Chunk #12 — Material and methods — I. Heroin self-administration, extinction, foot shock- and heroin- induced reinstatement — 4. Procedure — (A) Self-administration:

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Individual differences in gene expression of vasopressin, D2 receptor, POMC and orexin: vulnerability to relapse to heroin-seeking in rats.

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During heroin self-administration, we noted individual variability in heroin intake that was not attributable to any obvious technical issue since catheter patency was verified daily by drawing blood. Rather than excluding the “non-responders (NR)”, these rats were tested until the conclusion of the experiment and employed as “heroin control” for the rats that showed clear acquisition. The criterion used to classify the NR was as follows: less than a total of 20 infusions taken on days 5, 6 and 7 of self-administration testing.