The substantia gelatinosa Rolandi of the rat. Fine structure, cytochemistry (acid phosphatase) and changes after dorsal root section

Local NGF and GDNF levels modulate morphology and function of porcine DRG neurites, In Vitro

Nerve terminals of primary sensory neurons are influenced by their environment through target derived trophic factors, like nerve growth factor (NGF) or glial cell line-derived neurotrophic factor (GDNF). In mice, subpopulations of DRG neurons express receptors either for NGF or GDNF and therefore differentially respond to these neurotrophic factors. We probed neurite endings from porcine DRG neurons cultured in either NGF or GDNF and examined their shape, elongation and stimulus-evoked CGRP release. A compartmentalized culture system was employed allowing spatial separation of outgrown neurites from their somata and use of different growth factors in the compartments. We show that neurites of GDNF cultured somata extend into lateral compartments without added growth factor, unlike neurites of NGF cultured ones. Neurites of NGF cultured somata extend not only into NGF- but also into GDNF-containing compartments. GDNF at the site of terminals of NGF responsive somata led to a strong neurite arborization and formation of large growth cones, compared to neurites in medium with NGF. Functionally, we could detect evoked CGRP release from as few as 7 outgrown neurites per compartment and calculated release per mm neurite length. CGRP release was detected both in neurites from NGF and GDNF cultured somata, suggesting that also the latter ones are peptidergic in pig. When neurites of NGF cultured somata were grown in GDNF, capsaicin evoked a lower CGRP release than high potassium, compared to those grown in NGF. Our experiments demonstrate that the compartmented culture chamber can be a suitable model to assess neurite properties from trophic factor specific primary sensory neurons. With this model, insights into mechanisms of gain or loss of function of specific nociceptive neurites may be achieved.

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.

BMP2 expression in the adult rat brain

Bone morphogenetic protein-2 (BMP2) is a member of the transforming growth factor-β (TGF-β) superfamily and plays important roles in multiple biological events. Although BMP2 expression has been well described in the early development of the central nervous system (CNS), little information is available on its expression in the adult CNS. We thus investigated BMP2 expression in the adult rat CNS by using immunohistochemistry. Here we show that BMP2 is widely expressed throughout the adult CNS. In addition, besides intense BMP2 expression in almost all neurons, we found BMP2 expression in astrocytes and ependymal cells. Interestingly, we found that the axons of olfactory sensory neurons express BMP2. In addition, in the glomerular layer, BMP2 was very strongly expressed in some glomeruli, whereas the other glomeruli were weakly stained, suggesting that the variations in BMP2 expression level in each glomerus might be cues for each axon to find its adequate target and to keep its identity. Furthermore, we compared the expression patterns of BMP2 and BMP4. Interestingly, BMP4 was preferentially expressed in the dendrites of several neurons, whereas BMP2 was basically not expressed in the dendrites; however, it was detected in the axons. This means that in a single neuron the localizations of BMP2 and BMP4 are differentially regulated. These data indicate that BMP2 is more widely expressed throughout the adult CNS than previously reported, and its continued abundant expression in the adult brain strongly supports the idea that BMP2 also plays pivotal roles in the adult brain.

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.

IB4 afferent sprouting contributes to bladder dysfunction in spinal rats

We investigated the role that nonpeptidergic isolectin-B4 (IB4) positive, primary afferent sprouting plays in bladder dysfunction after spinal cord transection (SCT). Rats were implanted with an indwelling bladder cannula and subjected to a complete spinal cord transection at T9/T10. In one group of rats IB4-positive terminals increased below the level of the injury in L6 cord in laminae I and III-VI as early as 3 days after transection, and remained increased 8 and 21 days after transection. Growth associated protein 43 (Gap-43) was expressed on IB4-positive neurons 3 days post-transection and the number of L6 dorsal root ganglia (DRG) neurons expressing IB4 did not change after injury. In another set of experiments IB4-saporin or saporin alone was administered intrathecally to L6/S1 cord. IB4-positive afferents sprouted in L6 cord of saporin only treated rats but IB4 afferent labeling was decreased by 42 and 33% in L6 cord and DRG 21 days after IB4-saporin treatment. IB4-saporin treated rats voided with an efficiency of 28.3% 10-14 days after transection whereas one week later voiding efficiency increased to 86.1%. Inefficient voiding by saporin and 10-14 day IB4-saporin treated rats was linked to voiding that occurred after the peak in micturition pressure. On the other hand, increased voiding efficiency in 20-30 day IB4-saporin treated rats was associated with voiding occurring before the peak of the micturition pressure. These results suggest that IB4-positive afferent sprouting plays a role in the generation of bladder dysfunction following SCT.

Sprouting of CGRP primary afferents in lumbosacral spinal cord precedes emergence of bladder activity after spinal injury

Spinal cord injury (SCI) severely disrupts bladder function. What mediates bladder dysfunction after SCI is currently unknown. We investigated the role that primary afferent sprouting in lumbosacral cord may play in emergence of bladder activity after complete spinal cord transection. Rats had a bladder cannula chronically implanted. They were then subjected to complete surgical spinal cord transection at T9/T10. Cystometrographic analysis (0.1 ml/min) after injury revealed that bladder activity emerged in the form of nonvoiding contractions in all rats at approximately 5 days post transection. At 10-14 days after transection nonvoiding contractions remained and voiding contractions emerged that had increased maximal pressures (12-41 vs. 24-57 cmH(2)O) but were less efficient (6-15% vs. 79-100%) when compared to control implanted rats. We looked for sprouting 3 days and 8 days post transection, timepoints preceding the emergence of nonvoiding and voiding contractions respectively. Increases in CGRP density and distribution were seen in L6 and S1 spinal cord within lamina groupings of II-IV, V and VI, as well as lamina X at 8 days post transection. This increase remained in most lamina at 21 days post transection. Colocalization with the growth cone marker Gap-43 3 days and 5 days post transection at the level of the lumbosacral preganglionic nucleus verified that CGRP positive afferents were sprouting in L6/S1 spinal cord prior to emergence in bladder activity. These data provide support for the hypothesis that primary afferent sprouting contributes to emergence of bladder activity after spinal cord transection.

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.

Involvement of DRG11 in the development of the primary afferent nociceptive system

During development, dorsal root ganglia (DRG) neurons differentiate in various subpopulations, nociceptive neurons belonging in the small-diameter class. This study addresses the role played by DRG11, a transcription factor expressed in the spinal area of projection of small-diameter DRG neurons, in the development of the primary afferent system. The various subclasses of DRG neurons were compared between wild-type and Drg11(-/-) mice at embryonic and postnatal life. In Drg11(-/-) mice, numbers of small peptidergic and non-peptidergic DRG neurons were decreased at P7 concomitant with abnormal cell death. Innervation by small DRG neurons was impaired in cutaneous, visceral and deep tissues. Large DRG neurons were not affected. The data point to a role for DRG11 in early postnatal survival of normally generated small primary afferent neurons innervating various kinds of peripheral tissues, which would explain the nociceptive deficits observed in Drg11-null mutant mice.

Activation and retrograde transport of protein kinase G in rat nociceptive neurons after nerve injury and inflammation

Nerve injury elicits both universal and limited responses. Among the former is regenerative growth, which occurs in most peripheral neurons, and among the latter is the long-term hyperexcitability that appears selectively in nociceptive sensory neurons. Since positive injury signals communicate information from the site of an injury to the cell body, we hypothesize that a nerve injury activates both universal and limited positive injury signals. Studies in Aplysia indicate that protein kinase G is a limited signal that is responsible for the induction of long-term hyperexcitability. Given that long-term hyperexcitability contributes to chronic pain after axotomy in rodent neuropathic pain models, we investigated its underlying basis in the rat peripheral nervous system. Using biochemical assays, Western blots, and immunocytochemistry we found that the Type 1alpha protein kinase G is the predominant isoform in the rat periphery. It is present primarily in axons and cell bodies of nociceptive neurons, including populations that are isolectin B4-positive, isolectin B4-negative, and those that express transient receptor potential vanilloid receptor-1. Surprisingly, protein kinase G is not present in the facial nerve, which overwhelmingly contains axons of motor neurons. Crushing the sciatic nerve or a cutaneous sensory nerve activates protein kinase G in axons and results in its retrograde transport to the neuronal somata in the DRG. Preventing the activation of protein kinase G by injecting Rp-8-pCPT-cGMPS into the crush site abolished the transport. The protein kinase A inhibitor Rp-8-pCPT-cAMPS had no effect. Extracellular signal-related kinases 42/44 are also activated and transported after nerve crush, but in both motor and sensory axons. Chronic pain has been linked to long-term hyperexcitability following a nerve inflammation in several rodent models. We therefore injected complete Freund's adjuvant into the hindpaw to induce an inflammation and found that protein kinase G was activated in the sural nerve and transported to the DRG. In contrast, the extracellular signal-related kinases in the sensory axons were not activated by the complete Freund's adjuvant. These studies support the idea that the extracellular signal-related kinases are universal positive axonal signals and that protein kinase G is a limited positive axonal signal. They also establish the association between protein kinase G, the induction of long-term hyperexcitability, and chronic pain in rodents.

