Dopaminergic neuromodulation of synaptic transmission between mitral and granule cells in the teleost olfactory bulb

A growing body of evidence suggests that teleosts are important models for the study of neural processing of olfactory information, and the functional role of dopamine (DA), which is a potent neuromodulator endogenous to the mammalian olfactory bulb, has been one of the strongest focuses in this field. However, the cellular mechanisms of dopaminergic neuromodulation in olfactory bulbar neural circuits have not been fully understood. We investigated such mechanisms by using the goldfish, which offers several advantages for analyzing olfactory information processing by electrophysiological methods. First, we found in the olfactory bulb that numerous cell bodies of the dopaminergic neurons are mainly distributed in the mitral cell layer and extend fine processes to the glomerular layer. Next, we made in vitro field potential recordings and showed that synaptic transmissions from mitral to granule cells were suppressed by DA application. DA also increased the paired-pulse ratio, suggesting that the suppression of synaptic transmission is caused by a decrease in presynaptic glutamate release from the mitral cells. Furthermore, DA significantly suppressed the oscillatory activity of the olfactory bulb in response to olfactory stimuli. Although DA suppresses the synaptic inputs from the olfactory nerve to the olfactory bulbar neurons in mammals, this phenomenon was not observed in the goldfish. These findings indicate that suppression of the mitral to granule cell synaptic transmission in the reciprocal synapses plays an important role in the negative regulation of olfactory responsiveness in the goldfish olfactory bulb.     

An analysis of the effects of excitatory amino acid receptor antagonists on evoked field potentials in the olfactory bulb

The effects of excitatory amino acid antagonists on extracellular field potentials in the olfactory bulb produced by lateral olfactory tract stimulation were analysed in vivo. The compounds tested D-2-amino-5-phosphonovalerate, L-(+)2-amino-4-phosphonobutyrate, gamma-D-glutamylglycine, L-glutamic acid diethylester and cis-2,3-piperidine dicarboxylic acid, were administered by brain dialysis. Of the compounds tested, only cis-2,3 piperidine-dicarboxylic acid and gamma-D-glutamylglycine were able to suppress the synaptic excitation of granule cells. This pharmacological profile suggests the involvement of non-N-methyl-D-aspartate receptors. However, the suppression was accompanied by a reduction in the amplitude of the presynaptic volley. A second finding was that D-2-amino-5-phosphono-valerate and gamma-D-glutamyl glycine attenuated granule cell mediated inhibition of mitral cells, suggesting the involvement of N-methyl-D-aspartate-sensitive receptors. The possibility that mitral cells and that either centrifugal fibres, or an intrinsic olfactory bulb feedback loop might use an excitatory amino acid as its neurotransmitter is therefore discussed.     

Functional role of NMDA autoreceptors in olfactory mitral cells

The output of the olfactory bulb is governed by the interaction of synaptic potentials with the intrinsic conductances of mitral cells. While mitral cells often are considered as simple relay neurons, conveying activity in olfactory receptor cells to the piriform cortex, there is strong physiological and behavioral evidence that local synaptic interactions within the olfactory bulb modulate mitral cell discharges and facilitate odorant discrimination. Understanding the circuitry of the olfactory bulb is complicated by the fact that most dendrites in this region are both pre- and postsynaptic. Feedback inhibition is mediated through reciprocal dendrodendritic synapses between the secondary dendrites of mitral cells and GABAergic granule cells. Here we show that glutamate released from mitral cell dendrites also activates local N-methyl-D-aspartate (NMDA) autoreceptors, generating an inward tail current following depolarizing voltage steps. Autoreceptor-mediated self-excitation is calcium dependent, can be evoked by single action potentials in the presence of magnesium, and is graded with the number of spikes in a train. We find that dendrodendritic inhibition also is evoked by single action potentials but saturates rapidly during repetitive discharges. Self-excitation also underlies the prolonged afterdischarges apparent in mitral cells following potassium channel blockade. Both afterdischarges and autoreceptor-mediated tail currents persist in TTX, suggesting that they are produced by local rather than polysynaptic actions of glutamate. Blockade of NMDA autoreceptors with 2-amino-5-phosphonovaleric acid (APV) reduces the firing frequency within action potential cluster. The rapid kinetics of self-excitation suggests a functional role of NMDA autoreceptors in prolonging periods of phasic firing in mitral cells.     

