The effect of microinfusions of drugs into the accessory olfactory bulb on the olfactory block to pregnancy

Contactin-6-deficient male mice exhibit the abnormal function of the accessory olfactory system and impaired reproductive behavior

INTRODUCTION

Contactin-6 (CNTN6), also known as NB-3, is a neural recognition molecule and a member of the contactin subgroup of the immunoglobulin superfamily. Gene encoding CNTN6 is expressed in many regions of the neural system, including the accessory olfactory bulb (AOB) in mice. We aim to determine the effect of CNTN6 deficiency on the function of the accessory olfactory system (AOS).

METHODS

We examined the effect of CNTN6 deficiency on the reproductive behavior of male mice through behavioral experiments such as urine sniffing and mate preference tests. Staining and electron microscopy were used to observe the gross structure and the circuitry activity of the AOS.

RESULTS

Cntn6 is highly expressed in the vomeronasal organ (VNO) and the AOB, and sparsely expressed in the medial amygdala (MeA) and the medial preoptic area (MPOA), which receive direct and/or indirect projections from the AOB. Behavioral tests to examine reproductive function in mice, which is mostly controlled by the AOS, revealed that Cntn6^-/- adult male mice showed less interest and reduced mating attempts toward estrous female mice in comparison with their Cntn6^+/+ littermates. Although Cntn6^-/- adult male mice displayed no obvious changes in the gross structure of the VNO or AOB, we observed the increased activation of granule cells in the AOB and the lower activation of neurons in the MeA and the MPOA as compared with Cntn6^+/+ adult male mice. Moreover, there were an increased number of synapses between mitral cells and granule cells in the AOB of Cntn6^-/- adult male mice as compared with wild-type controls.

CONCLUSION

These results indicate that CNTN6 deficiency affects the reproductive behavior of male mice, suggesting that CNTN6 participated in normal function of the AOS and its ablation was involved in synapse formation between mitral and granule cells in the AOB, rather than affecting the gross structure of the AOS.

Arc-Expressing Accessory Olfactory Bulb Interneurons Support Chemosensory Social Behavioral Plasticity

The accessory olfactory system (AOS) is critical for the development and expression of social behavior. The first dedicated circuit in the AOS, the accessory olfactory bulb (AOB), exhibits cellular and network plasticity in male and female mice after social experience. In the AOB, interneurons called internal granule cells (IGCs) express the plasticity-associated immediate-early gene Arc following intermale aggression or mating. Here, we sought to better understand how Arc-expressing IGCs shape AOB information processing and social behavior in the context of territorial aggression. We used "ArcTRAP" (Arc-CreERT2) transgenic mice to selectively and permanently label Arc-expressing IGCs following male-male resident-intruder interactions. Using whole-cell patch-clamp electrophysiology, we found that Arc-expressing IGCs display increased intrinsic excitability for several days after a single resident-intruder interaction. Further, we found that Arc-expressing IGCs maintain this increased excitability across repeated resident-intruder interactions, during which resident mice increase or "ramp" their aggression. We tested the hypothesis that Arc-expressing IGCs participate in ramping aggression. Using a combination of ArcTRAP mice and chemogenetics (Cre-dependent hM4D(Gi)-mCherry AAV injections), we found that disruption of Arc-expressing IGC activity during repeated resident-intruder interactions abolishes the ramping aggression exhibited by resident male mice. This work shows that Arc-expressing AOB IGC ensembles are activated by specific chemosensory environments, and play an integral role in the establishment and expression of sex-typical social behavior. These studies identify a population of plastic interneurons in an early chemosensory circuit that display physiological features consistent with simple memory formation, increasing our understanding of central chemosensory processing and mammalian social behavior.SIGNIFICANCE STATEMENT The accessory olfactory system plays a vital role in rodent chemosensory social behavior. We studied experience-dependent plasticity in the accessory olfactory bulb and found that internal granule cells expressing the immediate-early gene Arc after the resident-intruder paradigm increase their excitability for several days. We investigated the roles of these Arc-expressing internal granule cells on chemosensory social behavior by chemogenetically manipulating their excitability during repeated social interactions. We found that inhibiting these cells eliminated intermale aggressive ramping behavior. These studies identify a population of plastic interneurons in an early chemosensory circuit that display physiological features consistent with simple memory formation, increasing our understanding of central chemosensory processing and mammalian social behavior.

The Bruce effect: Representational stability and memory formation in the accessory olfactory bulb of the female mouse

In the Bruce effect, a mated female mouse becomes resistant to the pregnancy-blocking effect of the stud. Various lines of evidence suggest that this form of behavioral imprinting results from reduced sensitivity of the female's accessory olfactory bulb (AOB) to the stud's chemosignals. However, the AOB's combinatorial code implies that diminishing responses to one individual will distort representations of other stimuli. Here, we record extracellular responses of AOB neurons in mated and unmated female mice while presenting urine stimuli from the stud and from other sources. We find that, while initial sensory responses in the AOB (within a timescale required to guide social interactions) remain stable, responses to extended stimulation (as required for eliciting the pregnancy block) display selective attenuation of stud-responsive neurons. Such temporal disassociation could allow attenuation of slow-acting endocrine processes in a stimulus-specific manner without compromising ongoing representations that guide behavior.

Paradoxically Sparse Chemosensory Tuning in Broadly Integrating External Granule Cells in the Mouse Accessory Olfactory Bulb

The accessory olfactory bulb (AOB), the first neural circuit in the mouse accessory olfactory system, is critical for interpreting social chemosignals. Despite its importance, AOB information processing is poorly understood compared with the main olfactory bulb (MOB). Here, we sought to fill gaps in the understanding of AOB interneuron function. We used 2-photon GCaMP6f Ca2+ imaging in an ex vivo preparation to study chemosensory tuning in AOB external granule cells (EGCs), interneurons hypothesized to broadly inhibit activity in excitatory mitral cells (MCs). In ex vivo preparations from mice of both sexes, we measured MC and EGC tuning to natural chemosignal blends and monomolecular ligands, finding that EGC tuning was sparser, not broader, than upstream MCs. Simultaneous electrophysiological recording and Ca2+ imaging showed no differences in GCaMP6f-to-spiking relationships in these cell types during simulated sensory stimulation, suggesting that measured EGC sparseness was not due to cell type-dependent variability in GCaMP6f performance. Ex vivo patch-clamp recordings revealed that EGC subthreshold responsivity was far broader than indicated by GCaMP6f Ca2+ imaging, and that monomolecular ligands rarely elicited EGC spiking. These results indicate that EGCs are selectively engaged by chemosensory blends, suggesting different roles for EGCs than analogous interneurons in the MOB.SIGNIFICANCE STATEMENT The mouse accessory olfactory system (AOS) interprets social chemosignals, but we poorly understand AOS information processing. Here, we investigate the functional properties of external granule cells (EGCs), a major class of interneurons in the accessory olfactory bulb (AOB). We hypothesized that EGCs, which are densely innervated by excitatory mitral cells (MCs), would show broad chemosensory tuning, suggesting a role in divisive normalization. Using ex vivo GCaMP6f imaging, we found that EGCs were instead more sparsely tuned than MCs. This was not due to weaker GCaMP6f signaling in EGCs than in MCs. Instead, we found that many MC-activating chemosignals caused only subthreshold EGC responses. This indicates a different role for AOB EGCs compared with analogous cells in the main olfactory bulb.

