Distribution of and organisation of dorsal horn neuronal cell bodies that possess the muscarinic m2 acetylcholine receptor

Histamine production by the gut microbiota induces visceral hyperalgesia through histamine 4 receptor signaling in mice

The gut microbiota has been implicated in chronic pain disorders, including irritable bowel syndrome (IBS), yet specific pathophysiological mechanisms remain unclear. We showed that decreasing intake of fermentable carbohydrates improved abdominal pain in patients with IBS, and this was accompanied by changes in the gut microbiota and decreased urinary histamine concentrations. Here, we used germ-free mice colonized with fecal microbiota from patients with IBS to investigate the role of gut bacteria and the neuroactive mediator histamine in visceral hypersensitivity. Germ-free mice colonized with the fecal microbiota of patients with IBS who had high but not low urinary histamine developed visceral hyperalgesia and mast cell activation. When these mice were fed a diet with reduced fermentable carbohydrates, the animals showed a decrease in visceral hypersensitivity and mast cell accumulation in the colon. We observed that the fecal microbiota from patients with IBS with high but not low urinary histamine produced large amounts of histamine in vitro. We identified Klebsiella aerogenes, carrying a histidine decarboxylase gene variant, as a major producer of this histamine. This bacterial strain was highly abundant in the fecal microbiota of three independent cohorts of patients with IBS compared with healthy individuals. Pharmacological blockade of the histamine 4 receptor in vivo inhibited visceral hypersensitivity and decreased mast cell accumulation in the colon of germ-free mice colonized with the high histamine-producing IBS fecal microbiota. These results suggest that therapeutic strategies directed against bacterial histamine could help treat visceral hyperalgesia in a subset of patients with IBS with chronic abdominal pain.

Neuron-specific responses to acetylcholine within the spinal dorsal horn circuits of rodent and primate

Excitatory and inhibitory neurotransmission within the spinal dorsal horn is tightly controlled to regulate transmission of nociceptive signals to the brain. One aspect of this control is modulation of neuronal activity through cholinergic signaling. Nociceptive neurons in the dorsal horn express both nicotinic and muscarinic cholinergic receptors and activation of these receptors reduces pain in humans, while inhibition leads to nociceptive hypersensitivity. At a cellular level, acetylcholine (ACh) has diverse effects on excitability which is dependent on the receptor and neuronal subtypes involved. In the present study we sought to characterize the electrophysiological responses of specific subsets of lamina II interneurons from rat and marmoset spinal cord. Neurons were grouped by morphology and by action potential firing properties. Whole-cell voltage-clamp recordings from lamina II dorsal horn neurons of adult rats showed that bath applied acetylcholine increased, decreased or had no effect on spontaneous synaptic current activity in a cell-type specific manner. ACh modulated inhibitory synaptic activity in 80% of neurons, whereas excitatory synaptic activity was affected in less than 50% of neurons. In whole-cell current clamp recordings, brief somatic application of ACh induced cell-type specific responses in 79% of rat lamina II neurons, which included: depolarization and action potential firing, subthreshold membrane depolarization, biphasic responses characterized by transient depolarization followed by hyperpolarization and membrane hyperpolarization alone. Similar responses were seen in marmoset lamina II neurons and the properties of each neuron group were consistent across species. ACh-induced hyperpolarization was blocked by the muscarinic antagonist atropine and all forms of acetylcholine-induced depolarization were blocked by the nicotinic antagonist mecamylamine. The cholinergic system plays an important role in regulating nociception and this study contributes to our understanding of how circuit activity is controlled by ACh at a cellular level in primate and rodent spinal cord.

Rhythmogenic networks are potently modulated by activation of muscarinic acetylcholine receptors in the rodent spinal cord

Electrical stimulation of the spinal cord is a potent means for activating mammalian stepping in the absence of the descending control from the brain. Previously, we have shown that stimulation of pain delivering (Aδ) sacrocaudal afferents (SCA) has a powerful capacity to activate the sacral and lumbar rhythmogenic networks in the neonatal rodent spinal cord. Relatively little is known about the neural pathways involved in activation of the locomotor networks by Aδ afferents, on their mechanism of action and on the possibility to modulate their activity. We have shown that elevation of the endogenous level of acetylcholine at the sacral cord by blocking cholinesterase could modulate the SCA-induced locomotor rhythm in a muscarinic receptor-dependent mechanism. Here, we review these and more recent findings and report that controlled stimulation of SCA in the presence of muscarine is a potent activator of the locomotor network. The possible mechanisms involved in the muscarinic modulation of the locomotor rhythm are discussed in terms of the differential projections of sacral relay neurons, activated by SCA stimulation, to the lumbar locomotor rhythm generators, and to their target motoneurons. Altogether, our studies show that manipulations of cholinergic networks offer a simple and powerful means to control the activity of locomotor networks in the absence of supraspinal control.

