal stimuli

Destruction of neurons in the superficial dorsal horn that express substance P receptor (NK-1R) has been reported to block development of behavioral hypersensitivity following peripheral sensitization of nociceptors. Baseline sensitivity was not altered in these rat models that assessed innate reflex responses (i.e. hind-paw withdrawal to thermal or mechanical stimulation). In the present study, we evaluated effects of intrathecal substance P-saporin (SP-sap), a toxin selective for cells expressing NK-1R, on operant escape responses of rats to thermal stimulation. For comparison, lick/guard reflex testing was performed. Injection of a modest dose (175 ng) of SP-sap into the lumbar subarachnoid space produced a partial loss of lamina I/II NK-1R-expressing dorsal horn neurons but did not affect NK-1R-expressing neurons in deeper laminae. Lick/guard responses to 0.3, 44 or 47 degrees C were not affected after SP-sap treatment, but escape responses to these temperatures were significantly attenuated. Three hours after application of mustard oil to the dorsal surface of both hind paws, escape from 44 degrees C was enhanced for controls but not SP-sap-treated rats. Lick/guard responses were enhanced by mustard oil for both SP-sap and control animals. Administration of morphine (1.0 mg/kg, s.c.) before testing decreased escape responding at 47 degrees C for both controls and SP-sap rats. Thus, partial loss of NK-1R-expressing neurons in the superficial dorsal horn attenuated thermal nociceptive sensitivity and prevented secondary hyperalgesia when studied with an operant algesia assay, in contrast to innate reflexes which were less sensitive to modification by intrathecal SP-sap.

Postsynaptic dorsal column neurons express NK1 receptors following colon inflammation

Recent clinical and experimental studies have suggested that the dorsal column pathway and specifically postsynaptic dorsal column neurons play an important role in the transmission of visceral pain. In our study we have mapped the distribution of postsynaptic dorsal column neurons in thoracic, lumbar and sacral spinal cord segments. The presence of immunoreactivity for neurokinin 1 receptors on these postsynaptic dorsal column neurons was examined under control conditions and after colon inflammation. The largest number of postsynaptic dorsal column neurons was found in the lumbar enlargement. They were mostly located in laminae III-IV, but depending on the spinal segment, about 7-15% of them were in the deep medial dorsal horn and in the central canal area. Under control conditions none of the 1438 postsynaptic dorsal column neurons examined expressed neurokinin 1 receptors. However, after induction of colon inflammation about 1.4% of the 2015 postsynaptic dorsal column neurons observed in the experimental group showed immunoreactivity for neurokinin 1 receptors. These neurons were preferentially found in the lower thoracic and lumbosacral spinal segments where they represented about 3-4% of the total population of postsynaptic dorsal column neurons examined. The de novo expression of neurokinin1 receptors on postsynaptic dorsal column neurons after colon inflammation suggests that substance P released from visceral primary afferents under inflammatory conditions could help produce central sensitization of these neurons.

Estrogen Receptors in the Spinal Cord, Sensory Ganglia, and Pelvic Autonomic Ganglia

Until relatively recently, most studies of the effects of estradiol in the nervous system focused on hypothalamic, limbic, and other brain centers involved in reproductive hormone output, feedback, and behaviors. Almost no studies addressed estradiol effects at the spinal cord or peripheral nervous system level. Prior to the mid-1960s-1970s, few studies examined neural components of reproductive endocrine organs (e.g., ovary or testis) or the genital organs (e.g., uterus or penis) because available data supported endocrine regulation of these structures. Over the last two decades interest in and studies on the innervation of the genital organs have burgeoned. Because of the responsiveness of genital organs to sex steroid hormones, these neural studies seeded interest in whether or not autonomic and sensory neurons that innervate these organs, along with their attendant spinal cord circuits, also are responsive to sex hormones. From the mid-1980s there has been a steady growth of interest in, and studies of the neuroanatomy, neurochemistry, neural connectivity, and neural functional aspects in reproductive organs and the response of these parameters to sex steroids. Thus, with the growth of probes and techniques, has come studies of anatomy, neurochemistry, and circuitry of sex hormone-responsive neurons and circuits in the spinal cord and peripheral nervous system. This review focuses on estrogen receptors in sensory, autonomic, and spinal cord neurons in locales that are associated with innervation of female reproductive organs.

Superficial NK1-expressing neurons control spinal excitability through activation of descending pathways

The increase in pain sensitivity that follows injury is regulated by superficially located projection neurons in the dorsal horn of the spinal cord that express the neurokinin-1 (NK1) receptor. After selective ablation of these neurons in rats, we identified changes in receptive field size, mechanical and thermal coding and central sensitization of deeper dorsal horn neurons that are important for both pain sensations and reflexes. We were able to reproduce these changes by pharmacological block of descending serotonergic facilitatory pathways. Using Fos histochemistry, we found changes in the activation of serotonergic neurons in the brainstem as well as evidence for a loss of descending control of spinal excitability. We conclude that NK1-positive spinal projection neurons, activated by primary afferent input, project to higher brain areas that control spinal excitability--and therefore pain sensitivity--primarily through descending pathways from the brainstem.

Long-term potentiation in spinothalamic neurons

Sensitization of nociceptive dorsal horn neurons, including spinothalamic tract (STT) cells, is thought to underlie the development of secondary hyperalgesia and allodynia following tissue injury. In central sensitization, responses to stimulation of sensory receptors are enhanced without any change in the excitability of the primary afferent neurons. We hypothesize that central sensitization of STT neurons is a variety of long-term potentiation (LTP). Evidence that LTP occurs in the spinal cord is reviewed. Neurotransmitters that trigger central sensitization include excitatory amino acids and peptides. Evidence for this is that co-activation of N-methyl-D-aspartate and NK1 receptors can produce long-lasting increases in the responses of STT cells, and antagonists of these receptors prevent central sensitization. Responses to excitatory amino acids increase and those to inhibitory amino acids decrease during central sensitization, presumably accounting for the changed excitability of STT cells. We believe these changes result from the activation of signal transduction pathways, including the protein kinase C, NO/protein kinase G and protein kinase A cascades. Recent evidence shows that calcium/calmodulin dependent kinase II (CaMKII) is also upregulated early in the process of central sensitization and that several types of ionotropic glutamate receptors become phosphorylated. It is proposed that the phosphorylation of neurotransmitter receptors leads to alterations in the sensitivity of these receptors and to central sensitization. Comparable events occur during LTP in brain structures.