Interneuron Functional Diversity in the Mouse Accessory Olfactory Bulb

In the mouse accessory olfactory bulb (AOB), inhibitory interneurons play an essential role in gating behaviors elicited by sensory exposure to social odors. Several morphological classes have been described, but the full complement of interneurons remains incomplete. In order to develop a more comprehensive view of interneuron function in the AOB, we performed targeted patch clamp recordings from partially overlapping subsets of genetically labeled and morphologically defined interneuron types. Gad2 (GAD65), Calb2 (calretinin), and Cort (cortistatin)-cre mouse lines were used to drive selective expression of tdTomato in AOB interneurons. Gad2 and Calb2-labeled interneurons were found in the internal, external, and glomerular (GL) layers, whereas Cort-labeled interneurons were enriched within the lateral olfactory tract (LOT) and external cellular layer (ECL). We found that external granule cells (EGCs) from all genetically labeled subpopulations possessed intrinsic functional differences that allowed them to be readily distinguished from internal granule cells (IGCs). EGCs showed stronger voltage-gated Na⁺ and non-inactivating voltage-gated K⁺ currents, decreased I_H currents, and robust excitatory synaptic input. These specific intrinsic properties did not correspond to any genetically labeled type, suggesting that transcriptional heterogeneity among EGCs and IGCs is not correlated with expression of these particular marker genes. Intrinsic heterogeneity was also seen among AOB juxtaglomerular cells (JGCs), with a major subset of Calb2-labeled JGCs exhibiting spontaneous and depolarization-evoked plateau potentials. These data identify specific physiological features of AOB interneurons types that will assist in future studies of AOB function.

Neural circuits regulating sexual behaviors via the olfactory system in mice

Reproduction is essential for any animal species. Reproductive behaviors, or sexual behaviors, are largely shaped by external sensory cues exchanged during sexual interaction. In many animals, including rodents, olfactory cues play a critical role in regulating sexual behavior. What exactly these olfactory cues are and how they impact animal behavior have been a central question in the field. Over the past few decades, many studies have dedicated to identifying an active compound that elicits sexual behavior from crude olfactory components. The identified substance has served as a tool to dissect the sensory processing mechanisms in the olfactory systems. In addition, recent advances in genetic engineering, and optics and microscopic techniques have greatly expanded our knowledge of the neural mechanisms underlying the control of sexual behavior in mice. This review summarizes our current knowledge about how sexual behaviors are controlled by olfactory cues.

α2-Adrenergic receptor activation promotes long-term potentiation at excitatory synapses in the mouse accessory olfactory bulb

The formation of mate recognition memory in mice is associated with neural changes at the reciprocal dendrodendritic synapses between glutamatergic mitral cell (MC) projection neurons and GABAergic granule cell (GC) interneurons in the accessory olfactory bulb (AOB). Although noradrenaline (NA) plays a critical role in the formation of the memory, the mechanism by which it exerts this effect remains unclear. Here we used extracellular field potential and whole-cell patch-clamp recordings to assess the actions of bath-applied NA (10 µM) on the glutamatergic transmission and its plasticity at the MC-to-GC synapse in the AOB. Stimulation (400 stimuli) of MC axons at 10 Hz but not at 100 Hz effectively induced N-methyl-d-aspartate (NMDA) receptor-dependent long-term potentiation (LTP), which exhibited reversibility. NA paired with subthreshold 10-Hz stimulation (200 stimuli) facilitated the induction of NMDA receptor-dependent LTP via the activation of α₂-adrenergic receptors (ARs). We next examined how NA, acting at α₂-ARs, facilitates LTP induction. In terms of acute actions, NA suppressed GC excitatory postsynaptic current (EPSC) responses to single pulse stimulation of MC axons by reducing glutamate release from MCs via G-protein coupled inhibition of calcium channels. Consequently, NA reduced recurrent inhibition of MCs, resulting in the enhancement of evoked EPSCs and spike fidelity in GCs during the 10-Hz stimulation used to induce LTP. These results suggest that NA, acting at α₂-ARs, facilitates the induction of NMDA receptor-dependent LTP at the MC-to-GC synapse by shifting its threshold through disinhibition of MCs.

Experience-Dependent Plasticity in Accessory Olfactory Bulb Interneurons following Male-Male Social Interaction

Chemosensory information processing in the mouse accessory olfactory system guides the expression of social behavior. After salient chemosensory encounters, the accessory olfactory bulb (AOB) experiences changes in the balance of excitation and inhibition at reciprocal synapses between mitral cells (MCs) and local interneurons. The mechanisms underlying these changes remain controversial. Moreover, it remains unclear whether MC-interneuron plasticity is unique to specific behaviors, such as mating, or whether it is a more general feature of the AOB circuit. Here, we describe targeted electrophysiological studies of AOB inhibitory internal granule cells (IGCs), many of which upregulate the immediate-early gene Arc after male-male social experience. Following the resident-intruder paradigm, Arc-expressing IGCs in acute AOB slices from resident males displayed stronger excitation than nonexpressing neighbors when sensory inputs were stimulated. The increased excitability of Arc-expressing IGCs was not correlated with changes in the strength or number of excitatory synapses with MCs but was instead associated with increased intrinsic excitability and decreased HCN channel-mediated I_H currents. Consistent with increased inhibition by IGCs, MCs responded to sensory input stimulation with decreased depolarization and spiking following resident-intruder encounters. These results reveal that nonmating behaviors drive AOB inhibitory plasticity and indicate that increased MC inhibition involves intrinsic excitability changes in Arc-expressing interneurons.SIGNIFICANCE STATEMENT The accessory olfactory bulb (AOB) is a site of experience-dependent plasticity between excitatory mitral cells (MCs) and inhibitory internal granule cells (IGCs), but the physiological mechanisms and behavioral conditions driving this plasticity remain unclear. Here, we report studies of AOB neuronal plasticity following male-male social chemosensory encounters. We show that the plasticity-associated immediate-early gene Arc is selectively expressed in IGCs from resident males following the resident-intruder assay. After behavior, Arc-expressing IGCs are more strongly excited by sensory input stimulation and MC activation is suppressed. Arc-expressing IGCs do not show increased excitatory synaptic drive but instead show increased intrinsic excitability. These data indicate that MC-IGC plasticity is induced after male-male social chemosensory encounters, resulting in enhanced MC suppression by Arc-expressing IGCs.

Neural mechanisms of social learning in the female mouse

Social interactions are often powerful drivers of learning. In female mice, mating creates a long-lasting sensory memory for the pheromones of the stud male that alters neuroendocrine responses to his chemosignals for many weeks. The cellular and synaptic correlates of pheromonal learning, however, remain unclear. We examined local circuit changes in the accessory olfactory bulb (AOB) using targeted ex vivo recordings of mating-activated neurons tagged with a fluorescent reporter. Imprinting led to striking plasticity in the intrinsic membrane excitability of projection neurons (mitral cells, MCs) that dramatically curtailed their responsiveness, suggesting a novel cellular substrate for pheromonal learning. Plasticity was selectively expressed in the MC ensembles activated by the stud male, consistent with formation of memories for specific individuals. Finally, MC excitability gained atypical activity-dependence whose slow dynamics strongly attenuated firing on timescales of several minutes. This unusual form of AOB plasticity may act to filter sustained or repetitive sensory signals.

DOI:http://dx.doi.org/10.7554/eLife.25421.001

To navigate social situations, humans and other animals need to remember who they have interacted with and how it went, and adjust their behavior in future encounters accordingly. For example, your physical actions, and even your body’s physiological responses, will be very different when you encounter the last person you kissed instead of the last person you fought with (assuming this is not the same person!).

Memories of social interactions can have dramatic consequences. For instance, male mice often kill the offspring of other males. Female mice appear to have adopted a countermeasure to avoid losing a litter of pups to such aggression: they will spontaneously abort a pregnancy when exposed to chemicals called pheromones from unfamiliar males. However, when the female mouse is exposed to the pheromones of the male she mated with she maintains her pregnancy. Exactly how the memories of previous social interactions with the males affect the female’s pheromone responses is not fully understood.

To investigate how the female is able to respond differently to different males, Gao et al. recorded the activity of individual neurons taken from the brain tissue of female mice who had recently mated. The recordings showed that previous social experiences produce learning-related changes in the brain of the female mouse that reduce how sensitively pheromone-detecting neurons respond to the chemical cues of the male mate. This suppresses the signals that the neurons would otherwise send to trigger an abortion in response to male pheromones.

Gao et al. also used fluorescent tags to identify which neurons in the female’s brain had been activated during mating. This revealed that only those neurons that had been activated by the mate become unresponsive when the cells again encountered his pheromones. This suggests that a set of neurons in the female’s brain records the chemical ‘fingerprint’ of the mate, and can then selectively filter out that mate’s pheromone signals.

