Learning enhances the survival of new neurons beyond the time when the hippocampus is required for memory

Abstract

Trace memories are formed when a stimulus event becomes associated with another event that occurs later in time and is discontinuous with the first event. The formation of trace memories enhances the survival of newly generated neurons in the dentate gyrus of the adult hippocampus (Gould et al., 1999a). Here we tested whether the acquisition of trace memories early during training is sufficient to enhance cell survival. We also examined whether the new neurons affected by trace memory formation persist indefinitely or only as long as the hippocampus is necessary for the expression of those memories. Groups of adult rats were injected with bromodeoxyuridine (BrdU), a marker of dividing cells, and trained 1 week later with paired stimuli using a trace eyeblink conditioning task or exposed to the same number of unpaired stimuli. Cell survival was assessed after different numbers of training trials and survival periods after training. Overall cell survival was not enhanced by exposure to 200 trials of paired stimuli during trace conditioning. However, there was a positive correlation between performance of individual animals and cell survival. In addition, exposure to 800 trials of paired stimuli during trace conditioning increased the number of BrdU-labeled cells 60 d after training. The vast majority of these cells were neurons and coexpressed the neuronal markers class IIIbeta-tubulin or neuronal nuclei. These data suggest that individual differences in associative learning predict whether new neurons will survive and that once affected, these neurons remain for months and beyond the time when they are required for the retention of trace memories.

Figure 1.

Schematic diagram of experimental design. Rats were injected with BrdU (200 mg/kg, i.p.) and 1 week later underwent 200 or 800 trials of trace eyeblink conditioning with paired stimuli or unpaired training. Two hundred trials of training were massed within 1 d, whereas 800 trials were distributed over 3 consecutive days (300, 300, and 200 per day). Rats were perfused on the 10th day after BrdU injection. Additional groups trained for 800 trials were killed 30 or 60 d after training (40 and 70 d after BrdU injection, respectively).

Figure 2.

Early learning in individual animals correlates with cell survival. Although animals exposed to 200 trials of paired stimuli during trace conditioning emitted more conditioned responses than those that were exposed to unpaired stimuli (A), 200 trials of training did not affect the number of BrdU-labeled cells in the dentate gyrus (B). However, there was a positive correlation between the performance of individual animals trained with paired stimuli and survival of new neurons in the dentate gyrus. Those animals that emitted more learned responses possessed more BrdU-labeled cells (C).

Figure 3.

Trace memory formation persistently enhances the survival of newly born cells in the dentate gyrus. A-C, Acquisition of the trace eyeblink conditioned response (trace paired) and the unpaired condition. D-F, Total numbers of BrdU-labeled cells in the dentate gyrus of animals 1 d (D), 30 d (E), or 60 d (F) after trace conditioning or unpaired training. Trace conditioning increased the number of BrdU-labeled cells when compared with exposure to unpaired stimuli at all survival times. Regardless of training conditions, the number of cells decreased between 1 and 30 d but not there after. Bars represent mean ± SEM. Significant differences are noted with asterisks.

Figure 4.

Most of the BrdU-labeled cells differentiate into neurons. Images depict a greater number of BrdU-labeled cells in animals exposed to paired stimuli during trace conditioning than animals exposed to unpaired stimuli at both 1 and 60 d after training (A). Regardless of training conditions, more BrdU-labeled cells are evident at 1 d (10 d after BrdU injection) than at 60 d (70 d after BrdU injection). The majority of BrdU-labeled cells (red) expressed the neuronal markers (green) TuJ1 (B) and NeuN (C).