Welcome to our Monthly Journal Club! Each month I post a paper or two that I have read and find interesting. I use this as a forum for open discussion about the paper in question. Anyone can participate in the journal club, and provide comments/critiques on the paper by leaving a comment below. I picked this month’s paper because it describes a novel approach to combat obesity by targeting dopamine signaling in the hypothalamus. Additionally, I love papers where connections between the brain and body are dissected, and this paper represents a great example of ‘holistic neuroscience’. The paper we are discussing is titled “Hypothalamic dopamine signaling regulates brown fat thermogenesis” (click the hyperlink to see the paper) by Ruben Nogueiras & colleagues at University Santiago de Compostela in Spain.
This is a large study involving converging pieces of evidence from mice, rats, and humans. The authors aimed to investigate how the neurotransmitter dopamine controls metabolism, energy balance, and feeding behavior. There are two primary aspects of feeding behavior that dopamine has been shown to influence. The first is the pleasurable feelings that come along with and reinforce the eating of tasty foods, even when you don’t need to eat (that is, ‘hedonic’ feeding). The other is a balancing of energy stores to ensure that you don’t starve to death and have the nutrients you need to live (that is, ‘homeostatic’ feeding).
The dopamine circuitry in the brain underlying hedonic feeding is somewhat similar to that involved in addictive drug-seeking behavior, and is very well understood. Alternatively, how dopamine regulates homeostatic feeding is much less well defined. There are 5 dopamine receptors (named D1R, D2R…D5R) expressed throughout the body and brain, each with distinct actions. This allows dopamine to have many different effects, depending on which receptor(s) is expressed in a given tissue/cell type. Of these, the D1R and D2R dopamine receptors have been shown to regulate food intake. These receptors are expressed in a major brain area important for maintaining physiological equilibrium within the body (that is, homeostasis), the hypothalamus. Keep that in mind as we move through this research paper.
To investigate how dopamine signaling alters whole-body metabolism and food intake, the authors infused bromocriptine, a drug that powerfully binds and activates D2R signaling, into the brain of rats. When they did this, rats receiving the drug gained significantly less weight over the following two weeks (see Figure 1). To see how this influences peripheral metabolism, the authors examined brown adipose tissue (BAT), which plays a critically important role in non-shivering thermogenesis (i.e., keeping you warm independent of shivering) and maintaining resting energy expenditure.
In BAT, bromocriptine treatment caused the up-regulation of several proteins associated with metabolic activation (e.g., UCP1, FGF21, PRDM16). Functionally, this was associated with an increase in the temperature of this brown fat (inter-scapular BAT), indicating energy utilization was increased in response to bromocriptine treatment in the brain. How could a signal from the brain make it down to brown fat to control energy balance? A primary candidate is a branch of the autonomic nervous system (ANS), the sympathetic nervous system (SNS). This system controls ‘automatic’ functions in your body that are not typically under your control (like pupil diameter, gut motility, sweat glands…), including brown fat heat production. When the researchers treated rats with a beta-3 receptor blocker (which blocks SNS function on brown fat), the rats no longer showed changes in metabolism and body weight when administered bromocriptine (see Figure 1). This suggests that the SNS relays the signal from the brain to brown fat to mediate these effects.
This answers how the signal makes it to the body from the brain, but does not let us know what brain region initiates or controls this (that is, where is the signal generated?). The researchers knew that the hypothalamus is a major area regulating homeostatic feeding, and therefore tested injecting bromocriptine in different hypothalamic areas to see if they could repeat the effect of ‘whole brain’ injections. After testing several areas, they demonstrated that bromocriptine injected into the lateral hypothalamus bordering the zona incerta had similar effects to that of brain-wide injections (see Figure 2).
Using a variety of techniques, they identified a specific cell type in this area (GABA-expressing) that seemed to be mediating these effects. To test this explicitly, they used a virus to express a designer receptor exclusively activated by designer drugs (DREADDs) in these neurons. This way, they could activate these cells with a simple injection of an inert compound (called CNO). When they did this, they observed the same effects as bromocriptine injections in to the lateral hypothalamus/zona incerta (see Figure 3). Additionally, when they used short hairpin RNAs to knockdown D2R, bromocriptine injections into this area no longer had any effect, demonstrating that GABA-expressing cells in this hypothalamic area mediate the effects of dopamine on body weight and metabolism via the D2-receptor!
The authors then spent a significant amount of time investigating what is happening in these GABA-expressing cells in the hypothalamus upon dopamine D2R signaling. They worked out a complicated molecular cascade that gets activated in response to dopamine, which mediates the downstream effects on body weight and brown fat heat production (involving PKA, rpS6, PDE3B). I think if I spent too much time on this, we’d get too lost in the weeds and no one would keep reading. If you’re interested in these details, please read the paper using the link at the beginning of this post!
Additionally, they uncovered that these D2R positive neurons actually act through their modulation of hypocretin/orexin neuron signaling. If you’ve been following the journal club for a while, you should be familiar with this cell type within the hypothalamus (it is my favorite!). When the authors used mice lacking hypocretin/orexin (genetic knockouts), bromocriptine no longer had an effect on metabolism and body weight! Additionally, when they activated GABA neurons using DREADD technology (as described above) along with a drug that inhibits hypocretin/orexin signaling, they no longer observed reductions in body weight or increases in BAT temperature! This strongly suggests that GABA neurons expressing D2R regulate whole body metabolism through their actions on hypocretin/orexin neurons.
All of this is important mechanistic work to understand circuitry underlying homeostatic hunger and energy balance. Following these extensive studies, the researchers took a retrospective approach to examine how another drug that binds the D2R, cabergoline, influences body weight in humans. This drug is not usually prescribed for obesity, but is given to combat a relatively common condition known as hyperprolactinemia as a result of tumors of the pituitary gland. They took data from a group of patients that had been given cabergoline for one year, and assessed body weight changes after 3 months and 12 months. They observed that patients treated with cabergoline experienced little to no side-effects and (on average) lost significant amounts of weight 3 months following starting treatment (see Figure 4). This effect persisted, to a degree, for a year following the start of treatment. They additionally demonstrated that patients had higher resting energy expenditure (similar to the BAT thermogenesis measures in rodents) than before they started treatment. Together , these data suggest that D2R signaling in the hypothalamus promotes weight loss by increasing resting energy expenditure through SNS innervation of brown adipose tissue.
This is a huge study that is promising as it provides complementary evidence for dopamine signaling in the hypothalamus regulating body weight from rats, mice, and humans. Importantly, the findings they observed in patients were using a drug already approved for human use (cabergoline), suggesting that it may be a relatively safe treatment to add to current standards of care for obesity and associated diseases. I think this study is a great example of how science should be done. The authors worked out an intricate mechanism, answering a key question in basic biology, and then confirmed their findings in a clinical setting using a retrospective human sample. I am excited to see how this work translates into clinical practice! Now, what do you think? Let me know if in the comment section below, and as always…stay curious!!