Neuronal swelling and surface area regulation: elevated intracellular calcium is not a requirement

Neurons aremechanically robust. During prolonged swelling, molluscan neurons cantriple their apparent membrane area. They gain surface area andcapacitance independent of extracellular Ca concentration([Ca]_e), but it isunknown if an increase in intracellular Ca concentration([Ca]_i) isnecessary. If Ca for stimulating exocytosis is unnecessary, it ispossible that swelling-induced membrane tension changes directlytrigger surface area readjustments. If, however, Ca-mediated but nottension-mediated membrane recruitment is responsible for surface areaincreases, swelling neurons should sustain elevated levels of[Ca]_i. The purpose ofthis investigation is to determine if the[Ca]_i in swellingneurons attains levels high enough to promote exocytosis and if anysuch increase is required. Lymnaeaneurons were loaded with the Ca concentration indicator fura 2. Calibration was performed in situ using 4-bromo-A-23187 and Ca-ethyleneglycol-bis(-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA), with free Ca concentration ranging from 0 to 5 µM. Swelling perturbations (medium osmolarity reduced to 25% for 5 min)were done at either a standard[Ca]_e or very low[Ca]_e level (0.9 mM or0.13 µM, respectively). In neither case did the[Ca]_i increase tolevels that drive exocytosis. We also monitored osmomechanically drivenmembrane dynamics [swelling, then formation and reversal ofvacuole-like dilations (VLDs)] with the[Ca]_i clamped below 40 nM via1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). [Ca]_idid not change with swelling, and VLD behavior was unaffected,consistent with tension-driven,[Ca]_i-independent surface area adjustments. In addition, neurons with[Ca]_i clamped at 0.1 µM via an ionophore could produce VLDs. We conclude that, undermechanical stress, neuronal membranes are compliant by virtue ofsurface area regulatory adjustments that operate independent of[Ca]_i. The findingssupport the hypothesis that plasma membrane area is regulated in partby membrane tension.

     

Electrophysiological and Morphological Properties of Neurons Acutely Isolated from the Rostral Gustatory Zone of the Rat Nucleus of the Solitary Tract

The biophysical and morphological characteristics of acutelyisolated neurons from the rostral nucleus of the solitary tract(rNST) were investigated under current clamp conditions andcompared with the results obtained from neurons recorded inbrain slices. The passive membrane properties of the isolatedneurons were similar to rNST neurons in brain slices and theneurons maintained their morphological characteristics althoughtheir dendritic tree was truncated. The isolated neurons alsoretained their characteristic repetitive firing properties.In addition we also noted developmental changes in the intrinsicmembrane properties of the isolated neurons, such as a shorteningin action potential duration, decrease in membrane time constantand input resistance, that occurred when these parameters werecompared in neurons isolated from young (5–10 days) andolder animals. These enzymatically dispersed neurons thereforeretained both the membrane properties and morphology observedin the intact brainstem and in vitro brain slice preparation.The use of this isolated neuron preparation provides a basisfor further study of rNST neurobiology. Chem. Senses 21: 729–737,1996     

CO2 chemosensitivity in Helix aspersa: three potassium currents mediate pH-sensitive neuronal spike timing

Elevated levels of carbon dioxide increase lung ventilation in Helix aspersa. The hypercapnic response originates from a discrete respiratory chemosensory region in the dorsal subesophageal ganglia that contains CO₂-sensitive neurons. We tested the hypothesis that pH-dependent inhibition of potassium channels in neurons in this region mediated the chemosensory response to CO₂. Cells isolated from the dorsal subesophageal ganglia retained CO₂ chemosensitivity and exhibited membrane depolarization and/or an increase in input resistance during an acid challenge. Isolated somata expressed two voltage-dependent potassium channels, an A-type and a delayed-rectifier-type channel (I_KA and I_KDR). Both conductances were inhibited during hypercapnia. The pattern of voltage dependence indicated that I_KA was affected by extracellular or intracellular pH, but the activity of I_KDR was modulated by extracellular pH only. Application of inhibitors of either channel mimicked many of the effects of acidification in isolated cells and neurons in situ. We also detected evidence of a pH-sensitive calcium-activated potassium channel (I_KCa) in neurons in situ. The results of these studies support the hypothesis that I_KA initiates the chemosensory response, and I_KDR and I_KCa prolong the period of activation of CO₂-sensitive neurons. Thus multiple potassium channels are inhibited by acidosis, and the combined effect of pH-dependent inhibition of these channels enhances neuronal excitability and mediates CO₂ chemosensory responses in H. aspersa. We did not find a single "chemosensory channel," and the chemosensitive channels that we did find were not unique in any way that we could detect. The protein "machinery" of CO₂ chemosensitivity is probably widespread among neurons, and the selection process whereby a neuron acts or does not act as a respiratory CO₂ chemosensor probably depends on the resting membrane potential and synaptic connectivity. carbon dioxide     

