Corticotropin releasing hormone antagonist does not prevent adrenalectomy-induced apoptosis in the dentate gyrus of the rat hippocampus

Functional differentiation in the transverse plane of the hippocampus: An update on activity segregation within the DG and CA3 subfields

Decades of neuroscience research in rodents have established an essential role of the hippocampus in the processing of episodic memories. Based on accumulating evidence of functional segregation in the hippocampus along the longitudinal axis, this role has been primarily ascribed to the dorsal hippocampus. More recent findings, however, demonstrate that functional segregation also occurs along transverse axis of the hippocampus, within the hippocampal subfields CA1, CA2, CA3, and the dentate gyrus (DG). Because the functional heterogeneity within CA1 has been addressed in several recent articles, here we discuss behavioral findings and putative mechanisms supporting generation of asymmetrical activity patterns along the transverse axis of DG and CA3. While transverse subnetworks appear to discretely contribute to the processing of spatial, non-spatial, temporal, and social components of episodic memories, integration of these components also occurs, especially in the CA3 subfield and possibly downstream, in the cortical targets of the hippocampus.

Adrenalectomy promotes a permanent decrease of plasma corticoid levels and a transient increase of apoptosis and the expression of Transforming Growth Factor beta1 (TGF-beta1) in hippocampus: effect of a TGF-beta1 oligo-antisense

BACKGROUND

Corticosterone reduction produced by adrenalectomy (ADX) induces apoptosis in dentate gyrus (DG) of the hippocampus, an effect related to an increase in the expression of the pro-apoptotic gene bax. However it has been reported that there is also an increase of the anti-apoptotic gene bcl-2, suggesting the promotion of a neuroprotective phenomenon, perhaps related to the expression of transforming growth factor beta1 (TGF-beta1). Thus, we have investigated whether TGF-beta1 levels are induced by ADX, and whether apoptosis is increased by blocking the expression of TGF-beta1 with an antisense oligonucleotide (ASO) administered intracerebrally in corticosterone depleted rats.

RESULTS

It was observed an increase of apoptosis in DG, 2 and 5 days after ADX, in agreement with a reduction of corticosterone levels. However, the effect of ADX on the number of apoptotic positive cells in DG was decreased 5 days after the lesion. In CA1-CA3 regions, the effect was only observed 2 days after ADX. TGF-beta1 mRNA levels were increased 2 days after ADX. The sustained intracerebro-ventricular administration of a TGF-beta1 ASO via an osmotic mini pump increased apoptosis levels in CA and DG regions 5 days after ADX as well as sham-operated control animals. No significant effect was observed following a scrambled-oligodeoxynucleotide treatment.

CONCLUSION

The changes in both the pattern and the magnitude of apoptotic-cell morphology observed 2 and 5 days after ADX suggest that, as a consequence of the reduction of corticosteroids, some trophic mechanisms restricting cell death to a particular time window are elicited. Sustained intracerebral administration of TGF-beta1 ASO increased the apoptosis promoted by ADX, suggesting that TGF-beta1 plays an anti-apoptotic role in vivo in hippocampus.

Lifelong corticosterone level determines age-related decline in neurogenesis and memory

Ageing is accompanied by an alteration of spatial memory, a decline in hippocampal neurogenesis and a dysregulation of the hypothalamic-pituitary axis (HPA) leading to elevated levels of circulating corticosterone. However, the role of the HPA axis in age-related decline in cognitive functions and in neurogenesis decline remains unclear. We found that suppression of glucocorticoids secretion from midlife to the rest of the animals' life increases neurogenesis in old animals and prevents the emergence of age-related memory disorders. Reciprocally, aged rats with a chronic upregulation of the HPA axis exhibit not only spatial memory impairments but also very low levels of hippocampal cell proliferation and survival. Altogether, these results indicate that the extent of lifetime exposure to glucocorticoids determines the extent of age-related decline in hippocampal neurogenesis and consequently age-related cognitive dysfunctions.

Hippocampal neurogenesis is not enhanced by lifelong reduction of glucocorticoid levels

Neurogenesis of dentate gyrus granule cells is generally considered to be negatively regulated by glucocorticoids. We tested the hypothesis that exposure to low plasma corticosteroid levels starting in the early postnatal period enhances granule cell proliferation rate during adulthood. Rat pups were adrenalectomized (ADX) on postnatal day 10 and were then "clamped" throughout life at low corticosterone levels via oral supplementation. Neurogenesis was determined using BrdU immunochemistry at 3 and 12 months in clamped rats as compared with age-matched, sham-operated controls. Rate of neurogenesis did not differ between the groups at either 3 or 12 months. It was significantly lower in 12-month-old compared with 3-month-old rats, despite the presence of an age-dependent increase of plasma corticosterone only in the sham-ADX rats. Granule cell layer volume, granule cell density, and granule cell degeneration (determined using apoptotic markers) were indistinguishable in the two groups, further supporting the comparable rate of neurogenesis under differing chronic glucocorticoid levels. In addition, whereas acute deprivation of plasma glucocorticoids (adrenalectomy) in adult rats evoked a burst of granule cell neurogenesis, complete elimination of these hormones (by stopping hormone supplementation) in adult, early-life ADX/clamped rats did not. These data do not support a simple inverse relationship between chronic plasma glucocorticoid levels and granule cell neurogenesis. Specifically, chronic modulation of glucocorticoid levels commencing early in life evokes additional, adaptive, and compensatory mechanisms that contribute to the regulation of granule cell proliferation.

