Regulation of main olfactory bulb mitral cell excitability by metabotropic glutamate receptor mGluR1

Firing Patterns of Mitral Cells and Their Transformation in the Main Olfactory Bulb

Mitral cells (MCs) in the main olfactory bulb relay odor information to higher-order olfactory centers by encoding the information in the form of action potentials. The firing patterns of these cells are influenced by both their intrinsic properties and their synaptic connections within the neural network. However, reports on MC firing patterns have been inconsistent, and the mechanisms underlying these patterns remain unclear. Using whole-cell patch-clamp recordings in mouse brain slices, we discovered that MCs exhibit two types of integrative behavior: regular/rhythmic firing and bursts of action potentials. These firing patterns could be transformed both spontaneously and chemically. MCs with regular firing maintained their pattern even in the presence of blockers of fast synaptic transmission, indicating this was an intrinsic property. However, regular firing could be transformed into bursting by applying GABAA receptor antagonists to block inhibitory synaptic transmission. Burst firing could be reverted to regular firing by blocking ionotropic glutamate receptors, rather than applying a GABAA receptor agonist, indicating that ionotropic glutamatergic transmission mediated this transformation. Further experiments on long-lasting currents (LLCs), which generated burst firing, also supported this mechanism. In addition, cytoplasmic Ca2+ in MCs was involved in the transformation of firing patterns mediated by glutamatergic transmission. Metabotropic glutamate receptors also played a role in LLCs in MCs. These pieces of evidence indicate that odor information can be encoded on a mitral cell (MC) platform, where it can be relayed to higher-order olfactory centers through intrinsic and dendrodendritic mechanisms in MCs.

Cannabinoids Regulate Sensory Processing in Early Olfactory and Visual Neural Circuits

Our sensory systems such as the olfactory and visual systems are the target of neuromodulatory regulation. This neuromodulation starts at the level of sensory receptors and extends into cortical processing. A relatively new group of neuromodulators includes cannabinoids. These form a group of chemical substances that are found in the cannabis plant. Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) are the main cannabinoids. THC acts in the brain and nervous system like the chemical substances that our body produces, the endogenous cannabinoids or endocannabinoids, also nicknamed the brain's own cannabis. While the function of the endocannabinoid system is understood fairly well in limbic structures such as the hippocampus and the amygdala, this signaling system is less well understood in the olfactory pathway and the visual system. Here, we describe and compare endocannabinoids as signaling molecules in the early processing centers of the olfactory and visual system, the olfactory bulb, and the retina, and the relevance of the endocannabinoid system for synaptic plasticity.

Cellular and Synaptic Mechanisms That Differentiate Mitral Cells and Superficial Tufted Cells Into Parallel Output Channels in the Olfactory Bulb

A common feature of the primary processing structures of sensory systems is the presence of parallel output “channels” that convey different information about a stimulus. In the mammalian olfactory bulb, this is reflected in the mitral cells (MCs) and tufted cells (TCs) that have differing sensitivities to odors, with TCs being more sensitive than MCs. In this study, we examined potential mechanisms underlying the different responses of MCs vs. TCs. For TCs, we focused on superficial TCs (sTCs), which are a population of output TCs that reside in the superficial-most portion of the external plexiform layer, along with external tufted cells (eTCs), which are glutamatergic interneurons in the glomerular layer. Using whole-cell patch-clamp recordings in mouse bulb slices, we first measured excitatory currents in MCs, sTCs, and eTCs following olfactory sensory neuron (OSN) stimulation, separating the responses into a fast, monosynaptic component reflecting direct inputs from OSNs and a prolonged component partially reflecting eTC-mediated feedforward excitation. Responses were measured to a wide range of OSN stimulation intensities, simulating the different levels of OSN activity that would be expected to be produced by varying odor concentrations in vivo. Over a range of stimulation intensities, we found that the monosynaptic current varied significantly between the cell types, in the order of eTC > sTC > MC. The prolonged component was smaller in sTCs vs. both MCs and eTCs. sTCs also had much higher whole-cell input resistances than MCs, reflecting their smaller size and greater membrane resistivity. To evaluate how these different electrophysiological aspects contributed to spiking of the output MCs and sTCs, we used computational modeling. By exchanging the different cell properties in our modeled MCs and sTCs, we could evaluate each property's contribution to spiking differences between these cell types. This analysis suggested that the higher sensitivity of spiking in sTCs vs. MCs reflected both their larger monosynaptic OSN signal as well as their higher input resistance, while their smaller prolonged currents had a modest opposing effect. Taken together, our results indicate that both synaptic and intrinsic cellular features contribute to the production of parallel output channels in the olfactory bulb.

Muscarinic Modulation of Antennal Lobe GABAergic Local Neurons Shapes Odor Coding and Behavior

In the antennal lobe (AL), the first olfactory relay of Drosophila, excitatory neurons are predominantly cholinergic. Ionotropic nicotinic receptors play a vital role in the effects of acetylcholine in the AL. However, the AL also has a high expression level of metabotropic muscarinic acetylcholine receptors type A (mAChRs-A). Nevertheless, the neurons expressing them and their role in the AL are unknown. Elucidating their function may reveal principles in olfactory modulation. Here, we show that mAChRs-A shape AL output and affect behavior. We localized mAChRs-A effects to a sub-population of GABAergic local neurons (iLNs), where they play a dual role: direct excitation of iLNs and stabilization of the synapse between receptor neurons and iLNs, which undergoes strong short-term depression. Our results reveal modulatory functions of the AL main excitatory neurotransmitter. Striking similarities to the mammalian olfactory system predict that mammalian glutamatergic metabotropic receptors could be associated with similar modulations.

Balancing Extrasynaptic Excitation and Synaptic Inhibition within Olfactory Bulb Glomeruli

Glutamatergic transmission in the brain typically occurs at well-defined synaptic connections, but increasing evidence indicates that neural excitation can also occur through activation of “extrasynaptic” glutamate receptors. Here, we investigated the underlying mechanisms and functional properties of extrasynaptic signals that are part of a feedforward path of information flow in the olfactory bulb. This pathway involves glutamatergic interneurons, external tufted cells (eTCs), that are excited by olfactory sensory neurons (OSNs) and in turn excite output mitral cells (MCs) extrasynaptically. Using pair-cell and triple-cell recordings in rat bulb slices (of either sex), combined with ultrastructural approaches, we first present evidence that eTC-to-MC signaling results from “spillover” of glutamate released at eTC synapses onto GABAergic periglomerular (PG) cells in glomeruli. Thus, feedforward excitation is an indirect result of and must cooccur with activation of inhibitory circuitry. Next, to examine the dynamics of the competing signals, we assayed the relationship between the number of spikes in eTCs and excitation of MCs or PG cells in pair-cell recordings. This showed that extrasynaptic excitation in MCs is very weak due to single spikes but rises sharply and supralinearly with increasing spikes, differing from sublinear behavior for synaptic excitation of PG cells. Similar dynamics leading to a preference for extrasynaptic excitation were also observed during recordings of extrasynaptic and inhibitory currents in response to OSN input of increasing magnitude. The observed alterations in the balance between extrasynaptic excitation and inhibition in glomeruli with stimulus strength could underlie an intraglomerular mechanism for olfactory contrast enhancement.

