Anatomical studies indicate that the ventral subiculum is in a prime position to mediate hippocampal inhibition of the hypothalamo-pituitary-adrenocortical (HPA) axis. The present study evaluated this hypothesis by assessing HPA function following ibotenic acid lesion of the ventral subiculum region. Rats with lesions of the ventral subiculum (vSUB) or ventral hippocampus (vHIPPO) did not show changes in basal corticosterone (CORT) secretion at either circadian peak or nadir time points when compared to sham-lesion rats (SHAM) or unoperated controls. However, rats with vSUB lesions exhibited a prolonged glucocorticoid stress response relative to all other groups. Baseline CRH mRNA levels were significantly increased in the medial parvocellular paraventricular nucleus (PVN) of the vSUB group relative to controls. CRH mRNA differences were particularly pronounced at caudal levels of the nucleus, suggesting topographic organization of vSUB interactions with PVN neurons. Notably, the vHIPPO group, which received large lesions of ventral CA1, CA3 and dentate gyrus without significant subicular damage, showed no change in stress-induced CORT secretion, suggesting that the ventral subiculum proper is principally responsible for ventral hippocampal actions on the HPA stress response. No differences in medial parvocellular PVN AVP mRNA expression were seen in either the vSUB or vHIPPO groups. The results indicate a specific inhibitory action of the ventral subiculum on HPA activation. The increase in CRH biosynthesis and stress-induced CORT secretion in the absence of changes in baseline CORT secretion or AVP mRNA expression suggests that the inhibitory actions of ventral subicular neurons affect the response capacity of the HPA axis.

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The aging process has been frequently associated with hippocampal neurodegeneration, loss of corticosteroid receptors, and, at the same time, dysfunction of the hypothalamo-pituitary-adrenal (HPA) axis. We were interested in characterizing simultaneously the activity of the HPA axis and status of both corticosteroid receptors (mineralocorticoid or MR and glucocorticoid or GR) in the hippocampus of aged male Fisher-344 rats. We compared intact, adrenalectomized (ADX), and corticosterone-replaced ADX young (5-6 months) and old (26-27 months) rats, examining all the parameters in the same animals. Aged rats exhibited an unaltered basal rhythm and initial corticosterone response to restraint stress. However, the same old animals showed a delayed turn-off of the stress response and did so at different points of the corticosterone circadian cycle. The aged hippocampus showed a 40-50% lower MR and GR binding under all the conditions studied. This aging effect was not attributable to changes in the kinetics, affinity, or nuclear translocation of MR or GR. Intact aged rats exhibited also a 30-40% reduction of hippocampal MR and GR steady-state mRNA levels. Interestingly, after 36 h ADX only the aged hippocampus showed upregulation of MR and GR mRNA content to levels comparable to those in young rats. However, this increase in MR and GR mRNA content was not accompanied by a proportional increase in the Bmax of these receptors, suggesting age-related translational or post-translational alterations. Moreover, corticosterone replacement was able to reverse the ADX-induced increase of MR and GR Bmax in young and old hippocampi but it only reversed the upregulated mRNA levels of MR (and not GR) in the older group. The fact that corticosterone was able to modulate the biosynthetic rate of MR and GR strongly suggests that the decrease of receptors is functional and not simply due to cell death in the aged hippocampus. We propose that in the aged Fisher rat the loss of hippocampal corticosteroid receptors is previous to any change in the circadian rhythm of circulating corticosterone. Furthermore, the altered turn-off of the corticosterone stress response observed in the same animals may be related to the reduction of functional MR and GR but it is not due to high basal levels of corticosterone.

A recombinant system was developed for generation of steroid-receptor complexes in vitro. The DNA- and steroid-binding domains of the rat mineralocorticoid receptor were expressed in Escherichia coli as a fusion protein with glutathione S-transferase. The identity of the expressed recombinant protein was confirmed by Western blot analysis. Protein preparations purified by affinity chromatography, avoiding the use of detergents or high ionic strength buffers, exhibited negligible steroid binding. However, after incubation of these preparations with rabbit reticulocyte lysate, known to promote the association of isolated steroid receptors with heat shock proteins, the [3H]aldosterone-binding activity gradually increased. This temperature-dependent effect reached a maximum after 1 h at 30 degrees C and was favored by ATP supplementation (Bmax = 22 +/- 3 pmol/mg of protein). The apparent Kd value for aldosterone (0.6 +/- 0.2 nM) and the steroid-binding specificity of the recombinant protein were in accordance with those reported for the native mineralocorticoid receptor. The sedimentation and DNA-cellulose-binding characteristics of the radioactive complexes were also in agreement with those reported for the native heteromeric receptor. Complexes sedimented at 8.9 +/- 0.2 or 4.2 +/- 0.2 S in sucrose gradients containing 20 mM sodium molybdate or 0.4 M KCl, respectively. Monoclonal antibody 8D3 against the 90-kDa heat shock protein (hsp90) was able to bind to the 8.9S complexes, increasing its sedimentation coefficient. Treatment of the complexes with 100 mM sodium thiocyanate, known to activate the native receptor to a DNA-binding state, caused a 79% increase in DNA-cellulose binding over the control values.(ABSTRACT TRUNCATED AT 250 WORDS)