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.

Distribution and injury-induced plasticity of cadherins in relationship to identified synaptic circuitry in adult rat spinal cord

Cadherins are synaptically enriched cell adhesion and signaling molecules. In brain, they function in axon targeting and synaptic plasticity. In adult spinal cord, their localization, synaptic affiliation, and role in injury-related plasticity are mostly unexplored. Here, we demonstrate in adult rat dorsal horn that E- and N-cadherin display unique patterns of localization to functionally distinct types of synapses of intrinsic and primary afferent origin. Within the nociceptive afferent pathway to lamina II, nonpeptidergic C-fiber synapses in the deeper half of lamina II (IIi) contain E-cadherin but mostly lack N-cadherin, whereas the majority of the peptidergic C-fiber synapses in the outer half of lamina II (IIo) contain N-cadherin but lack E-cadherin. Approximately one-half of the Abeta-fiber terminations in lamina III contain N-cadherin; none contain E-cadherin. Strikingly, the distribution and levels of these cadherins are differentially affected by sciatic nerve axotomy, a model of neuropathic pain in which degenerative and regenerative structural plasticity has been implicated. Within the first 7 d after axotomy, E-cadherin is rapidly and completely lost from the dorsal horn synapses with which it is affiliated, whereas N-cadherin localization and levels are unchanged; such patterns persist through 28 d postlesion. The loss of E-cadherin thus occurs before the onset of mechanical hyperalgesia (approximately 10-21 d postlesion), as reported previously. Together, the synaptic specificity displayed by these cadherins, coupled with their differential response to injury, suggests that they may proactively contribute to the maintenance of some, and incipient dismantling of other, synaptic circuits in response to nerve injury. Speculatively, such changes may ultimately contribute to subsequently emerging abnormalities in pain perception.

Inflammation induced increase of fluoride resistant acid phosphatase (FRAP) activity in the spinal dorsal horn in rats

Fluoride-resistant acid phosphatase (FRAP) has been suggested as an enzymatic marker for nociceptive primary afferent terminals in the spinal dorsal horn, however there has not been demonstrated a direct functional relation between FRAP activity and an increased nociceptive transmission. For this purpose, we quantitated FRAP activity in the spinal dorsal horn of the rat in a heat-induced cutaneous inflammatory model. Male Sprague-Dawley rats anaesthetised with thiopental were separated in two groups where the left hindpaw was submerged during 60 s either in water at room temperature (control group) or in water at 60 degrees C (inflammation group) which induce in this group a progressive hindpaw inflammation. After 8 h, the lumbar enlargement of the spinal cord was extracted, cut in slices and 1 mm micropunch fragments were obtained from the right and left dorsal horn. The activity of FRAP was determined using the Gomori colorimetric method and corrected by the protein concentrations. FRAP activity in the left dorsal horn was statistically higher than right dorsal horn in the inflammation group (3.05+/-0.54 versus 1.91+/-0.23 u/g per l; P<0.05). Also, FRAP activity from the left dorsal horn of the control and inflammation groups show a significant increase in the last group (3.05+/-0.54 versus 2.17+/-0.23 u/g per l; P<0.05). This results demonstrate that FRAP is not only an enzymatic marker for neuronal and fibre integrity of nociceptive primary afferents but also it is associated to the nociceptive afferent activation.

The Distribution of &mgr; and delta Opioid Binding Sites Belonging to a Single Cervical Dorsal Root in the Superficial Dorsal Horn of the Rat Spinal Cord: A Quantitative Autoradiographic Study

Numerous studies have demonstrated a dense concentration of opioid receptors in the superficial layers (laminae I - II) of the spinal cord. These receptors are located both pre- and postsynaptically at this level. The purpose of this study was to assess the distribution of opioid receptors belonging to a single (C7) dorsal root. Thus, quantitative autoradiography of &mgr; ([3H]Tyr-d-Ala-Gly-NMe-Phe-Gly-ol; [3H]DAMGO) and delta ([3H]Tyr-d-Thr-Gly-Phe-Leu-Thr; [3H]DTLET) opioid binding sites was performed for several experimental groups: control rats with intact dorsal roots and lesioned rats with a unilateral dorsal rhizotomy of (a) the C7 root alone, (b) the three successive roots rostral and caudal to the spared C7 root, and (c) the seven roots C4 - Th2. By subtracting results of the 'C7 cut' group from the 'intact' group or by subtracting results of the C4 - Th2 cut group from the C7 spaced group, it was possible to measure the distribution of &mgr; and delta opioid binding sites belonging to the C7 root. The combination of these two methods of calculation allowed us to demonstrate a significant distribution over two segments rostral and one segment caudal to the segment of entry. For [3H]DAMGO, the distribution was 10% (P < 0.05) in the C5, 27%, (P < 0.001) in the C6, 38% (P < 0.001) in the C7 and 14% (P < 0.05) in the C8 segment. For [3H]DTLET, the distribution was 11% (P=0.05) in the C5, 27%, (P < 0.01) in the C6, 37% (P < 0.001) in the C7 and 18% (P < 0.05) in the C8 segment. It is also noted that rostral distributions spread more densely and further than the caudal ones.

Vanilloid receptor 1 expression in the rat urinary tract

Previous findings have shown that the capsaicin sensitivity of sensory fibres is due to the expression of a specific membrane protein, the vanilloid receptor type 1 (VR1). In the present work we studied the distribution, morphology and the neurochemical content of nerve fibres expressing this receptor in the rat urinary tract. Immunolabelling was performed against the VR1 and the positive fibres were examined by light and electron microscopy. Colocalisation of VR1 and substance P or calcitonin gene-related peptide immunoreactivities, and isolectin B4 binding, was evaluated under the confocal microscope. In addition, the effect of intravesical administration of resiniferatoxin, an ultra-potent vanilloid receptor agonist, in the receptor expression in the bladder was also studied. Numerous VR1-immunoreactive fibres were found in the mucosa and muscular layer of the entire urinary tract except the kidney. In the bladder, most fibres were also substance P- or calcitonin gene-related peptide-immunoreactive but did not bind isolectin B4. Under the electron microscope VR1 immunoreactivity was confined to unmyelinated axons and varicosities containing small clear and large dense-core synaptic vesicles. They occurred beneath or among epithelial cells or closely apposed to smooth muscle cells. Intravesical resiniferatoxin decreased VR1 immunoreactivity transiently. These data indicate that primary sensory fibres expressing VR1 are extremely abundant in the rat urinary tract and that, in contrast to the skin, they belong almost exclusively to the peptide-containing sub-population of primary afferents. As capsaicin-sensitive bladder afferents are involved in nociception and reflex micturition control, the numerous free terminal nerve endings expressing VR1 in the mucosa seem more adequate to accomplish the former function. However, the close apposition between VR1-expressing fibres and smooth muscle cells suggests that they may also encode the tonus of the muscular layer.

