1,721,110 research outputs found
SYNAPTIC TRANSMISSION PROPERTIES OF THE CYTONEURAL JUNCTION IN THE ISOLATED FROG LABYRINTH.
No full textSubthreshold postsynaptic potentials (EPSPs) were recorded intracellularly at the cytoneural junction of the frog posterior canal. In single units EPSPs display highly variable size, so it is not clear whether they are generated by release of single quanta of transmitter and whether large ones represent giant events, multiquantal events or the random summation of independently occurring unitary events. Therefore their quantal nature was investigated by performing a statistical analysis of time intervals and peak amplitudes. In general the distributions of EPSP peak amplitudes were well fit by lognormal functions, and intervals were exponentially distributed. As EPSP rate rose above 100/s, peak amplitude histograms became broader and brief intervals became less frequent than expected. A procedure of Wiener filtering, based on estimating the event waveform from the autocorrelation of the synaptic recording was set up to sharpen the signal. After filtering, peak amplitude histograms were much less sensitive to EPSP rate and time intervals were exponentially distributed as expected for random release. The number of "multiquantal" events, as estimated from the area outside the main peak in amplitude histograms, was reduced by Wiener filtering and was in general consistent with the expectation that more than one independent event occurred within the time interval corresponding to the duration of the single event. This suggests that the events are uni-quantal, random and independent, i.e. miniature EPSPs (mEPSPs). mEPSP rate of occurrence was modified by mechanical stimulation of the canal, by activation of the efferent inhibitory or facilitatory systems and by changing extracellular Ca2+ concentration (0-9 mM). Peak amplitude histograms were almost unaffected after Wiener filtering also under the new experimental conditions and time interval histograms remained exponentially distributed. Following bath application of ATP (4x104-10-3 M) or its analog ATP-yS (10-3 M) a consistent though transient increase in resting mEPSP rate was observed in about 50% of the units examined. ATP effect was absent in all fibres where efferent stimulation produced inhibition and it was present in all fibres under the control of the facilitatory efferent system. In these fibres, efferent facilitation, measured after the effect of ATP (or ATP-yS) had vanished, was consistently reduced with respect to facilitation evoked in control solution. Results suggest that: 1) mEPSPs are true miniature, uniquantal events; 2) "giant" events do not occur at the cytoneural junction under the conditions here investigated; 3) experimental procedures capable of markedly modulating the rate of transmitter release do not alter its basic mechanism; 4) ATP may be the efferent facilitatory transmitter at the basal pole of the hair cell
Microgravity on frog labyrinth sensory function
No full textIL PROGETTO HA DURATA TRIENNALE, E' INIZIATO il 1° GENNAIO 2006 E TERMINERA' il 31 DICEMBRE 2009.
Vestibular information is directly concerned with gravity; thus changes in gravity conditions are expected to interfere with vestibular function. The project collects several researchers who have long been involved in studying the structural, morphological and functional aspects of vestibular organs, and is aimed at evaluating possible effects of microgravity on vestibular organ function. The project focusses on interference with: (i) sensory signal processing (transfer function and mathematical modelling of signal processing steps); (ii) ion distribution and membrane currents, especially calcium currents; (iii) structural and subcellular organization of the cytoneural junction; (iv) correlations between structural and functional alterations. The effects of microgravity are studied in the labyrinth, isolated from the frog head after exposing the animal to a conditioning period (4-6 hr) in simulated zero-gravity condition (random positioning machine)
Ionic mechanisms sustaining inhibition in the frog labyrinth
No full textActivation of the efferent system in the isolated frog labyrinth inhibits the afferent EPSP and spike discharge by releasing ACh. When Na-isethionate is substituted for external Cl-, inhibition disappears, thus indicating that ACh opens the Cl channel. Experiments in which increasing amounts of Cl- in the bath are replaced by equivalent amounts of Br-, NO2-, NO3-, CH3CO2-, SO4-- indicate that all these anions are able to penetrate the postsynaptic membrane, since full inhibition can invariably be evoked. At present it is possible to set a lover limit (4.52 A°) to the size of the inhibitory channel. It appears that Cl- is passively distributed across the hair cell membrane since long lasting treatments with metabolic inhibitors such as DNP (0.1mM) or NaN3 (up to 10-3 M) do not affect inhibition at all. In the presence of ammonium acetate (10mM), which is known to block active Cl-pumping in other systems, inhibition was similarly unaffected
