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K Dunlap

Publications and source records attributed to K Dunlap.

At least 19 recordsLinked to original sources
PubMed1992

Calcium channels coupled to glutamate release identified by omega-Aga-IVA.

Presynaptic calcium channels are crucial elements of neuronal excitation-secretion coupling. In mammalian brain, they have been difficult to characterize because most presynaptic terminals are too small to probe with electrodes, and available pharmacological tools such as dihydropyridines and omega-conotoxin are largely ineffective. Subsecond measurements of synaptosomal glutamate release have now been used to assess presynaptic calcium channel activity in order to study the action of peptide toxins from the venom of the funnel web spider Agelenopsis aperta, which is known to inhibit dihydropyridine and omega-conotoxin-resistant neuronal calcium currents. A presynaptic calcium channel important in glutamate release is shown to be omega-Aga-IVA sensitive and omega-conotoxin resistant.

Agatoxins↗
PubMed1992

Pharmacological discrimination of N-type from L-type calcium current and its selective modulation by transmitters.

GABA and norepinephrine inhibit high voltage-activated calcium current in chick sensory neurons. Using specific pharmacological tools, we have dissected this current into two components: the major one is omega-conotoxin sensitive and dihydropyridine resistant (N-type) while the minor one is dihydropyridine sensitively and omega-conotoxin resistant (L-type). The ability to selectively eliminate these two components has allowed us to determine whether the transmitters target the same or different channel types. Both GABA and norepinephrine modulate the N-type component as evidenced by their lack of effect on (1) omega-conotoxin-resistant current and (2) pure L-type tail current, prolonged by a dihydropyridine calcium channel agonist. This simple pharmacological profile will allow future tests of the significance of the two channel types in regulating sensory neuron functions.

Animals↗
PubMed1991

Multiple actions of extracellular ATP on calcium currents in cultured bovine chromaffin cells.

Hormone secretion from chromaffin cells is evoked by calcium influx through voltage-dependent channels in the plasma membrane. Previous studies have shown that ATP, cosecreted with catecholamines from chromaffin granules, can modulate the secretion resulting from depolarization by nicotinic agonists. The immediate effect of ATP is to enhance secretion; more prolonged exposure to the nucleotide results in inhibition. These receptor-mediated actions of ATP involve the activation of at least two separate classes of GTP-binding protein. Results from electrophysiological experiments reported here demonstrate that the modulatory actions of ATP can, in large part, be explained by the effects of the nucleotide on inward calcium current. ATP shows a rapid enhancement and a slower, persistent inhibition of the depolarization-induced inward current.

Adenosine Triphosphate↗
PubMed1991

Interleukin-1 augments gamma-aminobutyric acidA receptor function in brain.

Interleukin-1 (IL-1), a cytokine involved in the acute phase reaction to injury and infection, has multiple effects in the central nervous system, including induction of fever and sleep and the release of several neuropeptides. We evaluated effects of IL-1 beta on inhibitory postsynaptic function at the gamma-aminobutyric acidA (GABAA) receptor. IL-1 (100 pg/ml to 10 ng/ml) augmented GABAA receptor function in cortical synaptic preparations. This effect of IL-1 was largely prevented by incubation with a specific IL-1 receptor antagonist. The related cytokines interleukin-6 and tumor necrosis factor did not augment GABA-dependent chloride transport. Similar enhancement of GABAA receptor function was observed in tissue prepared from mice previously injected intraperitoneally with IL-1 (1 microgram). Electrophysiological studies in cultured primary cortical neurons demonstrated that IL-1 enhanced the GABA-mediated increase in chloride permeability, whereas IL-1 alone produced no alterations in resting conductance. Behavioral studies indicated that IL-1 is similarly active in vivo; mice treated with IL-1 showed a decrease in open-field activity and an increase in the threshold for pentylenetetrazol-induced seizures. The interaction of IL-1 with GABAA receptors might account for the somnogenic and motor-depressant effects of this cytokine.

Animals↗
PubMed1989

Intercellular signaling as visualized by endogenous calcium-dependent bioluminescence.

Bioluminescence in the hydrozoan coelenterate Obelia results from calcium activation of a photoprotein contained in light-emitting cells (photocytes) scattered in the animal's endoderm. The influx of calcium into nonluminescent endodermal cells through conventional voltage-dependent calcium channels is required for the excitation-luminescence coupling. Our results suggest that the subsequent diffusion of this calcium, via gap junctions, into the neighboring photocytes triggers a localized luminescence response. Following intense stimulation, the local rise in calcium elicits a secondary wave of luminescence that is supported by a voltage-independent calcium permeability mechanism in the photocyte plasma membrane. These two mechanisms for elevating internal calcium in light-emitting cells can account for the spatial and temporal features of intracellular luminescence in Obelia.

