Amino acid immunocytochemistry of primary afferent terminals in the rat dorsal horn

Rat odontoblasts may use glutamate to signal dentin injury

Accumulating evidence indicates that odontoblasts act as sensor cells, capable of triggering action potentials in adjacent pulpal nociceptive axons, suggesting a paracrine signaling via a currently unknown mediator. Since glutamate can mediate signaling by non-neuronal cells, and peripheral axons may express glutamate receptors (GluR), we hypothesized that the expression of high levels of glutamate, and of sensory receptors in odontoblasts, combined with an expression of GluR in adjacent pulpal axons, is the morphological basis for odontoblastic sensory signaling. To test this hypothesis, we investigated the expression of glutamate, the thermo- and mechanosensitive ion channels transient receptor potential vanilloid 1 (TRPV1), transient receptor potential ankyrin 1 (TRPA1), and TWIK-1-related K+channel (TREK-1), and the glutamate receptor mGluR5, in a normal rat dental pulp, and following dentin injury. We also examined the glutamate release from odontoblast in cell culture. Odontoblasts were enriched with glutamate, at the level as high as in adjacent pulpal axons, and showed immunoreactivity for TRPV1, TRPA1, and TREK-1. Pulpal sensory axons adjacent to odontoblasts expressed mGluR5. Both the levels of glutamate in odontoblasts, and the expression of mGluR5 in nearby axons, were upregulated following dentin injury. The extracellular glutamate concentration was increased significantly after treating of odontoblast cell line with calcium permeable ionophore, suggesting glutamate release from odontoblasts. These findings lend morphological support to the hypothesis that odontoblasts contain glutamate as a potential neuroactive substance that may activate adjacent pulpal axons, and thus contribute to dental pain and hypersensitivity.

Whiplash-like facet joint loading initiates glutamatergic responses in the DRG and spinal cord associated with behavioral hypersensitivity

The cervical facet joint and its capsule are a common source of neck pain from whiplash. Mechanical hyperalgesia elicited by painful facet joint distraction is associated with spinal neuronal hyperexcitability that can be induced by transmitter/receptor systems that potentiate the synaptic activation of neurons. This study investigated the temporal response of a glutamate receptor and transporters in the dorsal root ganglia (DRG) and spinal cord. Bilateral C6/C7 facet joint distractions were imposed in the rat either to produce behavioral sensitivity or without inducing any sensitivity. Neuronal metabotropic glutamate receptor-5 (mGluR5) and protein kinase C-epsilon (PKCε) expression in the DRG and spinal cord were evaluated on days 1 and 7. Spinal expression of a glutamate transporter, excitatory amino acid carrier 1 (EAAC1), was also quantified at both time points. Painful distraction produced immediate behavioral hypersensitivity that was sustained for 7 days. Increased expression of mGluR5 and PKCε in the DRG was not evident until day 7 and only following painful distraction; this increase was observed in small-diameter neurons. Only painful facet joint distraction produced a significant increase (p<0.001) in neuronal mGluR5 over time, and this increase also was significantly elevated (p≤0.05) over responses in the other groups at day 7. However, there were no differences in spinal PKCε expression on either day or between groups. Spinal EAAC1 expression was significantly increased (p<0.03) only in the nonpainful groups on day 7. Results from this study suggest that spinal glutamatergic plasticity is selectively modulated in association with facet-mediated pain.

Action potential-triggered somatic exocytosis in mesencephalic trigeminal nucleus neurons in rat brain slices

The neurons in the mesencephalic trigeminal nucleus (MeV) play essential roles in proprioceptive sensation of the face and oral cavity. The somata of MeV neurons are generally assumed to carry out neuronal functions but not to play a direct role in synaptic transmission. Using whole-cell recording and membrane capacitance (C(m)) measurements, we found that the somata of MeV neurons underwent robust exocytosis (C(m) jumps) upon depolarization and with the normal firing of action potentials in brain slices. Both removing [Ca(2+)](o) and buffering [Ca(2+)](i) with BAPTA blocked this exocytosis, indicating that it was completely Ca(2+) dependent. In addition, an electron microscopic study showed synaptic-like vesicles approximated to the plasma membrane in somata. There was a single Ca(2+)-dependent releasable vesicle pool with a peak release rate of 1912 fF s(-1). Importantly, following depolarization-induced somatic exocytosis, GABA-mediated postsynaptic currents were transiently reduced by 31%, suggesting that the somatic vesicular release had a retrograde effect on afferent GABAergic transmission. These results provide strong evidence that the somata of MeV neurons undergo robust somatic secretion and may play a crucial role in bidirectional communication between somata and their synaptic inputs in the central nervous system.

Nociceptive stimulation induces expression of Arc/Arg3.1 in the spinal cord with a preference for neurons containing enkephalin

Background

In pain processing, long term synaptic changes play an important role, especially during chronic pain. The immediate early gene Arc/Arg3.1 has been widely implicated in mediating long-term plasticity in telencephalic regions, such as the hippocampus and cortex. Accordingly, Arc/Arg3.1 knockout (KO) mice show a deficit in long-term memory consolidation. Here, we identify expression of Arc/Arg3.1 in the rat spinal cord using immunohistochemistry and in situ hybridization following pain stimuli.

Results

We found that Arc/Arg3.1 is not present in naïve or vehicle treated animals, and is de novo expressed in dorsal horn neurons after nociceptive stimulation. Expression of Arc/Arg3.1 was induced in an intensity dependent manner in neurons that were located in laminae I (14%) and II (85%) of the spinal dorsal horn. Intrathecal injection of brain derived neurotrophic factor (BDNF) also induced expression of Arc/Arg3.1. Furthermore, 90% of Arc/Arg3.1 expressing neurons also contained the activity marker c-Fos, which was expressed more abundantly. Preproenkephalin mRNA was found in the majority (68%) of the Arc/Arg3.1 expressing neurons, while NK-1 was found in only 19% and GAD67 mRNA in 3.6%. Finally, pain behavior in Arc/Arg3.1 KO mice was not significantly different from their wild type littermates after application of formalin or after induction of chronic inflammatory pain.

Conclusions

We conclude that Arc/Arg3.1 is preferentially expressed in spinal enkephalinergic neurons after nociceptive stimulation. Therefore, our data suggest that Arc/Arg3.1 dependent long term synaptic changes in spinal pain transmission are a feature of anti-nociceptive, i.e. enkephalinergic, rather than pro-nociceptive neurons.

Simulated whiplash modulates expression of the glutamatergic system in the spinal cord suggesting spinal plasticity is associated with painful dynamic cervical facet loading

The cervical facet joint and its capsule have been reported to be injured during whiplash scenarios and are a common source of chronic neck pain from whiplash. Both the metabotropic glutamate receptor 5 (mGluR5) and the excitatory amino acid carrier 1 (EAAC1) have pivotal roles in chronic pain. In this study, spinal mGluR5 and EAAC1 were quantified following painful facet joint distraction in a rat model of facet-mediated painful loading and were evaluated for their correlation with the severity of capsule loading. Rats underwent either a dynamic C6/C7 joint distraction simulating loading experienced during whiplash (distraction; n = 12) or no distraction (sham; n = 6) to serve as control. The severity of capsular loading was quantified using strain metrics, and mechanical allodynia was assessed after surgery. Spinal cord tissue was harvested at day 7 and the expression of mGluR5 and EAAC1 were quantified using Western blot analysis. Mechanical allodynia following distraction was significantly (p < 0.001) higher than sham. Spinal expression of mGluR5 was also significantly (p < 0.05) greater following distraction relative to sham. However, spinal EAAC1 was significantly (p = 0.0003) reduced compared to sham. Further, spinal mGluR5 expression was significantly positively correlated to capsule strain (p < 0.02) and mechanical allodynia (p < 0.02). Spinal EAAC1 expression was significantly negatively related to one of the strain metrics (p < 0.003) and mechanical allodynia at day 7 (p = 0.03). These results suggest that the spinal glutamatergic system may potentiate the persistent behavioral hypersensitivity that is produced following dynamic whiplash-like joint loading; chronic whiplash pain may be alleviated by blocking mGluR5 expression and/or enhancing glutamate transport through the neuronal transporter EAAC1.

