Dorsal root ganglion neurons innervating pelvic organs in the mouse express tyrosine hydroxylase

Genetic tools that target mechanoreceptors produce reliable labeling of bladder afferents and altered mechanosensation

Mechanosensitive neurons are important sensors of bladder distention, but their role in urologic disease remains unclear. Our current knowledge about how disease alters bladder sensation comes from studies that focus primarily on peptidergic nociceptors, leaving our understanding of neuropeptide-negative mechanoreceptors incomplete. In this study, we found that a substantial proportion of neurofilament heavy (NFH)-positive A-fibers innervating the bladder was calcitonin gene-related peptide (CGRP)-negative, potentially representing uncharacterized mechanoreceptors. We then identified two genetic strategies that label mechanoreceptors in mouse skin and confirmed that they likewise label bladder afferents. Cre-mediated tdTomato reporter expression driven by tropomyosin receptor kinase B (TrkB), which labels Aδ mechanoreceptors in the skin, successfully labeled bladder nerve terminals. The majority of TrkB bladder afferents were CGRP-negative and NFH-positive, with more characteristic staining patterns seen at the level of the cell body. The Ret proto-oncogene (Ret) also produced robust labeling of bladder afferents, where colocalization with CGRP and NFH was consistent with multiple afferent subtypes. Because TrkB labeling was more specific for putative mechanoreceptors, we directly tested the role of TrkB neurons in bladder mechanosensation in vivo. Using an intersectional genetic strategy, we selectively ablated TrkB afferents and measured bladder responses to mechanical distention using anesthetized cystometry. Compared with controls, mice with ablated TrkB afferents required higher distention pressure to elicit voids. Interestingly, after ablation, distention also increased the frequency of nonvoiding contractions, a poorly understood phenotype of several urologic diseases. These genetic strategies comprise critical new tools to advance the study of mechanoreceptors in bladder function and urologic disease pathophysiology.NEW & NOTEWORTHY Most mechanosensitive afferents do not express markers of peptidergic nociceptors and therefore remain largely overlooked in studies of bladder dysfunction and disease. TrkB-mediated labeling of putative Aδ mechanoreceptors emerged as a valuable tool for the study of neuropeptide-negative bladder afferents with a confirmed role in bladder mechanosensation. Targeted neuronal ablation likewise validated an intersectional genetic strategy that can now directly test the role of TrkB mechanoreceptors in bladder physiology and disease.

Comprehensive mapping of sensory and sympathetic innervation of the developing kidney

The kidneys act as finely tuned sensors to maintain physiological homeostasis. Both sympathetic and sensory nerves modulate kidney function through precise neural control. However, how the kidneys are innervated during development to support function remains elusive. Using light-sheet and confocal microscopy, we generated anatomical maps of kidney innervation across development. Kidney innervation commences on embryonic day 13.5 (E13.5) as network growth aligns with arterial differentiation. Fibers are synapsin I+, highlighting ongoing axonogenesis and potential signaling crosstalk. By E17.5, axons associate with nephrons, and the network continues to expand postnatally. CGRP+, substance P+, TRPV1+, and PIEZO2+ sensory fibers and TH+ sympathetic fibers innervate the developing kidney. TH+ and PIEZO2+ axons similarly innervate the human kidney, following the arterial tree to reach targets. Retrograde tracing revealed the primary dorsal root ganglia, T10-L2, from which sensory neurons project to the kidneys. Together, our findings elucidate the temporality and neuronal diversity of kidney innervation.

The differences in the anatomy of the thoracolumbar and sacral autonomic outflow are quantitative

PURPOSE

We have re-evaluated the anatomical arguments that underlie the division of the spinal visceral outflow into sympathetic and parasympathetic divisions.

METHODOLOGY

Using a systematic literature search, we mapped the location of catecholaminergic neurons throughout the mammalian peripheral nervous system. Subsequently, a narrative method was employed to characterize segment-dependent differences in the location of preganglionic cell bodies and the composition of white and gray rami communicantes.

RESULTS AND CONCLUSION

One hundred seventy studies were included in the systematic review, providing information on 389 anatomical structures. Catecholaminergic nerve fibers are present in most spinal and all cranial nerves and ganglia, including those that are known for their parasympathetic function. Along the entire spinal autonomic outflow pathways, proximal and distal catecholaminergic cell bodies are common in the head, thoracic, and abdominal and pelvic region, which invalidates the "short-versus-long preganglionic neuron" argument. Contrary to the classically confined outflow levels T1-L2 and S2-S4, preganglionic neurons have been found in the resulting lumbar gap. Preganglionic cell bodies that are located in the intermediolateral zone of the thoracolumbar spinal cord gradually nest more ventrally within the ventral motor nuclei at the lumbar and sacral levels, and their fibers bypass the white ramus communicans and sympathetic trunk to emerge directly from the spinal roots. Bypassing the sympathetic trunk, therefore, is not exclusive for the sacral outflow. We conclude that the autonomic outflow displays a conserved architecture along the entire spinal axis, and that the perceived differences in the anatomy of the autonomic thoracolumbar and sacral outflow are quantitative.

Urothelial bladder afferents selectively project to L6/S1 levels and are more peptidergic than those projecting to the T13/L1 levels in female rats

This neuroanatomical study in four, adult, Sprague-Dawley female rats quantified the number of Urothelial (labeled by intravesical DiI dye administration) and Non-Urothelial (labeled by intraparenchymal injection of Fast blue dye) bladder primary afferent neurons (bPANs) located in the T13, L1, L6 and S1 dorsal root ganglia. Additional immunohistochemical labeling using antibodies to detect either Substance P or CGRP further characterized the bPAN samples as peptidergic or non-peptidergic. Cell counts indicated that Urothelial bPANs were more common at the L6/S1 levels and more likely to be identified as peptidergic when compared with bPANs characterized at T13/L1 levels and with Non-Urothelial bPANs. These studies provide additional evidence that at least two distinct neuronal populations, with differing localization of sensory terminals, differing peptide content, and differing projections to the central nervous system, are responsible for bladder sensation.

Catechol-O-Methyltransferase Loss Drives Cell-Specific Nociceptive Signaling via the Enteric Catechol-O-Methyltransferase/microRNA-155/Tumor Necrosis Factor α Axis

BACKGROUND & AIMS

The etiology of abdominal pain in postinfectious, diarrhea-predominant irritable bowel syndrome (PI-IBS-D) is unknown, and few treatment options exist. Catechol-O-methyltransferase (COMT), an enzyme that inactivates and degrades biologically active catecholamines, plays an important role in numerous physiologic processes, including modulation of pain perception. Our objective was to determine the mechanism(s) of how decreased colonic COMT in PI-IBS-D patients contributes to the chronic abdominal pain phenotype after enteric infections.

METHODS

Colon neurons, epithelial cells, and macrophages were procured with laser capture microdissection from PI-IBS-D patients to evaluate cell-specific colonic COMT, microRNA-155 (miR-155), and tumor necrosis factor (TNF) α expression levels compared to recovered patients (infection cleared: did not develop PI-IBS-D) and control individuals. COMT-/-, colon-specific COMT-/-, and miR-155-/- mice and human colonoids were used to model phenotypic expression of COMT in PI-IBS-D patients and to investigate signaling pathways linking abdominal pain. Citrobacter rodentium and trinitrobenzene sulfonic acid animal models were used to model postinflammatory changes seen in PI-IBS-D patients.