Many other social interactions, such as parenting, are also strongly shaped by experience. The results presented by Gao et al. may therefore offer wider lessons for understanding how the brain targets different behaviors toward specific individuals. It will also be important to investigate how highly arousing experiences cause such powerful memories to form. This could ultimately help us to better understand – and potentially treat – conditions like post-traumatic stress disorder.

DOI:http://dx.doi.org/10.7554/eLife.25421.002

Heterogeneous effects of norepinephrine on spontaneous and stimulus-driven activity in the male accessory olfactory bulb

Norepinephrine (NE) release has been linked to experience-dependent plasticity in many model systems and brain regions. Among these is the rodent accessory olfactory system (AOS), which is crucial for detecting and processing socially relevant environmental cues. The accessory olfactory bulb (AOB), the first site of chemosensory information processing in the AOS, receives dense centrifugal innervation by noradrenergic fibers originating in the locus coeruleus. Although NE release has been linked to behavioral plasticity through its actions in the AOB, the impacts of noradrenergic modulation on AOB information processing have not been thoroughly studied. We made extracellular single-unit recordings of AOB principal neurons in ex vivo preparations of the early AOS taken from adult male mice. We analyzed the impacts of bath-applied NE (10 μM) on spontaneous and stimulus-driven activity. In the presence of NE, we observed overall suppression of stimulus-driven neuronal activity with limited impact on spontaneous activity. NE-associated response suppression in the AOB came in two forms: one that was strong and immediate (21%) and one other that involved gradual, stimulus-dependent monotonic response suppression (47%). NE-associated changes in spontaneous activity were more modest, with an overall increase in spontaneous spike frequency observed in 25% of neurons. Neurons with increased spontaneous activity demonstrated a net decrease in chemosensory discriminability. These results reveal that noradrenergic signaling in the AOB causes cell-specific changes in chemosensory tuning, even among similar projection neurons.NEW & NOTEWORTHY Norepinephrine (NE) is released throughout the brain in many behavioral contexts, but its impacts on information processing are not well understood. We studied the impact of NE on chemosensory tuning in the mouse accessory olfactory bulb (AOB). Electrophysiological recordings from AOB neurons in ex vivo preparations revealed that NE, on balance, inhibited mitral cell responses to chemosensory cues. However, NE's effects were heterogeneous, indicating that NE signaling reshapes AOB output in a cell- and stimulus-specific manner.

Computational modeling suggests distinct, location-specific function of norepinephrine in olfactory bulb and piriform cortex

Noradrenergic modulation from the locus coerulus is often associated with the regulation of sensory signal-to-noise ratio. In the olfactory system, noradrenergic modulation affects both bulbar and cortical processing, and has been shown to modulate the detection of low concentration stimuli. We here implemented a computational model of the olfactory bulb and piriform cortex, based on known experimental results, to explore how noradrenergic modulation in the olfactory bulb and piriform cortex interact to regulate odor processing. We show that as predicted by behavioral experiments in our lab, norepinephrine can play a critical role in modulating the detection and associative learning of very low odor concentrations. Our simulations show that bulbar norepinephrine serves to pre-process odor representations to facilitate cortical learning, but not recall. We observe the typical non-uniform dose—response functions described for norepinephrine modulation and show that these are imposed mainly by bulbar, but not cortical processing.

Roles for learning in mammalian chemosensory responses

This article is part of a Special Issue "Chemosignals and Reproduction". A rich variety of chemosignals have been identified that influence mammalian behaviour, including peptides, proteins and volatiles. Many of these elicit innate effects acting either as pheromones within species or allelochemicals between species. However, even innate pheromonal responses in mammals are not as hard-wired as the original definition of the term would suggest. Many, if not most mammalian pheromonal responses are only elicited in certain behavioural or physiological contexts. Furthermore, certain pheromones are themselves rewarding and act as unconditioned stimuli to link non-pheromonal stimuli to the pheromonal response, via associative learning. The medial amygdala, has emerged as a potential site for this convergence by which learned chemosensory input is able to gain control over innately-driven output circuits. The medial amygdala is also an important site for associating social chemosensory information that enables recognition of conspecifics and heterospecifics by association of their complex chemosensory signatures both within and across olfactory chemosensory systems. Learning can also influence pheromonal responses more directly to adapt them to changing physiological and behavioural context. Neuromodulators such as noradrenaline and oxytocin can plasticise neural circuits to gate transmission of chemosensory information. More recent evidence points to a role for neurogenesis in this adaptation, both at the peripheral level of the sensory neurons and via the incorporation of new neurons into existing olfactory bulb circuits. The emerging picture is of integrated and flexible responses to chemosignals that adapt them to the environmental and physiological context in which they occur.

Three-dimensional reconstruction of electron micrographs reveals intrabulbar circuit differences between accessory and main olfactory bulbs

Three-dimensional (3D) reconstruction of synaptic arrangement on a particular dendrite provides essential information regarding neuronal properties and neural microcircuits. Unconventional synapses are particularly good candidates for such steric attribution. In main and accessory olfactory bulbs (MOBs and AOBs), there are dendrodendritic reciprocal synapses (RSs) between excitatory projection neurons and inhibitory interneurons. Although the fine structure and configuration of these synapses have been investigated in MOB, their characteristics in AOB were unknown. In this study, we performed 3D AOB reconstruction using serial section transmission electron microscopy. We found numerous RSs on primary dendrites from glomeruli to mitral/tufted (MT) cell somas. These synapses formed between dendritic shafts of MT cells and large dendritic spines, or so-called gemmules, of granule (Gr) cells. This indicates that chemical signals received by a glomerulus are regulated in the primary dendrite of an MT cell before reaching its soma. In MOB, RSs are located on secondary dendrites and act as lateral and self-inhibiting following mitral cell depolarization. Our results indicate that AOB intrabulbar microcircuitry is quite different from that in the MOB.

Expression profiling reveals differential gene induction underlying specific and non-specific memory for pheromones in mice

Memory for the mating male's pheromones in female mice is thought to require synaptic changes in the accessory olfactory bulb (AOB). Induction of this memory depends on release of glutamate in response to pheromonal exposure coincident with release of norepinephrine (NE) in the AOB following mating. A similar memory for pheromones can also be induced artificially by local infusion of the GABA(A) receptor antagonist bicuculline into the AOB. The natural memory formed by exposure to pheromones during mating is specific to the pheromones sensed by the female during mating. In contrast, the artificial memory induced by bicuculline is non-specific and results in the female mice recognizing all pheromones as if they were from the mating male. Although protein synthesis has been shown to be essential for development of pheromone memory, the gene expression cascades critical for memory formation are not known. We investigated changes in gene expression in the AOB using oligonucleotide microarrays during mating-induced pheromone memory (MIPM) as well as bicuculline-induced pheromone memory (BIPM). We found the set of genes induced during MIPM and BIPM are largely non-overlapping and Ingenuity Pathway Analysis revealed that the signaling pathways in MIPM and BIPM also differ. The products of genes induced during MIPM are associated with synaptic function, indicating the possibility of modification at specific synapses, while those induced during BIPM appear to possess neuron-wide functions, which would be consistent with global cellular changes. Thus, these results begin to provide a mechanistic explanation for specific and non-specific memories induced by pheromones and bicuculline infusion respectively.

Nonlinear effects of noradrenergic modulation of olfactory bulb function in adult rodents

The mammalian main olfactory bulb receives a significant noradrenergic input from the locus coeruleus. Norepinephrine (NE) is involved in acquisition of conditioned odor preferences in neonatal animals, in some species-specific odor-dependent behaviors, and in adult odor perception. We provide a detailed review of the functional role of NE in adult rodent main olfactory bulb function. We include cellular, synaptic, network, and behavioral data and use computational simulations to tie these different types of data together.

