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.
The needs of the body focus attention on sensory cues associated with outcomes that can satisfy these needs. Dysregulation of this process contributes to pathological conditions including obesity, eating disorders, and addiction. Human neuroimaging studies suggest that insular cortex (InsCtx) plays a key role in these phenomena1,2. Indeed, InsCtx integrates interoceptive signals from throughout the body with taste information3–5. To begin to explore how physiological needs influence InsCtx, we investigated the role of hunger in InsCtx processing of learned food-predicting cues.
Hunger-dependent increases in visual food cue responses in human InsCtx are associated with increased incentive value1,6. In obesity and eating disorders, elevated responses persist after satiation7,8. Rodent InsCtx neurons respond to learned cues9–12, and InsCtx is required for food cues to elicit behavioural responses10,13. However, the mechanisms by which motivation-related subcortical circuits drive this process remain unknown.
Here we develop a novel approach for cellular-resolution imaging of InsCtx in behaving mice across hunger and satiety, combined with circuit mapping and pathway-specific manipulations. We uncover a specific pathway from hunger-related AgRP neurons14,15 to InsCtx, via the paraventricular thalamus (PVT) and basolateral amygdala (BLA). Our findings provide a framework for explorations of how natural and pathological need states converge in InsCtx to bias behavioural responses towards relevant cues.
Imaging InsCtx activity in behaving mice
We trained food-restricted mice to perform a go/no-go visual discrimination task in which licking after different learned visual cues led to delivery of liquid food (Ensure), aversive bitter solution (quinine), or no outcome16 (Fig. 1a). After performance during hunger, mice consumed unlimited quantities of Ensure to satiety (operationally defined as voluntary cessation of consumption; ~3–5 ml, ~30–75 min). Subsequent food-cue-evoked licking was greatly reduced (Fig. 1b and Extended Data Fig. 1a).
We first asked whether InsCtx is necessary for performance of this operant task. Pharmacological silencing of InsCtx, but not of adjacent secondary somatosensory cortex, reliably and reversibly impaired performance by reducing responses to food cues, without affecting responses to other cues or locomotion (Fig. 1c and Extended Data Fig. 1b–d). Despite encompassing primary visceral and gustatory cortices17, InsCtx was not required for home-cage feeding on chow (Fig. 1d), nor for ad libitum Ensure consumption in the task apparatus but in the absence of visual cues (Fig. 1e). Thus, InsCtx is important for operant behavioural responses to learned food-predicting cues (Supplementary Discussion).
InsCtx has been inaccessible to imaging in behaving animals because of its deep and lateral location behind essential skull and jaw bones18,19. We developed a novel preparation that spared these bones, using a reflective microprism20. Intact InsCtx was thus imaged without anaesthesia or head rotation18,19. We combined this approach with viral expression of GCaMP6f to simultaneously image the activity of 150–200 neurons in superficial layers of InsCtx in behaving mice (Fig. 1f, Extended Data Fig. 2a and Supplementary Video).
Many InsCtx neurons responded to the food cue, licking, Ensure4,12,21, or both the food cue and licking/Ensure (Fig. 1g). The sign (excitation/suppression) of these responses was uncorrelated (cue-licking: r = 0.01, P = 0.8; cue-Ensure: r = −0.1, P = 0.07; Extended Data Fig. 2b). Remarkably, 88% of neurons responded to the food cue and/or feeding (828 out of 941 neurons, six mice; Fig. 1h). A large subset (~30%) responded to the visual food cue, of which 85% also responded to licking/Ensure, suggesting that InsCtx food cue responses may represent predictions about gustatory and interoceptive consequences of upcoming consumption (Fig. 1h).
We next examined the spatial functional organization of InsCtx5,18,19. InsCtx was previously reported to contain a topographic map of gustatory features in naive, anaesthetized mice19. However, we did not observe any large-scale (hundreds of micrometres) or fine-scale (tens of micrometres) organization of neurons responding to the food cue or to licking/Ensure. Additionally, while previous work suggested functional differences between granular and dysgranular subregions of InsCtx3, we did not observe any (Extended Data Fig. 2c–g). Thus, in behaving mice, the cellular representation of food cues and subsequent food consumption is dense and spatially distributed throughout InsCtx.
Hunger gates InsCtx cue responses
We next considered the effects of homeostatic state on InsCtx cue responses by imaging the same neurons across hunger and satiety (Fig. 2a). Strikingly, the average population response across neurons excited (red) or suppressed (blue) by visual cues during hunger was abolished during satiety (Fig. 2b). During hunger, neurons were three times more likely to respond to the food cue than to others, and food-cue-responsive neurons rarely responded to other cues (Fig. 2c and d, left, middle). Responses were largest to food cues, intermediate to avoidable aversive cues, and smallest to neutral cues. Satiation eliminated the food cue bias by differentially attenuating food cue responses (Fig. 2c and d, right). These effects were more prominent in neurons excited compared with those suppressed by cues (Extended Data Fig. 3a–d). Cue responses did not show systematic attenuation across a session, although trial-to-trial response variability was higher than in visual cortical areas16 (Extended Data Fig. 3e, f). We calculated a ‘hunger modulation index’ (Fig. 2e), which revealed stronger responses during hunger versus satiety in 89% of neurons (244 out of 274). Thus, most InsCtx neurons showed selective responses to the food cue, which were attenuated during satiety.