Vanilloid receptor TRPV1-positive sensory afferents in the mouse ankle and knee joints

TRPV1, a cation channel on sensory nerves sensitive to heat and capsaicin, plays an important role in the transduction of noxious stimuli to the spinal cord. It is expressed by neurons in dorsal root ganglia (DRG) that may also express neuropeptides, which are important for the development of inflammation. Mice with genetic deletion of TRPV1 have been used to study the involvement of this receptor in the mediation of pain and inflammation in animal models of arthritis. However, the expression of TRPV1 in the mouse articular afferents has not been studied. We here provide numerical data on expression of TRPV1 in an identified population of sensory afferents to the mouse L3-L5 DRG that innervate joints, in comparison with that from bladder and skin. A combination of tracing and immunohistochemistry revealed that TRPV1-positive fibers innervate the mouse knee and ankle. At the level of DRG, approximately 40% of articular afferents from these joints express TRPV1 and the majority of them are peptidergic, as revealed by simultaneous immunostaining for the neuropeptide calcitonin gene-related peptide. These findings are consistent with the idea that activation of TRPV1 in peripheral axons of joint afferents may mediate the synovial release of neuropeptides in arthritis.

Expression of the vesicular glutamate transporters-1 and -2 in adult mouse dorsal root ganglia and spinal cord and their regulation by nerve injury

The expression of two vesicular glutamate transporters (VGLUTs), VGLUT1 and VGLUT2, was studied with immunohistochemistry in lumbar dorsal root ganglia (DRGs), the lumbar spinal cord and the skin of the adult mouse. About 12% and 65% of the total number of DRG neuron profiles (NPs) expressed VGLUT1 and VGLUT2, respectively. VGLUT1-immunoreactive (IR) NPs were usually medium- to large-sized, in contrast to a majority of small- or medium-sized VGLUT2-IR NPs. Most VGLUT1-IR NPs did not coexpress calcitonin gene-related peptide (CGRP) or bound isolectin B4 (IB4). In contrast, approximately 31% and approximately 42% of the VGLUT2-IR DRG NPs were also CGRP-IR or bound IB4, respectively. Conversely, virtually all CGRP-IR and IB4-binding NPs coexpressed VGLUT2. Moderate colocalization between VGLUT1 and VGLUT2 was also observed. Sciatic nerve transection induced a decrease in the overall number of VGLUT1- and VGLUT2-IR NPs (both ipsi- and contralaterally) and, in addition, a parallel, unilateral increase of VGLUT2-like immunoreactivity (LI) in a subpopulation of mostly small NPs. In the dorsal horn of the spinal cord, strong VGLUT1-LI was detected, particularly in deep dorsal horn layers and in the ventral horns. VGLUT2-LI was abundant throughout the gray spinal matter, 'radiating' into/from the white matter. A unilateral dorsal rhizotomy reduced VGLUT1-LI, while apparently leaving unaffected the VGLUT2-LI. Transport through axons for both VGLUTs was confirmed by their accumulation after compression of the sciatic nerve or dorsal roots. In the hind paw skin, abundant VGLUT2-IR nerve fibers were observed, sometimes associated with Merkel cells. Lower numbers of VGLUT1-IR fibers were also detected in the skin. Some VGLUT1-IR and VGLUT2-IR fibers were associated with hair follicles. Based on these data and those by Morris et al. [Morris JL, Konig P, Shimizu T, Jobling P, Gibbins IL (2005) Most peptide-containing sensory neurons lack proteins for exocytotic release and vesicular transport of glutamate. J Comp Neurol 483:1-16], we speculate that virtually all DRG neurons in adult mouse express VGLUTs and use glutamate as transmitter.

Glutamate and metabotropic glutamate receptors associated with innervation of the uterine cervix during pregnancy: Receptor antagonism inhibits c-fos expression in rat lumbosacral spinal cord at parturition

Dorsal root ganglia (DRG) neurons connect the spinal cord and uterine cervix, and are activated at parturition with subsequent stimulation of secondary neurons in the spinal dorsal horn and autonomic areas. Neuropeptide neurotransmitters and receptors have been studied in these areas, but amino acid transmitters, e.g., glutamate, an excitatory neurotransmitter involved in sensory and nociceptive processing, have not been characterized. To determine if glutamate is involved in innervation of the cervix, rats were examined for markers of glutamatergic neurons in the L6-S1 spinal cord, DRG and cervix. Metabotropic glutamate receptors mGluR5 in the spinal dorsal horn and their expression over pregnancy were examined in pregnant rats and pregnant rats treated continuously with an antagonist of mGluR5, 2-methyl-6-(phenylethynyl) pyridine (MPEP). Rats were allowed to deliver pups to determine if the antagonist altered the expression of an early response gene protein, Fos, in the L6-S1 cord. Immunohistochemistry showed glutamate- and vesicular glutamate transporter1 (VGluT1)-positive fibers in the cervix, glutamate- and VGluT1-expressing neurons in the DRG, some of which also exhibited retrograde tracer from cervical injections, and VGluT1 and mGluR5 immunoreactivities in the L6-S1 spinal dorsal horns. Expression of mGluR5 receptors increased over pregnancy. Fos-positive neurons were present among mGluR5-immunoreactivity in the spinal dorsal horn. Parturition-induced Fos-positive neurons in the spinal cords were abundant in control rats, but were reduced by 70% in MPEP-treated animals. These results suggest that glutamate is likely involved in the transmission of sensory signals, possibly pain, from the cervix to the spinal cord at parturition.

Two types of neurotransmitter release patterns in isolectin B4-positive and negative trigeminal ganglion neurons

Mammalian nociceptors have been classified into subclasses based on differential neurotrophin sensitivity and binding of the plant isolectin B4 (IB4). Most of the nerve growth factor-responsive IB4-negative (IB4 (-)) nociceptors contain neuropeptides such as substance P and calcitonin gene-related peptide, whereas the glial-derived neurotrophic factor-responsive IB4-positive (IB4 (+)) neurons predominantly lack such neuropeptides. We hypothesized that the differences in neuropeptide content between IB4 (+) and (-) neurons might be reflected in differences in stimulated exocytosis and/or endocytosis. To address this, we monitored the secretory activity of acutely dissociated neurons from adult rat trigeminal ganglia (TRG) using cell membrane capacitance (Cm) measurements and the fluorescent membrane-uptake marker N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino)phenyl)hexatrienyl)pyridinium dibromide (FM4-64). Cm measurements were performed under whole-cell voltage clamp and neurons were depolarized from -75 mV to +10 mV to elicit exocytosis. Both types of TRG neurons showed similarly-sized, calcium-dependent increases in Cm, demonstrating that both IB4 (+) and (-) TRG neurons are capable of stimulated exocytosis. However, the peak Cm of IB4 (+) neurons decayed faster toward baseline than that of IB4 (-) neurons. Also, IB4 (+) neurons had stable Cm responses to repeated stimuli whereas IB4 (-) neurons loss their secretory response during repeated stimulation. These data suggested that the IB4 (+) neurons possess a faster rate of endocytosis and vesicle replenishment than IB4 (-) neurons. To test this, we measured vesicle trafficking with the fluorescent membrane dye FM4-64. FM4-64 staining showed that IB4 (-) neurons exhibit a larger pool of endocytosed vesicles than IB4 (+) neurons because the peak fluorescence increases in IB4 (-) neurons were larger but slower than in IB4 (+) neurons. However, the recycled vesicles were released faster in IB4 (+) compared with IB4 (-) neurons. Taken together these data suggest that the IB4 (+) TRG neurons have faster exocytosis and endocytosis than the IB4 (-) neurons.

