The acquisition of olfactory information and its early processing in mammals are modulated by brain states through sniffing behavior and neural feedback. We imaged the spatiotemporal pattern of odor-evoked activity in a population of output neurons (mitral/tufted cells, MTCs) in the olfactory bulb (OB) of head-restrained mice expressing a genetically-encoded calcium indicator. The temporal dynamics of MTC population activity were relatively simple in anesthetized animals, but were highly variable in awake animals. However, the apparently irregular activity in awake animals could be predicted well using sniff timing measured externally, or inferred through fluctuations in the global responses of MTC population even without explicit knowledge of sniff times. The overall spatial pattern of activity was conserved across states, but odor responses had a diffuse spatial component in anesthetized mice that was less prominent during wakefulness. Multi-photon microscopy indicated that MTC lateral dendrites were the likely source of spatially disperse responses in the anesthetized animal. Our data demonstrate that the temporal and spatial dynamics of MTCs can be significantly modulated by behavioral state, and that the ensemble activity of MTCs can provide information about sniff timing to downstream circuits to help decode odor responses.
Sensory perception is not a simple feed-forward process, and higher brain areas can actively modulate information processing in "lower" areas. We used optogenetic methods to examine how cortical feedback projections affect circuits in the first olfactory processing stage, the olfactory bulb. Selective activation of back projections from the anterior olfactory nucleus/cortex (AON) revealed functional glutamatergic synaptic connections on several types of bulbar interneurons. Unexpectedly, AON axons also directly depolarized mitral cells (MCs), enough to elicit spikes reliably in a time window of a few milliseconds. MCs received strong disynaptic inhibition, a third of which arises in the glomerular layer. Activating feedback axons in vivo suppressed spontaneous as well as odor-evoked activity of MCs, sometimes preceded by a temporally precise increase in firing probability. Our study indicates that cortical feedback can shape the activity of bulbar output neurons by enabling precisely timed spikes and enforcing broad inhibition to suppress background activity.
Various technological advances have made imaging an increasingly useful tool in the life sciences. Imaging techniques that move away from the limitations of wide-field microscopy have allowed deeper, higher-resolution imaging of thick biological tissue. Even within wide-field microscopy, advancements such as structured and sheet illumination, as well as improvements of biological probes, have led to better visualization of cells and their subcellular structures. The illumination source for wide-field microscopy, however, has remained relatively unchanged for decades, relying mainly on xenon, mercury, and halogen lamps. Light-emitting diodes (LEDs) have existed for more than 80 years, but their spectral range and output light flux have only recently become large enough for use in biological applications. This article presents the basic information necessary to build and to use an LED-based illumination source for microscopy. It also provides some useful resources about LED advancements. Although commercial LED-based illuminators for microscopy are available, custom-built illumination systems can incorporate the latest LED chips into microscopes more quickly and inexpensively than is possible through the retail market.
Primary olfactory cortical areas receive direct input from the olfactory bulb, but also have extensive associational connections that have been mainly studied with classical anatomical methods. Here, we shed light on the functional properties of associational connections in the anterior and posterior piriform cortices (aPC and pPC) using optophysiological methods. We found that the aPC receives dense functional connections from the anterior olfactory nucleus (AON), a major hub in olfactory cortical circuits. The local recurrent connectivity within the aPC, long invoked in cortical autoassociative models, is sparse and weak. By contrast, the pPC receives negligible input from the AON, but has dense connections from the aPC as well as more local recurrent connections than the aPC. Finally, there are negligible functional connections from the pPC to aPC. Our study provides a circuit basis for a more sensory role for the aPC in odor processing and an associative role for the pPC.
Alterations in network activity trigger compensatory changes in excitation and inhibition that restore neuronal firing rate to an optimal range. One example of such synaptic homeostasis is the downregulation of inhibitory transmission by chronic inactivity, in part, through the reduction of vesicular transmitter content. The enzyme glutamic acid decarboxylase 67 (GAD67) is critical for GABA synthesis, but its involvement in homeostatic plasticity is unclear. We explored the role of GAD67 in activity-dependent synaptic plasticity using a mouse line (Gad1–/–) in which GAD67 expression is disrupted by genomic insertion of the green fluorescent protein (GFP). Homozygous deletion of Gad1 significantly reduced miniature inhibitory postsynaptic current (mIPSC) amplitudes and GABA levels in cultured hippocampal neurons. The fractional block of mIPSC amplitude by a low affinity, competitive GABAA receptor antagonist was higher in GAD67-lacking neurons, suggesting that GABA concentration in the synaptic cleft is lower in knockout animals. Chronic suppression of activity by the application of tetrodotoxin (TTX) reduced mIPSC amplitudes and the levels of GAD67 and GABA. Moreover, TTX reduced GFP levels in interneurons, suggesting that GAD67 gene expression is a key regulatory target of activity. These in vitro experiments were corroborated by in vivo studies in which olfactory deprivation reduced mIPSC amplitudes and GFP levels in glomerular neurons in the olfactory bulb. Importantly, TTX-induced downregulation of mIPSC was attenuated in Gad1–/– neurons. Altogether, these findings indicate that activity-driven expression of GAD67 critically controls GABA synthesis and, thus, vesicular filling of the transmitter.
