Immunohistochemical evidence that Met-enkephalin and GABA coexist in some neurones in rat dorsal horn

Ayahuasca and its major component harmine promote antinociceptive effects in mouse models of acute and chronic pain

ETHNOPHARMACOLOGICAL RELEVANCE

Ayahuasca (AYA) is a psychedelic brew used in religious ceremonies. It is broadly used as a sacred medicine for treating several ailments, including pain of various origins.

AIM OF THE STUDY

To investigate the antinociceptive effects of AYA and its mechanisms in preclinical models of acute and chronic pain in mice, in particular during experimental neuropathy.

MATERIALS AND METHODS

The antinociceptive effects of AYA administered orally were assessed in the following models of pain: formalin test, Complete Freund's Adjuvant (CFA)-induced inflammation, tail flick test, and partial sciatic nerve ligation model of neuropathic pain. Antagonism assays and Fos immunohistochemistry in the brain were performed. AYA-induced toxicity was investigated. AYA was chemically characterized. The antinociceptive effect of harmine, the major component present in AYA, was investigated.

RESULTS

AYA (24-3000 μL/kg) dose-dependently reduced formalin-induced pain-like behaviors and CFA-induced mechanical allodynia but did not affect CFA-induced paw edema or tail flick latency. During experimental neuropathy, single treatments with AYA (24-3000 μL/kg) reduced mechanical allodynia; daily treatments once or twice a day for 14 days promoted consistent and sustained antinociception. The antinociceptive effect of AYA (600 μL/kg) was reverted by bicuculline (1 mg/kg) and methysergide (5 mg/kg), but not by naloxone (5 mg/kg), phaclofen (2 mg/kg), and rimonabant (10 mg/kg), suggesting the roles of GABAA and serotonergic receptors. AYA increased Fos expression in the ventrolateral periaqueductal gray and nucleus raphe magnus after 1 h, but not after 6 h or 14 days of daily treatments. AYA (600 μL/kg) twice a day for 14 days did not alter mice's motor function, spontaneous locomotion, body weight, food and water intake, hematological, biochemical, and histopathological parameters. Harmine (3.5 mg/kg) promoted consistent antinociception during experimental neuropathy.

CONCLUSIONS

AYA promotes consistent antinociceptive effects in different mouse models of pain without inducing detectable toxic effects. Harmine is at least partially accountable for the antinociceptive properties of AYA.

Neuropathic pain modeling: Focus on synaptic and ion channel mechanisms

Animal models of pain consist of modeling a pain-like state and measuring the consequent behavior. The first animal models of neuropathic pain (NP) were developed in rodents with a total lesion of the sciatic nerve. Later, other models targeting central or peripheral branches of nerves were developed to identify novel mechanisms that contribute to persistent pain conditions in NP. Objective assessment of pain in these different animal models represents a significant challenge for pre-clinical research. Multiple behavioral approaches are used to investigate and to validate pain phenotypes including withdrawal reflex to evoked stimuli, vocalizations, spontaneous pain, but also emotional and affective behaviors. Furthermore, animal models were very useful in investigating the mechanisms of NP. This review will focus on a detailed description of rodent models of NP and provide an overview of the assessment of the sensory and emotional components of pain. A detailed inventory will be made to examine spinal mechanisms involved in NP-induced hyperexcitability and underlying the current pharmacological approaches used in clinics with the possibility to present new avenues for future treatment. The success of pre-clinical studies in this area of research depends on the choice of the relevant model and the appropriate test based on the objectives of the study.

Enkephalinergic Circuit Involved in Nociceptive Modulation in the Spinal Dorsal Horn

Enkephalin (ENK) has been implicated in pain modulation within the spinal dorsal horn (SDH). Revealing the mechanisms underlying ENK analgesia entails the anatomical and functional knowledge of spinal ENK-ergic circuits. Herein, we combined morphological and electrophysiological studies to unravel local ENK-ergic circuitry within the SDH. First, the distribution pattern of spinal ENK-ergic neurons was observed in adult preproenkephalin (PPE)-GFP knock-in mice. Next, the retrograde tracer tetramethylrhodamine (TMR) or horseradish peroxidase (HRP) was injected into the parabrachial nucleus (PBN) in PPE-GFP mice. Immunofluorescent staining showed I-isolectin B4 (IB4) labeled non-peptidergic afferents were in close apposition to TMR-labeled PBN-projecting neurons within lamina I as well as PPE-immunoreactivity (-ir) neurons within lamina II. Some TMR-labeled neurons were simultaneously in close association with both IB4 and PPE-ir terminals. Synaptic connections of these components were further confirmed by electron microscopy. Finally, TMR was injected into the PBN in adult C57BL/6 mice. Whole-cell patch recordings showed that δ-opioid receptor (DOR) agonist, [D-Pen^2,5]-enkephalin (DPDPE, 1 µM), significantly reduced the frequency of miniature excitatory postsynaptic current (mEPSC) and decreased the activity of TMR-labeled neurons. In conclusion, spinal ENKergic neurons receive direct excitatory inputs from primary afferents, which might be directly recruited to release ENK under the condition of noxious stimuli; ENK could inhibit the glutamatergic transmission towards projecting neurons via presynaptic and postsynaptic DORs. These morphological and functional evidence may explain the mechanisms underlying the analgesic effects exerted by ENK within the SDH.

GABAB receptors modulate morphine antinociception: Pharmacological and genetic approaches

Previous studies in our laboratory showed an interaction between the GABAergic and opioid systems involved in the analgesic effect of baclofen (BAC). Furthermore, it is known that sex differences exist regarding various pharmacological responses of morphine (MOR) and they are related to an increased sensitivity to MOR effects in males. The aims of the present study were to evaluate the possible involvement of the GABA_B receptors in the antinociceptive responses induced by MOR (1, 3 and 9 mg/kg, s.c.) administration using both pharmacological (BAC 2 mg/kg, i.p.; and 2-OH-saclofen, SAC 0.3 mg/kg, intra cisterna magna) and genetic approaches (GABA_B1 knockout mice; GABA_B1 KO) in mice of both sexes. In addition, we explored the alterations in c-Fos expression of different brain areas involved in the antinociceptive effect of MOR using both approaches. The pharmacological approach showed a higher dose-dependent antinociceptive effect of MOR in male mice compared to female mice. BAC and SAC pretreatment potentiated and attenuated the antinociceptive effect of MOR, respectively, in both sexes. The genetic approach revealed a dose-dependent antinociceptive effect of MOR in the wild type mice, but not in the GABA_B1 KO mice and no sex differences were observed. Additionally, BAC and SAC pretreatment and the lack of GABA_B1 subunit of the GABA_B receptor prevented the changes observed in c-Fos expression in the cingulate cortex and nucleus accumbens of male mice. Our results suggest that the GABA_B receptors are involved in the MOR antinociceptive effect of both male and female mice.

A Particular Medullary-Spinal Inhibitory Pathway is Recruited for the Expression of Muscle Atonia During REM Sleep

Muscle atonia is a major pathognomonic sign of paradoxical sleep (PS; coined REM Sleep), during which dreams mainly occur. In the 1980s, an idiopathic syndrome called REM sleep behavior disorder (RBD) was described in patients endowed with loss of PS paralysis concomitant to abnormal movements, suggesting a dysfunction of PS networks. Another major clinical RBD feature is its prevalent phenoconversion into synucleinopathies as Parkinson's disease in a delay of 10-15 years after diagnosis. Thus, we undertook experiments in rats to disentangle brainstem networks involved in PS, including atonia. We first identified a contingent of pontine glutamate neurons recruited during PS with inputs to the ventromedial medulla (vmM) where they contact γ-aminobutyric acid (GABA)/glycine inhibitory neurons also activated during PS. Here, we further show that these vmM inhibitory neurons send efferents to somatic spinal motoneurons until lumbar levels. As reported for the pontine generator, the genetic inactivation of the vmM inhibitory neurons abolishes atonia during PS without effects on waking locomotion and is sufficient to recapitulate major RBD symptoms. These original data suggest that RBD may reflect a severe dysfunction and/or degeneration linked to a developing synucleinopathic attack targeting specifically neurons that generate PS-specific atonia.

