BW284c51 blocks nicotinic acetylcholine receptors transplanted to Xenopus oocytes.

We have studied the effects of BW284c51 on the function of Torpedo nicotinic acetylcholine (Ach) receptors (nAchRs) transplanted to Xenopus laevis oocytes. BW284c51 reversible inhibited Ach-elicited currents (IAch) in a concentration-dependent manner, increased IAch desensitisation and changed the Ach concentration-dependence of the IAch from a two-site to a single-site Hill equation, without affecting the EC50. These effects were only present at hyperpolarising potentials, suggesting that nAchR blockade by BW284c51 is non-competitive and likely due to an open channel block as the principal mechanism.

Toxicity of binary mixtures of cadmium-copper and carbendazim-copper to the nematode Caenorhabditis elegans.

For ecological risk assessment, the additive model may be used to empirically predict toxic mixture effects. Detailed toxicity tests were performed to determine whether effects of mixtures of copper-cadmium and copper-carbendazim on Caenorhabditis elegans were similar to the effects of the individual compounds. Effects on the course of reproduction, the length of the juvenile period, the length of the reproductive period, and body length were analyzed. Dose-response data were compared to the additive model and tested for four deviation patterns from additivity: No deviation, synergistic/antagonistic deviation, dose ratio-dependent deviation, dose level-dependent deviation. During the exposure, the cadmium-copper effect on reproduction changed from a synergistic, to a dose ratio-dependent deviation from additivity. More cadmium in the mixture decreased the toxicity and more copper increased the toxicity. The effect of copper-carbendazim on reproduction was synergistic at low dose levels and antagonistic at high dose levels and independent of time. Mixture effects on the juvenile and reproductive period were similar to single component effects. It was concluded that the observed time-dependence of toxic interactions was small and that interactions on the timing of reproduction were not found. The additive model underestimated mixture effects on reproduction and body length.

Functional transplantation of chloride channels from the human syncytiotrophoblast microvillous membrane to Xenopus oocytes.

The materno-fetal transfer of metabolites and nutrients requires the operation of specific transport mechanisms through syncytiotrophoblast membranes. Electrophysiological studies on these cells are scarce and, because of their syncytial nature, whole-cell current recordings have not been carried out. We have now studied whether or not ion channels from the human syncytiotrophoblast microvillous (hSM) membrane can be transplanted to Xenopus oocytes. Sixty-two percent of hSM-injected oocytes displayed lower resting potential and higher membrane conductance than uninjected cells. The increased membrane conductance was due to the incorporation of Cl(-) channels, because neither replacing Na(+) in the bathing solution by N-methyl- D-glucamine or K(+), nor withdrawing Ca(2+) had any significant effect on the currents elicited by voltage pulses. In contrast, substitution of Cl(-) by different anions markedly affected the membrane conductance, giving an anion selectivity sequence of I(-)>Br(-)>Cl(-)>methanosulfonate congruent with gluconate. In addition, disulfonic stilbenes and gluconate, but not anthracene-9-carboxylic acid, blocked the transplanted channels. These properties are compatible with those of placental Cl(-) "maxi" channels. It is concluded that functional Cl(-) channels from the hSM become effectively incorporated into the Xenopus oocyte membrane, where their function can be studied in detail.

Protein orientation affects the efficiency of functional protein transplantation into the xenopus oocyte membrane.

Xenopus oocytes incorporate into their plasma membrane nicotinic acetylcholine receptors (nAChRs) after intracellular injection of lipid vesicles bearing this protein. The advantage of this approach over the classical oocyte expression system lies in the transplantation of native, fully processed proteins, although the efficiency of functional incorporation of nAChRs is low. We have now studied the incorporation into the oocyte membrane of the Torpedo chloride channel (ClC-0), a minor contaminant protein in some nAChR preparations. nAChR-injected oocytes incorporated functional ClC-0: i) in a higher number than functional nAChRs; ii) retaining their original properties; and iii) with a right-side-out orientation in the oocyte membrane. In an attempt to elucidate the reasons for the low efficiency in the functional incorporation of nAChRs into the oocyte membrane, we combined electrophysiological and [125I]alpha-bungarotoxin-binding experiments. Up to 3% of injected nAChRs were present in the oocyte plasma membrane at a given time. Thus, fusion of lipoproteosome vesicles to the oocyte plasma membrane is not the limiting factor for an efficient functional transplantation of foreign proteins. Accounting for the low rate of functional transplantation of nAChRs is their backward orientation in the oocyte membrane, since about 80% of them adopted an out-side-in orientation. Other factors, including differences in the susceptibility of the transplanted proteins to intracellular damage should also be considered.

