WADA, H.*; HAHN, T.P.; BREUNER, C.W.; Univ. of Texas at Austin; Univ. of California, Davis; Univ. of Texas at Austin: Ontogenetic pattern of intracellular corticosteroid receptors in white-crowned sparrow nestling brain

During the first stages of life, some species show a lack of reactivity to various stressors, failing to elicit glucocorticoid secretion. This is called the stress non-responsive period, which has been well documented in rats. It is thought to be adaptive, allowing organs to mature without glucocorticoids retarding growth. Previously, we have shown that altricial white-crowned sparrow nestlings also show a stress non-responsive period: handling stress did not increase total corticosterone during the first 3 days of 10-day nestling period. This lack of reactivity may be regulated at multiple physiological levels; here we focus on neural corticosteroid receptors. We investigated changes in the two intracellular corticosteroid receptors (the high-affinity mineralocorticoid receptor �MR, and the low-affinity glucocorticoid receptor�GR) with age. Brains were obtained from three age groups (1-3, 4-6, and 7-9 days post-hatch) of free living Nuttall�s white-crowned sparrow nestlings. We found that while GR showed no change in receptor number with age, MR had a trend in decline with age. The age group showing the non-responsive period also had the highest number of MR. These results suggest 3 points: 1) the non-responsive period seen in the youngest age group of this species may be caused by MR-mediated negative feedback, 2) different ontogenetic patterns of MR and GR may result in differential set-point (threshold) for the negative feedback causing contrasting sensitivity to a stressor with age, and 3) MR and GR numbers may be controlled separately during development. We are also currently investigating whether affinities of these receptors for corticosterone change with age.