Neither increased nor decreased availability of cortical serotonin (5HT) disturbs barrel field formation in isocaloric undernourished rat pups

Maternal protein restriction during gestation and lactation in the rat results in increased brain levels of kynurenine and kynurenic acid in their adult offspring

Early malnutrition is a risk factor for depression and schizophrenia. Since the offspring of malnourished dams exhibit increased brain levels of serotonin (5-HT), a tryptophan-derived neurotransmitter involved in the pathophysiology of these mental disorders, it is believed that the deleterious effects of early malnutrition on brain function are due in large part to altered serotoninergic neurotransmission resulting from impaired tryptophan (Trp) metabolism. However, tryptophan is also metabolized through the kynurenine (KYN) pathway yielding several neuroactive compounds including kynurenic (KA), quinolinic (QA) and xanthurenic (XA) acids. Nevertheless, the impact of perinatal malnutrition on brain kynurenine pathway metabolism has not been examined to date. Here, we used ultra-performance liquid chromatography-tandem mass spectrometry for the simultaneous quantification of tryptophan and a set of seven compounds spanning its metabolism through the serotonin and kynurenine pathways, in the brain of embryos and adult offspring of rat dams fed a protein-restricted (PR) diet. Protein-restricted embryos showed reduced brain levels of Trp, serotonin and KA, but not of KYN, XA, or QA. In contrast, PR adult rats exhibited enhanced levels of Trp in the brainstem and cortex along with increased concentrations of 5-HT, kynurenine and XA. The levels of XA and KA were also increased in the hippocampus of adult PR rats. These results show that early protein deficiency induces selective and long-lasting changes in brain kynurenine metabolism. Given the regulatory role of KYN pathway metabolites on brain development and function, these changes might contribute to the risk of developing psychiatric disorders induced by early malnutrition.

Development of 5-HT(1B), SERT and thalamo-cortical afferents in early nutrionally restricted rats: an emerging explanation for delayed barrel formation

Barrel formation is delayed in nutritionally restricted rats. The underlying cause of such delay is yet unclear. Because barrels appear upon the arrival of somatosensory thalamo-cortical afferents and the reorientation of the dendritic arborizations of cortical spiny stellate neurons, it is likely that at least one of these processes is altered by nutritional restriction. Also, the serotoninergic afferent system has been implicated in regulating barrel segregation and growth during early postnatal life. We then evaluated the pattern of immunostaining of the serotonin transporter (SERT) and of the serotonin receptor 1B (5-HT(1B)), as well as the growth and arrival time of somatosensory thalamo-cortical afferents, to infer the contribution of these elements in the delayed formation of barrels observed in nutritionally restricted rats. It was found that the rates of development and the segregation of thalamo-cortical fibers were normal in nutritionally restricted rats. SERT, but not 5-HT(1B) immunoreactivity, was decreased in the primary somatosensory cortex during barrel specification. The availability of both proteins in nutritionally restricted rats was lower than that observed in their well fed counterparts at later developmental times. It is concluded that the delayed formation of barrels observed in nutritionally restricted rats is due to a retarded reorientation of dendritic arbors of cortical neurons. This might happen as a secondary effect of decreasing the availability of SERT and/or increasing the availability of 5-HT(1B) receptor early in postnatal life.

Prenatal impairment of brain serotonergic transmission in infants

OBJECTIVE

To evaluate whether the free fraction of L-tryptophan (L-Trp) and the N1/P2 component of the auditory evoked potentials (AEPs) are associated with impaired brain serotonin neurotransmission in infants with intrauterine growth restriction (IUGR).

STUDY DESIGN

We measured free, bound, and total plasma L-Trp and recorded the N1/P2 component of AEP in a prospective, longitudinal, and comparative study comparing IUGR and control infants.

RESULTS

Plasma free L-Trp was increased and the amplitude of N1/P2 component was significantly decreased in IUGR relative to control infants. The free fraction of L-Trp and N1/P2 component had a negative association.

CONCLUSIONS

In newborns with IUGR, the changes in measured plasma free fraction of L-Trp and in the amplitude the N1/P2 component of the AEP suggest an inverse association between free L-Trp and components of the AEP. The changes observed in the free fraction of L-Trp and AEP may be causally associated with brain serotonergic activity in utero. In IUGR, epigenetic factors such as stress-induced disturbances in brain serotonin metabolism or serotonergic activity, identifiable by alterations in AEP, influence cerebral sensory cortex development and may be causally associated with serotonin-related disorders in adulthood.