Sensory processing of ambient CO₂ information in the brain of the moth Manduca sexta

Insects use information about CO₂ to perform vital tasks such as locating food sources. In certain moths, CO₂ is involved in oviposition behavior. The labial palps of adult moths that feed as adults have a pit organ containing sensory receptor cells that project into the antennal lobes, the sites of primary processing of olfactory information in the brain. In the moth Manduca sexta and certain other species of Lepidoptera, these receptor cells in the labial-palp pit organ have been shown to be tuned to CO ₂, and their axons project to a single, identified glomerulus in the antennal lobe, the labial-palp pit organ glomerulus. At present, however, nothing is known about the function of this glomerulus or how CO₂ information is processed centrally. We used intracellular recording and staining to reveal projection (output) neurons in the antennal lobes that respond to CO₂ and innervate the labial-palp pit organ glomerulus. Our results demonstrate that this glomerulus is the site of first-order processing of sensory information about ambient CO₂. We found three functional types of CO₂-responsive neurons (with their cell bodies in the antennal lobe or the protocerebrum) that provide output from the antennal lobe to higher centers in the brain. Some physiological characteristics of those neurons are described.