Amygdaloid control of alerting and behavioral arousal in rats: involvement of serotonergic mechanisms.

The role of 5-HT mechanisms in the amygdala in the modulation of sleep and arousal states and PGO waves was examined. Studies of the amygdala suggest that it provides a neural mechanism by which emotionally-relevant or significant stimuli may influence behavioral state and alerting mechanisms. The amygdala projects massively (via the central nucleus) into brainstem regions involved in alerting and in the generation of REM and PGO waves. Serotonergic innervation of the amygdala comes from DRN and to a lesser degree MRN. Microinjections of 5-HT into the amygdala produced short-latency changes of state from NREM and REM with the effect being relatively greater in REM. Microinjections of the 5-HT antagonist, methysergide, increased sleep efficiency and increased PGO wave frequency in waking and NREM. These results demonstrate an important role for the amygdala in the control of behavioral state and alerting mechanisms and suggest that 5-HT exerts some of its regulatory effects via an influence on forebrain regions.