Electrophysiological evidence for glutamate as a vomeronasal receptor cell neurotransmitter

The Neurotransmitter Receptor Architecture of the Mouse Olfactory System

The uptake, transmission and processing of sensory olfactory information is modulated by inhibitory and excitatory receptors in the olfactory system. Previous studies have focused on the function of individual receptors in distinct brain areas, but the receptor architecture of the whole system remains unclear. Here, we analyzed the receptor profiles of the whole olfactory system of adult male mice. We examined the distribution patterns of glutamatergic (AMPA, kainate, mGlu_2/3, and NMDA), GABAergic (GABA_A, GABA_A(BZ), and GABA_B), dopaminergic (D_1/5) and noradrenergic (α₁ and α₂) neurotransmitter receptors by quantitative in vitro receptor autoradiography combined with an analysis of the cyto- and myelo-architecture. We observed that each subarea of the olfactory system is characterized by individual densities of distinct neurotransmitter receptor types, leading to a region- and layer-specific receptor profile. Thereby, the investigated receptors in the respective areas and strata showed a heterogeneous expression. Generally, we detected high densities of mGlu_2/3Rs, GABA_A(BZ)Rs and GABA_BRs. Noradrenergic receptors revealed a highly heterogenic distribution, while the dopaminergic receptor D_1/5 displayed low concentrations, except in the olfactory tubercle and the dorsal endopiriform nucleus. The similarities and dissimilarities of the area-specific multireceptor profiles were analyzed by a hierarchical cluster analysis. A three-cluster solution was found that divided the areas into the (1) olfactory relay stations (main and accessory olfactory bulb), (2) the olfactory cortex (anterior olfactory cortex, dorsal peduncular cortex, taenia tecta, piriform cortex, endopiriform nucleus, entorhinal cortex, orbitofrontal cortex) and the (3) olfactory tubercle, constituting its own cluster. The multimodal receptor-architectonic analysis of each component of the olfactory system provides new insights into its neurochemical organization and future possibilities for pharmaceutic targeting.

Signal Detection and Coding in the Accessory Olfactory System

In many mammalian species, the accessory olfactory system plays a central role in guiding behavioral and physiological responses to social and reproductive interactions. Because of its relatively compact structure and its direct access to amygdalar and hypothalamic nuclei, the accessory olfactory pathway provides an ideal system to study sensory control of complex mammalian behavior. During the last several years, many studies employing molecular, behavioral, and physiological approaches have significantly expanded and enhanced our understanding of this system. The purpose of the current review is to integrate older and newer studies to present an updated and comprehensive picture of vomeronasal signaling and coding with an emphasis on early accessory olfactory system processing stages. These include vomeronasal sensory neurons in the vomeronasal organ, and the circuitry of the accessory olfactory bulb. Because the overwhelming majority of studies on accessory olfactory system function employ rodents, this review is largely focused on this phylogenetic order, and on mice in particular. Taken together, the emerging view from both older literature and more recent studies is that the molecular, cellular, and circuit properties of chemosensory signaling along the accessory olfactory pathway are in many ways unique. Yet, it has also become evident that, like the main olfactory system, the accessory olfactory system also has the capacity for adaptive learning, experience, and state-dependent plasticity. In addition to describing what is currently known about accessory olfactory system function and physiology, we highlight what we believe are important gaps in our knowledge, which thus define exciting directions for future investigation.

The rodent accessory olfactory system

The accessory olfactory system contributes to the perception of chemical stimuli in the environment. This review summarizes the structure of the accessory olfactory system, the stimuli that activate it, and the responses elicited in the receptor cells and in the brain. The accessory olfactory system consists of a sensory organ, the vomeronasal organ, and its central projection areas: the accessory olfactory bulb, which is connected to the amygdala and hypothalamus, and also to the cortex. In the vomeronasal organ, several receptors-in contrast to the main olfactory receptors-are sensitive to volatile or nonvolatile molecules. In a similar manner to the main olfactory epithelium, the vomeronasal organ is sensitive to common odorants and pheromones. Each accessory olfactory bulb receives input from the ipsilateral vomeronasal organ, but its activity is modulated by centrifugal projections arising from other brain areas. The processing of vomeronasal stimuli in the amygdala involves contributions from the main olfactory system, and results in long-lasting responses that may be related to the activation of the hypothalamic-hypophyseal axis over a prolonged timeframe. Different brain areas receive inputs from both the main and the accessory olfactory systems, possibly merging the stimulation of the two sensory organs to originate a more complex and integrated chemosensory perception.

