Electro-olfactogram Research Articles

Theophylline induces changes in the electro-olfactogram of the mouse

Options for the treatment of hyposmia are limited;available therapies do not provide a long-lasting effect.A recent study suggests that an unspecific phosphodiesterase inhibitor (PDE-I) increases olfactory sensitivity due to interaction with the signal transduction in the olfactory epithelium. The aim of the present study was to investigate whether theophylline, an unspecific PDE-I, evokes changes in the electro-olfactogram (EOG) which would support the hypothesis of a drug-related impact on signal transduction.In addition, the uptake of topically administered theophylline in the olfactory epithelium should be investigated. EOG was obtained in 29 samples of supravital mouse olfactory epithelia. Olfactory stimulation (phenylethyl alcohol, PEA and hydrogen sulfide, H2S) was performed using an air dilution olfactometer. Theophylline concentration in the olfactory epithelium of five samples was measured by means of high pressure liquid chromatography. Administration of theophylline resulted in a tendency towards smaller EOG amplitudes (p = 0.055), being reduced by 13 and 25% in response to stimulation with PEA or H2S,respectively. In comparison to the application of Ringer's solution, theophylline resulted in a significant (p = 0.031)decrease of the EOG amplitude. Latency was not significantly(p = 0.10) influenced by drug administration. The theophylline concentration in the olfactory epithelium ranged from 0.21 to 1.53 microg/mg. Theophylline seems to be taken up into the olfactory epithelium of supravital mice and to interact with the olfactory signal transduction.

Response of the hammerhead shark olfactory epithelium to amino acid stimuli

Sharks and rays are highly sensitive to chemical stimuli in their natural environment but several hypotheses predict that hammerhead sharks, with their expanded head and enlarged olfactory epithelium, have particularly acute olfactory systems. We used the electro-olfactogram (EOG) technique to compare the relative response of the scalloped hammerhead shark (Sphyrna lewini) olfactory epithelium to 20 proteinogenic amino acids and determine the sensitivity for 6 amino acids. At micromolar concentrations, cysteine evoked the greatest EOG response which was approximately twice as large as that of alanine. The weakest response was obtained for proline followed by aspartic acid and isoleucine. The olfactory epithelium showed adaptation to sequential stimulation, and recovery was related to the inter-stimulus time period. Estimated EOG response thresholds were in the sub-nanomolar range for both alanine (9.2 x 10(-11) M) and cysteine (8.4 x 10(-10) M) and in the micromolar range for proline and serine. These thresholds from 10(-10) to 10(-6) M for the scalloped hammerhead shark are comparable or lower than those reported for other teleost and elasmobranch species. Future work should focus on binary and more complex compounds to test for competition and cross-adaptation for different classes of peripheral receptors, and their responses to molecules found in biologically relevant stimuli.

Odorant Concentration Dependence in Electroolfactograms Recorded From the Human Olfactory Epithelium

Electroolfactograms (EOGs) are the summated generator potentials of olfactory receptor neurons measured directly from the olfactory epithelium. To validate the sensory origin of the human EOG, we set out to ask whether EOGs measured in humans were odorant concentration dependent. Each of 22 subjects (12 women, mean age = 23.3 yr) was tested with two odorants, either valeric acid and linalool (n = 12) or isovaleric acid and l-carvone (n = 10), each delivered at four concentrations diluted with warm (37 degrees C) and humidified (80%) odorless air. In behavior, increased odorant concentration was associated with increased perceived intensity (all F > 5, all P < 0.001). In EOG, increased odorant concentration was associated with increased area under the EOG curve (all F > 8, all P < 0.001). These findings substantiate EOG as a tool for probing olfactory coding directly at the level of olfactory receptor neurons in humans.

