New routes to early memories

A nasal chemosensation-dependent critical window for somatosensory development

Nasal chemosensation is considered the evolutionarily oldest mammalian sense and, together with somatosensation, is crucial for neonatal well-being before auditory and visual pathways start engaging the brain. Using anatomical and functional approaches in mice, we reveal that odor-driven activity propagates to a large part of the cortex during the first postnatal week and enhances whisker-evoked activation of primary whisker somatosensory cortex (wS1). This effect disappears in adult animals, in line with the loss of excitatory connectivity from olfactory cortex to wS1. By performing neonatal odor deprivation, followed by electrophysiological and behavioral work in adult animals, we identify a key transient regulation of nasal chemosensory information necessary for the development of wS1 sensory-driven dynamics and somatosensation. Our work uncovers a cross-modal critical window for nasal chemosensation-dependent somatosensory functional maturation.

Distinct forms of structural plasticity of adult-born interneuron spines in the mouse olfactory bulb induced by different odor learning paradigms

The morpho-functional properties of neural networks constantly adapt in response to environmental stimuli. The olfactory bulb is particularly prone to constant reshaping of neural networks because of ongoing neurogenesis. It remains unclear whether the complexity of distinct odor-induced learning paradigms and sensory stimulation induces different forms of structural plasticity. In the present study, we automatically reconstructed spines in 3D from confocal images and performed unsupervised clustering based on morphometric features. We show that while sensory deprivation decreased the spine density of adult-born neurons without affecting the morphometric properties of these spines, simple and complex odor learning paradigms triggered distinct forms of structural plasticity. A simple odor learning task affected the morphometric properties of the spines, whereas a complex odor learning task induced changes in spine density. Our work reveals distinct forms of structural plasticity in the olfactory bulb tailored to the complexity of odor-learning paradigms and sensory inputs.

Odor representations from the two nostrils are temporally segregated in human piriform cortex

The human olfactory system has two discrete channels of sensory input, arising from olfactory epithelia housed in the left and right nostrils. Here, we asked whether the primary olfactory cortex (piriform cortex [PC]) encodes odor information arising from the two nostrils as integrated or distinct stimuli. We recorded intracranial electroencephalogram (iEEG) signals directly from PC while human subjects participated in an odor identification task where odors were delivered to the left, right, or both nostrils. We analyzed the time course of odor identity coding using machine-learning approaches and found that uni-nostril odor inputs to the ipsilateral nostril are encoded ∼480-ms faster than odor inputs to the contralateral nostril on average. During naturalistic bi-nostril odor sampling, odor information emerged in two temporally segregated epochs, with the first epoch corresponding to the ipsilateral and the second epoch corresponding to the contralateral odor representations. These findings reveal that PC maintains distinct representations of odor input from each nostril through temporal segregation, highlighting an olfactory coding scheme at the cortical level that can parse odor information across nostrils within the course of a single inhalation.

Odor representations from the two nostrils are temporally segregated in human piriform cortex

The human olfactory system has two discrete channels of sensory input, arising from olfactory epithelia housed in the left and right nostrils. Here, we asked whether primary olfactory cortex (piriform cortex, PC) encodes odor information arising from the two nostrils as integrated or distinct stimuli. We recorded intracranial EEG signals directly from PC while human subjects participated in an odor identification task where odors were delivered to the left, right, or both nostrils. We analyzed the time-course of odor-identity coding using machine learning approaches, and found that uni-nostril odor inputs to the ipsilateral nostril are encoded ~480 ms faster than odor inputs to the contralateral nostril on average. During naturalistic bi-nostril odor sampling, odor information emerged in two temporally segregated epochs with the first epoch corresponding to the ipsilateral and the second epoch corresponding to the contralateral odor representations. These findings reveal that PC maintains distinct representations of odor input from each nostril through temporal segregation, highlighting an olfactory coding scheme at the cortical level that can parse odor information across nostrils within the course of a single inhalation.

Bilateral and unilateral odor processing and odor perception

Imagine smelling a novel perfume with only one nostril and then smelling it again with the other nostril. Clearly, you can tell that it is the same perfume both times. This simple experiment demonstrates that odor information is shared across both hemispheres to enable perceptual unity. In many sensory systems, perceptual unity is believed to be mediated by inter-hemispheric connections between iso-functional cortical regions. However, in the olfactory system, the underlying neural mechanisms that enable this coordination are unclear because the two olfactory cortices are not topographically organized and do not seem to have homotypic inter-hemispheric mapping. This review presents recent advances in determining which aspects of odor information are processed unilaterally or bilaterally, and how odor information is shared across the two hemispheres. We argue that understanding the mechanisms of inter-hemispheric coordination can provide valuable insights that are hard to achieve when focusing on one hemisphere alone.

Cell-Type-Specific Whole-Brain Direct Inputs to the Anterior and Posterior Piriform Cortex

The piriform cortex (PC) is a key brain area involved in both processing and coding of olfactory information. It is implicated in various brain disorders, such as epilepsy, Alzheimer’s disease, and autism. The PC consists of the anterior (APC) and posterior (PPC) parts, which are different anatomically and functionally. However, the direct input networks to specific neuronal populations within the APC and PPC remain poorly understood. Here, we mapped the whole-brain direct inputs to the two major neuronal populations, the excitatory glutamatergic principal neurons and inhibitory γ-aminobutyric acid (GABA)-ergic interneurons within the APC and PPC using the rabies virus (RV)-mediated retrograde trans-synaptic tracing system. We found that for both types of neurons, APC and PPC share some similarities in input networks, with dominant inputs originating from the olfactory region (OLF), followed by the cortical subplate (CTXsp), isocortex, cerebral nuclei (CNU), hippocampal formation (HPF) and interbrain (IB), whereas the midbrain (MB) and hindbrain (HB) were rarely labeled. However, APC and PPC also show distinct features in their input distribution patterns. For both types of neurons, the input proportion from the OLF to the APC was higher than that to the PPC; while the PPC received higher proportions of inputs from the HPF and CNU than the APC did. Overall, our results revealed the direct input networks of both excitatory and inhibitory neuronal populations of different PC subareas, providing a structural basis to analyze the diverse PC functions.

