Investigating the Effects of Probiotics on Synaptic Plasticity in the Follicular and Luteal Phases of the Menstrual Cycle

The human gastrointestinal (GI) tract is comprised of trillions of bacterial strains. Recent work has focused on investigating the influence of the gut microbiota on the processing and function of our brain; this is known as the 'gut-brain axis'. Our gut microbiota can be altered through diet, antibiotics, prebiotics and probiotics (Hemarajata et al., 2013). Probiotics are live microorganisms, which when delivered in adequate amounts, can offer beneficial health outcomes to the host (Hill et al., 2014). Modulation of our gut microbiota through probiotics, can improve our emotional state (Bagga et al., 2018; Messaoudi et al., 2011; Tillisch et al., 2013), decrease pain sensitivity (Weizman et al., 2016), improve cognitive processes (Chung et al., 2014; Kim et al., 2020), increase neurotransmitter levels (Mittal et al., 2017) and alter functional connectivity (Rode et al., 2022; Bagga et al., 2019). In addition, probiotics regulate estrogen levels (Guo et al., 2016; Urbaniak et al., 2016).

Estrogen levels have been recognized to influence the gut microbiota. The gut produces β-glucuronidases which is the enzyme required to breakdown conjugated estrogen into its active form (Dabek et al., 2008; Ervin et al., 2019). Low β-glucuronidases production occur in the presence of diminished bacterial diversity levels in the gut. The consequence of the low β-glucuronidases is a reduction in active forms of estrogen (Siddiqui et al., 2022; Pugh et al., 2021; Kwa et al., 2016, Baker et al., 2017). Further, gastrointestinal symptoms increase during menstruation (Pugh et al., 2021). Last, irritable bowel syndrome is more prevalent in females, indicating a possible role of sex hormones in relation to the gut microbiome (Pugh et al., 2021).

Estrogen has been found to impact neural activity, memory, and cognition (Inghilleri et al., 2004; Luine 2014). EEG brain activity was collected from women in their follicular and luteal phase (Brotzner et al., 2014). When estrogen levels were highest, individual alpha frequency oscillations (IAF) were lowest (Brotzner et al., 2014), the latter is associated with reduced cognition and working memory (Li et al., 2023). In addition, estrogen was found to reduce the firing threshold in some neurons which can promote excitability in the cortex (Gregory et al., 2019). Conversely, high progesterone levels are known to increase GABA activity which reduces neural activity (Guennoun et al., 2015). Furthermore, a 5Hz repetitive transcranial magnetic stimulation (rTMS) protocol was delivered over the motor cortex during the early and late follicular phase (Inghilleri et al., 2004). Corticospinal activity was increased when estradiol levels were highest (Inghilleri et al., 2004) showing the association between estradiol and synaptic plasticity.

Collectively, the literature suggests that women may have a reduced capacity for synaptic plasticity during the luteal phase when estradiol levels are low. Introducing a probiotic will increase β-glucuronidases which leads to greater availability of circulating estrogen which should promote synaptic plasticity. It is hypothesized that increases in synaptic plasticity will occur in the follicular and luteal phase in the presence of probiotics. This study aims 1) to determine whether probiotics can increase the propensity for synaptic plasticity in females and 2) to determine if there are differences in the propensity for plasticity depending on the phase of the menstrual cycle.

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