Decrease in neuronal nicotinic acetylcholine receptor subunit and PSD-93 transcript levels in the male mouse MPG after cavernous nerve injury or explant culture

TLR2 and TLR4 Modulate Mouse Ileal Motility by the Interaction with Muscarinic and Nicotinic Receptors

Irritable bowel syndrome (IBS) is a chronic functional bowel disorder characterized by intestinal dysmotility. Changes in intestinal microbiota (dysbiosis) can lead to alterations in neuro-muscular functions in the gut. Toll-like receptors (TLRs) 2 and 4 recognize intestinal bacteria and are involved in the motor response induced by gastrointestinal (GI) neurotransmitters. Acetylcholine (ACh) is a well-known neurotransmitter involved in the regulation of GI motility. This study aimed to evaluate the role of TLR2 and TLR4 in the intestinal motor-response induced by ACh in the mouse ileum, as well as the expression and function of the muscarinic and nicotinic ACh receptors. Muscle contractility studies showed that the contractions induced by ACh were significantly lower in TLR2^−/− and TLR4^−/− with respect to WT mice. In WT mice, the contractions induced by ACh were reduced in the presence of AF-DX AF-DX 116 (a muscarinic ACh receptor (mAChR) M2 antagonist), 4-DAMP (a mAChR M3 antagonist), mecamylamine (a nicotinic AChR receptor (nAChR) α3β4 antagonist) and α-bungarotoxin (a nAChR α7 antagonist). In TLR2^−/− mice, the contractions induced by ACh were increased by AF-DX 116 and mecamylamine. In TLR4^−/− mice, the contractions induced by ACh were reduced by α-bungarotoxin and 4-DAMP. The mRNA and protein expressions of M3 and α3 receptors were diminished in the ileum from TLR2^−/− and TLR4^−/− with respect to WT mice. However, the levels of mRNA and protein of β4 were diminished only in TLR4^−/− but not in TLR2^−/− mice. In conclusion, our results show that TLR2 and TLR4 modulates the motor responses to ACh in the mouse ileum. TLR2 acts on muscarinic M2 and M3 and nicotinic α3β4 ACh receptors, while TLR4 acts on muscarinic M3 and nicotinic α3β4 and α7 ACh receptors.

Nicotinic Acetylcholine Receptor Involvement in Inflammatory Bowel Disease and Interactions with Gut Microbiota

The gut-brain axis describes a complex interplay between the central nervous system and organs of the gastrointestinal tract. Sensory neurons of dorsal root and nodose ganglia, neurons of the autonomic nervous system, and immune cells collect and relay information about the status of the gut to the brain. A critical component in this bi-directional communication system is the vagus nerve which is essential for coordinating the immune system’s response to the activities of commensal bacteria in the gut and to pathogenic strains and their toxins. Local control of gut function is provided by networks of neurons in the enteric nervous system also called the ‘gut-brain’. One element common to all of these gut-brain systems is the expression of nicotinic acetylcholine receptors. These ligand-gated ion channels serve myriad roles in the gut-brain axis including mediating fast synaptic transmission between autonomic pre- and postganglionic neurons, modulation of neurotransmitter release from peripheral sensory and enteric neurons, and modulation of cytokine release from immune cells. Here we review the role of nicotinic receptors in the gut-brain axis with a focus on the interplay of these receptors with the gut microbiome and their involvement in dysregulation of gut function and inflammatory bowel diseases.

Synthesis and radiofluorination of novel fluoren-9-one based derivatives for the imaging of α7 nicotinic acetylcholine receptor with PET

By structure-activity relationship studies on the tilorone scaffold, the 'one armed' substituted dibenzothiophenes and the fluoren-9-ones were identified as the most potential α₇ nAChR ligands. While the suitability of dibenzothiophene derivatives as PET tracers is recognized, the potential of fluoren-9-ones is insufficiently investigated. We herein report on a series of fluoren-9-one based derivatives targeting α₇ nAChR with compounds 8a and 8c possessing the highest affinity and selectivity. Accordingly, with [¹⁸F]8a and [¹⁸F]8c we designed and initially evaluated the first fluoren-9-one derived α₇ nAChR selective PET ligands. A future application of these radioligands is facilitated by the herein presented successful implementation of fully automated radiosynthesis.

