Biomedical subjects

M Weckström

Publications and source records attributed to M Weckström.

At least 37 records · Page 2Linked to original sources

PubMed1995

Visual ecology and voltage-gated ion channels in insect photoreceptors.

That particular membrane conductances are selected for expression to enable the efficient coding of biologically relevant signals is illustrated by recent work on insect photoreceptors. These studies exploit the richness of insect vision and the accessibility of insect photoreceptors to cellular analysis in both intact animal and isolated cell preparations. The distribution of voltage-gated conductances among photoreceptors of different species correlates with visual ecology. Delayed-rectifier K+ channels are found in the rapidly responding photoreceptors of fast-flying flies. The conductance's activation range and dynamics match light-induced signals, and enable a rapid response by reducing the membrane time constant. Slow-moving flies have slowly responding photoreceptors that lack the delayed rectifier, but express an inactivating K+ conductance that is metabolically less demanding. Complementing these findings, locust photoreceptor membranes are modulated diurnally. The delayed rectifier is exhibited during the day and the inactivating K+ current is exhibited at night. Insect photoreceptors also demonstrate the amplification of signals by voltage-gated Na+ channels. In drone-bee photoreceptors, voltage-gated Na+ channels combine with K+ channels to enhance the small transient signals produced by the image of a queen bee passing over the retina. This subthreshold amplifier operates most effectively over the range of light intensities at which drones pursue queens.

Animals↗

PubMed1995

Transfer of graded potentials at the photoreceptor-interneuron synapse.

To characterize the transfer of graded potentials and the properties of the associated noise in the photoreceptor-interneuron synapse of the blowfly (Calliphora vicina) compound eye, we recorded voltage responses of photoreceptors (R1-6) and large monopolar cells (LMC) evoked by: (a) steps of light presented in the dark; (b) contrast steps; and (c) pseudorandomly modulated contrast stimuli at backgrounds covering 6 log intensity units. Additionally, we made recordings from photoreceptor axon terminals. Increased light adaptation gradually changed the synaptic signal transfer from low-pass to band-pass filtering. This was accompanied by decreased synaptic delay and increased contrast gain, but the overall synaptic gain and the intrinsic noise (i.e., transmission noise) were reduced. Based on these results, we describe a descriptive synaptic model, in which the kinetics of the tonic transmitter (histamine) release from the photoreceptor axon terminals change with mean photoreceptor depolarization. During signal transmission, tonic transmitter release is augmented by voltage-dependent contrast-enhancing mechanisms in the photoreceptor axons that produce fast transients from the rising phases of the photoreceptor responses and add these enhanced voltages to the original photoreceptor responses. The model can predict the experimental findings and it agrees with the recently proposed theory of maximizing sensory information.

Animals↗

PubMed1995

Graded responses and spiking properties of identified first-order visual interneurons of the fly compound eye.

1. We studied the graded and spiking properties of the "non-spiking" first-order visual interneurons of the fly compound eye in situ with the use of intracellular recordings. Iontophoretical QX-314 injections, Lucifer yellow marking, and (discontinuous) current-clamp method together with transfer function analysis were used to characterize the neural signal processing mechanisms in these neurons. 2. A light-OFF spike was seen in one identified anatomic subtype (L3, n = 6) of the three first-order visual interneurons (L1, L2, and L3, or LMCs) when recorded from synaptic region (i.e., in the 1st visual ganglion, lamina ganglionaris) in dark-adapted conditions. Hyperpolarization of the membrane potential by current caused the identified L1 (n = 4), as well as L3 (n = 6), to produce an OFF spike, a number of action potentials, and some subthreshold depolarizations after the light-ON response. In L2 the OFF spike or action potentials could not be elicited. 3. To produce action potentials in L1 and L3, it was found to be necessary to hyperpolarize the cells approximately 35-45 mV (n = 43) below the resting potential (RP) in the synaptic zone. Recordings from the axons of these cells revealed that near the second neuropil (chiasma) the threshold of these spikes was near to (approximately 10 mV below, n = 16) or even at the RP when an ON spike was also produced (n = 4). 4. The recorded spikes were up to 54 mV in amplitude, appeared with a maximum frequency of up to 120 impulses/s, and had a duration of approximately 8 ms. In L1 and L3 the spikes were elicited either after a light pulse (L3) or after a negative current step that was superimposed on a hyperpolarizing steady-state current (L3 and L1). A positive current step (similarly superimposed on a hyperpolarizing steady-state current) also triggered the spikes during the step. 5. Iontophoretic injection of a potent intracellularly effective blocker of voltage-gated sodium channels, QX-314, irreversibly eradicated the spikes and subthreshold depolarizations (n = 5). In addition, further injections elongated the light-ON responses and decreased or even abolished the light-OFF response. 6. Negative prepulses followed by positive current steps were applied from the RP, to test the activation-inactivation properties of the channels responsible for the OFF spike.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1995

Tonic transmitter release in a graded potential synapse.