Unique temperature-activated neurons from pit viper thermosensors

Rattlesnakes, copperheads, and other pit vipers have highly sensitive heat detectors known as pit organs, which are used to sense and strike at prey. However, it is not currently known how temperature change triggers cellular and molecular events that activate neurons supplying the pit organ. We dissociated and cultured neurons from the trigeminal ganglia (TG) innervating the pit organs of the Western Diamondback rattlesnake (Crotalus atrox) and the copperhead (Agkistrodon contortix) to investigate electrophysiological responses to thermal stimuli. Whole cell voltage-clamp recordings indicated that 75% of the TG neurons from C. atrox and 74% of the TG neurons from A. contortix showed a unique temperature-activated inward current (I_T). We also found an I_T-like current in 15% of TG neurons from the common garter snake, a species that does not have a specialized heat-sensing organ. A steep rise in the current-temperature relationship of I_T started just below 18°C, and cooling temperature-responsive TG neurons from 20°C resulted in an outward current, suggesting that I_T is on at relatively low temperatures. Ion substitution and Ca²⁺ imaging experiments indicated that I_T is primarily a monovalent cation current. I_T was not sensitive to capsaicin or amiloride, suggesting that the current did not show similar pharmacology to other mammalian heat-sensitive membrane proteins. Our findings indicate that a novel temperature-sensitive conductance with unique ion permeability and low-temperature threshold is expressed in TG neurons and may be involved in highly sensitive heat detection in snakes. snake; thermosensory; trigeminal; ion conductance     

Ultrastructural Evidence for Neuromuscular Systems in Coelenterates

Cellular interrelationships and synaptic connections in tentaclesof several species of coelenterates were examined by means ofelectron microscopy to determine if neuromuscular pathways werepresent. The presence of sensory cells, ganglion cells, epitheliomuscularcells, interneuronal synapses, and neuromuscular junctions suggeststhat neuromuscular pathways are present in coelenterates. Nakedaxons without sheath cells form several synapses en passantwith the same and with different epitheliomuscular cells aswell as with nematocytes and other neurons. Interneuronal synapsesand neuromuscular and neuronematocyte junctions have clear ordense-cored vesicles (700–1500 Å in diameter) associatedwith a dense cytoplasmic coat on the presynaptic membrane, acleft (100–300 Å in width) with intracleft filaments,and a subsynaptic membrane with a dense cytoplasmic coat. Atscyphozoan neuromuscular junctions there is a subsurface cisternaof endoplasmic reticulum, which is separated from the epitheliomuscularcell membrane by a narrow cytoplasmic gap (100–300 Åin width) . Neuromuscular junctions in coelenterates resembleen passant axonal junctions with smooth muscle in higher animals. Morphological evidence is presented for a simple reflex involvinga two-cell (sensory or ganglion-epitheliomuscular cell) or three-cell(sensory-ganglion-epitheliomuscular cell) pathway that may resultin the coordinated contraction of the longitudinal muscle intentacles of coelenterates.     