Functional incorporation of exogenous proteins into the Xenopus oocyte membrane does not depend on intracellular calcium increase.

Fusion of membranes occurs in diverse biological events and, in most cases, Ca2+ greatly augments its rate. The aim of this work was to study the role played by Ca2+ when transplanting exogenous proteins into Xenopus oocyte membranes. Lipid vesicles carrying nicotinic acetylcholine (ACh) receptors (nAChRs) from Torpedo electroplaques were injected into oocytes. The time course of nAChR incorporation was assessed by recording ACh-evoked currents at different times from injection. An incorporation peak was found at 16 h, but responses were maintained for over 48 h. To assess the role played by Ca2+, two groups were considered: control and chelator-loaded oocytes. In the latter group, cells were incubated with 50 microM 1,2-bis (2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetrakis(acetoxymethyl)ester (BAPTA-AM) or loaded with ca. 5 nmol ethyleneglycol-bis (beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) 2 h before nAChR injection. Both groups responded to ACh, although the current amplitude was smaller in chelator-loaded than in control cells. These results indicate that the slow fusion of lipoproteosome vesicles with the oocyte plasma membrane does not depend on intracellular Ca2+ increase and therefore belongs to the type called "constitutive". This membrane fusion process is thus different from those involved in resealing of disrupted oocyte membranes or in the fusion of cortical granules with the egg membrane.

Hemispheric asymmetry of macroscopic and elementary calcium signals mediated by InsP3 in Xenopus oocytes.

1. The mechanisms underlying hemispheric asymmetry of the inositol 1, 4,5-trisphosphate (InsP3)-calcium signalling pathway in Xenopus oocytes were examined by fluorescence imaging of calcium signals and recording calcium-activated Cl- currents (ICl,Ca) evoked by intracellular calcium injections and photorelease of InsP3. 2. The maximal ICl,Ca evoked by strong photorelease of InsP3 was 8 times greater in the animal than the vegetal hemisphere, but the average threshold amounts of InsP3 required to evoke detectable currents were similar in each hemisphere. 3. Currents evoked by injections of calcium were about 2.5 times greater near the animal pole than near the vegetal pole, whereas fluorescence signals evoked by injections were similar in each hemisphere. 4. Calcium waves were evoked by photolysis flashes of similar strengths in both hemispheres of albino oocytes, but peak calcium levels evoked by supramaximal stimuli were 70 % greater in the animal hemisphere. 5. Elementary calcium release events (puffs) in the animal hemisphere had amplitudes about double that in the vegetal hemisphere, and more often involved coupled release from adjacent sites. Calcium release sites were more closely packed in the animal hemisphere, with a mean spacing of about 1.5 micro m compared with 2.25 micro m in the vegetal hemisphere. 6. The larger amplitude of currents mediated by InsP3 in the animal hemisphere, therefore, involves an increased flux of calcium at individual release units, a more dense packing of release units and a higher density of Cl- channels.

Membrane currents in immature oocytes of the Rana perezi frog.

Immature oocytes of the Rana perezi frog were studied electrophysiologically to see if some of the unusual ionic channels found in Xenopus oocytes were also expressed in these cells. Growing oocytes showed a fairly linear current/voltage relationship (from -200 to +60 mV), whereas fully grown cells had several voltage-dependent conductances. Depolarizing pulses elicited a potassium current blocked by tetraethylammonium (TEA) and two kinetically different Ca2+-dependent Cl- currents (ICl(Ca)), both sensitive to niflumic acid. ICl(Ca), which have not been previously observed in Rana immature oocytes, were also found in response to acetylcholine or rabbit serum superfusion or intracellular injection of Ca2+. In addition, three different Cl- currents were activated in these cells by hyperpolarization: (1) a transient inward current dependent on a critical intracellular Ca2+ concentration; (2) an inward rectifier Cl- current, which was Ca2+ independent; and (3) a high threshold (over -140 mV), slow Cl- current, blocked by several divalent cations, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and 4-acetamido-4-isothiocyanatostilbene-2, 2'-disulphonic acid (SITS). The presence of most of these infrequent currents in immature oocytes of several frogs and toads suggests that they are not merely the result of random genomic expression but a programmed decision, probably related to a definite functional role.

