Role of Glutamate Metabotropic Receptors in Synaptic Function in the Spinal Cord

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9604654 Randic The perception of pain requires excitatory synaptic transmission from primary afferent sensory fibers to secondary projection neurons in the dorsal horn (DH) region of the spinal cord. It is now well established that the primary afferent fibers use glutamate, or related amino acid, to mediate this initial excitatory step in the pain pathway. Glutamate produces its effects through two broad categories of receptors called ionotropic and metabotropic. The activation of ionotropic glutamate receptors is essential for fast excitatory transmission in the brain and spinal cord. Multiple metabotropic glutamate receptors (mGluRs) are expressed in neurons of the spinal cord dorsal horn (DH), the site of processing of sensory signals, but their roles in synaptic function, and contribution to the spinal somatosensory transmission, including nociception, have yet to be elucidated. Until recently, progress in identifying the physiological and pathophysiological roles of mGluRs has been hindered by the lack of selective agonists and antagonists. The recent development of ligands that bind specifically to these receptors has provided a means of characterizing the important roles they may play in tuning of fast synaptic transmission, including the induction of long-term changes in synaptic efficacy. Three major objectives of the proposed study are: 1. To pharmacologically identify mGluR subtype(s) involved in modulation of primary afferent neurotransmission and synaptic plasticity in the spinal cord dorsal horn (laminae I-II). 2. To examine whether activation of the Group I mGluRs (mGluR1 and/or mGluR5) mobilizes Ca2+ from internal stores in LI-II neurons. 3. To study characteristics of Ca2+-induced Ca2+ release mechanism in the generation of Ca2+ signals in LI-II neurons. A combination of fura-2 imaging, extracellular, intracellular (sharp microelectrodes) and whole-cell and perforated-patch recordings, and pharmacological techniques, will be employed to study the effects of selective mGluR agonists and antagonists on excitatory synaptic responses and mobilization of Ca2+ from internal stores in the in vitro preparations of acutely isolated DH neurons (LI-II), and also in the LI-II neurons within rat spinal cord slices. LI-II cells in the spinal slices will be visualized using infra-red videomicroscopy and intracellular labeling with biocytin and rhodamine. Delineating the physiological roles of individual subtypes of the mGluR in the spinal DH, and cellular and molecular mechanisms underlying their actions, is an important step toward understanding of implications of glutamate-mediated transmission in the spinal cord DH synaptic function, but in particular for synaptic plasticity. Finally, this study may contribute to our understanding of pathophysiological processes in the spinal cord, especially pain, and thereby may lead to the development of rational therapies.