Amitriptyline reduces olfactory neuron adenylyl cyclase activity

Adenylyl cyclase plays an important role in olfactory signaltransduction. Recently, a novel type III adenylyl cyclase hasbeen localized in olfactory neurons (Pfeuffer et al., 1989;Bakalyar and Reed, 1990). Because amitriptyline (AMI), a tricyclicantidepressant, appears to have an inhibitory effect on adenylylcyclase activity in other in other neuronal tissue (Yamaokaet al., 1988; Wong et al., 1991), we measured the effect ofAMI on forskolin-stimulated adenylyl cyclase activity in membranepreparations of olfactory mucosa from adult rats. In the presenceof 5'-guanylyl-imidodiphosphate, AMI (0.5–8.0 µM)inhibited forskolin-stimulated adenylyl cyclase activity ina dose-dependent manner. To determine whether this effect wasspecific for olfactory neurons, as opposed to other cells inthe olfactory epithelium, rats were unilaterally bulbectomizedin order to reduce selectively the number of olfactory neuronson the side ipsilateral to the bulbectomy. In membrane preparationsfrom unilaterally bulbectomized animals we saw significantlylower adenylyl cyclase activity in ipsilateral olfactory mucosa,compared with adenylyl cyclase activity from non-bulbectomizedmucosa. These results indicate that AMI inhibition of adenylylcyclase activity is primariy localized in olfactory neurons.     

The Effect of Darkness on Active and Passive Transport in Chara corallina

In Chara corallina, the membrane potential may stay much morenegative than the equilibrium potential for potassium in thedark, indicating that the proton pump is operative. The highproton conductance which occurs at high external pH, as indicatedby a high membrane conductance and a membrane potential nearthe equilibrium potential for protons, is not seen in the darkat pH 11. This effect is likely to be related to inhibitionof photosynthesis since DCMU has the same effect. The effectis similar but not identical to the effect of a decreased internalpH. Key words: Light, dark, membrane potential, Chara     

Nitric Oxide as an Orthograde Cotransmitter at Central Synapses of Aplysia: Responses of Isolated Neurons in Culture

Nitric oxide serves as an orthograde synaptic cotransmitterbetween identified neurons in the cerebral ganglion of Aplysia.Nitric oxide synthase, the enzyme that produces nitric oxide,is localized in a few specific neurons in the ganglia, includingneuron C2. Guanylyl cyclase the target enzyme of nitric oxide,is found in neurons C4 and MCC, which are synaptic followersof C2. Stimulation of C2 causes a vsEPSP in these neurons thatis reduced to 50% of its amplitude by nitric oxide synthaseinhibitors and guanylyl cyclase inhibitors. The remaining portionof the vsEPSP is mediated by histamine. Thus, nitric oxide andhistamine act as orthograde cotransmitters in producing thevsEPSP. Both cotransmitters cause closure of a background potassiumchannel, which depolarizes the neuron and enhances its responseto synaptic inputs. Exogenous nitric oxide (released by nitricoxide donor molecules) and histamine mimic the vsEPSP's depolarizationand decreased membrane conductance. When neurons C4 or MCC areisolated in cell culture they respond just as they do in theganglion, i.e., the nitric oxide response but not the histamineresponse is blocked by guanylyl cyclase inhibitors, and themembrane conductance is decreased by both histamine and nitricoxide. Aplysia hemolymph partially suppresses the response tonitric oxide, due to nitric oxide scavenging by hemocyanin,which contains copper and is the equivalent of hemoglobin. NeuronC2 followers that are hyperpolarized by histamine are insensitiveto nitric oxide. Thus, only select follower neurons respondto both transmitters.     

NADH-Linked Ferricyanide and Cytochrome c Reduction Activities in Corn Root Plasma Membrane

Corn root plasma membrane catalyzed NADH reduction of ferricyanideand cytochrome c over a wide pH range. At pH 7.5, apparent K_msof NADH-cytochrome c pair were significantly lower than thoseof NADH-ferricyanide pair. FMN and polylysine respectively enhancedthe reduction of ferricyanide and cytochrome c. Yet, polyaspartatedecreased the ferricyanide reduction. NADH oxidation observedin the presence of both ferricyanide and cytochrome c was significantlyslower than the sum of rates obtained with individual acceptors.The results suggest that the membrane may contain differentbut not totally independent reduction sites for cytochrome cand ferricyanide. (Received April 13, 1993; Accepted August 23, 1993)     

