Rat Olfactory Bulb Research Articles

ZnO Nanoparticles-Induced MRI Alterations to the Rat Olfactory Epithelium and Olfactory Bulb after Intranasal Instillation.

Since zinc oxide (ZnO) nanoparticles (NPs) have been widely applied, the nano community and the general public have paid great attention to the toxicity of ZnO NPs. We detected 20-nm ZnO NPs biotoxicity following nasal exposure utilizing the non-invasive and real-time magnetic resonance imaging (MRI) technique. MR images were scanned in the rat olfactory epithelium (OE) and olfactory bulb (OB) on a 4.7 T scanner following the treatment (as early as 1 day and up to 21 days after), and the histological changes were evaluated. The influence of the size of the ZnO NPs and chemical components was also investigated. Our study revealed that 20-nm ZnO NPs induced obvious structural disruption and inflammation in the OE and OB at the acute stage. The results suggest that the real-time and non-invasive advantages of MRI allow it to observe and assess, directly and dynamically, the potential toxicity of long-term exposure to ZnO NPs in the olfactory system. These findings indicate the size-dependent toxicity of ZnO NPs with respect to the olfactory bulb. Further study is needed to reveal the mechanism behind ZnO NPs' toxicity.

EphA4 Targeting Peptide-Conjugated Extracellular Vesicles Rejuvenates Adult Neural Stem Cells and Exerts Therapeutic Benefits in Aging Rats.

Aging and various neurodegenerative diseases cause significant reduction in adult neurogenesis and simultaneous increase in quiescent neural stem cells (NSCs), which impact the brain's regenerative capabilities. To deal with this challenging issue, current treatments involve stem cell transplants or prevention of neurodegeneration; however, the efficacy or success of this process remains limited. Therefore, extensive and focused investigation is highly demanding to overcome this challenging task. Here, we have designed an efficient peptide-based EphA4 receptor-targeted ligand through an in silico approach. Further, this strategy involves chemical conjugation of the peptide with adipose tissue stem cell-derived EV (Exo-pep-11). Interestingly, our newly designed engineered EV, Exo-pep-11, targets NSC through EphA4 receptors, which offers promising therapeutic advantages by stimulating NSC proliferation and subsequent differentiation. Our result demonstrates that NSC successfully internalized Exo-pep-11 in both in vitro culture conditions as well as in the in vivo aging rats. We found that the uptake of Exo-pep-11 decreased by ∼2.3-fold when NSC was treated with EphA4 antibody before Exo-pep-11 incubation, which confirms the receptor-specific uptake of Exo-pep-11. Exo-pep-11 treatment also increases NSC proliferation by ∼1.9-fold and also shows ∼1.6- and ∼2.4-fold increase in expressions of Nestin and ID1, respectively. Exo-pep-11 also has the potential to increase neurogenesis in aging rats, which is confirmed by ∼1.6- and ∼1.5-fold increases in expressions of TH and Tuj1, respectively, in rat olfactory bulb. Overall, our findings highlight the potential role of Exo-pep-11 for prospective applications in combating age-related declines in NSC activity and neurogenesis.

Neurons in rat olfactory bulb ‘feel the pulse’

Neurons in rat olfactory bulb ‘feel the pulse’

Vasopressin differentially modulates the excitability of rat olfactory bulb neuron subtypes.

