Recordings from human myenteric neurons using voltage-sensitive dyes

Simulating the Voltage-Dependent Fluorescence of Di-8-ANEPPS in a Lipid Membrane

Voltage-sensitive fluorescent dyes such as di-8-ANEPPS (di-8-aminonaphthylethylenepyridinium propylsulfonate) are powerful tools to study biological membranes. Its fluorescence is affected by changes in the membrane potential and other factors, requiring extensive calibration to extract meaningful quantitative results. The amphiphilic di-8-ANEPPS molecule is expected to bind at the membrane-solution interface. However, atomic-level information is sparse about its position and orientation in the membrane, especially in regards to how the latter dynamically fluctuates to affect the observed fluorescence. In the present work, molecular dynamics simulations of the ground and excited states of di-8-ANEPPS embedded in a DPPC membrane as represented by classical force fields were used to investigate how the fluorescence is affected by externally applied potential. The calculations reproduce the shifts in the wavelength of emission as a function of voltage that are observed experimentally, indicating that the approach can help better understand the various factors that can affect the fluorescence of membrane-bound dyes.

Short- and Long-Term Effects of Cocaine on Enteric Neuronal Functions

Cocaine is one of the most consumed illegal drugs among (young) adults in the European Union and it exerts various acute and chronic negative effects on psychical and physical health. The central mechanism through which cocaine initially leads to improved performance, followed by addictive behavior, has already been intensively studied and includes effects on the homeostasis of the neurotransmitters dopamine, partly mediated via nicotinic acetylcholine receptors, and serotonin. However, effects on the peripheral nervous system, including the enteric nervous system, are much less understood, though a correlation between cocaine consumption and gastrointestinal symptoms has been reported. The aim of the present study was to gain more information on the effects of cocaine on enteric neuronal functions and the underlying mechanisms. For this purpose, functional experiments using an organ bath, Ussing chamber and neuroimaging techniques were conducted on gastrointestinal tissues from guinea pigs. Key results obtained are that cocaine (1) exhibits a stimulating, non-neuronal effect on gastric antrum motility, (2) acutely (but not chronically) diminishes responses of primary cultured enteric neurons to nicotinic and serotonergic stimulation and (3) reversibly attenuates neuronal-mediated intestinal mucosal secretion. It can be concluded that cocaine, among its central effects, also alters enteric neuronal functions, providing potential explanations for the coexistence of cocaine abuse and gastrointestinal complaints.

Imaging of mast cells

Mast cells are a part of the innate immune system implicated in allergic reactions and the regulation of host-pathogen interactions. The distribution, morphology and biochemical composition of mast cells has been studied in detail in vitro and on tissue sections both at the light microscopic and ultrastructural level. More recently, the development of fluorescent reporter strains and intravital imaging modalities has enabled first glimpses of the real-time behavior of mast cells in situ. In this review, we describe commonly used imaging approaches to study mast cells in cell culture as well as within normal and diseased tissues. We further describe the interrogation of mast cell function via imaging by providing a detailed description of mast cell-nerve plexus interactions in the intestinal tract. Together, visualizing mast cells has expanded our view of these cells in health and disease.

Excitability and Synaptic Transmission in the Enteric Nervous System: Does Diet Play a Role?

Changes in diet are a challenge to the gastrointestinal tract which needs to alter its processing mechanisms to continue to process nutrients and maintain health. In particular, the enteric nervous system (ENS) needs to adapt its motor and secretory programs to deal with changes in nutrient type and load in order to optimise nutrient absorption.The nerve circuits in the gut are complex, and the numbers and types of neurons make recordings of specific cell types difficult, time-consuming, and prone to sampling errors. Nonetheless, traditional research methods like intracellular electrophysiological approaches have provided the basis for our understanding of the ENS circuitry. In particular, animal models of intestinal inflammation have shown us that we can document changes to neuronal excitability and synaptic transmission.Recent studies examining diet-induced changes to ENS programming have opted to use fast imaging techniques to reveal changes in neuron function. Advances in imaging techniques using voltage- or calcium-sensitive dyes to record neuronal activity promise to overcome many limitations inherent to electrophysiological approaches. Imaging techniques allow access to a wide range of ENS phenotypes and to the changes they undergo during dietary challenges. These sorts of studies have shown that dietary variation or obesity can change how the ENS processes information-in effect reprogramming the ENS. In this review, the data gathered from intracellular recordings will be compared with measurements made using imaging techniques in an effort to determine if the lessons learnt from inflammatory changes are relevant to the understanding of diet-induced reprogramming.