Food cue bias was due to motivational salience rather than to inherent visual biases or overexposure to food cues early in training (Methods): cue responses in untrained, hungry mice were weaker, less prevalent, and not food cue-biased9 (Extended Data Fig. 3g), and InsCtx food cue bias was similar before and after switching the visual stimuli associated with food and neutral outcomes (Extended Data Fig. 3h–m). Further, while orofacial movements can modulate InsCtx activity4,21, these were unlikely to underlie food cue bias or hunger modulation (Extended Data Fig. 4a, b). Indeed, using simultaneous orofacial videography9 and InsCtx imaging, we found that food cue responses were only positively correlated with preparatory orofacial movements in 3% of neurons (Extended Data Fig. 4c–g and Supplementary Discussion).
Neurons in Fig. 2 were selected on the basis of responsiveness during hunger. In contrast, neurons significantly responsive during satiety were rare and did not demonstrate a significant bias to the food cue or to hunger modulation (Extended Data Fig. 5).
Hunger modulation was consistently observed throughout granular (0.55 ± 0.03) and dysgranular (0.45 ± 0.03) subregions of InsCtx, without clear spatial organization (Fig. 2e). In contrast, early visual cortex responses were not substantially modulated by hunger using the identical experimental setup16, arguing against global arousal driving InsCtx hunger modulation. To confirm this, we used moment-to-moment changes in pupil diameter as a proxy for changes in arousal22 (n = 266 neurons, three additional mice; Fig. 2f, g). We selected pairs of single-trial InsCtx cue responses across hunger and satiety with similar arousal levels (matched pupil diameter in the 1 s before each cue; Fig. 2f and Extended Data Fig. 5e, f). This analysis removed low-arousal trials during satiety, resulting in a small increase in cue response magnitude (Fig. 2g) and in a 35% reduction in hunger modulation (0.54 ± 0.05 versus 0.35 ± 0.05). Importantly, however, food cue bias was unaffected, and significant hunger modulation persisted (Fig. 2g; P = 4.3 × 10−7, Wilcoxon signed-rank test, n = 69 neurons, three mice).
AgRP activation mimics hunger in InsCtx
AgRP neurons integrate peripheral signals of caloric deficit, and their activation is both necessary and sufficient for home-cage and instrumental feeding14,15,23. We hypothesized that AgRP neuron activation in sated mice could partly restore InsCtx food cue responses. Because AgRP neuron activity drops upon consumption of a large quantity of food23–25, we first used fibre photometry24 to determine whether activity remained roughly constant during our task, which involved food cues and consumption of a very small quantity of food (5 μl Ensure per trial). During hunger, visual food cues evoked a small, transient (10–15 s) drop in AgRP neuron activity, demonstrating that activity mostly remained high during the task (Extended Data Fig. 6 and Supplementary Discussion). These data validated a chemogenetic approach mimicking hunger in sated mice, via persistent activation of AgRP neurons (AAV8-DIO-hM3Dq–mCherry injection in the arcuate nucleus (ARC) of AgRP-ires-Cre mice and intraperitoneal injection of clozapine N-oxide (CNO)15), combined with InsCtx imaging (Fig. 3a).
First, we imaged InsCtx during the behavioural task across hunger (Hungry-1) and satiety (Sated-1). After satiation in the behavioural apparatus, mice occasionally resumed feeding within 30–60 min (Extended Data Fig. 7A, B). Thus, we allowed mice to fully satiate on chow in their home cage overnight. The following day, we imaged the same InsCtx neurons during satiety (Sated-2) and after activation of AgRP neurons (Sated-2 + AgRP). Remarkably, after AgRP activation, mice selectively licked to the food cue but withheld licking to other cues, similarly to Hungry-1 (Fig. 3b, c). Thus, AgRP activation mimics caloric deficiency-induced hunger not only in restoring instrumental responding15,23,25, but also in restoring selective operant responding to learned food cues.
We next asked how AgRP activation affects InsCtx cue responses (n = 384 neurons, four mice; Fig. 3d–f). The average cue response across all neurons in Hungry-1 was similarly attenuated during Sated-1 and Sated-2. Strikingly, AgRP activation during satiety restored InsCtx cue responsiveness to levels observed during hunger (Fig. 3d). AgRP activation, but not control saline injections, also restored InsCtx food cue bias and response magnitude across cue-responsive neurons (Figs 3e–f and Extended Data Fig. 7C, D). However, not all InsCtx neurons regained responsivity (Fig. 3g): while same-neuron responses were positively correlated only between Hungry-1 and Sated-2 + AgRP, the correlation coefficient was relatively low (0.35; Fig. 3h). Thus, we considered the state dependence of neurons that were food-cue-responsive during Hungry-1 (Fig. 3i). While only 23% responded similarly to the food cue in Hungry-1 versus Sated-2, this doubled to 47% in Hungry-1 versus Sated-2 + AgRP (Fig. 3j). Surprisingly, this was comparable to response similarity across 2 hungry days (‘Hungry versus Hungry-next-day’; Fig. 3j and Extended Data Fig. 7E), suggesting that AgRP activation during satiety restored the previous day’s hunger pattern of cue-evoked responses as much as possible, given inherent day-to-day response variability of individual InsCtx neurons.
