FUNCTIONAL PROPERTIES OF NEURONAL NICOTINIC RECEPTORS

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A major distinction between nicotinic acetylcholine receptors located in the central nervous system (CNS) and the better characterized skeletal muscle nicotinic receptors is the remarkable heterogeneity of CNS receptors. Numerous cloned CNS nicotinic receptor subunits provide the basis for this heterogeneity, which has been confirmed in pharmacological studies of endogenous receptors. Heterologous expression has been useful for identifying potential receptor subtypes, but it is now apparent that the properties of neuronal receptors expressed in oocytes are not always equivalent to those of native receptors, and that the genetic composition of native CNS nicotinic receptors is more complex than previously thought. At present, endogenous CNS nicotinic receptors are poorly understood, due to a paucity of studies directed at characterizing these receptors in situ. The goal of this project is to use electrophysiological, pharmacological and biochemical techniques to define the properties of native presynaptic and postsynaptic CNS nicotinic receptors and to examine the consequences of nicotinic receptor activation in the CNS at the cellular level. Evidence from release studies and anatomical work indicates that a significant portion of CNS nicotinic receptors are located presynaptically, where they increase the release of neurotransmitters. Electrophysiological evidence confirming that nicotinic receptors enhance presynaptic transmitter release was obtained in the past funding period. Whole-cell patch-clamp electrophysiology in chick brain slices will be used to examine the nicotinic enhancement of gamma-amino-butyric acid (GABA) release, which occurs via separate mechanisms in the lateral spiriform nucleus (SPL) and the ventral lateral geniculate nucleus (GLV). Questions include: does nicotinic enhancement of transmitter release depend on Ca2+ flux through presynaptic nicotinic receptors? Are different subtypes of receptors involved? Which neurotransmitters are modulated by nicotinic receptors? Postsynaptic nicotinic receptors in the CNS will also be characterized using cell-attached patches to study the properties of single nicotinic receptor channels as well as whole-cell recordings. Neurons expressing different subtypes of nicotinic receptors will be patched in brain slices and following acute dissociation, and the heterogeneity of nicotinic receptors will be evaluated on the basis of single channel properties and pharmacological criteria. The development of presynaptic and postsynaptic nicotinic receptors in the SPL will be compared to that of functional innervation of the nucleus. Possible changes in receptor subtypes during development, which are known to occur in both skeletal muscle and autonomic ganglia, will be evaluated. Nicotinic receptors in the CNS are altered in a number of human diseases, including Alzheimer's and Parkinson's Disease. The addictive features of cigarette smoking, a major health concern, are due to the actions of nicotine at CNS nicotinic receptors. Understanding how these receptors regulate neurotransmission in the brain is thus important for human health.