John P. McGann

In Vivo Imaging of Neuronal Activity by Targeted Expression of a Genetically Encoded Probe in the Mouse

Genetically encoded probes show great promise in permitting functional imaging of specified neuronal populations in the intact nervous system, yet their in vivo application has been limited. Here, we have targeted expression of synapto-pHluorin, a pH-sensitive protein that reports synaptic vesicle fusion, to olfactory sensory neurons in mouse. Synapto-pHluorin selectively labeled presynaptic terminals of sensory neurons in glomeruli of the olfactory bulb. Odorant stimulation evoked large-amplitude fluorescence increases that were localized to individual glomeruli in vivo, correlated with presynaptic calcium influx, graded with stimulus intensity, and stable over a period of days. Spatial patterns of odorant-activated glomeruli were distributed and did not change systematically with increasing carbon chain length, in contrast to the finely organized chemotopy that has been reported using other imaging methods. Targeted expression of synapto-pHluorin in mouse will permit the analysis of previously inaccessible neuronal populations and chronic imaging from genetically identified neurons in vivo.

Odorant Representations Are Modulated by Intra- but Not Interglomerular Presynaptic Inhibition of Olfactory Sensory Neurons

Input to the central nervous system from olfactory sensory neurons (OSNs) is modulated presynaptically. We investigated the functional organization of this inhibition and its role in odor coding by imaging neurotransmitter release from OSNs in slices and in vivo in mice expressing synaptopHluorin, an optical indicator of vesicle exocytosis. Release from OSNs was strongly suppressed by heterosynaptic, intraglomerular inhibition. In contrast, inhibitory connections between glomeruli mediated only weak lateral inhibition of OSN inputs in slices and did not do so in response to odorant stimulation in vivo. Blocking presynaptic inhibition in vivo increased the amplitude of odorant-evoked input to glomeruli but had little effect on spatial patterns of glomerular input. Thus, intraglomerular inhibition limits the strength of olfactory input to the CNS, whereas interglomerular inhibition plays little or no role. This organization allows for control of input sensitivity while maintaining the spatial maps of glomerular activity thought to encode odorant identity.

Fear Learning Enhances Neural Responses to Threat-Predictive Sensory Stimuli

Fear Factor It has generally been assumed that the most basic aspects of peripheral sensory processing do not change even if they are paired with reward or punishment. A smell should still smell the same, a color should still look the same, and any changes observed would be likely to occur downstream of the primary sensory processing areas. However, using longitudinal in vivo neurophysiology in transgenic mice, Kass et al. (p. 1389) observed that neurotransmitter release from olfactory sensory neurons themselves was selectively enhanced for threat-predictive odors after fear conditioning. The response of mouse olfactory sensory neurons was selectively increased when an odor was linked to discomfort. The central nervous system rapidly learns that particular stimuli predict imminent danger. This learning is thought to involve associations between neutral and harmful stimuli in cortical and limbic brain regions, though associative neuroplasticity in sensory structures is increasingly appreciated. We observed the synaptic output of olfactory sensory neurons (OSNs) in individual mice before and after they learned that a particular odor indicated an impending foot shock. OSNs are the first cells in the olfactory system, physically contacting the odor molecules in the nose and projecting their axons to the brain’s olfactory bulb. OSN output evoked by the shock-predictive odor was selectively facilitated after fear conditioning. These results indicate that affective information about a stimulus can be encoded in its very earliest representation in the nervous system.

