PubMed Health⌕ Search

Biomedical subjects

D G McMahon

Publications and source records attributed to D G McMahon.

At least 19 recordsLinked to original sources
PubMed2001

Gating of retinal horizontal cell hemi gap junction channels by voltage, Ca2+, and retinoic acid.

PURPOSE: Hemi gap junction (HGJ) channels, precursors of gap junctional channels, are functionally expressed in retinal horizontal cells where they may play roles in osmoeregulation and ephaptic regulation of synaptic feedback to photoreceptors. In this study we examined mechanisms of gating of these channels by transmembrane voltage, Ca2+ and retinoic acid (RA). METHODS: Experiments were performed on cultured bass horizontal cells using the conventional whole cell patch clamp configuration. RESULTS: HGJ currents in isolated bass horizontal cells, revealed by perfusion with Ca2+ free media, were opened by positive holding potentials and inhibited by negative holding potentials. These currents were also inhibited by external application of either Ca2+ or RA. Using a rapid perfusion system, the latency of 2 mM Ca2+ to begin channel closure was unmeasurably brief, whereas the latency for 30 microM RA action was 177+/-9 ms (mean+/-standard error of the mean). The total inhibition of HGJ channel currents by coapplication of 0.3 microM RA and 100 microM Ca2+ was less than the sum of inhibition by RA alone and Ca2+ alone suggesting that the actions of RA and Ca2+ were not independent. In the presence of 0.3 microM RA, the half maximal concentration for Ca2+ inhibition was increased from a control value of 192 microM to 375 microM without affecting maximal inhibition. Similarly, the half maximal concentration for RA inhibition was increased from a control value of 0.44 microM to 1.1 microM without affecting maximal inhibition in the presence of 100 microM Ca2+. CONCLUSIONS: These results suggest that horizontal cell HGJ channels are closed by the normal negative resting potentials of these cells. Extracellular Ca2+ and the retinal neuromodulator RA also act to close HGJ channels through mechanisms or sites which are not independent.

Action Potentials↗
PubMed2000

Direct gating by retinoic acid of retinal electrical synapses.

Retinoic acid (RA), a signaling molecule derived from vitamin A, controls growth and differentiation of a variety of cell types through regulation of gene transcription. In the vertebrate retina, RA also regulates gap junction-mediated physiological coupling of retinal neurons through a nontranscriptional mechanism. Here we report that RA rapidly and specifically modulates synaptic transmission at electrical synapses of cultured retinal horizontal cells through an external RAR(beta)(/gamma)-like binding site, the action of which is independent of second messenger cascades. External application of all-trans retinoic acid (at-RA) reversibly reduced the amplitude of gap junctional conductance in a dose-dependent manner, but failed to affect non-gap-junctional channels, including glutamate receptors. In contrast, internal dialysis with at-RA was ineffective, indicating an external site of action. Selective RAR(beta)(/gamma) ligands, but not an RAR(alpha)-selective agonist, mimicked the action of at-RA, suggesting that gating of gap junctional channels is mediated through an RAR(beta)(/gamma)-like binding site. At-RA did not act on gap junctional conductance by lowering [pH](i) or by increasing [Ca(2+)](i). A G protein inhibitor and protein kinase inhibitors did not block at-RA uncoupling effects indicating no second messenger systems were involved. Direct action of at-RA on gap junction channels was further supported by its equivalent action on whole-cell hemi-gap-junctional currents and on cell-free excised patch hemichannel currents. At-RA significantly reduced single-channel open probability but did not change unitary conductance. Overall, the results indicate that RA modulates horizontal cell electrical synapses by activation of novel nonnuclear RAR(beta)(/gamma)-like sites either directly on, or intimately associated with, gap junction channels.

Adenosine Triphosphate↗
PubMed2000

GFP fluorescence reports Period 1 circadian gene regulation in the mammalian biological clock.