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.

Leaky Gate Model: Intensity-Dependent Coding of Pain and Itch in the Spinal Cord

Coding of itch versus pain has been heatedly debated for decades. However, the current coding theories (labeled line, intensity, and selectivity theory) cannot accommodate all experimental observations. Here we identified a subset of spinal interneurons, labeled by gastrin-releasing peptide (Grp), that receive direct synaptic input from both pain and itch primary sensory neurons. When activated, these Grp⁺ neurons generated rarely seen, simultaneous robust pain and itch responses that were intensity dependent. Accordingly, we propose a "leaky gate" model in which Grp⁺ neurons transmit both itch and weak pain signals; however, upon strong painful stimuli, the recruitment of endogenous opioids works to close this gate, reducing overwhelming pain generated by parallel pathways. Consistent with our model, loss of these Grp⁺ neurons increased pain responses while itch was decreased. Our new model serves as an example of non-monotonic coding in the spinal cord and better explains observations in human psychophysical studies.

Sympathetic preganglionic neurons: properties and inputs

The sympathetic nervous system comprises one half of the autonomic nervous system and participates in maintaining homeostasis and enabling organisms to respond in an appropriate manner to perturbations in their environment, either internal or external. The sympathetic preganglionic neurons (SPNs) lie within the spinal cord and their axons traverse the ventral horn to exit in ventral roots where they form synapses onto postganglionic neurons. Thus, these neurons are the last point at which the central nervous system can exert an effect to enable changes in sympathetic outflow. This review considers the degree of complexity of sympathetic control occurring at the level of the spinal cord. The morphology and targets of SPNs illustrate the diversity within this group, as do their diverse intrinsic properties which reveal some functional significance of these properties. SPNs show high degrees of coupled activity, mediated through gap junctions, that enables rapid and coordinated responses; these gap junctions contribute to the rhythmic activity so critical to sympathetic outflow. The main inputs onto SPNs are considered; these comprise afferent, descending, and interneuronal influences that themselves enable functionally appropriate changes in SPN activity. The complexity of inputs is further demonstrated by the plethora of receptors that mediate the different responses in SPNs; their origins and effects are plentiful and diverse. Together these different inputs and the intrinsic and coupled activity of SPNs result in the rhythmic nature of sympathetic outflow from the spinal cord, which has a variety of frequencies that can be altered in different conditions.

The coexistence of VGluT2 and neurotensin or leu-enkephalin in the medullary dorsal horn: a confocal and electron microscopic immunohistochemical study in the rat

Neuropeptides such as neurotensin (NT), and enkephalin (ENK) in the medullary dorsal horn (MDH) are involved in excitatory synaptic transmission to modulate nociceptive information. However, morphological evidence indicating that NT or ENK coexists with glutamate in the MDH is still meager. Using fluorescent immunohistochemistry, the results showed that double labeling of NT or ENK terminals with VGluT2 is mainly concentrated in the lamina II of the MDH, and many axon terminals exhibiting NT or ENK immunoreactivity in the superficial layers of the MDH showed VGluT2 immunoreactivity. Electron microscopy confirmed the coexpression of NT or ENK and VGluT2 in axon terminals within the laminae I and II of the MDH. These axon terminals make asymmetrical synapses with immunonegative neuronal cell bodies and dendrites. The findings suggest that glutamate is coreleased with NT or ENK from axon terminals of interneurons in the superficial layers of the MDH.

Birth-date dependent arrangement of spinal enkephalinergic neurons: evidence from the preproenkephalin-green fluorescent protein transgenic mice

Enkephalin (ENK) has been postulated to play important roles in modulating nociceptive transmission, and it has been proved that ENKergic neurons acted as a critical component of sensory circuit in the adult spinal cord. Revealing the developmental characteristics of spinal ENKergic neurons will be helpful for understanding the formation and alteration of the sensory circuit under pain status. However, the relationship between the embryonic birth date and the adult distribution of ENKergic neurons has remained largely unknown due to the difficulties in visualizing the ENKergic neurons clearly. Taking advantage of the preproenkephalin-green fluorescent protein (PPE-GFP) transgenic mice in identifying ENKergic neurons, we performed the current birth-dating study and examined the spinal ENKergic neurogenesis. The ENKergic neurons born on different developmental stages and their final location during adulthood were investigated by combining bromodeoxyuridine (BrdU) incorporation and GFP labeling. The spinal ENKergic neurogenesis was restricted at E9.5 to E14.5, and fitted in the same pattern of spinal neurogenesis. Further comparative analysis revealed that spinal ENKergic neurons underwent heterogeneous characteristics. Our study also indicated that the laminar arrangement of ENKergic neurons in the superficial spinal dorsal horn depended on the neurogenesis stages. Taken together, the present study suggested that the birth date of ENKergic neurons is one determinant for their arrangement and function.

GABAergic influence on temporomandibular joint-responsive spinomedullary neurons depends on estrogen status

Sensory input from the temporomandibular joint (TMJ) to neurons in superficial laminae at the spinomedullary (Vc/C1-2) region is strongly influenced by estrogen status. This study determined if GABAergic mechanisms play a role in estrogen modulation of TMJ nociceptive processing in ovariectomized female rats treated with high- (HE) or low-dose (LE) estradiol (E2) for 2days. Superficial laminae neurons were activated by ATP (1mM) injections into the joint space. The selective GABAA receptor antagonist, bicuculline methiodide (BMI, 5 or 50μM, 30μl), applied at the site of recording greatly enhanced the magnitude and duration of ATP-evoked responses in LE rats, but not in units from HE rats. The convergent cutaneous receptive field (RF) area of TMJ neurons was enlarged after BMI in LE but not HE rats, while resting discharge rates were increased after BMI independent of estrogen status. By contrast, the selective GABAA receptor agonist, muscimol (50μM, 30μl), significantly reduced the magnitude and duration of ATP-evoked activity, resting discharge rate, and cutaneous RF area of TMJ neurons in LE and HE rats, whereas lower doses (5μM) affected only units from LE rats. Protein levels of GABAA receptor β3 isoform at the Vc/C1-2 region were similar for HE and LE rats. These results suggest that GABAergic mechanisms contribute significantly to background discharge rates and TMJ-evoked input to superficial laminae neurons at the Vc/C1-2 region. Estrogen status may gate the magnitude of GABAergic influence on TMJ neurons at the earliest stages of nociceptive processing at the spinomedullary region.

Galanin receptor-expressing dorsal horn neurons: role in nociception

Galanin, along with enkephalins and neuropeptide Y, has been hypothesized to negatively modulate nociception in the superficial dorsal horn of the spinal cord. In the present study, we sought to determine the role of presumably excitatory dorsal horn galanin receptor-expressing neurons in nociception by selectively destroying GalR1-expressing superficial dorsal horn interneurons using lumbar intrathecal injections of the targeted cytotoxin, galanin-saporin (Gal-sap). Lumbar intrathecal injection of Gal-sap (500 ng) reduced immunoperoxidase staining for GalR1 in the superficial dorsal horn without affecting primary afferent neurons in lumbar dorsal root ganglia. Lumbar intrathecal Gal-sap also: 1--reduced nocifensive reflex responding on the thermal plate at 0.3 °C, 44 °C, and 47 °C; 2--increased hot side occupancy in a thermal preference task (15 °C vs 45 °C); and, 3--decreased escape from 44 °C and 47 °C, but not 20 °C. Thus, similar to lesions of mu opiate receptor-expressing dorsal horn interneurons, selective destruction of GalR1-expressing superficial dorsal horn neurons produces heat hypo-algesia, likely due to loss of GalR1-expressing excitatory interneurons leading to reduced activation of nociceptive projection neurons in response to aversive heat. These results are different than those seen with intrathecal neuropeptide Y-saporin and suggest the potential value of selectively targeting GalR1-expressing dorsal horn neurons to control pain.

Physiological properties of enkephalin-containing neurons in the spinal dorsal horn visualized by expression of green fluorescent protein in BAC transgenic mice

Background

Enkephalins are endogenous opiates that are assumed to modulate nociceptive information by mediating synaptic transmission in the central nervous system, including the spinal dorsal horn.

