Corticosterone influences on Mammalian neonatal sensitive-period learning

Abstract

Infant rats exhibit sensitive-period odor learning characterized by olfactory bulb neural changes and odor preference acquisitions critical for survival. This sensitive period is coincident with low endogenous corticosterone (CORT) levels and stress hyporesponsivity. The authors hypothesized that low corticosterone levels modulate sensitive-period learning. They assessed the effects of manipulating CORT levels by increasing and removing CORT during (Postnatal Day 8) and after (Postnatal Day 12) the sensitive period. Results show that (a) exogenous CORT prematurely ends sensitive-period odor-shock-induced preferences; (b) adrenalectomy developmentally extends the sensitive period as indicated by odor-shock-induced odor-preference learning in older pups, whereas CORT replacement can reinstate fear learning; and (c) CORT manipulation modulates olfactory bulb correlates of sensitive-period odor learning in a manner consistent with behavior.

Figure 1

Mean (± SEM) number of conditioned odor choices in a Y-maze test (total of five trials) for (A) sensitive-period Postnatal Day (PN) 8 pups and (B) post-sensitive-period PN12 pups. Mean (± SEM) level of odor-induced olfactory bulb focal ¹⁴C 2-deoxyglucose (2-DG) uptake in (C) sensitive-period PN8 pups and (D) post-sensitive-period PN12 pups. Asterisks represent significant differences from all others groups ( p < .05). CS = conditioned stimulus; CORT = corticosterone; ADX = adrenalectomy.

Figure 2

Mean (± SEM) number of responses (behavioral activitation) to the odor conditioned stimulus during odor–shock acquisition for (A) sensitive period Postnatal Day (PN) 8 pups and (B) post-sensitive-period PN12 pups. CORT = corticosterone; ADX = adrenalectomy.