Optogenetic Induction of Colonic Motility in Mice

Effects of optogenetic activation of the enteric nervous system on gastrointestinal motility in mouse small intestine

BACKGROUND AND AIMS

Recently, it was demonstrated that optogenetics could be used to stimulate enteric calretinin neurons, leading to increased colonic transit in vitro and in vivo. The aim of the current study was to determine if similar approaches could be used to stimulate the isolated mouse small intestine, with the aim of potentially also improving transit in the small bowel.

METHODS

Cre-Lox recombination was used to generate transgenic mice expressing the light-sensitive ion channel channelrhodopsin-2 (ChR2) in calretinin neurons.

RESULTS

Spontaneous migrating motor complexes were recorded from isolated terminal small intestine in both Cal^Cre+ mice expressing ChR2 in calretinin-expressing neurons and experimental controls, Cal^Cre-. Trains of blue light pulses (20 ms, 5 Hz, 20s) evoked a brief local contraction of circular muscle, but never a premature MMC, irrespective of light intensity (1-40 mV/mm²) or the region of ileum stimulated. However, MMCs were readily evoked by calretinin neuron activation in colon, consistent with our previous study. Light-evoked contractions of the terminal small bowel were hexamethonium-resistant (300 μM), but blocked by tetrodotoxin (0.6 μM). Light-evoked smooth muscle contraction did not change the pacemaker frequency underlying MMCs.

CONCLUSION

Focal illumination of the small intestine does not appear as effective at inducing propulsive motor activity as has been demonstrated in the colon of the same colony mice. This study suggests caution should be exercised when assuming optogenetic technology is equally effective at increasing GI transit in the small intestine as in the large intestine of mice.

Neural control of gut homeostasis

The gut-brain axis is a coordinated communication system that not only maintains homeostasis, but significantly influences higher cognitive functions and emotions, as well as neurological and behavioral disorders. Among the large populations of sensory and motor neurons that innervate the gut, insights into the function of primary afferent nociceptors, whose cell bodies reside in the dorsal root ganglia and nodose ganglia, have revealed their multiple crosstalk with several cell types within the gut wall, including epithelial, vascular, and immune cells. These bidirectional communications have immunoregulatory functions, control host response to pathogens, and modulate sensations associated with gastrointestinal disorders, through activation of immune cells and glia in the peripheral and central nervous system, respectively. Here, we will review the cellular and neurochemical basis of these interactions at the periphery, in dorsal root ganglia, and in the spinal cord. We will discuss the research gaps that should be addressed to get a better understanding of the multifunctional role of sensory neurons in maintaining gut homeostasis and regulating visceral sensitivity.

Selective optogenetic stimulation of efferent fibers in the vagus nerve of a large mammal

Background

Electrical stimulation applied to individual organs, peripheral nerves, or specific brain regions has been used to treat a range of medical conditions. In cardiovascular disease, autonomic dysfunction contributes to the disease progression and electrical stimulation of the vagus nerve has been pursued as a treatment for the purpose of restoring the autonomic balance. However, this approach lacks selectivity in activating function- and organ-specific vagal fibers and, despite promising results of many preclinical studies, has so far failed to translate into a clinical treatment of cardiovascular disease.

Objective

Here we report a successful application of optogenetics for selective stimulation of vagal efferent activity in a large animal model (sheep).

Methods and results

Twelve weeks after viral transduction of a subset of vagal motoneurons, strong axonal membrane expression of the excitatory light-sensitive ion channel ChIEF was achieved in the efferent projections innervating thoracic organs and reaching beyond the level of the diaphragm. Blue laser or LED light (>10 mW mm⁻²; 1 ms pulses) applied to the cervical vagus triggered precisely timed, strong bursts of efferent activity with evoked action potentials propagating at speeds of ∼6 m s⁻¹.

Conclusions

These findings demonstrate that in species with a large, multi-fascicled vagus nerve, it is possible to stimulate a specific sub-population of efferent fibers using light at a site remote from the vector delivery, marking an important step towards eventual clinical use of optogenetic technology for autonomic neuromodulation.

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Described is a method of selective efferent vagus nerve stimulation using light.

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Vagal preganglionic neurons are targeted to express light-sensitive channels.

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Specific efferent VNS by light delivery to the cervical vagus is achieved in a large animal model.

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Demonstrates feasibility of using optogenetic technology for autonomic neuromodulation.

