The influence of mild stressors on neurons containing interleukin-1β in the central (CeA) and medial (MeA) amygdala in the ageing process of rats

Proinflammatory cytokine - interleukin 1 beta (IL-1 beta) plays an important role in stress reactions in the structures of limbic system. The impact of stress on IL-1 beta may depend on the ontogenetic age. The study examined the influence of acute and chronic exposure to forced swim (FS) or high-light open-field (HL-OF) stressors on neurons containing IL-1 beta. Double immunofluorescence staining was used to reveal the density of IL-1 beta/NeuN (NeuN - a neuronal nuclear marker) immunoreactive (ir) cells in the amygdaloid central (CeA) and medial (MeA) nuclei, which are closely involved in the regulation of emotional stressors and hypothalamic-pituitary-adrenal axis (HPA) activation. Adult (P90; P - postnatal day), middle-aged (P360), and aged (P720) male Wistar Han rats were used in these experiments. We observed an age-dependent increase in the basal density of IL-1 beta/NeuN-ir cells in CeA and MeA in P90 vs. P360 and P360 vs. P720 rats. Neither acute nor chronic FS caused significant changes in the density of IL-1 beta-ir neurons in any of the investigated nuclei in P90, P360, and P720 rats as compared with the non-stressed groups. However, chronic but not acute HL-OF caused a marked increase in the density of IL-1 beta/NeuN-ir cells in the CeA and MeA of P360 rats and in MeA of the P720 animals. Moreover, chronic HL-OF led to an increase in the density of IL-1 beta-ir neurons in relation to acute HL-OF in the CeA and MeA of both P360 and P720 rats. Our results may indicate the involvement of IL-1 beta neurons in the development of ageing processes in CeA and MeA. Furthermore, our results point out that chronic HL-OF is an aggravating factor that induces an increase in the density of IL-1 beta/NeuN-ir cells in the MeA and/or CeA of middle-aged and aged rats. The increase is possibly due to insufficient control of the HPA axis associated with involutional ageing processes and seems to be a common denominator of the ageing process and stress.