Effects of morphine on the plus-maze discriminative avoidance task: role of state-dependent learning.
BACKGROUND
The amnesic effects of morphine may be related to its action on nociception, anxiety, or locomotion. This effect is also suggested to be related to state dependency.


OBJECTIVE
The aims of this study were to verify the effects of morphine on mice tested in the plus-maze discriminative avoidance task (DAT) that uses light and noise as aversive stimuli and allows the concomitant evaluation of learning, memory, anxiety, and locomotion and also to verify the possible role of state-dependent learning in the effects of morphine.


RESULTS
The DAT was conducted in a modified elevated plus-maze. In the training, the aversive stimuli were applied when mice entered in one of the enclosed arms, whereas in the test, no stimuli were applied. The main results showed that (1) pretraining morphine (5-20 mg/kg i.p.) induced retrieval deficits (evaluated by the time spent in the aversive arm in the test) but not acquisition deficits (evaluated by the decrease in aversive arm exploration along the training); (2) pretest morphine (5-10 but not 20 mg/kg) counteracted this deficit; (3) morphine induced hypolocomotion (decreased number of entries in the arms), irrespective of memory alterations; and (4) morphine did not alter anxiety-like behavior (evaluated by the time spent in the open arms) during the training.


CONCLUSIONS
Morphine given before training induces retrieval deficits in mice tested in the DAT, and these deficits could be related to morphine-induced state-dependent learning. Neither the memory deficit induced by pretraining morphine nor the reversal of this deficit by pretest morphine seems to be related to anxiety levels or locomotor alterations.
Patti
C L
CL
Department of Pharmacology, Universidade Federal de São Paulo, Rua Botucatu, 862-Ed. Leal Prado, 04023-062 São Paulo, Brazil.
Kameda
S R
SR
Carvalho
R C
RC
Takatsu-Coleman
A L
AL
Lopez
G B
GB
Niigaki
S T
ST
Abílio
V C
VC
Frussa-Filho
R
R
Silva
R H
RH
D000818:Animals; D001007:Anxiety; D001362:Avoidance Learning / Q000187:drug effects*; D004192:Discrimination, Psychological / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D008027:Light; D008297:Male; D018782:Maze Learning / Q000187:drug effects*; D008568:Memory / Q000187:drug effects; D051379:Mice; D009020:Morphine / Q000494:pharmacology*; D009622:Noise
D009020:Morphine