1. Synaptic Scaling in Anxiety Disorders
Mr NW McGregor1, Dr J Dimatelis2, Dr SMJ Hemmings1, Dr CJ Kinnear3,6, Prof DJ Stein4,5, Prof V RusselL2, Prof C Lochner1,5
1Department of Psychiatry, University of Stellenbosch; 2Department of Human Biology, University of Cape Town; 3Division of Molecular Biology and Human Genetics, University of Stellenbosch; 4Department of Psychiatry and Mental Health, University of Cape Town, 5MRC Unit on Anxiety and Stress Disorders, University of Stellenbosch, 6MRC Centre for Molecular and Cellular Biology
INTRODUCTION
The aetiology of anxiety disorders remains poorly understood. Nevertheless, there is clear evidence for a genetic component in the aetiology of anxiety (Hettema et al., 2001; van Grootheest et al., 2007). Previously genetic association studies focused on one particular disorder at a time, but currently preference is placed on multivariate genetic epidemiological approaches using large cohorts with phenotypes expanded beyond disorder-based classifications (Hettema et al., 2005, 2006; Kendler et al., 2003). These could allow for the identification of novel susceptibility genes amongst the anxiety disorders as a group.
Increasing focus has been placed on the interaction of genetic and neurobiological and environmental factors in mediating disorder pathogenesis (Hettema et al., 2005; Kendler et al., 1995, 2003).
Neural circuitry involving the striatum (a major site of synaptic plasticity in the basal ganglia of the brain) has been implicated in anxiety disorders like obsessive-compulsive disorder (OCD), social anxiety disorder (SAD) and panic disorder (PD) (Welch et al., 2007; Fineberg et al., 2011). A number of environmental factors have also been identified as risk factors for anxiety (Gross and Hen, 2004), including that trauma during early developmental stages can result in an increased susceptibility to anxiety. The hypothesis is therefore that a pre-existing genetic vulnerability may interact with adverse life events (e.g. childhood trauma) to result in the development of an anxiety disorder(-s).
THIS STUDY
Maternally- and/ or restraint-stressed rats (Sprague Dawley) were used to investigate the impact of early and adulthood stress on gene expression pathways involved in synaptic plasticity, and the development of anxiety-like behaviour.
Expression profiles obtained will be used to target novel candidate susceptibility genes for anxiety disorders in humans.
MATERIALS AND METHODS
*EPM = Elevated-Plus Maze; FST = Forced Swimming Test; OFT = Open Field Test; NGS = Next-Generation Sequencing
AIMS
To determine the effect of a developmental (maternal separation) and adult (restraint) stress on gene expression in the synaptic plasticity pathway of the rat dorsal striatal region.
To determine whether these stressors are associated with anxiety-like behaviour in male Sprague -Dawley rats.
To investigate whether the differentially expressed genes in rats with a trauma history could be candidate susceptibility genes for anxiety in humans with a history of early-life trauma.
RESULTS
Four different experimental groups were compared: unstressed controls (nRS nMS), restraint stressed (RS nMS), maternally separated (nRS MS) and combination restraint stressed and maternally separated (RS MS).
RS nMS showed significantly less stress or no significant stress levels compared to controls depending on the test (FST; P = and OFT; P = 0.22) (Figures 1 and 2).
nRS MS and RS MS rats showed similar stress levels on tests (FST: nRS MS vs RS MS; P = 0.73 and OFT: nRS MS vs RS MS; P = ) (Figures 1 and 2), and always higher anxiety-like tendencies compared to controls (FST: nRS MS - P = , RS MS - P = and OFT: nRS MS - P = , RS MS - P = 0.006).
Genes with fold-regulation greater than 1.7 were considered candidate genes for further investigation (Figure 3).
A tendency for an increased number of genes involved and severity of aberrant expression was observed in a RS  MS  RS MS ordered fashion, despite similar anxiety-like behaviour between MS and RS MS groups (total distance travelled and duration of immobility).
Human homologues for the susceptibility genes identified, yielded crucial domains (SMART bioinformatic analysis) which could influence gene expression if disrupted (Table 1).
DISCUSSION
Animal Model:
Behavioural data indicated significant differences in the behavioural patterns of the individual and combination stress groups compared to controls .
RS nMS rats showed significantly less stress (FST) or no significant (OFT) anxiety-like behaviour compared to controls.
Increased anxiety was expected in a Control  RS  MS  RS MS fashion (restraint stress is a mild-stress that should show lowest anxiety-like tendencies compared to maternal stress, an early-life trauma showing higher anxiety-like behaviour). The combination of restraint and maternal stress was expected to have an additive effect; however, maternal stress appeared to better equip stress management in the combination stress group i.e. ‘synaptic preconditioning or synaptic scaling’. This could be due to a reduction in receptors or neurotransmitter release post severe trauma minimising neuronal firing in response to trauma of smaller or similar magnitude. This is visualised by a lack of statistically significant increase (or decrease) in anxiety levels in the RS MS group compared to MS only rats relative to control anxiety levels.
Differential gene expression was observed in rats with a trauma history (RS nMS, nRS MS, RS MS) relative to non-traumatised controls (nRS nMS) indicating that maternal separation and restraint stress produced changes in the synaptic plasticity pathways of the rat striatum.
Human Correlation – based on rat data findings:
The BDNF, MMP9, ARC, EGR2, EGR4, NTF4 and GRM2 homologue genes in humans will be considered candidate susceptibility genes for anxiety disorders in humans with different degrees of trauma history and subjected to further investigation within a human cohort to see whether plasticity changes associated with anxiety-like behaviour in rats could also contribute to the development of anxiety disorders in humans.
Candidate genes possess important functional domains and conserved regions which, if disrupted, could contribute to the development and/ or progression of an anxiety disorder(-s)
Screening for the identified susceptibility loci may prove beneficial in better characterising anxiety disorders, and could ultimately contribute to treatments regarding new drug targets for novel susceptibility candidates identified.