RESULTS

Colonic COMT levels were significantly decreased and correlated with increased visual analog scale abdominal pain ratings in PI-IBS-D patients compared to recovered patients and control individuals. Colonic miR-155 and TNF-α were increased in PI-IBS-D patients with diminished colonic COMT. COMT-/- mice had significantly increased expression of miR-155 and TNF-α in both colon tissues and dorsal root ganglia. Introduction of cV1q antibody (anti-TNF-α) into mice reversed visceral hypersensitivity after C rodentium and trinitrobenzene sulfonic acid.

CONCLUSIONS

Decreased colonic COMT in PI-IBS-D patients drives abdominal pain phenotypes via the COMT/miR-155/TNF-α axis. These important findings will allow new treatment paradigms and more targeted and personalized medicine approaches for gastrointestinal disorders after enteric infections.

Oral Sensory Neurons of the Geniculate Ganglion That Express Tyrosine Hydroxylase Comprise a Subpopulation That Contacts Type II and Type III Taste Bud Cells

Oral sensory neurons of the geniculate ganglion (GG) innervate taste papillae and buds on the tongue and soft palate. Electrophysiological recordings of these neurons and fibers revealed complexity in the number of unique response profiles observed, suggesting there are several distinct neuronal subtypes. Molecular descriptions of these subpopulations are incomplete. We report here the identification of a subpopulation of GG oral sensory neurons in mice by expression of tyrosine hydroxylase (TH). TH-expressing geniculate neurons represent 10-20% of oral sensory neurons and these neurons innervate taste buds in fungiform and anterior foliate taste papillae on the surface of the tongue, as well as taste buds in the soft palate. While 35-50% of taste buds on the tongue are innervated by these TH+ neurons, 100% of soft palate taste buds are innervated. These neurons did not have extragemmal processes outside of taste buds and did not express the mechanosensory neuron-associated gene Ret, suggesting they are chemosensory and not somatosensory neurons. Within taste buds, TH-expressing fibers contacted both Type II and Type III cells, raising the possibility that they are responsive to more than one taste quality. During this analysis we also identified a rare TH+ taste receptor cell type that was found in only 12-25% of taste buds and co-expressed TRPM5, suggesting it was a Type II cell. Taken together, TH-expressing GG oral sensory neurons innervate taste buds preferentially in the soft palate and contact Type II and Type III taste bud receptor cells.

Quantification of CGRP-immunoreactive myenteric neurons in mouse colon

Quantitative data of biological systems provide valuable baseline information for understanding pathology, experimental perturbations, and computational modeling. In mouse colon, calcitonin gene-related peptide (CGRP) is expressed by myenteric neurons with multiaxonal (Dogiel type II) morphology, characteristic of intrinsic primary afferent neurons (IPANs). Analogous neurons in other species and gut regions represent 5-35% of myenteric neurons. We aimed to quantify proportions of CGRP-immunopositive (CGRP+) myenteric neurons. Colchicine-treated wholemount preparations of proximal, mid, and distal colon were labeled for HuC/D, CGRP, nitric oxide synthase (NOS), and peripherin (Per). The pan-neuronal markers (Hu+/Per+) co-labeled 94% of neurons. Hu+/Per- neurons comprised ∼6%, but Hu-/Per+ cells were rare. Thus, quantification was based on Hu+ myenteric neurons (8576 total; 1225 ± 239 per animal, n = 7). CGRP+ cell bodies were significantly larger than the average of all Hu+ neurons (329 ± 13 vs. 261 ± 12 μm² , p < .0001). CGRP+ neurons comprised 19% ± 3% of myenteric neurons without significant regional variation. NOS+ neurons comprised 42% ± 2% of myenteric neurons overall, representing a lower proportion in proximal colon, compared to mid and distal colon (38% ± 2%, 44% ± 2%, and 44% ± 3%, respectively). Peripherin immunolabeling revealed cell body and axonal morphology in some myenteric neurons. Whether all CGRP+ neurons were multiaxonal could not be addressed using peripherin immunolabeling. However, of 118 putatively multiaxonal neurons first identified based on peripherin immunoreactivity, all were CGRP+ (n = 4). In conclusion, CGRP+ myenteric neurons in mouse colon were comprehensively quantified, occurring within a range expected of a putative IPAN marker. All Per+ multiaxonal neurons, characteristic of Dogiel type II/IPAN morphology, were CGRP+.

The impact of C-tactile low-threshold mechanoreceptors on affective touch and social interactions in mice

Affective touch is necessary for proper neurodevelopment and sociability. However, it remains unclear how the neurons innervating the skin detect affective and social behaviors. The C low-threshold mechanoreceptors (C-LTMRs), a specific population of somatosensory neurons in mice, appear particularly well suited, physiologically and anatomically, to perceive affective and social touch. However, their contribution to sociability has not been resolved yet. Our observations revealed that C-LTMR functional deficiency induced social isolation and reduced tactile interactions in adulthood. Conversely, transient increase in C-LTMR excitability in adults, using chemogenetics, was rewarding, promoted touch-seeking behaviors, and had prosocial influences on group dynamics. This work provides the first empirical evidence that specific peripheral inputs alone can drive complex social behaviors. It demonstrates the existence of a specialized neuronal circuit, originating in the skin, wired to promote interactions with other individuals.

Cisplatin Neurotoxicity Targets Specific Subpopulations and K+ Channels in Tyrosine-Hydroxylase Positive Dorsal Root Ganglia Neurons

Among the features of cisplatin chemotherapy-induced peripheral neuropathy are chronic pain and innocuous mechanical hypersensitivity. The complete etiology of the latter remains unknown. Here, we show that cisplatin targets a heterogeneous population of tyrosine hydroxylase-positive (TH+) primary afferent dorsal root ganglion neurons (DRGNs) in mice, determined using single-cell transcriptome and electrophysiological analyses. TH+ DRGNs regulate innocuous mechanical sensation through C-low threshold mechanoreceptors. A differential assessment of wild-type and vitamin E deficient TH+ DRGNs revealed heterogeneity and specific functional phenotypes. The TH+ DRGNs comprise; fast-adapting eliciting one action potential (AP; 1-AP), moderately-adapting (≥2-APs), in responses to square-pulse current injection, and spontaneously active (SA). Cisplatin increased the input resistance and AP frequency but reduced the temporal coding feature of 1-AP and ≥2-APs neurons. By contrast, cisplatin has no measurable effect on the SA neurons. Vitamin E reduced the cisplatin-mediated increased excitability but did not improve the TH+ neuron temporal coding properties. Cisplatin mediates its effect by targeting outward K+ current, likely carried through K2P18.1 (Kcnk18), discovered through the differential transcriptome studies and heterologous expression. Studies show a potential new cellular target for chemotherapy-induced peripheral neuropathy and implicate the possible neuroprotective effects of vitamin E in cisplatin chemotherapy.

Analgesia and pain: Dual effect of dopamine on the peripheral nociceptive system is dependent on D2-or D1-like receptor activation

In this study, a pharmacological approach, together with the paw pressure test, was used to investigate the role of dopamine and its receptors in the peripheral processing of the nociceptive response in mice. Initially, the administration of dopamine (5, 20, and 80 ng/paw) in the hind paw of male Swiss mice (30-40 g) promoted antinociceptive effects in a dose-dependent manner. This was considered a peripheral effect, as it did not produce changes in the nociceptive threshold of the contralateral paw. The D₂, D₃, and D₄ dopamine receptor antagonists remoxipride (4 μg/paw), U99194 (16 μg/paw), and L-745,870 (16 μg/paw), respectively, reversed the dopamine-mediated antinociception in mice with PGE₂-induced hyperalgesia. The D₁ and D₅ dopamine receptor antagonists SKF 83566 (2 μg/paw) and SCH 23390 (1.6 μg/paw), respectively, did not alter dopamine antinociception. In contrast, dopamine at higher doses (0.1, 1, and 10 μg/paw) caused hyperalgesia in the animals, and the D₁ and D₅ receptor antagonists reversed this pronociceptive effect (10 μg/paw), whereas the D₂ receptor antagonist remoxipride did not. Our data suggest that dopamine has a dual effect that depends on the dose, as it causes peripheral antinociceptive effects at small doses via the activation of D₂-like receptors and nociceptive effects at higher doses via the activation of D₁-like receptors.