Two-photon microscopy has become an invaluable tool for visualizing the activity of neuronal populations at cellular resolution in vivo. Imaging typically requires restraining the head of the animal underneath the objective of a dedicated optical setup and experiments are therefore often performed under anesthesia. Here, we describe a method that allows imaging in awake mice with minimal motion artifacts and without the need for extensive training of the animal. We detail the necessary surgical procedures to chronically implant a small, lightweight headplate and to create a clear window for imaging. The design of a simple apparatus capable of stably accommodating the headplate while the mouse is positioned on a wheel with spring suspension is presented. When used in combination with a multiphoton microscope, this approach greatly facilitates optical recordings in nonanesthetized animals and opens the door to many projects that can bridge the gap between neural activity and behavior.
The vomeronasal organ (VNO) has a key role in mediating the social and defensive responses of many terrestrial vertebrates to species- and sex-specific chemosignals1. More than 250 putative pheromone receptors have been identified in the mouse VNO2, 3, but the nature of the signals detected by individual VNO receptors has not yet been elucidated. To gain insight into the molecular logic of VNO detection leading to mating, aggression or defensive responses, we sought to uncover the response profiles of individual vomeronasal receptors to a wide range of animal cues. Here we describe the repertoire of behaviourally and physiologically relevant stimuli detected by a large number of individual vomeronasal receptors in mice, and define a global map of vomeronasal signal detection. We demonstrate that the two classes (V1R and V2R) of vomeronasal receptors use fundamentally different strategies to encode chemosensory information, and that distinct receptor subfamilies have evolved towards the specific recognition of certain animal groups or chemical structures. The association of large subsets of vomeronasal receptors with cognate, ethologically and physiologically relevant stimuli establishes the molecular foundation of vomeronasal information coding, and opens new avenues for further investigating the neural mechanisms underlying behaviour specificity.
The responses of neural elements in many sensory areas of the brain vary systematically with their physical position, leading to a topographic representation of the outside world. Sensory representation in the olfactory system has been harder to decipher, in part because it is difficult to find appropriate metrics to characterize odor space and to sample this space densely. Recent experiments have shown that the arrangement of glomeruli, the elementary units of processing, is relatively invariant across individuals in a species, yet it is flexible enough to accommodate new sensors that might be added. Evidence supports the existence of coarse spatial domains carved out on a genetic or functional basis, but a systematic organization of odor responses or neural circuits on a local scale is not evident. Experiments and theory that relate the properties of odorant receptors to the detailed wiring diagram of the downstream olfactory circuits and to behaviors they trigger may reveal the design principles that have emerged during evolution.
The mitral-granule cell (MC-GC) reciprocal synapse is an important source of auto- and lateral inhibition in the olfactory bulb (OB), and this local inhibition is critical for odor discrimination. We may gain insight into the role of MC autoinhibition in olfaction by correlating the functional development of the autoinhibition with the postnatal development of olfactory function. We have studied the functional development of the MC-GC reciprocal synapse using whole-cell patch-clamp recordings from MCs and GCs in acute olfactory bulb slices from 3- to 30-day old rats. The magnitude of dendrodendritic inhibition (DDI) measured by depolarizing a single MC and recording recurrent inhibition in the same cell increased up to the fifteenth day of life (P15), but dropped between P15 and P30. The initial increase and later decrease in DDI was echoed by a similar increase and decrease in the frequency of miniature inhibitory postsynaptic currents (mIPSCs), suggesting an accompanying modulation in the number of synapses available to participate in DDI. The late decrease in DDI could also result, in part, from a decrease in GC excitability as well as an increase in relative contribution of N-methyl D-aspartate (NMDA) receptors to gamma-amino butyric acid (GABA) release from GC synapses. Changes in release probability of GABAergic synapses are unlikely to account for the late reduction in DDI, although they might contribute to the early increase during development. Our results demonstrate that the functional MC-GC circuit evolves over development in a complex manner that may include both construction and elimination of synapses.
Neural activity is intimately tied to blood flow in the brain. This coupling is specific enough in space and time that modern imaging methods use local hemodynamics as a measure of brain activity. In this review, we discuss recent evidence indicating that neuronal activity is coupled to local blood flow changes through an intermediary, the astrocyte. We highlight unresolved issues regarding the role of astrocytes and propose ways to address them using novel techniques. Our focus is on cellular level analysis in vivo, but we also relate mechanistic insights gained from ex vivo experiments to native tissue. We also review some strategies to harness advances in optical and genetic methods to study neurovascular coupling in the intact brain.