Poor human olfaction is a 19th-century myth

Humans have a good sense of smell In comparison to that of other animals, the human sense of smell is widely considered to be weak and underdeveloped. This is, however, an unproven hypothesis. In a Review, McGann traces the origins of this false belief back to comparative 19th-century neuroanatomical studies by Broca. A modern look at the human olfactory bulb shows that it is rather large compared with those of rats and mice, which are presumed to possess a superior sense of smell. In fact, the number of olfactory bulb neurons across 24 mammalian species is comparatively similar, with humans in the middle of the pack, and our sense of smell is similar to that of other mammals. Science, this issue p. eaam7263 BACKGROUND It is widely believed that the human sense of smell is inferior to that of other mammals, especially rodents and dogs. This Review traces the scientific history of this idea to 19th-century neuroanatomist Paul Broca. He classified humans as “nonsmellers” not owing to any sensory testing but because he believed that the evolutionary enlargement of the human frontal lobe gave human beings free will at the expense of the olfactory system. He especially emphasized the small size of the human brain’s olfactory bulb relative to the size of the brain overall, and noted that other mammals have olfactory bulbs that are proportionately much larger. Broca’s claim that humans have an impoverished olfactory system (later labeled “microsmaty,” or tiny smell) influenced Sigmund Freud, who argued that olfactory atrophy rendered humans susceptible to mental illness. Humans’ supposed microsmaty led to the scientific neglect of the human olfactory system for much of the 20th century, and even today many biologists, anthropologists, and psychologists persist in the erroneous belief that humans have a poor sense of smell. Genetic and neurobiological data that reveal features unique to the human olfactory system are regularly misinterpreted to underlie the putative microsmaty, and the impact of human olfactory dysfunction is underappreciated in medical practice. ADVANCES Although the human olfactory system has turned out to have some biological differences from that of other mammalian species, it is generally similar in its neurobiology and sensory capabilities. The human olfactory system has fewer functional olfactory receptor genes than rodents, for instance, but the human brain has more complex olfactory bulbs and orbitofrontal cortices with which to interpret information from the roughly 400 receptor types that are expressed. The olfactory bulb is proportionately smaller in humans than in rodents, but is comparable in the number of neurons it contains and is actually much larger in absolute terms. Thus, although the rest of the brain became larger as humans evolved, the olfactory bulb did not become smaller. When olfactory performance is compared experimentally between humans and other animals, a key insight has been that the results are strongly influenced by the selection of odors tested, presumably because different odor receptors are expressed in each species. When an appropriate range of odors is tested, humans outperform laboratory rodents and dogs in detecting some odors while being less sensitive to other odors. Like other mammals, humans can distinguish among an incredible number of odors and can even follow outdoor scent trails. Human behaviors and affective states are also strongly influenced by the olfactory environment, which can evoke strong emotional and behavioral reactions as well as prompting distinct memories. Odor-mediated communication between individuals, once thought to be limited to “lower animals,” is now understood to carry information about familial relationships, stress and anxiety levels, and reproductive status in humans as well, although this information is not always consciously accessible. OUTLOOK The human olfactory system is increasingly understood to be highly dynamic. Olfactory sensitivity and discrimination abilities can be changed by experiences like environmental odor exposure or even just learning to associate odors with other stimuli in the laboratory. The neurobiological underpinnings of this plasticity, including “bottom-up” factors like regulation of peripheral odor receptors and “top-down” factors like the sensory consequences of emotional and cognitive states, are just beginning to be understood. The role of olfactory communication in shaping social interactions is also actively being explored, including the social spread of emotion through olfactory cues. Finally, impaired olfaction can be a leading indicator of certain neurodegenerative diseases, notably Parkinson’s disease and Alzheimer’s disease. New experimentation will be required to understand how olfactory sequelae might also reflect problems elsewhere in the nervous system, including mental disorders with sensory symptomatology. The idea that human smell is impoverished compared to other mammals is a 19th-century myth. The human and rodent olfactory systems exploring the sensory world together. It is commonly believed that humans have a poor sense of smell compared to other mammalian species. However, this idea derives not from empirical studies of human olfaction but from a famous 19th-century anatomist’s hypothesis that the evolution of human free will required a reduction in the proportional size of the brain’s olfactory bulb. The human olfactory bulb is actually quite large in absolute terms and contains a similar number of neurons to that of other mammals. Moreover, humans have excellent olfactory abilities. We can detect and discriminate an extraordinary range of odors, we are more sensitive than rodents and dogs for some odors, we are capable of tracking odor trails, and our behavioral and affective states are influenced by our sense of smell.