Both electrical and chemical synapses mediate fast network oscillations in the olfactory bulb

Odor perception depends on a constellation of molecular, cellular, and network interactions in olfactory brain areas. Recently, there has been better understanding of the cellular and molecular mechanisms underlying the odor responses of neurons in the olfactory epithelium, the first-order olfactory area. In higher order sensory areas, synchronized activity in networks of neurons is known to be a prominent feature of odor processing. The perception and discrimination of odorants is associated with fast (20-70 Hz) electroencephalographic oscillations. The cellular mechanisms underlying these fast network oscillations have not been defined. In this study, we show that synchronous fast oscillations can be evoked by brief electrical stimulation in the rat olfactory bulb in vitro, partially mimicking the natural response of this brain region to sensory input. Stimulation induces periodic inhibitory synaptic potentials in mitral cells and prolonged spiking in GABAergic granule cells. Repeated stimulation leads to the persistent enhancement in both granule cell activity and mitral cell inhibition. Prominent oscillations in field recordings indicate that stimulation induces high-frequency activity throughout networks of olfactory bulb neurons. Network synchronization results from chemical and electrical synaptic interactions since both glutamate-receptor antagonists and gap junction inhibitors block oscillatory intracellular and field responses. Our results demonstrate that the olfactory bulb can generate fast oscillations autonomously through the persistent activation of networks of inhibitory interneurons. These local circuit interactions may be critically involved in odor processing in vivo.     

Effects of dopamine and fluphenazine on field potential amplitude in the salamander olfactory bulb

The effects of dopamine (DA) and fluphenazine (FLU), a phenothiazine DA receptor antagonist, were examined in the salamander olfactory bulb. Field potentials were recorded in the granule cell layer of superfused hemibrain preparations, and the amplitude of the large positive peak was compared following stimulation of the olfactory nerve (ON) and lateral olfactory tract (OT). In preparations superfused with normal amphibian Ringer's solution, the large peak occurred 14–21 ms after either ON or OT stimulation. It therefore appeared to reflect the activation of granule cell synapses with mitral cells, as in olfactory bulbs of other species. In three groups of preparations that were superfused with single concentrations of DA, significant decreases were observed in the amplitude of the large peak of ON- and OT-evoked potentials with increases in concentration from 5–200 M. Moreover, with 5 M DA and 50 M DA, significant decreases were observed in the amplitude of the large peak of ON-evoked potentials with increases in superfusion time from 1–15 min. With each DA concentration tested, the mean percentage decrease in the ON-evoked potentials was significantly larger than the mean percentage decrease in the OT-evoked potentials. In five groups of preparations that were superfused with single concentrations of FLU, significant decreases were also observed in the amplitude of the large peak of ON- and OT-evoked potentials with increases in concentration from 0.5–150 M. With 100 M FLU and 150 M FLU, significant decreases were observed in the amplitude of the large peak of both ON- and OT-evoked potentials with increases in superfusion time from 5–10 min. With each FLU concentration tested, the mean percentage change in the ON-evoked potentials was significantly larger than the mean percentage change in the OT-evoked potentials. The stronger effects of DA and FLU on the ON-evoked than OT-evoked potentials suggested that both drugs target receptors in the rostral (superficial) layers of the salamander olfactory bulb which have a higher density or affinity for DA and FLU than receptors in the more caudal (deep) layers of the bulb. When preparations were superfused with an equimolar mixture of DA and FLU at the ED₅₀ concentrations (50 M for both), FLU blocked approximately 50% of the decrease in the amplitude of the ON-evoked potentials relative to the decrease measured in preparations superfused with DA alone. Since FLU depressed the amplitude of ON-evoked potentials when it was tested alone, however, the rostral DA receptors could occur on both the olfactory receptor cell axons and their postsynaptic targets, or FLU could limit mitral/tufted cell excitation by affecting other types of receptors or voltage-sensitive Ca²⁺ channels. Results of this study which show that DA and FLU reduce the amplitude of evoked potentials in the salamander olfactory bulb provide evidence for the occurrence of DA receptors in the amphibian brain. More importantly, the stronger effects of DA and FLU on the ON-evoked than OT-evoked potentials suggest that the DA receptors could function to limit the excitation of cells at an early synaptic level in the salamander bulb. By modulating spatiotemporal patterns of synaptic activity in the glomerular layer, the receptors could profoundly influence the initial encoding of information about odors.     