Deciphering Spinal Endogenous Dopaminergic Mechanisms That Modulate Micturition Reflexes in Rats with Spinal Cord Injury

Spinal neuronal mechanisms regulate recovered involuntary micturition after spinal cord injury (SCI). It was recently discovered that dopamine (DA) is synthesized in the rat injured spinal cord and is involved in lower urinary tract (LUT) activity. To fully understand the role of spinal DAergic machinery in micturition, we examined urodynamic responses in female rats during pharmacological modulation of the DA pathway. Three to four weeks after complete thoracic SCI, the DA precursor L-DOPA administered intravenously during bladder cystometrogram (CMG) and external urethral sphincter (EUS) electromyography (EMG) reduced bladder overactivity and increased the duration of EUS bursting, leading to remarkably improved voiding efficiency. Apomorphine (APO), a non-selective DA receptor (DR) agonist, or quinpirole, a selective DR₂ agonist, induced similar responses, whereas a specific DR₂ antagonist remoxipride alone had only minimal effects. Meanwhile, administration of SCH 23390, a DR₁ antagonist, reduced voiding efficiency by increasing tonic EUS activity and shortening the EUS bursting period. Unexpectedly, SKF 38393, a selective DR₁ agonist, increased EUS tonic activity, implying a complicated role of DR₁ in LUT function. In metabolic cage assays, subcutaneous administration of quinpirole decreased spontaneous voiding frequency and increased voiding volume; L-DOPA and APO were inactive possibly because of slow entry into the CNS. Collectively, tonically active DR₁ in SCI rats inhibit urine storage and enhance voiding by differentially modulating EUS tonic and bursting patterns, respectively, while pharmacologic activation of DR₂, which are normally silent, improves voiding by enhancing EUS bursting. Thus, enhancing DA signaling achieves better detrusor-sphincter coordination to facilitate micturition function in SCI rats.

Bisphenol A affects vipergic nervous structures in the porcine urinary bladder trigone

Bisphenol A (BPA) is used in the production of plastics approved for contact with feed and food. Upon entering living organisms, BPA, as a potent endocrine disruptor, negatively affects various internal organs and regulatory systems, especially in young individuals. Although previous studies have described the neurotoxic effects of BPA on various tissues, it should be underlined that the putative influence of this substance on the chemical architecture of the urinary bladder intrinsic innervation has not yet been studied. One of the most important neuronal substances involved in the regulation of urinary bladder functions is vasoactive intestinal polypeptide (VIP), which primarily participates in the regulation of muscular activity and blood flow. Therefore, this study aimed to determine the influence of various doses of BPA on the distribution pattern of VIP-positive neural structures located in the wall of the porcine urinary bladder trigone using the double-immunofluorescence method. The obtained results show that BPA influence leads to an increase in the number of both neurons and nerve fibres containing VIP in the porcine urinary bladder trigone. This may indicate that VIP participates in adaptive processes of the urinary bladder evoked by BPA.

Sympathetic innervation of inguinal white adipose tissue in the mouse

Adipose tissue plays an important role in metabolic homeostasis and its prominent role as endocrine organ is now well recognized. Adipose tissue is controlled via the sympathetic nervous system (SNS). New viral, molecular-genetic tools will soon allow a more detailed study of adipose tissue innervation in metabolic function, yet, the precise anatomical extent of preganglionic and postganglionic inputs to the inguinal white adipose tissue (iWAT) is limited. Furthermore, several viral, molecular-genetic tools will require the use of cre/loxP mouse models, while the available studies on sympathetic iWAT innervation were established in larger species. In this study, we generated a detailed map for the sympathetic innervation of iWAT in male and female mice. We adapted iDISCO tissue clearing to process large, whole-body specimens for an unprecedented view of the natural abdominal SNS. Combined with pseudorabies virus retrograde tracing from the iWAT, we defined the preganglionic and postganglionic sympathetic input to iWAT. We used fluorescence-guided anatomical dissections of sympathetic nerves in reporter mice to further clarify that postganglionic axons connect to iWAT via lateral cutaneous rami (dorsolumbar iWAT portion) and the lumbar plexus (inguinal iWAT portion). Importantly, these rami carry axons that branch to iWAT, as well as axons that travel further to innervate the skin and vasculature, and their functional impact will require consideration in denervation studies. Our study may serve as a comprehensive map for future experiments that employ virally driven neuromodulation techniques to predict anatomy-based viral labeling.

Spatiotemporal mapping of sensory and motor innervation of the embryonic and postnatal mouse urinary bladder

The primary function of the urinary bladder is to store urine (continence) until a suitable time for voiding (micturition). These distinct processes are determined by the coordinated activation of sensory and motor components of the nervous system, which matures to enable voluntary control at the time of weaning. Our aim was to define the development and maturation of the nerve-organ interface of the mouse urinary bladder by mapping the organ and tissue distribution of major classes of autonomic (motor) and sensory axons. Innervation of the bladder was evident from E13 and progressed dorsoventrally. Increasing defasciculation of axon bundles to single axons within the muscle occurred through the prenatal period, and in several classes of axons underwent further maturation until P7. Urothelial innervation occurred more slowly than muscle innervation and showed a clear regional difference, from E18 the bladder neck having the highest density of urothelial nerves. These features of innervation were similar in males and females but varied in timing and tissue density between different axon classes. We also analysed the pelvic ganglion, the major source of motor axons that innervate the lower urinary tract and other pelvic organs. Cholinergic, nitrergic (subset of cholinergic) and noradrenergic neuronal cell bodies were present prior to visualization of these axon classes within the bladder. Examination of cholinergic structures within the pelvic ganglion indicated that connections from spinal preganglionic neurons to pelvic ganglion neurons were already present by E12, a time at which these autonomic ganglion neurons had not yet innervated the bladder. These putative preganglionic inputs increased in density prior to birth as axon terminal fields continued to expand within the bladder tissues. Our studies also revealed in numerous pelvic ganglion neurons an unexpected transient expression of calcitonin gene-related peptide, a peptide commonly used to visualise the peptidergic class of visceral sensory axons. Together, our outcomes enhance our understanding of neural regulatory elements in the lower urinary tract during development and provide a foundation for studies of plasticity and regenerative capacity in the adult system.