Colitis-induced neuroplasticity disrupts motility in the inflamed and post-inflamed colon

Effective colonic motility involves an intricate pattern of excitatory and inhibitory neuromuscular signals that arise from the enteric neural circuitry of the colon. Recent investigations have demonstrated that inflammation leads to a variety of changes in the physiological properties of the neurons in this circuitry, including hyperexcitability of neurons at the afferent end of the peristaltic reflex, synaptic facilitation, and attenuated inhibitory neuromuscular transmission. Furthermore, links have been established between these changes and disrupted motor activity in the colon, and we now know that some of these changes persist long after recovery from inflammation. It is highly likely that inflammation-induced neuroplasticity, which is not detectable by clinical diagnostics, contributes to disrupted motility in active and quiescent inflammatory bowel disease and in functional gastrointestinal disorders.

Human enteric neurons: morphological, electrophysiological, and neurochemical identification

BACKGROUND

Access to tissue, difficulties with dissection, and poor visibility of enteric ganglia have hampered electrophysiological recordings of human enteric neurons. Here, we report a method to combine intracellular recording with simultaneous morphological identification of neurons in the intact myenteric plexus of human colon ex vivo.

METHODS

Specimens of human colon were dissected into flat-sheet preparations with the myenteric plexus exposed. Myenteric neurons were impaled with conventional microelectrodes containing 5% 5,6-carboxyfluorescein in 20 mM Tris buffer and 1 M KCl.

KEY RESULTS

Electrophysiological recordings identified myenteric neurons with S and AH type properties (n = 13, N = 7) which were dye filled and classified during the recording as Dogiel type I (n = 10), Dogiel type II (n = 2), or filamentous (n = 1) cells. This classification was confirmed after fixation, in combination with immunohistochemical characterization.

CONCLUSIONS & INFERENCES

This method allows electrophysiological characterization with simultaneous identification of morphology. It can be used to identify recorded cells immediately after impalement and greatly facilitates recordings of human myenteric neurons in freshly dissected specimens of tissue. It can also be combined with immunohistochemical labeling of recorded cells.

Recent developments in VSD imaging of small neuronal networks

Voltage-sensitive dye (VSD) imaging is a powerful technique that can provide, in single experiments, a large-scale view of network activity unobtainable with traditional sharp electrode recording methods. Here we review recent work using VSDs to study small networks and highlight several results from this approach. Topics covered include circuit mapping, network multifunctionality, the network basis of decision making, and the presence of variably participating neurons in networks. Analytical tools being developed and applied to large-scale VSD imaging data sets are discussed, and the future prospects for this exciting field are considered.

Translational neuropharmacology: the use of human isolated gastrointestinal tissues

Translational sciences increasingly emphasize the measurement of functions in native human tissues. However, such studies must confront variations in patient age, gender, genetic background and disease. Here, these are discussed with reference to neuromuscular and neurosecretory functions of the human gastrointestinal (GI) tract. Tissues are obtained after informed consent, in collaboration with surgeons (surgical techniques help minimize variables) and pathologists. Given the difficulties of directly recording from human myenteric neurones (embedded between muscle layers), enteric motor nerve functions are studied by measuring muscle contractions/relaxations evoked by electrical stimulation of intrinsic nerves; responses are regionally dependent, often involving cholinergic and nitrergic phenotypes. Enteric sensory functions can be studied by evoking the peristaltic reflex, involving enteric sensory and motor nerves, but this has rarely been achieved. As submucosal neurones are more accessible (after removing the mucosa), direct neuronal recordings are possible. Neurosecretory functions are studied by measuring changes in short-circuit current across the mucosa. For all experiments, basic questions must be addressed. Because tissues are from patients, what are the controls and the influence of disease? How long does it take before function fully recovers? What is the impact of age- and gender-related differences? What is the optimal sample size? Addressing these and other questions minimizes variability and raises the scientific credibility of human tissue research. Such studies also reduce animal use. Further, the many differences between animal and human GI functions also means that human tissue research must question the ethical validity of using strains of animals with unproved translational significance.

