Increased elevated plus maze open-arm time in mice during naloxone-precipitated morphine withdrawal

Abstract

Opioid withdrawal is known to be anxiogenic in humans and, using the elevated plus maze (EPM), was demonstrated to also be anxiogenic in rats. Thus, this study characterizes EPM behaviors of mice during naloxone-precipitated morphine withdrawal. Naloxone did not significantly change EPM behaviors of drug-naïve mice. Additionally, morphine-dependent mice in which withdrawal was not precipitated (i.e. morphine-dependent mice receiving saline) spent less time in the open-arms compared to the controls. Surprisingly, increased open-arm time was observed in morphine-dependent mice undergoing naloxone-precipitated withdrawal. This increase was not because of total motor activity, as no significant differences in total activity were observed. Moreover, morphine dependency was necessary, given that there was not a significant increase in open-arm time for mice undergoing withdrawal from acute morphine. Increased open-arm time during withdrawal is unexpected, given that opioid withdrawal is usually associated with anxiety. Additionally, even in mice, naloxone-precipitated morphine withdrawal is known be aversive and increases plasma corticosterone levels. In conclusion, this study demonstrates somewhat unexpected EPM behavior in mice undergoing naloxone-precipitated morphine withdrawal. Possible interpretations of these EPM results, though somewhat speculative, raise the possibility that EPM behaviors might not be driven exclusively by anxiety levels but rather by other withdrawal-induced behaviors.

Fig. 1

The effect of naloxone on elevated plus maze behaviors. Five minutes after the saline or naloxone injection, mice were placed in the center of the maze facing toward an enclosed arm and their behaviors were recorded for 10 min. No significant effect of naloxone on open-arm time (a), total distance traveled in the open arms (b), and total activity (c) in drug-naïve mice was observed. Similarly, no significant effect of naloxone on open-arm time (g), total distance traveled in the open arms (h), and total activity (i) was observed in mice withdrawing from acute morphine. A significant decrease in open-arm time (d) and total distance traveled in the open arms (e) was observed in morphine-dependent mice receiving saline, but no significant effect on total activity (f). A significant increase in open-arm time (j) and total distance traveled in the open arms (k) was observed in morphine-dependent mice receiving naloxone, but no significant change in total activity (l). Results are presented as mean ± SEM, n = 8–15. *Significant difference from controls (P < 0.05).

Fig. 2

(a) Morphine-dependent mice receiving 0.4 and 2 mg/kg naloxone, but not 0.2 mg/kg naloxone, exhibited a significant increase in jumping behaviors. Saline-injected control mice, drug-naïve mice receiving naloxone, and morphine-dependent mice receiving saline did not jump at all. Jumping behavior was measured for 15 min following saline or naloxone injection. Results are presented as mean ± SEM, n = 5–7. *Significant difference compared to drug-naïve mice receiving saline (P < 0.001). (b) Morphine-dependent mice receiving 0.2 mg/kg naloxone had significantly higher plasma corticosterone levels as compared to both controls and morphine-dependent mice receiving saline (P < 0.05). Blood was collected 10 min after naloxone or saline injection. Results are presented as mean ± SEM, n = 3–7. *Significant difference compared to drug-naïve mice receiving saline (P < 0.05). §Significant difference compared to morphine-dependent mice receiving saline (P < 0.05).