Comparative Analysis of Dorsal Root, Nodose and Sympathetic Ganglia for the Development of New Analgesics

Interoceptive and exteroceptive signals, and the corresponding coordinated control of internal organs and sensory functions, including pain, are received and orchestrated by multiple neurons within the peripheral, central and autonomic nervous systems. A central aim of the present report is to obtain a molecularly informed basis for analgesic drug development aimed at peripheral rather than central targets. We compare three key peripheral ganglia: nodose, sympathetic (superior cervical), and dorsal root ganglia in the rat, and focus on their molecular composition using next-gen RNA-Seq, as well as their neuroanatomy using immunocytochemistry and in situ hybridization. We obtained quantitative and anatomical assessments of transmitters, receptors, enzymes and signaling pathways mediating ganglion-specific functions. Distinct ganglionic patterns of expression were observed spanning ion channels, neurotransmitters, neuropeptides, G-protein coupled receptors (GPCRs), transporters, and biosynthetic enzymes. The relationship between ganglionic transcript levels and the corresponding protein was examined using immunohistochemistry for select, highly expressed, ganglion-specific genes. Transcriptomic analyses of spinal dorsal horn and intermediolateral cell column (IML), which form the termination of primary afferent neurons and the origin of preganglionic innervation to the SCG, respectively, disclosed pre- and post-ganglionic molecular-level circuits. These multimodal investigations provide insight into autonomic regulation, nodose transcripts related to pain and satiety, and DRG-spinal cord and IML-SCG communication. Multiple neurobiological and pharmacological contexts can be addressed, such as discriminating drug targets and predicting potential side effects, in analgesic drug development efforts directed at the peripheral nervous system.

Morphological and neurochemical characterisation of anterogradely labelled spinal sensory and autonomic nerve endings in the mouse bladder

The bladder is innervated by axons of sympathetic and parasympathetic efferent nerves, and by spinal afferent neurons. The objective was to characterise anatomically and immunohistochemically the terminal endings of sensory and autonomic motor nerve endings in wholemount preparations of the mouse bladder. We used both anterograde labelling of pelvic and hypogastric nerves ex vivo and anterograde labelling from lumbosacral dorsal root ganglia (DRG) in vivo in male and female mice. These were combined with immunohistochemistry for major markers of sensory, sympathetic and parasympathetic nerves. Selective labelling of spinal afferent endings following dextran biotin-labelling from DRGs in vivo showed no co-localisation of VAChT or TH in sensory terminals in the detrusor and suburothelial plexus. Biotinamide was applied ex vivo to nerve trunks arising in the pelvic ganglion and running towards the bladder. Among the filled axons, 38% of detrusor fibres and 47% of suburothelial axons were immunoreactive for calcitonin-gene related peptide (CGRP). Vesicular acetylcholine transporter (VAChT) immunoreactivity was present in 26% of both detrusor and suburothelial axons. For tyrosine hydroxylase (TH), the proportions were 15% and 17%, respectively. Three major morphological types of CGRP-immunoreactive nerve endings were distinguished in the bladder wall: simple, branching and complex. VAChT-immunoreactive parasympathetic axons had simple and branching endings; TH immunoreactive axons all had simple morphologies. Our findings revealed that different subtypes of sensory and autonomic nerve endings can be reliably identified by combining anterograde labelling ex vivo with specific immunohistochemical markers, although morphologically some of these types of endings were indistinguishable.

Signaling Interaction between Facial and Meningeal Inputs of the Trigeminal System Mediates Peripheral Neurostimulation Analgesia in a Rat Model of Migraine

Peripheral neurostimulation within the trigeminal nerve territory has been used for pain alleviation during migraine attacks, but the mechanistic basis of this non-invasive intervention is still poorly understood. In this study, we investigated the therapeutic role of peripheral stimulation of the trigeminal nerve, which provides homosegmental innervation to intracranial structures, by assessing analgesic effects in a nitroglycerin (NTG)-induced rat model of migraine. As a result of neurogenic inflammatory responses in the trigeminal nervous system, plasma protein extravasation was induced in facial skin by applying noxious stimulation to the dura mater. Noxious chemical stimulation of the dura mater led to protein extravasation in facial cutaneous tissues and caused mechanical sensitivity. Trigeminal ganglion (TG) neurons were double-labeled via retrograde tracing to detect bifurcated axons. Extracellular recordings of wide dynamic range (WDR) neurons in the spinal trigeminal nucleus caudalis (Sp5C) demonstrated the convergence and interaction of inputs from facial tissues and the dura mater. Peripheral neurostimulation of homotopic facial tissues represented segmental pain inhibition on cephalic cutaneous allodynia in the migraine model. The results indicated that facial territories and intracranial structures were directly connected with each other through bifurcated double-labeled neurons in the TG and through second-order WDR neurons. Homotopic stimulation at the C-fiber intensity threshold resulted in much stronger inhibition of analgesia than the same intensity of heterotopic stimulation. These results provide novel evidence for the neurological bases through which peripheral neurostimulation may be effective in treating migraine in clinical practice.

Intrinsic sensory neurons provide direct input to motor neurons and interneurons in mouse distal colon via varicose baskets

Connections from intrinsic primary afferent neurons (IPANs), to ascending motor and interneurons have been described in guinea pig colon. These mono- and polysynaptic circuits may underlie polarized motor reflexes evoked by local gut stimulation. There is a need to translate findings in guinea pig to mouse, a species increasingly used in enteric neuroscience. Here, mouse distal colon was immunolabeled for CGRP, a marker of putative IPANs. This revealed a combination of large, intensely immunofluorescent axons in myenteric plexus and circular muscle, and thinner varicose axons with less immunofluorescence. The latter formed dense, basket-like varicosity clusters (CGRP+ baskets) that enveloped myenteric nerve cell bodies. Immunolabeling after 4-5 days in organ culture caused loss of large CGRP+ axons, but not varicose CGRP+ fibers and CGRP+ baskets. Baskets were characterized further by triple labeling with CGRP, nitric oxide synthase (NOS) and calretinin (CALR) antibodies. Approximately half (48%) of nerve cell bodies inside CGRP+ baskets lacked both NOS and CALR, while two overlapping populations containing NOS and/or CALR comprised the remainder. Quantitative analysis revealed CGRP+ varicosities were most abundant in baskets, followed by CALR+ varicosities, with a high degree of colocalization between the two markers. Few NOS+ varicosities occurred in baskets. Significantly higher proportions of CALR+ and CGRP+ varicosities colocalized in baskets than in circular muscle. In conclusion, CGRP+ baskets in mouse colon are formed by intrinsic enteric neurons with a neurochemical profile consistent with IPANs and have direct connections to both excitatory and inhibitory neurons.

The aging effects on phenylephrine-induced relaxation of bladder in mice

OBJECTIVE

We have demonstrated that phenylephrine (PE) activates the capsaicin-sensitive nerves, and then activates capsaicin-sensitive nerves to release an unknown substance that facilitates the release of norepinephrine (NE) from adrenergic nerves. Subsequently, NE stimulates β-ARs in the detrusor muscle in mice, leading to neurogenic relaxation of the urinary bladder (UB).

MATERIALS AND METHODS

We examined if there existed sensory-motor dysfunction in UB of aging mice. To investigate the change of PE-induced detrusor relaxation in aging male-C57BL/6 mice (12- vs. 24-month-old mice), UB strips from mice were isolated, cut into strips, and mounted in the organ bath.

RESULTS

The UB strip contractility responding to various agents was estimated using tissue bath wire myography. Acetylcholine (ACh) and KCl-induced UB strips contraction was not significantly different between 24- and 12-month mice. NE-induced UB strips relaxation was significantly lower in 24-month than 12-month mice. Denuded bladder strips showed similar decreased relaxation response to NE. This NE-induced relaxation was inhibited by silodosin and lidocaine. PE did not induce contraction in UB strips of aging mice. In contrast, PE-induced relaxation was weaker in 24-month than 12-month mice.

CONCLUSION

Our results suggested that the PE-induced relaxation was age related. Aging seemed to lead the sensory-motor dysfunction. More animal and human studies are required to prove this concept and its clinical usefulness in the future.