Spinal lamina I neurones that express neurokinin 1 receptors: II. Electrophysiological characteristics, responses to primary afferent stimulation and effects of a selective mu-opioid receptor agonist

Intracellular recordings were made from neurones in laminae I and II of the dorsal horn of a longitudinal, parasagittal spinal cord slice from the neonatal rat. Their responses to peripheral nerve stimulation were first tested. Then the responses to bath application of [Sar(9),Met(O(2))(11)]-substance P and [D-Ala(2),N-MePhe(4),Gly-ol(5)]-enkephalin, neurokinin 1 (NK(1)) and mu-opioid receptor agonists respectively, were studied. Finally, the structure of each neurone was investigated by injecting neurobiotin intracellularly following recording, and immunocytochemical studies were performed on post-fixed tissues to reveal whether they expressed the NK(1) receptor. Nine lamina I neurones where shown to express NK(1) receptor and these were depolarised by [Sar(9),Met(O(2))(11)]-substance P. These neurones typically received a powerful C-fibre input that was strongly inhibited, presynaptically, by the mu-opioid receptor agonist.The structure, afferent input, opioid sensitivity and intrinsic properties of these neurones are all consistent with the view that they are a major relay for nociceptive information leading to intense pain. The characteristics of 10 other neurones studied in which the NK(1) receptor was not found to be expressed at levels detectable by immunocytochemistry are briefly described for comparison. These results contribute to the emergent view that the large neurones in the most dorsal neuronal layer (lamina I) of the spinal cord, which express the principal receptor for substance P (NK(1)) over their entire soma and dendrites, are a major relay for information leading to intense pain. Inhibition of the relay of information by these neurones would be predicted to result in analgesia and hence, a detailed knowledge of their unique neurochemical characteristics is of paramount importance.

Projection neurons in lamina I of rat spinal cord with the neurokinin 1 receptor are selectively innervated by substance p-containing afferents and respond to noxious stimulation

Lamina I of the spinal cord is densely innervated by nociceptive primary afferents, many of which contain substance P. It contains numerous projection neurons: the majority of these respond to noxious stimuli, however some are activated by cooling. In the rat, approximately 80% of the projection neurons express the neurokinin 1 (NK1) receptor, on which substance P acts, and most cells with this receptor are activated by noxious stimuli. Lamina I neurons can be classified morphologically into pyramidal, multipolar, and fusiform types. It has been reported in the cat that pyramidal neurons are activated only by cooling and that in monkey relatively few pyramidal cells are NK1 receptor-immunoreactive. We have used immunocytochemistry to examine the innervation of lamina I projection neurons in the rat by substance P-containing primary afferents and their responses to a noxious stimulus (subcutaneous formalin injection). NK1 receptor-immunoreactive projection cells received a significantly higher density of contacts from substance P-containing afferents than neurons that lacked the receptor. Most contacts on NK1 receptor-immunoreactive cells were associated with synapses. Formalin injection induced c-Fos in approximately 80% of projection neurons with the NK1 receptor and in 25-45% of those without it. More than 80% of pyramidal neurons expressed the receptor, and for both substance P innervation and c-Fos expression there were no significant differences among different morphological types of NK1 receptor-immunoreactive neuron. We conclude that presence or absence of the NK1 receptor is a better indicator of function than morphology for lamina I projection neurons in the rat.

Selective innervation of lamina I projection neurones that possess the neurokinin 1 receptor by serotonin-containing axons in the rat spinal cord

Axons containing serotonin descend from brainstem to spinal cord and are thought to contribute to stimulation-produced and opioid analgesia, partly by a direct inhibitory action of serotonin on projection neurones. The density of serotoninergic innervation is highest in lamina I, which contains many nociceptive projection neurones. Two sets of anatomical criteria have been used to classify lamina I projection neurones: somatodendritic morphology and presence or absence of the neurokinin 1 receptor. To test whether the strength of serotoninergic innervation of lamina I projection neurones was related to morphology or neurokinin 1 receptor expression, we used confocal microscopy to determine the density of serotoninergic contacts on 60 cells retrogradely labelled from the caudal ventrolateral medulla. The contact density on neurones with the neurokinin 1 receptor was variable, with some cells receiving heavy input and others having few contacts. However, on average they received significantly more contacts (5.64 per 1000 microm(2) plasma membrane +/- 0.47, S.E.M.) than neurones which lacked the receptor (2.49 +/- .36). Among the neurokinin 1 neurones, serotoninergic innervation density was not related to morphology. Since the majority of serotoninergic boutons in lamina I of rat spinal cord do not appear to form synapses, we carried out electron microscopy on three heavily innervated neurokinin 1 receptor-immunoreactive projection neurones. Symmetrical synapses were found at 89% of serotoninergic contacts. These results indicate that serotoninergic innervation of lamina I projection neurones in the rat spinal cord is related to expression of neurokinin 1 receptors, but not to morphology, and that (at least on heavily innervated neurones) most serotonin-containing boutons which are in contact form synapses.

ERK MAP kinase activation in superficial spinal cord neurons induces prodynorphin and NK-1 upregulation and contributes to persistent inflammatory pain hypersensitivity

Activation of ERK (extracellular signal-regulated kinase) MAP (mitogen-activated protein) kinase in dorsal horn neurons of the spinal cord by peripheral noxious stimulation contributes to short-term pain hypersensitivity. We investigated ERK activation by peripheral inflammation and its involvement in regulating gene expression in the spinal cord and in contributing to inflammatory pain hypersensitivity. Injection of complete Freund's adjuvant (CFA) into a hindpaw produced a persistent inflammation and a sustained ERK activation in neurons in the superficial layers (laminae I-IIo) of the dorsal horn. CFA also induced an upregulation of prodynorphin and neurokinin-1 (NK-1) in dorsal horn neurons, which was suppressed by intrathecal delivery of the MEK (MAP kinase kinase) inhibitor U0126. CFA-induced phospho-ERK primarily colocalized with prodynorphin and NK-1 in superficial dorsal horn neurons. Although intrathecal injection of U0126 did not affect basal pain sensitivity, it did attenuate both the establishment and maintenance of persistent inflammatory heat and mechanical hypersensitivity. Activation of the ERK pathway in a subset of nociceptive spinal neurons contributes, therefore, to persistent pain hypersensitivity, possibly via transcriptional regulation of genes, such as prodynorphin and NK-1.

Tooth extraction-induced internalization of the substance P receptor in trigeminal nucleus and spinal cord neurons: imaging the neurochemistry of dental pain

Although pains arising from the craniofacial complex can be severe and debilitating, relatively little is known about the peripheral and central mechanisms that generate and maintain orofacial pain. To better understand the neurons in the trigeminal complex and spinal cord that are activated following nociceptive stimuli to the orofacial complex, we examined substance P (SP) induced internalization of substance P receptors (SPR) in neurons following dental extraction in the rat. Unilateral gingival reflection or surgical extraction of a rat maxillary incisor or molar was performed and tissues harvested at various time points post-extraction. Immunohistochemical analysis of brainstem and cervical spinal cord sections was performed using an anti-SPR antibody and confocal imaging. Both the number and location of neurons showing SPR internalization was dependent on the location and extent of tissue injury. Whereas extraction of the incisor induced internalization of SPR in neurons bilaterally in nucleus caudalis and the spinal cord, extraction of the molar induced strictly unilateral internalization of SPR-expressing neurons in the same brain structures. Minor tissue injury (retraction of the gingiva) activated SPR neurons located in lamina I whereas more extensive and severe tissue injury (incisor or molar extraction) induced extensive SPR internalization in neurons located in both laminae I and III-V. The rostrocaudal extent of the SPR internalization was also correlated with the extent of tissue injury. Thus, following relatively minor tissue injury (gingival reflection) neurons showing SPR internalization were confined to the nucleus caudalis while procedures which cause greater tissue injury (incisor or molar extraction), neurons showing SPR internalization extended from the interpolaris/caudalis transition zone through the C7 spinal level. Defining the population of neurons activated in orofacial pain and whether analgesics modify the activation of these neurons should provide insight into the mechanisms that generate and maintain acute and chronic orofacial pain.