1. We studied graded synaptic transmission in the fly photoreceptor-interneuron synapse by using intracellular in situ recordings from pre- and postsynaptic cells. 2. A large presynaptic hyperpolarization after light adaptation, caused by the activation of the electrogenic Na+/K+ pump, drastically reduced the conspicuous postsynaptic dark noise. At the same time, the postsynaptic neurons depolarized, with an increase of input resistance of 5-10 M omega. 3. The spectral characteristics of the postsynaptic membrane noise in dark and during noise reduction, together with the other results, suggested that the transmitter release decreased dramatically approximately 12 mV below the resting potential of the presynaptic photoreceptors. 4. During the postsynaptic noise reduction, the saturated and subsaturated first-order visual interneuron responses were increased up to 9 mV with a time constant of recovery of approximately 10 s. This increase was shown to be caused by the negative shift of the reversal potential of the transmitter-gated (mainly Cl-) conductance, caused apparently by the reduced transmitter input. 5. The results strongly suggest that the photoreceptor transmitter release in fly is tonic, even in dark, and further support the modulation of the synaptic voltage transfer by postsynaptic Cl- extrusion.

Animals↗

PubMed1995

Nonlinear models of the first synapse in the light-adapted fly retina.

1. Randomly modulated light stimuli were used to characterize the nonlinear dynamic properties of the synapse between photoreceptors and large monopolar neurons (LMC) in the fly retina. Membrane potential fluctuations produced by constant variance contrast stimuli were recorded at eight different levels of background light intensity. 2. Representation of the photoreceptor-LMC input-output data in the form of traditional characteristic curves indicated that synaptic gain was reduced by light adaptation. However, this representation did not include the time-dependent properties of the synaptic function, which are known to be nonlinear. Therefore nonlinear systems analysis was used to characterize the synapse. 3. The responses of photoreceptors and LMCs to random light fluctuations were characterized by second-order Volterra series, with kernel estimation by the parallel cascade method. Photoreceptor responses were approximately linear, but LMC responses were clearly nonlinear. 4. Synaptic input-output relationships were measured by passing the light stimuli to LMCs through the measured photoreceptor characteristics to obtain an estimate of the synaptic input. The resulting nonlinear synaptic functions were well characterized by second-order Volterra series. They could not be modeled by a linear-nonlinear-linear cascade but were better approximated by a nonlinear-linear-nonlinear cascade. 5. These results support two possible structural models of the synapse, the first having two parallel paths for signal flow between the photoreceptor and LMC, and the second having two distinct nonlinear operations, occurring before and after chemical transmission. 6. The two models were cach used to calculate the synaptic gain to a brief change in photoreceptor membrane potential. Both models predicted that synaptic gain is reduced by light adaptation.

Adaptation, Ocular↗

PubMed1995

2,3-Butanedione monoxime (BDM) increases initial yields and improves long-term survival of isolated cardiac myocytes.

When BDM is added to the perfusion solutions used during the preparation of single, enzymatically dispersed mammalian cardiomyocytes, higher yields of calcium-tolerant cells are obtained. As the principal component of a storage solution, BDM also improves the survival of myocytes maintained in cold storage. These data support the hypothesis that BDM can act as a cardioprotective agent under certain conditions.

Animals↗

PubMed1994

Block of stretch-activated atrial natriuretic peptide secretion by gadolinium in isolated rat atrium.

