Oxytocin receptors in the midbrain dorsal raphe are essential for postpartum maternal social and affective behaviors

Abstract

Oxytocin receptors (OTRs) in the midbrain dorsal raphe (DR; the source of most forebrain serotonin) have recently been identified as a potential pharmacological target for treating numerous psychiatric disorders. However, almost all research on this topic has been conducted on males and the role of DR OTRs in female social and affective behaviors is mostly unknown. This may be particularly relevant during early motherhood, which is a time of high endogenous oxytocin signaling, but also a time of elevated risk for psychiatric dysfunction. To investigate whether OTRs in the DR are necessary for postpartum female social and affective behaviors, we constructed and then injected into the DR an adeno-associated virus permanently expressing an shRNA targeting OTR mRNA. We then observed a suite of social and affective behaviors postpartum. OTR knockdown in the maternal DR led to pup loss after parturition, decreased nursing, increased aggression, and increased behavioral despair. These effects of OTR knockdown in the DR may be due to disrupted neuroplasticity in the primary somatosensory cortex (S1), which mediates maternal sensitivity to the tactile cues from young, as we found significantly more plasticity-restricting perineuronal nets (PNNs) in the S1 rostral barrel field and fewer PNNs in the caudal barrel field of OTR-knockdown mothers. These results demonstrate that OTRs in the midbrain DR are essential for postpartum maternal social and affective behaviors, are involved in postpartum cortical plasticity, and suggest that pharmacotherapies targeting OTRs in the DR could be effective treatments for some peripartum affective disorders.

Keywords: Anxiety; Behavioral despair; Dorsal raphe; Oxytocin; Perineuronal nets; Postpartum.

Figure 1.. Validation of viral construct.

Experimental timeline (A). In vitro (B) and in vivo (C) OTR expression in the dorsal raphe (DR) of scrambled-vector treated (Scrambled) and shRNA-vector treated (OTRKD) cells and recently parturient mothers, respectively. Vasopressin V1a receptor expression in the DR of Scrambled and OTRKD dams (D). Representative photomicrograph of serotonin-immunoreactive cells in the DR (E). Number of serotonin-ir cells in the DR of Scrambled and OTRKD dams (F). Representative photomicrograph and brain atlas diagram showing distribution of viral vector in the DR (G). * indicates significant difference between groups, p < 0.05. Scale bar in D = 100 μm.

Figure 2.. Effects of oxytocin receptor knockdown in the dorsal raphe on maternal caregiving behaviors.

Percentage of dams with pup loss (A) and dam body weight change (G) of scrambled-vector treated (Scrambled) and shRNA-vector treated (OTRKD) mothers. Frequency of mothers in the nest (B), nursing (C), nursing in a kyphotic posture (D), non-pup directed behavior (E), perioral activity (F), and ingestive behaviors (G) by Scrambled and OTRKD dams. All data are presented as Mean ± SEM. * above bars indicates significant differences between groups, p < 0.05.

Figure 3.. Effects of oxytocin receptor knockdown in the dorsal raphe on maternal affective behaviors.

Frequency of attacks (A) by scrambled-vector treated (Scrambled) and oxytocin receptor knockdown (OTRKD) dams. Total duration of attacks (B) and latency to attack (C) by Scrambled and OTRKD dams. Percentage of time floating in the forced swim test (D) and time in the open arm of an elevated plus maze (E) by Scrambled and OTRKD dams. All data are presented as Mean ± SEM. * above bars indicates significant differences between groups, p < 0.05.

Figure 4.. Effects of oxytocin receptor knockdown in the dorsal raphe on perineuronal nets in the primary somatosensory cortex.

Representative photomicrographs (A, B, C, H) and brain atlas plates (A, H) showing perineuronal net (PNN) expression in the primary somatosensory cortex (S1) of scrambled-vector treated (Scrambled) and shRNA-vector treated (OTRKD) mothers. Rostrocaudal extent of the S1 (D) and PNN density by map (E) in Scrambled and OTRKD mothers. (F) Statistical analysis of averaged PNN densities of combined rostral-caudal data revealed no significant difference between scrambled-vector treated and shRNA-vector treated mothers. Note: Each dot represents PNN density of one brain hemisphere, different colors represent different animals. PNN density from maps #16 and 17 (G) of the S1 in Scrambled and OTRKD mothers. PNN density in the ventrum (TR-Ventrum), dorsum (TR-Dorsum), and barrel field (BF) from maps #28 and 29 (H) of the S1 in Scrambled (S) and OTRKD (O) mothers. All data are presented as Mean ± SEM. * indicates significant difference between groups, p < 0.05. + indicates a trend, p < 0.01.