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The theta rhythm is a 3- 1 2 Hz electroencephalographic (EEG) oscillatory phenomenon that has been recorded from the hippocampus of small animals during large motor movements, periods of arousal, and memory tasks. Theta denotes the amount of brain synchrony occurring, with higher theta amplitude corresponding to more synchrony among individual neurons. The proposed pacemaker of the hippocampal theta rhythm is the medial septal area (MSA), which consists of neurons that fire rhythmically in frequencies corresponding to the theta rhythm. Despite the proven physical connection between the MSA and the amygdala, there is uncertainty as to whether the MSA serves as a pacemaker for the amygdala theta rhythm as well. To assess the effect of MSA neuronal firing on the amygdala theta rhythm, cholinergic antagonists and GABAergic agonists were infused into the MSA. Eight male Long-Evans rats were anesthetized with a ketamine/xylazine mixture. A guide cannula was inserted into the MSA and a recording electrode was inserted into the amygdala. After one week, rats were re-anesthetized and amygdala EEG was recorded. Baseline recordings were taken for 5 minutes, after which each rat was infused with 0.5 III of all of the following drug treatments: saline, scopolamine ( 10 µg or 20 µg), muscimol (30 ng or 60 ng), and ethanol (0.1 % or 1 .0%). A within-subjects ANOV A was used to analyze pre- and post-infusion differences in theta power. There were main effects for muscimol and ethanol, but no main effect for scopolamine. Subsequent T-tests revealed a significant difference in theta power between the saline treatment and both the high, t(7) = 4.015, P < .05, and low 1(7) = 2.934, P < .05, doses of muscimol. There was also a significant difference in theta power between the saline and the high, 1(6) = 2.602, P < .05, and low, t(4) = 3.240, P < .05, doses of ethanol. These results have strong implications for the respective roles of the cholinergic and GABAergic pathways in amygdala theta modulation.



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