As InsCtx activity was necessary for task performance, we considered whether downstream neurons could use single-trial population activity patterns to extract cue information. We trained a simple decoder using single-trial population cue responses (90–98 neurons per mouse, n = 4 mice) in the Hungry-1 state, and asked whether it could predict which cue was presented using single-trial responses from other states. Prediction accuracy when testing on Hungry-next-day data was comparable to within-day accuracy (assessed across subsets of trials; Extended Data Fig. 7F). In contrast, accuracy dropped to chance when testing on Sated-1 or Sated-2 (Fig. 3k). Remarkably, when testing on next-day Sated-2 + AgRP, prediction accuracy was comparable to within-day accuracy (Fig. 3k). The response pattern was essential for decoding, as shuffling across neurons decreased accuracy to chance. Additionally, accurate decoding preceded licking (Extended Data Fig. 7G, H). These results suggest that single-trial InsCtx population activity is sufficient to discriminate among different learned visual cues to predict upcoming food availability, but only during hunger. AgRP activation during satiety was sufficient to restore InsCtx response patterns, such that food availability information could potentially be read out by downstream neurons. Consistent with results from other cortical regions16,26, InsCtx preserves stable population response properties, despite variability in the average responses of individual neurons across days.
A pathway from AgRP neurons to InsCtx
InsCtx receives multiple ingestion-related visceral inputs, conveying diverse interoceptive information such as stomach stretch, visceral malaise, post-ingestive sugar absorption, and blood pressure3,5,27. These are conveyed to InsCtx via visceral thalamic, midbrain, and hindbrain nuclei17. Nevertheless, our findings suggest separate pathway(s) from AgRP neurons to InsCtx that could bypass and/or override visceral satiety pathways.
AgRP neurons do not project to InsCtx. Thus, we searched for pathways with one intermediate node (Fig. 4a). Retrogradely labelled InsCtx-projecting neurons (cholera toxin subunit B (CTB) conjugated to Alexa Fluor 488, injected in InsCtx) co-localized with AgRP axons at three sites: PVT, parasubthalamic nucleus, and lateral parabrachial nucleus (Fig. 4a). Using ex vivo channelrhodopsin-2-assisted circuit mapping28 (CRACM), we found negligible AgRP inputs onto CTB-labelled neurons, in contrast to unlabelled neurons (Fig. 4a). This suggests that no single intermediate node connects AgRP neurons to InsCtx (Supplementary Discussion). Next, we conducted a broad survey of sites that are one or more synapses downstream of AgRP neurons, by injecting Cre-dependent trans-synaptic anterograde herpes simplex virus (HSV)29 into the ARC of AgRP-ires-Cre mice. Three days later, labelled neurons (putatively one or two synapses downstream29) co-localized with AgRP axons in known targets regions. We also observed labelling in regions not innervated by AgRP axons, and the only such region projecting to InsCtx was BLA (Fig. 4b).
BLA encodes the value of learned cues12,16,30,31, and is necessary for InsCtx responses to these cues12. BLA neurons that project to nucleus accumbens (NAc) and central amygdala (CeA) are important for behaviours involving cues predicting reward and punishment32–34. Interestingly, using rabies-based axon collateral mapping35, we found that these and other BLA neurons all sent axon collaterals to InsCtx (Extended Data Fig. 8a).
We next searched for an intermediate node between AgRP neurons and BLA→InsCtx neurons, using projection-specific rabies monosynaptic tracing of inputs to BLA→InsCtx neurons. This labelled several sites (Extended Data Fig. 8b–d), but the major site that co-localized with AgRP axons was PVT (Fig. 4c). PVT is involved in motivated behaviours36 including feeding37,38, and AgRP→PVT stimulation induces feeding35. Using CRACM, we found that BLA→InsCtx neurons and BLA inhibitory interneurons received synaptic input from PVT (Extended Data Fig. 8e). Neurons were also labelled in bed nucleus of the stria terminalis (BNST; ruled out using CRACM) and ventrolateral periaqueductal grey (unlikely to be directly involved; Supplementary Discussion and Extended Data Fig. 8f).
These results implicate a pathway from AgRP neurons to BLA→InsCtx neurons via PVT. Using CRACM, we found that most PVT→BLA neurons received AgRP input (15 out of 22; Fig. 4d). Using rabies-based collateral mapping35, we found PVT→BLA axon collaterals in NAc/BNST39, but not in other PVT targets, including CeA40,41. In contrast, PVT→NAc/BNST neurons collateralized in all examined PVT targets (Extended Data Fig. 9A–C). Thus, while NAc/BNST is a major projection target of PVT36, distinct PVT subsets project to BLA or CeA and probably serve different behavioural functions41.
We next tested whether AgRP neurons preferentially target PVT→BLA neurons within the PVT, using CRACM of AgRP inputs onto different PVT populations (Fig. 4e). Remarkably, most PVT→BLA neurons received input from AgRP neurons (~70%; 15 out of 22), while lower connectivity rates were observed for other PVT subsets (for example, ~5% of PVT→InsCtx; Fig. 4e and Supplementary Discussion) and for randomly sampled PVT neurons (~20%; 5 out of 26).
To test the contribution of this pathway to behaviour and InsCtx activity, we performed several pathway-specific manipulations. First, as AgRP neurons are inhibitory, inhibition of PVT→BLA neurons in fed mice should increase feeding35. We achieved selective chemogenetic inhibition of PVT→BLA neurons by injecting retrogradely trafficking AAV6-Cre–GFP in BLA and AAV8-DIO-hM4Di–mCherry in PVT. Inhibition of PVT→BLA neurons significantly increased home-cage feeding (Fig. 4f ). Interestingly, this effect was smaller than that evoked by activating AgRP→PVT axons35, potentially owing to technical factors (partial penetrance/efficacy). However, because AgRP neurons also target other PVT neurons (Fig. 4e), we hypothesized that PVT→BLA neurons specifically control responses to predictive cues, thereby explaining this partial effect.