Seasonal Increase in Olfactory Receptor Neurons of the Japanese Toad, Bufo japonicus, is Paralleled by an Increase in Olfactory Sensitivity to Isoamyl Acetate

Japanese toads (Bufo japonicus) migrate to and from breeding sites in the early spring, possibly guided by olfactory cues. We previously showed that the electrical activity of olfactory receptor neurons (ORNs) in the toads was enhanced in the breeding period. We undertook morphological and physiological studies of the olfactory epithelium to determine whether any cellular substrate of the epithelium underlies the enhanced electrical activity of ORNs. The ORNs of the toads were labeled by antiserum to olfactory marker protein (OMP), and the morphology of the labeled cells and their distribution in the epithelium were examined throughout the year. The OMP-positive cells, distributed mainly in the basal and intermediate layers of the epithelium, were most numerous in the early breeding period. Cell proliferation in the epithelium detected by 5-bromo-2'-deoxyuridine labeling was most elevated in this period. The electrical activity of ORNs was examined by recording the electroolfactogram (EOG) in the toads throughout the year. Statistical analysis showed a positive correlation between the density of OMP-positive cells in the epithelium and the amplitude of the EOG responses. A greater number of ORNs in the breeding period possibly aids the toads in migrating to their breeding sites.

F-series prostaglandin function as sex pheromones in the Korean salamander, Hynobius leechii

In order to test whether prostaglandins (PGs) function as sex pheromones in Hynobius leechii, a salamander that externally fertilizes its eggs, we conducted electro-olfactogram (EOG) studies with 19 PGs, liquid chromatography tandem mass spectrometry (LC-MS/MS) analyses of female and male holding waters, and behavioral tests on selected PGs. Of the 19 PGs tested, only three induced strong EOG responses from both males and ovulated females: 15-epi-prostaglandin F2α (15(R)-PGF2α), 15-keto-prostaglandin F2α (15K-PGF2α), and 13,14-dihydro-15-keto-prostaglandin F2α (13,14-dh-15K-PGF2α). In the LC-MS/MS studies, samples of holding water from ovulated females contained higher concentrations of 15(R)-PGF2α, PGF2α, and 13,14-dh-15K-PGF2α than those from males or oviposited females. In the behavioral tests, only 15(R)-PGF2α and ovulated female holding water induced significant reproductive behavior from male salamanders. These results suggest that F-series prostaglandins function as sex pheromones in amphibians.

Modulation of Spontaneous and Odorant-Evoked Activity of Rat Olfactory Sensory Neurons by Two Anorectic Peptides, Insulin and Leptin

In mammals, the sense of smell is modulated by the status of satiety, which is mainly signaled by blood-circulating peptide hormones. However, the underlying mechanisms linking olfaction and food intake are poorly understood. Here we investigated the effects of two anorectic peptides, insulin and leptin, on the functional properties of olfactory sensory neurons (OSNs). Using patch-clamp recordings, we analyzed the spontaneous activity of rat OSNs in an in vitro intact epithelium preparation. Bath perfusion of insulin and leptin significantly increased the spontaneous firing frequency in 91.7% (n = 24) and 75.0% (n = 24) of the cells, respectively. When the activity was electrically evoked, both peptides shortened the latency to the first action potential by approximately 25% and decreased the interspike intervals by approximately 13%. While insulin and leptin enhanced the electrical excitability of OSNs in the absence of odorants, they surprisingly reduced the odorant-induced activity in the olfactory epithelium. Insulin and leptin decreased the peak amplitudes of isoamyl acetate-induced electroolfactogram (EOG) signals to 46 and 38%, respectively. When measured in individual cells by patch-clamp recordings, insulin and leptin decreased odorant-induced transduction currents and receptor potentials. Therefore by increasing the spontaneous activity but reducing the odorant-induced activity of OSNs, an elevated insulin and leptin level (such as after a meal) may result in a decreased global signal-to-noise ratio in the olfactory epithelium, which matches the smell ability to the satiety status.

