Synaptic complexes formed by functionally defined primary afferent units with fine myelinated fibers

Characterisation of deep dorsal horn projection neurons in the spinal cord of the Phox2a::Cre mouse line

Projection neurons belonging to the anterolateral system (ALS) underlie the perception of pain, skin temperature and itch. Many ALS cells are located in laminae III-V of the dorsal horn and the adjacent lateral white matter. However, relatively little is known about the excitatory synaptic input to these deep ALS cells, and therefore about their engagement with the neuronal circuitry of the region. We have used a recently developed mouse line, Phox2a::Cre, to investigate a population of deep dorsal horn ALS neurons known as "antenna cells", which are characterised by dense innervation from peptidergic nociceptors, and to compare these with other ALS cells in the deep dorsal horn and lateral white matter. We show that these two classes differ, both in the density of excitatory synapses, and in the source of input at these synapses. Peptidergic nociceptors account for around two-thirds of the excitatory synapses on the antenna cells, but for only a small proportion of the input to the non-antenna cells. Conversely, boutons with high levels of VGLUT2, which are likely to originate mainly from glutamatergic spinal neurons, account for only ∼5% of the excitatory synapses on antenna cells, but for a much larger proportion of the input to the non-antenna cells. VGLUT1 is expressed by myelinated low-threshold mechanoreceptors and corticospinal axons, and these innervate both antenna and non-antenna cells. However, the density of VGLUT1 input to the non-antenna cells is highly variable, consistent with the view that these neurons are functionally heterogeneous.

Defining a Spinal Microcircuit that Gates Myelinated Afferent Input: Implications for Tactile Allodynia

Chronic pain presents a major unmet clinical problem. The development of more effective treatments is hindered by our limited understanding of the neuronal circuits underlying sensory perception. Here, we show that parvalbumin (PV)-expressing dorsal horn interneurons modulate the passage of sensory information conveyed by low-threshold mechanoreceptors (LTMRs) directly via presynaptic inhibition and also gate the polysynaptic relay of LTMR input to pain circuits by inhibiting lamina II excitatory interneurons whose axons project into lamina I. We show changes in the functional properties of these PV interneurons following peripheral nerve injury and that silencing these cells unmasks a circuit that allows innocuous touch inputs to activate pain circuits by increasing network activity in laminae I-IV. Such changes are likely to result in the development of tactile allodynia and could be targeted for more effective treatment of mechanical pain.

Modulation of mechanosensory vibrissal responses in the trigeminocervical complex by stimulation of the greater occipital nerve in a rat model of trigeminal neuropathic pain

Background

Stimulation of the occipital or trigeminal nerves has been successfully used to treat chronic refractory neurovascular headaches such as migraine or cluster headache, and painful neuropathies. Convergence of trigeminal and occipital sensory afferents in the ‘trigeminocervical complex’ (TCC) from cutaneous, muscular, dural, and visceral sources is a key mechanism for the input-induced central sensitization that may underlie the altered nociception. Both excitatory (glutamatergic) and inhibitory (GABAergic and glycinergic) mechanisms are involved in modulating nociception in the spinal and medullary dorsal horn neurons, but the mechanisms by which nerve stimulation effects occur are unclear. This study was aimed at investigating the acute effects of electrical stimulation of the greater occipital nerve (GON) on the responses of neurons in the TCC to the mechanical stimulation of the vibrissal pad.

Methods

Adult male Wistar rats were used. Neuronal recordings were obtained in laminae II-IV in the TCC in control, sham and infraorbital chronic constriction injury (CCI-IoN) animals. The GON was isolated and electrically stimulated. Responses to the stimulation of vibrissae by brief air pulses were analyzed before and after GON stimulation. In order to understand the role of the neurotransmitters involved, specific receptor blockers of NMDA (AP-5), GABA_A (bicuculline, Bic) and Glycine (strychnine, Str) were applied locally.

Results

GON stimulation produced a facilitation of the response to light facial mechanical stimuli in controls, and an inhibition in CCI-IoN cases. AP-5 reduced responses to GON and vibrissal stimulation and blocked the facilitation of GON on vibrissal responses found in controls. The application of Bic or Str significantly reduced the facilitatory effect of GON stimulation on the response to vibrissal stimulation in controls. However, the opposite effect was found when GABAergic or Glycinergic transmission was prevented in CCI-IoN cases.

Conclusions

GON stimulation modulates the responses of TCC neurons to light mechanical input from the face in opposite directions in controls and under CCI-IoN. This modulation is mediated by GABAergic and Glycinergic mechanisms. These results will help to elucidate the neural mechanisms underlying the effectiveness of nerve stimulation in controlling painful craniofacial disorders, and may be instrumental in identifying new therapeutic targets for their prevention and treatment.

Central Nervous System Targets: Inhibitory Interneurons in the Spinal Cord

Pain is a percept of critical importance to our daily survival. In most cases, it serves both an adaptive function by helping us respond appropriately in a potentially hostile environment and also a protective role by alerting us to tissue damage. Normally, it is evoked by the activation of peripheral nociceptive nerve endings and the subsequent relay of information to distinct cortical and sub-cortical regions, but under pathological conditions that result in chronic pain, it can become spontaneous. Given that one in three chronic pain patients do not respond to the treatments currently available, the need for more effective analgesics is evident. Two principal obstacles to the development of novel analgesic therapies are our limited understanding of how neuronal circuits that comprise these pain pathways transmit and modulate sensory information under normal circumstances and how these circuits change under pathological conditions leading to chronic pain states. In this review, we focus on the role of inhibitory interneurons in setting pain thresholds and, in particular, how disinhibition in the spinal dorsal horn can lead to aberrant sensory processing associated with chronic pain states.

Low- and high-threshold primary afferent inputs to spinal lamina III antenna-type neurons

The dorsal horn of the spinal cord (laminae I-VI) processes diverse modalities of nociceptive and nonnociceptive sensory information. Antenna-type neurons with cell bodies located in lamina III and large dendritic trees extending from the superficial lamina I to deep lamina IV are best shaped for the integration of a wide variety of inputs arising from primary afferent fibers and intrinsic spinal circuitries. Although the somatodendritic morphology, the hallmark of antenna neurons, has been well studied, little is still known about the axon structure and basic physiological properties of these cells. Here, we did whole-cell recordings in a rat (P9-P12) spinal cord preparation with attached dorsal roots to examine the axon course, intrinsic firing properties, and primary afferent inputs of antenna cells. Nine antenna cells were identified from a large sample of biocytin-filled lamina III neurons (n = 46). Axon of antenna cells showed intensive branching in laminae III-IV and, in half of the cases, issued dorsally directed collaterals reaching lamina I. Antenna cells exhibited tonic and rhythmic firing patterns; single spikes were followed by hyperpolarization or depolarization. The neurons received monosynaptic inputs from the low-threshold Aβ afferents, Aδ afferents, as well as from the high-threshold Aδ, and C afferents. When selectively activated, C-fiber-driven monosynaptic and polysynaptic excitatory postsynaptic potentials were sufficiently strong to evoke firing in the neurons. Thus, lamina III antenna neurons integrate low-threshold and nociceptive high-threshold primary afferent inputs and can function as wide dynamic range neurons able to directly connect deep dorsal horn with the major nociceptive projection area lamina I.

