A quantitative and morphological study of projection neurons in lamina I of the rat lumbar spinal cord

Morphological characterization of spinal cord dorsal horn lamina I neurons projecting to the parabrachial nucleus in the rat

Many Rexed's lamina I neurons are nociceptive and project to the brain. Lamina I projection neurons can be classified as multipolar, fusiform, or pyramidal, based on cell body shape and characteristics of their proximal dendrites in the horizontal plane. There is also evidence that both multipolar and fusiform cells are nociceptive and pyramidal neurons nonnociceptive. In this investigation we identified which types of lamina I neurons belong to the spinoparabrachial tract in the rat and characterized them regarding the presence or absence of neurokinin-1 receptor (NK-1r) immunoreactivity. For this, cholera toxin subunit B (CTb), conjugated to a fluorescent marker was injected unilaterally into the parabrachial nucleus. Sections were additionally stained for the detection of NK-1r immunoreactivity and were examined using fluorescence and confocal microscopy. Serial confocal optical sections and 3D reconstructions were obtained for a considerable number of neurons per animal. Using immunofluorescence, we assessed the proportion of lamina I neurons belonging to the spinoparabrachial (SPB) tract and/or expressing NK-1r. The relative distribution of neurons belonging to the SPB tract was: 38.7% multipolar, 36.8% fusiform, 22.7% pyramidal, and 1.9% unclassified. Most of the SPB neurons expressing NK-1r were either multipolar or fusiform. Pyramidal SPB neurons were seldom immunoreactive for NK-1r, an observation that provides further support to the concept that most lamina I projection neurons of the pyramidal type are nonnociceptive. In addition, our study provides further evidence that these distinct morphological types of neurons differ in their phenotypic properties, but not in their projection patterns.

Spinal CGRP1 receptors contribute to supraspinally organized pain behavior and pain-related sensitization of amygdala neurons

CGRP receptor activation has been implicated in peripheral and central sensitization. The role of spinal CGRP receptors in supraspinal pain processing and higher integrated pain behavior is not known. Here we studied the effect of spinal inhibition of CGRP1 receptors on supraspinally organized vocalizations and activity of amygdala neurons. Our previous studies showed that pain-related audible and ultrasonic vocalizations are modulated by the central nucleus of the amygdala (CeA). Vocalizations in the audible and ultrasonic range and hindlimb withdrawal thresholds were measured in awake adult rats before and 5-6h after induction of arthritis by intra-articular injections of kaolin and carrageenan into one knee. Extracellular single-unit recordings were made from neurons in the latero-capsular division of the CeA (CeLC) in anesthetized rats before and after arthritis induction. CGRP1 receptor antagonists were applied to the lumbar spinal cord intrathecally (5 microl/min) 6h postinduction of arthritis. Spinal administration of peptide (CGRP8-37, 1 microM) and non-peptide (BIBN4096BS, 1 microM) CGRP1 receptor antagonists significantly inhibited the increased responses of CeLC neurons to mechanical stimulation of the arthritic knee but had no effect under normal conditions. In arthritic rats, the antagonists also inhibited the audible and ultrasonic components of vocalizations evoked by noxious stimuli and increased the threshold of hindlimb withdrawal reflexes. The antagonists had no effect on vocalizations and spinal reflexes in normal rats. These data suggest that spinal CGRP1 receptors are not only important for spinal pain mechanisms but also contribute significantly to the transmission of nociceptive information to the amygdala and to higher integrated behavior.

Cocaine- and amphetamine-regulated transcript peptide (CART) is present in peptidergic C primary afferents and axons of excitatory interneurons with a possible role in nociception in the superficial laminae of the rat spinal cord

Cocaine- and amphetamine-regulated transcript peptides (CART) have been implicated in the regulation of several physiological functions, including pain transmission. A dense plexus of CART-immunoreactive fibres has been described in the superficial laminae of the spinal cord, which are key areas in sensory information and pain processing. In this study, we used antibody against CART peptide, together with markers for various types of primary afferents, interneurons and descending systems to determine the origin of the CART-immunoreactive axons in the superficial laminae of the rat spinal cord. Calcitonin gene-related peptide (CGRP), a marker for peptidergic primary afferents in the dorsal horn, was present in 72.6% and 34.8% of CART-immunoreactive axons in lamina I and II, respectively. The majority of these fibres also contained substance P (SP), while a few were somatostatin (SOM)-positive. The other subpopulation of CART-immunoreactive boutons in lamina I and II also expressed SP and/or SOM without CGRP, but contained vesicular glutamate transporter 2, which is present mainly in excitatory interneuronal terminals. Our data demonstrate that the majority of CART-immunoreactive axons in the spinal dorsal horn originate from peptidergic nociceptive primary afferents, while the rest arise from excitatory interneurons that contain SP or SOM. This strongly suggests that CART peptide can affect glutamatergic neurotransmission as well as the release and effects of SP and SOM in nociception and other sensory processes.

