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PubMed2005

Elementary purinergic Ca2+ transients evoked by nerve stimulation in rat urinary bladder smooth muscle.

The translation of nerve transmission to Ca2+ signals in urinary bladder smooth muscle (UBSM) is incompletely understood. Thus, we sought to characterize Ca2+ signals in strips of UBSM loaded with the Ca2+-sensitive fluorescent dye, fluo-4, using laser scanning confocal microscopy. Two types of Ca2+ signals occurred spontaneously and could be evoked with field stimulation: large, rapid, global Ca2+ transients termed 'global Ca2+ flashes', and much smaller, localized Ca2+ transients. Global Ca2+ flashes were inhibited by the L-type voltage-dependent Ca2+ channel (VDCC) inhibitor, diltiazem and with P2X receptor blockade. Simultaneous intracellular recordings and Ca2+ measurements indicated that these events are caused by Ca2+ influx through VDCCs during action potentials. Small, local Ca2+ transients occurred spontaneously, and their frequency could be elevated with field stimulation. Atropine, an inhibitor of muscarinic receptors, did not affect these local Ca2+ transients. However, the desensitizing P2X receptor agonist alpha,beta-methylene ATP, and the purinergic antagonist, suramin, effectively inhibited the local Ca2+ transients. The frequency of these 'purinergic Ca2+ transients' was increased about 7-fold by a 10 s stimulus train (1 Hz). The amplitude, duration at one-half amplitude and the spatial spread of the evoked purinergic Ca2+ transients were F/F(o) = 2.4 +/- 0.13, 111.7 +/- 9.3 ms and 14.0 +/- 1.0 microm2, respectively. Tetrodotoxin inhibited evoked purinergic Ca2+ transients, indicating that they were dependent on nerve fibre activation. Purinergic Ca2+ transients were not dependent on VDCC activity. Neither 2-APB, an inhibitor of inositol 1,4,5-triphosphate (Ins(1,4,5)P3) (IP3)-induced Ca2+ release, nor ryanodine inhibited the purinergic Ca2+ transients. We have identified two novel Ca2+ signals in rat UBSM. Large, rapid, global Ca2+ flashes that represent Ca2+ influx through VDCCs during action potentials, and local, purinergic Ca2+ transients that represent Ca2+ entry through P2X receptors. Our results indicate that purinergic Ca2+ transients evoked by release of ATP from nerve varicosities are elementary signals in the process of nerve-smooth muscle communication.

Animals↗
PubMed2006

Regional differences in spontaneous Ca2+ spark activity and regulation in cat atrial myocytes.

Calcium sparks result from the concerted opening of a small number of Ca2+ release channels (ryanodine receptors, RyRs) organized in clusters in the membrane of the sarcoplasmic reticulum (SR). Calcium sparks represent the elementary events of SR Ca2+ release in cardiac myocytes, and their spatial and temporal summation results in whole-cell [Ca2+]i transients observed during excitation-contraction coupling (ECC). Atrial myocytes generally lack transverse tubules; however, during ECC Ca2+ release is initiated from junctional SR (j-SR) in the cell periphery from where activation propagates inwardly through Ca(2+)-induced Ca2+ release (CICR) from non-junctional SR (nj-SR). Despite the structural differences in the microdomains of RyRs of j-SR and nj-SR, spontaneous Ca2+ sparks are observed from both types of SR, albeit at different frequencies. In cells that showed spontaneous Ca2+ sparks from j-SR and nj-SR, subsarcolemmal (SS) Ca2+ sparks from the j-SR were 3-4 times more frequent than central (CTR) Ca2+ sparks occurring from nj-SR. Subsarcolemmal Ca2+ sparks had a slightly higher amplitude, but were essentially identical in their spatial spread and duration when compared to CTR Ca2+ sparks. Sensitization of RyRs with a low concentration (0.1 mM) of caffeine led to a 107% increase in the frequency of CTR Ca2+ sparks, whereas the SS Ca2+ spark frequency increased by only 58%, suggesting that the nj-SR is capable of much higher Ca2+ spark activity than observed normally in unstimulated cells. The L-type Ca2+ channel blocker verapamil reduced SS Ca2+ spark frequency to 38% of control values, whereas Ca2+ spark activity from nj-SR was reduced by only 19%, suggesting that SS Ca2+ sparks are under the control of Ca2+ influx from the extracellular space. Removal of extracellular Ca2+ eliminated SS Ca2+ sparks completely, whereas Ca2+ sparks from the nj-SR continued, albeit at a lower frequency. In membrane-permeabilized (saponin-treated) atrial myocytes, where [Ca2+] can be experimentally controlled throughout the entire myocyte, j-SR and nj-SR Ca2+ spark frequencies were identical, and Ca2+ sparks could be observed spaced at sarcomeric distances throughout the entire cell, suggesting that all release sites of the nj-SR can become active. Measurement of SR Ca2+ load (10 mM caffeine) revealed no difference between j-SR and nj-SR. The data suggest that in atrial myocytes, which lack a t-tubular system, the nj-SR is fully equipped with a three-dimensional array of functional SR Ca2+ release sites; however, in intact cells under resting conditions, peripheral RyR clusters have a higher probability of activation owing to their association with surface membrane Ca2+ channels, leading to higher spontaneous Ca2+ spark activity. In conclusion, Ca2+ sparks originating from both j-SR and nj-SR are rather stereotypical and show little differences in their spatiotemporal properties. In intact cells, however, the higher frequency of spontaneous SS Ca2+ sparks arises from the structural arrangement of sarcolemma and j-SR membrane and thus from the difference in the trigger mechanism.