Recent advances in neurotechnologies with broad potential for neuroscience research

Interest in deciphering the fundamental mechanisms and processes of the human mind represents a central driving force in modern neuroscience research. Activities in support of this goal rely on advanced methodologies and engineering systems that are capable of interrogating and stimulating neural pathways, from single cells in small networks to interconnections that span the entire brain. Recent research establishes the foundations for a broad range of creative neurotechnologies that enable unique modes of operation in this context. This review focuses on those systems with proven utility in animal model studies and with levels of technical maturity that suggest a potential for broad deployment to the neuroscience community in the relatively near future. We include a brief summary of existing and emerging neuroscience techniques, as background for a primary focus on device technologies that address associated opportunities in electrical, optical and microfluidic neural interfaces, some with multimodal capabilities. Examples of the use of these technologies in recent neuroscience studies illustrate their practical value. The vibrancy of the engineering science associated with these platforms, the interdisciplinary nature of this field of research and its relevance to grand challenges in the treatment of neurological disorders motivate continued growth of this area of study.

Human resident gut microbe Bacteroides thetaiotaomicron regulates colonic neuronal innervation and neurogenic function

BACKGROUND AND AIMS

As the importance of gut-brain interactions increases, understanding how specific gut microbes interact with the enteric nervous system (ENS), which is the first point of neuronal exposure becomes critical. Our aim was to understand how the dominant human gut bacterium Bacteroides thetaiotaomicron (Bt) regulates anatomical and functional characteristics of the ENS.

METHODS

Neuronal cell populations, as well as enteroendocrine cells, were assessed in proximal colonic sections using fluorescent immunohistochemistry in specific pathogen-free (SPF), germ-free (GF) and Bt conventionalized-germ-free mice (Bt-CONV). RNA expression of tight junction proteins and toll-like receptors (TLR) were measured using qPCR. Colonic motility was analyzed using in vitro colonic manometry.

RESULTS

Decreased neuronal and vagal afferent innervation observed in GF mice was normalized by Bt-CONV with increased neuronal staining in mucosa and myenteric plexus. Bt-CONV also restored expression of nitric oxide synthase expressing inhibitory neurons and of choline acetyltransferase and substance P expressing excitatory motor neurons comparable to those of SPF mice. Neurite outgrowth and glial cells were upregulated by Bt-CONV. RNA expression of tight junction protein claudin 3 was downregulated while TLR2 was upregulated by Bt-CONV. The enteroendocrine cell subtypes L-cells and enterochromaffin cells were reduced in GF mice, with Bt-CONV restoring L-cell numbers. Motility as measured by colonic migrating motor complexes (CMMCs) increased in GF and Bt-CONV.

CONCLUSION

Bt, common gut bacteria, is critical in regulating enteric neuronal and enteroendocrine cell populations, and neurogenic colonic activity. This highlights the potential use of this resident gut bacteria for maintaining healthy gut function.

Functional circuits and signal processing in the enteric nervous system

The enteric nervous system (ENS) is an extensive network comprising millions of neurons and glial cells contained within the wall of the gastrointestinal tract. The major functions of the ENS that have been most studied include the regulation of local gut motility, secretion, and blood flow. Other areas that have been gaining increased attention include its interaction with the immune system, with the gut microbiota and its involvement in the gut-brain axis, and neuro-epithelial interactions. Thus, the enteric circuitry plays a central role in intestinal homeostasis, and this becomes particularly evident when there are faults in its wiring such as in neurodevelopmental or neurodegenerative disorders. In this review, we first focus on the current knowledge on the cellular composition of enteric circuits. We then further discuss how enteric circuits detect and process external information, how these signals may be modulated by physiological and pathophysiological factors, and finally, how outputs are generated for integrated gut function.

Enlightening the frontiers of neurogastroenterology through optogenetics

Neurogastroenterology refers to the study of the extrinsic and intrinsic nervous system circuits controlling the gastrointestinal (GI) tract. Over the past 5-10 yr there has been an explosion in novel methodologies, technologies and approaches that offer great promise to advance our understanding of the basic mechanisms underlying GI function in health and disease. This review focuses on the use of optogenetics combined with electrophysiology in the field of neurogastroenterology. We discuss how these technologies and tools are currently being used to explore the brain-gut axis and debate the future research potential and limitations of these techniques. Taken together, we consider that the use of these technologies will enable researchers to answer important questions in neurogastroenterology through fundamental research. The answers to those questions will shorten the path from basic discovery to new treatments for patient populations with disorders of the brain-gut axis affecting the GI tract such as irritable bowel syndrome (IBS), functional dyspepsia, achalasia, and delayed gastric emptying.