Functional incorporation of P-glycoprotein into Xenopus oocyte plasma membrane fails to elicit a swelling-evoked conductance.

Microinjecton of Xenopus oocytes with P-glycoprotein-containing membranes from multidrug resistant cells following a recently published procedure resulted in the transplantation of the protein to the plasma membrane of the oocytes and was confirmed by Western blot analysis. These oocytes showed a reduced intracellular accumulation of daunomycin, when compared to uninjected oocytes or to those injected with membrane vesicles lacking P-glycoprotein, thus indicating that the protein had been incorporated in a transport-competent form. On the other hand, transplantation of P-glycoprotein to the oocyte membrane did not significantly change either the appearance or the properties of swelling-elicited membrane conductance with respect to those determined in oocytes either uninjected or injected with membranes lacking P-glycoprotein. These results do not support a role for P-glycoprotein as a swelling-activated chloride channel.

Incorporation of reconstituted acetylcholine receptors from Torpedo into the Xenopus oocyte membrane.

Xenopus oocytes are a valuable aid for studying the molecular structure and function of ionic channels and neurotransmitter receptors. Their use has recently been extended by the demonstration that oocytes can incorporate foreign membranes carrying preassembled receptors and channels. Here we show that when reconstituted in an artificial lipid matrix and injected into Xenopus oocytes, purified nicotinic acetylcholine receptors are efficiently inserted into the plasma membrane, where they form "clusters" of receptors that retain their native properties. This constitutes an innovative approach that, besides allowing the analyses of membrane fusion processes, is also a powerful technique for studying the characteristics and regulation of many membrane proteins (with their native stoichiometry and configuration) upon reinsertion into the membrane of a very convenient host cell system.

Confocal microfluorimetry of Ca2+ signals evoked in Xenopus oocytes by photoreleased inositol trisphosphate.

1. The subcellular characteristics of inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ liberation were studied in Xenopus oocytes by the use of confocal microfluorimetry to monitor Ca2+ signals from minutely localized region of the cell in response to photorelease of InsP3 from a caged precursor. 2. Photorelease of increasing amounts of InsP3 by progressively longer light flashes evoked transient Ca2+ responses that appeared abruptly at a certain threshold duration, and then grew steeply over a narrow range of flash durations to reach a maximum. Further lengthening of flash duration gave no increase in size of the Ca2+ signals, but their rate of rise continued to increase and their duration became longer. Simultaneous measurements of Ca(2+)-activated Cl- currents showed a slightly higher threshold than the Ca2+ signal, and a more graded dependence upon flash duration. 3. The threshold flash durations required to evoke Ca2+ and membrane current signals grew by more than 100-fold as the area of the oocyte exposed to photolysis light was reduced from a square of 140 microns to 5 microns. 4. Ca2+ signals evoked by photoreleased InsP3 began following a dose-dependent latency that was as long as several seconds with low intensity light, but shortened to about 50 ms at maximum intensity. The extrapolated minimum latency with infinite photorelease of InsP3 was about 30 ms. 5. InsP3-evoked membrane currents began 30 ms or longer after the corresponding Ca2+ signals, whereas currents evoked by photorelease of Ca2+ from a caged precursor began within 5 ms of the onset of the light flash. 6. No differences in duration of InsP3-evoked Ca2+ signals were apparent when the confocal measuring spot was positioned close to the plasma membrane or about 10 microns more deeply into the oocyte. At both locations the Ca2+ signals were more prolonged than the associated membrane current signals. 7. Ca2+ signals to a test light flash were suppressed for about 2 s following a conditioning suprathreshold flash, but recovered almost completely after 6 s. The associated membrane current signals were facilitated at short intervals, suppressed at intervals between 0.5 and 3 s, and subsequently recovered more slowly than the Ca2+ signals. 8. Photorelease of InsP3 during 30 s exposures of low intensity evoked trains of repetitive Ca2+ spikes. The overall amplitudes of these responses changed little with increasing in frequency, and became smaller and superimposed on a more sustained elevation of Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of membrane currents evoked by photoreleased inositol trisphosphate in Xenopus oocytes.

