Nonhuman primates offer unique opportunities to study the effects of genes, environments, and their interaction, on physiology and complex behavior. We examined genotype and early environment contributions to CNS function in a large sample of rhesus monkeys. In humans, length variation of the serotonin (5-HT) transporter (5-HTT) gene-linked polymorphic region (5-HTTLPR) that results in allelic variation in 5-HTT expression is associated with decreased serotonergic function and 5-HT-mediated psychopathology. We report that an analogous variation of the gene's regulatory region in monkeys interacts with early experience to affect central 5-HT functioning. Monkeys with deleterious early rearing experiences were differentiated by genotype in cerebrospinal fluid concentrations of the 5-HT metabolite, 5-hydroxyindoleacetic acid, while monkeys reared normally were not. These findings demonstrate an environment-dependent effect of the rh5-HTTLPR genotype on CNS 5-HT function and suggest nonhuman primates may provide an important avenue for investigating gene/environment interactions using candidate genes for physiological and behavioral traits.

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Serotonin (5-HT) is known to reduce apoptosis and in rodent models of brain ischemia. Modulation of programmed cell death during neural development was assessed in early postnatal brains of serotonin transporter (5-HTT) knockout mice, characterized by elevated extracellular 5-HT levels. The number of apoptotic cells visualized at postnatal day-1 (P1) by ISEL+ or TUNEL staining was significantly reduced in the striatum, thalamus/hypothalamus, cerebral cortex and hippocampus of 5-HTT knockout mice, compared to wild type and heterozygote mice, with differences displaying an increasing fronto-caudal gradient and regional specificity. These findings underscore 5-HT roles in the regulation of programmed cell death during brain development, and spur interest into pharmacological interventions aimed at relieving pathological apoptosis by potentiating serotoninergic neurotransmission.

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