Animals↗
PubMed2006

The inotropic effect of cardioactive glycosides in ventricular myocytes requires Na+-Ca2+ exchanger function.

Glycoside-induced cardiac inotropy has traditionally been attributed to direct Na(+)-K(+)-ATPase inhibition, causing increased intracellular [Na(+)] and consequent Ca(2+) gain via the Na(+)-Ca(2+) exchanger (NCX). However, recent studies suggested alternative mechanisms of glycoside-induced inotropy: (1) direct activation of sarcoplasmic reticulum Ca(2+) release channels (ryanodine receptors; RyRs); (2) increased Ca(2+) selectivity of Na(+) channels (slip-mode conductance); and (3) other signal transduction pathways. None of these proposed mechanisms requires NCX or an altered [Na(+)] gradient. Here we tested the ability of ouabain (OUA, 3 microm), digoxin (DIG, 20 microm) or acetylstrophanthidin (ACS, 4 microm) to alter Ca(2+) transients in completely Na(+)-free conditions in intact ferret and cat ventricular myocytes. We also tested whether OUA directly activates RyRs in permeabilized cat myocytes (measuring Ca(2+) sparks by confocal microscopy). In intact ferret myocytes (stimulated at 0.2 Hz), DIG and ACS enhanced Ca(2+) transients and cell shortening during twitches, as expected. However, prior depletion of [Na(+)](i) (in Na(+)-free, Ca(2+)-free solution) and in Na(+)-free solution (replaced by Li(+)) the inotropic effects of DIG and ACS were completely prevented. In voltage-clamped cat myocytes, OUA increased Ca(2+) transients by 48 +/- 4% but OUA had no effect in Na(+)-depleted cells (replaced by N-methyl-d-glucamine). In permeabilized cat myocytes, OUA did not change Ca(2+) spark frequency, amplitude or spatial spread (although spark duration was slightly prolonged). We conclude that the acute inotropic effects of DIG, ACS and OUA (and the effects on RyRs) depend on the presence of Na(+) and a functional NCX in ferret and cat myocytes (rather than alternate Na(+)-independent mechanisms).

Animals↗
PubMedDate not supplied

Dose descriptors for computed tomography.

A weakness in the dose descriptor, the computed tomography dosimetry index, is discussed and an alternate descriptor is proposed. The new descriptor, the equivalent rectangular width, conveniently expresses the spatial spread of dose and is simply derived from customary dose measurements. A measure of performance, which expresses the relative size of the volume being imaged with respect to the volume receiving the dose, is suggested for computed tomography. 87.70.Es, 87.60.Mv.

Radiation Dosage↗
PubMed2000

Dopamine transporter tryptophan mutants highlight candidate dopamine- and cocaine-selective domains.