To test this in the discrimination task, we performed chemogenetic stimulation of PVT→BLA neurons (Fig. 4g). During hunger, this should occlude persistent AgRP-mediated inhibition25 (Extended Data Fig. 6) of PVT→BLA neurons, thereby reducing behavioural performance. We used a 2-day protocol with two blocks after saline injections on day 1 (Saline-1.1, Saline-1.2). On day 2, the second block followed CNO injection (Saline-2.1, CNO-2.2). Stimulation of PVT→BLA neurons selectively reduced behavioural responses to the food cue, and did not affect licking behaviour (Fig. 4g and Extended Data Fig. 9D).
Pharmacological BLA silencing attenuates InsCtx learned cue responses in thirsty rats12. However, this could be mediated by indirect and/or direct pathways. To specifically test the contribution of BLA→InsCtx neurons to InsCtx cue responses, we combined selective chemogenetic inhibition of BLA→InsCtx neurons (injecting AAV6-Cre in InsCtx and AAV8-DIO-hM4Di–mCherry in BLA) with InsCtx imaging. We imaged the same neurons during the 2-day protocol described above (Fig. 5a; n = 350 neurons, four mice). In some neurons, food cue responses were stable across saline sessions, but decreased after BLA→InsCtx inhibition. In others, responses were unaffected (Fig. 5b). Across the population, responses were stable on day 1 (Saline-1.1 versus Saline-1.2), but were attenuated on day 2 after BLA→InsCtx inhibition (Saline-2.1 versus CNO-2.1; Fig. 5c, d), regardless of whether inhibition was bilateral (n = 2) or ipsilateral (n = 2). However, only bilateral inhibition reduced incidence (but not latency or vigour) of behavioural licking responses to food cues. InsCtx food cue responses were significantly attenuated, even when restricting analysis to trials with correct behavioural responses (Extended Data Fig. 10). In contrast, InsCtx responses to Ensure consumption were unaffected by BLA→InsCtx inhibition (Fig. 5e, f), consistent with pharmacological BLA silencing in thirsty rats12,42. BLA→InsCtx inhibition attenuated responses to all three cues. However, the strongest attenuation was of food cue responses, thereby reducing food cue bias in InsCtx (Fig. 5g).
Discussion
We propose the following model (Fig. 5h and Supplementary Discussion). Caloric deficiency increases AgRP neuron activity25, thereby inhibiting PVT, especially PVT→BLA neurons (Fig. 4e). BLA represents the valence of learned cues30,31, and may receive visual information from lateral-posterior thalamus and postrhinal cortex16 (mouse homologues of pulvinar and parahippocampal cortex; Extended Data Fig. 8c). Hunger-dependent enhancement of food cue responses may already occur in BLA12,16 (Figs 4 and 5). Therefore, during satiety, PVT may attenuate the food cue responses of BLA→InsCtx neurons, possibly by preferential excitation of BLA inhibitory interneurons (Extended Data Fig. 8e). In contrast, during hunger, AgRP inhibition of PVT→BLA neurons would disinhibit BLA→InsCtx neurons, allowing them to relay cue information to InsCtx12 (Fig. 5). InsCtx could then integrate cue information from BLA with other visceral and gustatory inputs from hindbrain, midbrain, and thalamus, to form an integrated representation of these cues and their predicted interoceptive outcomes (for example, stomach stretch and nutrient absorption).
We used a behavioural task involving both appetitive and aversive cues, and found that it required InsCtx, while feeding per se did not. We therefore suggest that InsCtx influences action selection by weighing the predicted interoceptive consequences associated with responding to learned cues in the context of current physiological needs2. Thus, state-specific gating of InsCtx, and the pathway we uncovered, provide a framework for future studies exploring how natural and pathological need states bias the weighing of positive and negative interoceptive consequences in InsCtx.
Online Content Methods, along with any additional Extended Data display items and Source Data, are available in the online version of the paper; references unique to these sections appear only in the online paper.
MethOdS
All animal care and experimental procedures were approved by the Beth Israel Deaconess Medical Center Institutional Animal Care and Use Committee. Animals were housed with standard mouse chow and water provided ad libitum, unless specified otherwise. We used male mice only. Sample sizes were chosen to reliably measure experimental parameters while keeping with standards in the relevant fields19,28,44,45, and remaining in compliance with ethical guidelines to minimize the number of animals used. Experiments did not involve experimenter-blinding, but randomization was used with respect to trial order and data collection. Animal subjects were not randomly allocated to experimental groups as all comparisons were performed within subject.
