Initial Characterization of a Subpopulation of Inherent Oscillatory Mammalian Olfactory Receptor Neurons.

Published evidence suggests that inherent rhythmically active or "bursting" primary olfactory receptor neurons (bORNs) in crustaceans have the previously undescribed functional property of encoding olfactory information by having their rhythmicity entrained by the odor stimulus. In order to determine whether such bORN-based encoding is a fundamental feature of olfaction that extends beyond crustaceans, we patch-clamped bORN-like ORNs in mice, characterized their dynamic properties, and show they align with the dynamic properties of lobster bORNs. We then characterized bORN-like activity by imaging the olfactory epithelium of OMP-GCaMP6f mice. Next, we showed rhythmic activity is not dependent upon the endogenous OR by patching ORNs in OR/GFP mice. Lastly, we showed the properties of bORN-like ORNs characterized in mice generalize to rats. Our findings suggest encoding odor time should be viewed as a fundamental feature of olfaction with the potential to be used to navigate odor plumes in animals as diverse as crustaceans and mammals.

Intermittency coding in the primary olfactory system: a neural substrate for olfactory scene analysis.

The spatial and temporal characteristics of the visual and acoustic sensory input are indispensable attributes for animals to perform scene analysis. In contrast, research in olfaction has focused almost exclusively on how the nervous system analyzes the quality and quantity of the sensory signal and largely ignored the spatiotemporal dimension especially in longer time scales. Yet, detailed analyses of the turbulent, intermittent structure of water- and air-borne odor plumes strongly suggest that spatio-temporal information in longer time scales can provide major cues for olfactory scene analysis for animals. We show that a bursting subset of primary olfactory receptor neurons (bORNs) in lobster has the unexpected capacity to encode the temporal properties of intermittent odor signals. Each bORN is tuned to a specific range of stimulus intervals, and collectively bORNs can instantaneously encode a wide spectrum of intermittencies. Our theory argues for the existence of a novel peripheral mechanism for encoding the temporal pattern of odor that potentially serves as a neural substrate for olfactory scene analysis.

Quantifying bursting neuron activity from calcium signals using blind deconvolution.

Advances in calcium imaging have enabled studies of the dynamic activity of both individual neurons and neuronal assemblies. However, challenges, such as unknown nonlinearities in the spike-calcium relationship, noise, and the often relatively low temporal resolution of the calcium signal compared to the time-scale of spike generation, restrict the accurate estimation of action potentials from the calcium signal. Complex neuronal discharge, such as the activity demonstrated by bursting and rhythmically active neurons, represents an even greater challenge for reconstructing spike trains based on calcium signals. We propose a method using blind calcium signal deconvolution based on an information-theoretic approach. This model is meant to maximise the output entropy of a nonlinear filter where the nonlinearity is defined by the cumulative distribution function of the spike signal. We tested our maximum entropy (ME) algorithm using bursting olfactory receptor neurons (bORNs) of the lobster olfactory organ. The advantage of the ME algorithm is that the filter can be trained online based only on the statistics of the spike signal, without any assumptions regarding the unknown transfer function characterizing the relation between the spike and calcium signal. We show that the ME method is able to more accurately reconstruct the timing of the first and last spikes of a burst compared to other methods and that it improves the temporal precision fivefold compared to direct timing resolution of calcium signal.

Gene expression and specificity in the mature zone of the lobster olfactory organ.

The lobster olfactory organ is an important model for investigating many aspects of the olfactory system. To facilitate study of the molecular basis of olfaction in lobsters, we made a subtracted cDNA library from the mature zone of the olfactory organ of Homarus americanus, the American lobster. Sequencing of the 5'-end of 5,184 cDNA clones produced 2,389 distinct high-quality sequences consisting of 1,944 singlets and 445 contigs. Matches to known sequences corresponded with the types of cells present in the olfactory organ, including specific markers of olfactory sensory neurons, auxiliary cells, secretory cells of the aesthetasc tegumental gland, and epithelial cells. The wealth of neuronal mRNAs represented among the sequences reflected the preponderance of neurons in the tissue. The sequences identified candidate genes responsible for known functions and suggested new functions not previously recognized in the olfactory organ. A cDNA microarray was designed and tested by assessing mRNA abundance differences between two of the lobster's major chemosensory structures: the mature zone of the olfactory organ and the dactyl of the walking legs, a taste organ. The 115 differences detected again emphasized the abundance of neurons in the olfactory organ, especially a cluster of mRNAs encoding cytoskeletal-associated proteins and cell adhesion molecules such as 14-3-3zeta, actins, tubulins, trophinin, Fax, Yel077cp, suppressor of profilin 2, and gelsolin.

Presynaptic inhibition of olfactory receptor neurons in crustaceans.

Presynaptic inhibition of transmitter release from primary sensory afferents is a common strategy for regulating sensory input to the arthropod central nervous system. In the olfactory system, presynaptic inhibition of olfactory receptor neurons has been long suspected, but until recently could not be demonstrated directly because of the difficulty in recording from the afferent nerve terminals. A preparation using the isolated but intact brain of the spiny lobster in combination with voltage-sensitive dye staining has allowed stimulus-evoked responses of olfactory receptor axons to be recorded selectively with optical imaging methods. This approach has provided the first direct physiological evidence for presynaptic inhibition of olfactory receptor neurons. As in other arthropod sensory systems, the cellular mechanism underlying presynaptic afferent inhibition appears to be a reduction of action potential amplitude in the axon terminal. In the spiny lobster, two inhibitory transmitters, GABA and histamine, can independently mediate presynaptic inhibition. GABA- and histaminergic interneurons in the lobster olfactory lobe (the target of olfactory receptor neurons) constitute dual, functionally distinct inhibitory pathways that are likely to play different roles in regulating primary olfactory input to the CNS. Presynaptic inhibition in the vertebrate olfactory system is also mediated by dual inhibitory pathways, but via a different cellular mechanism. Thus, it is possible that presynaptic inhibition of primary olfactory afferents evolved independently in vertebrates and invertebrates to fill a common, fundamental role in processing olfactory information.

A simple method for focally delivering multiple drugs or ligands to cells.

A simple venturi-based "spritzer" device is described that is capable of repeatedly and rapidly delivering small volumes of different solutions to cells or tissues. This is particularly advantageous in situations when the appropriate ligand or drug is not known a priori and has to be determined "on the fly" during the limited lifetime of a preparation. The utility of the device is demonstrated for applying odorants to olfactory receptor neurons (ORNs).