Results

To develop a new tool for the identification of in vitro enkephalinergic neurons and to analyze enkephalin promoter activity, we generated transgenic mice for a bacterial artificial chromosome (BAC). Enkephalinergic neurons from these mice expressed enhanced green fluorescent protein (eGFP) under the control of the preproenkephalin (PPE) gene (penk1) promoter. eGFP-positive neurons were distributed throughout the gray matter of the spinal cord, and were primarily observed in laminae I-II and V-VII, in a pattern similar to the distribution pattern of enkephalin-containing neurons. Double immunostaining analysis using anti-enkephalin and anti-eGFP antibodies showed that all eGFP-expressing neurons contained enkephalin. Incubation in the presence of forskolin, an activator of adenylate cyclase, increased the number of eGFP-positive neurons. These results indicate that eGFP expression is controlled by the penk1 promoter, which contains cyclic AMP-responsive elements. Sections obtained from sciatic nerve-ligated mice exhibited increased eGFP-positive neurons on the ipsilateral (nerve-ligated side) compared with the contralateral (non-ligated side). These data indicate that PPE expression is affected by peripheral nerve injury. Additionally, single-neuron RT-PCR analysis showed that several eGFP positive-neurons in laminae I-II expressed glutamate decarboxylase 67 mRNA and that some expressed serotonin type 3 receptors.

Conclusions

These results suggest that eGFP-positive neurons in laminae I-II coexpress enkephalin and γ-aminobutyric acid (GABA), and are activated by forskolin and in conditions of nerve injury. The penk1-eGFP BAC transgenic mouse contributes to the further characterization of enkephalinergic neurons in the transmission and modulation of nociceptive information.

Short-term modulation at synapses between neurons in laminae II-V of the rodent spinal dorsal horn

Unitary excitatory (EPSP) and inhibitory (IPSP) postsynaptic potentials (PSPs) were evoked between neurons in Rexed's laminae (L)II-V of spinal slices from young hamsters (7-24 days old) at 27°C using paired whole cell recordings. Laminar differences in synaptic efficacy were observed: excitatory connections were more secure than inhibitory connections in LII and inhibitory linkages in LII were less reliable than those in LIII-V. A majority of connections displayed paired-pulse facilitation or depression. Depression was observed for both EPSPs and IPSPs, but facilitation was seen almost exclusively for IPSPs. There were no frequency-dependent shifts between facilitation and depression. Synaptic depression was associated with an increased failure rate and decreased PSP half-width for a majority of connections. However, there were no consistent changes in failure rate or PSP time course at facilitating connections. IPSPs evoked at high-failure synapses had consistently smaller amplitude and showed greater facilitation than low-failure connections. Facilitation at inhibitory connections was positively correlated with synaptic jitter and associated with a decrease in latency. At many connections, the paired-pulse ratio varied from trial to trial and depended on the amplitude of the first PSP; dependence was greater for inhibitory synapses than excitatory synapses. Paired-pulse ratios for connections onto neurons with rapidly adapting, "phasic" discharge to depolarizing current injection were significantly greater than for connections onto neurons with tonic discharge properties. These results are evidence of diversity in synaptic transmission between dorsal horn neurons, the nature of which may depend on the types of linkage, laminar location, and intrinsic firing properties of postsynaptic cells.

Neurochemical properties of enkephalinergic neurons in lumbar spinal dorsal horn revealed by preproenkephalin-green fluorescent protein transgenic mice

Enkephalin (ENK) has been implicated in nociceptive transmission in the spinal cord while its functional role is not clear because of difficulties in ideally visualizing ENKergic neurons. We thus developed preproenkephalin-green fluorescent protein transgenic mice to better identify ENKergic neurons. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) together with immunohistochemistry and in situ hybridization were first employed to confirm the successful transgenic manipulation and its application in showing spinal ENKergic neurons. The proportions of ENKergic neurons in the spinal cord laminae I, II, III and IV-VI among dorsal horn neurons were 15.8 +/- 3.1%, 39.5 +/- 3.3%, 11.8 +/- 1.9% and 10.7 +/- 2.1%, respectively. Double labeling with other molecules was then performed to further clarify the neurochemical properties of spinal ENKergic neurons. GABA was found to exist in 42.9 +/- 2.8% of ENKergic neurons that were mainly located in lamina I-III. The proportions of parvalbumin-, calretinin-, calbindin- and neuronal nitric oxide synthase-positive cells among the ENKergic neurons were 5.2 +/- 0.7%, 42.6 +/- 2.3%, 25.8 +/- 2.2% and 11.1 +/- 1.6%, respectively. Compared with previously findings obtained with ENK antibody labeling, this line of newly generated mice can be a reliable tool for the study of specific spinal ENKergic neuronal population.

Enkephalins, dynorphins, and beta-endorphin in the rat dorsal horn: an immunofluorescence colocalization study

To characterize neuronal pathways that release opioid peptides in the rat dorsal horn, multiple-label immunohistochemistry, confocal microscopy, and computerized co-localization measures were used to characterize opioid-containing terminals and cells. An antibody that selectively recognized beta-endorphin labeled fibers and neurons in the ventral horn as well as fibers in the lateral funiculus and lamina X, but practically no fibers in the dorsal horn. An anti-enkephalin antibody, which recognized Leu-, Met-, and Phe-Arg-Met-enkephalin, labeled the dorsolateral funiculus and numerous puncta in laminae I-III and V of the dorsal horn. An antibody against Phe-Arg-Met-enkephalin, which did not recognize Leu- and Met-enkephalin, labeled the same puncta. Antibodies against dynorphin and prodynorphin labeled puncta and fibers in laminae I, II, and V, as well as some fibers in the rest of the dorsal horn. Dynorphin and prodynorphin immunoreactivities colocalized in some puncta and fibers, but the prodynorphin antibody additionally labeled cell bodies. There was no co-localization of dynorphin (or prodynorphin) with enkephalin (or Phe-Arg-Met-enkephalin). Enkephalin immunoreactivity did not colocalize with the C-fiber markers calcitonin gene-related peptide (CGRP), substance P, and isolectin B4. In contrast, there was some colocalization of dynorphin and prodynorphin with CGRP and substance P, but not with isolectin B4. Both enkephalin and dynorphin partly colocalized with vesicular glutamate transporter 2, a marker of glutamatergic terminals. The prodynorphin-positive neurons in the dorsal horn were distinct from neurons expressing mu-opioid receptors, neurokinin 1 receptors, and protein kinase C-gamma. These results show that enkephalins and dynorphins are present in different populations of dorsal horn neurons. In addition, dynorphin is present in some C-fibers.

Centralization of noxious stimulus-induced analgesia (NSIA) is related to activity at inhibitory synapses in the spinal cord

The duration of noxious stimulus-induced antinociception (NSIA) has been shown to outlast the pain stimulus that elicited it, however, the mechanism that determines the duration of analgesia is unknown. We evaluated the role of spinal excitatory and inhibitory receptors (NMDA, mGluR(5), mu-opioid, GABA(A), and GABA(B)), previously implicated in NSIA initiation, in its maintenance. As in our previous studies, the supraspinal trigeminal jaw-opening reflex (JOR) in the rat was used for nociceptive testing because of its remoteness from the region of drug application, the lumbar spinal cord. NSIA was reversed by antagonists for two inhibitory receptors (GABA(B) and mu-opioid) but not by antagonists for either of the two excitatory receptors (NMDA and mGluR(5)), indicating that NSIA is maintained by ongoing activity at inhibitory synapses in the spinal cord. Furthermore, spinal administration of the GABA(B) agonist baclofen mimicked NSIA in that it could be blocked by prior injection of the mu-opioid receptor antagonist H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP) in nucleus accumbens. CTAP also blocked baclofen antinociception when administered in the spinal cord. We conclude that analgesia induced by noxious stimulation is maintained by activity in spinal inhibitory receptors.