Behavioural training
See Supplementary Methods for a more detailed description. Behavioural training was performed as previously described16. Briefly, we food-restricted animals (to 85% of free-feeding body weight) and habituated them to head fixation. We trained animals to lick for drops of Ensure (5 μl, 0.0075 kcal, each). We then introduced the ‘go’ visual food cue, initially paired with unconditional (Pavlovian) delivery of Ensure. Once animals exhibited anticipatory licking, we transitioned them to operant delivery of Ensure, conditional on licking during the 2 s after food cue offset. After animals reliably licked to the operant food cue (in >80% of trials), we introduced ‘no-go’ trials involving presentation of an operant quinine-predicting cue (delivery of 5 μl of 1 mM quinine) and of a neutral cue (no outcome). Initially, we biased the total number of trials towards the food cue, and then gradually increased the fraction of other cue trials so that all visual cues were eventually presented in equal proportions. Animals typically learned to perform this task in ~2 weeks. Visual cues (presented for 2 s, every 8–10 s) were square-wave drifting gratings differing in orientation. Mice were considered well-trained on the basis of the following criteria: licking to >80% of food cue trials (usually about 90–95%) and licking to <50% of other cue trials (usually about 10–20%).
Switching cue–outcome associations in well-trained mice
We first trained mice with the same cue–outcome associations and training protocol described above, and imaged InsCtx once mice were well-trained. Then, we switched cue–outcome associations such that the visual grating associated with the neutral outcome became associated with the food outcome, and vice versa (the aversive cue remained unchanged). To address the potential effects of biased overexposure to one visual stimulus during early training, we kept the number of presentations of the three stimuli strictly equal throughout re-training on the new cue–outcome associations. Once mice were deemed successfully re-trained (on the basis of the criteria above), we re-imaged the same InsCtx fields of view in these mice.
Surgical procedures
See Supplementary Methods for a more detailed description of all surgical procedures described below.
Cannula implantation
Cannula implantation was performed using AgRP-ires-Cre mice (10–16 weeks old) as described previously10, and a custom-made headpost was glued to the skull.
Stereotactic injections
Stereotaxic injections were performed as previously described44. Mice were 8–14 weeks old at the time of injection, except for CRACM experiments, for which mice were 5–10 weeks old.
We used the following volumes of virus and injection coordinates: InsCtx (100–200 nl, Bregma: anteroposterior (AP) 0.0/0.8 mm, dorsoventral (DV) −4.1/−4.3mm, mediolateral (ML) ~4.0 mm); ARC (200 nl, Bregma: AP −1.45mm, DV −5.85 mm, ML ±0.25 mm); PVT (25–50 nl, Bregma: AP −1.0/−1.3 mm, DV −3.0/−3.0 mm, ML 0.0/0.0 mm); BLA (100 nl, Bregma: AP −1.6 mm, DV −4.5/−4.76mm, ML ±3.3mm); CeA (50nl, Bregma: AP −0.75mm, DV −5.1mm, ML ±2.3 mm); NAc (100 nl, Bregma: AP 1.4 mm, DV −4.7mm, ML ±0.85 mm).
We used the following viruses: AAV1-hSyn-GCaMP6f (UPenn), AAV1- hSyn-GCaMP6s (UNC), AAV8-DIO-hM3Dq–mCherry (UNC), AAV8-DIO- hM4Di–mCherr y (UNC), AAV8-DIO-ChR2(H134R)–mCherr y (UNC), AAV5-hSyn-ChR2(H134R)-EYFP (UNC), AAV8-FLEX-TVA–mCherry, (UNC), AAV8-FLEX-RG–mCherry, (UNC), SADΔG–EGFP (EnvA) rabies (Salk), AAV6-CAG-cre–GFP; (Boston Children’s Hospital), H129ΔTK-TT (Center for Neuroanatomy with Neurotropic Viruses, strain H356).
Optic fibre implantation for fibre photometry
Mice were first stereotactically injected with AAV1-hSyn-GCaMP6s into the ARC. An optic fibre (400 μm diameter core; numerical aperture 0.37; Thorlabs) with a metal ferrule was then implanted unilaterally over the ARC (AP −1.45 mm, DV −5.8 mm, ML 0.3 mm from Bregma). The fibre and a custom-made headpost were then glued to the skull.
Microprism assembly and surgery
Glass microprism assemblies were fabricated using standard 2 mm prisms (MCPH-1.0; Tower Optical) coated with aluminium along their hypotenuse20. Prisms were attached to a coverglass (#1 thickness), both along the hypotenuse (to prevent scratching of the reflective surface) and at the side of the prism facing InsCtx, using Norland Optical Adhesive.
Approximately 1–2 weeks after AAV-GCaMP6f46 injection into InsCtx, AgRP-ires-Cre or C57BL/6 mice (10–16 weeks old) were anaesthetized using isoflurane in 100% O2 (induction, 3%; maintenance, 1–1.5%). Using aseptic technique, a custom-made headpost was secured using C&B Metabond (Parkell). A 2.2 mm × 2.2 mm craniotomy was then performed over the left InsCtx (bottom edge of the craniotomy was just above the squamosal plate), centred around the AAV-GCaMP6f injection site. The microprism was then stereotactically lowered into the craniotomy, while verifying that the microprism’s bottom edge was inserted below the squamosal plate. Once the prism was in place, the window edges were affixed to the skull using Vetbond (3M), followed by C&B Metabond (Parkell). Meloxicam (0.5 mg per kg (body weight), subcutaneously) and a prophylactic antibiotic (cefazolin; 500 mg per kg (body weight), subcutaneously) were administered.
Pharmacological silencing
See Supplementary Methods for a more detailed description. Pharmacological silencing was performed as previously described10, except that 1 min after infusion, injection cannulae were replaced with dummy cannulae, and behavioural testing began 15 min later. We verified cannula location for every animal and included all animals with cannulae in InsCtx in subsequent analyses.