Synaptic Organization of VGLUT3 Expressing Low-Threshold Mechanosensitive C Fiber Terminals in the Rodent Spinal Cord

Low-threshold mechanosensitive C fibers (C-LTMRs) that express the vesicular glutamate transporter VGLUT3 are thought to signal affective touch, and may also play a role in mechanical allodynia. However, the nature of the central termination of C-LTMRs in the dorsal horn remains largely unexplored. Here, we used light and electron microscopy in combination with VGLUT3 immunolabeling as a marker of C-LTMR terminations to investigate this issue. VGLUT3⁺ C-LTMRs formed central terminals of Type II glomeruli in the inner part of lamina II of the dorsal horn, often establishing multiple asymmetric synapses with postsynaptic dendrites but also participating in synaptic configurations with presynaptic axons and dendrites. Unexpectedly, essentially all VGLUT3⁺ C-LTMR terminals showed substantial VGLUT1 expression in the rat, whereas such terminals in mice lacked VGLUT1. Most VGLUT3⁺ C-LTMR terminals exhibited weak-to-moderate VGLUT2 expression. Further, C-LTMR terminals formed numerous synapses with excitatory protein kinase Cγ (PKCγ) interneurons and inhibitory parvalbumin neurons, whereas synapses with calretinin neurons were scarce. C-LTMR terminals rarely if ever established synapses with neurokinin 1 receptor (NK1R)-possessing dendrites traversing lamina II. Thus, VGLUT3⁺ C-LTMR terminals appear to largely correspond to neurofilament-lacking central terminals of Type II glomeruli in inner lamina II and can thus be identified at the ultrastructural level by morphological criteria. The participation of C-LTMR terminals in Type II glomeruli involving diverse populations of interneuron indicates highly complex modes of integration of C-LTMR mediated signaling in the dorsal horn. Furthermore, differences in VGLUT1 expression indicate distinct species differences in synaptic physiology of C-LTMR terminals.

The histology, physiology, neurochemistry and circuitry of the substantia gelatinosa Rolandi (lamina II) in mammalian spinal cord

The substantia gelatinosa Rolandi (SGR) was first described about two centuries ago. In the following decades an enormous amount of information has permitted us to understand - at least in part - its role in the initial processing of pain and itch. Here, I will first provide a comprehensive picture of the histology, physiology, and neurochemistry of the normal SGR. Then, I will analytically discuss the SGR circuits that have been directly demonstrated or deductively envisaged in the course of the intensive research on this area of the spinal cord, with particular emphasis on the pathways connecting the primary afferent fibers and the intrinsic neurons. The perspective existence of neurochemically-defined sets of primary afferent neurons giving rise to these circuits will be also discussed, with the proposition that a cross-talk between different subsets of peptidergic fibers may be the structural and functional substrate of additional gating mechanisms in SGR. Finally, I highlight the role played by slow acting high molecular weight modulators in these gating mechanisms.

Identifying local and descending inputs for primary sensory neurons

Primary pain and touch sensory neurons not only detect internal and external sensory stimuli, but also receive inputs from other neurons. However, the neuronal derived inputs for primary neurons have not been systematically identified. Using a monosynaptic rabies viruses-based transneuronal tracing method combined with sensory-specific Cre-drivers, we found that sensory neurons receive intraganglion, intraspinal, and supraspinal inputs, the latter of which are mainly derived from the rostroventral medulla (RVM). The viral-traced central neurons were largely inhibitory but also consisted of some glutamatergic neurons in the spinal cord and serotonergic neurons in the RVM. The majority of RVM-derived descending inputs were dual GABAergic and enkephalinergic (opioidergic). These inputs projected through the dorsolateral funiculus and primarily innervated layers I, II, and V of the dorsal horn, where pain-sensory afferents terminate. Silencing or activation of the dual GABA/enkephalinergic RVM neurons in adult animals substantially increased or decreased behavioral sensitivity, respectively, to heat and mechanical stimuli. These results are consistent with the fact that both GABA and enkephalin can exert presynaptic inhibition of the sensory afferents. Taken together, this work provides a systematic view of and a set of tools for examining peri- and extrasynaptic regulations of pain-afferent transmission.

The sensory neurons of touch

The somatosensory system decodes a wide range of tactile stimuli and thus endows us with a remarkable capacity for object recognition, texture discrimination, sensory-motor feedback and social exchange. The first step leading to perception of innocuous touch is activation of cutaneous sensory neurons called low-threshold mechanoreceptors (LTMRs). Here, we review the properties and functions of LTMRs, emphasizing the unique tuning properties of LTMR subtypes and the organizational logic of their peripheral and central axonal projections. We discuss the spinal cord neurophysiological representation of complex mechanical forces acting upon the skin and current views of how tactile information is processed and conveyed from the spinal cord to the brain. An integrative model in which ensembles of impulses arising from physiologically distinct LTMRs are integrated and processed in somatotopically aligned mechanosensory columns of the spinal cord dorsal horn underlies the nervous system's enormous capacity for perceiving the richness of the tactile world.

Morphological, neurochemical and electrophysiological features of parvalbumin-expressing cells: a likely source of axo-axonic inputs in the mouse spinal dorsal horn

Perception of normal bodily sensations relies on the precise regulation of sensory information entering the dorsal horn of the spinal cord. Inhibitory, axoaxonic, synapses provide a mechanism for this regulation, but the source of these important inhibitory connections remains to be elucidated. This study shows that a subpopulation of spinal interneurons that expresses parvalbumin and have specific morphological, connectivity and functional characteristics are a likely source of the inhibitory inputs that selectivity regulate non-noxious tactile input in the spinal cord. Our findings suggest that a loss of normal function in parvalbumin positive dorsal horn neurons may result in the development of tactile allodynia, where non-painful stimuli gain the capacity to evoke the sensation of pain.

