The Neurobiology of Aggression, Mating, and Emotion States: From Mice to Humans

This Huberman Lab Essentials episode features Dr. David Anderson, an expert in the neurobiology of emotion and behavior, who delves into the fundamental distinctions between emotions and internal states. He posits that emotions are a class of internal states, akin to arousal, motivation, or sleep, which fundamentally alter the brain's input-output transformation. This perspective shifts the focus from subjective feelings, which are difficult to study causally, to measurable neurobiological processes. Key properties distinguishing emotion states from simple reflexes include persistence, where the state outlasts the evoking stimulus (e.g., fear after a rattlesnake encounter), and generalization, where a state triggered in one context influences reactions in another (e.g., a bad day at work affecting parental response). The discussion highlights how aggression, rather than being a singular internal state, is a description of behavior that can stem from various underlying states like anger, fear, or hunger.

Dr. Anderson's groundbreaking work, particularly with optogenetics in the ventromedial hypothalamus (VMH), reveals specific neural circuits for aggression. He distinguishes between offensive aggression, which can be rewarding to male mice, and defensive rage. Intriguingly, fear neurons are located cheek-to-jowl with offensive aggression neurons in the VMH, suggesting an evolutionary and functional interplay where fear can inhibit offensive aggression. The episode also explores the complex hormonal regulation of aggression, noting that many effects of testosterone are mediated by its conversion to estrogen via aromatization, and that the estrogen receptor is crucial for aggression in male mice. Furthermore, sex-specific neural circuits are discussed, such as female aggression being tied to nurturing pups and the existence of female-specific mating neurons in the VMH, distinct from male aggression circuits.

A significant practical insight emerges from the research on social isolation. Dr. Anderson explains that social isolation dramatically increases aggression and anxiety across species, from flies to mice, by upregulating the neuropeptide tachykinin 2. Crucially, drugs that block the tachykinin 2 receptor, such as osanotonant, have been shown to reverse these effects in socially isolated mice, making them 'chill' and allowing them to reintegrate without aggression. This finding has profound implications for human mental health, suggesting a potential therapeutic avenue for individuals experiencing social isolation stress, bereavement, or aggression, though economic barriers currently hinder human trials for these repurposed drugs.

Finally, the conversation touches upon the critical brain-body connection in emotion, referencing Antonio Damasio's somatic marker hypothesis and the bidirectional communication mediated by the peripheral nervous system, especially the vagus nerve. This intricate network explains how internal states manifest as physical sensations and how bodily feedback influences brain states. Dr. Anderson emphasizes that understanding these causal mechanisms of emotion systems is paramount for developing improved psychiatric treatments and attracting the next generation of neuroscientists to address the vast unknowns in the field, ultimately contributing to better mental health outcomes.

I see emotions as a type of internal state in the sense that arousal is also a type of internal state. Motivation is a type of internal state. Sleep is a type of internal state. They change the input to output transformation of the brain.

Emotions tend to outlast often the stimulus that evoke them.

The word aggression in my mind refers more to a description of behavior than it does to an internal state. Aggression could reflect an internal state that we would call anger in humans or could reflect fear or it could reflect hunger if it's predatory aggression.

male mice will learn to poke their nose or press a bar to get the opportunity to beat up a subordinate male mouse. It has a positive veilance.

at least hierarchically, it seems like fear is the dominant behavior over offensive aggression.

many of the effects of testosterone, although not all, many of them are mediated by its conversion to estrogen by a process called aromatization.

If you give those drugs to a socially isolated mouse, it blocks all of the effects of social isolation. It blocks the aggression. It blocks the increased fear and the increased anxiety. And in fact, Moriel described it. The mice just look chill.

putting a violent prisoner in solitary confinement is absolutely the worst most counterproductive thing you could do to them.

This brain body connection is critical not just for the gut but for the heart, for the lungs, for all kinds of other uh parts of your body. And Darwin recognized that as well.

We've got to figure out how emotion systems are controlled in a causal way uh if we ever want to improve on the psychiatric treatments that we have now.