Cannabinoid receptor-mediated modulation of inhibitory inputs to mitral cells in the main olfactory bulb

The endocannabinoid (eCB) signaling system has been functionally implicated in many brain regions. Our understanding of the role of cannabinoid receptor type 1 (CB1) in olfactory processing remains limited. Cannabinoid signaling is involved in regulating glomerular activity in the main olfactory bulb (MOB). However, the cannabinoid-related circuitry of inputs to mitral cells in the MOB has not been fully determined. Using anatomical and functional approaches we have explored this question. CB1 was present in periglomerular processes of a GAD65-positive subpopulation of interneurons but not in mitral cells. We detected eCBs in the mouse MOB as well as the expression of CB1 and other genes associated with cannabinoid signaling in the MOB. Patch-clamp electrophysiology demonstrated that CB1 agonists activated mitral cells and evoked an inward current, while CB1 antagonists reduced firing and evoked an outward current. CB1 effects on mitral cells were absent in subglomerular slices in which the olfactory nerve layer and glomerular layer were removed, suggesting the glomerular layer as the site of CB1 action. We previously observed that GABAergic periglomerular cells show the inverse response pattern to CB1 activation compared with mitral cells, suggesting that CB1 indirectly regulates mitral cell activity as a result of cellular activation of glomerular GABAergic processes . This hypothesis was supported by the finding that cannabinoids modulated synaptic transmission to mitral cells. We conclude that CB1 directly regulates GABAergic processes in the glomerular layer to control GABA release and, in turn, regulates mitral cell activity with potential effects on olfactory threshold and behavior.NEW & NOTEWORTHY Cannabinoid signaling with cannabinoid receptor type 1 (CB1) is involved in the regulation of glomerular activity in the main olfactory bulb (MOB). We detected endocannabinoids in the mouse MOB. CB1 was present in periglomerular processes of a GAD65-positive subpopulation of interneurons. CB1 agonists activated mitral cells. CB1 directly regulates GABAergic processes to control GABA release and, in turn, regulates mitral cell activity with potential effects on olfactory threshold and behavior.

AgRP/NPY Neuron Excitability Is Modulated by Metabotropic Glutamate Receptor 1 During Fasting

The potential to control feeding behavior via hypothalamic AgRP/NPY neurons has led to many approaches to modulate their excitability—particularly by glutamatergic input. In the present study using NPY-hrGFP reporter mice, we visualize AgRP/NPY neuronal metabotropic glutamate receptor 1 (mGluR1) expression and test the effect of fasting on mGluR1 function. Using the pharmacological agonist dihydroxyphenylglycine (DHPG), we demonstrate the enhanced capacity of mGluR1 to drive firing of AgRP/NPY neurons after overnight fasting, while antagonist 3-MATIDA reduces firing. Further, under synaptic blockade we demonstrate that DHPG acts directly on AgRP/NPY neurons to create a slow inward current. Using an in vitro approach, we show that emulation of intracellular signals associated with fasting by forskolin enhances DHPG induced phosphorylation of extracellularly regulated-signal kinase (1/2) in GT1-7 cell culture. We show in vivo that blocking mGluR1 by antagonist 3-MATIDA lowers fasting induced refeeding. In summary, this study identifies a novel layer of regulation on AgRP/NPY neurons integrated with whole body energy balance.

Activation of Group II Metabotropic Glutamate Receptors Suppresses Excitability of Mouse Main Olfactory Bulb External Tufted and Mitral Cells

Metabotropic glutamate receptors (mGluRs) are abundantly expressed in the rodent main olfactory bulb. The function of Group I mGluRs has been investigated in a number of studies, while the actions of Group II mGluRs, which include the mGluR2 and mGluR3 subtypes, have been less well explored. Here, we used electrophysiological approaches in mouse olfactory bulb slices to investigate how Group II mGluR activation and inactivation modifies the activity of external tufted (ET) and mitral cells. The Group II mGluR agonist DCG-IV was found to directly and uniformly reduce the spontaneous discharge of ET and mitral cells. The inhibitory effect of DCG-IV was absent in mitral cells with truncated apical dendrites, indicating a glomerular site of action. DCG-IV did not influence olfactory nerve-evoked monosynaptic responses in ET or mitral cells, indicating that Group II mGluRs do not presynaptically modulate glutamate release from olfactory nerve terminals. In contrast, DCG-IV suppressed polysynaptic responses in periglomerular cells evoked by olfactory nerve stimulation. DCG-IV also inhibited glutamate release from ET cells, and suppressed the spontaneous and olfactory nerve-evoked long-lasting depolarization in mitral cells. Applied alone, Group II receptor antagonists were without effect, suggesting that basal activation of these receptors is nil. These findings suggest that Group II mGluRs inhibit ET and mitral cell activity and further dampen intraglomerular excitatory circuits by suppressing glutamate release.

Distinct lateral inhibitory circuits drive parallel processing of sensory information in the mammalian olfactory bulb

Splitting sensory information into parallel pathways is a common strategy in sensory systems. Yet, how circuits in these parallel pathways are composed to maintain or even enhance the encoding of specific stimulus features is poorly understood. Here, we have investigated the parallel pathways formed by mitral and tufted cells of the olfactory system in mice and characterized the emergence of feature selectivity in these cell types via distinct lateral inhibitory circuits. We find differences in activity-dependent lateral inhibition between mitral and tufted cells that likely reflect newly described differences in the activation of deep and superficial granule cells. Simulations show that these circuit-level differences allow mitral and tufted cells to best discriminate odors in separate concentration ranges, indicating that segregating information about different ranges of stimulus intensity may be an important function of these parallel sensory pathways.

DOI:http://dx.doi.org/10.7554/eLife.16039.001

The brain often processes different features of sensory information in separate pathways. For example, when seeing an object, information about colour and movement are processed by separate types of neurons in the eye. These neurons in turn relay information to different sets of brain areas, all of which are active at the same time. Such parallel processing was originally not thought to apply to information about smell. This was because in mammals, the two types of neurons in the brain area that processes smell seemed to play the same role. However, more recent work suggests that there are in fact differences in the responses of these two neuron types (called mitral cells and tufted cells) to odors, suggesting that the brain might use parallel processing for information about smells too.

Information travels along neurons in the form of electrical signals, and this activity is often seen in the form of a series of “spikes”. In a process called lateral inhibition, the activity of one neuron can feed back and inhibit the activity of its neighbors. This is important for enhancing contrast; in terms of the sense of smell, lateral inhibition is thought to help distinguish between similar odors.

A technique called optogenetics allows the activity of particular neurons in an animal’s brain to be controlled by shining light onto them. Geramita et al. have now used this technique in mice to investigate whether there are differences in how lateral inhibition works in mitral cells and tufted cells. This revealed that lateral inhibition affects mitral cells only when they are spiking at intermediate firing rates, whereas tufted cells are only affected by lateral inhibition when spiking at low firing rates. Using computer simulations, Geramita et al. show that these different responses mean that mitral cells are best at distinguishing similar smells when they are present at high concentrations, while tufted cells are best at distinguishing similar smells that are present at low concentrations. These differences also mean that, by working together, mitral and tufted cells can distinguish between smells much better than either type of neuron on its own.

These results demonstrate that, as with the other senses, the brain processes information about smell using parallel pathways. Future work is now needed to see what effect switching off the activity of either mitral or tufted cells will have on an animal’s behavior.

DOI:http://dx.doi.org/10.7554/eLife.16039.002

Fear is the mother of invention: anuran embryos exposed to predator cues alter life-history traits, post-hatching behaviour, and neuronal activity patterns

Neurophysiological modifications associated to phenotypic plasticity in response to predators are largely unexplored, and there is a gap of knowledge on how the information encoded in predator cues is processed by prey sensory systems. To explore these issues, we exposed Rana dalmatina embryos to dragonfly chemical cues (kairomones) up to hatching. At different times after hatching (up to 40 days), we recorded morphology and antipredator behaviour of control and embryonic-treated tadpoles as well as their neural olfactory responses, by recording the activity of their mitral neurons before and after exposure to a kairomone solution. Embryonic-treated embryos hatched later and originated smaller hatchlings than control siblings. In addition, embryonic-treated tadpoles showed a stronger antipredator response than controls at 10 (but not at 30) days post-hatching, though the intensity of the contextual response to the kairomone stimulus did not differ between the two groups. Baseline neuronal activity at 30 days post-hatching, as assessed by the frequency of spontaneous excitatory postsynaptic events and by the firing rate of mitral cells, was higher among embryonic-treated tadpoles compared to controls. At the same time, neuronal activity showed a stronger increase among embryonic-treated tadpoles than among controls after a local kairomone perfusion. Hence, a different contextual plasticity between treatments at the neuronal level was not mirrored by the antipredator behavioural response. In conclusion, our experiments demonstrate ontogenetic plasticity in tadpole neuronal activity after embryonic exposure to predator cues, corroborating the evidence that early-life experience can contribute to shaping the phenotype at later life stages.