Phenotyping of sensory and sympathetic ganglion neurons of a galanin-overexpressing mouse--possible implications for pain processing

The distribution of galanin was studied in the lumbar 5 dorsal root ganglia (DRGs) and spinal cord, superior cervical ganglia (SCGs), and skin of transgenic mice overexpressing galanin under the dopamine beta-hydroxylase (DBH) promoter (GalOE-DBH mice) and in wild type (WT) mice. The DRGs and spinal cord were analysed before and after a unilateral, complete transection (axotomy) of the sciatic nerve and after dorsal rhizotomy. Both galanin protein and transcript were studied by, respectively, immunohistochemistry and in situ hybridization. Increased galanin expression was observed in several small, medium-sized and large DRG neuron profiles (NPs) in the naïve transgenic mouse, frequently in neurons lacking calcitonin gene-related peptide (CGRP) and isolectin B4-binding. This lack of coexistence was particularly evident in the medium-sized/large NPs. In the dorsal horn of the spinal cord, no differences were detected between GalOE-DBH and WT mice, both displaying a strong galanin-positive neuropil in the superficial laminae of the dorsal horn, but the transgenic mice showed a more abundant galanin-positive innervation of the ventral horn. A 12-day dorsal rhizotomy, surprisingly, failed to alter the galanin staining patterns in the dorsal (and ventral) dorsal horn. Unilateral axotomy induced upregulation of galanin in DRG NPs of all sizes in both types of mouse. In the hindpaw skin, a profuse galanin-positive fiber plexus was observed in sweat glands and around blood vessels of the transgenic mice, being much more restricted in WT mice. Finally, GalOE mice exhibited a strong galanin-like immunoreactivity in most SCG NPs. The overexpression of the peptide in DRGs and SCGs was paralleled by increased mRNA levels. The present results show that overexpression of galanin under the control of the DBH promoter does not only occur, as expected in these mice, in noradrenline/adrenaline neurons but also in DRG neurons, particularly in large and medium-sized NPs. To what extent and how this overexpression pattern is related to the previously shown elevated pain threshold under normal and lesion conditions is discussed [Grass, S., Crawley, J.N., Xu, X.J., Wiesenfeld-Hallin, Z., 2003a. Reduced spinal cord sensitization to C-fibre stimulation in mice over-expressing galanin. Eur. J. Neurosci. 17, 1829-1832; Hygge-Blakeman, K., Brumovsky, P., Hao, J.X., Xu, X.J., Hökfelt, T., Crawley, J.N., Wiesenfeld-Hallin, Z., 2004. Galanin over-expression decreases the development of neuropathic pain-like behaviour in mice after partial sciatic nerve injury. Brain Res. 1025, 152-158].

Distribution of vesicular glutamate transporters 1 and 2 in the rat spinal cord, with a note on the spinocervical tract

To evaluate whether the organization of glutamatergic fibers systems in the lumbar cord is also evident at other spinal levels, we examined the immunocytochemical distribution of vesicle glutamate transporters 1 and 2 (VGLUT1, VGLUT2) at several different levels of the rat spinal cord. We also examined the expression of VGLUTs in an ascending sensory pathway, the spinocervical tract, and colocalization of VGLUT1 and VGLUT2. Mainly small VGLUT2-immunoreactive varicosities occurred at relatively high densities in most areas, with the highest density in laminae I-II. VGLUT1 immunolabeling, including small and medium-sized to large varicosities, was more differentiated, with the highest density in the deep dorsal horn and in certain nuclei such as the internal basilar nucleus, the central cervical nucleus, and the column of Clarke. Lamina I and IIo displayed a moderate density of small VGLUT1 varicosities at all spinal levels, although in the spinal enlargements a uniform density of such varicosities was evident throughout laminae I-II in the medial half of the dorsal horn. Corticospinal tract axons displayed VGLUT1, indicating that the corticospinal tract is an important source of small VGLUT1 varicosities. VGLUT1 and VGLUT2 were cocontained in small numbers of varicosities in laminae III-IV and IX. Anterogradely labeled spinocervical tract terminals in the lateral cervical nucleus were VGLUT2 immunoreactive. In conclusion, the principal distribution patterns of VGLUT1 and VGLUT2 are essentially similar throughout the rostrocaudal extension of the spinal cord. The mediolateral differences in VGLUT1 distribution in laminae I-II suggest dual origins of VGLUT1-immunoreactive varicosities in this region.

Anatomical evidence for glutamatergic transmission in primary sensory neurons and onto postganglionic neurons controlling penile erection in rats: an ultrastructural study with neuronal tracing and immunocytochemistry

In male rats, the dorsal penile nerve (DPN) conveys sensory information from the genitals to the lumbosacral spinal segments of the spinal cord. DPN is the afferent limb of a reflex loop that supports reflexive erections, and that includes a network of spinal interneurons and autonomic and somatic motoneurons to the penis and perineal striated muscles. Autonomic efferent pathways to the penis relay in the major pelvic ganglion (MPG). Glutamate (Glu) is a likely candidate as a neurotransmitter of reflexive erections. Both AMPA and NMDA glutamatergic receptor subunits are present in the lumbosacral spinal cord, and AMPA and NMDA receptor antagonists block reflexive erections. In the present study, we used tract-tracing experiments combined with immunohistochemical and immunocytochemical techniques to ascertain the presence of Glu at two different levels of the network controlling reflexive erections. DPN afferents were localized in the dorsal horn of the lumbosacral cord and displayed the characteristics of either C-fibers or Adelta fibers. DPN terminals (some of them glutamatergic) were mainly distributed in the medial edge of the dorsal horn in the L6 spinal segment. GluR1 subunits were present in some DPN afferents, suggesting that they could be autoreceptors. DPN fibers were also present in the MPG, as were Glu terminals and GluR4 subunits. The results reveal the presence of Glu in DPN fibers and terminals and suggest that both the spinal cord and the MPG use glutamatergic transmission to control reflexive erections.

Excitatory amino acid concentrations increase in the spinal cord dorsal horn after repeated intramuscular injection of acidic saline

Chronic muscle pain is common and often difficult to treat. In this study, we further characterize a model of chronic muscle pain induced by repeated intramuscular injection of acidic saline. Two injections of acid into muscle separated by 5 days result in secondary mechanical hyperalgesia that lasts for up to 4 weeks. Blockade of spinal NMDA receptors prior to the second injection intramuscular acid injection delays the onset of hyperalgesia, where as the maintenance phase of hyperalgesia, evaluated 1 week after the second intramuscular injection, is dependent on activation of spinal AMPA/kainate and NMDA receptors. In order to determine if behavioral hyperalgesia and glutamate receptor involvement are associated with increased concentrations of excitatory amino acids (EAA), we utilized microdialysis to evaluate extracellular glutamate and aspartate concentrations in the spinal dorsal horn during the first and second intramuscular acid injections, and 1 week after the development of mechanical hyperalgesia. The second intramuscular injection evoked a calcium-dependent increase in both spinal glutamate and aspartate concentrations. Glutamate concentrations within the dorsal horn were also increased 1 week after the second acid injection. Our data suggest increased release of spinal EAAs in the dorsal horn contributes to the development and maintenance of hyperalgesia.

Different basal levels of CaMKII phosphorylated at Thr286/287 at nociceptive and low-threshold primary afferent synapses

Postsynaptic autophosphorylation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) at Thr286/287 is crucial for the induction of long-term potentiation at many glutamatergic synapses, and has also been implicated in the persistence of synaptic potentiation. However, the availability of CaMKII phosphorylated at Thr286/287 at individual glutamatergic synapses in vivo is unclear. We used post-embedding immunogold labelling to quantitatively analyse the ultrastructural localization of CaMKII phosphorylated at Thr286/287 (pCaMKII) at synapses formed by presumed nociceptive and low-threshold mechanosensitive primary afferent nerve endings in laminae I-IV of rat spinal cord. Immunogold labelling was enriched in the postsynaptic densities of such synapses, consistent with observations in pre-embedding immunoperoxidase-stained dorsal horn. Presynaptic axoplasm also exhibited sparse immunogold labelling, in peptidergic terminals partly associated with dense core vesicles. Analysis of single or serial pCaMKII-immunolabelled sections indicated that the large majority of synapses formed either by presumed peptidergic or non-peptidergic nociceptive primary afferent terminals in laminae I-II of the spinal cord, or by presumed low-threshold mechanosensitive primary afferent terminals in laminae IIi-IV, contained pCaMKII in their postsynaptic density. However, the postsynaptic levels of pCaMKII immunolabelling at low-threshold primary afferent synapses were only approximately 50% of those at nociceptive synapses. These results suggest that constitutively autophosphorylated CaMKII in the postsynaptic density is a common characteristic of glutamatergic synapses, thus potentially contributing to maintenance of synaptic efficacy. Furthermore, pCaMKII appears to be differentially regulated between high- and low-threshold primary afferent synapses, possibly reflecting different susceptibility to synaptic plasticity between these afferent pathways.

Characterization of N-methyl-D-aspartate receptor subunits responsible for postoperative pain

N-methyl-D-aspartate (NMDA) receptors have been suggested to be critical for the development of central sensitization, which may amplify postoperative pain. NMDA receptors are formed by GluRzeta (NR1) with any one of four GluRvarepsilon1-4 (NR2A-D) subunits. To clarify the involvement of NMDA receptors in postoperative pain, we examined the effect of the GluRepsilon2-selective antagonist (1S,2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenyl piperidino)-1-propanol (CP-101,606) on postoperative pain caused by plantar incision. We also applied the postoperative pain model to GluRepsilon1 and GluRepsilon4 knockout mice. CP-101,606 administered intrathecally 30 min prior to incision significantly increased mechanical withdrawal thresholds 2 h and 1-3 days after surgery and reduced postoperative pain dose-dependently. Neither GluRvarepsilon1 nor GluRepsilon4 knockout mice showed a difference in withdrawal thresholds as compared with wild-type mice. Pretreatment with CP-101,606 did not produce an additive analgesic effect in the mice. These results demonstrate that GluRepsilon2-containing NMDA receptors are involved in postoperative pain and that CP-101,606 may be effective in reducing it.

