Morphology and distribution of spinothalamic lamina I neurons in the monkey

Quantitative spatial analysis reveals that the local axons of lamina I projection neurons and interneurons exhibit distributions that predict distinct roles in spinal sensory processing

Our knowledge about the detailed wiring of neuronal circuits in the spinal dorsal horn (DH), where initial sensory processing takes place, is still very sparse. While a substantial amount of data is available on the somatodendritic morphology of DH neurons, the laminar and segmental distribution patterns and consequential function of individual axons are much less characterized. In the present study, we fully reconstructed the axonal and dendritic processes of 10 projection neurons (PNs) and 15 interneurons (INs) in lamina I of the rat, to reveal quantitative differences in their distribution. We also performed whole-cell patch-clamp recordings to test the predicted function of certain axon collaterals. In line with our earlier qualitative description, we found that lamina I INs in the lateral aspect of the superficial DH send axon collaterals toward the medial part and occupy mostly laminae I-III, providing anatomical basis for a lateromedial flow of information within the DH. Local axon collaterals of PNs were more extensively distributed including dorsal commissural axon collaterals that might refer to those reported earlier linking the lateral aspect of the left and right DHs. PN collaterals dominated the dorsolateral funiculus and laminae IV-VI, suggesting propriospinal and ventral connections. Indeed, patch-clamp recordings confirmed the existence of a dorsoventral excitatory drive upon activation of neurokinin-1 receptors that, although being expressed in various lamina I neurons, are specifically enriched in PNs. In summary, lamina I PNs and INs have almost identical dendritic input fields, while their segmental axon collateral distribution patterns are distinct. INs, whose somata reside in lamina I, establish local connections, may show asymmetry, and contribute to bridging the medial and lateral halves of the DH. PNs, on the other hand, preferably relay their integrated dendritic input to deeper laminae of the spinal gray matter where it might be linked to other ascending pathways or the premotor network, resulting in a putative direct contribution to the nociceptive withdrawal reflex.

Spinal ascending pathways for somatosensory information processing

The somatosensory system processes diverse types of information including mechanical, thermal, and chemical signals. It has an essential role in sensory perception and body movement and, thus, is crucial for organism survival. The neural network for processing somatosensory information comprises multiple key nodes. Spinal projection neurons represent the key node for transmitting somatosensory information from the periphery to the brain. Although the anatomy of spinal ascending pathways has been characterized, the mechanisms underlying somatosensory information processing by spinal ascending pathways are incompletely understood. Recent studies have begun to reveal the diversity of spinal ascending pathways and their functional roles in somatosensory information processing. Here, we review the anatomic, molecular, and functional characteristics of spinal ascending pathways.

Spinal cord projection neurons: a superficial, and also deep, analysis

Today there are extensive maps of the molecular heterogeneity of primary afferents and dorsal horn interneurons, yet there is a dearth of molecular and functional information regarding the projection neurons that transmit pain and itch information to the brain. Additionally, most contemporary research into the spinal cord and medullary projection neurons focuses on neurons in the superficial dorsal horn; the contribution of deep dorsal horn and even ventral horn projection neurons to pain and itch processing is often overlooked. In the present review we integrate conclusions from classical as well as contemporary studies and provide a more balanced view of the diversity of projection neurons. A major question addressed is the extent to which labeled-lines are maintained in these different populations or whether the brain generates distinct pain and itch percepts by decoding complex convergent inputs that engage projection neurons.

Hypnosis for cingulate-mediated analgesia and disease treatment

Hypnosis is a technique that induces changes in perceptual experience through response to specific suggestions. By means of functional neuroimaging, a large body of clinical and experimental studies has shown that hypnotic processes modify internal (self-awareness) as well as external (environmental awareness) brain networks. Objective quantifications of this kind permit the characterization of cerebral changes after hypnotic induction and its uses in the clinical setting. Hypnosedation is one such application, as it combines hypnosis with local anesthesia in patients undergoing surgery. The power of this technique lies in the avoidance of general anesthesia and its potential complications that emerge during and after surgery. Hypnosedation is associated with improved intraoperative comfort and reduced perioperative anxiety and pain. It ensures a faster recovery of the patient and diminishes the intraoperative requirements for sedative or analgesic drugs. Mechanisms underlying the modulation of pain perception under hypnotic conditions involve cortical and subcortical areas, mainly the anterior cingulate and prefrontal cortices as well as the basal ganglia and thalami. In that respect, hypnosis-induced analgesia is an effective and highly cost-effective alternative to sedation during surgery and symptom management.

Central neural substrates involved in temperature discrimination, thermal pain, thermal comfort, and thermoregulatory behavior

A phylogenetically novel pathway that emerged with primate encephalization is described, which conveys high-fidelity cutaneous thermosensory activity in "labeled lines" to a somatotopic map in the dorsal posterior insular cortex. It originates in lamina I of the superficial dorsal horn and ascends by way of the lateral spinothalamic tract and a distinct region in posterolateral thalamus. It evolved from the homeostatic sensory activity that represents the physiologic (interoceptive) condition of the body and drives the central autonomic network, which underlies all affective feelings from the body. Accordingly, human discriminative thermal sensations are accompanied by thermally motivated behaviors and thermal feelings of comfort or discomfort (unless neutral), which evidence suggests are associated with activity in the insular, cingulate, and orbitofrontal cortices, respectively. Yet, the substrates for thermoregulatory behavior have not been established, and several strong candidates (including the hypothalamus and the bed nucleus of the stria terminalis) are discussed. Finally, the neural underpinnings for relationships between thermal affect and social feelings (warm-positive/cold-negative) are addressed, including the association of hyperthermia with clinical depression.

Neuronal networks and nociceptive processing in the dorsal horn of the spinal cord

The dorsal horn (DH) of the spinal cord receives a variety of sensory information arising from the inner and outer environment, as well as modulatory inputs from supraspinal centers. This information is integrated by the DH before being forwarded to brain areas where it may lead to pain perception. Spinal integration of this information relies on the interplay between different DH neurons forming complex and plastic neuronal networks. Elements of these networks are therefore potential targets for new analgesics and pain-relieving strategies. The present review aims at providing an overview of the current knowledge on these networks, with a special emphasis on those involving interlaminar communication in both physiological and pathological conditions.

Limited changes in spinal lamina I dorsal horn neurons following the cytotoxic ablation of non-peptidergic C-fibers

BACKGROUND

Non-peptidergic nociceptive neurons are a sub-population of small diameter primary sensory neurons that comprise approximately 50 % of the C fiber population. Together with the peptidergic sub-population, they transmit nociceptive information from the periphery to the superficial dorsal horn of the spinal cord. Despite the numerous studies investigating the role of the non-peptidergic primary afferents, their role in normal nociception and in pain remains poorly understood. Our lab has previously demonstrated that, in rat models of neuropathic and inflammatory pain, there is a de novo expression of substance P receptors (NK-1r) by lamina I pyramidal projection neurons, a neuronal population that normally does not express these receptors.