Low doses of N-methyl-D-aspartate antagonists in superficial laminae of medulla oblongata facilitate wind-up of convergent neurones

In this study, a trigeminal model was used in which high threshold C-fibre-evoked activities of convergent neurones located in the subnucleus oralis of the trigeminal complex are modulated through the superficial part, the substantia gelatinosa, of the subnucleus caudalis. The two subnuclei are located 3 mm apart, therefore, it was possible to inject dizocilpine, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, into either the superficial or the deep parts of subnucleus caudalis without interfering with ongoing recording of convergent neurones in subnucleus oralis. A differential NMDA-dependent modulation of wind-up was observed according to the dose and the injection target. (1) The injections of small non-diffusible doses (0.12 microg) of dizocilpine into the superficial part of subnucleus caudalis facilitated wind-up. The effect peaked at 25 min with a mean increase above control of 173+/-31%. Injection (0.5 microg) of either the less active enantiomer dizocilpine or saline into superficial subnucleus caudalis had no significant effect on subnucleus oralis convergent neurones. This suggests that NMDA-dependent interneurones, probably located in substantia gelatinosa of subnucleus caudalis, exert, in normal conditions, an inhibitory control on wind-up of convergent subnucleus oralis neurones. (2) The injection of larger doses (0.5 microg) into the superficial part of subnucleus caudalis induced a predominant inhibitory effect on wind-up. The mean peak effect at 15 min was 46+/-7% compared to control (100%). Small and large doses of dizocilpine injected into the deep part of subnucleus caudalis had a predominant inhibitory effect. The inhibition of wind-up of subnucleus oralis neurones after injection of NMDA receptor antagonists in superficial or deep subnucleus caudalis indicates that wind-up may be due, at least in part, to NMDA activation at synapses that do not involve the recorded convergent neurones.

On the problem of lamination in the superficial dorsal horn of mammals: a reappraisal of the substantia gelatinosa in postnatal life

Although it is one of the most distinctive and earliest recognized features in the spinal cord, the substantia gelatinosa (SG) remains among the most enigmatic of central nervous system regions. The present neuroanatomical studies employed transganglionic transport of horseradish peroxidase conjugates of choleragenoid (B-HRP) and the B4 isolectin of Bandeiraea simplicifolia (IB4-HRP) on opposite sides to compare the projection patterns of myelinated and unmyelinated cutaneous primary afferents, respectively, within the superficial dorsal horn of the spinal cord in postnatal mice, from shortly after birth to adulthood. Putative unmyelinated afferents labeled with IB4-HRP gave rise to a dense sheet of terminal-like labeling restricted to the outer half of the SG. In contrast, myelinated inputs labeled with B-HRP gave rise to a similarly dense sheet of terminal-like labeling that occupied the inner half of the SG. This adult organization, with two dense sheets of terminal labeling in the superficial dorsal horn, was clearly evident shortly after birth using these markers, prior to the emergence of the SG. Furthermore, the location of the SG proper varied considerably within the dorsoventral plane of the dorsal horn according to mediolateral and segmental locations, a finding that was also seen in comparative studies in rat and cat. These findings caution against equating the SG in particular, and the superficial dorsal horn in general, with nociceptive processing; at minimum, the SG subserves a clear duality of function, with only a thin portion of its outermost aspect devoted to pain.

Isolectin B(4)-positive and -negative nociceptors are functionally distinct

Small-diameter sensory neurons that are primarily nociceptors can be divided neurochemically into two populations: isolectin B(4) (IB(4))-positive nonpeptidergic neurons, and IB(4)-negative peptidergic neurons. It has been shown that IB(4)-positive neurons depend on glial-derived neurotrophic factor (GDNF), whereas IB(4)-negative neurons depend on NGF for survival during postnatal development (Molliver et al., 1997). Furthermore, these two populations of nociceptors terminate in distinct regions of the superficial spinal cord. To date, however, no evidence exists that indicates whether these two groups of nociceptors have distinct functional roles in the process of nociception (Snider and McMahon, 1998). To search for functional differences, we performed whole-cell voltage and current-clamp recordings on acutely isolated adult mouse dorsal root ganglion neurons that were labeled with fluorescent IB(4). We found that IB(4)-positive neurons have longer-duration action potentials, higher densities of TTX-resistant sodium currents, and smaller noxious heat-activated currents than IB(4)-negative neurons. Furthermore, we show that NGF, but not GDNF, directly increases the number of neurons that respond to noxious heat. The different electrophysiological properties expressed by IB(4)-positive and -negative small neurons, including their different heat sensitivities, indicates that they may relay distinct aspects of noxious stimuli both acutely and after injury in vivo.

Inflammation-induced up-regulation of protein kinase Cgamma immunoreactivity in rat spinal cord correlates with enhanced nociceptive processing

Activation of various second messengers contributes to long-term changes in the excitability of dorsal horn neurons and to persistent pain conditions produced by injury. Here, we compared the time-course of decreased mechanical nociceptive thresholds and the density of protein kinase Cgamma immunoreactivity in the dorsal horn after injections of complete Freund's adjuvant in the plantar surface of the rat hindpaw. Complete Freund's adjuvant significantly increased paw diameter and mechanical sensitivity ipsilateral to the inflammation. The changes peaked one day post-injury, but endured for at least two weeks. In these rats, we recorded a 75-100% increase in protein kinase Cgamma immunoreactivity in the ipsilateral superficial dorsal horn of the L4 and L5 segments at all time-points. Electron microscopy revealed that the up-regulation was associated with a significant translocation of protein kinase Cgamma immunoreactivity to the plasma membrane. In double-label cytochemical studies, we found that about 20% of the protein kinase Cgamma-immunoreactive neurons, which are concentrated in inner lamina II, contain glutamate decarboxylase-67 messenger RNA, but none stain for parvalbumin or nitric oxide synthase. These results indicate that persistent changes in protein kinase Cgamma immunoreactivity parallel the time-course of mechanical allodynia and suggest that protein kinase Cgamma contributes to the maintenance of the allodynia produced by peripheral inflammation. The minimal expression of protein kinase Cgamma in presumed inhibitory neurons suggests that protein kinase Cgamma-mediated regulation of excitatory interneurons underlies the changes in spinal cord activity during persistent nociception.

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.

Plasticity of cutaneous primary afferent projections to the spinal dorsal horn

Reorganization of the somatotopic map in the spinal dorsal horn may be elicited by a variety of deafferenting lesions, including transection of peripheral nerves or dorsal roots, or the application of neurotoxins. While such lesions give rise to a variety of neurochemical and morphological changes in the dorsal horn, collateral sprouting of intact primary afferents appears to be minimal. Recently, intraaxonal injection of neurobiotin has allowed visualization of the entire spinal arborization of single A beta primary afferent fibers in animals where the somatotopy of the relevant region of dorsal horn has also been mapped. In contrast to the somatotopic precision of the terminal fields of peripheral nerves suggested by transganglionic tracing, these studies have shown that afferents make connections many millimeters rostral and caudal to the region where their receptive field is represented in the somatotopic map. Intracellular recording from dorsal horn neurons has further shown that these long-ranging projections make functional, but weak, synaptic connections. Thus the functional somatotopic reorganization that follows nerve lesions in mature animals might be explained simply by an increased synaptic efficacy of these existing projections. In contrast to the negligible sprouting of intact A beta primary afferents, those undergoing axonal regeneration exhibit dense collateral sprouting into deafferented regions of the dorsal horn, particularly the superficial laminae, where the terminal arbors of many small (A delta and C) nociceptive afferent fibres degenerate following peripheral nerve lesions. The inappropriate connections made by these collateral sprouts may partly underlie the painful sequelae of nerve injury in man.

Interaction between substance P-immunoreactive central terminals and gamma-aminobutyric acid-immunoreactive elements in synaptic glomeruli in the lamina II of the chicken spinal cord

We investigated the interaction between gamma-aminobutyric acid (GABA)-immunoreactive (IR) elements and substance P (SP)-IR central terminals in synaptic glomeruli in lamina II of the chicken spinal cord in order to ascertain how pain information is modulated in the spinal dorsal horn. We combined the peroxidase-antiperoxidase (PAP) technique and the protein A-gold (PAG) technique to observe the synaptic relationship between these two components. At the light microscopic level, we observed both GABA-IR and SP-IR elements in the lamina II. GABA-IR elements were also observed in the lamina III. At the electron microscopic level, the following three GABA-IR elements formed synapses with the SP-IR central terminals in synaptic glomeruli: (1) elements which appeared to be axon terminals containing tightly-packed pleomorphic clear vesicles; (2) elements which appeared to be vesicle-containing dendrites with loosely-packed clear and dense-cored vesicles (DCVs); and (3) dendrites without synaptic vesicles. The first type of element was always presynaptic to the SP-IR central terminal. The second type was postsynaptic, presynaptic or in some cases reciprocal to the SP-IR central terminals. The third type was postsynaptic to the SP-IR central terminal. These results suggest that the SP-containing primary afferents activate GABA-containing dendrites and that the SP-containing primary afferents are inhibited presynaptically by GABA-containing neurons through axo-axonic and dendro-axonic synapses.

