Light and electron microscope study of GABA-immunoreactive neurones in lamina III of rat spinal cord

The tracts, cytoarchitecture, and neurochemistry of the spinal cord

The human spinal cord can be described using a range of nomenclatures with each providing insight into its structure and function. Here we have comprehensively reviewed the key literature detailing the general structure, configuration of tracts, the cytoarchitecture of Rexed's laminae, and the neurochemistry at the spinal segmental level. The purpose of this review is to detail current anatomical understanding of how the spinal cord is structured and to aid researchers in identifying gaps in the literature that need to be studied to improve our knowledge of the spinal cord which in turn will improve the potential of therapeutic intervention for disorders of the spinal cord.

Morphological and neurochemical characterization of glycinergic neurons in laminae I-IV of the mouse spinal dorsal horn

A growing body of experimental evidence shows that glycinergic inhibition plays vital roles in spinal pain processing. In spite of this, however, our knowledge about the morphology, neurochemical characteristics, and synaptic relations of glycinergic neurons in the spinal dorsal horn is very limited. The lack of this knowledge makes our understanding about the specific contribution of glycinergic neurons to spinal pain processing quite vague. Here we investigated the morphology and neurochemical characteristics of glycinergic neurons in laminae I-IV of the spinal dorsal horn using a GlyT2::CreERT2-tdTomato transgenic mouse line. Confirming previous reports, we show that glycinergic neurons are sparsely distributed in laminae I-II, but their densities are much higher in lamina III and especially in lamina IV. First in the literature, we provide experimental evidence indicating that in addition to neurons in which glycine colocalizes with GABA, there are glycinergic neurons in laminae I-II that do not express GABA and can thus be referred to as glycine-only neurons. According to the shape and size of cell bodies and dendritic morphology, we divided the tdTomato-labeled glycinergic neurons into three and six morphological groups in laminae I-II and laminae III-IV, respectively. We also demonstrate that most of the glycinergic neurons co-express neuronal nitric oxide synthase, parvalbumin, the receptor tyrosine kinase RET, and the retinoic acid-related orphan nuclear receptor β (RORβ), but there might be others that need further neurochemical characterization. The present findings may foster our understanding about the contribution of glycinergic inhibition to spinal pain processing.

Injury of Muscular but not Cutaneous Nerve Drives Acute Neuropathic Pain in Rats

Acute pain is a common complication after injury of a peripheral nerve but the underlying mechanism is obscure. We established a model of acute neuropathic pain via pulling a pre-implanted suture loop to transect a peripheral nerve in awake rats. The tibial (both muscular and cutaneous), gastrocnemius-soleus (muscular only), and sural nerves (cutaneous only) were each transected. Transection of the tibial and gastrocnemius-soleus nerves, but not the sural nerve immediately evoked spontaneous pain and mechanical allodynia in the skin territories innervated by the adjacent intact nerves. Evans blue extravasation and cutaneous temperature of the intact skin territory were also significantly increased. In vivo electrophysiological recordings revealed that injury of a muscular nerve induced mechanical hypersensitivity and spontaneous activity in the nociceptive C-neurons in adjacent intact nerves. Our results indicate that injury of a muscular nerve, but not a cutaneous nerve, drives acute neuropathic pain.

Mutations affecting glycinergic neurotransmission in hyperekplexia increase pain sensitivity

See Dickenson (doi:10.1093/brain/awx334) for a scientific commentary on this article.Inhibitory interneurons in the spinal cord use glycine and GABA for fast inhibitory neurotransmission. While there is abundant research on these inhibitory pain pathways in animal models, their relevance in humans remains unclear, largely due to the limited possibility to manipulate selectively these pathways in humans. Hyperekplexia is a rare human disease that is caused by loss-of-function mutations in genes encoding for glycine receptors and glycine transporters. In the present study, we tested whether hyperekplexia patients display altered pain perception or central pain modulation compared with healthy subjects. Seven patients with genetically and clinically confirmed hyperekplexia were compared to 14 healthy age- and sex-matched controls. The following quantitative sensory tests were performed: pressure pain detection threshold (primary outcome), ice water tolerance, single and repeated electrical pain detection thresholds, nociceptive withdrawal reflex threshold, and conditioned pain modulation. Statistical analysis was performed using linear mixed models. Hyperekplexia patients displayed lower pain thresholds than healthy controls for all of the quantitative sensory tests [mean (standard deviation)]: pressure pain detection threshold [273 (170) versus 475 (115) kPa, P = 0.003], ice water tolerance [49.2 (36.5) versus 85.7 (35.0) s, P = 0.015], electrical single pain detection threshold [5.42 (2.64) versus 7.47 (2.62) mA, P = 0.012], electrical repeated pain detection threshold [3.76 (1.41) versus 5.8 (1.73) mA, P = 0.003], and nociceptive withdrawal reflex [7.42 (3.63) versus 14.1 (6.9) mA, P = 0.015]. Conditioned pain modulation was significantly reduced in hyperekplexia [increase to baseline: 53.2 (63.7) versus 105 (57) kPa, P = 0.030]. Our data demonstrate increased pain sensitivity and impaired central pain modulation in hyperekplexia patients, supporting the importance of glycinergic neurotransmission for central pain modulation in humans.

Modulation of C-nociceptive Activities by Inputs from Myelinated Fibers

To understand the mechanisms of neuropathic pain caused by demyelination, a rapid-onset, completed but reversible demyelination of peripheral A-fibers and neuropathic pain behaviors in adult rats by single injection of cobra venom into the sciatic nerve, was created. Microfilament recording revealed that cobra venom selectively blocked A-fibers, but not C-fibers. Selective blockade of A-fibers may result from A-fiber demyelination at the site of venom injection as demonstrated by microscope examination. Neuropathic pain behaviors including inflammatory response appeared almost immediately after venom injection and lasted about 3 weeks. Electrophysiological studies indicated that venom injection induced loss of conduction in A-fibers, increased sensitivity of C-polymodal nociceptors to innocuous stimuli, and triggered spontaneous activity from peripheral and central terminals of C-fiber nociceptors. Neurogenic inflammatory responses were also observed in the affected skin via Evans blue extravasation experiments. Both antidromic C-fiber spontaneous activity and neurogenic inflammation were substantially decreased by continuous A-fiber threshold electric stimuli applied proximally to the venom injection site. The data suggest that normal activity of peripheral A-fibers may produce inhibitory modulation of C-polymodal nociceptors. Removal of inhibition to C-fiber polymodal nociceptors following demyelination of A-fibers may result in pain and neurogenic inflammation in the affected receptive field.