Metabotropic glutamate receptors promote disinhibition of olfactory bulb glomeruli that scales with input strength

Increasing evidence indicates that the neural circuitry within glomeruli of the olfactory bulb plays a major role in affecting information flow between olfactory sensory neurons (OSNs) and output mitral cells (MCs). Glutamatergic external tufted (ET) cells, located at glomeruli, can act as intermediary cells in excitation between OSNs and MCs, whereas activation of MCs by OSNs is, in turn, suppressed by inhibitory synapses onto ET cells. In this study, we used patch-clamp recordings in rat olfactory bulb slices to examine the function of metabotropic glutamate receptors (mGluRs) in altering these glomerular signaling mechanisms. We found that activation of group II mGluRs profoundly reduced inhibition onto ET cells evoked by OSN stimulation. The mGluRs that mediated disinhibition were located on presynaptic GABAergic periglomerular cells and appeared to be activated by glutamate transients derived from dendrites in glomeruli. In terms of glomerular output, the mGluR-mediated reduction in GABA release led to a robust increase in the number of action potentials evoked by OSN stimulation in both ET cells and MCs. Importantly, however, the enhanced excitation was specific to when a glomerulus was strongly activated by OSN inputs. By being selective for strong vs. weak glomerular activation, mGluR-mediated disinhibition provides a mechanism to enhance the contrast in odor signals that activate OSN inputs into a single glomerulus at varying intensities.

Allosteric Modulation of GABAA Receptors by an Anilino Enaminone in an Olfactory Center of the Mouse Brain

In an ongoing effort to identify novel drugs that can be used as neurotherapeutic compounds, we have focused on anilino enaminones as potential anticonvulsant agents. Enaminones are organic compounds containing a conjugated system of an amine, an alkene and a ketone. Here, we review the effects of a small library of anilino enaminones on neuronal activity. Our experimental approach employs an olfactory bulb brain slice preparation using whole-cell patch-clamp recording from mitral cells in the main olfactory bulb. The main olfactory bulb is a key integrative center in the olfactory pathway. Mitral cells are the principal output neurons of the main olfactory bulb, receiving olfactory receptor neuron input at their dendrites within glomeruli, and projecting glutamatergic axons through the lateral olfactory tract to the olfactory cortex. The compounds tested are known to be effective in attenuating pentylenetetrazol (PTZ) induced convulsions in rodent models. One compound in particular, KRS-5Me-4-OCF₃, evokes potent inhibition of mitral cell activity. Experiments aimed at understanding the cellular mechanism underlying the inhibitory effect revealed that KRS-5Me-4-OCF₃ shifts the concentration-response curve for GABA to the left. KRS-5Me-4-OCF₃ enhances GABA affinity and acts as a positive allosteric modulator of GABA_A receptors. Application of a benzodiazepine site antagonist blocks the effect of KRS-5Me-4-OCF₃ indicating that KRS-5Me-4-OCF₃ binds at the classical benzodiazepine site to exert its pharmacological action. This anilino enaminone KRS-5Me-4-OCF₃ emerges as a candidate for clinical use as an anticonvulsant agent in the battle against epileptic seizures.

Resibufogenin and cinobufagin activate central neurons through an ouabain-like action

Cinobufagin and resibufogenin are two major effective bufadienolides of Chan su (toad venom), which is a Chinese medicine obtained from the skin venom gland of toads and is used as a cardiotonic and central nervous system (CNS) respiratory agent, an analgesic and anesthetic, and as a remedy for ulcers. Many clinical cases showed that Chan su has severe side-effects on the CNS, causing shortness of breath, breathlessness, seizure, coma and cardiac arrhythmia. We used whole-cell recordings from brain slices to determine the effects of bufadienolides on excitability of a principal neuron in main olfactory bulb (MOB), mitral cells (MCs), and the cellular mechanism underlying the excitation. At higher concentrations, cinobufagin and resibufogenin induced irreversible over-excitation of MCs indicating a toxic effect. At lower concentrations, they concentration-dependently increased spontaneous firing rate, depolarized the membrane potential of MCs, and elicited inward currents. The excitatory effects were due to a direct action on MCs rather than an indirect phasic action. Bufadienolides and ouabain had similar effects on firing of MCs which suggested that bufadienolides activated neuron through a ouabain-like effect, most likely by inhibiting Na⁺/K⁺-ATPase. The direct action of bufadienolide on brain Na⁺ channels was tested by recordings from stably Na_v1.2-transfected cells. Bufadienolides failed to make significant changes of the main properties of Na_v1.2 channels in current amplitude, current-voltage (I-V) relationships, activation and inactivation. Our results suggest that inhibition of Na⁺/K⁺-ATPase may be involved in both the pharmacological and toxic effects of bufadienolide-evoked CNS excitation.

Notch1 activity in the olfactory bulb is odour-dependent and contributes to olfactory behaviour

Notch signalling plays an important role in synaptic plasticity, learning and memory functions in both Drosophila and rodents. In this paper, we report that this feature is not restricted to hippocampal networks but also involves the olfactory bulb (OB). Odour discrimination and olfactory learning in rodents are essential for survival. Notch1 expression is enriched in mitral cells of the mouse OB. These principal neurons are responsive to specific input odorants and relay the signal to the olfactory cortex. Olfactory stimulation activates a subset of mitral cells, which show an increase in Notch activity. In Notch1cKOKln mice, the loss of Notch1 in mitral cells affects the magnitude of the neuronal response to olfactory stimuli. In addition, Notch1cKOKln mice display reduced olfactory aversion to propionic acid as compared to wildtype controls. This indicates, for the first time, that Notch1 is involved in olfactory processing and may contribute to olfactory behaviour.

Identification of both GABAA receptors and voltage-activated Na(+) channels as molecular targets of anticonvulsant α-asarone

Alpha (α)-asarone, a major effective component isolated from the Chinese medicinal herb Acorus tatarinowii, is clinically used as medication for treating epilepsy, cough, bronchitis, and asthma. In the present study, we demonstrated that α-asarone targets central nervous system GABAA receptor as well as voltage-gated Na(+) channels. Using whole-cell patch-clamp recording, α-asarone inhibited spontaneous firing of output neurons, mitral cells (MCs), in mouse olfactory bulb brain slice preparation and hyperpolarized the membrane potential of MCs. The inhibitory effect of α-asarone persisted in the presence of ionotropic glutamate receptor blockers but was eliminated after adding a GABAA receptor blocker, suggesting that GABAA receptors mediated the inhibition of MCs by α-asarone. This hypothesis was supported by the finding that α-asarone evoked an outward current, but did not influence inhibitory postsynaptic currents (IPSCs). In addition to inhibiting spontaneous firing, α-asarone also inhibited the Nav1.2 channel, a dominant rat brain Na(+) channel subtype. The effects of α-asarone on a defined Nav1.2 were characterized using transfected cells that stably expressed the Nav1.2 channel isoform. α-Asarone displayed strong tonic inhibition of Nav1.2 currents in a concentration- and membrane potential-dependent fashion. α-Asarone reduced channel availability in steady-state inactivation protocols by enhancing or stabilizing Na(+) channel inactivation. Both Na(+) channel blockade and activation of GABAA receptors provide a possible mechanism for the known anti-epileptic effects of α-asarone. It also suggests that α-asarone could benefit patients with cough possibly through inhibiting a Na(+) channel subtype to inhibit peripheral and/or central sensitization of cough reflexes.

Activation of group I metabotropic glutamate receptors enhances persistent sodium current and rhythmic bursting in main olfactory bulb external tufted cells

Rhythmically bursting olfactory bulb external tufted (ET) cells are thought to play a key role in synchronizing glomerular network activity to respiratory-driven sensory input. Whereas spontaneous bursting in these cells is intrinsically generated by interplay of several voltage-dependent currents, bursting strength and frequency can be modified by local intrinsic and centrifugal synaptic input. Activation of metabotropic glutamate receptors (mGluRs) engages a calcium-dependent cation current (I(CAN)) that increases rhythmic bursting, but mGluRs may also modulate intrinsic mechanisms involved in bursting. Here, we used patch-clamp electrophysiology in rat olfactory bulb slices to investigate whether mGluRs modulate two key intrinsic currents involved in ET cell burst initiation: persistent sodium (I(NaP)) and hyperpolarization-activated cation (Ih) currents. Using a BAPTA-based internal solution to block I(CAN), we found that the mGluR1/5 agonist DHPG enhanced I(NaP) but did not alter Ih. I(NaP) enhancement consisted of increased current at membrane potentials between -60 and -50 mV and a hyperpolarizing shift in activation threshold. Both effects would be predicted to shorten the interburst interval. In agreement, DHPG modestly depolarized (∼3.5 mV) ET cells and increased burst frequency without effect on other major burst parameters. This increase was inversely proportional to the basal burst rate such that slower ET cells exhibited the largest increases. This may enable ET cells with slow intrinsic burst rates to pace with faster sniff rates. Taken with other findings, these results indicate that multiple neurotransmitter mechanisms are engaged to fine-tune rhythmic ET cell bursting to context- and state-dependent changes in sniffing frequency.

