Developmental change in the access to olfactory memories

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.

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.

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.

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.

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.

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.

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.

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.

Temporary inactivation of the nucleus accumbens disrupts acquisition and expression of fear-potentiated startle in rats

Recent research suggests that in addition to its prominent role in appetitive learning, the nucleus accumbens (NAC) may also be involved in fear conditioning. In the present study, we investigated whether temporary inactivation of the NAC, by injection of tetrodotoxin (TTX), affects acquisition and expression of conditioned fear, as measured by fear-potentiated startle (FPS). TTX injection into the NAC totally blocked acquisition and markedly decreased expression of conditioned fear to a discrete visual conditioned stimulus (CS). Interestingly, temporary inactivation of the NAC did not affect shock sensitization of startle, indicating that both the perception of the shock and short-term contextual conditioning was not affected by intra-accumbal TTX injection. Taken together, these results show that the NAC is crucial for acquisition and expression of long-term conditioned fear, as measured by fear-potentiated startle, to discrete CSs, but not short-term conditioned fear to a context.

Olfactory and/or visual cues for spatial navigation through ontogeny: olfactory cues enable the use of visual cues

This study analyzed the spatial memory capacities of rats in darkness with visual and/or olfactory cues through ontogeny. Tests were conducted with the homing board, where rats had to find the correct escape hole. Four age groups (24 days, 48 days, 3-6 months, and 12 months) were trained in 3 conditions: (a) 3 identical light cues; (b) 5 different olfactory cues; and (c) both types of cues, followed by removal of the olfactory cues. Results indicate that immature rats first take into account olfactory information but are unable to orient with only the help of discrete visual cues. Olfaction enables the use of visual information by 48-day-old rats. Visual information predominantly supports spatial cognition in adult and 12-month-old rats. Results point out cooperation between vision and olfaction for place navigation during ontogeny in rats.

Conditioned changes in ultrasonic vocalizations to an aversive olfactory stimulus are lateralized in 6-day-old rats

Using a soft rubber plug to block airflow in one naris, Kucharski, Johanson, and Hall (1986) found that some forms of olfactory memory (e.g., odor preferences) were lateralized in young rats while other forms (e.g., conditioned activation and mouthing) were not. The present experiments extended that research by showing that conditioned increases in ultrasonic vocalizations were also lateralized. That is, when exposed to an odor that was previously paired with footshock, 6-day-old rats significantly increased their rate of vocalizing. This response only occurred, however, when the naris open at training was also open at test. The use of the developing rat as a natural split-brain preparation appears to be an effective procedure with which to broaden current approaches to the analysis of learning, memory, and emotion.

Odor recognition memory is better under bilateral than unilateral test conditions

Two odor memory tests were administered unilaterally (left and right) and bilaterally to the same set of 24 men and 24 women on two test occasions. These tests were (i) a "multiple-target" test (MTT) in which three target stimuli were selected, after 10-, 30-, and 60-sec retention intervals, from stimulus sets containing both target and distracter stimuli, and (ii) a 9-item "single-target" three-choice test (STT), in which single stimuli were selected from stimuli sets containing two distracters. Overall, odor memory scores were higher under bilateral than unilateral testing, and higher on the second than on the first test session. Unilateral testing resulted in a monotonic delay-related forgetting function. No differences were observed in the odor memory test scores of the left and right sides of the nose, and no significant correlations were present between these scores and scores on a battery non-olfactory memory tests. On the STT, the female, but not the male, subjects demonstrated better performance on the second, than on the first, test session. The results of this study imply that odor memory is facilitated centrally by bilateral activation, and that a memory system may exist for odors which is distinct from other memory systems.

Interhemispheric sharing of visual memory in macaques

(1) In macaques with the optic chiasm transected, and forebrain commissural communication limited to the anterior commissure or the posterior 5 mm of the splenium of the corpus callosum, visual patterns viewed initially by only one eye (hemisphere) are subsequently recognized by the other with normal accuracy. (2) The efficiency of these commissural paths is further indicated by the fact that even when as many as six "target" images are presented for memorization to only one hemisphere, it makes essentially no difference as to accuracy or latency of performance which hemisphere is then required to distinguish "target" from "non-target" images. (3) By electrically tetanizing structures in one or the other temporal lobe at various times in relation to visual input and/or mnemonic testing it could be shown: (a) that a memory trace restricted in its formation to a single hemisphere was available to the other via either forebrain commissure, and (b) that the memory is formed bilaterally despite unilateral input. (4) When the chiasm is split but the commissures are intact, simultaneous presentation of disparate images to each hemisphere severely perturbs performance, suggesting that the callosal system operates continuously to unify visual percepts; but when only the anterior commissure is intact, the two hemispheres accept incongruent images without perturbation. (5) In the fully "split-brain" condition, when one hemisphere cannot access memories held in the other, the accuracy of performance by each hemisphere is nevertheless burdened by the memory load of its neocortically disconnected partner. It can thus be inferred that the brainstem plays a critical, unifying role in this mnemonic process.