RESULTS

In this study, we used a ribosomal toxin, saporin, conjugated to the lectin IB4 to selectively ablate the non-peptidergic nociceptive C fibers, to investigate if the loss of these fibers was enough to induce a change in NK-1r expression by lamina I projection neurons. IB4-saporin treatment led to the permanent ablation of the IB4-positive afferents but also to a small non-significant reduction in CGRP-positive afferents. An overall increase in immunoreactivity for the NK-1r was observed in lamina I projection neurons, however, the lack of non-peptidergic afferents did not increase the number of lamina I pyramidal projection neurons immunoreactive for the receptor.

CONCLUSIONS

Our results demonstrate that the deletion of the non-peptidergic afferents, at the L4-L5 spinal levels, is not sufficient to trigger the de novo expression of NK-1r by projection pyramidal neurons but increases the expression of NK-1r in fusiform and multipolar projection neurons. Furthermore, our data suggest that a neuropathic component is essential to trigger the expression of NK-1r by pyramidal neurons.

De novo expression of neurokinin-1 receptors by spinoparabrachial lamina I pyramidal neurons following a peripheral nerve lesion

Lamina I of the spinal dorsal horn is a major site of integration and transmission to higher centers of nociceptive information from the periphery. One important primary afferent population that transmits such information to the spinal cord expresses substance P (SP). These fibers terminate in contact with lamina I projection neurons that express the SP receptor, also known as the neurokinin-1 receptor (NK-1r). Three types of lamina I projection neurons have been described: multipolar, fusiform, and pyramidal. Most neurons of the first two types are thought to be nociceptive and express the NK-1r, whereas most pyramidal neurons are nonnociceptive and do not express the NK-1r. In this immunocytochemical and behavioral study, we induced a neuropathic pain-like condition in the rat by means of a polyethylene cuff placed around in the sciatic nerve. We document that this lesion led to a de novo expression of NK-1r on pyramidal neurons as well as a significant increase in SP-immunoreactive innervation onto these neurons. These phenotypic changes were evident at the time of onset of neuropathic pain-related behavior. Additionally, we show that, after a noxious stimulus (intradermal capsaicin injection), these NK-1r on pyramidal neurons were internalized, providing evidence that these neurons become responsive to peripheral noxious stimulation. We suggest that the changes following nerve lesion in the phenotype and innervation pattern of pyramidal neurons are of significance for neuropathic pain and/or limb temperature regulation.

Spinal cord lesions in sporadic Parkinson's disease

In this autopsy-based study, α-synuclein immunohistochemistry and lipofuscin pigment-Nissl architectonics in serial sections of 100 μm thickness were used to investigate the spinal cords and brains of 46 individuals: 28 patients with clinically and neuropathologically confirmed Parkinson's disease, 6 cases with incidental Lewy body disease, and 12 age-matched controls. α-Synuclein inclusions (particulate aggregations, Lewy neurites/bodies) in the spinal cord were present between neuropathological stages 2-6 in all cases whose brains were staged for Parkinson's disease-related synucleinopathy. The only individuals who did not have Lewy pathology in the spinal cord were a single stage 1 case (incidental Lewy body disease) and all controls. Because the Parkinson's disease-related lesions were observable in the spinal cord only after Lewy pathology was seen in the brain, it could be concluded that, within the central nervous system, sporadic Parkinson's disease does not begin in the spinal cord. In addition: (1) α-Synuclein-immunoreactive axons clearly predominated over Lewy bodies throughout the spinal cord and were visible in medial and anterior portions of the anterolateral funiculus. Their terminal axons formed dense α-synuclein-immunoreactive networks in the gray matter and were most conspicuous in the lateral portions of layers 1, 7, and in the cellular islands of layer 9. (2) Notably, this axonopathy increased remarkably in density from cervicothoracic segments to lumbosacral segments of the cord. (3) Topographically, it is likely that the spinal cord α-synuclein immunoreactive axonal networks represent descending projections from the supraspinal level setting nuclei (locus coeruleus, lower raphe nuclei, magnocellular portions of the reticular formation). (4) Following the appearance of the spinal cord axonal networks, select types of projection neurons in the spinal cord gray matter displayed α-synuclein-immunoreactive inclusions: chiefly, nociceptive neurons of the dorsal horn in layer 1, sympathetic and parasympathetic preganglionic neurons in layer 7, the cellular pools of α-motoneurons in layer 9, and the smaller motoneurons in Onuf's nucleus in layer 9 (ventral horn). The spinal cord lesions may contribute to clinical symptoms (e.g., pain, constipation, poor balance, lower urinary tract complaints, and sexual dysfunction) that occur during the premotor and motor phases of sporadic Parkinson's disease.

A trigeminoreticular pathway: implications in pain

Neurons in the caudalmost ventrolateral medulla (cmVLM) respond to noxious stimulation. We previously have shown most efferent projections from this locus project to areas implicated either in the processing or modulation of pain. Here we show the cmVLM of the rat receives projections from superficial laminae of the medullary dorsal horn (MDH) and has neurons activated with capsaicin injections into the temporalis muscle. Injections of either biotinylated dextran amine (BDA) into the MDH or fluorogold (FG)/fluorescent microbeads into the cmVLM showed projections from lamina I and II of the MDH to the cmVLM. Morphometric analysis showed the retrogradely-labeled neurons were small (area 88.7 µm(2)±3.4) and mostly fusiform in shape. Injections (20-50 µl) of 0.5% capsaicin into the temporalis muscle and subsequent immunohistochemistry for c-Fos showed nuclei labeled in the dorsomedial trigeminocervical complex (TCC), the cmVLM, the lateral medulla, and the internal lateral subnucleus of the parabrachial complex (PBil). Additional labeling with c-Fos was seen in the subnucleus interpolaris of the spinal trigeminal nucleus, the rostral ventrolateral medulla, the superior salivatory nucleus, the rostral ventromedial medulla, and the A1, A5, A7 and subcoeruleus catecholamine areas. Injections of FG into the PBil produced robust label in the lateral medulla and cmVLM while injections of BDA into the lateral medulla showed projections to the PBil. Immunohistochemical experiments to antibodies against substance P, the substance P receptor (NK1), calcitonin gene regulating peptide, leucine enkephalin, VRL1 (TPRV2) receptors and neuropeptide Y showed that these peptides/receptors densely stained the cmVLM. We suggest the MDH- cmVLM projection is important for pain from head and neck areas. We offer a potential new pathway for regulating deep pain via the neurons of the TCC, the cmVLM, the lateral medulla, and the PBil and propose these areas compose a trigeminoreticular pathway, possibly the trigeminal homologue of the spinoreticulothalamic pathway.

