Glutamatergic pathways comprise the principal excitatory networks in the central nervous system, employing a combination of synapses on dendritic spines and shafts to position neurons in circuits. How synaptogenesis can be coordinated across neuronal populations during development to form these networks remains poorly understood. Our preliminary results suggest a profound and novel role for endogenous nicotinic signaling in this process;it appears to be a major force driving development of glutamatergic pathways and determining the kinds of connections formed. Nicotinic activity results from the transmitter acetylcholine (ACh) activating ionotropic nicotinic ACh receptors (nAChRs). The receptors appear early in postnatal life and initially mediate spontaneous waves of excitation in many brain regions. The two most abundant nAChR subtypes are ?7- containing homopentamers (?7-nAChRs) and ?2-containing heteropentamers (?2*-nAChRs). Our preliminary results with brain tissue from mice lacking ?7-nAChRs (constitutive knockouts) using multiple methods suggests that the receptors are required for neurons to receive proper numbers of functional glutamatergic synapses during early postnatal life. Preliminary results with mice lacking ?2*-nAChRs indicates the receptors provide a complementary role, increasing the number of dendritic spines available to receive glutamatergic synapses. In this proposal we combine genetic manipulation, imaging, patch-clamp recording, network labeling, and ultrastructural analysis to examine how nicotinic activity influences the synaptic composition of networks in vivo.
Three Specific Aims are proposed. The first tests the hypothesis that endogenous nicotinic cholinergic signaling determines the number, location, and possibly source of glutamatergic synapses on neurons in early postnatal brain and that the effects persist into the adult, determining the ratio of glutamatergic/GABAergic input a neuron receives. The second tests the hypothesis that early exposure to nicotine in vivo, at levels encountered by smokers, disrupts normal development, produces excessive glutamatergic contacts, and imposes long-lasting changes in neural circuits. The third tests the hypothesis that nicotinic control of spine formation is rapid and local, whereas nicotinic control of synapse formation involves microRNAs for broad coordination of gene expression. The experiments employ new methods, including mapping techniques in vivo to distinguish network changes and physiological methods to assess functional consequences. The findings are likely to have major import both for prevailing views about nicotinic cholinergic function during development and for understanding mechanisms guiding network construction. The results will have direct biomedical relevance in revealing mechanisms by which early nicotine exposure is likely to produce long-lasting behavioral impairments that continue in adults. Targets may be suggested for therapeutic intervention to protect against or compensate for prior nicotine exposure. These results will represent a major step forward in understanding nervous system development and identifying key vulnerabilities.
This proposal will examine signaling pathways in the developing brain that are co-opted by nicotine and will determine whether they play critical roles in shaping the formation of excitatory networks. Results obtained here are likely to provide new insight into mechanisms controlling brain development and may identify long- lasting detrimental effects of early nicotine exposure that account for reported behavioral deficits. Biomedically, the results may also suggest targets for therapeutic intervention and remedial action, offering some good news to counter the bad.
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