Regulation of synaptic timing in the olfactory bulb by an A-type potassium current

Although rapid synaptic transmission confers signal fidelity, the activity of some neuronal circuits depends on prolonged excitation or inhibition. Here we demonstrate that GABAergic granule cells in the rat olfactory bulb produce prolonged inhibition of mitral cells through a precise kinetic matching between transmitter-gated and voltage-gated channels in their dendritic membrane. A transient A-type potassium current (IA) specifically attenuated dendrodendritic inputs mediated by fast-acting AMPA receptors such that the excitation and subsequent inhibitory output of granule cells followed the prolonged kinetics of their NMDA receptors. Altering the weights of the AMPA and NMDA receptor-mediated inputs by modulating IA provides a mechanism to regulate the timing of inhibition according to the demands on the bulb network.     

The effects of anaesthetics on synaptic excitation and inhibition in the olfactory bulb

1. The effects of anaesthetics (pentobarbitone, hexobarbitone, halothane, urethane, chloralose, chloral hydrate and ethanol) on the extracellular field potentials of the olfactory bulb produced by lateral olfactory tract stimulation were analysed.2. Relatively large doses of all the anaesthetics (e.g. pentobarbitone, 40-70 mg/kg) depressed the synaptic excitation of granule cells.3. The antidromic invasion of mitral cell dendrites was only slightly less sensitive to the anaesthetics than was the synaptic excitation of granule cells.4. A wide dose range of anaesthetics (e.g. pentobarbitone, 3-60 mg/kg) prolonged the granule cell post-synaptic inhibition of mitral cells. All the anaesthetics, except ethanol, prolonged the inhibition.5. The action of anaesthetics on post-synaptic inhibition was due to a specific effect on the inhibitory synapses.6. Amino-oxyacetic acid, an inhibitor of GABA catabolism, had little effect on the synaptic inhibition or on the ability of hexobarbital to prolong the inhibition. This suggests that the prolongation seen with anaesthetics is not a result of interfering with GABA catabolism.7. The present results are compared with results obtained with anaesthetics in other areas of the nervous system and it is proposed that prolongation of ;gaba-ergic' inhibition might contribute to an agent's ability to produce general anaesthesia.     

Neurosteroid modulation of dendrodendritic inhibition in the mouse olfactory bulb

The present report describes neurosteroid modulation of olfactory bulb function by examining the effects of intrabulbar infusion of dehydroepiandrosterone sulfate (DHEAS), a neurohormone discovered in rat brain, on field potentials in the granule cell layer evoked by paired-pulse stimulation of the mouse lateral olfactory tract. Infusion of DHEAS (5 nmol) significantly decreased the test response without affecting the conditioning response. As a consequence, DHEAS selectively potentiated paired-pulse depression, which is believed to be due to granule cell-mediated inhibition of the mitral/tufted cells. The granule-to-mitral/tufted dendrodendritic synapse is GABAergic. Taken together, these results suggest that DHEAS potentiates the GABAergic dendrodendritic inhibition exerted by the granule cells on the mitral/tufted cells.     

Analysis of synaptic events in the mouse accessory olfactory bulb with current source-density techniques.