1. Isolated superfused rat atrial preparations were used to study the mechanism of stretch-induced atrial natriuretic peptide (ANP) secretion. The stretch of the atrial myocytes was induced by raising the intra-atrial pressure. The secretion rates were analysed by measuring ANP concentrations from the superfusate fractions by radioimmunoassay. 2. The effect of gadolinium, a blocker of stretch-activated ion channels, on stretch-induced and basal ANP secretion was investigated by superfusing the atrial preparation with 5, 20 or 80 microM GdCl3. Gadolinium decreased stretch-induced ANP secretion in a dose-dependent manner, but did not affect basal secretion. 3. Because high concentrations of gadolinium may block voltage-gated calcium channels, we tested whether the selective blockers of L-type (diltiazem) and T-type (NiCl2) calcium channels affect the stretch-stimulated ANP release. Neither diltiazem at 3 microM nor NiCl2 at 50 microM affected stretch-induced ANP release in paced atrial preparation. 4. Gadolinium, but not diltiazem, also inhibited stretch-stimulated ANP secretion in non-paced, quiescent atria. 5. The findings that ANP release is inhibited by Gd3+, but not by diltiazem or NiCl2, and that the stretch-induced secretion in quiescent atria is also inhibited by Gd3+, suggest that stretch-activated ion channels are involved in the regulation of stretch-induced ANP release.

Animals↗

PubMed1994

Effect of ryanodine on atrial natriuretic peptide secretion by contracting and quiescent rat atrium.

To elucidate the mechanism involved in the release of atrial natriuretic peptide (ANP), we studied the importance of ryanodine-sensitive Ca2+ release in stretch-secretion coupling. The experiments were made with a left atrial preparation, where the stretch of myocytes was induced by changing the intra-atrial pressure. When external pacing was not applied, the atrial preparation was not spontaneously contracting, and it was therefore possible to investigate the secretory mechanism in the quiescent atrium. The superfusate was collected in 2-min fractions and assayed for ANP immunoreactivity. Filtration analysis revealed that the major fraction in the superfusate in all experimental situations had a similar molecular weight as the ANP 1-28. Ryanodine (1.0 microM and 0.1 microM) inhibited stretch-stimulated ANP secretion dose dependently both in paced and non-paced atrium, but did not have any effect on basal secretion. The present results support the notion that intracellular Ca2+ transients from the intracellular stores are essential for stretch-stimulated ANP secretion, independently from excitation and contraction. Basal ANP secretion is not inhibited by blocking ryanodine-sensitive Ca2+ channels, either in contracting or in non-contracting atria. In addition our results confirm that the principal stimulus for ANP secretion in response to atrial distension is the stretch of myocytes. Length shortening of myocytes is not essential for ANP release.

Animals↗

PubMed1994

Contrast gain, signal-to-noise ratio, and linearity in light-adapted blowfly photoreceptors.

Response properties of short-type (R1-6) photoreceptors of the blowfly (Calliphora vicina) were investigated with intracellular recordings using repeated sequences of pseudorandomly modulated light contrast stimuli at adapting backgrounds covering 5 log intensity units. The resulting voltage responses were used to determine the effects of adaptational regulation on signal-to-noise ratios (SNR), signal induced noise, contrast gain, linearity and the dead time in phototransduction. In light adaptation the SNR of the photoreceptors improved more than 100-fold due to (a) increased photoreceptor voltage responses to a contrast stimulus and (b) reduction of voltage noise at high intensity backgrounds. In the frequency domain the SNR was attenuated in low frequencies with an increase in the middle and high frequency ranges. A pseudorandom contrast stimulus by itself did not produce any additional noise. The contrast gain of the photoreceptor frequency responses increased with mean illumination and the gain was best fitted with a model consisting of two second order and one double pole of first order. The coherence function (a normalized measure of linearity and SNR) of the frequency responses demonstrated that the photoreceptors responded linearly (from 1 to 150 Hz) to the contrast stimuli even under fairly dim conditions. The theoretically derived and the recorded phase functions were used to calculate phototransduction dead time, which decreased in light adaptation from approximately 5-2.5 ms. This analysis suggests that the ability of fly photoreceptors to maintain linear performance under dynamic stimulation conditions results from the high early gain followed by delayed compressive feed-back mechanisms.

Adaptation, Ocular↗

PubMed1994

The regulation of chloride homeostasis in the small nonspiking visual interneurons of the fly compound eye.

