Pregnenolone sulfate enhances neurogenesis and PSA-NCAM in young and aged hippocampus
Introduction
Aging is associated with cognitive dysfunction, which has been correlated to an alteration of hippocampal functioning [5], [6]. Indeed, the hippocampal formation (HF) plays a crucial role in controlling cognitive functions [18], and is the brain region most vulnerable to aging processes [50]. Both in humans and in animals, the severity of the cognitive disorders has been positively correlated with the extent of functional and structural modifications occurring in the hippocampal formation [11], [16], [23], [24], [25], [39].
Among the factors that could contribute to the reduced hippocampal function observed during aging, altered hormonal status [32], [40], [60], decreased neurotransmission [22] and more recently altered hippocampal levels of the neurosteroid pregnenolone sulfate (Preg-S) have been proposed. Preg-S belongs to a subclass of steroids synthesized de novo in the brain independently of peripheral synthesis [7], and is a potent memory enhancer in young animals [20], [49]. The levels of hippocampal Preg-S decrease during aging and the extent of this decrease is correlated positively with spatial memory disabilities [70]. The possible causal relationship between a decrease in Preg-S and the appearance of aging-related behavioral impairments is supported by the observation that an intra-hippocampal infusion of Preg-S in aged cognitively impaired rats reverses memory deficits [70]. Despite these striking relationships between Preg-S and hippocampal-dependant learning, the mechanisms by which Preg-S hippocampal levels influence learning abilities in aged animals remain largely unknown.
In this report, we analyzed the influence of the neurosteroid Preg-S on hippocampal neurogenesis. This novel form of structural plasticity occurs in the dentate gyrus of the HF and has been described in rodents, nonhuman primates and humans [19], [28]. The newly born cells express neuronal markers, emit axons, receive synaptic inputs [29], [48], [67] and their electrophysiological properties are very similar to those of mature dentate granule neurons [72]. Neurogenesis seems to play an important role in hippocampus-mediated learning [26], [34], [41], [65], [71].
The rationale for studying the interaction between neurosteroids and neurogenesis was threefold. Firstly, it has been proposed that the appearance of age-related cognitive dysfunctions stems from age-related decline in hippocampal neurogenesis [37], [63]. Secondly, neurogenesis has been shown to be sensitive to hormonal input and in particular to the effects of other steroids involved in aging such as glucocorticoids [13], [51]. Thirdly, Preg-S is a modulator of the gamma-aminobutyric acid receptor complex A (GABAA), which in turn has been shown to influence proliferation of progenitors derived from embryonic cortex [3], [44].
In order to determine the influence of Preg-S on hippocampal neurogenesis in vivo, we analyzed in young and old animals, the effect of intracerebroventricular (i.c.v.) infusions of Preg-S on neurogenesis using 5-bromo 2′ desoxyuridine (BrdU) to label dividing cells. We also examined the effects of Preg-S on the expression of PSA-NCAM, the polysialylated form of the Neuronal Cell Adhesion Molecule since PSA-NCAM is expressed by the newly born cells and plays a role in synaptic plasticity and learning processes [36], [63]. Finally, we analyzed the possible involvement of GABAA receptors in the regulation of adult neurogenesis by studying: (i) the influence of a GABAA agonist muscimol on Preg-S-induced changes of hippocampal plasticity, (ii) the role of another neurosteroid, allopregnanolone (AlloP), acting as a positive modulator of the GABAA complex and producing behavioral effects opposite to those of Preg-S [46], (iii) the effect of Preg-S in vitro on adult-derived spheres.
Section snippets
Housing conditions
Two or 18-month-old (n = 102 and 17, respectively) Sprague–Dawley male rats (Iffa Credo, France) were individually housed under a constant dark–light cycle (on 8.00 a.m., off 8.00 p.m.) with ad libitum access to food and water. Animals were allowed to acclimatize to the breeding room for at least two weeks before the beginning of the experiments.
Intracerebroventricular surgery
For i.c.v. implantation, rats were anaesthetized with ketamine (80 mg/kg i.p.) and secured in a Kopf stereotaxic instrument. A guide cannula (10 mm length,
Intracerebral injections of Preg-S in young and aged rats increase hippocampal plasticity
The effects of i.c.v. injections of Preg-S were examined in young adult rats using two indexes of hippocampal plasticity: neurogenesis and PSA-NCAM expression. To characterize neurogenesis, we studied cell proliferation, cell survival and the phenotype of the newly born cells. Progenitor cell proliferation in the dentate gyrus was examined by sacrificing rats 44 h after the last injection of BrdU. In these conditions, the injection of Preg-S induced a significant increase (63 % of control
Discussion
Our data demonstrate that neurosteroids exert a major influence on hippocampal plasticity. Thus, Preg-S increases neurogenesis and expression of PSA-NCAM in the dentate gyrus of young adult rats. More importantly, our results also show that the stimulatory effects of Preg-S on neurogenesis and PSA-NCAM expression are maintained in old animals, supporting the notion that neural plasticity can still be stimulated in the aged brain. These important findings could lead to the development of
Acknowledgements
The authors would like to thank Mrs. Aurousseau C., Mr. Hommolle V. (Perkin-Elmer, France) and Dr. J.-M. Daniel-Lamazière for their help with the confocal analysis. Supported by INSERM, CNRS (UR259-257-8BG01E), EC (EC QLK6 CT 2000-00179; EC QRT 2002-02187), la “Fondation pour la Recherche Médicale”, la “Région Aquitaine” and University of Bordeaux II, Institut Fédératif de Recherche no. 8.
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Have equally contributed to the work.