In the present study we showed that chronic corticosterone treatment, similarly to what previously reported by David et al. , induces an affective phenotype which was reversed by chronic administration of the Hypericum perforatum. The pre-treatment with corticosterone leads to a deterioration of the state of the coat that can not be reversed by chronic Hypericum perforatum administration. For instance, an anxiogenic effect was observed in CORT+ mice in the OF and the NSF. This effect was reversed by Hypericum perforatum treatment. In addition, CORT+ mice showed deficit in hippocampal progenitor cell proliferation and a reduced number of CA3 dendritic spines which were prevented by Hypericum perforatum chronic treatment.
Consistent with previous findings , our behavioural data demonstrate that increased corticosterone levels induce anxiety in CD1 mice as assessed by the decreased time in the center of the arena in OF paradigm and by the increase in latency to feed in NSF. Corticosterone-treated mice did not show depression-like phenotype in the FST. Hypericum perforatum administration significantly ameliorated the anxiety-like phenotype (OF and NSF) in CD1 mice. Our data are in accordance with those reporting an anti-anxiety effect induced by Hypericum perforatum administration in models of restrain stress and sleep deprivation [34, 35]. Studies conducted by Flausino et al. and Singewald et al. [36, 37] have shown that chronic administration of Hypericum perforatum induced an antidepressant-like effect in Mg-depleted mice in the forced swim test, as well as anxiolytic-like effects in both anxiety tests. In our study, we used a different chronic stress paradigm and performed different behavioral paradigms (OF and NSF). Nonetheless, our data further reaffirm a role for Hypericum perforatum in helping to cope with different stressful conditions.
The synaptic correlates of this anti-anxiety effect are still a matter of debate. There is some preliminary evidence of the mode of action of Hypericum extract such as inhibition of uptake of serotonin (5-HT), noradrenaline, dopamine [8, 38–42]. Based on the previous work hypericin was considered to be an inhibitor of both MAO type A and B  and inhibitor of synaptosomal reuptake of serotonin, noradrenaline and dopamine . Furthermore, Hypericum perforatum extract increases extracellular levels of dopamine, noradrenaline, serotonin in the rat locus coeruleus  and modulated adenosine, GABAA, GABAB and glutamate receptors . Nevertheless, whether these effects could account for the in-vivo anti-anxiety properties need still to be demonstrated.
To further verify the effects of Hypericum perforatum on hippocampal plasticity, we carried out an experiment on hippocampal progenitor cell proliferation. The hippocampus is a region that shows a remarkable capacity for structural reorganization. Preexisting neural circuits undergo modifications in dendritic complexity and synapse number, and entirely novel neural connections are formed through the process of neurogenesis. Stress exerts a significant modulation of hippocampal structural plasticity and is a potent negative regulator of neurogenesis. Different chronic stress suppress the rate of adult dentate gyrus proliferation and decreases the size of newborn cell clusters . In accordance with these data, we found a reduction in hippocampal progenitor cell proliferation in mice chronically treated with corticosterone. The effects of corticosterone administration on neurogenesis are limitated to the proliferation stage and not the maturation of newborn neurons. Interestingly, the effect of Hypericum perforatum extract on proliferation and maturation is more pronounced in corticosterone-treated mice than in controls. We can speculate that this model may increase the dynamic range in which Hypericum perforatum extract exerts its effects on different stages of neurogenesis. Adult hippocampal neurogenesis in the adult hippocampus has been implicated in cognitive function and is stimulated by antidepressant drugs, although its functional impact and contribution to the aetiology of anxiety/depression is unclear [20, 45, 46]. However, severe or chronic stress exposure, both in early life as well as in adulthood, affect hippocampal neurogenesis and plasticity [46, 47]. A reduction in neurogenesis can theoretically contribute to the cognitive symptoms of depression, even though by itself is unlikely to produce the full mood disorder . The molecular mechanisms by which glucocorticoids induce these changes are however still unclear. Conversely, we demonstrated that in corticosterone-treated mice these changes are reverted by 3 weeks Hypericum perforatum administration.
Hypericum perforatum administration reverted the negative effect of stress on dendritic spine in mature hippocampal neurons. Dendritic spines are small specialized membranous protrusions that contain the postsynaptic machinery (PSD, glutamate receptors, cytoskeleton) and play a crucial role in synaptic plasticity  and calcium signaling [51–53]. Dynamic changes in spine number and morphology are closely linked to changes in strength of synaptic connections . It has been suggested that a derangement of spine dynamics that favors loss of spines is a candidate mechanism for stress-evoked dendritic atrophy and associated synaptic dysfunction . Our data confirm the negative effects of stress on dendritic spine number and demonstrated for the first time that Hypericum perforatum administration significantly decreased stress-induced dendritic pathology. Live imaging studies showed that spines are remarkably dynamic, changing size and shape and number over timescales of seconds to minutes . Stress-induced changes in neurotransmitters, growth factors, hormones and oxidative stress could be responsible for the dendritic spines structural changes (size, number and shape). For instance, Hypericum perforatum administration significantly attenuated lipid peroxidation, nitrite concentration and partially restored GSH and catalase activity in chronic restrained mice suggesting a strong antioxidant effect . In this respect, administration of fluoxetine also reduces oxidative stress in restraint animals  and promotes resilience. It is thus conceivable that antioxidant properties of Hypericum perforatum may be protective against the deleterious effects of chronic stress on hippocampul dendritic spines.