Ultrastructural analysis of the central terminals of primary sensory neurones labelled by transganglionic transport of bandeiraea simplicifolia I-isolectin B4

In this study the ultrastructural appearance of primary sensory neurones labelled by the injection of the plant lectin Bandeiraea simplicifolia I-isolectin B(4) (BSI-B(4)) into a peripheral nerve has been examined in the rat. Electron microscopy of the somata of retrogradely labelled neurones showed the lectin to be associated with the inner surface of cytoplasmic vesicles, supporting the premise that the uptake of BSI-B(4) into sensory neurones is by the process of receptor-mediated endocytosis. Light and electron microscopic analysis of the spinal cord revealed transganglionically transported lectin in unmyelinated axons in the dorsolateral funiculus and axon terminals concentrated mainly within lamina II of the dorsal horn. Detailed analysis of 1377 of these axon terminals revealed that the majority were glomerular in shape and surrounded by up to 14 other unlabelled profiles. These findings suggest that primary sensory neurones which transganglionically transport BSI-B(4) have a synaptic ultrastructure similar to that which has been previously reported for unmyelinated primary sensory neurones. Moreover, it appears that the axon terminals of these neurones are subjected to extensive modulation. Examination of the vesicle content of lectin labelled axon terminals revealed that the majority contained small agranular vesicles while large granular vesicles were observed only occasionally. These findings support the suggestion that the populations of neurones expressing binding sites for BSI-B(4) are fairly distinct from those containing neuroactive peptides. In conclusion, the results of the current study suggest that the lectin BSI-B(4) can be used as a histological marker for a subpopulation of small diameter primary sensory neurones and provide further evidence for the potential of this lectin as a useful tool in the study of pain.

Glutamate, but not aspartate, is enriched in trigeminothalamic tract terminals and associated with their synaptic vesicles in the rat nucleus submedius

To examine the possible roles of glutamate and aspartate as neurotransmitters in the nucleus submedius (Sm) of rats, the distributions of these amino acids were examined by electron microscopic immunogold labeling. High levels of glutamate were detected in trigeminothalamic tract terminals anterogradely labeled with horseradish peroxidase conjugates. These terminals also displayed a positive correlation between the densities of synaptic vesicles and gold particles signaling glutamate. In contrast, aspartate levels in such terminals were low and displayed no correlation with the density of synaptic vesicles. Terminals of presumed cortical origin contained the highest estimated levels of glutamate, but the positive correlation between glutamate signal and synaptic vesicle density did not reach statistical significance, presumably due to technical factors. The latter terminals also contained relatively high levels of aspartate, though without any correlation to synaptic vesicle density. The present findings provide strong support for glutamate, but not aspartate, as a trigeminothalamic tract neurotransmitter responsible for the fast synaptic transmission of nociceptive signals to neurons in the rat nucleus submedius. Aspartate presumably serves metabolic roles in these terminals. With respect to terminals of presumed cortical origin, our data are not at odds with the notion that also these terminals use glutamate as their neurotransmitter. Our findings do not support a neurotransmitter role for aspartate in the latter terminals, although such a role cannot be entirely refuted.

Neurotrophins in spinal cord nociceptive pathways

Neurotrophins are a well-known family of growth factors for the central and peripheral nervous systems. In the course of the last years, several lines of evidence converged to indicate that some members of the family, particularly NGF and BDNF, also participate in structural and functional plasticity of nociceptive pathways within the dorsal root ganglia and spinal cord. A subpopulation of small-sized dorsal root ganglion neurons is sensitive to NGF and responds to peripheral NGF stimulation with upregulation of BDNF synthesis and increased anterograde transport to the dorsal horn. In the latter, release of BDNF appears to modulate or even mediate nociceptive sensory inputs and pain hypersensitivity. We summarize here the status of the art on the role of neurotrophins in nociceptive pathways, with special emphasis on short-term synaptic and intracellular events that are mediated by this novel class of neuromessengers in the dorsal horn. Under this perspective we review the findings obtained through an array of techniques in naïve and transgenic animals that provide insight into the modulatory mechanisms of BDNF at central synapses. We also report on the results obtained after immunocytochemistry, in situ hybridization, and monitoring intracellular calcium levels by confocal microscopy, that led to hypothesize that also NGF might have a direct central effect in pain modulation. Although it is unclear whether or not NGF may be released at dorsal horn endings of certain nociceptors in vivo, we believe that these findings offer a clue for further studies aiming to elucidate the putative central effects of NGF and other neurotrophins in nociceptive pathways.

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.

The chemo- and somatotopic architecture of the Galago cuneate and gracile nuclei

The pattern of peripheral nerve inputs into the dorsal column nuclei, cuneate and gracile, was investigated in the prosimian Galago garnetti. The major findings were, that there is a greater segregation of the inputs from the fingers/hand within the cuneate compared with input form the toes/foot within the gracile. In both nuclei, cell clusters can be identified as cytochrome oxidase dense blotches, reactive also for the activity-dependent enzyme nitric oxide synthase. In the cuneate, cell clusters were apparent as six main cytochrome oxidase/nitric oxide synthase-reactive ovals arranged in a medial to lateral sequence. In contrast in the gracile, a higher degree of parcellation was noted and several cytochrome oxidase/nitric oxide synthase blotches were distributed along the rostrocaudal axis of the nucleus. This different architecture parallels differences in the organization of the inputs from the hand and from the foot. In the cuneate, cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals from the glabrous and hairy skin of digits d1 to d5 segregated in each of the five most lateral cytochrome oxidase/nitric oxide synthase blotches. Afferents from the thenar, palmar pads and hypothenar overlapped with those from digit 1, digit 2 to digit 4 and digit 5, respectively. Inputs from wrist arm and shoulder were segregated in the most medial blotch. In the gracile, multiple foci of cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals were observed upon injections of single sites in the toes or plantar pads. Although in multiple foci, inputs from different toes segregated from one another as well. Terminals from the plantar pads appeared to converge on the same cytochrome oxidase/nitric oxide synthase blotches targeted by inputs from the toes. In both the cuneate and the gracile, cytochrome oxidase/nitric oxide synthase blotches also presented intense immunoreactivity for GABA, calbindin, parvalbumin, and brain derived neurotrophic factor. Finally, in the cuneate the cell cluster region presented similarities in prosimian galagos and four species of New World monkeys, whereas it appeared more differentiated and complex in the Old Word macaque monkeys. In conclusion, the different pattern of segregation of the inputs from the hand and from the foot can be related to the different metabolic organization of the cuneate and of the gracile, respectively.

Primary afferent terminals in spinal cord express presynaptic AMPA receptors

Larger dorsal root ganglion neurons are stained by an antibody for the C terminus of glutamate receptor subunit 2 (GluR2) and GluR3 (GluR2/3) rather than by an antibody for GluR4. In dorsal roots, anti-GluR2/3 stains predominantly myelinated fibers; anti-GluR4 or anti-GluR2/4 stains predominantly unmyelinated fibers. In the dorsal horn, puncta immunopositive for synaptophysin and GluR2/3 are predominantly in laminas III and IV, whereas puncta immunopositive for synaptophysin and GluR4 or GluR2/4 are predominantly in laminas I and II. Puncta immunopositive for GluR2/3 costain with the B subunit of cholera toxin, whereas puncta immunopositive for GluR2/4 costain with isolectin B4 after injections of these tracers in the sciatic nerve. No puncta costain with calcitonin gene-related peptide and AMPA receptor subunits. Electron microscopy indicates that AMPA receptor-immunopositive terminals are more numerous than suggested by confocal microscopy. Of all synapses in which immunostaining is presynaptic, postsynaptic, or both, the percentage of presynaptic immunostain is approximately 70% with anti-GluR4 or anti-GluR2/4 (in laminas I-III), 25-30% with anti-GluR2/3 (in laminas III and IV), and 5% with anti-GluR2 (in laminas I-III). Because of fixation constraints, the types of immunostained terminals could be identified only on the basis of morphological characteristics. Many terminals immunostained for GluR2/3, GluR4, or GluR2/4 have morphological features of endings of primary afferents. Terminals with morphological characteristics of presumed GABAergic terminals are also immunostained with anti-GluR2/4 and anti-GluR4 in laminas I and II and with anti-GluR2/3 in laminas III and IV. The conspicuous and selective expression of presynaptic AMPA receptor subunits may contribute to the characteristic physiological profile of different classes of primary afferents and suggests an important mechanism for the modulation of transmitter release by terminals of both myelinated and unmyelinated primary afferents.