Neuronal circuitry for pain processing in the dorsal horn

Neurons in the spinal dorsal horn process sensory information, which is then transmitted to several brain regions, including those responsible for pain perception. The dorsal horn provides numerous potential targets for the development of novel analgesics and is thought to undergo changes that contribute to the exaggerated pain felt after nerve injury and inflammation. Despite its obvious importance, we still know little about the neuronal circuits that process sensory information, mainly because of the heterogeneity of the various neuronal components that make up these circuits. Recent studies have begun to shed light on the neuronal organization and circuitry of this complex region.

Two populations of neurokinin 1 receptor-expressing projection neurons in lamina I of the rat spinal cord that differ in AMPA receptor subunit composition and density of excitatory synaptic input

Lamina I of the spinal cord contains many projection neurons that express the neurokinin 1 receptor (NK1r). It has been reported that these cells can undergo long-term potentiation (LTP), which may result from insertion of AMPA-type glutamate receptors (AMPArs) containing GluA1 or GluA4 subunits. We therefore investigated synaptic AMPAr expression on these cells with immunocytochemistry following antigen-retrieval. We also examined their density of glutamatergic input (by analysing AMPAr synaptic puncta and contacts from glutamatergic boutons), and phosphorylation of extracellular signal-regulated kinases (pERKs) following noxious stimulation. Our results indicate that there are two populations of NK1r-expressing projection neurons: large GluA4⁺/GluA1⁻ cells with a high density of glutamatergic input and small GluA1⁺/GluA4⁻ cells with a much lower input density. Results from pERK experiments suggested that the two groups may not differ in the types of noxious stimulus that activate them. Glutamatergic synapses on distal dendrites of the large cells were significantly longer than those on proximal dendrites, which presumably compensates for the greater attenuation of distally-generated excitatory postsynaptic currents (EPSCs). Both types of cell received contacts from peptidergic primary afferents, however, on the large cells these appeared to constitute over half of the glutamatergic synapses, and were often associated with elongated AMPAr puncta. This suggests that these afferents, which probably contain substance P, provide a powerful, secure synaptic input to large NK1r-expressing projection neurons. These results demonstrate the importance of GluA4-containing AMPArs in nociceptive transmission and raise the possibility that different forms of LTP in lamina I projection neurons may be related to differential expression of GluA1/GluA4.

Soma size distinguishes projection neurons from neurokinin 1 receptor-expressing interneurons in lamina I of the rat lumbar spinal dorsal horn

Lamina I of the spinal dorsal horn contains neurons that project to various brain regions, and ∼80% of these projection cells express the neurokinin 1 receptor (NK1r), the main receptor for substance P. Two populations of NK1r-immunoreactive neurons have been identified in lamina I: small weakly immunoreactive cells and large cells with strong immunolabelling [Cheunsuang O and Morris R (2000) Neuroscience 97:335–345]. The main aim of this study was to test the hypothesis that the large cells are projection neurons and that the small cells are interneurons. Projection neurons were identified by injection of tracers into the caudal ventrolateral medulla and lateral parabrachial area, and this was combined with immunostaining for NK1r. We found a bimodal size distribution for NK1r-immunoreactive neurons. The small cells (with somatic cross-sectional areas <200 μm²) showed weak immunoreactivity, while immunostaining intensity was variable among the large cells. Virtually all (99%) of the immunoreactive cells with soma areas >200 μm² were retrogradely labelled, while only 10% of retrogradely labelled cells were smaller than this. Soma sizes of retrogradely labelled neurons that lacked NK1r did not differ from those of NK1r-expressing projection neurons. It has been suggested that a population of small pyramidal projection neurons that lack NK1r may correspond to cells activated by innocuous cooling, and we therefore assessed the morphology of retrogradely labelled cells that were not NK1r-immunoreactive. Fifteen percent of these were pyramidal, but these did not differ in size from pyramidal NK1r-immunoreactive projection neurons. These results confirm that large NK1r-immunoreactive lamina I neurons are projection cells, and suggest that the small cells are interneurons. Since almost all of the NK1r-immunoreactive cells with soma size >200 μm² were retrogradely labelled, cells of this type can be identified as projection cells in anatomical studies.

Collateral projections of neurons in laminae I, III, and IV of rat spinal cord to thalamus, periaqueductal gray matter, and lateral parabrachial area

Projection neurons in lamina I, together with those in laminae III–IV that express the neurokinin 1 receptor (NK1r), form a major route through which nociceptive information reaches the brain. Axons of these cells innervate various targets, including thalamus, periaqueductal gray matter (PAG), and lateral parabrachial area (LPb), and many cells project to more than one target. The aims of this study were to quantify projections from cervical enlargement to PAG and LPb, to determine the proportion of spinothalamic neurons at lumbar and cervical levels that were labelled from PAG and LPb, and to investigate morphological differences between projection populations. The C7 segment contained fewer lamina I spinoparabrachial cells than L4, but a similar number of spino-PAG cells. Virtually all spinothalamic lamina I neurons at both levels were labelled from LPb and between one-third and one-half from PAG. This suggests that significant numbers project to all three targets. Spinothalamic lamina I neurons differed from those labelled only from LPb in that they were generally larger, were more often multipolar, and (in cervical enlargement) had stronger NK1r immunoreactivity. Most lamina III/IV NK1r cells at both levels projected to LPb, but few were labelled from PAG. The great majority of these cells in C7 and over one-fourth of those in L4 were spinothalamic, and at each level some projected to both thalamus and LPb. These results confirm that neurons in these laminae have extensive collateral projections and suggest that different neuronal subpopulations in lamina I have characteristic patterns of supraspinal projection. J. Comp. Neurol. 515:629–646, 2009.

Nonselective innervation of lamina I projection neurons by cocaine- and amphetamine-regulated transcript peptide (CART)-immunoreactive fibres in the rat spinal dorsal horn

Cocaine- and amphetamine-regulated transcript (CART) peptides have been implicated in spinal 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. We demonstrated previously that the majority of these fibres originate from nociceptive primary afferents. Using tract tracing, multiple immunofluorescent labelling and electronmicroscopy we determined the proportion of peptidergic primary afferents expressing CART, looked for evidence for coexistence of CART with galanin in these afferents in lamina I and examined their targets. Almost all (97.9%) randomly selected calcitonin gene-related peptide (CGRP)-immunoreactive terminals were substance P (SP)-positive (+) and CART was detected in approximately half (48.6%) of them. Most (81.4%) of the CGRP/SPergic boutons were galanin+ and approximately half (49.0%) of these contained CART. Many (72.9%) of the CARTergic boutons which expressed CGRP were also immunoreactive for galanin, while only 8.6% of the CARTergic terminals were galanin+ without CGRP. Electron microscopy showed that most of the CART terminals formed asymmetrical synapses, mainly with dendrites. All different morphological and neurochemical subtypes of spinoparabrachial projection neurons in the lamina I received contacts from CART-immunoreactive nociceptive afferents. The innervation density from these boutons did not differ significantly between either the different neurochemical or the morphological subclasses of these cells. This suggests a nonselective innervation of lamina I projection neurons from a subpopulation of CGRP/SP afferents containing CART peptide. These results provide anatomical evidence for involvement of CART peptide in spinal pain transmission.