Balanced cholinergic modulation of spinal locomotor circuits via M2 and M3 muscarinic receptors

Neuromodulation ensures that neural circuits produce output that is flexible whilst remaining within an optimal operational range. The neuromodulator acetylcholine is released during locomotion to regulate spinal motor circuits. However, the range of receptors and downstream mechanisms by which acetylcholine acts have yet to be fully elucidated. We therefore investigated metabotropic acetylcholine receptor-mediated modulation by using isolated spinal cord preparations from neonatal mice in which locomotor-related output can be induced pharmacologically. We report that M2 receptor blockade decreases the frequency and amplitude of locomotor-related activity, whilst reducing its variability. In contrast, M3 receptor blockade destabilizes locomotor-related bursting. Motoneuron recordings from spinal cord slices revealed that activation of M2 receptors induces an outward current, decreases rheobase, reduces the medium afterhyperpolarization, shortens spike duration and decreases synaptic inputs. In contrast, M3 receptor activation elicits an inward current, increases rheobase, extends action potential duration and increases synaptic inputs. Analysis of miniature postsynaptic currents support that M2 and M3 receptors modulate synaptic transmission via different mechanisms. In summary, we demonstrate that M2 and M3 receptors have opposing modulatory actions on locomotor circuit output, likely reflecting contrasting cellular mechanisms of action. Thus, intraspinal cholinergic systems mediate balanced, multimodal control of spinal motor output.

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.

Cholinergic mechanisms in spinal locomotion-potential target for rehabilitation approaches

Previous experiments implicate cholinergic brainstem and spinal systems in the control of locomotion. Our results demonstrate that the endogenous cholinergic propriospinal system, acting via M₂ and M₃ muscarinic receptors, is capable of consistently producing well-coordinated locomotor activity in the in vitro neonatal preparation, placing it in a position to contribute to normal locomotion and to provide a basis for recovery of locomotor capability in the absence of descending pathways. Tests of these suggestions, however, reveal that the spinal cholinergic system plays little if any role in the induction of locomotion, because MLR-evoked locomotion in decerebrate cats is not prevented by cholinergic antagonists. Furthermore, it is not required for the development of stepping movements after spinal cord injury, because cholinergic agonists do not facilitate the appearance of locomotion after spinal cord injury, unlike the dramatic locomotion-promoting effects of clonidine, a noradrenergic α-2 agonist. Furthermore, cholinergic antagonists actually improve locomotor activity after spinal cord injury, suggesting that plastic changes in the spinal cholinergic system interfere with locomotion rather than facilitating it. Changes that have been observed in the cholinergic innervation of motoneurons after spinal cord injury do not decrease motoneuron excitability, as expected. Instead, the development of a “hyper-cholinergic” state after spinal cord injury appears to enhance motoneuron output and suppress locomotion. A cholinergic suppression of afferent input from the limb after spinal cord injury is also evident from our data, and this may contribute to the ability of cholinergic antagonists to improve locomotion. Not only is a role for the spinal cholinergic system in suppressing locomotion after SCI suggested by our results, but an obligatory contribution of a brainstem cholinergic relay to reticulospinal locomotor command systems is not confirmed by our experiments.

Application of immunohistochemistry in stereology for quantitative assessment of neural cell populations illustrated in the Göttingen minipig

BACKGROUND

Stereology is the study of estimating geometric quantities. When successfully applied, the combination of immunohistochemistry (IHC) and stereology eliminates intra- and interobserver variability for cell type identification.

METHODOLOGY/PRINCIPAL FINDINGS

We propose a method to validate existing antibody based cell type markers for stereological application. Comparison was made on the 100-days-old Göttingen minipig (G-mini) neocortex between estimates of total neuron number derived from Giemsa staining using morphological criteria and immunohistochemistry-based cell counting with NeuN. The mean total neuron numbers estimated by the two staining methods were not significantly different. Estimated quantities, including glial cell number, neocortical volume, cell densities and glial-to-neuron ratio were also presented. Additionally, we assessed other commonly used glial markers and discussed how to evaluate the advantages and disadvantages of these markers for stereological estimation of cell number.