Effect of chronically elevated CO2 on CA1 neuronal excitability

To study the effect of chronically elevated CO₂ on the excitability and function of neurons, we exposed mice to 7.5–8% CO₂ for 2 wk (starting at 2 days of age) and examined the properties of freshly dissociated hippocampal neurons. Neurons from control mice (CON) and from mice exposed to chronically elevated CO₂ had similar resting membrane potentials and input resistances. CO₂-exposed neurons, however, had a lower rheobase and a higher Na⁺ current density (580 ± 73 pA/pF; n = 27 neurons studied) than did CON neurons (280 ± 51 pA/pF, n = 34; P < 0.01). In addition, the conductance-voltage curve was shifted in a more negative direction in CO₂-exposed than in CON neurons (midpoint of the curve was –46 ± 3 mV for CO₂ exposed and –34 ± 3 mV for CON, P < 0.01), while the steady-state inactivation curve was shifted in a more positive direction in CO₂-exposed than in CON neurons (midpoint of the curve was –59 ± 2 mV for CO₂ exposed and –68 ± 3 mV for CON, P < 0.01). The time constant for deactivation at –100 mV was much smaller in CO₂-exposed than in CON neurons (0.8 ± 0.1 ms for CO₂ exposed and 1.9 ± 0.3 ms for CON, P < 0.01). Immunoblotting for Na⁺ channel proteins (subtypes I, II, and III) was performed on the hippocampus. Our data indicate that Na⁺ channel subtype I, rather than subtype II or III, was significantly increased (43%, n = 4; P < 0.05) in the hippocampi of CO₂-exposed mice. We conclude that in mice exposed to elevated CO₂, 1) increased neuronal excitability is due to alterations in Na⁺ current and Na⁺ channel characteristics, and 2) the upregulation of Na⁺ channel subtype I contributes, at least in part, to the increase in Na⁺ current density. sodium ion channels; oxygen deprivation     

Neuropeptides in the Cnidaria

This chapter reviews experimental evidence for peptides actingas transmitters or morphogens in the Cnidaria. A wide rangeof brain/gut peptides have been localized immunohistochemicallyto specific neuronal populations in Hydra. These include gastrin/CCK,substance P, neurotensin, bombesin, oxytocin/vasopressin andFMRFamide. In most cases the amino-acid sequences of the cnidarianpeptides are somewhat different from their mammalian counterparts.The functions of these peptides in Hydra are unknown. A seriesof neuropeptides with the carboxy-terminus, Arg-Phe-amide, isubiquitous within the phylum. Immunoreactivity to antisera againstRFamide is seen in two characteristic neuronal subpopulations;a sensory and a ganglionic cell type. Two of these peptideshave now been isolated and sequenced. One has the structure,pGlu-Gly-Arg-Phe-amide (Antho-RFamide) and is found in severalanthozoans, a second with the sequence pGlu-Leu-Leu- Gly-Gly-Arg-Phe-amide(Poly A peptide) is found in the hydrozoan Polyorchis. Arg-Pheamidepeptides have excitatory effects on both muscles and neuronalsystems. In the anthozoans, RFamide peptides can increase thetone, contraction amplitude and frequency of a number of smoothmuscle systems. Additionally, in the anemone Calliactis, applicationof Antho-RFamide can dramatically increase the firing rate inone of the ectodermal conducting systems, the SSI. In Polyorchisseveral RFamides produce long duration spike trains in motorneurons that may or may not be associated with membrane depolarization.A peptide called ‘head activator’ (pGlu-(Pro)j-(Gly)8-Ser-Lys-Val-Ile-Leu-Phe)can induce the formation of a new head when it is released athigh concentrations at the cut surface of the column of Hydra.It acts by committing stem cells to become head specific neurons.     