In Vivo Optogenetic Modulation with Simultaneous Neural Detection Using Microelectrode Array Integrated with Optical Fiber

The detection of neuroelectrophysiology while performing optogenetic modulation can provide more reliable and useful information for neural research. In this study, an optical fiber and a microelectrode array were integrated through hot-melt adhesive bonding, which combined optogenetics and electrophysiological detection technology to achieve neuromodulation and neuronal activity recording. We carried out the experiments on the activation and electrophysiological detection of infected neurons at the depth range of 900-1250 μm in the brain which covers hippocampal CA1 and a part of the upper cortical area, analyzed a possible local inhibition circuit by combining opotogenetic modulation and electrophysiological characteristics and explored the effects of different optical patterns and light powers on the neuromodulation. It was found that optogenetics, combined with neural recording technology, could provide more information and ideas for neural circuit recognition. In this study, the optical stimulation with low frequency and large duty cycle induces more intense neuronal activity and larger light power induced more action potentials of neurons within a certain power range (1.032 mW-1.584 mW). The present study provided an efficient method for the detection and modulation of neurons in vivo and an effective tool to study neural circuit in the brain.

A Novel Mode of Sympathetic Reflex Activation Mediated by the Enteric Nervous System

Enteric viscerofugal neurons provide a pathway by which the enteric nervous system (ENS), otherwise confined to the gut wall, can activate sympathetic neurons in prevertebral ganglia. Firing transmitted through these pathways is currently considered fundamentally mechanosensory. The mouse colon generates a cyclical pattern of neurogenic contractile activity, called the colonic motor complex (CMC). Motor complexes involve a highly coordinated firing pattern in myenteric neurons with a frequency of ∼2 Hz. However, it remains unknown how viscerofugal neurons are activated and communicate with the sympathetic nervous system during this naturally-occurring motor pattern. Here, viscerofugal neurons were recorded extracellularly from rectal nerve trunks in isolated tube and flat-sheet preparations of mouse colon held at fixed circumferential length. In freshly dissected preparations, motor complexes were associated with bursts of viscerofugal firing at 2 Hz that aligned with 2-Hz smooth muscle voltage oscillations. This behavior persisted during muscle paralysis with nicardipine. Identical recordings were made after a 4- to 5-d organotypic culture during which extrinsic nerves degenerated, confirming that recordings were from viscerofugal neurons. Single unit analysis revealed the burst firing pattern emerging from assemblies of viscerofugal neurons differed from individual neurons, which typically made partial contributions, highlighting the importance and extent of ENS-mediated synchronization. Finally, sympathetic neuron firing was recorded from the central nerve trunks emerging from the inferior mesenteric ganglion. Increased sympathetic neuron firing accompanied all motor complexes with a 2-Hz burst pattern similar to viscerofugal neurons. These data provide evidence for a novel mechanism of sympathetic reflex activation derived from synchronized firing output generated by the ENS.

Enteric nervous system: sensory transduction, neural circuits and gastrointestinal motility

The gastrointestinal tract is the only internal organ to have evolved with its own independent nervous system, known as the enteric nervous system (ENS). This Review provides an update on advances that have been made in our understanding of how neurons within the ENS coordinate sensory and motor functions. Understanding this function is critical for determining how deficits in neurogenic motor patterns arise. Knowledge of how distension or chemical stimulation of the bowel evokes sensory responses in the ENS and central nervous system have progressed, including critical elements that underlie the mechanotransduction of distension-evoked colonic peristalsis. Contrary to original thought, evidence suggests that mucosal serotonin is not required for peristalsis or colonic migrating motor complexes, although it can modulate their characteristics. Chemosensory stimuli applied to the lumen can release substances from enteroendocrine cells, which could subsequently modulate ENS activity. Advances have been made in optogenetic technologies, such that specific neurochemical classes of enteric neurons can be stimulated. A major focus of this Review will be the latest advances in our understanding of how intrinsic sensory neurons in the ENS detect and respond to sensory stimuli and how these mechanisms differ from extrinsic sensory nerve endings in the gut that underlie the gut-brain axis.

