Dopamine D2 receptor-mediated presynaptic inhibition of olfactory nerve terminals.

Olfactory receptor neurons of the nasal epithelium project via the olfactory nerve (ON) to the glomeruli of the main olfactory bulb, where they form glutamatergic synapses with the apical dendrites of mitral and tufted cells, the output cells of the olfactory bulb, and with juxtaglomerular interneurons. The glomerular layer contains one of the largest population of dopamine (DA) neurons in the brain, and DA in the olfactory bulb is found exclusively in juxtaglomerular neurons. D2 receptors, the predominant DA receptor subtype in the olfactory bulb, are found in the ON and glomerular layers, and are present on ON terminals. In the present study, field potential and single-unit recordings, as well as whole cell patch-clamp techniques, were used to investigate the role of DA and D2 receptors in glomerular synaptic processing in rat and mouse olfactory bulb slices. DA and D2 receptor agonists reduced ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells. Spontaneous and ON-evoked spiking of mitral cells was also reduced by DA and D2 agonists, and enhanced by D2 antagonists. DA did not produce measurable postsynaptic changes in juxtaglomerular cells, nor did it alter their responses to mitral/tufted cell inputs. DA also reduced 1) paired-pulse depression of ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells and 2) the amplitude and frequency of spontaneous, but not miniature, excitatory postsynaptic currents in juxtaglomerular cells. Taken together, these findings are consistent with the hypothesis that activation of D2 receptors presynaptically inhibits ON terminals. DA and D2 agonists had no effect in D2 receptor knockout mice, suggesting that D2 receptors are the only type of DA receptors that affect signal transmission from the ON to the rodent olfactory bulb.     

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.     

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.     

Dopamine depresses glutamatergic synaptic transmission in the rat parabrachial nucleus in vitro

Nystatin-perforated patch recordings were made from rat parabrachial neurons in an in vitro slice preparation to examine the effect of dopamine on parabrachial cells and on excitatory synaptic transmission in this nucleus. In current clamp mode, dopamine reduced the amplitude of the evoked excitatory postsynaptic potential without significant change in membrane potential. In cells voltage-clamped at -65 mV, dopamine dose dependently and reversibly decreased evoked, pharmacologically isolated, excitatory postsynaptic currents with an EC50 of 31 microM. The reduction in excitatory postsynaptic current was accompanied by an increase in paired pulse ratio (a protocol used to detect presynaptic site of action) with no change in the holding current or in the decay of the evoked excitatory postsynaptic currents. In addition, dopamine altered neither postsynaptic (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate-induced currents, nor steady-state current voltage curves. Miniature excitatory postsynaptic current analysis revealed that dopamine caused a rightward shift of the frequency-distribution curve with no change in the amplitude-distribution curve, which is consistent with a presynaptic mechanism. The dopamine-induced attenuation of the excitatory postsynaptic current was almost completely blocked by the D1-like receptor antagonist SCH23390 (10 microM), although the D2-like antagonist sulpiride (10 microM) also partially blocked it. Combined application of both antagonists blocked all dopamine-induced synaptic effects. The synaptic effect of dopamine was mimicked by the D1-like agonist SKF38393 (50 microM), but the D2-1ike agonist quinpirole (50 microM) also had a small effect. Combined application of both agonists did not produce potentiated responses. Dopamine's effect on the excitatory postsynaptic current was independent of serotonin, GABA and adenosine receptors, but may have some interactions with adrenergic receptors. These results suggest that dopamine directly modulates excitatory synaptic events in the parabrachial nucleus predominantly via presynaptic D1-like receptors.     

