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.     

Tufted cell dendrodendritic inhibition in the olfactory bulb is dependent on NMDA receptor activity

Mitral and tufted cells constitute the primary output cells of the olfactory bulb. While tufted cells are often considered as "displaced" mitral cells, their actual role in olfactory bulb processing has been little explored. We examined dendrodendritic inhibition between tufted cells and interneurons using whole cell voltage-clamp recording. Dendrodendritic inhibitory postsynaptic currents (IPSCs) generated by depolarizing voltage steps in tufted cells were completely blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2amino-5-phosphonopentanoic acid (D,L-AP5), whereas the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 2-3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f] quinoxaline-7-sulfonamide (NBQX) had no effect. Tufted cells in the external plexiform layer (EPL) and in the periglomerular region (PGR) showed similar behavior. These results indicate that NMDA receptor-mediated excitation of interneurons drives inhibition of tufted cells at dendrodendritic synapses as it does in mitral cells. However, the spatial extent of lateral inhibition in tufted cells was much more limited than in mitral cells. We suggest that the sphere of influence of tufted cells, while qualitatively similar to mitral cells, is centered on only one or a few glomeruli.     

Short-axon cells in the olfactory bulb: dendrodendritic synaptic interactions.

1. In the rabbit olfactory bulb, analysis has been carried out of extracellular unitary responses in the glomerular layer to olfactory nerve volleys. 2. Units in the glomerular layer responded to single volleys with single, double, triple or longer repetitive spike discharges. The shortest initial latencies are consistent with monosynaptic excitation from the olfactory nerves; longer latencies may reflect longer nerve pathways or polysynaptic connexions in the glomerular layer. 3. Like mitral and tufted cells, some glomerular layer units gave evidence of activation by discrete nerve bundles. This correlates with recent anatomical evidence for projections of discrete olfactory nerve bundles to the glomeruli. 4. Facilitation of glomerular layer units took the form of lower spike thresholds and shorter latencies, when testing with paired olfactory nerve volleys of weak strength at relatively short intervals (less than 40 msec). Supression took the form of raised thresholds, longer latencies and briefer repetitive discharges; this was particularly evident with strong volleys at long testing intervals. 5. The early period of facilitation and later period of suppression did not correlate with the recovery cycle of the olfactory nerves; the nerves had an absolute refractory period of approximately 3 msec, relative refractory period of 15-30 msec, and a small supernormal period of several hundred msec or more. 6. The evidence that the facilitation and suppression are mediated by dendrodendritic pathways through the periglomerular short-axon cells is discussed in relation to recent electronmicroscopical studies. The results have implications for similar pathways through short-axon cell dendrites in other parts of the nervous system.     

Excitatory and inhibitory interactions in the olfactory bulb involving dendrodendritic synapses between mitral cells and granular cells

Zusammenfassung Die Größe der maximalen Anstiegsgeschwindigkeit des Ventrikeldruckesdp/dt _max kann durch 2 Grundmechanismen verändert werden. Erstens durch die Faservordehnung und zweitens durch die Änderung der Kontraktionsform der Herzmuskelfaser selbst. Letztere bezeichnet man als Kontraktilität. Durch Kontraktilitätsänderungen kann man vergleichsweise größere Änderungen vondp/dt _max erreichen als durch Variation hämodynamischer Bedingungen. Die Herzfrequenz hat einen relativ geringen Einfluß auf die Kontraktilität.Wird das Maximum der Druckanstiegsgeschwindigkeit in der isovolumetrischen Phase der Ventrikelkontraktion erreicht, so beeinflussen der enddiastolische Ventrikeldruck und der coronare Perfusionsdruck die Höhe vondp/dt _max. Ein erhöhter coronarer Perfusionsdruck steigert zusätzlich die Empfindlichkeit gegenüber Veränderungen des enddiastolischen Druckes. Dies kommt in einer Versteilerung der ventrikulären Funktionskurven zum Ausdruck.Unterhalb eines diastolischen Aortendruckes von 60 mm Hg liegt das Maximum der Druckanstiegsgeschwindigkeit im statistischen Mittel in der Austreibungsphase. Unter Infusion von Isoprenalin wirddp/dt _max ebenfalls nach Öffnung der Aortenklappe erreicht. Der Zeitpunkt vondp/dt _max ist mit der Öffnung der Aortenklappe nicht identisch. Hoher diastolischer Aortendruck, geringe Herzfrequenz und Propranolol verschieben dieses Maximum zeitlich vor die Öffnung der Aortenklappe. Aortendrucksenkung und Isoprenalinzufuhr verschieben dieses Maximum in die Austreibungsphase. Der coronare Perfusionsdruck und der enddiastolische Ventrikeldruck sind ohne Einfluß auf die Lage vondp/dt _max im Herzcyclus.Anspannungszeit und die Zeit vom Beginn der Ventrikelkontraktion bis zum Maximum der Druckanstiegsgeschwindigkeit (t–dp/dt) verhalten sich unterschiedlich. Veränderungen der enddiastolischen Faserspannung, des coronaren Perfusionsdruckes und des diastolischen Aortendruckes sind ohne Einfluß auf die Zeitt–dp/dt. Der diastolische Aortendruck bestimmt allein den Zeitpunkt der Öffnung der Aortenklappe und damit der Anspannungszeit. Bei einer Kontraktilitätsänderung durch Propranolol, Isoprenalin oder Variation der Herzfrequenz verändert sich die Zeitt–dp/dt, in der das Maximum der Druckanstiegsgeschwindigkeit erreicht wird. Erhöht man die Kontraktilität, so steigtdp/dt _max an, und gleichzeitig verkürzt sich die Zeit bis zum Maximum. Senkt man die Kontraktilität, so verlängert sich die Zeitt–dp/dt, und die Größe vondp/dt _max nimmt ab. Auf Grund dieser Befunde erscheint es möglich, Veränderungen desdp/dt _max aufzuteilen in solche, die durch eine Veränderung der Kontraktilität, und in solche, die durch eine Veränderung hämodynamischer Bedingungen bewirkt werden.     