Cocaine blocks the normal role of the dopamine transporter (DAT) in terminating dopamine signaling and in restricting its spatial spread through molecular interactions that remain largely obscure. Cocaine analog structure-activity studies suggest roles for cationic and hydrophobic interactions between DAT, dopamine, cocaine, and the sodium and chloride ions whose gradients power uptake processes. Tryptophan residues lying in putative DAT transmembrane domains could contribute to both aromatic and cationic interactions between DAT and dopamine or cocaine. We thus produced mutant DATs with alanine substitutions for tryptophans lying in or near putative DAT transmembrane domains. We have focused analyses on mutations that exert selective influences on affinities for dopamine or the cocaine analog CFT [(-)-2-beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane]. Substitutions W162A, W255A, and W310A reduced dopamine uptake affinities. 5W266A, 12W555A, and 12W561A each reduced dopamine superficial recognition affinities by more than 3-fold and all retained affinity for CFT. W406A, W496A and W523A each reduced CFT affinity, and W84A increased CFT affinity. None of these four mutations decreased dopamine uptake affinity. These data, current provisional DAT structural models, and results from parallel studies of other mutants identify candidate dopamine-selective DAT domains for transmembrane dopamine permeation and regions in which mutations selectively lower CFT affinities. Tryptophan residues may contribute more extensively to these selective domains than other previously studied DAT amino acids. These sites provide tempting targets for selective blockers of cocaine recognition by DAT.

Alanine↗
PubMed2006

Visual spatial summation in macaque geniculocortical afferents.

The spatial summation properties of visual signals were analyzed for geniculocortical afferents in the primary visual cortex (V1) of anesthetized paralyzed macaque monkeys. Afferent input responses were recorded extracellularly during cortical inactivation through superfusion of the cortex with muscimol, allowing investigation of lateral geniculate nucleus of the thalamus (LGN) cell properties in the absence of cortical feedback. Responses from afferent inputs were classified as magno-, parvo-, or koniocellular based on anatomical organization within the cortex, established through histological reconstructions, and visual response wavelength sensitivity. More than 80% of afferents showed strong surround suppression [suppression index (SI) >0.5] and 14% showed negligible surround suppression (SI < 0.2). Afferent responses with weak and strong surround suppression were found throughout cortical input layers 4C and 4A. High-contrast estimates of the spatial extent of the classical surround were similar to the nonclassical surround. The classical and nonclassical surrounds were, on average, 1.5-fold larger than the excitatory center. Unlike neurons within V1, the spatial extent of excitatory summation for geniculocortical afferents was contrast invariant. Nonclassical surround suppression showed slight contrast dependency with estimates larger (20%) at lower contrasts and stronger at higher contrasts (13%). Surround suppression is inherent in cortical input responses and likely derives from lateral inhibition in either the LGN or retina. Although surround suppression within afferent responses increases slightly with contrast, the spatial spread of excitation remains fixed with contrast. This argues for distinct mechanisms of action for contrast-dependent modulation in cortical and subcortical responses.

Action Potentials↗
PubMed1987

Cell size, projection path, and localization of estrogen-sensitive neurons in the rat ventromedial hypothalamus.

Antidromic action potentials were recorded in and around the ventromedial nucleus of the hypothalamus (VMN) of the ovariectomized female rat following stimulation of the central gray matter of the mesencephalon (CG). The animals had either the lateral or caudal trajectory of the VMN efferent disrupted by a small knife cut. Changes in the polarity, peak-to-peak amplitude, and the duration of the initial positive deflection of the antidromic action potentials were recorded for each response as the recording electrode was advanced at 10-micron intervals from the point of detection of the potential until its deterioration. Seventy-four recordings in which potentials exceeded 2 mV at the maxima were subjected to further analysis. The mean spatial spread of the extracellular antidromic action potentials, which was measured by moving the recording electrode at 10-micron intervals, was significantly larger in responses in 17 animals bearing the posterior knife cut (n = 41) than those in 15 animals with the lateral knife cut (n = 33). Mean peak-to-peak amplitude was also significantly greater in responses from the posterior-cut animals than in those from the lateral-cut ones. In the former, action potentials were completed with a shorter time lapse than the latter. In the posterior-cut animals, estrogen treatment of the ovariectomized rats significantly lowered antidromic activation thresholds and shortened absolute refractory periods for CG stimulation. Estrogen had no detectable effect on the excitability of cells that survived the lateral cut. Histological analyses located antidromically driven cells in the posterior-cut animals in the rostral extremity of the VMN and the adjacent retrochiasmatic area; those in the lateral-cut animals were scattered in and around the VMN, caudad to the former. These results suggest that rostral VMN neurons with large soma and laterally projecting axons are responsible for estrogen-dependent autonomic, neuroendocrine, and behavioral functions, such as reproduction and feeding.