Tonic and synaptically evoked presynaptic inhibition of sensory input to the rat olfactory bulb via GABA(B) heteroreceptors

Olfactory receptor neurons of the nasal epithelium send their axons, via the olfactory nerve (ON), to the glomeruli of the olfactory bulb (OB), where the axon terminals form glutamatergic synapses with the apical dendrites of mitral and tufted cells, the output cells of the OB, and with juxtaglomerular (JG) interneurons. Many JG cells are GABAergic. Here we show that, despite the absence of conventional synapses, GABA released from JG cells activates GABA(B) receptors on ON terminals and inhibits glutamate release both tonically and in response to ON stimulation. Field potential recordings and current-source density analysis, as well as intracellular and whole cell recording techniques were used in rat OB slices. Baclofen (2-5 microM), a GABA(B) agonist, completely suppressed ON-evoked synaptic responses of both mitral/tufted cells and JG cells, with no evidence for postsynaptic effects. Baclofen (0.5-1 microM) also reversed paired-pulse depression (PPD) of mitral/tufted cell responses to paired-pulse facilitation (PPF), and reduced depression of JG cell excitatory postsynaptic currents (EPSCs) during repetitive ON stimulation. These results suggest that baclofen reduced the probability of glutamate release from ON terminals. The GABA(B) antagonists CGP35348 or CGP55845A increased mitral/tufted cell responses evoked by single-pulse ON stimulation, suggesting that glutamate release from ON terminals is tonically suppressed via GABA(B) receptors. The same antagonists reduced PPD of ON-evoked mitral/tufted cell responses at interstimulus intervals 50-400 ms. This finding suggests that a single ON impulse evokes sufficient GABA release, presumably from JG cells, to activate GABA(B) receptors on ON terminals. Thus GABA(B) heteroreceptors on ON terminals are activated by ambient levels of extrasynaptic GABA, and by ON input to the OB. The time course of ON-evoked, GABA(B) presynaptic inhibition suggests that neurotransmission to M/T cells and JG cells will be significantly suppressed when ON impulses arrive in glomeruli at 2.5-20 Hz. GABA(B) receptor-mediated presynaptic inhibition of sensory input to the OB may play an important role in shaping the activation pattern of the OB glomeruli during olfactory coding.     

Excitatory synaptic transmission in cultures of rat olfactory bulb

1. Olfactory bulb neurons were dissociated from neonatal rats and plated at low density on a confluent layer of olfactory bulb astrocytes. Intracellular stimulation of presumptive mitral/tufted (M/T) cells evoked monosynaptic excitatory postsynaptic potentials (EPSPs) in adjacent neurons. Whole-cell recording techniques and a flow-pipe drug delivery system were used to compare EPSPs with voltage-clamp recordings of currents evoked by excitatory amino acids (EAA) including N-acetylaspartylglutamate (NAAG), a putative mitral cell transmitter. 2. Cultured olfactory bulb neurons were morphologically and physiologically distinct. Large pyramidal-shaped neurons were present, which were NAAG immunoreactive; stimulation of these neurons invariably evoked EPSPs, suggesting that they were M/T cells. The majority of small bipolar neurons were glutamic acid decarboxylase (GAD) immunoreactive consistent with granule or periglomerular gamma-aminobutyric acid (GABA)ergic interneurons. 3. Monosynaptic EPSPs between M/T cells could be separated into fast and slow components by the use of EAA receptor antagonists. A fast component with a time-to-peak of 7.7 +/- 1.0 (SE) ms and half-width of 31.8 +/- 7.4 ms was blocked by the non-NMDA receptor antagonist 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX, 2.5 microM). The slow component (time-to-peak = 41.4 +/- 7.2 ms; half-width = 218.9 +/- 40.4 ms) was blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP5, 100 microM). 4. Under voltage clamp, flow-pipe applications of NAAG (10-1,000 microM) evoked inward currents at a holding potential of -60 mV in Mg-free solutions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Olfactory nerve-evoked, metabotropic glutamate receptor-mediated synaptic responses in rat olfactory bulb mitral cells