Sensory inputs frequently converge on the brain in a spatially organized manner, often with overlapping inputs to multiple target neurons. Whether the responses of target neurons with common inputs become decorrelated depends on the contribution of local circuit interactions. We addressed this issue in the olfactory system using newly generated transgenic mice that express channelrhodopsin-2 in all of the olfactory sensory neurons. By selectively stimulating individual glomeruli with light, we identified mitral/tufted cells that receive common input (sister cells). Sister cells had highly correlated responses to odors, as measured by average spike rates, but their spike timing in relation to respiration was differentially altered. In contrast, non-sister cells correlated poorly on both of these measures. We suggest that sister mitral/tufted cells carry two different channels of information: average activity representing shared glomerular input and phase-specific information that refines odor representations and is substantially independent for sister cells.
Revealing the functional connectivity in natural neuronal networks is central to understanding circuits in the brain. Here, we show that silicon nanowire field-effect transistor (Si NWFET) arrays fabricated on transparent substrates can be reliably interfaced to acute brain slices. NWFET arrays were readily designed to record across a wide range of length scales, while the transparent device chips enabled imaging of individual cell bodies and identification of areas of healthy neurons at both upper and lower tissue surfaces. Simultaneous NWFET and patch clamp studies enabled unambiguous identification of action potential signals, with additional features detected at earlier times by the nanodevices. NWFET recording at different positions in the absence and presence of synaptic and ion-channel blockers enabled assignment of these features to presynaptic firing and postsynaptic depolarization from regions either close to somata or abundant in dendritic projections. In all cases, the NWFET signal amplitudes were from 0.3-3 mV. In contrast to conventional multielectrode array measurements, the small active surface of the NWFET devices, approximately 0.06 microm(2), provides highly localized multiplexed measurements of neuronal activities with demonstrated sub-millisecond temporal resolution and, significantly, better than 30 microm spatial resolution. In addition, multiplexed mapping with 2D NWFET arrays revealed spatially heterogeneous functional connectivity in the olfactory cortex with a resolution surpassing substantially previous electrical recording techniques. Our demonstration of simultaneous high temporal and spatial resolution recording, as well as mapping of functional connectivity, suggest that NWFETs can become a powerful platform for studying neural circuits in the brain.
Centrifugal serotonergic fibers innervate the olfactory bulb, but the importance of these projections for olfactory processing is unclear. We examined serotonergic modulation of sensory input to olfactory glomeruli using mice that express synaptopHluorin in olfactory receptor neurons (ORN). Odor-evoked synaptic input to glomeruli was attenuated by increased serotonin signaling through serotonin 2C (5-HT2C) receptors and amplified by decreased serotonergic activity. Intravital multiphoton calcium imaging revealed that 5-HT2C receptor activation amplified odor-evoked activity in a subset of juxtaglomerular cells and attenuated glutamate release from ORN terminals via GABA(B) receptors. Endogenous serotonin released by electrical stimulation of the dorsal raphe nucleus attenuated odor-evoked responses without detectable bias in glomerular position or odor identity. Weaker glomerular responses, however, were less sensitive to raphe stimulation than strong responses. Our data indicate that the serotonergic system regulates odor inputs in the olfactory bulb and suggest that behavioral states may alter odor processing at the earliest stages.
We explored the map of odor space created by glomeruli on the olfactory bulb of both rat and mouse. Identified glomeruli could be matched across animals by their response profile to hundreds of odors. Their layout in different individuals varied by only approximately 1 glomerular spacing, corresponding to a precision of 1 part in 1,000. Across species, mouse and rat share many glomeruli with apparently identical odor tuning, arranged in a similar layout. In mapping the position of a glomerulus to its odor tuning, we found only a coarse relationship with a precision of approximately 5 spacings. No chemotopic order was apparent on a finer scale and nearby glomeruli were almost as diverse in their odor sensitivity as distant ones. This local diversity of sensory tuning stands in marked distinction from other brain maps. Given the reliable placement of the glomeruli, it represents a feature, not a flaw, of the olfactory bulb.
Functional neuroimaging uses activity-dependent changes in cerebral blood flow to map brain activity, but the contributions of presynaptic and postsynaptic activity are incompletely understood, as are the underlying cellular pathways. Using intravital multiphoton microscopy, we measured presynaptic activity, postsynaptic neuronal and astrocytic calcium responses, and erythrocyte velocity and flux in olfactory glomeruli during odor stimulation in mice. Odor-evoked functional hyperemia in glomerular capillaries was highly correlated with glutamate release, but did not require local postsynaptic activity. Odor stimulation induced calcium transients in astrocyte endfeet and an associated dilation of upstream arterioles. Calcium elevations in astrocytes and functional hyperemia depended on astrocytic metabotropic glutamate receptor 5 and cyclooxygenase activation. Astrocytic glutamate transporters also contributed to functional hyperemia through mechanisms independent of calcium rises and cyclooxygenase activation. These local pathways initiated by glutamate account for a large part of the coupling between synaptic activity and functional hyperemia in the olfactory bulb.