Activation of MEK/ERK signaling contributes to the PACAP-induced increase in guinea pig cardiac neuron excitability

Pituitary adenylate cyclase (PAC)-activating polypeptide (PACAP) peptides (Adcyap1) signaling at the selective PAC1 receptor (Adcyap1r1) participate in multiple homeostatic and stress-related responses, yet the cellular mechanisms underlying PACAP actions remain to be completely elucidated. PACAP/PAC₁ receptor signaling increases excitability of neurons within the guinea pig cardiac ganglia, and as these neurons are readily accessible, this neuronal system is particularly amenable to study of PACAP modulation of ionic conductances. The present study investigated how PACAP activation of MEK/ERK signaling contributed to the peptide-induced increase in cardiac neuron excitability. Treatment with the MEK inhibitor PD 98059 blocked PACAP-stimulated phosphorylated ERK and, in parallel, suppressed the increase in cardiac neuron excitability. However, PD 98059 did not blunt the ability of PACAP to enhance two inward ionic currents, one flowing through hyperpolarization-activated nonselective cationic channels (I_h) and another flowing through low-voltage-activated calcium channels (I_T), which support the peptide-induced increase in excitability. Thus a PACAP- and MEK/ERK-sensitive, voltage-dependent conductance(s), in addition to I_h and I_T, modulates neuronal excitability. Despite prior work implicating PACAP downregulation of the K_V4.2 potassium channel in modulation of excitability in other cells, treatment with the K_V4.2 current blocker 4-aminopyridine did not replicate the PACAP-induced increase in excitability in cardiac neurons. However, cardiac neurons express the ERK target, the Na_V1.7 sodium channel, and treatment with the selective Na_V1.7 channel inhibitor PF-04856264 decreased the PACAP modulation of excitability. From these results, PACAP/PAC1 activation of MEK/ERK signaling may phosphorylate the Na_V1.7 channel, enhancing sodium currents near the threshold, an action contributing to repetitive firing of the cardiac neurons exposed to PACAP.

A Promising PET Tracer for Imaging of α₇ Nicotinic Acetylcholine Receptors in the Brain: Design, Synthesis, and in Vivo Evaluation of a Dibenzothiophene-Based Radioligand

Changes in the expression of α₇ nicotinic acetylcholine receptors (α₇ nAChRs) in the human brain are widely assumed to be associated with neurological and neurooncological processes. Investigation of these receptors invivo depends on the availability of imaging agents such as radioactively labelled ligands applicable in positron emission tomography (PET). We report on a series of new ligands for α₇ nAChRs designed by the combination of dibenzothiophene dioxide as a novel hydrogen bond acceptor functionality with diazabicyclononane as an established cationic center. To assess the structure-activity relationship (SAR) of this new basic structure, we further modified the cationic center systematically by introduction of three different piperazine-based scaffolds. Based on invitro binding affinity and selectivity, assessed by radioligand displacement studies at different rat and human nAChR subtypes and at the structurally related human 5-HT₃ receptor, we selected the compound 7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-fluorodibenzo-[b,d]thiophene 5,5-dioxide (10a) for radiolabeling and further evaluation invivo. Radiosynthesis of [¹⁸F]10a was optimized and transferred to an automated module. Dynamic PET imaging studies with [¹⁸F]10a in piglets and a monkey demonstrated high uptake of radioactivity in the brain, followed by washout and target-region specific accumulation under baseline conditions. Kinetic analysis of [¹⁸F]10a in pig was performed using a two-tissue compartment model with arterial-derived input function. Our initial evaluation revealed that the dibenzothiophene-based PET radioligand [¹⁸F]10a ([¹⁸F]DBT-10) has high potential to provide clinically relevant information about the expression and availability of α₇ nAChR in the brain.