Inward rectifier K+ channel Kir2.3 is localized at the postsynaptic membrane of excitatory synapses

Classical inwardly rectifyingK⁺ channels (Kir2.0) are responsible for maintaining theresting membrane potential near the K⁺ equilibriumpotential in various cells, including neurons. Although Kir2.3 is knownto be expressed abundantly in the forebrain, its precise localizationhas not been identified. Using an antibody specific to Kir2.3, weexamined the subcellular localization of Kir2.3 in mouse brain. Kir2.3immunoreactivity was detected in a granular pattern in restricted areasof the brain, including the olfactory bulb (OB). Immunoelectronmicroscopy of the OB revealed that Kir2.3 immunoreactivity wasspecifically clustered on the postsynaptic membrane of asymmetricsynapses between granule cells and mitral/tufted cells. Theimmunoprecipitants for Kir2.3 obtained from brain contained PSD-95 andchapsyn-110, PDZ domain-containing anchoring proteins. In vitro bindingassay further revealed that the COOH-terminal end of Kir2.3 isresponsible for the association with these anchoring proteins.Therefore, the Kir channel may be involved in formation of the restingmembrane potential of the spines and, thus, would affect the responseof N-methyl-D-aspartic acid receptor channels atthe excitatory postsynaptic membrane.

     

Oxidative stress decreases pHi and Na(+)/H(+) exchange and increases excitability of solitary complex neurons from rat brain slices

Putative chemoreceptors in the solitary complex (SC) are sensitive to hypercapnia and oxidative stress. We tested the hypothesis that oxidative stress stimulates SC neurons by a mechanism independent of intracellular pH (pH_i). pH_i was measured by using ratiometric fluorescence imaging microscopy, utilizing either the pH-sensitive fluorescent dye BCECF or, during whole cell recordings, pyranine in SC neurons in brain stem slices from rat pups. Oxidative stress decreased pH_i in 270 of 436 (62%) SC neurons tested. Chloramine-T (CT), N-chlorosuccinimide (NCS), dihydroxyfumaric acid, and H₂O₂ decreased pH_i by 0.19 ± 0.007, 0.20 ± 0.015, 0.15 ± 0.013, and 0.08 ± 0.002 pH unit, respectively. Hypercapnia decreased pH_i by 0.26 ± 0.006 pH unit (n = 95). The combination of hypercapnia and CT or NCS had an additive effect on pH_i, causing a 0.42 ± 0.03 (n = 21) pH unit acidification. CT slowed pH_i recovery mediated by Na⁺/H⁺ exchange (NHE) from NH₄Cl-induced acidification by 53% (n = 20) in -buffered medium and by 58% (n = 10) in HEPES-buffered medium. CT increased firing rate in 14 of 16 SC neurons, and there was no difference in the firing rate response to CT with or without a corresponding change in pH_i. These results indicate that oxidative stress 1) decreases pH_i in some SC neurons, 2) together with hypercapnia has an additive effect on pH_i, 3) partially inhibits NHE, and 4) directly affects excitability of CO₂/H⁺-chemosensitive SC neurons independently of pH_i changes. These findings suggest that oxidative stress acidifies SC neurons in part by inhibiting NHE, and this acidification may contribute ultimately to respiratory control dysfunction. hyperoxic hyperventilation; O₂ toxicity; pH regulation; brain stem; reactive oxygen species     

Membrane Lipids: What Membrane Physical Properties are Conserve during Physiochemically-Induced Membrane Restructuring?

SYNOPSIS. Current theories assert that organisms finely adjustthe order, or fluidity, of their cellular membranes in responseto changes in their physiochemical environment (e.g., pressure,temperature, salinity, etc.). However, membrane order may notbe the only property that is conserved. The most commonly observedalterations in cell membrane composition under conditions ofaltered physiochemical environment, namely changes in the phosphatidylethanolamine/phosphatidylcholine(PE/PC) ratio and the content of highly unsaturated acyl chains,are difficult to fully reconcile with the conservation of membraneorder alone. This report reviews the literature concerning twoproperties of membranes that may play vital roles in the adaptationof cellular membranes to changing environments: a) the tendencyof membranes to relax into the reversed hexagonal phase andb) the occurrence and structure of lipid-driven domains withinthe membrane. The tendency of a membrane to form the reversedhexagonal phase is a property central to a variety of importantcellular events. This tendency is tightly regulated by variationof the ratio of hexagonal phase-forming lipids to lamellar phase-forminglipids in the membrane. In most animal cells, this correspondsto the PE/PC ratio. Highly unsaturated acyl chains, in conjunctionwith cholesterol, modulate the occurrence and structure of lipid-drivenmembrane domains. These membrane domains are also criticallyinvolved in a number of key cellular processes. The changesin membrane lipid composition that occur during adaptation tothe environment may be required for the preservation of thetendency to form nonlamellar phases and of the occurrence andspecific structure of domains within the membrane, in additionto overall membrane order.     

