David H. Gire

Control of On/Off Glomerular Signaling by a Local GABAergic Microcircuit in the Olfactory Bulb

Odors are coded at the input level of the olfactory bulb by a spatial map of activated glomeruli, reflecting different odorant receptors (ORs) stimulated in the nose. Here we examined the function of local synaptic processing within glomeruli in transforming these input patterns into an output for the bulb, using patch-clamp recordings and calcium imaging in rat bulb slices. Two types of transformations were observed at glomeruli, the first of which produced a bimodal, “on/off” glomerular signal that varied probabilistically depending on olfactory receptor neuron (ORN) input levels. The bimodal response behavior was seen in glomerular synaptic responses, as well as in action potential (“spike”) firing, wherein all mitral cells affiliated with a glomerulus either engaged in prolonged spike bursts or did not spike at all. In addition, evidence was obtained that GABAergic periglomerular (PG) cells that surround a glomerulus can prevent activation of a glomerulus through inhibitory inputs targeted onto excitatory external tufted cells. The path of PG cell activation appeared to be confined to one glomerulus, such that ORNs at one glomerulus initiated inhibition of the same glomerulus. The observed glomerular “self-inhibition” provides a mechanism of filtering odor signals that would be an alternative to commonly proposed mechanisms of lateral inhibition between OR-specific glomeruli. In this case, selective suppression of weak odor signals could be achieved based on the difference in the input resistance of PG cells versus excitatory neurons at a glomerulus.

Mitral cells in the olfactory bulb are mainly excited through a multistep signaling path

Within the olfactory system, information flow from the periphery onto output mitral cells (MCs) of the olfactory bulb (OB) has been thought to be mediated by direct synaptic inputs from olfactory sensory neurons (OSNs). Here, we performed patch-clamp measurements in rat and mouse OB slices to investigate mechanisms of OSN signaling onto MCs, including the assumption of a direct path, using electrical and optogenetic stimulation methods that selectively activated OSNs. We found that MCs are in fact not typically activated by direct OSN inputs and instead require a multistep, diffuse mechanism involving another glutamatergic cell type, the tufted cells. The preference for a multistep mechanism reflects the fact that signals arising from direct OSN inputs are drastically shunted by connexin 36-mediated gap junctions on MCs, but not tufted cells. An OB circuit with tufted cells intermediate between OSNs and MCs suggests that considerable processing of olfactory information occurs before its reaching MCs.

Long-term enhancement of synchronized oscillations by adrenergic receptor activation in the olfactory bulb.

The noradrenergic system is widely thought to be important for associative learning in the olfactory system through actions in the first processing structure, the main olfactory bulb (MOB). Here, we used extracellular local field potential (LFP) and patch-clamp recordings in rat MOB slices to examine norepinephrine (NE)-induced long-term changes in circuit properties that might underlie learning. During responses to patterned olfactory nerve stimulation mimicking the breathing cycle, NE induced a long-term increase in gamma frequency (30-70 Hz) synchronized oscillations. The enhancement persisted long after washout of NE (<or=70 min), depended on the combined actions of NE and neuronal stimulation, and seemed to be caused by enhanced excitatory drive on the mitral/granule cell network that underlies rapid gamma oscillations. The last effect, increased excitation, was manifested as an increase in evoked long-lasting depolarizations (LLDs) in mitral cells. From a functional perspective, the observed long-term cellular and network changes could promote associative learning by amplifying odor-specific signals.

Information for decision-making and stimulus identification is multiplexed in sensory cortex

In recordings from anterior piriform cortex in awake behaving mice, we found that neuronal firing early in the olfactory pathway simultaneously conveyed fundamentally different information: odor value (is the odor rewarded?) and identity (what is the smell?). Thus, this sensory system performs early multiplexing of information reflecting stimulus-specific characteristics with that used for decision-making.

Adrenergic Receptor-Mediated Disinhibition of Mitral Cells Triggers Long-Term Enhancement of Synchronized Oscillations in the Olfactory Bulb

Norepinephrine (NE) is widely implicated in various forms of associative olfactory learning in rodents, including early learning preference in neonates. Here we used patch-clamp recordings in rat olfactory bulb slices to assess cellular actions of NE, examining both acute, short-term effects of NE as well as the relationship between these acute effects and long-term cellular changes that could underlie learning. Our focus for long-term effects was on synchronized gamma frequency (30-70 Hz) oscillations, shown in prior studies to be enhanced for up to an hour after brief exposure of a bulb slice to NE and neuronal stimulation. In terms of acute effects, we found that a dominant action of NE was to reduce inhibitory GABAergic transmission from granule cells (GCs) to output mitral cells (MCs). This disinhibition was also induced by clonidine, an agonist specific for alpha(2) adrenergic receptors (ARs). Acute NE-induced disinhibition of MCs appeared to be linked to long-term enhancement of gamma oscillations, based, first, on the fact that clonidine, but not agonists specific for other AR subtypes, mimicked NEs long-term actions. In addition, the alpha(2) AR-specific antagonist yohimbine blocked the long-term enhancement of the oscillations due to NE. Last, brief exposure of the slice to the GABA(A) receptor antagonist gabazine, to block inhibitory synapses directly, also induced the long-term changes. Acute disinhibition is a plausible permissive effect of NE leading to olfactory learning, because, when combined with exposure to a specific odor, it should lead to neuron-specific increases in intracellular calcium of the type generally associated with long-term synaptic modifications.

Precise Detection of Direct Glomerular Input Duration by the Olfactory Bulb

Sensory neuron input to the olfactory bulb (OB) was activated precisely for different durations with blue light in mice expressing channelrhodopsin-2 in olfactory sensory neurons. Behaviorally the mice discriminated differences of 10 ms in duration of direct glomerular activation. In addition, a subset of mitral/tufted cells in the OB of awake mice responded tonically therefore conveying information on stimulus duration. Our study provides evidence that duration of the input to glomeruli not synchronized to sniffing is detected. This potent cue may be used to obtain information on puffs in odor plumes.

Thresholding of sensory inputs by extrasynaptic glutamate receptors in olfactory bulb glomeruli

The mammalian olfactory bulb has presented a challenging system for understanding information processing, in part because the bulb largely lacks the topographical ordering of neurons that promotes processes such as lateral inhibition. Here we have used dual and triple-cell recordings in rodent bulb slices combined with ultrastructural methods to provide the first experimental evidence for a processing mechanism circumventing this problem that operates at the level of single glomeruli, the bulb’s odorant receptor-specific modules. A key feature is non-traditional, extrasynaptic glutamatergic signaling derived from excitatory interneurons and what it means for the local balance between excitation (E) and inhibition (I). We found that the distinct dynamic properties of extrasynaptic excitation versus synaptic inhibition create a thresholding effect whereby only strong stimuli produce a favorable E/I balance enabling an output. This single-glomerulus threshold could have a number of important functions during natural odor responses, for example enhancing stimulus tuning.