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381 Symposium Presidential: Why Have Neurogenesis inAdult Olfactory Systems?INTEGRATING NEW NEURONS INTO THE ADULTOLFACTORY SYSTEMLledo P. 1 1 Pasteur Institute, Paris, FranceIn the adult olfactory bulb, newly born neurons are constitutivelygenerated throughout life and form an integral part of normal functionalcircuitry. This process of late neurogenesis is subject, at various stages,to modulation and control by external influences, suggesting stronglythat it represents a plastic mechanism by which the brain´s performancecan be optimized according to the environment in which it finds itself.But optimized how? And why? This presentation will concentrate onsuch functional questions regarding neurogenic plasticity. Afteroutlining the processes of adult neurogenesis in the olfactory system,and after discussing their regulation by internal and environmentalinfluences, we shall ask how existing neuronal circuits can continue towork in the face of constant cell arrivals and departures, and explore thepossible functional roles that newborn neurons might subserve in theadult olfactory system. In particular, we shall report the degree ofsensitivity of the bulbar neurogenesis to the level of sensory inputs and,in turn, how the adult neurogenesis adjusts the neural networkfunctioning to optimize sensory information processing. We will bringtogether recently described properties and emerging principles of adultneurogenesis that support a much more complex role for the adultgeneratedcells than just providers of replaceable units. Throughout,and concentrating exclusively on mammalian systems, we shall stressthat adult neurogenesis constitutes another weapon in the brain´sarmory for dealing with a constantly changing world.382 Poster Central Taste and Chemosensory BehaviorCHORDA TYMPANI (CT), GREATER SUPERFICIALPETROSAL (GSP) AND IXTH NERVE TERMINAL FIELDS INHAMSTER SOLITARY NUCLEUS (NTS)Bradenham B.P. 1 , Harrison C.H. 1 , Stewart J.S. 2 , Stewart R. 2 1 Programin Neuroscience, Washington and Lee University, Lexington, VA;2 Psychology/Program in Neuroscience, Washington and Lee University,Lexington, VAIn mammals, GSP afferents innervate taste cells in palatal andnasoincisal mucosae, while CT and IXth nerves (IX) innervate tastecells of fungiform and circumvallate papillae, respectively. The nervesterminate centrally in the rostral pole of the NTS. In this study, wequantify anatomical overlap among terminal fields of these taste nervesin adult hamster NTS. We used triple fluorescent nerve labeling tovisualize CT, GSP, and LT-IX terminal fields in NTS. CT, GSP, and IXwere isolated, cut, and labeled with unique dextran amine conjugates.After 3-7 days survival, animals were sacrificed and perfused. Fixedbrains were sectioned horizontally and sections examined by confocalmicroscopy. Serial optical sections through physical sections ofcomplete terminal fields were analyzed offline. Data so far show that IXterminal field appears ~100 µm dorsal to GSP and CT, which enter therostral NTS nearly coincidently in the dorsal-ventral plane. GSPterminal field extends ~100 µm ventral to that of IX, while CT terminalfield extends ~ 50 µm ventral to that of GSP. IX and GSP terminal fieldvolumes are roughly similar and appear to be greater in volume than CTfield. This difference is attributable to the restricted caudal extent of CTterminal field. All three terminal fields overlap extensively throughoutthe dorsal-ventral plane. Nearly the entire volume of GSP field overlapswith IX or CT terminal fields, while IX and CT fields enjoy ~50 µm ofunique territory in dorsal and ventral NTS, respectively. These resultsprovide normative data for studies of development in the hamstercentral taste system. This work was supported by W&L R.E. LeeResearch Endowment (BPB, CHH)383 Poster Central Taste and Chemosensory BehaviorCA 2+ IMAGING OF PRIMARY GUSTATORY AFFERENTS INTHE VAGAL LOBE OF OF GOLDFISHHallock R. 1 , Ikenaga T. 2 , Finger T.E. 3 1 Psychology, State University ofNew York at Binghamton, Denver, CO; 2 Cell and DevelopmentalBiology, University of Colorado Health Sciences Center, Aurora, CO;3 Cellular and Structural Biology, University of Colorado HealthSciences Center, Aurora, COThe primary gustatory nucleus in goldfish has a characterizedanatomy whereby gustatory afferents terminate in distinct laminae. Weinjected Ca 2+ green dextran in the vagus nerve and allowed 3-daysrecovery to allow filling of the primary nerve terminals in the vagallobe. Vagal lobe slices were prepared for in vitro recording. Theprimary afferent fibers were electrically stimulated with pairs of 2.0 mspulses separated by 30 ms while optically recording Ca 2+ signals fromthe layers containing primary afferent terminals. Hence the Ca 2+ signalarose only from the primary afferent fibers. Results showed paired pulsefacilitation in that significantly more Ca 2+ was detected after the secondpulse than the first. This indicates more primary afferent