A population of large lamina I projection neurons with selective inhibitory input in rat spinal cord

Lamina I of the spinal dorsal horn contains a diverse mixture of neurons. Among these, a group of giant neurons (Waldeyer cells) has long been recognized. In this study we have used immunocytochemistry to characterize a population of Waldeyer cells which were identified by the presence of high levels of the glycine receptor-associated protein gephyrin on their cell bodies and proximal dendrites. Most of these cells (27/29) were retrogradely labelled after injection of cholera toxin B subunit into the parabrachial area, and the majority (26/30) expressed the protein product of immediate-early gene c-fos, Fos, following noxious stimulation. Unlike many lamina I projection neurons, these cells either lacked the neurokinin 1 receptor, or expressed it at a very low level. Most of the gephyrin puncta on the cells were adjacent to axons that contained glutamate decarboxylase (and were therefore presumably GABAergic), which suggests that the cells are under powerful inhibitory control. Only around 35% of the puncta were associated with axons that expressed the glycine transporter GLYT2 (a marker for glycinergic axons); however, the glycine receptor alpha1 subunit was present at all of the gephyrin puncta on these cells. The cells received synapses from axons that contained nitric oxide synthase, most of which were also GABAergic, and in some cases this input was so dense that it outlined the cell bodies and dendrites. The innervation by nitric oxide synthase-containing axons was selective for these cells, compared to other neurons in the dorsal horn. From the results of this study we suggest that the gephyrin-rich cells form a specific population of lamina I projection neurons which convey noxious information to the brain. These cells are under powerful inhibitory control, and the study provides further evidence that inhibitory circuits in the dorsal horn are organized in a specific manner.

Increased Fos induction in adult rats that experienced neonatal peripheral inflammation

The response to noxious stimulation was compared in adult rats that had peripheral inflammation as neonates and untreated rats. On postnatal day 1, rat pups experienced complete Freund's adjuvant (CFA)-induced inflammation of the left hind paw. At 8 weeks of age, these rats and neonatal untreated rats received a bilateral injection of CFA into their hind paws. Fos-like immunoreactivity (Fos-LI) was used as a measure of neuronal activity in dorsal horn nociceptive pathways. A significant increase in Fos-LI was found on the left side of the lumbar spinal cord of neonatal treated rats as compared to neonatal untreated rats. These results suggest that the experience of neonatal peripheral inflammation may result in an increase in the response of spinal cord neurons to peripheral inflammation as adults.

The molecular dynamics of pain control

Pain is necessary for survival, but persistent pain can result in anxiety, depression and a reduction in the quality of life. The discriminative and affective qualities of pain are both thought to be regulated in an activity-dependent fashion. Recent studies have identified cells and molecules that regulate pain sensitivity and the parallel pathways that distribute nociceptive information to limbic or sensory areas of the forebrain. Here, we emphasize the cellular and neurobiological consequences of pain, especially those that are involved in the generation and maintenance of chronic pain. These new insights into pain processing will significantly alter our approach to pain control and the development of new analgesics.

Intracellular labeling study of neurons in the superficial part of the magnocellular layer of the medullary dorsal horn of the rat

Morphology and electrical membrane properties of neurons in the superficial part of the magnocellular layer of the rat medullary dorsal horn (MDH: caudal subnucleus of the spinal trigeminal nucleus) were examined by using horizontal slice preparations. Intracellular recording and biocytin-injection combined with histochemical and immunohistochemical staining were done. Twenty-four neurons were examined successfully and classified into projection neurons (PNs) and intrinsic neurons (INs). The PNs were further divided into type I PNs (I-PNs) and type II PNs (II-PNs). The I-PNs sent axons to the medullary reticular formation; the II-PNs sent axons to the interpolar subnucleus of the spinal trigeminal nucleus but had no axons extending to the medullary reticular formation. The INs that sent no axons to the brain regions outside the MDH were also divided into small INs with spiny dendrites (INSSs) and large INs with aspiny dendrites (INLAs). The dendritic fields of the PNs extended to laminae I and II of the MDH and occasionally further to the spinal tract of the trigeminal nerve, whereas those of the INs were confined within the magnocellular layer of the MDH. The axonal branches of each IN formed a dense axonal mesh around the cell body of the parent neuron. Although the main bodies of the axonal fields of the INs were located in the magnocellular layer, some axonal branches extended to laminae I and II of the MDH. Immunoreactivity for NK1 receptor (substance P receptor) was found in approximately half of the PNs but not in the INs. Although no strong correlation was found between morphology and electrical membrane properties, there were some differences in electrical properties among the morphologically classified neuron groups, e.g., hyperpolarizing sag was observed in some PNs but not in the Ins; inward rectification was observed in some of the INSSs and INLAs but not in the PNs; the slow ramp depolarization and the slow afterdepolarization were observed in all INSSs examined but not in the PNs or INLAs.

Changes in tactile stimuli-induced behavior and c-Fos expression in the superficial dorsal horn and in parabrachial nuclei after sciatic nerve crush

Neurons in the superficial laminae of the dorsal horn are dominated by input from peripheral nociceptors. Following peripheral nerve injury, low threshold mechanoreceptive Abeta-fibers sprout from their normal termination site in laminae III/IV into laminae I-II and this structural reorganization may contribute to neuropathic tactile pain hypersensitivity. We have now investigated whether a sciatic nerve crush injury alters the behavioral response in rats to tactile stimuli and whether this is associated with a change in the pattern of c-Fos expression in the dorsal horn and the parabrachial area of the brainstem. Sciatic nerve crush resulted in a patchy but marked tactile allodynia manifesting first at 3 weeks and persisting for up to 52 weeks. C-Fos expression in the dorsal horn and parabrachial region was never observed on brushing the skin of the sciatic nerve territory in animals with intact nerves, but was found after sciatic nerve crush with peripheral regeneration. We conclude that after nerve injury, low threshold mechanoreceptor fibers may play a major role in producing pain-related behavior by activating normally nociceptive-specific regions of the central nervous system such as the superficial laminae of the dorsal horn and the parabrachial area.