Effects of Odor Stimulation on Antidromic Spikes in Olfactory Sensory Neurons

Spikes were evoked in rat olfactory sensory neuron (OSN) populations by electrical stimulation of the olfactory bulb nerve layer in pentobarbital anesthetized rats. The latencies and recording positions for these compound spikes showed that they originated in olfactory epithelium. Dual simultaneous recordings indicated conduction velocities in the C-fiber range, around 0.5 m/s. These spikes are concluded to arise from antidromically activated olfactory sensory neurons. Electrical stimulation at 5 Hz was used to track changes in the size and latency of the antidromic compound population spike during the odor response. Strong odorant stimuli suppressed the spike size and prolonged its latency. The latency was prolonged throughout long odor stimuli, indicating continued activation of olfactory receptor neuron axons. The amounts of spike suppression and latency change were strongly correlated with the electroolfactogram (EOG) peak size evoked at the same site across odorants and across stimulus intensities. We conclude that the curve of antidromic spike suppression gives a reasonable representation of spiking activity in olfactory sensory neurons driven by odorants and that the correlation of peak spike suppression with the peak EOG shows the accuracy of the EOG as an estimate of intracellular potential in the population of olfactory sensory neurons. In addition, these results have important implications about traffic in olfactory nerve bundles. We did not observe multiple peaks corresponding to stimulated and unstimulated receptor neurons. This suggests synchronization of spikes in olfactory nerve, perhaps by ephaptic interactions. The long-lasting effect on spike latency shows that action potentials continue in the nerve throughout the duration of an odor stimulus in spite of many reports of depolarization block in olfactory receptor neuron cell bodies. Finally, strong odor stimulation caused almost complete block of antidromic spikes. This indicates that a very large proportion of olfactory axons was activated by single strong odor stimuli.

Electroolfactogram Responses from Organotypic Cultures of the Olfactory Epithelium from Postnatal Mice

Organotypic cultures of the mouse olfactory epithelium connected to the olfactory bulb were obtained with the roller tube technique from postnatal mice aged between 13 and 66 days. To test the functionality of the cultures, we measured electroolfactograms (EOGs) at different days in vitro (DIV), up to 7 DIV, and we compared them with EOGs from identical acute preparations (0 DIV). Average amplitudes of EOG responses to 2 mixtures of various odorants at concentrations of 1 mM or 100 microM decreased in cultures between 2 and 5 DIV compared with 0 DIV. The percentage of responsive cultures was 57%. We also used the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) to trigger the olfactory transduction cascade bypassing odorant receptor activation. Average amplitudes of EOG responses to 500 microM IBMX were not significantly different in cultures up to 6 DIV or 0 DIV, and the average percentage of responsive cultures between 2 and 5 DIV was 72%. The dose-response curve to IBMX measured in cultures up to 7 DIV was similar to that at 0 DIV. Moreover, the percentage of EOG response to IBMX blocked by niflumic acid, a blocker of Ca-activated Cl channels, was not significantly different in cultured or acute preparations.

The impact of a brominated flame retardant on smoltification and olfactory function in Atlantic salmon (Salmo salar L.) smolts

Despite a reduction in exploitation of salmon stocks throughout the NE Atlantic, there continues to be a decline in many populations. The factors regulating these populations remain poorly understood, although there is evidence that environmental conditions experienced in freshwater can effect survival in the marine environment. The aim of this study was to investigate the impact of a brominated flame retardant (hexabromocyclododecane, HBCD) on the parr-smolt transformation in juvenile salmon. It is during this pre-adaptive period to marine life that olfactory imprinting to the natal river is considered to occur. Fish were exposed to low levels of HBCD for 30 days over the peak smoltification period in freshwater, and then transferred to clean seawater for 20 days. Fish were sampled weekly to assess changes in some of the physiological parameters associated with smoltification, and olfactory response to conspecific smolt urine was measured using an electro-olfactogram (EOG). Exposure to HBCD did not affect seawater adaptability, although there was some disruption of plasma thyroid hormone levels, as well as a reduction in olfactory function to conspecific smolt urine. The results are discussed in relation to the marine survival and successful homing of adult salmon.

