In mice, prolonged social isolation causes a dramatic increase in the neuropeptide tachykinin. This neurochemical surge is directly responsible for increased aggression, fear, and anxiety. A drug that blocks the tachykinin receptor can completely reverse these isolation-induced effects.
Stimulating specific aggression neurons in the ventromedial hypothalamus (VMH) of male mice elicits offensive aggression that they find rewarding. Mice will learn to perform tasks, like pressing a lever, for the opportunity to attack a subordinate male, indicating the behavior has a positive valence.
Neurons for fear and offensive aggression are located closely together in the hypothalamus. Activating these fear neurons can immediately stop a fight, causing the animals to freeze. This reveals a functional hierarchy where the fear state is dominant and can override aggressive impulses.
Dr. Anderson defines emotions as internal states that change the brain's input-output transformation. This perspective shifts the focus from subjective feelings (the "tip of the iceberg") to the underlying neurobiological processes that control behavior, making them more scientifically tractable.
Contrary to popular belief, testosterone's effects on aggression in male mice are often mediated by its conversion to estrogen via the enzyme aromatase. Researchers found that estrogen implants alone can restore aggression in castrated mice, completely bypassing the direct need for testosterone.
Distinct neuron populations control mating and aggression. Activating mating neurons in a male mouse's medial preoptic area during a fight causes it to immediately stop attacking and instead attempt to mate with the other male, demonstrating a clear neural override mechanism between competing social behaviors.