Jeffry S. Isaacson - US grants

Project Summary/Abstract In humans, the sense of smell contributes significantly to our quality of life. Odors are strong cues for evoking memories of past events and olfaction plays an important role in our perception of flavor. However, the mechanisms underlying the processing and encoding of olfactory information in the brain are not clear. To address this question, we study the properties of neuronal circuits formed by excitatory and inhibitory neurons in the rodent olfactory bulb, a site where olfactory information is initially encoded in the brain. We also examine the properties of circuits in the primary olfactory (piriform) cortex, the next major level at which olfactory information is processed. Our long term goal is to understand how neural circuits ultimately give rise to sensory perception The experiments proposed employ optical and electrophysiological techniques to determine the response properties of identified neuronal populations in both the olfactory bulbs and cortex. Specific Aim 1 proposes the use of chronic, in vivo two-photon recording of activity from identified principal cells and inhibitory interneurons in the olfactory bulb of awake, head-fixed mice. We will use this approach to determine how associative learning modifies odor representations in behaving mice. We hypothesize that local interneurons play an important role in learning-related changes in olfactory bulb activity. Specific Aim 2 proposes to investigate how particular subtypes of GABAergic inhibitory interneurons modulate odor coding in piriform cortex. We combine optogenetics, brain slice and in vivo electrophysiological recordings to reveal the operations by which distinct interneuron classes govern odor-evoked activity of cortical principal cells.Together, these experiments will show how local inhibitory interneurons in the olfactory bulb and cortex regulate sensory information processing in the brain.

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. 1. Optimize a BLV syncytium-forming assay for virus titration, and develop additional methods to study expression of BLV mRNA and proteins. Most published studies comparing various stages of BLV infection use peripheral blood mononuclear cells (PBMCs) isolated from the blood of naturally or experimentally infected cattle. The use of an in vitro model of BLV infection using various cell lines will allow better control of variables inherent in studying live animals (duration of infection, viral load, concomitant infections). In addition, being able to infect cells at a single time point will allow determination of the chronology of various effects in cells following viral infection. We are also optimizing a reverse-transcriptase PCR (RT-PCR) assay to study levels of various BLV RNAs in vitro, and we will also use ELISA and flow cytometry to study expression of BLV-encoded proteins. 2. Test various substances as potential modulators (inhibitors or enhancers) of BLV replication. No satisfactory treatment is available for HTLV, which infects about 20 million people worldwide (Gillet et al., 2007). Since BLV is a close genetic relative of HTLV, BLV should be a good model for testing potential inhibitors of HTLV, while avoiding the inherent risks of working with the human virus. Additionally, other compounds can be screened to determine effects on BLV replication (inhibition or enhancement), for use in further studies of BLV on host cells. 3. Use the compounds identified as inhibitor or enhancers of BLV replication in vitro as tools for studying of BLV effects on host immune cells. Prior to our development of the BLV-induced syncytia assay, we were mainly focused on investigating the mechanism(s) of immune activation by BLV. It has long been observed that peripheral blood mononuclear cells from BLV-infected animals undergo spontaneous proliferation in vitro (Trueblood et al., 1998). Moreover, the occurrence of elevated numbers of circulating B cells in about 30% of infected animals suggests that BLV may have an immunostimulatory effect. Additionally, several investigators have reported enhanced antibody responses (Isaacson et al., 1996a), increased B cell expression of MHC-II molecules (Isaacson et al., 1996b), and several alterations in cytokine secretion (Stone et al., 1994, Trueblood et al., 1998) associated with BLV infection. In the later part of this project, we plan to use any reagents identified as reliable inhibitors or enhancers of BLV replication in vitro to further elucidate the mechanism(s) of BLV-induced immunostimulation.

DESCRIPTION (provided by applicant): The sense of smell plays an important role in our quality of life. However, the mechanisms governing the processing and representation of olfactory information in the brain are not well understood. In mammals, the olfactory bulb is a critical brain region responsible for the initial processing of olfactory information. Much of our current understanding of olfactory bulb function is based on previous studies using in vitro brain slices or acute preparations of anesthetized animals. While these approaches have provided valuable insight, much less is known about odor coding in awake, behaving animals. For example, what are the dynamics of odor representations in the same animal over long time scales (i.e. days, weeks, months) and how is odor coding in the olfactory bulb shaped by experience and learning in awake, behaving animals? To address these questions, we propose an experimental strategy using chronic 2-photon calcium imaging to study odor-evoked activity in the olfactory bulbs of awake mice. Specific Aim 1 proposes the development of an approach for the selective expression of the genetically-encoded calcium indicator GCaMP3 in principal (mitral/tufted) cells or local interneurons using a Cre-dependent, adeno-associated virus system. We hypothesize that this approach will provide single cell resolution of action potential-dependent calcium signals in large populations of neural ensembles that can be imaged chronically in awake, head fixed mice. Specific Aim 2 proposes imaging experiments to determine how odor-evoked responses of mitral cells and interneurons (granule cells) differ between the awake and anesthetized state. We will also use chronic imaging in awake animals to determine whether patterns of odor-evoked activity in neural ensembles are stable or dynamic over days of repeated testing. These experiments will provide new insight into the nature of odor coding in the awake brain.

Project Summary/Abstract The sense of hearing contributes significantly to our quality of life. Indeed, the auditory system plays a fundamental role in our awareness of our local environment, communication, and our ability to enjoy music. However, the mechanisms underlying the processing and encoding of sensory information in the brain are not clear. To address this question, we study the properties of neuronal circuits in the auditory cortex, a higher brain region important for sound perception. The long-term objective of our research is to understand how the interplay of different types of excitatory and inhibitory circuits regulate sensory information processing in the brain. The experiments proposed employ optical recording techniques to study the response properties of large ensembles of neurons in auditory cortex. Specific Aim 1 proposes the development of a strategy for chronic, in vivo two-photon recording of activity from identified cell populations in the primary auditory cortex of awake, head-fixed mice. We will use this approach to determine the tuning properties and spatial organization of principal cells in different cortical layers as well as distinct subtypes of inhibitory interneurons. Specific Aim 2 proposes to investigate how sensory representations in adult auditory cortex are shaped by experience. We hypothesize that brief daily experience to sounds causes a long-lasting habituation of cortical principal cell responses mediated by local inhibitory interneurons. Specific Aim 3 proposes to investigate how cortical activity is modulated by sound-guided behavior. We hypothesize that engagement in an auditory task rapidly enhances cortical representations to sounds compared to passive listening. Moreover, this modulation by sound-guided behavior is also due to changes in the activity of local interneurons. Together, these experiments will show that cortical sensory representations are highly flexible and bidirectionally modulated by local inhibitory circuits that regulate the salience of acoustic stimuli.