De novo expression of the neurokinin 1 receptor in spinal lamina I pyramidal neurons in polyarthritis

Spinal lamina I (LI) neurons play a major role in the transmission and integration of pain-related information that is relayed to higher centers. Alterations in the excitability of these neurons influence chronic pain development, and expression of the neurokinin 1 receptor (NK-1r) is thought to play a major role in such changes. Novel expression of NK-1r may underlie hyperexcitability in new populations of LI neurons. LI projection neurons can be classified morphologically into fusiform, pyramidal, and multipolar cells, differing in their functional properties, with the pyramidal type being nonnociceptive. In agreement with this, we have shown that spinoparabrachial pyramidal neurons seldom express NK-1r, in contrast with the other two cell types. In this study we investigated in the rat the long-term changes in NK-1r expression by spinoparabrachial LI neurons following the unilateral injection in the hindpaw plantar surface of complete Freund's adjuvant (CFA). Cholera toxin subunit B (CTb) was injected unilaterally into the parabrachial nucleus. Our results revealed that, ipsilaterally, pyramidal neurons were seldom immunoreactive for NK-1r both in saline-injected and in CFA-injected rats, up to 10 days post-CFA. However, a considerable number of pyramidal cells were immunoreactive for NK-1r at 15, 21, and 30 days post-CFA. Our data raise the possibility -- which needs to be confirmed by electrophysiology -- that most LI projection neurons of the pyramidal type are likely nonnociceptive in naive animals but might become nociceptive following the development of arthritis.

A quantitative study of spinothalamic neurons in laminae I, III, and IV in lumbar and cervical segments of the rat spinal cord

The major ascending outputs from superficial spinal dorsal horn consist of projection neurons in lamina I, together with neurons in laminae III–IV that express the neurokinin 1 receptor (NK1r) and have dendrites that enter the superficial laminae. Some neurons in each of these populations belong to the spinothalamic tract, which conveys nociceptive information via the thalamus to cortical areas involved in pain. A projection from the cervical superficial dorsal horn to the posterior triangular nucleus (PoT) has recently been identified. PoT is at the caudal end of the thalamus and was not included in injection sites in many previous retrograde tracing studies. We have injected various tracers (cholera toxin B subunit, Fluoro-Gold, and fluorescent latex microspheres) into the thalamus to estimate the number of spinothalamic neurons in each of these two populations, and to investigate their projection targets. Most lamina I and lamina III/IV NK1r-immunoreactive spinothalamic neurons in cervical and lumbar segments could be labeled from injections centered on PoT. Our results suggest that there are 90 lamina I spinothalamic neurons per side in C7 and 15 in L4 and that some of those in C7 only project to PoT. We found that 85% of the lamina III/IV NK1r-immunoreactive neurons in C6 and 17% of those in L5 belong to the spinothalamic tract, and these apparently project exclusively to the caudal thalamus, including PoT. Because PoT projects to second somatosensory and insular cortices, our results suggest that these are major targets for information conveyed by both these populations of spinothalamic neurons.

Retrograde analysis of the cerebellar projections to the posteroventral part of the ventral lateral thalamic nucleus in the macaque monkey

The organization of cerebellothalamic projections was investigated in macaque monkeys using injections of retrograde tracers (cholera toxin B and fluorescent dextrans) in the posteroventral part of the ventrolateral thalamic nucleus (VLpv), the main source of thalamic inputs to the primary motor cortex. Injections that filled all of VLpv labeled abundant neurons that were inhomogeneously distributed among many unlabeled cells in the deep cerebellar nuclei (DCbN). Single large pressure injections made in face-, forelimb-, or hindlimb-related portions of VLpv using physiological guidance labeled numerous neurons that were broadly dispersed within a coarse somatotopographic anteroposterior (foot to face) gradient in the dentate and interposed nuclei. Small iontophoretic injections labeled fewer neurons with the same somatotopographic gradient, but strikingly, the labeled neurons in these cases were as broadly dispersed as in cases with large injections. Simultaneous injections of multiple tracers in VLpv (one tracer per somatic region with no overlap between injections) confirmed the general somatotopography but also demonstrated clearly the overlapping distributions and the close intermingling of neurons labeled with different tracers. Significantly, very few neurons (<2%) were double-labeled. This organizational pattern contrasts with the concept of a segregated "point-to-point" somatotopy and instead resembles the complex patterns that have been observed throughout the motor pathway. These data support the idea that muscle synergies are represented anatomically in the DCbN by a general somatotopography in which intermingled neurons and dispersed but selective connections provide the basis for plastic, adaptable movement coordination of different parts of the body. Indexing terms:

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.

The somatosensory system, with emphasis on structures important for pain

Santiago Ramón y Cajal described a number of somatosensory structures, including several associated with pain, in his major work on the Histology of the Nervous System of Man and Vertebrates. Our knowledge of such structures has been considerably expanded since Cajal because of the introduction of a number of experimental approaches that were not available in his time. For example, Cajal made several drawings of peripheral mechanoreceptors, as well as of bare nerve endings, but later work by others described additional somatosensory receptors and investigated the ultrastructure of bare nerve endings. Furthermore, the transducer molecules responsible for responses to nociceptive, thermal or chemical stimuli are now becoming known, including a series of TRP (transient receptor potential) receptor molecules, such as TRPV1 (the capsaicin receptor). Cajal described the development of dorsal root and other sensory ganglion cells and related the disposition of their somata and neurites to his theory of the functional polarity of neurons. He described the entry of both large and small afferent fibers into the spinal cord, including the projections of their collaterals into different parts of the gray matter and into different white matter tracts. He described a number of types of neurons in the gray matter, including ones in the marginal zone, substantia gelatinosa and head and neck of the dorsal horn. He found neurons in the deep dorsal horn whose dendrites extend dorsally into the superficial dorsal horn. Some of these neurons have since been shown by retrograde labeling to be spinothalamic tract cells. Cajal clearly described the dorsal column/medial lemniscus pathway, but the presence and course of the spinothalamic tract was unknown at the time.

