Further evaluation of morphine aversion: maintenance of a taste aversion using a low, nonaversive morphine dose.
Previously, in an investigation of morphine-conditioned taste aversion (CTA), we found that limited preexposure to a low, nonaversive (non-CTA-inducing) dose of morphine (2.5 mg/kg) was as effective as preexposure to a higher, CTA-inducing dose (15 mg/kg) in blocking the formation of a subsequent morphine CTA. In the present study, we examined the capacity of this low, 2.5-mg/kg morphine dose to maintain a CTA initially induced by the 15-mg/kg dose. A standard CTA procedure was used. Results indicated that rats given three initial taste-drug pairings with 15 mg/kg morphine followed on subsequent pairing days by treatment with the low, non-CTA-inducing, 2.5-mg/kg dose continued to exhibit a strong CTA over 8 pairing days. A similar pattern was observed for animals continuing to receive taste-drug pairings with the 15-mg/kg dose. Animals receiving only one taste-drug pairing with the 15-mg/kg dose, followed on subsequent pairing days by 2.5-mg/kg conditioning, failed to show such a pattern of CTA. An intermediate CTA pattern was seen with animals conditioned with 15, 10, 5, and repeated 2.5-mg/kg doses over consecutive pairing days. These data suggest that exposure to a low dose of morphine, with no apparent CTA-inducing properties, is sufficient to maintain a previously established morphine taste aversion. Potential implications for understanding the apparent discriminative complexity of morphine's motivational properties are discussed.
D000818:Animals; D001362:Avoidance Learning / Q000187:drug effects*; D003214:Conditioning, Classical / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D005795:Generalization, Stimulus / Q000187:drug effects; D008297:Male; D009020:Morphine / Q000008:administration & dosage*; D051381:Rats; D011919:Rats, Inbred Strains; D012439:Saccharin / Q000008:administration & dosage; D013997:Time Factors
D009020:Morphine; D012439:Saccharin