OSCILLATIONS AND GASEOUS OXIDES IN INVERTEBRATE OLFACTION

Olfactory systems combine an extraordinary molecular sensitivity with robust synaptic plasticity. Central neuronal circuits that perform pat tern recognition in olfaction typically discriminate between hundreds of molecular species and form associations between odor onsets and beh avioral contingencies that can last a lifetime. Two design features in the olfactory system of the terrestrial mollusk Limax maximus may be common elements of olfactory systems that display the twin features of broad molecular sensitivity and rapid odor learning: spatially cohere nt oscillations in the second-order circuitry that receives sensory in put; and involvement of the interneuronal messengers nitric oxide (NO) and carbon monoxide (GO) in sensory responses and circuit dynamics of the oscillating olfactory network. The principal odor processing cent er in Limax, the procerebrum (PC) of the cerebral ganglion, contains o n the order of 10(5) local interneurons and receives both direct and p rocessed input from olfactory receptors. Field potential recordings in the PC show an oscillation at approximately 0.7 Hz that is altered by odor input, Optical recordings of voltage changes in local regions of the PC show waves of depolarization that originate at the distal pole and propagate to the base of the PC, Weak odor stimulation transientl y switches PC activity from a propagating mode to a spatially uniform mode, The field potential oscillation in the PC lobe depends on interc ellular communication via NO, based on opposing effects of reagents th at decrease or increase NO levels in the PC, Inhibition of NO synthase slows the field potential oscillation, while application of exogenous NO increases the oscillation frequency. A role for CO in PC dynamics is suggested by experiments in which CO liberation increases the PC os cillation frequency. These design features of the Limax PC lobe odor p rocessing circuitry may relate to synaptic plasticity that subserves b oth connection of new receptors throughout the life of the slug and it s highly developed odor learning ability. (C) 1996 John Wiley & Sons, Inc.