Muscarinic receptor type 1 (M1) stimulation, probably through KCNQ/Kv7 channel closure, increases spontaneous GABA release at the dendrodendritic synapse in the mouse accessory olfactory bulb

Cholinergic modulation of spontaneous GABAergic currents (mIPSC) was studied using whole-cell patch methods in mouse accessory olfactory bulb slices. Carbachol (above 100 microM) administration produced an increase in the mIPSC frequency in mitral cells, but did not affect the responses of mitral cells to GABA. The carbachol effect persisted in the presence of combined ionotropic and metabotropic glutamatergic receptor antagonists. The carbachol effect was reduced by the muscarinic receptor type-1 and -4 (M1 and M4) antagonist pirenzepine (10 microM), but not by the M2 and M4 antagonist himbacine (10 microM). The KCNQ/Kv7 potassium channel openers retigabine (80 microM) and diclofenac (300 microM) blocked the carbachol action, while the KCNQ potassium channel blocker XE-911 (20 microM) increased the mIPSC frequency. XE-911's action persisted in the presence of glutamate receptor blockers. In the presence of carbachol, mIPSCs were abolished by Ni (200 microM), while being insensitive to the calcium channel blocker nimodipine (30 microM), suggesting a role for R-type calcium channels in the GABA release. These results suggest that carbachol closed KCNQ channels by stimulating M1 receptors on granule cell dendrites, and the resulting depolarized and unstable membrane promoted calcium influx, thus increasing the GABA release. The possible role of acetylcholine in facilitating formation of a pheromone memory in mice is also discussed.

Excitatory actions of noradrenaline and metabotropic glutamate receptor activation in granule cells of the accessory olfactory bulb

Modulation of dendrodendritic synapses by the noradrenergic system in the accessory olfactory bulb (AOB) plays a key role in the formation of memory in olfactory-mediated behaviors. We have recently shown that noradrenaline (NA) inhibits mitral cells by increasing gamma-aminobutyric acid inhibitory input onto mitral cells in the AOB, suggesting an excitatory action of NA on granule cells (GCs). Here, we show that NA (10 microM) elicits a long-lasting depolarization of GCs. This effect is mediated by activation of alpha(1)-adrenergic receptors as the depolarization is mimicked by phenylephrine (PE, 30 microM) and completely blocked by the alpha(1)-adrenergic receptor antagonist prazosin (300 nM). In addition to this depolarization, application of NA induced the appearance of a slow afterdepolarization (sADP) following a stimulus-elicited train of action potentials. Similarly, the group I metabotropic glutamate receptor (mGluR1) agonist DHPG (10-30 microM) also produced a depolarization of GCs and the appearance of a stimulus-induced sADP. The ionic and voltage dependence and sensitivity to blockers of the sADP suggest that it is mediated by the nonselective cationic conductance I(CAN). Thus the excitatory action resulting from the activation of these receptors could be mediated by a common transduction target. Surprisingly, the excitatory effect of PE on GCs was completely blocked by the mGluR1 antagonist LY367385 (100 microM). Conversely, the effect of DHPG was not antagonized by the alpha(1)-adrenergic receptor antagonist prazosin (300 nM). These results suggest that most of the noradrenergic effect on GCs in the AOB is mediated by potentiation of a basal activity of mGluR1s.

A computer-assisted odorized hole-board for testing olfactory perception in mice

The present paper describes a behavioral setup, designed and built in our laboratory, allowing the systematic and automatic recording of performances in a large number of olfactory behavioral tests. This computerized monitoring system has the capability of measuring different aspects of olfactory function in mice using different paradigms including threshold evaluation, generalization tasks, habituation/dishabituation, olfactory associative learning, short-term olfactory memory with or without a spatial component, and olfactory preferences. In this paper, we first describe the hole-board apparatus and its software and then give the experimental results obtained using this system. We demonstrate that one single, easy-to-run experimental setup is a powerful tool for the study of olfactory behavior in mice that has many advantages and broad applications.

Odor perception and olfactory bulb plasticity in adult mammals

The adult mammalian olfactory bulb (OB) is unique in that olfactory sensory neurons project directly, without prior thalamic relay, to the OB. This review discusses evidence for the direct involvement of the OB in odor perception and its modulation by olfactory experience. We first discuss recent data showing that the OB exhibits a high level of plasticity in response to olfactory experience including exposure, enrichment, and learning. We next review evidence showing that, in return, experimental manipulation of the OB neural network changes how odorants are processed and perceived. We finally review in more detail a few experiments showing a tight correlation between the modulation of OB neural processing and odor perception. We argue that the OB has evolved to be an adapting network, allowing animals to adjust olfactory computations to changing environments.

Noradrenergic neuromodulation in the olfactory bulb modulates odor habituation and spontaneous discrimination

Noradrenergic projections from the locus coeruleus (LC) project to the olfactory bulb (OB), a cortical structure implicated in odor learning and perceptual differentiation among similar odorants. The authors tested the role of OB noradrenaline (NA) in short-term olfactory memory using an animal model of LC degeneration coupled with intrabulbar infusions of NA. Specifically, the authors lesioned cortical noradrenergic fibers in mice with the noradrenergic neurotoxin N-Ethyl-N-(2-chloroethyl)-2-bromobenzylamine hydrochloride (DSP4) and measured the effects on an olfactory habituation/spontaneous discrimination task. DSP4-treated mice failed to habituate to repeated odor presentations, indicating that they could not remember odors over the 5-min intertrial interval. The authors then infused NA bilaterally into the OBs of both DSP4-treated and nonlesioned control animals at two concentrations (10(-3)M and 10(-5)M, 2 microl/side). In DSP4-treated animals, NA administration at either concentration restored normal habituation and spontaneous discrimination performance, indicating that noradrenergic neuromodulation mediates these aspects of perceptual learning and that its efficacy does not require activity-dependent local regulation of NA release. Functional OB learning mechanisms may be necessary for normal odor recognition and differentiation among physically similar odorants.

Signal transduction and gene expression in cultured accessory olfactory bulb neurons

Glutamate and norepinephrine (NE) are believed to mediate the long-lasting synaptic plasticity in the accessory olfactory bulb (AOB) that underlies pheromone recognition memory. The mechanisms by which these neurotransmitters bring about the synaptic changes are not clearly understood. In order to study signals that mediate synaptic plasticity in the AOB, we used AOB neurons in primary culture as a model system. Because induction of pheromone memory requires coincident glutamatergic and noradrenergic input to the AOB, and requires new protein synthesis, we reasoned that glutamate and NE must induce gene expression in the AOB. We used a combination of agonists that stimulate alpha1 and alpha2 adrenergic receptors in combination with N-methyl-d-aspartic acid and tested expression of the immediate-early gene (IEG) c-Fos. We found that the glutamatergic and noradrenergic stimulation caused significant induction of c-Fos mRNA and protein. Induction of c-Fos was significantly reduced in the presence of inhibitors of protein kinase C, mitogen-activated protein kinase (MAPK) and phospholipase C. These results suggest that glutamate and NE induce gene expression in the AOB through a signaling pathway mediated by protein kinase C and MAPK.

Oxytocin facilitates the induction of long-term potentiation in the accessory olfactory bulb

When female mice are mated, they form a memory to the pheromonal signal of their male partner. Several lines of evidence indicate that the neural changes underlying this memory occur in the accessory olfactory bulb (AOB) at the first stage of the vomeronasal system. The formation of this memory depends on the mating-induced release of noradrenaline in the AOB. In addition to noradrenaline, the neuropeptide oxytocin (OT) is also released within the central nervous system during mating. Because OT has been implicated in social memory and its receptors are expressed in the AOB, we hypothesized that OT might promote the strength of synaptic transmission from mitral to granule cells in the AOB. To test this hypothesis, we analyzed the lateral olfactory tract-evoked field potential that represents the granule cell response to mitral cell activation and its plasticity in parasagittal slices of the AOB. Of the 10-, 20-, 50-, and 100-Hz stimulations tested, the 100-Hz stimulation was optimal for inducing long-term potentiation (LTP). OT paired with 100-Hz stimulation that only produced short-term potentiation enhanced LTP induction in a dose-dependent manner. OT-paired LTP was blocked by both the selective OT antagonist desGly-NH(2),d(CH(2))(5)[Tyr(Me)(2),Thr(4)]-ornithine vasotocin and the N-methyl-d-aspartate (NMDA) receptor antagonist dl-2-amino-5-phosphonovaleric acid. These results indicate that OT can function as a gate to modulate the establishment of NMDA receptor-dependent LTP at the mitral-to-granule cell synapse in the AOB.