Action Potentials↗
PubMed1995

Calcium imaging of motoneuron activity in the en-bloc spinal cord preparation of the neonatal rat.

1. This paper describes the use of calcium imaging to monitor patterns of activity in neonatal rat motoneurons retrogradely labeled with the calcium-sensitive dye, calcium green-dextran. 2. Pressure ejection of calcium green-dextran into ventral roots and into the surgically peeled ventrolateral funiculi (VLF) at the lumbar cord labeled spinal motoneurons and interneurons. The back labeled motoneurons often formed two or three discrete clusters of cells. 3. Fluorescent changes (10-20%) could be detected in labeled motoneurons after a single antidromic stimulus of the segmental ventral root. These changes progressively increased in amplitude during stimulus trains (1-5 s) at frequencies from 5 to 50 Hz, presumably reflecting a frequency-dependent increase in free intracellular calcium. 4. Stimulation of the ipsilateral VLF at the caudal lumbar level (L6), elicited frequency-dependent, synaptically induced motoneuronal discharge. Frequency-dependent fluorescent changes could be detected in calcium green-labeled motoneurons during the VLF-induced synaptic activation. 5. The spatial spread of synaptic activity among calcium green-labeled clusters of motoneurons could be resolved after dorsal root stimulation. Low-intensity stimulation of the roots produced fluorescence changes restricted to the lateral clusters of motoneurons. With increasing stimulation intensity the fluorescence change increased in the lateral cells and could spread into the medial motoneuronal group. After a single supramaximal stimulus a similar pattern was observed with activity beginning laterally and spreading medially. 6. Substantial changes in fluorescence of calcium green-labeled motoneurons were also observed during motoneuron bursting induced by bath application of the glycine receptor antagonist strychnine or the potassium channel blocker 4-aminopyridine (4-AP). 7. Our results show that membrane-impermeant fluorescent calcium indicators can be used as a tool to study the activity of specific populations of spinal neurons during execution of motor functions in the developing mammalian spinal cord. They also suggest that lateral clusters of motoneurons in the developing spinal cord of the rat are more recruitable or excitable than more medial clusters. Further understanding of these findings requires identification of these clusters.

Animals↗
PubMed2005

Endothelin-1-induced arrhythmogenic Ca2+ signaling is abolished in atrial myocytes of inositol-1,4,5-trisphosphate(IP3)-receptor type 2-deficient mice.

Recent studies have suggested that inositol-1,4,5-trisphosphate-receptor (IP3R)-mediated Ca2+ release plays an important role in the modulation of excitation-contraction coupling (ECC) in atrial tissue and the generation of arrhythmias, specifically chronic atrial fibrillation (AF). IP3R type-2 (IP3R2) is the predominant IP3R isoform expressed in atrial myocytes. To determine the role of IP3R2 in atrial arrhythmogenesis and ECC, we generated IP3R2-deficient mice. Our results revealed that endothelin-1 (ET-1) stimulation of wild-type (WT) atrial myocytes caused an increase in basal [Ca2+]i, an enhancement of action potential (AP)-induced [Ca2+]i transients, an improvement of the efficacy of ECC (increased fractional SR Ca2+ release), and the occurrence of spontaneous arrhythmogenic Ca2+ release events as the result of activation of IP3R-dependent Ca2+ release. In contrast, ET-1 did not alter diastolic [Ca2+]i or cause spontaneous Ca2+ release events in IP3R2-deficient atrial myocytes. Under basal conditions the spatio-temporal properties (amplitude, rise-time, decay kinetics, and spatial spread) of [Ca2+]i transients and fractional SR Ca2+ release were not different in WT and IP3R2-deficient atrial myocytes. WT and IP3R2-deficient atrial myocytes also showed a significant and very similar increase in the amplitude of AP-dependent [Ca2+]i transients and Ca2+ spark frequency in response to isoproterenol stimulation, suggesting that both cell types maintained a strong inotropic reserve. No compensatory changes in Ca2+ regulatory protein expression (IP3R1, IP3R3, RyR2, NCX, SERCA2) or morphology of the atria could be detected between WT and IP3R2-deficient mice. These results show that lack of IP3R2 abolishes the positive inotropic effect of neurohumoral stimulation with ET-1 and protects from its arrhythmogenic effects.

