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M B Djamgoz

Publications and source records attributed to M B Djamgoz.

At least 19 recordsLinked to original sources
PubMed1992

Cytochalasin inhibits light-dependent synaptic plasticity of horizontal cells in teleost retina.

Previous studies have shown that the horizontal cell-->cone photoreceptor negative feedback synapse in teleost fish retinae is 'plastic', being suppressed in the dark and potentiated by light adaptation. The possible involvement of filamentous actin in ultrastructural and electrophysiological aspects of this plasticity has been investigated using cytochalasins, which inhibit actin turnover, in the cyprinid fish (roach) retinae. Cytochalasin B or D (40 microM) inhibited both the light-dependent formation and maintenance of spinules, and enhancement of the feedback interaction involved in generation of biphasic spectral responses in horizontal cells. The results suggest that actin turnover is essential for both ultrastructural and electrophysiological plasticity of horizontal cell feedback and that spinules could mediate this dynamic interaction.

Animals↗
PubMed1992

Chromaticity of synaptic inputs to H1 horizontal cells in carp retina: analysis by voltage-clamp and spectral adaptation.

Cone photoreceptor inputs to H1 horizontal cells (H1 HCs) in carp retina were studied by measuring light-modulated currents (IL) to monochromatic stimuli (460, 533, 688 nm) under a voltage-clamp condition. By using double-barrelled micro-electrodes H1 HCs were voltage-clamped whilst perfusing with dopamine to uncouple the cells. The IL of the H1 HCs driven by each cone input was segregated by selective chromatic adaptation, and differences in the kinetics of the IL of the H1 HCs were revealed. Thus, all together, three types of IL were observed: (1) a 'fast outward' current to the long-wavelength stimulus; (2) a 'slow outward' current to the middle-wavelength stimulus; and (3) a 'delayed inward' current that followed the peak of 'slow outward' current to the short-wavelength stimulus. The reversal potentials of the three currents were estimated to be at least 20 mV more positive than the dark resting potential by extrapolation of the IL-V curve. These observations are consistent with the idea that the H1 HCs receive sign-inverting, conductance decreasing synaptic input(s) from at least one other cone mechanism, in addition to the main conventional EPSP type synaptic input from red-sensitive cones.

Animals↗
PubMed1992

Gamete development in Plasmodium berghei regulated by ionic exchange mechanisms.

Ionic regulation in the induction of exflagellation of Plasmodium berghei was investigated by culturing the parasites in various isotonic media. Of the salts tested, NaHCO3 exhibited the highest activity in inducing exflagellation, whereas KHCO3 showed no activity. In the absence of HCO3-, media containing monovalent cation (Na+, K+, Cs+, Rd+, choline+, lysine+, arginine+) and Cl- also induced exflagellation, but their activities were lower than that of NaHCO3. Anions of Br- or NO3- could be substituted with Cl-, whereas other anions such as I-, NO2-, SO4(2-), SCN-, H2PO4-, or HPO4(2-) failed to induce exflagellation, as did tetramethylammonium-Cl, CaCl2, MgSO4, MgCl2 and sucrose as well. These results suggest that the induction of exflagellation requires the presence of Na+ and HCO3- or monovalent, membrane-permeable cation and Cl- in the medium. Measurements of the efflux of H[14C]O3- or Cl- indicated that these anions were released from the cells into the NaCl or the NaHCO3 medium, respectively, probably by exchange in HCO3-/Cl-. Determination of intracellular ionic concentrations by electron microscopic X-ray microanalysis of cryopreserved specimens revealed that in the NaHCO3 medium, external Na+ (and probably HCO3-) enters the gametocytes by exchange with internal Cl- (and probably H+), whereas in Cl(-)-containing media, external unspecified cation and Cl- influx by exchange, probably with H+ and HCO3-. It is therefore suggested that two separate ion exchangers, i.e., Na(+)-dependent HCO3-(in)/Cl-(out) and nonspecific monovalent-cation-dependent Cl-(in)/HCO3-(out) exchangers, are involved in the induction of gametogenesis in P. berghei.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Protease activities in carp retina.

