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

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.

The types of neuron which contain protein kinase C gamma in rat spinal cord

Protein kinase C (PKC) is thought to have a role in sensitization of dorsal horn neurons in certain pain states, and a recent study has reported that mice which lack the gamma isoform (PKCgamma) show reduced neuropathic pain after peripheral nerve injury. Although PKCgamma is present at high levels in the ventral part of lamina II we have limited information concerning the types of neuron in which it is located. In this study we have used immunocytochemistry to characterise the neurons which contain PKCgamma. Immunoreactive neurons were concentrated in ventral lamina II, but were also present in lamina III. Some weakly-immunoreactive neurons were located in the dorsal part of lamina II and in lamina I. The great majority (92%) of cells with PKCgamma were not GABA-immunoreactive, and these cells are likely to be excitatory interneurons. Dual-immunofluorescence labelling showed that PKCgamma was not randomly distributed amongst non-GABAergic neurons, since it was present in 76% of cells with neurotensin and 45% of those with somatostatin, but only 5% of those with the mu-opioid receptor (MOR-1). Cells with the neurokinin 1 receptor are found in lamina I and lamina III, and PKCgamma was present in 22% and 37% of these populations, respectively. These results suggest that excitatory interneurons in laminae II and III which lack the micro-opioid receptor may have a significant role in generating neuropathic pain.

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.

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.

Nitric oxide-producing islet cells modulate the release of sensory neuropeptides in the rat substantia gelatinosa

The substantia gelatinosa of the spinal cord (lamina II) is the major site of integration for nociceptive information. Activation of NMDA glutamate receptor, production of nitric oxide (NO), and enhanced release of substance P and calcitonin gene-related peptide (CGRP) from primary afferents are key events in pain perception and central hyperexcitability. By combining reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase histochemistry for NO-producing neurons with immunogold labeling for substance P, CGRP, and glutamate, we show that (1) NO-producing neurons in lamina IIi are islet cells; (2) these neurons rarely form synapses onto peptide-immunoreactive profiles; and (3) NADPH diaphorase-positive dendrites are often in close spatial relationship with peptide-containing terminals and are observed at the periphery of type II glomeruli showing glutamate-immunoreactive central endings. By means of confocal fluorescent microscopy in acute spinal cord slices loaded with the Ca2+ indicator Indo-1, we also demonstrate that (1) NMDA evokes a substantial [Ca2+]i increase in a subpopulation of neurons in laminae I-II, with morphological features similar to those of islet cells; (2) a different neuronal population in laminae I-IIo, unresponsive to NMDA, displays a significant [Ca2+]i increase after slice perfusion with either substance P and the NO donor 3morpholinosydnonimine (SIN-1); and (3) the responses to both substance P and SIN-1 are either abolished or significantly inhibited by the NK1 receptor antagonist sendide. These results provide compelling evidence that glutamate released at type II glomeruli triggers the production of NO in islet cells within lamina IIi after NMDA receptor activation. The release of substance P from primary afferents triggered by newly synthesized NO may play a crucial role in the cellular mechanism leading to spinal hyperexcitability and increased pain perception.

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.

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-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.

An immunocytochemical investigation of the relationship between substance P and the neurokinin-1 receptor in the lateral horn of the rat thoracic spinal cord

The relationship between substance P-containing axons and sympathetic preganglionic neurons possessing the neurokinin-1 receptor was investigated in the lateral horn of the rat thoracic spinal cord. Sympathetic preganglionic neurons were labelled retrogradely with Fluorogold. Sections containing labelled cells were reacted with antibodies against choline acetyltransferase, substance P and the neurokinin-1 receptor and examined with three-colour confocal laser scanning microscopy. In all, 95 sympathetic preganglionic neurons were examined and 79% of these were immunoreactive for the neurokinin-1 receptor. Substance P-immunoreactive axons not only made contacts with preganglionic neurons which were immunoreactive for the receptor but also made contacts with cells which did not express the receptor. Dendrites, labelled with immunoreactivity for choline actyltransferase, also received contacts from substance P-immunoreactive varicosities but this was not related to the presence or the absence of receptor. An electron microscopic analysis was performed to investigate the relationship between substance P-containing boutons and dendrites possessing the neurokinin-1 receptor. Immunoreactivity for substance P was detected with peroxidase immunocytochemistry and immunoreactivity for the receptor was detected with the silver-intensified gold method. Substance P-containing boutons made synapses with dendrites which were positively and negatively labelled for the receptor. Receptor immunoreactivity was not usually present at synapses formed by substance P boutons with neurokinin-1-immunoreactive dendrites. It is concluded that substance P may modulate much of the activity of sympathetic preganglionic neurons through an indirect non-synaptic mechanism.

