FENS-Kavli Network of Excellence: Bridging levels and model systems in neuroscience: Challenges and solutions(2023) The European journal of neuroscience. 58, 12, p. 4460-4465 Abstract
"But what's the mechanism? And what's the behavioral relevance?" These very common questions reflect that obvious fact that neural systems can be described at multiple levels. They further reflect the fact that many neuroscientists view the achievement of such multilevel descriptions as an important accomplishment. Neuroscientists have achieved a remarkable level of understanding at each different level, yet comprehensive descriptions that bridge across multiple levels remain a substantial challenge in neuroscience. Many of us may take the importance and the considerable difficulty of this endeavour for granted and, therefore, expect that it will be somehow solved in the future as we make more progress. In contrast, I argue here that concerted action is needed to address this outstanding challenge. I discuss the need to bridge different levels and model systems in neuroscience. I briefly review key concepts from philosophy of science that can create a conceptual framework to do so. Finally, I suggest concrete "bottom-up" and "top-down" steps the neuroscience community can take to make progress in this direction. I hope these suggestions will serve an initial basis for further fruitful discussions that will advance us towards achieving this important goal.
(2023) BioRxiv. 2023.11.13. Abstract
The insular cortex is involved in diverse processes including bodily homeostasis, emotions, and cognition. Yet little is known about how it processes information at the level of neuronal populations, precluding an understanding of the computations it performs. We leveraged recent advances in machine learning to study insular cortex population activity patterns (i.e., neuronal manifold) in mice performing goal-directed behaviors. We find that the insular cortex activity manifold is remarkably consistent across different animals and under different motivational states. Activity dynamics within the neuronal manifold are highly stereotyped during rewarded trials, enabling robust prediction of trial outcomes across different mice, and across various natural and artificial motivational states. Comparing goal-directed behavior with self-paced free consumption, we find that the stereotyped activity patterns reflect goal-directed reward anticipation, and not licking, taste, or valence. These findings reveal a core computation in insular cortex that could explain its involvement in pathologies involving aberrant motivations.Competing Interest StatementThe authors have declared no competing interest.
(2022) Physiology (Bethesda, Md.). 38, 2, p. 1-9 2022. Abstract
Physiological needs create powerful motivations (e.g., thirst and hunger). Studies in humans and animal models have implicated the insular cortex in the neural regulation of physiological needs and need-driven behavior. We review prominent mechanistic models of how the insular cortex might achieve this regulation, and present a conceptual and analytical framework for testing these models in healthy and pathological conditions.
(2022) Frontiers for young minds. 10, 867981. Abstract
When we are hungry and smell delicious food, things start to happen in our bodies—our mouths water, our stomachs release digestive juices, hormones, and more… all before we even taste the food. How does this happen? Our brains can predict the future. Not through magic, but by learning from past experiences. Our brains are constantly making predictions about the world around us—that is what allows us to catch a ball, ride a bike, and estimate how fast a car is coming so we can safely cross the road. Our brains also make predictions about things inside our bodies. In this article, I will describe how the brain uses information from past experiences to predict what the body will need in the future. These predictions are closely related to the way we experience emotions and feelings, so understanding them may help us better understand and treat various mental health conditions.
Cellular activity in insular cortex across seconds to hours: Sensations and predictions of bodily states(2021) Neuron. 109, 22, p. 3576-3593 Abstract
Our wellness relies on continuous interactions between our brain and body: different organs relay their current state to the brain and are regulated, in turn, by descending visceromotor commands from our brain and by actions such as eating, drinking, thermotaxis, and predator escape. Human neuroimaging and theoretical studies suggest a key role for predictive processing by insular cortex in guiding these efforts to maintain bodily homeostasis. Here, we review recent studies recording and manipulating cellular activity in rodent insular cortex at timescales from seconds to hours. We argue that consideration of these findings in the context of predictive processing of future bodily states may reconcile several apparent discrepancies and offer a unifying, heuristic model for guiding future work.