Cannabinoid receptor-mediated regulation of neuronal activity and signaling in glomeruli of the main olfactory bulb

Cannabinoid receptors (CB1Rs) are present in glomeruli of the main olfactory bulb. The functions of CB1Rs and their endogenous activators, endocannabinoids, for glomerular signaling are unknown. Glomeruli contain at least three types of neurons: periglomerular (PG), external tufted (ET), and short-axon (SA) cells. PG cells form inhibitory GABAergic dendrodendritic synapses with ET cells. ET cells form excitatory glutamatergic dendrodendritic synapses with PG and SA cells. In mouse brain slices, we used whole-cell patch-clamp recordings to study the role of CB1Rs in regulating PG and ET cells. Cannabinoids displayed strong, direct inhibitory effects on PG cells and weak effects on ET cells. Single pulses or a train of pulses of depolarizing current injected into an ET cell evoked suppression of IPSCs. This suggests retrograde endocannabinoid signaling, namely, depolarization-induced suppression of inhibition (DSI) in ET cells. Our results support the hypothesis that burst firing of ET cells triggers the release of endocannabinoids which in turn directly inhibit PG cells and reduce GABA release from PG cells. This, in turn, can result in a transient reduction of PG cell inhibitory input to ET cells.

Expression of transient receptor potential (TRP) channel mRNAs in the mouse olfactory bulb

Transient receptor potential (TRP) channels are a large family of cation channels. The 28 TRP channel subtypes in rodent are divided into 6 subfamilies: TRPC1-7, TRPV1-6, TRPM1-8, TRPP2/3/5, TRPML1-3 and TRPA1. TRP channels are involved in peripheral olfactory transduction. Several TRPC channels are expressed in unidentified neurons in the main olfactory bulb (OB), but the expression of most TRP channels in the OB has not been investigated. The present study employed RT-PCR as an initial survey of the expression of TRP channel mRNAs in the mouse OB and in 3 cell types: external tufted, mitral and granule cells. All TRP channel mRNAs except TRPV5 were detected in OB tissue. Single cell RT-PCR revealed that external tufted, mitral and granule cell populations expressed in aggregate 14 TRP channel mRNAs encompassing members of all 6 subfamilies. These different OB neuron populations expressed 7-12 channel mRNAs. Common channel expression was more similar among external tufted and mitral cells than among these cells and granule cells. These results indicate that a large number of TRP channel subtypes are expressed in OB neurons, providing the molecular bases for these channels to regulate OB neuron activity and central olfactory processing.

Ginseng derivative ocotillol enhances neuronal activity through increased glutamate release: a possible mechanism underlying increased spontaneous locomotor activity of mice

Ginsenosides are the main active ingredients in ginseng and have recently been reported to have beneficial effects on the CNS. Ocotillol is a derivate of pseudoginsenoside-F11, which is an ocotillol-type ginsenoside found in American ginseng. We examined the effects of ocotillol (a) on neuronal activity of projection neurons, mitral cells (MC), in a mouse olfactory bulb brain slice preparation using whole-cell patch-clamp recording, and (b) on animal behavior by measuring locomotor activity of mice in vivo. Ocotillol displayed an excitatory effect on spontaneous action potential firing and depolarized the membrane potential of MCs. The effect was concentration-dependent, with an EC(50) of 4 μM. In the presence of blockers of ionotropic glutamatergic and GABAergic synaptic transmission (6-cyano-7-nitroquinoxaline-2,3-dione [CNQX], 10 μM; D-AP5, 50 μM; gabazine, 5 μM), the excitatory effect of ocotillol on firing was abolished. Further experiments showed that the ocotillol-induced neuronal excitation persisted in the presence of GABA(A) receptor antagonist gabazine but was eliminated by applying AMPA/kainate receptor antagonist CNQX and N-methyl-d-aspartate (NMDA) receptor antagonist D-AP5, suggesting that ionotropic glutamate transmission was involved in mediating the effects of ocotillol. Bath application of ocotillol evoked an inward current as well as an increased frequency of spontaneous glutamatergic excitatory postsynaptic currents (EPSCs). Both the inward current and sEPSCs could be blocked by ionotropic glutamate receptor antagonists CNQX and D-AP5. These results indicate that the excitatory action of ocotillol on MCs was mediated by enhanced glutamate release. Behavioral experiments demonstrated that ocotillol increased locomotor activities of mice. Our results suggest that ocotillol-evoked neuronal excitability was mediated by increased release of glutamate, which may be responsible for the increased spontaneous locomotor activities in vivo.

Low-magnesium medium induces epileptiform activity in mouse olfactory bulb slices

Magnesium-free medium can be used in brain slice studies to enhance glutamate receptor function, but this manipulation causes seizure-like activity in many cortical areas. The rodent olfactory bulb (OB) slice is a popular preparation, and potentially ictogenic ionic conditions have often been used to study odor processing. We studied low Mg(2+)-induced epileptiform discharges in mouse OB slices using extracellular and whole cell electrophysiological recordings. Low-Mg(2+) medium induced two distinct types of epileptiform activity: an intraglomerular delta-frequency oscillation resembling slow sniff-induced activity and minute-long seizure-like events (SLEs) consisting of large negative-going field potentials accompanied by sustained depolarization of output neurons. SLEs were dependent on N-methyl-D-aspartate receptors and sodium currents and were facilitated by α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors. The events were initiated in the glomerular layer and propagated laterally through the external plexiform layer at a slow time scale. Our findings confirm that low-Mg(2+) medium should be used with caution in OB slices. Furthermore, the SLEs resembled the so-called slow direct current (DC) shift of clinical and experimental seizures, which has recently been recognized as being of great clinical importance. The OB slice may therefore provide a robust and unique in vitro model of acute seizures in which mechanisms of epileptiform DC shifts can be studied in isolation from fast oscillations.

A substituted anilino enaminone acts as a novel positive allosteric modulator of GABA(A) receptors in the mouse brain

A small library of anilino enaminones was analyzed for potential anticonvulsant agents. We examined the effects of three anilino enaminones on neuronal activity of output neurons, mitral cells (MC), in an olfactory bulb brain slice preparation using whole-cell patch-clamp recording. These compounds are known to be effective in attenuating pentylenetetrazol-induced convulsions. Among the three compounds tested, 5-methyl-3-(4-trifluoromethoxy-phenylamino)-cyclohex-2-enone (KRS-5Me-4-OCF₃) showed potent inhibition of MC activity with an EC₅₀ of 24.5 μM. It hyperpolarized the membrane potential of MCs accompanied by suppression of spontaneous firing. Neither ionotropic glutamate receptor blockers nor a GABA(B) receptor blocker prevented the KRS-5Me-4-OCF(3)-evoked inhibitory effects. In the presence of GABA(A) receptor antagonists, KRS-5Me-4-OCF(3) completely failed to evoke inhibition of MC spiking activity, suggesting that KRS-5Me-4-OCF₃-induced inhibition may be mediated by direct action on GABA(A) receptors or indirect action through the elevation of tissue GABA levels. Neither vigabatrin (a selective GABA-T inhibitor) nor 1,2,5,6-tetrahydro-1-[2-[[(diphenylmethylene)amino]oxy]ethyl]-3-pyridinecarboxylic acid hydrochloride (NNC-711) (a selective inhibitor of GABA uptake by GABA transporter 1) eliminated the effect of KRS-5ME-4-OCF₃ on neuronal excitability, indicating that the inhibitory effect of the enaminone resulted from direct activation of GABA(A) receptors. The concentration-response curves for GABA are left-shifted by KRS-5Me-4-OCF₃, demonstrating that KRS-5Me-4-OCF₃ enhanced GABA affinity and acted as a positive allosteric modulator of GABA(A) receptors. The effect of KRS-5Me-4-OCF₃ was blocked by applying a benzodiazepine site antagonist, suggesting that KRS-5Me-4-OCF₃ binds at the classic benzodiazepine site to exert its pharmacological action. The results suggest clinical use of enaminones as anticonvulsants in seizures and as a potential anxiolytic in mental disorders.

