Abstract
Self-administration of addictive drugs is a widely used tool for studying behavioral, neurobiological, and genetic factors in addiction. However, how locomotor activity is affected during self-administration of addictive drugs has not been extensively studied. In our present study, we tested the locomotor activity levels during acquisition, extinction and reinstatement of morphine self-administration in rats. We found that compared with saline self-administration (SA), rats that trained with morphine SA had higher locomotor activity. Rats that successfully acquired SA also showed higher locomotor activity than rats that failed in acquiring SA. Moreover, locomotor activity was correlated with the number of drug infusions but not with the number of inactive pokes. We also tested the locomotor activity in the extinction and the morphine-primed reinstatement session. Interestingly, we found that in the first extinction session, although the number of active pokes did not change, the locomotor activity was significantly lower than in the last acquisition session, and this decrease can be maintained for at least six days. Finally, morphine priming enhanced the locomotor activity during the reinstatement test, regardless of if the active pokes were significantly increased or not. Our results clearly suggest that locomotor activity, which may reflect the pharmacological effects of morphine, is different from drug seeking behavior and is a distinctive index in drug self-administration.
Highlights
After Weeks [1] reported that rats voluntarily administered addictive drugs intravenously, self-administration (SA) of addictive drugs has been widely used as a tool for studying behavioral, neurobiological, and genetic factors in addiction for decades [2]
One-way analysis of variance (ANOVA) confirmed that the locomotor activity progressively increased in the morphine SA group (F 13, 598 = 19.47, P
It is concluded that morphine SA effectively enhanced locomotor activity
Summary
After Weeks [1] reported that rats voluntarily administered addictive drugs intravenously, self-administration (SA) of addictive drugs has been widely used as a tool for studying behavioral, neurobiological, and genetic factors in addiction for decades [2]. Locomotor activity is a distinctive index in morphine self-administration This is a relevant question, since hyperlocomotion is a critical index of the effects induced by drug abuse. The motor stimulant effects of opioids are closely related to the addictive properties of these drugs [12], and the locomotor enhancing action induced by morphine share similar receptor-regulated mechanisms with rewarding effects [12,13,14]. Locomotor activity is commonly measured in behavioral sensitization tests, which has been proposed as a model for the development of drug dependence [16] and craving. This may be one of the underlying mechanisms responsible for high rates of relapse [17]
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