Optogenetic analysis of neuromuscular transmission in the colon of ChAT-ChR2-YFP BAC transgenic mice

Propulsion of luminal content along the gut requires coordinated contractions and relaxations of gastrointestinal smooth muscles controlled by the enteric nervous system. Activation of excitatory motor neurons (EMNs) causes muscle contractions, whereas inhibitory motor neuron (IMN) activation causes muscle relaxation. EMNs release acetylcholine (ACh), which acts at muscarinic receptors on smooth muscle cells and adjacent interstitial cells of Cajal, causing excitatory junction potentials (EJPs). IMNs release ATP (or another purine) and nitric oxide to cause inhibitory junction potentials (IJPs) and muscle relaxation. We used commercially available choline acetyltransferase (ChAT)-channelrhodopsin-2 (ChR2)-yellow fluorescent protein (YFP) bacterial artificial chromosome (BAC) transgenic mice, which express ChR2 in cholinergic neurons, to study cholinergic neuromuscular transmission in the colon. Intracellular microelectrodes were used to record IJPs and EJPs from circular muscle cells. We used blue light stimulation (BLS, 470 nm, 20 mW/mm²) and electrical field stimulation (EFS) to activate myenteric neurons. EFS evoked IJPs only, whereas BLS evoked EJPs and IJPs. Mecamylamine (10 µM, nicotinic cholinergic receptor antagonist) reduced BLS-evoked IJPs by 50% but had no effect on electrically evoked IJPs. MRS 2179 (10 µM, a P2Y₁ receptor antagonist) blocked BLS-evoked IJPs. MRS 2179 and N^ω-nitro-l-arginine (100 µM, nitric oxide synthase inhibitor) isolated the EJP, which was blocked by scopolamine (1 µM, muscarinic ACh receptor antagonist). Immunohistochemistry revealed ChAT expression in ~88% of enhanced YFP (eYFP)-expressing neurons, whereas 12% of eYFP neurons expressed nitric oxide synthase. These data show that cholinergic interneurons synapse with EMNs and IMNs to cause contraction and relaxation of colonic smooth muscle.NEW & NOTEWORTHY Electrical stimulation of interganglionic connectives has been used widely to study synaptic transmission in the enteric nervous system. However, electrical stimulation will activate many types of neurons and nerve fibers, which complicates data interpretation. Optogenetic activation of enteric neurons using genetically modified mice expressing channelrhodopsin-2 in cholinergic neurons offers a new approach that provides more specificity for nerve stimulation when studying myenteric plexus nerve circuitry.

Optogenetic control of the enteric nervous system and gastrointestinal transit

There are limited effective therapies available for improving gastrointestinal (GI) transit in mammals with intractable or chronic constipation. Current therapeutics to improve GI-transit usually require oral ingestion of therapeutic drugs, such as the serotonin receptor agonist prucalopride. However, most receptors are distributed all over the body and unsurprisingly drugs like prucalopride stimulate multiple organs, often leading to unwanted side effects. There is a desperate need in the community to improve GI-transit selectively without effects on other organs. Areas covered: We performed a systematic review of the literature on Pubmed and report significant technical advances in optogenetic control of the GI-tract. We discuss recent demonstrations that optogenetics can be used to potently control the activity of subsets of enteric neurons. Special focus is made of the first recent demonstration that wireless optogenetics can be used to stimulate the colon in conscious, freely-moving, untethered mice causing a significant increase in fecal pellet output. This is a significant technical breakthrough with a major therapeutic potential application to improve GI-transit. Expert opinion: The ability to selectively stimulate the ENS to modulate GI-transit in live mammals using light, avoids the need for oral consumption of any drugs and side effects; by stimulating only the GI-tract.

Regional complexity in enteric neuron wiring reflects diversity of motility patterns in the mouse large intestine

The enteric nervous system controls a variety of gastrointestinal functions including intestinal motility. The minimal neuronal circuit necessary to direct peristalsis is well-characterized but several intestinal regions display also other motility patterns for which the underlying circuits and connectivity schemes that coordinate the transition between those patterns are poorly understood. We investigated whether in regions with a richer palette of motility patterns, the underlying nerve circuits reflect this complexity. Using Ca²⁺ imaging, we determined the location and response fingerprint of large populations of enteric neurons upon focal network stimulation. Complemented by neuronal tracing and volumetric reconstructions of synaptic contacts, this shows that the multifunctional proximal colon requires specific additional circuit components as compared to the distal colon, where peristalsis is the predominant motility pattern. Our study reveals that motility control is hard-wired in the enteric neural networks and that circuit complexity matches the motor pattern portfolio of specific intestinal regions.

