Research report
Type 4 phosphodiesterase inhibition impairs detection of low odor concentrations in mice

https://doi.org/10.1016/j.bbr.2005.02.011Get rights and content

Abstract

The cAMP-specific phosphodiesterase PDE4A is abundant in the dendrites, soma and axons of olfactory receptor neurons of the mouse, but it is not present in the cilia, where olfactory transduction initiates. Although the function of PDE4A in mammalian olfaction is unknown, patch clamp studies on deciliated olfactory receptor cells in the newt have shown that adrenaline or cAMP analogs can increase the contrast sensitivity to current injection. We used mice to ask whether increasing the levels of cAMP in sensory neurons by inhibiting PDE4A activity with rolipram could lead to changes in the perception of odorants that correspond to the in vitro cellular responses seen in newts. In an automated olfactometer, rolipram treatment (1 mg/kg, i.p.) significantly impaired the detection accuracy of 1-propanol at relatively high dilutions but did not affect detection at lower dilutions. Meanwhile, the ability to discriminate amyl acetate alone from a mixture of amyl acetate + citronellal was not affected by rolipram at any odor dilution. In a different task in which mice were trained to discriminate between cups of scented versus unscented sand, rolipram treatment resulted in poorer discrimination at high and better discrimination at low, odor dilutions. In sum, PDE4 inhibition resulted in a consistent decrement in the ability of mice to detect low concentrations of odorants, but the effects of rolipram on detection of higher concentrations were task-dependent.

Introduction

Olfactory transduction begins when an odorant molecule binds to receptor sites on the cilia of receptor neurons. Odorant-receptor binding activates adenylyl cyclase through a second messenger cascade involving the G protein Golf, causing an increase in the level of adenosine 3′,5′-cyclic monophosphate (cAMP) [22]. Elevated levels of cAMP cause cyclic nucleotide gated channels to open, resulting in calcium influx and depolarization of the neuron [10], [18]. Cyclic nucleotide removal is mediated by phosphodiesterases (PDEs), which consist of a large 11-family group of enzymes found in virtually all tissues [9]. The predominant PDEs that occur in the rodent olfactory epithelium are members of the calcium-dependent PDE1 and cAMP-specific PDE4 families [3], [5], [28]. Within olfactory receptor neurons, these enzymes are subcellularly segregated: whereas PDE1 is found in the cilia, a PDE4 isoform (PDE4A) occurs in the cell bodies, axons and dendrites but is absent from the cilia [3], [4], [12].

The specific role of non-ciliary cAMP in general, and PDE4A in particular, in olfactory function is unclear. One possibility is that non-ciliary cAMP may modulate sensory perception. In one study, adrenaline or cAMP analogues applied to isolated, deciliated newt olfactory receptor cells in vitro suppressed spike generation in response to weak stimuli, yet increased spike frequency in response to moderate to strong stimuli [13]. The steepened slope of the intensity-response curve reflected cAMP-induced changes in contrast sensitivity, leading to the conclusion that cAMP localized outside of the olfactory sensory neuron cilia could contribute to certain aspects of olfactory perception [8], [13].

We sought to evaluate the potential involvement of PDE4A in odorant contrast by measuring olfactory discrimination in mice treated with rolipram, which is a specific inhibitor of mammalian PDE4s [23]. We asked whether increasing cAMP levels beyond the cilia by inhibition of PDE4A with rolipram could produce a behavioral manifestation of the stimulus–response properties seen by Kawai et al. [13] in olfactory neurons. Specifically, we examined whether rolipram treatment would decrease olfactory sensitivity of mice to increased dilutions of an odorant, yet enhance sensitivity at moderate to low dilutions. To investigate this question, mice were tested in two separate, appetitively motivated behavioral paradigms. In the first, an automated liquid dilution olfactometer was developed to measure the ability of water-deprived mice to: (a) distinguish a pure odorant from an odor mixture and (b) to detect increasing dilutions of a single odorant. In the second, food-deprived mice were tested in a food-motivated, two-choice discrimination task using cups of sand that were unscented or scented with different dilutions of an odorant.

Section snippets

Subjects

Adult male Swiss Webster mice (between 7 and 10 weeks of age at the beginning of testing) were obtained from Taconic Farms (Germantown, NY) and group-housed on a 12-h light:12-h dark cycle. On the two-choice discrimination task using scented sand, subjects (n = 9) were fed sufficient rodent chow daily to maintain 80–90% pre-testing ad libitum body weight, with feeding always following testing on test days. Water was continuously available. For olfactometer testing, a different set of subjects (n = 

Odor mixtures

Mice treated with rolipram or vehicle readily distinguished AA-only from AA mixed with any of the three lowest dilutions of CIT (Fig. 2). However, performance began to decline under both rolipram and vehicle treatment once the dilution of CIT in the S− was brought to 0.01%, as indicated by two animals that failed to pass criterion on both the rolipram and vehicle tests. At the highest CIT dilution tested, all animals failed on both treatment conditions. Repeated measures ANOVA corroborated

Discussion

Electrophysiological observations on cAMP-induced spiking in newt olfactory sensory neurons led us to examine whether treatment of mice with the PDE4 inhibitor rolipram could increase the contrast between high and low odor concentrations. Kawai et al. [13] found that adrenaline could reversibly suppress responses of deciliated cells in vitro under weakly stimulated conditions, and strengthen responses to moderate and higher levels of input. These responses were mimicked by application of the

Acknowledgements

Supported by MH59200. We thank Dr. Michael Baum for a critical reading of an earlier version of the paper. We also appreciate the generous assistance of Dr. Howard Eichenbaum and members of his laboratory for helping us set up and establish the olfactory testing protocols.

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