Reduction of spinal glycine receptor-mediated miniature inhibitory postsynaptic currents in streptozotocin-induced diabetic neuropathic pain

Diabetic neuropathic pain (DNP) is a common clinical problem, and the mechanisms underlying the onset and progression of this complication are poorly understood. The present study examined the glycine receptors (GlyR) in the control of synaptic input to dorsal horn neurons in diabetes. Male Sprague-Dawley rats with or without streptozotocin (STZ) intraperitoneal injections were used. Tactile sensitivities were assessed by measuring paw withdrawal thresholds to von Frey filaments for four weeks. The extent of GlyR-mediated inhibition controlling primary afferent-evoked excitation in dorsal horn neurons was examined by using the whole cell patch clamp recording technique in isolated adult rat spinal cord slices. The content of the spinal dorsal horn glycine levels was measured by microdialysis. An intrathecal glycine agonist injection was used to test whether mimicking endogenous glycine-receptor-mediated inhibition reduces DNP. We found that persistent hyperglycemia induced by the administration of STZ caused a decrease in the paw withdrawal latency to mechanical stimuli. The miniature inhibitory post-synaptic current (mIPSC) rise, decay kinetics and mean GlyR-mediated mIPSC amplitude were not affected in DNP. The mean frequency of GlyR-mediated mIPSC of lamina I neurons from DNP rats was, however, significantly reduced when compared with neurons from control rats. Principal passive and active membrane properties and the firing patterns of spinal lamina I neurons were not changed in DNP rats. Spinal microdialysis rats had a significantly decreased glycine level following its initial elevation. The intrathecal administration of glycine diminished tactile pain hypersensitivity in DNP rats. In conclusion, these results indicate that long-lasting hyperglycemia induced by STZ injections leads to a reduced glycinergic inhibitory control of spinal lamina I neurons through a presynaptic mechanism.

Three-dimensional organization of local excitatory and inhibitory inputs to neurons in laminae III-IV of the spinal dorsal horn

Laser scanning photostimulation was used to map the distribution of the synaptic input zones (sites that give local synaptic inputs) for dorsal horn laminae III-IV neurons, in parasagittal and transverse slices of the rat lumbar spinal cord, and examine how these inputs differed for neurons of different morphologies. All neurons received local excitatory and inhibitory synaptic inputs from within laminae III-IV, while a subset of neurons also received excitatory input from the superficial laminae, especially lamina IIi, as well as the II/III border region. Two anatomical properties were found to be predictive of the dorsoventral position of a neuron's input zone relative to its soma: (1) both excitatory and inhibitory input zones were more dorsal for neurons with longer dorsal dendrites, and (2) excitatory, but not inhibitory, input zones were more dorsal (relative to the soma) for more ventral neurons, with the transition between the dorsal input zones of laminae III-IV neurons and the ventral input zones of lamina II neurons occurring at the II/III border. The observed morphophysiological correlations support the idea that interlaminar connectivity is mediated via translaminar dendritic extensions and that, more generally, local connectivity within the dorsal horn is governed by rules relating the position of a neuron's soma and dendrites to the position of the local presynaptic neurons from which it receives inputs, which are specific to the axis and direction (dorsal vs. ventral), whether the input is excitatory or inhibitory, and the laminar position of the postsynaptic neuron.

Properties of spinal lamina III GABAergic neurons in naïve and in neuropathic mice

BACKGROUND

Nerve injury leads to Aβ-fibre-mediated mechanical allodynia that is in part due to an impaired GABAergic inhibition in the spinal cord dorsal horn. The properties and function of GABAergic neurons in spinal cord lamina III, an area where low-threshold mechanosensitive Aβ-fibres terminate are, however, largely unknown.

METHODS

We used transgenic mice, which express enhanced green fluorescent protein (EGFP) under control of the promoter GAD67. The morphology and neurochemical characteristics of GABAergic, EGFP-expressing neurons were characterized. We assessed active and passive membrane properties of spinal lamina III GABAergic neurons in naïve animals and animals with a chronic constriction injury (CCI) of the sciatic nerve.

RESULTS

EGFP-expressing neurons in lamina III were predominantly islet cells (47%), whereas non-EGFP-expressing neurons were largely inverted stalked cells (40%). EGFP-expressing neurons accounted for about 25% of GABAergic neurons in lamina III. Forty-four percent co-expressed glycine, 10% neuronal nitric oxide synthase and 3% co-expressed parvalbumin. We found costaining with protein kinase CβII in 42% of EGFP-expressing neurons but no expression of protein kinase Cγ. Membrane properties and excitability of EGFP-and non-EGFP-expressing neurons from naïve and neuropathic animals were indistinguishable. The most frequent firing pattern was tonic firing (naïve: 35%, neuropathic: 37%) followed by gap firing (naïve: 33%, neuropathic: 25%). Delayed, initial burst and single-spike firing patterns made up the remainder in both groups.

CONCLUSION

A change in membrane excitability or discharge pattern of this group of lamina III GABAergic neurons is unlikely the cause for mechanical allodynia in animals with CCI.

Fast synaptic inhibition in spinal sensory processing and pain control

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

GABA-receptor control of the amplitude and duration of the neuronal responses to formalin in the rat spinal cord

The GABAergic inhibitory system in the dorsal horn of the spinal cord has been implicated in the modulation of pain, including the control of nociceptive transmission during inflammation. This electrophysiological study examined the effects of the GABAA and GABAB receptor antagonists, bicuculline and CGP35348, on the magnitude and duration of the formalin response. The responses of spinal nociceptive dorsal horn neurones to subcutaneous injection of formalin into the hindpaw in the anaesthetized rat were recorded. Both phases of the formalin response were monitored, and the antagonists were administered either simultaneously with formalin or 50 min after injection of formalin. Bicuculline (50 microg), the GABAA antagonist, administered simultaneously with formalin significantly increased the magnitude of the overall response, especially the second phase, and also abolished the silent interphase period. In addition, 50 min after injection of formalin, bicuculline increased the duration of the second phase in a dose-dependent manner. CGP35348 (250 microg), the GABAB antagonist, administered 50 min after injection of formalin also increased the duration of the second phase significantly, but had no effect on the magnitude of the response or the silent interphase when administered simultaneously with formalin. These results show that GABAA- and GABAB-receptor-mediated inhibitions are involved in controlling the duration of the second phase of the formalin response, and that GABAA-receptor-mediated inhibition also contributes to the manifestation of the silent interphase period and the magnitude of the second phase. Thus, GABA neurones are critical in determining the level and duration of nociceptive information transmitted through the spinal cord during inflammation.

Early demyelination of primary A-fibers induces a rapid-onset of neuropathic pain in rat

Some types of peripheral neuropathic pain are associated with damage to myelin rather than to axons of primary sensory neurons. It is extremely important to develop selective demyelination animal models for understanding neuropathic pain caused by demyelination. We induced a rapid-onset and reversible demyelination of peripheral A-fibers and neuropathic pain behaviors in adult rats by a single injection of cobra venom into the sciatic nerve. The relation between A-fiber demyelination and the abnormal pain behaviors was investigated using this model. Microfilament recordings revealed that cobra venom selectively blocked A-fibers, but not C-fibers. Selective blockade of A-fibers may result from A-fiber demyelination at the site of venom injection as demonstrated by microscope examination. The axons of the demyelinated A-fibers appeared to be otherwise normal. Neuropathic pain behaviors appeared almost immediately after venom injection and lasted about 3 weeks. Electrophysiological studies indicated that venom injection induced loss of conduction in A-fibers, increased sensitivity of C-polymodal nociceptors to innocuous stimuli, and triggered spontaneous activity from both peripheral and central terminals of C-fiber nociceptors. Neurogenic inflammatory responses were also observed in the affected skin via Evan's Blue extravasation experiments. Both antidromic C-fiber spontaneous activity and neurogenic inflammation were substantially decreased by continuous A-fiber threshold electric stimuli applied proximally to the venom injection site. The data suggest that normal activity of peripheral A-fibers may produce inhibitory modulation of C-fiber polymodal nociceptors. Removal of inhibition to C-fiber polymodal nociceptors following demyelination of A-fibers may result in pain and neurogenic inflammation in the affected receptive field.