The group I metabotropic glutamate receptor (mGluR) subtype, mGluR1, is highly expressed on the apical dendrites of olfactory bulb mitral cells and thus may be activated by glutamate released from olfactory nerve (ON) terminals. Previous studies have shown that mGluR1 agonists directly excite mitral cells. In the present study, we investigated the involvement of mGluR1 in ON-evoked responses in mitral cells in rat olfactory bulb slices using patch-clamp electrophysiology. In voltage-clamp recordings, the average EPSC evoked by single ON shocks or brief trains of ON stimulation (six pulses at 50 Hz) in normal physiological conditions were not significantly affected by the nonselective mGluR antagonist LY341495 (50-100 microM) or the mGluR1-specific antagonist LY367385 (100 microM); ON-evoked responses were attenuated, however, in a subset (36%) of cells. In the presence of blockers of ionotropic glutamate and GABA receptors, application of the glutamate uptake inhibitors THA (300 microM) and TBOA (100 microM) revealed large-amplitude, long-duration responses to ON stimulation, whereas responses elicited by antidromic activation of mitral/tufted cells were unaffected. Magnitudes of the ON-evoked responses elicited in the presence of THA-TBOA were dependent on stimulation intensity and frequency, and were maximal during high-frequency (50-Hz) bursts of ON spikes, which occur during odor stimulation. ON-evoked responses elicited in the presence of THA-TBOA were significantly reduced or completely blocked by LY341495 or LY367385 (100 microM). These results demonstrate that glutamate transporters tightly regulate access of synaptically evoked glutamate from ON terminals to postsynaptic mGluR1s on mitral cell apical dendrites. Taken together with other findings, the present results suggest that mGluR1s may not play a major role in phasic responses to ON input, but instead may play an important role in shaping slow oscillatory activity in mitral cells and/or activity-dependent regulation of plasticity at ON-mitral cell synapses.     

Synaptic inputs to granule cells of the dorsal cochlear nucleus

The mammalian dorsal cochlear nucleus (DCN) integrates auditory nerve input with nonauditory signals via a cerebellar-like granule cell circuit. Although granule cells carry nonauditory information to the DCN, almost nothing is known about their physiology. Here we describe electrophysiological features of synaptic inputs to granule cells in the DCN by in vitro patch-clamp recordings from P12 to P22 rats. Granule cells ranged from 6 to 8 microm in cell body diameter and had high-input resistance. Excitatory postsynaptic currents consisted of both AMPA receptor-mediated and N-methyl-D-aspartate receptor-mediated currents. Synaptically evoked excitatory postsynaptic currents ranged from -25 to -140 pA with fast decay time constants. Synaptic stimulation evoked both short- and long-latency synaptic responses that summated to spike threshold, indicating the presence of a polysynaptic excitatory pathway in the granule cell circuit. Synaptically evoked inhibitory postsynaptic currents in Cl(-)-loaded cells ranged from -30 to -1,021 pA and were mediated by glycine and, to a lesser extent, GABA(A) receptors. Unlike cerebellar granule cells, DCN granule cells lacked tonic inhibition by GABA. The glycinergic synaptic conductance was mediated by heteromeric glycine receptors and was far stronger than the glutamatergic conductance, suggesting that glycinergic neurons may act to gate nonauditory signals in the DCN.     

Properties of external plexiform layer interneurons in mouse olfactory bulb slices

In the external plexiform layer (EPL) of the main olfactory bulb, apical dendrites of inhibitory granule cells form large numbers of synapses with mitral and tufted (M/T) cells, which regulate the spread of activity along the M/T cell dendrites. The EPL also contains intrinsic interneurons, the functions of which are unknown. In the present study, recordings were obtained from cell bodies in the EPL of mouse olfactory bulb slices. Biocytin-filling confirmed that the recorded cells included interneurons, tufted cells, and astrocytes. The interneurons had fine, varicose dendrites, and those located superficially bridged the EPL space below several adjacent glomeruli. Interneuron activity was characterized by high frequency spontaneous excitatory postsynaptic potential/currents that were blocked by the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and largely eliminated by the voltage-sensitive Na+ channel blocker, tetrodotoxin. Interneuron activity differed markedly from that of tufted cells, which usually exhibited spontaneous action potential bursts. The interneurons produced few action potentials spontaneously, but often produced them in response to depolarization and/or olfactory nerve (ON) stimulation. The responses to depolarization resembled responses of late- and fast-spiking interneurons found in other cortical regions. The latency and variability of the ON-evoked responses were indicative of polysynaptic input. Interneurons expressing green fluorescent protein under control of the mouse glutamic acid decarboxylase 65 promoter exhibited identical properties, providing evidence that the EPL interneurons are GABAergic. Together, these results suggest that EPL interneurons are excited by M/T cells via AMPA/kainate receptors and may in turn inhibit M/T cells within spatial domains that are topographically related to several adjacent glomeruli.     