A mathematical model to investigate the key drivers of the biogeography of the colon microbiota

The gut microbiota, mainly located in the colon, is engaged in a complex dialogue with the large intestinal epithelium through which important regulatory processes for the health and well-being of the host take place. Imbalances of the microbial populations, called dysbiosis, are related to several pathological status, emphasizing the importance of understanding the gut bacterial ecology. Among the ecological drivers of the microbiota, the spatial structure of the colon is of special interest: spatio-temporal mechanisms can lead to the constitution of spatial interactions among the bacterial populations and of environmental niches that impact the overall colonization of the colon. In the present study, we introduce a mathematical model of the colon microbiota in its fluid environment, based on the explicit coupling of a population dynamics model of microbial populations involved in fibre degradation with a fluid dynamics model of the luminal content. This modeling framework is used to study the main drivers of the spatial structure of the microbiota, specially focusing on the dietary fibre inflow, the epithelial motility, the microbial active swimming and viscosity gradients in the digestive track. We found 1) that the viscosity gradients allow the creation of favorable niches in the vicinity of the mucus layer; 2) that very low microbial active swimming in the radial direction is enough to promote bacterial growth, which sheds a new light on microbial motility in the colon and 3) that dietary fibres are the main driver of the spatial structure of the microbiota in the distal bowel whereas epithelial motility is preponderant for the colonization of the proximal colon; in the transverse colon, fibre levels and chemotaxis have the strongest impact on the distribution of the microbial communities.

Opportunities and Challenges for Single-Unit Recordings from Enteric Neurons in Awake Animals

Advanced electrode designs have made single-unit neural recordings commonplace in modern neuroscience research. However, single-unit resolution remains out of reach for the intrinsic neurons of the gastrointestinal system. Single-unit recordings of the enteric (gut) nervous system have been conducted in anesthetized animal models and excised tissue, but there is a large physiological gap between awake and anesthetized animals, particularly for the enteric nervous system. Here, we describe the opportunity for advancing enteric neuroscience offered by single-unit recording capabilities in awake animals. We highlight the primary challenges to microelectrodes in the gastrointestinal system including structural, physiological, and signal quality challenges, and we provide design criteria recommendations for enteric microelectrodes.

Identification of a Rhythmic Firing Pattern in the Enteric Nervous System That Generates Rhythmic Electrical Activity in Smooth Muscle

The enteric nervous system (ENS) contains millions of neurons essential for organization of motor behavior of the intestine. It is well established that the large intestine requires ENS activity to drive propulsive motor behaviors. However, the firing pattern of the ENS underlying propagating neurogenic contractions of the large intestine remains unknown. To identify this, we used high-resolution neuronal imaging with electrophysiology from neighboring smooth muscle. Myoelectric activity underlying propagating neurogenic contractions along murine large intestine [also referred to as colonic migrating motor complexes, (CMMCs)] consisted of prolonged bursts of rhythmic depolarizations at a frequency of ∼2 Hz. Temporal coordination of this activity in the smooth muscle over large spatial fields (∼7 mm, longitudinally) was dependent on the ENS. During quiescent periods between neurogenic contractions, recordings from large populations of enteric neurons, in mice of either sex, revealed ongoing activity. The onset of neurogenic contractions was characterized by the emergence of temporally synchronized activity across large populations of excitatory and inhibitory neurons. This neuronal firing pattern was rhythmic and temporally synchronized across large numbers of ganglia at ∼2 Hz. ENS activation preceded smooth muscle depolarization, indicating rhythmic depolarizations in smooth muscle were controlled by firing of enteric neurons. The cyclical emergence of temporally coordinated firing of large populations of enteric neurons represents a unique neural motor pattern outside the CNS. This is the first direct observation of rhythmic firing in the ENS underlying rhythmic electrical depolarizations in smooth muscle. The pattern of neuronal activity we identified underlies the generation of CMMCs.SIGNIFICANCE STATEMENT How the enteric nervous system (ENS) generates neurogenic contractions of smooth muscle in the gastrointestinal (GI) tract has been a long-standing mystery in vertebrates. It is well known that myogenic pacemaker cells exist in the GI tract [called interstitial cells of Cajal (ICCs)] that generate rhythmic myogenic contractions. However, the mechanisms underlying the generation of rhythmic neurogenic contractions of smooth muscle in the GI tract remains unknown. We developed a high-resolution neuronal imaging method with electrophysiology to address this issue. This technique revealed a novel pattern of rhythmic coordinated neuronal firing in the ENS that has never been identified. Rhythmic neuronal firing in the ENS was found to generate rhythmic neurogenic depolarizations in smooth muscle that underlie contraction of the GI tract.