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Updated: Aug 22, 2026
Assessment of Morphine-induced Hyperalgesia and Analgesic Tolerance in Mice Using Thermal and Mechanical Nociceptive Modalities
Published on: July 29, 2014
Effects of endogenous morphine deprivation on memory retention of passive avoidance learning in mice
Massimo Guarna1, Carla Ghelardini, Nicoletta Galeotti
1Department of Anatomical and Biomedical Sciences, University of Siena, Italy.
Abstract:
Memory and the processes of learning in mammals are well known to be affected by opioid agonists such as morphine, which has been proven to interfere and cause amnesia. The presence of endogenous morphine has been demonstrated in various tissues from mammals to invertebrates. In this study, we have investigated the effects caused by in-vivo immunodepletion of endogenous morphine on working memory under different experimental conditions. When mice were submitted to fasting, a stress condition, acquisition and consolidation of memory were significantly impaired compared to controls. This was demonstrated by a decrease in entry latency into the dark room in the retention session of the passive avoidance test. This effect was significantly reversed to baseline values when endogenous morphine was depleted from the extracellular brain space. These findings support a role for endogenous morphine in weakening memory processes under stress conditions.
Insights
Endogenous morphine weakens memory during stress. Depleting this opioid reversed memory impairment in fasting mice, revealing its role in stress-related memory decline.
Area of Science:
- Neuroscience
- Behavioral Neuroscience
- Molecular Biology
Background:
- Opioid agonists like morphine affect learning and memory, potentially causing amnesia.
- Endogenous morphine, naturally occurring in mammals and invertebrates, has been detected in various tissues.
- The role of endogenous morphine in memory, particularly under stress, remains largely unexplored.
Purpose of the Study:
- To investigate the effects of in-vivo immunodepletion of endogenous morphine on working memory.
- To examine how endogenous morphine influences memory acquisition and consolidation under stress conditions, specifically fasting.
- To determine if reducing endogenous morphine levels can reverse stress-induced memory deficits.
Main Methods:
- In-vivo immunodepletion of endogenous morphine in mice.
- Utilizing fasting as a stress condition.
- Assessing memory performance using the passive avoidance test, measuring entry latency into the dark room.
Main Results:
- Fasting significantly impaired memory acquisition and consolidation in control mice, indicated by decreased entry latency.
- In-vivo immunodepletion of endogenous morphine reversed these memory impairments in fasting mice, restoring performance to baseline levels.
- These findings highlight a significant impact of endogenous morphine on memory processes under stress.
Conclusions:
- Endogenous morphine plays a crucial role in impairing memory processes during stress conditions.
- The study provides evidence that endogenous morphine contributes to stress-induced memory deficits.
- Targeting endogenous morphine pathways may offer therapeutic potential for stress-related memory disorders.