Glutamatergic transmission and plasticity between olfactory bulb mitral cells

In the olfactory bulb the sets of mitral cells that project their apical dendrite to the same glomerulus represent unique functional networks. While it is known that mitral cells release vesicular glutamate from their apical tuft it is believed that the resultant self-excitation (SE), transmitted via dendritic gap junctions, is the main form of lateral transmission within the mitral cell assembly. In this study we used simultaneous whole-cell recordings from mitral cell pairs to show that a direct form of chemical lateral excitation (LE) provides a means of mitral cell-mitral cell communication. In contrast to the ubiquitous expression and robust nature of SE, the efficacy of glutamatergic LE between mitral cells is highly variable and mediated by calcium-impermeable AMPA receptors. We also find that the strength of LE is bi-directionally modulated, in a homeostatic manner, by sniffing-like patterns of presynaptic activity. Since these changes last many minutes we suggest that such mitral cell-mitral cell interactions provide the glomerular network with a locus for olfactory plasticity and a potential mechanism for receptive field modulation.

Noradrenergic control of odor recognition in a nonassociative olfactory learning task in the mouse

The present study examined the influence of pharmacological modulations of the locus coeruleus noradrenergic system on odor recognition in the mouse. Mice exposed to a nonrewarded olfactory stimulation (training) were able to memorize this odor and to discriminate it from a new odor in a recall test performed 15 min later. At longer delays (30 or 60 min), the familiar odor was no longer retained, and both stimuli were perceived as new ones. Following a post-training injection of the alpha(2)-adrenoceptor antagonist dexefaroxan, the familiar odor was still remembered 30 min after training. In contrast, both the alpha(2)-adrenoceptor agonist UK 14304 and the noradrenergic neurotoxin DSP-4 prevented the recognition of the familiar odor 15 min after the first exposure. Noradrenaline release in the olfactory bulb, assessed by measurement of the extracellular noradrenaline metabolite normetanephrine, was increased by 62% following dexefaroxan injection, and was decreased by 38%-44% after treatment with UK 14304 and DSP-4. Performance of mice in the recall test was reduced by a post-training injection of the beta-adrenoceptor antagonist propranolol or the alpha(1)-antagonist prazosin, thus implicating a role for beta- and alpha(1)-adrenoceptors in the facilitating effects of noradrenaline on short-term olfactory recognition in this model.

Adrenergic modulation of olfactory bulb circuitry affects odor discrimination

A rodent's survival depends upon its ability to perceive odor cues necessary to guide mate selection, sexual behavior, foraging, territorial formation, and predator avoidance. Arguably, the need to discriminate odor cues in a complex olfactory environment requires a highly adaptable olfactory system. Indeed, it has been proposed that context-dependent modulation of the initial sensory relay could alter olfactory perception. Interestingly, 40% of the adrenergic innervation from the locus coeruleus, fibers that are activated by contextual cues, innervates the first relay station in the olfactory system (the main olfactory bulb). Here we utilize restricted pharmacological inhibition of olfactory bulb noradrenergic receptors in awake-behaving animals. We show that combined blockade of alpha and beta adrenergic receptors does not impair two-odor discrimination behavior per se but does impair the ability to discriminate perceptually similar odors. Thus, contextual cues conveyed by noradrenergic fibers alter processing before the second synapse in the olfactory cortex, resulting in tuning of the ability to discriminate between similar odors.

Mammalian social odours: attraction and individual recognition

Mammalian social systems rely on signals passed between individuals conveying information including sex, reproductive status, individual identity, ownership, competitive ability and health status. Many of these signals take the form of complex mixtures of molecules sensed by chemosensory systems and have important influences on a variety of behaviours that are vital for reproductive success, such as parent-offspring attachment, mate choice and territorial marking. This article aims to review the nature of these chemosensory cues and the neural pathways mediating their physiological and behavioural effects. Despite the complexities of mammalian societies, there are instances where single molecules can act as classical pheromones attracting interest and approach behaviour. Chemosignals with relatively high volatility can be used to signal at a distance and are sensed by the main olfactory system. Most mammals also possess a vomeronasal system, which is specialized to detect relatively non-volatile chemosensory cues following direct contact. Single attractant molecules are sensed by highly specific receptors using a labelled line pathway. These act alongside more complex mixtures of signals that are required to signal individual identity. There are multiple sources of such individuality chemosignals, based on the highly polymorphic genes of the major histocompatibility complex (MHC) or lipocalins such as the mouse major urinary proteins. The individual profile of volatile components that make up an individual odour signature can be sensed by the main olfactory system, as the pattern of activity across an array of broadly tuned receptor types. In addition, the vomeronasal system can respond highly selectively to non-volatile peptide ligands associated with the MHC, acting at the V2r class of vomeronasal receptor. The ability to recognize individuals or their genetic relatedness plays an important role in mammalian social behaviour. Thus robust systems for olfactory learning and recognition of chemosensory individuality have evolved, often associated with major life events, such as mating, parturition or neonatal development. These forms of learning share common features, such as increased noradrenaline evoked by somatosensory stimulation, which results in neural changes at the level of the olfactory bulb. In the main olfactory bulb, these changes are likely to refine the pattern of activity in response to the learned odour, enhancing its discrimination from those of similar odours. In the accessory olfactory bulb, memory formation is hypothesized to involve a selective inhibition, which disrupts the transmission of the learned chemosignal from the mating male. Information from the main olfactory and vomeronasal systems is integrated at the level of the corticomedial amygdala, which forms the most important pathway by which social odours mediate their behavioural and physiological effects. Recent evidence suggests that this region may also play an important role in the learning and recognition of social chemosignals.

Adrenergic enhancement of inhibitory transmission in the accessory olfactory bulb

Noradrenergic modulation of dendrodendritic synapses between the mitral and granule cells in the accessory olfactory bulb (AOB) is postulated to play a key role in the formation of memory in olfactory-mediated behaviors. Current models propose that noradrenaline (NA) increases excitation of mitral/tufted cells (M/TCs) by decreasing the release of GABA from granule cells. However, surprisingly little is known about the actions of NA at the cellular level in the AOB. Here, in recordings from AOB slices, we show that NA decreases the firing frequency of M/TCs in response to stimulation. This effect is attributable to an increase in the GABA inhibitory input to M/TCs. Application of NA (10 microM) produced an approximately 20-fold increase in the frequency of GABA-induced miniature IPSCs (mIPSCs) without changing their amplitude. A pharmacological analysis indicated that the increase in mIPSCs frequency results from activation of alpha1 adrenergic receptors. In addition to increasing the mIPSC frequency, NA also potentiated GABA inhibitory currents induced by direct stimulation of granule cells. Together, our results suggest that NA increases the release of GABA from granule cells by acting on presynaptic receptors. Thus, the role of the noradrenergic activity in the AOB may be opposite than suggested previously: we find that the overall effect of NA in the AOB is inhibition of M/TCs.