Electroacupuncture relieved visceral and referred hindpaw hypersensitivity in colitis rats by inhibiting tyrosine hydroxylase expression in the sixth lumbar dorsal root ganglia

Irritable bowel syndrome patients frequently complain of pain in body regions somatotopically distinct from the gut, suggesting the involvement of an exaggerated signaling process in both visceral and somatic sensory pathways. Increasing evidence has shown that sprouting of tyrosine hydroxylase immunoreactive (TH-IR) fibers toward sensory neurons in dorsal root ganglia maintains and exacerbates the neuropathic and inflammatory pain, as well as colonic inflammation. The aim of the present study was to determine whether electroacupuncture could alleviate the visceral and secondary somatic hyperalgesia in colitis rats by suppressing the TH-IR expression in related dorsal root ganglia. After trinitrobenzene sulfonic acid irritation, rats developed inflammatory tissue damage in the distal colon, which was accompanied by visceral hypersensitivity and secondary hind paw hyperalgesia, as indicated by enhanced visceromotor response to colorectal distension and decreased mechanical and thermal withdrawal latency of the hind paw. Additionally, excessive TH-IR fibers sprouted toward calcitonin gene-related peptide immunoreactive sensory neurons, and TH-IR neurons also increased in the sixth lumbar dorsal root ganglia of colitis rats. Both electroacupuncture and guanethidine attenuated visceral and referred hind paw hyperalgesia by inhibiting the over-expression of TH-IR neurons and fibers in the sixth lumbar dorsal root ganglia. Moreover local inflammatory damage in the distal colon was restored after 7 days of electroacupuncture intervention. These results suggest that electroacupuncture relieved visceral and referred hind paw hypersensitivity in colitis rats by inhibiting TH expression in the sixth lumbar dorsal root ganglia.

Functional Anatomy of the Sensory Nervous System: Updates From the Neuroscience Bench

The simple tripartite classification of sensory neurons as A-beta, A-delta, and C fibers fails to convey the complexity of the neurons that encode stimuli as diverse as the texture of a surface, the location of a pinprick, or the direction of hair movement as a breeze moves across the skin. It has also proven to be inadequate when investigating the molecular mechanisms underlying pain, which can encompass any combination of chemical, tactile, and thermal modalities. Beginning with a brief overview of visceral and sensory neuroanatomy, this review expands upon sensory innervation of the skin as a prime example of the heterogeneity and complexity of the somatosensory nervous system. Neuroscientists have characterized defining features of over 15 subtypes of sensory neurons that innervate the skin of the mouse. This has enabled the study of cell-specific mechanisms of pain, which suggests that diverse sensory neuron subtypes may have distinct susceptibilities to toxic injury and different roles in pathologic mechanisms underlying altered sensation. Leveraging this growing body of knowledge for preclinical trials and models of neurotoxicity can vastly improve our understanding of peripheral nervous system dysfunction, advancing the fields of toxicologic pathology and neuropathology alike.

Single-cell RNAseq reveals seven classes of colonic sensory neuron

OBJECTIVE

Integration of nutritional, microbial and inflammatory events along the gut-brain axis can alter bowel physiology and organism behaviour. Colonic sensory neurons activate reflex pathways and give rise to conscious sensation, but the diversity and division of function within these neurons is poorly understood. The identification of signalling pathways contributing to visceral sensation is constrained by a paucity of molecular markers. Here we address this by comprehensive transcriptomic profiling and unsupervised clustering of individual mouse colonic sensory neurons.

DESIGN

Unbiased single-cell RNA-sequencing was performed on retrogradely traced mouse colonic sensory neurons isolated from both thoracolumbar (TL) and lumbosacral (LS) dorsal root ganglia associated with lumbar splanchnic and pelvic spinal pathways, respectively. Identified neuronal subtypes were validated by single-cell qRT-PCR, immunohistochemistry (IHC) and Ca²⁺-imaging.

RESULTS

Transcriptomic profiling and unsupervised clustering of 314 colonic sensory neurons revealed seven neuronal subtypes. Of these, five neuronal subtypes accounted for 99% of TL neurons, with LS neurons almost exclusively populating the remaining two subtypes. We identify and classify neurons based on novel subtype-specific marker genes using single-cell qRT-PCR and IHC to validate subtypes derived from RNA-sequencing. Lastly, functional Ca²⁺-imaging was conducted on colonic sensory neurons to demonstrate subtype-selective differential agonist activation.

CONCLUSIONS

We identify seven subtypes of colonic sensory neurons using unbiased single-cell RNA-sequencing and confirm translation of patterning to protein expression, describing sensory diversity encompassing all modalities of colonic neuronal sensitivity. These results provide a pathway to molecular interrogation of colonic sensory innervation in health and disease, together with identifying novel targets for drug development.

Inhibition of Cav 3.2 calcium channels: A new target for colonic hypersensitivity associated with low-grade inflammation

BACKGROUND AND PURPOSE

Abdominal pain associated with low-grade inflammation is frequently encountered in irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD) during remission. Current treatments are not very effective and new therapeutic approaches are needed. The role of Ca_V 3.2 channels, which are important in other chronic pain contexts, was investigated in a murine model of colonic hypersensitivity (CHS) associated with low-grade inflammation.

EXPERIMENTAL APPROACH

Low doses of dextran sulfate sodium (DSS; 0.5%) were chronically administered to C57BL/6j mice in drinking water. Their inflammatory state was assessed by systemic and local measures of IL-6, myeloperoxidase, and lipocalin-2 using elisa. Colonic sensitivity was evaluated by the visceromotor responses to colorectal distension. Functional involvement of Ca_V 3.2 channels was assessed with different pharmacological (TTA-A2, ABT-639, and ethosuximide) and genetic tools.

KEY RESULTS

DSS induced low-grade inflammation associated with CHS in mice. Genetic or pharmacological inhibition of Ca_V 3.2 channels reduced CHS. Cav3.2 channel deletion in primary nociceptive neurons in dorsal root ganglia (Ca_V 3.2^Nav1.8 KO mice) suppressed CHS. Spinal, but not systemic, administration of ABT-639, a peripherally acting T-type channel blocker, reduced CHS. ABT-639 given intrathecally to Ca_V 3.2^Nav1.8 KO mice had no effect, demonstrating involvement of Ca_V 3.2 channels located presynaptically in afferent fibre terminals. Finally, ethosuximide, which is a T-type channel blocker used clinically, reduced CHS.

CONCLUSIONS AND IMPLICATIONS

These results suggest that ethosuximide represents a promising drug reposition strategy and that inhibition of Ca_V 3.2 channels is an attractive therapeutic approach for relieving CHS in IBS or IBD.

The hypothalamic-spinal dopaminergic system: a target for pain modulation

Nociceptive signals conveyed to the dorsal horn of the spinal cord by primary nociceptors are subject to extensive modulation by local neurons and by supraspinal descending pathways to the spinal cord before being relayed to higher brain centers. Descending modulatory pathways to the spinal cord comprise, among others, noradrenergic, serotonergic, γ-aminobutyric acid (GABA)ergic, and dopaminergic fibers. The contributions of noradrenaline, serotonin, and GABA to pain modulation have been extensively investigated. In contrast, the contributions of dopamine to pain modulation remain poorly understood. The focus of this review is to summarize the current knowledge of the contributions of dopamine to pain modulation. Hypothalamic A11 dopaminergic neurons project to all levels of the spinal cord and provide the main source of spinal dopamine. Dopamine receptors are expressed in primary nociceptors as well as in spinal neurons located in different laminae in the dorsal horn of the spinal cord, suggesting that dopamine can modulate pain signals by acting at both presynaptic and postsynaptic targets. Here, I will review the literature on the effects of dopamine and dopamine receptor agonists/antagonists on the excitability of primary nociceptors, the effects of dopamine on the synaptic transmission between primary nociceptors and dorsal horn neurons, and the effects of dopamine on pain in rodents. Published data support both anti-nociceptive effects of dopamine mediated by D2-like receptors and pro-nociceptive effects mediated by D1-like receptors.