Animals↗
PubMed1989

Specific inhibitors of protein kinase C block transmitter-induced modulation of sensory neuron calcium current.

Modulation of neuronal, voltage-dependent calcium current has been described for a number of transmitters and peptides, but the biochemical basis for this phenomenon has not been completely identified. In several cases protein kinase C (PKC) is thought to mediate transmitter inhibition of calcium current; however, a lack of specific PKC inhibitors has hampered a direct physiological test of this idea. We have used the whole-cell, tight-seal configuration of the patch-clamp technique to apply intracellularly two specific PKC inhibitors to the cell bodies of embryonic chick sensory neurons. Both inhibitors, a 17 kd protein purified from bovine brain and a synthetic 13 amino acid "pseudosubstrate" peptide, blocked inhibition of calcium current by either norepinephrine or an exogenously applied PKC activator. These results provide strong evidence that activation of PKC is a prerequisite for the modulation of sensory neuron calcium current by norepinephrine.

Animals↗
PubMed1989

G proteins couple alpha-adrenergic and GABAb receptors to inhibition of peptide secretion from peripheral sensory neurons.

Regulation of neuronal calcium channels by GTP-binding proteins (G proteins) is likely to be an important mechanism by which inhibitory transmitters influence excitation-secretion coupling in presynaptic nerve endings. Here, we report that in peripheral sensory neurons from embryonic chick dorsal root ganglia (DRG), the G protein-mediated inhibition of voltage-dependent calcium channels may best explain how norepinephrine (NE) and GABA inhibit the electrically evoked, calcium-dependent release of substance P (SP). As is the case for the previously reported inhibitory actions of these transmitters on DRG cell calcium channels, we demonstrate that NE and GABA inhibit peptide secretion through activation of alpha-adrenergic and GABAb receptors that are functionally coupled to pertussis toxin (PTX)-sensitive G proteins. Pretreatment of DRG cell cultures with PTX blocked the ability of NE and GABA to inhibit the release of SP, an action correlated with PTX-catalyzed ADP-ribosylation of membrane proteins with apparent molecular weight (Mr) of 40-41 kDa. Western immunoblot analysis of chick DRG cell membrane proteins using antisera directed against synthetic peptides corresponding to amino acid sequences predicted from cDNAs for PTX-sensitive G protein alpha subunits revealed a minimum of 2 Gi-like proteins (Mr 40 and 41 kDa) and a third Go-like protein (Mr 40 kD). Significantly, these findings implicate Gi- and/or Go-like GTP-binding proteins as mediators of presynaptic inhibition in peripheral sensory neurons.

Adenosine Diphosphate Ribose↗
PubMed1988

Characterization of the electrically evoked release of substance P from dorsal root ganglion neurons: methods and dihydropyridine sensitivity.

The mechanism by which dihydropyridines (DHPs) modulate the electrically evoked or KCI-induced release of substance P (SP) from embryonic chick dorsal root ganglion (DRG) neurons was investigated in the present study. The release of SP, as measured by radioimmunoassay (RIA), was characterized in terms of its dependence on extracellular calcium ion, its stimulus-response relationship, its sensitivity to the calcium-channel blocker omega conus toxin (omega-CgTx), and its modulation by the DHPs Bay K 8644 and nifedipine. Here it is reported that omega-CgTx (1 microM) blocked the electrically evoked release of SP. In contrast, the calcium-channel agonist Bay K 8644 (5 microM) facilitated the release of SP (by 45%), whereas the calcium-channel antagonist nifedipine (5 microM) was without effect. When the release of SP was triggered by depolarization of cultures with 60 mM KCI, the actions of the DHPs became much more pronounced. Under these conditions, Bay K 8644 facilitated (by 115%), whereas nifedipine inhibited (by 58%), peptide secretion. Voltage-clamp analysis of DRG cell calcium currents demonstrated that these actions of omega-CgTx, Bay K 8644, and nifedipine are explicable in terms of their effects on the slowly inactivating (L-type) calcium current. On the basis of these findings, it is suggested that the SP release mechanism exhibits DHP sensitivity due to the involvement of L-type calcium channels in the neurosecretory process. This model predicts that the voltage and time-dependent antagonist actions of nifedipine are sufficient to explain its failure to inhibit the electrically evoked release of SP.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1987

Dihydropyridine inhibition of neuronal calcium current and substance P release.