Moving from an averaged to specific view of spinal cord pain processing circuits

Neurons in the superficial dorsal horn (SDH) of the spinal cord play a critical role in processing potentially painful or noxious signals from skin, muscle, and viscera. Many acute pain therapies are based on the notion that altering the excitability of SDH neurons can block or gate these signals and reduce pain. This same notion also underlies treatments for certain chronic pain states. Basic scientists are now beginning to identify a number of potential molecular targets for spinal cord-based pain therapies with a focus on ion channels and receptors that can alter neuronal excitability. The current challenge in pain research is to identify which are the most promising targets and how their manipulation alters pain processing. In this review, we propose that our understanding of spinal pain processing mechanisms and translation of these discoveries into pain therapies could be improved by 1) better appreciating and understanding neuronal heterogeneity in the SDH; 2) establishing connectivity patterns among SDH neuron types; and 3) testing and extending findings made in vitro to intact (in vivo) animal models. As this information becomes available, it will be possible to determine the precise distribution of potential therapeutic targets on various SDH neuron types within specific circuits known to be functionally important in spinal pain processing.

Expression of c-Fos in the parabrachial nucleus following peripheral nerve injury in rats

Chronic constriction injury (CCI) of the sciatic nerve in rats evokes c-Fos expression at spinal cord level. Using immunohistochemical methods we studied changes in c-Fos expression in the brain stem area, which is suggested as one of the major targets of projection neurons in the superficial dorsal horn laminae, i.e., the parabrachial area. During the first week following injury, the animals developed tactile allodynia. At this time we found an increase of c-Fos positive neurons in the parabrachial area, mainly in the pontine part where the group of c-Fos immunoreactive neurons was present in the dorsal part of lateral parabrachial subnuclei. The number of c-Fos positive neurons gradually decreased up to 14 days following CCI. The specific activation of brain stem neurons during onset of mechanical allodynia could underlie the changes in central nociceptive processing following peripheral nerve injury.

Upregulation of substance P in low-threshold myelinated afferents is not required for tactile allodynia in the chronic constriction injury and spinal nerve ligation models

It has been proposed that substance P and calcitonin gene-related peptide (CGRP) are upregulated in low-threshold myelinated primary afferents after certain types of nerve injury, and that release of substance P from these afferents contributes to the resulting tactile allodynia. To test this hypothesis, we looked for neuropeptides in Abeta primary afferent terminals in the ipsilateral gracile nucleus and spinal dorsal horn in three nerve injury models: sciatic nerve transection (SNT), spinal nerve ligation (SNL), and chronic constriction injury (CCI). We also looked for evidence of neurokinin 1 (NK1) receptor internalization in the dorsal horn after electrical stimulation of Abeta afferents. We found no evidence of either substance P or CGRP expression in injured Abeta terminals in the spinal cord in any of the models. Although substance P was not detected in terminals of injured afferents in the gracile nucleus, CGRP was expressed in between 32 and 68% of these terminals, with a significantly higher proportion in the SNL and CCI models, compared with SNT. In addition, we did not detect any Abeta-evoked NK1 receptor internalization in neurons from laminas I, III, or IV of the dorsal horn in the CCI or SNL models. These results do not support the proposal that substance P is present at significant levels in the terminals of injured Abeta primary afferents in neuropathic models. They also suggest that any release of substance P from injured Abeta afferents is unlikely to activate NK1 receptors in the dorsal horn or contribute to neuropathic pain.

Phosphorylation of ERK in neurokinin 1 receptor-expressing neurons in laminae III and IV of the rat spinal dorsal horn following noxious stimulation

Background

There is a population of large neurons with cell bodies in laminae III and IV of the spinal dorsal horn which express the neurokinin 1 receptor (NK1r) and have dendrites that enter the superficial laminae. Although it has been shown that these are all projection neurons and that they are innervated by substance P-containing (nociceptive) primary afferents, we know little about their responses to noxious stimuli. In this study we have looked for phosphorylation of extracellular signal-regulated kinases (ERKs) in these neurons in response to different types of noxious stimulus applied to one hindlimb of anaesthetised rats. The stimuli were mechanical (repeated pinching), thermal (immersion in water at 52°C) or chemical (injection of 2% formaldehyde).