Oxytocin enhances presynaptic and postsynaptic glutamatergic transmission between rat olfactory bulb neurones in culture

Although oxytocin (OT) within the olfactory bulb has been implicated in maternal behaviour and olfactory recognition, the cellular mechanisms of action remain to be clarified. We examined the effects of OT on glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) in cultured granule cells with the use of whole-cell patch-clamp recordings. OT reversibly increased both the frequency and amplitude of sEPSCs. The effects of OT on sEPSCs were blocked by the selective OT receptor antagonist desGly-NH(2)(9),d (CH(2))(5)-[Thy(Me)(2),Thr(4)]-ornithine vasotocin. OT had no detectable effect, however, on high voltage-activated Ca2+ currents in mitral/tufted cells, suggesting that OT acts presynaptically on step(s) in the release process downstream from calcium influx. OT augmented the membrane current in granule cells evoked by exogenous application of glutamate, indicating a postsynaptic site of action. These results indicate that OT facilitates sEPSCs in granule cells by both pre- and postsynaptic mechanisms.     

Histamine suppresses non-NMDA excitatory synaptic currents in rat supraoptic nucleus neurons

Whole cell patch-clamp recordings were obtained from supraoptic neurons to investigate the effects of histamine on excitatory postsynaptic currents evoked by electrical stimulation of areas around the posterior supraoptic nucleus. When cells were voltage-clamped at -70 mV, evoked excitatory postsynaptic currents had amplitudes of 88.4 +/- 9.6 pA and durations of 41.1 +/- 3.0 ms (mean +/- SE; n = 43). With twin stimulus pulses (20 Hz) used, paired-pulse facilitation ratios were 1.93 +/- 0.12. Bath application of 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX) abolished synaptic currents. Histamine at concentrations approximately 0.1-10 microM reversibly suppressed excitatory postsynaptic currents in all supraoptic neurons tested. Within 2 min after application of (10 microM) histamine, current amplitudes and durations decreased by 61. 5 and 31.0%, respectively, with little change in the paired-pulse facilitation ratio. Dimaprit or imetit (H(2) or H(3) receptor agonists) did not reduce synaptic currents, whereas pyrilamine (H(1) receptor antagonist) blocked histamine-induced suppression of synaptic currents. When patch electrodes containing guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) were used to record cells, histamine still suppressed current amplitudes by 49.1% and durations by 41.9%. Similarly, intracellular diffusion of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) and H(7) did not abolish histamine-induced suppression of synaptic currents, either. Bath perifusion of 8-bromo-quanosine 3',5'-cyclic monophosphate reduced current amplitudes by 32.3% and durations by 27.9%. After bath perfusion of slices with N(omega)-nitro-L-arginine methyl ester (L-NAME), histamine injection decreased current amplitudes only by 31.9%, much less than the inhibition rate in control (P < 0.01). In addition, histamine induced little change in current durations and paired-pulse facilitation ratios, representing a partial blockade of histamine effects on synaptic currents by L-NAME. In supraoptic neurons recorded using electrodes containing BAPTA and perifused with L-NAME, the effects of histamine on synaptic currents were completely abolished. Norepinephrine injection reversibly decreased current amplitudes by 39.1% and duration by 64.5%, with a drop in the paired-pulse facilitation ratio of 47.9%. Bath perifusion of L-NAME, as well as intracellular diffusion of GDP-beta-S, 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine, or BAPTA, failed to block norepinephrine-induced suppression of evoked synaptic currents. The present results suggest that histamine suppresses non-N-methyl-D-aspartate synaptic currents in supraoptic neurons through activation of H(1) receptors. It is possible that histamine first acts at supraoptic cells (perhaps both neuronal and nonneuronal) and induces the production of nitric oxide, which then diffuses to nearby neurons and modulates synaptic transmission by a postsynaptic mechanism.     

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.     

