Amplitude Of Receptor Potential Research Articles

Neuromuscular, retinal, and reproductive impact of low-dose polystyrene microplastics on Drosophila

Facing the challenge of global microplastics (MPs) pollution, full characterization of MPs biohazards is urgent. Recent intensive studies revealed that the toxicity depends on the material, size, and exposure concentration of MP. To better elucidate MPs biohazards, we investigated the impact of polystyrene-MPs of size 0.1 μm at a low dose of 50 μg/L on the neuromuscular, retinal, and reproductive phenotypes of fruit fly model, by voltage-clamped electrophysiology, electroretinogram, and reproductive assay, respectively. We found that MPs decreased the frequency of spontaneous junction currents of synapse and altered the receptor potential amplitude of the retina. Furthermore, MPs lowered the rate of embryo-laying of fruit flies. The differential gene expression of ligand-receptor interaction, endocytosis, phototransduction, and Toll/Imd signaling pathways might underlie these MPs-induced phenotypes. These findings call for further investigation on the potential biohazards of low-dose MPs.

Mechanisms of synaptic depression at the hair cell ribbon synapse that support auditory nerve function

Inner hair cells (IHCs) in the cochlea are the mammalian phono-receptors, transducing sound energy into graded changes in membrane potentials, the so called "receptor potentials." Ribbon synapses between IHCs and auditory nerve neurons are responsible for converting receptor potentials into spike rates. The characteristics of auditory nerve responses to sound have been described extensively. For instance, persistent acoustic stimulation produces sensory adaptation, which is revealed as a reduction in neuronal spike rate with time constants in the range of milliseconds to seconds. Since the amplitude of IHC receptor potentials is invariant during this period, the classic hypothesis pointed to vesicle depletion at the IHC as responsible for auditory adaptation. In this study, we observed that fast synaptic depression occurred in responses to stimuli of varying intensities. Nevertheless, release continued after this initial depression, via synaptic vesicles with slower exocytotic kinetics. Heterogeneity in kinetic elements, therefore, favored synaptic responses with an early peak and a sustained phase. The application of cyclothiazide (CTZ) revealed that desensitization of postsynaptic receptors contributed to synaptic depression, which was more pronounced during stronger stimulation. Thus, desensitization had a twofold effect: It abbreviated signaling between IHC and the auditory nerve and also balanced differences in decay kinetics between responses to different stimulation strengths. We therefore propose that both pre- and postsynaptic mechanisms at the IHC ribbon synapse contribute to synaptic depression at the IHC ribbon synapse and spike rate adaptation in the auditory nerve.

MT 2-like melatonin receptor modulates amplitude receptor potential in visual cells of crayfish during a 24-hour cycle

Retinular photoreceptors are structures involved in the expression and synchronization of the circadian rhythm of sensitivity to light in crayfish. To determine whether melatonin possesses a differential effect upon the receptor potential (RP) amplitude of retinular photoreceptors circadian time (CT)-dependent, we conducted experiments by means of applying melatonin every 2 h during a 24-hour cycle. Melatonin with 100 nM increased RP amplitude during subjective day to a greater degree than during subjective night. To determine whether MT 2 melatonin receptors regulate the melatonin-produced effect, we carried out two experiments, circadian times (CTs) 6 and 18, which included the following: (1) application of the MT 2 receptor selective agonist 8-M-PDOT and antagonist DH97, and (2) the specific binding of [ 125I]-melatonin in eyestalk membranes. The amount of 10 nM of 8-M-PDOT increased RP amplitude in a similar manner to melatonin, and 1 nM DH97 abolished the increase produced by melatonin and 8-M-PDOT. Binding of [ 125I]-melatonin was saturable and specific. Scatchard analysis revealed an affinity constant ( K d) of 1.1 nM and a total number of binding sites ( B max) of 6 fmol/mg protein at CT 6, and a K d of 1.46 nM and B max of 7 fmol/mg protein at CT 18. Our results indicate that melatonin increased RP amplitude of crayfish retinular photoreceptors through MT 2-like melatonin receptors. These data support the idea that melatonin is a signal of darkness for the circadian system in crayfish retinular cells.

