Electrophysiological characterization of rat and mouse olfactory receptor neurons from an intact epithelial preparation

Mechanisms determining the dynamic range of the bullfrog olfactory receptor cell

Spike discharges of single olfactory receptor cells (ORCs) were recorded with the whole cell patch-clamp method applied to slice preparation. In parallel, activities of transduction channels were measured under the voltage-clamp condition. When cells were stimulated by odorants, 54 out of 306 cells exhibited inward current responses (10 mM cineole in the puffer pipette). The amplitude of the inward current was dependent on the stimulus period, reflecting the time integration for the stimulus dose, and the relation could be fitted by the Hill equation. Under the current-clamp condition, current injection induced spike discharges. In cells showing repetitive firings, the firing frequency was dependent on the amount of injected current. The relation was fitted by the Michaelis-Menten equation showing saturation. When cells were responsive to the odorant and had abilities to discharge repetitive spikes, the depolarizing responses were accompanied by repetitive spikes. In those cells, the spike frequency was dose-dependent, expressing saturation similar to the result obtained by current injection. Since both transduction channel and spike generative steps expressed saturation in their dose dependences, we explored what step(s) actually determines saturation in ORC signaling processes. By examining dose-response relations of both the current and spikes in the same cells, saturating dose was found to be dependent largely on that of the transduction step. This suggests that the dynamic range is fundamentally determined by the transduction system. In addition, a simple model derived from the nonlinearity of the plasma membrane could explain that a critical level of dynamic range was, at least in part, modified by the membrane nonlinearity.

Evidence for the disproportionate mapping of olfactory airspace onto the main olfactory bulb of the hamster

Olfactory receptor neurons (ORNs) project to the rodent main olfactory bulb (MOB) from spatially distinct air channels in the olfactory recesses of the nose. The relatively smooth central channels of the dorsal meatus map onto the dorsal MOB, whereas the highly convoluted peripheral channels of the ethmoid turbinates project to the ventral MOB. Medial and lateral components of each projection stream innervate the medial and lateral MOB, respectively. To ascertain whether such topography entails the disproportionate representation seen in other sensory maps, we used disector-based stereological techniques in hamsters to estimate the number of ORNs associated with each channel in the nose and the number of their targets (glomeruli and mitral and tufted cells) in corresponding divisions of the MOB. Each circumferential half of the MOB (dorsal/ventral, medial/lateral) contained about 50% of the 3,100 glomeruli and about 50% of the 160,000 mitral and tufted cells per bulb. We found equivalent numbers of ORNs with dendritic knobs in the medial and lateral channels (4.5 million each). However, the central channels had only 2 million knobbed ORNs, whereas the peripheral channels had 7 million. Thus, there is a disproportionate mapping of the central-peripheral axis of olfactory airspace onto the dorsal-ventral axis of the MOB, encompassing a greater than threefold variation in the average convergence of ORNs onto MOB secondary neurons. We hypothesize that the disproportionate projections help to optimize chemospecific processing by compensating, with differing sensitivity, for significant variation in the distribution and concentration of odorant molecules along the olfactory air channels during sniffing.

Relation between stimulus and response in frog olfactory receptor neurons in vivo

The spiking activity of receptor neurons was recorded extracellularly in the frog olfactory epithelium in response to four odourants applied at precisely controlled concentrations. A set of criteria was formulated to define the spikes in the response. Four variables - latency, duration, number of interspike intervals and frequency - were determined to quantify the responses. They were studied at the single neuron, neuron population and ciliary membrane levels. The dose-response curves were determined using specific functions and their characteristics were evaluated. The characteristic molar concentrations at threshold or at maximum duration and the characteristics of variables, e.g. minimum latency or maximum frequency, have asymmetric histograms with peaks close to the origin and long tails. Dynamic ranges have even more asymmetric histograms, so that a significant fraction of neurons presents a much wider range than their one-decade peak. From these histograms, response properties of the whole neuron population can be inferred. In general, location along the concentration axis (thresholds), width (dynamic ranges) and heights of dose-response curves are independent, which explains the diversity of curves, prevents their global categorization and supports the qualitative coding of odourants. No evidence for odourant-independent types of neurons was found. Finally, receptor activation and ciliary membrane conductance were reconstructed in the framework of a model based on firing data, known mucus biochemical and neuron morpho-electrical characteristics. It is in agreement with independent determinations of Kd of odourant-receptor interaction and of conductance characteristics, and describes their statistical distributions in the neuron population.

