Brief communication.  Generalization of [DAla2]-enkephalinamide but not of substance P to the morphine cue.
Rats were trained to discriminate morphine (7.5 mg/kg, IP) from saline in a two bar positively reinforced lever pressing paradigm on a FR4 schedule. Morphine (IP) showed a naloxone reversible dose-related generalization to the training dose. [DAla2]-Methionine enkephalinamide (DAE) at 1 mg/kg and Substance P (SP) at 0.1 and 0.25 mg/kg showed vehicle appropriate responding after IP injection. DAE (5 mg/kg) disrupted responding completely; SP (0.5 and 0.1 mg/kg) disrupted responding in 50% of the rats. The disruption caused by IP injection of DAE was not naloxone reversible. Intraventricular injection of morphine (5 microgram/rat) and DAE (5 microgram/rat) produced generalization to the opiate cue. The effect of DAE was reversed by naloxone (1 mg/kg, SC). SP (500 and 750 ng/rat, IVT) produced saline-like responding; 1 microgram/rat disrupted responding completely. These data demonstrate that morphine and enkephalin, but not Substance P, share similar discriminative properties.
D000818:Animals; D003463:Cues; D004193:Discrimination Learning / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004723:Endorphins / Q000494:pharmacology*; D004745:Enkephalins / Q000494:pharmacology*; D005795:Generalization, Stimulus / Q000187:drug effects*; D007274:Injections, Intraperitoneal; D007276:Injections, Intraventricular; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D051381:Rats; D013373:Substance P / Q000494:pharmacology*
D004723:Endorphins; D004745:Enkephalins; D013373:Substance P; D009270:Naloxone; D009020:Morphine