Differential effects of unilateral olfactory deprivation on noradrenergic and cholinergic systems in the main olfactory bulb of the rat

The lack of environmental olfactory stimulation produced by sensory deprivation causes significant changes in the deprived olfactory bulb. Olfactory transmission in the main olfactory bulb (MOB) is strongly modulated by centrifugal systems. The present report examines the effects of unilateral deprivation on the noradrenergic and cholinergic centrifugal systems innervating the MOB. The morphology, distribution, and density of positive axons were studied in the MOBs of control and deprived rats, using dopamine-beta-hydroxylase (DBH)-immunohistochemistry and acetylcholinesterase (AChE) histochemistry in serial sections. Catecholamine content was compared among the different groups of MOBs (control, contralateral, and ipsilateral to the deprivation) using high-performance liquid chromatography analysis. Sensory deprivation revealed that the noradrenergic system developed adaptive plastic changes after olfactory deprivation, including important modifications in its fiber density and distribution, while no differences in cholinergic innervation were observed under the same conditions. The noradrenergic system underwent an important alteration in the glomerular layer, in which some glomeruli showed a dense noradrenergic innervation that was not detected in control animals. The DBH-positive glomeruli with the highest noradrenergic fiber density were compared with AChE-stained sections and it was observed that the strongly noradrenergic-innervated glomeruli were always atypical glomeruli (characterized by their strong degree of cholinergic innervation). In addition to the morphological findings, our biochemical data revealed that olfactory deprivation caused a decrease in the content of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid in the ipsilateral MOB in comparison to the contralateral and control MOBs, together with an increase in noradrenaline levels in both the ipsilateral and contralateral MOBs. Our results show that regulation of the noradrenergic centrifugal system in the MOB depends on environmental olfactory stimulation and that it is highly reactive to sensory deprivation. By contrast, the cholinergic system is fairly stable and does not exhibit clear changes after the loss of sensory inputs.

Changes in electrophysiological activity in the accessory olfactory bulb and medial amygdala associated with mate recognition in mice

The ability of female mice to recognize their mate's pheromonal identity is critical for the maintenance of their pregnancy and is hypothesized to involve increases in the inhibitory control of mitral/tufted projection neurons in the accessory olfactory bulb. Local field potential recordings from this region of freely behaving female mice showed oscillating neural activity over a wide range of frequencies, which was affected by chemosensory input and prior experience. Mating caused lasting increases in the baseline neural activity in the accessory olfactory bulb, with large increases in the amplitude of local field potential oscillations across a range of frequencies. Exposure to the mate's urinary cues remained effective in increasing the power of these oscillations following mating, but urinary cues from an unfamiliar male were ineffective. A differential response to the familiar and unfamiliar chemosignals was also observed at the level of the amygdala following mating. Individual neurons in the medial amygdala responded more strongly to urine from an unfamiliar male than from the mating male. These findings are consistent with the selective enhancement of inhibition of the familiar pheromonal signal at the level of the accessory olfactory bulb, which is proposed to underlie recognition of the mating male.

Heterogeneous targeting of centrifugal inputs to the glomerular layer of the main olfactory bulb

The centrifugal systems innervating the olfactory bulb are important elements in the functional regulation of the olfactory pathway. In this study, the selective innervation of specific glomeruli by serotonergic, noradrenergic and cholinergic centrifugal axons was analyzed. Thus, the morphology, distribution and density of positive axons were studied in the glomerular layer of the main olfactory bulb of the rat, using serotonin-, serotonin transporter- and dopamine-beta-hydroxylase-immunohistochemistry and acetylcholinesterase histochemistry in serial sections. Serotonin-, serotonin transporter-immunostaining and acetylcholinesterase-staining revealed a higher heterogeneity in the glomerular layer of the main olfactory bulb than previously reported. In this sense, four types of glomeruli could be identified according to their serotonergic innervation. The main distinctive feature of these four types of glomeruli was their serotonergic fibre density, although they also differed in their size, morphology and relative position throughout the rostro-caudal main olfactory bulb. In this sense, some specific regions of the glomerular layer were occupied by glomeruli with a particular morphology and a characteristic serotonergic innervation pattern that was consistent from animal to animal. Regarding the cholinergic system, we offer a new subclassification of glomeruli based on the distribution of cholinergic fibres in the glomerular structure. Finally, the serotonergic and cholinergic innervation patterns were compared in the glomerular layer. Sexual differences concerning the density of serotonergic fibres were observed in the atypical glomeruli (characterized by their strong cholinergic innervation). The present report provides new data on the heterogeneity of the centrifugal innervation of the glomerular layer that constitutes the morphological substrate supporting the existence of differential modulatory levels among the entire glomerular population.

Information processing in the mammalian olfactory system

Recently, modern neuroscience has made considerable progress in understanding how the brain perceives, discriminates, and recognizes odorant molecules. This growing knowledge took over when the sense of smell was no longer considered only as a matter for poetry or the perfume industry. Over the last decades, chemical senses captured the attention of scientists who started to investigate the different stages of olfactory pathways. Distinct fields such as genetic, biochemistry, cellular biology, neurophysiology, and behavior have contributed to provide a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. So far, the combination of these approaches has been most effective at the cellular level, but there are already signs, and even greater hope, that the same is gradually happening at the systems level. This review summarizes the current ideas concerning the cellular mechanisms and organizational strategies used by the olfactory system to process olfactory information. We present findings that exemplified the high degree of olfactory plasticity, with special emphasis on the first central relay of the olfactory system. Recent observations supporting the necessity of such plasticity for adult brain functions are also discussed. Due to space constraints, this review focuses mainly on the olfactory systems of vertebrates, and primarily those of mammals.

Olfactory regulation of maternal behavior in mammals

In mammals, olfactory cues are extensively used in many aspects of maternal care to ensure the coordination of mother-infant interactions and consequently the normal development of the offspring. Outside the period of parturition and lactation, when the young are not a behavioral priority, olfactory cues play an inhibitory role on maternal responsiveness since in most mammalian species studied so far, nonpregnant females find the odor of young aversive. On the contrary at the time of parturition, a shift in the hedonic value of infantile odors occurs so that the young now become a very potent stimulus and this sensorial processing constitutes an important part of the maternal motivational system. Moreover, infants' odors provide a basis for individual recognition by their mothers and some species (ungulates) have developed highly specialized mechanisms for processing of the infant signals. Perception of the smell of the young also regulates various aspects of maternal behavior. Dodecyl propionate, a compound released by of pup's preputial glands, has been shown to influence anogenital licking behavior, a fundamental pattern of maternal behavior in rodents. While there is no functional specificity of either the main or the accessory olfactory systems in the development of maternal behavior amongst species, it appears that only the main olfactory system is implicated when individual odor discrimination of the young is required. Neural structures, such as the main olfactory bulb, undergo profound changes when exposed to offspring odors at parturition. These changes in synaptic circuitry contribute both to maternal responsiveness to these odors, to their memorization, and to effects of long-term maternal experience.

Bicuculline induces synapse formation on primary cultured accessory olfactory bulb neurons

To investigate the roles of the GABAergic inhibitory system of accessory olfactory bulb (AOB) in pheromonal memory formation, we have developed a primary culture system of AOB neurons, which had numerous excitatory and inhibitory synapses. Using this culture system of AOB neurons, we examined the correlation in rats between neuronal excitation and synaptic morphology by bicuculline-induced disinhibition of cultured AOB neurons. The exposure to bicuculline induced long-lasting oscillatory changes in the intracellular calcium level ([Ca2+]in) of cultured non-GABAergic multipolar neurons, which were identified as mitral/tufted cells (MT cells). These MT cells exhibited the appearance of dendritic filopodia structures after a 10-min treatment with bicuculline. By labelling presynaptic terminals with FM4-64, the appearance of new presynaptic terminals was clearly observed on newly formed filopodia after 120 min treatment with bicuculline. These results suggest that bicuculline-induced [Ca2+]in oscillation of MT cells induces the growth of filopodia and subsequently the formation of new presynaptic terminals. Furthermore, tetrodotoxin or the deprivation of extracellular calcium blocked bicuculline-induced synapse formation. The present results indicate that the long-lasting [Ca2+]in oscillation caused by bicuculline-induced disinhibition of cultured MT cells is significantly implicated in the mechanism underlying synapse formation on cultured AOB neurons. Our established culture system of AOB neurons will aid in clarifying the mechanism of synapse formation between AOB neurons and the molecular mechanism of pheromonal memory formation.