Non-cholinergic synaptic potentials mediated by lumbar colonic nerve in the guinea-pig inferior mesenteric ganglion in vitro

Non-cholinergic slow synaptic potentials mediated by the lumbar colonic nerve have been investigated using an in vitro preparation of the guinea-pig inferior mesenteric ganglion attached to a distal colonic segment. Non-cholinergic potential responses to colonic nerve stimulation, colonic distension and chemical activation of sensory afferents were recorded intracellularly from neurons in the inferior mesenteric ganglion. Electrical stimulation of the lumbar colonic nerve produced either a slow excitatory postsynaptic potential, or a slow inhibitory postsynaptic potential followed by a slow excitatory postsynaptic potential. The extrapolated reversal potential of the slow excitatory postsynaptic potential was in the range of 0 to -20 mV and that of the slow inhibitory postsynaptic potential was -90 to 110 mV. The slow excitatory postsynaptic potential and the slow inhibitory postsynaptic potential were reversibly abolished by perfusion of the ganglion with tetrodotoxin (1 microM), or perfusion with low calcium (200 microM), high magnesium (12 mM) containing solution. Capsaicin (1 microM) evoked a reversible depolarization of inferior mesenteric ganglion cells after which desensitization occurred and the slow excitatory postsynaptic potential was abolished but the slow inhibitory postsynaptic potential was enhanced in amplitude and prolonged in duration. Bath application of substance P (2 microM) evoked a prolonged depolarization of inferior mesenteric ganglion neurons, during which the slow excitatory postsynaptic potential but not the slow inhibitory postsynaptic potential was abolished. Distensions of the colon to pressures in the range of 2-25 cm of water produced a stimulus graded non-cholinergic slow depolarization which was occasionally followed by a late slow hyperpolarization. Both types of response were abolished by tetrodotoxin.(ABSTRACT TRUNCATED AT 250 WORDS)     

An electrophysiological study of the accessory olfactory bulb in the rabbit--II. Input-output relations as assessed from analysis of intra- and extracellular unit recordings

The input-output relations of the rabbit accessory olfactory bulb were studied by intra- and extracellular single unit recordings following electrical stimulation of the vomeronasal nerves, the lateral olfactory tract and the corticomedial amygdala. Cellular activity of accessory bulb mitral cells evoked by stimulation of the vomeronasal nerves consisted of a brief excitation with a latency of 16 ms. This initial response was followed by a period of reduced firing probability which was due to an inhibitory postsynaptic potential. In many cases this secondary response was followed by a second excitatory postsynaptic potential on which action potentials were generated at higher stimulus intensities. Deeper cells in the granule cell layer responded with a long latency, long duration, excitation, often consisting of bursts of 2-3 spikes. The majority of mitral cells were antidromically invaded by amygdala stimulation. The latencies of the antidromic spikes showed a wide range of variation (12-80 ms). Due to this great variation in antidromic latency the inhibitory postsynaptic potential following the antidromic action potential was rather modest but prolonged in duration. In many cases the onset of the inhibitory postsynaptic potential preceded the antidromic response. The majority of cells did not respond to lateral olfactory tract stimulation. Only 10% of the tested cells were invaded antidromically by stimulation at this site. These neurons were also driven antidromically by amygdala stimulation. We conclude that, although the physiological characteristics of mitral cells of the main and accessory olfactory bulb are very similar, there are important differences. The efferent fibres of the accessory bulb conduct at very slow and variable rates and project directly to the corticomedial amygdala.     

Significance of slow synaptic potentials for transmission of excitation in guinea-pig myenteric plexus