Excitatory amino acid release in the spinal cord caused by plantar incision in the rat

There is extensive evidence that spinal excitatory amino acids (EAAs) like glutamate (Glu) and aspartate (Asp) are important in the processing of nociceptive behaviors caused by incisions. To better understand EAA-induced dorsal horn sensitization caused by surgery, we examined the time course and extent of spinal amino acid (AA) release during and after a plantar incision utilizing in vivo microdialysis. We also examined the role of primary afferent input and axonal conduction by measuring spinal EAAs in rats after hindpaw denervation and in rats treated with spinal tetrodotoxin (TTX). In halothane-anesthetized rats, a microdialysis filament (200 microm diameter, 45,000 MW cut off, Hospal AN69) was passed transversely through the deep dorsal horn of the spinal cord (L4-L6). After 18-24h, the dialysis filament was perfused with artificial cerebrospinal fluid (ACSF), dialysate samples collected and analyzed for AAs (Glu, Asp, asparagine, glutamine, serine, and glycine). Rats underwent anesthetic induction with halothane followed by a plantar incision (n=8) or sham operation (n=8). AAs were also measured in incised rats that underwent hindpaw denervation (n=4), in rats that had the filament placed outside the L4-L6 spinal segments (n=7), in rats with a microdialysis catheter placed in the ventral horn at L4-L6 (n=7) and in rats treated with spinally administered TTX (n=5). AAs were measured during recovery from anesthesia and for the next 8h. In the sham-operated group, Asp and Glu did not change throughout the experiment. In rats undergoing plantar incision, Asp and Glu increased from 10 to 30 min after incision to 200+/-30 and 138+/-12 percentage of control, respectively. The EAAs returned to baseline by 1h after incision. For the other AAs, only serine and asparagine increased after incision. No increase in AA release by incision was observed after hindpaw denervation, TTX treatment or placement of the filament outside the L4-L6 segments. In rats with a filament implanted in the ventral horn (L4-L6), EAAs increased during halothane induction and sham preparation. Thus, the EAA release required an intact afferent nerve barrage and segmental excitatory nerve transmission. The incision-induced nociceptive afferent barrage increased the release of Glu and Asp in the lumbar dorsal horn for 45 min. The concentrations of AAs returned to baseline by 1h. The percentage increase is in some cases less and for a shorter period of time compared to other models of persistent pain, perhaps because the incision injury is less severe compared to others models. This profile of EAA release further explains why models of inflammation and chemical irritation do not translate well to human postoperative pain.

Synaptic relationships between hair follicle afferents and neurones expressing GABA and glycine-like immunoreactivity in the spinal cord of the rat

gamma-Aminobutyric acid (GABA) and glycine have been implicated in the inhibition of sensory pathways in the dorsal horn of the spinal cord. The object of this study is to investigate the interactions between neurones immunoreactive for GABA and/or glycine and hair follicle afferent terminals labelled by intracellular injection with neurobiotin. GABA and glycine-like immunoreactivity in axons and dendrites in synaptic contact with the afferent terminals was demonstrated by using a postembedding immunogold method, and serial section reconstruction was used to show the distribution and nature of these interactions in lamina III of the dorsal horn. Most afferent boutons (94%) were postsynaptic at axo-axonic synapses: 67% of presynaptic boutons presynaptic to the afferent terminals were immunoreactive for GABA and glycine, 24% for GABA alone, and 7% for glycine alone. Only a small percentage of dendrites postsynaptic to afferent boutons appeared to belong to inhibitory interneurones: 3% were immunoreactive for GABA and glycine, 10% for glycine alone, but 87% were immunoreactive for neither antibody. Many afferent boutons were the central terminals of what appeared to be type IIb glomeruli and were involved triadic synaptic arrangements at which boutons presynaptic to an afferent terminal also made axodendritic contacts with dendrites postsynaptic to the afferent. Many of the presynaptic boutons involved in the triads were immunoreactive for GABA and glycine. Because afferent terminals do not themselves express glycine receptors (Mitchell et al. [1993] J. Neurosci. 13:2371-2381), glycine may therefore act on dendrites postsynaptic to hair follicle afferent terminals at these triads.

AMPA glutamatergic receptor-immunoreactive subunits are expressed in lumbosacral neurons of the spinal cord and neurons of the dorsal root and pelvic ganglia controlling pelvic functions in the rat

Sacral preganglionic neurons innervate the pelvic organs via a relay in the major pelvic ganglion. Pudendal motoneurons innervate striated muscles and sphincters of the lower urinary, genital and digestive tracts. The activity of these spinal neurons is regulated by sensory afferents of visceral and somatic origins. Glutamate is released by sensory afferents in the spinal cord, and interacts with a variety of receptor subtypes. The aim of the present study was to investigated the presence of AMPA glutamate receptor subunits (GluR1-GluR4) in the neural network controlling the lower urogenital and digestive tracts of male rats. We performed double-immunohistochemistry directed against a neuronal tracer, the cholera toxin beta subunit (Ctbeta) and each of the four receptor subunits. GluR1, GluR2 and GluR3 subunits were present in many sacral preganglionic neurons retrogradely labelled with Ctbeta applied to the pelvic nerve, and in some dorsolateral and dorsomedian motoneurons retrogradely labelled with Ctbeta injected in ischiocavernosus and bulbospongiosus muscles. The four subunits were detected in postganglionic neurons of the major pelvic ganglion retrogradely labelled with Ctbeta injected in the corpus cavernosum, and in some somata of sensory afferents of the L6 dorsal root ganglion labelled with Ctbeta applied to the dorsal penile nerve or injected in corpus cavernosum. The results provide a detailed knowledge of the neural targets expressing the various AMPA receptor subunits and suggest that part of the neural network that controls pelvic organs, including sensory afferents and postganglionic neurons, is sensitive to glutamate through the whole family of AMPA subunits.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Intraspinal amino acid neurotransmitter activities are involved in the generation of rhythmic sympathetic nerve discharge in newborn rat spinal cord

Endogenous neurotransmitter activities underlying the sympathetic nerve discharge (SND) generated by newborn rat spinal cord in vitro were investigated using glutamatergic, glycinergic, and GABAergic antagonists. Under control conditions, the SND power spectrum had two major frequency components: synchronous bursting SND (bSND) with power dominant at < 0.1 Hz and quasiperiodic SND (qSND) oscillating at 1-2 Hz. Using high Mg2+ solution (12-24 mM) to block Ca2+-dependent synaptic transmission reversibly abolished SND. An interruption of glutamatergic neurotransmission by CNQX (non-NMDA receptor blocker) or L-AP4 (reducing the synaptic release of glutamate) failed to affect qSND, but consistently reduced bSND. Application of kynurenate, a broad-spectrum ionotropic glutamate receptor blocker, only caused an unstable SND but did not reduce SND. In contrast, strychnine (Stry, glycine receptor antagonist) consistently reduced qSND in a dose-dependent manner. Bicuculline (Bic, GABA(A) receptor antagonist) induced a synchronous bSND of irregular rhythm, which could be further regularized by adding Stry. Bic-induced bSND was reversibly abolished by CNQX or L-AP4. In conclusion, intraspinal glycinergic, GABAergic, and glutamatergic activities are involved in the generation of the spinal cord-derived SND in newborn rats. Intraspinal GABAergic interneurons may tonically inhibit the glutamatergic bursting neurons that generate a synchronous bSND. Activities of these glutamatergic bursting neurons may also be modulated by intraspinal glycinergic interneurons.