Results

Five minutes after each type of stimulus we observed numerous cells with phosphorylated ERK (pERK) in laminae I and IIo, together with scattered positive cells in deeper laminae. We found that virtually all of the lamina III/IV NK1r-immunoreactive neurons contained pERK after each of these stimuli and that in the great majority of cases there was internalisation of the NK1r on the dorsal dendrites of these cells. In addition, we also saw neurons in lamina III that were pERK-positive but lacked the NK1r, and these were particularly evident in animals that had had the pinch stimulus.

Conclusion

Our results demonstrate that lamina III/IV NK1r-immunoreactive neurons show receptor internalisation and ERK phosphorylation after mechanical, thermal or chemical noxious stimuli.

Inflammation reduces the contribution of N-type calcium channels to primary afferent synaptic transmission onto NK1 receptor-positive lamina I neurons in the rat dorsal horn

N-type calcium channels contribute to the release of glutamate from primary afferent terminals synapsing onto nocisponsive neurons in the dorsal horn of the spinal cord, but little is known of functional adaptations to these channels in persistent pain states. Subtype-selective conotoxins and other drugs were used to determine the role of different calcium channel types in a rat model of inflammatory pain. Electrically evoked primary afferent synapses onto lumber dorsal horn neurons were examined three days after induction of inflammation with intraplantar complete Freund's adjuvant. The maximal inhibitory effect of the N-type calcium channel blockers, omega-conotoxins CVID and MVIIA, were attenuated in NK1 receptor-positive lamina I neurons after inflammation, but the potency of CVID was unchanged. This was associated with reduced inhibition of the frequency of asynchronous-evoked synaptic events by CVID studied in the presence of extracellular strontium, suggesting reduced N-type channel contribution to primary afferent synapses after inflammation. After application of CVID, the relative contributions of P/Q and L channels to primary afferent transmission and the residual current were unchanged by inflammation, suggesting the adaptation was specific to N-type channels. Blocking T-type channels did not affect synaptic amplitude under control or inflamed conditions. Reduction of N-type channel contribution to primary afferent transmission was selective for NK1 receptor-positive neurons identified by post hoc immunohistochemistry and did not occur at synapses in laminae II(o) or II(i), or inhibitory synapses. These results suggest that inflammation selectively downregulates N-type channels in the terminals of primary afferents synapsing onto (presumed) nociceptive lamina I NK1 receptor-positive neurons.

Local and descending circuits regulate long-term potentiation and zif268 expression in spinal neurons

Long-term potentiation (LTP), a use dependent long-lasting modification of synaptic strength, was first discovered in the hippocampus and later shown to occur in sensory areas of the spinal cord. Here we demonstrate that spinal LTP requires the activation of a subset of superficial spinal dorsal horn neurons expressing the neurokinin-1 receptor (NK1-R) that have previously been shown to mediate certain forms of hyperalgesia. These neurons participate in local spinal sensory processing, but are also the origin of a spino-bulbo-spinal loop driving a 5-hydroxytryptamine 3 receptor (5HT3-R)- mediated descending facilitation of spinal pain processing. Using a saporin-substance P conjugate to produce site-specific neuronal ablation, we demonstrate that NK1-R expressing cells in the superficial dorsal horn are crucial for the generation of LTP-like changes in neuronal excitability in deep dorsal horn neurons and this is modulated by descending 5HT3-R-mediated facilitatory controls. Hippocampal LTP is associated with early expression of the immediate-early gene zif268 and knockout of the gene leads to deficits in long-term LTP and learning and memory. We found that spinal LTP is also correlated with increased neuronal expression of zif268 in the superficial dorsal horn and that zif268 antisense treatment resulted in deficits in the long-term maintenance of inflammatory hyperalgesia. Our results support the suggestion that the generation of LTP in dorsal horn neurons following peripheral injury may be one mechanism whereby acute pain can be transformed into a long-term pain state.

Synaptic amplifier of inflammatory pain in the spinal dorsal horn

Inflammation and trauma lead to enhanced pain sensitivity (hyperalgesia), which is in part due to altered sensory processing in the spinal cord. The synaptic hypothesis of hyperalgesia, which postulates that hyperalgesia is induced by the activity-dependent long-term potentiation (LTP) in the spinal cord, has been challenged, because in previous studies of pain pathways, LTP was experimentally induced by nerve stimulation at high frequencies ( approximately 100 hertz). This does not, however, resemble the real low-frequency afferent barrage that occurs during inflammation. We identified a synaptic amplifier at the origin of an ascending pain pathway that is switched-on by low-level activity in nociceptive nerve fibers. This model integrates known signal transduction pathways of hyperalgesia without contradiction.