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.

Profile of neuronal excitation following selective activation of the neurokinin-1 receptor in rat deep dorsal horn in vitro

The excitatory actions of the selective neurokinin-1 receptor (NK1R) agonist [Sar9,Met(O2)11]substance P (SP) were tested on a sample (n = 50) of deep dorsal horn neurones in the isolated and hemisected young rat spinal cord. Superfusion of the NK1R agonist (2 microM) elicited a prolonged membrane depolarisation (6.6 +/- 0.5 mV) and an increase in action potential firing in 41/50 (82%) neurones. These [Sar9,Met(O2)11]SP-induced depolarisations were attenuated by the selective NK1R antagonist GR82334 (1 microM). An increased neuronal excitability after [Sar9,Met(O2)11]SP application was indicated by an augmented spike frequency generated in response to long duration, step depolarisations. In order to assess whether a direct excitatory action existed, [Sar9,Met(O2)11]SP was re-tested on a sample of TTX-treated neurones (n = 14). The majority (9/14) retained agonist sensitivity although the amplitude of the depolarisation was reduced to 48% of the control value. A sample of neurones (n = 7) that responded to the NK1R agonist were morphologically characterised after filling with the intracellular dye, biocytin. Dorsal dendrites that clearly penetrated lamina II and that could receive a direct C-afferent input, were identified in only 2/7 neurones. These electrophysiological and neuroanatomical data demonstrate that deep dorsal horn neurones possess functional NK1Rs. The implications of the existence of these NK1Rs in the context of spinal somatosensory systems and SP is considered.

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.

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.

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.

Tachykinin actions on deep dorsal horn neurons in vitro: an electrophysiological and morphological study in the immature rat

To assess whether functional neurokinin receptors exist in the deep dorsal horn of the rat, the actions of the selective neurokinin-1 receptor (NK1R) agonist [Sar9,Met(O2)11]substance P ([Sar9,Met(O2)11]SP), the neurokinin-2 receptor (NK2R) agonists [beta-Ala8]NKA(4-10) and GR64349 and the neurokinin-3 receptor (NK3R) agonist senktide were examined intracellularly in vitro. [Sar9,Met(O2)11]SP (1-4 microM) and senktide (1-2 microM) elicited slow depolarizations (<10 mV) associated with increased synaptic activity and cell firing. [beta-Ala8]NKA(4-10) (10-20 microM) and GR64349 (0.25-10 microM) caused small depolarizations (<2.0 mV) and no firing. Neurons were categorized as either 'tonic' or 'phasic' depending on their firing response to direct current step depolarizations. Tonic neurons, which, unlike phasic neurons, display no spike firing accommodation, generated a significantly larger depolarization to the NK1R and NK3R agonists. The putative contribution of these receptors to primary afferent-mediated synaptic transmission was assessed by testing the NK1R antagonist GR82334 (1 microM), the NK2R antagonist MEN10,376 (1 microM) and the NK3R antagonist [Trp7,beta-Ala8]NKA(4-10) (1 microM) against the dorsal root-evoked excitatory postsynaptic potential (DR-EPSP). GR82334 and [Trp7,beta-Ala8]NKA(4-10) significantly reduced (P < or = 0.05) the duration but not the amplitude of the DR-EPSP. MEN10,376 (1 microM) had no effect on DR-EPSP amplitude or duration. Morphological detail was obtained for seven biocytin-filled deep dorsal horn neurons tested with [Sar9,Met(O2)11]SP. Five neurons responded to the NK1R agonist, and two of these had dorsally directed dendrites into the substantia gelatinosa. The other three [Sar9,Met(O2)11]SP-sensitive neurons had dendrites within deeper laminae. These data support the existence of functional NK1Rs and NK3Rs in the deep dorsal horn which may be involved in mediating sensory afferent inputs from nociceptors.

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.