Quantitative analysis of immunogold labeling indicates low levels and non-vesicular localization of L-aspartate in rat primary afferent terminals

The role of L-aspartate as an excitatory neurotransmitter in primary afferent synapses in the spinal cord dorsal horn is disputed. To further investigate this issue, we examined the presence of aspartate-like immunoreactivity in primary afferent nerve terminals and other tissue components of the dorsal horn. We also examined the relationship between aspartate and glutamate immunogold labeling density and the density of synaptic vesicles in primary afferent terminals and presumed inhibitory terminals forming symmetric synapses. Weak aspartate immunosignals, similar to or lower than those displayed by presumed inhibitory terminals, were detected in both C-fiber primary afferent terminals in lamina II (dense sinusoid axon terminals, identified by morphological criteria) and in A-fiber primary afferent terminals in laminae III-IV (identified with anterograde transport of choleragenoid-horseradish peroxidase conjugate). The aspartate immunogold signal in primary afferent terminals was only about one-fourth of that in deep dorsal horn neuronal cell bodies. Further, whereas significant positive correlations were evident between synaptic vesicle density and glutamate immunogold labeling density in both A- and C-fiber primary afferent terminals, none of the examined terminal populations displayed a significant correlation between synaptic vesicle density and aspartate immunogold labeling density. Thus, our results indicate relatively low levels and a non-vesicular localization of aspartate in primary afferent terminals. It is therefore suggested that aspartate, rather than being a primary afferent neurotransmitter, serves a role in the intermediary metabolism in primary afferent terminals.

Labeling of vagal motoneurons and central afferents after injection of cholera toxin B into the airway lumen

We tested the hypothesis that application of the subunit B of cholera toxin (CTB) to the airway mucosa would produce labeling of neuronal somata and sensory fibers in the medulla oblongata. Using (125)I-CTB as a tracer, we demonstrated first that CTB is transported across the tracheal epithelium, but once in the airway wall, it remains confined to the subepithelial space and lamina propria. Despite the rarity of intrinsic neurons in these areas, intraluminal CTB labeled approximately 10-60 neurons/rat in the nucleus ambiguus and a smaller number of neurons in the dorsal motor nucleus of the vagus. Well-defined sensory fiber terminals were also labeled in the commissural, medial, and ventrolateral subnuclei of the nucleus of the tractus solitarius. Approximately 50 and 90% of the neurons labeled by intraluminal CTB were also labeled by injections of FluoroGold into the tracheal adventitia and lung parenchyma, respectively. These findings demonstrate that a substantial number of medullary vagal motoneurons innervate targets in the vicinity of the airway epithelium. These neurons do not appear to be segregated anatomically from vagal motoneurons that project to deeper layers of the airway wall or lung parenchyma.

Amino acid release into the knee joint: key role in nociception and inflammation

This study examined the release of several amino acids after induction of knee joint inflammation in rats using kaolin and carrageenan. During the initial 10-min collection after knee joint injection with the irritants, the concentration of glutamate and the nitric oxide metabolites, arginine and citrulline, doubled. This increase persisted for at least two hours. During the same time period aspartate concentrations remained unchanged. Direct knee joint administration of lidocaine prevented the increases in amino acid concentration measurable by microdialysis probe inserted into the joint. These data suggest the possibility that glutamate may be released by neuronal endings in the joint.

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

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

AMPA receptors at primary afferent synapses in substantia gelatinosa after sciatic nerve section

Increased excitability of superficial laminae of the spinal cord may contribute to the pathological pain consequent to peripheral nerve injury. Among several mechanisms that may be responsible for this occurrence is upregulation of receptors for glutamate in the spinal cord. To explore this possibility, we investigated changes in AMPA receptors in substantia gelatinosa of rats after section of the sciatic nerve. Immunofluorescence was performed on sections from the fourth lumbar segment. Quantitative analysis of digitally captured images suggested that staining for an antibody to a sequence shared by GluR2 and GluR3 (GluR2/3) was increased on the side ipsilateral to the lesion. To determine whether antigen accumulation was at synaptic sites and to probe whether it was selective for primary afferent terminals, we performed electron microscopy on immunogold-labelled material. Gold particles coding for GluR2/3 subunits were counted from synaptic active zones of glomerular terminals in substantia gelatinosa that originate from small calibre afferent fibres, and from active zones of terminals of probable intrinsic origin. Counts were significantly increased on the side ipsilateral to the lesion only at synapses of primary afferent terminals. These results document selective upregulation of receptor protein at the synapse. This upregulation may contribute to the increased sensitivity of dorsal horn neurons following peripheral nerve injury.

Immunocytochemical localization of the AMPA receptor subunits in the mesencephalic trigeminal nucleus of the rat

The mesencephalic trigeminal nucleus is composed of large (35-50 microns) pseudo-unipolar neurons. Closely associated with them are small (< 20 microns) multipolar neurons. An unique peculiarity of the pseudo-unipolar perikarya is that they receive synaptic input from various sources, which sets them apart from the dorsal root and cranial nerves sensory ganglia neurons. Whereas glutamate is the best neurotransmitter candidate in pseudo-unipolar neurons, glutamatergic input into them has not yet been reported. AMPA glutamate receptors are implicated in fast excitatory glutamatergic synaptic transmission. They have been localized ultrastructurally at postsynaptic sites. This study demonstrates that the pseudo-unipolar neurons of the mesencephalic trigeminal nucleus express AMPA glutamate receptor subunits, which indicates that these neurons receive glutamatergic input. Serial sections from the rostral pons and midbrain of Sprague-Dawley rats were immunostained with antibodies against C-terminus of AMPA receptor subunits: GluR1, GluR2/3, and GluR4. The immunoreaction was visualized with avidin-biotin-peroxidase/DAB for light and electron microscopy. With GluR1 antibody only the smallest multipolar neurons were recognized as immunopositive within the mesencephalic trigeminal nucleus. GluR2/3 stained the pseudo-unipolar neurons intensely within the entire rostro-caudal extent of the nucleus. In addition the former antibody stained small multipolar neurons within the mesencephalic trigeminal nucleus, though with somewhat larger dimensions than those immunoreactive for GluR1. Whereas the overall staining with GluR4 antibody was scant, those pseudo-unipolar neurons that were stained, were strongly stained. Furthermore, a considerable number of microglial cells within and surrounding the mesencephalic trigeminal nucleus displayed very intense immunoreactivity for GluR4. These results are discussed in the light of the glutamate receptor subunit composition.

Identification of gamma-aminobutyric acid-immunoreactive axon endings associated with mesencephalic periodontal afferent terminals and morphometry of the two types of terminals in the cat supratrigeminal nucleus

A previous study has shown that mesencephalic periodontal afferent terminals receive contacts more frequently from axonal endings containing pleomorphic, synaptic vesicles (P-endings) in the supratrigeminal nucleus (Vsup) than in the trigeminal motor nucleus, suggesting that interneurons in Vsup play an important role in modulating the jaw-closing reflex. The present study was attempted to identify neurotransmitters in P-endings associated with mesencephalic periodontal afferents in cat Vsup through the use of intracellular staining of horseradish peroxidase combined with the postembedding immunogold methods. A morphometric analysis was carried out to compare the ultrastructural features of these two types of terminals. Serial sections of 31 labeled boutons and of their associated 38 P-endings were examined. They were processed for postembedding immunogold labeling with antibodies to the neurotransmitter gamma-aminobutyric acid (GABA). The 38 P-endings presynaptic to periodontal afferents showed GABA-like immunoreactivity, but the afferent terminals were free from the labeling. The morphometric analysis indicated that bouton volume, apposed surface area, total active zone size, and mitochondrial volume were smaller in GABA-immunoreactive P-endings than in periodontal afferents, but the pooled data of the two types of terminals showed that each synaptic parameter was highly correlated in a positive, linear manner with bouton volume. These observations provide evidence that P-endings presynaptic to mesencephalic periodontal afferents contain the neurotransmitter GABA and that their axoaxonic synapses are organized in accordance with the ultrastructural "size principle" proposed by Pierce and Mendell (Pierce and Mendell [1993] J. Neurosci. 13:4748-4763) on Ia-motoneuron synapses.

Glutamate-enriched cholinergic synaptic terminals in the entopeduncular nucleus and subthalamic nucleus of the rat

Several lines of evidence suggest that the cholinergic neurons of the mesopontine tegmentum contain elevated levels of glutamate and are the source of cholinergic terminals in the subthalamic nucleus and entopeduncular nucleus. The object of this study was to test whether cholinergic terminals in the entopeduncular nucleus and subthalamic nucleus, also express relatively high levels of glutamate. To address this, double immunocytochemistry was performed at the electron microscopic level. Perfuse-fixed sections of rat brain were immunolabelled to reveal choline acetyltransferase by the pre-embedding avidin-biotin-peroxidase method. Serial ultrathin sections of cholinergic terminals in both the entoped uncular nucleus and subthalamic nucleus were then subjected to post-embedding immunocytochemistry to reveal glutamate and GABA. Quantification of the immunogold labelling showed that choline acetyltransferase-immunopositive terminals and boutons in both regions were significantly enriched in glutamate immunoreactivity and had significantly lower levels of GABA immunoreactivity in comparison to identified GABAergic terminals. Furthermore, the presumed transmitter pool of glutamate i.e. that associated with synaptic vesicles, was significantly greater in the choline acetyltransferase-positive terminals than identified GABA terminals, albeit significantly lower than in established glutamatergic terminals. In the entopeduncular nucleus, a small proportion of cholinergic terminals displayed high levels of GABA immunoreactivity. Taken together with other immunocytochemical and tracing data, the elevated levels of glutamate in cholinergic terminals in the entopeduncular nucleus and subthalamic nucleus, is further evidence adding weight to the suggestion that acetylcholine and glutamate may be co-localized in both the perikarya and terminals of at least a proportion of neurons of the mesopontine tegmentum.