Pigment dispersing hormone generates a circadian response to light in the crayfish, Procambarus clarkii

Photoreceptor cells have been identified as important structures in the organization of the circadian system responsible for the generation and expression of the electroretinogram (ERG) circadian rhythm. They are the structures where the circadian periodicity is expressed (effectors) and which transform information from external light signals to be conducted to the pacemaker in order to induce adjustments of the rhythm (synchronizers). After isolation, eyestalks perfused in a pigment dispersing hormone (PDH) solution, show significant changes in receptor potential (RP) amplitude and duration. Exogenous PDH injected into intact crayfish induces a migration of retinal shielding pigments to a light-adapted state. A single dose of PDH produces advances or delays in the circadian rhythm of response to light of visual photoreceptors. All these effects depend on the circadian phase of PDH application. Consequently, the determination of the action of exogenous PDH on photoreceptor cells proved to be very helpful in understanding some mechanisms underlying the circadian organization of crayfish.

Activation and inhibition of the transduction process in silkmoth olfactory receptor neurons.

Electrophysiological responses of olfactory receptor neurons in both male and female silkmoths (Bombyx mori) were investigated. In both sexes, the G-protein activator sodium fluoride and 1,2-dioctanoyl-sn-glycerol, a membrane-permeable analog of the protein kinase C activator diacylglycerol, elicited nerve impulse responses similar to those elicited by weak continuous stimulation with odorants. Therefore, G(q)-proteins and diacylglycerol-activated ion channels seem to be involved in the transduction process in both pheromone-sensitive neurons in males and general odorant-sensitive neurons in females. Decyl-thio-trifluoro-propanone is known to inhibit electrophysiological responses of male moths to pheromones, but has no effect in females. Application of this inhibitor reduced the frequency, but not the amplitude of elementary receptor potentials. It had no inhibitory effect on nerve impulse responses elicited by sodium fluoride or 1,2-dioctanoyl-sn-glycerol. This supports the idea that decyl-thio-trifluoro-propanone acts on a prior step of the transduction cascade, e.g. on the pheromone receptor molecules. General odorants, such as (+/-)-linalool and 1-heptanol, excite olfactory receptor neurons in females, but inhibit the pheromone-sensitive neurons in males. Both (+/-)-linalool and 1-heptanol inhibited the responses of male neurons elicited by sodium fluoride or 1,2-dioctanoyl-sn-glycerol. (+/-)-Linalool reduced the amplitude of elementary receptor potentials. In contrast to decyl-thio-trifluoro-propanone, (+/-)-linalool and 1-heptanol seem to interfere with later processes of the transduction cascade, possibly the opening of ion channels.

Octopamine modulates the sensitivity of silkmoth pheromone receptor neurons.

Effects of octopamine and its antagonist epinastine on electrophysiological responses of receptor neurons of Antheraea polyphemus specialised to the pheromone components (E,Z)-6,11-hexadecadienyl acetate and (E,Z)-6,11-hexadecadienal were investigated. Injections of octopamine and epinastine into the moths had no effect on the transepithelial potential of the antennal-branch preparation nor on the spontaneous nerve impulse frequency in either type of receptor neuron. However, in the presence of continuous low-intensity pheromone stimulation, octopamine significantly increased the nerve impulse frequency in the acetate receptor neuron, but not in the aldehyde receptor neuron. Octopamine and epinastine had no significant effect on the receptor potential amplitudes elicited in both receptor neuron types by pheromone stimulation. However, the peak nerve impulse frequency in the response of both receptor neuron types to pheromone was significantly affected: decreased by epinastine and increased by octopamine over a broad range of pheromone concentrations. In control experiments, injection of physiological saline did not significantly alter the peak nerve impulse frequency. The effect of octopamine was established within 1 h after injection and persisted for about 4 h. The possibility of a direct action of octopamine on the nerve impulse generation by the receptor neurons is discussed.

Inhibitors of sensillar esterase block reversibly the responses of moth pheromone receptor cells.