MECCANISMI IONICI DELL'INIBIZIONE EFFERENTE NEL LABIRINTO DI RANA
No full textNel labirinto isolato di rana l'attivazione del sistema efferente, ottenuta mediante stimolazione dei monconi dei nervi anteriore ed orizzontale, inibisce la scarica afferente di EPSPs e potenziali d'azione derivabile da singole fibre del canale posteriore. L'effetto é mediato dalla liberazione di ACh, che aumenta la conduttanza della membrana delle cellule cigliate allo ione Cl; infatti la sostituzione del Cl- con l'anione isetionato provoca la scomparsa dell'inibizione. Per determinare le dimensioni dei canali sinaptici aperti dalla ACh, il Cl viene sostituito nel liquido extracellulare con anioni di diametro idrato crescente: Br-, NO2- , NO3 -, CH3 CO-2, SO4--, CH3 CH2 CO2 -, CH3(CH2)2CO2- , CH3 (CH2 )3 CO2-, benzoato. I risultati sperimentali hanno dimostrato che questi ioni passano liberamente attraverso la membrana postsinaptica, infatti la loro presenza non influenza l'effetto inibitorio. D'altra parte, quando il Cl viene sostituito con il citrato, l'inibizione scompare (in presenza di ioni che chelano il Calcio, la concentrazione di questo ione era adeguatamente aumentata). Dal momento che gli ioni benzoato e citrato misurano rispettivamente 6.5 e 7.5 A°, il canale della membrana postsinaptica ha probabilmente una dimensione compresa tra questi due valori. L'azione di inibitori metabolici quali DNP (0.1 mM) e NaN3 (1 mM) o di bloccanti selettivi della pompa del Cl quali acetazolamide (5 mM) o acetato di ammonio (10 mM) non modifica l'efficacia della sinapsi inibitoria. Appare quindi difficile interferire con i meccanismi che mantengono il gradiente di concentrazione del Cl- nelle cellule cigliate
Cholinergic-through fibers in the rat superior cervical ganglion
No full textIn questo lavoro, nell'ambito della descrizione delle connessioni tra fibre pregangliari e neuroni principali gangliari, è stata messa in evidenza la presenza di un contingente di fibre pregangliari che attraversano senza interrompersi il ganglio cervicale superiore di ratto, proiettando nei tronchi postgangliari. La stimolazione antidromica di un tronco post¬gangliare produce infatti l'attivazione sinaptica di un numero molto esteso di neuroni gangliari. Tale attivazione si manifes¬ta con la comparsa di un potenziale d'azione composito, mediato sinapticamente, negli altri tronchi postgangliari e con la liberazione di una notevole quantità di acetilcolina dal gan¬glio eserinizzato, pari al 25% della quantità di mediatore liberata nelle stesse condizioni dalla stimolazione convenzion¬ale delle fibre pregangliari del tronco simpatico
Effect of barium ions on the sensory activity of the isolated frog labyrinth
No full textBarium ions are known to interact with the K+ conductance in several preparations as well as to substitute for Ca2+ in resting and evoked transmitter release. The primary afferent resting activity ìn the frog labyrinth is a Ca2+-dependent process, which is extremely sensitive to the external K level; it seems thus interesting to investigate the effect of Ba2+ on the EPSP and spike discharges recorded from single fibres of the posterior nerve in the isolated preparation. Control recordings were performed in Ringer solution of the following ionic composition (mM): NaCl 116, KCl 2.5, CaCl2 1.8, TrisCl 5. The changes in bathing saline were usually accompanied by appropriate modifications to the NaCI concentration to keep the solution isoosmolar.
When BaCl2 was added to a Calcium free Ringer solution the resting activity was preserved; moreover, on increasing Ba2+ concentration (0.9 - 4 mM), an obvious facílitation of the EPSP and spike discharges was observed after 3 - 5 minutes. The spike shape and duration were unaffected by this treatment. In 15 mM Mg2+- Calcium-free Ringer solution the resting activity was completely abolished and no recovery was observed by gradually rising the Ca2+ concentration (1.8 - 18.6 mM). However, when Ba2+ (1.8 mM) was added to the bathing medium the Mg2+ block was fully releaved, independently of the Ca2+ levels, though no facilitatìon was observed. A sìmìlar recovery was not obtaìned in 15 mM Mg2+ - 18.6 mM Ca2+ - 7.5 mM K+ solution, whích rules out the possibility that recovery in spike activity might be simply related to a Ba2+ induced depolarization of the hair cell.
Finally, when the resting activity was completely abolished by a 3 - 5 mM CoCl2-Calcium-free Ringer solution, some recovery was obtained in the presence of BaCl2 (1.8 mM), whereas an isosmolar amount of CaCl2 was completely uneffective. The time of recovery increases and facilitation decreases on increasing CoCl2 concentration.