Lateral inhibition in the olfactory bulb and in olfaction

Lateral inhibition in the olfactory bulb is mediated by circuits that involve reciprocal dendrodendritic connections between mitral and granule cells. Because of the properties of these connections and also because odor stimuli are not represented in an obviously topographic fashion, questions have been raised about whether the function of local inhibition in the olfactory bulb can be compared to the function of inhibition in other brain areas. Here, I propose an analysis of local inhibition in the olfactory bulb based on the simplification that olfactory bulb circuitry can be thought of as implementing a simple linear two-dimensional filter. This analysis highlights some important characteristics of the circuitry of the olfactory bulb and suggests that the function of lateral inhibition in the olfactory bulb may be to compensate for generalized, spatially distributed activation that otherwise may obscure the specific, discrete patterns of glomerular activation seen across the olfactory bulb.     

Early X-irradiation of rats--II. Effect on granule cells and their dendrodendritic synapses in the olfactory bulb

Low, repeated doses of X-rays from a Co60 source were used to impair the development of the granule cells and their dendritic terminals in the olfactory bulb, and the resulting effect was studied under light and electron microscopes at 9 days of age. Irradiation of rats from embryonic day 18 (in utero) to postnatal day 5 resulted, among others, in maldevelopment of the (internal) granule cell and external plexiform layers. This was accompanied by a decrease in the number and the density of the granule cells, and the remaining granule cells contained less ribosomes, regardless of their position within the layer. This implies that both supposed subtypes of granule cells were effected. In the external plexiform layer, a reduced number of mature dendrodendritic synapses and signs of harmed granule gemmules were observed. The results suggest that intrauterinal plus postnatal irradiation with low, repeated doses of X-rays may be an effective tool impairing the development of prenatally forming neurons.     

Frequency-dependent modulation of inhibition in the rat olfactory bulb

We analyzed postmortem tissues of sporadic amyotrophic lateral sclerosis (SALS) for mRNA levels of two inflammatory proteins, complement C1qB and clusterin (apoJ). By Northern blot hybridization, SALS was associated with increased mRNA for C1qB and clusterin in the motor cortex (Brodmann area A4), but not in superior temporal cortex (A17), relative to neurologically normal controls. By in situ hybridization, SALS spinal cords showed increased C1qB and clusterin mRNA in areas undergoing neurodegeneration. This evidence implicates inflammatory mechanisms during neurodegenerative processes in SALS.     

Activity-dependent gating of lateral inhibition in the mouse olfactory bulb

Lateral inhibition is a circuit motif found throughout the nervous system that often generates contrast enhancement and center-surround receptive fields. We investigated the functional properties of the circuits mediating lateral inhibition between olfactory bulb principal neurons (mitral cells) in vitro. We found that the lateral inhibition received by mitral cells is gated by postsynaptic firing, such that a minimum threshold of postsynaptic activity is required before effective lateral inhibition is recruited. This dynamic regulation allows the strength of lateral inhibition to be enhanced between cells with correlated activity. Simulations show that this regulation of lateral inhibition causes decorrelation of mitral cell activity that is evoked by similar stimuli, even when stimuli have no clear spatial structure. These results show that this previously unknown mechanism for specifying lateral inhibitory connections allows functional inhibitory connectivity to be dynamically remapped to relevant populations of neurons.     

Reconstruction of dendritic growth cones in neonatal mouse olfactory bulb

Summary An improved and simplified method of serial sectioning for electron microscopy was used to prepare material from which were reconstructed dendritic growth cones of mitral cells in neonatal mouse olfactory glomerulus. Growth cones are characterized by: (1) one or more filopodia, each approximately 0.2 m in diameter, projecting from an expanded dendritic terminal or pre-terminal region; (2) a polygonal array of microfilaments approximately 50 Å in diameter, filling the enlarged terminal and filopodia; (3) an absence of microtubules and a paucity of mitochondria; (4) a few profiles of smooth endoplasmic reticulum or vesicles; (5) occasional axodendritic synapses. These characteristics are compared with those of other growing processes in the nervous system and a consistent picture of the appearance of growing neuronal processes obtained, suitable to use as a guide in a search for additional growing processes in developing and mature brains.     

