Developmental disruption of serotonin transporter function impairs cerebral responses to whisker stimulation in mice

There is growing evidence that serotonin (5-hydroxtryptamine, 5-HT) has major influences on brain development in mammals. Genetic and pharmacological disruption of 5-HT signaling during early postnatal development in rodents causes neuroanatomical cortical abnormalities, including malformations in the somatosensory cortex. Possible functional consequences of this developmental perturbation by 5-HT are not yet understood. We have examined the effects of deletion of the 5-HT transporter (5-HTT) gene on somatosensory responses to sensory stimulation in mice. Local cerebral glucose utilization (1CMR(glc)) was measured by the quantitative 2-deoxy[C-14]glucose method during unilateral whisker stimulation in awake adult mice. 1CMR(glc) was increased by stimulation but to a markedly lesser extent in 5-HTT-/- mice than in 5-HTT+/+ controls in each of four major stations in the whisker-to-barrel cortex pathway (the spinal and principal sensory trigeminal nuclei, the ventral posteromedial thalamic nucleus, and the barrel region of the somatosensory cortex). Lowering brain 5-HT levels by administration of the selective tryptophan hydroxylase inhibitor p-chlorophenylalanine on postnatal days 0 and 1 restored the metabolic responses to functional activation in the whisker-to-barrel cortex pathway in adult 5-HTT-/- mice. These results indicate that functional deficits in this pathway in 5-HTT-/- mice may be due to excessive postnatal 5-HT activity. With or without postnatal p-chlorophenylalanine treatment, 5-HTT-/- mice exhibited lower resting (unstimulated) 1CMR(glc) than did 5-HTT+/+ controls in the whisker-to-barrel cortex pathway and throughout the brain. These findings have implications for understanding the potential long-term consequences of genetic and pharmacological disruption of 5-HT neurotransmission on cerebral functions during critical periods of postnatal development.