Fast synaptic inhibition in spinal sensory processing and pain control

The two amino acids GABA and glycine mediate fast inhibitory neurotransmission in different CNS areas and serve pivotal roles in the spinal sensory processing. Under healthy conditions, they limit the excitability of spinal terminals of primary sensory nerve fibers and of intrinsic dorsal horn neurons through pre- and postsynaptic mechanisms, and thereby facilitate the spatial and temporal discrimination of sensory stimuli. Removal of fast inhibition not only reduces the fidelity of normal sensory processing but also provokes symptoms very much reminiscent of pathological and chronic pain syndromes. This review summarizes our knowledge of the molecular bases of spinal inhibitory neurotransmission and its organization in dorsal horn sensory circuits. Particular emphasis is placed on the role and mechanisms of spinal inhibitory malfunction in inflammatory and neuropathic chronic pain syndromes.

Ultrastructural analysis of the synaptic connectivity of TRPV1-expressing primary afferent terminals in the rat trigeminal caudal nucleus

Trigeminal primary afferents that express the transient receptor potential vanilloid 1 (TRPV1) are important for the transmission of orofacial nociception. However, little is known about how the TRPV1-mediated nociceptive information is processed at the first relay nucleus in the central nervous system (CNS). To address this issue, we studied the synaptic connectivity of TRPV1-positive (+) terminals in the rat trigeminal caudal nucleus (Vc) by using electron microscopic immunohistochemistry and analysis of serial thin sections. Whereas the large majority of TRPV1+ terminals made synaptic contacts of an asymmetric type with one or two postsynaptic dendrites, a considerable fraction also participated in complex glomerular synaptic arrangements. A few TRPV1+ terminals received axoaxonic contacts from synaptic endings that contained pleomorphic synaptic vesicles and were immunolabeled for glutamic acid decarboxylase, the synthesizing enzyme for the inhibitory neurotransmitter γ-aminobutyric acid (GABA). We classified the TRPV1+ terminals into an S-type, containing less than five dense-core vesicles (DCVs), and a DCV-type, containing five or more DCVs. The number of postsynaptic dendrites was similar between the two types of terminals; however, whereas axoaxonic contacts were frequent on the S-type, the DCV-type did not receive axoaxonic contacts. In the sensory root of the trigeminal ganglion, TRPV1+ axons were mostly unmyelinated, and a small fraction was small myelinated. These results suggest that the TRPV1-mediated nociceptive information from the orofacial region is processed in a specific manner by two distinct types of synaptic arrangements in the Vc, and that the central input of a few TRPV1+ afferents is presynaptically modulated via a GABA-mediated mechanism.

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.

Molecular architecture of endocannabinoid signaling at nociceptive synapses mediating analgesia

Cannabinoid administration suppresses pain by acting at spinal, supraspinal and peripheral levels. Intrinsic analgesic pathways also exploit endocannabinoids; however, the underlying neurobiological substrates of endocannabinoid-mediated analgesia have remained largely unknown. Compelling evidence shows that, upon exposure to a painful environmental stressor, an endocannabinoid molecule called 2-arachidonoylglycerol (2-AG) is mobilized in the lumbar spinal cord in temporal correlation with stress-induced antinociception. We therefore characterized the precise molecular architecture of 2-AG signaling and its involvement in nociception in the rodent spinal cord. Nonradioactive in situ hybridization revealed that dorsal horn neurons widely expressed the mRNA of diacylglycerol lipase-alpha (DGL-alpha), the synthesizing enzyme of 2-AG. Peroxidase-based immunocytochemistry demonstrated high levels of DGL-alpha protein and CB(1) cannabinoid receptor, a receptor for 2-AG, in the superficial dorsal horn, at the first site of modulation of the ascending pain pathway. High-resolution electron microscopy uncovered postsynaptic localization of DGL-alpha at nociceptive synapses formed by primary afferents, and revealed presynaptic positioning of CB(1) on excitatory axon terminals. Furthermore, DGL-alpha in postsynaptic elements receiving nociceptive input was colocalized with metabotropic glutamate receptor 5 (mGluR(5)), whose activation induces 2-AG biosynthesis. Finally, intrathecal activation of mGluR(5) at the lumbar level evoked endocannabinoid-mediated stress-induced analgesia through the DGL-2-AG-CB(1) pathway. Taken together, these findings suggest a key role for 2-AG-mediated retrograde suppression of nociceptive transmission at the spinal level. The striking positioning of the molecular players of 2-AG synthesis and action at nociceptive excitatory synapses suggests that pharmacological manipulation of spinal 2-AG levels may be an efficacious way to regulate pain sensation.

Myelinated skin sensory neurons project extensively throughout adult mouse substantia gelatinosa

The substantia gelatinosa (SG) of the dorsal horn of the spinal cord is a recipient zone for unmyelinated sensory neurons in adults. Recent studies of the central anatomy of physiologically identified skin sensory neurons in neonatal mice have shown that this region also receives substantial inputs from a variety of myelinated afferents. The present experiments were performed to determine whether these neonatal inputs represent a transient phenotype that retracts from the SG. Studies were conducted in an in vivo spinal cord preparation from adult mice; thoracic levels were targeted to facilitate comparisons with previous in vitro findings. We show that the SG continues to receive substantial projections from myelinated skin sensory neurons throughout life. A large population of myelinated nociceptors conducting in the upper A delta and low A beta range maintained extensive projections throughout all areas of the SG well into adulthood; the latter gave rise to dorsally recurving "flame"-shaped arbors extending into the marginal layer that were identical to afferents described in neonates and after nerve injury in adult rats. Furthermore, exquisitely sensitive down hair follicle afferents projected throughout the inner half of the SG (i.e., lamina IIi) and sent dense clusters of terminals well into the outer SG (IIo), where they intermingled with those of unmyelinated nociceptors. Arguments are presented that the SG likely plays a predominant role in tactile processing under normal conditions, but that this role switches rapidly to nociceptive-only during environmental exigencies imposed by temperature extremes.

Distinct populations of spinal cord lamina II interneurons expressing G-protein-gated potassium channels

Noxious stimuli are sensed and carried to the spinal cord dorsal horn by A delta and C primary afferent fibers. Some of this input is relayed directly to supraspinal sites by projection neurons, whereas much of the input impinges on a heterogeneous population of interneurons in lamina II. Previously, we demonstrated that G-protein-gated inwardly rectifying potassium (GIRK) channels are expressed in lamina II of the mouse spinal cord and that pharmacologic ablation of spinal GIRK channels selectively blunts the analgesic effect of high but not lower doses of intrathecal mu-opioid receptor (MOR) agonists. Here, we report that GIRK channels formed by GIRK1 and GIRK2 subunits are found in two large populations of lamina II excitatory interneurons. One population displays relatively large apparent whole-cell capacitances and prominent GIRK-dependent current responses to the MOR agonist [D-Ala2,N-MePhe4,Gly-ol5] -enkephalin (DAMGO). A second population shows smaller apparent capacitance values and a GIRK-dependent response to the GABA(B) receptor agonist baclofen, but not DAMGO. Ultrastructural analysis revealed that GIRK subunits preferentially label type I synaptic glomeruli, suggesting that GIRK-containing lamina II interneurons receive prominent input from C fibers, while receiving little input from A delta fibers. Thus, excitatory interneurons in lamina II of the mouse spinal cord can be subdivided into different populations based on the neurotransmitter system coupled to GIRK channels. This important distinction will afford a unique opportunity to characterize spinal nociceptive circuitry with defined physiological significance.

