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Dose-dependent impairing effects of morphine on avoidance acquisition and performance in male mice
M A Aguilar1, J Miñarro, V M Simón
1Area de Psicobiología, Facultad de Psicología, Universitat de Valencia, Spain. asuncion.aguilar@uv.es
Abstract:
The effects of morphine (6.3, 12.6, and 25.2 mg/kg) on active avoidance behavior of BALB/C mice are explored in three acquisition sessions and in two subsequent performance sessions. Morphine-treated animals showed an increase in avoidance acquisition with respect to control group without differences in performance. However, a dramatical, concomitant rise in the locomotor activity of the animals (increase in the number of crossings during the intertrial intervals) prompted us to transform the data employing a formula with which a measure of actual learning was obtained. Applying this formula, we have observed that morphine administration impairs, dose-dependently, acquisition and performance of avoidance. Thus, the impairing effects of morphine on avoidance could be masked by their stimulant effects on locomotor activity.
Insights
Morphine initially appears to improve avoidance learning in mice, but this effect is misleading. Higher doses actually impair both learning and performance by increasing locomotor activity, masking true cognitive effects.
Area of Science:
- Neuroscience
- Behavioral Pharmacology
- Animal Models
Background:
- Morphine is a potent opioid analgesic with known central nervous system effects.
- Opioid administration can influence cognitive functions, including learning and memory.
- Locomotor activity is a common behavioral measure affected by psychoactive substances.
Purpose of the Study:
- To investigate the dose-dependent effects of morphine on active avoidance behavior in BALB/C mice.
- To differentiate between direct effects on learning and indirect effects mediated by locomotor activity.
- To clarify the complex relationship between morphine, learning, and motor function.
Main Methods:
- BALB/C mice were administered varying doses of morphine (6.3, 12.6, 25.2 mg/kg).
- Active avoidance behavior was assessed across acquisition and performance sessions.
- Locomotor activity was measured by counting intertrial interval crossings.
- Data were transformed using a formula to isolate actual learning from motor effects.
Main Results:
- Initial observations suggested increased avoidance acquisition with morphine treatment.
- A significant, dose-dependent increase in locomotor activity was observed.
- After correcting for locomotor activity, morphine was found to impair both acquisition and performance of avoidance behavior.
- The stimulant effect on locomotion masked the underlying cognitive impairment.
Conclusions:
- Morphine's effects on active avoidance behavior are complex and dose-dependent.
- Increased locomotor activity can confound the interpretation of learning deficits.
- The apparent enhancement of avoidance learning by morphine is an artifact of its stimulant properties.