Resilience against food cravings via environmental enrichment (EE): neuronal ensemble mechanisms

1 in 4 people in the UK are now obese and recent NHS figures indicate ~£6.1 billion is spent on treatment of overweight and obesity-related ill-health. Obesity may result from excessive eating and increases the risk of debilitating diseases (e.g. cancer). Stimuli called 'cues' (e.g. the sights/smells of food, food advertisements) trigger us to crave, seek, and eat foods. Thus, we need to identify how we can increase our resilience against these effects of food cues to better manage eating-related behaviours. Although many studies have revealed how cues trigger cravings in the brain, little is known about how it produces 'anti-craving' effects.

Interestingly, a brief experience of physical and cognitive stimulation (e.g. walking, playing Tetris) reduces cravings and the attractiveness of food cues. Similarly, in laboratory animals such stimulation via brief exposure to items such as exercise wheels and toys called 'environmental enrichment' (EE) powerfully reduces food seeking triggered by food cues. As such, EE is a useful approach to investigate increased resilience against reactions to cues and cravings.

The prefrontal cortex is implicated in suppressing urges and regulating food cravings, which are impaired in obese patients. In animals, food cues mobilise or 'recruit' specific groups or 'ensembles' of neurons to suppress or promote food seeking in this brain area. Also, when food seeking levels change, ensembles change their 'electrical' properties. In turn, this controls the rapid 'activity' of neurons which interprets information about cues. These properties are regulated by ion channels, which are viewed as important therapeutic targets for various diseases.

We recently found that brief EE exposure robustly reduced food seeking and ensemble recruitment in the prefrontal cortex triggered by food cues in hungry mice. But what is the precise identity of this dynamically regulated ensemble following EE? Our aim here is to reveal how EE suppresses food seeking triggered by food cues through two possible prefrontal cortex actions by: i) adjusting the recruitment, excitability, or rapid activity properties of an 'existing' food seeking ensemble; and/or ii) actively recruiting a 'new' ensemble during cue exposure with unique excitability or rapid activity properties. We will examine the properties of prefrontal cortex ensembles activated by food seeking before and after EE. Notably, since this deep brain area is difficult to image, we will take advantage of a glass 'microprism' to image how ensembles are recruited and rapidly activated. We will also selectively activate an ensemble and determine the causal link between ensemble activity and suppression of food seeking. Furthermore, we will use electrophysiology to characterise electrical state alterations of ensembles. We will reveal how specific groups of neurons with altered physiological properties reduce reactions to food cues. Such knowledge will provide the crucial building blocks to develop novel therapeutic strategies that engage anti-craving mechanisms to combat obesity.

Environmental enrichment (EE) in animal housing provides cognitive and physical stimulation. In hungry mice, brief EE exposure powerfully attenuates food seeking triggered by food-associated cues. Also, EE reduces medial prefrontal cortex (mPFC) activity, indicated by decreases in 'Fos', an activity marker that labels behaviourally-relevant neuronal ensembles. This neurobehavioural attenuation is thought to be mediated by reductions in the motivational impact of food cues. Shifts in recruitment, excitability, and encoding (real-time activity) properties of ensembles play a critical role in interpreting such changes.

In this project, we will elucidate how EE suppresses cue-reactive food seeking via the mPFC by: i) modulating the recruitment, excitability or encoding properties of an 'existing' food cue reactive ensemble; and/or ii) actively recruiting a 'new' ensemble during cue exposure with unique excitability or encoding properties. We will reveal these novel mechanisms using advanced tools that enable us to examine precise changes in neuronal recruitment, excitability, and cue-encoding. These include: in vivo 2-photon imaging in awake and behaving mice that express calcium indicators of neural activity and fluorescent reporters of Fos-promoter activation; and whole-cell electrophysiology of cue-activated, Fos-expressing neurons. Notably, the imaging method uses a specialised 'microprism' to image the medial PFC that was previously difficult to access due to its location within a brain fissure. These studies will enhance our understanding of how neuronal ensembles can interpret changes in the motivational attributes of food cues by using existing and/or new ensemble representations.

The proposed work will reveal brain activity states that contribute to the resilience against undesirable reactions to food cues, such as food cravings that may lead to excessive eating and obesity. Elucidating such states are important for understanding brain mechanisms that help us better cope with emotional states that promote unhealthy eating. Our findings from this basic research will have significant implications for a diverse group of beneficiaries below.

1. Academics: Our work examines how the activity patterns of neuronal ensembles, the basic computational units in the brain, and how changes in their physiological properties could play a role in reducing behavioural reactivity against food cues. Thus, academics such as psychologists, neuroscientists, and clinicians may utilise our findings to design studies that could potentially reveal why certain individuals might be more resilient or vulnerable to cue reactivity. Also, since the neurophysiological changes we examine are related to alterations in ion channel functioning, our data may guide drug discovery researchers in academia (e.g. Sussex Drug Discovery Centre) and industry to novel drug targets that potentially confer resilience to cue reactivity. Of note, our institute has designated 'impact funds' in place to pursue these types of studies on a small-scale. In terms of capacity building, the post-doctoral research fellow and research technician will directly benefit by receiving high-quality training in advanced neuroscience tools such as high-resolution brain imaging, large-scale data analysis, and neuronal ensemble manipulations, that few UK labs currently utilise. Hence, they will acquire a unique skill set, making them competitive in the job market.

2. Health science/policy-related organisations: The UK has one of the highest incidences of obesity in the EU, with one in four adults who are obese. Although our studies are performed in laboratory rodents, rodent and human brains are remarkably similar in terms of neuronal circuits that process food cues. Such organisations may use evidence from our studies (especially since we are examining neurons or molecules that cannot be studied in humans) to build convincing cases and/or guide research priorities regarding the benefits of mentally and physically active lifestyles on the brain. For example, the Pew Charitable Trust, a public policy institute, recently published an article from mice studies about the beneficial effects of exercise on the brain and how growth factor molecules play a role. Also, I will continue to engage with networks such as the Association for the Study of Obesity, which will serve as a platform for discussing our findings with those in the relevant areas of epidemiology, clinical trials, and public health.

3. The general public: Our work will raise awareness about the positive impact of healthy lifestyles. Issues related to healthy lifestyles and food craving management are highly newsworthy material for public media outlets (e.g. health and fitness magazines). Also, one of brain's biggest mysteries is how neurons process information related to sensory stimuli in the environment. Since our findings will illuminate how neurons process presentations of food-associated stimuli, our work will be of great interest to those who are fascinated about brain functioning. Finally, our research impact can be enhanced locally by engaging with organisations such as Brighton and Sussex Universities Food Network, which act as a knowledge exchange hub for researchers to share ideas, explore collaborative opportunities, and to launch a dialogue with the key members of the public (e.g. local city council) about food-related health issues. Policymakers can exploit outcomes from these meetings to design more effective evidence-based campaigns against craving and weight management.