Persistent, generalized hypersensitivity of olfactory bulb interneurons after olfactory fear generalization

Abstract

Generalization of fear from previously threatening stimuli to novel but related stimuli can be beneficial, but if fear overgeneralizes to inappropriate situations it can produce maladaptive behaviors and contribute to pathological anxiety. Appropriate fear learning can selectively facilitate early sensory processing of threat-predictive stimuli, but it is unknown if fear generalization has similarly generalized neurosensory consequences. We performed in vivo optical neurophysiology to visualize odor-evoked neural activity in populations of periglomerular interneurons in the olfactory bulb 1 day before, 1 day after, and 1 month after each mouse underwent an olfactory fear conditioning paradigm designed to promote generalized fear of odors. Behavioral and neurophysiological changes were assessed in response to a panel of odors that varied in similarity to the threat-predictive odor at each time point. After conditioning, all odors evoked similar levels of freezing behavior, regardless of similarity to the threat-predictive odor. Freezing significantly correlated with large changes in odor-evoked periglomerular cell activity, including a robust, generalized facilitation of the response to all odors, broadened odor tuning, and increased neural responses to lower odor concentrations. These generalized effects occurred within 24 hours of a single conditioning session, persisted for at least 1 month, and were detectable even in the first moments of the brain’s response to odors. The finding that generalized fear includes altered early sensory processing of not only the threat-predictive stimulus but also novel though categorically-similar stimuli may have important implications for the etiology and treatment of anxiety disorders with sensory sequelae.

Aversive learning-induced plasticity throughout the adult mammalian olfactory system: insights across development

“…While pairing odors with an aversive outcome during conditioning and measuring startle or freezing responses are common across most current studies involving adult mammalian fear learning, there are many variations in experimental paradigms such as aversive stimulus intensity and duration, number of pairings within a session, and conditioned odorant used. These discrepancies likely impact the magnitude of learning-induced effects but do not appear to greatly alter behavior or plasticity (Kass and McGann 2017;Kudo et al 2004;Morrison et al 2016;Pavesi et al 2012;Ross and Fletcher 2018); however, there are a number of factors which might. One such parameter is whether other odors are presented during training.…”

“…Studies utilizing calcium imaging confirmed early reports of altered OB processing of conditioned odors following fear conditioning in several cell types. In anesthetized mice, enhanced responses of the conditioned stimulus have been reported in dendrites of both excitatory OB output cells (Fletcher 2012) and inhibitory periglomerular cells (Kass and McGann 2017), that persist as long as one month following single-day fear conditioning. Similar, but more robust associative learning-induced enhancements are seen in glomeruli of awake mice (Ross and Fletcher 2018), though whether these dendritic changes reflect altered coding at the soma of output cells remains a critical question.…”

“…Olfactory aversive learning induces changes in sensory coding in the adult mammalian olfactory pathway, a fact we have known for several decades (Bressler 1988;Coopersmith et al 1986;Freeman and Schneider 1982;Pager and Royet 1976), yet we have barely begun to scratch the surface as to the mechanisms underlying neural plasticity responsible for such changes. In recent years, altered coding has been reported across all stages of the adult olfactory pathway including olfactory sensory neurons (OSN) (Kass et al 2013), olfactory bulb (OB) cells (Fletcher 2012;Kass and McGann 2017;Ross and Fletcher 2018), and piriform cortex (PCx) cells (Chen et al 2011;Sevelinges et al 2004); however, extensive investigation of electrophysiological or molecular mechanisms or widespread network alterations underlying the observed coding changes is still lacking in adults. Instead, much of the work regarding mechanisms of olfactory associative learning comes from neonates; however, most of this information regarding odor learning in pups comes from a time point before which the brain or olfactory system is fully developed.…”

Aversive Learning Increases Release Probability of Olfactory Sensory Neurons

“…In genetically modified mice in which OSNs express the fluorescent exocytosis indicator synaptopHluorin (35), discriminant fear conditioning (CS+ with US and CS-without US) increases the glomerular responses to CS + odors but not to CS-odors (5). Similarly, classical fear conditioning (CS-US pairing) also increases the activity from mitral and tufted cells imaged at their dendrites in individual glomeruli (4, 7) as well as from periglomerular cells in the OB (33). In this study, we provide direct electrophysiological evidence to support that fear conditioning changes peripheral olfactory inputs from OSNs to the OB neurons.…”

“…This finding suggests that the mechanisms that are involved in downregulating GABA B receptors may not selectively target the conditioned glomerulus only, but rather a small area covering multiple glomeruli. Together with other fear conditioning induced changes in the OB circuit and neuromodulation (3,5), this phenomenon may contribute to olfactory fear generalization (4,33). To test this possibility, future experiments would require activating adjacent glomeruli individually while recording from their postsynaptic cells.…”