Bicuculline infusion into the accessory olfactory bulb facilitates the induction of maternal behavior in rats

The GABAA antagonist bicuculline, intracranially infused into the accessory olfactory bulb (AOB), facilitated the expression of maternal behavior (MB) in virgin Wistar female rats. Behavioral effects were observed 24 hours after infusion and were injection dependent. Pheromonal stimuli, generated by the pups, are thought to exert an inhibitory effect on vomeronasal nuclei involved in MB in virgin rats. The present study investigated the possibility that a decrement in AOB output, resulting from long-term compensatory synaptic changes to chronic bicuculline infusion, would facilitate the expression of MB. The implications of our findings for the mechanisms involved in the induction of MB and the maternal experience effect are discussed.

Distinct distribution of four Ca2+-dependent subtypes of protein kinase C in rat olfactory bulb; definite expression of betaII-subtype in the accessory olfactory bulb

The localization of four subtypes of Ca2+-dependent protein kinase C (PKC) in the main and accessory olfactory bulb was examined by immunocytochemistry by using specific antibodies against each PKC subtype. In the main olfactory bulb, alpha-PKC was densely localized in a large number of granule cells and in a few tufted cells, and faint immunoreactivity was seen in some periglomerular cells. betaI-PKC was intensely found in periglomerular cells and tufted cells. gamma-PKC immunoreactivity was present in the external plexiform layer, the internal plexiform layer, and the granular layer, but the immunoreactivity was found only in the neuropils. Little, if any, betaII-PKC was seen in the main olfactory bulb. On the other hand, the intense immunoreactivity for betaII-PKC was seen in periglomerular cells of the accessory olfactory bulb. The betaI-PKC and gamma-PKC were also present in periglomerular cells of the accessory olfactory bulb, while alpha-PKC was localized only in granule cells. Double staining study in the accessory olfactory bulb showed that betaII-PKC was present in the GABAergic periglomerular cells, while betaI-PKC localized to the non-GABAergic periglomerular cells; gamma-PKC was expressed in both GABAergic and non-GABAergic cells. These findings suggest that four calcium-dependent subtypes of PKC play different roles in the olfactory bulb and definite expression of betaII-PKC strongly suggested the involvement of this subtype in a specific function in the accessory olfactory bulb.

Modulation of dendrodendritic interactions and mitral cell excitability in the mouse accessory olfactory bulb by vaginocervical stimulation

When female mice are mated, they form a memory to the pheromonal signal of their male partner. The neural changes underlying this memory occur in the accessory olfactory bulb, depend upon vaginocervical stimulation at mating and involve changes at the reciprocal synapses between mitral and granule cells. However, the action of vaginocervical stimulation on the reciprocal interactions between mitral and granule cells remains to be elucidated. We have examined the effects of vaginocervical stimulation on paired-pulse depression of amygdala-evoked field potentials recorded in the external plexiform layer of the accessory olfactory bulb (AOB) and the single-unit activity of mitral cells antidromically stimulated from the amygdala in urethane-anaesthetized female mice. Artificial vaginocervical stimulation reduced paired-pulse depression (considered to be due to feedback inhibition of the mitral cell dendrites from the granule cells via reciprocal dendrodendritic synapses) recorded in the AOB external plexiform layer. As would be expected from this result, vaginocervical stimulation also enhanced the spontaneous activity of a proportion of the mitral cells tested. These results suggest that vaginocervical stimulation reduces dendrodendritic feedback inhibition to mitral cells and enhances their activity.

Activation of GABA(A) receptors in the accessory olfactory bulb does not prevent the formation of an olfactory memory in mice

When female mice are mated, they form a memory to the pheromonal signal of their male partner. The neural mechanisms underlying this memory involve changes at the reciprocal dendrodendritic synapses between glutamatergic mitral cells and gamma-aminobutyric acid (GABA)-ergic granule cells in the accessory olfactory bulb (AOB). Blockade of GABA(A) receptors in the AOB leads to the formation of an olfactory memory. In an attempt to disrupt memory formation at mating, we used local infusions of the GABA(A) receptor agonist muscimol into the AOB during the critical period for memory formation. Muscimol across a wide range of doses (1-1000 pmol) did not prevent memory formation. The resistance of this memory to GABA(A) receptor activation may reflect the complexity of synaptic microcircuits in the AOB.

Noradrenergic modulation of calcium currents and synaptic transmission in the olfactory bulb of Xenopus laevis tadpoles

Norepinephrine (NE) has various modulatory roles in both the peripheral and the central nervous systems. Here we investigate the function of the locus coeruleus efferent fibres in the olfactory bulb of Xenopus laevis tadpoles. In order to distinguish unambiguously between mitral cells and granule cells of the main olfactory bulb and the accessory olfactory bulb, we used a slice preparation. The two neuron types were distinguished on the basis of their location in the slice, their typical branching pattern and by electrophysiological criteria. At NE concentrations lower than 5 microM there was only one effect of NE upon voltage-gated conductances; NE blocked a high-voltage-activated Ca(2+)-current in mitral cells of both the main and the accessory olfactory bulbs. No such effect was observed in granule cells. The effect of NE upon mitral cell Ca(2+)-currents was mimicked by the alpha(2)-receptor agonists clonidine and alpha-methyl-NE. As a second effect, NE or clonidine blocked spontaneous synaptic activity in granule cells of both the main and the accessory olfactory bulbs. NE or clonidine also blocked the spontaneous synaptic activity in mitral cells of either olfactory bulb. The amplitude of glutamate-induced currents in granule cells was modulated neither by clonidine nor by alpha-methyl-NE. Taken together, the main effect of the noradrenergic, presynaptic, alpha(2)-receptor-mediated block of Ca(2)+-currents in mitral cells appeared to be a wide-spread disinhibition of mitral cells in the accessory olfactory bulb as well as in the main olfactory bulb.

Oxytocin induces preservation of social recognition in male rats by activating alpha-adrenoceptors of the olfactory bulb

In this report, a series of four experiments was performed to evaluate the relationship between the olfactory bulb norepinephrine system and intra-olfactory bulb infusion of oxytocin in the preservation of social memory responses. The present data indicate that oxytocin exerts this preservation of social recognition through a specific, receptor-mediated mechanism within the olfactory bulb (experiment 1). The involvement of the olfactory bulb norepinephrine system is revealed by the demonstration that retrodialysis of oxytocin into the olfactory bulb increases norepinephrine release (experiment 4). Our data suggest that the increased output of olfactory bulb norepinephrine resulting from oxytocin appears to activate alpha-adrenoceptors to produce this preservation in recognition because infusions of clonidine into the olfactory bulb preserve recognition responses in a manner similar to that observed with oxytocin (experiment 2). In addition, a co-infusion of oxytocin with phentolamine abolishes recognition responses (experiment 3). Accordingly, this model affords the opportunity to study neuropeptide-catecholamine interactions, link these interactions with a specific behavioural outcome and identify a novel function/site of action for oxytocin in the male.

Norepinephrine increases rat mitral cell excitatory responses to weak olfactory nerve input via alpha-1 receptors in vitro

A rat olfactory bulb in vitro slice preparation was used to investigate the actions of norepinephrine on spontaneous and afferent (olfactory nerve) evoked activity of mitral cells. Single olfactory nerve shocks elicited a characteristic mitral cell response consisting of distinct, early and late spiking components separated by a brief inhibitory epoch. Bath-applied norepinephrine (1 microM) increased the early spiking component elicited by perithreshold (79% increase, P<0.02), but not by suprathreshold (3% decrease, P>0.05), intensity olfactory nerve shocks. The facilitatory effect of norepinephrine was due to a reduction in the incidence of response failures to perithreshold intensity shocks. Norepinephrine also decreased the inhibitory epoch separating the early and late spiking components by 44% (P<0.05). By contrast, norepinephrine had no consistent effect on the spontaneous discharge rate of the mitral cells. The effects of norepinephrine were mimicked by the al receptor agonist phenylephrine (1 microM, P<0.001). Both norepinephrine and phenylephrine modulation of mitral cell responses were blocked by the al adrenergic antagonist WB-4101 (1 microM). These findings are consistent with observations that the main olfactory bulb exhibits the highest density of alpha1 receptors in the brain. The alpha2 receptor agonist clonidine (100 nM) and the beta receptor agonist isoproterenol (1 microM) had inconsistent effects on mitral cell spontaneous and olfactory nerve-evoked activity. These results indicate that norepinephrine increases mitral cell excitatory responses to weak but not strong olfactory nerve inputs in vitro via activation of al receptors. This is consistent with recent findings in vivo that synaptically released norepinephrine preferentially increases mitral cell excitatory responses to weak olfactory nerve inputs. Taken together, these results suggest that the release of norepinephrine in the olfactory bulb may increase the sensitivity of mitral cells to weak odors. Olfactory cues evoke norepinephrine release in the main olfactory bulb, and norepinephrine plays important roles in early olfactory learning and reproductive/maternal behaviors. By increasing mitral cell responses to olfactory nerve input, norepinephrine may play a critical role in modulating olfactory function, including formation and/or recall of specific olfactory memories.

