Baclofen Modulates Dopamine Responses

Animals were implanted with chronic, carbon fiber microsensors targeted at the nucleus accumbens core (1.3-mm lateral and 1.3-mm rostral from bregma) and with bilateral guide cannulae (26 gauge; Plastics One, VA) directed at the VTA (0.5-mm lateral and −5.6-mm caudal to bregma, lowered 7.0 mm ventral from dura mater). Dummy canulae (Plastics One, VA) were installed in the guide cannulae and removed during testing. On test days (~2 months after implantation), injectors (33 gauge; Plastics One, VA) were inserted through the guide cannulae so they protruded 1 mm beyond the guide cannulae to a final depth of 8.0 mm ventral from dura mater. Injections (0.5 µl) of ACSF (in mM: 154.7 Na⁺, 2.9 K⁺, 132.49 Cl⁻, 1.1 Ca²⁺ at pH = 7.4) or baclofen (50 ng) dissolved in ACSF were visually monitored for accuracy and were completed within four min. Dopamine responses to reward delivery were recorded immediately prior to injections and 5 min after injections. Voltammetric responses were analyzed by calculating the area under the curve of the change in current at the peak dopamine oxidation potential which was normalized to the percentage of that for the pre-injection reward delivery.