Possible sources and sites of action of the nitric oxide involved in synaptic plasticity at spinal lamina I projection neurons

The synaptic long-term potentiation between primary afferent C-fibers and spinal lamina I projection neurons is a cellular model for hyperalgesia [Ikeda H, Heinke B, Ruscheweyh R, Sandkühler J (2003) Synaptic plasticity in spinal lamina I projection neurons that mediate hyperalgesia. Science 299:1237-1240]. In lamina I neurons with a projection to the periaqueductal gray, this long-term potentiation is dependent on nitric oxide. In the present study, we used immunohistochemistry to detect possible sources and sites of action of the nitric oxide necessary for the long-term potentiation at lamina I spino-periaqueductal gray neurons in rats. None of the three isoforms of the nitric oxide synthase was expressed in a significant number of lamina I spino-periaqueductal gray neurons or primary afferent C-fibers (as evaluated by staining of their cell bodies in the dorsal root ganglia). However, endothelial and inducible nitric oxide synthase were found throughout the spinal cord vasculature and neuronal nitric oxide synthase was present in a number of neurons in laminae II and III. The nitric oxide target soluble guanylyl cyclase was detected in most lamina I spino-periaqueductal gray neurons and in approximately 12% of the dorsal root ganglion neurons, all of them nociceptive as evaluated by coexpression of substance P. Synthesis of cyclic 3',5'-guanosine monophosphate upon stimulation by a nitric oxide donor confirmed the presence of active guanylyl cyclase in at least a portion of the spino-periaqueductal gray neuronal cell bodies. We therefore propose that nitric oxide generated in neighboring neurons or blood vessels acts on the spino-periaqueductal gray neuron and/or the primary afferent C-fiber to enable long-term potentiation. Lamina I spino-parabrachial neurons were stained for comparison and yielded similar results.

Characterization of sensory neuron subpopulations selectively expressing green fluorescent protein in phosphodiesterase 1C BAC transgenic mice

BACKGROUND

The complex neuronal circuitry of the dorsal horn of the spinal cord is as yet poorly understood. However, defining the circuits underlying the transmission of information from primary afferents to higher levels is critical to our understanding of sensory processing. In this study, we have examined phosphodiesterase 1C (Pde1c) BAC transgenic mice in which a green fluorescent protein (GFP) reporter gene reflects Pde1c expression in sensory neuron subpopulations in the dorsal root ganglia and spinal cord.

RESULTS

Using double labeling immunofluorescence, we demonstrate GFP expression in specific subpopulations of primary sensory neurons and a distinct neuronal expression pattern within the spinal cord dorsal horn. In the dorsal root ganglia, their distribution is restricted to those subpopulations of primary sensory neurons that give rise to unmyelinated C fibers (neurofilament 200 negative). A small proportion of both non-peptidergic (IB4-binding) and peptidergic (CGRP immunoreactive) subclasses expressed GFP. However, GFP expression was more common in the non-peptidergic than the peptidergic subclass. GFP was also expressed in a subpopulation of the primary sensory neurons immunoreactive for the vanilloid receptor TRPV1 and the ATP-gated ion channel P2X3. In the spinal cord dorsal horn, GFP positive neurons were largely restricted to lamina I and to a lesser extent lamina II, but surprisingly did not coexpress markers for key neuronal populations present in the superficial dorsal horn.

CONCLUSION

The expression of GFP in subclasses of nociceptors and also in dorsal horn regions densely innervated by nociceptors suggests that Pde1c marks a unique subpopulation of nociceptive sensory neurons.

Disinhibition opens the gate to pathological pain signaling in superficial neurokinin 1 receptor-expressing neurons in rat spinal cord

Blockade of local spinal cord inhibition mimics the behavioral hypersensitivity that manifests in chronic pain states. This suggests that there is a pathway capable of mediating allodynia/hyperalgesia that exists but is normally under strong inhibitory control. Lamina I and III neurokinin 1 (NK1) receptor expressing (NK1R+) dorsal horn neurons, many of which are projection neurons, are required for the development of this hypersensitivity and are therefore likely to be a component of this proposed pathway. To investigate, whole-cell patch-clamp recordings were made from lamina I and III NK1R+ neurons in the spinal cord slice preparation with attached dorsal root. Excitatory postsynaptic currents were recorded in response to electrical stimulation of the dorsal root. Lamina I NK1R+ neurons were shown to receive high-threshold (Adelta/C fiber) monosynaptic input, whereas lamina III NK1R+ neurons received low-threshold (Abeta fiber) monosynaptic input. In contrast, lamina I neurons lacking NK1 receptor (NK1R-) received polysynaptic A fiber input. Blockade of local GABAergic and glycinergic inhibition with bicuculline (10 microm) and strychnine (300 nm), respectively, revealed significant A fiber input to lamina I NK1R+ neurons that was predominantly Abeta fiber mediated. This novel A fiber input was polysynaptic in nature and required NMDA receptor activity to be functional. In lamina I NK1R- and lamina III NK1R+ neurons, disinhibition enhanced control-evoked responses, and this was also NMDA receptor dependent. These disinhibition-induced changes, in particular the novel polysynaptic low-threshold input onto lamina I NK1R+ neurons, may be an underlying component of the hypersensitivity present in chronic pain states.

