The effect of mild stress stimulation on the nerve growth factor (NGF) and tyrosine kinase receptor A (TrkA) immunoreactivity in the paraventricular nucleus (PVN) of the hypothalamus and hippocampus in aged vs. adult rats.

Ontogenetic life and stress can have different effects on the nerve growth factor (NGF) and its tyrosine kinase receptor A (TrkA) in the structures of the limbic system. This study aimed to explore the influence of two different stressors, acute and chronic exposure to forced swim (FS) stress or high-light open-field (HL-OF) stress, on cells containing NGF and TrkA. Immunofluorescence staining was used to reveal the density of NGF and TrkA immunoreactive (ir) cells in the paraventricular nucleus (PVN) of the hypothalamus or hippocampal subfields CA1, CA3 and dentate gyrus (DG) in adult (postnatal day 90; P90) and aged (P720) rats. Data revealed that neither acute nor chronic FS caused any alteration in NGF-ir and TrkA-ir cells in any of the structures investigated in P90 and P720 rats. However, a significant increase in NGF-ir was detected in the CA1 and CA3 after acute but not after chronic HL-OF in both age groups. The TrkA-ir remained unchanged after exposure to HL-OF in the PVN and hippocampus. Despite lack of change in the density of NGF-ir and TrkA-ir cells between P90 and P720 non-stressed rats, a significant age-related decrease in NGF-ir and TrkA-ir cells in the PVN of FS- and HL-OF-stressed rats was noted. However, in the hippocampus, an age-related decrease in NGF-ir or TrkA-ir cells was observed in all rats except acute FS-stressed rats. The changes are possibly associated with involutional aging processes caused by insufficient control of hypothalamic-pituitary-adrenal (HPA) axis functioning in P720 rats and may contribute to disturbances in NGF signaling.