The antinociceptive effect of intrathecal administration of glycine transporter-2 inhibitor ALX1393 in a rat acute pain model

BACKGROUND

Glycinergic neurons in the spinal dorsal horn have been implicated in the inhibition of spinal pain processing in peripheral inflammation and chronic pain states. Neuronal isoform glycine transporter-2 (GlyT2) reuptakes presynaptically released glycine and regulates the glycinergic neurotransmission. In this study, we examined whether a selective GlyT2 inhibitor, ALX1393, elicits an antinociceptive effect in a rat acute pain model.

METHODS

Male Sprague-Dawley rats were implanted with a catheter intrathecally. The effects of intrathecal administration of ALX1393 (4, 20, or 40 microg) on thermal, mechanical, and chemical nociception were evaluated by tail flick, hot plate, paw pressure, and formalin tests. Furthermore, to explore whether ALX1393 affects motor function, a rotarod test was performed.

RESULTS

ALX1393 exhibited antinociceptive effects on the thermal and mechanical stimulations in a dose-dependent manner. The maximal effect of ALX1393 was observed at 15 min after administration, and a significant effect lasted for about 60 min. These antinociceptive effects were reversed completely by strychnine injected immediately after the administration of ALX1393. In the formalin test, ALX1393 inhibited pain behaviors in a dose-dependent manner, both in the early and late phases, although the influence was greater in the late phase. In contrast to antinociceptive action, ALX1393 did not affect motor function up to 40 microg.

CONCLUSIONS

This study demonstrates the antinociceptive action of ALX1393 on acute pain. These findings suggest that the inhibitory neurotransmitter transporters are promising targets for the treatment of acute pain and that the selective inhibitor of GlyT2 could be a novel therapeutic drug.

Pre- and postsynaptic localization of the 5-HT7 receptor in rat dorsal spinal cord: Immunocytochemical evidence

Several lines of evidence indicate that 5-HT7 receptors are involved in pain control at the level of the spinal cord, although their mechanism of action is poorly understood. To provide a morphological basis for understanding the action of 5-HT on this receptor, we performed an immunocytochemical study of 5-HT7 receptor distribution at the lumbar level. 5-HT7 immunolabelling is localized mainly in the two superficial laminae of the dorsal horn and in small and medium-sized dorsal root ganglion cells, which is consistent with a predominant role in nociception. In addition, moderate labelling is found in the lumbar dorsolateral nucleus (Onuf's nucleus), suggesting involvement in the control of pelvic floor muscles. Electron microscopic examination of the dorsal horn revealed three main localizations: 1) a postsynaptic localization on peptidergic cell bodies in laminae I-III and in numerous dendrites; 2) a presynaptic localization on unmyelinated and thin myelinated peptidergic fibers (two types of axon terminals are observed, large ones, presumably of primary afferent origin, and smaller ones partially from intrinsic cells; this presynaptic labelling represents 60% and 22% of total labelling in laminae I and II, respectively); and 3) 16.9% of labelling in lamina I and 19.8% in lamina II are observed in astrocytes. Labeled astrocytes are either intermingled with neuronal elements or make astrocytic "feet" on blood vessels. In dendrites, the labelling is localized on synaptic differentiations, suggesting that 5-HT may act synaptically on the 5-HT7 receptor. This localization is compared with other 5-HT receptor localizations, and their physiological consequences are discussed.

NK1 receptor activation leads to enhancement of inhibitory neurotransmission in spinal substantia gelatinosa neurons of mouse

Substance P (SP) is a well-established pain messenger in the spinal cord, although its role in substantia gelatinosa (lamina II) still remains elusive. We carried out patch-clamp recordings on lamina II neurons from transverse mouse spinal cord slices (P8-12), using the selective NK1 receptor agonist [Sar9,Met(O2)11]-SP (SM-SP, 3-5 microM) in the presence of NBQX. Activation of NK1 receptors was confirmed after pre-incubation with selective NK1 antagonist L732,138 (4 microM) that consistently blocked the effects of SM-SP (nine neurons). After SM-SP challenge and spontaneous inhibitory post-synaptic current (sIPSC) analysis, 50% of recorded neurons (15 out of 30) were found to display a transient increase in frequency; in five neurons this was also associated with increase of peak amplitude. Five out of eight neurons displayed pure GABAA microM) receptor-mediated sIPSCs, whereas the remaining ones showed mixed GABAergic/glycinergic events. After miniature IPSC analysis, a significant increase in frequency was observed in three out of 14 SM-SP responsive neurons. At least four different morphological types were apparent among NK1-responsive neurons after filling with Lucifer Yellow/biocytin: fusiform with dorso-ventral dendritic arbors (i); round-to-oval with dendritic arborization mainly directed to lamina I (ii) or III (iii), and round-to-oval with dendrites sparsely distributed all around the cell body (iv). Thus, there was no correlation between morphology and electrophysiological properties of responsive neurons. Our observations provide new insights on the processing of sensory neurotransmission in spinal cord, and indicate that activation of NK1 receptors is involved in the maintenance of the inhibitory tone of substantia gelatinosa interneurons.

Kappa opioid receptors in rat spinal cord: sex-linked distribution differences

Activation of kappa opioid receptors (KORs) in the spinal cord can diminish nociception. Humans and rodents show sex differences in the analgesia produced by KOR agonists, and female rats show fluctuations in KOR density and sensitivity across the estrous cycle. However, it is unclear whether there are sex differences in the amount and/or distribution of spinal KORs. In the present study, immunocytochemically labeled KORs were examined in laminae I and II of the lumbosacral spinal dorsal horn of male and normally cycling female Sprague-Dawley rats. The basic pattern of KOR labeling was determined in both sexes using qualitative electron microscopy (EM), and sex-linked differences in the density and subcellular distribution of KOR immunoreactivity were determined with quantitative EM and light microscopy. KOR labeling was visualized with immunoperoxidase for optimally sensitive detection, or with immunogold for precise subcellular localization. By EM, the general pattern of KOR immunoreactivity was similar in males and females. KOR immunoreactivity was common in dendrites, axons, and axon terminals, and was in a few glia and neuronal somata. Most KOR-immunoreactive (-ir) axons were fine-diameter and unmyelinated. Most KOR-ir terminals were small or medium-sized, and a minority formed asymmetric or symmetric synapses with unlabeled dendrites. KOR immunoreactivity was associated both with the plasma membrane and with cytoplasmic organelles, notably including dense core vesicles in terminals. Light microscopic densitometry revealed that KOR immunoreactivity was significantly denser in estrus and proestrus females than in males. By EM, the distribution of KOR-immunogold labeling within axon terminals differed, with a greater proportion of cytoplasmic KOR labeling in estrus females compared with males. In contrast, the abundance and types of KOR-immunoperoxidase-labeled profiles did not show sex-linked differences. We conclude that in both sexes, KORs are positioned to influence both pre- and postsynaptic neurotransmission and are present in morphologically heterogeneous neuron populations. These findings are consistent with complex consequences of KOR activation in the spinal cord. In addition, the presence of increased KOR density and proportionally elevated intracellular KORs in proestrus/estrus females suggests a basis for sex-linked differences in KOR-mediated antinociception.