Differential expression of NK1 and GABAB receptors in spinal neurones projecting to antinociceptive or pronociceptive medullary centres

The balance between excitatory and inhibitory input exerted upon spinal cord neurones that belong to spinofugal pathways determines the ultimate type of information transmitted to the brain. We compared the relative expression of NK1 and GABAB receptors in two spinomedullary pathways targeting an antinociceptive area and a pronociceptive centre, respectively, the lateral part of the caudal ventrolateral medulla (VLMlat) and the dorsal reticular nucleus (DRt). Spinal cord sections of rats injected in the VLMlat or DRt with the retrograde tracer cholera toxin subunit B were triple-immunoreacted for the tracer, NK1 receptors and GABAB receptors. The dorsal horn neurones labelled from the VLMlat mainly co-localized the two receptors while those labelled from the DRt mainly expressed GABAB receptors, which was particularly evident in neurones of laminae IV-V. The morphological classification of lamina I neurones projecting to the VLMlat showed that fusiform, flattened and pyramidal cells mainly co-localized NK1 and GABAB receptors. As to lamina I neurones projecting to the DRt, multipolar neurones mainly expressed GABAB receptors while the majority of flattened and pyramidal neurones co-localized NK1 and GABAB receptors. The present results suggest that the expression of NK1 and GABAB receptors varies in neurones participating to different spinofugal pathways. The importance of the present findings in the knowledge of the endogenous supraspinal pain control system is discussed.

Characterization of neurons that express preprotachykinin B in the dorsal horn of the rat spinal cord

Although it is established that neurokinin B is expressed by some neurons in laminae I-III of the rat spinal dorsal horn, little is known about the proportions of cells in these laminae that express neurokinin B, or whether these are excitatory or inhibitory neurons. Neurokinin B is derived from preprotachykinin B, and we have used an antibody against preprotachykinin B to address these issues. We found that preprotachykinin B-immunoreactive neurons were present throughout laminae I-III, constituting 10-11% of the neuronal population in laminae I-II, and 4% of that in lamina III. They formed a prominent band in the ventral half of lamina II (where they made up 16% of the population) and the dorsalmost part of lamina III. The great majority (99%) of preprotachykinin B-immunoreactive axonal boutons contained the vesicular glutamate transporter 2, while none contained glutamic acid decarboxylase. Since most of these boutons are likely to be derived from local preprotachykinin B-expressing cells, these observations suggest that most of the latter are excitatory interneurons. Although 9% of preprotachykinin B-labeled axonal varicosities were substance P-immunoreactive, none contained calcitonin gene-related peptide, which is consistent with reports that neurokinin B is not expressed by primary afferent axons. Many of the preprotachykinin B-immunoreactive cells contained compounds that are present in putative excitatory neurons in laminae I-III: calbindin (84%), protein kinase Cgamma (76%) or somatostatin (31%). However, there was little or no overlap between preprotachykinin B and three other markers associated with excitatory neurons in these laminae: the mu opioid receptor MOR-1, the neurokinin 1 receptor and neurotensin. These results suggest that neurokinin B is expressed by specific populations of excitatory neurons in the superficial dorsal horn. By examining expression of Fos protein in response to intraplantar injection of formaldehyde we provide evidence that many of the preprotachykinin B cells in lamina I and the outer part of lamina II respond to noxious stimulation.

Morphology and neurokinin 1 receptor expression of spinothalamic lamina I neurons in the rat spinal cord

Distinct morphological types of spinothalamic tract (STT) lamina I (LI) neurons have been identified in the cat and monkey spinal dorsal horn. Because these morphological types appear to differ in functional properties and receptor expression, we examined their distribution in the rat to test how their identification relates to earlier classification schemes. LI STT cells were retrogradely labeled with cholera toxin subunit b (CTb). Three types were recognized on the basis of cell body shape and proximal dendrites in the horizontal plane: fusiform, multipolar, and pyramidal. The relative distribution of these types was: 43, 26, and 28%, respectively, similar to that observed in the cat and monkey. 3D reconstructions were used to view each cell in all three major projection planes: horizontal, parasagittal, and transverse. Most LI STT neurons appeared fusiform in the parasagittal plane even though they belonged to different types based on their appearance in the horizontal plane, except in the most lateral portion of the dorsal horn, where LI curves ventrally. The proportion of STT neurons within LI was quantified by using the optical dissector method. To label all LI neurons, we used an anti-neuron-specific nuclear protein (NeuN) antibody. We found that approximately 9% of LI neurons projected to the thalamus. We also investigated neurokinin 1 receptor (NK-1r) expression in LI STT neurons. As in the monkey, most pyramidal STT neurons did not express NK-1r. These results provide further evidence that distinct morphological types of neurons differ in phenotype but not in their projection pattern.

