This large insect species is easily raised in the laboratory and exhibits complex behavior generated by a (relatively) simple CNS. The nervous system can be kept alive outside the animal and individual neurons can be repeatably identified. In addition to this physiological robustness, several important Manduca genes have recently been cloned that provide new tools for manipulating neural signaling. The present focus of our research is on the physiological and behavioral roles of two transmitter systems; nitric oxide (NO), a short-lived soluble gas that may act in a non-synaptic fashion in the brain (see link), and acetylcholine (ACh), a classical synaptic signal.

Insect acetylcholine receptors as "sensory gateways"

     Sensory neurons in insects use ACh as their primary central transmitter. When visual, olfactory, or mechanical information has been collected by peripheral sense organs it is relayed to neurons in the central nervous system by the release of ACh. This signal is detected by ACh receptors (AChRs) on interneurons and motoneurons which in turn integrate the information and incorporate it in the animal's behavior. There are two families of receptors involved in this process, the multisubunit ion channels (nicotinic; nAChRs) and the biochemically signaling proteins (muscarinic; mAChRs).

Nicotinic acetylcholine receptors

     The nAChRs typically mediate very fast electrical responses. However, associated with this transient electrical event the nAChRs in Manduca neurons can also generate large and comparatively long-lasting changes in intracellular calcium. We are studying the function of this calcium signal in populations of neurons in culture and in specific neurons that control reflex movements. At least one of the actions of nAChR stimulation is to activate nitric oxide synthase and produce cGMP in a subset of abdominal neurons.

     An additional goal is to understand how information from these receptors is used during normal behavior. We have used injections of double stranded RNA (RNAi) block the production of individual gene products (such as subunits of the nAChR) and plan to use this to "knock out" specific receptors in fully grown animals. The behavior of these animals can then be analyzed (see soft-bodied locomotion link) and compared to animals with the normal compliment of gene products.

Muscarinic acetylcholine receptors

     In addition to rapid synaptic signaling ACh also elicits slower, long-lasting effects via muscarinic acetylcholine receptors (mAChRs). In Manduca, one of these effects is to increase the excitability of a specific motoneuron (PPR) so that it fires action potentials in response to inputs that were previously ineffective. This change outlasts the sensory stimulation and therefore modulates subsequent behavior. The biochemical changes evoked by Manduca mAChRs include an increased turnover of signaling phospholipids in the membrane (PIP2), the mobilization of calcium, and an increase in the production of cGMP.

Significance of these studies

     Many of these research findings have biomedical significance: in particular, Manduca's nicotine-resistance is of interest commercially (in developing pesticides), ecologically (in insect-plant interactions), and medically (in treating nicotine addiction). Changes in neuronal biochemistry and excitability such as those mediated by mAChRs are likely to be critical in the etiology of epilepsy and in alterations in behavior caused by drugs or experience. By focusing on identified neurons with known functions, we are hopeful that our research will help identify the mechanisms and define the roles of particular receptors in the central nervous system.