Effects of morphine on associative memory and locomotor activity in the honeybee (Apis mellifera)

Morphine can modulate the processes underlying memory in vertebrates. However, studies have shown various modulations by morphine: positive, negative and even neutral. The honeybee is a potential platform for evaluating the effects of drugs, especially addictive drugs, on the nervous system. However, the involvement of morphine in learning and memory in insects or other invertebrates is poorly understood. The current work evaluated whether morphine affects memory acquisition, consolidation and retrieval in honeybees, using the proboscis extension response (PER) paradigm. We demonstrated that morphine treatment (5 μg/bee) before training decreased the percentage of correct PERs and the response latency related to aversive rather than rewarding odors when tested after 1 or 24 h. Morphine treatment after training also caused a decrease in this latency when tested after 24 h. Meanwhile, morphine treatment reduced the ambulation distance when tested after 30 min. Our findings suggest that morphine impairs the acquisition of short- and long-term associative memory and slightly disrupts the consolidation of long-term memory in honeybees. These negative effects cannot be explained by reduced locomotion but by impaired memory associated with aversion.