Presence or absence of TrkA protein distinguishes subsets of small sensory neurons with unique cytochemical characteristics and dorsal horn projections

Investigations into the biological actions of nerve growth factor (NGF) have shown that dorsal root ganglion (DRG) neurons subserving nociception require NGF for survival and maintenance of phenotype. This discovery suggests that the signaling NGF receptor, TrkA, can be used as a marker for nociceptive neurons. In this study, we have used antibodies to TrkA, in conjunction with cell biological markers that show a restricted distribution in the DRG, to further characterize subsets of DRG neurons that are dependent upon NGF. Staining for TrkA labeled small and medium-sized neurons that composed 47% of all neurons in thoracic ganglia. Double-labeling with antibodies to the high molecular weight neurofilament protein (NFH), a marker for neurons with myelinated axons, demonstrated that TrkA staining is found in only a small subset of myelinated neurons. Surprisingly, many DRG neurons were not labeled by either TrkA or NFH. These neurons had small soma areas, contained the intermediate filament protein peripherin, and were labeled by the lectin BSI, identifying them as neurons likely to have unmyelinated axons. In addition, small TrkA-negative neurons were extensively labeled by antibodies to the intermediate filament protein alpha-internexin, the delta isoform of protein kinase C, and by the BSI isolectin BSI-B4. In order to assess the potential functions of TrkA-negative small neurons, we examined their projections to the dorsal horn of the spinal cord. TrkA-immunoreactivity in the spinal cord was restricted to lamina I and the outer region of lamina II (IIo), similar to staining for calcitonin gene-related peptide. In contrast, the central projections of TrkA-negative neurons, as visualized by BSI-B4 staining, were particularly dense in lamina IIi. Our results suggest that TrkA-expressing and non-TrkA-expressing small neurons compose functionally distinct populations of DRG neurons.

Plasticity of complex terminals in lamina II in partially deafferented spinal cord: the cat spared root preparation

Projections to the dorsal horn change in adult mammals in response to complete or partial deafferentation. The number of synaptic terminals remains constant after complete lumbosacral deafferentation, indicating replacement of lost dorsal root terminals by newly formed terminals from spared intrinsic systems. The density of a spared central projection of a dorsal root is increased in dorsal horn after partial deafferentation, consistent with sprouting by the axons in the spared root. In this study, we have used electron microscopy to study morphological changes in a specific class of terminals in the dorsal horn induced by partial deafferentation. Complex terminals (CTs) in the dorsal horn originate exclusively from dorsal roots and are readily distinguished morphologically. The CTs and the postsynaptic densities (PSDs) associated with CTs were measured in lamina II at L5 and L6 in cats subjected to unilateral spared root (L6) dorsal rhizotomies and compared to CTs in the control side. Acutely following partial deafferentation, the number of CT profiles decreased. At more chronic survivals, the number of CT profiles were restored to normal levels, and both the number and the length of PSDs were increased. The changes in CTs and PSDs suggest sprouting and synaptogenesis by the spared dorsal root fibers that produce changes in the postsynaptic neuron. Spared root deafferentation thus elicits compensatory changes in presynaptic terminals of the spared root and also in their postsynaptic target neurons.

Early morphological changes of primary afferent neurons and their processes in newborn mice after treatment with capsaicin

Degenerating figures of dorsal root ganglion (DRG) neurons and their central and peripheral processes (dorsal root and saphenous nerve) and terminals (central terminals in the superficial dorsal horn and cutaneous nerve of the hind paw dorsal skin) of neonatal mice were examined 30 min, 1, 2 and 5 h, and 2, 3, 5, and 10 days after subcutaneous injection of capsaicin on post-natal day 2. Many small DRG neurons showed degeneration 1 h after treatment. Scarcely any features of degeneration were seen in the DRG and dorsal root 10 days after treatment. The degenerating aspects of terminal axons in the marginal layer of the superficial dorsal horn were characterized by enlarged round axons with closely packed osmiophilic materials, lamellar bodies, and loss of axoplasmic organelles. Two types of central terminals (C-terminals) showed degeneration in the substantia gelatinosa from 30 min after treatment onward. One type consisted of small, round, sinuous or slender dark terminals (CI-terminals), and the other of large, pale, round or angular terminals (CII-terminals). Those that degenerated markedly had homogeneously electron-dense axoplasm with dilated synaptic vesicles and inclusion bodies. Extensive degeneration of terminal axons in the marginal layer occurred 5 h after treatment, whereas conspicuous degeneration of C-terminals occurred from 30 min to 10 days after treatment in the substantia gelatinosa. CI-terminals showed marked degeneration during the first 3 days, whereas marked degeneration of CII-terminals occurred between 5 and 10 days after treatment. This time difference between the peaks of degeneration of CI- and CII-terminals indicates an important difference in the origins of these two types of capsaicin-sensitive, nociceptive fibers in the superficial dorsal horn; CI-terminals are derived from small DRG cells, whereas CII-terminals are derived from larger DRG cells. Unmyelinated axons in the dorsal root, saphenous nerve, and dorsal skin of the hind paw showed similar degeneration patterns 2 h after treatment to those of terminal axons in the marginal layer. Thus, the degenerating profiles in the marginal layer suggest that these axons arose from collaterals of unmyelinated primary axons descending or ascending within the marginal layer. Numerous enlarged degenerating axons showing vacuolation were conspicuous in the dorsal skin 3 days after treatment. The synchronous degeneration of the smaller DRG neurons, their central and peripheral processes, and their CI-terminals in the substantia gelatinosa supports the idea that the smaller DRG neurons are directly influenced by capsaicin, and that their degeneration is followed by centrifugal degeneration.

Long range afferents in rat spinal cord. III. Failure of impulse transmission in axons and relief of the failure after rhizotomy of dorsal roots

Dorsal root afferents entering the spinal cord form a T-junction with a caudal branch descending many segments and giving off side branches terminating in the dorsal horn. This anatomical finding contrasts with the physiological observation that the postsynaptic effects of arriving afferents in the dorsal horn are limited to a few segments on either side of the root carrying the input. This paper explores the possibility that one explanation for this paradox is that orthodromic impulse conduction fails to penetrate the long range parts of the caudal branch. The experiments show that when all roots are intact, very few fibres can be detected carrying orthodromic impulses as far as 20 mm caudal to the entry point. After section of neighbouring dorsal roots, however, large numbers of conducting fibres can be recorded at that point. Signs of orthodromic conduction begin 7 days after root section. These results were confirmed by another method which compared the relative refractory period of the membrane of the descending branch produced either after a local stimulus had evoked an action potential or after a rostral distant stimulus had produced an orthodromic action potential. Again, in the intact cord, the results indicate that impulses fail to penetrate long distances into the descending branches but that, as soon as 2 days after rhizotomy in the area of suspected conduction failure, orthodromic conduction is restored. It is proposed that the failure and release of conduction may depend on the control of membrane potential in the primary afferents, which would form a pre-presynaptic control mechanism.