(2020) Neuron. 105, 6, p. 1094-1111 Abstract
Interoception, the sense of internal bodily signals, is essential for physiological homeostasis, cognition, and emotions. While human insular cortex (InsCtx) is implicated in interoception, the cellular and circuit mechanisms remain unclear. We imaged mouse InsCtx neurons during two physiological deficiency states: hunger and thirst. InsCtx ongoing activity patterns reliably tracked the gradual return to homeostasis but not changes in behavior. Accordingly, while artificial induction of hunger or thirst in sated mice via activation of specific hypothalamic neurons (AgRP or SFOGLUT) restored cue-evoked food- or water-seeking, InsCtx ongoing activity continued to reflect physiological satiety. During natural hunger or thirst, food or water cues rapidly and transiently shifted InsCtx population activity to the future satiety-related pattern. During artificial hunger or thirst, food or water cues further shifted activity beyond the current satiety-related pattern. Together with circuit-mapping experiments, these findings suggest that InsCtx integrates visceral-sensory signals of current physiological state with hypothalamus-gated amygdala inputs that signal upcoming ingestion of food or water to compute a prediction of future physiological state.
(2018) Current Opinion in Neurobiology. 49, p. 16-23 Abstract
Physiological need states and associated motivational drives can bias visual processing of cues that help meet these needs. Human neuroimaging studies consistently show a hunger-dependent, selective enhancement of responses to images of food in association cortex and amygdala. More recently, cellular-resolution imaging combined with circuit mapping experiments in behaving mice have revealed underlying neuronal population dynamics and enabled tracing of pathways by which hunger circuits influence the assignment of value to visual objects in visual association cortex, insular cortex, and amygdala. These experiments begin to provide a mechanistic understanding of motivation-specific neural processing of need-relevant cues in healthy humans and in disease states such as obesity and other eating disorders.
(2017) Journal of Neuroscience. 37, 49, p. 12018-12030 Abstract
In nature, animals normally perceive sensory information on top of backgrounds. Thus, the neural substrate to perceive under background conditions is inherent in all sensory systems. Where and how sensory systems process backgrounds is not fully understood. In olfaction, just a few studies have addressed the issue of odor coding on top of continuous odorous backgrounds. Here, we tested how background odors are encoded by mitral cells (MCs) in the olfactory bulb (OB) of male mice. Using in vivo two-photon calcium imaging, we studied how MCs responded to odors in isolation versus their responses to the same odors on top of continuous backgrounds. Weshow that MCs adapt to continuous odor presentation and that mixture responses are different when preceded by background. In a subset of odor combinations, this history-dependent processing was useful in helping to identify target odors over background. Other odorous backgrounds were highly dominant such that target odors were completely masked by their presence. Our data are consistent in both low and high odor concentrations and in anesthetized and awake mice. Thus, odor processing in the OB is strongly influenced by the recent history of activity, which could have a powerful impact on how odors are perceived.
(2017) Nature Neuroscience. 20, 10, p. 1321-1322 Abstract
Central amygdala directs behavioral responses to emotionally salient stimuli. While most studies have focused on aversive responses, some central amygdala neurons promote feeding and are positively reinforcing.
Aldosterone-Sensing Neurons in the NTS Exhibit State-Dependent Pacemaker Activity and Drive Sodium Appetite via Synergy with Angiotensin II Signaling(2017) Neuron. 96, 1, p. 190-206 Abstract
Sodium deficiency increases angiotensin II (ATII) and aldosterone, which synergistically stimulate sodium retention and consumption. Recently, ATII-responsive neurons in the subfornical organ (SFO) and aldosterone-sensitive neurons in the nucleus of the solitary tract (NTSHSD2 neurons) were shown to drive sodium appetite. Here we investigate the basis for NTSHSD2 neuron activation, identify the circuit by which NTSHSD2 neurons drive appetite, and uncover an interaction between the NTSHSD2 circuit and ATII signaling. NTSHSD2 neurons respond to sodium deficiency with spontaneous pacemaker-like activity—the consequence of “cardiac” HCN and Nav1.5 channels. Remarkably, NTSHSD2 neurons are necessary for sodium appetite, and with concurrent ATII signaling their activity is sufficient to produce rapid consumption. Importantly, NTSHSD2 neurons stimulate appetite via projections to the vlBNST, which is also the effector site for ATII-responsive SFO neurons. The interaction between angiotensin signaling and NTSHSD2 neurons provides a neuronal context for the long-standing “synergy hypothesis” of sodium appetite regulation.
