Behavioral and neurochemical alterations induced by reversible conductive hearing loss in aged male rats.

Eighteen months old male rats of the Sprague-Dawley strain were subjected to a reversible conductive hearing loss (HL) or a sham operation. A series of behavioral tests performed 3 months after surgery, revealed a sustained deficit in learning and memory capacity and a marked depressive attitude of rats with HL. At this time, a group of these animals were allowed to recover from HL for 1 month and were again tested behaviorally compared to those with persistent auditory deficit. A better performance at the active and passive avoidance tests and normal responsiveness to the despair test was found in animals with recovered hearing capacity as compared to those with persistent HL. A biochemical analysis revealed a decrease of dopamine and homovanillic acid content and of choline-acetyltransferase and acetylcholinesterase activity in the striatum and hippocampus of animals with persistent HL as compared to those with recovered auditory deficit. These data suggest that persistent auditory deafferentation affects cognitive mechanism in old rats in a reversible manner.