Fuqiang Xu

Simultaneous activation of mouse main and accessory olfactory bulbs by odors or pheromones

It is generally believed that the main olfactory system processes common odors and the accessory olfactory system is specifically for pheromones. The potential for these two systems to respond simultaneously to the same stimuli has not been fully explored due to methodological limitations. Here we examine this phenomenon using high‐resolution functional magnetic resonance imaging (fMRI) to reveal simultaneously the responses in the main (MOB) and accessory olfactory bulbs (AOB) to odors and pheromones. Common odorants elicited strong signals in the MOB and weak signals in the AOB. 2‐Heptanone, a known mouse pheromone, elicited strong signals in both the MOB and AOB. Urine odor, a complicated mixture of pheromones and odorants, elicited significant signals in limited regions of the MOB and large regions of the AOB. The fMRI results demonstrate that both the main and the accessory olfactory systems may respond to volatile compounds but with different selectivity, suggesting a greater integration of the two olfactory pathways than traditionally believed. J. Comp. Neurol. 489:491–500, 2005.

Odor maps of aldehydes and esters revealed by functional MRI in the glomerular layer of the mouse olfactory bulb

Odorant identity is believed to be encoded in the olfactory bulb (OB) by glomerular activity patterns. It has not yet been possible to visualize and compare entire patterns for different odorants in the same animal because of technical limitations. For this purpose we used high-resolution functional MRI at 7 T, combined with glomerular-layer flat maps, to reveal responses to aliphatic homologues in the mouse OB. These odorants elicited reproducible patterns in the OB, with the medial and lateral regions containing the most intense signals. Unexpectedly, in view of the symmetrical projections of olfactory receptor neurons to medial and lateral glomeruli, the activity patterns in these regions were asymmetrical. The highly activated medial and lateral areas were shared by homologous members, generating a conserved “family signature” for a homologous series. The moderately active areas, including the dorsal region that has been extensively studied by optical imaging, were more sensitive to the length of the carbon chain, producing more subtle features of individual members and different changing trends among homologues. The global mapping with functional MRI not only extended previous studies but also revealed additional rules for representation of homologues in the OB. Insights into possible relations between the functional patterns, molecular projections, and odor perception may now be obtained based on the global from the olfactory epithelium to the OB glomerular activity patterns.

A unique urinary constituent, 6-hydroxy-6-methyl-3-heptanone, is a pheromone that accelerates puberty in female mice.

BACKGROUND Olfactorily mediated puberty acceleration in female mice (measured by an increase in uterine weight) has been observed since the 1960s without the active chemosignal being structurally identified. There are many controversies in the literature as to whether this male-originated pheromone is a volatile substance. We investigated the chemical nature of the urinary fractions that are responsible for the characteristic uterine weight increases. RESULTS The active pheromone was identified as 5,5-dimethyl-2-ethyltetrahydrofuran-2-ol and/or its open-chain tautomer (6-hydroxy-6-methyl-3-heptanone). A series of cyclic vinyl ethers were isolated from chromatographically active fractions of the urine. Because these compounds did not accelerate puberty, we postulated that these ethers were degradation products of a lactol (5,5-dimethyl-2-ethyltetrahydrofuran-2-ol). The lactol was then detected directly in the mouse urine extract using a silylation agent. Synthetic 6-hydroxy-6-methyl-3-heptanone had strong biological activity, whereas its close structural analogs did not. CONCLUSIONS The male house mouse excretes into its urine a large quantity of a volatile substance that has a unique lactol/hydroxyketone structure. This substance is capable of binding to the less volatile urinary constituents, such as proteins or peptides, and is active in puberty-acceleration bioassays. The controversies regarding the volatility of the puberty-accelerating pheromones can now be explained by considering a complex of volatile lactol/hydroxyketone and urinary proteins.