Olfactory Laterality Is Valence-Dependent in Mice

(1) Background: Although olfaction is the predominant sensory modality in rodents, studies focusing on lateralisation of olfactory processing remain scarce, and they are limited to the exploration of brain asymmetries. This study aimed to test whether outbred and inbred mice (NMRI and C57BL/6J mice strains) show nostril-use preference in processing olfactory stimuli differing in terms of emotional valence under unrestrained conditions. (2) Methods: Five odour stimuli were used in the study: vanilla, female urine, garlic, rat, distilled water. We measured the number of times mice used their left or right nostril for each testing session. (3) Results: We here showed that mice preferentially used their right nostril when sniffing attractive stimuli (female urine, vanilla), and their left nostril when sniffing aversive stimuli (rat odour). Results were consistent for both strains. (4) Conclusions: Surprisingly, the results obtained seem opposite to the valence theory assessing that the left and the right hemispheres are dominant in processing stimuli with a positive and a negative valence, respectively. It remains to be determined whether this valence-dependent pattern is specific or not to olfaction in mice. These new findings will be important to better understand how both hemispheres contribute to odour processing in rodents.

Nostril-specific and structure-based olfactory learning of chiral discrimination in human adults

Practice makes perfect. In human olfaction, such plasticity is generally assumed to occur at the level of cortical synthetic processing that shares information from both nostrils. Here we present findings that challenge this view. In two experiments, we trained human adults unirhinally for the discrimination between odor enantiomers over a course of about 10 to 11 days. Results showed that training-induced perceptual gain was restricted to the trained nostril yet partially generalized to untrained odor enantiomers in a structure- rather than quality- based manner. In other words, learning enhanced the differentiation of chirality (molecular configuration) as opposed to overall odor quality (odor object) per se. These findings argue that, unlike earlier beliefs, one nostril does not readily know what the other learns. Moreover, the initial analytical processing of the structural features of uninarial olfactory input remains plastic in human adults.

Although we may only become consciously aware of our sense of smell when we encounter something pungent, it can greatly influence our quality of life. Smells are processed by our olfactory system, a collection of receptors and nerve cells in the nose and brain.

Odor molecules activate the olfactory system when they bind to receptors in the nostrils. These molecules can have a wide range of chemical and physical properties. However, some odor molecules are mirror images of each other. These variants are known as enantiomers. Some people can naturally smell the difference between enantiomers; others can be taught how to tell them apart. Studying this training process could help us to understand how the olfactory system adapts to new circumstances.

Feng and Zhou trained volunteers to distinguish between odor enantiomers – but only using one nostril. After training, the volunteers were better able to tell the difference between different enantiomers – even for certain scents they had not been trained to discriminate – when sniffing through the trained nostril. However, they got no better at distinguishing between enantiomers when they sniffed them using the other nostril.

Overall, the results reported by Feng and Zhou confirm that human adults can learn how to process the structural features of odorants. However, in contrast to earlier beliefs, they also suggest that one nostril does not readily know what the other learns. This new understanding of how the olfactory system adapts could ultimately help us to develop therapies to restore a lost sense of smell.

A Mirror-Symmetric Excitatory Link Coordinates Odor Maps across Olfactory Bulbs and Enables Odor Perceptual Unity

Sensory input reaching the brain from bilateral and offset channels is nonetheless perceived as unified. This unity could be explained by simultaneous projections to both hemispheres, or inter-hemispheric information transfer between sensory cortical maps. Odor input, however, is not topographically organized, nor does it project bilaterally, making olfactory perceptual unity enigmatic. Here we report a circuit that interconnects mirror-symmetric isofunctional mitral/tufted cells between the mouse olfactory bulbs. Connected neurons respond to similar odors from ipsi- and contra-nostrils, whereas unconnected neurons do not respond to odors from the contralateral nostril. This connectivity is likely mediated through a one-to-one mapping from mitral/tufted neurons to the ipsilateral anterior olfactory nucleus pars externa, which activates the mirror-symmetric isofunctional mitral/tufted neurons glutamatergically. This circuit enables sharing of odor information across hemispheres in the absence of a cortical topographical organization, suggesting that olfactory glomerular maps are the equivalent of cortical sensory maps found in other senses.

Myelination of the developing lateral olfactory tract and anterior commissure

Both the lateral olfactory tract (LOT) and anterior limb of the anterior commissure (AC) carry olfactory information. The LOT forms the projection from the olfactory bulb to the ipsilateral olfactory cortices, while the AC carries odor information across the midline to the contralateral olfactory cortex and bulb. The LOT and AC differ on a number of dimensions, including early development and functional onset. The present work, examining their myelination in mice, reveals additional important differences. For example, the LOT initiates myelination 3-4 days earlier than the AC, evidenced by both an earlier increase in myelin basic protein staining seen with immunohistochemistry and an earlier appearance of myelinated fibers using electron microscopy. While both exhibit a period of rapid myelination, it occurs 4-5 days earlier in the LOT than the AC. The tracts also respond differently to early sensory restriction. Unilateral naris occlusion from the day after birth to postnatal day 30 had no consistent effects on the AC but resulted in significantly thinner myelin sheaths relative to axon caliber in the LOT. Finally, the two tracts differ structurally (the LOT contains larger, more densely packed axons with significantly thicker myelin sheaths resulting in a conduction velocity that is more than twice as fast as the AC). The findings indicate that these two large, accessible tracts provide an important means for studying brain maturation due to basic differences in both the timing of their maturation and general organization.

Neural correlates of side-specific odour memory in mushroom body output neurons

Humans and other mammals as well as honeybees learn a unilateral association between an olfactory stimulus presented to one side and a reward. In all of them, the learned association can be behaviourally retrieved via contralateral stimulation, suggesting inter-hemispheric communication. However, the underlying neuronal circuits are largely unknown and neural correlates of across-brain-side plasticity have yet not been demonstrated. We report neural plasticity that reflects lateral integration after side-specific odour reward conditioning. Mushroom body output neurons that did not respond initially to contralateral olfactory stimulation developed a unique and stable representation of the rewarded compound stimulus (side and odour) predicting its value during memory retention. The encoding of the reward-associated compound stimulus is delayed by about 40 ms compared with unrewarded neural activity, indicating an increased computation time for the read-out after lateral integration.

Diverse Representations of Olfactory Information in Centrifugal Feedback Projections

Although feedback or centrifugal projections from higher processing centers of the brain to peripheral regions have long been known to play essential functional roles, the anatomical organization of these connections remains largely unknown. Using a virus-based retrograde labeling strategy and 3D whole-brain reconstruction methods, we mapped the spatial organization of centrifugal projections from two olfactory cortical areas, the anterior olfactory nucleus (AON) and the piriform cortex, to the granule cell layer of the main olfactory bulb in the mouse. Both regions are major recipients of information from the bulb and are the largest sources of feedback to the bulb, collectively constituting circuits essential for olfactory coding and olfactory behavior. We found that, although ipsilateral inputs from the AON were uniformly distributed, feedback from the contralateral AON had a strong ventral bias. In addition, we observed that centrifugally projecting neurons were spatially clustered in the piriform cortex, in contrast to the distributed feedforward axonal inputs that these cells receive from the principal neurons of the bulb. Therefore, information carried from the bulb to higher processing structures by anatomically stereotypic projections is likely relayed back to the bulb by organizationally distinct feedback projections that may reflect different coding strategies and therefore different functional roles.