Brain stem control of arterial pressure in chronic arterial baroreceptor-denervated rats

Interruption of the baroreceptor reflex by transection of afferent nerves (sinoaortic denervation; SAD) or lesions of nucleus tractus solitarius (NTS) elevates sympathetic nerve activity (SNA) and arterial pressure (AP). However, within 1 wk, mean AP returns to normal despite the absence of baroreflexes. In this study, we examine central mechanisms that control AP in chronic baroreceptor-denervated rats. In urethane-anesthetized rats (1.5g/kg iv) after autonomic ganglionic blockade (5 mg/kg iv chlorisondamine), α1-adrenergic-mediated pressor responses (1–100 μg/kg iv phenylephrine) were not altered by chronic lesions of NTS, indicating vascular reactivity to sympathetic stimulation is normal. Transection of the spinal cord at T1 profoundly decreased AP and was not further reduced by chlorisondamine in control or denervated rats. Inhibition of the rostral ventrolateral medulla (RVLM) by microinjections of muscimol (100 pmol/side) decreased AP to levels not further reduced by chlorisondamine in control rats, rats with SAD, and rats with NTS lesions. Blockade of GABAA receptors in the RVLM (50 pmol/side bicuculline) increased AP similarly in control rats and denervated rats. In agreement, inhibition of the caudal ventrolateral medulla (CVLM) by microinjections of muscimol or blockade of glutamatergic inputs (2.7 nmol/side kynurenate) produced comparable increases in AP in control and denervated rats. These data suggest the RVLM continues to drive the SNA that regulates AP in the chronic absence of baroreceptor inputs. In addition, despite the absence of a tonic excitatory input from NTS, in chronic baroreceptor-denervated rats glutamatergic inputs drive the CVLM to tonically inhibit the RVLM. Baroreceptor-independent regulation of the ventrolateral medulla may underlie central mechanisms contributing to the long-term control of AP.

Properties of mouse spinal lamina I GABAergic interneurons

Lamina I is a sensory relay region containing projection cells and local interneurons involved in thermal and nociceptive signaling. These neurons differ in morphology, sensory response modality, and firing characteristics. We examined intrinsic properties of mouse lamina I GABAergic neurons expressing enhanced green fluorescent protein (EGFP). GABAergic neuron identity was confirmed by a high correspondence between GABA immunolabeling and EGFP fluorescence. Morphologies of these EGFP+/GABA+ cells were multipolar (65%), fusiform (31%), and pyramidal (4%). In whole cell recordings, cells fired a single spike (44%), tonically (35%), or an initial burst (21%) in response to current steps, representing a subset of reported lamina I firing properties. Membrane properties of tonic and initial burst cells were indistinguishable and these neurons may represent one functional population because, in individual neurons, their firing patterns could interconvert. Single spike cells were less excitable with lower membrane resistivity and higher rheobase. Most fusiform cells (64%) fired tonically while most multipolar cells (56%) fired single spikes. In summary, lamina I inhibitory interneurons are functionally divisible into at least two major groups both of which presumably function to limit excitatory transmission.

Synaptic input of rat spinal lamina I projection and unidentified neurones in vitro