Naris occlusion alters the electro-olfactogram: Evidence for compensatory plasticity in the olfactory system

Unilateral naris occlusion (NO) has been widely used as a method of olfactory deprivation to study the role of stimulus-driven activity in olfactory development [P.C. Brunjes, Unilateral naris closure and olfactory system development, Brain Res. Rev. 19 (1994) 146-160]. Recent immunochemical studies of the olfactory epithelium (OE) following NO provide evidence for a previously unknown compensatory response to deprivation [D.M. Coppola, A. Waguespack, M. Reems, M.L. Butman, J. Cherry, Naris occlusion alters transductory protein immunoreactivity in olfactory epithelium, Histol. Histopathol. 21 (2006) 487-501; A. Waguespack, M. Reems, M.L. Butman, J. Cherry, D.M. Coppola, Olfactory receptor neurons have enhanced olfactory marker protein immunoreactivity in the nasal cavity ipsilateral to naris occlusion, Brain Res. 1044 (2005) 1-7]. To further investigate this phenomenon we measured electro-olfactograms (EOG) from the open and occluded OE of adult mice that had undergone NO as newborns. EOG waveforms from the open side OE of NO mice were indistinguishable from those obtained from untreated animals. However, amplitudes of EOGs from the occluded OE of NO mice were greater, on average, than those recorded at matched locations from the open side. This result was consistent across turbinates, odors, and all but the highest odor concentration. In addition, EOGs recorded from the occluded OE had significantly slower onset and recovery kinetics. Responses in a double-pulse protocol confirmed that the kinetics of the cellular or population processes that underlie the EOG are slowed by NO. These results provide the most direct support, to date, for compensatory plasticity in the olfactory system.

Mechanisms of neuronal chloride accumulation in intact mouse olfactory epithelium

When olfactory receptor neurons respond to odours, a depolarizing Cl(-) efflux is a substantial part of the response. This requires that the resting neuron accumulate Cl(-) against an electrochemical gradient. In isolated olfactory receptor neurons, the Na(+)-K(+)-2Cl(-) cotransporter NKCC1 is essential for Cl(-) accumulation. However, in intact epithelium, a robust electrical olfactory response persists in mice lacking NKCC1. This response is largely due to a neuronal Cl(-) efflux. It thus appears that NKCC1 is an important part of a more complex system of Cl(-) accumulation. To identify the remaining transport proteins, we first screened by RT-PCR for 21 Cl(-) transporters in mouse nasal tissue containing olfactory mucosa. For most of the Cl(-) transporters, the presence of mRNA was demonstrated. We also investigated the effects of pharmacological block or genetic ablation of Cl(-) transporters on the olfactory field potential, the electroolfactogram (EOG). Mice lacking the common Cl(-)/HCO(3)(-) exchanger AE2 had normal EOGs. Block of NKCC cotransport with bumetanide reduced the EOG in epithelia from wild-type mice but had no effect in mice lacking NKCC1. Hydrochlorothiazide, a blocker of the Na(+)-Cl(-) cotransporter, had only a small effect. DIDS, a blocker of some KCC cotransporters and Cl(-)/HCO(3)(-) exchangers, reduced the EOG in epithelia from both wild-type and NKCC1 knockout mice. A combination of bumetanide and DIDS decreased the response more than either drug alone. However, no combination of drugs completely abolished the Cl(-) component of the response. These results support the involvement of both NKCC1 and one or more DIDS-sensitive transporters in Cl(-) accumulation in olfactory receptor neurons.

Differential detection of conspecific-derived odorants by the two olfactory epithelia of the Senegalese sole ( Solea senegalensis)

The two olfactory epithelia of members of the family Soleidae sample two distinct water sources; the upper (right) side is in contact with the open water column whilst the lower (left) side is in contact with interstitial water. To evaluate whether there are differences in the sensitivities, and therefore functional roles, of the two epithelia the olfactory activity of conspecific-derived odorants was assessed in both using the electro-olfactogram (EOG). The upper nostril was significantly more sensitive to conspecific bile fluid, intestinal fluid and mucus than the lower nostril. Crude fractionation of these samples (solid-phase extraction with C18 and C2/ENV+ cartridges) revealed that olfactory activity in each body fluid was likely due to a mixture of compounds. In each case, the upper olfactory epithelium was significantly more sensitive than the lower. Similarly, olfactory sensitivity to a range of C 24 and C 27 bile acids was greater in the upper epithelium. These results suggest that intra-specific chemical communication is mediated mainly, if not entirely, by the upper olfactory epithelium. The odorants involved, and their functional roles, remain to be established.