SIGNIFICANCE STATEMENT

Principles of anatomical organization, sometimes instantiated as "maps" in the mammalian brain, have provided key insights into the structure and function of circuits in sensory systems. Generally, these characterizations focus on projections from early sensory processing areas to higher processing structures despite considerable evidence that feedback or centrifugal projections often constitute major conduits of information flow. Our results identify structure in the organization of centrifugal feedback projections to the olfactory bulb that is fundamentally different from the organization of feedforward circuits. Our study suggests that understanding computations performed in the olfactory bulb, and more generally in the olfactory system, requires understanding interactions between feedforward and feedback "maps" both structurally and functionally.

Gamma Oscillations in the Rat Ventral Striatum Originate in the Piriform Cortex

Local field potentials (LFPs) recorded from the human and rodent ventral striatum (vStr) exhibit prominent, behaviorally relevant gamma-band oscillations. These oscillations are related to local spiking activity and transiently synchronize with anatomically related areas, suggesting a possible role in organizing vStr activity. However, the origin of vStr gamma is unknown. We recorded vStr gamma oscillations across a 1.4 mm² grid spanned by 64 recording electrodes as male rats rested and foraged for rewards, revealing a highly consistent power gradient originating in the adjacent piriform cortex. Phase differences across the vStr were consistently small (<15°) and current source density analysis further confirmed the absence of local sink-source pairs in the vStr. Reversible occlusions of the ipsilateral (but not contralateral) nostril, known to abolish gamma oscillations in the piriform cortex, strongly reduced vStr gamma power and the occurrence of transient gamma-band events. These results imply that local circuitry is not a major contributor to gamma oscillations in the vStr LFP and that piriform cortex is an important driver of gamma-band oscillations in the vStr and associated limbic areas.SIGNIFICANCE STATEMENT The ventral striatum (vStr) is an area of anatomical convergence in circuits underlying motivated behavior, but it remains unclear how its inputs from different sources interact. A major proposal about how neural circuits may switch dynamically between convergent inputs is through temporal organization reflected in local field potential (LFP) oscillations. Our results show that, in the rat, the mechanisms controlling gamma-band oscillations in the vStr LFP are primarily located in the in the adjacent piriform cortex rather than in the vStr itself, providing a novel interpretation of previous rodent work on gamma oscillations in the vStr and related circuits and an important consideration for future work seeking to use oscillations in these areas as biomarkers for behavioral and neurological disorders.

Determination of the connectivity of newborn neurons in mammalian olfactory circuits

The mammalian olfactory bulb is a forebrain structure just one synapse downstream from the olfactory sensory neurons and performs the complex computations of sensory inputs. The formation of this sensory circuit is shaped through activity-dependent and cell-intrinsic mechanisms. Recent studies have revealed that cell-type specific connectivity and the organization of synapses in dendritic compartments are determined through cell-intrinsic programs already preset in progenitor cells. These progenitor programs give rise to subpopulations within a neuron type that have distinct synaptic organizations. The intrinsically determined formation of distinct synaptic organizations requires factors from contacting cells that match the cell-intrinsic programs. While certain genes control wiring within the newly generated neurons, other regulatory genes provide intercellular signals and are only expressed in neurons that will form contacts with the newly generated cells. Here, the olfactory system has provided a useful model circuit to reveal the factors regulating assembly of the highly structured connectivity in mammals.

Dynamic cortical lateralization during olfactory discrimination learning

Bilateral cortical circuits are not necessarily symmetrical. Asymmetry, or cerebral lateralization, allows functional specialization of bilateral brain regions and has been described in humans for such diverse functions as perception, memory and emotion. There is also evidence for asymmetry in the human olfactory system, although evidence in non-human animal models is lacking. In the present study, we took advantage of the known changes in olfactory cortical local field potentials that occur over the course of odour discrimination training to test for functional asymmetry in piriform cortical activity during learning. Both right and left piriform cortex local field potential activities were recorded. The results obtained demonstrate a robust interhemispheric asymmetry in anterior piriform cortex activity that emerges during specific stages of odour discrimination learning, with a transient bias toward the left hemisphere. This asymmetry is not apparent during error trials. Furthermore, functional connectivity (coherence) between the bilateral anterior piriform cortices is learning- and context-dependent. Steady-state interhemispheric anterior piriform cortex coherence is reduced during the initial stages of learning and then recovers as animals acquire competent performance. The decrease in coherence is seen relative to bilateral coherence expressed in the home cage, which remains stable across conditioning days. Similarly, transient, trial-related interhemispheric coherence increases with task competence. Taken together, the results demonstrate transient asymmetry in piriform cortical function during odour discrimination learning until mastery, suggesting that each piriform cortex may contribute something unique to odour memory.

Visualizing the engram: learning stabilizes odor representations in the olfactory network

The nature of memory is a central issue in neuroscience. How does our representation of the world change with learning and experience? Here we use the transcription of Arc mRNA, which permits probing the neural representations of temporally separated events, to address this in a well characterized odor learning model. Rat pups readily associate odor with maternal care. In pups, the lateralized olfactory networks are independent, permitting separate training and within-subject control. We use multiday training to create an enduring memory of peppermint odor. Training stabilized rewarded, but not nonrewarded, odor representations in both mitral cells and associated granule cells of the olfactory bulb and in the pyramidal cells of the anterior piriform cortex. An enlarged core of stable, likely highly active neurons represent rewarded odor at both stages of the olfactory network. Odor representations in anterior piriform cortex were sparser than typical in adult rat and did not enlarge with learning. This sparser representation of odor is congruent with the maturation of lateral olfactory tract input in rat pups. Cortical representations elsewhere have been shown to be highly variable in electrophysiological experiments, suggesting brains operate normally using dynamic and network-modulated representations. The olfactory cortical representations here are consistent with the generalized associative model of sparse variable cortical representation, as normal responses to repeated odors were highly variable (∼70% of the cells change as indexed by Arc). Learning and memory modified rewarded odor ensembles to increase stability in a core representational component.

