Neurogenesis and helplessness are mediated by controllability in males but not in females

Abstract

Background: Numerous studies have implicated neurogenesis in the hippocampus in animal models of depression, especially those related to controllability and learned helplessness. Here, we tested the hypothesis that uncontrollable but not controllable stress would reduce cell proliferation in the hippocampus of male and female rats and would relate to the expression of helplessness behavior.

Methods: To manipulate controllability, groups of male and female rats were trained in one session (acute stress) or over seven sessions (repeated stress) to escape a footshock, whereas yoked control subjects could not escape but were exposed to the same amount of stress. Cell proliferation was assessed with immunohistochemistry of bromodeoxyuridine (BrdU) and immunofluorescence of BrdU and neuronal nuclei (NeuN). Separate groups were exposed to either controllable or uncontrollable stress, and their ability to learn to escape during training on a more difficult task was used as a behavioral measure of helplessness.

Results: Acute stress reduced cell proliferation in males but did not affect proliferation in the female hippocampus. When animals were given the opportunity to learn to control the stress over seven days, males produced more cells than the yoked males without control. Repeated training with controllable stress did not influence proliferation in females. Under all conditions, males were more likely than females to express helplessness behavior, even males that were not previously stressed.

Conclusions: The modulation of neurogenesis by controllability was evident in males but not in females, as was the expression of helplessness behavior, despite the fact that men are less likely than women to experience depression.

Figure 1. Acute stress decreases cell proliferation in the male but not the female hippocampus

(a) Escape latencies in the FR1 task (means of five trials + SEM) for males exposed to one session (30 trials) of controllable stress. (b) Mean number (means + SEM) of BrdU labeled cells in the dentate gyrus from the same groups, along with a group of naives. (c) Escape latencies in the FR1 task (means of five trials + SEM) for females exposed to one session of controllable stress. (d) Mean number (means + SEM) of BrdU labeled cells in the dentate gyrus from the same groups, along with a group of naives. [* indicates significant differences (p<0.05)].

Figure 2. BrdU and NeuN labeling in the dentate gyrus of hippocampus

Images illustrate BrdU-labeled cells (green) that expressed NeuN (red), a marker of mature neurons in animals that were sacrificed 3 weeks after the BrdU injection. Arrows indicate cells that were immunoreactive for both BrdU and NeuN.

Figure 3. Controllability over a repeated stressor modulates neurogenesis in the male but not the female hippocampus

(a) Escape latencies in the FR1 task (means of 30 trials + SEM) for males exposed to seven sessions of controllable stress and (b) the number of BrdU labeled cells in the dentate gyrus of 15 yoked pairs of the males. Pairs were sorted into those demonstrating an increase in proliferation with controllable stress, from the left and ending with the opposite response to the right. (c) Escape latencies in the FR1 task (means of 30 trials + SEM) for females exposed to seven sessions of controllable stress and (d) the number of BrdU labeled cells in the dentate gyrus of 12 female yoked pairs. Pairs were sorted into those demonstrating an increase in proliferation with controllable stress, from the left and ending with the opposite response to the right.

Figure 4. New cells generated in the male and female hippocampus in response to controllable versus uncontrollable stress

BrdU labeled cells in the dentate gyrus of a (a) male and (female) hippocampus after training with seven sessions of controllable or uncontrollable stress. Larger images were magnified ×400 and the smaller images ×200.

Figure 5. Performance during training with the FR1 task in males versus females

(a) Escape latencies in the FR1 task (means of five trials + SEM) for males trained with one session of controllable stress. (b) Escape latencies in the FR1 task (means of 30 trials + SEM) for males trained with seven sessions of controllable stress. (c) Escape latencies in the FR1 task (means of five trials + SEM) for females trained with one session of controllable stress. (d) Escape latencies in the FR1 test (means of thirty trials + SEM) for females exposed to seven days of controllable stress.

Figure 6. Males express learned helplessness behavior, whereas females do not

(a) Escape latencies in males that were only trained on the operant task in which they had to cross the shuttle-box apparatus twice to escape (FR2) (means of five trials + SEM). (b) Escape latencies during training on the FR2 task (means of five trials + SEM) after exposure to one session of controllable or uncontrollable stress (30 trials). (c) Escape latencies in males that were trained on the FR2 task (means of five trials + SEM) after exposure to seven sessions of controllable or uncontrollable stress. (d) Escape latencies in females that were only trained on the operant task in which they had to cross the shuttle-box twice to escape (FR2) (means of five trials + SEM). (e) Escape latencies during training on the FR2 task (means of five trials + SEM) in females after exposure to one session of controllable or uncontrollable stress. (f) Escape latencies in the FR2 test (means of five trials + SEM) after exposure to seven sessions of controllable or uncontrollable stress.