The contribution of amino acids to the odour of a prey species in the Senegalese sole ( Solea senegalensis)

For many fish, olfaction is important in food search and consumption. Amino acids are known to elicit feeding behaviour in several species. The aim of the current study was to evaluate the contribution of amino acids to the odour of a natural prey organism of the Senegalese sole ( Solea senegalensis). Both whole-body macerates and substances released to the water by living ragworms ( Hediste diversicolor) were fractionated by molecular weight filtration followed by solid-phase extraction (SPE), and the olfactory activity of the resultant fractions was assessed by the electro-olfactogram (EOG) in the sole. The amino acid concentrations of the macerate and water were determined by gas chromatography and mass spectrometry (GC–MS). In the macerate, the majority of odorants were small molecular weight compounds (< 500 Da) which were not retained by C-18 SPE cartridges. An artificial mixture of amino acids at the same concentrations as found in the macerate had similar olfactory potency. The odorants released to the water by living ragworms were also small molecular weight compounds (< 500 Da) but the majority of olfactory activity could be extracted by C-18 SPE cartridges. The concentrations of amino acids in these samples were too low to contribute greatly to its olfactory potency. These results suggest that, whilst olfactory sensitivity to amino acids may explain most of the potency of the macerate, living ragworms are releasing additional odorants other than amino acids which may be equally important in chemo-sensory food location in the sole.

Linuron and carbaryl differentially impair baseline amino acid and bile salt olfactory responses in three salmonids

For salmon, amino acid and bile salt detection form the basis for important behaviors including predator evasion and conspecific recognition, respectively. For this reason, decreases in olfactory sensory neuron responses to the amino acid l-serine and the bile salt taurocholic acid (TChA) have been used in studies as indicators of acute olfactory pesticide toxicity to environmental contaminants such as metals and pesticides. In this study, we first compare baseline responses to these two odorant classes across three salmonids, and then explore how two currently used pesticides alter these responses. We found baseline differences in electro-olfactogram (EOG) responses and their sensitivity to pesticide exposure between rainbow trout, coho and sockeye salmon. For example, rainbow trout had lower baseline EOGs than either coho or sockeye (e.g. 10 −5 M TChA EOGs of 1.34 ± 0.17 versus 2.57 ± 0.46 and 2.72 ± 0.43 mV, respectively). At 15 min after exposure to 10 μg/L of the herbicide linuron, rainbow l-serine-evoked EOGs were 49.6% of control versus 78.5 and 69.8% for sockeye and coho, indicating rainbow were more sensitive to linuron. In contrast, at 30 min of exposure to 100 μg/L carbaryl, l-serine-evoked EOGs of sockeye were 49.7% of control versus 60.3 and 62.3% for rainbow and coho, suggesting sockeye were more sensitive to carbaryl. In all species the l-serine-evoked EOGs did not return to baseline by 15 min after 100 μg/L carbaryl exposure, suggesting persisting impairment of amino acid detection. The TChA-evoked EOGs were less affected by carbaryl exposure (i.e. EOGs were 83.3, 84.9 and 66.0% of control 15 min after exposure) and not affected at all by 100 μg/L linuron exposure. Species-specific differences in pesticide sensitivity may limit extrapolation of toxicity across salmonids while the generally greater sensitivity of amino acid olfaction may lead to selective impairment of behaviors such as predator evasion.

Relating olfactory neurotoxicity to altered olfactory-mediated behaviors in rainbow trout exposed to three currently-used pesticides