Widespread expression of the AMPA receptor GluR2 subunit at glutamatergic synapses in the rat spinal cord and phosphorylation of GluR1 in response to noxious stimulation revealed with an antigen-unmasking method

Glutamate, the principal excitatory neurotransmitter in the spinal cord, acts primarily through AMPA receptors. Although all four AMPA subunits are expressed by spinal neurons, we know little about their distribution at glutamatergic synapses. We used an antigen-unmasking technique to reveal the synaptic distribution of glutamate receptor (GluR) 1-4 subunits with confocal microscopy. After pepsin treatment, punctate staining was seen with antibodies against each subunit: GluR2-immunoreactive puncta were distributed throughout the gray matter, whereas GluR1-immunoreactive puncta were restricted to the dorsal horn and were most numerous in laminas I-II. Punctate staining for GluR3 and GluR4 was found in all laminas but was weak in superficial dorsal horn. Colocalization studies showed that GluR2 was present at virtually all (98%) puncta that were GluR1, GluR3, or GluR4 immunoreactive and that most (>90%) immunoreactive puncta in laminas IV, V, and IX showed GluR2, GluR3, and GluR4 immunoreactivity. Evidence that these puncta represented synaptic receptors was obtained with electron microscopy and by examining the association of GluR2- and GluR1-immunoreactive puncta with glutamatergic boutons (identified with vesicular glutamate transporters or markers for unmyelinated afferents). The great majority (96%) of these boutons were associated with GluR2-immunoreactive puncta. Our findings suggest that GluR2 is almost universally present at AMPA-containing synapses, whereas GluR1 is preferentially associated with primary afferent terminals. We also found a substantial, rapid increase in staining for synaptic GluR1 subunits phosphorylated on the S845 residue in the ipsilateral dorsal horn after peripheral noxious stimulation. This finding demonstrates plastic changes, presumably contributing to central sensitization, at the synaptic level.

NGF controls axonal receptivity to myelination by Schwann cells or oligodendrocytes

Axons dictate whether or not they will become myelinated in both the central and peripheral nervous systems by providing signals that direct the development of myelinating glia. Here we identify the neurotrophin nerve growth factor (NGF) as a potent regulator of the axonal signals that control myelination of TrkA-expressing dorsal root ganglion neurons (DRGs). Unexpectedly, these NGF-regulated axonal signals have opposite effects on peripheral and central myelination, promoting myelination by Schwann cells but reducing myelination by oligodendrocytes. These findings indicate a novel role for growth factors in regulating the receptivity of axons to myelination and reveal that different axonal signals control central and peripheral myelination.

Electrophysiological characterisations of rat lamina I dorsal horn neurones and the involvement of excitatory amino acid receptors

Lamina I of the spinal cord plays a key role in sensory transmission between afferent activity and the CNS. Studies have shown lamina I neurones to have distinct response properties compared to deep dorsal horn neurones, but little is known regarding excitatory amino acid mechanisms in their responses. Spinal electrophysiological recordings of lamina I neurones confirmed that the majority of these neurones (74%) are nociceptive specific (NS) in their responses, of which 18% can be termed polymodal nociceptive (HPC) (13% of the total population). The remainder (26%) were wide dynamic range. Lamina I neurones had smaller mechanical and heat-evoked responses compared to deeper dorsal horn neurones. The electrically evoked responses were also smaller, with a distinct lack of an NMDA-mediated 'wind-up' effect. NBQX (AMPA receptor antagonist, 0.5, 5, 50 microg/50 microl) produced dose-dependent inhibitions of the electrically evoked neuronal responses, but APV (NMDA receptor antagonist, 50, 100, 500 microg/50 microl) had minimal effects on their responses. These results implicate mainly AMPA receptors in the responses of lamina I neurones. Bicuculline (GABA(A) receptor antagonist, 0.5, 5, 50 microg/50 microl) demonstrated a role exerted by GABA(A) receptors in the control of A-delta fibre-mediated mechanical responses in lamina I. Overall, this study describes a high threshold, AMPA receptor possessing population of lamina I neurones, which seem to lack functional NMDA receptors, and are partially controlled by GABA(A) receptor activity.

Vesicular glutamate transporters in the spinal cord, with special reference to sensory primary afferent synapses

Spinal cord sensory synapses are glutamatergic, but previous studies have found a great diversity in synaptic vesicle structure and have suggested additional neurotransmitters. The identification of several vesicular glutamate transporters (VGLUTs) similarly revealed an unexpected molecular diversity among glutamate-containing terminals. Therefore, we quantitatively investigated VGLUT1 and VGLUT2 content in the central synapses of spinal sensory afferents by using confocal and electron microscopy immunocytochemistry. VGLUT1 localization (most abundant in LIII/LIV and medial LV) is consistent with an origin from cutaneous and muscle mechanoreceptors. Accordingly, most VGLUT1 immunoreactivity disappeared after rhizotomy and colocalized with markers of cutaneous (SSEA4) and muscle (parvalbumin) mechanoreceptors. With postembedding colloidal gold, intense VGLUT1 immunoreactivity was found in 88-95% (depending on the antibody used) of C(II) dorsal horn glomerular terminals and in large ventral horn synapses receiving axoaxonic contacts. VGLUT1 partially colocalized with CGRP in some large dense-core vesicles (LDCVs). However, immunostaining in neuropeptidergic afferents was inconsistent between VGLUT1 antibodies and rather weak with light microscopy. VGLUT2 immunoreactivity was widespread in all spinal cord laminae, with higher intensities in LII and lateral LV, complementing VGLUT1 distribution. VGLUT2 immunoreactivity did not change after rhizotomy, suggesting a preferential intrinsic origin. However, weak VGLUT2 immunoreactivity was detectable in primary sensory nociceptors expressing lectin (GSA-IB4) binding and in 83-90% of C(I) glomerular terminals in LII. Additional weak VGLUT2 immunoreactivity was found over the small clear vesicles of LDCV-containing afferents and in 50-60% of C(II) terminals in LIII. These results indicate a diversity of VGLUT isoform combinations expressed in different spinal primary afferents.

