The CRF system mediates increased passive stress-coping behavior following the loss of a bonded partner in a monogamous rodent

Abstract

Social relationships significantly influence physiology and behavior, including the hypothalamo-pituitary-adrenal axis, anxiety, and mental health. Disruption of social bonds through separation or death often results in profound grieving, depression, and physical illness. As the monogamous prairie vole forms enduring, selective pair bonds with the mating partner, they provide an animal model to study the physiological consequences of pair bonding and, thus, the loss of the bonded partner. Male prairie voles were paired with a novel female or male sibling. After 5 days, half of the males of each group were separated from the partner. Elevated plus-maze, forced swim, and tail suspension tests were used to assess anxiety-like and passive stress-coping behaviors indicative of depressive-like behavior. Following 4 days of separation from the female but not the male partner, experimental males displayed increased passive stress-coping. This effect was abolished by long-term intracerebroventricular infusion of a nonselective corticotropin-releasing factor (CRF) receptor antagonist without disrupting the bond itself. Both CRF type 1 and 2 receptors were involved in the emergence of passive stress-coping behavior. Furthermore, pairing with a female was associated with elevated CRF mRNA in the bed nucleus of the stria terminalis, and partner loss elicited a pronounced increase in circulating corticosteroid and adrenal weight. We speculate that the CRF system may mediate an aversive affect following separation from the female partner, which may facilitate proximity seeking between the pair-bonded individuals. Hence, the prairie vole model may provide insights into brain mechanisms involved in the psychopathological consequences of partner loss.

Figure 1

Testing schedules of the five experimental approaches (A–E). Male prairie voles were co-housed and allowed to form a partner bond. After 5 days, half of the males of each group were separated from the partner; animals from paired groups stayed with their partner. On the ninth day after pairing, ie the fourth day after separation (A, B) or on the eighth day after pairing, ie the third day after separation (D, E), testing for behavioral effects started, or voles were left undisturbed until the tenth day after first pairing, ie the fifth day after separation in the particular groups, to take blood samples and brains under basal conditions (C). In Experiments (D, E), osmotic minipumps were implanted after 3 days of pairing delivering Ringer's solution until day 5 of pairing (grey bars) when half of the animals were separated followed by CRF receptor antagonist lasting until the end of the experiment (black bars). EPM = elevated plus-maze; FS = forced swim test; HPA = hypothalamo–pituitary–adrenal axis; PP = partner preference test; TS = tail suspension test.

Figure 2

Effect of partner separation on passive stress-coping behavior in (a) the forced swim test and (b) the tail suspension test as well as (c, d) anxiety-like behavior on the elevated plus-maze. Male voles were paired for 5 days with a female (fp) or male sibling (sp). Half of the males were then separated from their cage-mate (black bars) whereas the remaining half remained pair housed (grey bars) for 4–5 days before behavioral testing (from Experiment A (a, b) or Experiment B (c, d; see Figure 1 for details). Passive stress-coping behavior is reflected as the amount of time the animal spends inactive, ie floating (a) or immobile (b). Anxiety-like behavior is reflected as percentage of time spent in and percentage of entries into the open arms vs all arms (c, d). Numbers of animals included in the statistics were (a) female partner: n=9 in each group; sibling partner: n=10 in each group; (b) female partner: n=9 in each group; sibling partner paired: n=9; separated: n=10; (c, d) female partner paired: n=10; separated: n=12; sibling partner: n=12 in each group. Data are expressed as mean + SEM. **p<0.01 vs all other groups.

Figure 3

Effect of partner separation on basal plasma levels of corticosterone (a) and ACTH (b). Male voles were paired for 5 days with a female (fp) or male sibling (sp). Half of the males were then separated from their cage-mate (black bars) whereas the remaining half remained pair housed (grey bars) for 5 days (Experiment C; see Figure 1 for details). The voles were left undisturbed until the initial separation from the partner and no behavioral tests were performed. Numbers of animals included o in the statistics were 10 per group. Data are expressed as mean + SEM. *p<0.05 vs respective female-paired group.

Figure 4

CRF mRNA expression in the mBNST. Clusters of grains in the region of the mBNST (a). Male voles (Experiment C; see Figure 1 for details) paired with a female (grey bars) or separated from a female (black bars) demonstrate higher CRF mRNA grains in the mBNST relative to males paired or o separated from a sibling male (b). Data are expressed as mean + SEM. *p<0.05.

Figure 5

Effect of i.c.v. CRF-R antagonist (d-phe-CRF) treatment unspecific for receptor type 1 and 2 on passive stress-coping in (a) the forced swim test, (b) the tail suspension test, and (c) the partner preference test in female-paired male voles from Experiment D (see Figure 1 for details). Male voles were group-housed for 5 days with a female and constantly infused with CRF-A the following days while being still with the partner (grey bars) or separated 3–5 days (black bars). The amount of time the animals spend on passive stress-coping strategy, ie floating (a) or immobility (b). The partner preference (c) is represented by the time the male voles spent on huddling with either a female stranger or the bonding partner. Numbers of animals included in the statistics were paired VEH: 5; CRF-A: 4; separated VEH: 7; CRF-A: 6. Data are expressed as mean + SEM. **p0.01 vs all other groups. *p<0.05 vs huddling with stranger in same group.

Figure 6

Effect of i.c.v. CRF-R1 (CP-154526) or CRF-R2 (Astressin-2B) antagonist treatment on passive stress-coping in (a) the forced swim test and (b) the tail suspension test in female-paired male voles from Experiment E (see Figure 1 for details). Male voles were group-housed for 5 days with a female and constantly infused with CRF-R1 or -R2 antagonist the following days when still with the partner (grey bars) or separated 3–4 days (black bars). The amount of time the animals spend on passive stress-coping strategy is reflected as floating (a) or immobility (b). Numbers of animals included in the statistics were paired VEH: 10; CRF-R1: 9; CRF-R2: 9; separated were 10 per group. Data are expressed as mean + SEM. **p<0.01 vs respective female-paired group; ^##p<0.01, ^#p<0.05 vs vehicle-treated separated group.