It follows that Ba2+ions have a powerful and persistent effect on the transmitter release that may reflect mainly their ability to penetrate the Ca-channel more easily than Ca2+ ions. Ba2+ action might also be related to a depolarizing effect on the hair cell membrane, possibly due to the block of a Ca2+- dependent K+ conductance. On the other hand, Ba2+ ions proved not to interfere with the K+ channels of the myelinated sensory fibres
THE EFFECT OF HIGH POTASSIUM AND RUBIDIUM CONCENTRATIONS ON EPSP AND SPIKE DISCHARGES OF THE FROG LABYRINTH POSTERIOR CANAL
No full textThe effects of high perilymphatic K and Rb concentrations (20 mM) on the posterior canal resting discharge have been examined in the isolated frog labyrinth. Both K and Rb usually increase the resting EPSP and spike frequencies recorded intra-axonally at the level of the posterior nerve. The afferent K-Rb facilitation is accompanied by a reduction in size and broadening of the spikes; no repetitive action potentials, however, occur in the fibre. Therefore, the higher resting receptor activity is exclusively related to the increase in the EPSP emission rate. The afferent discharge facilitation was inversely related to the fibre's initial resting activity. The K effect was larger than the Rb effect, and both K and Rb actions were more conspicuous in some units (high sensitive) than in others (low sensitive). High K and Rb media steadily depolarize the basolateral receptor membrane. The increase in EPSP rate observed under these conditions indicates that the reduction in hair celi membrane potential sustains an increased transmitter release at the junction. This presynaptic effect is most likely related to larger amounts of Ca entering the cell during maintained depolarization
Meccanismo quantale di liberazione del mediatore alla giunzione citoneurale nel labirinto di rana
No full textThe cytoneural junctions of the frog labyrinth posterior canal usually show different (m)EPSP resting rates (10-300/s). A statistical analysis of (m)EPSPs, recorded intracellularly from the posterior nerve, is carried out to investigate the basic mechanisms of transmitter release. In units with low resting rates (<100/s) and during mechanical as well as efferent inhibitory stimulation, where a direct evaluation of the event parameters is possible, peak amplitude distributions are continuous, unimodal and fitted by lognormal functions, while time interval distributions between events are monoexponential. For higher resting rates, and systematically during mechanical excitation or activation of the facilitatory efferent system, single events extensively overlap. To reduce summation, a Wiener filter is built to sharp the original signal. The Wiener filter transfer function is calculated from the EPSP waveform estimated by the autoregressive fit to the autocorrelation of the original signal. This procedure yields peak size distributions well fitted by lognormal functions in all the situations tested. After filtering, time intervals are still monoexponential at rest as well as during the evoked release. Results suggest that only the rate of transmitter release, but not its basic mechanism, is altered during sustained synaptic activity. Furthermore, the Ca2+-free EGTA-solution slightly affects efferent inhibition and depresses facilitation. The high Ca2+-level enhances inhibition and depresses facilitation
The effect of barium and some channel blockers on sensory discharge of the frog labyrinth posterior canal recorded at rest and during rotation
No full textThe effect on the afferent synaptic transmission of Ba2+, Sr2+, tetraethylammonium (TEA) and 4-aminopyridine (4-AP) has been investigated in the isolated frog labyrinth by intracellularly recording the posterior canal resting and evoked receptor discharge. BaCl2 (0.3 mM) or SrCl2 (1.8 mM) substitution for normal external CaCl2 restored the afferent activity without affecting the membrane potential of the sensory fibres. On further increasing Ba2+ concentration (0.5-5 mM) a dose-dependent increase in the EPSP and spike discharges was observed in all the units examined. Ba2+ (1.8-4 mM) removed the depression of the sensory activity operated by CoCl2 (3 mM), while its facilitatory effect was completely antagonized by raising Ca2+ concentration (up to 10 mM). TEA (20 mM) elicited a clear-cut increase in the EPSP and spike discharges which, however, was less consistent than that produced by Ba2+ (1 mM). The increment in spike frequency produced by TEA and Ba2+ proved to be inversely related to the initial resting firing level of the different units. The 4-AP (4-20 mM) effect resulted in a decrease of the sensory activity, which was fully restored by TEA or Ba2+. In normal saline a linear relationship was found between the mean unit resting discharge and the respective excitatory peak response during sinusoidal rotation (0.1-0.3 Hz). This result suggest that the mechanical response is mainly determined by the unit resting level. Consistent evoked responses were obtained under TEA and Ba2+ treatment which proved to depend linearly on the new mean resting discharge of the different units. Conversely, a reduced evoked response was invariably observed in all the fibres tested in the presence of 4-AP. The present results suggest that Ba2+ and Sr2+ may substitute for Ca2+ in the transmitter release process at the cyto-neural junction, the ability of Ba2+ being even larger than that of Sr2+ and Ca2+ itself. The effects of TEA and 4-AP are discussed in the light of their possible interaction with the presynaptic K+-currents recently described in hair cells
High perilymphatic potassium and rubidium levels modulate the sensory resting discharge in the isolated frog labyrinth
No full textThe effects of high perilymphatic K+ and Rb+ concentrations (20 mM) on the posterior canal resting discharge have been examined in the isolated frog labyrinth. Both K+ and Rb+ produced an increase in spike frequency which is sustained by a parallel increase in the excitatory postsynaptic potential (EPSP) emission rate. High K+ and Rb+ levels reduce the size of the afferent spike, broaden its time course but do not induce repetitive activity at the axon level. The K+ and Rb+ facilitation proved to be inversely related to the fibre's initial resting activity. The facilitation produced by increased K+ was usually larger than that in Rb+ solution. High sensitive and low sensitive units with similar low resting discharge were detected in relation to their response either to K+ or Rb+. The effects of both ions can be explained on the basis of their interactions with the presynaptic hair cell currents
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