Morphological and neurochemical differences in peptidergic nerve fibers of the mouse vagina

The vagina is innervated by a complex arrangement of sensory, sympathetic, and parasympathetic nerve fibers that contain classical transmitters plus an array of neuropeptides and enzymes known to regulate diverse processes including blood flow and nociception. The neurochemical characteristics and distributions of peptide-containing nerves in the mouse vagina are unknown. This study used multiple labeling immunohistochemistry, confocal maging and analysis to investigate the presence and colocalization of the peptides vasoactive intestinal polypeptide (VIP), calcitonin-gene related peptide (CGRP), substance P (SP), neuropeptide tyrosine (NPY), and the nitric oxide synthesizing enzyme neuronal nitric oxide synthase (nNOS) in nerve fibers of the murine vaginal wall. We compared cervical and vulvar areas of the vagina in young nullipara and older multipara C57Bl/6 mice, and identified differences including that small ganglia were restricted to cervical segments, epithelial fibers were mainly present in vulvar segments and most nerve fibers were found in the lamina propria of the cervical region of the vagina, where a higher number of fibers containing immunoreactivity for VIP, CGRP, SP, or nNOS were found. Two populations of VIP-containing fibers were identified: fibers containing CGRP and fibers containing VIP but not CGRP. Differences between young and older mice were present in multiple layers of the vaginal wall, with older mice showing overall loss of innervation of epithelium of the proximal vagina and reduced proportions of VIP, CGRP, and SP containing nerve fibers in the distal epithelium. The distal vagina also showed increased vascularization and perivascular fibers containing NPY. Immunolabeling of ganglia associated with the vagina indicated the likely origin of some peptidergic fibers. Our results reveal regional differences and age- or parity-related changes in innervation of the mouse vagina, effecting the distribution of neuropeptides with diverse roles in function of the female genital tract.

The role of calcitonin gene-related peptide in peripheral and central pain mechanisms including migraine

Calcitonin gene-related peptide (CGRP) is a 37-amino acid peptide found primarily in the C and Aδ sensory fibers arising from the dorsal root and trigeminal ganglia, as well as the central nervous system. Calcitonin gene-related peptide was found to play important roles in cardiovascular, digestive, and sensory functions. Although the vasodilatory properties of CGRP are well documented, its somatosensory function regarding modulation of neuronal sensitization and of enhanced pain has received considerable attention recently. Growing evidence indicates that CGRP plays a key role in the development of peripheral sensitization and the associated enhanced pain. Calcitonin gene-related peptide is implicated in the development of neurogenic inflammation and it is upregulated in conditions of inflammatory and neuropathic pain. It is most likely that CGRP facilitates nociceptive transmission and contributes to the development and maintenance of a sensitized, hyperresponsive state not only of the primary afferent sensory neurons but also of the second-order pain transmission neurons within the central nervous system, thus contributing to central sensitization as well. The maintenance of a sensitized neuronal condition is believed to be an important factor underlying migraine. Recent successful clinical studies have shown that blocking the function of CGRP can alleviate migraine. However, the mechanisms through which CGRP may contribute to migraine are still not fully understood. We reviewed the role of CGRP in primary afferents, the dorsal root ganglion, and in the trigeminal system as well as its role in peripheral and central sensitization and its potential contribution to pain processing and to migraine.

Dynamic Expression of Serotonin Receptor 5-HT3A in Developing Sensory Innervation of the Lower Urinary Tract

Sensory afferent signaling is required for normal function of the lower urinary tract (LUT). Despite the wide prevalence of bladder dysfunction and pelvic pain syndromes, few effective treatment options are available. Serotonin receptor 5-HT3A is a known mediator of visceral afferent signaling and has been implicated in bladder function. However, basic expression patterns for this gene and others among developing bladder sensory afferents that could be used to inform regenerative efforts aimed at treating deficiencies in pelvic innervation are lacking. To gain greater insight into the molecular characteristics of bladder sensory innervation, we conducted a thorough characterization of Htr3a expression in developing and adult bladder-projecting lumbosacral dorsal root ganglia (DRG) neurons. Using a transgenic Htr3a-EGFP reporter mouse line, we identified 5-HT3A expression at 10 days post coitus (dpc) in neural crest derivatives and in 12 dpc lumbosacral DRG. Using immunohistochemical co-localization we observed Htr3a-EGFP expression in developing lumbosacral DRG that partially coincides with neuropeptides CGRP and Substance P and capsaicin receptor TRPV1. A majority of Htr3a-EGFP+ DRG neurons also express a marker of myelinated Aδ neurons, NF200. There was no co-localization of 5-HT3A with the TRPV4 receptor. We employed retrograde tracing in adult Htr3a-EGFP mice to quantify the contribution of 5-HT3A+ DRG neurons to bladder afferent innervation. We found that 5-HT3A is expressed in a substantial proportion of retrograde traced DRG neurons in both rostral (L1, L2) and caudal (L6, S1) axial levels that supply bladder innervation. Most bladder-projecting Htr3a-EGFP+ neurons that co-express CGRP, Substance P, or TRPV1 are found in L1, L2 DRG, whereas Htr3a-EGFP+, NF200+ bladder-projecting neurons are from the L6, S1 axial levels. Our findings contribute much needed information regarding the development of LUT innervation and highlight the 5-HT3A serotonin receptor as a candidate for future studies of neurally mediated bladder control.

Nociceptive and inflammatory mediator upregulation in a mouse model of chronic prostatitis

Chronic nonbacterial prostatitis, characterized by genitourinary pain in the pelvic region in the absence of an identifiable cause, is common in adult males. Surprisingly, the sensory innervation of the prostate and mediators that sensitize its innervation have received little attention. We thus characterized a mouse model of chronic prostatitis, focusing on the prostate innervation and how organ inflammation affects gene expression of putative nociceptive markers in prostate afferent somata in dorsal root ganglia (DRG) and mediators in the prostate. Retrograde tracing (fast blue) from the prostate revealed that thoracolumbar and lumbosacral DRG are the principal sources of somata of prostate afferents. Nociceptive markers (eg, transient receptor potential, TREK, and P2X channels) were upregulated in fast blue-labeled thoracolumbar and lumbosacral somata for up to four weeks after inflaming the prostate (intraprostate injection of zymosan). Prostatic inflammation was evident histologically, by monocyte infiltration and a significant increase in mast cell tryptase activity 14, 21, and 28 days after zymosan injection. Interleukin 10 and NGF were also significantly upregulated in the prostate throughout the 4 weeks of inflammation. Open-field pain-related behaviors (eg, rearing) were unchanged in prostate-inflamed mice, suggesting the absence of ongoing nociception, but withdrawal thresholds to lower abdominal pressure were significantly reduced. The increases in IL-10, mast cell tryptase, and NGF in the inflamed prostate were cotemporaneous with reduced thresholds to probing of the abdomen and upregulation of nociceptive markers in DRG somata innervating the prostate. The results provide insight and direction for the study of mechanisms underlying pain in chronic prostatitis.

