Odor transduction

Alternative second-messenger pathways may be at work in olfactory transduction. The role of cAMP in olfactory transduction is well established. Are there alternative pathways, such as those involving phospholipids and Ca2+ Several groups have reported that certain odorants can elicit an increase in the phosphoinositde second messenger inositol 1,4,5,-trisphosphate (IP3) (Ch. 20). However, there is no clear evidence that IP3 directly mediates an electrical response in OSNs, nor is there a clear rationale for two parallel excitatory odor transduction cascades. However, more recent data support the idea that phos-phoinositides or enzymes related to their metabolism may play a modulatory role, shaping the OSN output by... 

Fadool, D. A., Estey, S. J., and Ache, B. W., Evidence that a Gq-protein mediates excitatory odor transduction in lobster olfactory receptor neurons, Chem. Senses, 20, 489, 1995. 

Odorant recognition initiates a second-messenger cascade leading to the depolarization of the neuron and the generation of action potentials 821 Negative-feedback processes mediate adaptation of the olfactory transduction apparatus to prolonged or repetitive stimulation 823 Alternative second-messenger pathways maybe at work in olfactory transduction 823... 

FIGURE 50-5 A model for the transduction of odors in OSNs. The individual steps are detailed in the text. Note that several feedback loops modulate the odor response, including inhibition of the CNG channel by Ca2+ ions (purple balls) that permeate the channel, and a Ca2+/calmodulin (CaM) -mediated desensitization of the channel that underlies rapid odor adaptation. Several other mechanisms, including phosphodiesterase-mediated hydrolysis of the second messenger, cAMP, and phosphorylation of the OR by various kinases, have also been described. 

Zufall, F. and Munger, S. D. From odor and pheromone transduction to the organization of the sense of smell. Trends Neurosci. 24 191-193, 2001. 

It is apparent that multiple recognition mechanisms are active in taste and odor recognition. There is now emerging evidence of multiple receptors and transduction mechanisms. The complex olfactory component of a flavor requires a mechanism with multiple receptors and several transduction pathways to be sufficiently sensitive ai discriminating to distinguish among closely relat materials. 

These are exciting times in the field of chemosensory reception in general and olfaction in particular. In the decade since the landmark identification of a novel class of candidate odorant receptors (ORs) in rats (Buck and Axel, 1991), we have seen an explosion of similar studies involving other vertebrate as well as several insect species. In addition to an ever-increasing wealth of behavioral and physiological studies, insect systems provide arguably the most robust experimental system for the study of olfaction as well as a profound demonstration of the universal conservation of olfactory signal transduction mechanisms. 

As is the case for all sensory pathways, the capacity to perceive and respond to olfactory cues (odorants) is the combined result of events that take place in both peripheral and central processing centers. These steps, which will be discussed in detail below, begin with the molecular transduction of chemical signals in the form of odorants into electrical activity by olfactory receptor neurons (ORNs) in the periphery whose axonal projections form characteristic synaptic connections with elements of the central nervous system (CNS). Within the CNS, complex patterns of olfactory signals are integrated and otherwise processed to afford recognition and ultimately, the behavioral responses to the insect s chemical environment. Within the context of pheromone recognition these responses would likely be centered on various elements of the insect s reproductive cycle. 

This chapter describes some of the principles and mechanisms underlying the primary processes of olfactory signaling, the chemo-electrical signal transduction. We will focus on molecular events that follow the interaction of odorants with olfactory sensory neurons, and leave aside perireceptor events including odorant... 

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