terminals areactivated by the second pulse than the first. In addition, the area ofincrease in Ca 2+ signal after the second stimulating pulse was broaderthan that observed after the first pulse, suggesting that the first pulseactivates the internal circuitry of the sensory layer of the vagal lobe.Since the recorded signal arises entirely from primary afferents, thesefindings suggest that the primary afferent terminals may be under tonicinhibition which is released by the initial stimulus pulse. Supported byNIDCD Grant DC00147(T.E.F.).384 Poster Central Taste and Chemosensory BehaviorTEMPORAL PATTERNS OF NEURAL ACTIVITY IN THENUCLEUS OF THE SOLITARY TRACT OF C57BL/6BYJ MICEMcCaughey S. 1 1 Monell Chemical Senses Center, Philadelphia, PAThe spontaneous and evoked activity of taste-responsive neurons canbe characterized in terms of mean response rates over periods withspecific durations, but the temporal patterns of activity within thoseperiods may also contain important information. The goal of this workwas to examine temporal patterns of single-unit activity in the rostralnucleus of the solitary tract (NST) of C57BL/6ByJ mice. Theextracellular activity of 39 NST cells was measured in anesthetizedanimals. The spontaneous firing patterns of neurons were investigatedby plotting the distributions of interspike intervals (ISIs). ISIs less than10 ms were especially common in some neurons, which gave evidenceof a preferred firing interval. In general, the presence of a preferredinterval was a characteristic of a cell and was not changed byapplication of taste stimuli, and preferred intervals were more likely tobe found in neurons with salt- or acid-oriented response profiles than inthose with sugar-oriented profiles. Temporal patterns of taste-evokedresponses were also examined and were found to vary acrosscompounds that are thought to taste sweet to mice. This variation meantthat the sweeteners evoked similar across-neuron patterns of activityacross a 5-sec evoked period, but did not when evoked periods less than1 sec were used. Given that perceptions of sweetness are likely to occurin less than a second, these results suggest that factors besides acrossneuronpatterning make a substantial contribution to taste qualityperception. This work was supported by NIH grant R03 DC005929.96
385 Central Taste and Chemosensory BehaviorCOMPUTATIONAL MODELS OF TEMPORAL FIRINGPROPERTIES OF SINGLE NEURONS IN THE NUCLEUS OFTHE SOLITARY TRACTChen J. 1 , Di Lorenzo P.M. 1 1 Psychology, State University of New Yorkat Binghamton, Binghamton, NYElectrophysiological responses to taste in the brain stem have mostoften been characterized by their mean firing rate rather than by thetemporal structure of the spike train. However, previous data from ourlab have shown that firing rate across stimulus repetitions can varywidely in some NTS cells and further, that those cells which showedmost variable firing rates nevertheless conveyed information about tastestimuli via spike timing. In the present project, we tested the hypothesisthat taste-sensitive neurons could generate non-random spike trainsgiven a random spike train as input. Numerous computational models oftaste-sensitive neurons were built based on physiological propertiesobtained from in vitro recordings reported in the literature. Strength ofinput and various morphological parameters were systematically variedand the results were assessed by metric-space analysis (Victor andPurpura, 1996) and by statistical tests for randomness. Results suggestthat taste-sensitive neurons can generate more reliable temporal patternsof response with higher frequency or stronger inputs. However, thelength of the dendrite, the number of dendritic branching points and thedistribution of the synapses do not play a significant role in determininga neuron´s temporal firing properties. These simulation results suggestthat precise spike timing can be determined by intracellular biophysicalparameters and the distribution and strength of the excitatory inputs,without the need for inhibitory feedback. Supported by NDCD grantRO1-DC005219.386 Poster Central Taste and Chemosensory BehaviorPRESYNAPTIC NICOTINIC RECEPTORS REGULATEGLUTAMATE RELEASE IN THE NUCLEUS OF THESOLITARY TRACT OF THE RATUteshev V. 1 , Smith D. 1 1 Anatomy and Neurobiology, University ofTennessee, Memphis, TNThe nucleus of the solitary tract (NST) is the first relay in theprocessing of gustatory and sensory visceral information. We haveshown previously that NST somata express nicotinic (nAChRs) andmuscarinic receptors that may shape the information processing in theNST. Here, we report that in rat brainstem slices, activation ofpresynaptic nAChRs by picospritzer applications of nicotine (500 µM,70 ms) to NST somata facilitates spontaneous release of glutamate. Theeffect of presynaptic facilitation lasted for ~1 minute, upon a singlepicospritzer application; and it could be evoked as often as every 3minutes. Analysis has shown that the effect was characterized by asignificant