The accessory olfactory bulb of the mouse was studied by current source-density analysis of field potentials to determine the laminar and temporal distribution of synaptic currents evoked by electrical stimulation of the vomeronasal organ. The one-dimensional current source-density analysis revealed two major spatially and temporally distinct inward membrane currents (sinks): one in the glomerular layer and the other in the external plexiform layer. The glomerular layer sink preceded the external plexiform layer sink by a mean of 5.5 ms. Local infusions of the broad-spectrum excitatory amino acid antagonist, kynurenate, into the accessory olfactory bulb blocked the external plexiform layer sink without an obvious effect on the glomerular layer sink. The selective non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione produced a dose-dependent blockade of the external plexiform layer sink, whereas the selective N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovalerate was without effect. These results, taken together with the cytoarchitecture of the accessory olfactory bulb, suggest that the glomerular layer sink results mainly from synaptic excitation evoked in the glomerular dendritic branches of mitral cells by the vomeronasal afferent fibres and the external plexiform layer sink mainly from non-N-methyl-D-aspartate receptor-mediated synaptic excitation in the peripheral processes of granule cells via the mitral to granule cell dendrodendritic synapse.     

Complementary postsynaptic activity patterns elicited in olfactory bulb by stimulation of mitral/tufted and centrifugal fiber inputs to granule cells

Main olfactory bulb (MOB) granule cells receive spatially segregated glutamatergic synaptic inputs from the dendrites of mitral/tufted cells as well as from the axons of centrifugal fibers (CFFs) originating in olfactory cortical areas. Dendrodendritic synapses from mitral/tufted cells occur on granule cell distal dendrites in the external plexiform layer (EPL), whereas CFFs preferentially target the somata/proximal dendrites of granule cells in the granule cell layer (GCL). In the present study, tract tracing, and recordings of field potentials and voltage-sensitive dye optical signals were used to map activity patterns elicited by activation of these two inputs to granule cells in mouse olfactory bulb slices. Stimulation of the lateral olfactory tract (LOT) produced a negative field potential in the EPL and a positivity in the GCL. CFF stimulation produced field potentials of opposite polarity in the EPL and GCL to those elicited by LOT. LOT-evoked optical signals appeared in the EPL and spread subsequently to deeper layers, whereas CFF-evoked responses appeared in the GCL and then spread superficially. Evoked responses were reduced by N-methyl-d-aspartate (NMDA) receptor antagonists and completely suppressed by AMPA receptor antagonists. Reduction of extracellular Mg(2+) enhanced the strength and spatiotemporal extent of the evoked responses. These and additional findings indicate that LOT- and CFF-evoked field potentials and optical signals reflect postsynaptic activity in granule cells, with moderate NMDA and dominant AMPA receptor components. Taken together, these results demonstrate that LOT and CFF stimulation in MOB slices selectively activate glutamatergic inputs to the distal dendrites versus somata/proximal dendrites of granule cells.     

Gamma-frequency excitatory input to granule cells facilitates dendrodendritic inhibition in the rat olfactory Bulb

Recurrent and lateral inhibition play a prominent role in patterning the odor-evoked discharges in mitral cells, the output neurons of the olfactory bulb. Inhibitory responses in this brain region are mediated through reciprocal synaptic connections made between the dendrites of mitral cells and GABAergic interneurons. Previous studies have demonstrated that N-methyl-D-aspartate (NMDA) receptors on interneurons play a critical role in eliciting GABA release at reciprocal dendrodendritic synapses. In acute olfactory bulb slices, these receptors are tonically blocked by extracellular Mg2+, and recurrent inhibition is disabled. In the present study, we examined the mechanisms by which this tonic blockade could be reversed. We demonstrate that near-coincident activation of an excitatory pathway to the proximal dendrites of GABAergic interneurons relieves the Mg2+ blockade of NMDA receptors at reciprocal dendrodendritic synapses and greatly facilitates recurrent inhibition onto mitral cells. Gating of recurrent and lateral inhibition in the presence of extracellular Mg2+ requires gamma-frequency stimulation of glutamatergic axons in the granule cell layer. Long-range excitatory axon connections from mitral cells innervated by different subpopulations of olfactory receptor neurons may provide a gating input to granule cells, thereby facilitating the mitral cell lateral inhibition that contributes to odorant encoding.     

