NEONATAL ISOLATION ENHANCES HIPPOCAMPAL DENTATE RESPONSE TO TETANIZATION IN FREELY MOVING JUVENILE MALE-RATS

The impact of early neonatal isolation on measures of hippocampal neur onal plasticity was examined in freely moving male rats at 30 days of age. Beginning on Postnatal (PN) Day 2, one-half of pups from each exp erimental litter were individually isolated from the nest, dam, and si blings for a period of I h per day over PN Days 2-9, while their sibli ngs remained in the nest. In addition, randomly selected litters serve d as unhandled controls. On PN Day 26 all pups were weaned and chronic ally implanted for recording of evoked held potentials and induction o f hippocampal long-term potentiation, At 30 days of age, pups from the three treatment groups (isolated, nonisolated siblings, and unhandled controls) were tested for their ability to establish and maintain lon g-term potentiation across the perforant path/hippocampal dentate gran ule cell synapse. Changes in population EPSP slope and population spik e amplitude (PSA) recorded following tetanization were used to assess the effects of neonatal isolation on hippocampal response measures. No significant between-group differences were obtained for input/output response curves constructed prior to tetanization. All three groups sh owed immediate and significant enhancement of the PSA measure at 15 mi n posttetanization. The level of PSA enhancement obtained from previou sly isolated pups was significantly greater than that obtained from bo th the nonisolated sibling and unhandled control groups. PSA enhanceme nt in both the nonisolated and unhandled animals decayed to baseline 4 8-72 h after tetanization, while isolates showed no significant decay of the PSA measure at this period. Neonatal isolation was found to sig nificantly enhance both the magnitude and duration of dentate granule cell response to tetanizing stimulations applied during early adolesce nce, a full 3 weeks after termination of the isolation paradigm. The r esults suggest that regularly repeated isolation stress occurring duri ng infancy has an enduring impact on dentate neuroplasticity. (C) 1995 Academic Press, Inc.