Chronic in vivo sequelae of repetitive acute mfb-DBS on accumbal dopamine and midbrain neuronal activity.

Medial Forebrain Bundle Deep Brain Stimulation (MFB-DBS) can have rapid and long lasting antidepressant effects in Treatment Resistant Depression (TRD) patients. The mechanisms are not well understood, but one hypothesis stipulates that modulation of the dopaminergic (DAergic) fibers contribute to the therapeutic outcome. Acute DBS effects on DA release have been studied; however, longitudinal studies with acute-repetitive DBS are lacking. Long-Evans accumbal DA release and Ventral Tegmental Area (VTA) calcium tonic and phasic signaling to different mfb-DBS parameters were measured using fiber photometry over 8 weeks, following acute and repetitive stimulation in behaving and non-behaving animals. DBS-induced release was observed in both targets, with increased frequency and DBS duration. 130 Hz stimulation increased phasic and tonic DA response over time, with the latter being a potential mechanism for its long-term clinical effectiveness. VTA calcium transients decreased, while phasic activity increased with frequency. Pulse width (PW)-mediated differential peak release timing also suggests potential parallel activation of diverse fiber types. Additionally, decreased DA transients rate during Elevated Plus Maze (EPM) suggests context and stimulation duration-dependent DA release. The data confirm chronic antidromic/orthodromic DAergic responses with stimulation parameter dependent variability, providing novel insights into temporal adaptations, connectivity and fiber recruitment on mfb DBS.