Transient activity induces a long-lasting increase in the excitability of olfactory bulb interneurons

Little is known about the cellular mechanisms that underlie the processing and storage of sensory in the mammalian olfactory system. Here we show that persistent spiking, an activity pattern associated with working memory in other brain regions, can be evoked in the olfactory bulb by stimuli that mimic physiological patterns of synaptic input. We find that brief discharges trigger persistent activity in individual interneurons that receive slow, subthreshold oscillatory input in acute rat olfactory bulb slices. A 2- to 5-Hz oscillatory input, which resembles the synaptic drive that the olfactory bulb receives during sniffing, is required to maintain persistent firing. Persistent activity depends on muscarinic receptor activation and results from interactions between calcium-dependent afterdepolarizations and low-threshold Ca spikes in granule cells. Computer simulations suggest that intrinsically generated persistent activity in granule cells can evoke correlated spiking in reciprocally connected mitral cells. The interaction between the intrinsic currents present in reciprocally connected olfactory bulb neurons constitutes a novel mechanism for synchronized firing in subpopulations of neurons during olfactory processing.     

Neuromodulatory effect of GnRH on the synaptic transmission of the olfactory bulbar neural circuit in goldfish, Carassius auratus

Gonadotropin-releasing hormone (GnRH) is well known as a hypophysiotropic hormone that is produced in the hypothalamus and facilitates the release of gonadotropins from the pituitary gonadotropes. On the other hand, the functions of extrahypothalamic GnRH systems still remain elusive. Here we examined whether the activity of the olfactory bulbar neural circuits is modulated by GnRH that originates mainly from the terminal nerve (TN) GnRH system in goldfish (Carassius auratus). As the morphological basis, we first observed that goldfish TNs mainly express salmon GnRH (sGnRH) mRNA and that sGnRH-immunoreactive fibers are distributed in both the mitral and the granule cell layers. We then examined by extracellular recordings the effect of GnRH on the electrically evoked in vitro field potentials that arise from synaptic activities from mitral to granule cells. We found that GnRH enhances the amplitude of the field potentials. Furthermore, these effects were observed in both cases when the field potentials were evoked by stimulating either the lateral or the medial olfactory tract, conveying functionally different sensory information, separately, and suggesting that GnRH may modulate the responsiveness to wide categories of odorants in the olfactory bulb. Because GnRH also changed the paired-pulse ratio, it is suggested that the increased amplitude of the field potential results from changes in the presynaptic glutamate release of mitral cells rather than the increase in the glutamate receptor sensitivity of granule cells. These results suggest that TN regulates the olfactory responsiveness of animals appropriately by releasing sGnRH peptides in the olfactory bulbar neural circuits.     

Intracellular responses of identified rat olfactory bulb interneurons to electrical and odor stimulation

1. Intracellular recordings were made from 28 granule cells and 6 periglomerular cells of the rat olfactory bulb during odor stimulation and electrical stimulation of the olfactory nerve layer (ONL) and lateral olfactory tract (LOT). Neurons were identified by injection of horseradish peroxidase (HRP) or biocytin and/or intracellular response characteristics. Odorants were presented in a cyclic sniff paradigm, as reported previously. 2. All interneurons could be activated from a wide number of stimulation sites on the ONL, with distances exceeding their known dendritic spreads and the dispersion of nerve fibers within the ONL, indicating that multisynaptic pathways must also exist at the glomerular region. All types of interneurons also responded to odorant stimulation, showing a variety of responses. 3. Granule cells responded to electrical stimulation of the LOT and ONL as reported previously. However, intracellular potential, excitability, and conductance analysis suggested that the mitral cell-mediated excitatory postsynaptic potential (EPSP) is followed by a long inhibitory postsynaptic potential (IPSP). An early negative potential, before the EPSP, was also observed in every granule cell and correlated with component I of the extracellular LOT-induced field potential. We have interpreted this negativity as a "field effect," that may be diagnostic of granule cells. 4. Most granule cells exhibited excitatory responses to odorant stimulation. Odors could produce spiking responses that were either nonhabituating (response to every sniff) or rapidly habituating (response to first sniff only). Other granule cells, while spiking to electrical stimulation, showed depolarizations that did not evoke spikes to odor stimulation. These depolarizations were transient with each sniff or sustained across a series of sniffs. These physiological differences to odor stimulation correlated with granule cell position beneath the mitral cell layer for 12 cells, suggesting that morphological subtypes of granule cells may show physiological differences. Some features of the granule cell odor responses seem to correlate with some of the features we have observed in mitral/tufted cell intracellular recordings. Only one cell showed inhibition to odors. 5. Periglomerular (PG) cells showed a response to ONL stimulation that was unlike that found in other olfactory bulb neurons. There was a long-duration hyperpolarization after a spike and large depolarization or burst of spikes (20-30 ms in duration). Odor stimulation produced simple bursts of action potentials, Odor stimulation produced simple bursts of action potentials, suggesting that PG cells may simply follow input from the olfactory nerve.(ABSTRACT TRUNCATED AT 400 WORDS)     

