['Serotonin (5-HT) plays a complex regulatory role in processes like anxiety, depression, aggression, and impulse control. Due to the large amount of serotonergic receptors, knockout mice offer an important opportunity to investigate the role of specific receptors. The 5-HT(1B) receptor is thought to mediate aggression and impulse control. This was studied here in mice lacking 5-HT(1B) receptors (5-HT(1B) KO).', 'Wild type and 5-HT(1B) KO mice were exposed to several types of entrained and nonentrained stimuli. With telemetry, body temperature, heart rate, and locomotor activity were measured continuously during the different experiments.', 'To nonentrained stimuli like disturbance stress and confrontation with an intruder, 5-HT(1B) KO mice showed exaggerated physiologic and behavioral responses. These mice displayed behavioral disinhibition, measured as increased social interest and aggression to an intruder mouse. However, in response to well-entrained stimuli like daily light transitions, responses were smaller in 5-HT(1B) KO than in wild type mice, suggesting that hyperreactivity is stimulus specific.'] Is absence of 5-HT ( 1B ) receptors associated with impaired impulse control in male 5-HT ( 1B ) knockout mice?