Photorelease of inositol 1,4,5-trisphosphate (InsP3) from a caged precursor was used to study characteristics of Ca(2+)-activated Cl- currents activated in Xenopus oocytes by the InsP3-Ca2+ signaling pathway. Photolysis flashes shorter than a threshold duration evoked no response, but the current amplitude then grew about linearly as the flash duration was further lengthened. Currents directly evoked by photorelease of Ca2+ from a caged precursor grew linearly with increasing flash duration and showed a small threshold before they were activated. However, the major part of the threshold of InsP3-evoked responses appears to arise because a certain concentration of InsP3 (estimated to be approximately 60 nM) is required to evoke Ca2+ liberation. Subthreshold conditioning flashes potentiated responses to subsequent flashes, and the potentiation increased linearly with increasing conditioning flash duration before abruptly declining. The potentiation decayed exponentially with a time constant of approximately 17 s with increasing interflash interval. Currents evoked by photoreleased InsP3 began after a latency that shortened from 10 s or longer to 100 ms as the photolysis intensity was increased. This dose dependence of the latency could be quantitatively explained by the time required for the InsP3 concentration to rise above threshold. Intracellular injection of heparin (a competitive antagonist at the InsP3 receptor) increased the threshold for InsP3 action, as did increased temperature. We conclude that several characteristics of InsP3-evoked responses, including their dose dependence, latency, and facilitation with paired stimuli, arise because a distinct threshold level of InsP3 is required to evoke release of Ca2+ from intracellular stores.

Inositol tetrakisphosphate liberates stored Ca2+ in Xenopus oocytes and facilitates responses to inositol trisphosphate.

1. The actions of the putative second messenger inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) were studied by injecting it into voltage-clamped oocytes while recording Ca(2+)-dependent chloride membrane currents and, in some experiments, fluorescence signals from Ca2+ indicators. 2. Ins(1,3,4,5)P4 evoked a rise in intracellular Ca2+ and associated chloride current in oocytes bathed in normal or Ca(2+)-free Ringer solutions. The fluorescence Ca2+ signal showed a prolonged rise with superimposed oscillations, whereas the current reflected only the oscillatory component. 3. Injections of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) evoked currents showing an initial transient, followed by oscillations. Ins(1,3,4,5)P4 evoked similar oscillations, but the transient component was usually small or absent. Ins(1,3,4,5)P4 was about 20-fold less potent than Ins(1,4,5)P3, as measured by comparing doses required to elicit currents with the same integral. The most sensitive oocytes responded to about 1 fmol Ins(1,3,4,5)P4 and 0.1 fmol Ins(1,4,5)P3. 4. Injections of Ins(2,4,5)P4 evoked oscillatory currents, with a potency about three times greater than Ins(1,4,5)P3. Ins(1,3,4)P4 was ineffective in some oocytes even at doses of several picomoles, but in other oocytes evoked small transient and oscillatory currents with a potency 100 times or more less than Ins(1,3,4,5)P4. 5. Injections of Ins(1,3,4,5)P4 made into the animal hemisphere of the oocyte evoked larger currents than injections into the vegetal hemisphere. 6. Photo-release of Ins(1,4,5)P3 from caged Ins(1,4,5)P4 loaded into the oocyte was used to examine interactions between Ins(1,4,5)P3 and Ins(1,3,4,5)P4. Injection of low (ca 1 fmol) doses of Ins(1,3,4,5)P4 shortly before a light flash greatly facilitated currents evoked by photo-release of near-threshold amounts of Ins(1,4,5)P3. This facilitation was unaffected by removal of extracellular Ca2+ and arose because Ins(1,3,4,5)P4 reduced the threshold amount of Ins(1,4,5)P3 required to evoke a response. 7. Larger amounts (several femtomoles) of Ins(1,3,4,5)P4 depressed responses evoked by photo-release of Ins(1,4,5)P3. This may arise because Ca2+ liberated by Ins(1,3,4,5)P4 inhibits the ability of Ins(1,4,5)P3 to release further Ca2+. 8. We conclude that Ins(1,3,4,5)P4 liberates intracellular Ca2+ in the oocyte in a manner similar to that of Ins(1,4,5)P3, and suggest that a physiological role for Ins(1,3,4,5)P4 may be to facilitate responses to Ins(1,4,5)P3.