The pheromone of Antherae polyphemus is slowly (within min) degraded by a sensillum lymph specific esterase. The effects of two volatile alkyl-thio-trifluoro-methyl-ketones with alkyl chain length of six (HTFP) and ten carbons (DTFP), which selectively inhibit the activity of the sensillar esterase, were tested. Single sensillum recordings from sensilla trichodea were performed on isolated antennae either held in a continuous stream of clean air, or exposed to an airstream containing one of the esterase inhibitors. Afterwards the antennae were stimulated with the pheromone components (E,Z)-6,11-hexadecadienyl acetate and (E,Z)-6,11-hexadecadienal. Both esterase inhibitors caused a reduction of electrophysiological responses to each of the two pheromone compounds. The DTFP was slightly more effective than HTFP. An almost complete block of the receptor potential amplitude and nerve impulse response was achieved within seconds of exposure to the inhibitors. Responses of both pheromone receptor cells were reduced to the same extent. The half-times of rise and decline of the receptor potential remained unaffected in most cells. The responses to pheromone partially recovered to approximately 50-70% within several min in clean air. An inhibition of the sensillar esterase cannot explain the observed effects. The inhibitors could either react directly with the receptor molecules, thereby inhibiting the action of the pheromone, or bind to the pheromone-binding protein (PBP) and prevent the formation of the pheromone-PBP complex stimulating the receptor.

Inhibitors of sensillar esterase reversibly block the responses of moth pheromone receptor cells

Three volatile alkyl-thio-trifluoro propanones inhibiting the esterase in olfactory sensilla of the silkmoths Antheraea polyphemus and A. pernyi were used to test the hypothesis that enzymatic pheromone degradation is responsible for the decline of the receptor potential after pheromone stimulation. Test stimuli were the pheromone components (E,Z)-6,11-hexadecadienyl acetate, a substrate for the sensillar esterase, and (E,Z)-6,11-hexadecadienal, not degraded by the esterase. Each compound acts on a separate type of receptor cell. In both receptor cell types the trifluoro propanones caused a partially reversible reduction of sensitivity as indicated by smaller receptor potential amplitudes and lower nerve impulse frequencies. Since application of the esterase inhibitors did not prolong the decline of the receptor potential of the acetate cell, the esterase is not responsible for the rapid pheromone deactivation. When the trifluoro propanones were applied after the pheromone at high concentrations, they rapidly inhibited (repolarized) both receptor cell types. Experiments with local application of trifluoro propanones revealed that the inhibitory effect spreads within seconds along the length of the sensillum. The inhibition of the electrophysiological responses might be due to an antagonistic action of the trifluoro propanones at the pheromone-binding sites, either at the receptor molecules or at the pheromone-binding protein.

Bidirectional synaptic transmission in Necturus taste buds

Pairs of taste cells were impaled with intracellular recording microelectrodes in intact taste buds in slices of Necturus lingual epithelium. Applying short pulses of 140 mM KCl or 200 mM CaCl2 solutions to the apical pore elicited receptor potentials in taste receptor cells. Chemostimulation of receptor cells elicited postsynaptic responses in basal cells in the taste bud. Postsynaptic responses in basal cells had a threshold for activation and did not saturate with increasing doses of chemical stimulus applied to the receptor cells. We directly depolarized individual receptor cells and tested whether this would evoke postsynaptic responses in basal cells. Depolarizing receptor cells to approximately 0 mV evoked small depolarizing responses in basal cells in 16% of the experiments. The properties of these responses were consistent with their being mediated by a chemical synapse. A comparison of the responses in basal cells evoked by depolarizing single receptor cells, with responses evoked by stimulating the entire receptor cell population with KCl suggests that there is extensive synaptic convergence from receptor cells onto each basal cell. We also tested whether electrical excitation of basal cells would elicit (retrograde) synaptic responses in receptor cells. Single depolarizing pulses (up to 1 sec duration) applied to basal cells through the intracellular recording microelectrode never evoked synaptic responses in receptor cells. However, when repetitive electrical stimuli were applied to basal cells (four to six 1 sec depolarizations to approximately 0 mV every 12 sec) we observed prolonged effects on receptor cells in 11 of 23 experiments. These effects included an increase in the amplitude of receptor potentials elicited by KCI (mean +/- SD = +19 +/- 5%), an increase in membrane input resistance of receptor cells (+27 +/- 11%), and a hyperpolarization of receptor cells (3-10 mV). In control experiments, repetitive stimulation of one receptor cell never elicited such effects in another receptor cell. We investigated the possibility that serotonin (5-HT), released from basal cells, mediated the above modulatory effects on receptor cells. Bath-applied 5-HT (100 microM) mimicked the effects produced by repetitive basal cell stimulation (KCI responses increased by 23 +/- 12%; input resistance increased by 24 +/- 11%; hyperpolarization of 5-15 mV; N = 14). We conclude that basal cells release 5-HT onto adjacent taste receptor cells and that this enhances the electrotonic propagation of receptor potentials from the apical (chemosensitive) tip to the basal (synaptic) processes of receptor cells. The net effect is that activation of basal cells effectively increases the chemosensitivity of taste receptor cells.