Functional and Structural Parallels in Crustacean and Drosophila Neuromuscular Systems

Comparison of morphological and physiological phenotypes ofrepresentative crustacean motor neurons, and selected motorneurons of Drosophila larval abdominal muscles, shows severalfeatures in common. Crustacean motor nerve terminals, and thoseof Drosophila, possess numerous small synapses with well-definedactive zones. In crustaceans, neurons that are more tonicallyactive have markedly varicose terminals; synapses and mitochondriaare selectively localized in the varicosities. Phasic motoraxons have filiform terminals, sometimes with small varicosities;mitochondrial content is less than for tonic axons, and synapsesare distributed along the terminals. Tonic axons generate smallexcitatory potentials which facilitate strongly at higher frequencies,and which are resistant to depression. Thephasic neurons generatelarge excitatory potentials which exhibit relatively littlefrequency facilitation, and depress rapidly. In Drosophila,counterparts of crustacean phasic and tonic motor neurons havebeen found, but the differentiation is less pronounced. It isinferredthat cellular factors regulating the number of participatingsynapses and the probability of quantal release are similarin crustaceans and Drosophila, and that advantage can be takenof this in future to develop experiments addressing the regulationof synaptic plasticity.     

Formation of Axon to Myocyte Contacts in Drosophila Cell Cultures

Cultures of Drosophila embryonic cells offer new opportunitiesfor studying myoneural junctions. In culture, neuroblasts andmyoblasts differentiate and yield neurons and myocytes. Someaxons grow across the surface of the culture vessel and attachto myocytes, forming functional myoneural junctions. Therefore,all stages in junction formation may be examined in vitro underconditions where pharmacological, electrophysiological, andother commonplace approaches are facilitated. This method offersan additional, most powerful approach for studying the junctions,that of genetic analysis. Drosophila mutations may be soughtwhich affect junction formation and function. Altered cellsand junctions from mutants may then be compared to those fromwild-type animals in order to dissect the gene-directed stepsunderlying junction phenomena.     

Specialized receptor neurons for pheromones and host plant odors in the boll weevil, Anthonomus grandis Boh. (Coleoptera: Curculionidae)

Antennal olfactory receptor neurons in the boll weevil, Anthonomusgrandis, were investigated through single neuron recordings.Receptor neurons for both pheromone components and host plantodors were associated with type I sensilla within the sensoryband regions. Nine types of receptor neurons were identified,based on their responsiveness to the four aggregation pheromonecomponents and selected host plant odors. Three receptor neurontypes responded to either compound I, II or IV of the aggregationpheromone. Dose—response curves were similar for eachof these receptor neuron types, which differed only in theirkey compound. In each instance, I neurons responded primarilyto (+)-I, the optical isomer produced by the boll weevil whichwas found to be active in field tests. Receptor neurons forII also responded to a lesser degree to III, its aldehydic analog,at the same stimulus load. Six additional receptor neuron typesresponded to selected host plant odors: ß-caryophyllene,trans-2-hexen-l-o1 and other six carbon alcohols and aldehydes,trans-ß-ocimene, benzaldehyde, linalool, and B-bisabolol.These neurons were as responsive as, or in some cases more responsiveat the same stimulus load as receptor neurons for pheromonecomponents. Receptor neurons responsive to six-carbon alcoholsand aldehydes were generally most responsive to trans-2-hexen-l-ol.Receptor neurons for other plant odors responded principallyto only one compound among the odorants tested. However, responsesof these neurons were not uniform, suggesting possible specializationfor other unidentified key odorants. Comparisons were also madebetween single neuron and electroantennogram responses. Theresults indicate that the boll weevil, a narrowly oligaphagousinsect, detects its host plant at some distance, and utilizesinformation about a wide range of chemical structures in itsolfactory-mediated behavior.     