Facilitatory actions of serotonin type 3 receptors on GABAergic inhibitory synaptic transmission in the spinal superficial dorsal horn

Analgesic effects of serotonin (5-hydroxytryptamine [5-HT]) type 3 (5-HT3) receptors may involve the release of gamma-aminobutyric acid (GABA) in the spinal dorsal horn. However, the precise synaptic mechanisms for 5-HT3 receptor-mediated spinal analgesia are not clear. In this study, we investigated whether GABAergic neurons in the superficial dorsal horn (SDH) express functional 5-HT3 receptors and how these 5-HT3 receptors affect GABAergic inhibitory synaptic transmission in the SDH, by using slice preparations from adult glutamate decarboxylase 67-green fluorescent protein (GAD67-GFP) knock-in mice. Tight-seal whole cell recordings from GFP-positive and -negative neurons showed that 5-HT3 receptor-specific agonist 2-methyl-serotonin (2-Me-5-HT) induced inward currents in a substantial population of both GFP-positive and -negative neurons. Additionally, we confirmed expression of 5-HT3 receptors in both types of neurons by single-cell reverse transcription-polymerase chain reaction (RT-PCR) analysis. Further, GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs)-both those evoked by electrical stimulation and those occurring spontaneously in tetrodotoxin (i.e., miniature IPSCs [mIPSCs])-were recorded from GFP-negative neurons. 2-Me-5-HT increased the amplitude of the evoked IPSCs and the frequency of mIPSCs. The amplitude of mIPSCs was not affected by 2-Me-5-HT, suggesting that 5-HT augments GABAergic synaptic transmission via presynaptic mechanisms. The present observations indicate that 5-HT3 receptors are expressed on both somadendritic regions and presynaptic terminals of GABAergic neurons and regulate GABAA receptor-mediated inhibitory synaptic transmission in the SDH. Taken together, these results provide clues for the underlying mechanisms of the antinociceptive actions of 5-HT3 receptors in the spinal dorsal horn.

Acute inflammation induces segmental, bilateral, supraspinally mediated opioid release in the rat spinal cord, as measured by mu-opioid receptor internalization

The objective of this study was to measure opioid release in the spinal cord during acute and long-term inflammation using mu-opioid receptor (MOR) internalization. In particular, we determined whether opioid release occurs in the segments receiving the noxious signals or in the entire spinal cord, and whether it involves supraspinal signals. Internalization of neurokinin 1 receptors (NK1Rs) was measured to track the intensity of the noxious stimulus. Rats received peptidase inhibitors intrathecally to protect opioids from degradation. Acute inflammation of the hind paw with formalin induced moderate MOR internalization in the L5 segment bilaterally, whereas NK1R internalization occurred only ipsilaterally. MOR internalization was restricted to the lumbar spinal cord, regardless of whether the peptidase inhibitors were injected in a lumbar or thoracic site. Formalin-induced MOR internalization was substantially reduced by isoflurane anesthesia. It was also markedly reduced by a lidocaine block of the cervical-thoracic spinal cord (which did not affect the evoked NK1R internalization) indicating that spinal opioid release is mediated supraspinally. In the absence of peptidase inhibitors, formalin and hind paw clamp induced a small amount of MOR internalization, which was significantly higher than in controls. To study spinal opioid release during chronic inflammation, we injected complete Freund's adjuvant (CFA) in the hind paw and peptidase inhibitors intrathecally. Two days later, no MOR or NK1R internalization was detected. Furthermore, CFA inflammation decreased MOR internalization induced by clamping the inflamed hind paw. These results show that acute inflammation, but not chronic inflammation, induces segmental opioid release in the spinal cord that involves supraspinal signals.

Hypothalamospinal oxytocinergic antinociception is mediated by GABAergic and opiate neurons that reduce A-delta and C fiber primary afferent excitation of spinal cord cells

Recent results implicate a new original mechanism involving oxytocin (OT), as a mediator via descending fibers of the paraventricular hypothalamic nucleus (PVN), in antinociception and analgesia. In rats electrical stimulation of the PVN or topical application of OT selectively inhibits A-delta and C fiber responses in superficial dorsal horn neurons, and this inhibition is reversed by a selective OT antagonist. However, little is known about the mechanisms and the spinal elements participating in this phenomenon. Here we show that topical application of bicuculline blocks the effects produced by PVN electrical stimulation or OT application. PVN electrical stimulation also activates a subpopulation of neurons in lamina II. These PVN-On cells are responsible for the amplification of local GABAergic inhibition. This result reinforces the suggestion that a supraspinal descending control of pain processing uses a specific neuronal pathway in the spinal cord in order to produce antinociception involving a GABAergic interneuron. Moreover, the topical administration of naloxone or a mu-opiate receptor antagonist beta-funaltrexamine only partially blocks the inhibitory effects produced by OT application or PVN electrical stimulation. Thus, this OT mechanism only involves opiate participation to a minor extent. The OT-specific, endogenous descending pathway represents an interesting mechanism to resolve chronic pain problems in special the neuropathic pain.

A transcriptional network coordinately determines transmitter and peptidergic fate in the dorsal spinal cord

Dorsal horn neurons express many different neuropeptides that modulate sensory perception like the sensation of pain. Inhibitory neurons of the dorsal horn derive from postmitotic neurons that express Pax2, Lbx1 and Lhx1/5, and diversify during maturation. In particular, fractions of maturing inhibitory neurons express various neuropeptides. We demonstrate here that a coordinate molecular mechanism determines inhibitory and peptidergic fate in the developing dorsal horn. A bHLH factor complex that contains Ptf1a acts as upstream regulator and initiates the expression of several downstream transcription factors in the future inhibitory neurons, of which Pax2 is known to determine the neurotransmitter phenotype. We demonstrate here that dynorphin, galanin, NPY, nociceptin and enkephalin expression depends on Ptf1a, indicating that these neuropeptides are expressed in inhibitory neurons. Furthermore, we show that Neurod1/2/6 and Lhx1/5, which act downstream of Ptf1a, control distinct aspects of peptidergic differentiation. In particular, the Neurod1/2/6 factors are essential for dynorphin and galanin expression, whereas the Lhx1/5 factors are essential for NPY expression. We conclude that a transcriptional network operates in maturing dorsal horn neurons that coordinately determines transmitter and peptidergic fate.

Ptf1a, Lbx1 and Pax2 coordinate glycinergic and peptidergic transmitter phenotypes in dorsal spinal inhibitory neurons

Inhibitory neurons in the dorsal horn synthesize a variety of neurotransmitters, including GABA, glycine and a set of peptides. Here we show that three transcription factors, Ptf1a, Pax2, and Lbx1, which have been reported to promote a GABAergic cell fate, also specify glycinergic and peptidergic transmitter phenotypes. First, Ptf1a appears to be a master regulator, as indicated by a requirement of Ptf1a for the expression of glycinergic marker GlyT2 and a set of peptides, including neuropeptide Y (NPY), nociceptin/orphanin FQ (N/OFQ), somatostatin (SOM), enkephalin (ENK), dynorphin (DYN) and galanin (GAL). Second, Pax2 is a downstream target of Ptf1a and controls subsets of transmitter phenotypes, including the expression of GlyT2, NPY, N/OFQ, DYN, and GAL, but is dispensable for SOM or ENK expression. Third, for Lbx1, due to neuronal cell loss at late stages, our analyses focused on early embryonic stages, and we found that Lbx1 is required for the expression of GlyT2, NPY, N/OFQ and is partially responsible for SOM expression. Our studies therefore suggest a coordinated and hierarchical specification of a variety of neurotransmitters in dorsal spinal inhibitory neurons.