Dorsal horn neurons firing at high frequency, but not primary afferents, release opioid peptides that produce micro-opioid receptor internalization in the rat spinal cord

To determine what neural pathways trigger opioid release in the dorsal horn, we stimulated the dorsal root, the dorsal horn, or the dorsolateral funiculus (DLF) in spinal cord slices while superfusing them with peptidase inhibitors to prevent opioid degradation. Internalization of mu-opioid receptors (MOR) and neurokinin 1 receptors (NK1R) was measured to assess opioid and neurokinin release, respectively. Dorsal root stimulation at low, high, or mixed frequencies produced abundant NK1R internalization but no MOR internalization, indicating that primary afferents do not release opioids. Moreover, capsaicin and NMDA also failed to produce MOR internalization. In contrast, dorsal horn stimulation elicited MOR internalization that increased with the frequency, being negligible at <10 Hz and maximal at 500 Hz. The internalization was abolished by the MOR antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), in the presence of low Ca2+ and by the Na+ channel blocker lidocaine, confirming that it was caused by opioid release and neuronal firing. DLF stimulation in "oblique" slices (encompassing the DLF and the dorsal horn of T11-L4) produced MOR internalization, but only in areas near the stimulation site. Moreover, cutting oblique slices across the dorsal horn (but not across the DLF) eliminated MOR internalization in areas distal to the cut, indicating that it was produced by signals traveling in the dorsal horn and not via the DLF. These findings demonstrate that some dorsal horn neurons release opioids when they fire at high frequencies, perhaps by integrating signals from the rostral ventromedial medulla, primary afferents, and other areas of the spinal cord.

Effects of GABA receptor antagonist on trigeminal caudalis nociceptive neurons in normal and neonatally capsaicin-treated rats

We have recently demonstrated that significant increases in cutaneous mechanoreceptive field (RF) size and spontaneous activity occur in nociceptive neurons of trigeminal subnucleus caudalis (Vc, the medullary dorsal horn) of adult rats depleted of C-fiber afferents by neonatal treatment with capsaicin. These neuronal changes in capsaicin-treated (CAP) rats are suggestive of central neuroplasticity and involve N-methyl-D-aspartic acid (NMDA) receptor mechanisms. The present study examined whether the GABA(A) receptor antagonist bicuculline (BIC) or the GABA(B) receptor antagonist 2-hydroxysaclofen (SAC) can influence the RF properties and activity of Vc nociceptive neurons classified as either nociceptive-specific or wide-dynamic range in CAP adult rats or in neonatally vehicle-treated (CON) rats. C-fiber depletion was confirmed in the CAP rats by a significant decrease in plasma extravasation of Evans blue dye in a skin area receiving topical application of mustard oil, a small-fiber excitant and inflammatory irritant. As previously reported, marked increases in cutaneous RF size and spontaneous activity occurred in Vc nociceptive neurons of adult CAP rats, compared with CON rats. GABA(A) receptor blockade by BIC (i.t.) in CON rats produced a significant increase in spontaneous activity and in pinch RF size and tactile RF size (or appearance of a tactile area in the RF of nociceptive-specific neurons), as well as a significant lowering of the mechanical threshold and a significant enhancement of responses to pinch stimuli applied to the RF. In CAP rats, GABA(A) receptor blockade also produced significant changes similar to those documented in CON rats, except for a paradoxical and significant decrease in pinch RF size and no noticeable changes in responses to pinch stimuli. GABA(B) receptor blockade by SAC (i.t. ) did not produce any significant changes in Vc nociceptive neurons in either CON or CAP rats. These results suggest that GABA(A) receptor-mediated inhibition may be involved in maintaining the functional expression of Vc nociceptive neuronal properties in normal conditions, and that in animals depleted of their C-fiber afferents, some features of this GABA(A) receptor-mediated modulation may be disrupted such that a GABA(A) receptor-mediated excitation is manifested.

Nitric oxide-mediated spinal disinhibition contributes to the sensitization of primate spinothalamic tract neurons

This study concentrated on whether an increase in spinal nitric oxide (NO) diminishes inhibition of spinothalamic tract (STT) cells induced by activating the periaqueductal gray (PAG) or spinal glycinergic and GABAergic receptors, thus contributing to the sensitization of STT neurons. A reduction in inhibition of the responses to cutaneous mechanical stimuli induced by PAG stimulation was seen in wide dynamic range (WDR) STT cells located in the deep layers of the dorsal horn when these neurons were sensitized during administration of a NO donor, 3-morpholinosydnonimine (SIN-1), into the dorsal horn by microdialysis. In contrast, PAG-induced inhibition of the responses of high-threshold (HT) and superficial WDR STT cells was not significantly changed by spinal infusion of SIN-1. A reduction in PAG inhibition when STT cells were sensitized after intradermal injection of capsaicin could be nearly completely blocked by pretreatment of the dorsal horn with a NO synthase inhibitor, 7-nitroindazole. Moreover, spinal inhibition of nociceptive activity of deep WDR STT neurons elicited by iontophoretic release of glycine and GABA agonists was attenuated by administration of SIN-1. This change paralleled the change in PAG-induced inhibition. However, the inhibition of HT and superficial WDR cells induced by glycine and GABA release did not show a significant change when SIN-1 was administered spinally. Combined with our recent results, these data show that the effectiveness of spinal inhibition can be reduced by the NO/cGMP pathway. Thus disinhibition may constitute one mechanism underlying central sensitization.

Spinal effects of bicuculline: modulation of an allodynia-like state by an A1-receptor agonist, morphine, and an NMDA-receptor antagonist

