Neurobehavioral assessment of maternal odor in developing rat pups: implications for social buffering

Abstract

Social support can attenuate the behavioral and stress hormone response to threat, a phenomenon called social buffering. The mother's social buffering of the infant is one of the more robust examples; yet we understand little about the neurobiology. Using a rodent model, we explore the neurobiology of social buffering by assessing neural processing of the maternal odor, a major cue controlling social buffering in rat pups. We used pups before (postnatal day (PN) 7) and after (PN14, PN23) the functional emergence of social buffering. Pups were injected with ¹⁴C 2-deoxyglucose (2-DG) and presented with the maternal odor, a control preferred odor incapable of social buffering (acetophenone), or no odor. Brains were removed, processed for autoradiography and brain areas identified as important in adult social buffering were assessed, including the amygdala basolateral complex (Basolateral Amygdala [BLA]), medial prefrontal cortex (mPFC), and anterior cingulate cortex (ACC). Results suggest dramatic changes in the processing of maternal odor. PN7 pups show mPFC and ACC activation, although PN14 pups showed no activation of the mPFC, ACC, or BLA. All brain areas assessed were recruited by PN23. Additional analysis suggests substantial changes in functional connectivity across development. Together, these results imply complex nonlinear transitions in the neurobiology of social buffering in early life that may provide insight into the changing role of the mother in supporting social buffering.

Keywords: Social buffering; amygdala; infant; maternal odor; prefrontal cortex.

Figure 1

Behavioral responses toward maternal odor and acetophenone odor in developing rat pups. Y-maze tests show the number of choices toward odors during five trials (±S.E.M.) for PN7, PN14, and PN23 pups (A). Odor choices were determined in separate tests consisting of maternal odor versus familiar odor (clean bedding) or acetophenone versus. familiar odor. (B) The nipple attachment test shows latency to attach to the nipple (±S.E.M.) by PN14 and PN23 pups exposed to a lactating female with maternal odor removed (“no odor”), or with presentation of maternal odor or acetophenone in an airstream above the mother’s ventrum. *p < 0.050 and ***p < 0.001. A total of 94 pups were used, with N = 7–8/group.

Figure 2

Regions of Interest (ROI) analysis of relative 2-DG uptake in response to maternal odor, acetophenone, and no odor presentations in prefrontal cortex (PFC) areas of the anterior cingulate cortex (ACC; A), medial prefrontal cortex (mPFC; B), and basolateral amygdala (BLA; C) of pups at postnatal day (PN)7, PN14, and PN23. Bars represent the mean level of 2-DG uptake (±S.E.M.) in the three ROI. Significance indicated by *, with ** used to indicate the two odors are significantly different from one another. A total of 73 pups were used, with N = 7–9/group.

Figure 3

Functional connectivity for acetophenone (A) and maternal odor (B) between ROI’s within the amygdala, the PFC, and between the amygdala and PFC (joint) at PN7, PN14, and PN23. Maternal odor modified functional connectivity between the amygdala and prefrontal cortex (PFC) relative to a no odor control condition at all ages. Data is derived from bivariate correlation matrices representing the difference in mean 2-DG uptake for maternal odor and acetophenone relative to no odor control (see Methods). Error bars represent S.E.M. (N = 15/group). Asterisks denote significant difference from 0 for the Joint module, p < 0.05.

Figure 4

Schematic of potential circuits underlying social buffering of pups by the mother across development. The first period in PN7 pups is characterized by the stress hyporesponsive period (SHRP), when adult-like social buffering does not occur, although maternal cues maintain the SHRP. The ability of maternal odor to activate the prefrontal cortex (PFC) suggests that PFC–paraventricular nucleus of the hypothalamus (PVN) connections may be involved in the mother’s maintenance of pups’ low corticosterone levels. By PN14, maternal cues socially buffers pups, with data suggesting that this primarily occurs by directly blocking the hypothalamic–pituitary axis (HPA) at the level of the PVN. Maternal odor did not activate the PFC or amygdala, suggesting that these routes for social buffering are not active at this age. Finally, in the PN23 pups, the circuitry underlying the processing of maternal cues seems to expand to include areas of the PFC and the amygdala, suggesting that both direct (PVN) and indirect (PFC, amygdala) routes might underlie social buffering.