Glutamate- and GABA-immunoreactive synapses on sympathetic preganglionic neurons caudal to a spinal cord transection in rats

Spinal cord injury destroys bulbospinal amino acid-containing pathways to sympathetic preganglionic neurons and severely disrupts blood pressure control, resulting in resting or postural hypotension and episodic hypertension. Almost all immunoreactivity for the excitatory amino acid L-glutamate has been reported to disappear from autonomic areas of the cord caudal to a transection, apparently depriving autonomic neurons of their major excitatory input. However, the magnitude of the neurogenic episodic hypertension after cord injury suggests that excitatory inputs to sympathetic preganglionic neurons must still be present. Moreover, the hypotension associated with high spinal injuries may reflect a enhanced role for inhibitory transmitters, such as GABA. This apparent contradiction regarding the presence of glutamate and lack of information about GABA prompted the present investigation. In rats seven days after spinal cord transection, we examined identified sympathetic preganglionic neurons caudal to the injury for the presence of synapses or direct contacts from varicosities that were immunoreactive for the amino acids, L-glutamate and GABA. Adrenal sympathetic preganglionic neurons were retrogradely labelled with cholera toxin B subunit and amino acid immunoreactivity was revealed with post-embedding immunogold labelling. In single ultrathin sections, 46% (98/212) of the synapses or direct contacts on adrenal sympathetic preganglionic neurons were immunoreactive for glutamate and 39% (83/214) were immunoreactive for GABA. Analysis of inputs with the physical disector yielded similar results for the two amino acids. The proportions of glutamatergic or GABAergic synapses on cell bodies and dendrites were similar. When alternate ultrathin sections were stained to reveal glutamate or GABA immunoreactivity, either one or the other amino acid occurred in 78.4% (116/148) of inputs; 4.1% (6/148) of inputs contained both amino acids and 17.5% (26/148) of inputs contained neither. These results demonstrate that nerve fibres immunoreactive for the neurotransmitter amino acids, glutamate and GABA, provide most of the input to sympathetic preganglionic neurons caudal to a spinal cord transection. Synapses containing glutamate and GABA could provide the anatomical substrate for the exaggerated sympathetic reflexes and the low sympathetic tone that result from spinal cord injury.

Ultrastructural relationships of spinal primary afferent fibres with neuronal and non-neuronal cells in the myenteric plexus of the cat oesophago-gastric junction

Spinal primary afferent fibres innervating the myenteric area in the oesophago-gastric junction of the cat were selectively labelled by anterogradely transported cholera toxin B subunit-horseradish peroxidase conjugate injected into thoracic dorsal root ganglia. The ultrastructure of these labelled primary afferent fibres was studied in order to determine whether they display close relationships with specific cell types in the myenteric plexus. Horseradish peroxidase was revealed with tetramethylbenzidine stabilized with ammonium heptamolybdate or with the tetramethylbenzidine/tungstate reaction in order to visualize the cytoplasmic organelles and the axolemma, respectively. The labelled primary afferent fibres were unmyelinated. Two kinds of profiles of labelled fibres containing vesicles and mitochondrial accumulations were found: (i) fibres running in myenteric connectives in isolated nerve bundles, and (ii) fibres within the myenteric ganglia. The first kind had small areas of axolemma with no glial cell covering, whereas the second kind had little or no glial cell covering (termed naked primary afferent fibres). In addition, labelled fibres containing few vesicles and mitochondria and running in nerve bundles surrounded by perineurium were numerous. Within the myenteric ganglia, naked primary afferent fibres contacted myenteric neurons. The contacts were mainly axosomatic. No synaptic specializations were distinguished. In the interganglionic area, some labelled fibres terminated close to blood vessels. The intraganglionic naked primary afferent fibres are suggested to be mechanoreceptors. Their exposed axolemma might allow both mechanotransduction and release of neurotransmitters which could act on myenteric neurons. Because they are protected by their glial cell sheath and by bundles of collagen fibrils, interganglionic primary afferent fibres are likely to be less exposed to deformation.

Heterogeneity of AMPA receptors in the dorsal column nuclei of the rat

We have combined immunocytochemistry with retrograde tracing to demonstrate that projecting neurons in the gracile and cuneate nuclei express predominantly the GluR3 subunit of the AMPA receptor while interneurons in these nuclei express predominantly the GluR1 subunit. Interneurons expressing the GluR2 subunit are also present. It is speculated that the two classes of interneurons may release different inhibitory transmitters.

Ultrastructural evidence for prominent distribution of the mu-opioid receptor at extrasynaptic sites on noradrenergic dendrites in the rat nucleus locus coeruleus

Physiological studies have indicated that agonists at the mu-opioid receptor (mu OR), such as morphine or the endogenous peptide methionine5-enkephalin, can markedly decrease the spontaneous activity of noradrenergic neurons in the locus coeruleus (LC). Messenger RNA and protein for mu OR are also densely expressed by LC neurons. During opiate withdrawal, increased discharge rates of LC neurons coincide with the expression of behavioral features associated with the opiate withdrawal syndrome. To better define the cellular sites for the physiological activation of mu OR in the LC and its relation to afferent terminals, we examined the ultrastructural localization of mu OR immunoreactivity in sections dually labeled for the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Immunogold-silver labeling for mu OR (i-mu OR) was localized to parasynaptic and extrasynaptic portions of the plasma membranes of perikarya and dendrites, many of which also contained immunolabeling for TH. The dendrites containing exclusively i-mu OR were more numerous in the rostral pole of the LC. The i-mu OR in dendrites with and without detectable TH immunoreactivity were usually postsynaptic to unlabeled axon terminals containing heterogeneous types of synaptic vesicles and forming asymmetric synaptic specializations characteristic of excitatory-type synapses. These results provide the first direct ultrastructural evidence that mu OR is strategically localized to modulate the postsynaptic excitatory responses of catecholamine-containing neurons in the LC.

Glutamate-enriched inputs from the mesopontine tegmentum to the entopeduncular nucleus in the rat

In order to clarify the origin and to examine the synaptology of the projection from the mesopontine tegmentum to the entopeduncular nucleus, rats received discrete deposits of anterograde tracers in different regions of the mesopontine tegmentum. Anterogradely labelled fibres in the entopeduncular nucleus were analysed at the light and electron microscopic levels. To determine the neurochemistry of the projection, the distributions of GABA and glutamate immunoreactivity in anterogradely labelled boutons in the entopenducular nucleus were studied by postembedding immunocytochemistry. The morphological characteristics of anterogradely labelled structures were compared to those of choline acetyltransferase-immunopositive structures. The anterograde tracing demonstrated that the projection to the entopeduncular nucleus arises from the area defined by the cholinergic neurons of the pedunculopontine region and from the more medial and largely non-cholinergic, midbrain extrapyramidal area. The anterogradely labelled terminals formed asymmetrical synaptic contacts with dendritic shafts, cell bodies and more rarely spines in the entopeduncular nucleus, and they were significantly enriched in glutamate immunoreactivity compared to identified GABAergic terminals in the same region. The morphology, trajectory and synaptology of the anterogradely labelled fibres showed similarities to those of choline acetyltransferase-immunopositive fibres and terminals, providing indirect evidence in support of previous suggestions that at least part of the projection is cholinergic. The structures postsynaptic to the anterogradely labelled boutons also received input from other classes of terminals that had the morphological and neurochemical characteristics of boutons derived from the neostriatum, globus pallidus and subthalamic nucleus. These findings imply that the mesopontine tegmentum sends a projection to the entopeduncular nucleus that is heterogeneous with respect to its origin and also possibly its neurochemistry. The synaptology of the projection underlies one route through which the mesopontine tegmentum can exert effects on movement by modulating the direct and indirect pathways of information flow through the basal ganglia.