Single-unit recordings were made in the intact anesthetized rat of the responses of dorsal horn neurons to C-, Adelta-, and Abeta-fiber stimulation. The postdischarge and windup responses of the same cells along with responses to innocuous stimuli, prod and brush, also were measured. The effects of (-)-bicuculline-methobromide (0.5, 5, 50, and 250 microg) were observed on these neuronal responses. The C- and Adelta-fiber-evoked responses were facilitated significantly in a dose-dependent manner. The input was facilitated, but as the final overall response was not increased by the same factor, windup appeared to be reduced. However, postdischarge, resulting from the increase in the excitability produced by windup, tended to be facilitated. After doses of >/=5 microg bicuculline, stimulation at suprathreshold Abeta-fiber-evoked activity caused enhanced firing, mainly at later latencies corresponding to Adelta-fiber-evoked activity in normal animals. Few cells responded consistently to brush and so no significant change was observed. Responses evoked by innocuous pressure (prod) always were observed in cells that concurrently responded to electrical stimulation with a C-fiber response. This tactile response was facilitated significantly by bicuculline. The effects of N6-cyclopentyladenosine (N6-CPA), an adenosine A1-receptor agonist, was observed after pretreatment with 50 microg bicuculline, as were the effects of morphine and 7-chlorokynurenate (7-CK). N6-CPA inhibited prod, C- and Adelta-fiber-evoked responses as well as the initial and overall final response to the train of C-fiber strength stimuli. Inhibitions were reversed with 8(p-sulphophenyl) theophylline. Morphine, the mu-receptor agonist, also inhibited the postbicuculline responses to prod, C-, and Adelta-fiber responses and initial and final responses to a train of stimuli. Inhibitory effects of morphine were reversed partly by naloxone. 7-CK, an antagonist at the glycine site on the N-methyl-D-aspartate-receptor complex, inhibited the responses to C- and Adelta-fiber-evoked activity as well as prod. The postdischarges were inhibited by this drug. Again both the initial and overall responses of the cell were inhibited. To conclude, bicuculline caused an increase in the responses of deep dorsal horn cells to prod, Adelta-fiber-evoked activity, increased C-fiber input onto these cells along with the appearance of responses at latencies normally associated with Adelta fibers, but evoked by suprathreshold Abeta-fiber stimulation. These alterations may be responsible for some aspects of the clinical phenomenon of allodynia and hyperalgesia. These altered and enhanced responses were modulated by the three separate classes of drugs, the order of effectiveness being 7-CK, N6-CPA, and then morphine.

GluR1 and GluR2/3 subunits of the AMPA-type glutamate receptor are associated with particular types of neurone in laminae I-III of the spinal dorsal horn of the rat

GluR1 and GluR2 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor are expressed at high levels by neurones in laminae I-III of rat spinal dorsal horn, an area which contains numerous, densely packed small neurones. In order to determine whether these subunits are expressed by inhibitory or excitatory neurones, we combined pre-embedding immunocytochemistry with antibodies that recognize either GluR1, or an epitope common to GluR2 and 3, with postembedding detection of gamma-aminobutyric acid (GABA) and glycine. Most (78%) of the neurones with GluR1-immunoreactivity were GABA-immunoreactive, and some of these were also glycine-immunoreactive, whereas nearly all (97%) of the GluR2/3-immunoreactive neurones were not GABA- or glycine-immunoreactive. We carried out double-immunofluorescence and confocal microscopy to provide further information on the neurochemistry of cells that express these subunits. As expected, all neurotensin- and virtually all somatostatin-immunoreactive cells (which are thought to be excitatory interneurones) were GluR2/3- but not GluR1-immunoreactive, whereas parvalbumin-containing cells (most of which are GABAergic) possessed GluR1-, but usually not GluR2/3-immunoreactivity. Neurones that contained nitric oxide synthase (most of which are GABAergic) were more variable, with 57% GluR1-immunoreactive and 41% GluR2/3-immunoreactive. Cholinergic neurones in lamina III (which are also GABAergic) invariably showed each type of GluR-immunoreactivity. These results suggest that neuronal populations in laminae I-III have characteristic patterns of GluR expression: GluR1 is particularly associated with inhibitory neurones, and GluR2 with excitatory neurones. This makes it likely that some of the AMPA receptors present on the inhibitory interneurones lack the GluR2 subunit, and may therefore have significant Ca2+-permeability.

Neuronal model of tactile allodynia produced by spinal strychnine: effects of excitatory amino acid receptor antagonists and a mu-opiate receptor agonist

Touch evoked agitation (allodynia) can be induced by spinal delivery of strychnine and this effect is antagonized by intrathecal NMDA and non-NMDA receptor antagonists, but not by mu-opiate receptor agonists. In this study, we sought to characterize the effect of focal glycine-receptor inhibition on spontaneous and evoked activity in dorsal horn neurons of the chloralose-anesthetized cat. Strychnine (1 mM) applied near the neurons through a dialysis fiber caused an enhanced response to hair deflection, enlargement of the low threshold receptive fields and in some cells, an increase in afterdischarge. These changes were observed only in cells that were activated by both hair deflection and high intensity mechanical stimulation. Subsequent co-administration of an NMDA receptor antagonist (AP-7, 2.0 mM) preferentially blocked strychnine-associated effects without changing the original receptive field characteristics. Co-administration of a non-NMDA excitatory amino acid receptor antagonist (CNQX, 1 mM) with the strychnine served to block low (brush) and high intensity (pinch) afferent input. In contrast, addition of a mu-opiate receptor agonist (alfentanil 2.4 mM) to the strychnine perfusate selectively reduced responsiveness to high intensity stimulation, while having no effect on the exaggerated response to hair deflection. Given the functional and pharmacological similarity of the effects of spinal strychnine to post-nerve injury states in man, disinhibition due to a loss of glycinergic input may be associated with large myelinated fiber-mediated nociceptive states. Consistent with these data is the contention that under normal circumstances, afferent hair follicle input onto convergent neurons is regulated by a tonic glycinergic circuit. Removal of this regulatory influence leads to a magnification of low threshold tactile throughput in dorsal horn. This model may help to provide pharmacological insights into more efficacious treatments for such pain states that are relatively refractory to opioid therapies.

Role of opioid receptors (mu, delta 1, delta 2) in modulating responses of nociceptive neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in the rat

This report describes the effects of intravenously administered agonists and antagonists at mu-, delta 1- and delta 2-opioid receptors on the A delta- and C-fiber-evoked responses of trigeminal nociceptive neurons in anesthetized rats. Extracellular single unit recordings were made from 61 nociceptive neurons (23 NS, 38 WDR) in the superficial and 37 nociceptive neurons (3 NS, 34 WDR) in the deeper dorsal horn of the medulla (trigeminal nucleus caudalis). Administration of either the delta 1-receptor agonist [D-Pen2,5]enkephalin (DPDPE; 0.05-2 mg/kg), the delta 2-receptor agonist [D-Ala2, Glu4]deltorphin (DELT; 1-2 mg/kg) or the mu-receptor agonist [D-Ala2, N-MePhe4, Gly5-ol]enkephalin (DAMGO; 0.05-1 mg/kg) inhibited the A delta- and C-fiber-evoked responses of nociceptive neurons in the superficial and deeper dorsal horn. The inhibitory effect was more pronounced on the C-fiber-evoked responses than on the A delta-fiber-evoked responses. In other neurons, DPDPE also produced facilitation, or inhibition followed by facilitation, or differential effects (inhibition of the C-fiber-evoked responses and facilitation of the A delta-fiber-evoked responses) on the A delta- and C-fiber-evoked responses. The effects of DPDPE were antagonized by 7-benzylidenenaltrexone (BNTX, 0.4-1 mg/kg), a delta 1-receptor antagonist, in 88% (7/8) of neurons. Naltriben (NTB, 0.7-1 mg/kg), a delta 2-receptor antagonist, antagonized the effect of both DELT and DPDPE. A smaller dose of NTB (0.3 mg/kg), which failed to reverse the effects of DPDPE in 100% (4/4) of neurons, effectively antagonized the effects of DELT in 100% (6/6) of neurons. The inhibitory action of DAMGO was completely antagonized by naloxone (0.2 mg/kg) in 100% (6/6) of neurons. The results of the present investigation suggest that: (1) mu-, delta 1- and delta 2-opioid receptors play an important role in the inhibitory modulation of the A delta- and C-fiber-evoked responses of nociceptive neurons in the superficial and deeper dorsal horn of the medulla; (2) selective inhibition of the C-fiber-evoked responses by activation of opioid receptors may account for the opioid-mediated selective suppression of second or persistent pain as compared to first pain; and (3) NTB, in a limited dose range, can discriminate between delta 1- and delta 2-opioid receptor subtypes.