An ex vivo preparation of the intact mouse vomeronasal organ and accessory olfactory bulb

The accessory olfactory system (AOS) in mammals detects and processes information from liquid-phase environmental odorants, including pheromones. The AOS carries out tasks such as individual recognition, learning, and decision-making with relatively few stages of neural processing; it thus represents an attractive system for investigating the neural circuits that carry out these functions. Progress in understanding the AOS has long been impeded by its relative inaccessibility to standard physiological approaches. In this report, we detail a novel dissection and tissue perfusion strategy that improves access to the accessory olfactory bulb (AOB) while maintaining afferent connections from sensory neurons in the vomeronasal organ (VNO). Mitral cells demonstrated spontaneous and evoked firing patterns consistent with recent in vivo reports. We assayed cell degradation in the AOB tissue using Fluoro-Jade C and found that the VNO and AOB glomerular, external plexiform, and mitral cell layers showed minimal signs of degeneration for up to 6h. Whereas histology indicated some degeneration in the deep inhibitory granule cell layer over time, electrophysiological assays demonstrated intact inhibitory function on mitral cells. Pharmacological blockade of GABA(A) receptors with 3microM SR95531 (gabazine) resulted in increased evoked mitral cell activity. Furthermore, mitral cells displayed suppression of responses to preferred urine stimuli when preferred and non-preferred stimuli were mixed, an effect thought to involve functional laterally connected inhibition. These results demonstrate the utility of whole mount ex vivo preparations for studying sensory processing in the AOS, and suggest that similar strategies may improve experimental access to other difficult-to-study neural circuits.

Embryonic and postnatal differentiation of olfactory epithelium and vomeronasal organ in the Syrian hamster

The details of the embryonic and postnatal differentiation of the olfactory epithelium (OE) and vomeronasal organ (VNO) were examined by light and electron microscopy in the Syrian hamster. At 10 days of gestation, the nasal placode is invaginated to form the olfactory pit on either side at the rostral end of the embryo. Abundant mitotic figures are observed near the free surface of the epithelium lining the olfactory pit. At 11 days of gestation, the mass of the epithelium lining a recess is separated from the medial wall of the olfactory pit to form the VNO. At 13 days of gestation, mitotic figures become observable in the basal layer of the vomeronasal sensory epithelium (VSE) in addition to the superficial to middle layers, while in the OE mitotic figures are observed mainly in the middle to basal layer. At 1 day after birth, the OE is almost complete in differentiation. On the other hand, the VSE differentiate slowly to retain some immature properties even at 10 days after birth. These findings suggest that the olfactory function seems to be solely ascribed to the OE for a while after birth. The significance of mitotic figures are discussed in the course of development with special reference to the origin of the nasal placode from the central nervous system.

Immunohistochemical Localization of Glutamate in the Gerbil Main Olfactory Bulb Using an Antiserum Directed against Glutamate

Information on the localization and the roles of glutamate in the nervous system is becoming valuable because the axon terminals of the olfactory sensory neurons and the synapses of the mitral and tufted output cells appear to be glutamatergic. In this study, we have analysed the distribution of glutamate immunoreactivity in the main olfactory bulb (MOB) of the Mongolian gerbil using an antiserum directed against glutamate. Glutamate immunoreactivity in the MOB was present in the olfactory nerve layer (Onl), glomerular layer (GL), external plexiform layer (EPL) and mitral cell layer (ML), but not in the granule cell layer (GCL). Glutamate immunoreactivity detected in the Onl was thought to be terminal ramifications of glomeruli. Some neurons in the periglomerular region showed glutamate immunoreactivity. In the EPL, glutamate immunoreactivity was found in some neuronal somata (tufted cells) and processes. In addition, mitral cells in the ML were labelled by the glutamate antibody. The pattern of glutamate immunoreactivity in the mitral cells was similar to that in the tufted cells. In brief, glutamate in the gerbil MOB is the neurotransmitter used by primary afferents and output neurons.

Effects of N-methyl-D-aspartate glutamate receptor antagonists on oscillatory signal propagation in the guinea-pig accessory olfactory bulb slice: characterization by optical, field potential and patch clamp recordings

