Dissociation in the modulatory effects of environmental novelty on the locomotor, analgesic, and eating response to acute and repeated morphine in the rat.
BACKGROUND
We have previously shown that environmental novelty can potentiate the activating effects of morphine and the development of sensitization to this effect. OBJECTIVES. Our main goal was to determine whether environmental novelty can also modulate the prophagic (time spent eating and food intake; experiment 1) and/or the analgesic (tail-flick test; experiments 2 and 3) effect of morphine, as well as the development of tolerance or sensitization to these effects.


METHODS
In experiment 1, two groups of rats were administered seven intraperitoneal (i.p.) injections of either saline or morphine (4.0 mg/kg) either in their home cages (home groups) or in a distinct environment (novelty groups). After 7 days of withdrawal, both groups underwent a morphine challenge (4.0 mg/kg, i.p.). In experiment 2, home and novelty rats were administered four doses of morphine (0.0, 2.0, 4.0, and 8.0 mg/kg, i.p.) following a counterbalanced order. In experiment 3, home and novelty rats were administered eight intraperitoneal (i.p.) injections of either saline or morphine (8.0 mg/kg) and then given a morphine challenge (4.0 mg/kg).


RESULTS
Environmental novelty enhanced the locomotor activating effect of morphine and the expression of sensitization to this effect (even after a period of withdrawal). Environmental novelty had relatively little effect on morphine-induced eating, and no effect on morphine-induced analgesia.


CONCLUSIONS
Environmental context can have very different consequences on distinct drug effects as well as on distinct neurobehavioral adaptations to the same drug treatment (e.g., psychomotor sensitization versus analgesic tolerance).
Paolone
Giovanna
G
Department of Human Physiology and Pharmacology Vittorio Erspamer, University of Rome La Sapienza, Rome, Italy.
Burdino
Rosetta
R
Badiani
Aldo
A
D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004777:Environment; D005247:Feeding Behavior / Q000187:drug effects*; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009043:Motor Activity / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley
D000701:Analgesics, Opioid; D009020:Morphine