(2017) Nature. 546, 7660, p. 611-616 Abstract
Physiological needs bias perception and attention to relevant sensory cues. This process is 'hijacked' by drug addiction, causing cue-induced cravings and relapse. Similarly, its dysregulation contributes to failed diets, obesity, and eating disorders. Neuroimaging studies in humans have implicated insular cortex in these phenomena. However, it remains unclear how 'cognitive' cortical representations of motivationally relevant cues are biased by subcortical circuits that drive specific motivational states. Here we develop a microprism-based cellular imaging approach to monitor visual cue responses in the insular cortex of behaving mice across hunger states. Insular cortex neurons demonstrate food-cue-biased responses that are abolished during satiety. Unexpectedly, while multiple satiety-related visceral signals converge in insular cortex, chemogenetic activation of hypothalamic 'hunger neurons' (expressing agouti-related peptide (AgRP)) bypasses these signals to restore hunger-like response patterns in insular cortex. Circuit mapping and pathway-specific manipulations uncover a pathway from AgRP neurons to insular cortex via the paraventricular thalamus and basolateral amygdala. These results reveal a neural basis for state-specific biased processing of motivationally relevant cues.
(2017) Nature Neuroscience. 20, 1, p. 42-51 Abstract
Arcuate nucleus (ARC) neurons sense the fed or fasted state and regulate hunger. Agouti-related protein (AgRP) neurons in the ARC (ARCAgRP neurons) are stimulated by fasting and, once activated, they rapidly (within minutes) drive hunger. Pro-opiomelanocortin (ARCPOMC) neurons are viewed as the counterpoint to ARCAgRP neurons. They are regulated in an opposite fashion and decrease hunger. However, unlike ARCAgRP neurons, ARCPOMC neurons are extremely slow in affecting hunger (many hours). Thus, a temporally analogous, rapid ARC satiety pathway does not exist or is presently unidentified. Here we show that glutamate-releasing ARC neurons expressing oxytocin receptor, unlike ARCPOMC neurons, rapidly cause satiety when chemo- or optogenetically manipulated. These glutamatergic ARC projections synaptically converge with GABAergic ARCAgRP projections on melanocortin-4 receptor (MC4R)-expressing satiety neurons in the paraventricular hypothalamus (PVH MC4R neurons). Transmission across the ARC Glutamatergic →PVH MC4R synapse is potentiated by the ARCPOMC neuron-derived MC4R agonist, α-melanocyte stimulating hormone (α-MSH). This excitatory ARC→PVH satiety circuit, and its modulation by α-MSH, provides insight into regulation of hunger and satiety.
(2014) Frontiers in Neural Circuits. 8, 129. Abstract
Sensory inputs from the nasal epithelium to the olfactory bulb (OB) are organized as a discrete map in the glomerular layer (GL). This map is then modulated by distinct types of local neurons and transmitted to higher brain areas via mitral and tufted cells. Little is known about the functional organization of the circuits downstream of glomeruli. We used in vivo two-photon calcium imaging for large scale functional mapping of distinct neuronal populations in the mouse OB, at single cell resolution. Specifically, we imaged odor responses of mitral cells (MCs), tufted cells (TCs) and glomerular interneurons (GL-INs). Mitral cells population activity was heterogeneous and only mildly correlated with the olfactory receptor neuron (ORN) inputs, supporting the view that discrete input maps undergo significant transformations at the output level of the OB. In contrast, population activity profiles of TCs were dense, and highly correlated with the odor inputs in both space and time. Glomerular interneurons were also highly correlated with the ORN inputs, but showed higher activation thresholds suggesting that these neurons are driven by strongly activated glomeruli. Temporally, upon persistent odor exposure, TCs quickly adapted. In contrast, both MCs and GL-INs showed diverse temporal response patterns, suggesting that GL-INs could contribute to the transformations MCs undergo at slow time scales. Our data suggest that sensory odor maps are transformed by TCs and MCs in different ways forming two distinct and parallel information streams.
(2014) Neuron. 81, 5, p. 1097-1110 Abstract
The adult mammalian brain is continuously supplied with adult-born neurons in the olfactory bulb (OB) and hippocampus, where they are thought to be important for circuit coding and plasticity. However, direct evidence for the actual involvement of these neurons in neural processing is still lacking. We recorded the spiking activity of adult-born periglomerular neurons in the mouse OB in vivo using two-photon-targeted patch recordings. We show that odor responsiveness reaches a peak during neuronal development and then recedes at maturity. Sensory enrichment during development enhances the selectivity of adult-born neurons after maturation, without affecting neighboring resident neurons. Thus, in the OB circuit, adult-born neurons functionally integrate into the circuit, where they acquire distinct response profiles in an experience-dependent manner. The constant flow of these sensitive neurons into the circuit provides it with a mechanism of long-term plasticity, wherein new neurons mature to process odor information based on past demands.