Transducer properties of the rapidly adapting stretch receptor neurone in the crayfish (Pacifastacus leniusculus).

1. The transducer properties of the rapidly adapting stretch receptor neurone of the crayfish (Pacifastacus leniusculus) were studied using a two-microelectrode voltage clamp technique. 2. The impulse response to ramp-and-hold extensions of the receptor muscle typically consisted of a high frequency burst followed by cessation of impulses within a relatively short time depending on the amplitude of extension. The type of adaptation was consistent with earlier studies. The stimulus-response relationship for the impulse frequency was non-linear and had a slope in a log-log plot of 2.9. 3. When impulse generation was blocked by tetrodotoxin (TTX), (block of Na+ channels) the receptor potential was extension dependent and similar to that found in the slowly adapting receptor. For small extensions there was an initial peak followed by a fall to a steady potential level. For large extensions the potential response during the ramp phase consisted of a peak followed by a constant potential level lasting to the end of the ramp. When the extension changed to the hold phase the potential fell towards a steady state. The relation between extension and amplitude of receptor potential was non-linear and saturated at -40 to -30 mV (extensions > 15% of zero length, lo). 4. When potassium channels were blocked by TEA (50 mM) and 4-aminopyridine (4-AP, 5 mM) (and Na+ channels blocked by TTX) the shape of the generator potential become less complex with an increased amplitude for large extensions. 5. When the receptor neurone was voltage clamped at the resting potential, extension of the receptor muscle produced an inwardly directed receptor current, the stretch-induced current (SIC). The response consisted of a fast transient phase which decayed towards a steady state. The SIC peak amplitude was dependent on extension in a sigmoidal fashion and saturated at 190 nA (extensions > 25% of lo). The slope of the steepest part of the stimulus-response relation (between 10 and 20% extension) was 4.7 +/- 0.25 (mean +/- S.E.M.) in a log-log plot. 6. The peak amplitude of the SIC increased with increasing extension speed (ramp steepness), the relation between the slope of the ramp and current amplitude being a first order (hyperbolic) function. The amplitude of the receptor current was voltage dependent and had a reversal potential of +16.2 +/- 1.8 mV (mean +/- S.E.M., 32 cells). From the reversal potential the permeability ratio, PNa/PK, of the transducer permeability system was calculated to be 1.5. The I-V curve of SIC was non-linear.(ABSTRACT TRUNCATED AT 400 WORDS)

Contribution of proton transporter to acid-induced receptor potential in frog taste cells

1. 1. The generative mechanism of the acid-induced receptor potential in frog taste cells was re-examined with micro-electrodes. 2. 2. The amplitude of receptor potentials in a taste cell induced by 1 mM acetic acid increased as the concentration of Ca 2+, Sr 2+ and B 2+ on the tongue surface was increased. 3. 3. The amplitude of the acid-induced receptor potential was inhibited by about 40% by a proton pump blocker DCCD ( N,N′-dicyclohexylcarbodiimide) added to superficial normal saline. 4. 4. It is concluded that most of the acid-induced response in a frog taste cell was generated by current carried through proton-gated C 2+ channel of the apical receptive membrane. The remaining part of the acid response was generated by current carried through a DCCD-sensitive proton transporter of the receptive membrane.