Caffeine inhibits inositol trisphosphate-mediated liberation of intracellular calcium in Xenopus oocytes.

1. Voltage-clamp recording of Ca(2+)-activated chloride currents in Xenopus oocytes was used to study the effects of caffeine on the liberation of intracellular Ca2+ induced by photo-release of inositol 1,4,5-trisphosphate (InsP3) from caged InsP3. Bath application of caffeine, at concentrations between 0.1 and 10 mM, reduced or abolished the current evoked by photo-release of InsP3 and by microinjection of InsP3. 2. Caffeine did not appreciably reduce currents evoked by injection of Ca2+ into oocytes, whereas measurements using the Ca2+ indicator Rhod-2 showed that it instead inhibited the liberation of Ca2+ by InsP3. 3. Caffeine increased the threshold amount of InsP3 required to evoke a current response and proportionally reduced the currents evoked by suprathreshold levels of InsP3. 4. Theophylline and 3-isobutyl-1-methylxanthine (IBMX) were much less potent than caffeine, and few changes were seen in the InsP3 responses following application of forskolin or intracellular injection of cyclic AMP. Thus, inhibition of InsP3 responses by caffeine does not arise through inhibition of phosphodiesterase enzymes. 5. Even at high (10 mM) concentrations, caffeine did not itself elicit any clear Ca(2+)-activated current. It is therefore unlikely that inhibition of the InsP3 responses arise because caffeine itself liberates Ca2+ from intracellular stores. 6. The site of action of caffeine is intracellular, because injections of caffeine into the oocyte strongly inhibited responses to InsP3, whereas local extracellular applications of similar amounts were almost without effect.

Inositol 1,3,4,6-tetrakisphosphate mobilizes calcium in Xenopus oocytes with high potency.

Injection of Ins(1,3,4,6)P4 into Xenopus oocytes evoked Ca2(+)-dependent membrane currents with a potency 5-10 times less than Ins(1,4,5)P3, whereas Ins(1,3,4)P3 and Ins(1,3,4,5,6)P5 were almost ineffective. Responses to Ins(1,3,4,6)P4 arose through liberation of intracellular Ca2+ and through entry of extracellular Ca2+. These results, together with the observation that Ins(1,3,4,6)P4 facilitated responses to Ins(1,4,5)P3, suggests that both of these compounds may act on the same intracellular receptors.

Localized all-or-none calcium liberation by inositol trisphosphate.

Laser confocal microscopy was used to monitor calcium ion (Ca2+) liberation from highly localized (micrometer) regions of intact Xenopus oocytes in response to photo-released inositol 1,4,5-trisphosphate (InsP3). Local Ca2+ release varied in an all-or-none manner with increasing amount of InsP3, in contrast to signals recorded from larger areas, which grew progressively as the concentration of InsP3 was raised above a threshold. Liberation of Ca2+ was restricted to within a few microns of the site of InsP3 release and, in response to agonist activation, localized regions of the oocyte showed asynchronous oscillations in cytoplasmic Ca2+ release. Results obtained with this technique provided direct evidence that InsP3-induced Ca2+ liberation was quantized and suggest that the InsP3-sensitive Ca2+ pool may be a collection of independent, localized compartments that release Ca2+ in an all-or-none manner.

A slowly inactivating potassium current in native oocytes of Xenopus laevis.

Membrane currents were recorded in voltage-clamped oocytes of Xenopus laevis in response to voltage steps. We describe results obtained in oocytes obtained from one donor frog, which showed an unusually large outward current upon depolarization. Measurements of reversal potentials of tail currents in solutions of different K+ concentration indicated that this current is carried largely by K+ ions. It was strongly reduced by extracellular application of tetraethylammonium, though not by Ba2+ or 4-aminopyridine. Removal of surrounding follicular cells did not reduce the K+ current, indicating that it arises across the oocyte membrane proper. Activation of the K+ conductance was first detected with depolarization to about -12 mV, increased with a limiting voltage sensitivity of 3 mV for an e-fold change in current, and was half-maximally activated at about +10 mV. The current rose following a single exponential timecourse after depolarization, with a time constant that shortened from about 400 ms at -10 mV to about 15 ms at +80 mV. During prolonged depolarization the current inactivated with a time constant of about 4 s, which did not alter greatly with potential. The K+ current was independent of Ca2+, as it was not altered by addition of 10 mM Mn2+ to the bathing medium, or by intracellular injection of EGTA. Noise analysis of K+ current fluctuations indicated that the current is carried by channels with a unitary conductance of about 20 ps and a mean open lifetime of about 300 ms (at room temperature and potential of +10 to +20 mV).

