Developmental impairment of auditory evoked N1/P2 component in rats undernourished in utero: its relation to brain...

In utero undernourishment produces an elevation of L-tryptophan and serotonin in the brain, including the auditory cortex (A1), such changes seem to be related to an increase in the free fraction (FFT) of plasma L-tryptophan that is transported into the brain through the blood-brain barrier, where it is taken up by serotonergic neurons for serotonin synthesis. Our observations support that FFT has a positive correlation with L-tryptophan (L-Trp) and serotonin levels in the auditory cortex (r=0.95 and 0.82, respectively). Interestingly, a decreased intensity dependence of the auditory evoked N1/P2 component was found in gestationally undernourished animals during their postnatal development. The N1/P2 component had a negative correlation (r=0.81) with A1 serotonin, such that it reflects changes in the neurotransmitter concentration. The present observations suggest a relevant role of serotonin in modulating the activity of the auditory cortex. Since the N1/P2 component is mainly associated with the activity of A1 neurons, it may well be that perception of auditory information is impaired during this developmental period, in the early undernourished animals, possibly affecting cognitive processes. This may be relevant to humans since low birth weight babies that also suffered gestational undernourishment (fetal-placental insufficiency) present an increase in plasma FFT from birth up to 3 months of age. These findings support that the plasma FFT and the intensity dependence of the auditory evoked N1/P2 component relate one another and may be markers of changes of the brain serotonergic activity.