Stress is a highly prevalent risk factor for several neuropsychiatric disorders including depression and anxiety.  Reports show that 37% of adults experience high levels of stress globally, which has a major economic impact, costing around $1 trillion globally each year (Gallup, 2025). Acute stress exposure leads to increased anxiety symptoms and is commonly modelled in rodents to study anxiety-like behaviours relevant to trauma-related disorders (Verbitsky et al., 2020).

Selective serotonin reuptake inhibitors (SSRIs) are the primary treatment for depression and anxiety. SSRIs are suggested to work by blocking the reuptake of serotonin at the presynaptic neuron, increasing the synaptic levels of serotonin (Angoa-Pérez et al. 2014). Although SSRIs rapidly increase extracellular serotonin, the therapeutic effect is delayed, suggesting the involvement of long-term neuronal and synaptic remodelling (Harmer et al., 2009).

Astrocytes, a type of brain glia cells, respond to stress and antidepressant treatment. Astrocytes express several 5-HT receptors, including 5-HT1A, 5-HT2A, 5-HT2B, 5-HT4R, hence could potentially respond to changing levels of serotonin following stress and antidepressant treatment (Muller et al., 2020). However, this has not been shown yet.

We investigated how serotonin deficiency and stress exposure differentially affect behaviour and astrocyte morphology in the medial lateral prefrontal cortex, amygdala and hippocampus of mice. To investigate this, we used tryptophan hydroxylase 2 knockout (Tph2^-/-) mice. Tph2 is the rate limiting enzyme for the conversion of tryptophan to serotonin centrally. Knockout of Tph2 is used to produce mice with a complete absence of serotonin centrally. We also used a 2-hour acute restraint stress procedure to generate the stressed groups.  

Here we employed 4 groups of mice: Tph2^-/- stressed, Tph2^-/- control, wildtype (WT) stressed and WT control (n=8 per group). To analyse the anxiety-like behaviours, the mice were subjected to the elevated plus maze (EPM) test for 5 mins, 24 hrs following the last restraint session. Astrocytes were visualised using GFAP immunohistochemistry then confocal microscopy was used to image fluorescently labelled sections. Quantitative analysis was performed using an image analysis pipeline to evaluate astrocyte number and morphology, including process length, branch complexity and somal volumes. All animal procedures were conducted in accordance with UK legislation and approved by the University of Bristol Animal Welfare and Ethical Review Body.

WT stressed mice showed increased anxiety-like behaviour at 1- and 14-days post stress, shown by a 15.39% reduction in open arm entries (p=0.0053) and a 39.87 second reduction of time in the distal open arms (p<0.001). Anxiety index values were also greater for the stressed animals, by 0.15 and 0.16, at 1- and 14-days post stress respectively (p<0.001). We hypothesise that stress induces morphological changes to astrocytes, and that in the absence of serotonin these changes are altered, indicating an interaction between genotype and stress.

These findings will aid our understanding of astrocyte response to serotonergic dysfunction and stress exposure, which may reveal glial mechanisms contributing to human stress susceptibility.