Dihydropyridine (DHP) calcium channel antagonists, which inhibit the slowly inactivating or L-type cardiac calcium (Ca) current, have been shown to be ineffective in blocking 45Ca influx and Ca-dependent secretion in a number of neuronal preparations. In the studies reported here, however, the antagonist DHP nifedipine inhibited both the L-type Ca current and potassium-evoked substance P (SP) release from embryonic chick dorsal root ganglion (DRG) neurons. These results suggest that, in DRG neurons, Ca entry through L-type channels is critical to the control of secretion. The inhibition of Ca current by nifedipine was both voltage and time-dependent, significant effects being observed only on currents evoked from relatively positive holding potentials maintained for several seconds. As expected from these results, nifedipine failed to inhibit L-type Ca current underlying the brief plateau phase of the action potential generated from the cell's normal resting potential; likewise, no significant effect of the drug was observed on action potential-stimulated SP release evoked by electrical field stimulation. The results of this work are discussed in terms of an assessment of the role of L-type Ca channels in neurosecretion.

Action Potentials↗
PubMed1986

A comparison of the cost of various treatment methods for early cancer of the prostate.

The relative costs have been determined for treating prostate cancer by radical prostatectomy, lymph node dissection with I125 implant, and external beam radiation in a large community practice. About 15% of patients were treated with the options involving surgery. The median cost of radical prostatectomy was $14,400, lymph dose dissection and I125 implant $12,000, and external beam radiation $6750 prior to October 1984 and $5600 after October 1984. None of the data indicate superior outcome by any one of these methods for Stage A or B prostate cancer. Therefore, the surgical approaches are usually not recommended except for the patient highly motivated to maintain potency who may select the I125 implant. In an era of diminishing funds for health care, the federal government, industry, and perceptive HMO's may elect to pay only for the less expensive method.

Brachytherapy↗
PubMed1986

Kinase C activator 1,2-oleoylacetylglycerol attenuates voltage-dependent calcium current in sensory neurons.

The diacylglycerol analogue 1,2-oleoylacetylglycerol (OAG) and the phorbol ester 12-deoxyphorbol 13-isobutyrate (DPB) were tested for their effects on the voltage-dependent calcium (Ca) current in embryonic chicken dorsal root ganglion neurons in vitro. OAG (0.6-60 microM) and DPB (0.01-50 microM) produced reversible decreases in Ca current. Neither drug affected resting membrane conductance, the voltage-dependent potassium current, or the Ca current-voltage relationship. The concentrations of OAG and DPB that reduced Ca current correlate well with those concentrations that have been shown, in other systems, to activate protein kinase C-dependent phosphorylation. The time course for OAG action on Ca current is also consistent with an involvement of kinase C. Incubation of dorsal root ganglion cells in 60 microM OAG prevented further reductions in Ca current by either 50 microM DPB or 10 microM norepinephrine, a known modulator of the voltage-dependent Ca channel in these cells. This evidence suggests that protein kinase C may play a role in modulating Ca channel function.

Animals↗
PubMed1985

Forskolin prolongs action potential duration and blocks potassium current in embryonic chick sensory neurons.

To determine if alterations in internal cyclic adenosine monophosphate (cAMP) play a role in modulation of voltage-dependent channels in embryonic chick sensory neurons in vitro, forskolin (a direct activator of adenylate cyclase) was tested on the cells. Forskolin, in concentrations between 1 and 100 microM, produced dose-dependent, reversible increases in action potential duration. This effect of forskolin was blocked by incubation of the neurons in 1 mM 2',5'-dideoxyadenosine, an inhibitor of forskolin-induced activation of cyclase in other cells. This suggests that the increase in action potential duration is likely to be mediated by activation of adenylate cyclase. Cholera toxin, another cyclase activator, also increased action potential duration when applied to the sensory neurons in a concentration of 10 micrograms/ml. Forskolin applied to voltage-clamped neurons decreased a voltage-dependent outward current, a result consistent with its effect on the action potential. These effects of forskolin are mimicked by capsaicin, but are in marked contrast to those previously reported for norepinephrine on the action potential and membrane currents (Dunlap and Fischbach 1981). Furthermore, forskolin does not block (or attenuate) the effects of norepinephrine, suggesting that increases in adenylate cyclase activity are most likely not involved in norepinephrine's action on the calcium channel.

Action Potentials↗
PubMed1984

Functional and pharmacological differences between two types of GABA receptor on embryonic chick sensory neurons.