Gonadotropin-releasing hormone inhibits ether-à-go-go-related gene K+ currents in mouse gonadotropes

Secretion of LH from gonadotropes is initiated by a GnRH-induced increase in intracellular Ca(2+) concentration ([Ca(2+)](i)). This increase in [Ca(2+)](i) is the result of Ca(2+) release from intracellular stores and Ca(2+) influx through voltage-dependent Ca(2+) channels. Here we describe an ether-à-go-go-related gene (erg) K(+) current in primary mouse gonadotropes and its possible function in the control of Ca(2+) influx. To detect gonadotropes, we used a knock-in mouse strain, in which GnRH receptor-expressing cells are fluorescently labeled. Erg K(+) currents were recorded in 80-90% of gonadotropes. Blockage of erg currents by E-4031 depolarized the resting potential by 5-8 mV and led to an increase in [Ca(2+)](i), which was abolished by nifedipine. GnRH inhibited erg currents by a reduction of the maximal erg current and in some cells additionally by a shift of the activation curve to more positive potentials. In conclusion, the erg current contributes to the maintenance of the resting potential in gonadotropes, thereby securing a low [Ca(2+)](i) by restricting Ca(2+) influx. In addition, the erg channels are modulated by GnRH by an as-yet unknown signal cascade.

All-or-none population bursts temporally constrain surround inhibition between mouse olfactory glomeruli

With each sniff, the olfactory bulbs of the brain generate a neural activity pattern representing the odour environment, transmitting this to higher brain centres in the form of mitral cell output. Inhibitory circuits in the olfactory bulb glomerular and external plexiform layers may amplify contrast in these patterns, through surround inhibition of mitral cells. These circuits may operate in series, but their respective roles are unclear. A single sniff is sufficient for odour discrimination, but is not clear that the inhibitory circuits act within this timeframe. We used microdissected slices of mouse olfactory bulb to study each circuit in isolation. We found that unlike surround inhibition mediated in the external plexiform layer, surround inhibition mediated in the glomerular layer was activated by sensory synaptic input, but not by mitral cell output. The results also suggest that interactions between olfactory glomeruli are exclusively inhibitory, unlike in antennal lobe, and that surround inhibition mediated within the external plexiform layer may involve neural circuit elements not preserved in slice preparations. Surround inhibition was effective only after an interval corresponding to a single sniff in vivo. Surplus excitation, initiated by sensory input but generated by collective all-or-none responses of mitral cells, may delay surround inhibition and allow the synchronous activation of multiple glomeruli without each suppressing the other. Surround inhibition in the glomerular layer may subsequently allow a fresh representation of the odour environment to be generated with each sniff. These findings are consistent with combinatorial odour coding based on all-or-none glomerular responses.

Metabotropic glutamate receptors and dendrodendritic synapses in the main olfactory bulb

The main olfactory bulb (MOB) is the first site of synaptic processing in the central nervous system for odor information that is relayed from olfactory receptor neurons in the nasal cavity via the olfactory nerve (ON). Glutamate and ionotropic glutamate receptors (iGluRs) play a dominant role at ON synapses. Similarly, glutamate and iGluRs mediate dendrodendritic transmission between several populations of neurons within the MOB network. Neuroanatomical studies demonstrate that metabotropic glutamate receptors (mGluRs) are densely expressed through the MOB network, and they are particularly abundant at dendrodendritic synapses. Until recently, the physiological roles of mGluRs in the MOB were poorly understood. Over the past several years, mGluRs have been shown to play surprisingly powerful neuromodulatory roles at ON synapses and in dendrodendritic neurotransmission in the MOB. This chapter focuses on recent advances in our understanding of mGluR-mediated signaling components at dendrodendritic synapses.

Group I mGluR activation enhances Ca(2+)-dependent nonselective cation currents and rhythmic bursting in main olfactory bulb external tufted cells

In the main olfactory bulb, activation of group I metabotropic glutamate receptors (mGluRs) by olfactory nerve stimulation generates slow (2 Hz) oscillations near the basal respiratory frequency. These oscillations arise in the glomerular layer and may be generated, in part, by the intrinsic neurons, the juxtaglomerular neurons. We investigated the physiological effects of group I mGluR agonists on one population of juxtaglomerular neurons, external tufted (ET) cells, which rhythmically burst at respiratory frequencies and synchronize the intraglomerular network. Electrophysiological studies in rat main olfactory bulb slices demonstrated that the mGluR agonist 3,4-dihydroxyphenylglycine (DHPG) amplified the strength of ET cell spike bursts, principally by increasing the number of spikes per burst. Voltage-clamp and Ca(2+)-imaging studies showed that DHPG elicits a Ca(2+)-dependent nonselective cation current (I(CAN)) in the dendrites of ET cells triggered by Ca(2+) release from internal stores. The DHPG effects on bursting and membrane current were attenuated by flufenamic acid and SKF96365, agents known to antagonize I(CAN) in a variety of neurons. DHPG also elicited slow membrane current oscillations and spikelets in ET cells when synaptic transmission and intrinsic membrane channels were inoperative. These findings indicate that DHPG may passively (by increasing burst strength) or actively (by increasing conductance of gap junctions) enhance the strength of electrical synapses between ET cells. Together, these findings indicate that activation of group I mGluRs on the dendrites of ET cells play a key role in the generation of slow rhythmic oscillation in the glomerular network, which is in turn tuned to sniffing of the animal in vivo.

Optical imaging of postsynaptic odor representation in the glomerular layer of the mouse olfactory bulb

Olfactory glomeruli are the loci where the first odor-representation map emerges. The glomerular layer comprises exquisite local synaptic circuits for the processing of olfactory coding patterns immediately after their emergence. To understand how an odor map is transferred from afferent terminals to postsynaptic dendrites, it is essential to directly monitor the odor-evoked glomerular postsynaptic activity patterns. Here we report the use of a transgenic mouse expressing a Ca(2+)-sensitive green fluorescence protein (GCaMP2) under a Kv3.1 potassium-channel promoter. Immunostaining revealed that GCaMP2 was specifically expressed in mitral and tufted cells and a subpopulation of juxtaglomerular cells but not in olfactory nerve terminals. Both in vitro and in vivo imaging combined with glutamate receptor pharmacology confirmed that odor maps reported by GCaMP2 were of a postsynaptic origin. These mice thus provided an unprecedented opportunity to analyze the spatial activity pattern reflecting purely postsynaptic olfactory codes. The odor-evoked GCaMP2 signal had both focal and diffuse spatial components. The focalized hot spots corresponded to individually activated glomeruli. In GCaMP2-reported postsynaptic odor maps, different odorants activated distinct but overlapping sets of glomeruli. Increasing odor concentration increased both individual glomerular response amplitude and the total number of activated glomeruli. Furthermore, the GCaMP2 response displayed a fast time course that enabled us to analyze the temporal dynamics of odor maps over consecutive sniff cycles. In summary, with cell-specific targeting of a genetically encoded Ca(2+) indicator, we have successfully isolated and characterized an intermediate level of odor representation between olfactory nerve input and principal mitral/tufted cell output.