5-HT(3A) receptor subunit is expressed in a subpopulation of GABAergic and enkephalinergic neurons in the mouse dorsal spinal cord

5-Hydroxytryptamine (5-HT)(3) receptors have been proposed to modulate nociception and pain responses at the spinal level. To gain insight into the cellular mechanism of 5-HT(3) receptors, we examined their expression in GABAergic and enkephalinergic (ENKergic) neurons in the spinal dorsal horn (SDH) using single-cell reverse transcription-polymerase chain reaction (RT-PCR). The glutamic acid decarboxylase (GAD)(67)-green fluorescent protein (GFP) knock-in mouse was used in which all GABAergic neurons were fluorescent. The general tissue RT-PCR results showed that 5-HT(3A) receptor subunit mRNA was present in the mouse SDH, while 5-HT(3B) receptor subunit was absent. Single-cell RT-PCR results showed that 76.2% (16/21) and 33.3% (7/21) of the total 5-HT(3A)-expressing neurons were positive for GAD(67) and preproenkephalin (PPE, a precursor of ENK), respectively. 5-HT(3A) receptor subunit was detected in 28.1% (16/57) of GABAergic neurons and 22.6% (7/31) of ENKergic neurons. About 40.4% (23/57) of GABAergic neurons expressed PPE mRNA. Of the neurons that co-express GAD(67) mRNA and PPE mRNA, about 22% expressed 5-HT(3A) mRNA. These observations indicate that 5-HT(3A) receptor co-localizes with GABA and ENK in the SDH, suggesting that serotonin may activate GABAergic and ENKergic neurons via 5-HT(3A) receptor subunit and therefore affect the release of GABA and ENK. The different cellular localization of 5-HT(3A) receptor subunit suggest the complex participation of this receptor in the inhibitory neuronal circuits of the SDH neurons.

Local circuit connections between hamster laminae III and IV dorsal horn neurons

To better understand the role of intrinsic spinal cord circuits in the integration of mechanosensory information, we studied synaptic transmission between neurons in Rexed's laminae III-IV, a major termination zone for cutaneous mechanoreceptor afferents, using dual, simultaneous whole cell electrophysiological recordings in young hamsters. Synaptic connections were detected between 32 of 106 cell pairs (linkage probability of 0.3) and were predominantly unidirectional (91%). Inhibitory connections outnumbered excitatory connections by 2:1. Amplitude of single-axon postsynaptic potentials (PSPs) was independent of postsynaptic cell input resistance. Intracellular labeling suggested that recordings were obtained from local axon interneurons. In connected cell pairs, the percentage of presynaptic action potentials that failed to evoke a postsynaptic response was 44 +/- 29%. Shape indices of PSPs suggested that synaptic contacts were widely distributed along the postsynaptic membrane. Linkage probability was unrelated to intrinsic firing properties, laminar position of the cells or the distance (<160 mum) separating them. However, PSPs in target cells following action potentials in neurons with phasic firing patterns had longer duration and lower failure rates than PSPs activated by neurons with tonic firing patterns. Thus transmission reliability at synapses between lamina III/IV interneurons overall is low, and efficacy of these connections is related to firing properties of the presynaptic cells. The observations also suggest that synaptic organization in LIII-IV is fundamentally different from the superficial dorsal horn (LI-II) where neural circuits may be composed of stereotyped units made from connections between a few functional types of neurons.

Morphology and electrophysiological properties of hamster spinal dorsal horn neurons that express VGLUT2 and enkephalin

The excitatory amino acid glutamate mediates transmission at spinal synapses, including those formed by sensory afferent fibers and by intrinsic interneurons. The identity and physiological properties of glutamatergic dorsal horn neurons are poorly characterized despite their importance in spinal sensory circuits. Moreover, many intrinsic spinal glutamatergic synapses colocalize the opioid peptide enkephalin (ENK), but the neurons to which they belong are yet to be identified. Therefore, we used immunohistochemistry and confocal microscopy to investigate expression of the VGLUT2 vesicular glutamate transporter, an isoform reported in nonprimary afferent spinal synapses, and ENK in electrophysiologically identified neurons of hamster spinal dorsal horn. VGLUT2 immunoreactivity was localized in restricted fashion to axon varicosities of neurons recorded from laminae II-V, although the occurrence of immunolabeling in individual varicosities varied widely between cells (39 +/- 36%, n = 31 neurons). ENK colocalized with VGLUT2 in up to 77% of varicosities (17 +/- 21%, n = 21 neurons). The majority of neurons expressing VGLUT2 and/or ENK had axons with dense local terminations or projections consistent with propriospinal functions. VGLUT2 and ENK labeling were not correlated with cellular morphology, intrinsic membrane properties, firing patterns, or synaptic responses to sensory afferent stimulation. However, VGLUT2 expression was significantly higher in neurons with depolarized resting membrane potential. The results are new evidence for a population of dual-function dorsal horn interneurons that might provide another mechanism for limiting excitation within dorsal horn circuits during periods of strong sensory activation.

Immunohistochemical study of delta and mu opioid receptors on synaptic glomeruli with substance P-positive central terminals in chicken dorsal horn

In an attempt to clarify the mechanism underlying the regulation of the release of substance P (SP) from the central axon terminals of the synaptic glomeruli in lamina II of the dorsal horn, we examined the expression patterns of delta and mu opioid receptors (DOR and MOR) in relation to those of enkephalin (ENK) and SP in the synaptic glomeruli. DOR, MOR, ENK and SP immunoreactivities in lamina II of the dorsal horn in the chicken were examined by confocal laser scanning and electron microscopies. DOR immunoreactivity was localized in both SP-positive central terminals and peripheral elements, while MOR immunoreactivity was only localized in the peripheral elements of the synaptic glomeruli. Both of the peripheral DOR- and MOR-immunoreactive elements were shown to be vesicle-containing dendrites by electron microscopy. Dual immunohistochemistry indicated that DOR, MOR and ENK immunoreactivities were located in distinct peripheral elements. On the basis of present results, the possible roles of DOR and MOR in the regulation of the release of SP from the central axon terminals in the synaptic glomeruli are discussed.

Embryogenesis of GABAergic elements in the nervous system of Eisenia fetida (Annelida, Oligochaeta)

The appearance and development of the GABA-immunoreactive nervous elements in the central nervous system of Eisenia fetida were studied by immunocytochemistry. The nervous system originates from the neuroectoderm situated on the ventral side of the embryo. The organization of the circumpharyngeal ring starts earlier than that of the ventral cord. In the elementary ring the first GABA-immunopositive neurons can be observed (E1 stage) around the mouth. Later the cell number gradually increases and parallel to this process the elementary ring is separeted into a superficial and a deeper portion. The brain and the subesophageal ganglion will be organized from the superficial ring, while the nervous elements of the deeper ring will give rise for the first GABA-immunoreactive elements of the stomatogastric nervous system. In the early stages of the embryogenesis the immunoreactive cells of the developing brain appear solitary, while from the stage E4 they gradually are observed in groups. According to their position, these cell groups are similar to those observed in the brain of the adult earthworms. During embryogenesis the level of the ventral cord ganglia depends on their position in the ectodermal germ bands. It means, that the more organized ganglia are near the circumpharyngeal ring, mean while less developed ganglia are situated caudally from them. By the end of the embryogenesis all ganglia of the ventral cord will be equally well organized. The nerve tracts of the ganglia are built up from contra- and ipsilateral by projected fibres. From E3 stage the medial tracts, mean while from the E4 stage the lateral tracts begin to be formed. During the next stages, more and more fibres connect to the both tracts. At hatching, the development of the central nervous system of Eisenia fetida is not completed, the process is continued during the postembryonic development.

Distribution of antinociceptive adenosine A1 receptors in the spinal cord dorsal horn, and relationship to primary afferents and neuronal subpopulations

Adenosine can reduce pain and allodynia in animals and man, probably via spinal adenosine A1 receptors. In the present study, we investigate the distribution of the adenosine A1 receptor in the rat spinal cord dorsal horn using immunohistochemistry, in situ hybridization, radioligand binding, and confocal microscopy. In the lumbar cord dorsal horn, dense immunoreactivity was seen in the inner part of lamina II. This was unaltered by dorsal root section or thoracic cord hemisection. Confocal microscopy of the dorsal horn revealed close anatomical relationships but no or only minor overlap between A1 receptors and immunoreactivity for markers associated with primary afferent central endings: calcitonin gene-related peptide, or isolectin B4, or with neuronal subpopulations: mu-opioid receptor, neuronal nitric oxide synthase, met-enkephalin, parvalbumin, or protein kinase Cgamma, or with glial cells: glial fibrillary acidic protein. A few adenosine A1 receptor positive structures were double-labeled with alpha-amino-3-hydroxy-5-methyl-4-isoaxolepropionic acid glutamate receptor subunits 1 and 2/3. The results indicate that most of the adenosine A1 receptors in the dorsal horn are located in inner lamina II postsynaptic neuronal cell bodies and processes whose functional and neurochemical identity is so far unknown. Many adenosine A1 receptor positive structures are in close contact with isolectin B4 positive C-fiber primary afferents and/or postsynaptic structures containing components of importance for the modulation of nociceptive information.