New developments in fluorophores as well as in detection methods have fueled the rapid growth of optical imaging in the life sciences. Commercial widefield microscopes generally use arc lamps, excitation/emission filters and shutters for fluorescence imaging. These components can be expensive, difficult to maintain and preclude stable illumination. Here, we describe methods to construct inexpensive and easy-to-use light sources for optical microscopy using light-emitting diodes (LEDs). We also provide examples of its applicability to biological fluorescence imaging.
Experience-dependent changes in neural circuits have traditionally been investigated several synapses downstream of sensory input. Whether experience can alter the strength of primary sensory synapses remains mostly unknown. To address this issue, we investigated the consequences of odor deprivation on synapses made by olfactory sensory axons in the olfactory bulb of rats. Odor deprivation triggered an increase in the probability of glutamate release from olfactory sensory neuron synapses. Deprivation also increased the amplitude of quantal synaptic currents mediated by AMPA- and NMDA-type glutamate receptors, as well as the abundance of these receptors in the glomerular region. Our results demonstrate that sensory experience is capable of modulating synaptic strength at the earliest stages of information transfer between the environment and an organism. Such compensatory experience-dependent changes may represent a mechanism of sensory gain control.
The ability of synapses to sustain signal propagation relies on rapid recycling of transmitter-containing presynaptic vesicles. Clathrin- and dynamin-mediated retrieval of vesicular membrane has an undisputed role in synaptic vesicle recycling. There is also evidence for other modes of vesicle retrieval, including bulk retrieval and the so-called kiss-and-run recycling. Whether dynamin in required for these other modes of synaptic vesicle endocytosis remains unclear. Here, we have tested the role of dynamin in synaptic vesicle endocytosis by using a small molecule called dynasore, which rapidly inhibits the GTPase activity of dynamin with high specificity. Endocytosis after sustained or brief stimuli was completely and reversibly blocked by dynasore in cultured hippocampal neurons expressing the fluorescent tracer synaptopHluorin. By contrast, dynasore had no effect on exocytosis. In the presence of dynasore, low-frequency stimulation led to sustained accumulation of synaptopHluorin and other vesicular proteins on the surface membrane at a rate predicted from net exocytosis. These vesicular components remained on surface membranes even after the stimulus was terminated, suggesting that all endocytic events rely on dynamin during low-frequency activity as well as in the period after it. Ultrastructural analysis revealed a reduction in the density of synaptic vesicles and the presence of endocytic structures only at synapses that were stimulated in the presence of dynasore. In sum, our data indicate that dynamin is essential for all forms of compensatory synaptic vesicle endocytosis including any kiss-and-run events.
Neural activity regulates the number and properties of GABAergic synapses in the brain, but the mechanisms underlying these changes are unclear. We found that blocking spike activity globally in developing hippocampal neurons from rats reduced the density of GABAergic terminals as well as the frequency and amplitude of miniature inhibitory postsynaptic currents (mIPSCs). Chronic inactivity later in development led to a reduction in the mIPSC amplitude, without any change in GABAergic synapse density. By contrast, hyperpolarizing or abolishing spike activity in single neurons did not alter GABAergic synaptic inputs. Suppressing activity in individual presynaptic GABAergic neurons also failed to decrease synaptic output. Our results indicate that GABAergic synapses are regulated by the level of activity in surrounding neurons. Notably, we found that the expression of GABAergic plasticity involves changes in the amount of neurotransmitter in individual vesicles.
Genetically encoded fluorescent probes have become indispensable tools in the biological sciences. Studies of synaptic vesicle recycling have been facilitated by a group of GFP-derived probes called pHluorins. These probes exploit changes in pH that accompany exocytosis and recapture of synaptic vesicles. Here we describe how these synaptic tracers can be used in rodent hippocampal neurons to monitor the synaptic vesicle cycle in real time and to obtain mechanistic insights about it. Synapses can be observed in living samples using a wide-field fluorescence microscope and a cooled charge-coupled device camera. A simple specimen chamber allows electrical stimulation of synapses to evoke exocytosis in a precisely controlled manner. We present protocols to measure various parameters of the synaptic vesicle cycle. This technique can be easily adapted to study different classes of synapses from wild-type and mutant mice. Once cultured neurons expressing synaptopHluorin are available, the whole procedure should take about 2 h.