For testing performance on the visual discrimination task, food-restricted mice (~85% of free-feeding weight) performed two runs per day. The first run was always a saline infusion run and the second run was either a drug infusion run or another saline run (controlling for time elapsed and Ensure consumed).
For testing locomotion in the home cage, food-restricted mice were head-fixed and infused with saline/drug (on separate days), and then returned to their home cage. Fifteen minutes after infusion, a 30 min video recording began using a camera (Point Grey, Flea3 FL3-U3-13Y3M) above the home cage. For each frame, mice were segmented to obtain coordinates of centre-of-mass, used to compute position and locomotion.
For testing feeding on chow in the home cage, food-restricted mice were head-fixed and infused with saline/drug (on separate days), and then returned to their home cage. Fifteen minutes after infusion, a large food pellet (regular chow, ~4 g) was inserted into the home cage. The food pellet was weighed every 30 min.
For testing feeding on Ensure while head-fixed on the trackball, food-restricted mice were head-fixed and infused with saline/drug (on separate days). Uniform grey illumination was presented instead of visual stimuli. Fifteen minutes after infusion, we began a 30 min run in which licking triggered delivery of Ensure (5 μl; 2.5 s minimum interval between Ensure deliveries). Mice usually consumed 2–3 ml of Ensure during this period.
Two-photon imaging across hunger, satiety, activation of AgRP neurons, inhibition of BLA→InsCtx neurons, and in naive mice
See Supplementary Methods for a more detailed description. Two-photon imaging was performed using a resonant-scanning two-photon microscope with tiltable scanhead (Neurolabware; 31 frames per second; 1,154 pixels × 512 pixels). All imaging was performed with a 20×, 0.45 numerical aperture air objective (Olympus) with a 540 μm × 360 μm field of view. All fields of view were imaged at a depth of 90–150 μm, using a Mai Tai DeepSee laser (Newport) with laser power at 920–960 nm of 35–80 mW at the front aperture of the objective (power at the sample was probably substantially less because of partial transmission via the microprism).
To assay how changes in hunger state affected behavioural and neural activity, we imaged in two blocks of ~180 trials within a session (trial duration 8–10 s), one during hunger (food-restriction) and a subsequent block immediately after re-feeding (satiety; after ad libitum access to Ensure for 45–75 min, consumption of ~3–5 ml, eventually causing voluntarily cessation of licking16). Using this satiation protocol, we found that mice could occasionally resume feeding within 30–60 min. In such cases, the imaging run was aborted, and mice were allowed to consume more Ensure ad libitum before restarting the imaging run. ‘Sated’ runs did not have licking in >70% of trials. This re-satiation was necessary only in a subset of mice and only in ~30% of post-satiation sessions (see examples in Extended Data Fig. 7A, B).
For experiments involving chemogenetic activation of AgRP neurons, after imaging during hunger and satiety, mice were returned to their home cage with ad libitum access to regular chow. The next morning, we imaged the same InsCtx field of view in this satiety state (100–110% of normal body weight) during ~180 trials (30 min). We then injected CNO (1–3 mg per kg (body weight), intraperitoneally). Ten minutes later, we initiated an additional imaging run of ~180 trials. The effects of CNO injections were not due to the actual pain caused by the injection, as saline injections did not restore behavioural responses or neuronal responses (Extended Data Fig. 7D). For every mouse used for these experiments, post-mortem histology and immunohistochemistry (see below) confirmed hM3Dq–mCherry expression in the ARC.
For experiments involving chemogenetic inhibition of BLA→InsCtx neurons, we first performed bilateral injections of AAV6-Cre47 into InsCtx and AAV8-DIO-hM4Di–mCherry into BLA of C57BL/6 mice. We injected AAV1- hSyn-GCaMP6f into InsCtx 1–2 weeks later. We then performed InsCtx microprism surgeries 1–2 weeks later. After an additional 4–6 weeks, we ran a 2-day protocol, two blocks per day, each after injection of either saline or CNO (10 mg per kg (body weight)). We started imaging 10 min after injection. We only analysed mice with hits either bilateral or ipsilateral to the InsCtx microprism, assessed using post-mortem histology.
For experiments involving naive mice (before learning the behavioural task), mice were habituated to head-restraint, and food-restricted (~85% normal body weight). Mice were then head-fixed under the two-photon microscope in the absence of a lick-spout, and underwent one 30-min habituation session with presentation of the visual cue sequence described above. InsCtx was then imaged during a second identical 30 min session.
Pupil and orofacial videography during two-photon imaging
We acquired data using a GigE Vision video-rate camera (Dalsa; 15 Hz) with a 60-mm lens (Nikon) from a pre-selected region of interest around the eye ipsilateral to visual cue presentation (contralateral to the InsCtx microprism), or around the orofacial region. The pupil was illuminated by spread of two-photon excitation infrared light from within the brain.
Post-mortem identification of imaging field location
Mice were terminally anaesthetized with an overdose of chloral hydrate (Sigma Aldrich), decapitated after several hours to reduce blood loss, and heads were post-fixed in 10% formalin (Fisher Scientific). We used light and fluorescence microscopy for visualization of surface vasculature and GCaMP6f fluorescence. Microprism location was evident by a minor indentation of the tissue. We aligned the post-mortem surface vasculature to in vivo microprism epifluorescence images and then to vascular landmarks from in vivo two-photon imaging. We used this to localize imaging fields, relative to the middle cerebral artery and rhinal vein. We broadly classified imaging fields either in granular or dysgranular subregions of InsCtx on the basis of proximity to the rhinal vein and of subsequent examination of coronal sections.