GABA- and glycine-like immunoreactivity in axons and dendrites contacting the central terminals of rapidly adapting glabrous skin afferents in rat spinal cord

The object of the present study was to determine the nature and distribution of synaptic contacts on the terminals of rapidly adapting mechanosensory afferents innervating the glabrous skin of the rat foot. Afferents were physiologically characterized by intracellular recording, before injection with neurobiotin and preparation for electron microscopy. Axon terminals were serially sectioned and immunolabeled with antibodies against GABA and glycine using a postembedding immunogold method. Afferent boutons in lamina III were often surrounded by several presynaptic axons and postsynaptic dendrites (thus forming type II glomeruli), while boutons in laminae IV-V had only simple, nonglomerular interactions. In both regions triadic synaptic arrangements where presynaptic interneurons contact both afferent boutons and their postsynaptic dendrites were present in 50-75% of boutons. Approximately three-quarters of presynaptic axons were immunoreactive for both GABA and glycine and most of the remainder for GABA alone. Most postsynaptic dendrites were not immunoreactive. Comparisons are made with information from similar studies of other rat and cat afferents conducting in the Aalphabeta range. This demonstrates that although the principles of control may be similar for cutaneous afferents of this type there are significant differences between cutaneous and 1a muscle afferents in the rat. There are also differences in detail between the interactions of afferents of the same modality in rat and cat; in the rat there are greater numbers of presynaptic axons per bouton and a greater proportion of boutons receive axo-axonic contacts and are involved in synaptic triads.

Opioid peptides modulate the response of neurons of the superficial laminae of the rat spinal dorsal horn to GABA

The modulatory effects of methionine-enkephalin (M-ENK) and selective opioid-receptor agonists on GABA-activated whole-cell currents were investigated in neurons acutely dissociated from the superficial laminae of the rat spinal dorsal horn using nystatin-perforated patch recording configuration under voltage-clamp conditions. The results show that: (1). GABA acted on GABA(A) receptors and elicited inward Cl(-) currents (I(GABA)) at -60 mV; (2). M-ENK depressed I(GABA) in approximately 65% of the tested neurons and potentiated I(GABA) in approximately 15% of the neurons tested; (3). the agonists of mu-, kappa-, and delta-opioid receptors-[D-AIa(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), dynorphin-A (Dyn-A), and [D-Pen(2),D-Pen(5)]-enkephalin (DPDPE) also depressed the I(GABA), and the order of agonist potency was DAMGO>Dyn-A>DPDPE; and (4) naloxone blocked the inhibitory effects of M-ENK on I(GABA). The antagonists of mu-, kappa-, and delta-opioid receptors-beta-funaltrexamine (beta-FNA), nor-binaltorphimine (nor-BNI), and naltrindole (NTI) prevented the DAMGO-, Dyn-A-, and DPDPE-induced depression of I(GABA). The results suggest that M-ENK downregulates I(GABA) principally through mu- and kappa-opioid receptors, and thus exerts its modulating effects indirectly on the transmission of noxious information at the spinal level.

Antinociceptive potentiation and attenuation of tolerance by intrathecal electric stimulation in rats

We tested whether intrathecal electric stimulation would reduce the tolerance to chronic morphine use and the severity of precipitated morphine withdrawal. Rats received intrathecal electrode catheter implantation and a continuous intrathecal infusion of morphine (2 nmol/h) or saline for 7 days. Intrathecal electric stimulations (0, 20, or 200 V) were performed once daily during the same period. Daily tail-flick and intrathecal morphine challenge tests were performed to assess the effect of intrathecal electric stimulation on antinociception and tolerance to morphine. Naloxone withdrawal (2 mg/kg) was performed to assess morphine dependence, and changes in spinal neurotransmitters were monitored by microdialysis. The antinociceptive effect of intrathecal morphine was increased by 200 V of electric stimulation. The magnitude of tolerance was decreased in the rats receiving the 2 nmol/h infusion with 200 V of intrathecal electric stimulation compared with the control group (morphine 2 nmol/h alone) (AD(50), 13.6 vs 124.7 nmol). The severity of naloxone-induced withdrawal was less in the rats receiving 200 V of stimulation. Intrathecal stimulation thus enhances analgesia and attenuates naloxone-induced withdrawal symptoms in rats receiving chronic intrathecal morphine infusion. Increases in spinal glycine release may be the underlying mechanism. This method may merit further investigation in the context of the long-term use of intrathecal opioids for controlling chronic pain.

IMPLICATIONS

Control of chronic pain is a major health problem. We show here that direct electrical stimulation of the spinal cord in rats enhances analgesia and attenuates naloxone-induced withdrawal symptoms. This may warrant further investigation in the context of long-term use of intrathecal opioids for controlling chronic pain.

Immunocytochemical localization of glutamic acid decarboxylase in physiologically identified interneurons of hamster spinal laminae III-V

Neurons in Rexed's laminae III-V of an isolated spinal cord-skin patch preparation from hamsters were recorded in whole-cell mode and stained intracellularly with biocytin. Evidence of inhibitory synaptic function was obtained via post-hoc immunofluorescent labeling with a monoclonal antibody directed against an axon terminal isoform of brain glutamic acid decarboxylase. For a subset of neurons, examination with laser scanning confocal microscopy revealed punctate accumulations of glutamic acid decarboxylase immunoreactivity within axon enlargements (1-3 microm diameter), as imaged in single optical sections and confirmed by subsequent optical scans in the orthogonal plane. Axons of glutamic acid decarboxylase-immunoreactive neurons were found to exhibit dense local terminations overlapping the soma and dendrites or bifurcated into lengthy rostrocaudal daughter branches ventral to the cell body. The degree and uniformity of immunolabeling in axonal enlargements varied considerably, even amongst boutons belonging to the same cell. Glutamic acid decarboxylase-positive neurons received input from myelinated (A) afferent fibers and responded to natural stimuli appropriate for activating responses in low threshold mechanoreceptors. These results provide evidence that two different populations of GABAergic inhibitory interneurons are involved in local and intersegmental circuits that mediate integration of mechanosensory information in the deep spinal dorsal horn.

The expression of vesicular glutamate transporters VGLUT1 and VGLUT2 in neurochemically defined axonal populations in the rat spinal cord with emphasis on the dorsal horn

Two vesicular glutamate transporters, VGLUT1 and VGLUT2, have recently been identified, and it has been reported that they are expressed by largely nonoverlapping populations of glutamatergic neurons in the brain. We have used immunocytochemistry with antibodies against both transporters, together with markers for various populations of spinal neurons, in an attempt to identify glutamatergic interneurons in the dorsal horn of the mid-lumbar spinal cord of the rat. The great majority (94-100%) of nonprimary axonal boutons that contained somatostatin, substance P or neurotensin, as well as 85% of those that contained enkephalin, were VGLUT2-immunoreactive, which suggests that most dorsal horn neurons that synthesize these peptides are glutamatergic. In support of this, we found that most somatostatin- and enkephalin-containing boutons (including somatostatin-immunoreactive boutons that lacked calcitonin gene-related peptide and were therefore probably derived from local interneurons) formed synapses at which AMPA receptors were present. We also investigated VGLUT expression in central terminals of primary afferents. Myelinated afferents were identified with cholera toxin B subunit; most of those in lamina I were VGLUT2-immunoreactive, whereas all those in deeper laminae were VGLUT1-immunoreactive, and some (in laminae III-VI) appeared to contain both transporters. However, peptidergic primary afferents that contained substance P or somatostatin (most of which are unmyelinated), as well as nonpeptidergic C fibres (identified with Bandeiraea simplicifolia isolectin B4) showed low levels of VGLUT2-immunoreactivity, or were not immunoreactive with either VGLUT antibody. As all primary afferents are thought to be glutamatergic, this raises the possibility that unmyelinated afferents, most of which are nociceptors, express a different vesicular glutamate transporter.