Vasopressin (VP) plays a crucial role in social memory even at the level of the olfactory bulb (OB), where OB VP cells are activated during social interactions. However, it remains unclear how VP modulates olfactory processing to enable enhanced discrimination of very similar odors, e.g., rat body odors. Thus far, it has been shown that VP reduces firing rates in mitral cells (MCs) during odor presentation in vivo and decreases the amplitudes of olfactory nerve-evoked excitatory postsynaptic potentials (ON-evoked EPSPs) in external tufted cells in vitro. We performed whole-cell patch-clamp recordings and population Ca2+ imaging on acute rat OB slices. We recorded ON-evoked EPSPs as well as spontaneous inhibitory postsynaptic currents (IPSCs) from two types of projection neurons: middle tufted cells (mTCs) and MCs. VP bath application reduced the amplitudes of ON-evoked EPSPs and the frequencies of spontaneous IPSCs in mTCs but did not change those in MCs. Therefore, we analyzed ON-evoked EPSPs in inhibitory interneurons, i.e., periglomerular cells (PGCs) and granule cells (GCs), to search for the origin of increased inhibition in mTCs. However, VP did not increase the amplitudes of evoked EPSPs in either type of interneurons. We next performed two-photon population Ca2+ imaging in the glomerular layer and the superficial GC layer of responses to stronger ON stimulation than during patch-clamp experiments that should evoke action potentials in the measured cells. We observed that VP application increased ON-evoked Ca2+ influx in juxtaglomerular cells and GC somata. Thus, our findings indicate inhibition by VP on projection neurons via strong ON input-mediated inhibitory interneuron activity. This neural modulation could improve representation of odors, hence, better discriminability of similar odors, e.g., conspecific body odors.

Odor modality is transmitted to cortical brain regions from the olfactory bulb.

Odor perception is the impetus for important animal behaviors with two predominate modes of processing: odors pass through the front of the nose (orthonasal) while inhaling and sniffing, or through the rear (retronasal) during exhalation and while eating. Despite the importance of olfaction for an animal's well-being and that ortho and retro naturally occur, it is unknown how the modality (ortho vs. retro) is even transmitted to cortical brain regions, which could significantly affect how odors are processed and perceived. Using multielectrode array recordings in tracheotomized anesthetized rats, which decouples ortho-retro modality from breathing, we show that mitral cells in rat olfactory bulb can reliably and directly transmit orthonasal versus retronasal modality with ethyl butyrate, a common food odor. Drug manipulations affecting synaptic inhibition via GABAA lead to worse decoding of ortho versus retro, independent of whether overall inhibition increases or decreases, suggesting that the olfactory bulb circuit may naturally favor encoding this important aspect of odors. Detailed data analysis paired with a firing rate model that captures population trends in spiking statistics shows how this circuit can encode odor modality. We have not only demonstrated that ortho/retro information is encoded to downstream brain regions but also used modeling to demonstrate a plausible mechanism for this encoding; due to synaptic adaptation, it is the slower time course of the retronasal stimulation that causes retronasal responses to be stronger and less sensitive to inhibitory drug manipulations than orthonasal responses.NEW & NOTEWORTHY Whether ortho (sniffing odors) versus retro (exhalation and eating) is encoded from the olfactory bulb to other brain areas is not completely known. Using multielectrode array recordings in anesthetized rats, we show that the olfactory bulb transmits this information downstream via spikes. Altering inhibition degrades ortho/retro information on average. We use theory and computation to explain our results, which should have implications on cortical processing considering that only food odors occur retronasally.

Towards detection of cancer biomarkers in human exhaled air by transfer-learning-powered analysis of odor-evoked calcium activity in rat olfactory bulb

Detection of volatile organic compounds in exhaled air is a promising approach to non-invasive and scalable gastric cancer screening. This work proposes a new approach for the detection of volatile organic compounds by analyzing odor-evoked calcium responses in the rat olfactory bulb. We estimate the feasibility of gastric cancer biomarker detection added to the exhaled air of healthy participants. Our detector consists of a convolutional encoder and a similarity-based classifier over encoder outputs. To minimize overfitting on a small available training set, we involve a pre-training where the encoder is trained on synthetic data representing spatiotemporal patterns similar to real calcium responses in the olfactory bulb. We estimate the classification accuracy of exhaled air samples by matching their encodings with encodings of calibration samples of two classes: 1) exhaled air and 2) a mixture of exhaled air with the cancer biomarker. On our data, the accuracy increased from 0.68 on real data up to 0.74 if pre-training on synthetic data is involved. Our work is focused on proving the feasibility of proposed new approach rather than on comparing its efficiency with existing methods. Such detection is often performed with an electronic nose, but its output becomes unstable over time due to a sensor drift. In contrast to the electronic nose, rats can robustly detect low concentrations of biomarkers over lifetime. The feasibility of gastric cancer biomarker detection in exhaled air by bio-hybrid system is shown. Pre-training of neural models for images analysis increases the accuracy of detection.