Brain tissue preparation and immunohistochemistry
Histology was performed as previously described44. We used the following primary antibodies: rabbit anti-dsRed, Clonetech (632496) 1:1,000; chicken anti-GFP, Life Technologies (A10262) 1:1,000; goat anti-AgRP, Neuromics (GT15023) 1:1,000; rabbit anti-cfos, Santa- Cruz (sc-52) 1:1,000.
Anterograde HSV, rabies collateral mapping, projection-specific monosynaptic rabies tracing, and CRACM
Experiments were all performed similarly to previously described procedures44,48,49, but with slight modifications. See Supplementary Methods for a more detailed description.
Food intake studies after chemogenetic inhibition of PVT→BLA neurons
See Supplementary Methods for a more detailed description. Briefly, we assessed food intake after mice received injections of saline on day 1 and CNO (10 mg per kg (body weight)) on day 2. A complete experiment involved repetition of these measurements once a week for 3 weeks. Data for each mouse were an average of the three repetitions of each condition.
Behavioural studies during the visual discrimination task after chemogenetic excitation of PVT→BLA neurons
See Supplementary Methods for a more detailed description. Briefly, the procedure was similar to the pharmacological silencing described above, but using intraperitoneal injections of saline or CNO (10 mg per kg (body weight)).
Fibre photometry in the home cage and during the visual discrimination task
See Supplementary Methods for a more detailed description. Fibre photometry was conducted as previously described24,44. For home-cage feeding, ad libitum fed mice were fasted for 24 h and then put in their home cage. We collected baseline data and then dropped a 0.2 g pellet into the home cage every 7–10 min. Fibre photometry during the visual discrimination task was performed using the same procedure described above for two-photon imaging of InsCtx.
Statistics
Statistical tests were performed using standard Matlab (MathWorks) functions. Differences across mice (for example, behaviour) were tested using a t-test because of relatively small sample sizes. Differences in neural activity across large populations of InsCtx neurons were tested using non-parametric tests (Kruskal–Wallis and Mann–Whitney tests) because of the non-normal distributions of the data. We did not assume equal/unequal variance in parametric t-tests, as all t-tests were paired.
Data analysis
All data analyses were performed using custom scripts in Matlab (MathWorks) and ImageJ (NIH). See Supplementary Methods for a more detailed description of the analyses described below.
Single-neuron response analyses
Initial image registration, time course extraction, and alignment of cell masks across runs and across days were performed as previously described16. Cells were then categorized as responsive to visual cues and/or licking and/or Ensure delivery, by independently testing evoked responses of each cell for each day the cell was identified. For visual cue responses, we compared mean 1-s pre-stimulus activity to activity in a 200-ms sliding window from stimulus onset until 100 ms before licking onset, to minimize contamination by licking-related activity. We only compared time points that preceded licking onset by >100 ms in at least ten trials, using a Wilcoxon signed-rank test with a false discovery rate correction for multiple comparisons (P < 0.05). We also separately repeated this analysis using data up to 200 ms or 300 ms before licking onset and observed similar results (Extended Data Fig. 4a, b). We assessed a neuron’s response magnitude using the maximal absolute value of the average cue response, and trial-to-trial variability using the Fano factor (variance/mean).
Pupil diameter and its effects on cue-evoked responses
To measure pupil diameter across hunger and satiety, we used pupil-tracking movies from both states acquired within the same imaging session. We concatenated data from ‘Hungry’ and ‘Sated’ sessions and performed all analyses on this concatenated movie. We warped and rotated the movie to achieve a circular pupil shape, and then used the Matlab function ‘imfindcircles.m’ to detect the pupil circumference in every frame separately, from which we extracted pupil diameter (scaled to correct for warping).
We calculated the average pre-cue pupil diameter in the 1-s interval before each cue during hunger and satiety. To identify pairs of trials with matched pupil diameter across states, we first searched for all ‘satiety trials’ with pre-cue diameter within ±10% of a ‘hunger trial’. Of these, we then selected the ‘satiety trial’ that had the value nearest to the ‘hunger trial’ (matching ~50% of trials this way).
Orofacial movements and their effects on cue responses
We analysed orofacial movements as previously described9. We used the same procedure to analyse both licking-independent orofacial movements and neuronal cue responses (using only orofacial/neuronal data up to 100 ms before the first lick). We examined trial-to-trial variability in neuronal responses versus orofacial responses by calculating the Pearson correlation between the absolute value of neuronal response and orofacial response per trial (average of 0–1.5s after cue onset), testing for a positive correlation coefficient.
Evaluation of spatial clustering of neurons with similar functional properties
We calculated pairwise distances between all neurons and examined the distribution of distances between neurons that were either similar or different in their responses type (food-cue-responsive versus licking/Ensure-responsive).
Comparisons across natural or artificial hunger states
We first aligned data from the 2 days of the experiment and only used neurons that were active and could be reliably identified on both days. We normalized the responses of each neuron within a day across states, using a single transformation (z-score) that was applied to all cue response trials. Z-scoring was performed by (xi x̄ )/S, where xi is ΔF/F at time-point i, x̄ is the average ΔF/F of all visual cue data from that day (all time points, all trials, all visual cues, all states), and S is the standard deviation of ΔF/F from all visual cue presentations from that day (all time points, all trials, all visual cues, all states).