Spinal lamina I neurons that express neurokinin 1 receptors: morphological analysis

The morphology of neurons in lamina I of the dorsal horn of the lumbar spinal cord which express neurokinin 1 receptors in the rat has been investigated. On the basis of soma and dendritic measurements, these neurons form two populations. One group consists of large neurons that stain intensely for the neurokinin 1 receptor with the immunochemical methods employed. They have a large soma, typically giving rise to between three and five thick principal dendrites. The dendritic tree, however, is relatively sparse, with the principal dendrites giving rise to small numbers of second- and third-order branches. All these dendrites are almost spine free. The dendritic tree spreads extensively in the rostrocaudal (approximately 550microm) and mediolateral (approximately 30microm) orientations, with few ventrally directed branches. These cells give rise to a single axon from their soma or a principal dendrite that generates a few local branches and also ramifies sparsely in deeper laminae (II-IV). The details of axonal morphology were established from intracellularly labelled material. Ultrastructural analysis of the synaptic input to these neurons reveals that they receive synapses with both clear round, flattened and dense-core vesicles; however, they do not form components of glomerular synapses. The second neuron type stains less intensely and typically has a small fusiform soma, giving rise to dendrites at its rostral and caudal poles. The dendritic tree is long in the rostrocaudal orientation (approximately 350microm), but restricted mediolaterally (approximately 40microm). The primary dendrites of these neurons bifurcate and soon give rise to third-order branches that are spiny. No pattern of organization could be detected for the distribution of either neuron type. These observations are discussed in the light of other recent studies indicating a central role for lamina I neurons expressing neurokinin 1 in the perception of severe pain.

Visualization of lamina I of the dorsal horn in live adult rat spinal cord slices

The superficial dorsal horn of the spinal cord, particularly lamina I, plays a key role in the integration and relay of pain related sensory input. To study the physiology of lamina I neurons in slices, a clear delineation of this layer can be greatly advantageous. Yet, it has remained difficult to distinguish this layer in live tissue in conventional transverse spinal slices because of its very narrow thickness at the edge of the dorsal horn. We describe here the criteria we used to delineate lamina I in live tissue using gradient contrast videomicroscopy in 400 microm-thick parasagittal spinal cord slices from adult rats (30-60-day-old). Because of the longitudinal orientation of the neurons in this layer, the resulting distinctive reticulated appearance of lamina I made it possible to readily distinguish it from lamina II. The usefulness of this distinguishing parameter is demonstrated by our ability to contrast synaptic properties of neurons in lamina I from those in lamina II. Complete morphological identification of lamina I neurons however also requires visualization of the cell in the horizontal plane. To maintain compatibility with the parasagittal slice, we used 3D reconstructions from confocal images of the recorded neurons. Rotation of the neuron in space allowed for its morphological characterization in all three planes (horizontal, parasagittal, and transverse). This approach therefore presents optimal conditions for systematic electrophysiological recording from visually identified lamina I neurons.

Neurokinin 1 receptor expression by neurons in laminae I, III and IV of the rat spinal dorsal horn that project to the brainstem

Large neurons in laminae III and IV of the spinal cord which express the neurokinin 1 receptor and have dendrites that enter the superficial laminae are a major target for substance P (SP)-containing (nociceptive) primary afferents. Although some of these neurons project to the thalamus, we know little about other possible projection targets. The main aim of this study was to determine whether all cells of this type are projection neurons and to provide information about brainstem sites to which they project. Injections of cholera toxin B subunit were made into four brainstem areas that receive input from the spinal cord, and the proportion of cells of this type in the L4 spinal segment that were retrogradely labelled was determined in each case. The results suggest that most of these cells (>90%) project to the contralateral lateral reticular nucleus (or to a nearby region), while many (>60%) send axons to the lateral parabrachial area and some to the dorsal part of the caudal medulla. However, few of these cells project to the periaqueductal grey matter. As lamina I neurons with the neurokinin 1 receptor appear to be important in the generation of hyperalgesia, we also examined projection neurons in this lamina and found that for each injection site the great majority possessed the receptor. These results demonstrate that dorsal horn neurons which express the neurokinin 1 receptor contribute to several ascending pathways that are thought to be important in pain mechanisms.

Neurons in laminae III and IV of the rat spinal cord with the neurokinin-1 receptor receive few contacts from unmyelinated primary afferents which do not contain substance P

We have previously demonstrated that neurons in laminae III and IV of the spinal dorsal horn which possess the neurokinin-1 receptor and have long dorsal dendrites receive a major synaptic input from substance P-containing primary afferents and a more limited input from myelinated afferents. In the present study we have carried out a quantitative analysis of the contacts which cells of this type receive from two other classes of unmyelinated primary afferent: those which contain somatostatin and those without neuropeptides. We found that although boutons belonging to both of these types of afferent do form contacts with neurons of this type, the contacts are far less numerous than those formed by substance P-containing afferents. In laminae I and II, the density of contacts which dendrites of these cells received from somatostatin-containing afferents was 1.2/100 microm and that from non-peptidergic C afferents was 2.0/100 microm, which is far lower than our previous estimate of 18.8/100 microm from substance P-containing fibres in these laminae. These results indicate that although the dendrites of large neurons in laminae III and IV which possess the neurokinin-1 receptor pass through regions of the dorsal horn in which many types of primary afferent terminate, their synaptic input from primary afferents is organized in a highly selective manner.

Transmission of chronic nociception by spinal neurons expressing the substance P receptor

Substance P receptor (SPR)-expressing spinal neurons were ablated with the selective cytotoxin substance P-saporin. Loss of these neurons resulted in a reduction of thermal hyperalgesia and mechanical allodynia associated with persistent neuropathic and inflammatory pain states. This loss appeared to be permanent. Responses to mildly painful stimuli and morphine analgesia were unaffected by this treatment. These results identify a target for treating persistent pain and suggest that the small population of SPR-expressing neurons in the dorsal horn of the spinal cord plays a pivotal role in the generation and maintenance of chronic neuropathic and inflammatory pain.

Subpopulations of GABAergic and non-GABAergic rat dorsal horn neurons express Ca2+-permeable AMPA receptors

Subpopulations of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors that are either permeable or impermeable to Ca2+ are expressed on dorsal horn neurons in culture. While both mediate synaptic transmission, the Ca2+ -permeable AMPA receptors provide a Ca2+ signal that may result in a transient change in synaptic strength [Gu, J.G., Albuquerque, C., Lee, C.J. & MacDermott, A.B. (1996) Nature, 381, 793]. To appreciate the relevance of these receptors to dorsal horn physiology, we have investigated whether they show selective expression in identified subpopulations of dorsal horn neurons. Expression of Ca2+-permeable AMPA receptors was assayed using the kainate-induced cobalt loading technique first developed by Pruss et al. [Pruss, R.M., Akeson, R.L., Racke, M.M. & Wilburn, J.L. (1991) Neuron, 7, 509]. Subpopulations of dorsal horn neurons were identified using immunocytochemistry for gamma-aminobutyric acid (GABA), glycine, substance P receptor (NK1 receptor) and the Ca2+-binding proteins, calretinin and calbindin D28K. We demonstrate that, in dorsal horn neurons in culture, kainate-induced cobalt uptake is selectively mediated by Ca2+-permeable AMPA receptors, and that a majority of GABA and NK1 receptor-expressing neurons express Ca2+-permeable AMPA receptors. GABAergic dorsal horn neurons are important in local inhibition as well as in the regulation of transmitter release from primary afferent terminals. NK1 receptor-expressing dorsal horn neurons include many of the projection neurons in the nociceptive spino-thalamic pathway. Thus, we have identified two populations of dorsal horn neurons representing important components of dorsal horn function that express Ca2+-permeable AMPA receptors. Furthermore, we show that several subpopulations of putative excitatory interneurons defined by calretinin and calbindin expression do not express Ca2+-permeable AMPA receptors.