Developing a functional urinary bladder: a neuronal context

The development of organs occurs in parallel with the formation of their nerve supply. The innervation of pelvic organs (lower urinary tract, hindgut, and sexual organs) is complex and we know remarkably little about the mechanisms that form these neural pathways. The goal of this short review is to use the urinary bladder as an example to stimulate interest in this question. The bladder requires a healthy mature nervous system to store urine and release it at behaviorally appropriate times. Understanding the mechanisms underlying the construction of these neural circuits is not only relevant to defining the basis of developmental problems but may also suggest strategies to restore connectivity and function following injury or disease in adults. The bladder nerve supply comprises multiple classes of sensory, and parasympathetic or sympathetic autonomic effector (motor) neurons. First, we define the developmental endpoint by describing this circuitry in adult rodents. Next we discuss the innervation of the developing bladder, identifying challenges posed by this area of research. Last we provide examples of genetically modified mice with bladder dysfunction and suggest potential neural contributors to this state.

Bladder outlet obstruction causes up-regulation of nicotinic acetylcholine receptors in bladder-projecting pelvic ganglion neurons

Pelvic ganglion (PG) neurons relay sympathetic and parasympathetic signals to the lower urinary tract, comprising the urinary bladder and bladder outlet, and are thus essential for both storage and voiding reflexes. Autonomic transmission is mediated by activation of the nicotinic acetylcholine receptor (nAChR) in PG neurons. Previously, bladder outlet obstruction (BOO), secondary to benign prostatic hyperplasia, was found to increase soma sizes of bladder-projecting PG neurons. To date, however, it remains unknown whether these morphological changes are accompanied by functional plasticity in PG neurons. In the present study, we investigated whether BOO alters acetylcholine receptor (nAChR) transcript expression and current density in bladder PG neurons. Partial ligation of the rat urethra for six weeks induced detrusor overactivity (DO), as observed during cystometrical measurement. In rats exhibiting DO, membrane capacitance of parasympathetic bladder PG neurons was selectively increased. Real-time PCR analysis revealed that BOO enhanced the expression of the transcripts encoding the nAChR α3 and β4 subunits in PG neurons. Notably, BOO significantly increased ACh-evoked current density in parasympathetic bladder PG neurons, whereas no changes were observed in sympathetic bladder and parasympathetic penile PG neurons. In addition, other ligand-gated ionic currents were immune to BOO in bladder PG neurons. Taken together, these data suggest that BOO causes upregulation of nAChR in parasympathetic bladder PG neurons, which in turn may potentiate ganglionic transmission and contribute to the development of DO.

Nickel suppresses the PACAP-induced increase in guinea pig cardiac neuron excitability

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a potent intercellular signaling molecule involved in multiple homeostatic functions. PACAP/PAC1 receptor signaling increases excitability of neurons within the guinea pig cardiac ganglia, making them a unique system to establish mechanisms underlying PACAP modulation of neuronal function. Calcium influx is required for the PACAP-increased cardiac neuron excitability, although the pathway is unknown. This study tested whether PACAP enhancement of calcium influx through either T-type or R-type channels contributed to the modulation of excitability. Real-time quantitative polymerase chain reaction analyses indicated transcripts for Cav3.1, Cav3.2, and Cav3.3 T-type isoforms and R-type Cav2.3 in cardiac neurons. These neurons often exhibit a hyperpolarization-induced rebound depolarization that remains when cesium is present to block hyperpolarization-activated nonselective cationic currents (Ih). The T-type calcium channel inhibitors, nickel (Ni(2+)) or mibefradil, suppressed the rebound depolarization, and treatment with both drugs hyperpolarized cardiac neurons by 2-4 mV. Together, these results are consistent with the presence of functional T-type channels, potentially along with R-type channels, in these cardiac neurons. Fifty micromolar Ni(2+), a concentration that suppresses currents in both T-type and R-type channels, blunted the PACAP-initiated increase in excitability. Ni(2+) also blunted PACAP enhancement of the hyperpolarization-induced rebound depolarization and reversed the PACAP-mediated increase in excitability, after being initiated, in a subset of cells. Lastly, low voltage-activated currents, measured under perforated patch whole cell recording conditions and potentially flowing through T-type or R-type channels, were enhanced by PACAP. Together, our results suggest that a PACAP-enhanced, Ni(2+)-sensitive current contributes to PACAP-induced modulation of neuronal excitability.

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.