Effects of GABA and glycine receptor antagonists on the activity and PAG-induced inhibition of rat dorsal horn neurons

The effects of bicuculline and strychnine on the activity and periaqueductal gray (PAG)-induced inhibition of rat dorsal horn neurons of the lumbar spinal cord were tested. Extracellular single unit recordings were from 36 dorsal horn neurons near a microdialysis fiber passed through the spinal cord for drug application. The GABAA receptor antagonist, bicuculline, was tested on 19 cells, whereas the glycine receptor antagonist, strychnine, was tested on 17 cells. Both bicuculline and strychnine increased the background activity and responses to mechanical stimulation (BRUSH, PRESS, and PINCH) of the skin.06 They also significantly blocked the PAG-induced inhibition of responses to peripheral mechanical stimuli. This experiment suggests that the mechanism of PAG-induced descending inhibition of dorsal horn neuron activity involves GABA and/or glycine release in the spinal cord and that there is tonic release of these inhibitory neurotransmitters.

Distribution of glycine-immunoreactive profiles in the monkey spinal cord: a light microscopic and ultrastructural study

The present study analyzed the relationships of glycine (GLY)-immunoreactive (-IR) and unlabeled profiles in the primate spinal cord. Light microscopic analysis demonstrated GLY-IR profiles in laminae III-VII, with fewer labeled profiles in laminae I, II, VIII, IX and X. The dorsal part of the lateral funiculus and the dorsal funiculus contained few labeled axons, in contrast to all other areas of white matter, which were heavily labeled. At the electron microscopic level, GLY-IR terminals in monkeys contained mainly round, with occasional pleomorphic, clear vesicles; however, F-type GLY-IR terminals synapsing on motoneurons contained pleomorphic vesicles. This seems to be an important species difference because vesicles in GLY-IR terminals in rat and cat are predominantly oval or elliptical. GLY-IR terminals synapsed on unlabeled as well as GLY-IR cell bodies and dendrites. This is morphological evidence that GLY may be both an inhibitor (GLY-IR terminals synapse on and presumably inhibit non-GLY cell bodies and dendrites) and a disinhibitor (GLY-IR terminals synapse on and presumably inhibit other GLY elements) of spinal activity. Also noteworthy was the conspicuous absence of axoaxonic interactions involving GLY-IR terminals. A related finding was that GLY profiles were always postsynaptic, never presynaptic, to glomerular primary afferent terminals. The functional implications would seem to be that primary afferent input can activate the spinal GLY system but that there is little GLY presynaptic control of afferent input in monkeys. This is in contrast to rats and cats, in which axoaxonic interactions involving GLY-IR terminals have been observed and where it is common to find GLY-IR terminals presynaptic to glomerular primary afferent terminals.

The contribution of GABAB receptor-mediated events to inflammatory pain processing: carrageenan oedema and associated spinal c-Fos expression in the rat

In this pharmacological study we have assessed the effect of baclofen, a selective GABAB receptor agonist, on spinal expression of the immediate early gene c-Fos and the peripheral oedema evoked by a prolonged peripheral inflammation due to intraplantar carrageenan. Baclofen was administered intravenously 30 min before intraplantar injection of carrageenan in freely moving rats. Three hours after carrageenan the number of spinal c-Fos protein-like immunoreactive neurons and peripheral (ankle and paw) oedema were assessed. For the two series of experiments the total number of control carrageenan-evoked c-Fos protein-like immunoreactive neurons in segments L4-L5 of the spinal cord was 176 +/- 6 and 177 +/- 9 c-Fos protein-like immunoreactive neurons per section, for carrageenan control with intravenous and intraplantar saline, respectively. c-Fos protein-like immunoreactive neurons were predominantly located in laminae I-II and V-VI of the dorsal horn of the spinal cord in carrageenan controls receiving intravenous (68 +/- 3 and 69 +/- 2 c-Fos protein-like immunoreactive neurons, respectively) and intraplantar (62 +/- 4 and 71 +/- 5 c-Fos protein-like immunoreactive neurons, respectively) saline. Pre-administered systemic baclofen (0.05, 1.5 and 3 mg/kg i.v.) dose dependently reduced the total number of c-Fos protein-like immunoreactive neurons (81 +/- 3, 66 +/- 4 and 49 +/- 4% of control total number of c-Fos protein-like immunoreactive neurons, respectively), with strongest effects on the number of deep (74 +/- 3, 60 +/- 3 and 43 +/- 4% of control, respectively) as compared with superficial (90 +/- 4, 77 +/- 5 and 59 +/- 5% of control, respectively) c-Fos protein-like immunoreactive neurons. The effects of systemic baclofen on the carrageenan-induced spinal c-Fos expression and both the paw and ankle oedema were positively correlated (r = 0.479, P < 0.05 and r = 0.733, P < 0.001, respectively). Intraplantar baclofen (50 and 100 micrograms in 50 microliters of saline), simultaneously injected with intraplantar carrageenan, did not significantly influence carrageenan-evoked spinal c-Fos expression or ankle oedema. Despite the fact that the highest dose of intraplantar baclofen significantly reduced paw oedema (23 +/- 3% reduction of control paw oedema), our results are clearly in favour of a spinal site of action of systemic baclofen.

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.

Substance P- and GABA-like immunoreactivities are co-localized in axonal varicosities in the superficial laminae of cat but not rat spinal cord

In the present study, we applied a combination of pre-embedding peroxidase-based immunocytochemistry and post-embedding immunogold staining to examine the synaptic interactions of substance P (SP) and gamma-aminobutyric acid (GABA) in the superficial laminae of the dorsal horn of cat and rat spinal cord. We demonstrate for the first time the co-existence of SP and GABA immunoreactivities in axonal boutons in laminae I-III of cat spinal dorsal horn. In cat, most SP + GABA immunoreactive (IR) axonal boutons established synapses with SP-IR or non-IR dendrites. These synapses were exclusively symmetric. Quantitative analysis showed that the percentage of SP/GABA double labelled bouton profiles was higher (7%) in lamina I but was considerably lower in laminae IIo, IIi and III. Similarly, the density (number of bouton profiles per 100 microns2) of SP + GABA-IR bouton profiles was highest in lamina I. However, in agreement with previous studies, the co-localization of SP and GABA immunoreactivities was never detected in the rat dorsal horn. In both species, SP + GABA-IR or GABA-IR axonal bouton profiles were never seen presynaptic to SP-IR boutons. These findings provide a morphological basis for the interaction of excitatory and inhibitory agents in the nociceptive circuits in the dorsal horn of the cat and rat spinal cord.