Role for the Membrane Receptor Guanylyl Cyclase-C in Attention Deficiency and Hyperactive Behavior

A receptor for gut hormones also functions in the brain, where its loss affects attention. Midbrain dopamine neurons regulate many important behavioral processes, and their dysfunctions are associated with several human neuropsychiatric disorders such as attention deficit hyperactivity disorder (ADHD) and schizophrenia. Here, we report that these neurons in mice selectively express guanylyl cyclase-C (GC-C), a membrane receptor previously thought to be expressed mainly in the intestine. GC-C activation potentiates the excitatory responses mediated by glutamate and acetylcholine receptors via the activity of guanosine 3′,5′-monophosphate–dependent protein kinase (PKG). Mice in which GC-C has been knocked out exhibit hyperactivity and attention deficits. Moreover, their behavioral phenotypes are reversed by ADHD therapeutics and a PKG activator. These results indicate important behavioral and physiological functions for the GC-C/PKG signaling pathway within the brain and suggest new therapeutic targets for neuropsychiatric disorders related to the malfunctions of midbrain dopamine neurons.

Processing of visually evoked innate fear by a non-canonical thalamic pathway

The ability of animals to respond to life-threatening stimuli is essential for survival. Although vision provides one of the major sensory inputs for detecting threats across animal species, the circuitry underlying defensive responses to visual stimuli remains poorly defined. Here, we investigate the circuitry underlying innate defensive behaviours elicited by predator-like visual stimuli in mice. Our results demonstrate that neurons in the superior colliculus (SC) are essential for a variety of acute and persistent defensive responses to overhead looming stimuli. Optogenetic mapping revealed that SC projections to the lateral posterior nucleus (LP) of the thalamus, a non-canonical polymodal sensory relay, are sufficient to mimic visually evoked fear responses. In vivo electrophysiology experiments identified a di-synaptic circuit from SC through LP to the lateral amygdale (Amg), and lesions of the Amg blocked the full range of visually evoked defensive responses. Our results reveal a novel collicular–thalamic–Amg circuit important for innate defensive responses to visual threats.

Whole-brain mapping of the direct inputs and axonal projections of POMC and AgRP neurons

Pro-opiomelanocortin (POMC) neurons in the arcuate nucleus (ARC) of the hypothalamus and nucleus tractus solitarius (NTS) of the brainstem play important roles in suppressing food intake and maintaining energy homeostasis. Previous tract-tracing studies have revealed the axonal connection patterns of these two brain areas, but the intermingling of POMC neurons with other neuron types has made it challenging to precisely identify the inputs and outputs of POMC neurons. In this study, we used the modified rabies virus to map the brain areas that provide direct inputs to the POMC neurons in the ARC and NTS as well as the inputs to the ARC AgRP neurons for comparison. ARC POMC neurons receive inputs from dozens of discrete structures throughout the forebrain and brainstem. The brain areas containing the presynaptic partners of ARC POMC neurons largely overlap with those of ARC AgRP neurons, although POMC neurons receive relatively broader, denser inputs. Furthermore, POMC neurons in the NTS receive direct inputs predominantly from the brainstem and show very different innervation patterns for POMC neurons in the ARC. By selectively expressing fluorescent markers in the ARC and NTS POMC neurons, we found that almost all of their major presynaptic partners are innervated by POMC neurons in the two areas, suggesting that there are strong reciprocal projections among the major POMC neural pathways. By comprehensively chartering the whole-brain connections of the central melanocortin system in a cell-type-specific manner, this study lays the foundation for dissecting the roles and underlying circuit mechanisms of specific neural pathways in regulating energy homeostasis.

Mapping at glomerular resolution: fMRI of rat olfactory bulb.

The rat olfactory bulb contains ∼2000 functional units called glomeruli which are used to recognize specific characteristics of odorants. Activity localization of individual glomerulae (∼0.002 μL) has important consequences for understanding mechanisms in olfactory information encoding. High‐resolution functional MRI (fMRI) data from the rat olfactory bulb are presented using the blood oxygenation level dependent (BOLD) method at 7 T. Either individual or clusters of fMRI voxels suggestive of activity in the olfactory nerve and glomerular layers were reproducibly detected with repeated 2‐min exposures of iso‐amyl acetate at spatial resolution of 0.001–0.003 μL. The importance of glomerular clustering for olfaction and the implications of BOLD mapping with even higher spatial resolution (i.e., ≪0.001 μL voxels) are discussed. High‐resolution in vivo mapping of the rat olfactory bulb with fMRI at high magnetic field promises to provide novel data for understanding olfaction. Magn Reson Med 48:570–576, 2002.