Endogenous cyclic activation of a specific set of genes, including Period 1 (Per1), drive circadian rhythms in the suprachiasmatic nucleus (SCN), a biological clock nucleus of the brain. We have produced transgenic mice in which a degradable form of recombinant jellyfish green fluorescent protein (GFP) is driven by the mouse Period 1 (mPer1) gene promoter. GFP protein is expressed in the circadian neural structures of the retina and SCN. Fluorescent signals are resolved at the level of individual neurons. mPer1-driven GFP fluorescence intensity reports light-induction and circadian rhythmicity in SCN neurons. This circadian reporter transgene captures the gene expression dynamics of living biological clock neurons and ensembles, providing a novel view of this brain function.

Animals↗
PubMed1999

Serotonin modulates glutamate responses in isolated suprachiasmatic nucleus neurons.

Two input pathways to the suprachiasmatic nucleus (SCN) of the hypothalamus are the glutamatergic retinohypothalamic tract and the serotonergic afferent from the midbrain raphe nucleus. To determine whether these two temporal signaling pathways can converge at the cellular level, we have investigated the effects of serotonin on glutamate-induced calcium responses of individual SCN neurons isolated in cell culture. Dispersed cultures were formed from the SCN of neonatal rats. The calcium indicator Fura-2 acetoxymethyl ester was used to assess the changes in [Ca(2+)](i) by recording the 340-nm/380-nm excitation ratio. Application of glutamate (5 microM) to the culture caused a rapid (within 10 s) increase in the fluorescence ratio of neurons indicating a marked increase in the concentration of intracellular free calcium. However, when 5-hydroxytryptamine (5-HT; 5 microM) was coapplied with glutamate, 31% of neurons showed an overall 61% reduction in the peak of the glutamate-induced calcium increase. Application of the 5-HT(7/1A) receptor agonist, (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin [(+/-)-8-OH-DPAT] (1 microM), also reduced the calcium elevation this time by 80% in 18% of the neurons tested. When the 5-HT(7/2/1C) receptor antagonist, ritanserin (800 nM), was coapplied with serotonin, it blocked modulation of the glutamate responses. Further support for the involvement of the 5-HT(7) receptor was provided by the ability of the adenylate cyclase activator, forskolin (10 microM), and the cAMP analogue, 8-Br cAMP (0.5 mM), to mimic the suppressive effect of serotonin. Blocking spike-mediated cell communication with tetrodotoxin (1 microM) did not prevent the serotonergic suppression of glutamate-induced responses. These results support the hypothesis that the serotonergic modulation of photic entraining signals can occur in SCN neurons.

Animals↗
PubMed1997

Stimulus-secretion coupling in porcine adrenal chromaffin cells: effect of dexamethasone.

Recent studies from this laboratory established that dexamethasone (DEX) potentiates Ca2+ current via voltage-gated Ca2+ channels (VGCC), and as a consequence potentiates agonist-induced cytosolic Ca2+ transients in rat adrenal chromaffin cells. The present study examined whether DEX can also modulate VGCC activity and agonist-induced cytosolic Ca2+ transients in porcine adrenal medullary chromaffin (PAMC) cells, and if so whether this results in alterations in catecholamine secretion. Forty-eight-hr exposure to 1 microM DEX significantly increased peak Ca2+ current (delta + 138%; n = 6; P < 0.05) in PAMC cells. DEX treatment also significantly potentiated the increase in cytosolic Ca2+ in response to membrane depolarization with KCl (delta + 20%; n = 29; P < 0.05), but did not affect the amplitude of Ca2+ transients elicited by nicotine or acetylcholine. Despite the potentiation of intracellular Ca2+, DEX treatment had no effect on KCl-induced secretion of either norepinephrine or epinephrine. These data demonstrate that as in the rat chromaffin cell, DEX can also increase VGCC activity in PAMC cells. However, the subsequent potentiation of selected agonist-induced increases in intracellular Ca2+ does not appear to be sufficient to alter catecholamine secretion.

Acetylcholine↗
PubMed1997

Modulation of hybrid bass retinal gap junctional channel gating by nitric oxide.