Maturation of neuronal excitability in hippocampal neurons of mice chronically exposed to cyclic hypoxia

To examine the effects of chronic cyclichypoxia on neuronal excitability and function in mice, we exposed miceto cyclic hypoxia for 8 h daily (9 cycles/h) for ~2 wk (startingat 2-3 days of age) and examined the properties of freshlydissociated hippocampal neurons obtained from slices. Compared withcontrol (Con) hippocampal CA1 neurons, exposed neurons (CYC) hadsimilar resting membrane potentials (V_m) andaction potentials (AP). CYC neurons, however, had a lower rheobase thanCon neurons. There was also an upregulation of the Na⁺current density (333 ± 84 pA/pF, n = 18) in CYCcompared with that of Con neurons (193 ± 20 pA/pF,n = 27, P < 0.03). Na⁺channel characteristics were significantly altered by hypoxia. Forexample, the steady-state inactivation curve was significantly morepositive in CYC than in Con (60 ± 6 mV, n = 8, for CYC and 71 ± 3 mV, n = 14, for Con,P < 0.04). The time constant for deactivation(_d) was much shorter in CYC than in Con (at 100 mV,_d=0.83 ± 0.23 ms in CYC neurons and 2.29 ± 0.38 ms in Con neurons, P = 0.004). We conclude thatthe increased neuronal excitability in mice neurons treated with cyclichypoxia is due to alterations in Na⁺ channelcharacteristics and/or Na⁺ channel expression. Wehypothesize from these and previous data from our laboratory (Gu XQ andHaddad GG. J Appl Physiol 91: 1245-1250, 2001) that thisincreased excitability is a reflection of an enhanced central nervoussystem maturation when exposed to low O₂ conditions inearly postnatal life.

     

The Role of Serotonin in Tritonia diomedea

The within-swim pattern of cycle periods in Tritonia swimmingchanged when the behavior was repeatedly elicited suggestingthat an excitatory process reaches a ceiling or wanes over repeatedtrials. Exposure to subthreshold stimuli enhanced swimming inresponse to a subsequent super-threshold stimulus, perhaps usinga similar excitatory process. In reduced preparations, subthresholdstimuli increased action potential activity in identified serotonergicneurons. Finally, stimulating serotonergic neurons enhanceda fictive swimming pattern, much like subthreshold stimuli enhancedthe swimming behavior. Both within-swim and across-swim changesin the swimming behavior may be caused by increased activityin identified serotonergic neurons. Comparative study suggeststhat ancestral serotonergic systems facilitated network oscillationsfor the production of rhythmic behaviors such as feeding andlocomotion. This concept of serotonin as oscillatizer is usedto explain the role of serotonergic neurons in Tritonia. Implicationsfor human mental health are discussed.     

Neurophysiological responses of pheromone-sensitive receptor neurons on the antenna of Trichoplusia ni (Hubner) to pulsed and continuous stimulation regimens

Both the frequency and the temporal pattern of action potentialproduction in an insect olfactory receptor neuron are stronglyaffected by odorant composition and the time course over whichstimulus concentration varies. To investigate the temporal characteristicsof the neurophysiological responses of these neurons, we deviseda stimulus delivery system that allows us to repeatedly presentwell-mixed, constant concentration odor pulses with relativelysharp onsets and offsets. Here we compare neurophysiologicalresponses to several different stimulation regimens, includingpulses of different durations and repetition rates. During stimulationwith high concentrations of pheromone, the temporal patternof neural activity from olfactory receptor neurons on the antennaof Trichoplusia ni (Hübner) is characterized by an initialphasic period (100–200 ms), followed by a tonic periodwhich is typically maintained for the remaining duration ofthe stimulus. Different olfactory receptor neurons appear tovary among themselves in the relative distribution between thephasic and tonic portions of the overall discharge. During stimulationregimens involving rapid repeated pulses of odorants, a portionof the phasic response levels is preserved during each pulse.Consequently, T. ni males probably detect much of the fluctuationin concentration of pheromone that may normally occur downwindfrom the site of pheromone release.