Curcumin-activated Olfactory Ensheathing Cells Improve Functional Recovery After Spinal Cord Injury by Modulating Microglia Polarization Through APOE/TREM2/NF-κB Signaling Pathway

Transplantation of curcumin-activated olfactory ensheathing cells (aOECs) improved functional recovery in spinal cord injury (SCI) rats. Nevertheless, little is known considering the underlying mechanisms. At the present study, we investigated the promotion of regeneration and functional recovery after transplantation of aOECs into rats with SCI and the possible underlying molecular mechanisms. Primary OECs were prepared from the olfactory bulb of rats, followed by treatment with 1µM CCM at 7–10 days of culture, resulting in cell activation. Concomitantly, rat SCI model was developed to evaluate the effects of transplantation of aOECs in vivo. Subsequently, microglia were isolated, stimulated with 100 ng/mL lipopolysaccharide (LPS) for 24 h to polarize to M1 phenotype and treated by aOECs conditional medium (aOECs-CM) and OECs conditional medium (OECs-CM), respectively. Changes in the expression of pro-inflammatory and anti-inflammatory phenotypic markers expression were detected using western blotting and immunofluorescence staining, respectively. Finally, a series of molecular biological experiments including knock-down of triggering receptor expressed on myeloid cells 2 (TREM2) and analysis of the level of apolipoprotein E (APOE) expression were performed to investigate the underlying mechanism of involvement of CCM-activated OECs in modulating microglia polarization, leading to neural regeneration and function recovery. CCM-activated OECs effectively attenuated deleterious inflammation by regulating microglia polarization from the pro-inflammatory (M1) to anti-inflammatory (M2) phenotype in SCI rats and facilitated functional recovery after SCI. In addition, microglial polarization to M2 elicited by aOECs-CM in LPS-induced microglia was effectively reversed when TREM2 expression was downregulated. More importantly, the in vitro findings indicated that aOECs-CM potentiating LPS-induced microglial polarization to M2 was partially mediated by the TREM2/nuclear factor kappa beta (NF-κB) signaling pathway. Besides, the expression of APOE significantly increased in CCM-treated OECs. CCM-activated OECs could alleviate inflammation after SCI by switching microglial polarization from M1 to M2, which was likely mediated by the APOE/TREM2/NF-κB pathway, and thus ameliorated neurological function. Therefore, the present finding is of paramount significance to enrich the understanding of underlying molecular mechanism of aOECs-based therapy and provide a novel therapeutic approach for treatment of SCI.

The method for assessment of local permutations in the glomerular patterns of the rat olfactory bulb by aligning interindividual odor maps.

The comparison of odor functional maps in rodents demonstrates a high degree of inter-individual variability in glomerular activity patterns. There are substantial methodological difficulties in the interindividual assessment of local permutations in the glomerular patterns, since the position of anatomical extracranial landmarks, as well as the size, shape and angular orientation of olfactory bulbs can vary significantly. A new method for defining anatomical coordinates of active glomeruli in the rat olfactory bulb has been developed. The method compares the interindividual odor functional maps and calculates probabilistic maps of glomerular activity with adjustment. This adjustment involves rotation, scaling and shift of the functional map relative to its expected position in probabilistic map, computed according to the anatomical coordinates. The calculation of the probabilistic map of the odorant-specific response compensates for potential anatoamical errors due to individual variability in olfactory bulb dimensions and angular orientation. We show its efficiency on real data from a large animal sample recorded by two-photon calcium imaging in dorsal surface of the rat olfactory bulb. The proposed method with probabilistic map calculation enables the spatial consistency of the effects of individual odorants in different rats to be assessed and allow stereotypical positions of odor-specific clusters in the glomerular layer of the olfactory bulb to be identified.

