Hippocampal mossy fibers, motivational states, and contextual retrieval

Neontal rats were irradiated to prevent dentate granule cell genesis and thereby remove mossy fibers from hippocampal circuitry. When mature, these animals were much less able than their nonirradiated siblings to behave differentially in a T-maze in which food and water could be found in opposite arms in response to variations of motivational state. A lack of differentiation was manifested by (i) high tendencies to make only one response during initial trials, (ii) increased numbers of trials to criterion, and (iii) negative covariation between performance levels in obtaining food and water as learning trials proceeded. The results are interpreted as indicating that the irradiated animals learn in a manner quite different from normal ones and is consistent with a theory that the dentate granule cells and mossy fibers are involved in information retrieval. Speculations on the exact role of the mossy fibers are advanced.