Graeme Lowe - US grants

The long term goal of this project is to determine the cellular mechanisms of olfaction. This proposal will focus on the initial steps leading to olfactory perception: transduction and adaptation in the olfactory receptor cells. Recent progress on the molecular biology, biochemistry, and electrophysiology of olfactory receptor cells have identified a number of intracellular messengers that may be involved in the transduction of odorous stimuli into an electrophysiological response. Nevertheless, many key questions remain unanswered regarding the importance of each putative messenger and it's role in generating or modulating odorant responses. These questions will be addressed by using patch clamp recordings from solitary receptor cells. First, the properties of the odorant-induced transduction current will be characterized. The currents evoked by all of the putative second messengers will be characterized and compared with the currents evoked by odorants. These data, together with observations on the effects of pharmacologic agents that interfere with the putative transduction mechanisms, will establish the functions of each of the intracellular messengers in olfactory transduction. Adaptation will be studied in a similar manner. A major strength of these experiments will be the use of a mammalian animal model, the rat, which will permit direct comparisons between the electrophysiological data obtained and the growing body of biochemical and molecular genetic data from rodents. An additional strength will be the use, of a novel approach: photolytically activated second messengers and odorants. This project will provide novel data about processes occurring within mammalian olfactory receptor cells, information that can have immediate relevance to human olfaction, as we have shown for induction of olfactory sensitivity in mice. In addition, this work will serve as foundation for relating physiological studies on a variety of species to the physiology and pathology of human olfactory receptor cells.

The mammalian olfactory system is unrivalled in its ability to detect, identify and discriminate an enormous variety of odor stimuli with exquisite sensitivity. What neural processing mechanisms underlie this remarkable feat? Odor information is relayed to the brain as spatially patterned activity in the glomerular layer of the olfactory bulb. The bulb transforms these patterns into the coordinated firing of ensembles of output neurons, the mitral cells. The long term objective of this research is to determine the dendritic and synaptic mechanisms that shape these firing patterns. Mitral cells radiate long secondary dendrites which are coupled, via reciprocal synapses, to granule cells. The deceptively simple structure of these dendrites belies their complex, multifunctional roles in signal processing. The aim of this project is to analyze the spatial organization of signaling in these dendrites. Our working model divides the dendrite into two dynamic domains: a proximal somatodendritic domain for temporal coding, and a distal dendritic domain for spatial coding. In the proximal domain, action potential timing is postulated to be controlled by: (i) integration of GABAergic input from reciprocal synapses, and (ii) modulation of intrinsic conductances by glutamate autoreceptors. In the distal domain, backpropagating action potentials activate calcium channels, triggering dendrodendritic transmission. It is postulated that spatial patterns of transmission depend on: (i) local modulation of action potentials, and (ii) differential distribution of calcium channels and AMPA or NMDA-type glutamate autoreceptors. We propose that calcium signaling is under dual feedback control: positive feedback amplification by NMDA receptors is balanced against negative feedback inhibition by GABA receptors. These mechanisms determine the spatiotemporal patterns of neurotransmission and electrical activity in the olfactory bulb that are central to odor information coding and processing. We analyze these mechanisms by combining brain slice patch-clamp, optical imaging, and laser photostimulation using caged compounds. This work has broad significance for understanding the control of dendritic transmission by patterns of electrical and calcium signaling, and may provide fundamental insights into the cellular bases of CNS pathologies involving the excitatory- inhibitory control of neural network activity, such as epilepsy.