CONCLUSION/SIGNIFICANCE

The concordance in quantitative estimates of total neuron number derived from NeuN- and Giemsa-stained sections provides evidence for the sensitivity and specificity of NeuN as a neuronal marker in the G-mini. Although time-consuming, quantitative validation of IHC should always be considered in stereological studies if there is doubt of the sensitivity, specificity, or reproducibility of cell type markers. Inaccurate staining may cause both over- and underestimation of the total cell number and inflict considerable limitation when analyzing the results.

Quantitative caveats of standard immunohistochemical procedures: implications for optical disector-based designs

Immunohistochemistry is a ubiquitous technique in histology. Often, the goal of such studies is the quantification of some parameter associated with a particular antigen. When used correctly, the optical disector offers a statistically relevant approach to achieve this goal without bias from cell size, shape, or orientation. This three-dimensional counting probe is virtually embedded within the depth of the tissue section, thus avoiding sampling near the cut surfaces of the section, where cells are often lost during the cutting and subsequent processing steps. It follows that the probability that a cell could be immunolabeled should be equal throughout the section depth to correctly employ the optical disector. In this report, we demonstrate that parameters commonly used in immunohistochemistry often leave the middle of the section unlabeled. Furthermore, the degree of incomplete penetration varies among antibodies but can be overcome in some cases by extending the incubation time of the secondary antibody. The detection of this phenomenon in immunofluorescence preparations and the implications of these findings for quantitative stereology using the optical disector are discussed.

Muscarinic receptor antagonism at the spinal cord level causes inhibitory effects on male rat sexual behavior

The role of cholinergic neurotransmission in male rat sexual behavior at the brain level has been studied by several researchers. However, little is known about its role at the spinal cord level. In this study, the effects of the intrathecal (IT) administration of the muscarinic receptor antagonist subtypes (MRAs) methoctramine (Meth), tropicamide (Trop) and 4-DAMP on male rat sexual behavior were evaluated during three ejaculatory series. Meth and Trop are preferring antagonists for the M2/M4 receptor subtypes, and 4-DAMP is a preferring antagonist for the M3 receptor subtype. All the MRAs tested noticeably inhibited male rat copulatory behavior, reflected by a reduction in the number of animals engaging in sexual behavior and a gradual decrease in the number of animals able to ejaculate. Significant increases in intromission latency (IL), ejaculation latency (EL) and post-ejaculatory interval (PEI) were observed. The ranking of inhibitory potency in all recorded parameters was Meth>/=4-DAMP>Trop. In theory, the effects of Meth and Trop could be a result of interaction with M2/M4 receptors. However, given that the M2 receptor constitutes the greatest population of muscarinic receptors at all spinal cord sites and given the high affinity for Meth on M2 receptors, the high potency in the inhibitory effects of Meth is indicative of the special role of M2 spinal receptors in the implementation of this behavior. The weaker effects of Trop could be linked to the smaller population of M4 receptors in the spinal cord, but some interaction with M2 receptors is probable. Since some differences in the pattern of inhibitory response between Meth and 4-DAMP were observed in this and a previous study, a possible role for M3 receptors must be considered. The data obtained in this study confirm the facilitating effect of acetylcholine (ACh) at the spinal cord level on male rat sexual behavior through muscarinic mechanisms, with an important influence on ejaculatory processes. These data support the hypothesis of the modulating role of ACh on male rat sexual behavior at the spinal cord level.

Effects of cholinesterase inhibition in supraspinal and spinal neural pathways on the micturition reflex in rats

OBJECTIVE

To investigate whether activation of brain and spinal cholinergic pathways affects the micturition reflex in rats.

MATERIALS AND METHODS

The effects of intracerebroventricular (i.c.v.) or intrathecal (i.t.) administration of neostigmine as a cholinesterase inhibitor and oxotremorine-M (OXO-M) as a muscarinic acetylcholine receptor (mAChRs) agonist, on the micturition reflex were evaluated by infusion cystometrography (CMG) in urethane-anaesthetized untreated rats or rats pretreated with capsaicin.