Odor-evoked neurophysiological responses can form the basis for behavioral responses. Here we first characterized olfactory-mediated behavioral and neurophysiological responses of juvenile rainbow trout to the amino acid l-histidine, then looked at whether there were similar responses to the carbamate antisapstain IPBC and the herbicides atrazine and Roundup ®, and lastly explored how exposures to these pesticides modified the l-histidine responses. Trout were behaviorally attracted to 10 −7 M l-histidine (as assayed in a counter-current olfactometer), but this preference behavior switched to indifference with higher histidine concentrations. Neurophysiologically, the summed electrical responses of peripheral olfactory neurons, as measured using electro-olfactogram (EOG), was 0.843 ± 0.252 mV to 10 −7 M l-histidine. Of the pesticides, only Roundup ® evoked EOGs, indicating the amino acid-based pesticide may have acted as an odorant, and generated a behavioral response: it was avoided at active ingredient [AI; glyphosate isopropyl amine] concentrations ≥ 10 mg/l. With 30 min pesticide exposures, 10 −7 M l-histidine preference behavior was eliminated following exposure to 1 μg/l IPBC and atrazine, and 100 μg/l AI Roundup ®. Similarly, 10 −7 M l-histidine-evoked EOGs were significantly reduced by exposure to 1 μg/l IPBC, 10 μg/l atrazine, and 100 μg/l AI Roundup ®. When combined together, the results demonstrate that typical preference behavior can be abolished when neurophysiological responses are reduced by >60% of control. This asymmetry in response thresholds suggests that behavioral responses may be more sensitive toxicological endpoints than neurophysiological responses.

Loss of CNGB1 Protein Leads to Olfactory Dysfunction and Subciliary Cyclic Nucleotide-gated Channel Trapping

Olfactory receptor neurons (ORNs) employ a cyclic nucleotide-gated (CNG) channel to generate a receptor current in response to an odorant-induced rise in cAMP. This channel contains three types of subunits, the principal CNGA2 subunit and two modulatory subunits (CNGA4 and CNGB1b). Here, we have analyzed the functional relevance of CNGB1 for olfaction by gene targeting in mice. Electro-olfactogram responses of CNGB1-deficient (CNGB1-/-) mice displayed a reduced maximal amplitude and decelerated onset and recovery kinetics compared with wild-type mice. In a behavioral test, CNGB1-/- mice exhibited a profoundly decreased olfactory performance. Electrophysiological recordings revealed that ORNs of CNGB1-/- mice weakly expressed a CNG current with decreased cAMP sensitivity, very rapid flicker-gating behavior and no fast modulation by Ca2+-calmodulin. Co-immunoprecipitation confirmed the presence of a CNGA2/CNGA4 channel in the olfactory epithelium of CNGB1-/- mice. This CNGA2/CNGA4 channel was targeted to the plasma membrane of olfactory knobs, but failed to be trafficked into olfactory cilia. Interestingly, we observed a similar trafficking defect in mice deficient for the CNGA4 subunit. In conclusion, these results demonstrate that CNGB1 has a dual function in vivo. First, it endows the olfactory CNG channel with a variety of biophysical properties tailored to the specific requirements of olfactory transduction. Second, together with the CNGA4 subunit, CNGB1 is needed for ciliary targeting of the olfactory CNG channel.

Changes in juvenile coho salmon electro‐olfactogram during and after short‐term exposure to current‐use pesticides

For anadromous salmonids, olfaction is a critical sense, enabling return migration. In recent years, several pesticides have been identified that interfere with salmonid olfaction at concentrations in the microg/L range; thus, they may pose a risk to species longevity. In the present study, we investigated the acute effects of five agricultural pesticides on juvenile coho salmon (Oncorhynchus kisutch) olfaction using the electro-olfactogram (EOG), a measure of odorant-evoked field potentials. Electro-olfactogram responses to the odorant L-serine were measured during and following a 30-min exposure of the left olfactory rosette to chlorothalonil, endosulfan, glyphosate acid, iodocarb (IPBC), trifluralin, and 2,4-dichlorophenoxyacetic acid. With the relatively insoluble pesticides endosulfan and trifluralin, decreases in EOG amplitude were only apparent at relatively high concentrations (100 and 300 microg/L, respectively) following 20 min of exposure and were absent for chlorothalonil (1 mg/L). With the water-soluble herbicide glyphosate, significant EOG reductions occurred within 10 min of exposure to 1 mg/L and more rapidly with higher concentrations. Recovery of EOG post-glyphosate exposure was concentration-dependent, and complete recovery was not observed with some concentrations at 60 min postexposure. Dichlorophenoxyacetic acid only affected EOG at high concentration (100 mg/L), where it eliminated EOG within 2 min of exposure. With IPBC, EOG was decreased at 25 min of exposure to 1 microg/L; higher concentrations caused decreases to occur more rapidly. Excluding IPBC and glyphosate, all EOG reductions occurred at concentrations greater than the current Canadian water-quality guidelines and reported 96-h lethality values. Our results show that olfactory neurons can be impaired rapidly by some current-use pesticides, even at exposures in the low-microg/L range.

