Breaking bonds in male prairie vole: long-term effects on emotional and social behavior, physiology, and neurochemistry

Abstract

Social relationships are essential for many fundamental aspects of life while bond disruption can be detrimental to mental and physical health. Male prairie voles form enduring social bonds with their female partners, allowing the evaluation of partner loss on behavior, physiology, and neurochemistry. Males were evaluated for partner preference formation induced by 24h of mating, and half were separated from their partner for 4 wk. In Experiment 1, partner loss significantly increased anxiety-like behaviors in the elevated plus maze and light-dark box tests and marginally increased depressive-like behaviors in the forced swim test. In addition, while intruder-directed aggression is common in pair bonded prairie voles, separated males were affiliative and lacked aggression toward an unfamiliar female and an intruding male conspecific. Partner loss increased the density of oxytocin-immunoreactivity (-ir), vasopressin-ir, and corticotrophin-releasing hormone-ir cells in the paraventricular nucleus of the hypothalamus and oxytocin-ir cells in the supraoptic nucleus. Tyrosine hydroxylase-ir was not affected. In Experiment 2, partner preference was observed after 2 wk of partner loss but eliminated after 4 wk partner loss. Body weight gain and plasma corticosterone concentrations were elevated throughout the 4 wk. No effects were observed for plasma oxytocin or vasopressin. Together, partner loss elicits anxiety-like and depression-like behaviors, disrupts bond-related behaviors, and alters neuropeptide systems that regulate such behaviors. Thus, partner loss in male prairie voles may provide a model to better understand the behavior, pathology, and neurobiology underlying partner loss and grief.

Keywords: Bond loss; Corticotrophin releasing hormone; Oxytocin; Social stress; Tyrosine hydroxylase; Vasopressin.

Figure 1

Anxiety-like behaviors. (A–D) Elevated plus maze (EPM) test. Partner loss led to a (A) delay in the latency of males to enter the open arm, (B) decrease in the percentage of open arm entries vs total arm entries by the males, and (C) increase in the percentage of time the males spent in the closed arm in the EPM test. D, No effects on total arm entries were observed. (E–F) Light-dark box (LDB) test. E, Partner loss led to an increase in time spent in the dark box and a decrease in time spent in the light box in the LDB test. F, No effects on line crossings were observed. Bars labeled with asterisks indicate a significant difference between the separated males (Separated) and paired males (Paired) for a specific measure as determined by Independent Sample’s T-test (p < 0.05). A–F, Data are expressed as mean ± SEM.

Figure 2

Social affiliation and aggression behaviors. (A–B) Affiliation (AFF) test. Partner loss (A) did not affect the amount of time males spent in either the conspecific or empty cage, (B) but it did led to an increase in non-agonistic body contact and a decrease in attack behavior. (C–F) Resident-intruder test (RIT). Partner loss (C–D) increased total affiliation and decreased aggression in both frequency and duration and (E–F) increased nose-to-nose (Nose) olfactory investigation frequency and duration and nose-to-anogenital (Anogenital) olfactory investigation frequency. Bars labeled with asterisks indicate a significant difference between the separated males (Separated) and paired males (Paired) for a specific measure as determined by Independent Sample’s T-test (p < 0.05). A–F, Data are expressed as mean ± SEM.

Figure 3

Immunohistochemistry Representative photo images illustrating immunoreactive staining of CRH-ir (A–B), OT-ir (C–D) and AVP-ir (E–F) cells in the PVN as well as TH-ir cells in the VTA (G–H) from male voles that were either paired with (A, C, E & G) or separated from (B, D, F & H) their female partner. Scale bar = 100µm.

Figure 4

Immunohistochemistry. Partner loss (A) decreased CRH-ir density in the PVN but not the SON, (B) decreased Oxt-ir density in the PVN and SON, (C) decreased AVP-ir density in the PVN but not the SON, and (D) had no effect on TH-ir density in the ZIR or VTA. Bars labeled with asterisks indicate a significant difference between the separated males (Separated) and paired males (Paired) for a specific measure as determined by Independent Sample’s T-test (p < 0.05). A–D, Data are expressed as mean ± SEM.

Figure 5

Partner preference test (PPT). Paired males displayed a preference for their familiar partner (black bars) over stranger females (white bars) after both 2 wk or 4 wk pairing. However, males separated from their female only displayed a partner preference after 2 wk separation, and showed no preference after 4 wk separation. Bars labeled with different letters differ significantly by SNK's post-hoc test in which a significant interaction was detected in the mixed-model ANOVA (p < 0.05). Data are expressed as mean ± SEM.