Intracellular recordings were made from neurones in myenteric ganglia of the guinea-pig ileum in vitro. Synaptic potentials were evoked by electrically stimulating presynaptic fibres as they entered the ganglion, using a small focal electrode. Slow synaptic depolarizations (excitatory postsynaptic potentials) were evoked in most myenteric neurones of both types. A single stimulus was more likely to evoke a slow excitatory postsynaptic potential in cells with nicotinic synaptic input (S cells; 50%) than in cells with long-lasting after-hyperpolarizations following the soma action potential (AH cells; 20%). Two pulses often evoked a slow excitatory postsynaptic potential in AH cells when one pulse was ineffective. The optimally effective time between the pulses was about 100 ms. Ten pulses resulted in slow excitatory postsynaptic potentials even when delivered at frequencies as low as 0.5 Hz. For the same frequency of presynaptic stimulation, the duration of the slow excitatory postsynaptic potential was greater in AH cells than in S cells and the amplitude of the slow excitatory postsynaptic potential was slightly greater in S than AH cells. Spontaneous depolarizations were observed which had time-courses and amplitudes similar to the evoked slow excitatory postsynaptic potential. They were not blocked by tetrodotoxin or atropine. The calcium-dependent after-hyperpolarization which follows one or more action potentials in AH cells was reduced or even abolished during the slow excitatory postsynaptic potential. Presynaptic nerve stimulation at intensities lower than those required to cause a slow excitatory postsynaptic potential caused a reduction in the calcium dependent after-hyperpolarization. It is concluded that the slow excitatory postsynaptic potential is generated by an intracellular intermediate process which is sensitive to the intracellular calcium concentration. The results suggest that the postsynaptic action of the synaptic transmitter is to interfere with the intracellular process which couples the entry of calcium to the increase in potassium conductance.     

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.     

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.     

Potentiating effects of 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluoro-phenoxyaceta mide (PEPA) on excitatory synaptic transmission in dentate granule cells

A novel sulfonylamino compound, 4-[2-(phenylsulfonylamino)-ethylthio]-2,6-difluoro-phenoxyaceta mide (PEPA) has been shown to selectively potentiate glutamate-induced currents in Xenopus oocytes expressing recombinant AMPA receptor subunits, GluR1-GluR4, by attenuation of desensitization. Here, we examined the effects of PEPA on responses to excitatory amino acids as well as on excitatory synaptic transmission in dentate granule cells of rat hippocampal slices using the whole-cell patch clamp technique. PEPA at 100 microM produced a 3-4-fold increases in the peak amplitude of current responses to AMPA and glutamate applied iontophoretically in the dentate granule cells, whereas it showed no effect on NMDA-induced currents. Excitatory postsynaptic currents (EPSCs) evoked in these neurons by stimulation of the perforant path had fast and slow components mediated by AMPA and NMDA receptors, respectively. PEPA at concentrations between 10 and 100 microM potentiated only the AMPA component of the EPSC (AMPA EPSC) in a dose-dependent manner without affecting the NMDA component. Although the potentiating effect of PEPA on the amplitude of the AMPA EPSC was weaker than that on the AMPA-induced current, it clearly prolonged the duration of the EPSC. PEPA at 100 microM increased the peak amplitude of the AMPA EPSC by 17%, and increased the area enclosed by the AMPA EPSC by 72%.     

Dendritic sodium spikelets and low-threshold calcium spikes in turtle olfactory bulb granule cells

Active dendritic membrane properties were investigated by whole cell recordings from adult turtle olfactory bulb granule cells. The laminar structure of the olfactory bulb allowed differential polarization of the distal apical dendrites versus the somatic part of the cells by an external electric field. Dendritic depolarization evoked small (approximately 10 mV) all-or-none depolarizing events of approximately 10-ms duration. These spikelets often occurred in bursts at high frequency (< or = 250 Hz); they were present despite the application of synaptic and gap junction antagonists, but were abolished by TTX and intracellularly applied QX314. The spikelets were interpreted as attenuated sodium spikes initiated in different branches of the granule cells dendrites. They occurred spontaneously, but could also be evoked by excitatory postsynaptic potentials (EPSPs) to the distal dendrites. Spikelets initiated by distal excitation could function as prepotentials for full sodium spikes, in part depending on the level of proximal depolarization. Somatic depolarization by the electric field evoked full sodium spikes as well as low-threshold calcium spikes (LTSs). Calcium imaging revealed that the electrophysiologically identified LTS evoked from the soma was associated with calcium transients in the proximal and the distal dendrites. Our data suggest that the LTS in the soma/proximal dendrites plays a major role in boosting excitability, thus contributing to the initiation of sodium spiking in this compartment. The results furthermore suggest that the LTS and the sodium spikes may act independently or cooperatively to regulate dendritic calcium influx.     

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.