A lateralized odor learning model in neonatal rats for dissecting neural circuitry underpinning memory formation

Rat pups during a critical postnatal period (≤ 10 days) readily form a preference for an odor that is associated with stimuli mimicking maternal care. Such a preference memory can last from hours, to days, even life-long, depending on training parameters. Early odor preference learning provides us with a model in which the critical changes for a natural form of learning occur in the olfactory circuitry. An additional feature that makes it a powerful tool for the analysis of memory processes is that early odor preference learning can be lateralized via single naris occlusion within the critical period. This is due to the lack of mature anterior commissural connections of the olfactory hemispheres at this early age. This work outlines behavioral protocols for lateralized odor learning using nose plugs. Acute, reversible naris occlusion minimizes tissue and neuronal damages associated with long-term occlusion and more aggressive methods such as cauterization. The lateralized odor learning model permits within-animal comparison, therefore greatly reducing variance compared to between-animal designs. This method has been used successfully to probe the circuit changes in the olfactory system produced by training. Future directions include exploring molecular underpinnings of odor memory using this lateralized learning model; and correlating physiological change with memory strength and durations.

Unlearning: NMDA receptor-mediated metaplasticity in the anterior piriform cortex following early odor preference training in rats

Here we demonstrate metaplastic effect of a change in NMDA receptor (NMDAR) number in the anterior piriform cortex (aPC) in rat induced by a 10 min pairing of peppermint odor + stroking, which significantly modifies later learning and memory. Using isolated synaptoneurosomes, we found NR1 receptor downregulation 3 h after training and upregulation at 24 h. Consistent with the NR1 pattern, the NMDAR-mediated EPSP was smaller at 3 h and larger at 24 h. Subunit composition was unchanged. Whereas LTP was reduced at both times by training, LTD was facilitated only at 3 h. Behaviorally, pups, given a pairing of peppermint + stroking 3 h after an initial peppermint + stroking training, lost the normally acquired peppermint preference 24 h later. To probe the pathway specificity of this unlearning effect, pups were trained first with peppermint and then, at 3 h, given a second training with peppermint or vanillin. Pups given peppermint training at both times lost the learned peppermint preference. Pups given vanillin retraining at 3 h had normal peppermint preference. Downregulating NR1 with siRNA prevented odor preference learning. Finally, the NMDAR antagonist MK-801 blocked the LTD facilitation seen 3 h after training, and giving MK-801 before the second peppermint training trial eliminated the loss of peppermint odor preference. A training-associated reduction in NMDARs facilitates LTD 3 h later; training at the time of LTD facilitation reverses an LTP-dependent odor preference. Experience-dependent, pathway-specific metaplastic effects in a cortical structure have broad implications for the optimal spacing of learning experiences.

The mouse olfactory peduncle. 3. Development of neurons, glia, and centrifugal afferents

The present series of studies was designed to provide a general overview of the development of the region connecting the olfactory bulb to the forebrain. The olfactory peduncle (OP) contains several structures involved in processing odor information with the anterior olfactory nucleus (cortex) being the largest and most studied. Results indicate that considerable growth occurs in the peduncle from postnatal day (P)10–P20, with reduced expansion from P20 to P30. No evidence was found for the addition of new projection or interneurons during the postnatal period. GABAergic cells decreased in both number and density after P10. Glial populations exhibited different patterns of development, with astrocytes declining in density from P10 to P30, and both oligodendrocytes and microglia increasing through the interval. Myelination in the anterior commissure emerged between P11 and P14. Dense cholinergic innervation was observed at P10 and remained relatively stable through P30, while considerable maturation of serotonergic innervation occurred through the period. Unilateral naris occlusion from P1 to P30 resulted in about a 30% reduction in the size of the ipsilateral peduncle but few changes were observed on the contralateral side. The ipsilateral peduncle also exhibited higher densities of GAD67-containing interneurons and cholinergic fibers suggesting a delay in normal developmental pruning. Lower densities of interneurons expressing CCK, somatostatin, and NPY and in myelin basic protein staining were also observed. Understanding variations in developmental trajectories within the OP may be an important tool for unraveling the functions of the region.

Mechanisms underlying early odor preference learning in rats

Early odor preference training in rat pups produces behavioral preferences that last from hours to lifetimes. Here, we discuss the molecular and circuitry changes we have observed in the olfactory bulb (OB) and in the anterior piriform cortex (aPC) following odor training. For normal preference learning, both structures are necessary, but learned behavior can be initiated by initiating local circuit change in either structure. Our evidence relates dynamic molecular and circuit changes to memory duration and storage localization. Results using this developmental model are consistent with biological memory theories implicating N-methyl-D-aspartate (NMDA) receptors and β-adrenoceptors, and their associated cascades, in memory induction and consolidation. Finally, our examination of the odor preference model reveals a primary role for increases in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor synaptic strength, and in network strength, in the creation and maintenance of preference memory in both olfactory structures.

Lateralized odor preference training in rat pups reveals an enhanced network response in anterior piriform cortex to olfactory input that parallels extended memory

The present study examines synaptic plasticity in the anterior piriform cortex (aPC) using ex vivo slices from rat pups given lateralized odor preference training. In the early odor preference learning model, a brief 10 min training session yields 24 h memory, while four daily sessions yield 48 h memory. Odor preference memory can be lateralized through naris occlusion as the anterior commissure is not yet functional. AMPA receptor-mediated postsynaptic responses in the aPC to lateral olfactory tract input, shown to be enhanced at 24 h, are no longer enhanced 48 h after a single training session. Following four spaced lateralized trials, the AMPA receptor-mediated fEPSP is enhanced in the trained aPC at 48 h. Calcium imaging of aPC pyramidal cells within 48 h revealed decreased firing thresholds in the pyramidal cell network. Thus multiday odor preference training induced increased odor input responsiveness in previously weakly activated aPC cells. These results support the hypothesis that increased synaptic strength in olfactory input networks mediates odor preference memory. The increase in aPC network activation parallels behavioral memory.

