ANTIDEPRESSANT ACTIVITY OF LEFT UNILATERAL TRANSCRANIAL DIRECT STIMULATION OF EXCITABLE GABAERGIC NEURONS

Abstract Background Transcranial direct current stimulation (tDCS) is a non-invasive method of neuromodulation that employs a mild electrical discharge to stimulate brain regions beneath a cathode-anode electrode ensemble. tDCS increases cell excitability without drastically modifying bursting and firing frequency ( Pelletier 2014). As the medial prefrontal cortex (mPFC) is crucial for the antidepressant response, mPFC-tDCS has been explored as a potential therapeutic strategy for major depression (Thibault 2021). Previous research suggests that direct stimulation of the mPFC has antidepressant effects in a clinical subpopulation (Chrisykou 2022). However, the precise molecular mechanisms underlying tDCS effects remain unknown. Aims Using a rat model of mPFC-directed tDCS, we assessed the therapeutically relevant impact of bilateral (BL) and unilateral left (UL) or right (UR) stimulation in behavioural tests for antidepressant-related properties (sucrose preference test [SPT] and forced swim test [FST]) and anxiolytic-like properties (novelty-suppressed feeding test [NSFT] and open-field test [OFT]). We sought to test the involvement of cortical gamma-amino butyric acid (GABA) circuits, as with other treatment modalities (Fogaca 2019). Results The stress-inducing paradigm employed was olfactory bulbectomy (OBX). Twenty-one days post-OBx surgery, animals were submitted to acute (20-min) or subchronic (5-7 days) cathodic stimulations with or without an injection of the small-conductance, calcium-activated potassium channel 3-preferring (SKC3) antagonist, NS8593 (1 mg/kg, i.p.). Low (0.05 or 0.1 mA) or zero intensity stimulation was delivered over the mPFC via the cathode, followed by behavioural testing, brain extraction and slicing. RNAscope in situ-hybridization (ISH) was done to examine the differential activation of discrete GABAergic cell types (somatostatin [SST+ GABAergic], parvalbumin [PV+ GABAergic] and CAMKII mRNA probes) in the mPFC-limbic projections, using an mRNA probe for zif268. Method OBx+NS8593-treated males showed a therapeutic-like response to UL stimulation only. In the OFT, OBx-males exhibited a significant antidepressant/anxiolytic-like reduction of motor activity compared to drug-naï ve/OBx animals, and similar to control, non-OBx levels (p<.05). In the SPT, these animals showed a reversal of anhedonia-like reduction of SP scores. ISH imaging of zif268 revealed significant increases in neural activity within the targeted and projections regions. Specifically, there was a significant activation (p<.01) in the cingulate, prelimbic and orbital regions, insular cortex and claustrum. RNAscope ISH for discrete GABAergic cell types (SST, PV and CAMKII) is underway. Discussion A combination of tDCS and NS8593 led to a potent antidepressant-like response. In principle, NS8593 inhibits SKC3 in interneurons, enhancing their excitability, consequently leading to tDCS induced GABAergic activation. In effect, a pathological mPFC hyperactivity is alleviated, yielding therapeutic- like effect. Conclusion: These findings suggest that tDCS in combination with excitability priming achieves selective patterns of brain regional and cell-specific activation. This novel strategy offers a potentially effective alternative to psychotropic drug treatments. References 1.Pelletier SJ, Cicchetti F. 2014, ‘Cellular and molecular mechanisms of action of transcranial direct current stimulation: evidence from in vitro and in vivo models’, International Journal of Neuropsychopharmacology, vol. 18, issue 2, <https://academic.oup.com/ijnp/article/18/2/pyu047/690895> 2.Bittar P., Labonte B. 2021, ‘Functional Contribution of the Medial Prefrontal Circuitry in Major Depressive Disorder and Stress-Induced Depressive-Like Behaviors’, Vol 15, <https://www.frontiersin.org/articles/10.3389/fnbeh.2021.699592/full> 3.Chrysikou E., Wing E., van Dam W. 2022, ‘Transcranial Direct Current Stimulation Over the Prefrontal Cortex in Depression Modulates Cortical Excitability in Emotion Regulation Regions as Measured by Concurrent Functional Magnetic Resonance Imaging: An Exploratory Study’, Biological Psychiatry: Cognitive Neuroscience and Neuroimaging, Vol 7, Issue 1, pp 85-94, <https://www.frontiersin.org/articles/10.3389/fnbeh.2021.699592/full> 4.Fogaç a V., Duman S. 2019, ‘Cortical GABAergic Dysfunction in Stress and Depression: New Insights for Therapeutic Interventions’, Frontiers in Cellular Neuroscience, Vol 13, <https://www.frontiersin.org/articles/10.3389/fncel.2019.00087>