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.     

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.     

Modification of olfactory bulb synaptic inhibition by early unilateral olfactory deprivation

Early unilateral olfactory deprivation produces large structural and neurochemical changes in the olfactory bulb, the first central relay for olfactory information. The functioning of deprived bulbs was examined in the present report by using paired-pulse stimulation of the lateral olfactory tract. Paired-pulse stimulation reflects interactions between mitral/tufted cells and granule cells, as well as the modulatory effects of centrifugal and intra-bulbar association fibers. Paired-pulse stimulation produced inhibition of mitral/tufted cells in control animals at PN20-PN22. This inhibition was significantly enhanced in littermates deprived of olfactory input from PN1 to PN20-PN22. Suppression of mitral/tufted cell single-unit spontaneous activity following single-pulse stimulation of the lateral olfactory tract (LOT) was similarly enhanced in deprived bulbs. These results suggest that early olfactory deprivation significantly modifies subsequent olfactory system function.     

Zinc and copper influence excitability of rat olfactory bulb neurons by multiple mechanisms

Zinc and copper are highly concentrated in several mammalian brain regions, including the olfactory bulb and hippocampus. Whole cell electrophysiological recordings were made from rat olfactory bulb neurons in primary culture to compare the effects of zinc and copper on synaptic transmission and voltage-gated ion channels. Application of either zinc or copper eliminated GABA-mediated spontaneous inhibitory postsynaptic potentials. However, in contrast to the similarity of their effects on inhibitory transmission, spontaneous glutamate-mediated excitatory synaptic activity was completely blocked by copper but only inhibited by zinc. Among voltage-gated ion channels, zinc or copper inhibited TTX-sensitive sodium channels and delayed rectifier-type potassium channels but did not prevent the firing of evoked single action potentials or dramatically alter their kinetics. Zinc and copper had distinct effects on transient A-type potassium currents. Whereas copper only inhibited the A-type current, zinc modulation of A-type currents resulted in either potentiation or inhibition of the current depending on the membrane potential. The effects of zinc and copper on potassium channels likely underlie their effects on repetitive firing in response to long-duration step depolarizations. Copper reduced repetitive firing independent of the initial membrane voltage. In contrast, whereas zinc reduced repetitive firing at membrane potentials associated with zinc-mediated enhancement of the A-type current (-50 mV), in a significant proportion of neurons, zinc increased repetitive firing at membrane potentials associated with zinc-mediated inhibition of the A-type current (-90 mV). Application of zinc or copper also inhibited voltage-gated Ca(2+) channels, suggesting a possible role for presynaptic modulation of neurotransmitter release. Despite similarities between the effects of zinc and copper on some ligand- and voltage-gated ion channels, these data suggest that their net effects likely contribute to differential modulation of neuronal excitability.     

Three-dimensional analysis of dendritic trees of mitral cells in the rabbit olfactory bulb

Physiologically identified mitral and displaced mitral cells were intracellularly stained with horseradish peroxidase (HRP), and analysed three-dimensionally. One to three primary dendrites of the mitral cell were found to enter into a glomerulus. Secondary dendrites spread for the most part in a circular area with radius of 1 mm in the inner half of the external plexiform layer (EPL), tagentially to the mitral cell layer. Dendrites of displaced mitral cells were observed mainly in the outer half of the EPL.     

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.     

Regulation of granule cell excitability by a low-threshold calcium spike in turtle olfactory bulb

Granule cells excitability in the turtle olfactory bulb was analyzed using whole cell recordings in current- and voltage-clamp mode. Low-threshold spikes (LTSs) were evoked at potentials that are subthreshold for Na spikes in normal medium. The LTSs were evoked from rest, but hyperpolarization of the cell usually increased their amplitude so that they more easily boosted Na spike initiation. The LTS persisted in the presence of TTX but was antagonized by blockers of T-type calcium channels. The voltage dependence, kinetics, and inactivation properties of the LTS were characteristic of a low-threshold calcium spike. The threshold of the LTS was slightly above the resting potential but well below the Na spike threshold, and the LTS was often evoked in isolation in normal medium. Tetraethylammonium (TEA) and 4-aminopyridine (4-AP) had only minimal effects on the LTS but revealed the presence of a high-threshold Ca2+ spike (HTS), which was antagonized by Cd2+. The LTS displayed paired-pulse attenuation, with a timescale for recovery from inactivation of about 2 s at resting membrane potential. The LTS strongly boosted Na spike initiation; with repetitive stimulation, the long recovery of the LTS governed Na spike initiation. Thus the olfactory granule cells possess an LTS, with intrinsic kinetics that contribute to sub- and suprathreshold responses on a timescale of seconds. This adds a new mechanism to the early processing of olfactory input.