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)

Neurobiology of associative learning in the neonate: early olfactory learning

Mammalian neonates have been simultaneously described as having particularly poor memory, as evidenced by infantile amnesia, and as being particularly excellent learners with unusually plastic nervous systems that are easily influenced by experience. An understanding of the neurobiological constraints and mechanisms of early learning may contribute to a unified explanation of these two disparate views. Toward that end, we review here our work on the neurobiology of learning and memory in neonates. Specifically, we have examined the neurobiology of early learning using an olfactory classical conditioning paradigm. Olfactory classical conditioning in neonates at the behavioral level conforms well with the requirements and outcomes of classical conditioning described in adults. Furthermore, specific neural correlates of this behavioral conditioning have been described including anatomical and physiological changes, neural pathways, and modulatory systems. In this Review, we outline the behavioral paradigm, the identified neural correlates, and apparent mechanisms of this learning. Finally, we compare the neurobiology of early learning with that reported for mature animals, with specific reference to the role of US-CS convergence, memory modulation, consolidation, and distributed memory.

The locus coeruleus, norepinephrine, and memory in newborns

Use of learned odor cues by newborn rats is critical for pup survival. Rat pups acquire approach responses to maternal odors through an associative conditioning mechanism. This learned behavioral response is accompanied by a modification of olfactory bulb neural response patterns to the learned odor. Both the behavioral and neural response changes involved and require norepinephrine release in the olfactory bulb. The source of this norepinephrine is the locus coeruleus. It is proposed that the unique response properties of the locus coeruleus during the early postnatal period in the rat may facilitate acquisition of these critical early memories.

Neural correlates of memory for odor detection conditioning in adult rats

Adult male Wistar rats were trained in a simple odor detection task, with peppermint odor serving as either an S+, S- or as a randomly presented odor. Twenty-four hours after the last training session, rats were injected with [14C]2-deoxyglucose and exposed to the odor. Mean relative 2-deoxyglucose uptake to the odor was enhanced in the pars dorsalis of the anterior olfactory nucleus of S+ and S- trained rats compared to controls. In contrast, no differences in uptake were detected in either odor-specific focal regions of the olfactory bulb glomerular layer, the pyriform cortex, or the hippocampus.

The role of olfactory bulb norepinephrine in early olfactory learning

Wistar rat pups were implanted with bilateral olfactory bulb cannulas on postnatal day 5 (PN5). On PN6, pups were trained in an olfactory classical conditioning task with peppermint odor as the CS and tactile stimulation/stroking as the UCS. Pups were randomly assigned to either PAIRED, BACKWARD or ODOR-only conditions. Half the pups in each group received intrabulbar infusions of 100 microM propranolol and half received intrabulbar infusions of saline during the training session. Propranolol infusions blocked acquisition of the learned odor preference expressed by PAIRED saline-infused pups. Diffusion of the infusate was checked in additional pups by infusing [3H]NE and performing LSC analysis. Infusate concentration did not significantly differ between the anterior and posterior halves of the bulb, but were sharply lower in the olfactory peduncle and more posterior areas. The results suggest that olfactory bulb NE is critical for early olfactory learning.

Neonatal exposure to triethyltin disrupts olfactory discrimination learning in preweanling rats

Triethyltin is an organotin compound that is known to produce neurotoxicity in both adult and developing organisms. Although this neurotoxicity has been documented with a variety of behavioral and biological measures, the effects of this compound on learning during early development have been less extensively studied. The present study reports four experiments that examined this question with an odor aversion learning paradigm in which pups received presentations of one odor paired with footshock and an alternate odor without shock. In Experiment 1, Long-Evans rat pups were injected IP on postnatal day 5 (PND 5) with either 0, 3 or 5 mg/kg TET and then tested for olfactory discrimination learning on PND 18. Only the 5-mg/kg dose impaired discrimination learning. In Experiment 2, PND 5 exposure to TET (5 mg/kg) disrupted olfactory learning on PND 18 but not on PND 12, whereas exposure on PND 10 disrupted learning at both ages of testing. In Experiment 3, PND 16 exposure to TET (5 mg/kg) also disrupted acquisition of olfactory learning on PND 18 but had no effect on retention of an olfactory discrimination that was acquired prior to TET exposure (i.e., on PND 14 and PND 15). Unconditioned responses to footshock were also unaffected by TET (Experiment 4). These findings indicate that neonatal exposure to TET impairs associative learning in developing rats and are discussed in relation to other studies of the developmental neurotoxicity of this compound.

Changes in excitatory amino acid modulation of phosphoinositide metabolism during development

Two different developmental patterns of stimulation of phosphoinositide (PI) metabolism by excitatory amino acid (EAA) receptors were observed during the postnatal maturation of various brain regions. A 'burst' in PI metabolism was seen at postnatal day 6 (PND6) in olfactory bulb and cerebellum and at PND9 in hippocampus. In cortex and thalamus/hypothalamus high levels of PI metabolism were observed initially, and then began to decline at PND15 and PND18, respectively. NMDA inhibition of PI metabolism was generally found to parallel the EAA activation but the persistence of inhibition varied in the different brain regions.

New routes to early memories

Stimulation of one side of the olfactory system during training with odor-milk pairings in neonatal rats results in their ability to recall an odor memory by using the trained but not the untrained side of the brain. In 12-day-old rats, olfactory learning can be recalled by stimulation of either the trained or untrained side. The development of bilateral recall reflects the maturation of olfactory commissural pathways that provide access to the olfactory memory stored on the contralateral side. Furthermore, the commissural pathways need not be present at the time of memory formation but can establish new and specific access to already existing olfactory memories.