The Role of the Brain’s Reward System in Expectation of Pain

Post by Kasey Hemington

What's the science?

Pain is a subjective experience that can be modulated by many factors, including its anticipation. The brain’s reward system works by detecting the difference between events that we experience and their prior anticipation and is known to be involved in how we perceive pain. The ventral tegmental area (VTA) projects to the rostral anterior cingulate cortex and nucleus accumbens via the mesocorticolimbic pathways and is a key part of the brain’s reward system. How these pathways might play a role in encoding our expectations of pain is not clear. This week in The Journal of Neuroscience, Tu and colleagues studied the structure and function of the mesocorticolimbic pathways using magnetic resonance imaging (MRI) during a task in which humans anticipated a painful experience.

How did they do it?

Twenty-nine young adults (14 females) participated in the experiment, which involved a calibration phase, a conditioning phase, and a test phase. During the calibration phase, electrical stimulation was delivered to the forearm to identify the level at which each individual reported low pain (2/10), moderate pain (4/10) and high pain (6/10). In the conditioning phase, participants saw a + sign or – sign on a screen, which they were told was ‘associated with a painful stimulus’. Fifteen seconds after seeing the + sign or – sign, the high pain or low pain level of electrical stimulation respectively was delivered. In other words, the participants were conditioned to associate the + with more pain and the – with less pain. During the test phase, participants again viewed the + or – sign prior to experiencing the painful stimuli, however, unbeknownst to participants, the same moderately painful stimuli were delivered after every image shown, in order to test conditioning effects. Participants were also shown an ‘o’ symbol on some trials during the test phase that they did not see during the conditioning phase, which could be assumed to be ‘neutral’. After receiving the painful stimulus, participants rated the pain.

During the test phase, functional MRI (fMRI) scanning was performed and the connectivity of the VTA with other brain regions was analyzed while participants viewed the images and anticipated the painful stimuli. The authors studied a ‘positive expectancy effect’, the difference between pain perception in response to the – sign versus the o sign, and a ‘negative expectancy effect’, the difference between pain perception in response to the + sign versus the o sign. Structural MRI data (assessing the brain’s grey matter volume) was also collected.

What did they find?

On average, pain following the -, o and + cues was rated as 2.69/10, 3.32/10 and 4.10/10 respectively during the test phase, indicating that the conditioning phase was effective. The VTA was more tightly functionally connected with the rostral anterior cingulate cortex and the nucleus accumbens during – cues compared to o cues, and less tightly connected during + cues compared to o cues. There was also a negative relationship across trials between perceived pain intensity and VTA connectivity with the aforementioned brain regions. In statistical mediation analyses, VTA – nucleus accumbens functional connectivity and VTA – rostral anterior cingulate cortex functional connectivity were found to mediate the effect of expectancy (due to a cue) on pain perception. For example, if someone had an expectation of high pain and low VTA – nucleus accumbens functional connectivity, this might result in them reporting higher pain perception than they otherwise might have.

When the authors compared VTA functional connectivity across subjects, they found that it did not predict pain responses. However, when they analyzed the structural MRI data, they found that grey matter volumes of the VTA, rostral anterior cingulate cortex, and nucleus accumbens predicted the positive expectancy effect - individuals with larger volumes in these areas were likely to experience a larger effect. Grey matter volume of the rostral anterior cingulate cortex predicted a larger negative expectancy effect.

What's the impact?

This study demonstrates that the function and structure of the VTA and mesocorticolimbic pathways are related to one’s anticipation of a painful experience. These results emphasize the role of the brain’s reward system in shaping how expectancy of pain can alter the way we feel pain.

Tu et al. Mesocorticolimbic pathways encode cue-based expectancy effects on pain. Journal of Neuroscience (2019). Access the original scientific publication here.

Identity Domains: A Computational Framework for Personality Analysis

Post by Deborah Joye

What's the science?