Developmental regulation of metabotropic glutamate receptor 1 splice variants in olfactory bulb mitral cells

Alternative splicing of the metabotropic glutamate receptor 1 (mGluR1) receptor gene generates two major receptor isoforms, mGluR1a and mGluR1b, differing in intracellular function and distribution. However, little is known on the expression profiles of these variants during development. We examined the mRNA expression profile of mGluR1a/b in microdissected layers and acutely isolated mitral cells in the developing mouse olfactory bulb. This analysis showed that the two mGluR1 variants are differentially regulated within each bulb layer. During the first postnatal week, the mGluR1a isoform replaces GluR1b in the microdissected mitral cell layer (MCL) and in isolated identified mitral cells, coinciding with a developmental epoch of mitral cell dendritic reorganization. Although mGluR1a mRNA is expressed at high levels in both the adult external plexiform layer (EPL) and MCL, Western blotting analysis reveals a marked reduction of the mGluR1a protein in the MCL, where mitral cell bodies are located, and strong labeling in the EPL, which contains mitral cell dendrites. This suggests that there is increased dendritic trafficking efficiency of the receptor in adult. The temporal and spatial shift in mGluR1b/a expression suggests distinct roles of the mGluR1 isoforms, with mGluR1b potentially involved in the early mitral cell maturation and mGluR1a in dendritic and synapse function.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

The relationship between blood flow and neuronal activity in the rodent olfactory bulb

In the brain, neuronal activation triggers an increase in cerebral blood flow (CBF). Here, we use two animal models and several techniques (two-photon imaging of CBF and neuronal calcium dynamics, intracellular and extracellular recordings, local pharmacology) to analyze the relationship between neuronal activity and local CBF during odor stimulation in the rodent olfactory bulb. Application of glutamate receptor antagonists or tetrodotoxin directly into single rat olfactory glomeruli blocked postsynaptic responses but did not affect the local odor-evoked CBF increases. This suggests that in our experimental conditions, odor always activates more than one glomerulus and that silencing one of a few clustered glomeruli does not affect the vascular response. To block synaptic transmission more widely, we then superfused glutamate antagonists over the surface of the olfactory bulb in transgenic G-CaMP2 mice. This was for two reasons: (1) mice have a thin olfactory nerve layer compared to rats and this will favor drug access to the glomerular layer, and (2) transgenic G-CaMP2 mice express the fluorescent calcium sensor protein G-CaMP2 in mitral cells. In G-CaMP2 mice, odor-evoked, odor-specific, and concentration-dependent calcium increases in glomeruli. Superfusion of glutamate receptor antagonists blocked odor-evoked postsynaptic calcium signals and CBF responses. We conclude that activation of postsynaptic glutamate receptors and rises in dendritic calcium are major steps for neurovascular coupling in olfactory bulb glomeruli.

Activation of group I metabotropic glutamate receptors on main olfactory bulb granule cells and periglomerular cells enhances synaptic inhibition of mitral cells

Granule and periglomerular cells in the main olfactory bulb express group I metabotropic glutamate receptors (mGluRs). The group I mGluR agonist 3,4-dihydroxyphenylglycine (DHPG) increases GABAergic spontaneous IPSCs (sIPSCs) in mitral cells, yet the presynaptic mechanism(s) involved and source(s) of the IPSCs are unknown. We investigated the actions of DHPG on sIPSCs and TTX-insensitive miniature IPSCs (mIPSCs) recorded in mitral and external tufted cells in rat olfactory bulb slices. DHPG, acting at mGluR1 and mGluR5, increased the rate but not amplitude of sIPSCs and mIPSCs in both cell types. The increase in mIPSCs depended on voltage-gated Ca2+ channels but persisted when ionotropic glutamate receptors and sodium spikes were blocked. Focal DHPG puffs onto granule cells or bath application after glomerular layer (GL) excision failed to increase mIPSCs in mitral cells. Additionally, GL excision reduced sIPSCs in mitral cells by 50%, suggesting that periglomerular cells exert strong tonic GABAergic inhibition of mitral cells. In contrast, GL DHPG puffs readily increased mIPSCs. These findings indicate that DHPG-evoked GABA release from granule cells requires spikes, whereas in the GL, DHPG facilitates periglomerular cell GABA release via both spike-dependent and spike-independent presynaptic mechanisms. We speculate that mGluRs amplify spike-driven lateral inhibition through the mitral-to-granule cell circuit, whereas GL mGluRs may play a more important role in amplifying intraglomerular inhibition after subthreshold input.

Recurrent dendrodendritic inhibition of accessory olfactory bulb mitral cells requires activation of group I metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) modulate neural excitability and network tone in many brain regions. Expression of mGluRs is particularly high in the accessory olfactory bulb (AOB), a CNS structure critical for detecting chemicals that identify kin and conspecifics. Because of its relative simplicity and its direct projection to the hypothalamus, the AOB provides a model system for studying how mGluRs affect the flow of encoded sensory information to downstream areas. We investigated the role of group I mGluRs in synaptic processing in AOB slices and found that under control conditions, recurrent inhibition of principal neurons (mitral cells) was completely eliminated by the mGluR1 antagonist LY367385 [(S)-(+)-alpha-amino-4-carboxy-2 methylbenzeneacetic acid]. In addition, the group I mGluR agonist DHPG [(S)-3,5-dihydroxyphenylglycine; 20 microM] induced a dramatic increase in the rate of spontaneous IPSCs. This increase was dependent on voltage-gated calcium channels but persisted even after blockade of ionotropic glutamatergic transmission and sodium channels. Together, these results indicate that mGluR1 plays a critical role in controlling information flow through the AOB and suggest that mGluR1 may be an important locus for experience-dependent changes in synaptic function.

Disynaptic amplification of metabotropic glutamate receptor 1 responses in the olfactory bulb

Sensory systems often respond to rapid stimuli with high frequency and fidelity, as perhaps best exemplified in the auditory system. Fast synaptic responses are fundamental requirements to achieve this task. The importance of speed is less clear in the olfactory system. Moreover, olfactory bulb output mitral cells respond to a single stimulation of the sensory afferents with unusually long EPSPs, lasting several seconds. We examined the temporal characteristics, developmental regulation, and the mechanism generating these responses in mouse olfactory bulb slices. The slow EPSP appeared at postnatal days 10-11 and was mediated by metabotropic glutamate receptor 1 (mGluR1) and NMDA receptors. mGluR1 contribution was unexpected because its activation usually requires strong, high-frequency stimulation of inputs. However, dendritic release of glutamate from the intraglomerular network caused spillover-mediated recurrent activation of metabotropic glutamate receptors. We suggest that persistent responses in mitral cells amplify the incoming sensory information and, along with asynchronous inputs, drive odor-evoked slow temporal activity in the bulb.

Group I metabotropic glutamate receptors are differentially expressed by two populations of olfactory bulb granule cells

In the main olfactory bulb, several populations of granule cells (GCs) can be distinguished based on the soma location either superficially, interspersed with mitral cells within the mitral cell layer (MCL), or deeper, within the GC layer (GCL). Little is known about the physiological properties of superficial GCs (sGCs) versus deep GCs (dGCs). Here, we used patch-clamp recording methods to explore the role of Group I metabotropic glutamate receptors (mGluRs) in regulating the activity of GCs in slices from wildtype and mGluR-/- mutant mice. In wildtype mice, bath application of the selective Group I mGluR agonist DHPG depolarized and increased the firing rate of both GC subtypes. In the presence of blockers of fast synaptic transmission (APV, CNQX, gabazine), DHPG directly depolarized both GC subtypes, although the two GC subtypes responded differentially to DHPG in mGluR1-/- and mGluR5-/- mice. DHPG depolarized sGCs in slices from mGluR5-/- mice, although it had no effect on sGCs in slices from mGluR1-/- mice. By contrast, DHPG depolarized dGCs in slices from mGluR1-/- mice but had no effect on dGCs in slices from mGluR5-/- mice. Previous studies showed that mitral cells express mGluR1 but not mGluR5. The present results therefore suggest that sGCs are more similar to mitral cells than dGCs in terms of mGluR expression.

Sensory-evoked intrinsic optical signals in the olfactory bulb are coupled to glutamate release and uptake

Functional imaging signals arise from metabolic and hemodynamic activity, but how these processes are related to the synaptic and electrical activity of neurons is not well understood. To provide insight into this issue, we used in vivo imaging and simultaneous local pharmacology to study how sensory-evoked neural activity leads to intrinsic optical signals (IOS) in the well-defined circuitry of the olfactory glomerulus. Odor-evoked IOS were tightly coupled to release of glutamate and were strongly modulated by activation of presynaptic dopamine and GABA-B receptors. Surprisingly, IOS were independent of postsynaptic transmission through ionotropic or metabotropic glutamate receptors, but instead were inhibited when uptake by astrocytic glutamate transporters was blocked. These data suggest that presynaptic glutamate release and uptake by astrocytes form a critical pathway through which neural activity is linked to metabolic processing and hence to functional imaging signals.

