Essentials: The Biology of Aggression, Mating & Arousal | Dr. David Anderson

TL;DR

Dr. David Anderson explains how distinct hypothalamic neuron populations drive aggression, mating, and arousal, revealing that estrogen—not testosterone—is the key aggressor hormone.

Key Points

• 1.Emotions are neurobiological states, not just feelings. Dr. Anderson defines emotions as internal states—like hunger or arousal—that change brain input-output transformations, with subjective feeling being only the tip of the iceberg.
• 2.Aggression neurons and fear neurons sit side-by-side in the ventromedial hypothalamus (VMH). VMH is shaped like a pear: aggression neurons occupy the base, fear neurons the top—and stimulating fear neurons instantly stops a fight dead in its tracks.
• 3.Offensive aggression is rewarding to male mice. Research by Da Lin showed male mice will nose-poke or press a bar to get the opportunity to fight a subordinate, confirming offensive aggression has positive valence.
• 4.Estrogen, not testosterone, is the key hormone driving aggression. The molecular marker of VMH aggression neurons is the estrogen receptor; castrated mice whose fighting is lost can have it restored with an estrogen implant, bypassing testosterone entirely via aromatization.
• 5.Female VMH contains separate neuron subsets for fighting and mating. Work by Mongyu Leu identified two distinct estrogen-receptor neuron populations in female VMH—one controls fighting (active only during nursing), the other controls mating—and the mating neurons are female-specific and absent in males.
• 6.'Make love, not war' neurons in the medial preoptic area directly suppress aggression. Activating mating neurons mid-fight causes a male mouse to stop attacking, sing to, and attempt to mount its rival—demonstrating hard-wired mutual antagonism between aggression and mating circuits.
• 7.Social isolation massively upregulates tachykinin 2, driving aggression, fear, and anxiety. Moriel Zelikowsky found two weeks of isolation turns mice's brains visibly green with tachykinin 2; blocking its receptor with osanotonant (a drug with a good human safety profile) reverses all isolation-induced aggression and allows reintegration with littermates.
• 8.The vagus nerve bidirectionally links brain emotion states to body sensations. Vagal fibers carry signals between the brain and the heart, lungs, and gut in both directions, forming the physiological basis for somatic emotional experiences described in Damasio's somatic marker hypothesis and Finias and Adolf's body heat maps.