[D-Pen2-D-Pen5]enkephalin, a delta opioid agonist, given intracerebroventricularly in the mouse produces antinociception through medication of spinal GABA receptors

Intracerebroventricular (ICV) administration of [D-Pen2-D-Pen5]enkephalin (DPDPE), a delta opioid receptor agonist, activates a descending antinociceptive pathway that inhibits the tail-flick response in mice. Involvement of spinal GABA receptors in this response was studied by giving GABA antagonist intrathecally. First, antinociception produced by intrathecally administered isoguvacine, a GABAA agonist, was inhibited by intrathecal bicuculline (GABA receptor antagonist) or picrotoxin (chloride channel antagonist). Then, antinociception induced by ICV DPDPE was antagonized by intrathecal picrotoxin and bicuculline in a dose-and time-dependent manner. Second, intrathecal administration of 2-hydroxysaclofen, a GABAB antagonist (which inhibited antinociception induced by a GABAB agonist, baclofen, given IT), produced a shift of the dose-response curve for ICV DPDPE to the right. GABAA agonist, baclofen, given IT), produced a shift of the dose-response curve for ICV DPDPE to the right. GABAA and B antagonists given together intrathecally produced a greater than additive antagonistic effect against ICV DPDPE-induced antinociception. Thus, the delta agonist action of DPDPE in the brain leads to activation of descending spinal pathways which involve mediation by spinal GABAA and GABAB receptors in the antinociceptive response.

Glycine and GABAA antagonists reduce the inhibition of primate spinothalamic tract neurons produced by stimulation in periaqueductal gray

Amino acids are demonstrated to be important neurotransmitters mediating the inhibitory transmission from nucleus raphe magnus to spinal nociceptive dorsal horn neurons. In this study, the role of glycine and GABA in the inhibitory processes evoked by stimulation in periaqueductal gray (PAG) of responses of primate spinothalamic tract (STT) neurons to cutaneous mechanical and thermal stimuli was investigated by examining the effects of strychnine and bicuculline, antagonists of glycine and GABAA receptors, respectively, introduced into the dorsal horn through a microdialysis fiber. The inhibitory effects of iontophoretic application of glycine and GABAA agonists on STT cell activity evoked by noxious mechanical stimulation of the skin were selectively blocked by their specific antagonist, strychnine or bicuculline, infused into the dorsal horn. Similarly, intra-spinal application of strychnine or bicuculline resulted in a significant reduction in the PAG stimulation-induced inhibition of responses of STT cells to cutaneous stimuli. This reduction was mainly on the PAG-induced inhibition of the responses to noxious mechanical stimuli. Our results suggest that glycinergic and GABAergic inhibitory interneurons in the spinal cord dorsal horn synapsing on STT cells are activated during stimulation in PAG and contribute to descending antinociceptive actions.

Subpopulations of GABAergic neurons in laminae I-III of rat spinal dorsal horn defined by coexistence with classical transmitters, peptides, nitric oxide synthase or parvalbumin

GABAergic neurons in laminae I-III of the spinal dorsal horn may contain one or more of the following compounds: glycine, acetylcholine, neuropeptide Y, enkephalin, nitric oxide synthase or parvalbumin. Although the pattern of co-localization of some of these compounds is understood, it is not known which types of GABAergic neurons contain parvalbumin, or whether nitric oxide synthase coexists with peptides, acetylcholine or parvalbumin in any of these neurons, and in this study we have used immunocytochemistry and enzyme histochemistry to resolve these issues. Parvalbumin-immunoreactivity was restricted to those GABA-immunoreactive neurons that also showed glycine-immunoreactivity and was not co-localized with neuropeptide Y-immunoreactivity or NADPH diaphorase activity. By combining NADPH diaphorase histochemistry with immunocytochemistry with an antiserum to nitric oxide synthase, we were able to show that NADPH diaphorase activity was a reliable marker for nitric oxide synthase in the spinal cord. Neurons that possess GABA- but not glycine-immunoreactivity may contain neuropeptide Y, enkephalin, acetylcholine or NADPH diaphorase, and all of the cholinergic neurons appear to contain NADPH diaphorase. By combining immunofluorescent detection of neuropeptide Y or enkephalin with NADPH diaphorase histochemistry, we showed that peptide-immunoreactivity did not coexist with NADPH diaphorase. This suggests that neither of these peptides coexists with nitric oxide synthase or with acetylcholine in neurons in the superficial dorsal horn. Several phenotypically distinct groups of GABA-immunoreactive neuron can therefore be identified in laminae I-III of the dorsal horn, and these may represent different functional types of inhibitory neuron.

Cobalt accumulation in neurons expressing ionotropic excitatory amino acid receptors in young rat spinal cord: morphology and distribution

Excitatory amino acids (EAA) acting on N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate receptors play an important role in synaptic transmission in the spinal cord. Quantitative autoradiography and physiological experiments suggest that NMDA receptors are localized mainly in lamina II while kainate and AMPA receptors are found on both dorsal and ventral horn neurons. However the cell types expressing EAA receptors and their laminar distribution is not known. We have used a cobalt uptake method to study the morphology and distribution of spinal cord neurons expressing AMPA, kainate, or NMDA excitatory amino acid receptors in the lumbar enlargement of the rat spinal cord. The technique involved superfusion of hemisected spinal cords of 14 day-old rat pups in vitro with excitatory amino acid receptor ligands in the presence of CoCl2. Cobalt has been shown to enter cells through ligand-gated ion channels in place of Ca2+. Cells which accumulated cobalt ions following activation by ionotropic excitatory amino acid receptors were visualized histochemically. The cobalt uptake generated receptor-specific labeling of cells, as the NMDA receptor antagonist D-(-)-2-amino-(5)-phosphonovaleric acid (D-AP-5) (20 microM) blocked the NMDA, but not kainate-induced cobalt uptake. The kainate-induced cobalt labeling was reduced by the non-selective excitatory amino acid receptor antagonist kynurenic acid (4 mM). Passive opening of the voltage-gated Ca(2+)-channels by KCl (50 mM) did not result in cobalt uptake, indicating that cobalt enters the cells through ligand-gated Ca(2+)-channels. AMPA (500 microM), kainate (500 microM), or NMDA (500 microM) each induced cobalt uptake with characteristic patterns and distributions of neuronal staining. Overall, kainate induced cobalt uptake in the greatest number of neuronal staining. Overall, kainate induced cobalt uptake in the greatest number of neuronal perikarya while NMDA-induced uptake was the lowest. AMPA and kainate, but not NMDA superfusion, resulted in cobalt labeling of glial cells. Our results show that the cobalt uptake technique is a useful way to study the morphology and distribution of cells expressing receptors with ligand-gated Ca2+ channels.