Ketamine and MK801 attenuate paired pulse inhibition in the olfactory bulb of the rat

Summary We have investigated the effects of the phencyclidine like-compounds ketamine and MK801 on the evoked field potentials of rat olfactory bulb. Low doses of ketamine (3–6 mg/kg) blocked the inhibition of mitral cells by granule cells evoked by stimulation of lateral olfactory tract fibres or by stimulation of olfactory nerve. This blockade was not accompanied by a decrease in granule cell excitation as revealed by field potential recording. MK801 had a similar effect on the inhibition of mitral cells evoked by stimulation of the lateral olfactory tract. As ketamine does not influence the inhibitory action of GABA (Anis et al. 1983) these results suggest that both ketamine and MK801 block inhibition by an action on intrinsic excitatory feed-back circuits in the olfactory bulb.     

Dopamine modulates synaptic transmission between rat olfactory bulb neurons in culture

The glomerular layer of the olfactory bulb (OB) contains synaptic connections between olfactory sensory neurons and OB neurons as well as connections among OB neurons. A subpopulation of external tufted cells and periglomerular cells (juxtaglomerular neurons) expresses dopamine, and recent reports suggest that dopamine can inhibit olfactory sensory neuron activation of OB neurons. In this study, whole cell electrophysiological and primary culture techniques were employed to characterize the neuromodulatory properties of dopamine on glutamatergic transmission between rat OB mitral/tufted (M/T) cells and interneurons. Immunocytochemical analysis confirmed the expression of tyrosine hydroxylase, the rate-limiting enzyme for dopamine synthesis, in a subpopulation of cultured neurons. D2 receptor immunoreactivity was also observed in cultured M/T cells. Dopamine reduced spontaneous excitatory synaptic events recorded in interneurons. Although the D1 receptor agonist SKF38393 and the D2 receptor agonist bromocriptine mesylate mimicked this effect, evoked excitatory postsynaptic potentials (EPSPs) recorded from monosynaptically coupled neuron pairs were attenuated by dopamine and bromocriptine but not by SKF38393. Neither glutamate-evoked currents nor the membrane resistance of the postsynaptic interneuron were affected by dopamine. However, evoked calcium channel currents in the presynaptic M/T cell were diminished during the application of either dopamine or bromocriptine, but not SKF38393. Dopamine suppressed calcium channel currents even after nifedipine blockade of L-type channels, suggesting that inhibition of the dihydropyridine-resistant high-voltage activated calcium channels implicated in transmitter release may mediate dopamine's effects on spontaneous and evoked synaptic transmission. Together, these data suggest that dopamine inhibits excitatory neurotransmission between M/T cells and interneurons via a presynaptic mechanism.     

Effect of stimulating the nucleus of the horizontal limb of the diagonal band on single unit activity in the olfactory bulb

The effects of centrifugal afferents on single unit discharge in the main olfactory bulb were studied in anaesthetized rats. Recording with extracellular micropipettes revealed spontaneous firing in all bulb layers. Units were located to different laminae using evoked field-potential profiles and histological verification. Output neurons were identified by antidromic response to stimulation of the lateral olfactory tract. Single- or brief multiple-pulse stimulation in the nucleus of the horizontal limb of the diagonal band, but not in adjacent regions, facilitated 17 out of 27 mitral cells with no effect on 10, but inhibited 21 out of 33 granule cell layer units with no effect on 12. Of 13 presumed tufted cells, six were facilitated and the rest unaffected. In contrast, stimulation of olfactory cortex inhibited mitral cells and facilitated most granule layer cells. The results are consistent with an inhibition of tonic granule cell discharge by the horizontal diagonal band nucleus, with resultant disinhibition of mitral cells via the dendrodendritic synapses of granule cells on mitral cell secondary dendrites.     