Ultrastructural studies on peptides in the dorsal horn of the spinal cord--I. Co-existence of galanin with other peptides in primary afferents in normal rats

The aim of the present study was to investigate galanin-like immunoreactivity in primary afferent terminals and its relationship to other neuropeptides in laminae I and II of the fourth and fifth lumbar segments of normal rat spinal cord using immunofluorescence and pre- and post-embedding electron-microscopic immunocytochemistry. Triple-immunofluorescence staining showed that galanin-like immunoreactivity co-localized with substance P- and calcitonin gene-related peptide-like immunoreactivities in many nerve fibres and terminals in laminae I and II of the dorsal horn. At the ultrastructural level, using pre-embedding immunocytochemistry, galanin-like immunoreactivity was found in type I glomeruli with an electron-dense central terminal containing many densely packed synaptic vesicles and several large dense-core vesicles. Both the cytoplasm and the core of the large vesicles were immunoreactive. In type II glomeruli with an electron-lucent central terminal and loosely packed synaptic vesicles the large dense-core vesicles and the cytoplasm were only weakly galanin-positive. Post-embedding immunocytochemistry revealed that galanin-like immunoreactivity co-existed with substance P- and calcitonin gene-related peptide-like immunoreactivities in many terminals and in individual large dense-core vesicles in lamina II. These terminals were considered to represent primary afferents, since there is evidence that calcitonin gene-related peptide in the dorsal horn only occurs in nerve endings originating in dorsal root ganglia. Evidence was also unexpectedly obtained for the occurrence of several other peptides in calcitonin gene-related peptide-positive terminals, i.e. in presumably primary afferents. Thus galanin-like immunoreactivity sometimes also co-localized with cholecystokinin- and neuropeptide tyrosine-like immunoreactivities in calcitonin gene-related peptide-immunoreactive terminals and in some large dense-core vesicles in such terminals. A small number of calcitonin gene-related peptide immunoreactive, presumably primary afferent terminals contained enkephalin-, neurotensin- (and galanin-)like immunoreactivities. These results indicated that galanin can be co-stored with several other neuropeptides in large dense-core vesicles in primary afferent terminals and may presumably be released together with them in the superficial layer of the dorsal horn. Since various combinations of peptides, presumably at varying concentrations, occur in the large dense-core vesicles in a given nerve ending, it is likely that the individual large dense-core vesicles produced in a neuron are heterogenous with regard to peptide content and thus to the message that they transmit upon release.

Selective labelling of primary sensory afferent terminals in lamina II of the dorsal horn by injection of Bandeiraea simplicifolia isolectin B4 into peripheral nerves

The I-B4 isolectin from Bandeiraea simplicifolia exhibits specific binding to a subpopulation of rat dorsal root ganglion neurons of small diameter which terminate in the substantia gelatinosa of the dorsal horn. Recent double-labelling experiments in the rat have demonstrated that only primary afferents which innervate the skin are recognized by the I-B4 lectin [Plenderleith and Snow (1993) Neurosci. Lett. (in press)]. As the I-B4 lectin appears to bind selectively to a subset of small-diameter primary afferents with cutaneous peripheral projections, we sought to determine whether it could be used as a transganglionic tracer which selectively labels the spinal terminations of cutaneous afferents in superficial dorsal horn. We now report that the I-B4-horseradish peroxidase conjugate labels synaptic terminals in lamina II of the dorsal horn following the injection of the conjugate into the sciatic and saphenous nerves in the rat. Electron-microscopic examination of the dorsal horn revealed many examples of labelled synaptic terminals and unmyelinated axons, but in no cases was label observed in myelinated axons. No label was observed outside of the substantia gelatinosa; thus the I-B4 isolectin is unique among lectins used for transganglionic tracing in that it does not retrogradely label motoneurons. These results, together with previous studies of lectin binding properties of primary sensory afferents, suggest that injection of I-B4 conjugates into peripheral nerves enables the visualization of the central terminations of cutaneous C-fibres. Transganglionic labelling with the I-B4 isolectin from Bandeiraea simplicifolia should facilitate further examination of synaptic relationships of nociceptive cutaneous afferents in the superficial dorsal horn.

Ultrastructural morphology, synaptic relationships, and CGRP immunoreactivity of physiologically identified C-fiber terminals in the monkey spinal cord

The spinal cord terminations of two electrophysiologically identified single C-fibers (one identified as a C-nociceptor) were intra-axonally labeled with horseradish peroxidase and analyzed with both light and electron microscopy. Serial section ultrastructural analysis and postembedding immunocytochemical techniques for calcitonin gene-related peptide (CGRP), substance P (SP), and GABA were used to study the synaptology, and neuropeptide content. All C-terminal synapses were in laminae I and II. The terminals sampled (n = 73) from these two C-fibers rarely established glomerular synaptic complexes, but rather, simple terminals, usually measuring 1-4 microns in length and 1-3 microns in diameter. They most often established 1 or 2 (range 1 to 5) quite large asymmetric axodendritic synaptic contacts. Postsynaptic structures included dendritic spines and shafts with and without vesicles. C-terminals were filled with small round synaptic vesicles (45-60 nm) and also contained variable numbers of large dense-core vesicles (LDCVs, 80-110 nm). LDCVs inside identified C-terminals frequently displayed CGRP immunoreactivity. We were unable to detect SP immunoreactivity inside our sample of C-fiber LDCVs. C-terminals were never found postsynaptic to other profiles. Thus, the C-fiber terminals sampled in this study have simple synaptology, do not receive presynaptic control and contain CGRP immunoreactivity. They differ greatly from the terminals of A delta nociceptors studied previously by our group that had glomerular endings, often received presynaptic input and did not contain CGRP immunoreactivity. This suggests the existence of different processing mechanisms, at the level of the first synapse, for nociceptive inputs arriving to lamina I and II through different types of primary afferents.

Periterminal synaptic organization of primary afferents in laminae I and IIo of the rat spinal cord, as shown after anterograde HRP labelling

The fine structure and periterminal synaptology of the primary afferent terminations in laminae I and IIo are examined in the rat, following anterograde labelling with horseradish peroxidase applied to the right C5-dorsal root. Labelled varicosities observed along the terminal arbors in parasagittal thick sections were relocated in ultrathin sections by electron microscopy. The labelled terminal profiles generated by the three primary afferent plexuses which can be identified by light microscopy in laminae I-IIo had similar fine structural features, except that axo-axonal contacts, although rare, were more frequent in the medial network plexus. Primary boutons were packed with agranular spherical vesicles and some large granular vesicles, and were mostly presynaptic to profiles of dendritic trunks of marginal cells. Unlabelled axonal profiles, either light with some flattened vesicles, or dense with round vesicles, were also presynaptic at symmetrical or asymmetrical contacts, respectively, to those dendritic profiles. It is suggested that such knobs of intrinsic origin are responsible for postsynaptic modulation of the primary noxious input. Although the 20 microns wide lamina IIo belongs cytoarchitectonically to lamina II and can be distinguished from lamina I by a decreased amount of myelinated fibres and large dendritic profiles, the periterminal synaptology was here found to be the same as in lamina I.

An ultrastructural study of the binding of an alpha-D-galactose specific lectin from Griffonia simplicifolia to trigeminal ganglion neurons and the trigeminal nucleus caudalis in the rat

The pattern of binding by the isolectin I-B4 from Griffonia simplicifolia to trigeminal ganglion neurons and the trigeminal nucleus caudalis has been investigated at the ultrastructural level in the rat. This lectin bound to small ganglion neurons with two different binding patterns. The majority of the ganglion cells labelled had reaction product throughout their cytoplasm and this was associated with the Golgi apparatus and endoplasmic reticulum. In a second group of small ganglion neurons the binding was only found on the surface plasma membrane of the cells. In the trigeminal tract the cytoplasm of many unmyelinated axons and a few small myelinated axons was found to bind this lectin. A very thin band of staining was also found on the inner and outer edges of the myelin sheaths of other myelinated axons. Staining of synapses was found throughout laminae I and II with the highest frequency in the inner part of laminae II. These synapses made both simple and complex connections with one or more dendrites, contained clear round vesicles and had asymmetric synaptic densities. Some of the glomerular synapses stained were observed to receive presynaptic synapses containing small clear flattened vesicles. Synapses containing both clear round and large dense core vesicles were unstained. Some staining was also found in dendrites. In weakly fixed tissue, staining was also found around some glial cells and on the luminal membranes of capillary endothelial cells. This lectin is a valuable tool for studies of the "non-peptide" group of C-fibre primary afferents.