Calcium permeable AMPA receptors and autoreceptors in external tufted cells of rat olfactory bulb

Glomeruli are functional units of the olfactory bulb responsible for early processing of odor information encoded by single olfactory receptor genes. Glomerular neural circuitry includes numerous external tufted (ET) cells whose rhythmic burst firing may mediate synchronization of bulbar activity with the inhalation cycle. Bursting is entrained by glutamatergic input from olfactory nerve terminals, so specific properties of ionotropic glutamate receptors on ET cells are likely to be important determinants of olfactory processing. Particularly intriguing is recent evidence that AMPA receptors of juxta-glomerular neurons may permeate calcium. This could provide a novel pathway for regulating ET cell signaling. We tested the hypothesis that ET cells express functional calcium-permeable AMPA receptors. In rat olfactory bulb slices, excitatory postsynaptic currents (EPSCs) in ET cells were evoked by olfactory nerve shock, and by uncaging glutamate. We found attenuation of AMPA/kainate EPSCs by 1-naphthyl acetyl-spermine (NAS), an open-channel blocker specific for calcium permeable AMPA receptors. Cyclothiazide strongly potentiated EPSCs, indicating a major contribution from AMPA receptors. The current-voltage (I-V) relation of uncaging EPSCs showed weak inward rectification which was lost after > approximately 10 min of whole-cell dialysis, and was absent in NAS. In kainate-stimulated slices, Co(2+) ions permeated cells of the glomerular layer. Large AMPA EPSCs were accompanied by fluorescence signals in fluo-4 loaded cells, suggesting calcium permeation. Depolarizing pulses evoked slow tail currents with pharmacology consistent with involvement of calcium permeable AMPA autoreceptors. Tail currents were abolished by Cd(2+) and (+/-)-4-(4-aminophenyl)-2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX), and were sensitive to NAS block. Glutamate autoreceptors were confirmed by uncaging intracellular calcium to evoke a large inward current. Our results provide evidence that calcium permeable AMPA receptors reside on ET cells, and are divided into at least two functionally distinct pools: postsynaptic receptors at olfactory nerve synaptic terminals, and autoreceptors sensitive to glutamate released from dendrodendritic synapses.

Metabotropic glutamate receptors in the main olfactory bulb drive granule cell-mediated inhibition

Main olfactory bulb (MOB) granule cells (GCs) express high levels of the group I metabotropic glutamate receptor (mGluR), mGluR5. We investigated the role of mGluRs in regulating GC activity in rodent MOB slices using whole cell patch-clamp electrophysiology. The group I/II mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) or the selective group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) depolarized ( approximately 20 mV) and increased the firing rate of GCs. In the presence of ionotropic glutamate and GABA receptor antagonists, DHPG evoked a more modest depolarization ( approximately 8 mV). In voltage clamp, DHPG, but not group II [(2S,2'R,3)-2-(2',3'-dicarboxycyclopropyl)glycine, DCG-IV] or group III [L(+)-2-amino-4-phosphonobutyric acid, L-AP4] mGluR agonists, induced an inward current. The inward current reversed polarity near the potassium equilibrium potential, suggesting mediation by closure of potassium channels. The DHPG-evoked inward current was unaffected by the mGluR1 antagonist (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385), was blocked by the group I/II mGluR antagonist (alphaS)-alpha-amino-alpha-[(1S,2S)-2-carboxycyclopropyl]-9H-xanthine-9-propanoic acid (LY341495), and was absent in GCs from mGluR5 knockout mice. LY341495 also attenuated mitral cell-evoked voltage-sensitive dye signals in the external plexiform layer and mitral cell-evoked spikes in GCs. These results suggest that activation of mGluR5 increases GC excitability, an effect that should increase GC-mediated GABAergic inhibition of mitral cells. In support of this: DHPG increased the frequency of spontaneous GABAergic inhibitory postsynaptic currents in mitral cells and LY341495 attenuated the feedback GABAergic postsynaptic potential elicited by intracellular depolarization of mitral cells. Our results suggest that activation of mGluR5 participates in feedforward and/or feedback inhibition at mitral cell to GC dendrodendritic synapses, possibly to modulate lateral inhibition and contrast in the MOB.

Complementary postsynaptic activity patterns elicited in olfactory bulb by stimulation of mitral/tufted and centrifugal fiber inputs to granule cells

Main olfactory bulb (MOB) granule cells receive spatially segregated glutamatergic synaptic inputs from the dendrites of mitral/tufted cells as well as from the axons of centrifugal fibers (CFFs) originating in olfactory cortical areas. Dendrodendritic synapses from mitral/tufted cells occur on granule cell distal dendrites in the external plexiform layer (EPL), whereas CFFs preferentially target the somata/proximal dendrites of granule cells in the granule cell layer (GCL). In the present study, tract tracing, and recordings of field potentials and voltage-sensitive dye optical signals were used to map activity patterns elicited by activation of these two inputs to granule cells in mouse olfactory bulb slices. Stimulation of the lateral olfactory tract (LOT) produced a negative field potential in the EPL and a positivity in the GCL. CFF stimulation produced field potentials of opposite polarity in the EPL and GCL to those elicited by LOT. LOT-evoked optical signals appeared in the EPL and spread subsequently to deeper layers, whereas CFF-evoked responses appeared in the GCL and then spread superficially. Evoked responses were reduced by N-methyl-d-aspartate (NMDA) receptor antagonists and completely suppressed by AMPA receptor antagonists. Reduction of extracellular Mg(2+) enhanced the strength and spatiotemporal extent of the evoked responses. These and additional findings indicate that LOT- and CFF-evoked field potentials and optical signals reflect postsynaptic activity in granule cells, with moderate NMDA and dominant AMPA receptor components. Taken together, these results demonstrate that LOT and CFF stimulation in MOB slices selectively activate glutamatergic inputs to the distal dendrites versus somata/proximal dendrites of granule cells.

Dynamical mechanisms of odor processing in olfactory bulb mitral cells

In the olfactory system, the contribution of dynamical properties such as neuronal oscillations and spike synchronization to the representation of odor stimuli is a matter of substantial debate. While relatively simple computational models have sufficed to guide current research in large-scale network dynamics, less attention has been paid to modeling the membrane dynamics in bulbar neurons that may be equally essential to sensory processing. We here present a reduced, conductance-based compartmental model of olfactory bulb mitral cells that exhibits the complex dynamical properties observed in these neurons. Specifically, model neurons exhibit intrinsic subthreshold oscillations with voltage-dependent frequencies that shape the timing of stimulus-evoked action potentials. These oscillations rely on a persistent sodium conductance, an inactivating potassium conductance, and a calcium-dependent potassium conductance and are reset via inhibitory input such as that delivered by periglomerular cell shunt inhibition. Mitral cells fire bursts, or clusters, of spikes when continuously stimulated. Burst properties depend critically on multiple currents, but a progressive deinactivation of I(A) over the course of a burst is an important regulator of burst termination. Each of these complex properties exhibits appropriate dynamics and pharmacology as determined by electrophysiological studies. Additionally, we propose that a second, inconsistently observed form of infrathreshold bistability in mitral cells may derive from the activation of ATP-activated potassium currents responding to hypoxic conditions. We discuss the integration of these cellular properties in the larger context of olfactory bulb network operations.

Detecting activity in olfactory bulb glomeruli with astrocyte recording

In the olfactory bulb, axons of olfactory sensory neurons (OSNs) expressing the same olfactory receptor converge on specific glomeruli. These afferents form axodendritic synapses with mitral/tufted and periglomerular cell dendrites, whereas the dendrites of mitral/tufted cells and periglomerular interneurons form dendrodendritic synapses. The two types of intraglomerular synapses appear to be spatially isolated in subcompartments delineated by astrocyte processes. Because each astrocyte sends processes into a single glomerulus, we used astrocyte recording as an intraglomerular detector of neuronal activity. In glomerular astrocytes, a single shock in the olfactory nerve layer evoked a prolonged inward current, the major part of which was attributable to a barium-sensitive potassium current. The K+ current closely reflected the time course of depolarization of mitral/tufted cells, indicating that K+ accumulation mainly reflects the activity of mitral/tufted cells. The astrocyte K+ current was dependent on AMPA and NMDA receptors in mitral/tufted cells as well as on a previously undescribed metabotropic glutamate receptor 1 component. Block of the K+ current with barium unmasked a synaptic glutamate transporter current. Perhaps surprisingly, the transporter current had components caused by glutamate released at both olfactory nerve terminals and mitral/tufted cell dendrites. The time course of the transporter currents suggested that rapid synchronous glutamate release at OSN terminals triggers asynchronous glutamate release from mitral/tufted cells. Glomerular astrocyte recording provides a sensitive means to examine functional compartmentalization within and between olfactory bulb glomeruli.