Biohybrid Technology for the Detection of Ultralow Concentrations of Trinitrotoluene in Air

The technology is based on recording the focal activity (FA) of the olfactory bulb (OB) of rats upon the exposure rats to trinitrotoluene (TNT) vapors in the concentration 4.7 × 10–15 g/cm3, separately and in a mixture with saturated vapors of a complex interference composed of spices. The focal activity of the rat olfactory bulb was recorded using an array of 16 special electrodes implanted in the dorsal part of the rat olfactory bulb. The setup contained a box with a rat, sources of vapors (odorants), a multichannel digital system for recording electrical signals from an electrode array, and software performing algorithms for recognizing and classifying odorants presented to the rat. One source of TNT vapors was a device for batch volumetric preparation with a concentration of about 10–15 g/cm3, and another source was a source with a concentration of 4.7 × 10–15 g/cm3 in an air flow presented to rats. Data from 25 tests showed a 100% probability of detecting TNT vapors of the specified concentrations in pure air and also in the presence of a complex odor interference in the form of vapors from a mixture of red pepper, coriander, tobacco, etc., which, as a rule, mask the smell of TNT for animals.The technology is based on recording the focal activity (FA) of the olfactory bulb (OB) of rats upon the exposure rats to trinitrotoluene (TNT) vapors in the concentration 4.7 × 10–15 g/cm3, separately and in a mixture with saturated vapors of a complex interference composed of spices. The focal activity of the rat olfactory bulb was recorded using an array of 16 special electrodes implanted in the dorsal part of the rat olfactory bulb. The setup contained a box with a rat, sources of vapors (odorants), a multichannel digital system for recording electrical signals from an electrode array, and software performing algorithms for recognizing and classifying odorants presented to the rat. One source of TNT vapors was a device for batch volumetric preparation with a concentration of about 10–15 g/cm3, and another source was a source with a concentration of 4.7 × 10–15 g/cm3 in an air flow presented to rats. Data from 25 tests showed a 100% probability of detecting TNT vapors of the specified concentrations in pure air and also in the presence of a complex odor interference in the form of vapors from a mixture of red pepper, coriander, tobacco, etc., which, as a rule, mask the smell of TNT for animals.

Biohybrid Technology for the Detection of Ultralow Concentrations of Trinitrotoluene in Air

The technology is based on recording the focal activity (FA) of the olfactory bulb (OB) of rats upon the exposure rats to trinitrotoluene (TNT) vapors in the concentration 4.7 × 10–15 g/cm3, separately and in a mixture with saturated vapors of a complex interference composed of spices. The focal activity of the rat olfactory bulb was recorded using an array of 16 special electrodes implanted in the dorsal part of the rat olfactory bulb. The setup contained a box with a rat, sources of vapors (odorants), a multichannel digital system for recording electrical signals from an electrode array, and software performing algorithms for recognizing and classifying odorants presented to the rat. One source of TNT vapors was a device for batch volumetric preparation with a concentration of about 10–15 g/cm3, and another source was a source with a concentration of 4.7 × 10–15 g/cm3 in an air flow presented to rats. Data from 25 tests showed a 100% probability of detecting TNT vapors of the specified concentrations in pure air and also in the presence of a complex odor interference in the form of vapors from a mixture of red pepper, coriander, tobacco, etc., which, as a rule, mask the smell of TNT for animals.

Changes in the Spectral Characteristics and Level of Coherence of Local Field Potentials in the Rat Olfactory Bulb during Xylazine-Tiletamine-Zolazepam Anesthesia

Changes in the Spectral Characteristics and Level of Coherence of Local Field Potentials in the Rat Olfactory Bulb during Xylazine-Tiletamine-Zolazepam Anesthesia

N-(levodopa) chitosan derivative based on click chemistry shows biological functionality in brain cells