AMPA receptor subunits underlying terminals of fine-caliber primary afferent fibers

Postembedding immunogold electron microscopy was used to determine the relation of primary afferent terminals in superficial laminae of the spinal dorsal horn with AMPA receptor subunits. Immunogold particles coding for GluR1 and GluR2/3 were concentrated at synaptic sites, between 30 nm outside and 40 nm inside the postsynaptic membrane. Immunopositive synapses displayed round vesicles and asymmetric specializations, characteristic of terminals releasing excitatory neurotransmitters; symmetric synapses, characteristic of terminals releasing inhibitory amino acids, were immunonegative. In superficial laminae, large terminals of two main types at the center of a synaptic glomerulus originate from primary afferents: C1 terminals are mainly endings of unmyelinated afferent fibers; C2 terminals are mainly endings of thinly myelinated afferent fibers. Terminals of both types were presynaptic to AMPA subunits, but in different proportions: C1 terminals were related more to GluR1 than to GluR2/3, whereas the reverse was true for C2 terminals. These results suggest that functional properties of peripheral afferents to the spinal cord may be specified by the density and combination of receptor subunits in the postsynaptic membrane, and raise the possibility that calcium-permeable AMPA channels may play a special role in the mediation of sensory input by unmyelinated fibers.

Electron microscopy of immunoreactivity patterns for glutamate and gamma-aminobutyric acid in synaptic glomeruli of the feline spinal trigeminal nucleus (Subnucleus Caudalis)

We studied the ultrastructure of the synaptic organization in the feline spinal trigeminal nucleus, emphasizing specific neurotransmitter patterns within lamina II of the pars caudalis/medullary dorsal horn. Normal adults were perfused, and Vibratome sections from pars caudalis were processed for electron microscopy. Ultrathin sections were reacted with antibodies for the excitatory neurotransmitter glutamate (Glu) and for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) by using postembedding immunogold techniques. Both single- and double-labeled preparations were examined. Results with single labeling show that Glu-immunoreactive terminals have round synaptic vesicles and form asymmetric synaptic contacts onto dendrites. GABA-immunoreactive axon terminals and vesicle-containing dendrites have pleomorphic vesicles, and the axon terminals form symmetric contacts onto dendrites and other axons. Double labeling on a single section shows glomeruli with central Glu-immunoreactive terminals that are presynaptic to dendrites, including GABA+ vesicle-containing dendrites. These Glu+ terminals are also postsynaptic to GABA+ axon terminals, and these GABA-immunoreactive terminals may also be presynaptic to the GABA+ vesicle-containing dendrites. Quantitative analyses confirm the specificity of the Glu and GABA immunoreactivities seen in the various glomerular profiles. The results suggest that a subpopulation of Glu-immunoreactive primary afferents (excitatory) may be under the direct synaptic influence of a GABA-immunoreactive intrinsic pathway (inhibitory) by both presynaptic and postsynaptic mechanisms.

Monosynaptic connections between primary afferents and giant neurons in the turtle spinal dorsal horn

This paper reports the occurrence of monosynaptic connections between dorsal root afferents and a distinct cell type-the giant neuron-deep in the dorsal horn of the turtle spinal cord. Light microscope studies combining Nissl stain and transganglionic HRP-labeling of the primary afferents have revealed the occurrence of axosomatic and axodendritic contacts between labeled boutons and giant neurons. The synaptic nature of these contacts has been confirmed by use of electron microscope procedures involving the partial three-dimensional reconstruction of identified giant neurons. Intracellular recording in spinal cord slices provided functional evidence indicating the monosynaptic connections between dorsal root afferents and giant neurons. The recorded neurons were morphologically identified by means of biocytin injection and with avidin conjugates. Electrical stimulation of the ipsilateral dorsal roots evoked synaptic responses with short, fixed latencies (1.6-5.6 ms), which remained unchanged at high frequencies (10 Hz). Excitatory polysynaptic potentials were also observed. By means of pharmacological procedures the short-latency response was dissected in two components: one insensitive to tetrodotoxin, the other abolished by the drug. The toxin-resistant component was presumed to be sustained by small-diameter C fibers. The synaptic response was mainly mediated by the glutamate-AMPA receptor subtype; however, a small component mediated by NMDA receptor was also present.

Central connections of the motor and sensory vagal systems innervating the trachea

Cholera toxin beta-subunit was used as both a transganglionic and retrograde cell body tracer to determine respectively the central sensory and motor systems innervating the trachea in three mammalian species, dog, ferret and rat. A basic pattern was found in all three animals. Sensory fibers terminated in three subnuclei of the nucleus tractus solitarius (NTS) with the densest concentration localized in a restricted part of the medial part of the rostral NTS. Weaker projections were identified in the ventrolateral NTS subnucleus and sparse labeling was seen in the commissural NTS subnucleus. No labeling was identified in the area postrema. The pattern of retrograde cell-body labeling was also similar in all three species. Two main sites were labeled: the rostralmost part of the dorsal vagal nucleus and the rostral nucleus ambiguus (NA). In the NA, cell labeling was found in mainly in the ventral (or external) portion of the nucleus, but some labeled neurons were consistently found in the compact NA as well. In addition, labeled neurons were also seen in the dorsomedial part of the C1-C2 ventral horn. In summary, the central sites of termination of the sensory fibers and cells of origin innervating the trachea were similar in all three species.

The glutamate-enriched cortical and thalamic input to neurons in the subthalamic nucleus of the rat: convergence with GABA-positive terminals

Neurons of the subthalamic nucleus play a key role in the normal physiology and the pathophysiology of the basal ganglia. In order to understand better how the activity of subthalamic neurons and hence the output of the basal ganglia are controlled, we have reexamined the topography and examined in detail the synaptology and neurochemical nature of the two major excitatory projections to the subthalamic nucleus, that from the cortex and from the parafascicular nucleus of the thalamus. The approach was to use anterograde neuronal tracing and postembedding immunocytochemistry for amino acid transmitters. In confirmation of previous findings the cortical and thalamic projections were topographically organized, although the topography was more finely organized, and the projections more extensive, than previously demonstrated. Cortical and thalamic terminals made asymmetrical synaptic contacts with the dendrites and spines of subthalamic neurons. The thalamic terminals contacted larger postsynaptic targets, and therefore presumably more proximal regions of subthalamic neurons, than did the cortical terminals. Quantitative analysis of the postembedding immunolabelled sections revealed that the cortical and thalamic terminals were significantly enriched in glutamate-immunoreactivity when compared to identified gamma-aminobutyric acid (GABA)-positive terminals, supporting physiological studies that suggest that these projections use glutamate as their neurotransmitter. In addition a small population of nonanterogradely labelled terminals that formed asymmetrical synapses and were immunopositive for GABA were identified. A larger population of terminals that formed symmetrical synapses were also immunopositive for GABA and were probably derived from the globus pallidus. The latter type of terminal was found to make convergent synaptic input with cortical or thalamic terminals on the dendrites and spines of subthalamic neurons, indicating that the "indirect pathways" by which information flows through the basal ganglia converge at the level of individual neurons in the subthalamic nucleus.

Glutamatergic and GABAergic input to rat spinothalamic tract cells in the superficial dorsal horn

The distribution of synaptic terminals onto spinothalamic tract cells (types I and II) of the superficial dorsal horn was determined with special reference to the amino acid transmitters glutamate and gamma-aminobutyric acid. Fifteen spinothalamic cells retrogradely labeled from the thalamus with the neuroanatomical tracer wheatgerm agglutinin conjugated to horseradish peroxidase were sectioned for electron microscopy. Serial sections from several levels through each cell were immunostained for glutamate and gamma-aminobutyric acid using a postembedding immunogold technique. Perimeter measurements of spinothalamic cell somata and dendrites and the lengths of apposition for all terminal profiles in contact with the spinothalamic cells were obtained from electron micrographs using a digitizing tablet. These data were used to determine the density of terminals on the soma and dendrites. In addition, the terminal population on these cells was categorized by transmitter content (glutamate, gamma-aminobutyric acid, or unlabeled). The results demonstrate that terminal density increased on dendrites relative to their distance from the soma. Glutamatergic and GABAergic input composed 37% and 20% of the terminal population, respectively, and these percentages remained uniform for the soma and dendrites. There were no significant differences among the 15 cells analyzed for this study. The results, therefore, suggest that both type I and type II STT cells of the superficial DH have similar synaptic organizations.