Protease of carp retina were examined by electrophoresis and fluorogenic assays. A 70 kD serine protease with an alkaline pH optimum was detected in gelatin-containing polyacrylamide gels. A similar enzyme was found in carp brain and muscle, but not in lens. Using aminomethylcoumarin (MCA) substrates, activities that hydrolysed Z-Phe-Arg-MCA, Boc-Ala-Gly-Pro-Arg-MCA and various aminoacyl-MCAs were detected. The Z-Phe-Arg-MCA hydrolase was an acidic cysteine protease, whereas the Boc-Ala-Gly-Pro-Arg-MCA hydrolase was an alkaline cysteine protease. All aminoacyl hydrolase activities tested were inhibited by bestatin and o-phenanthroline, but not by inhibitors of serine, cysteine and aspartic proteases, suggesting they are metalloaminopeptidases. Of the substrates tested, Tyr-MCA was the most readily hydrolysed aminoacyl substrate. Preliminary evidence was obtained suggesting that levels of these activities do not differ between light- and dark-adapted retinae. The proteases have a potential involvement in retinal functioning and show similarities to other proteases known to act in the central nervous system. In particular, the Tyr-MCA hydrolase may be related to an enzyme known to remove the N-terminal tyrosine residue from enkephalin.

Amino Acid Sequence↗
PubMed1992

Localization and function of dopamine in the adult vertebrate retina.

Dopamine (DA) has satisfied many of the criteria for being a major neurochemical in vertebrate retinae. It is synthesized in amacrine and/or interplexiform cells (depending on species) and released upon membrane depolarization in a calcium-dependent way. Strong evidence suggests that it is normally released within the retina during light adaptation, although flickering and not so much steady light stimuli have been found to be most effective in inducing endogenous dopamine release. DA action is not restricted to those neurones which appear to be in "direct" contact with pre-synaptic dopaminergic terminals. Neurones that are several microns away from such terminals can also be affected, presumably by short diffusion of the chemical. DA thus affects the activity of many cell types in the retina. In photoreceptors, it induces retinomotor movements, but inhibits disc shedding acting via D2 receptors, without significantly altering their electrophysiological responses. DA has two main effects upon horizontal cells: it uncouples their gap junctions and, independently, enhances the efficacy of their photoreceptor inputs, both effects involving D1 receptors. In the amphibian retina, where horizontal cells receive mixed rod and cone inputs, DA alters their balance in favour of the cone input, thus mimicking light adaptation. Light-evoked DA release also appears to be responsible for potentiating the horizontal cell-->cone negative feed-back pathway responsible for generation of multi-phasic, chromatic S-potentials. However, there is little information concerning action of DA upon bipolar and amacrine cells. DA effects upon ganglion cells have been investigated in mammalian (cat and rabbit) retinae. The results suggest that there are both synaptic and non-synaptic D1 and D2 receptors on all physiological types of ganglion cell tested. Although the available data cannot readily be integrated, the balance of evidence suggests that dopaminergic neurones are involved in the light/dark adaptation process in the mammalian retina. Studies of the DA system in vertebrate retinae have contributed greatly to our understanding of its role in vision as well as DA neurobiology generally in the central nervous system. For example, the effect of DA in uncoupling horizontal cells is one of the earliest demonstrations of the uncoupling of electrotonic junctions by a neurally released chemical. The many other, diverse actions of DA in the retina reviewed here are also likely to become model modes of neurochemical action in the nervous system.(ABSTRACT TRUNCATED AT 400 WORDS)

Amphibians↗
PubMed1991

Melatonin receptor mRNA expression in Xenopus oocytes: inhibition of G-protein-activated response.

Melatonin is the major endocrine product of the pineal gland in the mammalian brain and plays a variety of roles in photoperiodic functions. In order to investigate melatonin receptors, poly(A)+ RNA was extracted from pars tuberalis of the ovine pituitary and injected into oocytes of Xenopus laevis. After 3-5 days of incubation, functional melatonin receptors were expressed. Receptors were revealed by their inhibitory effect upon oscillatory currents resulting from AlF4-induced activation of G-proteins in the oocyte membrane under voltage clamp conditions. The effect of melatonin was dose-dependent, non-desensitizing and was not observed in uninjected oocytes.

Aluminum↗
PubMed1991

An interplexiform cell in the goldfish retina: light-evoked response pattern and intracellular staining with horseradish peroxidase.

The light-evoked response pattern and morphology of one interplexiform cell were studied in the goldfish retina by intracellular recording and staining. The membrane potential of the cell spontaneously oscillated in the dark. In response to a brief light stimulus, the membrane potential initially gave a slow transient depolarization. During maintained light, the oscillations showed a tendency to be suppressed; the response of the cell to the offset of the stimulus was not so prominent. The perikaryon of the interplexiform cell was positioned at the proximal boundary of the inner nuclear layer. The cell had two broad layers of dendrites; one was diffuse in the inner plexiform layer, the other was more sparse in the outer plexiform layer. The morphological and electrophysiological characteristics of the cell are discussed in relation to dopaminergic interplexiform cells and the light-evoked release pattern of dopamine in the teleost retina.

Animals↗
PubMed1991

Dopamine and plasticity of horizontal cell function in the teleost retina: regulation of a spectral mechanism through D1-receptors.