Objective To develop N-(levodopa) chitosan derivatives through click chemistry to study their effect in brain cells. Significance: This study presents a proof-of-concept that macromolecules such as N-(Levodopa) chitosan derivatives traverse brain cell membranes and induce biomedical functionalities. Methods Through click chemistry, we developed N-(levodopa) chitosan derivatives. They were physically and chemically characterized by FT-IR, 1H-NMR, TGA and Dynamic Light Scattering analyses. Solution and nanoparticles of N-(levodopa) chitosan derivatives were tested in primary cell cultures from the postnatal rat olfactory bulb, substantia nigra and corpus callosum. Ca2+ imaging and UPLC experiments were used to investigate if the biomaterial modulated the brain cell physiology. Results N-(levodopa) chitosan derivatives induced intracellular Ca2+ responses in primary cell cultures of the rat brain. UPLC experiments indicated that levodopa attached to chitosan was converted into dopamine by brain cells. Conclusion The present study shows that N-(levodopa) chitosan may be useful to develop new treatment strategies, which could serve as molecular reservoirs of biomedical drugs to treat degenerative disorders of the nervous system.

Measurement of neuro-energetics and neurotransmission in the rat olfactory bulb using 1H and 1H-[13C] NMR spectroscopy.

The olfactory bulb (OB) plays a fundamental role in the sense of smell and has been implicated in several pathologies, including Alzheimer's disease. Despite its importance, high metabolic activity and unique laminar architecture, the OB is not frequently studied using MRS methods, likely due to the small size and challenging location. Here we present a detailed metabolic characterization of OB metabolism, in terms of both static metabolite concentrations using 1H MRS and metabolic fluxes associated with neuro-energetics and neurotransmission by tracing the dynamic 13C flow from intravenously administered [1,6-13C2]-glucose, [2-13C]-glucose and [2-13C]-acetate to downstream metabolites, including [4-13C]-glutamate, [4-13C]-glutamine and [2-13C]-GABA. The unique laminar architecture and associated metabolism of the OB, distinctly different from that of the cerebral cortex, is characterized by elevated GABA and glutamine levels, as well as increased GABAergic and astroglial energy metabolism and neurotransmission. The results show that, despite the technical challenges, high-quality 1H and 1H-[13C] MR spectra can be obtained from the rat OB in vivo. The derived metabolite concentrations and metabolic rates demonstrate a unique metabolic profile for the OB. The metabolic model provides a solid basis for future OB studies on functional activation or pathological conditions.

Fasting alters p75NTR and AgRP mRNA expression in rat olfactory bulb and hippocampus

Classic non-homeostatic structures involved in food intake regulation are reciprocally influenced by metabolic signals. Orexigenic peptides expressed in the olfactory bulb (OB) and hippocampus (HP) modulate olfactory processing and memory, respectively. Hypothalamic circuits also modulate feeding behavior by activating and releasing Agouti-related peptide (AgRP) in response to orexigenic signals. An adequate response to fasting requires the expression of p75 neurotrophin receptor (p75NTR) in AgRP neurons. The present study aimed to determine whether there is a role for p75NTR and AgRP in the OB and HP on the feeding behavior of fasted rats. A group of fasted rats (FG) was confronted with a decision-making paradigm in a T-maze containing a standard chow pellet (CP), and the same pellet coated with a phenolic-rich avocado paste extract (AVO) on either end; their OB and HP were then analyzed with histological and molecular tools. FG rats had briefer feeding latencies, as compared to control rats fed ad libitum (median latencies: 55.4 vs 191.7 min, p = 0.032). They also had reduced cell counts in both brain structures, as compared to satiated rats. AgRP mRNA was not expressed in the HP of either group, however, it was found in the OB. p75NTR mRNA was expressed in both brain structures of FG rats. These results suggest that contrasting metabolic states (fasted or satiated) motivate different feeding responses, which are influenced by p75NTR and AgRP mRNA expression in non-homeostatic food intake brain structures.