Regulation of opioid binding sites in the superficial dorsal horn of the rat spinal cord following loose ligation of the sciatic nerve: comparison with sciatic nerve section and lumbar dorsal rhizotomy

The aim of the present study was to quantify time-related modifications in mu and delta opioid binding sites in the superficial layers (laminae I and II) of the L4 lumbar segment in a rat model of mononeuropathy induced by loose ligation of the sciatic nerve. We have shown a 28% (P < 0.01) and 24% (P < 0.01) decrease in ipsi/contralateral side binding ratios for tritiated (Tyr*-D-Ala-Gly-NMe-Phe-Gly-ol) ([3H]DAMGO) and tritiated (Tyr*-D-Thr-Gly-Phe-Leu-Thr) ([3H]DTLET) respectively, at two weeks postlesion which correspond to the delay of maximal hyperalgesia and of maximal alteration of fine diameter primary afferent fibers. In contrast, no change in [3H]U.69593 specific binding could be detected at this postlesion delay. For longer survival delays (four, eight and 15 weeks postlesion), mu and delta binding ratios return towards control values (approximately equal to 1), probably reflecting the occurrence of a long-term neuroplasticity (i.e. a new equilibrium in the metabolism of primary neurons, or collateral sprouting from intact primary afferents) following loose nerve ligation. In addition, a comparison of the results obtained in this model with those measured after sciatic nerve section and lumbar dorsal rhizotomy was performed in order to compare the degree of loss in opioid binding sites in these three types of lesion. The section of the sciatic nerve induced at eight days postlesion an 18% (P < 0.01) and 28% (P < 0.01) decrease in binding ratio for [3H]DAMGO and [3H]DTLET, respectively. At two weeks postlesion the loss was 24% (P < 0.01) for the two ligands, and at longer delays (four and 12 weeks), a progressive recovery in binding ratio was observed. Thus, it appears that both sciatic nerve lesions we have studied result in mu and delta binding modifications which have similar intensity and similar time course from two to 12-15 weeks postlesion. In contrast, the unilateral rhizotomy of nine consecutive dorsal roots (T13-S2), which is known to induce a massive degeneration of fine diameter primary afferent fibers, is followed by a dramatic decrease in binding ratios for [3H]DAMGO (53%, P < 0.001) and [3H]DTLET (45%, P < 0.001) at two weeks postlesion. These data suggest that the more deprived the dorsal horn is of fine diameter primary afferent fibers, the more dramatic is the opioid binding loss in the ipsilateral side as compared to the contralateral side.(ABSTRACT TRUNCATED AT 400 WORDS)

Fos-like immunoreactivity in the rat superficial dorsal horn induced by formalin injection in the forepaw: effects of dorsal rhizotomies

As previously described at the lumbar spinal level, we found that 2 h after subcutaneous formalin injection in the distal part of the fore-limb, Fos-like immunoreactivity (FLI) was induced in the ipsilateral cervical enlargement. Not surprisingly, as the injection site corresponds to the distal part of the C6-C8 dorsal root dermatomes, maximal labelling which predominated in the superficial laminae, was observed in the C6-C8 segments and to a lesser extent in C5. Similar experiments were performed on rats which underwent various types of unilateral dorsal rhizotomies (DRh) 7 days before formalin injection. In animals with C4, C5, T1 and T2 DRh sparing C6-C8 the rostrocaudal distribution was similar to the intact one. But, in animals having C4-T2 DRh sparing one single root, C7, the segmental FLI distribution was modified: it was slightly increased in C7, decreased in C6 and significantly decreased in C8. As expected, no FLI was found in animals with C4 to T2 DRh. The spared root model provides information about the segmental distribution in the cervical spinal cord of the input brought by a single root following stimulation of the distal forelimb, i.e., maximal distribution in the entry segment, but also in the two rostral and one caudal segments.

Time-related decreases in mu and delta opioid receptors in the superficial dorsal horn of the rat spinal cord following a large unilateral dorsal rhizotomy

The aim of the present study was to measure the time-related modifications of mu and delta opioid binding sites in the superficial layers of the dorsal horn of the rat spinal cord after a C4-T2 unilateral dorsal rhizotomy. Using specific ligands, namely [3H]DAMGO for mu sites and [3H]DTLET for delta sites, and a quantitative autoradiographic analysis, we have observed: (a) a decrease in binding on the ipsilateral side to the lesion as early as the first day postrhizotomy, the maximal loss being attained at 8 days postlesion, (b) after 8 days postlesion, the residual binding remains stable over the period of analysis (90 days), (c) the loss of mu receptors (71-74%) is significantly more pronounced than the loss of delta receptors (57-62%) and (d) affinities of postsynaptic mu and delta receptors are similar to those of the total receptor population in the superficial layers of the dorsal horn. Comparison of these results with the degeneration of primary afferent fibers reported in literature favors the localization of the majority of mu and delta opioid binding sites on fine diameter primary afferent fibers.

Intraspinal sprouting of calcitonin gene-related peptide containing primary afferents after deafferentation in the rat

The occurrence of sprouting in the spinal cord in response to denervation has been a subject of debate. To test for sprouting of primary afferent fibers after denervation, rats were unilaterally deafferented for 35 days (chronic side) by dorsal rhizotomies performed from T2 to T8 and T10 to L5, thus isolating or sparing the T9 root. The contralateral T9 root was spared by a similar surgery 5 days (acute side) prior to sacrifice. The survival time on the chronic side presumably allows intraspinal sprouting of T9 primary afferents to occur whereas the time on the acute side does not. To test for sprouting of primary afferents, it is necessary to identify these nerve processes. Calcitonin gene-related peptide (CGRP) immunoreactivity has been localized to a subpopulation of primary afferent nerve processes and their terminals within the dorsal horn. Therefore, immunohistochemical methods were used to determine the distribution of CGRP immunoreactivity in laminae I and II on both sides of the spinal cord. Using image analysis, there was an increase of 153 to 704% in the density of CGRP immunoreaction product on the chronic side compared to the acute side in the spared segment. This difference is statistically significant. Furthermore, the increased density on the chronic side extended two segments cranial and two segments caudal to the spared root segment. No difference was found in the laminar distribution between sides. These data support the hypothesis of primary afferent sprouting following spinal cord denervation.

Developmental alterations in nociceptive threshold, immunoreactive calcitonin gene-related peptide and substance P, and fluoride-resistant acid phosphatase in neonatally capsaicin-treated rats

This study examined the effect of neonatal administration of capsaicin on nociceptive threshold and the distribution of calcitonin gene-related peptide (CGRP), substance P (SP), and fluoride-resistant acid phosphatase (FRAP) in the dorsal horn of the spinal cord during the course of development (10 days to 12 weeks of age) in the rat. As early as 10 days of age, CGRP-like immunoreactivity was reduced in laminae I, II, and V, as well as in the bundles of fibers situated dorsal and ventral to the central canal. However, beginning on or about 6 weeks of age, the density of CGRP-like immunoreactivity in the superficial laminae and in the bundles dorsal and ventral to the central canal increased. Moreover, thick, nonvaricose CGRP-like immunoreactive fibers appeared in laminae III and IV. These recurring fibers were of primary afferent origin as demonstrated by their disappearance after multiple, unilateral rhizotomies. A similar age-dependent alteration in the density of FRAP activity was also observed. Although virtually absent at 10 days of age after neonatal administration of capsaicin, the density of FRAP activity increased in lamina II by 8 weeks of age. This activity disappeared after multiple, unilateral rhizotomies, indicating that the FRAP activity that reappeared was of primary afferent origin. Neonatal administration of capsaicin also reduced the density of SP-like immunoreactivity in the dorsal horn as early as 10 days of age, although the density of SP-like immunoreactivity showed some recovery after 6 weeks of age. However, unlike CGRP-like immunoreactivity or FRAP activity, the density of SP-like immunoreactivity in capsaicin-treated rats was not detectably altered by multiple, unilateral rhizotomies, indicating that it originated principally from intrinsic dorsal horn neurons. Age-dependent alterations in both thermal and mechanical, but not chemical, nociceptive thresholds were also observed in these same animals. Thus, tail flick latency, hot plate latency, and paw withdrawal threshold were maximally increased at 6 weeks of age, after which time thresholds declined to vehicle-treated values. In contrast, capsaicin-treated animals were uniformly insensitive to ophthalmic administration of capsaicin. The correspondence between developmental alterations in CGRP-like immunoreactivity or FRAP activity and in thermal and mechanical nociceptive thresholds is suggestive of a role of CGRP- or FRAP-containing primary afferents in thermal and mechanical nociception.

