Mature and Precursor Brain-Derived Neurotrophic Factor Have Individual Roles in the Mouse Olfactory Bulb

Background: Sensory deprivation induces dramatic morphological and neurochemical changes in the olfactory bulb (OB) that are largely restricted to glomerular and granule layer interneurons. Mitral cells, pyramidal-like neurons, are resistant to sensory-deprivation-induced changes and are associated with the precursor to brain-derived neurotrophic factor (proBDNF); here, we investigate its unknown function in the adult mouse OB. Principal Findings: As determined using brain-slice electrophysiology in a whole-cell configuration, brain-derived neurotrophic factor (BDNF), but not proBDNF, increased mitral cell excitability. BDNF increased mitral cell action potential firing frequency and decreased interspike interval in response to current injection. In a separate set of experiments, intranasal delivery of neurotrophic factors to awake, adult mice was performed to induce sustained interneuron neurochemical changes. ProBDNF, but not BDNF, increased activated-caspase 3 and reduced tyrosine hydroxylase immunoreactivity in OB glomerular interneurons. In a parallel set of experiments, short-term sensory deprivation produced by unilateral naris occlusion generated an identical phenotype. Conclusions: Our results indicate that only mature BDNF increases mitral cell excitability whereas proBDNF remains ineffective. Our demonstration that proBDNF activates an apoptotic marker in vivo is the first for any proneurotrophin an

A unique olfactory bulb microcircuit driven by neurons expressing the precursor to glucagon-like peptide 1

Abstract: The presence of large numbers of local interneurons in the olfactory bulb has demonstrated an extensive local signaling process, yet the identification and purpose of olfactory microcircuits is poorly explored. Because the discrimination of odors in a complex environment is highly dependent on the tuning of information by local interneurons, we studied for the first time the role of preproglucagon (PPG) neurons in the granule cell layer of the olfactory bulb. Combining electrophysiological recordings and confocal microscopy, we discovered that the PPG neurons are a population of cells expressing the precursor of glucagon-like peptide 1 and are glutamatergic; able to modulate the firing pattern of the mitral cells (M/TCs). Optogenetic activation of PPG neurons resulted in a mixed excitation and inhibition that created a multiphasic response shaping the M/TCs firing pattern. This suggests that PPG neurons could drive neuromodulation of the olfactory output and change the synaptic map regulating olfactory coding

The incretin hormone glucagon-like peptide 1 increases mitral cell excitability by decreasing conductance of a voltage-dependent potassium channel.

KEY POINTS: The gut hormone called glucagon-like peptide 1 (GLP-1) is a strong moderator of energy homeostasis and communication between the peripheral organs and the brain. GLP-1 signalling occurs in the brain; using a newly developed genetic reporter line of mice, we have discovered GLP-synthesizing cells in the olfactory bulb. GLP-1 increases the firing frequency of neurons (mitral cells) that encode olfactory information by decreasing activity of voltage-dependent K channels (Kv1.3). Modifying GLP-1 levels, either therapeutically or following the ingestion of food, could alter the excitability of neurons in the olfactory bulb in a nutrition or energy state-dependent manner to influence olfactory detection or metabolic sensing. The results of the present study uncover a new function for an olfactory bulb neuron (deep short axon cells, Cajal cells) that could be capable of modifying mitral cell activity through the release of GLP-1. This might be of relevance for the action of GLP-1 mimetics now widely used in the treatment of diabetes. ABSTRACT: The olfactory system is intricately linked with the endocrine system where it may serve as a detector of the internal metabolic state or energy homeostasis in addition to its classical function as a sensor of external olfactory information. The recent development of transgenic mGLU-yellow fluorescent protein mice that express a genetic reporter under the control of the preproglucagon reporter suggested the presence of the gut hormone, glucagon-like peptide (GLP-1), in deep short axon cells (Cajal cells) of the olfactory bulb and its neuromodulatory effect on mitral cell (MC) first-order neurons. A MC target for the peptide was determined using GLP-1 receptor binding assays, immunocytochemistry for the receptor and injection of fluorescence-labelled GLP-1 analogue exendin-4. Using patch clamp recording of olfactory bulb slices in the whole-cell configuration, we report that GLP-1 and its stable analogue exendin-4 increase the action potential firing frequency of MCs by decreasing the interburst interval rather than modifying the action potential shape, train length or interspike interval. GLP-1 decreases Kv1.3 channel contribution to outward currents in voltage clamp recordings as determined by pharmacological blockade of Kv1.3 or utilizing mice with Kv1.3 gene-targeted deletion as a negative control. Because fluctuations in GLP-1 concentrations monitored by the olfactory bulb can modify the firing frequency of MCs, olfactory coding could change depending upon nutritional or physiological state. As a regulator of neuronal activity, GLP-1 or its analogue may comprise a new metabolic factor with a potential therapeutic target in the olfactory bulb (i.e. via intranasal delivery) for controlling an imbalance in energy homeostasis.This work was supported by NIH R01 DC013080 and DC003387 from the NIDCD, an American Heart Association (AHA) Postdoctoral Grant Award 14POST20380615, a Creative Research Council (CRC) award from FSU, a grant from the Medical Research Council, UK (MR/J013293/1) and support from the National Health and Medical Research Council of Australia, Project Grant #1025031. PPG-YFP mice, expressing the YFP variant Venus under the control of the mouse proglucagon promoter (mGLU124 line), were generated with grant support from the Wellcome Trus

Mitral Cells of the Olfactory Bulb Perform Metabolic Sensing and Are Disrupted by Obesity at the Level of the Kv1.3 Ion Channel