The negative feed-back interaction between horizontal cells (HCs) and cones in the cyprinid fish retina is thought to be mediated by horizontal cell spinules. These are "plastic" structures, largely absent from the dark-adapted retina and formed anew during light adaptation. We have previously shown that horizontal cell feed-back is similarly enhanced by light adaptation. The role of the interplexiform cell transmitter dopamine in both processes has been studied in the roach retina. Application of dopamine to dark-adapted retinae induced spinule formation in a dose-dependent way. The effect of dopamine was mimicked by dibutyryl-cAMP and suppressed selectively by D1 receptor antagonists. The effect of light in inducing spinule formation was lost in retinae depleted of endogenous dopamine. However, application of exogenous dopamine to these retinae triggered normal spinule formation. For all pharmacological treatments used, there was a strong correlation between spinule number and degree of feed-back activity in biphasic horizontal cells. Thus, when the spinule content of the cone pedicles was high, biphasic horizontal cell responses exhibited strong depolarizing components and vice versa. It is concluded that light-evoked formation of spinules in HC dendrites involves the action of dopamine upon D1 receptors. Spinules, in turn, are likely to be presynaptic terminals mediating the dynamic negative feed-back effect of horizontal cells upon cones.

Animals↗
PubMed1991

Voltage clamp study of electrophysiologically-identified horizontal cells in carp retina.

Passive membrane properties and electromotive force of light modulated currents of L-, R/G-type and rod-driven horizontal cells were studied by voltage-clamp using double-barrelled micro-electrodes whilst perfusing with 5 microM dopamine to uncouple the gap junctions. Input impedances of horizontal cells in darkness were 31 +/- 1.4 M omega (mean +/- SE, n = 63); the resting potentials were -37 +/- 1.3 mV. Current-voltage relationships had regions of both inward and outward rectification and a region of negative resistance was commonly observed. Reversal potentials of light modulated currents were estimated on average to be -7 +/- 4 mV (n = 14), which is consistent with the involvement of K+ and Na+ and/or Ca2+ gradients. Importantly in R/G cells both depolarizing and hyperpolarizing components of the response had essentially the same reversal potential.

Animals↗
PubMed1991

Ganglion cells in the goldfish retina: correlation of light-evoked response and morphology.

Goldfish retinal ganglion cells were intracellularly stained with horseradish peroxidase after recording their responses to a predetermined set of test stimuli. Depolarizing responses were elicited by cells differing in shapes and sizes of their somata and dendritic fields; these cells were mostly bistratified in the inner plexiform layer (sublamina b and distal sublamina a). Hyperpolarizing responses were generated by cells monostratified in a, and by cells bistratified in a and at the a/b border. Responses that were hyperpolarizing to long wavelengths and involving large superimposed depolarizations for short wavelengths were recorded from cells with somata displaced in the inner nuclear layer. The latter cell group had wide, elliptical dendritic fields (confined to the distal sublamina a) and very fine axons. The ganglion cell types recorded are compared with morphological classification schemes proposed from earlier studies. Possible "structure-function" relations are also discussed.

Animals↗
PubMed1991

Biocytin: intracellular staining, dye-coupling and immunocytochemistry in carp retina.

Correlation of electrophysiological and morphological, including ultrastructural, characteristics of neurones is important for understanding the functional organization of neuronal systems. Further correlation with neurotransmitter content is essential for determining the neurochemical(s) used by a given neurone for propagating its signal. The two main neuronal markers presently available (lucifer yellow and horseradish peroxidase) are not satisfactory for correlating all three aspects. We have devised a new simple procedure whereby retinal interneurones can be labelled with biocytin by positive ionophoresis of an unbuffered solution. Biocytin readily crosses gap junctions thus revealing extensive networks of coupled cells. In the case of H1 horizontal cells, which are known to be GABAergic, the neurotransmitter can also be demonstrated by superimposed immunocytochemistry.

Animals↗
PubMed1990

Variability of light-evoked response pattern and morphological characterization of amacrine cells in goldfish retina.

Amacrine cells of the goldfish retina were characterized electrophysiologically and subsequently labelled by intracellular injection of horseradish peroxidase. An attempt was made to broaden the electrophysiological classification of the cells. Light-evoked sustained amacrine cell responses were divided into two subtypes depending on colour opponency. Colour-coded responses (red/depolarizing and green/hyperpolarizing) were found to arise in amacrine cells possessing highly polarized dendritic fields; the dendrites were monostratified in the proximal half (sublamina b) of the inner plexiform layer. Non-colour-opponent sustained responses also arose in monostratified units, but the level of dendritic ramification was in sublamina a or b (hyperpolarizing or depolarizing units, respectively). Transient (ON-OFF) responses were associated mainly with bi- or multi-stratified or diffuse amacrine cells. Some variability was observed in the sizes of the dendritic fields in different sublaminae. There was a tendency for units with brisk components of responses to be narrowly stratified in the inner plexiform layer. Some units possessed "distant" dendrites. Several aspects of structure-function correlation in amacrine cells are discussed.

