The hippocampus is necessary for enhancements and impairments of learning following stress

Abstract

The hippocampus is often considered to be an important site for stress and learning interactions; however, it has never been demonstrated whether these effects require the hippocampus. In the current study, hippocampal lesions prevented both enhancements of learning after stress in male rats and impairments of learning after stress in female rats without disrupting learning itself in either sex. Thus, the hippocampus is necessary for modifying learning in males and females after acute stressful experience.

Figure 1

Rats in the experimental group were given excitotoxic lesions of the complete hippocampus. (a) The largest lesion (in black) and smallest lesion (in gray) were selected from all animals included in the study, and are shown here according to a rat brain atlas (Supplementary Results online). (b) Representative examples of the smallest and largest lesion are shown. Scale bar, 1 mm. These experiments were conducted in accordance with procedures outlined by the Animal Care and Facilities Committee at Rutgers University.

Figure 2

Hippocampal lesions prevent the modulation of learning by stress, without disrupting learning itself or corticosterone release. (a) Hippocampal lesions prevented the stress-induced enhancement of conditioning in males and the stress-induced impairment of conditioning in females. Data are represented as the mean ± s.e.m. percentage of conditioned responses over 600 training trials. (b) Stress increased the percentage of male rats that learned, whereas it decreased the percentage of female rats that learned. However, both effects were prevented by hippocampal lesions. The graphs show the percentage of the subjects that emitted at least 60% conditioned responses for a minimum of one trial block (± standard error of the proportion), and asterisks indicate significant differences from sham-operated counterparts (P ≤ 0.05). (c) Hippocampal lesions did not prevent the release of corticosterone after stressor exposure. Data are represented as the mean ± s.e.m. plasma levels of corticosterone (ng ml⁻¹). Asterisks indicate increases in corticosterone levels relative to unstressed controls (P < 0.05).