Human NT2 neurons express a large variety of neurotransmission phenotypes in vitro

The NT2 cell line, which was derived from a human teratocarcinoma, exhibits properties that are characteristic of a committed neuronal precursor at an early stage of development. NT2 cells can be induced by retinoic acid to differentiate in vitro into postmitotic central nervous system (CNS) neurons (NT2-N cells). The commitment of NT2-N cells to a stable neuronal phenotype is irreversible. Because it may be possible to transplant these human neurons to compensate for neuronal loss after traumatic injuries or neurodegenerative diseases of the CNS, knowledge of their phenotype is essential. This study aimed to characterize in detail the neurotransmission phenotype of NT2-N cells by using immunocytochemical methods. Single peroxidase immunostaining demonstrated that NT2-N cells expressed the gamma-aminobutyric acidergic (GABAergic), catecholaminergic, and cholinergic phenotypes to a large extent and expressed the serotonergic phenotype to a minor extent. NT2-N cells also expressed different neuropeptides, such as neuropeptide Y, oxytocin, vasopressin, calcitonin gene-related peptide, and Met- and Leu-enkephalin. Double fluorescence immunostaining further indicated that a large number of NT2-N cells could express GABA and another neurotransmitter or neuropeptide at the same time. Finally, electron microscopy demonstrated that these NT2 neurons elaborate classical synaptic contacts. The multipotentiality of these neurons, combined with their apparent functionality, suggests that they may represent useful material for a variety of therapeutic approaches aimed at replacing dead neurons after neurodegenerative diseases or lesions of the CNS.

GABA- and glycine-immunoreactive terminals contacting motoneurons in lamprey spinal cord

Double postembedding GABA- and glycine-immunostaining was performed on the lamprey (Lampetra fluviatilis) spinal cord after previous HRP labeling of motoneurons. Immunopositive boutons contacting motoneurons were counted and distinguished as GABA (39%), glycine (30%) and both GABA+glycine-immunopositive (31%). Densely-packed, flattened synaptic vesicles were only observed in glycine-immunopositive boutons while GABA-immunoreactive and GABA+glycine-immunoreactive boutons contained rounded or oval synaptic vesicles. Dense-core vesicles of different diameters were associated with conventional synaptic vesicles in 74% of GABA-only-immunopositive boutons, 50% of double GABA+glycine-immunopositive boutons, but were only observed in 9% of glycine-only-immunopositive boutons. The presence of terminals immunoreactive to either GABA or glycine contacting the motoneurons suggests that there is a morphological substrate for both GABAergic and glycinergic postsynaptic inhibition of motoneurons in the lamprey spinal cord.

The interaction of antinociceptive effects of morphine and GABA receptor agonists within the rat spinal cord

Previous reports indicate that there may be an interaction between gamma-aminobutyric acid receptors and opioid receptors systems within the spinal cord, the antinociceptive effects of which have not been elucidated. We examined the effects of intrathecally coadministered morphine and muscimol or baclofen on somatic and visceral antinociception in rats. The tail flick (TF) test and colorectal distension (CD) test were used to assess somatic and visceral antinociceptive effects, respectively. Motor function was also assessed. The measurements were performed for 180 min after the intrathecal administration of morphine (0.1-10 micrograms), muscimol (0.2-10 micrograms), baclofen (0.03-1 microgram), combination of morphine and muscimol or baclofen, or saline. Morphine, muscimol, or baclofen increased both TF latency and CD threshold in a dose-dependent fashion. Although morphine 0.1 microgram, muscimol 0.2 microgram, or baclofen 0.03 microgram alone did not significantly increase TF latency and CD threshold, the combination of morphine 0.1 microgram and muscimol 0.2 microgram or baclofen 0.03 microgram significantly increased both TF latency and CD threshold. The coadministration of muscimol or baclofen increased the antinociceptive effects of morphine in intensity and duration. None of the rats showed motor dysfunction after the coadministration of morphine and muscimol 0.2 microgram, although muscimol produced motor paralysis of the lower limbs in a dose-dependent fashion. Those results suggest a clinical relevance of the coadministration of mu-opioids and GABA receptor agonists for pain control.

IMPLICATIONS

We examined the antinociceptive interaction between morphine and muscimol or baclofen at the spinal level in rats. Intrathecal muscimol or baclofen potentiated both somatic and visceral antinociceptive effects of morphine.

Electron microscopic analysis of gamma-aminobutyric acid and glycine colocalization in rat trigeminal subnucleus caudalis

Postembedding immunogold methods were used to examine the distribution of gamma-aminobutyric acid (GABA) and glycine and especially their colocalization in glomerular neuronal profiles adjacent to trigeminal primary afferent profiles in lamina II of rat subnucleus caudalis. We found that 60% of the profiles adjacent to the trigeminal primary afferent terminals exhibited colocalization of GABA and glycine. GABA alone was found to localize in 17% of the adjacent profiles. Glycine alone was found to localize in 18% of the adjacent profiles. Of interest, 10% of the trigeminal primary afferent fibers showed glycine localization. All the profiles with colocalization of GABA and glycine were identified as presynaptic axonal terminals, suggesting a possible cumulative effect by these two inhibitory neurotransmitters in presynaptic inhibition. These findings show that GABA and glycine colocalize in a subpopulation of presynaptic axonal terminals within lamina II of the subnucleus caudalis. The possible origins of these axons are discussed, as well as their potential involvement in presynaptic inhibition of orofacial nociception.

Morphological substrates underlying opioid, epinephrine and gamma-aminobutyric acid inhibitory actions in the rat locus coeruleus

The locus coeruleus (LC) has been implicated in attentional processes related to orienting behaviors, learning and memory, anxiety, stress, the sleep-wake cycle, and autonomic control, as well as to contributing to the affective state. Direct activation of LC neurons causes desynchronization of the electroencephalogram, suggesting that the LC is an important modulator of the behavioral state. The LC has been an intensely studied neuronal system, as the physiology and pharmacology of this nucleus is well understood. This is mainly because of the similarity in neurochemical composition of LC cells which all contain norepinephrine in the rat. However, the homogeneity in neurotransmitter content in LC neurons is sharply contrasted by the heterogeneity of neurochemicals found in its afferent processes. Among these are axon terminals that contain inhibitory and excitatory amino acids, monoamines, and neuropeptides, many of which have been shown to exert differential physiological effects on LC discharge activity. Although much attention has focused on physiological activation of LC neurons, substantial evidence indicates that diverse afferents prominently inhibit noradrenergic cellular activity. Such inhibitory neurochemicals, which arise from local and extrinsic sources, include gamma-aminobutyric acid (GABA) and epinephrine as well as the neuropeptides methionine5-enkephalin and leucine5-enkephalin. Inhibitory transmission in the LC has widespread implications for norepinephrine release at diverse postsynaptic targets, and clinically useful pharmacological agents such as clonidine, an alpha2 adrenergic receptor agonist that potently inhibits the firing of LC neurons, alleviate some negative physical symptoms observed following withdrawal from opiates. In the present review, the synaptic and functional organization of selected inhibitory-type neurotransmitters in the LC obtained from immunoelectron microscopic data will be discussed.

Oxytocin modulates glutamatergic synaptic transmission between cultured neonatal spinal cord dorsal horn neurons

The functional characteristics of binding sites for the neuropeptide oxytocin (OT) detected by radioautography in laminae I and II of the dorsal horn (DH) and on cultured neonatal DH neurons were studied on the latter using perforated patch-clamp recordings. The neurons were identified by their spike discharge properties and on the basis of the presence of met-enkephalin-like and glutamate decarboxylase-like immunoreactivities. OT (100 nM) never induced any membrane current at a holding potential of -60 mV but increased the frequency of spontaneously occurring AMPA receptor-mediated EPSCs or the mean amplitude of electrically evoked EPSCs in a subset (35%) of neurons. The frequency of miniature EPSCs (m-EPSCs) recorded in the presence of 0.5 microM tetrodotoxin was also increased by OT (100 nM) without any change in their mean amplitude, indicating an action at a site close to the presynaptic terminal. The decay kinetics of any type of EPSC were never modified by OT. The effect of OT was reproduced by [Thr4, Gly7]-OT (100 nM), a selective OT receptor agonist, and blocked by d(CH2)5-[Tyr(Me)2,Thr4,Tyr-NH29]-ornithine vasotocin (100 nM), a specific OT receptor antagonist. Reducing the extracellular Ca2+ concentration from 2.5 to 0.3 mM in the presence of Cd2+ (100 microM) reversibly blocked the effect of OT on m-EPSCs. The OT receptors described here may represent the substrate for modulatory actions of descending hypothalamo-spinal OT-containing pathways on the nociceptive system.