RESULTS

Neostigmine injected i.c.v. increased bladder capacity (BC) and pressure threshold (PT) dose-dependently, with an increase in maximum voiding pressure (MVP) and a decrease in voiding efficiency (VE) at higher doses. Also, neostigmine injected i.t. increased the BC and PT dose-dependently without changing MVP or VE, and these effects were not apparent in capsaicin-pretreated rats. In both routes, atropine as an antagonist of mAChRs, but not mecamylamine as a nicotinic-AChR antagonist, almost completely antagonized the effects of neostigmine. The rank order of potencies of the antagonists for increasing effects of BC induced by 1 nmol of neostigmine was: pirenzepine (an M(1) mAChR antagonist) = atropine > 4-DAMP (an M(3) mAChR antagonist) " methoctramine (an M(2) mAChR antagonist) and tropicamide (an M(4) mAChR antagonist) via the i.c.v. route; and atropine > methoctramine > pirenzepine > tropicamide and 4-DAMP via the i.t. route, respectively. OXO-M injected via i.c.v. and i.t. had the same effects on BC, PT, MVP and VE as neostigmine by i.c.v. and i.t., respectively.

CONCLUSIONS

These results indicate that activation of muscarinic cholinergic mechanisms by the cholinesterase inhibitor in the brain and spinal cord can inhibit the micturition reflex, mainly by affecting afferent pathways. These mAChR-induced inhibitory effects seem to be mediated through M(1)/M(3) receptor subtypes in the brain, while in the spinal cord, the M(1)/M(2) receptor subtypes might be involved in inhibitory effects, which are mediated via inhibition of mechanoceptive C-fibre afferent pathways.

Cholinergic mechanisms involved in the pain relieving effect of spinal cord stimulation in a model of neuropathy

The mechanisms underlying the pain relieving effect of spinal cord stimulation (SCS) on neuropathic pain remain unclear. We have previously demonstrated that suppression of tactile hypersensitivity produced by SCS may be potentiated by i.t. clonidine in a rat model of mononeuropathy. Since the analgesic effect of this drug is mediated mainly via cholinergic mechanisms, a study exploring the possible involvement of the spinal cholinergic system in SCS was undertaken. The effect of SCS was assessed with von Frey filaments in rats displaying tactile hypersensitivity after partial ligation of the sciatic nerve and both SCS-responding and non-responding as well as normal rats were subjected to microdialysis in the dorsal horn. Acetylcholine (ACh) was analyzed with HPLC before, during and after SCS. SCS produced significantly increased release of ACh in the dorsal horn in rats responding to SCS whereas the release was unaffected in the non-responding animals. Furthermore, the basal release of ACh was significantly lower in nerve lesioned than in normal rats. In another group of rats it was found that the response to SCS was completely eliminated by i.t. atropine and a muscarinic M(4) receptor antagonist while a partial attenuation was produced by M(1) and M(2) antagonists. Blocking of nicotinic receptors did not influence the SCS effect. In conclusion, the attenuating effect of SCS on pain related behavior is associated with the activation of the cholinergic system in the dorsal horn and mediated via muscarinic receptors, particularly M(4,) while nicotinic receptors appear not to be involved.

The anti-inflammatory effect of peripheral bee venom stimulation is mediated by central muscarinic type 2 receptors and activation of sympathetic preganglionic neurons

The anti-inflammatory effect (AI) induced by peripheral injection of diluted bee venom (dBV) involves activation of spinal cord circuits and is mediated by catecholamine release from adrenal medulla, but the precise neuronal mechanisms involved are not fully understood. In a recent study, we demonstrated that an increase in spinal acetylcholine is involved in mediating the anti-inflammatory effect of dBV and that this mediation also involves adrenomedullary activation. The present study utilized the mouse air pouch inflammation model to evaluate the involvement of spinal acetylcholine receptors and sympathetic preganglionic neurons (SPNs) in dBV's anti-inflammatory effect (dBVAI). Intrathecal (IT) pretreatment with atropine (muscarinic cholinergic antagonist) but not hexamethonium (nicotinic cholinergic antagonist) significantly suppressed dBVAI on zymosan-evoked leukocyte migration. Subsequent experiments showed that IT pretreatment with methoctramine (a muscarinic receptor type 2; M(2) antagonist), but not pirenzepine (an M(1) antagonist) or 4-DAMP (an M(3) antagonist), suppressed the dBVAI. In addition, dBV stimulation specifically increased Fos expression in SPNs of the T7-T11, but not the T1-T6 or T12-L2 spinal cord segments, in animals with zymosan-induced inflammation. Moreover, IT methoctramine pretreatment suppressed this dBV-induced Fos expression specifically in SPNs of T7-T11 level. Peripheral sympathetic denervation using 6-hydroxydopamine (6-OHDA) treatment (which spares sympathetic adrenal medullary innervation) did not alter dBVAI. Collectively these results indicate that dBV stimulation leads to spinal cord acetylcholine release that in turn acts on spinal M(2) receptors, which via a hypothesized disinhibition mechanism activates SPNs that project to the adrenal medulla. This activation ultimately leads to the release of adrenal catecholamines that contribute to dBVAI.