Dopamine modulation of transient receptor potential vanilloid type 1 (TRPV1) receptor in dorsal root ganglia neurons

The transient receptor potential vanilloid type 1 (TRPV1) receptor plays a key role in the modulation of nociceptor excitability. To address whether dopamine can modulate the activity of TRPV1 channels in nociceptive neurons, the effects of dopamine and dopamine receptor agonists were tested on the capsaicin-activated current recorded from acutely dissociated small diameter (<27 μm) dorsal root ganglia (DRG) neurons. Dopamine or SKF 81297 (an agonist at D1/D5 receptors), caused inhibition of both inward and outward currents by ∼60% and ∼48%, respectively. The effect of SKF 81297 was reversed by SCH 23390 (an antagonist at D1/D5 receptors), confirming that it was mediated by activation of D1/D5 dopamine receptors. In contrast, quinpirole (an agonist at D2 receptors) had no significant effect on the capsaicin-activated current. Inhibition of the capsaicin-activated current by SKF 81297 was mediated by G protein coupled receptors (GPCRs), and highly dependent on external calcium. The inhibitory effect of SKF 81297 on the capsaicin-activated current was not affected when the protein kinase A (PKA) activity was blocked with H89, or when the protein kinase C (PKC) activity was blocked with bisindolylmaleimide II (BIM). In contrast, when the calcium-calmodulin-dependent protein kinase II (CaMKII) was blocked with KN-93, the inhibitory effect of SKF 81297 on the capsaicin-activated current was greatly reduced, suggesting that activation of D1/D5 dopamine receptors may be preferentially linked to CaMKII activity. We suggest that modulation of TRPV1 channels by dopamine in nociceptive neurons may represent a way for dopamine to modulate incoming noxious stimuli.

The impact of L5 dorsal root ganglion degeneration and Adamkiewicz artery vasospasm on descending colon dilatation following spinal subarachnoid hemorrhage: An experimental study; first report

CONTEXT

Somato-sensitive innervation of bowels are maintained by lower segments of spinal cord and the blood supply of the lower spinal cord is heavily dependent on Adamkiewicz artery. Although bowel problems are sometimes seen in subarachnoid hemorrhage neither Adamkiewicz artery spasm nor spinal cord ischemia has not been elucidated as a cause of bowel dilatation so far.

AIMS

The goal of this study was to study the effects Adamkiewicz artery (AKA) vasospasm in lumbar subarachnoid hemorrhage (SAH) on bowel dilatation severity.

SETTINGS AND DESIGN

An experimental rabbit study.

MATERIALS AND METHODS

The study was conducted on 25 rabbits, which were randomly divided into three groups: Spinal SAH (N = 13), serum saline (SS) (SS; N = 7) and control (N = 5) groups. Experimental spinal SAH was performed. After 21 days, volume values of descending parts of large bowels and degenerated neuron density of L5DRG were analyzed.

STATISTICAL ANALYSIS USED

Statistical analysis was performed using the PASW Statistics 18.0 for Windows (SPSS Inc., Chicago, Illinois). Two-tailed t-test and Mann-Whitney U-tests were used. The statistical significance was set at P < 0.05.

RESULTS

The mean volume of imaginary descending colons was estimated as 93 ± 12 cm(3) in the control group and 121 ± 26 cm(3) in the SS group and 176 ± 49 cm(3) in SAH group. Volume augmentations of the descending colons and degenerated neuron density L5DRG were significantly different between the SAH and other two groups (P < 0.05).

CONCLUSION

An inverse relationship between the living neuronal density of the L5DRG and the volume of imaginary descending colon values was occurred. Our findings will aid in the planning of future experimental studies and determining the clinical relevance on such studies.

Axotomy of tributaries of the pelvic and pudendal nerves induces changes in the neurochemistry of mouse dorsal root ganglion neurons and the spinal cord

Using immunohistochemical techniques, we characterized changes in the expression of several neurochemical markers in lumbar 4-sacral 2 (L4-S2) dorsal root ganglion (DRG) neuron profiles (NPs) and the spinal cord of BALB/c mice after axotomy of the L6 and S1 spinal nerves, major tributaries of the pelvic (targeting pelvic visceral organs) and pudendal (targeting perineum and genitalia) nerves. Sham animals were included. Expression of cyclic AMP-dependent transcription factor 3 (ATF3), calcitonin gene-related peptide (CGRP), transient receptor potential cation channel subfamily V, member 1 (TRPV1), tyrosine hydroxylase (TH) and vesicular glutamate transporters (VGLUT) types 1 and -2 was analysed seven days after injury. L6-S1 axotomy induced dramatic de novo expression of ATF3 in many L6-S1 DRG NPs, and parallel significant downregulations in the percentage of CGRP-, TRPV1-, TH- and VGLUT2-immunoreactive (IR) DRG NPs, as compared to their expression in uninjured DRGs (contralateral L6-S1-AXO; sham mice); VGLUT1 expression remained unaltered. Sham L6-S1 DRGs only showed a small ipsilateral increase in ATF3-IR NPs (other markers were unchanged). L6-S1-AXO induced de novo expression of ATF3 in several lumbosacral spinal cord motoneurons and parasympathetic preganglionic neurons; in sham mice the effect was limited to a few motoneurons. Finally, a moderate decrease in CGRP- and TRPV1-like-immunoreactivities was observed in the ipsilateral superficial dorsal horn neuropil. In conclusion, injury of a mixed visceral/non-visceral nerve leads to considerable neurochemical alterations in DRGs matched, to some extent, in the spinal cord. Changes in these and potentially other nociception-related molecules could contribute to pain due to injury of nerves in the abdominopelvic cavity.

Distribution across tissue layers of extrinsic nerves innervating the mouse colorectum - an in vitro anterograde tracing study

BACKGROUND

Anterograde in vitro tracing of the pelvic nerve (PN) and visualization in the horizontal plane in whole mount preparations has been fundamental in the analysis of distribution of peripheral nerves innervating the colorectum. Here, we performed a similar analysis, but in cryostat sections of the mouse colorectum, allowing for a more direct visualization of nerve distribution in all tissue layers.

METHODS

Colorectum with attached PNs was dissected from adult male BalbC mice. Presence of active afferents was certified by single fiber recording of fine PN fibers. This was followed by 'bulk' (all fibers) anterograde tracing using biotinamide (BTA). Histo- and immunohistochemical techniques were used for visualization of BTA-positive nerves, and evaluation of co-localization with calcitonin gene-related peptide (CGRP), respectively. Tissue was analyzed using confocal microscopy on transverse or longitudinal colorectum sections.

KEY RESULTS

Abundant BTA-positive nerves spanning all layers of the mouse colorectum and contacting myenteric plexus neurons, distributing within the muscle layer, penetrating deeper into the organ and contacting blood vessels, submucosal plexus neurons or even penetrating the mucosa, were regularly detected. Several traced axons co-localized CGRP, supporting their afferent nature. Finally, anterograde tracing of the PN also exposed abundant BTA-positive nerves in the major pelvic ganglion.

CONCLUSIONS & INFERENCES

We present the patterns of innervation of extrinsic axons across layers in the mouse colorectum, including the labile mucosal layer. The proposed approach could also be useful in the analysis of associations between morphology and physiology of peripheral nerves targeting the different layers of the colorectum.