A role for the anterior piriform cortex in early odor preference learning: evidence for multiple olfactory learning structures in the rat pup

cFos activation in the anterior piriform cortex (aPC) occurs in early odor preference learning in rat pups (Roth and Sullivan 2005). Here we provide evidence that the pairing of odor as a conditioned stimulus and β-adrenergic activation in the aPC as an unconditioned stimulus generates early odor preference learning. β-Adrenergic blockade in the aPC prevents normal preference learning. Enhancement of aPC cAMP response element-binding protein (CREB) phosphorylation in trained hemispheres is consistent with a role for this cascade in early odor preference learning in the aPC. In vitro experiments suggested theta-burst-mediated long-term potentiation (LTP) at the lateral olfactory tract (LOT) to aPC synapse depends on N-methyl-D-aspartate (NMDA) receptors and can be significantly enhanced by β-adrenoceptor activation, which causes increased glutamate release from LOT synapses during LTP induction. NMDA receptors in aPC are also shown to be critical for the acquisition, but not expression, of odor preference learning, as would be predicted if they mediate initial β-adrenoceptor-promoted aPC plasticity. Ex vivo experiments 3 and 24 h after odor preference training reveal an enhanced LOT-aPC field excitatory postsynaptic potential (EPSP). At 3 h both presynaptic and postsynaptic potentiations support EPSP enhancement while at 24 h only postsynaptic potentiation is seen. LOT-LTP in aPC is excluded by odor preference training. Taken together with earlier work on the role of the olfactory bulb in early odor preference learning, these outcomes suggest early odor preference learning is normally supported by and requires multiple plastic changes at least at two levels of olfactory circuitry.

Olfactory bulb α2-adrenoceptor activation promotes rat pup odor-preference learning via a cAMP-independent mechanism

In this study, three lines of evidence suggest a role for α(2)-adrenoreceptors in rat pup odor-preference learning: olfactory bulb infusions of the α(2)-antagonist, yohimbine, prevents learning; the α(2)-agonist, clonidine, paired with odor, induces learning; and subthreshold clonidine paired with subthreshold β-adrenoceptor activation also recruits learning. Increased mitral cell layer pCREB occurs with clonidine-infusion, but cAMP is not increased. Similar results using a GABAa-antagonist suggest that disinhibition may support clonidine-induced learning. We suggest that norepinephrine can act through multiple bulbar adrenoceptor subtypes to induce odor learning and that cAMP-dependent, as well as cAMP-independent, signals may act as unconditioned stimuli.

What a nostril knows: olfactory nerve-evoked AMPA responses increase while NMDA responses decrease at 24-h post-training for lateralized odor preference memory in neonate rat

Increased AMPA signaling is proposed to mediate long-term memory. Rat neonates acquire odor preferences in a single olfactory bulb if one nostril is occluded at training. Memory testing here confirmed that only trained bulbs support increased odor preference at 24 h. Olfactory nerve field potentials were tested at 24 h in slices from trained and untrained bulbs. A larger AMPA component and a smaller NMDA component characterized responses in the bulb receiving odor preference training. Field potential changes were not seen in a bulbar region separate from the lateral odor-encoding area. These results support models in which memory is mediated by increased olfactory nerve-mitral cell AMPA signaling, and memory stability is promoted by decreased NMDA-mediated signaling.

Membrane and synaptic properties of pyramidal neurons in the anterior olfactory nucleus

The anterior olfactory nucleus (AON) is positioned to coordinate activity between the piriform cortex and olfactory bulbs, yet the physiology of AON principal neurons has been little explored. Here, we examined the membrane properties and excitatory synapses of AON principal neurons in brain slices of PND22-28 mice and compared their properties to principal cells in other olfactory cortical areas. AON principal neurons had firing rates, spike rate adaptation, spike widths, and I-V relationships that were generally similar to pyramidal neurons in piriform cortex, and typical of cerebral cortex, consistent with a role for AON in cortical processing. Principal neurons in AON had more hyperpolarized action potential thresholds, smaller afterhyperpolarizations, and tended to fire doublets of action potentials on depolarization compared with ventral anterior piriform cortex and the adjacent epileptogenic region preendopiriform nucleus (pEN). Thus, AON pyramidal neurons have enhanced membrane excitability compared with surrounding subregions. Interestingly, principal neurons in pEN were the least excitable, as measured by a larger input conductance, lower firing rates, and more inward rectification. Afferent and recurrent excitatory synapses onto AON pyramidal neurons had small amplitudes, paired pulse facilitation at afferent synapses, and GABA(B) modulation at recurrent synapses, a pattern similar to piriform cortex. The enhanced membrane excitability and recurrent synaptic excitation within the AON, together with its widespread outputs, suggest that the AON can boost and distribute activity in feedforward and feedback circuits throughout the olfactory system.

Cerebral asymmetry of emotion and its relationship to olfaction in infancy

To examine the relationship between emotional development and laterality, smells differing in affective valence (predetermined by adults) were presented to 10 neonates. Each infant was presented with two positive and two negative food-related odorants, each paired with a neutral stimulus. Each pair was presented twice, once with the odorant to the right nostril and once to the left nostril. These eight trials were repeated in two blocks. In addition, control trials, where a puff of air was administered to both nostrils, were included. Head turning from midline position to presentation of the odorants as videotaped while the infant slept. Direction of initial head turn served as the dependent measure. A significant interaction with Smell x Nostril x Direction was found. Post hoc analyses revealed that significant effects occurred only for the positive smells. When the positive smells were presented to the left hemisphere, neonates made significantly more head turns towards the smell. This raises the possibility that approach behaviours may develop earlier than withdrawal behaviours.

Centrifugal innervation of the mammalian olfactory bulb

Although it has been known for decades that the mammalian olfactory bulb receives a substantial number of centrifugal inputs from other regions of the brain, relatively few data have been available on the function of the centrifugal olfactory system. Knowing the role of the centrifugal projection and how it works is of critical importance to fully understanding olfaction. The centrifugal fibers can be classified into two groups, a group that release neuromodulators, such as noradrenaline, serotonin, or acetylcholine, and a group originating in the olfactory cortex. Accumulating evidence suggests that centrifugal neuromodulatory inputs are associated with acquisition of odor memory. Because the distribution of the terminals on these fibers is diffuse and widespread, the neuromodulatory inputs must affect diverse subsets of bulbar neurons at the same time. In contrast, knowledge of the role of centrifugal fibers from the olfactory cortical areas is limited. Judging from recent morphological evidence, these fibers may modify the activity of neurons located in sparse and discrete loci in the olfactory bulb. Given the modular organization of the olfactory bulb, centrifugal fibers from the olfactory cortex may help coordinate the activities of restricted subsets of neurons belonging to distinct functional modules in an odor-specific manner. Because the olfactory cortex receives inputs from limbic and neocortical areas in addition to inputs from the bulb, the centrifugal inputs from the cortex can modulate odor processing in the bulb in response to non-olfactory as well as olfactory cues.

