The type and duration of stress stimulation are postulated to affect the expression of the brain derived neurotrophic factor (BDNF) differentially during ontogenetic life. The aim of our study was to investigate the influence of two different stressors, i.e. chronic (15 min daily for 21 days) exposure to the forced swim (FS) test or the high light open field (HL-OF) test, on the BDNF contained in magnocellular (PVm) and parvocellular (PVp) neurons of the hypothalamic paraventricular (PV) and the supraoptic (SO) nuclei. The immunofluorescence (-ir) method was used to detect BDNF-ir cells. The research showed that only the PVp part of the PV in juvenile (P28; P-postnatal day) control rats had a significantly lower density of BDNF-ir neurons than that in middle-aged (P360) control subjects. After chronic FS, a significant decrease in BDNF-ir cells was observed in the studied hypothalamic nuclei of the juvenile rats, but no changes were noted in the middle-aged individuals. The PV (PVm, PVp) and the SO nuclei in juvenile rats showed a significantly lower density of BDNF-ir neurons than the corresponding area of the hypothalamus in middle-aged rats. However, following the HL-OF test, the density of BDNF-ir neurons remained unaltered both in the P28 and the P360 groups. The data suggest that the type of the stressor applied was the factor that differentiated the number of BDNF-ir cells in the PVm and the SO only in juvenile rats: chronic HL-OF was more severe than FS. The age of the animals was the main factor that conditioned the BDNF hypothalamic PV (PVm, PVp) and the SO response to FS stimulation. The different density of BDNF-ir containing cells in the PVp of juvenile versus middle-aged rats can be explained by a functional, age-related change in the demand of PVp neurons for BDNF.
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