Ectopic substance P-immunoreactive boutons are preferentially presynaptic to neurokinin-1 receptor immunoreactive dendrites in the spinal white matter of transgenic mice

A recent immunocytochemical study has shown that substance P (SP) preferentially innervates targets expressing the neurokinin-1 receptor (NK-1r) in the superficial spinal dorsal horn of the rat. Based on these findings, we decided to further investigate the relationship between SP and the NK-1r in a transgenic mouse model in which SP fibres are ectopically located. Double-labelling immunocytochemistry at both the light and electron microscopic levels was performed to study the association between SP and the NK-1r in the spinal white matter of both control and transgenic mice. Light microscopy revealed NK-1r-immunoreactive (IR) dendrites in the white matter of the dorsolateral funiculus in both control and transgenic mice. In transgenic mice, but not in controls, SP-IR fibres were observed in close proximity to the NK-1r-IR dendrites in the white matter. At the ultrastructural level, SP-IR boutons were apposed to NK-1r-IR dendrites in the dorsolateral funiculus of transgenic mice, and a synapse was frequently observed as well. These results indicate that, even in conditions in which SP fibres are ectopically located, they still preferentially innervate targets expressing the NK-1r.

Distinct neurochemical features of acute and persistent pain

To address the neurochemistry of the mechanisms that underlie the development of acute and persistent pain, our laboratory has been studying mice with deletions of gene products that have been implicated in nociceptive processing. We have recently raised mice with a deletion of the preprotachykinin-A gene, which encodes the peptides substance P (SP) and neurokinin A (NKA). These studies have identified a specific behavioral phenotype in which the animals do not detect a window of "pain" intensities; this window cuts across thermal, mechanical, and chemical modalities. The lowered thermal and mechanical withdrawal thresholds that are produced by tissue or nerve injury, however, were still present in the mutant mice. Thus, the behavioral manifestations of threshold changes in nociceptive processing in the setting of injury do not appear to require SP or NKA. To identify relevant neurochemical factors downstream of the primary afferent, we are also studying the dorsal horn second messenger systems that underlie the development of tissue and nerve injury-induced persistent pain states. We have recently implicated the gamma isoform of protein kinase C (PKCgamma) in the development of nerve injury-induced neuropathic pain. Acute pain processing, by contrast, is intact in the PKCgamma-null mice. Taken together, these studies emphasize that there is a distinct neurochemistry of acute and persistent pain. Persistent pain should be considered a disease state of the nervous system, not merely a prolonged acute pain symptom of some other disease conditions.

A role for spinal lamina I neurokinin-1-positive neurons in cold thermoreception in the rat

Lamina I neurons of the spinal cord convey specific nociceptive activity to the brain. A subpopulation of lamina I cells bears substance P receptors (neurokinin-1) and recent studies have shown that these neurons encode for the intensity of noxious peripheral stimulation. Here, we report that cool thermal stimuli, applied to the hindpaw of anaesthetized rats, induce Fos expression in lamina I neurokinin-1 neurons that is graded with respect to the intensity of the thermal stimulus. Thus, as the temperature of the stimulus was reduced, both the total number of neurokinin-l-positive neurons expressing Fos and the proportion of Fos nuclei present within neurokinin-1 cells showed a significant increase. These data show that lamina I neurokinin-1 cells encode the intensity of noxious cooling of the skin. In laminae III and IV, although there was no correlation between neurokinin-1 cell activation and stimulus intensity, the total Fos count in these layers was inversely related to the depth of cooling. Thus, neurons in laminae III and IV may also play a role in thermoreception.

Cellular properties of lateral spinal nucleus neurons in the rat L6-S1 spinal cord

Conventional intracellular recordings were made from 26 lateral spinal nucleus (LSN) neurons in slices of L6-S1 spinal cord from 10- to 15-day-old rats. At rest, LSN neurons did not fire spontaneous action potentials. With injection of a positive current pulse, action potentials had an amplitude of 72 +/- 7 (SD) mV and duration at half-peak height of 0.75 +/- 0.22 ms. Action potentials were followed by an afterpotential. Most LSN neurons (13/17) exhibited only an afterhyperpolarization (AHP); four neurons exhibited both a fast and a slow AHP separated by an afterdepolarization (ADP). For LSN neurons that exhibited only an AHP, a slow ADP could be identified during bath application of apamin (100 nM). Four of 11 LSN neurons showed a postinhibitory rebound (PIR). Two types of PIR were noted, one with high threshold and low amplitude and the other with low threshold and high amplitude. The PIR with high amplitude was partially blocked in 0 mM Ca2+/high Mg2+ (10 mM) recording solution. Repetitive firing properties were examined in 17 LSN neurons. On the basis of the ratio of the slopes between initial instantaneous firing and steady-state firing frequencies, neurons with low spike frequency adaptation (SFA, 8/17) and high SFA (4/17) were identified. In addition, 2/17 LSN neurons exhibited biphasic repetitive firing patterns, which were composed of a fast SFA, delayed excitation, and low SFA; another two neurons showed only delayed excitation. Plateau potentials also were found in two LSN neurons. Dorsal root stimulation revealed that most LSN neurons (12/13) had polysynaptic postsynaptic potentials (PSP); only one neuron exhibited a monosynaptic PSP. Electrical stimulation of the dorsal root evoked prolonged discharges in low SFA neurons and a short discharge in high SFA neurons. Intrinsic properties were modulated by bath application of substance P (SP). Membrane potentials were depolarized in all eight LSN neurons tested, and membrane resistance was either increased (n = 3) or decreased (n = 2). Both instantaneous firing and steady-state firing were facilitated by SP. In addition, oscillation of membrane potentials were induced in three LSN neurons. These results demonstrate that LSN neurons exhibit a variety of intrinsic properties, which may significantly contribute to sensory processing, including nociceptive processing.

The distribution of substance P receptor (NK1)-like immunoreactive neurons in the newborn and adult human spinal cord

Substance P receptor (i.e. NK1)-like immunoreactive (SPR-LI) neurons were observed in the newborn and adult human spinal cord. Substance P receptor-like immunoreactive neuronal cell bodies were seen most frequently in lamina I, and were scattered throughout the remaining laminae of the dorsal horn and the area around the central canal. Some neurons in the intermediolateral nucleus also showed weak immunoreactivity. The pattern of distribution of SPR-LI neurons in the adult spinal cord was essentially the same as that in the newborn spinal cord. However, SPR-LI neurons cell bodies were seen much more frequently in the newborn than in the adult dorsal horn, especially in lamina II.