1. To elucidate the role of the nitric oxide (NO) transmitter system in the regulation of gap junctional channel gating, we have examined the effects of the NO donor sodium nitroprusside (SNP) on the electrical synapses of hybrid bass H2-type horizontal cells. 2. SNP reversibly reduced the macroscopic junctional conductance without significantly changing voltage sensitivity. 3. Kinetic analyses showed that SNP made the voltage-dependent decay of junctional currents more rapid. 4. Single-channel data showed that SNP reduced channel open probability by reducing channel open frequency. 5. The action of SNP can be prevented or largely reduced by the NO scavenger, haemoglobin. NO release by SNP solutions was detected directly by a NO sensor. 6. NO appears to modulate the gap junctional conductance by activating the cGMP-cGMP-dependent protein kinase G (PKG) pathway. A membrane-permeable cGMP analogue, 8-Br-cGMP, mimics the action of SNP. A soluble guanylate cyclase inhibitor (LY-83583) and a highly specific cGMP-dependent protein kinase inhibitor (RKRARKE) blocked the action of NO. 3-Isobutyl-1-methylxanthine (IBMX), a non-specific phosphodiesterase inhibitor, potentiated the effect of SNP. 7. [Ca2+]i image studies showed that NO donors did not change [Ca2+]i in horizontal cells, suggesting that the regulation of junctional channels by NO is [Ca2+]i independent.

1-Methyl-3-isobutylxanthine↗
PubMed1996

Horizontal cell electrical coupling in the giant danio: synaptic modulation by dopamine and synaptic maintenance by calcium.

Electrical synapses, and their structural manifestation, gap junctions, are critical elements of retinal circuitry. These synapses are subject to both rapid modulation and slower structural changes by physiological signals which mediate changes in the adaptational state of the retina. The electrical synapses of fish retinal horizontal cells are an excellent preparation for in vitro studies of electrical synapses. We have examined the rapid modulation of electrical coupling by dopamine and effects on the expression and maintenance of electrical synapses by cell calcium in pairs of horizontal cells isolated from retinas of the giant danio (Danio aquipinnatus). We report that rapid modulation by dopamine reduces junctional conductance by modifying gap junction channel gating, while maintaining cells in reduced calcium medium, and lowering intracellular calcium concentration, results in the loss of electrical coupling. The effects of calcium on synaptic maintenance may be related to structural changes observed in horizontal cell electrical synapses during light adaptation.

Animals↗
PubMed1996

Nitric oxide and cGMP modulate retinal glutamate receptors.

1. In the retina, as in other regions of the vertebrate central nervous system, glutamate receptors mediate excitatory chemical synaptic transmission and are a critical site for the regulation of cellular communication. In this study, retinal horizontal cells from the hybrid less were dissociated in cell culture, voltage clamped by the whole cell recording technique, and the currents evoked by application of excitatory amino acids recorded. 2. Responses to glutamate and its agonist kainate were reduced by approximately 50% in the presence of the nitric oxide (NO) donors sodium nitroprusside and S-nitroso-N-acetylpenicillamine. The effect of these compounds was blocked by the NO scavenger hemoglobin. 3. This effect of NO donors on kainate currents could be mimicked by the application of a membrane permeable guanosine 3',5'-cyclic monophosphate (cGMP) analogue, 8-Br-cGMP. The NO effect was also blocked by application of the guanylate cyclase inhibitor LY-83583, and by a protein kinase G inhibitor peptide. 4. In H1-type horizontal cells, stimulation of endogenous nitric oxide synthase with L-arginine reduced kainate responses, whereas application of D-arginine had no effect. 5. This receptor modulation mechanism may act in concert with other pre- and postsynaptic mechanisms to modify horizontal cell synaptic function according to the adaptational state of the retina and also may protect horizontal cells from glutamate excitotoxicity.

Animals↗
PubMed1994

Protein content and cAMP-dependent phosphorylation of fractionated white perch retina.