Functions and imaging of mast cell and neural axis of the gut

Close association between nerves and mast cells in the gut wall provides the microanatomic basis for functional interactions between these elements, supporting the hypothesis that a mast cell-nerve axis influences gut functions in health and disease. Advanced morphology and imaging techniques are now available to assess structural and functional relationships of the mast cell-nerve axis in human gut tissues. Morphologic techniques including co-labeling of mast cells and nerves serve to evaluate changes in their densities and anatomic proximity. Calcium (Ca(++)) and potentiometric dye imaging provide novel insights into functions such as mast cell-nerve signaling in the human gut tissues. Such imaging promises to reveal new ionic or molecular targets to normalize nerve sensitization induced by mast cell hyperactivity or mast cell sensitization by neurogenic inflammatory pathways. These targets include proteinase-activated receptor (PAR) 1 or histamine receptors. In patients, optical imaging in the gut in vivo has the potential to identify neural structures and inflammation in vivo. The latter has some risks and potential of sampling error with a single biopsy. Techniques that image nerve fibers in the retina without the need for contrast agents (optical coherence tomography and full-field optical coherence microscopy) may be applied to study submucous neural plexus. Moreover, the combination of submucosal dissection, use of a fluorescent marker, and endoscopic confocal microscopy provides detailed imaging of myenteric neurons and smooth muscle cells in the muscularis propria. Studies of motility and functional gastrointestinal disorders would be feasible without the need for full-thickness biopsy.

Activity of protease-activated receptors in primary cultured human myenteric neurons

Activity of the four known protease-activated receptors (PARs) has been well studied in rodent enteric nervous system and results in animal models established an important role for neuronal PAR2. We recently demonstrated that, unlike in rodents, PAR1 is the dominant neuronal protease receptor in the human submucous plexus. With this study we investigated whether this also applies to the human myenteric plexus. We used voltage sensitive dye recordings to detect action potential discharge in primary cultures of human myenteric neurons in response to PAR activating peptides (APs). Application of the PAR1-AP (TFLLR) or PAR4-AP (GYPGQV) evoked spike discharge in 79 or 23% of myenteric neurons, respectively. The PAR1-AP response was mimicked by the endogenous PAR1 activator thrombin and blocked by the PAR1 antagonists SCH79797. Human myenteric neurons did not respond to PAR2-AP. This was not due to culture conditions because all three PAR-APs evoked action potentials in cultured guinea pig myenteric neurons. Consecutive application of PAR-APs revealed coexpression (relative to the population responding to PAR-APs) of PAR1/PAR2 in 51%, PAR1/PAR4 in 43%, and of PAR2/PAR4 in 29% of guinea pig myenteric neurons. Our study provided further evidence for the prominent role of neuronal PAR1 in the human enteric nervous system.

Optophysiological approach to resolve neuronal action potentials with high spatial and temporal resolution in cultured neurons

Cell to cell communication in the central nervous system is encoded into transient and local membrane potential changes (ΔVm). Deciphering the rules that govern synaptic transmission and plasticity entails to be able to perform V_m recordings throughout the entire neuronal arborization. Classical electrophysiology is, in most cases, not able to do so within small and fragile neuronal subcompartments. Thus, optical techniques based on the use of fluorescent voltage-sensitive dyes (VSDs) have been developed. However, reporting spontaneous or small ΔV_m from neuronal ramifications has been challenging, in part due to the limited sensitivity and phototoxicity of VSD-based optical measurements. Here we demonstrate the use of water soluble VSD, ANNINE-6plus, with laser-scanning microscopy to optically record ΔV_m in cultured neurons. We show that the sensitivity (>10% of fluorescence change for 100 mV depolarization) and time response (sub millisecond) of the dye allows the robust detection of action potentials (APs) even without averaging, allowing the measurement of spontaneous neuronal firing patterns. In addition, we show that back-propagating APs can be recorded, along distinct dendritic sites and within dendritic spines. Importantly, our approach does not induce any detectable phototoxic effect on cultured neurons. This optophysiological approach provides a simple, minimally invasive, and versatile optical method to measure electrical activity in cultured neurons with high temporal (ms) resolution and high spatial (μm) resolution.