PHOTOSENSORY TRANSDUCTION IN CILIATES. III. THE TEMPORAL RELATION BETWEEN MEMBRANE POTENTIALS AND PHOTOMOTILE RESPONSES IN Blepharisma japonic urn

Abstract— Blepharisma japonicum exhibits a step‐up photophobic response when subjected to an increase in light stimulus intensity. This response is characterized by the stop reaction after a period of delay followed by backward swimming (lateral rotation). The latency of the stop response decreased and duration of the lateral rotation increased as the intensity of light stimuli was raised. A step‐increase in light intensity elicited a graded membrane depolarization (photic receptor potential), as measured by intracellular microelectrode. When the amplitude of receptor potential exceeded a threshold depolarization for membrane excitation (15–25 mV), an all‐or‐none action potential of 50–65 mV in amplitude was evoked which also occurred with some latency. Light stimuli of higher intensity (suprathreshold) elicited action potential which was followed by a membrane after‐depolarization. Increasing the intensity of stimuli caused generation of an action potential with shorter lag period and prolonged after‐depolarization. The action spectra for the latency of stop reaction, receptor potential amplitude and cell photoresponsiveness showed maxima at 460, 530 and 580 nm. The analysis of temporal relationships between the electrophysiological responses and the motile events showed that latency of an action potential, induced by the receptor potential, correlates well with the latency of a cell stop response. Also the duration of membrane after‐depolarization resembled the time period of the cell's backward swimming (cell rotation). The data obtained indicate that the primary reaction initiated by light absorption in the photoreceptor pigment (blepharismin) is converted into the observed electrical potential changes, which in turn results in the photomotile response of Blepharisma cells.

Electrophysiological study of the effects of deltamethrin, bioresmethrin, and DDT on the activity of pheromone receptor neurones in two moth species

Electrophysiological techniques were used to study the effects of deltamethrin, bioresmethrin, and DDT on the olfactory system of Mamestra suasa and M. brassicae male moths (Noctuidae). Direct applications on the antenna of sublethal doses of pesticides were found not to modulate the activity of olfactory neurones. Thus lethal doses of the insecticides were applied topically in acetone solution on the abdomen of individuals. None of the three insecticides assayed modified the amplitude of EAG or receptor potentials in response to pheromone stimulations. In return, action potential initiation was susceptible to the insecticides. Deltamethrin inhibited both the spontaneous firing activity and that following pheromone or electrical stimulations. Under the influence of DDT olfactory neurones emitted bursts of impulses spontaneously in place of single spikes and discharged bursts of spikes in high frequency in response to pheromone stimulations instead of the usual regular volley. Despite the change of the firing pattern induced by DDT, the sensitivity and the specificity of the olfactory system remained unchanged. Bioresmethrin modified neither the neurone activity at rest nor the specificity of pheromone receptor neurones. In contrast, applications of this pyrethroid significantly reduced the number of action potentials emitted in response to pheromone stimulations. The differences in effects of deltamethrin, DDT, and bioresmethrin on the activity of pheromone receptor neurones are discussed on the basis of studies carried out both on the central and the peripheral nervous system.

The voltage responses of hair cells in the basal turn of the guinea-pig cochlea.

1. Intracellularly recorded voltage responses to tones and to current injection were measured from inner (IHC) and outer (OHC) hair cells in the basal turn of the guinea-pig cochlea. 2. The voltage responses of IHCs to tones are increased by negative current and decreased by positive current. For any given current strength, the decrease in the receptor potential amplitude caused by positive current is less than the increase caused by negative current. This was attributed to the voltage-dependent rectification of the membrane conductance. 3. Estimates of the reversal potential of the receptor current were based on extrapolation of the slopes of the current-voltage relations for potentials more negative than -60 mV. The estimated reversal potentials were close to the measured endolymphatic potential. 4. Negative current increased the IHC membrane time constant and increased the transducer driving voltage. When receptor potentials to low-frequency tones were adjusted for constant driving voltage and for the membrane time constant, the positive DC component of the receptor potential was decreased relative to the AC component by current injection but the DC component in response to high-frequency tones near the best frequency of the IHC (16 kHz) was unchanged by negative current. 5. When compensated for constant driving voltage and low-pass filtering due to the basolateral membrane conductance, the proportion of the transducer conductance open in IHCs at rest is increased by negative current. 6. Negative current injection reduced the positive DC components of OHC voltage responses to low-frequency tones and may make them negative. In some cases negative current injection decreased the amplitude of the OHC receptor potential while positive current injection enhanced it. From these observations it is proposed that negative current injection shifts the operating point of the OHC transducer functions towards a positive, saturating region of the relationship and positive current decreases the proportion of the transducer conductance which is open at rest.