The ‘hunger modulation index’ was calculated for each neuron as (Rhungry − Rsated)/(Rhungry + Rsated), where R is a neuron’s average cue response, as described above. We assessed similarity across two given states using a three-step approach. First, we calculated a ‘state modulation index’ (similar to the hunger modulation index). Second, we compared the state modulation index with the similarity within-state of Hungry-1 by assessing each neuron’s reliability (or ‘self-similarity’ in subsets of trials) by randomly splitting up trials into two halves and calculating the state modulation index between the two halves, repeated 100 times. Third, we compared the actual state modulation index across states/days to the neuron’s ‘self-similarity’ and classified it as similar if (1) both state modulation indices (across-state and within-state) were between the 10th and 90th percentiles of the ‘self-similarity’ distribution and (2) both state modulation indices had the same sign (excitation/suppression).
Comparisons across saline and CNO injections during inhibition of BLA→InsCtx neurons
We first aligned data from the 2 days, as described above. For subsequent analyses, we included all neurons that were cue- and/or Ensure-responsive either on Saline-1.1 and/or on Saline-2.1. To facilitate comparisons across experimental conditions, we used the same within-day z-scoring procedure described above. The ‘modulation index’ was calculated for each neuron per day as (Rsession 2 − Rsession 1)/(Rsession 2 + Rsession 1), where R is a neuron’s average cue response, as described above.
Population decoding
For each mouse and session, we used all simultaneously imaged neurons. We used within-day, across-state, z-scored time courses of responses to each cue, re-zeroed using the 1-s pre-cue period. For each trial, we then obtained a population ‘template vector’ for each cue by calculating the average cue-evoked response per neuron and normalized it to obtain a vector of unit magnitude so as to classify only on the basis of the pattern (not magnitude) of population responses. For each trial, we obtained a ‘single-trial vector’ that was also normalized to unit magnitude. We then calculated the cosine similarity between ‘trial vectors’ and ‘template vectors’ for each cue. The decoder’s prediction of which cue was presented during that trial was the cue whose ‘template vector’ was most similar to the ‘trial vector’ (highest cosine similarity). Decoder accuracy was the fraction of food cue trials with a correct prediction of food cue presentation (chance: 33%). We assessed maximal decoder accuracy by creating a ‘template vector’ from a randomly selected subset of 75% of trials for each cue, and tested the decoder on the remaining 25%. This was repeated 1,000 times, and the average of these repetitions was used as maximal decoding accuracy. Of note, while we intentionally used a simple linear decoder, nonlinear decoders might achieve higher accuracy.
Fibre photometry data analysis
Traces were downsampled from 1kHz to 100Hz and smoothed using a 1-s running average. We calculated ΔF/F = (F − F0)/F0. In home-cage pellet drop experiments, F0 was the average of 30s before the first pellet drop. In the visual discrimination task, F0 was the average of 1 s before each cue.
Data availability
The original and/or analysed data sets generated during the current study, and the code used to analyse them, are available from the corresponding authors on reasonable request.
Extended Data
Supplementary Material
Acknowledgments
We thank S. Subramanian, N. Patel, M. Gyetvan, G. Niyazov, D. Anderson, and M. Dello Russo (mouse training), and A. Sugden (electronics). We thank the Lowell laboratory, Andermann laboratory, D. Nachmani, T. Anthony, and J. Assad for discussions. We thank the GENIE Project, Howard Hughes Medical Institute, for GCaMP6. The HSV129ΔTK-TT was provided by the Center for Neuroanatomy with Neurotropic Viruses (grant P40RR018604). Authors were supported by a European Molecular Biology Organization postdoctoral fellowship; Edmond and Lily Safra Center for Brain Sciences postdoctoral award (Y.L.); Davis Family Foundation postdoctoral fellowship (C.R.B.); National Science Foundation Graduate Research Fellowship Program and the Sackler Scholars Program (N.J.); National Institutes of Health (NIH), National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) NRSA F31 DK105678 (R.N.R.); F32 DK103387 (J.M.R.); an NIH New Innovator Award DP2 DK105570 and R01 DK109930, the Klarman Family Foundation, a McKnight Scholar Award, a Pew Scholar Award and a Smith Family Foundation Award (M.L.A.); NIH R01s DK075632, DK096010, DK089044, DK111401, and P30s DK046200 and DK057521 (B.B.L.).
Footnotes
Supplementary Information is available in the online version of the paper.
Author Contributions Y.L., B.B.L., and M.L.A. designed the experiments and wrote the manuscript. Y.L. performed all imaging, feeding studies, pharmacological silencing, and data analyses. Y.L. and K.M.L. performed circuit mapping. Y.L., G.J.G., and M.L.A. developed the InsCtx microprism surgery. Y.L. and V.E.D. performed fibre photometry. J.C.M. and H.F. performed slice electrophysiology. Y.L. and N.J. performed locomotion experiments. R.N.R. assisted with data analysis and provided conceptual input. C.R.B. assisted with initial imaging and AgRP activation, and provided conceptual input. J.M.R. assisted with initial pharmacological silencing.
The authors declare no competing financial interests.
Readers are welcome to comment on the online version of the paper.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
The original and/or analysed data sets generated during the current study, and the code used to analyse them, are available from the corresponding authors on reasonable request.