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.

The Night's Watch: How Microglia Protect and Shape Our Brain as We Sleep

Post by Flora Moujaes 

What's the science? 

The brain is not just comprised of neurons: it contains many other types of cells, such as microglia and astrocytes, which play a fundamental role in the brain. Microglia are best known for their role in the immune response, yet they are also involved in a number of other key brain functions including plasticity: the process through which the brain changes and adapts to new experiences. Microglia interact closely with neurons at the synapse: the junction between two neurons. To date, research on microglia has focused on anesthetized animals, leaving open the possibility that microglial dynamics may be different during awake-states. It also remains unknown how neurotransmitters regulate microglial functioning. However, microglia have a higher expression of a specific type of receptor for the neurotransmitter norepinephrine compared to any other type of brain cell, suggesting norepinephrine may be a key modulator. This week in Nature Neuroscience, Stowell et al. use advanced imaging technology to show that microglial dynamics (1) differ between awake and anesthetized mice and (2) are modulated by norepinephrine.

How did they do it? 

To determine whether microglial behavior is affected by anesthesia, the researchers first imaged microglial dynamics in healthy adult mice while they were awake and after having been anesthetized with a fentanyl cocktail. Given the role of microglia in the immune response, they also imaged microglial dynamics in awake and anesthetized mice that had suffered an acute brain injury.

In order to uncover the underlying mechanisms responsible for the differences in microglia in awake and anesthetized mice, the researchers explored the role of norepinephrine in microglial functioning. Norepinephrine was of particular interest as (1) it is known to be a powerful mediator of wakefulness, and (2) microglia have a very high number of beta2 adrenergic receptors (which norepinephrine bind to). The researchers modulated the level of noradrenergic signalling in microglia, either by stimulating the microglia’s beta2 adrenergic receptors using the agonist clenbuterol to increase norepinephrine levels or by inhibiting the microglia’s beta2 adrenergic receptors using the antagonist ICI-118,551 to decrease norepinephrine levels. They examined whether this (1) affected microglial functioning in both anesthetized and awake mice, (2) affected microglial response to injury, or (3) impaired synaptic plasticity. 

What did they find?

Wakefulness vs. Anaesthesia: The researchers found that microglia in the awake brain differ from those in the anesthetized brain with regards to (1) surveillance monitoring and (2) their injury response. They showed that anesthesia rapidly increased microglial surveillance and increased the microglial response to injury compared to the awake condition. This suggests that wakefulness exerts a primary inhibitory effect on microglial dynamics.

Norepinephrine Modulation: The researchers then replicated the findings from awake vs. anesthetized mice by pharmacologically modulating microglial noradrenergic signalling. They found high levels of norepinephrine in awake mice led to reduced microglial functioning, while low levels of norepinephrine in anesthetized mice led to increased microglial functioning. Increased norepinephrine levels also led to a significant reduction in microglial response to injury. Finally, they found that a chronic increase in microglial noradrenergic signalling impairs experience-dependent plasticity in the developing visual system of mice. 

What's the impact?

Overall this study suggests that wakefulness exerts a primarily inhibitory effect on microglial dynamics. It also shows that microglial roles in surveillance and synaptic plasticity in the healthy brain are modulated by norepinephrine. This suggests that the enhanced remodeling of the neural circuits that occurs during sleep may be mediated by the increase in the ability of microglia to dynamically interact with the brain. This is an especially interesting finding as it demonstrates that simply by modulating immune cells, it’s possible to alter synaptic plasticity. 

Stowell et al. Noradrenergic signaling in the wakeful state inhibits microglial surveillance and synaptic plasticity in the mouse visual cortex. Nature Neuroscience (2019). Access the original scientific publication here.

Stress-Induced Lateral Habenula Changes and the Association with Depressive Behavior in Mice

Post by Lincoln Tracy

What's the science?

Chronic stress has been identified as a prominent risk factor for the development of depression in humans. Several animal models of depression exist that involve exposing the animals to chronic stressors, leading to behavioral changes that mimic a subset of core depression symptoms in humans. The problem with this approach is that while depression often presents in different ways in different people, patients are often grouped into one category. Recent studies have identified that the lateral habenula—a small section of the brain near the pineal gland—is hyperactive in depression. This week in Neuron, Cerniauskas and colleagues developed a novel approach to examine the molecular, synaptic, and circuit basis of unique chronic stress-induced behavioral characteristics in mice.

How did they do it?

First, the authors used a chronic mild stress model to induce depression-like symptoms in mice. This involved exposing the mice to a series of stressors over an eight-week period. The authors then used a series of behavioral tasks to assess anxiety-related behaviors (elevated plus maze), interest in rewarding stimuli (sucrose preference test), responses to being placed in an inescapable situation (tail suspension test), and deficits in sociability behavior (social interaction test). Second, they used receiver operating characteristic (ROC) curves to make an unbiased decision as to whether each individual mouse displayed a certain set of behavioral characteristics. Third, they used histology, microscopy, single-cell RNA sequencing, whole-brain input mapping, and electrophysiological techniques to analyze the molecular, synaptic, and circuit adaptions in the lateral habenula after chronic stress.

What did they find?

First, the authors found that mice exposed to chronic stress and the control mice both showed considerable variability in three of the four behavioral tasks. Second, the stress-induced hyperactivity in the lateral habenula was directly associated with connections to the ventral tegmental area and the rostromedial tegmental nucleus—two areas of the brain involved in dopaminergic (or reward) signalling in the brain—rather than the dorsal raphe nucleus, where the serotoninergic neurons are located. Specifically, lateral habenula excitability was associated with increased passive coping rather than anhedonia (an inability to feel pleasure) or anxiety, which are other common symptoms of depression in humans. The authors also identified a subset of genes that together can be used as biomarkers to identify mice that display increased passive coping and allow for the differentiation of lateral habenula neurons that project to the ventral tegmental area or the dorsal raphe nucleus.   

LHb = lateral habenula, EP = entopeduncular nucleus, DR = dorsal raphe, VTA = ventral tegmental area

What's the impact?

This study demonstrated that mice, like humans, have considerable individual variability in how they respond to chronic stress. It is the first study to link a specific behavioral phenotype (reduced motivated behavior), which is commonly observed in depression in humans to specific molecular, cellular, and circuit changes in the lateral habenula. Even though the study was performed in mice, lateral habenula hyperactivity has also been observed in humans with depression, suggesting that there may be — at least in part — a translational aspect of these findings. This study may serve as a foundation for future research investigating symptom-specific therapeutic interventions as well as predictive biomarkers for depression. 

Cerniauskas et al. Chronic Stress Induces Activity, Synaptic, and Transcriptional Remodeling of the Lateral Habenula Associated with Deficits in Motivated Behaviors. Neuron (2019). Access the original scientific publication here.