Physiological, Morphological and Neurochemical Characterization of Neurons Modulated by Movement

Abstract

The role of individual neurons and their function in neuronal circuits is fundamental to understanding the neuronal mechanisms of sensory and motor functions. Most investigations of sensorimotor mechanisms rely on either examination of neurons while an animal is static 1,2 or record extracellular neuronal activity during a movement. 3,4 While these studies have provided the fundamental background for sensorimotor function, they either do not evaluate functional information which occurs during a movement or are limited in their ability to fully characterize the anatomy, physiology and neurochemical phenotype of the neuron. A technique is shown here which allows extensive characterization of individual neurons during an in vivo movement. This technique can be used not only to study primary afferent neurons but also to characterize motoneurons and sensorimotor interneurons. Initially the response of a single neuron is recorded using electrophysiological methods during various movements of the mandible followed by determination of the receptive field for the neuron. A neuronal tracer is then intracellularly injected into the neuron and the brain is processed so that the neuron can be visualized with light, electron or confocal microscopy ( Fig. 1). The detailed morphology of the characterized neuron is then reconstructed so that neuronal morphology can be correlated with the physiological response of the neuron (Figs. 2,3). In this communication important key details and tips for successful implementation of this technique are provided. Valuable additional information can be determined for the neuron under study by combining this method with other techniques. Retrograde neuronal labeling can be used to determine neurons with which the labeled neuron synapses; thus allowing detailed determination of neuronal circuitry. Immunocytochemistry can be combined with this method to examine neurotransmitters within the labeled neuron and to determine the chemical phenotypes of neurons with which the labeled neuron synapses. The labeled neuron can also be processed for electron microscopy to determine the ultrastructural features and microcircuitry of the labeled neuron. Overall this technique is a powerful method to thoroughly characterize neurons during in vivo movement thus allowing substantial insight into the role of the neuron in sensorimotor function. Check the animal to assure that a surgical level of level of anesthesia has been obtained by testing for the absence of a withdrawal reflex and vocalization when the toes are pinched as well as the absence of a palpebral reflex. Check the level of anesthesia every 15 minutes and maintain a surgical level of anesthetisa by injections of sodium pentobarbital 15mg/kg every 45 minutes. 2. Use aseptic technique and make an incision in the inguinal region just distal to the crease formed by the abdomen and inner thigh and insert a cannula (1mm diameter, Clay Adams) into the femoral vein, and femoral artery. Make a midline incision in the submandibular region, reflect th...