Long-term depression at olfactory nerve synapses

The synapses formed by the olfactory nerve (ON) convey sensory information to olfactory glomeruli, the first stage of central odor processing. Morphological and behavioral studies suggest that glomerular odor processing is plastic in neonate rodents. However, long-term synaptic plasticity, a cellular correlate of functional and structural plasticity, has not yet been demonstrated in this system. Here, we report that ON-->mitral cell (MC) synapses of 5- to 8-d-old mice express long-term depression (LTD) after brief low-frequency ON stimulation. Pharmacological techniques and imaging of presynaptic calcium signals demonstrate that ON-MC LTD is expressed presynaptically and requires the activation of metabotropic glutamate receptors but does not require fast synaptic transmission. LTD at the ON--> MC synapse is potentially relevant for the establishment, maintenance, and experience-dependent refinement of odor maps in the olfactory bulb.

Olfactory nerve stimulation-evoked mGluR1 slow potentials, oscillations, and calcium signaling in mouse olfactory bulb mitral cells

Fast synaptic transmission between olfactory receptor neurons and mitral cells (MCs) is mediated through AMPA and NMDA ionotropic glutamate receptors. MCs also express high levels of metabotropic glutamate receptor 1 (mGluR1) whose functional significance is less understood. Here we characterized a slow mGluR1-mediated potential that was evoked by high-frequency (100-Hz) olfactory nerve (ON) stimulation in the presence of NBQX and D-APV, blockers of ionotropic glutamate receptors, and that was associated with a local Ca2+ transient in the MC dendritic tuft. High-frequency ON stimulation in the presence of NBQX and D-APV also evoked a slow, nearly 2-Hz oscillation of MC membrane potential that was abolished by the mGluR1 antagonist LY367385 (50 microM). Both mGluR slow potential and slow oscillation persisted in the presence of gabazine (10 microM), a GABA(A) receptor antagonist, and intracellular QX-314 (10 mM), a Na+ channel blocker. In contrast to a slow mGluR1 potential in cerebellar Purkinje neurons, the MC mGluR1 potential was not depressed by SKF96365 (< or =250 microM) and thus is likely not mediated by TRPC1 cation channels, nor was it potentiated by an elevation of intracellular Ca2+ level. Imaging with the Na+ indicator SBFI revealed a Na+ transient in the MC dendrite accompanying the mGluR1 slow potential. We conclude that the MC mGluR1 potential triggered by glutamate released from the ON supports oscillations and synchronizations of MCs associated within one glomerulus.

Olfactory nerve-evoked, metabotropic glutamate receptor-mediated synaptic responses in rat olfactory bulb mitral cells

The group I metabotropic glutamate receptor (mGluR) subtype, mGluR1, is highly expressed on the apical dendrites of olfactory bulb mitral cells and thus may be activated by glutamate released from olfactory nerve (ON) terminals. Previous studies have shown that mGluR1 agonists directly excite mitral cells. In the present study, we investigated the involvement of mGluR1 in ON-evoked responses in mitral cells in rat olfactory bulb slices using patch-clamp electrophysiology. In voltage-clamp recordings, the average EPSC evoked by single ON shocks or brief trains of ON stimulation (six pulses at 50 Hz) in normal physiological conditions were not significantly affected by the nonselective mGluR antagonist LY341495 (50-100 microM) or the mGluR1-specific antagonist LY367385 (100 microM); ON-evoked responses were attenuated, however, in a subset (36%) of cells. In the presence of blockers of ionotropic glutamate and GABA receptors, application of the glutamate uptake inhibitors THA (300 microM) and TBOA (100 microM) revealed large-amplitude, long-duration responses to ON stimulation, whereas responses elicited by antidromic activation of mitral/tufted cells were unaffected. Magnitudes of the ON-evoked responses elicited in the presence of THA-TBOA were dependent on stimulation intensity and frequency, and were maximal during high-frequency (50-Hz) bursts of ON spikes, which occur during odor stimulation. ON-evoked responses elicited in the presence of THA-TBOA were significantly reduced or completely blocked by LY341495 or LY367385 (100 microM). These results demonstrate that glutamate transporters tightly regulate access of synaptically evoked glutamate from ON terminals to postsynaptic mGluR1s on mitral cell apical dendrites. Taken together with other findings, the present results suggest that mGluR1s may not play a major role in phasic responses to ON input, but instead may play an important role in shaping slow oscillatory activity in mitral cells and/or activity-dependent regulation of plasticity at ON-mitral cell synapses.

Comparative analysis of the subcellular and subsynaptic localization of mGluR1a and mGluR5 metabotropic glutamate receptors in the shell and core of the nucleus accumbens in rat and monkey

Group I metabotropic glutamate receptors (mGluRs) play critical roles in synaptic plasticity and drug addiction. To characterize potential sites whereby these receptors mediate their effects in the ventral striatum, we studied the subcellular and subsynaptic localization of mGluR1a and mGluR5 in the shell and core of the nucleus accumbens in rat and monkey. In both species, group I mGluRs are mainly postsynaptic in dendrites and spines, with rare presynaptic labeling in unmyelinated axons. Minor, yet significant, differences in proportions of specific immunoreactive elements were found between the accumbens shell and the accumbens core in monkey. At the subsynaptic level, significant differences were found in the proportion of plasma membrane-bound mGluR5 labeling between species. In dendrites, spines, and unmyelinated axons, a significantly larger proportion of mGluR5 labeling was bound to the plasma membrane in rats (50-70%) than in monkeys (30-50%). Conversely, mGluR1a displayed the same pattern of immunogold labeling in the two species. Electron microscopic colocalization studies revealed 30% colocalization of mGluR1a and mGluR5 in dendrites and as much as 50-65% in spines in both compartments of the rat accumbens. Both group I mGluRs were significantly expressed in D1-immunoreactive dendritic processes (60-75% colocalization) and spines (30-50%) of striatal projection neurons as well as dendrites of cholinergic (30-70%) and parvalbumin-containing (70-85%) interneurons. These findings highlight the widespread expression of group I mGluRs in projection neurons and interneurons of the shell and core of the nucleus accumbens, providing a solid foundation for regulatory and therapeutic functions of group I mGluRs in reward-related behaviors and drug addiction.

Olfactory nerve stimulation-induced calcium signaling in the mitral cell distal dendritic tuft

Olfactory receptor neuron axons form the olfactory nerve (ON) and project to the glomerular layer of the olfactory bulb, where they form excitatory synapses with terminal arborizations of the mitral cell (MC) tufted primary dendrite. Clusters of MC dendritic tufts define olfactory glomeruli, where they involve in complex synaptic interactions. The computational function of these cellular interactions is not clear. We used patch-clamp electrophysiology combined with whole field or two-photon Ca2+ imaging to study ON stimulation-induced Ca2+ signaling at the level of individual terminal branches of the MC primary dendrite in mice. ON-evoked subthreshold excitatory postsnaptic potentials induced Ca2+ transients in the MC tuft dendrites that were spatially inhomogeneous, exhibiting discrete "hot spots." In contrast, Ca2+ transients induced by backpropagating action potentials occurred throughout the dendritic tuft, being larger in the thin terminal dendrites than in the base of the tuft. Single ON stimulation-induced Ca2+ transients were depressed by the NMDA receptor antagonist D-aminophosphonovaleric acid (D-APV), increased with increasing stimulation intensity, and typically showed a prolonged rising phase. The synaptically induced Ca2+ signals reflect, at least in part, dendrodendritic interactions that support intraglomerular coupling of MCs and generation of an output that is common to all MCs associated with one glomerulus.