The Actions of Proctolin, Octopamine and Serotonin on Crustacean Proprioceptors Show Species and Neurone Specificity

ABSTRACT A comparative survey is presented of the responsiveness of crustacean mechanoreceptors to the neurohormones proctolin, octopamine and serotonin. Seven identifiable primary afferents were examined in the crayfish Cherax destructor and the lobster Homarus americanus: three from the oval organ (OO) of the second maxilla, two from the non-spiking stretch receptor (NSSR) of the swimmeret and two from the muscle receptor organ (MRO) of the abdomen. Proctolin modulation was observed in 10 of the 14 fibres tested and was invariably potentiating, resulting in enhanced receptor potential amplitudes and increased firing. Octopamine and serotonin each modulated 8 of the 14 fibres and their effects were excitatory or depressive depending upon the target fibre. In the latter case the receptor potentials became attenuated, often to subthreshold levels, with loss of spiking. A comparison of results from Cherax with those of Homarus shows that there is species specificity in the responses of homologous neurones. Neurohormones that are excitatory in one species may be ineffective or depressive in the other. The broad distribution of modulatory effects observed here suggests that sensitivity to biogenic amines and peptides is a general property of proprioceptors.

Dimethyl sulfoxide elevates intracellular Ca2+ and mimics effects of increased light intensity in a photoreceptor

A 1% (v/v) solution of dimethyl sulfoxide (DMSO) added to the saline bath of isolated Balanus eburneus photoreceptors increased receptor potential amplitude by 40-50% and shortened time to peak amplitude and latency by 20-25%. The light-sensitive membrane current of voltage-clamped cells was increased systematically as DMSO concentration was increased from 1% to 10%. The null potential of the light sensitive current was unaffected by DMSO with short pulses of light, indicating that DMSO has no direct effect on ion selectivity of the light-sensitive channel. Absorbance changes of cell injected with the calcium indicator arsenazo III show that DMSO elevates intracellular Ca2+ (Cai). Current-voltage relations in darkness reveal that DMSO induces a small sustained inward current (approximately 5 nA) which has a null potential similar to the light-induced current. DMSO may activate the light-sensitive conductance via the increase in Cai. However, the altered kinetics and increased amplitude of the receptor current are opposite to the desensitizing effects normally observed with increased Cai.

The responses of cochlear hair cells to tonic displacements of the sensory hair bundle

Hair bundle displacements and receptor potentials were recorded from outer hair cells (OHCs) in organotypic cultures of the mouse cochlea during force steps applied to the bundles with a silica probe of known stiffness. The receptor potentials of some OHCs adapt for excitatory displacements and the time constants of receptor potential adaptation and hair bundle force relaxation for excitatory displacements are very similar. Thus in these OHCs, the receptor potentials correspond to the applied force for excitatory displacements. For inhibitory displacements, the receptor potentials correspond to hair bundle displacement. Some OHC receptor potentials are nonadapting and follow displacement in both the excitatory and inhibitory directions. The hair bundles of nonadapting OHCs are less stiff than those of adapting OHCs and nonadapting OHCs are an order of magnitude less sensitive to hair bundle displacement than adapting OHCs. In response to a combination of excitatory, tonic, hair bundle displacement and current injection, the receptor potentials of nonadapting OHCs decline as the membrane potential is made more positive and reverse near 0 mV. When the receptor potentials of adapting OHCs measured during current injection are compensated for constant input resistance and driving voltage across the transducer conductance, the receptor potential amplitude at the offset of the step displacement is independent of the level and polarity of the injected current. Before adaptation, at the onset of the step displacement of the hair bundle, the amplitude of the receptor potential increases as the injected current becomes more positive. For adapting OHCs, the receptor potential amplitude is a linear function of excitatory bundle displacement for amplitudes less than 50 nm. With negative, but not positive, current injection the receptor potentials at the onset of the displacement tend to saturate and the slope of the function decreases. This voltage dependent control of OHC transducer operating range is proposed to have a role in regulating the sensitivity of the cochlea.