A confirmation of Rexed's laminar hypothesis using the Sholl linear method complemented by nonparametric statistics

Images of Golgi-impregnated neurons from laminae I to VI in the dorsal horn of the cat spinal cord were subjected to the linear Sholl analysis of concentric circles to support Rexed's hypothesis on the laminar organization of spinal gray matter in mammals. Since Rexed's determination of the laminae is based upon size, location, and grouping of cell bodies, neglecting one of the principal morphologic attributes of the neuron-the dendritic tree, the purpose of the present study was to evaluate Rexed's hypothesis testing the structure of dendritic arborization patterns of neurons. The differences in the complexity of dendritic trees between the groups of neurons from different laminae were evaluated by nonparametric statistics. Data obtained using Sholl's method is not always subjected to complete statistical analysis. The problem becomes particularly apparent in the quantitative examination of dendritic structures. Our aim was also to perform a careful analysis of our data for normality, in order to choose the appropriate statistical method for data processing. In the linear Sholl analysis, it is important to properly represent and interpret the frequency functions. The objective of this study was also to investigate the problems of determining the frequency functions, plotting the corresponding lines of regression, and measuring the degree of fluctuation of experimental data points around these lines. The main result of our testing is a confirmation of Rexed's laminar scheme: we have proved that there are 6 out of 10 possible pairs of samples where one member significantly differs from the other, i.e. one lamina is significantly distinguishable from the other.

Retrograde analyses of spinothalamic projections in the macaque monkey: input to posterolateral thalamus

The distribution of retrogradely labeled spinothalamic tract (STT) neurons was analyzed in macaque monkeys following variously sized, physiologically guided pressure or iontophoretic injections of cholera toxin subunit B (CTb) in order to determine whether different STT termination sites receive input selectively from different sets of STT cells. This report focuses on posterolateral thalamus, where prior anterograde tracing observations identified the posterior part of the ventromedial nucleus (VMpo) as the major projection target of lamina I STT neurons. Large injections in posterolateral thalamus labeled predominantly STT cells in lamina I throughout the spinal cord. In cases with medium-sized or small injections centered in VMpo, almost all labeled STT cells ( approximately 90%) were lamina I neurons. Small injections revealed a posteroanterior (foot to hand) somatotopographic organization consistent with that observed in prior anterograde tracing work; injections in posterior VMpo labeled primarily lumbosacral lamina I cells, whereas injections placed more anteriorly in VMpo labeled primarily cervical lamina I cells. These findings support the concept that VMpo is a primate lamina I spinothalamocortical relay nucleus important for pain, temperature, itch, muscle ache, sensual touch, and other interoceptive feelings from the body, and they provide strong evidence for the general hypothesis that the STT consists of several functionally and anatomically differentiable components.

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.

Retrograde analyses of spinothalamic projections in the macaque monkey: Input to ventral posterior nuclei

The distribution of retrogradely labeled spinothalamic tract (STT) neurons was analyzed in monkeys following variously sized injections of cholera toxin subunit B (CTb) in order to determine whether different STT termination sites receive input from different sets of STT cells. This report focuses on STT input to the ventral posterior lateral nucleus (VPL) and the subjacent ventral posterior inferior nucleus (VPI), where prior anterograde tracing studies identified scattered STT terminal bursts and a dense terminal field, respectively. In cases with small or medium-sized injections in VPL, labeled STT cells were located almost entirely in lamina V (in spinal segments consistent with the mediolateral VPL topography); few cells were labeled in lamina I (<8%) and essentially none in lamina VII. Large and very large injections in VPL produced marked increases in labeling in lamina I, associated first with spread into VPI and next into the posterior part of the ventral medial nucleus (VMpo), and abundant labeling in lamina VII, associated with spread into the ventral lateral (VL) nucleus. Small injections restricted to VPI labeled many STT cells in laminae I and V with an anteroposterior topography. These observations indicate that VPL receives STT input almost entirely from lamina V neurons, whereas VPI receives STT input from both laminae I and V cells, with two different topographic organizations. Together with the preceding observation that STT input to VMpo originates almost entirely from lamina I, these findings provide strong evidence that the primate STT consists of anatomically and functionally differentiable components.

Fractal analysis of the laminar organization of spinal cord neurons

Images of Golgi impregnated neurons from different laminae of the human and rat dorsal horns were subjected to a quantitative analysis to support the Rexed's laminar scheme in mammals. Four methods of fractal analysis were performed in the proceedings: box-counting, mass-radius, cumulative intersection, and vectorized intersection. The results show that the box-counting method is more precise than the other fractal methods performed, and offers support for the conclusion that fractal analysis can successfully discriminate the neuron populations among different laminae. The analysis supports the concept of Rexed's cytoarchitectonic lamination of the dorsal horn.

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.

Distribution of trigeminothalamic and spinothalamic lamina I terminations in the macaque monkey

Thalamic terminations from trigeminal, cervical, and lumbosacral lamina I neurons were investigated with Phaseolus vulgaris leucoagglutinin (PHA-L) and labeled dextrans. Iontophoretic injections guided by physiological recordings were restricted to lamina I or laminae I-II. PHA-L-labeled trigemino- and spinothalamic (TSTT) terminations were identified immunohistochemically. TRITC- and FITC-labeled dextrans were injected at different levels to confirm topography. Terminations consistently occurred in two main locations: a distinguishable portion of posterolateral thalamus identified cytoarchitectonically as the posterior part of the ventral medial nucleus (VMpo) and a portion of posteromedial thalamus designated as the ventral caudal part of the medial dorsal nucleus (MDvc). In addition, isolated fibers bearing boutons of passage were observed in the ventral posterior medial and lateral (VPM and VPL) nuclei, and spinal terminations occurred in the ventral posterior inferior nucleus (VPI). Isolated terminations occasionally occurred in other sites (e.g., suprageniculate, zona incerta, hypothalamic paraventricular n.). Terminations in MDvc occurred in concise foci that were weakly organized topographically (posteroanterior = rostrocaudal). Terminations in VMpo consisted of dense clusters of ramified terminal arbors bearing multiple large boutons that were well organized topographically (anteroposterior = rostrocaudal). Terminations in VMpo colocalized with a field of calbindin-immunoreactive terminal fibers; double-labeled terminals were documented at high magnification. This propitious marker was especially useful at anterior levels, where VMpo can easily be misidentified as VPM. These findings demonstrate phylogenetically novel primate lamina I TSTT projections important for sensory and motivational aspects of pain, temperature, itch, muscle ache, sensual touch, and other interoceptive feelings from the body.

Afferent pain pathways: a neuroanatomical review

Painful experience is a complex entity made up of sensory, affective, motivational and cognitive dimensions. The neural mechanisms involved in pain perception acts in a serial and a parallel way, discriminating and locating the original stimulus and also integrating the affective feeling, involved in a special situation, with previous memories. This review examines the concepts of nociception, acute and chronic pain, and also describes the afferent pathways involved in reception, segmental processing and encephalic projection of pain stimulus. The interaction model of the cerebral cortex areas and their functional characteristics are also discussed.