Properties of reciprocal synapses in the mouse accessory olfactory bulb.

Reciprocal dendrodendritic synapses between mitral and granule cells in the accessory olfactory bulb have been implicated in a specialized form of olfactory learning in mice, in which a female forms a memory to the pheromonal signal of the male that mates with her. Relatively little is known, however, about the mechanism of synaptic transmission at the reciprocal synapses. We analyzed synaptic currents generated in accessory olfactory bulb mitral cells in slice preparations with the patch-clamp technique in nystatin-perforated whole-cell configuration. A brief (5-20-ms) depolarizing voltage step from -70 to 0 mV applied to a single mitral cell evoked GABA(A) receptor-mediated inhibitory postsynaptic currents. The inhibitory postsynaptic currents persisted in the presence of tetrodotoxin, indicating that the inhibitory postsynaptic current in mitral cells can be elicited through purely dendritic interactions. The inhibitory postsynaptic currents were greatly enhanced by washout of extracellular Mg(2+). In Mg(2+)-free solution, the N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonovaleric acid greatly reduced the inhibitory postsynaptic currents, whereas the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-(1H,4H)-dione (CNQX) slightly reduced them.These data demonstrate that NMDA receptors play an important role in the generation of dendrodendritic inhibition in mitral cells of the mouse accessory olfactory bulb.     

Opposing inward and outward conductances regulate rebound discharges in olfactory mitral cells

The olfactory bulb, a second-order sensory brain region, relays afferent input from olfactory receptor neurons to piriform cortex and other higher brain centers. Although large inhibitory postsynaptic potentials (IPSPs) are evident in in vivo intracellular recordings from mitral cells, the functional significance of these synaptic responses has not been defined. In many brain regions, IPSPs can function to either inhibit spiking by transiently suppressing activity or can evoke spiking directly by triggering rebound discharges. We used whole cell patch-clamp recordings from mitral cells in olfactory bulb slices to investigate the mechanisms by which IPSPs regulate mitral cell spike discharges. Mitral cells have unusual intrinsic membrane properties that support rebound spike generation in response to small-amplitude (3-5 mV) but not large-amplitude hyperpolarizing current injections or IPSPs. Rebound spiking occurring in mitral cells was dependent on recovery of subthreshold Na currents, and could be blocked by tetrodotoxin (TTX, 1 microM) or the subthreshold Na channel blocker riluzole (10 microM). Surprisingly, larger-amplitude hyperpolarizing stimuli impeded spike generation by recruiting a transient outward I(A)-like current that was sensitive to high concentrations of 4-aminopyridine and Ba. The interplay of voltage-gated subthreshold Na channels and transient outward current produces a narrow range of IPSP amplitudes that generates rebound spikes. We also found that subthreshold Na channels boost subthreshold excitatory stimuli to produce membrane voltages where granule-cell-mediated IPSPs can produce rebound spikes. These results demonstrate how the intrinsic membrane properties of mitral cells enable inhibitory inputs to bidirectionally control spike output from the olfactory bulb.     

Synaptology of the olfactory bulb of an elasmobranch fish, Sphyrna tiburo

The ultrastructure of the elasmobranch olfactory bulb was examined in order to determine the synaptology of the olfactory circuitry in the bonnethead shark, Sphyrna tiburo. The compartmentalization of the bulb, together with the lack of mitral cell basal dendrites, suggests a different way of performing lateral communication between mitral cells of the olfactory bulb. The results show that granule cells assume an important role by directly interlinking mitral cells. A corollary of this is the segregation of the input onto the mitral cell dendritic arborization: afferent fibers synapse onto the intraglomerular mitral terminals, whereas most local circuit interactions utilize extraglomerular synapses located on the shafts and the somas of the mitral dendrites. Therefore, the elasmobranch synaptic pattern is different from that of higher vertebrates; This might represent the use of a different neural route to achieve the same processing task.