Changes in neurotransmitter release in the main olfactory bulb following an olfactory conditioning procedure in mice

Olfactory learning is associated with substantial neural changes at the level of the accessory and main olfactory bulb, during both pheromonal learning in mated mice and lamb odour recognition in post partum sheep. These forms of learning occur during "sensitive periods" and an important question is whether similar neural changes occur in the olfactory bulb at other times. We used a classical conditioning procedure to establish an olfactory discrimination in adult mice and then measured changes in neurotransmitter levels in the main olfactory bulb in response to the presentation of the conditioned odours. Presentation of the conditioned, but not the non-conditioned, odour resulted in significant increases in the levels of certain transmitters, including glutamate from the mitral/tufted cells, GABA from the granule and periglomerular cells and noradrenaline from the centrifugal projection from the locus coeruleus. Overall, there was a decrease in the ratio of excitatory to inhibitory neurotransmitters in the olfactory bulb in response to the conditioned, but not the non-conditioned odour. Moreover, the magnitude of the decrease in this ratio was correlated with the level of behavioural response to the conditioned odour. These findings support the hypothesis that changes in the gain of the reciprocal synapses between mitral/tufted neurons and their inhibitory interneurons are a general feature of olfactory learning.

Impaired maternal behavior in mice lacking norepinephrine and epinephrine

The roles of norepinephrine (NE) and epinephrine in behavior were investigated by targeted disruption of the dopamine beta-hydroxylase (Dbh) gene, thereby eliminating these compounds in vivo. Most heterozygous pups born to Dbh-/- females died within several days of birth and were often found scattered within the bedding. Potential causes including deficits in olfaction and lactation were not apparent. A deficit in maternal behavior was confirmed by the lack of pup retrieval exhibited by Dbh-/- virgin females. Restoration of NE shortly before but not after birth induced females that previously abandoned their litters to act maternally. Our results suggest that NE is responsible for long-lasting changes that promote maternal behavior during both development and parturition in mice.

Amygdala but not hippocampal lesions impair olfactory memory for mate in prairie voles (Microtus ochrogaster)

Exposure to an unfamiliar male conspecific results in pregnancy interruption (i.e., the Bruce effect) in rodents. Unlike most laboratory rodents, female prairie voles (Microtus ochrogaster) are induced into estrus by chemosensory stimuli contained in the urine of male conspecifics while grooming the anogenital (A-G) region of unfamiliar males. Female prairie voles maintain a brief "memory" for the stud male for 8-10 days after mating. Subsequent exposure to the same mate within this 8- to 10-day window does not elicit A-G investigation by the female and pregnancy block does not result. However, exposure to the original male after 10 days evokes A-G investigation and pregnancy block. To determine the neuroanatomic area(s) involved in olfactory memory for mate, female voles received bilateral electrolytic lesions of either the amygdala or hippocampus. Females were subsequently exposed to males for 48 h, separated for 3 days, then reintroduced to their original mate for 24 h. Although pregnancy rate did not differ among the experimental groups, a greater proportion of amygdala-lesioned females displayed pregnancy block when reexposed to their previous mates compared with hippocampal- or sham-lesioned voles. Amygdala-lesioned voles also displayed a greater number of A-G investigations compared with the other groups. Performance on olfactory tests was not impaired. Taken together, these results suggest that the amygdala plays an important role in olfactory memory for mate in prairie voles.

Analgesia induced by repeated exposure to low dose x-rays in mice, and involvement of the accessory olfactory system in modulation of the radiation effects

The effects of low-dose x-rays on mouse nociceptive behavior were examined using a formalin injected test that rated the amount of time the animals spent licking the injected hind paw. Male ICR White Swiss mice showed a marked suppression of licking behavior after repeated low-dose x-irradiation (5 cGy/day, 6 consecutive days). The most profound effect was observed on the day 30 after irradiation. The decline of licking behavior, however, was not observed at all following olfactory bulbectomy or vomeronasal tract cut. The analgesic effects could be observed in writhing animals administered acetic acid intraperitoneally. Moreover, analgesia was totally blocked by the administration of N-nitro-L-arginine, a nitric oxide synthase inhibitor, to accessory olfactory bulbs prior to the exposure. The present results indicate that the olfactory system plays an important role in modulation of radiation-induced analgesia, and a possible involvement of nitric oxide in the formation of recognition memory subjected to repeated x-rays. Relatively higher doses (5 cGy x 9 days, 5 cGy x 12 days), however, did not induce such effects, namely, the decline of nociceptive response was limited to the animals irradiated with the smaller dose.

Neural mechanisms of mammalian olfactory learning

In this review, we compare the neural basis of olfactory learning in three specialized contexts that occur during sensitive periods of enhanced neural plasticity. Although they involve very different behavioural contexts, they share several common features, including a dependence on noradrenergic transmission in the olfactory bulb. The most extensively characterized of these examples is the learning of pheromonal information by female mice during mating. While this form of learning is unusual in that the neural changes underlying the memory occur in the accessory olfactory bulb at the first stage of sensory processing, it involves similar neural mechanisms to other forms of learning and synaptic plasticity. The learning of newborn lamb odours after parturition in sheep, and the olfactory conditioning in neonatal animals such as rats and rabbits, are mediated by the main olfactory system. Although the neural mechanisms for learning in the main olfactory system are more distributed, they also involve changes occurring in the olfactory bulb. In each case, odour learning induces substantial structural and functional changes, including increases in inhibitory neurotransmission. In the main olfactory bulb, this probably represents a sharpening of the odour-induced pattern of activity, due to increases in lateral inhibition. In contrast, the different morphology of mitral cells in the accessory olfactory bulb results in increased self-inhibition, disrupting the transmission of pheromonal information. Although these examples occur in highly specialized contexts, comparisons among them can enhance our understanding of the general neural mechanisms of olfactory learning.

Activation of locus coeruleus enhances the responses of olfactory bulb mitral cells to weak olfactory nerve input

The main olfactory bulb (MOB) receives a dense projection from the pontine nucleus locus coeruleus (LC), the largest collection of norepinephrine (NE)-containing cells in the brain. LC is the sole source of NE innervation of MOB. Previous studies of the actions of exogenously applied NE on mitral cells, the principal output neurons of MOB, are contradictory. The effect of synaptically released NE on mitral cell activity is not known, nor is the influence of NE on responses of mitral cells to olfactory nerve inputs. The goal of the present study was to assess the influence of LC activation on spontaneous and olfactory nerve-evoked activity of mitral cells. In methoxyflurane-anesthetized rats, intracoerulear microinfusions of acetyicholine (ACh) (200 mM; 90-120 nl) evoked a four- to fivefold increase in LC neuronal discharge, and a transient EEG desynchronization and decrease in mitral cell discharge. LC activation increased excitatory responses of mitral cells evoked by weak (i.e., perithreshold) nasal epithelium shocks (1.0 Hz) in 17/18 cells (mean Increase = 67%). The discharge rate of mitral cells at the time that epithelium-evoked responses were increased did not differ significantly from pre-LC activation baseline values. Thus, changes in mitral baseline activity do not account for the increased response to epithelium stimulation. These findings suggest that increased activity in LC-NE projections to MOB may enhance detection of relatively weak odors.