A critical review of the role of the proposed VMpo nucleus in pain

The evidence presented by Craig and his colleagues for an important projection from lamina I spinothalamic tract neurons to a renamed thalamic nucleus (the posterior part of the ventral medial nucleus or VMpo), as well as to the ventrocaudal medial dorsal and the ventral posterior inferior thalamic nuclei, is critically reviewed. Of particular concern is the denial of an important nociceptive lamina I projection to the ventrobasal complex. Contrary evidence is reviewed that strongly favors a role of spinothalamic projections from both lamina I and deep layers of the dorsal horn to the ventrobasal complex and other thalamic nuclei and from there to the SI and SII somatosensory cortices in the sensory-discriminative processing of pain and temperature information.

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

In the rat lumbar spinal cord the major supraspinal targets for lamina I projection neurons are the caudal ventrolateral medulla (CVLM), lateral parabrachial area (LPb) and periaqueductal grey matter (PAG). In this study we have estimated the number of lamina I neurons retrogradely labelled from each of these sites in the L4 segment, as well as the proportion that can be labelled by injecting different tracers into two separate sites. Our results suggest that this segment contains approximately 400 lamina I projection neurons on each side, and that approximately 85% of these can be labelled from either the CVLM or the LPb on the contralateral side. Around 120 lamina I cells in L4 project to the PAG, and over 90% of these cells can also be labelled from the CVLM or LPb. Most lamina I neurons projecting to CVLM or LPb are located in the contralateral dorsal horn, but in each case some cells were found to have bilateral projections. We also examined horizontal sections to investigate morphology and the expression of the neurokinin 1 (NK1) receptor in cells labelled from CVLM, LPb or PAG. There were no consistent morphological differences between these groups, however, while cells with strong or moderate NK1 receptor-immunostaining were labelled from LPb or CVLM, they seldom projected to the PAG. These results suggest that many lamina I cells project to more than one site in the brain and that those projecting to PAG may represent a distinct subclass of lamina I projection neuron.

Differentiation of lamina I spinomedullary and spinothalamic neurons in the cat

We characterized spinomedullary neurons that project to the ventrolateral portion of the medulla that receives lamina I terminations in two sets of experiments in the cat. First, their distribution was examined using single unilateral iontophoretic injections of cholera toxin subunit B. The injection sites were characterized by microelectrode recordings from nociceptive- and thermoreceptive-specific units, indicative of lamina I input. The spinomedullary neurons were symmetrically distributed bilaterally, predominantly (63-69%) in lamina I but also in laminae V-VIII and the thoracic lateral horn (intermediolateral cell column). In horizontal sections, spinomedullary lamina I neurons included all three main morphological types described earlier. Second, spinomedullary and spinothalamic neurons were compared in retrograde double-labeling experiments. Different combinations of tracers were injected in the right thalamus and the left or right ventrolateral medulla (guided by recordings). The numbers of spinomedullary and spinothalamic neurons on the left side were comparable, and the segmental and laminar distributions were similar, except that a greater proportion of spinomedullary neurons originated from thoracic segments. However, the proportion of double-labeled neurons was consistently approximately 1%, indicating that spinomedullary and spinothalamic pathways arise from separate subpopulations. Spinomedullary neurons were more ventrally located within lamina I than spinothalamic neurons. A significantly greater proportion of spinomedullary neurons had fusiform somata (49% vs. 36%). These observations indicate that lamina I is the major source of spinal input to this portion of the ventrolateral medulla, that the projection includes several morphological types of inputs, and that this projection is distinct from the spinothalamic projection. These findings are consistent with the concept that lamina I projections constitute an ascending homeostatic afferent pathway relating the physiological condition of the body.

The central autonomic nervous system: conscious visceral perception and autonomic pattern generation

The overall organization of the peripheral autonomic nervous system has been known for many decades, but the mechanisms by which it is controlled by the central nervous system are just now coming to light. In particular, two major issues have seen considerable progress in the past decade. First, the pathways that provide visceral sensation to conscious perception at a cortical level have been elucidated in both animals and humans. The nociceptive system runs in parallel to the pathways carrying visceral sensation from the cranial nerves and may be considered in itself a component of visceral sensation. Second, structures in the central nervous system that generate patterns of autonomic response have been identified. These pattern generators are located at multiple levels of the central nervous system, and they can be combined in temporal and spatial patterns to subserve a wide range of behavioral needs.

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.

Association of spinothalamic lamina I neurons and their ascending axons with calbindin-immunoreactivity in monkey and human

The calbindin-immunoreactivity of spinothalamic (STT) lamina I neurons and their ascending axons was examined in two experiments. In the first experiment, lamina I STT neurons in macaque monkeys were double-labeled for calbindin and for retrogradely transported WGA*HRP following large (n=2) or small (n=1) injections that included the posterior thalamus. Most, but not all (78%) of the contralateral retrogradely labeled lamina I STT cells were positive for calbindin. Calbindin-immunoreactivity was not selectively associated with any particular anatomical type of lamina I STT cell; 82% of the fusiform cells, 78% of the pyramidal cells and 67% of the multipolar cells were double-labeled. In the second experiment, oblique transverse sections from upper cervical spinal segments of three macaque monkeys, one squirrel monkey and five humans were stained for calbindin-immunoreactivity. In each case, a distinct bundle of fibers was densely stained in the middle of the lateral funiculus. This matches the location of anterogradely labeled ascending lamina I axons observed in prior work in cats and monkeys, and it matches the location of the classically described 'lateral spinothalamic tract' in humans. This bundle had variable shape across cases, an observation that might have clinical significance. These findings support the view that lamina I STT neurons are involved in spinal cordotomies that reduce pain, temperature and itch sensations.