Perirhinal-amygdala circuit-level computational model of temporal encoding in fear conditioning

Here we present a real-time model of fear conditioning in which the functional anatomy and neurophysiology of the lateral amygdala and perirhinal cortex provide a mechanism for temporal learning during Pavlovian conditioning. The model uses realistic neuronal and circuit dynamics to map time onto space and relies on a conventional Hebbian learning rule that requires strict temporal contiguity for synaptic modification. The input—output relationships of the model neurons simulate our physiological recordings with respect to latency to fire, firing frequency, and accommodation tendency. Chains of these neurons form a spectrum of activity windows delayed by various amounts from the conditioned stimulus onset. Simulations reveal that learning occurs only when the conditioned and unconditioned stimuli are explicitly paired, that the interstimulus interval (ISI) is accurately learned over a time range from 0.5 to 16 sec, and that low-frequency noise causes the accuracy of temporal learning to decrease as the ISI increases, in accordance with a Weber-type law.

Presynaptic Inhibition of Olfactory Sensory Neurons: New Mechanisms and Potential Functions

Presynaptic inhibition is the suppression of neurotransmitter release from a neuron by inhibitory input onto its presynaptic terminal. In the olfactory system, the primary sensory afferents from the olfactory neuroepithelium to the brains olfactory bulb are strongly modulated by a presynaptic inhibition that has been studied extensively in brain slices and in vivo. In rodents, this inhibition is mediated by γ-amino butyric acid (GABA) and dopamine released from bulbar interneurons. The specialized GABAergic circuit is now well understood to include a specific subset of GAD65-expressing periglomerular interneurons that stimulate presynaptic GABAB receptors to reduce presynaptic calcium conductance. This inhibition is organized to permit the selective modulation of neurotransmitter release from specific populations of olfactory sensory neurons based on their odorant receptor expression, includes specialized microcircuits to create a tonically active inhibition and a separate feedback inhibition evoked by sensory input, and can be modulated by centrifugal projections from other brain regions. Olfactory nerve output can also be modulated by dopaminergic circuitry, but this literature is more difficult to interpret. Presynaptic inhibition of olfactory afferents may extend their dynamic range but could also create state-dependent or odorant-specific sensory filters on primary sensory representations. New directions exploring this circuits role in olfactory processing are discussed.

In vivo visualization of olfactory pathophysiology induced by intranasal cadmium instillation in mice

Intranasal exposure to cadmium has been related to olfactory dysfunction in humans and to nasal epithelial damage and altered odorant-guided behavior in rodent models. The pathophysiology underlying these deficits has not been fully elucidated. Here we use optical imaging techniques to visualize odorant-evoked neurotransmitter release from the olfactory nerve into the brains olfactory bulbs in vivo in mice. Intranasal cadmium chloride instillations reduced this sensory activity by up to 91% in a dose-dependent manner. In the olfactory bulbs, afferents from the olfactory epithelium could be quantified by their expression of a genetically encoded fluorescent marker for olfactory marker protein. At the highest dose tested, cadmium exposure reduced the density of these projections by 20%. In a behavioral psychophysical task, mice were trained to sample from an odor port and make a response when they detected an odorant against a background of room air. After intranasal cadmium exposure, mice were unable to detect the target odor. These experiments serve as proof of concept for a new approach to the study of the neural effects of inhaled toxicants. The use of in vivo functional imaging of the neuronal populations exposed to the toxicant permits the direct observation of primary pathophysiology. In this study optical imaging revealed significant reductions in odorant-evoked release from the olfactory nerve at a cadmium chloride dose two orders of magnitude less than that required to induce morphological changes in the nerve in the same animals, demonstrating that it is a more sensitive technique for assessing the consequences of intranasal neurotoxicant exposure. This approach is potentially useful in exploring the effects of any putative neurotoxicant that can be delivered intranasally.