Synaptic organization of tooth pulp afferent terminals in the rat trigeminal sensory nuclei

Previous studies provide evidence that a structure/function correlation exists in the distinct zones of the trigeminal sensory nuclei. To evaluate this relationship, we examined the ultrastructure of afferent terminals from the tooth pulp in the rat trigeminal sensory nuclei: the principalis (Vp), the dorsomedial part of oral nucleus (Vdm), and the superficial layers of caudalis (Vc), by using transganglionic transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). A total of 93 labeled boutons were serially sectioned, in which some sections were incubated with gamma-aminobutyric acid (GABA) antiserum. Almost all labeled boutons formed asymmetric contact with nonprimary dendrites, in which more than half of labeled boutons in the Vc made synapses with their spines. The labeled boutons could be divided into two types on the basis of numbers of dense-cored vesicles (DCVs) in a boutons: S-type and DCV-type. Almost all labeled boutons in the Vp and Vdm were S-type, whereas two types were distributed evenly in the Vc. In contrast to DCV-type boutons, the S-type was frequently postsynaptic to unlabeled axon terminals containing a mixture of round, oval, and flattened vesicles (p-endings) and forming symmetrical synapses. Most p-endings examined were immunoreactive to GABA. The frequency of axoaxonic contacts was higher for labeled boutons in the Vp than in the Vdm and Vc. These results suggest that the three structures of trigeminal sensory nuclei serve distinct functions in nociceptive processing.

Central projection of unmyelinated (C) primary afferent fibers from gastrocnemius muscle in the guinea pig

We have demonstrated the central projections of muscle C or group IV afferent fibers in the guinea pig by tracing arborizations in the spinal cord. C afferent fibers from the gastrocnemius muscle (GCM) were electrophysiologically identified by conduction velocity (less than 1 m/second). A single neuron in the lumbar 5 dorsal root ganglion (L5 DRG) was intracellularly labeled with Phaseolus vulgaris leucoagglutinin (PHA-L). After iontophoretic injection of PHA-L, we processed the lumbar cord and L5 DRG for PHA-L immunohistochemistry. Six muscle C afferent fibers from 40 animals were labeled, and whole trajectories were recovered. Labeled fibers were reconstructed by tracing of the arbor in serial parasagittal sections. The GCM C afferents projected rostrocaudally for two or three segments and ran at the surface of the dorsal funiculus, giving off collaterals into laminae I and II and sometimes into parts of lamina III. We determined, based on the branching pattern and form of the terminal plexus, that the branching of muscle C afferent fibers showed an intermediate pattern that fell morphologically between the terminal patterns of somatic and visceral afferents. The numbers and sizes of fiber swellings and terminal swellings were measured on all collateral branches. We found that the area of distribution of the terminal swellings of muscle C afferent fibers is larger than that of somatic terminals but that the density of terminal swellings in the terminal area was lower than that of the somatic terminals.

Widespread projections from myelinated nociceptors throughout the substantia gelatinosa provide novel insights into neonatal hypersensitivity

Skin sensory neurons have long been thought to undergo major changes in anatomy and physiology over the first few weeks of postnatal life. Low-threshold mechanoreceptors (LTMRs) are believed to project extensively throughout superficial dorsal horn laminas initially and provide the afferent limb for hyperactive nocifensive reflexes. However, our recent studies revealed that neonatal LTMRs do not project into "pain-specific" regions; instead, they exhibit adult-like anatomy shortly after birth. We sought to determine whether the same might be true for myelinated high-threshold mechanoreceptors (HTMRs). We used an intact, ex vivo somatosensory system preparation from neonatal mice to allow intrasomal recording and neurobiotin labeling of individual sensory neurons characterized via natural skin stimuli. Neonatal HTMRs displayed a number of key hallmarks of their adult counterparts; relative to LTMRs, they exhibited broader, inflected somal spikes and higher mechanical thresholds and/or responded in an increasingly vigorous manner to incrementally graded forces in a manner capable of encoding stimulus intensity. Two types were discerned on the basis of central anatomy: one subset projected to superficial laminas (I/II); the other gave rise to diffuse, dorsally recurving collateral arbors extending throughout the entire dorsal horn (I-V). The latter represent a novel cutaneous afferent morphology that persists in older animals. These studies reveal that inputs from myelinated afferents to superficial pain-specific laminas in neonates arise from HTMRs and not LTMRs as commonly thought. This frequently overlooked population is in a position, therefore, to contribute substantially to paradoxical nocifensive behaviors in neonates and various pain states in adults.

Nitric oxide as an anterograde neurotransmitter in the trigeminal motor pool

We demonstrate the presence of nitric oxide synthase containing fibers within the guinea pig trigeminal motor nucleus and describe the effects of nitric oxide (NO) on trigeminal motoneurons. Using immunohistochemical techniques, we observed nitrergic fibers displaying varicosities and giving rise to bouton-like structures in apposition to retrogradely labeled motoneuron processes, most of which were dendrites. NO-donors evoked a membrane depolarization (mean 7.5 mV) and a decrease in rheobase (mean 38%). These substances also evoked an apparent increase in an hyperpolarization-activated cationic current (I(H)). These changes were not accompanied by any modification of the motoneurons' input resistance or time constant. The effects were suppressed by blocking the cytosolic guanlyate cyclase. A membrane-permeant cyclic guanosine 3,5'-monophosphate (cGMP) analogue mimicked the effects of NO. There was a considerable increase in synaptic activity following NO-donors or db-cGMP application. Tetrodotoxin supressed the increase in synaptic activity evoked by NO-donors. The histological and electrophysiological evidence, taken together, indicates the existence of a nitrergic system able to modulate trigeminal motoneurons under yet unknown physiological conditions.

Quantitative ultrastructure of slowly adapting lingual afferent terminals in the principal and oral nuclei in the cat

Previous studies provide evidence that a structure/function correlation exists in the cytoarchitectonically different zones of the trigeminal sensory nuclei. To extend this relationship, we examined the ultrastructural features of trigeminal primary afferent neurons in the cat dorsal principal nucleus (Vpd) and the rostrodorsomedial oral nucleus (Vo.r) using intra-axonal labeling with horseradish peroxidase and morphometric analyses. All labeled boutons contained round synaptic vesicles. Eighty-two percent of the boutons in the Vo.r and 99% of the boutons in the Vpd were presynaptic to nonprimary dendrites. The remaining boutons in the Vo.r were presynaptic to somata (8%) or primary dendrites (10%). The average number of postsynaptic profiles per labeled bouton did not differ in the Vpd and Vo.r. Most labeled boutons in the two nuclei were postsynaptic to unlabeled axon terminals with pleomorphic vesicles (p-ending). The number of p-endings per labeled bouton was higher in the Vpd than Vo.r A morphometric analysis indicated that labeled bouton volume and apposed surface area were larger in the Vpd than Vo.r while active zone area and vesicle number did not differ. All these parameters were larger than those of p-endings in each nucleus. In both labeled boutons and p-endings, the parameters were positively correlated with bouton size. These results suggest that sensory information conveyed through trigeminal afferents is more strongly controlled at the level of the first synapse by presynaptic mechanisms in the Vpd than in the Vo.r, while the efficacy of transmission at primary afferent synapses does not differ.