Responses of spinothalamic lamina I neurons to maintained noxious mechanical stimulation in the cat

Noxious mechanical stimuli that are maintained for minutes produce a continuous sensation of pain in humans that augments during the stimulus. It has recently been shown with systematic force-controlled stimuli that, while all mechanically responsive nociceptors adapt to these stimuli, the basis for such pain can be ascribed to A-fiber rather than C-fiber nociceptors, based on distinctions in their respective response profiles and stimulus-response functions. The present experiments investigated whether similar distinctions could be made in subsets of nociceptive lamina I spinothalamic tract (STT) neurons using similar maintained stimuli. Twenty-eight lamina I STT neurons in the lumbosacral dorsal horn of barbiturate-anesthetized cats were tested with noxious mechanical stimuli applied with a probe of 0.1 mm(2) contact area at forces of 25, 50, and 100 g for 2 min. The neurons were classified as nociceptive-specific (NS, n = 14) or polymodal nociceptive (HPC, n = 14) based on their responses to quantitative thermal stimuli. The NS neurons had greater responses and showed less adaptation than the HPC neurons in response to these stimuli, and they encoded stimulus intensity better. Comparison of the normalized response profiles of all 28 nociceptive lamina I STT neurons, independent of cell classification, revealed 2 subgroups that differed significantly: "Maintained" cells with responses that remained above 50% of the initial peak rate during stimulation and "Adapting" cells with responses that quickly declined to <50%. The Maintained neurons encoded the intensity of the mechanical stimuli better than the Adapting neurons, based on ratiometric functions. A k-means cluster analysis of all 28 cells distinguished the identical two subgroups. These categories corresponded closely to the NS and HPC categories: Maintained cells were mostly NS neurons (10 NS, 3 HPC), and Adapting cells were mostly HPC neurons (4 NS, 11 HPC). Thus the present data are consistent with the distinctions between A-fiber and C-fiber nociceptors observed previously, because A-fiber nociceptors are the predominant input to NS lamina I STT neurons and C-fiber nociceptors are the predominant input to HPC neurons. These findings support the view that NS, but perhaps not HPC, lamina I STT neurons have a role in the pain caused by maintained mechanical stimuli and contribute to the sensations of "first" pain and "sharpness." Nonetheless, none of the units studied showed increasing responses during the stimuli, suggesting a role for other ascending neurons or forebrain integration in the augmenting pain produced by maintained mechanical stimulation.

Quantitative response characteristics of thermoreceptive and nociceptive lamina I spinothalamic neurons in the cat

The physiological characteristics of antidromically identified lamina I spinothalamic (STT) neurons in the lumbosacral spinal cord were examined using quantitative thermal and mechanical stimuli in barbiturate-anesthetized cats. Cells belonging to the three main recognized classes were included based on categorization with natural cutaneous stimulation of the hindpaw: nociceptive-specific (NS), polymodal nociceptive (HPC), or thermoreceptive-specific (COOL) cells. The mean central conduction latencies of these classes differed significantly; NS = 130.8 +/- 55.5 (SD) ms (n = 100), HPC = 72.1 +/- 28.0 ms (n = 128), and COOL = 58.6 +/- 25.3 ms (n = 136), which correspond to conduction velocities of 2.5, 4.6, and 5.6 m/s. Based on recordings made prior to any noxious stimulation, the mean spontaneous discharge rates of these classes also differed: NS = 0.5 +/- 0.7 imp/s (n = 47), HPC = 0.9 +/- 0.7 imp/s (n = 59), and COOL = 3.3 +/- 2.6 imp/s (n = 107). Standard, quantitative, thermal stimulus sequences applied with a Peltier thermode were used to characterize the stimulus-response functions of 76 COOL cells, 47 HPC cells, and 37 NS cells. The COOL cells showed a very linear output from 34 degrees C down to approximately 15 degrees C and a maintained plateau thereafter. The HPC cells showed a fairly linear but accelerating response to cold below a median threshold of approximately 24 degrees C and down to 9 degrees C (measured at the skin-thermode interface with a thermode temperature of 2 degrees C). The HPC cells and the NS cells both showed rapidly increasing, sigmoidal response functions to noxious heat with a fairly linear response between 45 and 53 degrees C, but they had significantly different thresholds; half of the HPC cells were activated at ~45.5 degrees C and half of the NS cells at approximately 43 degrees C. The 20 HPC lamina I STT cells and 10 NS cells tested with quantitative pinch stimuli showed fairly linear responses above a threshold of approximately 130 g/mm(2) for HPC cells and a threshold of approximately 100 g/mm(2) for NS cells. All of these response functions compare well (across species) with the available data on the characteristics of thermoreceptive and nociceptive primary afferent fibers and the appropriate psychophysics in humans. Together these results support the concept that these classes of lamina I STT cells provide discrete sensory channels for the sensations of temperature and pain.

Immunohistochemical profiles of spinal lamina I neurones retrogradely labelled from the nucleus tractus solitarii in rat suggest excitatory projections

Three morphologically distinct types of lamina I neurones, fusiform, flattened and pyramidal, project from the spinal cord to the caudal part of the nucleus tractus solitarii in the rat, and may represent a pathway whereby peripheral stimuli can modify autonomic functions. The neurochemistry of these three types of projection neurones was investigated using retrograde neuronal tracing with cholera toxin B-subunit combined with dual and triple immunofluorescence labelling for different neuroactive substances. None of the lamina I neurones with immunoreactivity for GABA or glycine were found to project to the nucleus tractus solitarii, whereas high levels of glutamate immunoreactivity, which may indicate a glutamatergic phenotype, were found in 18.4% of fusiform, 9.6% of pyramidal and 2.1% of flattened projection neurones. Immunoreactivity for calbindin-D28K was present in 34.9% of fusiform cells, 18.3% of pyramidal cells and 10.5% of flattened cells, and nitric oxide synthase immunoreactivity was detected in 13.8% of fusiform cells, 1.1% of pyramidal cells and 4.2% of flattened cells that had projections to the nucleus tractus solitarii. Calbindin immunoreactivity was co-localised in major subpopulations of projection neurones of each morphological type that contained glutamate immunoreactivity, whereas co-localisation of nitric oxide synthase immunoreactivity in these neurones was relatively uncommon. The pyramidal cell was the only retrogradely labelled cell type found to be immunoreactive for substance P, but few (<5%) of these neurones were immunolabelled. These data are consistent with the hypothesis that lamina I neurones projecting to the dorsal vagal complex are not inhibitory, and that some of them, belonging mostly to the fusiform and pyramidal types, may exert excitatory, glutamate- or substance P-mediated effects upon inhibitory interneurones in the nucleus tractus solitarii. Such excitatory pathways could be involved in the attenuation of the reflex control of blood pressure by both painful and innocuous peripheral stimuli, such as those arising in injury and exercise.

Projections from the marginal zone and deep dorsal horn to the ventrobasal nuclei of the primate thalamus

It has been concluded recently that if a projection from the marginal zone to the ventral posterior lateral (VPL) nucleus exists, it is sparse. Given the importance of the marginal zone in nociception, this conclusion has raised doubts about the significance of the role of the ventrobasal complex in nociception. We have reexamined this projection using injections of the retrograde tracer, cholera toxin subunit B, into one side of the lateral thalamus in macaque monkeys. The injections were confined to the ventrobasal complex (with minimal spread to adjacent nuclei that do not receive spinal projections) in two animals. Many retrogradely labeled neurons were found in lamina I (as well as in lamina V) of the contralateral spinal and medullary dorsal horn. The results are consistent with the view that neurons in the marginal zone contribute prominently to the spinothalamic and trigeminothalamic projections to the VPL and ventral posterior medial (VPM) nuclei. This pathway is likely to be important for the sensory-discriminative processing of nociceptive information with respect to the location and intensity of painful stimuli.

