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The firing rate of VTA DA is time-dependently changed in response to subsequent challenge after a single morphine exposure. Rats received morphine (20 mg/kg, s.c.) or saline, and then challenged with morphine (5 mg/kg, i.v.) 24, 72 or 120 hours later. Data are summary based on at least 5 subjects at each time point in each group and are expressed as % change induced by the challenge. A: firing rate. B: bursting level. C: CV. *p < 0.05, t-test.
