Muscarinic acetylcholine receptors are required for nestmate-odor memory retrieval but not for olfaction in harvester ants (Pogonomyrmex barbatus)

Abstract

The ability of social insects such as ants to distinguish between nestmates and non-nestmates is odor-based, learned and critical to the survival of the colony. However, little is known about how the brain processes the olfactory cues associated with nestmate recognition. Previous studies have shown that inhibition of muscarinic acetylcholine receptors with a competitive inhibitor, scopolamine affects nestmate recognition in honeybees, causing them to mistakenly attack nestmates. This suggests that muscarinic receptors may be required for either olfaction and/or retrieval of odor memory. The aim of this study was to further investigate the role of muscarinic receptors in nestmate recognition in another social insect, the harvester ant (Pogonomyrmex barbatus). Focal ants were fed either scopolamine dissolved in sucrose or sucrose alone (control) for approximately 72 hours, then placed in an arena with either a nestmate or a genetically unrelated non-nestmate. We videotaped their interactions and determined the frequency with which focal ants exhibited aggressive behaviors toward the introduced ants. Focal ants treated with scopolamine exhibited inappropriate aggressive behaviors toward nestmates with a significantly higher frequency than did the controls, but with the same frequency that they exhibited aggressive behaviors toward non-nestmates. These results suggest that scopolamine-treated ants can't distinguish nestmates from non-nestmates. However, scopolamine-treated focal ants did respond appropriately to the presence of an olfactory repellent, indicating that scopolamine does not disrupt olfaction. The data support a role for muscarinic receptors in odor memory retrieval rather than in odor perception, and indicate that this role is conserved in P. barbatus.