Action Potentials↗
PubMed2001

Calcium puffs are generic InsP(3)-activated elementary calcium signals and are downregulated by prolonged hormonal stimulation to inhibit cellular calcium responses.

Elementary Ca(2+) signals, such as "Ca(2+) puffs", which arise from the activation of inositol 1,4,5-trisphosphate receptors, are building blocks for local and global Ca(2+) signalling. We characterized Ca(2+) puffs in six cell types that expressed differing ratios of the three inositol 1,4,5-trisphosphate receptor isoforms. The amplitudes, spatial spreads and kinetics of the events were similar in each of the cell types. The resemblance of Ca(2+) puffs in these cell types suggests that they are a generic elementary Ca(2+) signal and, furthermore, that the different inositol 1,4,5-trisphosphate isoforms are functionally redundant at the level of subcellular Ca(2+) signalling. Hormonal stimulation of SH-SY5Y neuroblastoma cells and HeLa cells for several hours downregulated inositol 1,4,5-trisphosphate expression and concomitantly altered the properties of the Ca(2+) puffs. The amplitude and duration of Ca(2+) puffs were substantially reduced. In addition, the number of Ca(2+) puff sites active during the onset of a Ca(2+) wave declined. The consequence of the changes in Ca(2+) puff properties was that cells displayed a lower propensity to trigger regenerative Ca(2+) waves. Therefore, Ca(2+) puffs underlie inositol 1,4,5-trisphosphate signalling in diverse cell types and are focal points for regulation of cellular responses.

Adenosine Triphosphate↗
PubMed2006

Nonparaxial fields with maximum joint spatial-directional localization. II. Vectorial case.

The monochromatic nonparaxial vector fields that achieve a minimum spatial spread for a given directional spread are found. The derivation of these fields is analogous to the one presented in part I of this series for the case of scalar fields. This derivation is based on a variational treatment and multipolar expansion. The resulting lower bounds for the spreads of vector fields turn out to be considerably more restrictive than for scalar fields.

Journal Article↗
PubMed2005

Video-rate near-infrared optical tomography using spectrally encoded parallel light delivery.

A novel parallel source implementation approach to near-infrared tomography is demonstrated through spectral encoding of the light delivery. This new technique allows many sources to be input into the tissue at the same time, and a high-resolution spectrometer is used to spatially spread out the signals from each spectrally encoded source. The parallel sampling of all sources at all detection locations renders rapid imaging. Acquisition of complete tomographic data sets at a video rate of 35 frames/s is achieved for imaging of a 6.35 mm diameter inclusion with an absorption coefficient of 0.01 mm(-1) and a reduced scattering coefficient of 1.5 mm(-1) that is moving along a circular path inside a 1% Intralipid solution.

Computer Systems↗
PubMed2006

Modeling the statistics of elementary calcium release events.

Elementary Ca(2+) signals, such as "Ca(2+) puffs", which arise from the release of Ca(2+) from endoplasmic reticulum through small clusters of inositol 1,4,5-trisphosphate receptors, are the building blocks for intracellular Ca(2+) signaling. The small number of release channels involved during a Ca(2+) puff renders the puffs stochastic, with distributed amplitudes, durations, and frequency, well characterized experimentally. We present a stochastic model that accurately describes simultaneously the statistical properties of the duration, amplitudes, frequencies, and spatial spread with a single set of parameters.

Calcium↗
PubMed2006

The use of genetic clines to estimate dispersal distances of marine larvae.