Spike encoding of olfactory receptor cells

Olfaction begins with the transduction of the information carried by odorants into electrical signals in olfactory receptor cells (ORCs). The binding of odor molecules to specific receptor proteins on the ciliary surface of ORCs induces the receptor potentials. This initial excitation causes a slow and graded depolarizing voltage change, which is encoded into a train of action potentials. Action potentials of ORCs are generated by voltage-gated Na+ currents and T-type Ca2+ currents in the somatic membrane. Isolated ORCs, which have lost their cilia during the dissociation procedure, are known to exhibit spike frequency accommodation by injecting the steady current. This raises the possibility that somatic ionic channels in ORCs may serve for odor adaptation at the level of spike encoding, although odor adaptation is mainly accomplished by the ciliary transduction machinery. This review discusses current knowledge concerning the mechanisms of spike generation in ORCs. It also reviews how neurotransmitters and hormones modulate ionic currents and action potentials in ORCs.

Olfactory signal transduction in the mouse septal organ

The septal organ, a distinct chemosensory organ observed in the mammalian nose, is essentially a small island of olfactory neuroepithelium located bilaterally at the ventral base of the nasal septum. Virtually nothing is known about its physiological properties and function. To understand the nature of the sensory neurons in this area, we studied the mechanisms underlying olfactory signal transduction in these neurons. The majority of the sensory neurons in the septal organ express olfactory-specific G-protein and adenylyl cyclase type III, suggesting that the cAMP signaling pathway plays a critical role in the septal organ as in the main olfactory epithelium (MOE). This is further supported by patch-clamp recordings from individual dendritic knobs of the sensory neurons in the septal organ. Odorant responses can be mimicked by an adenylyl cyclase activator and a phosphodiesterase inhibitor, and these responses can be blocked by an adenylyl cyclase inhibitor. There is a small subset of cells in the septal organ expressing a cGMP-stimulated phosphodiesterase (phosphodiesterase 2), a marker for the guanylyl cyclase-D subtype sensory neurons identified in the MOE. The results indicate that the septal organ resembles the MOE in major olfactory signal transduction pathways, odorant response properties, and projection to the main olfactory bulb. Molecular and functional analysis of the septal organ, which constitutes approximately 1% of the olfactory epithelium, will provide new insights into the organization of the mammalian olfactory system and the unique function this enigmatic organ may serve.

Whole-cell recording from honeybee olfactory receptor neurons: ionic currents, membrane excitability and odourant response in developing workerbee and drone

Whole-cell recording techniques were used to characterize ionic membrane currents and odourant responses in honeybee olfactory receptor neurons (ORNs) in primary cell culture. ORNs of workerbee (female) and drone (male) were isolated at an early stage of development before sensory axons connect to their target in the antennal lobe. The results collectively indicate that honeybee ORNs have electrical properties similar, but not necessarily identical to, those currently envisaged for ORNs of other species. Under voltage clamp at least four ionic currents could be distinguished. Inward currents were made of a fast transient, tetrodotoxin-sensitive sodium current. In some ORNs a cadmium-sensitive calcium current was detected. ORNs showed heterogeneity in their outward currents: either outward currents were made of a delayed rectifier type potassium current, which was partially blocked by tetraethyl ammonium or quinidine, or were composed of a delayed rectifier type and a transient calcium-dependent potassium current, which was cadmium-sensitive and abolished by removal of external calcium. The proportion of each of the two outward currents, however, was different within the ORNs of the two sexes suggesting a gender-specific functional heterogeneity. ORNs showed heterogeneity in action potential firing properties: depolarizing current steps elicited either one action potential or, as in most of the cells, it led to repetitive spiking. Action potentials were tetrodotoxin-sensitive suggesting they are carried by sodium. Odourant stimulation with different mixtures and pure substances evoked depolarizing receptor potentials with superimposed action potentials when spike threshold was reached. In summary, honeybee ORNs are remarkably mature at early stages in their development.

A psychophysical study of the relationship between temporal processing in odor mixtures and transduction pathways

Depending on the odorant, transduction during the olfactory reception process is reported to be mediated by the second messengers cyclic adenosine 3',5'-monophosphate (cAMP) or inositol 1,4,5-triphosphate (IP(3)). The present study with humans investigates the relationship between temporal processing in mixtures and the type of transduction process used. The most common outcomes were reciprocal temporal interactions which were primarily dependent on odorant concentration and independent of the type of transduction process. The results are consistent with the bulk of evidence that each receptor neuron commonly has only one type of receptor.

Spiking properties of olfactory receptor cells in the slice preparation

The whole-cell, patch clamp [corrected] method was applied to olfactory receptor cells in slice preparations made from bullfrog olfactory epithelium. Under voltage-clamp conditions, olfactory receptor cells showed a transient inward current followed by a steady outward current in response to depolarizing voltage steps, as has been shown in the isolated preparation. The input resistance was 5.4 +/- 3.9 GOmega and capacitance 21.9 +/- 9.7 pF. Under current-clamp conditions, depolarization of cells by current injection induced action potentials. In 13 out of 20, spike generation was repetitive with a maximum frequency of 24 Hz. The frequency of the repetitive discharges increased as the injected current was increased. The relationship between the size of the injected current and firing frequency could be well fitted by the Michaelis-Menten equation, indicating that the spike generation site lacks the non-linear boosting system. The slice preparation developed here would provide a powerful tool to study the spike encoding system of the olfactory receptor cells.