In the retinas of teleost fish dopamine, released from interplexiform cells, modulates synaptic transmission at both the chemical and electrical synapses of retinal horizontal cells. This modulation is due to activation of adenylate cyclase and phosphorylation by protein kinase A, perhaps of the synaptic ion channel proteins themselves. In this study we have fractionated the white perch retina by Percoll density gradient centrifugation in order to identify proteins which coenrich with horizontal cells. In addition we have tested retinal fractions for phosphorylation by native cAMP-dependent kinase. Our findings indicate that there are at least 3 proteins of molecular weights 28, 43/44 and 50 kDa which coenrich with horizontal cells and 3 proteins of 30/31 kDa, 35 kDa (putative rhodopsin) and 48 kDa (putative arrestin) which coenrich with photoreceptor fractions. The 43/44 kDa phosphoprotein is a target for cAMP-dependent protein phosphorylation and thus is apparently an element of the dopaminergic modulatory pathway in perch horizontal cells.

Animals↗
PubMed1994

Modulation of gap-junction channel gating at zebrafish retinal electrical synapses.

1. Transmission at electrical synapses is modulated by a variety of physiological signals, and this modulation is a potentially general mechanism for regulating signal integration in neural circuits and networks. In the outer plexiform layer of the retina, modulation of horizontal-cell electrical coupling by dopamine alters the extent of spatial integration in the horizontal-cell network. By analyzing the activity of individual gap-junction channels in low-conductance electrical synapses of zebrafish retinal horizontal cells, we have defined the properties of these synaptic ion channels and characterized the functional changes in them during modulation of horizontal-cell electrical synapses. 2. Zebrafish horizontal-cell gap-junction channels have a unitary conductance of 50-60 pS and exhibit open times of several tens of milliseconds. The kinetic process of channel closure is best described by the sum of two rate constants. 3. Dopamine, and its agonist, (+/-)-6,7-dihydroxy-2-amino-tetralin (ADTN), modulates electrical synaptic transmission between horizontal cells predominantly by affecting channel-gating kinetics. These agents reduced the open probability of gap-junction channels two- to threefold by reducing both the duration and frequency of channel openings. Both time constants for channel open duration were reduced, whereas the duration of shut periods was increased. Similar changes in open-time kinetics were observed in power spectra of higher conductance gap junctions. 4. These results provide a description of rapid electrical synaptic modulation at the single channel level. The description should be useful in understanding the mechanisms of plasticity at these synapses throughout the vertebrate central nervous system.

Animals↗
PubMed1994

Modulation of electrical synaptic transmission in zebrafish retinal horizontal cells.

Electrical synaptic transmission is widespread in the vertebrate CNS and its modulation plays a critical role in altering the properties of coupled neural networks. In order to define further the mechanisms of electrical synaptic plasticity in the vertebrate retina, the electrophysiological characteristics of solitary horizontal cells and horizontal cell pairs from the zebrafish (Brachydanio rerio) were examined by whole-cell patch-clamp recordings from cells in primary cell culture. In solitary cells, the current-voltage relation exhibited inward current at potentials negative to -60 mV, a linear region of high resistance from -50 mV to 0 mV, and outward current positive to +20 mV. The inward current at negative potentials was blocked by substituting Cs+ for K+ in the extracellular medium, while the outward current at positive potentials was blocked by substitution of Cs+ for K+ in the pipette solution. Measurements of gap junctional conductance from electrically coupled cell pairs revealed that zebrafish horizontal cells expressed a mean junctional conductance of considerably smaller magnitude than other teleost retinal horizontal cells. Junctional conductance was found to be voltage dependent, exhibiting time-dependent closure with increasing transjunctional voltage. Voltage sensitivity was symmetrical about 0 mV junctional potential. At +/- 90 mV the ratio of steady state to peak current was approximately 0.5 and the time constant for inactivation of the junctional current was approximately 120 msec. Junctional conductance was also modulated by dopamine and cAMP. Pairs of horizontal cells responded to puff application of dopamine with a two- to threefold reduction in junctional conductance, but there was no discernible effect on extrajunctional conductances. The action of dopamine on coupling was mimicked by application of the dopamine agonist (+/-)-6,7-dihydroxy-2-amino-tetralin (ADTN) and the membrane permeable cAMP analog 8-bromo-cAMP. The selective D1 dopamine receptor antagonist SCH23390 blocked uncoupling by dopamine. These data provide a primary description of the electrophysiological characteristics of solitary horizontal cells and the electrical coupling between pairs of horizontal cells dissociated from the zebrafish retina. They indicate that zebrafish horizontal cells are distinct from the horizontal cells of other teleosts in their coupling characteristics. The results suggest that zebrafish horizontal cells exhibit differences in the regulation of synaptic assembly and maintenance that have important implications for the function of the zebrafish horizontal cell network in vivo.