Spinal lamina I projection neurones that transmit nociceptive information to the brain play a pivotal role in hyperalgesia in various animal models of inflammatory and neuropathic pain. Consistently, activity-dependent long-term potentiation can be induced at synapses between primary afferent C-fibres and lamina I projection neurones but not unidentified neurones in lamina I. The specific properties that enable projection neurones to undergo long-term potentiation and mediate hyperalgesia are not fully understood. Here, we have tested whether lamina I projection neurones differ from unidentified neurones in types or strength of primary afferent input and/or action potential-independent excitatory and inhibitory input. We used the whole-cell patch-clamp technique to record synaptic currents in projection and unidentified lamina I neurones in a transverse lumbar spinal cord slice preparation from rats between postnatal day 18 and 37. Lamina I neurones with a projection to the parabrachial area or the periaqueductal grey were identified by retrograde labelling with a fluorescent tracer. The relative contribution of NMDA receptors versus AMPA/kainate receptors to C-fibre-evoked excitatory postsynaptic currents of lamina I neurones significantly decreased with age between postnatal day 18 and 27, but was independent of the supraspinal projection of the neurones. We did not find a significant contribution of kainate receptors to C-fibre-evoked excitatory postsynaptic currents. Lamina I projection and unidentified neurones possessed functional GABAA and glycine receptors but received scarce action potential-independent spontaneous GABAergic and glycinergic inhibitory input as measured by miniature inhibitory postsynaptic currents. The miniature excitatory postsynaptic current frequencies were five times higher in projection than in unidentified neurones. The predominance of excitatory synaptic input to projection neurones, taken together with the previous finding that their membranes are more easily excitable than those of unidentified neurones, may facilitate the induction of synaptic long-term potentiation.

Fos induction in lamina I projection neurons in response to noxious thermal stimuli

Lamina I of the spinal cord contains many projection neurons: the majority of these are activated by noxious stimulation, although some respond to other stimuli, such as innocuous cooling. In the rat, approximately 80% of lamina I projection neurons express the neurokinin 1 (NK1) receptor, on which substance P acts. Lamina I neurons can be classified into three main morphological classes: pyramidal, fusiform and multipolar cells. It has been reported that in the cat, pyramidal cells respond to innocuous cooling, and whilst both fusiform and multipolar cells are activated by noxious mechanical and heat stimuli, only cells in the latter group respond to noxious cold [Nat Neurosci 1 (1998) 218]. However, we have previously shown that NK1 receptor-immunoreactive projection neurons belonging to each morphological class are equally likely to up-regulate the transcription factor Fos after noxious chemical stimulation, and that the density of innervation by substance P-containing (nociceptive) afferents is similar for cells of each type [J Neurosci 22 (2002) 4103]. This suggests that the morphological-physiological correlation that has been reported in the cat may not apply in the rat. We have tested this further by examining Fos expression in lamina I spinoparabrachial neurons in the rat after application of noxious heat or noxious cold stimuli under general anesthesia. Following noxious heat, 57-69% of NK1 receptor-immunoreactive spinoparabrachial neurons expressed Fos, and the proportion did not differ significantly between morphological groups. However, after noxious cold stimulation Fos was present in 63% of multipolar neurons, but only 19-26% of fusiform or pyramidal cells. These results suggest that although most NK1 receptor-expressing spinoparabrachial neurons are activated by noxious stimuli, responsiveness to noxious cold is significantly more common in those of the multipolar type. There therefore appears to be a correlation between morphology and function for lamina I projection neurons in the rat.

In cat four times as many lamina I neurons project to the parabrachial nuclei and twice as many to the periaqueductal gray as to the thalamus

The spinothalamic tract, and especially its fibers originating in lamina I, is the best known pathway for transmission of nociceptive information. On the other hand, different studies have suggested that more lamina I cells project to the parabrachial nuclei (PBN) and periaqueductal gray (PAG) than to the thalamus. The exact ratio of the number of lamina I projections to PBN, PAG and thalamus is not known, because comprehensive studies examining these three projections from all spinal segments, using the same tracers and counting methods, do not exist. In the present study, the differences in number and distribution of retrogradely labeled lamina I cells in each segment of the cat spinal cord (C1-Coc2) were determined after large wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) injections in either PBN, PAG or thalamus. We estimate that approximately 6000 lamina I cells project to PBN, 3000 to PAG and less than 1500 to the thalamus. Of the lamina I cells projecting to thalamus or PAG more than 80%, and of the lamina I-PBN cells approximately 60%, were located on the contralateral side. In all cases, most labeled lamina I cells were found in the upper two cervical segments and in the cervical and lumbar enlargements.

Imbalance between the expression of NK1 and GABAB receptors in nociceptive spinal neurons during secondary hyperalgesia: A c-fos study in the monoarthritic rat