Embryonic chick sensory neurons grown in dissociated cell culture exhibit two functional responses to GABA: an increase in resting membrane permeability to chloride (Cl) ions (resulting in membrane depolarization) and a decrease in voltage-dependent calcium (Ca) channel current (resulting in a decreased action potential duration). These two functional effects differ in a number of ways. (1) The increase in resting membrane permeability desensitizes in the maintained presence of GABA, while the decrease in action potential duration does not. (2) Muscimol is a selective agonist for the increase in resting conductance, while baclofen is a selective agonist for the decrease in action potential duration. (3) Bicuculline inhibits the GABA- or muscimol-induced increase in Cl permeability, but it does not block the GABA- or baclofen-induced decrease in action potential duration. These functional and pharmacological differences between the two effects of GABA suggest that two separate receptors are involved.

Animals↗
PubMed1984

Pharmacological characterization of amine receptors on embryonic chick sensory neurones.

The effects of noradrenaline, dopamine and 5-hydroxytryptamine were investigated on the duration of the action potential of embryonic chick sensory neurones in vitro. All three amines, like gamma-aminobutyric acid, decreased the duration of the action potential evoked by current injection. The onset of the noradrenaline-induced decrease in action potential duration was fast (less than 1s) and the recovery phase was dependent upon the dose of noradrenaline applied. Rapid washout of the noradrenaline revealed a minimum 30s recovery time which was independent of the initial noradrenaline concentration. Dopamine and 5-hydroxytryptamine could mimic the effects of noradrenaline on action potential duration. The ED50 for all three amines was approximately 1 microM. At a saturating concentration of 10 microM, noradrenaline was more potent than dopamine and 5-hydroxytryptamine. Saturating doses of noradrenaline and dopamine or 5-hydroxytryptamine were not additive. Responses to all three amines were affected similarly by antagonists: they were antagonized by yohimbine, phentolamine, haloperidol and mianserin but not by propranolol, prazosin, domperidone, spiperone or methysergide. Clonidine and xylazine (alpha 2-adrenoceptor agonists) were also without effect. In contrast to the amines, saturating concentrations of gamma-aminobutyric acid were additive with those of noradrenaline. Responses to GABA were not antagonized by the amine receptor antagonists. The evidence described here suggests that the amines and gamma-aminobutyric acid acid decrease sensory neurone action potential duration via pharmacologically-distinct membrane receptors. In addition, it is likely that the amines are acting via a single class of receptor whose pharmacology is different from classical adrenoceptors, dopamine receptors and 5-hydroxytryptamine receptors.

Action Potentials↗
PubMed1981

Two types of gamma-aminobutyric acid receptor on embryonic sensory neurones.

1 Embryonic sensory neurones of the chick grown in dissociated cell culture respond to application of low concentrations of gamma-aminobutyric acid (GABA) with a change in resting membrane resistance (R(in)) and/or a change in action potential duration (APD) (Dunlap & Fischbach, 1978; Choi & Fischbach, 1981). Intracellular microelectrode recording techniques were employed to determine if these two effects are mediated by the same, or different, GABA receptors.2 Cells responded, for the most part, with a change in either R(in) or APD, but 10% of the cells exhibited both effects. In the latter cells the two responses were clearly distinguishable as discussed below.3 The proportion of neurones exhibiting a GABA-induced decrease in R(in) declined during the first week in vitro while the proportion exhibiting a decrease in APD increased during that time.4 The two effects were pharmacologically distinct. Muscimol, a GABA analogue, produced only the change in R(in) (ED(50) = 5.5 muM) while baclofen, another analogue of GABA, produced only the change in APD (ED(50) = 1 muM). The analogues were approximately equipotent with GABA. Bicuculline, a GABA antagonist, blocked the muscimol-induced change in R(in) (but not the baclofen-induced change in APD) in a dose-dependent fashion with an ID(50) = 0.7 muM.5 The time courses of the two effects were different. The change in APD resulting from a brief application of GABA (or baclofen) was prolonged relative to the rapid return to control associated with the GABA- (or muscimol-) induced change in R(in).6 Desensitization of the two responses exhibited separate time courses. In the continual presence of the agonists, GABA- and muscimol-induced decreases in R(in) completely desensitized in ca. 10 s while GABA- and baclofen-induced decreases in APD persisted undiminished throughout a prolonged (1 min) application of the drugs and returned to control only after cessation of application.7 It is concluded that embryonic chick sensory neurones in culture exhibit two types of GABA receptor that differ in their functional and pharmacological properties. Implications of these results are discussed.

Action Potentials↗