The amygdala is an essential brain region responsible for social behaviors and anxiety. The lateral amygdaloid nucleus (LAN), basal amygdaloid nucleus (BAN) and medial amygdaloid nucleus (MAN) are important nuclei in the amygdaloid complex. There is a change of emotional responses such as anxiety over the course of the day, thus called circadian (circa: about, diem: day) as well as during the process of aging indicating a regulation of the amygdalar function via the biological clock, which is also affected by aging. Thus, we investigated the circadian changes between the early light phase and early dark phase of the day in addition to age-related changes between young and middle-aged mice on the expression of the neuronal nuclear protein (NeuN) and the neural plasticity marker phosphorylated mitogen-activated protein kinase 1/2 (p42/44-MAPK) in the LAN, BAN and MAN subregions of the amygdala using immunohistochemistry.




               We show higher expression levels of NeuN and p42/44-MAPK during the light phase indicating a strong correlation between the neuronal activity in the amygdala and the circadian system. From young to middle-aged mice, the expression levels of these neuronal proteins in the amygdala decrease, suggesting a pronounced interaction between the circadian system and aging in the regulation of the neuronal functional state in the amygdala. Our data may present a better understanding of the role of circadian timing and the effect of age on the neuronal functions within the amygdala and related behavioral changes such as fear and anxiety in health and disease.