Spinal neurons activated with the urethrogenital reflex in the male rat

The urethrogenital (UG) reflex is a spinal ejaculatory-like reflex. The location of spinal neurons activated by the UG reflex was examined in the male rat using the immediate early gene, c-fos. In addition, co localization of neurons containing galanin and choline acetyl transferase (ChAT) and serotonin fibers with fos-immunoreactive (fos-I) nuclei was examined. Activation of the UG reflex resulted in a significant increase in fos positive nuclei in segments T13-S1, compared to controls in which the UG reflex was not activated. Spinal circuits involved in the UG reflex include neurons relaying afferent information from the pudendal sensory nerve, in the dorsal horn and medial cord of L5-S1. Interneurons specifically activated with the UG reflex were identified in the medial, intermediate and lateral gray. A small proportion of parasympathetic and sympathetic preganglionic neurons in the intermediolateral cell column (IML) of L5-S1 and IML and medial gray of T13-L2, respectively, was activated with the UG reflex. A significant increase in the number of galanin containing neurons expressing c-fos in the medial gray of L3-L4 was also observed with the UG reflex. Serotonin fibers and varicosities were found throughout the spinal cord and were especially dense in the ventral horn, IML and medial gray. Fos activated neurons were found in close apposition to serotonin fibers in the IML and medial gray. These studies demonstrate the multisegmental intraspinal circuitry responsible for ejaculatory-like responses and demonstrate the potential involvement of galanin, acetylcholine and serotonin in mediation of the UG reflex.

Spinal dorsal horn neurone targets for nociceptive primary afferents: do single neurone morphological characteristics suggest how nociceptive information is processed at the spinal level

It has become increasingly clear that nociceptive information is signalled by several anatomically distinct populations of primary afferents that target different populations of neurones in the spinal cord. It is probable that these different systems all give rise to the sensation pain and hence, an understanding of their separate roles and the processes that they employ, may offer ways of selectively targeting pain arising from different causes. The review focuses on what is known of the anatomy of neurones in LI-III of the spinal dorsal horn that are implicated in nociception. The dendritic geometry and synaptic input of the large LI neurones that receive input from primary afferents containing substance P that express neurokinin 1 (NK(1)) receptors suggests that these neurones may monitor the extent of injury rather than the specific localisation of a discrete noxious stimulus. This population of neurones is also critically involved in hyperalgesia. In contrast neurones in LII with the morphology of stalked cells that receive primary afferent input from glomerular synapses may be more suitable for fine discrimination of the exact location of a noxious event such as a sting or parasite attack. The review focuses as far as possible on precisely defined anatomy in the belief that only by understanding these anatomical relationships will we eventually be able to interpret the complex processes occurring in the dorsal horn. The review attempts to be an accessible guide to a sometimes complex and highly specialised literature in this field.

Role of muscarinic and nicotinic cholinergic receptors in an experimental model of epilepsy-induced analgesia

The blockade of GABA-mediated Cl(-) influx with pentylenetetrazol (PTZ) was used in the present work to induce seizures in animals. The neurotransmission in the postictal period has been the focus of many studies, and there is evidence suggesting antinociceptive mechanisms following tonic-clonic seizures in both animals and men. The aim of this work was to study the involvement of acetylcholine in the antinociception induced by convulsions elicited by peripheral administration of PTZ (64 mg/kg). Analgesia was measured by the tail-flick test in eight albino Wistar rats per group. Convulsions were followed by significant increases in tail-flick latencies (TFLs) at least for 120 min of the postictal period. Peripheral administration of atropine (0.25, 1 and 4 mg/kg) caused a significant dose-dependent decrease in the TFL in seizing animals, as compared to controls. These data were corroborated by peripheral administration of mecamylamine, a nicotinic cholinergic receptor blocker, at the same doses (0.25, 1 and 4 mg/kg) used for the muscarinic cholinergic receptor antagonist. The recruitment of the muscarinic receptor was made 10 min postconvulsions and in subsequent periods of postictal analgesia, whereas the involvement of the nicotinic cholinergic receptor was implicated only after 30 min postseizures. The cholinergic antagonists caused a minimal reduction in body temperature, but did not impair baseline TFL, spontaneous exploration or motor coordination in the rotarod test at the maximal dose of 4 mg/kg. These results indicate that acetylcholine may be involved as a neurotransmitter in postictal analgesia.