(2014) PLoS Pathogens. 10, 2, 1003963. Abstract
The human cytomegalovirus (HCMV) is extremely prevalent in the human population. Infection by HCMV is life threatening in immune compromised individuals and in immune competent individuals it can cause severe birth defects, developmental retardation and is even associated with tumor development. While numerous mechanisms were developed by HCMV to interfere with immune cell activity, much less is known about cellular mechanisms that operate in response to HCMV infection. Here we demonstrate that in response to HCMV infection, the expression of the short form of the RNA editing enzyme ADAR1 (ADAR1-p110) is induced. We identified the specific promoter region responsible for this induction and we show that ADAR1-p110 can edit miR-376a. Accordingly, we demonstrate that the levels of the edited-miR-376a (miR-376a(e)) increase during HCMV infection. Importantly, we show that miR-376a(e) downregulates the immune modulating molecule HLA-E and that this consequently renders HCMV infected cells susceptible to elimination by NK cells.
(2012) Nature Neuroscience. 15, 1, p. 26-28 Abstract
The adult olfactory bulb and hippocampus are continuously supplied with newborn neurons that are thought to possess a capacity for plasticity only at a young neuronal age, mainly during the early stages of integration into the network. We find that the two main types of adult-born neurons in the mouse olfactory bulb undergo experience-dependent plasticity long after maturation and integration, as evidenced by stabilization of synaptic turnover rates. Thus, the potential time window for plasticity of adult-born neurons extends well into maturity.
(2011) Journal of Comparative Neurology. 519, 11, p. 2212-2224 Abstract
The adult mammalian olfactory bulb (OB) is continuously supplied with adult-born neurons. While some new neurons die shortly after arrival into the OB, others persist throughout the life of the animal. Here we followed the long-term morphological changes in adult-born periglomerular neurons and granule cells from the mouse OB well after they mature. We present a dataset of dendritic morphology and synaptic distributions from >100 adult-born neurons as imaged in vivo and reconstructed in 3D. The dataset currently includes a substantial range of neuronal ages (0.5-11 months old). Using this dataset, we show that the morphological steady-state which adult-born periglomerular neurons reach soon after maturation is not maintained in older neurons. Rather, total dendritic length decreases after 6 months of age. We find that this morphological decrease in "old" periglomerular neurons is regulated by the age of the animal, and is independent of neuronal age. This suggests that morphological development of adult-born neurons is regulated extrinsically. Our dendritic morphology dataset of 3D reconstructions is made available to the scientific community so it may serve as a useful resource for comparative morphological studies of the OB, and in particular of adult neurogenesis.
(2010) Frontiers in Systems Neuroscience. 3, 17. Abstract
Advances in neuroanatomy and computational power are leading to the construction of new digital brain atlases. Atlases are rising as indispensable tools for comparing anatomical data as well as being stimulators of new hypotheses and experimental designs. Brain atlases describe nervous systems which are inherently plastic and variable. Thus, the levels of brain plasticity and stereotypy would be important to evaluate as limiting factors in the context of static brain atlases. In this review, we discuss the extent of structural changes which neurons undergo over time, and how these changes would impact the static nature of atlases. We describe the anatomical stereotypy between neurons of the same type, highlighting the differences between invertebrates and vertebrates. We review some recent experimental advances in our understanding of anatomical dynamics in adult neural circuits, and how these are modulated by the organism's experience. In this respect, we discuss some analogies between brain atlases and the sequenced genome and the emerging epigenome. We argue that variability and plasticity of neurons are substantially high, and should thus be considered as integral features of high-resolution digital brain atlases.
(2009) Journal of Neuroscience. 29, 1, p. 86-97 Abstract
The adult mammalian brain maintains a prominent stem cell niche in the subventricular zone supplying new neurons to the olfactory bulb. We examined the dynamics of synaptogenesis by imaging the formation and elimination of clusters of a postsynaptic marker (PSD95), genetically targeted to adult-born neurons. We imaged in vivo adult-born periglomerular neurons (PGNs) during two phases of development, immaturity and maturity. Immature PGNs showed high levels of PSD95 puncta dynamics during 12-72 h intervals. Mature PGNs were more stable compared with immature PGNs but still remained dynamic, suggesting that synaptogenesis persists long after these neurons integrated into the network. By combining intrinsic signal and two photon imaging we followed PSD95 puncta in sensory enriched glomeruli. Sensory input upregulated the development of adult-born PGNs only in enriched glomeruli. Our data provide evidence for an activity-based mechanism that enhances synaptogenesis of adult-born PGNs during their initial phases of development.