Expression of glucocorticoid receptors in the hippocampal region of the rat brain during postnatal development

Circulating glucocorticoids, of which their concentration is largely under the control of the hypothalamic-pituitary adrenal (HPA) axis, acting through the glucocorticoid receptors (GR) regulate a large variety of pivotal functions of the organism such as growth, development, immune- and stress-response. The main mechanism of regulation of the HPA axis activity is via negative feedback at all levels of the HPA axis itself as well as at the extra-hypothalamic level, a central part of which is the hippocampus. During neonatal development, the HPA axis of rats undergoes a period of hyporesponsiveness (SHRP)-when most stress stimuli fail to induce stress-response. Here, we describe the pattern of GR proteins expression in the hippocampal area of the rat brain during postnatal development and in adulthood. We demonstrated that the GR protein, of which its expression level is gradually enhanced in the hippocampus during postnatal life, exists in three different molecular sized forms. A larger molecular form was expressed at rather high levels at all studied time periods. A second smaller variant of GR was transiently expressed during the first one and a half weeks that corresponds with SHRP and then appeared again only in the adulthood. By the end of SHRP on PD 13, third smallest protein form of GR started to be detected in the hippocampal area. Thus, it remains to be disclosed in the nearest future, how the hippocampal GR isoforms may be involved in regulation of the neonatal HPA axis hyporesponsiveness as well as in functions of this system during the ensuing period of the brain maturation.