Olfactory inhibition and recovery in chum salmon (Oncorhynchus keta) following copper exposure

Copper can be an effective hatchery treatment to control certain parasitic and fungal infections in salmonids, yet it is also highly toxic to the fish's olfactory system. Numerous reports have described the early inhibitory impacts of copper on fish olfaction, but very few have addressed processes of recovery. Here we show that the chum salmon (Oncorhynchus keta) olfactory system can recover from short-term (4 h) exposures to copper (3–58 µg·L–1) within 1 day, as indicated by electro-olfactogram (EOG) recordings. Recovery followed initial EOG reductions of up to 88%. We also applied a novel cytophysiological approach to monitor changes in the olfactory receptor cells using a fluorescent styryl dye, AM1-43. Copper's influence on the number of receptor cells that labelled with the dye was biphasic, initially increasing the number at the two lowest copper concentrations (3 and 8 µg·L–1) and decreasing the number at the two highest concentrations (24 and 58 µg·L–1). Changes in the ability of olfactory receptor neurons to internalize the dye were monitored up to 10 days in fish recovering from copper exposure.

Air Movement Evokes Electro-Olfactogram Oscillations in the Olfactory Epithelium and Modulates Olfactory Processing in a Slug

In many animals, neurons in the olfactory system have been shown to respond not only to odorants but also to air movements. However, the manner in which the mechanical dynamics of odor stimulation affect olfactory processing remains poorly understood. Using a series of flow rates and odor concentrations from clean air to high-concentration vapors, we systematically analyzed the effects of air movement and odor concentration on olfactory processing. We extracellularly recorded local field potentials and spike units from the olfactory epithelium (OE) and tentacular nerve (TN), which connects the first and second relay centers of olfactory information, in the terrestrial slug Limax marginatus. We found that clean air puffs at a flow rate of 0.18 ml/s (gentle wind), but not high-concentration odor puffs at lower flow rates, induced electro-olfactogram (EOG) oscillations in the OE with a constant frequency (2.5 Hz), regardless of the odor. Surgically isolated OE preparations also showed these EOG oscillations, indicating that the oscillations arose from the OE independently of the downstream circuits. The EOG oscillations entrained the slower spontaneous TN oscillations (1-2 Hz) to the fixed rhythm (2.5 Hz). Spontaneous and odor-evoked units were phase-locked to the TN oscillation peaks. This TN oscillation entrainment by the EOG oscillations caused stronger phase-locking, specifically TN oscillation peaks and EOG oscillation troughs. Taken together, these results suggest that when odors are carried by a gentle wind, the air movement induces EOG oscillations and modulates rhythmic spike patterning of olfactory outputs to the second olfactory relay center in Limax.

Effects of Concentration and Sniff Flow Rate on the Rat Electroolfactogram

Previous reports using the electroolfactogram (EOG) to study the spatial and temporal aspects of response in the rodent olfactory epithelium had focused on high odorant concentrations that gave large responses. This investigation has used lower concentrations to test the difference between responses in the rat dorsomedial and lateral recesses with a range of nasal flow rates and a range of chemical properties. The responses to a highly polar, more hydrophilic odorant changed more steeply with flow rate than responses to a very nonpolar, hydrophobic odorant. With low flow rates there was a response delay in the lateral recess, which is consistent with the models indicating lower flow rates in that region. We observed significant volume conduction effects in which large responses in the dorsomedial region obscured smaller initial portions of the lateral responses. These effects could be removed by destroying the dorsomedial response with a high concentration of a low molecular weight ester. We caution that investigators of EOG recordings from the intact epithelium must attend to the possible presence of volume conduction, which can be assessed by attention to the selectivity of odorant response, response waveform, and response latency.