Neurobiology of maternal regulation of infant fear: the role of mesolimbic dopamine and its disruption by maltreatment

Abstract

Child development research highlights caregiver regulation of infant physiology and behavior as a key feature of early life attachment, although mechanisms for maternal control of infant neural circuits remain elusive. Here we explored the neurobiology of maternal regulation of infant fear using neural network and molecular levels of analysis in a rodent model. Previous research has shown maternal suppression of amygdala-dependent fear learning during a sensitive period. Here we characterize changes in neural networks engaged during maternal regulation and the transition to infant self-regulation. Metabolic mapping of 2-deoxyglucose uptake during odor-shock conditioning in postnatal day (PN)14 rat pups showed that maternal presence blocked fear learning, disengaged mesolimbic circuitry, basolateral amygdala (BLA), and plasticity-related AMPA receptor subunit trafficking. At PN18, when maternal presence only socially buffers threat learning (similar to social modulation in adults), maternal presence failed to disengage the mesolimbic dopaminergic system, and failed to disengage both the BLA and plasticity-related AMPA receptor subunit trafficking. Further, maternal presence failed to block threat learning at PN14 pups following abuse, and mesolimbic dopamine engagement and AMPA were not significantly altered by maternal presence-analogous to compromised maternal regulation of children in abusive relationships. Our results highlight three key features of maternal regulation: (1) maternal presence blocks fear learning and amygdala plasticity through age-dependent suppression of amygdala AMPA receptor subunit trafficking, (2) maternal presence suppresses engagement of brain regions within the mesolimbic dopamine circuit, and (3) early-life abuse compromises network and molecular biomarkers of maternal regulation, suggesting reduced social scaffolding of the brain.

Fig. 1

Amygdala-dependent threat learning and amygdala protein expression: sensitive period maternal presence blockade of learning and plasticity molecules. Twenty-four hours following a Pavlovian odor-shock conditioning procedure that paired neutral peppermint odor presentations with 0.5 mA shocks to the tail, rat pups were tested for long-term memory using a Y-maze odor choice test containing the conditioned stimulus odor (peppermint) and a familiar odor (clean wood shavings) (a); pups were conditioned either alone or with the mother present. In pups conditioned alone (P-A), threat learning and memory was expressed at PN14 and PN18, as indicated by a significant reduction in choices towards the conditioned stimulus (CS) peppermint odor in the Y-maze choice test (b). Maternal presence blocked learning at PN14 and buffered learning at PN18 [ANOVA, age × condition, F_(3,63) = 3.357, p = 0.002; M.E. of condition, F_(3,63) = 21.28, p < 0.001]. In PN18 pups, choices toward the CS were lower when conditioned with paired CS-US in the presence of the mother, compared to unpaired presentations (p = 0.003); this was a reversal of the pattern observed in PN14 pups conditioned with the mother compared to unpaired (P-M vs. U-A, p = 0.05). Furthermore, choices towards the CS when conditioned with the mother are lower than chance at PN18 (P-M, p = 0.027), whereas they are higher than chance at PN14 (P-M, p = 0.047). Threat memory and corresponding avoidance behavior were associated with increases in GluA2/3 expression in the amygdala at PN14 and PN18 (d, e) [GluA2; two-way ANOVA (age × condition), main effect of condition: F_(3,40) = 7.699, p < 0.001; GluA3: age × condition, F_(3,45) = 15.04, age (F_(1,45) = 10.65, p = 0.002), condition (F_(3,45) = 25.29, p < 0.001)]. GluA1 decreased in PN12 pups, while it increased in PN18 pups (d); these effects were blocked by maternal presence at PN14 but not PN18 [age × condition: F_(3,48) = 8.576, p < 0.001, age: F_(1,48) = 24.02, p < 0.001]. P-A/U-A/O-A/P-M: paired alone, unpaired alone, odor only alone, paired with mother. *p < 0.05, **p < 0.01, ***p < 0.001, ^#p = 0.05; “⁰” denotes significant difference from chance performance on Y-maze (dotted line), p < 0.05. Error bars indicate mean ± SEM. Dashed line: average protein expression in untrained “odor only” group (control). The same tubulin-corrected values were used for all markers probed. Comparisons were made across gels processed in parallel using samples derived from the same experiment. Full length blots/gels are presented in Supplementary Figs. 4–7. Behavioral and protein expression data from additional controls (unpaired with mother, odor only with mother) are presented in Supplementary Fig. 2a–d

Fig. 2

Maternal presence suppresses regions in threat circuit during and after sensitive period. 2-DG uptake within individual brain regions is suppressed in an age-specific manner when pups are odor-shock conditioned alone or with mom. *p < 0.05, **p < 0.01, ***p < 0.001, n.s., not significant. Error bars indicate mean ± SEM. All comparisons performed with two-way ANOVA (age × condition); F, df, and p presented in Supplementary Table 1 and Supplemental Results

Fig. 3

Low shavings model of rodent Scarcity-Adversity impairs maternal suppression of threat learning. a Timeline of experimental procedure with LS or control rearing from PN8-12, followed by conditioning alone or with the mother at PN14 and PN18 and Y maze testing 24 h later. b Y maze performance shows that mother fails to suppress threat learning (ANOVA, condition: F_(3,59) = 5.36, p = 0.003). P-A/U-A/O-A/P-M: paired alone, unpaired alone, odor only alone, paired with mother. *p < 0.05, **p < 0.01, ***p < 0.001, n.s., not significant; “⁰” denotes significant difference from chance performance on Y-maze (dotted line), p < 0.05. c 2-DG uptake within individual brain regions is no longer suppressed when pups are conditioned with the mother if they have been abused. *p < 0.05, **p < 0.01, ***p < 0.001. Error bars indicate mean ± SEM. All comparisons performed with two-way ANOVA (age × condition); F, df and p values for post hoc comparisons are reported in Supplemental Table 2. Error bars indicate mean ± SEM. All p values for post hoc comparisons are reported in Supplemental Results

Fig. 4

Functional connectivity within infant neural threat network. Bivariate correlation matrices were constructed across ROIs assessed in Experiments 1–2. Matrices are shown for two ages, with and without maternal presence, and after control versus abusive rearing (LS). White squares denote example nodules (groups of ROIs) that show dramatic changes in connectivity across condition. Color bar shows Pearson’s r values with high correlations in red and low correlations in blue. a–d Correlations within modules were transformed to Fisher z scores and compared using three-way ANOVA. a VTA-NAc module: age × maternal presence, F_(1,24) = 6.635, p = 0.017; rearing, F_(1,24) = 10.19, p = 0.004. B, VTA-BLA-NAc: rearing, F_(1,56) = 14.26, p < 0.001. C, PLH-VTA-NAc: age, F_(1,24) = 14.31, p < 0.001. D, PLH-VTA-BLA: age × rearing, F_(1,32) = 6.283, p = 0.018. *p < 0.05, **p < 0.01, ***p < 0.001, ^#p = 0.06. Error bars indicate mean ± SEM. All p values for post hoc comparisons are reported in Supplemental Results