Inhibition by Ca2+ of inositol trisphosphate-mediated Ca2+ liberation: a possible mechanism for oscillatory release of Ca2+.

Light-flash photolysis of caged inositol 1,4,5-trisphosphate (InsP3) was used to generate reproducible transients of free InsP3 in Xenopus oocytes, and the resulting liberation of Ca2+ from intracellular stores was monitored by recording Ca2+-activated membrane currents and by use of the fluorescent Ca2+ indicator fluo-3. InsP3-mediated Ca2+ release was inhibited by elevating the intracellular free Ca2+ level, either by microinjecting Ca2+ into the cell or by applying conditioning light flashes to liberate Ca2+. This inhibition followed a slow time course, being maximal after about 2 s and subsequently declining over several seconds. Negative feedback of Ca2+ ions on InsP3-mediated Ca2+ liberation may explain the oscillatory release of Ca2+ seen during activation of inositol phospholipid signaling in the oocyte, and the time course of the inhibition is consistent with the period of the oscillations.

Effects of central or peripheral axotomy on membrane properties of sensory neurones in the petrosal ganglion of the cat.

1. The properties of sensory neurones in the petrosal ganglion of the cat were examined in vitro with intracellular electrodes 8 days after section of the central (bulbar roots) or peripheral process. Two types of cells, both with conduction velocities faster than 2 m/s and with humps on the falling phases of their action potentials (H-neurones), were studied: glossopharyngeal neurones arising from the tongue and pharynx, and carotid neurones originating in the carotid body and carotid sinus. 2. Peripheral axotomy produced an increase in action potential duration and a marked decrease in the amplitude and duration of the spike after-hyperpolarization in both glossopharyngeal and carotid neurones. 3. The maximum rate of depolarization of the action potential increased after peripheral axotomy in glossopharyngeal cells but did not change in carotid neurones. 4. The time-dependent inward rectification in response to hyperpolarizing pulses was markedly reduced in both types of cells after peripheral axotomy. 5. Section of the peripheral process produced a decrease of the rheobase of glossopharyngeal cells, but not of carotid neurones. After axotomy the proportion of cells giving tonic discharges in response to long depolarizing pulses increased from 13 to 54% among carotid neurones but did not change in glossopharyngeal cells. 6. No significant changes in membrane potential or input resistance of either group of cells were found after peripheral axotomy. 7. Central axotomy did not produce any changes in the electrophysiological properties of glossopharyngeal or carotid neurones. 8. Peripheral conduction velocity was decreased in both types of cells after peripheral axotomy, but did not change after section of the bulbar roots. 9. It is concluded that the electrical properties of sensory neurones are modified after peripheral axotomy but not after central axotomy. Furthermore, the changes produced by peripheral axotomy are different in neurones innervating different peripheral targets. 10. The possibility that some electrical properties of sensory neurones are maintained by their peripheral targets is discussed.

Membrane properties of glossopharyngeal sensory neurons in the petrosal ganglion of the cat.

The active and passive properties of petrosal ganglion sensory neurons with axons in the glossopharyngeal nerve were examined with intracellular microelectrodes in an in vitro preparation. Glossopharyngeal neurons could be classified into two groups: H-cells showing an inflexion or hump on the falling phase of the spike and F-cells, generating a short action potential without a hump. Most of the neurons found (85%) were H-cells. The axonal conduction velocity of both types of cells fell into the A delta range, although the average value for F-cells (13 m/s) was higher than that found for H-cells (10 m/s). H- and F-cells had similar resting membrane potentials and input resistances, but different action potential characteristics. F-cells showed a smaller action potential with a faster rate of depolarization, followed by a shorter after-hyperpolarization. The response to depolarizing current pulses applied through the microelectrode was also different in both types of cells. About half of the H-cells could not be depolarized to threshold while 85% of F-cells generated spikes. It is concluded that two different populations of petrosal ganglion neurons send axons into the glossopharyngeal nerve.