Morphologic features and electrical membrane properties of projection neurons in the marginal layer of the medullary dorsal horn of the rat

Possible correspondence between morphologic features and electrical membrane properties of projection neurons in lamina I [the marginal zone (MZ)] of the caudal subnucleus of the spinal trigeminal nucleus [the medullary dorsal horn (MDH)] was examined by using intracellular recordings and biocytin-injections combined with histochemical and immunohistochemical staining techniques. The experiments were done in horizontal slice preparations of the rat brain. Thirteen MZ neurons were recorded stably and stained successfully. These neurons were confirmed to send their axons to the brain regions outside the MDH by camera lucida reconstruction. They were divided into two types on the basis of branching patterns of their axons within the MDH: Type I projection (P-I) neurons (n = 7 neurons) had main axons that rarely emitted axon collaterals within the MDH, whereas type II projection (P-II) neurons (n = 6 neurons) had main axons that emitted many axon collaterals within laminae I, II (substantia gelatinosa), and III (magnocellular part) of the MDH and also to the spinal tract of the trigeminal nerve; these axon collaterals usually constituted a dense mesh of axonal processes within laminae I and II of the MDH, especially in lamina II. About half of the neurons of each type showed immunoreactivity for the neurokinin-1 receptor. Resting membrane potentials were significantly more positive in P-I neurons than in P-II neurons. The P-II neurons had higher input resistance, a longer membrane time constant, and a higher threshold for spike than P-I neurons. In response to weak, long depolarizing current pulses, P-II neurons often showed slow ramp depolarization; the same neurons exhibited delayed repolarization to the resting potential (slow after depolarization) after the offset of the long depolarizing current pulses. Neither the slow-ramp depolarization nor the slow after depolarization was observed in P-I neurons. Slow return to resting membrane potential after offset of hyperpolarizing current pulses also was observed frequently in P-II neurons but not in P-I neurons. The results indicate that P-II neurons differ in their membrane properties compared with P-I neurons, and P-II neurons may be involved in the local circuit mechanism within the MDH more deeply than P-I neurons.

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.

Physiological properties of the lamina I spinoparabrachial neurons in the rat

Single-unit extracellular recordings of spino-parabrachial (spino-PB) neurons (n = 53) antidromically driven from the contralateral parabrachial (PB) area were performed in the lumbar cord in anesthetized rats. All the spino-PB neurons were located in the lamina I of the dorsal horn. Their axons exhibited conduction velocities between 2.8 and 27.8 m/s, in the thin myelinated fibers range. They had an extremely low spontaneous activity (median = 0. 064 Hz) and a small excitatory receptive field (</=2 toes or pads). They were all activated by both peripheral A (mainly Adelta) and C fibers after intense transcutaneous electrical stimulation. Their discharge always increased in response to noxious natural stimuli of increasing intensities. The great majority (75%) of spino-PB neurons were nociceptive specific, i.e., they were excited only by noxious stimuli. The remaining (25%) still were excited primarily by noxious stimuli but also responded moderately to innocuous stimuli. Almost all spino-PB neurons (92%, 49/53) were activated by both mechanical and heat noxious stimuli. Among them, 35% were in addition moderately activated by noxious cold (thresholds between +20 and -10 degrees C). Only (8%, 4/53) responded exclusively to noxious heat. Spino-PB neurons clearly encoded the intensity of mechanical (n = 39) and thermal (n = 38) stimuli in the noxious range, and most of the individual stimulus-response functions were monotonic and positive up to 40/60 N. cm(-2) and 50 degrees C, respectively. For the mechanical modality, the mean threshold was 11.5 +/- 1.25 N. cm(-2) (mean +/- SE), the response increased almost linearly with the logarithm of the pressure between 10 and 60 N. cm(-2), the mean p(50) (pressure evoking 50% of the maximum response) and the maximum responsiveness were: 30 +/- 2.4 N. cm(-2) and 40.5 +/- 5 Hz, respectively. For the thermal modality, the mean threshold was 43.6 +/- 0.5 degrees C, the mean curve had a general sigmoid aspect, the steepest portion being in the 46-48 degrees C interval, the mean t(50) and the maximum responsiveness were: 47.4 +/- 0.3 degrees C and 40 +/- 4.4 Hz, respectively. Most of the spino-PB neurons tested (13/16) had their noxiously evoked responses clearly inhibited by heterotopic noxious stimuli. The mean response to noxious stimuli during heterotopic stimuli was 31.7 +/- 6.1% of the control response. We conclude that the nociceptive properties of the lamina I spino-PB neurons are reflected largely by those of PB neurons that were suggested to be involved in autonomic and emotional/aversive aspects of 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.

Junctional versus extrajunctional glycine and GABA(A) receptor-mediated IPSCs in identified lamina I neurons of the adult rat spinal cord

Colocalization of GABA and glycine in synaptic terminals of the superficial dorsal horn raises the question of their relative contribution to inhibition of different classes of neurons in this area. To address this issue, miniature IPSCs (mIPSCs) mediated via GABA(A) receptors (GABA(A)Rs) and glycine receptors (GlyRs) were recorded from identified laminae I-II neurons in adult rat spinal cord slices. GABA(A)R-mediated mIPSCs had similar amplitude and rise times, but significantly slower decay kinetics than GlyR-mediated mIPSCs. Lamina I neurons appeared to receive almost exclusively GlyR-mediated mIPSCs, even after application of hypertonic solutions. Yet, all neurons responded to exogenous applications of both GABA and glycine, indicating that they expressed both GABA(A)Rs and GlyRs. Given that virtually all glycinergic interneurons also contain GABA, the possibility was examined that GABA(A)Rs may be located extrasynaptically in lamina I neurons. A slow GABA(A)R-mediated component was revealed in large, but not minimally evoked monosynaptic IPSCs. Administration of the benzodiazepine flunitrazepam unmasked a GABA(A)R component to most mIPSCs, suggesting that both transmitters were released from the same vesicle. The isolated GABA(A)R component of these mIPSCs had rising kinetics 10 times slower than that of the GlyR component (or of GABA(A)R mIPSCs in lamina II). The slow GABA(A)R components were prolonged by GABA uptake blockers. It is concluded that, whereas GABA and glycine are likely released from the same vesicle of transmitter in lamina I, GABA(A)Rs appear to be located extrasynaptically. Thus, glycine mediates most of the tonic inhibition at these synapses. This differential distribution of GABA(A)Rs and GlyRs confers distinct functional properties to inhibition mediated by these two transmitters in lamina I.