Extinction of behavior in infant rats: Development of functional coupling between septal, hippocampal, and ventral tegmental regions

Learning of a behavior at a particular age during the postnatal period pres umably occurs when the functional brain circuit mediating the behavior matu res. The inability to express a learned behavior, such as inhibition, may b e accounted for by the functional dissociation of brain regions comprising the circuit. In this study we tested this hypothesis by measuring brain met abolic activity, as revealed by fluorodeoxyglucose (FDG) autoradiography, d uring behavioral extinction in 12- and 17-d-old rat pups. Subjects were fir st trained on a straight alley runway task known as patterned single altern ation (PSA), wherein reward and nonreward trials alternate successively. Th ey were then injected with FDG and given 50 trials of continuous nonreward (i.e., extinction). Pups at postnatal day 12 (P12) demonstrated significant ly slower extinction rates compared to their P17 counterparts, despite the fact that both reliably demonstrated the PSA effect, i.e., both age groups distinguished between reward and nonreward trials during acquisition. Covar iance analysis revealed that the dentate gyrus, hippocampal fields CA1-3, s ubiculum, and lateral septal area were significantly correlated in P17 but not P12 pups. Significant correlations were also found between the lateral septal area, ventral tegmental area, and the medial septal nucleus in P17 p ups. Similar correlative patterns were not found in P12 and P17 handled con trol animals. Taken together, these results suggest that septal, hippocampa l, and mesencephalic regions may be functionally dissociated at P12, and th e subsequent maturation of functional connectivity between these regions al lows for the more rapid expression of behavioral inhibition during extincti on at P17.