Robin A.J. Lester - US grants

DESCRIPTION (Applicant's Abstract): Why neuronal nicotinic receptors (nAChRs) comprise such a structurally diverse group of ligand-gated ion channels is unknown. However, it likely reflects their diverse roles in both the modulation and mediation o synaptic transmission throughout the nervous system. nAChRs have been implicated in learning and memory, chronic addiction and neurodegenerative diseases - all of which will be influenced by factors which regulate nAChR function. Thus, a greater understanding of the participation of particular nAChRs in these processe could be achieved by determining the link between the subtype of nicotinic acetylcholine receptor (nAChR) and its short and long-term regrulation. Within the framework of addiction, this proposal addresses the relationship between subunit composition of nAChRs and their function. The specific aims will be directed towards although not limited to, answering the question: Is nAChR desensitization both necessary and sufficient to explain the alterations in nAChR number and function observed following chronic exposure to nicotine? This aim will be pursued by examining nAChRs of known subunit composition in various expression systems. The study will focus on the interaction of nicotine, at tobacco-related concentrations, with nAChR subtypes the have properties consistent with their presence in the central nervous system (CNS). It will provide evidence for the following three hypotheses: (1) The functional properties of nAChRs can be predicted from thei subunit composition. The activation and desensitization characteristics of nAChRs will be examined with the idea that certain subunits will confer dominant properties upon receptors. (2) Biochemical regulation o receptor properties occurs in a nAChR subtype-specific manner. Individual subunits will be differentially amenable to regulation by other cellular factors (e.g., Ca2+ and phosphorylation). (3) The outcome o prolonged exposure of nAChRs to nicotine will be related to the functional propeffies and the biochemical state of particular nAChRs. A full understanding of the subunit-specific properties of nAChRs will predict how different nAChRs in the CNS should be affected under conditions of long-term exposure to tobacco-related concentrations of nicotine. Such data will suggest which types of receptors are likely to be important in the addiction process. In addition to addressing issues related to the neurobiology of addiction, these studies will provide further insights into the subunit composition and regulation of nAChRs in the CNS. Overall, this knowledge will be important, not only for the design of selective and potentially therapeutic agents, but also for understanding why a single class of receptors should have such a diverse molecular basis.

DESCRIPTION: (Applicant's Abstract) Neuronal mechanisms that underlie cognitive processing are likely to involve modulation of synaptic function. Neuronal acetylcholine receptors (nAChRs) form a diverse group of channels that have been implicated in higher brain functions such as selective attention and learning and memory. Because, in particular, nAChRs are differentially permeable to the intracellular signal Ca2+, these receptors are ideally suited to contribute to a variety of long-lasting cellular events. In order to understand the role of nAChRs it is essential to understand their molecular properties. The goals of this proposal are to gain insight into: The synaptic functions of CNS nAChRs in terms of receptor composition and regulation. These aims will be addressed by testing two specific hypotheses: 1. Select nAChR subtypes serve dominant functions within CNS nuclei We suggest that the large number of nAChR subunits exist, not to form a vast number of nAChR subtypes, but to create a basic set of receptors, each of which has a minimal essential core of subunits. Inclusion of additional subunits would serve to modify their basic function. Thus we would predict that in areas of the brain where a large variety of nAChR subunits exist, certain types would dominate. 2. The level of intracellular Ca2+ controls the sensitivity of neuronal nAChRs We suggest that the function of one subtype of nAChR is to sense to the level of intracellular Ca2+ by adjusting its responsiveness to its transmitter. Thus, its function may be enhanced in the presence of ongoing synaptic activity. In turn, because nAChRs allow Ca2+ into cells they will contribute dynamically to intracellular Ca2+ signaling. Provided that this positive-feedback is kept regulated, nAChRs could contribute to long-lasting changes at synapses. If however, this process becomes unregulated, increased Ca2+ flux through potentiated nAChR channels could lead towards cell damage. By addressing these hypotheses, we will be able to offer neuronal mechanisms that could underlie higher brain functions and may help explain cholinergic-nicotinic dysfunction in certain diseases.

DESCRIPTION (provided by applicant): Nicotine addiction must develop as a down-stream consequence of the interaction of nicotine with its receptors. Thus, knowledge of how this initial molecular step produces long-term changes in CNS function will be critical to understanding how drug abuse behavior develops. With respect to nicotine-induced molecular changes our previous research has shown that in heterologous expression systems nicotine causes an apparent increase in the number of nicotinic acetylcholne receptors (nAChRs) on the plasma membrane through a specific interaction with high affinity desensitized confirmations of the receptor. We now want to extend these studies to native nAChRs in the CNS in the context of a synaptic network. Because nicotine is present in the CSF of tobacco users a t c oncentrations t hat primarily desensitize n AChRs, our observations Iead t o t he general hypothesis that: Desensitization of nAChRs is the primary mechanism through which chronic nicotine modifies synaptic function. We will explore this hypothesis by examining how nicotine-induced desensitization of alpha7* and alpha4beta2* nAChRs regulates both nAChR function and synaptic transmission in the hippocampus. There are three aims: Specific Aim 1: Examine the regulation of synaptic transmission via the endogenous activation of nAChRs. Specific Aim 2: Study the regulation of synaptic plasticity by prolonged nicotine-induced desensitization of nAChRs. Specific Aim 3: Assess the regulation of nAChR number and function by chronic exposure to nicotine.