Gaseous Oxides and Olfactory Computation

The gaseous neurotransmitters nitric oxide (NO) and carbon monoxide (CO) are prominent and universal components of the array of neurotransmitters found in olfactory information processing systems. These highly mobile communication compounds have effects on both second messenger signaling and directly on ion channel gating in olfactory receptors and central synaptic processing of receptor input. Olfactory systems are notable for the plasticity of their synaptic connections, revealed both in higher-order associative learning mechanisms using odor cues and developmental plasticity operating to maintain function during addition of new olfactory receptors and new central olfactory interneurons. We use the macrosmatic terrestrial mollusk Limax maximus to investigate the role of NO and CO in the dynamics of central odor processing and odor learning. The major central site of odor processing in the Limax CNS is the procerebral (PC) lobe of the cerebral ganglion, which displays oscillatory dynamics of its local field potential and periodic activity waves modulated by odor input. The bursting neurons in the PC lobe are dependent on local NO synthesis for maintenance of bursting activity and wave propagation. New data show that these bursting PC interneurons are also stimulated by carbon monoxide. The synthesizing enzyme for carbon monoxide, heme oxygenase 2, is present in the neuropil of the PC lobe. Since the PC lobe exhibits two forms of synaptic plasticity related to both associative odor learning and continual connection of new receptors and interneurons, the use of multiple gaseous neurotransmitters may be required to enable these multiple forms of synaptic plasticity.