Compensatory rapid switching of binasal inputs in the olfactory cortex

Odors are inhaled through the nostrils into two segregated nasal passages and detected by sensory neurons in the bilateral olfactory epithelia. Airflow through the two nasal passages is usually asymmetrical because of alternating changes in nasal mucosal congestion. Here we show that neurons in the anterior olfactory nucleus (AON) of the adult rat olfactory cortex are ordinarily dominated by ipsi-nasal inputs and that binasal neurons in the AON respond to ipsilateral and contralateral nasal inputs with nearly equivalent odorant category selectivity. Deprivation of ipsilateral nasal inputs by unilateral nostril obstruction greatly enhanced the response to contralateral odor stimulation, in a reversible manner, in approximately 33% of AON neurons within only several minutes. In 27% of AON neurons that showed spike responses induced by the inspiration of room air, ipsilateral nasal obstruction initially suppressed respiration phase-locked spike discharges and, several minutes later, induced respiration phase-locked discharges with longer delays between inspiration and response. Recordings from AON neurons in rats with anterior commissure (AC) transection indicated that the resumed respiration phase-locked discharges with longer delays were mediated by the contralateral pathway via the AC. The ipsi-nasal occlusion-induced switching of nasal inputs to individual AON neurons shows that a subset of AON neurons in the adult rat has neuronal mechanisms for rapid nostril dominance plasticity, which may enable both right and left olfactory cortices to preserve their responsiveness to the external odor world, despite reciprocal changes in nasal airflow.

Precise circuitry links bilaterally symmetric olfactory maps

Olfactory sensory neurons expressing a common receptor gene converge onto one or a few glomeruli with stereotyped positions within the mouse main olfactory bulb (MOB), producing a map of approximately 1800 olfactory columns representing approximately 1000 odorant receptors. Here, we report that this precise olfactory map is maintained over several synapses that ultimately cross MOB hemispheres to link bilateral isofunctional olfactory columns. Focal injection of tracer into genetically identified glomeruli revealed an exquisite topography that involves a bilateral connection via the anterior olfactory nucleus pars externa (AONpE) that links isofunctional olfactory columns in the contralateral MOB. Physiological and behavioral assays revealed an important role for the AONpE in bilateral exchange of odorant-specific information. These results indicate that the interbulbar link through the AONpE integrates bilateral olfactory sensory maps and exchanges olfactory information, positioning it as a unique model system for studying interhemispheric connections.

Interhemispheric olfactory circuit and the memory beyond

In mammals, olfactory sensory neurons project their axons exclusively to the ipsilateral olfactory bulb. It remains unclear how odor information interacts between the two hemispheres of the brain. In this issue of Neuron, Yan et al. describe the precise interbulbar connection through the anterior olfactory nucleus pars externa (AONpE), which links contralateral isotypic olfactory columns.

Orbital spaceflight during pregnancy shapes function of mammalian vestibular system

Pregnant rats were flown on the NASA Space Shuttle during the early developmental period of their fetuses' vestibular apparatus and onset of vestibular function. The authors report that prenatal spaceflight exposure shapes vestibular-mediated behavior and central morphology. Postflight testing revealed (a) delayed onset of body righting responses, (b) cardiac deceleration (bradycardia) to 70 degrees head-up roll, (c) decreased branching of gravistatic afferent axons, but (d) no change in branching of angular acceleration receptor projections with comparable synaptogenesis of the medial vestibular nucleus in flight relative to control fetuses. Kinematic analyses of the dams' on-orbit behavior suggest that, although the fetal otolith organs are unloaded in microgravity, the fetus' semicircular canals receive high levels of stimulation during longitudinal rotations of the mother's weightless body. Behaviorally derived stimulation from maternal movements may be a significant factor in studies of vestibular sensory development. Taken together, these studies provide evidence that gravity and angular acceleration shape prenatal organization and function within the mammalian vestibular system.

A field guide to the anterior olfactory nucleus (cortex)

While portions of the mammalian olfactory system have been studied extensively, the anterior olfactory nucleus (AON) has been relatively ignored. Furthermore, the existing research is dispersed and obscured by many different nomenclatures and approaches. The present review collects and assembles the relatively sparse literature regarding the portion of the brain situated between the olfactory bulb and primary olfactory (piriform) cortex. Included is an overview of the area's organization, the functional, morphological and neurochemical characteristics of its cells and a comprehensive appraisal of its efferent and afferent fiber systems. Available evidence suggests the existence of subdivisions within the AON and demonstrates that the structure influences ongoing activity in many other olfactory areas. We conclude with a discussion of the AON's mysterious but complex role in olfactory information processing.

A postnatal sensitive period for plasticity of cortical afferents but not cortical association fibers in rat piriform cortex

Male and female rats underwent unilateral naris occlusion or sham surgery on either post-natal day (PN) 1 or after PN30. Following at least 30 days of unilateral olfactory deprivation, rats were urethane anesthetized and recordings were made from anterior piriform cortex (aPCX). Shock stimulation of afferent fibers (lateral olfactory tract) and association/commissural fibers evoked field potentials in aPCX that were analyzed across groups and between ages. The results demonstrate that early-onset unilateral olfactory deprivation depresses field potentials evoked by stimulation of the deprived cortical afferent, while late-onset deprivation did not. In contrast, intracortical association fiber mediated field potentials in the deprived cortex were enhanced after both early-onset and late-onset deprivation. These results suggest differential developmental plasticity of afferent and association fiber pathways in paleocortex that mirrors that previously described in neocortical sensory systems.

Effects of prenatal spaceflight on vestibular responses in neonatal rats

Ten pregnant Norway rats (Rattus norvegicus) were flown for 11 days on board the NASA space shuttle from gestational day 9 (launch) until gestational day 20 (landing) of the rats' 22-day pregnancy. After the birth of the pups, vestibular responses were analyzed from postnatal day (P) 0 until P5. In the first test, P0 neonates were supported on a platform in a side-lying position. Skyward head movements (i.e., movements performed against the gravity vector) were more frequent than head movements toward Earth in both flight and control neonates. In the second test, the contact-righting reflex, composed of stereotyped movements that rotate the body from supine to prone on a solid surface, was analyzed in P0 neonates. The frequency and latency of contact-righting responses did not differ in flight and control neonates. In the third test, vestibular head righting, with tactile and proprioceptive cues removed, was tested in neonates on P1, P3, and P5 by using a water-immersion test. Righting responses were observed less frequently in P1 and P3 flight neonates compared with controls. However, this deficit was transient, as evidenced by complete response recovery on P5. Collectively, these findings provide evidence for a selective disruption of vestibular-mediated responses after prenatal exposure to spaceflight.