Changes in the spectral characteristics and the coherence of the rat olfactory bulb local field potentials under xylazine-tiletamine-zolazepam anesthesia

Local field potentials (LFP) recorded in the olfactory bulb (OB) are known to be largely generated in local neural networks, are directly related to the processing of olfactory information, and are influenced by various factors, including anesthetics. Using 8-electrode arrays implanted in the dorsal regions of 6 adult male Norway rats OBs, the effects of xylazine-tiletamine-zolazepam (XTZ) anesthesia on the spectral characteristics and coherence of the LFP in the frequency range 1–150 Hz were studied in 2 hours long chronic experiments. It is shown that the most significant changes in the rat OB LFP under XTZ-anesthesia are observed in the high γ frequency band. A statistically significant increase in the power (2–4 times) and coherence (up to 50%) of this band was observed in the LFP of all animals within 10–15 minutes from the anesthesia onset. At the same time, during wakefulness, the dominant frequencies of this band were 70–80 Hz, and within 10–15 minutes from the anesthesia onset they increased to 110–130 Hz. During anesthesia, a gradual shift of these dominant frequencies to a lower range (90–110 Hz) was observed, while their total power, in contrast to the coherence, was statistically significantly reduced when the animal started recovering from anesthesia

Diabetes Induces Permanent Deleterious Effects in the Olfactory Bulb Associated with Increased Tyrosine Hydroxylase Expression and ERK1/2 Phosphorylation.

Diabetes mellitus type 2 (T2D) complications include brain damage which increases the risk of neurodegenerative diseases and dementia. An early manifestation of neurodegeneration is olfactory dysfunction (OD), which is also presented in diabetic patients. Previously, we demonstrated that OD correlates with IL-1β and miR-146a overexpression in the olfactory bulb (OB) on a T2D rodent model, suggesting the participation of inflammation on OD. Here, we found that OD persists on a long-term T2D condition after the downregulation of IL-1β. Remarkably, OD was associated with the increased expression of the dopaminergic neuronal marker tyrosine hydroxylase, ERK1/2 phosphorylation, and reduced neuronal activation on the OB of diabetic rats, suggesting the participation of the dopaminergic tone on the OD derived from T2D. Dopaminergic neurons are susceptible in neurodegenerative diseases such as Parkinson's disease; therefore further studies must be performed to completely elucidate the participation of these neurons and ERK1/2 signaling on olfactory impairment.

Changes in spontaneous and odorant-induced single-unit activity of mitral/tufted neurons of the rat olfactory bulb during xylazine-tiletamine-zolazepam anesthesia.

The nature and severity of mitral/tufted (M/T) cells reactions to odorants presented in anesthesia depend on various factors, and, above all, the nature and concentration of the odor, anesthesia, and the functional state of the olfactory bulb (OB). Compared to wakefulness, under anesthesia, the intensity of OB M/T cells responses to the odorants presented increases. However, the influence of anesthesia dynamics on the intensity of such responses has not been studied. To address this problem in rats, the activity of M/T cells and the local field potentials (LFP) in OB were recorded in the course of xylazine-tiletamine-zolazepam (XTZ) anesthesia. It has been shown that in the course of the anesthesia, the average frequency of background and odorant-induced single-unit activity of M/T cells increases, while the dominant frequency value of LFP in the gamma frequency range (90–170 Hz), on the contrary, decreases. The observed effects are assumed to be associated with changes in the functional state of the OB and systems for processing olfactory information in anesthesia.

Extraglomerular Excitation of Rat Olfactory Bulb Mitral Cells by Depolarizing GABAergic Synaptic Input.