Animals↗
PubMed1993

Modulation of calcium current, intracellular calcium levels and cell survival by glucose deprivation and growth factors in hippocampal neurons.

Basic fibroblast growth factor (bFGF) and nerve growth factor (NGF) can protect CNS neurons against ischemic/excitotoxic insults, but the mechanism of action is unknown. Imaging of the calcium indicator dye fura-2 and whole-cell patch clamp recordings of calcium currents were used to examine the mechanisms whereby hypoglycemia damages and growth factors protect cultured rat hippocampal neurons. When cultures were deprived of glucose, massive neuronal death occurred 16-24 h following the onset of hypoglycemia. Early hypoglycemia-induced changes included calcium current inhibition and a reduction in intracellular free calcium levels ([Ca2+]i) without morphological signs of neuronal damage. Later changes included a large elevation of [Ca2+]i which was causally involved in neuronal damage. NGF and bFGF prevented or reduced both the early and later responses to hypoglycemia. The growth factors increased calcium (barium) current and [Ca2+]i to normal limits during the early stages of hypoglycemia and prevented the later elevation in [Ca2+]i and neuronal damage. Nifedipine, but not omega-conotoxin, blocked calcium currents. The increased calcium current caused by the growth factors was apparently not sufficient to protect neurons against hypoglycemic damage since K+ depolarization during the early stages of hypoglycemia did not prevent and, in fact exacerbated, the subsequent neuronal damage. In addition, exposure of neurons to K+, NGF or bFGF only during the first 1 h of hypoglycemia did not protect against hypoglycemic damage. Taken together, the data suggest that neurons initially respond to hypoglycemia with a reduction in calcium currents which may provide a means to maintain [Ca2+]i within a concentration range conducive to cell survival. Prolonged energy deprivation eventually results in a failure of calcium extrusion systems, glutamate receptor activation and a loss of neuronal calcium homeostasis. Taken together, the data indicate that the mechanism of growth factor protection against energy deprivation involves prevention of the late rise in [Ca2+]i.

Animals↗
PubMed1992

Visual responses in teleosts. Electroretinograms, eye movements, and circadian rhythms.

We have recorded ocular potentials in response to brief flashes of light from two teleosts, the white perch (Roccus americana) and the green sunfish (Lepomis cyanellus). The animals were respired and maintained in an alert state for up to 2 d. Responses were recorded with corneal and transcleral electrodes. The responses of green sunfish were composed of electroretinogram (ERG) and eye movement potentials, whereas the responses in white perch contained only the ERG. Injection of curare abolished the sunfish eye movement potentials, unmasking the ERG. Observation under infrared illumination established a direct relationship between eye movements and the fast potentials which could be abolished by curare. We found no evidence of circadian changes in the amplitude of the ERG b-wave of either species. However, our results combined with those of a previous study of sunfish ocular potentials (Dearry, A., and B. Barlow, Jr. 1987. J. Gen. Physiol. 89: 745-770) suggest that the sunfish visual system exhibits rhythmic changes in oculomotor responses, which appear to be controlled by a circadian oscillator.

Animals↗
PubMed1989

Horizontal cell gap junctions: single-channel conductance and modulation by dopamine.