The neurochemical changes that operate in nociceptive spinal cord circuits during secondary hyperalgesia are largely unknown, in particular with respect to the balance between excitatory and inhibitory neurotransmission. In this study we evaluated the expression of NK1 and GABA(B) receptors in nociceptive spinal neurons in a model of secondary hyperalgesia consisting of noxious mechanical stimulation of the hindlimb skin close to a joint chronically inflamed by complete Freund's adjuvant. In spinal segments receiving input from that skin area, Fos-immunodetection was combined with immunocytochemistry for NK1 receptors, GABA(B) receptors or both receptors. In control and monoarthritic animals, neurons double-labeled for Fos and each receptor occurred mainly in laminae I and IV-V. In lamina I, the percentage of NK1 neurons expressing Fos was higher in monoarthritics while lower percentages of GABA(B) neurons expressed Fos. The percentage of Fos-positive cells expressing NK1 immunoreaction did not change in monoarthritics but that of Fos cells with GABA(B) immunoreaction was lower in these animals. In laminae IV-V, a large increase in Fos expression was detected in monoarthritic rats but the relative proportions of Fos-positive neurons expressing each receptor were similar in the two groups. Co-localization of NK1 and GABA(B) receptors occurred only in lamina I neurons in both experimental groups with no differences between control and monoarthritic animals in the percentages of Fos-positive neurons that expressed the receptors. Considering the participation of lamina I neurons bearing NK1 and GABA(B) receptors in several spinofugal systems, it is possible that the imbalance between excitatory and inhibitory actions exerted, respectively, by substance P and GABA may subserve secondary hyperalgesia by increasing ascending transmission of nociceptive input.

Setting the tone: superficial dorsal horn projection neurons regulate pain sensitivity

The neurokinin-1 receptor is expressed by lamina I projection neurons of the spinal cord that are crucial for regulating pain behavior. These neurons are nocispecific, support long-term potentiation and appear to downregulate the K(+)-Cl(-) exporter channel KCC2 following peripheral nerve damage, leading to increased excitability. These lamina I neurons project to the brainstem and thalamus and modulate descending inhibitory and excitatory pathways to the dorsal horn that regulate nociceptive traffic.

Glial nitric oxide-mediated long-term presynaptic facilitation revealed by optical imaging in rat spinal dorsal horn

We investigated a presynaptic form of long-term potentiation (LTP) in horizontal slices of the rat spinal cord by visualizing presynaptic and postsynaptic excitation with a voltage-sensitive dye. To record presynaptic excitation, we stained primary afferent fibers anterogradely from the dorsal root. A single-pulse test stimulation of C fiber-activating strength to the dorsal root elicited action potential (AP)-like or compound AP-like optical signals throughout the superficial dorsal horn. After conditioning (240 pulses at 2 Hz for 2 min), the presynaptic excitation was augmented. Furthermore, new excitation was elicited in the areas that were silent before conditioning. For postsynaptic recording, projection neurons in spinal lamina I were stained retrogradely from the periaqueductal gray in the brain stem. The test stimulation elicited AP-like or EPSP-like optical signals in the stained neurons. After conditioning, the EPSP-like responses were augmented, and previously silent neurons were converted to active ones. Results obtained with a nitric oxide (NO) donor, NO synthase inhibitors, metabotropic glutamate receptor (mGluR) agonist and mGluR1 antagonist, and a glial metabolism inhibitor suggest that after conditioning, presynaptic excitation is facilitated by NO released from glial cells via the activation of mGluR1. The results also indicate the possible presence of additional presynaptic and postsynaptic mechanism(s) for the LTP induction. Activity-dependent LTP of nociceptive afferent synaptic transmission in the spinal cord is believed to underlie central sensitization after inflammation or nerve injury. This glial NO-mediated control of presynaptic excitation may contribute to the induction at least in part.

Distinctive membrane and discharge properties of rat spinal lamina I projection neurones in vitro

Most lamina I neurones with a projection to the brainstem express the neurokinin 1 receptor and thus belong to a small subgroup of lamina I neurones that are necessary for the development of hyperalgesia in rat models of persisting pain. These neurones are prone to synaptic plasticity following primary afferent stimulation in the noxious range while other nociceptive lamina I neurones are not. Here, we used whole-cell patch-clamp recordings from lamina I neurones in young rat spinal cord transverse slices to test if projection neurones possess membrane properties that set them apart from other lamina I neurones. Neurones with a projection to the parabrachial area or the periaqueductal grey (PAG) were identified by retrograde labelling with the fluorescent tracer DiI. The properties of lamina I projection neurones were found to be fundamentally different from those of unidentified, presumably propriospinal lamina I neurones. Two firing patterns, the gap and the bursting firing pattern, occurred almost exclusively in projection neurones. Most spino-parabrachial neurones showed the gap firing pattern while the bursting firing pattern was characteristic of spino-PAG neurones. The underlying membrane currents had the properties of an A-type K(+) current and a Ca(2+) current with a low activation threshold, respectively. Projection neurones, especially those of the burst firing type, were more easily excitable than unidentified neurones and received a larger proportion of monosynaptic input from primary afferent C-fibres. Intracellular labelling with Lucifer yellow showed that projection neurones had larger somata than unidentified neurones and many had a considerable extension in the mediolateral plane.