Personality is the collection of individual behaviors or traits that differ from one human to the next. The ability to organize individual differences in behavior into distinct categories is important for understanding the biological underpinnings of both healthy and pathological behaviors. In humans, many individual differences have been categorized by psychologists into varying personality traits, resulting in the widespread use of personality tests to determine the trait make-up of individuals. However, personality tests tend to rely on self-report questionnaires and do not track actual behaviors as they occur. Like humans, mice also exhibit individual differences in their behaviors. Some mice prefer to stay close to the nest, whereas others leave the nest to explore the environment. Some mice will readily approach a stranger, while others are more reserved. Organizing individual differences amongst other species has been challenging since there are few conceptual frameworks with which to comprehensively categorize behaviors into consistent traits. This week in Nature Neuroscience, Forkosh, Karamihalev, and colleagues present a computational framework that organizes individual behaviors into trait-like dimensions that are stable across development, consistent across social settings, and correlated with gene expression differences within the brain.

How did they do it?

The authors built a semi-naturalistic arena and filled it with different features for mice to interact with such as ramps, feeders, dividing walls, and hiding places. The authors then tracked the behavior of individual mice as they interacted with their cage-mates and environment over several days. They classified both individual behaviors, like movement and foraging and interactions between the mice, like dominance behaviors and other social contacts. The authors then trained their linear discriminant analysis algorithm to look for 60 unique behaviors within the behavioral dataset. Their algorithm was specifically designed to isolate dimensions of the dataset that are the best at discriminating one mouse from another, which they called identity domains. The algorithm does this by maximizing trait variability between each mouse, while also maximizing trait consistency within one mouse. To ensure that their algorithm functioned as planned, the authors validated the analysis on two separate groups of mice and found consistent results. To determine if identity domains were consistent within mice across development, the authors profiled identity domains of juvenile mice, then profiled the same mice as adults. To test whether identity domains remained consistent in different social settings, the authors profiled groups of mice, then mixed the mice up into different groups and profiled them again. Finally, to determine whether identity domains correlated with changing gene expression in the brain, the authors performed RNA sequencing three brain regions (basolateral amygdala, insular cortex, and medial prefrontal cortex) of profiled mice.

What did they find?

The authors’ algorithm captured four identity domains – consistent dimensions of the data that described the stable behavior of individual mice over time. Interestingly, the authors found that when mice were profiled as both juveniles and adults, three of the four assigned identity domains remained stable, suggesting that identity domains capture traits that are consistent across development. The authors then mixed up groups after they had been profiled and found that while mice changed some behaviors in new social settings, their assigned identity domains remained stable. Using RNA sequencing, the authors demonstrated that gene expression variability in 3 different regions could be predicted using identity domain scores, suggesting that behavioral differences captured by identity domains and gene expression in the brain are associated. Finally, the authors investigated the identity domains of mice with known behavioral phenotypes, such as mice that are known to exhibit high anxiety behavior. The authors found that their assigned identity domain scores were highly associated with expected personality traits, suggesting real-world relevance of the identity domain scores.

What's the impact?

Individual differences in behavior are quite difficult to study. This work presents a novel framework that offers a more objective study of personality by tracking real behavioral output and categorizing it into trait-like identity domains in mice. Interestingly, identity domains capture differences that are stable over time and in different social contexts. Moreover, the correlation between identity domain scores and gene expression differences in several brain areas suggests that this tool can capture stable behavioral differences that are reflective of fundamental differences in brain function.

Forkosh et al., Identity domains capture individual differences from across the behavioral repertoire, Nature Neuroscience (2019). Access the original scientific publication here.

Ghrelin Increases Food Intake via Descending Hippocampus to Hindbrain Signalling Pathway

Post by Amanda McFarlan

What's the science?

Ghrelin is a gut-derived hormone that plays a critical role in feeding behavior by stimulating the appetite. The ghrelin receptor can be found in regions of the hippocampus and recently, it has been shown that activating the ghrelin receptor in the ventral hippocampus results in an increase in food intake. However, the underlying mechanisms that mediate feeding behavior are still unknown. This week in Biological Psychiatry, Suarez and colleagues investigated the role of ghrelin signalling in the ventral hippocampus in mediating food intake.

How did they do it?

