Brain serotonin determines maternal behavior and offspring survival

Abstract

Maternal care is an indispensable component of offspring survival and development in all mammals and necessary for reproductive success. Although brain areas regulating maternal behaviors are innervated by serotonergic afferents, very little is known about the role of this neurotransmitter in these behaviors. To evaluate the contribution of serotonin to maternal care, we used mice with a null mutation in the gene for tryptophan hydroxylase-2 (TPH2), which results in a genetic depletion of brain serotonin, and tested them in a wide range of maternal behavior paradigms. We found that litters born to and reared by TPH2(-/-) mothers showed decreased survival, lower weaning weights and increased cannibalization. In addition, TPH2(-/-) mothers performed poorly in pup retrieval, huddling, nest construction and high-arched back nursing. Aggression in TPH2(-/-) dams was not triggered by lactation and was steadily high. Survival and weaning weight deficits of TPH2(-/-) pups were rescued by cross-fostering and in litters of mixed genotype (TPH2(-/-) and TPH2(-/+) ). However, the maternal behaviors of TPH2(-/-) dams did not improve when rearing either TPH2(+/+) pups or mixed-genotype litters. In addition, TPH2(-/-) pups significantly worsened the behavior of TPH2(+/+) dams with respect to cannibalism, weaning weight and latency to attack. Olfactory and auditory functions of TPH2(-/-) females or anxiety-like behaviors did not account for these maternal alterations as they were equal to their TPH2(+/+) counterparts. These findings illustrate a profound influence of brain serotonin on virtually all elements of maternal behavior and establish that TPH2(-/-) pups can engender maladaptive mothering in dams of both genotypes.

Keywords: Maternal behavior; TPH2; TPH2 knockout; pup survival; serotonin.

Figure 1. Litters born to TPH2^−/− dams show decreased survival, increased cannibalism and reduced body weight at weaning and are rescued by cross-fostering

Litters born to TPH2^+/+ (WT) or TPH2^−/− (KO) dams were reared by their biological mothers or they were cross-fostered to receptive dams of the opposite genotype immediately after birth. The genotypes of dams and pups (in parentheses) are designated as follows: WT(WT), WT dam rearing their own WT pups; WT(KO), WT dams rearing cross-fostered KO pups; KO(KO), KO dams rearing their own KO pups and KO(WT), KO dams rearing cross-fostered WT pups. Litters were assessed for (a) % pup survival, (b) % litter cannibalized, (c) body weight at weaning and (d) body size at weaning. Data are expressed as the mean ± SEM. *P < 0.05, **P < 0.001 and ***P < 0.0001.

Figure 2. TPH2^−/− dams show defective instinctual maternal behaviors that are resistant to rescue by cross-fostering

TPH2^+/+ (WT) and TPH2^−/− (KO) dams were assessed for maternal behaviors to include (a) % of litter retrieved, (b) pup retrieval rate, (c) pup huddling after retrieval, (d) nest rating, (e) high-arched back nursing and (f) low-arched back nursing. Dam and litter genotypes are designated as described in the legend of Fig. 1. Data are expressed as the mean ± SEM. *P < 0.05, **P < 0.001 and ***P < 0.0001.

Figure 3. Lactating TPH2^−/− dams do not show increased aggression to male intruders in the presence of their own or cross-fostered TPH2^+/+ pups

Aggressive behavior shown by lactating TPH2^+/+ (WT) or TPH2^−/− (KO) dams in the presence of the indicated litters was compared to that of non-lactating WT and KO (Ctrl WT or Ctrl KO) females upon the introduction into their cages of a socialized male intruder. Dam and litter genotypes are designated as described in the legend of Fig. 1. Aggressive behaviors were scored as (a) number of attacks on the intruder and (b) latency to attack the intruder. Data are expressed as the mean ± SEM. *P < 0.05, **P < 0.001.

Figure 4. Litters of mixed-genotype born to TPH2^−/− dams show decreased survival, increased cannibalism and decreased weaning weights

Matings between TPH2^−/+ (HET) females and TPH2^−/− (KO) males (HET × KO) or between KO females and HET males (KO × HET) were arranged to produce litters containing pups of mixed genotypes (i.e. 50% KO and 50% HET). Independent matings of KO females to KO males were also carried out to produce litters containing 100% KO pups (KO × KO). The maternal performance of HET and KO dams was scored for (a) % pup survival, (b) % of litter cannibalized, (c) % of pups cannibalized by HET or KO dams; (d) body weight at weaning and (e) body size at weaning. Data are expressed as the mean ± SEM. *P < 0.05, **P < 0.001 and ***P < 0.0001.

Figure 5. TPH2^−/− dams show defective instinctual maternal behaviors when rearing litters of mixed genotype

Matings were arranged so that TPH2^−/+ (HET) or TPH2^−/− (KO) dams reared litters of mixed-genotype pups (50% HET, 50% KO) as described in the legend of Fig. 4. HET and KO dams were assessed for maternal behaviors to include (a) % of litter retrieved, (b) pup retrieval rate, (c) pup huddling after retrieval, (d) nest rating, (e) high-arched back nursing and (f) low-arched back nursing. Data are expressed as the mean ± SEM. **P < 0.001 and ***P < 0.0001.

Figure 6. TPH2^−/− dams do not show maternal aggression when rearing litters of mixed genotype

Matings were arranged so that TPH2^−/+ (HET) or TPH2^−/− (KO) dams reared litters of mixed-genotype pups (50% HET, 50% KO) as described in the legend of Figure 4. Aggressive behaviors were scored as (a) number of attacks on the intruder and (b) latency to attack the intruder. Data are expressed as the mean ± SEM for groups containing 10–11 mice per group.

Figure 7. General olfactory and auditory sensory function is normal in TPH2^−/− mice

General olfactory function was assessed in TPH2^+/+ (WT) and TPH2^−/− (KO) female mice using (a) the olfactory habituation test and (b, c) the pup odorant/pheromone preference test. The ABR-threshold served as a measure of auditory responsiveness in TPH2^−/− and TPH2^+/+ females (d). Data are expressed as the mean ± SEM. *P < 0.05.

Figure 8. TPH2^−/− dams do not show anxiety-like behavior upon separation from their own or cross-fostered pups

TPH2^+/+ (WT) and TPH2^−/− (KO) dams raising their own or cross-fostered pups were assessed for anxiety-like behaviors using the EPM (a–c) or the LD (d–f). Dams and pup genotypes are designated in the legend of Figure 1. In the elevated plus maze, dams were assessed for (a) time spent in the open arms, (b) time spent in the closed arms and (c) distance traveled. In the light–dark box, dams were assessed for (a) time spent in the lighted compartment, (b) time spent in the dark compartment and (f) distance traveled. Data are expressed as the mean ± SEM. *P < 0.05.

Figure 9. TPH2^−/− dams do not show anxiety-like behavior upon separation from litters of mixed genotype

Matings between TPH2^−/+ (HET) females and TPH2^−/− (KO) males (HET × KO) or between KO females and HET males were arranged to produce litters containing pups of mixed genotypes (i.e. 50% KO and 50% HET). Tests of anxiety-like behaviors were carried out using the elevated plus maze (a–c) and the light–dark box (d–f). In the elevated plus maze, dams were assessed for (a) time spent in the open arms, (b) time spent in the closed arms and (c) distance traveled. In the light–dark box, dams were assessed for (d) time spent in the lighted compartment, (e) time spent in the dark compartment and (f) distance traveled. Data are expressed as the mean ± SEM. *P < 0.05.