NK-1 receptor immunoreactivity in distinct morphological types of lamina I neurons of the primate spinal cord

In cat and monkey, lamina I cells can be classified into three basic morphological types (fusiform, pyramidal, and multipolar), and recent intracellular labeling evidence in the cat indicates that fusiform and multipolar lamina I cells are two different types of nociceptive cells, whereas pyramidal cells are innocuous thermoreceptive-specific. Because earlier observations indicated that only nociceptive dorsal horn neurons respond to substance P (SP), we examined which morphological types of lamina I neurons express receptors for SP (NK-1r). We categorized NK-1r-immunoreactive (IR) lamina I neurons in serial horizontal sections from the cervical and lumbar enlargements of four monkeys. Consistent results were obtained by two independent teams of observers. Nearly all NK-1r-IR cells were fusiform (42%) or multipolar (43%), but only 6% were pyramidal (with 9% unclassified). We obtained similar findings in three monkeys in which we used double-labeling immunocytochemistry to identify NK-1r-IR and spinothalamic lamina I neurons retrogradely labeled with cholera toxin subunit b from the thalamus; most NK-1r-IR lamina I spinothalamic neurons were fusiform (48%) or multipolar (33%), and only 10% were pyramidal. In contrast, most (approximately 75%) pyramidal and some (approximately 25%) fusiform and multipolar lamina I spinothalamic neurons did not display NK-1r immunoreactivity. These data indicate that most fusiform and multipolar lamina I neurons in the monkey can express NK-1r, consistent with the idea that both types are nociceptive, whereas only a small proportion of lamina I pyramidal cells express this receptor, consistent with the previous finding that they are non-nociceptive. However, these findings also indicate that not all nociceptive lamina I neurons express receptors for SP.

GABAergic neurons that contain neuropeptide Y selectively target cells with the neurokinin 1 receptor in laminae III and IV of the rat spinal cord

Neuropeptide Y (NPY) is contained in a population of GABAergic interneurons in the spinal dorsal horn and, when administered intrathecally, can produce analgesia. We previously identified a strong monosynaptic link between substance P-containing primary afferents and cells in lamina III or IV with the neurokinin 1 (NK1) receptor. Because some of these cells belong to the spinothalamic tract, they are likely to have an important role in pain mechanisms. In this study, we used confocal microscopy to examine the input to lamina III/IV NK1 receptor-immunoreactive neurons from NPY-containing axons. All of the cells studied received a dense innervation from NPY-immunoreactive axons, and electron microscopy revealed that synapses were often present at points of contact. Most NPY-immunoreactive boutons were also GABAergic, which supports the suggestion that they are derived from local neurons. The association between NPY-containing axons and NK1 receptor-immunoreactive neurons was specific, because postsynaptic dorsal column neurons (which were located in laminae III-V but did not possess NK1 receptors) and lamina I neurons with the NK1 receptor received significantly fewer contacts from NPY-immunoreactive axons. In addition, the NK1 receptor-immunoreactive lamina III/IV cells received few contacts from nitric oxide synthase-containing axons (which belong to a different population of GABAergic dorsal horn neurons). The NPY-containing axons appeared to be targeted to the NK1 receptor-immunoreactive neurons themselves rather than to their associated substance P-immunoreactive inputs. The dense innervation of these cells by NPY-containing axons suggests that they may possess receptors for NPY and that activation of these receptors may contribute to NPY-mediated analgesia.

The distribution of neurons expressing calcium-permeable AMPA receptors in the superficial laminae of the spinal cord dorsal horn

The superficial dorsal horn is a major site of termination of nociceptive primary afferents. Fast excitatory synaptic transmission in this region is mediated mainly by release of glutamate onto postsynaptic AMPA and NMDA receptors. NMDA receptors are known to be Ca2+-permeable and to provide synaptically localized Ca2+ signals that mediate short-term and long-term changes in synaptic strength. Less well known is a subpopulation of AMPA receptors that is Ca2+-permeable and has been shown to be synaptically localized on dorsal horn neurons in culture (Gu et al., 1996) and expressed by dorsal horn neurons in situ (Nagy et al., 1994; Engelman et al., 1997). We used kainate-induced cobalt uptake as a functional marker of neurons expressing Ca2+-permeable AMPA receptors and combined this with markers of nociceptive primary afferents in the postnatal rat dorsal horn. We have shown that cobalt-positive neurons are located in lamina I and outer lamina II, a region strongly innervated by nociceptors. These cobalt-positive neurons colocalize with afferents labeled by LD2, and with the most dorsal region of capsaicin-sensitive and IB4- and LA4-positive afferents. In contrast, inner lamina II has a sparser distribution of cobalt-positive neurons. Some lamina I neurons expressing the NK1 receptor, the receptor for substance P, are also cobalt positive. These neurons are likely to be projection neurons in the nociceptive pathway. On the basis of all of these observations, we propose that Ca2+-permeable AMPA receptors are localized to mediate transmission of nociceptive information.

Substance P receptor (neurokinin-1)-expressing neurons in lamina I of the spinal cord encode for the intensity of noxious stimulation: a c-Fos study in rat

The substance P receptor neurokinin-1 is expressed by a subset of neurons in the rat spinal cord. We have combined immunostaining for Fos, a marker of noxious peripheral stimulation, and neurokinin-1 to examine whether nociceptive signals from particular peripheral tissues (skin, muscle or knee joint) or activity generated by nerve injury or formalin-induced inflammation are preferentially modulated by substance P. Our results indicate that superficial and deep spinal neurokinin-1-positive neurons process nociceptive information in markedly different ways. In lamina I, the number of double-labelled neurons was positively correlated with the intensity of the stimulus (defined by the total Fos count) and was not directly related to any particular peripheral target. However, in the deeper layers of the spinal cord (V-X), there was no such correlation, and stimulation of joint nociceptors and formalin-induced inflammation produced the greatest proportion of Fos/neurokinin-1 co-localization, suggesting a particular role for substance P in the mediation of joint pain and inflammatory hyperalgesia. Thus, lamina I neurokinin-1 receptor-bearing neurons appear to be involved in intensity discriminative aspects of pain, whereas the deep neurokinin-1 cells are involved in spatial localization or the detection of particular nociceptive submodalities.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Preferential synaptic relationships between substance P-immunoreactive boutons and neurokinin 1 receptor sites in the rat spinal cord

Substance P plays an important role in the transmission of pain-related information in the dorsal horn of the spinal cord. Recent immunocytochemical studies have shown a mismatch between the distribution of substance P and its receptor in the superficial laminae of the dorsal horn. Because such a mismatch was not observed by using classical radioligand binding studies, we decided to investigate further the issue of the relationship between substance P and its receptor by using an antibody raised against a portion of the carboxyl terminal of the neurokinin 1 receptor and a bispecific monoclonal antibodies against substance P and horseradish peroxidase. Light microscopy revealed a good correlation between the distributions of substance P and the neurokinin 1 receptor, both being localized with highest densities in lamina I and outer lamina II of the spinal dorsal horn. An ultrastructural double-labeling study, combining preembedding immunogold with enzyme-based immunocytochemistry, showed that most neurokinin 1 receptor immunoreactive dendrites were apposed by substance P containing boutons. A detailed quantitative analysis revealed that neurokinin 1 receptor immunoreactive dendrites received more appositions and synapses from substance P immunoreactive terminals than those not expressing the neurokinin 1 receptor. Such preferential innervation by substance P occurred in all superficial dorsal horn laminae even though neurokinin 1 receptor immunoreactive dendrites were a minority of the total number of dendritic profiles in the above laminae. These results suggest that, contrary to the belief that neuropeptides act in a diffuse manner at a considerable distance from their sites of release, substance P should act on profiles expressing the neurokinin 1 receptor at a short distance from its site of release.

