Functional MRI investigation of brain-wide olfactory networks in rats

Abstract

Brain networks, composed of interconnected neurons and neural circuits, are crucial for information processing underlying various cognitive and physiological functions. Previous studies have primarily focused on examining anatomical projections and targets of local olfactory microcircuits, such as the olfactory bulb (OB), anterior olfactory nucleus (AON), and piriform cortex (Pir), including the synaptic interactions between these three connected regions. However, it is increasingly recognized that neural interplay between small numbers of anatomically connected regions (i.e., local micro-circuits) is insufficient to describe the dynamic properties of information processing that are pivotal for brain functions. Furthermore, the emerging associations between olfactory-related dysfunctions and neurological disorders such as aging, neurodegenerative diseases, and COVID-19, have highlighted the need for a better understanding of olfactory networks at the systems level. Blood-oxygenation-level-dependent (BOLD) functional MRI (fMRI) is a versatile noninvasive imaging platform capable of mapping brain structure-function relationships through neurovascular coupling in vivo with sub-millimeter spatial resolution. The purpose of this doctoral work is to develop state-of-the-art neuromodulation and large-scale fMRI approaches, for in vivo investigation of the brain-wide spatiotemporal activation characteristics in the olfactory system initiated from the primary olfactory regions in rodent models. Using multi-modal optogenetic and fMRI techniques, complemented by electrophysiological recordings and histology, the spatiotemporal properties of long-range olfactory networks were examined. Spatially, optogenetic stimulation at three primary olfactory regions evoked distributed BOLD activations across both hemispheres in known primary olfactory, limbic, hippocampal, striatal, and sensorimotor networks, indicating the association of the primary olfactory network with high-order functions. Electrophysiological recordings confirmed the orthodromic neural activity propagation to downstream targets in the olfactory network, revealing the involvement of both local and long-range projections. OB stimulation activated primary olfactory network regions, while AON and Pir stimulation preferentially activated hippocampal and striatal networks, and limbic network, respectively, indicating their region-specific recruitment properties. Temporally, repeated OB or AON stimulations diminished long-range orthodromic neural activity propagation, while Pir stimulations did not, likely due to robust AON inhibitory outputs to striatal and limbic network regions, as indicated via effective connectivity modeling. The systemic dysfunction of olfactory networks in an aged rat model induced by D-galactose was additionally investigated. Optogenetic stimulation of the olfactory bulb reveals intact neural activity propagation in long-range olfactory networks spatially. However, it is shown that the activations in primary olfactory and limbic networks decreased, along with increased inhibitory outputs from AON to Pir compared to healthy rats. This indicates significant impairment of a key primary olfactory circuit, affecting downstream neural activity propagation. Our study presents a comprehensive delineation and characterization of the dynamic properties of neural activity propagation in long-range olfactory networks, highlighting the roles of primary olfactory cortical, AON, and Pir outputs, in shaping neural interactions at the systems level. Critical insights are provided into an integrated understanding of olfactory networks and their fundamental properties in both healthy and diseased brains, and thus potential therapeutic strategies may arise for neurological diseases linked to dysfunctions in the olfactory system.