Brain serotonin depletion and nipple attachment in rat pups

Brain serotonin determines maternal behavior and offspring survival

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.

Immobility in the swim test and observations of maternal behavior in lactating flinders sensitive line rats

In order to elucidate the relationship between maternal behavior and depression, the Flinders sensitive line (FSL) model of depression was studied and compared to Sprague-Dawley (SD) controls. Immobility in the swim test was measured, as an index for depressive-like behavior, and frequencies of maternal and non-maternal behaviors were recorded using short un-intrusive observations in the home cage. Lactating FSL rats displayed higher levels of immobility in the swim test compared to controls, indicating depressive-like behavior. In addition, compared to SD rats, FSL dams showed less frequent pup licking and non-nutritive contact with pups during the first and third weeks of lactation. In the third postpartum week, FSL dams showed less frequent nursing postures and more frequent self-directed behaviors. Thus, lactating FSL dams exhibit both depressed-like behavior and some abnormalities in maternal behavior.

Huntingtin-associated protein 1 (Hap1) mutant mice bypassing the early postnatal lethality are neuroanatomically normal and fertile but display growth retardation

Huntingtin-associated protein 1 (Hap1) is the first huntingtin interacting protein identified in a yeast two-hybrid screen. Although Hap1 expression has been demonstrated in neuronal and non-neuronal tissues, its molecular role is poorly understood. Recently, it has been shown that targeted disruption of Hap1 in mice results in early postnatal death as a result of depressed feeding behavior. Although this result clearly demonstrates an essential role of Hap1 in postnatal feeding, the mechanisms leading to this deficiency, as well as the role of Hap1 in adults, remain unclear. Here we show that Hap1 null mutants display suckling defects and die within the first days after birth due to starvation. Upon reduction of the litter size, some mutants survive into adulthood and display growth retardation with no apparent brain or behavioral abnormalities, suggesting that Hap1 function is essential only for early postnatal feeding behavior. Using a conditional gene repair strategy, we also show that the early lethality can be rescued if Hap1 expression is restored in neuronal cells before birth. Furthermore, no synergism was observed between Hap1 and huntingtin mutation during mouse development. Our results demonstrate that Hap1 has a fundamental role in regulating postnatal feeding in the first 2 weeks after birth and a non-essential role in the adult mouse.

Neural and hormonal consequences of neonatal 5,7-dihydroxytryptamine may not be associated with serotonin depletion

The neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) is often used in neonatal rats to induce specific, rapid, and permanent depletion of brain serotonin (5-HT). One assumed benefit of using this drug in neonates is that it is well-tolerated, with pups exhibiting few side effects normally attributed to 5-HT depletion. Here, we present evidence that 5,7-DHT administered neonatally induces seizure-like behavior, decreases weight gain, and increases plasma corticosterone without depletion of brain 5-HT.

Neonatal alcohol exposure alters suckling behavior in neonatal rat pups

Prenatal alcohol exposure has been associated with a variety of suckling deficits in both humans and animals. In this study, the effect of neonatal alcohol exposure on suckling performance was examined in 15-day-old rat pups. Neonatal alcohol exposure has been used as a model to study the effects of alcohol exposure during a period equivalent to the human third trimester with respect to brain growth. Subjects were Long-Evans rats which had been artificially reared (AR) and fed through gastrostomy tubes from postnatal day (PN) 4-PN 12. The AR groups included two groups given ethanol doses of 6 g/kg/day or 4 g/kg/day and an isocaloric maltose-dextrin control group. A suckled control group raised by their natural mothers was also included to control for artificial rearing. Fifteen-day-old pups were individually placed with an anesthetized dam for a 1-h videotaped test session. Pups in the 6 g/kg alcohol group took longer to attach to the nipple and spent less time suckling than pups from all other treatment groups. Nipple-shifting behavior was disrupted in all artificially reared groups, but it was most severely affected in the 6 g/kg group. These findings suggest that neonatal alcohol exposure interferes with suckling performance and these altered behaviors may contribute to the postnatal growth deficits that have been reported following alcohol exposure in utero.

Peripheral serotonergic inhibition of suckling

Systemically administered serotonin, which does not pass the blood-brain barrier, inhibited nipple attachment behavior in 20- and 30-day-old rat pups. Xylamidine, a peripheral serotonin antagonist, attenuated the effects of serotonin, quipazine, and fenfluramine on nipple attachment behavior. Thus, serotonin receptors in the periphery may play an important role in the serotonergic inhibitory mechanism that has been hypothesized as the developing system leading to weaning. However, unlike more general 5-HT antagonists, xylamidine given alone failed to facilitate suckling, suggesting different sites of action for facilitation and inhibition of this infantile behavior.

Injection of 5,7-dihydroxytryptamine into the B3 raphe region of neonatal rat pups induces hyperalgesia but only slight alterations in ingestion-related behaviors

The effects of intrabrainstem injections of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into the B3 raphe region (nucleus raphe magnus and nucleus reticularis paragigantocellularis) on early ingestive behavior and nociception were assessed in Sprague-Dawley rat pups during the first postnatal week. Lesions resulted in a marked depletion of serotonin (5HT) in hindbrain without influencing 5HT levels in forebrain. Pretreatment with desipramine (DMI) resulted in a sparing of noradrenergic neurons from neurotoxic effects. The B3 lesion resulted in significant hyperalgesia as reflected by decreased latencies in tail flick testing. Although nipple attachment latencies in suckling tests were slightly increased by the lesion, no notable effects on mouthing or other ingestive-related behaviors were observed in testing conducted in an independent ingestion paradigm. These results suggest that whereas B3 serotonergic neurons may be functioning in an adult-typical manner to regulate analgesia during the early postnatal period, this raphe region may play only a slight role in the modulation of ingestion-related behaviors early in life.