Many unresolved issues in the ecology and evolution of marine populations center on how far planktonic larvae disperse away from their parents. Genetic tools provide a promising way to define the spatial spread of larvae, yet their accurate interpretation depends on the extent to which genetic loci are under selection. Genetic clines, geographic zones in which genetically differentiated populations interbreed, provide opportunities to explicitly and simultaneously quantify the relative roles of selection and dispersal. Here, we review the theory and analysis of genetic clines and apply these techniques to published studies of multilocus clines in the sea. The geographic width of a stable genetic cline is determined by a balance between the homogenizing effects of dispersal and the diversifying effects of selection. For marine researchers, the power of genetic clines is that, if selection and clinal width are quantified, then the average geographic distances that larvae move can be inferred. Measuring selection or dispersal through laboratory or field-based experimentation is possible, though logistically difficult, for pelagically dispersed organisms. Instead, dispersal may be more robustly quantified from the degree of linkage disequilibrium between two or more loci, because linkage disequilibrium integrates selection across multiple life stages and generations. It is also relatively insensitive to whether exogenous or endogenous selection operates. Even without quantifying linkage disequilibrium, the theory of genetic clines indicates that the average dispersal distance of larvae is a fraction (i.e., generally <35%) of the clinal width. Because cline theory is based on several underlying assumptions, including near-equilibrium between selection and migration, the dispersal distances inferred from empirical data should be of the correct order but may not be precise. Even so, such estimates of larval dispersal are valuable, as they can be utilized to design appropriate scales for future investigations and provide some guidance to conservation efforts.

Animal Migration↗
PubMedDate not supplied

[Active epidemiological surveillance and prevention of diseases caused by biomechanical overload of the upper limbs: experience at a territorial service of occupational medicine].

Nowadays in the western countries, the Work Musculoscheletal Disorders (WMSDs) are the most relevant work-related diseases either for the large number of at-risk workers and for the high prevalence of cases among them. Among WMSDs, the Carpal Tunnel Syndrome (CTS) is considered a sentinel event in occupational medicine and epidemiological surveillance systems were set up to study its temporal occurrence and spatial spread. Surprisingly enough, in our country the epidemiology of the WMSDs is still unknown; although observations of important clusters of cases are increasing, these pathologies are still largely underreported by medical doctors. In the high industrialised province of Brescia, Northwestern Italy, only 1% of the 25.000 occupational disease, collected by the occupational medicine and prevention service of the National Health Service (SPSAL) since 1989 to 1997, were coded as WMSDs. Even if the notification of these work-related disorders is compulsory by low since 1973, up-to-now very few cases were reported to SPSALs, and consequently active prevention programs were very few too. For that reason, a project of active surveillance of WMSDs diagnosed on the general population was carried out to improve their identification and to evaluate the work-related etiological fraction as well as to address primary prevention projects. During 1997-1999 618 upper limb musculoskeletal disorders were collected from 2 orthopaedic hospital divisions where cases underwent surgical treatment. 369 of them were interviewed by telephone (253 female, 116 man) and 128 cases (34%) were classified as probable professionally exposed to some known hazard. Female were prevalent (71%), involved particularly in textile, garments, plastic, food production and domestic services. Among men, construction, metallurgic and mechanic were the more frequent jobs. 116 WMSDs were reported to the National Insurance Institute (INAIL). The collection of these WMSDs was the basis for specific communications to the firm manager(s) and to the factory's medical doctor(s) with the aim to make it easy: i) a good health surveillance to exposed workers; ii) a specific risk assessment; iii) carrying out prevention programs and adopting personal protection equipments. Some problems are focused concerning the underreporting of cases, the anamnesis information, the fate of interventions to face the problem in the workplaces. A possible implementation of this project is also discussed to avoid a WMSds surveillance unlinked prevention programs of occupational risks.

Adolescent↗
PubMed2005

GIS and epidemiology.