To characterize the role of N-methyl-d-aspartate glutamate receptors in oscillations induced by a single electrical stimulation of the vomeronasal nerve layer, optical, field potential and patch clamp recordings were carried out in guinea-pig accessory olfactory bulb slice preparations. Bath application of the N-methyl-D-aspartate receptor antagonists, 2-amino-5-phosphonovaleric acid or MK-801, produced an increase in frequency of oscillating waves (oscillation) in external plexiform and mitral cell layers. The removal of Mg2+ from perfusate abolished oscillations, while subsequent application of 2-amino-5-phosphonovaleric acid or MK-801 restored oscillations. Vomeronasal nerve layer-evoked postsynaptic currents were analyzed by whole-cell clamp recordings from mitral and granule cells. A long-lasting excitatory postsynaptic current and periodic inhibitory postsynaptic currents, which were superimposed on the long excitatory postsynaptic current, were observed in mitral cells. The frequency of the periodic inhibitory postsynaptic currents correlated with the frequency of oscillations observed in the optical and field potential recordings. Furthermore, periodic inhibitory postsynaptic currents were blocked by puff application of bicuculline to the external plexiform layer/mitral cell layer, where mitral cells make dendrodendritic synapses with granule cells. In addition, puff application of the non-N-methyl-D-aspartate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, to the external plexiform layer/mitral cell layer suppressed an early phase of periodic inhibitory postsynaptic currents (membrane oscillation), whereas 2-amino-5-phosphonovaleric acid suppressed the late phase of periodic inhibitory postsynaptic currents. These data indicate that periodic excitatory postsynaptic currents of granule cells induce relevantly periodic inhibitory postsynaptic currents in mitral cells via dendrodendritic synapses and suggest that feedback inhibition regulates generation of oscillation via activation of non-N-methyl-d-aspartate glutamate receptors and gradual attenuation of oscillation via activation of N-methyl-D-aspartate receptors on granule cells.

A subset of periglomerular neurons in the rat accessory olfactory bulb may be excited by GABA through a Na(+)-dependent mechanism

The periglomerular (PG) cells of the accessory olfactory bulb (AOB) are GABAergic interneurons which receive input from the vomeronasal sensory neurons and form dendrodendritic synapses with each other and with mitral cells. Their electrophysiological properties have not been investigated. We have developed a novel method of isolating PG cells from the AOB, and used the whole-cell patch and gramicidin-perforated patch clamp techniques to measure their basic electrophysiological characteristics and their response to GABA. PG cells were found to be excitable neurons with voltage-gated Na(+) and K(+) currents, though it was very difficult to get PG cells to fire an action potential. The voltage-gated Na(+) currents of PG cells activate at more positive potentials than those of typical CNS neurons. PG cells respond to GABA with currents in which GABA(A) receptors play a significant role. A subset ( approximately 40%) of PG cells respond to GABA with currents which have unusually high reversal potentials, indicating that GABA may be excitatory to these neurons. This phenomenon cannot be explained entirely by elevation of intracellular chloride concentrations, and is dependent on the presence of extracellular sodium.

Effects of GABAergic agonists and antagonists on oscillatory signal propagation in the guinea-pig accessory olfactory bulb slice revealed by optical recording

To investigate the action of GABAergic agents on oscillatory signal propagation induced by electrical stimulation of the vomeronasal nerve layer, optical and electrophysiological recordings were carried out in slice preparations of the guinea-pig accessory olfactory bulb. In response to electrical stimuli, characteristic optical signals appeared in each layer: in the vomeronasal nerve layer, a transient presynaptic response; in the glomerular layer, pre- and postsynaptic responses; in the external plexiform, mitral cell and granule cell layers, a damped oscillatory response. Application of the GABAergic agonists, that is, GABA, muscimol (a GABAA receptor agonist) and baclofen (a GABAB receptor agonist), suggested that the GABAB action existed mainly in the glomeruli, whereas the GABAA action was present in both the glomeruli and the external plexiform layer. Bicuculline (a GABAA receptor antagonist) produced long-lasting but nonoscillating excitation in the external plexiform and mitral cell layers, indicating that the GABAA action contributes to the formation of oscillatory responses. When double-pulse stimulation was applied to the vomeronasal nerve layer, the test responses in the glomerular layer and external plexiform and mitral cell layers were depressed, but those in the vomeronasal nerve layer were not. Application of 2-hydroxysaclofen (a GABAB receptor antagonist) mostly blocked paired-pulse depression occurring in the glomerular layer and restored the reduced transmission to mitral cells, but had only a small effect on the depressed oscillatory response in the external plexiform and mitral cell layers. These observations suggest that GABAB action in the glomerular layer might, at least, regulate information flow from vomeronasal afferents to apical dendrites of mitral cells, like a gate inhibition. However, actions other than GABAB could also be involved in the depression of the oscillation in the external plexiform and mitral cell layers.