Sixty-five percent of Americans are over-weight. While the neuroendocrine controls of energy homeostasis are well known, how sensory systems respond to and are impacted by obesity is scantily understood. The main accepted function of the olfactory system is to provide an internal depiction of our external chemical environment, starting from the detection of chemosensory cues. We hypothesized that the system additionally functions to encode internal chemistry via the detection of chemicals that are important indicators of metabolic state. We here uncovered that the olfactory bulb (OB) subserves as an internal sensor of metabolism via insulin-induced modulation of the potassium channel Kv1.3. Using an adult slice preparation of the olfactory bulb, we found that evoked neural activity in Kv1.3-expressing mitral cells is enhanced following acute insulin application. Insulin mediated changes in mitral cell excitability are predominantly due to the modulation of Kv1.3 channels as evidenced by the lack of effect in slices from Kv1.3-null mice. Moreover, a selective Kv1.3 peptide blocker (ShK186) inhibits more than 80% of the outward current in parallel voltage-clamp studies, whereby insulin significantly decreases the peak current magnitude without altering the kinetics of inactivation or deactivation. Mice that were chronically administered insulin using intranasal delivery approaches exhibited either an elevation in basal firing frequency or fired a single cluster of action potentials. Following chronic administration of the hormone, mitral cells were inhibited by application of acute insulin rather than excited. Mice made obese through a diet of ∼32% fat exhibited prominent changes in mitral cell action potential shape and clustering behavior, whereby the subsequent response to acute insulin stimulation was either attenuated or completely absent. Our results implicate an inappropriate neural function of olfactory sensors following exposure to chronic levels of the hormone insulin (diabetes) or increased body weight (obesity)

A unique olfactory bulb microcircuit driven by neurons expressing the precursor to glucagon-like peptide 1

Abstract: The presence of large numbers of local interneurons in the olfactory bulb has demonstrated an extensive local signaling process, yet the identification and purpose of olfactory microcircuits is poorly explored. Because the discrimination of odors in a complex environment is highly dependent on the tuning of information by local interneurons, we studied for the first time the role of preproglucagon (PPG) neurons in the granule cell layer of the olfactory bulb. Combining electrophysiological recordings and confocal microscopy, we discovered that the PPG neurons are a population of cells expressing the precursor of glucagon-like peptide 1 and are glutamatergic; able to modulate the firing pattern of the mitral cells (M/TCs). Optogenetic activation of PPG neurons resulted in a mixed excitation and inhibition that created a multiphasic response shaping the M/TCs firing pattern. This suggests that PPG neurons could drive neuromodulation of the olfactory output and change the synaptic map regulating olfactory coding

Neither proBDNF nor denatured BDNF increase mitral cell excitability.

<p>(A–B) Representative action potential trains in mitral cells in response to 75 pA current injections of 1000 ms duration before (left) and approximately 10 min after (right) bath application of either 10 ng/ml proBDNF (A) or 10 ng/ml denatured BDNF (B, denBDNF). The resting membrane potential was held near −65 mV. (C–D) Line graphs of the mean (± s.e.m.) spike frequency (Hz) and of the mean (± s.e.m.) ISI at each current injection as recorded in A and B before (closed circles; black) and after (open circles; red) proBDNF (C) or denatured BDNF (D). Data were fit with linear regressions to facilitate visualization. Neither treatment had a significant effect upon either rate or ISI; two-way ANOVA (N = 4; p≥0.05).</p

BDNF increases mitral cell excitability.

<p>(A) Representative action potential trains in a mitral cell in response to various current injections of 1000 ms duration before (left) and approximately 10 min after 10 ng/ml BDNF bath application (right). The resting membrane potential was held near −65 mV. (B) Line graph of the mean (± s.e.m.) spike frequency (Hz) evoked at each current injection as recorded in A. Closed circles (black) represent data before BDNF stimulation and open circles (red) represent the same neurons after BDNF stimulation. (C) Line graph of the mean (± s.e.m.) interspike interval (ISI) at each current injection. Data were fit with linear regressions to facilitate visualization. BDNF had a significant treatment effect for both rate and ISI; two-way ANOVA (N = 4; p<0.05).</p

Five days of naris occlusion increases activated-caspase 3 immunoreactivity and reduces tyrosine hydroxylase immunoreactivity.

<p>(A) Representative photomicrographs taken of the mouse glomerular layer following five days of sham (control), contralateral to the naris occlusion (open), and ipsilateral to the naris occlusion (occluded) treatment. Cryosections were incubated with activated-caspase 3 antiserum (1∶100) (red) and DAPI nuclear stain to visualize immunoreactive glomerular cells. Bar plot of the mean (± s.e.m.) number of activated-caspase 3 immunoreactive glomerular cells per treatment condition per 93,500 µm² field of view. (B) Same as in A but cryosections were incubated with tyrosine hydroxylase (TH) antiserum (1∶4,000) (red). * = Significantly-different by a one-way ANOVA with a Student-Newman-Keuls post-hoc test (<i>snk</i>) (N = 3, α≤0.05).</p

ProBDNF intranasal delivery (IND) increases activated-caspase 3 immunoreactivity and reduces tyrosine hydroxylase immunoreactivity.

<p>(A) Representative photomicrographs taken of the mouse glomerular layer following a five day treatment of: PBS, mature BDNF, or proBDNF as administered by IND. Cryosections were incubated with activated-caspase 3 antiserum (1∶100) (red) and with DAPI nuclear stain (blue) to visualize immunoreactive glomerular cells. Bar plot of the mean (± s.e.m.) number of activated-caspase 3 immunoreactive glomerular cells per treatment condition per 93,500 µm² field of view. (B) Same as in A but cryosections were incubated with tyrosine hydroxylase (TH) antiserum (1∶4,000) (red). * = Significantly-different by a one-way ANOVA with a Student-Newman-Keuls post-hoc test (<i>snk</i>) (N = 3, α≤0.05).</p