Odor-Specific, Olfactory Marker Protein-Mediated Sparsening of Primary Olfactory Input to the Brain after Odor Exposure

Long-term plasticity in sensory systems is usually conceptualized as changing the interpretation of the brain of sensory information, not an alteration of how the sensor itself responds to external stimuli. However, here we demonstrate that, in the adult mouse olfactory system, a 1-week-long exposure to an artificially odorized environment narrows the range of odorants that can induce neurotransmitter release from olfactory sensory neurons (OSNs) and reduces the total transmitter release from responsive neurons. In animals heterozygous for the olfactory marker protein (OMP), this adaptive plasticity was strongest in the populations of OSNs that originally responded to the exposure odorant (an ester) and also observed in the responses to a similar odorant (another ester) but had no effect on the responses to odorants dissimilar to the exposure odorant (a ketone and an aldehyde). In contrast, in OMP knock-out mice, odorant exposure reduced the number and amplitude of OSN responses evoked by all four types of odorants equally. The effect of this plasticity is to preferentially sparsen the primary neural representations of common olfactory stimuli, which has the computational benefit of increasing the number of distinct sensory patterns that could be represented in the circuit and might thus underlie the improvements in olfactory discrimination often observed after odorant exposure (Mandairon et al., 2006a). The absence of odorant specificity in this adaptive plasticity in OMP knock-out mice suggests a potential role for this protein in adaptively reshaping OSN responses to function in different environments.

Functional Rehabilitation of Cadmium-Induced Neurotoxicity Despite Persistent Peripheral Pathophysiology in the Olfactory System

Intranasal exposure to the heavy metal cadmium has been linked to olfactory dysfunction and neurotoxicity. Here, we combine optical imaging of in vivo neurophysiology, genetically defined anatomical tract tracing, mass spectrometry, and behavioral psychophysical methods to evaluate the persistent harmful effects of acute intranasal exposure to cadmium in a mouse model and to investigate the functional consequences of sensory rehabilitation training. We find that an acute intranasal instillation of cadmium chloride leads to an accumulation of cadmium in the brains olfactory bulb that persists for at least 4 weeks. This is accompanied by persistent severe pathophysiology of the olfactory nerve, a gradual reduction in axonal projections from the olfactory epithelium, and complete impairment on an olfactory detection task. Remarkably, 2 weeks of odorant-guided operant conditioning training proved sufficient to restore olfactory detection performance to control levels in cadmium-exposed mice. Optical imaging from rehabilitated mice showed that this training did not cause any detectable restoration of olfactory nerve function, suggesting that the recovery of function was mediated by central neuroplasticity in which the brain learned to interpret the degraded sensory input. These data demonstrate that sensory learning can mask even severe damage from neurotoxicants and suggest that explicit sensory training may be useful in rehabilitation of olfactory dysfunction.

Intranasal exposure to manganese disrupts neurotransmitter release from glutamatergic synapses in the central nervous system in vivo.

Chronic exposure to aerosolized manganese induces a neurological disorder that includes extrapyramidal motor symptoms and cognitive impairment. Inhaled manganese can bypass the blood-brain barrier and reach the central nervous system by transport down the olfactory nerve to the brains olfactory bulb. However, the mechanism by which Mn disrupts neural function remains unclear. Here we used optical imaging techniques to visualize exocytosis in olfactory nerve terminals in vivo in the mouse olfactory bulb. Acute Mn exposure via intranasal instillation of 2-200 μg MnCl(2) solution caused a dose-dependent reduction in odorant-evoked neurotransmitter release, with significant effects at as little as 2 μg MnCl(2) and a 90% reduction compared to vehicle controls with a 200 μg exposure. This reduction was also observed in response to direct electrical stimulation of the olfactory nerve layer in the olfactory bulb, demonstrating that Mns action is occurring centrally, not peripherally. This is the first direct evidence that Mn intoxication can disrupt neurotransmitter release, and is consistent with previous work suggesting that chronic Mn exposure limits amphetamine-induced dopamine increases in the basal ganglia despite normal levels of dopamine synthesis (Guilarte et al., J Neurochem 2008). The commonality of Mns action between glutamatergic neurons in the olfactory bulb and dopaminergic neurons in the basal ganglia suggests that a disruption of neurotransmitter release may be a general consequence wherever Mn accumulates in the brain and could underlie its pleiotropic effects.