Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:To block the entire time-course of morphine reward and not just the short time that the animals were in the place-conditioning apparatus immediately after morphine or saline injection, the TPP was reversibly inactivated with the local anesthetic bupivacaine, which has an effect that lasts for at least 2 h (Swerdlow & Jones, 1970; Covino, 1986), compared with the time-course (approximately 30–60 min) of the faster acting anesthetic lidocaine (Covino, 1986).
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