Gelperin A, Kleinfeld D, Denk W, Cooke I R
Biological Computation Research Department, Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974, USA.
J Neurobiol. 1996 May;30(1):110-22. doi: 10.1002/(SICI)1097-4695(199605)30:1<110::AID-NEU10>3.0.CO;2-Q.
Olfactory systems combine an extraordinary molecular sensitivity with robust synaptic plasticity. Central neuronal circuits that perform pattern recognition in olfaction typically discriminate between hundreds of molecular species and form associations between odor onsets and behavioral 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 coherent oscillations in the second-order circuitry that receives sensory input; and involvement of the interneuronal messengers nitric oxide (NO) and carbon monoxide (CO) in sensory responses and circuit dynamics of the oscillating olfactory network. The principal odor processing center in Limax, the procerebrum (PC) of the cerebral ganglion, contains on the order of 10(5) local interneurons and receives both direct and processed 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 transiently switches PC activity from a propagating mode to a spatially uniform mode. The field potential oscillation in the PC lobe depends on intercellular communication via NO, based on opposing effects of reagents that 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 oscillation frequency. These design features of the Limax PC lobe odor processing circuitry may relate to synaptic plasticity that subserves both connection of new receptors throughout the life of the slug and its highly developed odor learning ability.
嗅觉系统将非凡的分子敏感性与强大的突触可塑性结合在一起。在嗅觉中执行模式识别的中枢神经回路通常能区分数百种分子种类，并在气味出现与可能持续一生的行为偶发事件之间形成关联。陆生软体动物大蛞蝓嗅觉系统中的两个设计特征可能是具有广泛分子敏感性和快速气味学习这两个特征的嗅觉系统的共同要素：在接收感觉输入的二级回路中的空间相干振荡；以及中间神经元信使一氧化氮（NO）和一氧化碳（CO）参与振荡嗅觉网络的感觉反应和回路动力学。大蛞蝓主要的气味处理中心，即脑神经节的嗅叶，包含约10^5个局部中间神经元，并接收来自嗅觉感受器的直接和经过处理的输入。嗅叶中的场电位记录显示出约0.7Hz的振荡，该振荡会因气味输入而改变。对嗅叶局部区域电压变化的光学记录显示，去极化波起源于远端极并传播到嗅叶基部。弱气味刺激会暂时将嗅叶活动从传播模式切换到空间均匀模式。基于降低或增加嗅叶中NO水平的试剂的相反作用，嗅叶中的场电位振荡依赖于通过NO进行的细胞间通信。抑制一氧化氮合酶会减缓场电位振荡，而施加外源性NO会增加振荡频率。CO释放增加嗅叶振荡频率的实验表明CO在嗅叶动力学中起作用。大蛞蝓嗅叶气味处理回路的这些设计特征可能与突触可塑性有关，突触可塑性既有助于蛞蝓一生中新感受器的连接，也有助于其高度发达的气味学习能力。