Cytochemical relationships and central terminations of a unique population of primary afferent neurons in rat

Light and electron microscopic immunohistochemical techniques were used to investigate the central projections and colocalization relationships of a subpopulation of primary afferent neurons that were immunolabelled with an antibody (AB893) against rat liver gap junctions. In lumbar dorsal root ganglia AB893-immunoreactivity was seen in 14.5% of all cells and in both small and large size neurons. Colocalization analysis showed that 78% of all AB893-immunoreactive (AB893-IR) neurons contained calcitonin gene-related peptide, while only 7 to 10% contained the calcium binding proteins parvalbumin or calbindin D28k. Among small type B AB893-IR ganglion cells, it was calculated that over 90% contained fluoride-resistant acid phosphatase, while only 1 to 2% contained substance P or somatostatin. Cytochrome oxidase histochemistry revealed light staining in the vast majority of AB893-IR cells. In the dorsal horn of the spinal cord the antibody labelled fibers in the dorsal root, Lissauer's tract, lamina I and lamina II. Isolated immunoreactive fiber bundles were arranged in sheets spanning most of lamina II. Immunoreactive fibers were depleted from the dorsal horn after dorsal rhizotomy or neonatal capsaicin treatment. Ultrastructural examination showed that AB893-IR fibers were composed of closely associated clusters of 2 to 5 unmyelinated fibers each ranging from 0.1-0.4 microns in diameter. Immunoreactivity was distributed intermittently along the cytoplasmic membrane of axons and en passant sinusoid terminals located centrally within the fiber clusters, as well as along axonal membranes adjacent to the central axon or terminal. The results suggest that the immunoreactive fibers in lamina II of the dorsal horn originate from a subpopulation of AB893-IR neurons that contain FRAP and give rise to unmyelinated axons.

Fine structure and synaptic architecture of HRP-labelled primary afferent terminations in lamina IIi of the rat dorsal horn

The fine structure and synaptic architecture of the afferent terminations in dorsal horn lamina II are studied using a combined light and electron microscopic procedure after anterograde labelling with horseradish peroxidase. Vibratome parasagittal sections, stained with heavy metal intensified diaminobenzidine after tracer application to the dorsal roots, were flat-embedded in Epon. The five types of labelled terminal arbors occurring in lamina IIi (Cruz et al., '87: J. Comp. Neurol. 261:221-236) were drawn and relocated in 5-microns sections cut serially from the thick sections. Ultrathin sections were then cut from the 5-microns sections so that the terminal fibers and swellings observed in the light microscope could be traced in the electron microscope. The flame-shaped arbors arose from fine myelinated stem fibers. Terminal strands generated large oval central terminals of type II synaptic glomeruli (CII), which established frequent axoaxonal contacts. Similar terminals have been labelled in the cat after tracer injections into hair-follicle fibers (Réthelyi et al., '82: J. Comp. Neurol. 207:381-393). The other four plexuses arose from unmyelinated stem fibers. The swarms of ultrafine boutons consisted of extremely thin terminal fibers generating very small, round, or polygonal glomerular terminals containing tightly packed agranular synaptic vesicles of variable size and one mitochondrion at best. The terminal strands of the bouquet plexus bore long and scalloped central varicosities of type I synaptic glomeruli (CI) with pleomorphic agranular vesicles and a relative abundance of dendroaxonal contacts. These features, together with the location in dorsal lamina IIi, suggest their belonging to the fluoride resistant acid phosphatase (FRAP)-reactive population. The boutons of the undulating fibers and those of the lateral plexus were, like those of the bouquets, scalloped and elongated rostrocaudally (CI), but contained a few large granular vesicles. The occurrence of the swarm, undulating, and lateral plexuses in ventral lamina IIi, which seems to lack FRAP or peptidergic terminals, suggests an origin from other, still unidentified neurochemical populations of fine primary afferents.

Plasticity of spinal systems after unilateral lumbosacral dorsal rhizotomy in the adult rat

Plasticity of spinal systems in response to lumbosacral deafferentation has previously been described for the cat, by using immunocytochemistry to demonstrate plasticity of tachykinin systems and degeneration methods to demonstrate plasticity of descending systems. In this study, we describe the response to lumbosacral deafferentation in the adult rat. Application of immunocytochemical methods to visualize tachykinins (predominantly substance P magnitude of SP), serotonin (5-HT), and dopamine B-hydroxylase (DBH), the synthesizing enzyme for norepinephrine, permits us to compare the response of SP systems in rat and cat spinal cord and to examine the response of two descending systems, serotoninergic and noradrenergic, to deafferentation. We used image analysis of light microscopic preparations to quantify the immunoreaction product in the spinal cord in order to estimate the magnitude, time course and localization of changes induced by the lesion. The distribution of SP, serotoninergic (5-HT), and noradrenergic staining in the spinal cord of rat is very similar to that of the cat. Unilateral lumbosacral rhizotomy elicits a partial depletion, followed by a partial replacement of tachykinin immunoreactivity in laminae I and II. This response was similar to that described for the cat, although characterized by a longer time course, and, as in the cat, is likely due to plasticity of tachykinin containing interneurons. The same lesion elicits no depletion but a marked and permanent increase in 5-HT immunoreactivity in laminae I and II, which develops more rapidly than the response by the SP system. These results indicate sprouting or increased production of SP and 5-HT in response to deafferentation. No change was seen in DBH immunoreactivity, indicating that the noradrenergic system does not show plasticity in response to deafferentation. Our results demonstrate that dorsal rhizotomy evokes different effects in different systems in the adult spinal cord of the rat and thus suggests that the response of undamaged pathways to partial denervation of their target is regulated rather than random.

Ultrastructural visualization of glutamate and aspartate immunoreactivities in the rat dorsal horn, with special reference to the co-localization of glutamate, substance P and calcitonin-gene related peptide

Antisera raised against the fixation products of L-glutamate and L-aspartate were used, singly or in combination, to study the ultrastructural localization of the amino acids in the rat dorsal horn, with post-embedding immunogold techniques. Immunostaining for each of the amino acids was also combined with immunolocalization of GABA, an important inhibitory neurotransmitter in the spinal cord, or synaptophysin, a synaptic vesicle glycoprotein. In addition, we examined the localization of glutamate immunoreactivity in relation to that of calcitonin-gene related peptide and substance P, two neuropeptides present in high concentrations in the dorsal horn. Glutamate- and aspartate-immunoreactive neuronal cell bodies, dendrites, axons and terminals were apparent in the first three laminae of the dorsal horn. In somatic and dendritic profiles, the immunolabel was present over the general cytoplasm and mitochondria; in the terminals, it was found over small, agranular vesicles, mitochondria and, at times, synaptic densities. Quantitative estimation indicated that the colloidal gold density in the glutamate-immunoreactive terminals was five-fold more than in any other neuronal profile. Both glutamate- and aspartate-immunopositive terminals made asymmetric synaptic contacts onto unlabelled dendrites; glutamate-positive terminals often formed the core of type I and II glomeruli. After double labelling of the same sections, glutamate and aspartate immunoreactivities consistently occurred in different axonal and terminal profiles. In these preparations, it was clearly seen that glutamate-immunoreactive terminals were far more numerous than (more than 10-fold) those immunoreactive for aspartate. Double labelling for glutamate or aspartate and GABA also revealed distinct staining of different terminals. Simultaneous immunolocalization of each of the amino acids and synaptophysin showed the amino acid and glycoprotein immunoreactivities co-localized in small, agranular vesicles in immunoreactive terminals. Finally, triple labelling of the same sections for glutamate, calcitonin gene-related peptide and substance P revealed that glutamate was often co-localized with either of the two neuropeptides in the same axonal boutons; terminals that showed simultaneous labelling for glutamate, calcitonin gene-related peptide and substance P were also noted. In all cases, the glutamate immunoreactivity was restricted to small, clear vesicles whereas the neuropeptide immunoreactivities were present in larger, dense-cored vesicles. Our observations demonstrate that there is an abundant glutamate immunoreactivity in the superficial layers of the rat dorsal horn, localized in neuronal profiles distinct from those containing aspartate or GABA.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization and localization of a putative morphine-modulating peptide, FLFQPQRFamide, in the rat spinal cord: biochemical and immunocytochemical studies