1. We have used intracellular recordings and ionophoretic injections in vivo to investigate the ion exchange mechanisms responsible for the maintenance of the ion gradients in the large monopolar cells (LMCs) of the first optic ganglion of the blowfly, Calliphora vicinia. 2. Ionophoretic chloride injections caused a rapid approximately 20-mV depolarization of the resting potential (Erp) and abolished or even reversed the light-ON response (OR), which is caused by histamine-gated chloride conductance, as the chloride equilibrium potential (ECl) was increased beyond the Erp, i.e., 50 mV upward. Ionophoretic sodium injections were found to mimic the action of the ionophoretic chloride injections and thus also to cause chloride accumulation inside the cell. 3. Ionophoretic injections of bicarbonate only had the effect of hyperpolarizing the Erp by 5-15 mV for 1-25 s, but chloride gradient, i.e., ECl remained unchanged. Intracellular proton load caused depolarization of the Erp by 15 +/- 5 mV (mean +/- SE) for 20-25 s and a slight 15 +/- 5-mV decrease of the peak OR. Ionophoretic injections of potassium, acetate, and furosemide failed to cause any physiological effect. 4. The time constant for the recovery of the peak OR after sodium load increased linearly as a function of injected charge whereby the time constant for the recovery after chloride accumulation increased slowly up to 50 nC of injected charge, after which it increased rapidly, possibly indicating substrate inhibition. The time constant for the recovery of peak OR after sodium load was from 5 to 65 nC greater than that of chloride.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphatases↗

PubMed1994

The rpa (receptor potential absent) visual mutant of the blowfly (Calliphora erythrocephala) is deficient in phospholipase C in the eye.

The rpa (receptor potential absent) mutation of the blowfly, Calliphora erythrocephala, reduces the light-evoked responses of photoreceptor cells and renders the fly blind. This phenotype is similar to the phenotype caused by norpA mutations in Drosophila which have been shown to occur within a gene encoding phospholipase C. In Western blots, norpA antiserum stains a protein in homogenates of wild-type Calliphora eye and head that is similar in molecular weight to the norpA protein. Very little staining of this protein is observed in similar homogenates of rpa mutant. Moreover, norpA antiserum strongly stains retina in immunohistochemical assays of wild-type adult head, but not in rpa mutant. Furthermore, eyes of rpa mutant have a reduced amount of phospholipase C activity compared to eye of wild-type Calliphora. These data suggest that the rpa mutation occurs in a phospholipase C gene of the blowfly that is homologous to the norpA gene of Drosophila.

Animals↗

PubMed1993

Band-pass filtering by voltage-dependent membrane in an insect photoreceptor.

The membrane properties of short type blowfly photoreceptors (R1-6) were investigated in dark and light adaptation with single electrode current and voltage clamp techniques. The impedance of the cells was defined in frequency domain by using discontinuous current clamp and white-noise-modulated current injection. We found that the slow activation and relaxation of the voltage-dependent K+ conductance transform the photoreceptor membrane effectively into a band-pass filter. This behaviour could be observed under current clamp as voltage-dependent outward and inward rectification of the membrane. The voltage-dependent band-pass filtering is likely to be present in all neurons with graded potentials and voltage-dependent membrane conductances.

Adaptation, Physiological↗

PubMed1993

The dynamic nonlinear behavior of fly photoreceptors evoked by a wide range of light intensities.

Fly photoreceptor cells were stimulated with steps of light over a wide intensity range. First- and second-order Volterra kernels were then computed from sequences of combined step responses. Diagonal values of the second-order Volterra kernels were much greater than the off-diagonal values, and the diagonal values were roughly proportional to the corresponding first-order kernels, suggesting that the response could be approximated by a static nonlinearity followed by a dynamic linear component (Hammerstein model). The amplitudes of the second-order kernels were much smaller in light-adapted than in dark-adapted photoreceptors. Hammerstein models constructed from the step input/output measurements gave reasonable approximations to the actual photoreceptor responses, with light-adapted responses being relatively better fitted. However, Hammerstein models could not account for several features of the photoreceptor behavior, including the dependence of the step response shape on step amplitude. A model containing an additional static nonlinearity after the dynamic linear component gave significantly better fits to the data. These results indicate that blowfly photoreceptors have a strong early gain control nonlinearity acting before the processes that create the characteristic time course of the response, in addition to the nonlinearities caused by membrane conductances.

Animals↗

PubMed1993

A digital feedback controller application: a light response control system to reveal photoreceptor adaptation.

We present a new digital feedback application for the study of sensitivity characteristics of biological photoreceptors. The voltage response of a cell is controlled by feeding back to the stimulating LED--via the controller--the recorded membrane voltage. The light intensity changes represent the change in the sensitivity of the photoreceptors, revealing the time course of adaptation.

Adaptation, Physiological↗

PubMed1993

Spike-like potentials in the axons of nonspiking photoreceptors.