Neurotransmitters in subcortical somatosensory pathways

Investigations during recent years indicate that many different neuroactive substances are involved in the transmission and modulation of somesthetic information in the central nervous system. This review surveys recent developments within the field of somatosensory neurotransmission, emphasizing immunocytochemical findings. Increasing evidence indicates a widespread role for glutamate as a fast-acting excitatory neurotransmitter at different levels in somatosensory pathways. Several studies have substantiated a role for glutamate as a neurotransmitter in primary afferent neurons and in corticofugal projections, and also indicate a neurotransmitter role for glutamate in ascending somatosensory pathways. Other substances likely to be involved in somatosensory neurotransmission include the neuropeptides. Many different peptides have been detected in primary afferent neurons with unmyelinated or thinly myelinated axons, and are thus likely to be directly involved in primary afferent neurotransmission. Some neurons giving rise to ascending somatosensory pathways, primarily those with cell bodies in the dorsal horn, are also immunoreactive for peptides. Recent investigations have shown that the expression of neuropeptides, both in primary afferent and ascending tract neurons, may change as a result of various kinds of peripheral manipulation. The occurrence of neurotransmitters in intrinsic neurons and neurons providing modulating inputs to somatosensory relay nuclei (the dorsal horn, the lateral cervical nucleus, the dorsal column nuclei and the ventrobasal thalamus) is also reviewed. Neurotransmitters and modulators in such neurons include acetylcholine, monoamines, GABA, glycine, glutamate, and various neuropeptides.

Coexistence of NADPH diaphorase with GABA, glycine, and acetylcholine in rat spinal cord

The enzyme NADPH diaphorase is present in many spinal neurons, and is thought to correspond to nitric oxide synthase. In order to determine which types of neuron in the spinal cord contain this enzyme, we have carried out a combined enzyme histochemical and immunocytochemical study with antibodies to GABA, glycine, and choline acetyltransferase. Two hundred rats were tested for GABA- and glycine-like immunoreactivity. The majority of these neurons (207/224) were GABA-immunoreactive and 139 were also glycine-immunoreactive. NADPH diaphorase-positive neurons in laminae I and II generally showed both types of immunoreactivity, while those in deeper laminae of the dorsal horn and around the central canal either showed both types or else were only GABA-immunoreactive. Since GABA and acetylcholine are thought to coexist in spinal neurons, NADPH diaphorase staining was combined with immunostaining for choline acetyltransferase. Immunoreactive neurons in laminae III and IV were all NADPH diaphorase-positive, while only some of those around the central canal and in the deeper laminae of the dorsal horn were positive. Choline acetyltransferase-immunoreactive neurons in the intermediolateral cell column (presumed sympathetic preganglionic neurons) were often NADPH diaphorase-positive, whereas those in the ventral horn (presumed motoneurons) were not. NADPH diaphorase-positive cells in the intermediolateral cell column were not immunoreactive with GABA or glycine antibodies.

Evidence that neuropeptide Y is present in GABAergic neurons in the superficial dorsal horn of the rat spinal cord

In order to determine whether or not neuropeptide Y coexists with GABA or glycine in rat dorsal horn, we have examined 84 neuropeptide Y-immunoreactive neurons in laminae I-III with a combined pre- and postembedding immunocytochemical method. All of the neuropeptide Y-immuno-reactive neurons were also GABA-immunoreactive, but they were either non-immunoreactive or weakly immunoreactive with the glycine antiserum. In addition, a double-label immunofluorescence method was used to search for co-localization of neuropeptide Y and [Met]enkephalin in spinal cord. Although the two types of peptide immunoreactivity often coexisted in varicosities around the central canal and in the ventral horn, such coexistence was not seen in the superficial dorsal horn. These results suggest that neuropeptide Y is present in GABAergic neurons in laminae I-III of rat dorsal horn, but that it is largely or completely restricted to those neurons which do not contain glycine. In addition, the cells that contain GABA and neuropeptide Y appear to form a different population from those that contain GABA and [Met]enkephalin. Neuropeptide Y administered by intrathecal injection causes analgesia, and there is evidence that this may involve a presynaptic mechanism. The results of the present study suggest that neuropeptide Y may act in conjunction with GABA to produce presynaptic inhibition of nociceptive primary afferents.

Immunocytochemical study of somatostatin, neurotensin, GABA, and glycine in rat spinal dorsal horn

In order to determine whether somatostatin coexists with GABA or glycine in neurones in rat spinal dorsal horn, a combined pre- and post-embedding immunocytochemical study was carried out. One hundred six somatostatin-immunoreactive neurones located in lamina II and the dorsal half of lamina III were tested with antiserum or monoclonal antibody to GABA and none of these cells showed GABA-like immunoreactivity. However, 8 out of 13 somatostatin-immunoreactive neurones located deeper in the dorsal horn (ventral lamina III and lamina IV) showed glycine-like immunoreactivity, and 6 of these were also GABA-immunoreactive. We have previously shown that neurotensin-immunoreactive neurones in laminae II and III are also not immunoreactive when tested with GABA antiserum (Todd et al.: Neuroscience 47:685-691, 1992), and a double-labelling fluorescence method was therefore used to compare the distribution of somatostatin and neurotensin within the superficial dorsal horn. The two types of peptide-immunoreactivity were never found in the same profile. These results suggest that somatostatin and neurotensin are present in different populations of non-GABAergic neurones in rat superficial dorsal horn, but that some somatostatin-containing neurones in the deeper part of the dorsal horn contain glycine, with or without GABA.

Ultrastructural and immunocytochemical study of lamina II islet cells in rat spinal dorsal horn

In order to compare the ultrastructure of GABA-immunoreactive and nonimmunoreactive islet cells in lamina II of the rat dorsal horn, a combined ultrastructural and immunocytochemical study of nine Golgi-stained neurones was performed. Cell bodies of these neurones were tested with antiserum to GABA, and in most cases with antiserum to glycine, while parts of the cell body and dendritic tree were examined with the electron microscope. Four of the neurones had cell bodies that were immunoreactive with GABA antiserum, and 2 of these were also glycine-immunoreactive, while 2 were not. Cell bodies of the remaining five neurones were not immunoreactive with GABA antiserum, nor, in the 3 cases tested, with glycine antiserum. Three of the GABA-immunoreactive cells possessed vesicle-containing dendrites and were presynaptic at dendrodendritic synapses, whereas no vesicles were observed in the dendrites of any of the neurones that were not GABA-immunoreactive. The axon of one of the nonimmunoreactive cells was found with the electron microscope. It gave rise to boutons that contained round agranular vesicles and a few dense-cored vesicles. Three synapses formed by this axon were identified and all were asymmetric. No obvious differences were detected in the types of profile that were presynaptic to GABA-immunoreactive and nonimmunoreactive cells. These results suggest that GABAergic islet cells are a source of presynaptic dendrites in lamina II of the rat and that some presynaptic dendrites contain GABA and glycine, while others contain GABA without glycine. The nonimmunoreactive islet cells presumably represent a distinct functional class of neurones and some of these may release an excitatory amino acid transmitter, possibly in addition to one or more neuropeptides.

Immunocytochemical evidence that GABA and neurotensin exist in different neurons in laminae II and III of rat spinal dorsal horn

Pre-embedding immunocytochemistry with antiserum to neurotensin was combined with post-embedding immunocytochemistry with GABA antiserum, in order to identify neurotensin- and GABA-containing neurons in laminae I-III of rat spinal dorsal horn. The distribution of cell bodies containing these two compounds was similar to that which has been described previously. None of the 88 neurotensin-immunoreactive neurons which were tested showed GABA-like immunoreactivity, which suggests that GABA and neurotensin exist in different cells in this region. Since both compounds are thought to be present in islet cells, it is likely that there are two neurochemically distinct populations of islet cells in lamina II of rat spinal cord.