The bovine octapeptide Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2 (FLFQPQRFamide), originally detected by antisera raised against the invertebrate peptide, Phe-Met-Arg-Phe-NH2, is a neuropeptide which antagonizes the actions of endogenous and exogenous opiates. Using a sensitive radioreceptor assay, we show that rat spinal cord extracts were able to inhibit binding of FLFQPQRFamide, suggesting that a biologically active FLFQPQRFamide-like material exists in the rat spinal cord. We also raised antibodies against the peptide and used them, together with radioimmunological and immunohistochemical methods, to characterize this material further and analyse its cellular and subcellular localization in this area of the central nervous system. Radioimmunoassay showed that extracts from cervical and thoracolumbar levels contained measurable amounts of FLFQPQRFamide-immunoreactive material (about 3 ng/g tissue), present essentially in the dorsal horn. Analytical reverse-phase chromatography revealed that this material existed in several molecular forms. One of these fractions (about 20% of the total immunoreactivity) had the elution characteristics of synthetic FLFQPQRFamide. Light microscopic immunohistochemistry showed FLFQPQRFamide immunoreactivity at all spinal levels, localized mainly in a dense plexus of fibers in the superficial layers of the dorsal horn. Immunoreactive profiles were also seen in the lateral funiculi and around the central canal at all levels and in the intermediolateral columns; some rare immunoreactive fibers were also noted in the ventral horn at cervical and thoracic levels. FLFQPQRFamide-positive cell bodies were never detected in any of our sections. Electron microscopy of ultrathin sections of the dorsal horn and central gray treated with our antisera and a post-embedding immunogold procedure revealed that the immunoreactivity, at least within these areas, was restricted to dense-cored vesicles (90-120 nm in diameter) in axonal and terminal profiles. As seen by radioimmunoassay and immunohistochemistry, unilateral rhizotomy of all dorsal roots between segments C4 and T2 did not change the levels of FLFQPQRFamide immunoreactivity in the ipsilateral C6-C8 segments. Taken together with our recent data showing the existence of specific FLFQPQRFamide receptors at the spinal cord level, our present observations suggest that the dorsal horn of the rat spinal cord may be a site where vertebrate Phe-Met-Arg-Phe-like peptides, and in particular, FLFQPQRFamide, may exert opiate modulating activities.

Oxytocinergic innervation of the rat spinal cord. An electron microscopic study

Pre- and postembedding immunocytochemical procedures were used, together with antisera raised against oxytocin or its neurophysin, to characterize oxytocinergic pathways in the rat spinal cord, at the electron microscopic level. Pre-embedding immunoperoxidase staining performed on vibratome sections revealed oxytocin- and neurophysin-positive axonal profiles and terminals scattered predominantly in laminae I and II of the dorsal horn and in the central gray (lamina X). They were also visible, but to a lesser extent, in the intermediolateral columns, at thoracic and lumbar levels. Postembedding immunogold staining performed directly on ultrathin sections of the same areas, fixed in osmium and embedded in resin, permitted to show clearly that the oxytocinergic axons made symmetrical and asymmetrical synaptic contacts onto dendritic profiles. It also allowed subcellular localization of the neuropeptide immunoreactivities which were restricted to relatively large, electron-dense vesicles in the immunopositive terminals. Oxytocinergic terminals were never seen to participate in glomerular configurations in the superficial layers of the dorsal horn nor were immunoreactive cell bodies visible in any spinal area. Our results provide direct morphological evidence that oxytocinergic pathways make synapses in several regions of the spinal cord, thus supporting the contention that oxytocin may exert neurotransmitter/neuromodulator actions in this area of the CNS.

Choline acetyltransferase-immunoreactive profiles are presynaptic to primary sensory fibers in the rat superficial dorsal horn

The specific aim of this study was to search for morphological counterparts to the known antinociceptive effects of cholinomimetic drugs at the spinal cord level. For this, the light microscopic and ultrastructural distribution of choline acetyltransferase immunoreactivity was studied in laminae I-III of the rat cervical spinal cord. Immunoreactivity was present in cell bodies in lamina III, and in dendrites and axons of all three laminae. Immunoreactive axonal varicosities were often presynaptic to the central varicosities of type II synaptic glomeruli in lamina II and lamina III, less often presynaptic to the central elements of type I glomeruli in lamina II, and often presynaptic to dendrites in both type I and type II glomeruli. In addition, immunoreactive dendrites were often postsynaptic to the central varicosities of glomeruli of all morphological types. These results indicate that 1) primary sensory fibers excite cholinergic interneurons; 2) the acetylcholine released by the axon terminals of these interneurons modulates both nociceptive and non-nociceptive sensory information at the spinal cord level through both pre- and postsynaptic mechanisms. Furthermore, our results reinforce current ideas on reciprocal sensory interaction between thick and fine afferent fibers.

Computerized ultrastructural analysis of the shape of the active synaptic zones in rat spinal cord

Active synaptic zones are cytoplasmic specializations that indicate where synaptic transmission occurs. We have used computerized three-dimensional reconstructions from serial ultrathin sections to define certain features of the geometry of these zones in mammalian spinal cord. Our main finding is that the active zones in the dorsal portion of the spinal cord can be placed in one of two categories with respect to curvature: (1) uncurved or slightly curved and (2) very curved. The very curved category is associated with simple axodendritic type synapses in which the axonal terminal arises from primary afferent fibers.

GABA-like immunoreactivity in type I glomeruli of rat substantia gelatinosa

A postembedding immunogold study of type I synaptic glomeruli in lamina II of rat dorsal horn was carried out using antiserum to gamma-aminobutyric acid (GABA). Gold particles were concentrated over some peripheral axons and vesicle-containing dendrites within these glomeruli and both types of profile were presynaptic to central axons. These results suggest that GABA is involved in presynaptic inhibition of unmyelinated primary afferents and is released by some presynaptic dendrites within lamina II.

Central boutons of glomeruli in the spinal cord of the cat are enriched with L-glutamate-like immunoreactivity

Several lines of evidence indicate that L-glutamate may be a neurotransmitter of fine myelinated and unmyelinated primary afferent fibres in the spinal cord. The aim of the present study was to determine if L-glutamate was enriched in the terminals of these fibres. We performed the post-embedding immunogold technique on sections taken from the superficial regions of the lumbar cord in two cats. An antiserum, raised against protein-conjugated L-glutamate, was employed. Several tests on tissue and on a model system indicated that the antiserum recognized a glutaraldehyde-fixed L-glutamate-like substance. Terminals of fine afferent fibres were identified in the substantia gelatinosa as central boutons of synaptic glomeruli. Central boutons were examined through serial sections following immunogold reactions and were found to be heavily labelled with gold particles in consecutive sections. Quantitative analysis indicated that central boutons were more than two and a half times as densely labelled with gold particles than the tissue average. It was concluded that this represents a genuine enrichment of L-glutamate in these structures. Comparisons were made between L-glutamate-immunoreactive properties of central terminals and immunoreactivity for GABA, aspartate and glutamine. Statistical analysis revealed that central boutons in sections incubated in GABA antiserum and glutamine antiserum were associated with significantly lower densities of gold particle labelling than the average for the same tissue. Particle densities of central boutons in sections incubated in aspartate antiserum were not significantly different from average tissue densities. It was concluded that central boutons were not enriched with these three amino acids. Central boutons of synaptic glomeruli were classified into three groups on morphological criteria: (1) dense sinusoidal boutons; (2) large dense-core vesicle-containing boutons; and (3) regular synaptic vesicle-containing boutons. Quantitative analysis revealed that all of these groups were enriched in glutamate immunoreactivity, however, there were differences between the groups; large dense-core vesicle-containing boutons were associated with significantly lower densities of particles than regular synaptic vesicle-containing and dense sinusoidal terminals. The evidence indicates that central boutons, which most probably originate from fine myelinated and unmyelinated primary afferent fibres, are enriched with L-glutamate which may serve as a neurotransmitter in such fibres.