Understanding the spatial patterns of infectious diseases can provide insight as to their causes and controls. Geographic information systems (GIS) and related technologies like remote sensing are increasingly used to analyze geographical distribution of diseases as well as relationships between pathogenic factors (causative agents, patients, vectors and hosts) and their geographic environments. Basic and analytical applications of GIS in epidemiology can help in visualizing and analyzing geographic distribution of diseases through time, thus revealing spatio-temporal trends, patterns, and relationships that would be more difficult or obscure to discover in tabular or other formats. GIS can provide a means to meet the demands of outbreak investigation and response, where understanding the spatial spread and dynamics of an outbreak is central to the design of prevention and control strategies.

Communicable Diseases↗
PubMed1991

Spatial restriction of light adaptation and mutation-induced inactivation in fly photoreceptors.

The spatial spread within fly photoreceptors of 2 forms of desensitization by bright light have been investigated: the natural process of light adaptation in normal Musca photoreceptors and a receptor-potential inactivation in the no-steady-state (nss) mutant of the sheep blowfly Lucilia. The suction-electrode method used for recording from vertebrate rods was applied to fly ommatidia. A single ommatidium in vitro was partially sucked into a recording pipette. Illumination of the portion of the ommatidium within the pipette resulted in a flow of current having a wave form similar to that of the receptor potential and polarity consistent with current flow into the illuminated region of the photoreceptors. Two 5-microns slits of light, positioned at right angles to the ommatidial axis, were employed to determine the spread of light adaptation or inactivation along the ommatidium. The intensity of a flash of light delivered to one (adapting) slit was adjusted until it produced a criterion fractional reduction in the response to the other (test) slit. The reciprocal of this intensity of the adapting slit was taken as a measure of the effectiveness of the slit in causing light adaptation or inactivation. The effectiveness of the slit in causing light adaptation in normal Musca ommatidia fell as the adapting and test slits were moved farther apart along the ommatidial axis, declining to half its maximal value at a distance of 13 +/- 2 microns. Similar measurements of the effectiveness of a slit in causing light-induced inactivation in the nss mutant of Lucilia also demonstrated localization, declining to half its maximal value at a distance between the slits of 9 +/- 1 microns. Neither light adaptation nor inactivation by the nss mutation, therefore, appear to be mediated by voltage or by a highly diffusible agent. The results are consistent with the idea that inactivation by the nss mutation replaces adaptation in the mutant photoreceptors.

Adaptation, Physiological↗
PubMed1989

Amacrine cell interactions underlying the response to change in the tiger salamander retina.

The neural circuitry and pharmacology underlying transient signal formation at the bipolar-amacrine cell interface were studied. Synaptic currents were measured with whole cell patch clamp in retinal slices. Cell types were identified with Lucifer yellow staining. Activity was initiated with puffs of kainate of known time course and spatial spread delivered at bipolar dendrites. OFF bipolar cells responded to kainate with a sustained inward current, but ON bipolar cells were silent. Two types of amacrine cell were found: (1) narrow field cells, with processes that extended laterally less than 200 microns, responding with a sustained inward current, and (2) wide field cells, with processes that extended laterally by up to 1 mm, responding with a brief transient inward current followed by a more sustained outward current. We pharmacologically dissected the synaptic interactions underlying the transient current in the wide field amacrine cell. In the presence of 5-aminovaleric acid (AVA), the time course of this transient current was increased so that it resembled the response of bipolar cells. Because AVA is a GABAB antagonist, it appears to block an opposing signal that truncates the sustained excitatory bipolar input, thereby generating the transient. GABAB specificity is confirmed by (1) block of the transient inward current by baclofen, a GABAB agonist, and (2) block of the baclofen effect by AVA. The site of GABAB action appears to be presynaptic to the amacrine cell membrane because neither baclofen nor AVA, in combination with picrotoxin, had a direct effect at the amacrine cell membrane. GABAB receptors are often found at presynaptic terminals where they modulate calcium or potassium conductances. It has been shown that bipolar cell terminals receive a GABAergic synaptic input (Vaughn et al., 1981; Wu et al., 1981; Tachibana and Kaneko, 1987). The narrow field sustained-responding amacrine cells appear to be GABAergic (Werblin et al., 1988). This suggests that transient activity measured in wide field amacrine cells is formed at a population of bipolar cell terminals by GABAergic feedback from narrow field amacrine cells at GABAB receptors.

Animals↗