Brain oxytocin correlates with maternal aggression: link to anxiety

Abstract

The oxytocinergic system is critically involved in the regulation of maternal behavior, which includes maternal aggression. Because aggression has been linked to anxiety, we investigated the maternal aggression and the role of brain oxytocin in lactating Wistar rats selectively bred for high anxiety-related behavior (HAB) or low anxiety-related behavior (LAB) during the 10 min maternal defense test. HAB dams displayed more maternal aggression against a virgin intruder compared with LAB dams, resulting in more defensive behavior and higher anxiety of HAB-defeated virgins. The different levels of aggression were accompanied by opposite oxytocin release patterns within the paraventricular nucleus (PVN; HAB, increase; LAB, decrease). Furthermore, oxytocin release was higher within the central nucleus of the amygdala (CeA) of HAB dams compared with LABs. A direct correlation between the offensive behavior displayed during the maternal defense test and local oxytocin release was found in both the PVN and CeA. Using retrodialysis, blockade of endogenous oxytocin action by infusion of an oxytocin receptor antagonist (des-Gly-NH2,d(CH2)5[Tyr(Me)2,Thr4]OVT) into the PVN or CeA reduced maternal aggression of HAB dams, whereas infusion of synthetic oxytocin into the PVN tended to increase aggression toward the intruder in LAB dams. There were no significant differences in oxytocin receptor mRNA expression or oxytocin receptor binding between lactating HAB and LAB dams. Therefore, differences in intracerebral release patterns of oxytocin, rather than differences at the level of oxytocin receptors, are critical for the regulation of maternal aggressive behavior.

Figure 1.

Behavior of HAB and LAB residents (a) and their virgin intruders unselected for anxiety (b) during the 10 min maternal defense test. a, The occurrence of attacks and total offensive behavior as well as total defensive, explorative, and maternal behavior were monitored in HAB and LAB residents on day 3 of lactation. b, The occurrence of total offensive, freezing, and total defensive behavior, as well as cage exploration and total explorative behavior, were monitored in the virgin intruder rats. The number of rats included in the statistics was n = 7 per group. Data are expressed as mean ± SEM. ^**p < 0.01; ^*p < 0.05 versus LAB resident or intruders defeated by LAB residents. res, Residents.

Figure 2.

Anxiety-related behavior on the elevated plus-maze of residents and intruders 10 min after termination of the maternal defense test. Comparison of either the lactating HAB and LAB residents (a) or the respective virgin intruders (unselected for anxiety) defeated by HAB or LAB residents (b) as reflected by the percentage of entries into the open arms of the maze, the percentage of time spent on the open arms of the maze, as well as the total number of full entries into the open arms during a 5 min exposure time. In b, a group of nondefeated virgin controls is included. The numbers in parentheses indicate group size. Data are expressed as mean ± SEM. ^**p < 0.01; ^*p < 0.05 versus all other groups. res, Residents.

Figure 3.

Oxytocin release within the PVN (a) and the CeA (b) of lactating HAB and LAB residents in response to maternal defense and exemplary reconstructions of the exact placement of the microdialysis probes inside (“insider”) or outside (“outsider”) the target area drawn on to plates from the Paxinos and Watson (1998) atlas. Thirty minute dialysates were sampled under basal conditions (samples 1 and 2). During the third dialysis sampling period, a virgin intruder rat was placed into the cage of the lactating resident for 10 min (maternal defense) with continued microdialysis (insider). Also shown are individual oxytocin contents in sample 3 collected outside the respective target areas (outsider; ×, HAB; +, LAB). Data are expressed as percentage of baseline (mean of sample 1 and 2; =100%; dotted line) ± SEM. Insets demonstrate mean basal oxytocin (samples 1 and 2) content in picograms per 100 μl sample and δ values (sample 3 minus minimum basal). The numbers on the right of each plate indicate the distance in millimeters posterior to bregma. Number of rats included in the statistics: a, HAB, eight; LAB, six; b, HAB, six; LAB, six. ^*p < 0.05, ^**p < 0.01 versus sample 2 of respective group; ^##p < 0.01 versus LAB.

Figure 4.

Correlation of maternal aggression with local oxytocin release. The total amount of offensive behavior displayed by the lactating HAB and LAB residents during maternal defense was correlated with the release of oxytocin in either the PVN (a) or the CeA (b). The numbers in parentheses indicate group size. Each data point represents one individual dam. The dotted line indicates the regression line.

Figure 5.

Behavioral consequences of local infusion of the oxytocin receptor antagonist and exemplary reconstructions of the exact placement of the microdialysis probes. The antagonist was applied via retrodialysis bilaterally into (insider) the PVN (a) and the CeA (b) of lactating HAB and LAB residents, and the occurrence of attacks, offensive, and defensive behaviors during the 10 min maternal defense test was monitored. The exact placement of the microdialysis probes inside (insider; c) or outside (outsider; d) the target area was drawn on to plates from the Paxinos and Watson (1998) atlas. Number of rats included in the statistics: a, HAB: VEH, six; OXT-A, eight; LAB: VEH, six; OXT-A, six; b, HAB: VEH, seven; OXT-A, eight; LAB: VEH, six; OXT-A, eight. Data are expressed as mean ± SEM. ^*p < 0.05 versus respective vehicle-treated group. VEH, Vehicle; OXT-A, oxytocin receptor antagonist.

Figure 6.

Behavioral consequences of application of synthetic oxytocin or vehicle infused via retrodialysis bilaterally into (insider) the paraventricular nucleus of lactating LAB residents. a, The amount of attacks and lateral threats and of offensive, defensive, and maternal behaviors during the 10 min maternal defense test was monitored. b, Exemplary reconstructions of the exact placement of the microdialysis probes inside (insider) or outside (outsider) the target area was drawn on to plates from the Paxinos and Watson (1998) atlas. The numbers in parentheses indicate group size. Data are expressed as mean ± SEM.

Figure 7.

Evaluation of oxytocin receptors. Comparison of oxytocin receptor (OXT-R) mRNA expression in the magnocellular (magno) and parvocellular (parvo) part of the PVN and the CeA of lactating (lact) HAB and LAB dams, as well as of pooled virgin (virg) female HAB and LAB rats versus pooled lactating HAB and LAB rats (gray columns) (a) and OXT-R binding in the CeA of lactating HAB and LAB dams (b). The number of rats included in the statistics: lactating, HAB, six, LAB, eight; pooled HAB/LAB, lactating, 15; virgin, 16. Data are expressed as mean ± SEM. ^*p < 0.05 versus pooled virgin group.