Pain impairs consolidation, but not acquisition or retrieval of a declarative memory.

Among the physical conditions that impair memory performance, pain is one of the most prevalent. However, the mechanisms by which pain impairs memory are largely unknown. In this study, we asked whether pain affects memory acquisition, consolidation and retrieval as well as whether memory impairment depends on pain intensity. Wistar rats received a hind paw injection of formalin (1%) at different phases of object recognition test. The injection of formalin after training but not before training or testing impaired object recognition memory. We concluded that pain impairs the consolidation but not acquisition or retrieval of object recognition memory, which is a subtype of declarative memory. Morphine, at a dose that did not affect object recognition memory in control rats, drastically reduced formalin-induced nociceptive behavior without reversing memory impairment. A lower dose of formalin (0.25%) induced less nociceptive behavior, but similar memory impairment. There is no statistical correlation between the intensity of nociceptive response and the performance in object recognition test. However, when formalin-induced nociceptive response was blocked by a local anesthetic, memory impairment was prevented. These findings suggest that pain-induced impairment in the consolidation of object recognition memory does not directly depend on the intensity of nociceptive activity.