Electrophysiological properties of cultured neonatal rat dorsal horn neurons containing GABA and met-enkephalin-like immunoreactivity

We have developed a culture of neurons dissociated from the most superficial laminae of the neonatal rat spinal cord dorsal horn (DH). By using the perforated patch-clamp technique, we distinguished four types of neurons based on their firing properties in response to intracellular injection of 900 ms lasting current pulses. Type 1 neurons were characterized by a tonic firing. Type 2 neurons displayed marked spike accommodation and fired brief (<500 ms) bursts of action potentials, whereas type 3 neurons fired a single spike. Type 4 neurons exhibited different types of firing patterns, but all of them possessed a time-dependent inwardly rectifying current activated by membrane hyperpolarization. Met-enkephalin-like immunoreactivity (met-ENK-LI) and glutamic acid decarboxylase-like immunoreactivity (GAD-LI) were colocalized in 42% of the neurons (n = 59), which were previously identified electrophysiologically. Type 1-4 neurons represented respectively 4, 64, 20, and 12% of the population of neurons colocalizing met-ENK-LI and GAD-LI. We conclude that the electrophysiological properties of DH neurons present in our cultures are similar to those described in acute slice or hemisected spinal cord preparations and that met-ENK-LI and GABA-LI are preferentially colocalized in type 2 neurons.

GABAergic synapses on mu-opioid receptor-expressing neurons in the superficial dorsal horn: an electron microscope study in the cat spinal cord

A double-immunocytochemical electron microscope study was performed in the cat to examine whether GABAergic axons might be in synaptic contact with spinal neurons expressing mu-opioid receptor (MOR) in laminae I and II of the spinal dorsal horn at the lumbar cord segments. Structures showing MOR-like immunoreactivity (-LI) and those showing GABA-LI were labeled, respectively, with diaminobenzidine/peroxidase-reaction products and immunogold particles. Approximately one-third of dendritic profiles with MOR-LI in laminae I and II were postsynaptic to axon terminals with GABA-LI; about one-fourth of somatic profiles with MOR-LI were also postsynaptic to axon terminals with GABA-LI. The results suggest that activation of MOR on postsynaptic neurons may modulate effects which are induced by GABA released from presynaptic neurons.

The pharmacology of excitatory and inhibitory amino acid-mediated events in the transmission and modulation of pain in the spinal cord

1. The aim of this review is to consider the relative roles of inhibitory and excitatory amino acid receptor-mediated events in the processes leading to pain transmission in the spinal cord. 2. Emphasis will be on the roles of the inhibitory and excitatory amino acids, GABA and glutamate, and how the relative balance between activity in these systems appears to determine the level of pain transmission. 3. The N-methyl-D-aspartate (NMDA) receptor for glutamate has been implicated in the generation and maintenance of central (spinal) states of hypersensitivity. It has been shown that activation of this receptor underlies wind-up, whereby the level of transmission of noxious messages is potentiated. Antagonists at this receptor-channel complex prevent or block enhanced (hyperalgesic) pain states induced by tissue damage, inflammation, nerve damage and ischemia. 4. Information concerning amplification systems in the spinal cord, such as the NMDA receptor, is a step toward understanding why and how a painful response is not always matched to the stimulus. Such events have parallels with other plastic events such as long-term potentiation (LTP) in the hippocampus. 5. However, the roles of inhibitory transmitter systems can also change insofar as opioid, adenosine and GABA transmission in the spinal cord can vary in different pain states. 6. Changes in GABA systems have been well-documented and discussion will center on whether this has clinical implications. 7. In addition to behavioral and electrophysiological approaches to the pharmacology of pain the current status of the use of markers of early onset genes such as c-fos, as monitors of activity, will be discussed. 8. Hyperalgesia would appear to be balanced by inhibitions during inflammatory conditions but not in neuropathic states, pains due to nerve damage. In the latter case, events reminiscent of LTP may predominate, whereas they are held in check by inhibitions under conditions of inflammation.

Electron microscopic evidence for coexistence of leucine5-enkephalin and gamma-aminobutyric acid in a subpopulation of axon terminals in the rat locus coeruleus region

We recently described ultrastructural evidence for morphologically heterogeneous axon terminals containing the endogenous opioid peptide, methionine5-enkephalin (ENK), that formed synapses with neurons containing the catecholamine synthesizing enzyme, tyrosine hydroxylase, in the locus coeruleus (LC) of the rat brain. The morphological characteristics of these terminals suggested that ENK may be co-localized with either an excitatory or inhibitory amino acid. To further test this hypothesis, we combined immunogold-silver localization of gamma-aminobutyric acid (GABA) and immunoperoxidase labeling for ENK in single sections through the LC, in the present study, to determine whether ENK and GABA were contained within single axon terminals. Light microscopic analysis of ENK and GABA immunoreactivities in the LC indicated that both transmitters were enriched in the dorsal pons. Although electron microscopy revealed that ENK and GABA were located primarily in axon terminals, some dendrites also contained immunolabeling for GABA. The dense core vesicles were consistently the most immunoreactive in ENK containing axon terminals and were identified toward the periphery of the axon terminal distal to the synaptic specialization. Axon terminals containing either ENK or GABA immunoreactivities contained pleomorphic vesicles as well as large dense core vesicles, varied in size and formed heterogeneous types of synaptic specializations (i.e. asymmetric vs. symmetric). Approximately 38% (n = 76) of the axon terminals containing ENK immunoreactivity (n = 200) also contained GABA. Some axon terminals containing peroxidase labeling for ENK (22%; n = 44) converged on common targets with GABA-labeled axon terminals. Finally, a few ENK-labeled axon terminals (14%; n = 28) formed asymmetric (excitatory-type) synapses with dendrites containing gold-silver labeling for GABA. The results, therefore, indicate that the opioid peptide, ENK, and the inhibitory amino acid, GABA, may influence LC neurons by concerted actions via (1) release from a common axon terminal, and (2) via separate sets of afferents converging on similar portions of the plasmalemma of target neurons. Furthermore, these studies also suggest a cellular substrate for opioid inhibition of LC neurons via activation (i.e. asymmetric synapses) of inhibitory GABAergic neurons. Future studies are required to determine whether the receptive sites for ENK and GABA are located at similar sites on the plasma membranes of LC neurons pre- or postsynaptically and whether there is differential release of either transmitter from single terminals in the LC.

GABA-like immunoreactivity in the thalamic nucleus submedius of the cat

The distribution of GABAergic elements and their synaptic contacts in the nucleus submedius, a specific nociceptive relay in the medial thalamus of the cat, was studied using light and electron-microscopic postembedding immunohistochemical methods. About one-fourth of the neurons in nucleus submedius were GABA immunoreactive. These neurons were generally smaller than the unlabeled neurons and are probably local circuit neurons. Electron microscopy showed GABA immunoreactivity in two types of vesicle-containing profiles, F-terminals and presynaptic dendrites. F-terminals formed simple synapses with the dendrites of presumed thalamocortical relay cells. Presynaptic dendrites were involved in more complex synaptic arrangements that included ascending trigeminothalamic and spinothalamic tract terminals and thalamocortical relay cell dendrites. Analysis of single sections showed that about 40% of the trigeminothalamic and spinothalamic tract terminals, identified by anterograde transport of horseradish peroxidase, were presynaptic to GABAergic presynaptic dendrites. These results show that GABAergic neurons are frequent in nucleus submedius and that the GABAergic elements make synaptic connections similar to those described for other sensory relay nuclei, including the somatosensory ventroposterior nucleus. This suggests that GABAergic mechanisms play an important role in the processing of nociceptive and thermoreceptive information.