Cells in laminae III and IV of the rat spinal cord which possess the neurokinin-1 receptor receive monosynaptic input from myelinated primary afferents

We have previously demonstrated that neurons which have cell bodies in laminae III or IV of the rat spinal cord, dendrites that enter the superficial laminae and which possess the neurokinin-1 receptor receive a major synaptic input from substance P-containing primary afferent axons. In this study we set out to determine whether these cells also receive monosynaptic input from myelinated primary afferents by using transganglionic transport of the B subunit of cholera toxin to identify the central terminals of myelinated afferents from the sciatic nerve. Dual-immunofluorescence and confocal microscopy revealed apparent contacts between labelled primary afferent terminals and all of the neurokinin-1 receptor-immunoreactive cells examined, although these contacts were much less numerous than those which the cells receive from substance P-containing primary afferents. By using a combined confocal and electron microscopic technique we were able to confirm that synapses were present at some of the contacts between primary afferents and neurokinin-1 receptor-immunoreactive neurons. These results suggest that cells of this type will have wide-dynamic range receptive fields, but with a relatively strong input from nociceptors.

Nociceptive and thermoreceptive lamina I neurons are anatomically distinct

Pain and temperature stimuli activate neurons of lamina I within the dorsal horn of the spinal cord, and although these neurons can be classified into three basic morphological types and three major physiological classes, earlier studies did not establish a structure/function correlation between their morphology and their physiological responses. We recorded and intracellularly labeled 38 cat lamina I neurons. All 12 fusiform cells were nociceptive-specific, responsive only to pinch and/or heat. All 11 pyramidal cells were thermoreceptive-specific, responsive only to innocuous cooling. Of ten multipolar cells, six were polymodal, responsive to heat, pinch and cold, and four were nociceptive-specific. Five unclassified cells had features consistent with this pattern. These results support the view that central pain and temperature pathways contain anatomically discrete sets of modality-selective neurons.

A quantitative study of neurons which express neurokinin-1 or somatostatin sst2a receptor in rat spinal dorsal horn

The neurokinin-1 and somatostatin sst2a receptors have both been identified on spinal cord neurons. In this study we have estimated the proportions of neurons in different parts of the spinal cord which express these receptors, by using a monoclonal antibody against a neuronal nuclear protein named NeuN and combining the optical disector method with confocal microscopy. The NeuN antibody was initially tested on over 3200 neurons identified with antisera against a variety of compounds, including neuropeptides, enzymes and receptors, and also on astrocytes and oligodendrocytes. All of the neurons, but none of the glial cells that were examined possessed NeuN-immunoreactivity, which suggests that NeuN is a reliable marker for all spinal cord neurons. We found that approximately 45% of neurons in lamina I, 23-29% of those in laminae IV-VI and 18% in lamina X possessed the neurokinin-1 receptor, while the receptor was present on a smaller proportion of neurons in laminae II and III (6% and 11%, respectively). Thirteen percent of lamina I neurons and 15% of those in lamina II expressed the sst2a receptor. To provide further information about the types of neuron which possess the sst2a receptor, we searched for possible co-existence with the neurokinin-1 receptor as well as with GABA and glycine. sst2a and neurokinin-1 receptors were not co-localized on neurons in laminae I and II. All of the sst2a-immunoreactive neurons examined were also GABA-immunoreactive, and 83.5% were glycine-immunoreactive, indicating that the receptor is located on inhibitory neurons in the superficial dorsal horn. These results demonstrate the proportions of neurons in each region of the spinal cord which can be directly activated by substance P or somatostatin acting through these receptors. Levels of receptors can change in pathological states, and this method could be used to determine whether or not these changes involve alterations in the number of neurons which express receptors. In addition, the method can be used to estimate the sizes of neurochemically-defined populations of spinal cord neurons.

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.

Substance P receptor (NK1)-immunoreactive neurons projecting to the periaqueductal gray: distribution in the spinal trigeminal nucleus and the spinal cord of the rat

Substance P receptor (SPR)-immunoreactive neurons projecting to the periaqueductal gray (PAG) were examined in the rat spinal trigeminal nucleus and spinal cord by a retrograde tracing method combined with immunofluorescence histochemistry. After injection of Fluoro-gold (FG) into the PAG, SPR-immunoreactive neurons labeled with FG were observed mainly in the lateral spinal nucleus and lamina I of the medullary and spinal dorsal horns and additionally in laminae V and X of the spinal cord.

Substance P receptor-expressing neurons in the medullary and spinal dorsal horns projecting to the nucleus of the solitary tract in the rat

By using substance P receptor (SPR) immunofluorescence histochemistry combined with fluorescent retrograde labeling, we examined the distribution of the trigeminal and spinal neurons with SPR-like immunoreactivity (-LI) projecting to the nucleus of the solitary tract in the rat. After injection of Fluoro-Gold (FG) into the nucleus of the solitary tract, FG-labeled neurons showing SPR-LI were mainly seen in lamina I of the medullary and spinal dorsal horns, lamina V and the lateral spinal nucleus of the spinal cord. The present results suggest that the trigeminal and spinal neurons with SPR-LI, especially those in lamina I may be involved in the transmission of somatic and/or visceral nociceptive information from the medullary and spinal dorsal horns to the nucleus of the solitary tract.

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.

Inhibition of hyperalgesia by ablation of lamina I spinal neurons expressing the substance P receptor

Substance P is released in the spinal cord in response to painful stimuli, but its role in nociceptive signaling remains unclear. When a conjugate of substance P and the ribosome-inactivating protein saporin was infused into the spinal cord, it was internalized and cytotoxic to lamina I spinal cord neurons that express the substance P receptor. This treatment left responses to mild noxious stimuli unchanged, but markedly attenuated responses to highly noxious stimuli and mechanical and thermal hyperalgesia. Thus, lamina I spinal cord neurons that express the substance P receptor play a pivotal role in the transmission of highly noxious stimuli and the maintenance of hyperalgesia.

Noxious cutaneous thermal stimuli induce a graded release of endogenous substance P in the spinal cord: imaging peptide action in vivo

Dorsal root ganglia (DRG) neurons synthesize and transport substance P (SP) to the spinal cord where it is released in response to intense noxious somatosensory stimuli. We have shown previously that SP release in vivo causes a rapid and reversible internalization of SP receptors (SPRs) in dorsal horn neurons, which may provide a pharmacologically specific image of neurons activated by SP. Here, we report that noxious heat (43 degrees, 48 degrees, and 55 degrees C) and cold (10 degrees, 0 degrees, -10 degrees, and -20 degrees C) stimuli, but not innocuous warm (38 degrees C) and cold (20 degrees C) stimuli, applied to the hindpaw of anesthetized rats induce SPR internalization in spinal cord neurons that is graded with respect to the intensity of the thermal stimulus. Thus, with increasing stimulus intensities, both the total number of SPR+ lamina I neurons showing SPR internalization and the number of internalized SPR+ endosomes within each SPR immunoreactive neuron showed a significant increase. These data suggest that thermal stimuli induce a graded release of SP from primary afferent terminals and that agonist dependent receptor endocytosis provides evidence of a spatially and pharmacologically unique "neurochemical signature" after specific somatosensory stimuli.