Principal cells in the olfactory bulb (OB), mitral and tufted cells, receive direct sensory input and generate output signals that are transmitted to downstream cortical targets. Excitatory input from glutamatergic receptor neurons are the primary known sources of rapid excitation to OB principal cells. Principal cells also receive inhibitory input from local GABAergic interneurons in both the glomerular and plexiform layers. Previous work suggests that the functional effect of these inhibitory inputs, including numerous dendrodendritic synapses with GABAergic granule cells, is to reduce firing probability. In this study, we use in vitro patch-clamp recordings to demonstrate that rat (of both sexes) OB mitral cells also can be excited by GABAergic synapses formed outside the glomerular layer. Depolarizing GABAergic responses to focal extracellular stimulation were revealed when fast ionotropic glutamate receptors were blocked, and occurred with short, monosynaptic latencies. These novel synaptic responses were abolished by gabazine, bicuculline, and picrotoxin, three structurally dissimilar GABAA receptor antagonists. The likely location of depolarizing GABAergic input to mitral cells was the proximal axon based on the actions of focally applied gabazine and GABA near this region. Excitatory GABAergic synaptic responses, commonly studied in cortical brain regions, have not been reported previously in OB principal cells. Excitatory GABAergic responses promote action potential firing and provide a mechanism for mitral cells to be excited independently of olfactory sensory input.SIGNIFICANCE STATEMENT Odor stimuli generate distinctive activity patterns in olfactory bulb neurons through a combination of excitatory and inhibitory synaptic interactions. Most of the excitatory drive to each principal cell is assumed to arise from a highly restricted subset of sensory neurons. This study describes a novel second source of synaptic excitation to principal cells to arise from GABAergic inputs to the proximal axon, a common site of action potential initiation. This new pathway provides a synaptic mechanism to excite OB principal cells that is independent of the canonical excitatory sensory input contained in the glomerular layer.

Anatomical and Functional Connectivity at the Dendrodendritic Reciprocal Mitral Cell-Granule Cell Synapse: Impact on Recurrent and Lateral Inhibition.

In the vertebrate olfactory bulb, reciprocal dendrodendritic interactions between its principal neurons, the mitral and tufted cells, and inhibitory interneurons in the external plexiform layer mediate both recurrent and lateral inhibition, with the most numerous of these interneurons being granule cells. Here, we used recently established anatomical parameters and functional data on unitary synaptic transmission to simulate the strength of recurrent inhibition of mitral cells specifically from the reciprocal spines of rat olfactory bulb granule cells in a quantitative manner. Our functional data allowed us to derive a unitary synaptic conductance on the order of 0.2 nS. The simulations predicted that somatic voltage deflections by even proximal individual granule cell inputs are below the detection threshold and that attenuation with distance is roughly linear, with a passive length constant of 650 μm. However, since recurrent inhibition in the wake of a mitral cell action potential will originate from hundreds of reciprocal spines, the summated recurrent IPSP will be much larger, even though there will be substantial mutual shunting across the many inputs. Next, we updated and refined a preexisting model of connectivity within the entire rat olfactory bulb, first between pairs of mitral and granule cells, to estimate the likelihood and impact of recurrent inhibition depending on the distance between cells. Moreover, to characterize the substrate of lateral inhibition, we estimated the connectivity via granule cells between any two mitral cells or all the mitral cells that belong to a functional glomerular ensemble (i.e., which receive their input from the same glomerulus), again as a function of the distance between mitral cells and/or entire glomerular mitral cell ensembles. Our results predict the extent of the three regimes of anatomical connectivity between glomerular ensembles: high connectivity within a glomerular ensemble and across the first four rings of adjacent glomeruli, substantial connectivity to up to eleven glomeruli away, and negligible connectivity beyond. Finally, in a first attempt to estimate the functional strength of granule-cell mediated lateral inhibition, we combined this anatomical estimate with our above simulation results on attenuation with distance, resulting in slightly narrowed regimes of a functional impact compared to the anatomical connectivity.

Olfaction and Melatonin: The Use of the Olfactory Discrimination Test.

In order to investigate the role of melatonin in olfactory function, we present the olfactory discrimination test as a simple and low-cost behavioral assessment. The test consists in evaluating the time that each rat spent in two compartments: one has a familiar odor (sawdust with the smell from the animal) and the other one with an unfamiliar odor (clean sawdust). Animals with the normal olfactory functions will discriminate between these two odors and will spend more time in the familiar compartment. We used the olfactory discrimination test to evaluate the role of melatonin receptors expressed in the olfactory bulb of rats. In a previous study, our results have successfully detected an olfactory modulation, by mean of the olfactory discrimination test, promoted by the infusion of melatonin receptor ligands into the olfactory bulb of rats.