Immunocytochemical localization of glutamate and gamma-aminobutyric acid in the accessory olfactory bulb of the rat

The synaptic organization of the accessory olfactory bulb (AOB) was studied in the rat with antibodies against the excitatory neurotransmitter glutamate (Glu) and the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). To a large extent, the immunoreactivity patterns produced by the two antibodies were complementary. Glu-like immunoreactivity (-LI) was observed in the glomerular neuropil, in the mitral cells, and in large neurons located in the periglomerular region. Immunogold electron microscopy revealed particularly high levels of Glu-LI in the axon terminals of vomeronasal neurons. GABA-LI was present in granule and periglomerular cells and in their processes. The dendritic spines of granule cells, which were presynaptic to mitral cells, were strongly labelled by the antiserum against GABA. Labelling of serial semithin sections showed that the GABA-positive and Glu-positive neurons of the periglomerular region are generally distinct, and colocalization of Glu and GABA occurred only in a few cells. These results are consistent with electrophysiological studies indicating that the synaptic organization of the AOB is similar to that of the main olfactory bulb. In both systems, Glu is the neurotransmitter used by primary afferents and output neurons, whereas GABA is involved in the circuits underlying lateral and feed-back inhibition.

Synaptic organization and neurotransmitters in the rat accessory olfactory bulb

The accessory olfactory bulb (AOB) is the first relay station in the vomeronasal system and may play a critical role in processing pheromone signals. The AOB shows similar but less distinct lamination compared with the main olfactory bulb (MOB). In this study, synaptic organization of the AOB was analyzed in slice preparations from adult rats by using both field potential and patch-clamp recordings. Stimulation of the vomeronasal nerve (VN) evoked field potentials that showed characteristic patterns in different layers of the AOB. Current source density (CSD) analysis of the field potentials revealed spatiotemporally separated loci of inward current (sinks) that represented sequential activation of different neuronal components: VN activity (period I), synaptic excitation of mitral cell apical dendrites (period II), and activation of granule cells by mitral cell basal dendrites (period III). Stimulation of the lateral olfactory tract also evoked field potentials in the AOB, which indicated antidromic activation of the mitral cells (period I and II) followed by activation of granule cells (period III). Whole cell patch recordings from mitral and granule cells of the AOB supported that mitral cells are excited by VN terminals and subsequently activate granule cells through dendrodendritic synapses. Both CSD analysis and patch recordings provided evidence that glutamate is the neurotransmitter at the vomeronasal receptor neuron; mitral cell synapses and both NMDA and non-NMDA receptors are involved. We also demonstrated electrophysiologically that reciprocal interaction between mitral and granule cells in the AOB is through the dendrodendritic reciprocal synapses. The neurotransmitter at the mitral-to-granule synapses is glutamate and at the granule-to-mitral synapse is gamma-aminobutyric acid. The synaptic interactions among receptor cell terminals, mitral cells, and granule cells in the AOB are therefore similar to those in the MOB, suggesting that processing of chemosensory information in the AOB shares similarities with that in the MOB.

Excitatory amino acid neurotransmission in the primary gustatory nucleus of the goldfish Carassius auratus

The vagal lobe in goldfish is a laminated structure in the midmedulla responsible for processing vagal gustatory input from the oropharynx. The anatomical arrangement of the vagal lobe is conducive to an in vitro slice preparation for investigating the physiology and pharmacology of primary gustatory fibers. Postsynaptic population responses (N2 and N3) were evoked from sensory layers of the vagal lobe following stimulation of the incoming vagal fibers. Application of 100 microM kynurenic acid, a broad spectrum glutamate receptor antagonist, abolished or significantly decreased the evoked responses. These results indicate that excitatory amino acids are the neurotransmitter at the first relay in the taste pathway in the central nervous system.

Prenatal differentiation of mouse vomeronasal neurones

The vomeronasal organ (VNO) subserves basic chemosensory functions in rodents, mainly related to sexual behaviour. In order to understand early stages of the VNO structural maturation, we have undertaken an immunocytochemical analysis of the VNO of fetal mice. Our results demonstrate that Olfactory Marker Protein (OMP), a marker of differentiated chemosensory cells, is already expressed in vomeronasal neurones and their fibres projecting to the accessory olfactory bulb during the last week of gestation. However, in contrast to the adult, where its expression is restricted to the medial sensory neuronal component of the VNO, during fetal development OMP is also present in cells located in the lateral non-sensory epithelial component. Some other markers of nasal chemosensory neurones, such as GAP-43/B-50, Protein Gene Product 9.5 (PGP 9.5) and carnosine are also transiently expressed in this ectopic site. These results indicate that (i) significant morphological and biochemical maturation of the VNO is achieved before birth; (ii) transient cell populations, sharing the biochemical profile of the vomeronasal chemosensory receptors, occur in ectopic areas during fetal development.

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.