Cells in laminae III and IV of the rat spinal cord that possess the neurokinin-1 receptor and have dorsally directed dendrites receive a major synaptic input from tachykinin-containing primary afferents

Many neurons with cell bodies in laminae III or IV of the spinal dorsal horn possess the neurokinin 1 receptor and have dorsal dendrites that arborize in the superficial dorsal horn. We have performed a confocal microscopic study to determine whether these cells receive inputs from substance P-containing primary afferents. All neurons of this type received contacts from substance P-immunoreactive axons, and in most cases the contacts onto dorsal dendrites were very numerous. A great majority (90-100%) of substance P-immunoreactive varicosities in contact with these cells were also immunoreactive with antibody to calcitonin gene-related peptide, indicating that they were of primary afferent origin. The density of contacts from substance P-immunoreactive varicosities onto these cells was significantly higher than that seen on cholinergic neurons in lamina III (which do not possess the receptor). Electron microscopy revealed that synapses were present at points of contact between substance P-immunoreactive boutons and dorsal dendrites of cells with the neurokinin 1 receptor. Some cells of this type belong to the spinothalamic tract, and we therefore examined neurons with cell bodies in laminae III or IV that possessed the neurokinin 1 receptor and were labeled retrogradely after thalamic injection of cholera toxin B subunit. These cells also received contacts from substance P-immunoreactive axons on their dorsal dendrites. The results of this study indicate that neurons of this type are a major target for substance P-containing primary afferents.

A method for combining confocal and electron microscopic examination of sections processed for double- or triple-labelling immunocytochemistry

Double-labelling immunocytochemical techniques are important for revealing synaptic connections between different populations of neurons within the central nervous system. This article describes a new method in which confocal laser scanning microscopy and electron microscopy are performed on the same Vibratome section which has been processed for immunocytochemistry. Two or three primary antibodies are initially detected with fluorescent secondary antibodies and observed with the confocal microscope. The primary antibodies are then revealed by an immunoperoxidase technique (with diaminobenzidine), and the material is prepared for electron microscopy. By comparing the resulting electron micrographs with the images acquired from the confocal microscope, it is possible to recognise each immunoreactive structure seen with the electron microscope in the original confocal images, and therefore to determine which type(s) of immunoreactivity each structure contains. This method has been used to demonstrate that some neurons in the spinal dorsal horn which possess the neurokinin-1 receptor receive axosomatic synapses from boutons that contain substance P and calcitonin gene-related peptide.

Absence of prostaglandin E2-induced hyperalgesia in NMDA receptor epsilon subunit knockout mice

1. We have previously found that intrathecal administration of prostaglandins E2 (PGE2) and D2 (PGD2) into conscious mice induced hyperalgesia by the hot plate test. The present study investigated the involvement of N-methyl-D-aspartate (NMDA) receptor in the prostaglandin-induced hyperalgesia by use of mice tacking NMDA receptor epsilon 1, epsilon 4, or epsilon 1/epsilon 4 subunits. 2. PGE2 induced hyperalgesia over a wide range of doses from 50 pg to 500 ng kg-1 in wild-type mice. But PGE2 could not induce hyperalgesia in epsilon 1, epsilon 4, or epsilon 1/epsilon 4 subunit knockout mice. 3. The NMDA receptor antagonist D-(-)-2-amino-5-phosphonovaleric acid (D-AP5), the non-NMDA receptor antagonist 7-D-glutamylaminomethyl sulphonic acid (GAMS), and the nitric oxide synthase inhibitor N epsilon-nitro-L-arginine methyl ester (L-NAME) inhibited the PGE2-induced hyperalgesia in wild-type mice. 4. PGD2 induced hyperalgesia at doses of 25 ng to 250 ng kg-1 in both wild-type and epsilon 1/epsilon 4 subunit knockout mice. The substance P receptor antagonist OP 96.345 blocked the PGD2-induced hyperalgesia in wild-type and epsilon 1/epsilon 4 subunit knockout mice. 5. These results demonstrate that the pathways leading to hyperalgesia are different between PGD2 and PGE2, and that both epsilon 1 and epsilon 4 subunits of the NMDA receptor are involved in the PGE2-induced hyperalgesia.

The glycine site of the NMDA receptor contributes to neurokinin1 receptor agonist facilitation of NMDA receptor agonist-evoked activity in rat dorsal horn neurons

We have investigated the role of the glycine recognition site of the N-methyl-D-aspartate receptor (the GlyNMDA site) in the facilitation of NMDA receptor agonist-evoked activity in rat dorsal horn neurons that is brought about by neurokinin1 (NK1) receptor agonist and the contribution of protein kinase C (PKC) activation to this phenomenon. Ionophoresis of the selective NMDA receptor agonist 1-aminocyclobutane-cis-1,3-dicarboxylic acid (ACBD) produced a sustained increase in the firing rate of single laminae III-V neurons recorded extracellularly using multibarrelled glass electrodes. The highly selective NK1 receptor agonist acetyl-[Arg6,Sar9,Met(O2)11]-SP6-11 (Sar9-SP) greatly facilitated this response, but under the present conditions had no effect when applied alone or with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor agonist) at the same current. In the presence of the GLyNMDA site antagonists 2-carboxy-4,6-dichloro-(1H)-indole-3-propanoic acid (MDL 29951), 7-chloro-3-(cyclopropylcarbonyl)-4-hydroxy-2(1H)-quinoline (L701,252), 5,7-dinitroquinaxoline-2,3-dione (MNQX) or 7-chlorothiokynurenic acid (7-CTK), or the PKC inhibitors, chelerythrine or GF109203X, the Sar9-SP-induced facilitation of ACBD-evoked activity was prevented, generally restoring activity to a level similar to that in the presence of ACBD alone, whilst an AMPA receptor antagonist, 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (NBQX) did not inhibit the facilitation. At the same ionophoretic currents these compounds had no effect on ACBD-evoked activity in the absence of Sar9-SP but were inhibitory at significantly greater currents. To further substantiate the importance of the GlyNMDA site in the interaction, the effects of NMDA receptor antagonists selective for alternative recognition sites on the NMDA receptor were investigated. MK-801, a non-competitive NMDA receptor antagonist and arcaine, a competitive inhibitor at the polyamine site, were applied to the facilitated activity seen in the presence of Sar9-SP and ACBD, and to ACBD-evoked activity alone. Unlike the GlyNMDA site antagonists and PKC inhibitors, these compounds reduced both facilitated and ACBD-evoked activity at similar currents. Furthermore, like the NK1 receptor agonist, a selective GlyNMDA site agonist 1-aminocyclopropane carboxylic acid (ACPC) caused facilitation of ACBD-evoked activity which was also blocked by currents of L701,252 that did not alter activity evoked by ACBD alone. These data suggest that activation of the GlyNMDA site (perhaps as a consequence of glycine release or modification of its influence by intracellular signalling cascades) is an essential component of the means by which NK1 receptor activation results in facilitated responsiveness of dorsal horn neurons towards NMDA receptor agonists.