Lateral Entorhinal Modulation of Piriform Cortical Activity and Fine Odor Discrimination

The lateral entorhinal cortex (LEC) receives direct input from olfactory bulb mitral cells and piriform cortical pyramidal cells and is the gateway for olfactory input to the hippocampus. However, the LEC also projects back to the piriform cortex and olfactory bulb. Activity in the LEC is shaped by input from the perirhinal cortices, hippocampus, and amygdala, and thus could provide a rich contextual modulation of cortical odor processing. The present study further explored LEC feedback to anterior piriform cortex by examining how LEC top-down input modulates anterior piriform cortex odor evoked activity in rats. Retrograde viral tracing confirmed rich LEC projections to both the olfactory bulb and piriform cortices. In anesthetized rats, reversible lesions of the ipsilateral LEC increased anterior piriform cortical single-unit spontaneous activity. In awake animals performing an odor discrimination task, unilateral LEC reversible lesions enhanced ipsilateral piriform cortical local field potential oscillations during odor sampling, with minimal impact on contralateral activity. Bilateral LEC reversible lesions impaired discrimination performance on a well learned, difficult odor discrimination task, but had no impact on a well learned simple odor discrimination task. The simple discrimination task was impaired by bilateral reversible lesions of the anterior piriform cortex. Given the known function of LEC in working memory and multisensory integration, these results suggest it may serve as a powerful top-down modulator of olfactory cortical function and odor perception. Furthermore, the results provide potential insight into how neuropathology in the entorhinal cortex could contribute to early olfactory deficits seen in Alzheimers disease.

Whole-Brain Mapping of Inputs to Projection Neurons and Cholinergic Interneurons in the Dorsal Striatum

The dorsal striatum integrates inputs from multiple brain areas to coordinate voluntary movements, associative plasticity, and reinforcement learning. Its projection neurons consist of the GABAergic medium spiny neurons (MSNs) that express dopamine receptor type 1 (D1) or dopamine receptor type 2 (D2). Cholinergic interneurons account for a small portion of striatal neuron populations, but they play important roles in striatal functions by synapsing onto the MSNs and other local interneurons. By combining the modified rabies virus with specific Cre- mouse lines, a recent study mapped the monosynaptic input patterns to MSNs. Because only a small number of extrastriatal neurons were labeled in the prior study, it is important to reexamine the input patterns of MSNs with higher labeling efficiency. Additionally, the whole-brain innervation pattern of cholinergic interneurons remains unknown. Using the rabies virus-based transsynaptic tracing method in this study, we comprehensively charted the brain areas that provide direct inputs to D1-MSNs, D2-MSNs, and cholinergic interneurons in the dorsal striatum. We found that both types of projection neurons and the cholinergic interneurons receive extensive inputs from discrete brain areas in the cortex, thalamus, amygdala, and other subcortical areas, several of which were not reported in the previous study. The MSNs and cholinergic interneurons share largely common inputs from areas outside the striatum. However, innervations within the dorsal striatum represent a significantly larger proportion of total inputs for cholinergic interneurons than for the MSNs. The comprehensive maps of direct inputs to striatal MSNs and cholinergic interneurons shall assist future functional dissection of the striatal circuits.

Adaptation in the rodent olfactory bulb measured by fMRI

Effective evaluation of the odor environment necessitates the ability to attenuate responses to potent background odors in favor of novel and less robust stimuli. Olfactory receptor neuron studies suggest that some of this adaptation takes place in the primary sensory neurons, but the more extensive adaptation seen in higher cortical areas implies the involvement of additional neural mechanisms. At 7.0 T, high‐resolution fMRI was used to assess the response of the rodent olfactory bulb, the most peripheral cortical structure involved in olfactory processing, to a variety of odor stimuli. The results suggest that there are additional regulatory mechanisms in the olfactory bulb that result in greater adaptation in deeper areas than that seen in sensory receptors alone and that the resultant adaptation is positively affected by increasing stimulus duration and concentration and decreasing recovery time. The implications of these findings for the integration of peripheral input with perception are discussed. Magn Reson Med 54:443–448, 2005.