To investigate the role of hippocampal ghrelin signalling, the authors implanted cannulas bilaterally in the ventral hippocampus in adult male rats. After food-restricting the rats for 24 hours, they delivered an injection of artificial cerebrospinal fluid (as a control) or a subthreshold dose of ghrelin (no effect on feeding behaviours alone) through the cannulas. Approximately 45 minutes after the injection, they performed either an intraperitoneal injection of a satiation-promoting drug or a nonnutritive oral gavage of fiber to physically expand the stomach. The rats were given access to food following the intraperitoneal injection or gavage and their food intake was recorded for up to 4 hours. Then, the authors used a deprivation intensity discrimination task to train two groups of rats to use internal cues about their hunger and satiety levels as a discriminative stimulus for a sugar reward. The first group of rats was trained to anticipate a reward after 24 hours of food restriction, while the second group was trained to anticipate a reward with no food restriction. After the training, both groups of rats underwent two test days following a period of no food restriction. On the test days, the rats received a delivery of artificial cerebrospinal fluid (control) or ghrelin in the ventral hippocampus via the cannulas (with the order counterbalanced) 1 hour prior to being placed in an operant chamber to record their appetitive behavior. Next, to identify the downstream targets of the ventral hippocampal → lateral hypothalamic area projections, the authors targeted the expression of Cre-recombinase to neurons in the ventral hippocampus and their first-order targets and targeted the expression of an anterograde tracer (traces neurons from their source to their target) tagged with tdTomato in the lateral hypothalamic area. They performed immunohistochemistry and fluorescence in situ hybridization to map the axons from lateral hypothalamic area projections and determine their neuronal targets.

What did they find?

The authors determined that administering a subthreshold dose of ghrelin to the ventral hippocampus in 24-hour food-restricted rats significantly attenuated the reduction in food intake following the administration of satiation-promoting drugs or oral fiber gavage compared to controls. Since the satiation-promoting drugs targeted either the paracrine (vagal) or non-vagal endocrine pathways, these findings indicate that ghrelin signalling within the ventral hippocampus may act on both paracrine and endocrine pathways to mediate food intake. Next, the authors revealed that, following the administration of ghrelin in the ventral hippocampus, the group of rats that were trained to anticipate a reward after 24 hours of food restriction had an increase in appetitive behaviors compared to controls. In contrast, the group of rats that were trained to anticipate a reward with no food restriction had a decrease in appetitive behaviors compared to controls. These findings suggest that ghrelin signalling in the ventral hippocampus may override satiety-inducing mechanisms by producing internal cues that signal a low energy state similar to being food-restricted for 24 hours. The authors also found that the ghrelin receptor is most commonly expressed in glutamatergic neurons in the pyramidal layer of the ventral hippocampus. Further, they showed that a subset of tdTomato-positive projection neurons in the lateral hypothalamic area was co-localized with the expression of orexin (a neuropeptide that promotes food intake) and had axon terminals that targeted the laterodorsal tegmental nucleus. They used fluorescent in situ hybridization to determine that a large population of neurons in the laterodorsal tegmental nucleus in the hindbrain have a co-localized expression of the orexin-1 receptor and the glucagon-like peptide-1 receptor (previously shown to be important for reducing food intake). Together, these findings suggest that glutamatergic neurons in the ventral hippocampus that express the ghrelin receptor project to orexin neurons in the lateral hypothalamic area, which in turn target neurons in the laterodorsal tegmental nucleus in the hindbrain to regulate food intake.

What's the impact?

This is the first study to show that ghrelin signalling in the ventral hippocampus decreases the efficacy of satiation and satiety-promoting signals that target endocrine pathways in the periphery. The authors also identified orexin neurons in the lateral hypothalamic area as the downstream targets of ghrelin receptor-expressing neurons in the ventral hippocampus. Together, these findings provide insight into the neural pathways and mechanisms involved in hippocampal-mediated control of feeding behavior via ghrelin signalling.

Suarez et al. Ghrelin and orexin interact to increase meal size through a descending hippocampus to hindbrain signalling pathway (2019). Access the original scientific publication here.