Primary afferent responses of a crustacean mechanoreceptor are modulated by proctolin, octopamine, and serotonin

Modulation of sensory responses recorded intracellularly in primary sensory afferents of a crustacean proprioceptor is described. The neuropeptide proctolin enhances the sensory response, whereas the bioamines octopamine and serotonin depress it. The lobster oval organ of the second maxilla, a simple stretch receptor lacking centrifugal control, provides a useful model for studies on nonsynaptic modulation at peripheral sensory loci. Its three large afferents, X, Y, and Z, were prepared for intracellular recording and tested under five experimental conditions: (1) when fully rested, (2) when adapted to maintained stretch and firing tonically, (3) when showing reduced responses after habituation to repetitive stimulation, (4) not stretched but depolarized with current injections, (5) after TTX blockade. The results, taken together, indicate that conductances contributing to the overall amplitude of the receptor potential are major targets for modulators. Thus proctolin increased receptor potential amplitudes with consequent augmentation of spiking, whereas serotonin and octopamine depressed the receptor potentials, often to subthreshold levels with loss of spiking. Octopamine was a less potent agent than serotonin and failed to act upon fibers under TTX blockade. Fibers Y and Z consistently showed sensitivity to the modulators tested. The largest fiber, X, typically was resistant to proctolin, octopamine, and serotonin. Threshold concentrations of 10(-10)-10(-11) M determined in vitro are well below the circulating levels for serotonin and octopamine found in vivo. Proctolin, however, is usually not detectable in the hemolymph, and it is suggested that a significant site of proctolin release may be the oval organ itself.

Habituation in Stentor: a response-dependent process

The contractile protozoan Stentor coeruleus habituates during repetition of the mechanical stimulus used to elicit the initial contractions. This decrement in response probability was found to be highly correlated with a reduction in receptor potential amplitude, while the amplitude of the action potentials that triggered the contractions did not change. When mechanical stimulation elicited receptor potentials and not action potentials, the receptor potential did not habituate significantly. Conversely, action potentials repetitively elicited by current pulses habituated animals to mechanical stimuli. Similarly, voltage steps used to simulate action potentials produced pronounced decrements in receptor currents recorded from voltage-clamped cells, while mechanical stimulation produced only small decrements. Thus, habituation depends primarily on action potential production, while mechanical stimulation itself makes a much smaller, but significant, contribution. The temporal relation between mechanical stimuli and action potentials, when both occur, is inconsequential in determining the rate and degree of habituation produced.

Habituation in Stentor: produced by mechanoreceptor channel modification

Habituation in Stentor was shown in the previous paper (Wood, 1988) to result from a progressive diminution in mechanoreceptor potential amplitude during the course of repetitive mechanical stimulation. The source of this diminution was analyzed in the present studies by eliciting mechanoreceptor currents from control and habituated cells under voltage clamp control. The I-V plots obtained have a voltage-dependent region between -60 and -20 mV, and a linear voltage-independent region between -20 and 20 mV, with a reversal potential near 20 mV. The slope of the linear region did not change as a result of habituation indicating that the maximum mechanoreceptor current, and hence the total number of mechanoreceptor channels, was the same in control and habituated cells. The mechanoreceptor current reversal potential was changed by only a few millivolts, from 21.6 to 17.2 mV, leaving the driving force term for the mechanoreceptor current at resting potential relatively unchanged. On the other hand, the voltage-dependent region of the I-V plot became significantly steeper after habituation (9.6 mV/e-fold change) compared to its value before habituation (12.3 mV/e-fold change). This shift in voltage dependence is sufficient to explain the diminution in receptor potential amplitude seen during normal recordings at resting potential. The importance of the voltage-dependent characteristic of the mechanoreceptor channels to the habituation process was corroborated by the observation that the amplitude of receptor potentials elicited from cells depolarized by current pulses habituated to only a very limited degree while receptor potentials elicited at resting potential underwent marked habituation.(ABSTRACT TRUNCATED AT 250 WORDS)