1. The voltage responses to light of dark-adapted cockroach photoreceptors were recorded from the somata in the retina and the axons below the two basement membranes. 2. One or more spike-like fast depolarizations superimposed on the graded receptor potential were recorded in photoreceptor axons identified by Lucifer yellow injections. These spikes are voltage dependent in as much as they could be elicited with depolarizing current pulses as well as with light stimuli. In photoreceptor somata only graded receptor potentials were recorded. 3. The physiological function of these axonal spikes may be to serve as an amplification mechanism that counteracts the unfavorable combination of photoreceptor geometry and electrical properties.

Algorithms↗

PubMed1992

Measurement of cell impedance in frequency domain using discontinuous current clamp and white-noise-modulated current injection.

A method is described for the determination of cellular input impedance of non-spiking neurones. The input impedance is important when cellular geometry and the effects of voltage-dependent channels are considered. Cells are impaled with a single glass microelectrod and current is injected using a time-sharing technique. The cell's impedance is measured by randomly modulating the injected current and calculating the impedance as a transfer function between current and recorded membrane voltage. Corresponding coherence functions can also be calculated for estimating the signal-to-noise ratio, and also linearity (i.e. possible activation of voltage-dependent conductances) of the membrane.

Animals↗

PubMed1991

Voltage-activated potassium channels in blowfly photoreceptors and their role in light adaptation.

1. The membrane properties of the photoreceptors of the blowfly (Calliphora vicina) were investigated in situ by making intracellular recordings in the intact retina, using discontinuous single-electrode current and voltage clamp techniques. Single channels were investigated using inside-out patches from dissociated photoreceptors. 2. Photoreceptors have a resting potential in darkness of -60.4 +/- 6.6 mV (mean +/- S.D.; n = 43), a resting input resistance of 32 +/- 3 M omega (n = 11) and membrane time constant of 4.1 +/- 1 ms (n = 9). These values give a total cell capacitance of 0.13 nF and an effective membrane area of 1.3 x 10(-4) cm2. 3. Single-electrode voltage clamp reveals a voltage-sensitive outward current with an activation threshold at approximately -75 mV. This conductance has two kinetic components, the slower component activating at more depolarized levels. On the basis of its kinetics, a reversal potential of -85 +/- 6 mV (n = 6), sensitivity to intracellularly injected tetraethylammonium chloride (TEA), and its slow and partial inactivation (approximately 25%) this mechanism is classified as a delayed rectifier potassium conductance. 4. Voltage-sensitive potassium channels showing similar properties were found in excised inside-out patches from dissociated photoreceptors. Single-channel conductances are ca 20 pS for both fast and slow kinetic components, indicating a channel density in the intact cell of ca 2 microns -2. The reversal potential follows the Nernst slope for potassium ions. 5. The voltage dependence of the conductance was determined in patches containing channels of predominantly one or the other kinetic component. The midpoint of the activation curve is -65 mV for the fast and -50 mV for the slow component. Activation time constants (measured from a holding potential of -100 mV) are voltage dependent, and in the range 1-10 ms for the fast and 5-40 ms for the slow component. Both kinetic components are blocked by TEA (greater than 2.5 mM). The slow component is more sensitive to quinidine (greater than 200 microM), and the fast component to 4-aminopyridine (4-AP; greater than 200 microM). 6. In the intact preparation the outward current shows no dependence on light stimulation in the studied ranges of voltage (up to -25 mV) and intensity (up to 5.5 x 10(4) effective photons). Ensemble averages of channel openings in perfused inside-out patches show no dependence on calcium concentration in the range 10 nM-1.8 mM.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Ocular↗

PubMed1989

Light and dark adaptation in fly photoreceptors: duration and time integral of the impulse response.

The properties of impulse responses of blowfly photoreceptors in light and dark adaptation were studied with brief (30 mu sec) white light flashes applied with different repetition frequencies, thereby producing a range of interstimulus periods (ISPs) from 200 msec to 100 sec. The amplitude of the impulse response as intracellularly recorded membrane voltage recovered fully with ISP greater than 300 msec. The duration of the impulse response (as half-repolarization time) increased beyond that, saturating with ISPs of about 30 sec with durations of about 60 msec, which is to be compared with a duration of 14 msec with ISP of 200 msec. This finding is not associated with saturation of the membrane potential, as it is also found with subsaturating responses. It is also different from the prolonged depolarizing afterpotential (PDA). The findings could in principle be explained by calcium-activated potassium conductance in the photoreceptor membrane.

Animals↗