Evidence for the presence of neurokinin-1 receptors on dorsal horn spinocerebellar tract cells in the rat

Dorsal horn spinocerebellar tract cells of adult rats were labelled by retrograde axonal transport with the B subunit of cholera toxin. Sections were prepared from lumbar and thoracic spinal segments and incubated with antisera which specifically recognise neurokinin-1 receptor protein and substance P. Labelled cells and immunoreactivity for the receptor and substance P were identified by using three different fluorophores and the relationships between them were assessed in single optical sections with three-colour confocal laser scanning microscopy. Forty-eight cells were examined and 23 of them displayed immunoreactivity for the receptor. Many substance P-immunoreactive profiles were present in lamina V and some formed contacts with spinocerebellar tract cells possessing neurokinin-1 receptor immunoreactivity. The evidence suggests that substance P may influence the activity of a subpopulation of dorsal horn spinocerebellar tract cells by acting through neurokinin-1 receptors.

Quantitative analysis of substance P-immunoreactive boutons on physiologically characterized dorsal horn neurons in the cat lumbar spinal cord

A quantitative analysis of substance P (SP)-immunoreactive (IR) terminals contacting physiologically characterized dorsal horn neurons was performed. Three types of neuron were studied: nociceptive specific (NS) from lamina I (n = 3), wide dynamic range (WDR) from laminae II-IV (n = 3), and nonnociceptive (NN) from lamina IV (n = 3). The nociceptive response of focus was a slow, prolonged depolarization to noxious stimuli, because this response was previously shown to be blocked by selective neurokinin-1 (NK-1) receptor antagonists. Ultrastructural immunocytochemistry was used to quantify the relative number of SP-IR boutons apposed to the intracellularly labeled cell per unit of length (density). Densities of the total population (SP immunoreactive+nonimmunoreactive) of apposed boutons were similar in all three regions (cell body, proximal and distal dendrites) for the three functional types of neuron. NS neurons received a significantly higher density of appositions from SP-IR boutons than NN cells in all three regions. However, compared to WDR cells, NS cells possessed a significantly higher density of appositions from SP-IR boutons only in the cell body and proximal dendrites. WDR cells had a higher density of appositions from SP-IR boutons than NN cells, but only in the proximal and distal dendrites. On average, 33.5% of the SP-IR boutons apposed to the cells displayed a synaptic contact. Finally, 30-45% of the SP-IR boutons apposed to the cells colocalized calcitonin gene-related protein (CGRP) immunoreactivity, indicating their primary sensory origin. The data indicate a direct correlation between the amount of SP-IR input and the nociceptive nature of the cells and suggest that SP acts on NK-1 receptors at a short distance from its release site.

GABA and its receptors in the spinal cord

The importance of the inhibitory neurotransmitter, GABA, within higher centres of the mammalian brain is unquestionable. However, its role within the spinal cord is of equal significance. There have been numerous studies over the past two decades that have established GABA as a neurotransmitter at both post- and presynaptic sites in the cord. Here, Marzia Malcangio and Norman Bowery review the current status of GABA in relation to nociception and skeletal muscle tone, and indicate that its contribution to spinal cord function should not be overlooked.

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

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

Neurokinin-1 receptors on lumbar spinothalamic neurons in the rat

In order to determine whether spinothalamic neurons in the lumbar spinal cord of the rat process neurokinin-1 (substance P) receptors, we injected cholera toxin B subunit into the thalamus and carried out dual-labelling immunocytochemistry to search for neurons that were immunoreactive with antibodies to cholera toxin and neurokinin-1 receptor. We examined 356 spinothalamic neurons in transverse sections and found that 35% of these were neurokinin-1 receptor-immunoreactive. Double-labelled cells made up the majority of the spinothalamic population in lamina I and the lateral spinal nucleus, and were also present in laminae III-V and the area around the central canal. On the side contralateral to the injection site, 77% of spinothalamic neurons in lamina I also showed neurokinin-1 receptor immunoreactivity, while 33% of those in laminae III-V and 14% of the ventromedial group possessed the receptor. Several of the double-labelled neurons with cell bodies in laminae III and IV had dendrites which could be followed dorsally into the superficial dorsal horn. These results indicate that substance P released from nociceptive primary afferents into the superficial dorsal horn is likely to act on spinothalamic tract neurons in lamina I, and also on those with cells bodies in laminae III-IV and long dorsal dendrites.