Co-expression patterns of cocaine- and amphetamine-regulated transcript (CART) with neuropeptides in dorsal root ganglia of the pig

In the present study the neuronal distribution of CART was evaluated immunohistochemically in porcine dorsal root ganglia (DRGs). In co-localization studies the co-expression patterns of CART with SP, CGRP, galanin, CALB and LENK were investigated by means of triple immunohistochemical stainings. In porcine DRGs, the expression of CART was found in approximately 5% of primary sensory neurons. The vast majority (ca. 95%) of CART-immunoreactive (IR) neurons were small and middle sized, and only 5% were categorized as large. CART-IR neurons additionally exhibiting the presence of SP/CGRP (ca. 12%), SP/CALB (ca. 12%), SP/LENK (ca. 5%) were found. The vast majority of CART-IR/CGRP-IR neurons did not display immunoreaction to SP (ca. 60%). Subclasses of CART-IR/LENK-IR/SP-negative (ca. 5%), as well as CART-IR/CALB-IR/SP-negative neurons (ca. 10%), were also visualized. In addition, CART-IR neurons with no immunoreactivities to any of the neuropeptides studied were also shown. In porcine DRGs none of the CART-IR neurons exhibited the presence of galanin. The results obtained in the study suggest that CART may functionally modulate the activity of the porcine primary sensory neurons. It is concluded that co-expression of CART with CGRP, SP, LENK and CALB in subsets of the pig L1-L6 DRGs neurons provide anatomical evidence for a CART role in pain processing.

Inhibition of tetrodotoxin-resistant sodium current in dorsal root ganglia neurons mediated by D1/D5 dopamine receptors

Background

Dopaminergic fibers originating from area A11 of the hypothalamus project to different levels of the spinal cord and represent the major source of dopamine. In addition, tyrosine hydroxylase, the rate-limiting enzyme for the synthesis of catecholamines, is expressed in 8-10% of dorsal root ganglia (DRG) neurons, suggesting that dopamine may be released in the dorsal root ganglia. Dopamine has been shown to modulate calcium current in DRG neurons, but the effects of dopamine on sodium current and on the firing properties of small DRG neurons are poorly understood.

Results

The effects of dopamine and dopamine receptor agonists were tested on the tetrodotoxin-resistant (TTX-R) sodium current recorded from acutely dissociated small (diameter ≤ 25 μm) DRG neurons. Dopamine (20 μM) and SKF 81297 (10 μM) caused inhibition of TTX-R sodium current in small DRG neurons by 23% and 37%, respectively. In contrast, quinpirole (20 μM) had no effects on the TTX-R sodium current. Inhibition by SKF 81297 of the TTX-R sodium current was not affected when the protein kinase A (PKA) activity was blocked with the PKA inhibitory peptide (6–22), but was greatly reduced when the protein kinase C (PKC) activity was blocked with the PKC inhibitory peptide (19–36), suggesting that activation of D1/D5 dopamine receptors is linked to PKC activity. Expression of D1and D5 dopamine receptors in small DRG neurons, but not D2 dopamine receptors, was confirmed by Western blotting and immunofluorescence analysis. In current clamp experiments, the number of action potentials elicited in small DRG neurons by current injection was reduced by ~ 30% by SKF 81297.

Conclusions

We conclude that activation of D1/D5 dopamine receptors inhibits TTX-R sodium current in unmyelinated nociceptive neurons and dampens their intrinsic excitability by reducing the number of action potentials in response to stimulus. Increasing or decreasing levels of dopamine in the dorsal root ganglia may serve to adjust the sensitivity of nociceptors to noxious stimuli.

VGLUTs in Peripheral Neurons and the Spinal Cord: Time for a Review

Vesicular glutamate transporters (VGLUTs) are key molecules for the incorporation of glutamate in synaptic vesicles across the nervous system, and since their discovery in the early 1990s, research on these transporters has been intense and productive. This review will focus on several aspects of VGLUTs research on neurons in the periphery and the spinal cord. Firstly, it will begin with a historical account on the evolution of the morphological analysis of glutamatergic systems and the pivotal role played by the discovery of VGLUTs. Secondly, and in order to provide an appropriate framework, there will be a synthetic description of the neuroanatomy and neurochemistry of peripheral neurons and the spinal cord. This will be followed by a succinct description of the current knowledge on the expression of VGLUTs in peripheral sensory and autonomic neurons and neurons in the spinal cord. Finally, this review will address the modulation of VGLUTs expression after nerve and tissue insult, their physiological relevance in relation to sensation, pain, and neuroprotection, and their potential pharmacological usefulness.

Sympathetic axonopathies and hyperinnervation in the small intestine smooth muscle of aged Fischer 344 rats

It is well documented that the intrinsic enteric nervous system of the gastrointestinal (GI) tract sustains neuronal losses and reorganizes as it ages. In contrast, age-related remodeling of the extrinsic sympathetic projections to the wall of the gut is poorly characterized. The present experiment, therefore, surveyed the sympathetic projections to the aged small intestine for axonopathies. Furthermore, the experiment evaluated the specific prediction that catecholaminergic inputs undergo hyperplastic changes. Jejunal tissue was collected from 3-, 8-, 16-, and 24-month-old male Fischer 344 rats, prepared as whole mounts consisting of the muscularis, and processed immunohistochemically for tyrosine hydroxylase, the enzymatic marker for norepinephrine, and either the protein CD163 or the protein MHCII, both phenotypical markers for macrophages. Four distinctive sympathetic axonopathy profiles occurred in the small intestine of the aged rat: (1) swollen and dystrophic terminals, (2) tangled axons, (3) discrete hyperinnervated loci in the smooth muscle wall, including at the bases of Peyer's patches, and (4) ectopic hyperplastic or hyperinnervating axons in the serosa/subserosal layers. In many cases, the axonopathies occurred at localized and limited foci, involving only a few axon terminals, in a pattern consistent with incidences of focal ischemic, vascular, or traumatic insult. The present observations underscore the complexity of the processes of aging on the neural circuitry of the gut, with age-related GI functional impairments likely reflecting a constellation of adjustments that range from selective neuronal losses, through accumulation of cellular debris, to hyperplasias and hyperinnervation of sympathetic inputs.

Expression of vesicular glutamate transporters in sensory and autonomic neurons innervating the mouse bladder

PURPOSE

VGLUTs, which are essential for loading glutamate into synaptic vesicles, are present in various neuronal systems. However, to our knowledge the expression of VGLUTs in neurons innervating the bladder has not yet been analyzed. We studied VGLUT1, VGLUT2 and VGLUT3 in mouse bladder neurons.

MATERIALS AND METHODS

We analyzed the expression of VGLUT1, VGLUT2 and calcitonin gene-related peptide by immunohistochemistry in the retrograde labeled primary afferent and autonomic neurons of BALB/c mice after injecting fast blue in the bladder wall. To study VGLUT3 we traced the bladder of transgenic mice, in which VGLUT3 is identified by enhanced green fluorescent protein detection.

RESULTS

Most bladder dorsal root ganglion neurons expressed VGLUT2. A smaller percentage of neurons also expressed VGLUT1 or VGLUT3. Co-expression with calcitonin gene-related peptide was only observed for VGLUT2. Occasional VGLUT2 immunoreactive neurons were seen in the major pelvic ganglia. Abundant VGLUT2 immunoreactive nerves were detected in the bladder dome and trigone, and the urethra. VGLUT1 immunoreactive nerves were discretely present.

CONCLUSIONS

We present what are to our knowledge novel data on VGLUT expression in sensory and autonomic neurons innervating the mouse bladder. The frequent association of VGLUT2 and calcitonin gene-related peptide in sensory neurons suggests interactions between glutamatergic and peptidergic neurotransmissions, potentially influencing commonly perceived sensations in the bladder, such as discomfort and pain.