Animals↗
PubMed1990

Dopamine and 2-amino-4-phosphonobutyrate differentially modify spectral responses of H1 horizontal cells in carp retina.

Cone-driven external H1 horizontal cells (H1 HCs) in the cyprinid fish retina hyperpolarize in response to all visible lights, and their synaptic inputs have been widely believed to be excitatory. Recent experiments indicate, however, that short- and long-lambda (wavelength)-sensitive cone photoreceptors have different types of synaptic mechanisms; a conductance-decreasing, sign-reversing and short-lambda-mediating type, and a more conventional conductance-increasing class of excitatory (sign-conserving) synapse transmitting mainly long-lambda signals to H1 HCs. Here, a new set of evidence is presented for such spectrally segregated synaptic multiplicity, which also supports the notion that H1 HCs are actually color-opponent units where the depolarizing response component due to short-lambda-sensitive cones is normally overshadowed by the dominant hyperpolarizing component ascribed to long-lambda-mediating synaptic inputs. Application of dopamine to the retina preferentially enhanced the H1 HC responses to long-lambda flashes, and also depolarized the resting membrane potential in the dark. The spectral response was also examined after applying APB (2-amino-4-phosphonobutyric acid), in the presence of dopamine included to avoid polysynaptic effects of APB. This treatment enhanced the H1 HC responses to short-lambda stimuli and hyperpolarized the resting potential. These results are consistent with the suggestion that dopamine potentiates the conductance-increasing and long-lambda-mediating excitatory transmitter action, whereas APB acts as an agonist at the receptor involved in the conductance-decreasing and short-lambda-mediating transmitter action.

Aminobutyrates↗
PubMed1990

Amino acid receptors from insect muscle: electrophysiological characterization in Xenopus oocytes following expression by injection of mRNA.

Poly(A)+ Messenger ribonucleic acid (mRNA) was extracted from leg muscles of the locust Schistocerca gregaria and injected into oocytes of Xenopus laevis. After 5-10 days incubation, receptors for L-glutamate, L-quisqualate, DL-ibotenate and gamma-aminobutyric acid (GABA) were expressed. Agonist-induced currents were dose-dependent, and, in the concentration range 1 microM to 1 mM, generally had peak values of 50 nA. The responses to all agonists, apart from GABA, exhibited desensitization which could not be reversed even by prolonged washing with Ringer. Application of 100 microM GABA to oocytes voltage clamped at -60 mV produced a smooth inward current with a reversal potential of -22 +/- 1 mV, which is consistent with the involvement of chloride ions. At 100 microM, picrotoxin reversibly abolished this current, while 100 microM bicuculline had no effect. L-Glutamate elicited a smooth current with a reversal potential of -52 +/- 3 mV. L-Quisqualate elicited an inward current at -60 mV with a reversal potential of -9 +/- 2 mV; this current occasionally had an oscillatory component. The response to ibotenate comprised a smooth inward current with a reversal potential of -21 +/- 3 mV which was probably mediated by chloride ions.

Amino Acids↗
PubMed1990

Retinal dopamine.

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Adaptation, Ocular↗
PubMed1989

Haloperidol suppresses light-induced spinule formation and biphasic responses of horizontal cells in fish (roach) retina.

In retinae of lower vertebrates, negative feed-back interactions between horizontal cells (second-order neurones), and cone photoreceptors lead to generation of spectrally multi-phasic light-evoked responses (S-potentials) in horizontal cells. Spinules (finger-like extensions of horizontal cell dendrites) have been suggested to mediate these interactions in retinae of teleost fish. We have studied whether prevention of light-dependent spinule formation would indeed affect an S-potential component (the red-sensitive depolarization in H2 horizontal cells), known to depend on such negative feed-back. Haloperidol was used as a dopamine antagonist to suppress light-induced formation of spinules in retinae of the cyprinid fish, the roach. In normal (untreated) retinae, biphasic S-potentials were strongly depolarizing and horizontal cell dendrites possessed abundant spinules. However, following application of haloperidol to the vitreous prior to light adaptation, spinule formation was suppressed, and concomitantly, red-sensitive depolarizing S-potentials remained significantly under-developed. The results are consistent, therefore, with the idea that spinules mediate the negative feed-back interaction between horizontal cells and cones.

Animals↗