Association of an odor with activation of olfactory bulb noradrenergic beta-receptors or locus coeruleus stimulation is sufficient to produce learned approach responses to that odor in neonatal rats

These experiments examined the sufficiency of pairing an odor with either intrabulbar activation of noradrenergic beta-receptors or pharmacological stimulation of the locus coeruleus to support learned odor preferences in Postnatal Day 6-7 rat pups. The results showed that pups exposed to odor paired with beta-receptor activation limited to the olfactory bulb (isoproterenol, 50 microM) displayed a conditioned approach response on subsequent exposure to that odor. Furthermore, putative stimulation of the locus coeruleus (2 microM idazoxan or 2 mM acetylcholine) paired with odor produced a subsequent preference for that odor. The effects of locus coeruleus stimulation could be blocked by a pretraining injection of the beta-receptor antagonist propranolol (20 mg/kg). Together these results suggest that convergence of odor input with norepinephrine release from the locus coeruleus terminals within the olfactory bulb is sufficient to support olfactory learning.

Trans-neuronal modification of anterior piriform cortical circuitry in the rat

Long-Evans rats received unilateral naris closure on postnatal day 1 (PN1) or sham surgery. On PN30, brains were processed for anterograde horseradish peroxidase (HRP) labeling of lateral olfactory tract (LOT) fibers in anterior piriform cortex (aPCX) Layer Ia, Timm staining of association/commissural fibers in aPCX Layer Ib, or Golgi staining for reconstruction of aPCX semilunar cell dendrites. The results demonstrate that the width of aPCX Layer Ia was reduced ipsilateral to the sealed naris compared to undeprived controls. No significant change in Layer Ib was detected. Furthermore, semilunar cell dendrites were reduced by unilateral deprivation compared to undeprived controls. The reduction in dendritic tree size was localized to distal dendritic segments, roughly corresponding to Layer Ia. These results suggest an activity-dependent change in both the distribution of cortical afferents and in the dendritic field of their target cells. While these results are similar to those reported for other sensory systems, the relatively simple architecture and laminated organization of bilateral inputs to the aPCX make the olfactory system an ideal model system to examine experience-dependent synaptic reorganization and its functional consequences.

Ontogeny of the conditioned eyeblink response in rats: acquisition or expression?

Eyeblink conditioning depends critically on an identified brainstem-cerebellar circuit and is modulated under some circumstances by the hippocampus, amygdala, and other forebrain regions. Developmental studies of eyeblink conditioning could help elucidate questions concerning the behavioral expression of plasticity within these brain circuits and regions, and of their functional interactions, as they unfold during ontogeny. Recently, this laboratory has shown that conditioning of the eyeblink reflex develops dramatically between Postnatal Days (PND) 17 and PND 24 in the rat. The present study asked whether the developmental emergence of the eyeblink conditioned response (CR) occurs gradually or abruptly over this age range, and whether it reflects developmental changes in acquisition or expression of the learned eyeblink reflex. In Experiment 1, rat pups received two consecutive days of training beginning on PND 17, 20, or 24. Conditioned responses occurred at low levels on PND 17-18, intermediate levels on PND 20-21, and high levels on PND 24-25. In Experiment 2, 17-day-old rats received 2 days of training, 72 h apart, so that effects of training on PND 17 could be examined at an age, PND 20, when expression of the eyeblink CR was clearly possible. On PND 20, rat pups that had received paired training on PND 17 showed significantly faster conditioning than controls that had received unpaired training or no training on PND 17. These findings suggest that neural plasticity underlying associative learning developmentally precedes its overt expression in behavior. Hypotheses concerning the nature and locus of this learning are discussed.

The effects of variable periods of functional deprivation on olfactory bulb development in rats

Dramatic alterations occur in the developing olfactory bulb when air flow is reduced through one-half of the nasal cavity. Naris closure on the day after the day of birth (P1) in rats, for example, results in reduced cell survival in the ipsilateral bulb by P20 and a substantial (25%) decrease in bulb size by P30. Almost immediate changes in protein synthesis and cell metabolism are also observed, and one prevalent theory suggests that these changes may be important in specifying which cells are subsequently eliminated. In the present study we used a reversible technique for unilateral naris closure to examine the sensitive period for the effects of olfactory deprivation on bulb size and cell survival. This technique involves the insertion of removable plugs into a rat pup's external naris. We occluded the naris for increasing periods of time (P1-P10, P1-P15, or P1-P20), reared all animals to P30, and measured volumes of bulb laminae. In addition, we examined the duration of naris closure needed to affect cell survival by injecting animals with the thymidine analogue bromodeoxyuridine to label cells born soon after the onset of olfactory deprivation. Results indicate that relatively long periods of naris occlusion (P1-P15 or longer) are required to produce a substantial reduction in experimental bulb size. Cell survival was decreased following olfactory deprivation from P1 to P10, but not after deprivation from P1 to P3. These data support the hypothesis that changes that occur within 48 h of naris closure are not sufficient to affect cell survival.

Olfactory bulb recovery after early sensory deprivation

Olfactory bulbs retain the ability to acquire new neurons throughout life. Unilateral olfactory deprivation during the first postnatal month in rats results in a dramatic reduction in the size of the experimental olfactory bulb. Part of this reduction is attributable to the death of neurons and glia. To examine the regenerative capacity of the juvenile olfactory bulb, we developed a technique for reversible olfactory deprivation. Reversible blockade from postnatal day 1 (P1) to P20 or P30 results in reduced bulb volume and tyrosine hydroxylase immunostaining, and decreased depth in the olfactory mucosa. In another experiment, normal stimulation was restored for varying periods of time, and experimental and control bulb volumes were measured. Recovery of bulb size occurs after 40 d of normal stimulation. Rats injected with a thymidine analog to label dividing cells during the recovery period revealed that rescue results at least in part from the addition of new neurons and glia. Thus, cells born after the return of normal levels of environmental stimulation can replace some of the neurons and glia that are lost during olfactory deprivation. This system can be used to study mechanisms that underlie neuronal regeneration in the maturing mammalian brain.