Implication of corticosteroid receptors in the regulation of hippocampal structural plasticity

The dentate gyrus is one of the few areas of the adult brain that continues to produce neurons and to express the embryonic polysialylated isoforms of neuronal cell adhesion molecules (PSA-NCAM). The stress hormone corticosterone exerts a complex modulation on neurogenesis and PSA-NCAM, and previous studies have shown that mature granule cells require corticosterone for their survival. Thus, the aim of our work was to investigate the respective role of the different corticosteroid receptors on these three parameters in adrenalectomized rats. It was found that treatment with a low dose of the mineralocorticoid receptor agonist, aldosterone, prevents only the adrenalectomy-induced increase in cell death. Treatment with a higher dose of aldosterone normalized cell proliferation whereas PSA-NCAM expression was normalized only by treatment with the glucocorticoid receptor agonist, RU 28362. It is concluded that stimulation of the mineralocorticoid receptor is sufficient to mediate the effects of corticosterone on neurogenesis and to protect mature cells from cell death whereas stimulation of the glucocorticoid receptor is necessary to modulate PSA-NCAM expression.

Circulating corticosterone alters the rate of neuropathological and behavioral changes induced by trimethyltin in rats

When trimethyltin (TMT) is administered to rats, the plasma corticosterone concentration rises transiently 3 to 4 days later. We examined whether plasma corticosterone plays a causative role in the TMT-induced impairment of the hippocampus as assessed by pathological and behavioral tests. TMT-administered rats were supplementally treated with either adrenalectomy or metyrapone (twice daily for the first 7 days after TMT) in order to permanently deplete or transiently suppress circulating corticosterone. Loss of pyramidal cells in the CA1 and CA3 fields, mossy fiber sprouting, and impairment of spatial memory were observed after TMT intoxication. Adrenalectomy apparently aggravated both the hippocampal damage and the spatial memory impairment induced by TMT treatment. The TMT+metyrapone treatment groups exhibited a significant reduction in pyramidal cells in both the CA1 and the CA3 regions. However, the neuronal damage in CA1 was significantly different between the TMT and the TMT+metyrapone groups. Metyrapone significantly reduced the TMT-induced damage to pyramidal cells in CA1, but not CA3, and it also abolished mossy fiber sprouting. TMT-induced learning impairment and hyperactivity were alleviated by metyrapone treatment. It is thus concluded that both the high levels of corticosterone induced by TMT and the pathologically low levels of corticosterone induced by adrenalectomy will worsen the consequences of TMT.

Androgens are neuroprotective in the dentate gyrus of adrenalectomized female rats

Neuroprotective effects of androgens have not been well-characterized, but there is evidence that 5 alpha-androstane-3 alpha, 17 beta-diol (3 alpha-diol) has anti-seizure effects. To further examine androgens' neuroprotective effects, testosterone (T), dihydrotestosterone (DHT), 3 alpha-diol (1.0 mg/kg SC daily), or sesame oil vehicle was administered to adrenalectomized or sham-operated, young, female Long Evans rats (N = 52). After seven days, animals were perfused and trunk blood was collected for radioimmunoassay of plasma corticosterone and androgens. No pyknotic cells were seen in the dentate of the sham-operated animals or those animals that had incomplete adrenalectomies (n = 20); however, cresyl violet and TUNEL stains revealed pyknotic cells in the granule layer of the dentate gyrus of adrenalectomized rats (n = 28). Testosterone, DHT, or 3 alpha-diol significantly reduced the number of pyknotic cells in the dentate gyrus compared to vehicle administered, adrenalectomized rats. Steroid-administered animals had levels of T, DHT, or 3 alpha-diol within physiological concentrations. These findings suggest that T, DHT, or 3 alpha-diol may have neuroprotective effects via a common mechanism of action.

Selective vulnerability to radiation in the hippocampal dentate granule cells

BACKGROUND

Radiation therapy is an effective approach in the treatment of highly radiosensitive brain tumors such as germinomas. However, recent studies have reported intellectual disturbances in patients who underwent whole-brain irradiation as children. We detected apoptosis in the infantile murine cerebrum after systemic X-ray irradiation.

METHODS

Subjects were 100 ICR mice 4 weeks old, of which 90 were systemically exposed to 18 Gy X-rays (0.45 Gy/min); 10 each were decapitated and the cerebrums were removed 1, 3, 6, 9, 12, 18, 24, 48, and 72 hours after irradiation. Controls were 10 unirradiated mice. DNA fragmentation analysis was carried out by agarose gel electrophoresis, and morphological analysis was by the TUNEL method.

RESULTS

According to agarose gel electrophoresis, the cerebral DNA ladders were detected only over 6 to 24 hr, peaking in 9 hr. Even at the peak, band intensity was nearly double that of the unirradiated normal thymus. According to the TUNEL analysis, radiation-induced apoptosis increased, with a peak at 9 hours, but decreased 24 hours after irradiation. Apoptotic cells were always localized exclusively in the hippocampal dentate granule cells.

CONCLUSIONS

We found that vulnerability to radiation existed in the hippocampal dentate granule cells. Intellectual disturbances in patients who have undergone whole-brain irradiation may be caused by injury to the hippocampus.

The neurosteroid, 3alpha-androstanediol, prevents inhibitory avoidance deficits and pyknotic cells in the granule layer of the dentate gyrus induced by adrenalectomy in rats

Testosterone (T), dihydrotestosterone (DHT), 3alpha-diol (5alpha-androstane-3alpha,17beta-diol), or vehicle was administered daily to gonadectomized, adrenalectomized male Long-Evans rats. Four days after adrenalectomy (ADX), animals were tested for inhibitory avoidance. Vehicle-treated rats had shorter latencies than 3alpha-diol-treated rats and non-corticosterone-depleted rats. All three androgens significantly reduced the number of pyknotic cells in corticosterone-depleted rats. Neurosteroids may mediate androgens' functional and morphological neuroprotective effects.