Ventral subiculum regulates hypothalamo-pituitary-adrenocortical and behavioural responses to cognitive stressors

The hippocampus plays an important role in central stress integration. The present study tests the hypothesis that the ventral subiculum, as a principal source of hippocampal efferents, is involved in co-ordination of hypothalamo-pituitary-adrenocortical and behavioural responses to cognitively-processed information. Basal hypothalamo-pituitary-adrenocortical activation appears to be normal in ventral subiculum lesion rats, as basal corticosterone and adrenocorticotropic hormone secretion, anterior pituitary pro-opiomelanocortin and type 1 corticotropin-releasing hormone receptor messenger RNA expression, adrenal and thymus weight, and splenic mitogen activity are not affected by lesion. Lesions of the ventral subiculum induce glucocorticoid hypersecretion following restraint stress or open field exposure, whereas responses to ether inhalation are unaffected. Interestingly, ventral subiculum lesion does not affect fast glucocorticoid negative feedback inhibition of restraint-induced adrenocorticotropic hormone release. Corticotropin-releasing hormone immunoreactivity is increased in the hypothalamic paraventricular nucleus of ventral subiculum lesion rats, and is differentially depleted by acute stress exposure (relative to sham-lesion rats). However, ventral subiculum lesion does not affect basal and stress-induced corticotropin-releasing hormone, arginine vasopressin and cFOS messenger RNA expression in paraventricular nucleus neurons. Behavioural analysis reveals that ventral subiculum lesion rats are hyper-responsive to open field exposure, showing decreased total ambulation and reduced incidence of central square entry. The results suggest that the ventral subiculum plays a specific role in integrating cognitively-processed stimuli (e.g., restraint and open field exposure) into appropriate neuroendocrine and behavioural responses to stress. Enhanced stress-induced glucocorticoid secretion and increased corticotropin-releasing hormone biosynthesis are likely due to removal of oligosynaptic inhibitory input to the paraventricular nucleus subsequent to ventral subiculum lesion.