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

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

Distribution of orexin-A and orexin-B (hypocretins) in the rat spinal cord

Orexin-A and orexin-B (also known as hypocretin-1 and hypocretin-2) are hypothalamic peptides that regulate feeding behavior, energy metabolism, and sleep-wake cycle. We determined the distribution of orexin-A and -B in the rat spinal cord, using sensitive radioimmunoassays and an immunohistochemical technique with three antisera specific for these orexins. Orexins were distributed throughout the spinal cord, and their contents were highest in the cervical region. Orexin fibers were concentrated in lamina I of the dorsal horn and area X surrounding the central canal of the spinal cord. Abundant orexin fibers also were present in the preganglionic sympathetic and parasympathetic cell columns. These findings suggest that orexins in the spinal cord may be involved in the modulation of sensory informations and the autonomic system as neurotransmitters or neuromodulators, both.

Injury to dorsal root ganglia alters innervation of spinal cord dorsal horn lamina involved in nociception

STUDY DESIGN

A study of the relation between the development of mechanical allodynia and the reorganization of primary afferent terminals in the sensory lamina of the rat spinal cord dorsal horn after partial dorsal root ganglion injury in rats.

OBJECTIVES

To investigate the pathologic mechanisms of mechanical allodynia after partial dorsal root ganglion injury.

SUMMARY OF BACKGROUND DATA

After experimental peripheral nerve injury causing neuropathic pain, myelinated afferent fibers sprout into lamina II of the dorsal horn. This lamina is associated with nociceptive-specific neurons that generally are not stimulated by myelinated fiber input from mechanical receptors. These morphologic changes are suggested to have significance in the pathogenesis of chronic mechanical allodynia, although it is not known whether this kind of morphologic change occurs after dorsal root ganglion injury.

METHODS

After partial dorsal root ganglion crush injury, the mechanical force causing footpad withdrawal was measured with von Frey hairs, and myelinated primary afferents were labeled with cholera toxin B subunit horseradish peroxidase, a selective myelinated fiber tracer that identifies transganglionic synapses.

RESULTS

After partial dorsal root ganglion injury, mechanical allodynia developed in the corresponding footpad within 3 days and persisted throughout the experimental period. At 2 and 4 weeks after the injury, B subunit horseradish peroxidase-positive fibers, presumably myelinated afferents, were observed to be sprouting into lamina II of the dorsal horn on the injured side, but not on the contralateral control side.

CONCLUSIONS

Morphologic change in spinal cord dorsal horn lamina II occurs after partial dorsal root ganglion injury. This change may have significance in the pathogenesis of chronic mechanical allodynia after partial dorsal root ganglion injury.

Spinal laminae I-II neurons in rat recorded in vivo in whole cell, tight seal configuration: properties and opioid responses

Using the in vivo whole cell recording procedure described previously, we recorded 73 neurons in laminae I and II in the lumbar spinal cord of the rat. Input impedances averaged 332 MOmega, which indicated that prior sharp electrode recordings contained a significant current shunt. Characterization of the adequate stimuli from the excitatory hindlimb receptive field indicated that 39 of 73 neurons were nociceptive, 6 were innocuous cooling cells, 20 responded maximally to brush, and 8 cells were not excited by stimulation of the skin of the hindlimb. The locations of 15 neurons were marked with biocytin. Nociceptive neurons were mostly found in lamina I and outer II, cooling cells in lamina I, and innocuous mechanoreceptive cells were mostly found in inner II or in the overlying white matter. The mu-opioid agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-Enkephalin (DAMGO) hyperpolarized 7 of 19 tested neurons with a conductance increase. This hyperpolarization was reversed by naloxone in the neurons in which it was applied. DAMGO also decreased the frequency of spontaneous PSPs in 13 neurons, 7 of which were also hyperpolarized by DAMGO. Five of the seven hyperpolarized neurons were nociceptive, responding to both heat and mechanically noxious stimuli, whereas two responded to slow, innocuous brush. These results indicate that whole cell, tight seal recordings sample a similar population of lamina I and II neurons in the rat as those found with sharp electrode recordings in cat and monkey. They further indicate that DAMGO hyperpolarizes a subset of the nociceptive neurons that have input from both heat and mechanical nociceptors and that presynaptic DAMGO effects can be observed in nociceptive neurons that are not hyperpolarized by DAMGO.

Ultrastructural analysis of ectopic synaptic boutons arising from peripherally regenerated primary afferent fibers

The central axons of peripherally regenerated Abeta primary sensory neurons were impaled in the dorsal columns of alpha-chloralose-anesthetized cats 9-12 mo after axotomy. The adequate peripheral stimulus was determined, and the afferent fibers intracellularly stimulated while simultaneously recording the resulting cord dorsum potentials (CDPs). Fibers that successfully had reinnervated the skin responded to light tactile stimulation, and evoked CDPs that suggested dorsally located boutons were stained intracellularly with horseradish peroxidase (HRP). Two HRP-stained regenerated Abeta afferent fibers were recovered that supported large numbers of axon collaterals and swellings in laminae I, IIo, and IIi. Sections containing the ectopic collateral fibers and terminals in the superficial dorsal horn were embedded in plastic. Analyses of serial ultrathin sections revealed that ectopic projections from both regenerated fibers supported numerous synaptic boutons filled with clear round vesicles, a few large dense core vesicles (LDCVs) and several mitochondria (>3). All profiles examined in serial sections (19) formed one to three asymmetric axo-dendritic contacts. Unmyelinated portions of ectopic fibers giving rise to en passant and terminal boutons often contained numerous clear round vesicles. Several boutons (47%) received asymmetric contacts from axon terminals containing pleomorphic vesicles. These results strongly suggest that regenerated Abeta fibers activated by light tactile stimuli support functional connections in the superficial dorsal horn that have distinct ultrastructural features. In addition, the appearance of LDCVs suggests that primary sensory neurons are capable of changing their neurochemical phenotype.

