RECRUITMENT AND REPLACEMENT OF HIPPOCAMPAL-NEURONS IN YOUNG AND ADULTCHICKADEES - AN ADDITION TO THE THEORY OF HIPPOCAMPAL LEARNING

We used [H-3]thymidine to document the birth of neurons and their recr uitment into the hippocampal complex (HC) of juvenile (4.5 months old) and adult black-capped chickadees (Parus atricapillus) living in thei r natural surroundings, Birds received a single dose of [H-3]thymidine in August and were recaptured and killed 6 weeks later, in early Octo ber, All brains were stained with Cresyl violet, a Nissl stain, The bo undaries of the HC were defined by reference to the ventricular wall, the brain surface, or differences in neuronal packing density. The HC of juveniles was as large as or larger than that of adults and packing density of HC neurons was 31% higher in juveniles than in adults, Alm ost all of the H-3-labeled HC neurons were found in a 350-mu m-wide la yer of tissue adjacent to the lateral ventricle, Within this layer the fraction of H-3-labeled neurons was 50% higher in juveniles than in a dults. We conclude that the HC of juvenile chickadees recruits more ne urons and has more neurons than that of adults, We speculate that juve niles encounter greater environmental novelty than adults and that the greater number of HC neurons found in juveniles allows them to learn more than adults, At a more general level, we suggest that (i) long-te rm learning alters HC neurons irreversible (ii) sustained hippocampal learning requires the periodic replacement of HC neurons; (iii) memori es coded by hippocampal neurons are transferred elsewhere before the n eurons are replaced.