OSCILLATIONS OF THE TRANSEPITHELIAL POTENTIAL OF MOTH OLFACTORY SENSILLA ARE INFLUENCED BY OCTOPAMINE AND SEROTONIN

The biogenic amine octopamine is known to enhance the sensitivity of male moths to their species-specific pheromones in flight-tunnel experiments. This sensitization of pheromone-guided upwind flight is at least partly due to octopamine-dependent increases in the peak nerve impulse frequency of the pheromone response of olfactory receptor neurons. It is not known, however,whether octopamine exerts its effects directly on the electrical properties of the olfactory receptor neurons or indirectly, via modulation of the accessory cells of the sensillum.

In extracellular tip recordings of pheromone-dependent trichoid sensilla on the antennae of male Manduca sexta moths, we investigated the effects of octopamine and serotonin on the transepithelial potential, which is generated by the activity of V-ATPases in sensillar accessory cells. In addition, the action potential activity of unstimulated olfactory receptor neurons was examined in the presence of biogenic amines. Under constant environmental conditions, the transepithelial potential oscillated regularly with periods of 2-8 min and with a 1-25 mV peak-to-peak amplitude over periods of several hours. These oscillatory intervals were interrupted by periods of relatively stable transepithelial potential, correlated with flight activity by the moth. Octopamine reduced the amplitude of the transepithelial potential oscillation and decreased the resistance of the sensillum preparation in a dose-dependent manner. Serotonin altered the waveform of the transepithelial potential, but did not change the resistance of the preparation. Thus, both amines affect the accessory cells, but have different targets in the regulation of the transepithelial potential. Neither amine significantly influenced the spontaneous action potential activity of the olfactory receptor neurons.

Response properties of isolated mouse olfactory receptor cells

Response properties of isolated mouse olfactory receptor cells were investigated using the suction pipette technique. Cells were exposed to the odour cineole or to solutions of modified ionic content by rapidly changing the solution superfusing the cilia. All experiments were performed at 37 degrees C. Mouse olfactory receptor cells displayed a steep dependence of action potential frequency on stimulus concentration, a 3-fold increase in stimulus concentration often saturating the firing frequency at 200-300 Hz. The receptor current increased more gradually with increasing cineole concentration and did not saturate within the 100-fold range of cineole concentrations applied. When stimulated for 30 s with a low odour concentration, cells responded with sporadic spike firing. Higher concentrations led to the generation of a large receptor current at the onset of stimulation which returned to baseline levels within a few seconds, accompanied during its rising phase by a short burst of action potentials. Thereafter an oscillating response pattern was observed during the remainder of the stimulus, consisting of repetitive increases in receptor current of around 1 s duration accompanied by short bursts of action potentials. Olfactory adaptation was studied by comparing the responses to two closely spaced odour stimuli. The response to the second odour stimulus recovered to 80% of its original magnitude when the cell was superfused with Ringer solution during the 5 s interval between odour exposures. In contrast, exposure to a choline-substituted low Na+ solution between odour stimuli had two effects. First, the receptor current response to the first odour stimulus did not terminate as quickly as in the presence of Na+, suggesting the presence of a Na+ -Ca2+ exchanger. Second, the response to the second stimulus only recovered to 55% of its original magnitude, demonstrating the involvement of Na+-Ca2+ exchange in the recovery of sensitivity in mouse olfactory receptor cells following stimulation.

Functional mosaic organization of mouse olfactory receptor neurons

In contrast to rapid progress in the molecular biology of olfaction, there are few physiological data to characterize the odor response properties of different populations of olfactory receptor neurons (ORNs) and their spatial distributions across the epithelium, which is essential for understanding the coding mechanisms underlying odor discrimination and recognition. We have tested the hypothesis that the ORNs are arranged in a functional mosaic, using an intact epithelial preparation from the mouse, in which odor responses of many ORNs in situ can be monitored simultaneously with calcium imaging techniques. ORNs responding to a given odor were widely distributed across epithelium and intermingled with ORNs responding to other odors. Tight clusters of ORNs responding to the same odor were observed. For a given odor, more ORNs were recruited when the concentration was increased. ORNs were able to distinguish between pairs of enantiomers by showing distinct but somewhat overlapping patterns. The results provide evidence regarding the response spectra of ORNs in situ, supporting the combinatorial coding of odor quality and intensity by different ORN subsets.