Electrophysiological Field Potential Recordings

Field potential recordings were performed as previously described (Adermark et al., 2011 (link)). Briefly, local field population spikes (PSs) were activated with a frequency of 0.05 Hz in subregions of the: ventral striatum nucleus accumbens core (NAcC) and shell (NAcSh); dorsal striatum [dorsomedial striatum (DMS); dorsolateral striatum (DLS)]; amygdala [central nucleus of the amygdala (CeA) and basolateral amygdala; Beridze et al., 2020 (link); Figs. 2A,F, 3A, 4A,F; Extended Data Figs. 2-1, 3-1, 4-1]. Only rats that obtained ≥15 infusions in the last 10 self-administration sessions were considered in the electrophysiological recordings. Recordings in different brain areas were conducted in separate brain slices retrieved from the same rat, thereby minimizing the risk of individual variation between brain regions and treatments. Stimulation electrodes (World Precision Instruments; type TM33B) were positioned locally, 0.2–0.3 mm from the recording electrode (borosilicate glass, 2.5–4.5 MΩ, World Precision Instruments; Extended Data Figs. 2-1, 3-1, 4-1), and the amplitude of the population spike (PS) were measured. Signals were amplified with a custom-made amplifier, filtered at 3 kHz, and digitized at 8 kHz.
During field potential recordings, slices were perfused with prewarmed aCSF (30°C) containing the GABAA receptor antagonist bicuculline to isolate excitatory transmission to monitor putative effects by treatment on neurotransmission, stimulus/response curves were created by stepwise increasing the stimulation strength, and by monitoring the paired pulse ratio (PPR). For PPR, responses were evoked with a paired pulse stimulation (50-ms interpulse interval) at approximately half max stimulation strength, and PPR was calculated by dividing the second pulse (PS2) with the first pulse (PS1).