Early Increase in Serotonin Axons in Autism Brain Results in Dystrophic Fibers and Glial Reactivity

Background: Studies of autistic boys aged 2-5 years using PET indicate that serotonin in the frontal cortex and thalamus is lower than in control subjects (Chugani et al., 1999). There are reports that serotonin drugs may be helpful in alleviating some of the symptoms of autism (Hollander et al, 2007). If serotonin levels are increased, anxiety and repetitive behaviors are improved (Kolevzon et al 2006); and if reduced, the symptoms worsen in children with autism (McDougle et al., 1996). There may be an association between autism and particular polymorphisms of the serotonin transporter gene (Singh et al, 2007). An increase in blood serotonin levels of children with autism has been long noted (Schain and Freedman 1961; Minderaa et al, 1989; Janusonis 2008). There are a number of theories of how damage to the serotonin system may lead to autism (Casanova, 2007) and several animal autism models are based on manipulation of the serotonin system early in development (Whitaker-Azmitia, 2001; Boylan et al, 2007). “Overall, serotonin appears to have the most empirical evidence for a role in autism.” Lam et al (2006). However, to date, no direct studies of serotonin have been reported in postmortem brain. Objectives: Label serotonin axons in medial forebrain bundle and in subcortical and cortical telencephalic structures and determine if there are age-related changes in autism and typical control donors. Study any accompanying changes in astrocytes and microglial cells with dual 5-HTT labeling. Methods: The age-related (2-33yrs) profiles of serotonin axons (5-HTT antibody) and supporting astrocytes (GFAP antibody) and microglial (IBA-1 antibody) cells are immunoreactive (IR) with a transporter antibody in post-mortem tissue from autistic (2-34 years; n=7) and typical control (2-33 years; n=7) donors. All tissue was obtained from Autism Tissue Program. Diamonio-benzidine and HRP procedure, with or without Nickel enhance was used to visualize cells. Immunoreactive density and morphometric studies using NIH Image J system to estimate the number and occurrence of dystrophic fibers. Results: 5-HTT-IR axons in autism, in contrast to typical control, show region specific changes. 5-HTT-IR axons in MFB, septum and preoptic area are increased in autism compared to typical controls, while axons in temporal cortex and hippocampus are reduced at all ages studied. Dystrophic 5-HTT-IR fibers were seen in amygdala and temporal cortex in the brains from older autistic patients (12-29 yrs.). In temporal cortex of autism donor, astrocytes are reactive at all ages while microglial cells are reactive only at the youngest age. Conclusions: The current hypothesis is that in brains from autistic donors, serotonin axons in telencephalon peak early in subcortical structures but maintain depressed levels in cortical structures due to loss of astrocytic trophic functions. Dystrophic 5-HTT-IR fibers appear similar to those in neurodegenerative diseases (Azmitia and Nixon, 2008). Diet, exercise, light and drug treatments to decrease serotonin actions early in childhood but increase serotonin action in adolescence-adult periods may prove beneficial.