Electrophysiological evidence for tetrodotoxin-resistant sodium channels in slowly conducting dural sensory fibers

A tetrodotoxin (TTX)-resistant sodium channel was recently identified that is expressed only in small diameter neurons of peripheral sensory ganglia. The peripheral axons of sensory neurons appear to lack this channel, but its presence has not been investigated in peripheral nerve endings, the site of sensory transduction in vivo. We investigated the effect of TTX on mechanoresponsiveness in nerve endings of sensory neurons that innervate the intracranial dura. Because the degree of TTX resistance of axonal branches could potentially be affected by factors other than channel subtype, the neurons were also tested for sensitivity to lidocaine, which blocks both TTX-sensitive and TTX-resistant sodium channels. Single-unit activity was recorded from dural afferent neurons in the trigeminal ganglion of urethan-anesthetized rats. Response thresholds to mechanical stimulation of the dura were determined with von Frey monofilaments while exposing the dura to progressively increasing concentrations of TTX or lidocaine. Neurons with slowly conducting axons were relatively resistant to TTX. Application of 1 microM TTX produced complete suppression of mechanoresponsiveness in all (11/11) fast A-delta units [conduction velocity (c.v.) 5-18 m/s] but only 50% (5/10) of slow A-delta units (1.5 <c.v.<5 m/s) and 13% (2/15) of C units (c.v. </=1.5 m/s). The mean TTX concentration that produced complete suppression of mechanoresponsiveness was approximately 270-fold higher in C units than in fast A-delta units. In contrast, no significant difference was found between C and A-delta units in the concentration of lidocaine required for complete suppression of mechanoresponsiveness, indicating that the greater TTX resistance of mechanoresponsiveness in C units is not attributable to differences in safety factor unrelated to channel subtype. These data offer indirect evidence that a TTX-resistant channel subtype is expressed in the terminal axonal branches of many of the more slowly conducting (C and slow A-delta) dural afferents. The channel appears to be present in these fibers, but not in the faster A-delta fibers, in sufficient numbers to support the initiation and propagation of mechanically induced impulses. Comparison with previous data on the absence of TTX resistance in peripheral nerve fibers suggests that the TTX-resistant sodium channel may be a distinctive feature of the receptive rather than the conductive portion of the sensory neuron's axonal membrane.

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

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

Ultrastructural observations of synaptic connections of vibrissa afferent terminals in cat principal sensory nucleus and morphometry of related synaptic elements

Previous work suggests that slowly adapting (SA) periodontal afferents have different synaptic arrangements in the principal (Vp) and oral trigeminal nuclei and that the synaptic structure associated with transmitter release may be related directly to bouton size. The present study examined the ultrastructures of SA and fast adapting (FA) vibrissa afferents and their associated unlabeled axonal endings in the cat Vp by using intra-axonal labeling with horseradish peroxidase and a morphometric analysis. All SA and FA afferent boutons contained clear, round, synaptic vesicles. All the FA and most SA boutons were presynaptic to dendrites, but a few SA boutons were axosomatic. Both types of bouton were frequently postsynaptic to unlabeled axonal ending(s) containing pleomorphic, synaptic vesicles (P-ending). The size of labeled boutons was larger in FA than SA afferents, but the size of dendrites postsynaptic to labeled boutons was larger for SA than FA afferents. Large-sized FA and SA boutons made synaptic contacts with small-diameter dendrites. The size of FA and SA boutons was larger than that of their associated P-endings. A morphometric analysis made on the pooled data of SA and FA boutons indicated that apposed surface area, active zone number, total active zone area, vesicle number, and mitochondrial volume were highly correlated in a positive linear manner with labeled bouton volume. These relationships were also applicable to unlabeled P-endings, but the range of each parameter was smaller than that of the labeled boutons. These observations provide evidence that the two functionally distinct types of vibrissa afferent manifest unique differences but share certain structural features in the synaptic organization and that the ultrastructural "size principle" proposed by Pierce and Mendell ([1993] J. Neurosci. 13:4748-4763) for Ia-motoneuron synapses is applicable to the somatosensory system.

Large dorsal horn neurons which receive inputs from numerous substance P-like immunoreactive axon terminals in the laminae I and II of the chicken spinal cord

Large neurons outlined with numerous substance P (SP)-like immunoreactive (LI) boutons were detected immunocytochemically in the dorsal horn of the chicken spinal cord at the light microscopic level. The cervical enlargement was mainly used for observations. By electron microscopy, asymmetrical synapses were observed between the SP-LI axon terminals and the soma and dendrites of the large neurons. Cell bodies of the large neurons were mostly localized in the lamina I and the region lateral to the lamina I. Some of the cell bodies were also located in the lamina II. Their dendrites extended in the lamina I, in the region lateral to the lamina I, and deeply in the lamina II. In the lamina II, dendrites of these neurons formed synapses with SP-containing central terminals in synaptic glomeruli known to originate from primary afferents. The findings suggest that these large neurons receive nociceptive information directly from primary afferents. In the light of previous investigations, these neurons are considered to be pain-transmitting long ascending tract neurons.

Interaction between neurons in different laminae of the dorsal horn of the spinal cord. A correlation study in normal and neuropathic rats

Simultaneous recordings of 135 pairs of units, located respectively in the superficial (I-IIo) and deep (V) laminae of the dorsal horn of the lumbar spinal cord of anaesthetized and paralysed animals, were performed both from normal (62 pairs) and from peripherally injured (chronically constricted sciatic nerve) rats (73 pairs). In each pair, one neuron was classified as nociceptive, responding only to noxious stimuli, and the other as a wide dynamic range neuron, responding to both non-noxious and noxious stimuli. To understand if some interaction was present between diverse neurons modulated by noxious inputs, we used cross-correlation techniques. The responses of simultaneously recorded pairs of units to suprathreshold (5 mA, 0.5 ms) electrical stimuli were used. A clearly delayed peak in the cross-correlograms of recordings from normal animals was present, indicating connectivity of superficial and deep-layer cells. This feature was not present in the cross-correlograms obtained from nerve-injured animals. Even if a specific pathway cannot be explicitly assigned to support these functional results, an overall connection between superficial and deep layers of the spinal cord is suggested. These connections are supposed to be either inactive or rearranged in the nerve-injured rats, thus suppressing a well timed coordinated connectivity. This anomaly could underlie a reduced degree of functional coherence in the modulation of nociceptive spinal inputs in cases of chronic pain.