increase in the mean miniature postsynaptic current (mPSC)frequency (p < 0.004, paired one-tailed) and an insignificant increase inthe mPSC amplitude (p < 0.07, paired two-tailed). The effect wasresistant to tetrodotoxin (0.5 µM), a blocker of sodium action potentials,and 20 nM methyllcaconitine, a blocker of α7 nAChRs; but it wasblocked by 10 µM mecamylamine, a broad spectrum nAChR blocker.The effect was Ca 2+ -dependent, because it was eliminated when 2 mMCa 2+ in the extracellular solution was replaced with 0 mM Ca 2+ +5 mMEGTA; but it was resistant to 200 µM Cd 2+ +200 µM Ni 2+ , blockers ofvoltage-gated Ca 2+ channels. Intriguingly, the effect was observed inonly ~20 % of NST neurons, suggesting that it defines a subpopulationof NST neurons confined to a certain, unknown at this point, function.We conclude that the observed effect of presynaptic facilitation resultsfrom elevations in [Ca 2+ ] i in presynaptic glutamatergic terminals due toa direct Ca 2+ influx through non-α7 nAChRs. Supported by DC000066to DVS.387 Poster Central Taste and Chemosensory BehaviorCHARACTERISTICS OF INHIBITORY POSTSYNAPTICACTIVITY OF RAT INFERIOR SALIVATORY NUCLEUSNEURONSSuwabe T. 1 , Kim M. 2 , Bradley R.M. 1 1 Biologic & Materials Sciences,University of Michigan, Ann Arbor, MI; 2 Nursing, Chonnam UniversityMedical School, Gwangju, South KoreaNeural information derived from stimulating taste buds initiatesreflex salivary secretion. The efferent limb of this reflex is composed ofsecretomotor neurons contained in the salivatory nucleus situated alongthe medial border of the nucleus of the solitary tract (NST). We haveinvestigated synaptic activity of neurons of the inferior salivatorynucleus (ISN) that control the parotid and von Ebner salivary glands.Stimulation of the NST evokes mixed excitatory and inhibitorspostsynaptic potentials in the ISN neurons. To characterize theinhibitory synaptic activity whole-cell recordings andimmunocytochemical staining for GABA and glycine was performed onidentified ISN neurons in rat brainstem slices. ISN neurons respondedto both GABA and glycine with membrane hyperpolarization and adecrease in input resistance in the presence of 2 µM tetrodotoxin (n = 7)indicating that ISN neurons have both GABA and glycine receptors.Immunocytochemical labeling also revealed that about a 50% of theISN neurons were positive for GABA and glycine. Inhibitorypostsynaptic potentials (IPSP) evoked by electrical stimulation of theNST were studied under glutamate receptor block. The amplitude of theIPSPs was not significantly altered by the glycine receptor antagoniststrychnine (2 µM, n = 9, P > 0.05), but was eliminated by the GABA Areceptor antagonist bicuculline (10 µM, n = 8). This result indicates thatinhibition of ISN neurons is mediated by GABA A receptors driven viasynaptic input from the NST, but glycine receptors receive input fromthe other brain regions. Support contributed by: NIH grant DC000288to RMB.388 Poster Central Taste and Chemosensory BehaviorEXCITATORY POSTSYNAPTIC ACTIVITY OF THE RATINFERIOR SALIVATORY NUCLEUS NEURONSKim M. 1 , Suwabe T. 2 , Chiego D.J. 3 , Bradley R.M. 2 1 Nursing, ChonnamUniv Medical School, Gwangju, South Korea; 2 Biologic & MaterialsSciences, Univ of Michigan, Ann Arbor, MI; 3 Cariology, Univ ofMichican, Ann Arbor, MIStimulation of taste buds results in a number of reflex activitiesorganized at the brainstem level. Important to taste transduction is thereflex secretion of saliva. The output limb of this reflex arc originates ina column of parasympathetic motor neurons (the salivatory nucleus)closely associated with the brainstem taste relay nucleus—the nucleusof the solitary tract (NST). To characterize this reflex we have focusedon the inferior salivatory nucleus (ISN) responsible for the control ofthe parotid and von Ebner salivary glands. Stimulation of the NSTevokes postsynaptic potentials (PSP) in the ISN neurons which haveboth an excitatory and inhibitory components. To characterize theexcitatory component we have used whole-cell recordings andimmunocytochemical staining for ionotropic glutamate receptorsubtypes on identified ISN neurons in rat brainstem slices. Theinhibitory component of the PSPs was blocked by superfusion of theGABA A receptor antagonist, bicuculline. ISN neurons were stronglypositive for all the glutamate receptor subtypes including NMDA (NR1,NR2A, NR2B), AMPA (GluR1, GluR2, GluR3, GluR4), and kainate(GluR5-7, KA2). In whole cell recordings the NMDA receptorantagonist APV (50 µM) and the AMPA/kainate receptor antagonistCNQX (10 µM) both decreased the amplitude of the EPSPs. Mixturesof CNQX and APV eliminated the EPSPs. These results suggest thatexcitatory postsynaptic activity of ISN neurons induced by synapticinput from the NST is mediated by NMDA, AMPA and kainatereceptors. Support contributed by: NIDCD grant DC000288 to RMB.97
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sniffing behavior. Furthermore, we
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