14,15,19 Besides reduced neurogenesis

in DG, there is also evidence for reduced size of principal neuron cell bodies in hippocampus, which is consistent with reduced size of the dendritic tree.20 Synaptic reorganization is also a likely consequence of these rather drastic CP-690550 purchase structural changes, and the animal models cited above provide evidence that synapses can be rapidly formed as a result of stress. Taken together, such structural changes seem likely to play a major role in the volume loss in Inhibitors,research,lifescience,medical the human hippocampus and the related effects on cognitive function and affect.18 This article will review underlying mechanisms Inhibitors,research,lifescience,medical and consider their applicability to furthering our understanding of the pathophysiology of mood and anxiety disorders. Allostasis and mechanisms for behavioral adaptation The amygdala and hippocampus are both involved in contextual fear conditioning and in passive avoidance learning. In fear conditioning, glucocorticoids enhance learned fear21 and they play Inhibitors,research,lifescience,medical an important role in forming the memory of context

in contextual fear conditioning, but not of the actual effect of footshock in rats that are already familiar with the context where the shock is administered.22,23 This suggests that the hippocampal role in contextual fear conditioning is enhanced by moderate levels of glucocorticoids, but the fear conditioning

is either not so dependent on glucocorticoids or is so strong that glucocorticoid Inhibitors,research,lifescience,medical influences are hard to demonstrate. Yet there is evidence for an influence of glucocorticoids on the flow of information within the amygdala. Glucocorticoids potentiate serotonin inhibition of the processing of excitatory input to the lateral amygdala from Inhibitors,research,lifescience,medical the thalamus, suggesting that there is a mechanism for containing, or limiting, the sensory input that is important for fear conditioning.24 Thus, adrenal steroids may regulate the nature of the signals that reach the amygdala and allow for greater discrimination next of the most salient cues for learning. Moreover, in passive avoidance, both catecholamines and glucocorticoids play a role in facilitating learning.25,26 Catecholamines work outside of the blood–brain barrier and their effects can be blocked by β-adrencrgic–blocking agents, which do not cross the blood–brain barrier.26 Glucocorticoids enter the brain, and local implants of exogenous corticosterone into hippocampus, amygdala, and nucleus tractus solitarii arc all able to enhance passive avoidance learning.25 Adrenal steroids also play a supporting role in the learning of a spatial navigation task in mice.