Identification of Specific Sensory Neuron Populations for Study of Expressed Ion Channels
Summary
Afferent sensory neurons signal sensory information from the periphery to the central nervous system. Identifying specific afferent neurons will help in understanding their physiology. We describe a method of retrograde labeling to identify afferent neurons, and study the voltage-gated ion channels in these neurons using patch clamp electrophysiology and immunocytochemistry.
Abstract
Sensory neurons transmit signals from various parts of the body to the central nervous system. The soma for these neurons are located in the dorsal root ganglia that line the spinal column. Understanding the receptors and channels expressed by these sensory afferent neurons could lead to novel therapies for disease. The initial step is to identify the specific subset of sensory neurons of interest. Here we describe a method to identify afferent neurons innervating the muscles by retrograde labeling using a fluorescent dye DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate). Understanding the contribution of ion channels to excitation of muscle afferents could help to better control excessive excitability induced by certain disease states such as peripheral vascular disease or heart failure. We used two approaches to identify the voltage dependent ion channels expressed by these neurons, patch clamp electrophysiology and immunocytochemistry. While electrophysiology plus pharmacological blockers can identify functional ion channel types, we used immunocytochemistry to identify channels for which specific blockers were unavailable and to better understand the ion channel distribution pattern in the cell population. These techniques can be applied to other areas of the nervous system to study specific neuronal groups.
Introduction
Dorsal root ganglia (DRG) are comprised of soma from afferent neurons innervating various parts of the body, and important insights can be gained from studying sensory neurons with different innervation targets1. Various indirect or invasive methods are used to isolate and identify afferent neurons for e.g. injecting the nerve with markers like fluorogold1, injecting dye into the muscles by surgical method2. Of the different dyes that have been successfully used to label neurons and neuronal tracts, fluorescent dyes like DiI are widely used in neuroanatomical tracings owing to their rapid uptake by neuron terminals, long term retention in neurons, and the lack of transfer between neurons3. The afferent neurons labeled with DiI can be easily visualized using a fluorescent microscope for detailed study. However, one limitation is that markers such as DiI can be lost from cells during cell permeabilization needed for immunocytochemistry with intracellular epitopes4. Here we describe a method that uses DiI injected into skeletal muscle to identify muscle afferent neurons in the DRG. We also describe a cell permeabilization method that retains DiI in the neuron while permitting antibody access to intracellular epitopes. We demonstrate use of both patch clamp electrophysiology and immunocytochemistry to study these identified neurons, as well as describe the distinct advantages of each technique in studying ion channels that control neuronal excitability. Finally, we describe potential problems that can hinder interpretation of electrophysiological data. We have used these methods to identify muscle and cutaneous afferent neurons and determine the voltage-gated sodium (NaV) channels expressed in these identified neurons8. However, these methods can be adapted to a variety of preparations for studying identified subsets of neurons innervating a particular target tissue or brain nucleus.
Protocol
Representative Results
Discussion
Muscle afferents originating from triceps surae muscles (e.g. gastrocnemius) have been used in multiple studies to investigate the exercise pressor reflex (EPR)13. Given the interest in studying sensory neurons mediating the EPR, the gastrocnemius muscles were selected for dye injection. DiI was chosen due to its lipophilic nature and since it is not lost from the neuron by diffusion through the intact plasma membrane3,14. Using this method as a guide, one can inject the dye into any target…
Divulgazioni
The authors have nothing to disclose.
Acknowledgements
This work was funded by National Institutes of Health Grant AR059397 (KSE).
Materials
| NaV 1.8 | Abcam | Ab93616 | |
| NaV 1.1 / 1.6/ 1.7 and 1.9 | Alamone Labs, Jerusalem, Israel | ASC- 001/ 009 /008 /017 | |
| Alexa Fluor secondary antibodies | Invitrogen | ||
| Normal goat serum | |||
| Fluoro-Gel | Electron Microscopy Sciences | 17985 |
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