Horizontal cells form an electrically coupled network for the transmission of inhibitory signals in the outer retina. In teleosts, horizontal cell coupling is modulated by the neurotransmitter dopamine. Using voltage-clamped pairs of teleost horizontal cells, we have examined the effects of dopamine on the conductance and gating properties of the cell-to-cell channels that mediate electrical synaptic transmission. Variance analysis of the junctional current noise showed that dopamine substantially reduced the open probability of gap junction channels, from 0.75 to 0.14. Direct observation of unitary junctional gating events in poorly coupled cell pairs indicated that these channels have a unitary conductance of 50-60 pS. The elementary conductance of channels in cell pairs treated with dopamine (48.7 +/- 6.6 pS) was statistically indistinguishable from channels in untreated cells (53.2 +/- 7.2 pS). Uncoupling with octanol also yielded a similar unitary conductance (61.1 +/- 11.1 pS). Our results suggest that dopamine reduces the open probability of gap junctional channels by decreasing their open duration.

Animals↗
PubMed1987

The Bulla ocular circadian pacemaker. I. Pacemaker neuron membrane potential controls phase through a calcium-dependent mechanism.

In an effort to understand the cellular basis of entrainment of circadian oscillators we have studied the role of membrane potential changes in the neurons which comprise the ocular circadian pacemaker of Bulla gouldiana in mediating phase shifts of the ocular circadian rhythm. We report that: 1. Intracellular recording was used to measure directly the effects of the phase shifting agents light, serotonin, and 8-bromo-cAMP on the membrane potential of the basal retinal neurons. We found that light pulses evoke a transient depolarization followed by a smaller sustained depolarization. Application of serotonin produced a biphasic response; a transient depolarization followed by a sustained hyperpolarization. Application of a membrane permeable analog of the intracellular second messenger cAMP, 8-bromo-cAMP, elicited sustained hyperpolarization, and occasionally a weak phasic depolarization. 2. Changing the membrane potential of the basal retinal neurons directly and selectively with intracellularly injected current phase shifts the ocular circadian rhythm. Both depolarizing and hyperpolarizing current can shift the phase of the circadian oscillator. Depolarizing current mimics the phase shifting action of light, while hyperpolarizing current produces phase shifts which are transposed approximately 180 degrees in circadian time to depolarization. 3. Altering BRN membrane potential with ionic treatments, depolarizing with elevated K+ seawater or hyperpolarizing with lowered Na+ seawater, produces phase shifts similar to current injection. 4. The light-induced depolarization of the basal retinal neurons is necessary for phase shifts by light. Suppressing the light-induced depolarization with injected current inhibits light-induced phase shifts. 5. The ability of membrane potential changes to shift oscillator phase is dependent on extracellular calcium. Reducing extracellular free Ca++ from 10 mM to 1.3 X 10(-7) M inhibits light-induced phase shifts without blocking the photic response of the BRNs. The results indicate that changes in the membrane potential of the pacemaker neurons play a critical role in phase shifting the circadian rhythm, and imply that a voltage-dependent and calcium-dependent process, possibly Ca++ influx, shifts oscillator phase in response to light.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1987

The Bulla ocular circadian pacemaker. II. Chronic changes in membrane potential lengthen free running period.

We have used intracellular recording to directly measure the effects of three experimental agents, light, elevated potassium seawater, and lowered sodium seawater on the membrane potential of the putative circadian pacemaker neurons of the Bulla eye. These agents were subsequently tested for effects on the free running period of the circadian pacemaker. We report that: 1. When applied to the eye, light and elevated potassium seawater depolarized the putative pacemaker neurons, while lowered sodium seawater hyperpolarized them. The membrane potential changes induced by these agents are sustained for at least one hour, suggesting that they produce persistent changes in the average membrane potential of the putative pacemaker neurons. 2. The amplitude of the membrane potential response to the depolarizing agents varies with the phase of the circadian cycle. Depolarizations induced by light and elevated potassium seawater are twice as large during the subjective night than they are during the subjective day. No significant difference was found in the response to lowered sodium seawater at different phases. 3. Continuous application of each of these agents caused a lengthening of the free running period of the Bulla eye. Constant light increased the period by 0.9 h, while the other depolarizing treatment (elevated potassium seawater) increased the free running period by 0.6 h. Both treatments increased the mean peak impulse frequency of treated eyes. The hyperpolarizing treatment also increased the period of the ocular pacemaker (+0.8 h), but had little effect on peak impulse frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