An ultrastructural study of the glycine transporter GLYT2 and its association with glycine in the superficial laminae of the rat spinal dorsal horn

The glycine transporter GLYT2 is present in axonal boutons throughout the spinal cord, and its laminar distribution matches that of glycine-enriched axons, which are presumed to be glycinergic. In order to determine whether boutons which possess GLYT2 are glycine-enriched, we have carried out pre-embedding immunocytochemistry with antibody raised against GLYT2, and combined this with post-embedding detection of glycine, in the rat. GLYT2 immunoreactivity was present in boutons which formed symmetrical axodendritic, axosomatic or axoaxonic synapses, and was often seen in peripheral axons of type II synaptic glomeruli. One hundred and fifty GLYT2-immunoreactive boutons were analysed quantitatively, and in 142 (94.6%) of these the density of gold particles representing glycine-like immunoreactivity exceeded the background level (over presumed glutamatergic boutons) by at least a factor of two. Within immunoreactive boutons, the GLYT2 reaction product was associated with the plasma membrane, but often appeared as discrete clumps and was generally excluded from the region of the active sites of synapses. These results confirm that GLYT2 is associated with glycine-enriched axonal boutons in the superficial dorsal horn. They also suggest that GLYT2 is unevenly distributed on the plasma membrane of these boutons, and raise the possibility that it may be excluded from synaptic clefts.

GABA and glycine in synaptic glomeruli of the rat spinal dorsal horn

The superficial dorsal horn of rat spinal cord contains two types of synaptic glomerulus, which are centred around the terminals of unmyelinated and myelinated primary afferents respectively. Both types of glomerulus contain GABAergic axons and dendrites, which are thought to originate from local inhibitory interneurons. Some of the dendrites contain synaptic vesicles, and may be presynaptic to the central axon of dendroaxonic synapses. In order to determine whether GABAergic structures in the two types of glomerulus are derived from different populations of interneurons, a quantitative post-embedding immunogold study of GABA- and glycine-immunoreactivity in a rat dorsal horn was performed. In type I glomeruli, all of the peripheral axons and most vesicle-containing dendrites were GABA-immunoreactive, but only one of 32 axons and none of the vesicle-containing dendrites was glycine-immunoreactive. In contrast, most of the peripheral axons and some of the vesicle-containing dendrites in type II glomeruli possessed both GABA- and glycine-immunoreactivity. This strongly suggests that different types of inhibitory interneuron in the dorsal horn are associated with the two types of glomerulus. It is therefore likely that different populations of interneurons mediate presynaptic inhibition of unmyelinated and myelinated primary afferents.

Electron microscopic observation of synaptic connections of jaw-muscle spindle and periodontal afferent terminals in the trigeminal motor and supratrigeminal nuclei in the cat

Previous studies indicate that the trigeminal motor nucleus (Vmo) and supratrigeminal nucleus (Vsup) receive direct projections from muscle spindle (MS) and periodontal ligament (PL) afferents. The aim of the present study is to examine the ultrastructural characteristics of the two kinds of afferent in both nuclei using the intracellular horseradish peroxidase (HRP) injection technique in the cat. Our observations are based on complete or near-complete reconstructions of 288 MS (six fibers) and 69 PL (eight fibers) afferent boutons in Vmo, and of 93 MS (four fibers) and 188 PL (four fibers) afferent boutons in Vsup. All the labeled boutons contained spherical synaptic vesicles and were presynaptic to neuronal elements, and some were postsynaptic to axon terminals containing pleomorphic, synaptic vesicles (P-endings). In Vmo neuropil, MS afferent boutons were distributed widely from soma to distal dendrites, but PL afferent boutons predominated on distal dendrites. Most MS afferent boutons (87%) formed synaptic specialization(s) with one postsynaptic target while some (13%) contacting two or three dendritic profiles; PL afferents had a higher number of boutons (43%) contacting two or more dendritic profiles. A small but significant number of MS afferent boutons (12%) received contacts from P-endings, but PL afferent boutons (36%) received three times as many contacts from P-endings as MS afferents. In Vsup neuropil, most MS (72%) and PL (87%) afferent boutons formed two contacts presynaptic to one dendrite and postsynaptic to one P-ending, and their participation in synaptic triads was much more frequent than in Vmo neuropil. The present study indicates that MS and PL afferent terminals have a distinct characteristic in synaptic arrangements in Vmo and Vsup and provides evidence that the synaptic organization of primary afferents differs between the neuropils containing motoneurons and their interneurons.

Synapses from labeled type II axons in the mouse cochlear nucleus

This study investigates the ultrastructure and central targets in the cochlear nucleus of axonal swellings of type II primary afferent neurons. Type II axons comprise only 5-10% of the axons of the auditory nerve of mammals, but they alone provide the afferent innervation of the outer hair cells. In this study, type II axons were labeled with horseradish peroxidase, and serial-section electron microscopy was used to examine their swellings in: (1) the granule-cell lamina at its boundary with posteroventral cochlear nucleus, (2) the rostral anteroventral cochlear nucleus, and (3) the auditory nerve root. Only some (18%) of the type II terminal and en-passant swellings formed synapses. The synapses were asymmetric and contained clear round synaptic vesicles, suggesting that they are excitatory. Type II synapses were compared to those from type I fibers providing the afferent innervation of the inner hair cells. Type II synapses tended to have slightly smaller and fewer synaptic vesicles, had a greater proportion of the membrane apposition accompanied by a postsynaptic density, and often had densities that were discontinuous or 'perforated'. In all cochlear nucleus regions examined, the postsynaptic targets of type II synapses had characteristics of dendrites; in most cases these dendrites could not be traced to their cell bodies of origin. Some evidence suggests, however, that targets may include granule cells, spherical cells, and other cells in the nerve root. These results suggest afferent information from outer hair cells reaches diverse regions and targets within the cochlear nucleus.

Sprouting of A beta fibers into lamina II of the rat dorsal horn in peripheral neuropathy

Cholera toxin beta-subunit conjugated to horseradish peroxidase was used to label the large myelinated (A beta) fiber input to the dorsal horn in a model of peripheral neuropathy induced by tight ligation of the L5 and L6 spinal nerves. Following induction of neuropathy, A beta fibers were present in lamina II of the ipsilateral dorsal horn, a region normally devoid of A beta input. This reorganization of large fiber input to the superficial dorsal horn provides some anatomical basis for sensory changes found in this model of neuropathic pain.

Evidence that large myelinated primary afferent fibers make synaptic contacts in lamina II of neonatal rats

Choleragenoid horseradish peroxidase (B-HRP) is a retrogradely transported marker that selectively labels large cutaneous myelinated primary afferent fibers. In adults, B-HRP labelled large afferent fibers are seen to enter laminae III-V, and to a lesser extent lamina I, whereas lamina II, which is the major termination site of unmyelinated primary afferents, remains unlabelled. In the neonate, however, there is extensive B-HRP label in lamina II. The present study shows that the B-HRP labelled fibers in the neonate make many synaptic contacts in lamina II. This supports the idea that large primary afferent fibers in neonatal animals make synaptic contact with post-synaptic targets that presumably process nociceptive information. Accordingly to ameliorate pain in neonates it may be more important to block low threshold sensory input whereas in adults it would be more important to block the high threshold inputs.