Mitral/tufted cell activity is attenuated and becomes uncoupled from respiration following naris closure

Patterned neural activity helps to establish neuronal connectivity, produce coding of sensory information, and shape synaptic strengths. Here we demonstrate that normal olfactory bulb development might rely on spatial and temporal patterns of afferent neural activity. Neonatal naris occlusion profoundly impacts the development of the ipsilateral olfactory bulb, including reduced bulb volume, decreased protein synthesis, and increased cell death. Relatively few morphologic changes occur if closure is performed postweaning. We examined the immediate electrophysiological consequences of occlusion across this developmentally sensitive period by recording spontaneous and odor-driven mitral/tufted cell responses while the naris was open, closed, and then reopened. In 1-week-old animals, occlusion severely attenuated spontaneous activity, and presentation of the broad-spectrum odorant amyl acetate failed to evoke responses. In 2- and 4-week old rats, spontaneous activity was also reduced by naris closure. However, some cells remained responsive to concentrated odors, even in animals with transected anterior commissures, suggesting passage of odors across the septal window or retronasal pathways. In all age groups, cellular activity became uncoupled from the respiratory cycle. Approximately 47% (18 of 38) of the mitral/tufted cells exhibited activity that was correlated with respiration in the open-naris state, while only 5% (2 of 38) were coupled during naris closure. These data (a) indicate that naris closure reduces both spontaneous and odor-evoked responses, and (b) provide an electrophysiological correlate to a sensitive period in bulb development. The loss of respiration-related synchrony and the reduced activity of mitral/tufted cells may synergistically contribute to the diverse consequences of naris closure on bulb development.

Binaral interactions in the rat piriform cortex

Single-unit recordings were made from layer II/III anterior piriform cortex (aPCX) neurons in adult Wistar rats to examine odor response patterns to unilaterally and bilaterally delivered stimuli. Isoamyl acetate odor stimulation was presented either unilaterally through tubes inserted into the external nares, or bilaterally during unilateral olfactory bulb lidocaine infusions. Olfactory bulb multiunit or slow-wave activity was recorded simultaneously bilaterally to monitor selectivity of unilateral odor stimulation. The results demonstrate that 1) commissural input to aPCX neurons is sufficient to drive odor responses, and 2) aPCX neurons can be classified on the basis of spatial receptive field type. These receptive fields include cells that respond 1) selectively to ipsilateral stimulation, 2) selectively to contralateral stimulation, 3) to either ipsilateral or contralateral stimulation, and 4) selectively to bilateral stimulation. The potential functions of binaral convergence in the piriform cortex are discussed, and may include enhancement of perceived odor intensity and bilateral access to olfactory memory.

Oligodendrocyte/myelin-immunoreactivity in the developing olfactory system

Immunocytochemistry was used to characterize oligodendrocyte maturation in the developing mammalian olfactory system. Postnatal day 10-16, 20, 30 and adult rats were examined, as well as postnatal day 20, 30, 40 and adult Monodelphis domestica (the grey, short-tailed opossum). In rats, oligodendrocyte/myelin-immunoreactivity first appears in the accessory olfactory bulb by day 11, with labeling rapidly increasing throughout the entire bulb over the next five days. An adult pattern of immunoreactivity, characterized by dense labeling in the granule cell layer, sparse immunoreactivity in the external plexiform layer, and staining along the periphery of glomeruli, is attained by day 30. Staining is apparent in both the lateral olfactory tract and anterior commissure by day 11, and becomes heavy by day 20. While patterns of oligodendrocyte/myelin-immunoreactivity in the adult Monodelphis and rat bulb are similar, staining first appears much later in the opossum (around day 30), and maturation occurs more slowly. For example, rostral-caudal gradients in the development of staining in the anterior commissure were noted which were not seen in the rat. These differences emerge because Monodelphis' slower growth allows more resolution into developmental sequences. Finally, in rats, unilateral naris closure on the day after birth, which significantly alters normal patterns of bulb development, has no effect on the pattern and level of immunoreactivity even after long (30 day) survival periods. In both normal and naris occluded rats, oligodendrocyte/myelin-immunoreactivity is found in caudal aspects of the rat bulb on day 11 and subsequently progresses throughout the entire bulb over the next five days. Patterns in the Monodelphis bulb mirror those observed in the rat, however, staining appears later and progresses more slowly, suggesting Monodelphis is a useful animal for examining early myelin formation.

Retronasal or internasal olfaction can mediate odor-guided behaviors in newborn mice

Studies of olfactory deprivation have most frequently used unilateral naris occlusion to effect deprivation. Recent psychophysical evidence suggests that adult rodents with either acute or chronic naris occlusion show little decrement in olfactory ability. In this study the effect of naris occlusion coupled with ipsilateral or contralateral olfactory bulb deafferentation on odor-guided behaviors was tested in neonatal mice. Animals that received bilateral bulbectomy or control manipulation were also included. In Experiment 1, olfactory lesions were produced by bulb aspiration on the second day after birth (P2). Daily weight gain, a reliable measure of suckling success, was recorded until P21. In Experiment 2, olfactory lesions consisted of bulb transection on P2. Daily weights were recorded until subjects were P10. Animals with bilateral bulbectomy had the highest mortality rate and slowest growth rate. Both naris occlusion groups grew more slowly than controls but were not significantly different at P5 or P10. They diverged, thereafter, such that at P20 the group with naris occlusion ipsilateral to bulbectomy was similar to controls, while the contralateral group was similar to the bilateral bulbectomy group. In Experiment 2, the weights of the naris occlusion groups diverged by the day after surgery, with the contralateral group suffering the most arrested growth. Behavioral tests were combined from the two experiments for analysis. Subjects were tested at P5 for their ability to find the nipple, at P8 for their ability to find the nest, and at P10 for unwashed vs. washed nipple preferences. Only the contralateral group had a significantly depressed ability to find the nipple, while all lesions groups had a significant but similar decline in nest-finding ability.(ABSTRACT TRUNCATED AT 250 WORDS)

Olfactory disturbance induced by deafferentation of serotonergic fibers in the olfactory bulb

The serotonergic neurons of the brain stem project widely throughout the central nervous system, and the olfactory bulb is one of the major forebrain targets of the ascending serotonin pathway. According to physiological studies, neurons of the olfactory bulb were found to reduce their spontaneous discharge rates by electrophoretically applied serotonin. However, roles of the bulbar serotonin in the sense of smell remain unanswered. In the present study, using 5,7-dihydroxytryptamine, a specific neurotoxin for serotonin, we found that the conditioned rats who learned to avoid a repellent by olfaction lost ability of discrimination by deafferentation of the bulbar serotonergic fibers. Such olfactory dysfunction did not occur in the early stage (three days after injection of the toxin) when the serotonergic fibers disappeared in the bulb, but developed a few weeks later. Interestingly, histological examination revealed marked shrinkage of the bulbar glomerulus which is a major termination site of the bulbopetal serotonergic fibers, and also a synaptic site of olfactory receptor cells and bulbar output neurons. The results indicate that depletion of the serotonergic fibers in the olfactory bulb causes glomerular atrophy and olfactory disturbance in the rat.