Biomedical subjects

C Allgaier

Publications and source records attributed to C Allgaier.

At least 55 records · Page 3Linked to original sources

PubMed1991

A detailed study on the effects of protein kinase C activation on alpha-2 adrenoceptor-coupled modulation of norepinephrine release in hippocampus.

The question was studied whether there is a direct link between protein kinase C and presynaptic alpha-2 adrenoceptors regulating depolarization-induced norepinephrine (NE) release. Effects of the protein kinase C activator 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB) on electrically evoked [3H]NE release were investigated in rabbit and rat hippocampus. Release evoked with 4 pulses/100 Hz (POP stimulation; i.e. under conditions virtually free of autoinhibition), was increased by 4 beta-PDB in a comparable manner in both species. Conversely, the alpha-2 adrenoceptor agonist clonidine diminished POP-induced [3H]NE release in a concentration-dependent manner. The net effects of clonidine were of a similar magnitude up to near maximal concentrations, irrespective of whether or not the 4 beta-PDB was present. Correspondingly, the net effect of 4 beta-PDB remained unchanged under these conditions. An impairment of the net effect of 4 beta-PDB was only seen at higher concentrations of clonidine. Concurrent addition of the alpha-2 adrenoceptor antagonist yohimbine and 4 beta-PDB enhanced release elicited with 36 pulses/3 Hz (i.e., in presence of autoinhibition), in a manner which was at least additive. Taken together, the above data exclude a direct link between presynaptic alpha-2 adrenoceptors and protein kinase C and restrict a functional interaction to very distinctive conditions.

Animals↗

PubMed1991

3,4-Diaminopyridine-evoked noradrenaline release in rat hippocampus: role of Na+ entry on Ca2+ pools and of protein kinase C.

Slices of rat hippocampus, preincubated with [3H]noradrenaline [(3H]NA), were superfused continuously and stimulated by addition of 3,4-diaminopyridine (3,4-DAP; 100 microM) for 10 min to the superfusion medium. An overflow of 3H evoked by 3,4-DAP (representing [3H]NA release) was measurable not only in the presence but also in the absence of extracellular Ca2+. Both the protein kinase C (PKC) activator 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB) and the PKC inhibitor polymyxin B, affected mainly the evoked release in the absence of extracellular Ca2+ in a facilitatory or inhibitory manner, respectively. Moreover, in the absence of extracellular Ca2+, both the 3,4-DAP-evoked [3H]NA release and the facilitatory effect of 4 beta-PDB were abolished in the presence of tetrodotoxin or in the absence of Na+ in the superfusion medium. Ruthenium red, a blocker of mitochondrial Ca2+ reuptake, potently increased 3,4-DAP-evoked [3H]NA release in Ca(2+)-free EGTA-containing medium. The facilitatory effects of ruthenium red and 4 beta-PDB were additive. From these and earlier observations we conclude (1) that the mechanism of 3,4-DAP-evoked [3H]NA release involves both Ca2+ influx into the nerve terminals and mobilization of intraneuronal Ca2+ pools. Most probably Ca2+ release from cytoplasmic Ca2+ stores (e.g. endoplasmic reticular pools or mitochondria) is induced by Na+ ions entering the nerve endings during 3,4-DAP-evoked repetitive action potentials. (2) The facilitatory effect of phorbol ester on 3,4-DAP-evoked NA release appears to be mediated not by changes in Ca2+ influx, but by enhancement of intraneuronal events distal to Na+ ion entry and increased intracellular Ca2+ availability.

4-Aminopyridine↗

PubMed1991

Studies on the interaction between presynaptic alpha 2-adrenoceptors and adenosine A1 receptors located on noradrenergic nerve terminals.

The aim of the present study was to obtain a more detailed understanding of the interaction between presynaptic alpha 2-adrenoceptors and A1 adenosine receptors mediating inhibition of noradrenaline release in the central nervous system. Slices of rabbit hippocampus, prelabelled with [3H]noradrenaline, were superfused in the presence of the re-uptake inhibitor (+)-oxaprotiline and electrically stimulated during superfusion. During stimulation with 36 pulses at 3 Hz the alpha 2-adrenoceptor antagonist yohimbine induced a five-fold increase of noradrenaline release indicating a pronounced autoinhibition of approximately 80%. In these experiments the inhibition of release caused by R-PIA, a preferential A1 agonist, as well as its facilitation caused by DPCPX, a selective A1 antagonist, were smaller in comparison to the effects of these compounds on release virtually free of autoinhibition (i.e. by stimulating the tissue with 4 pulses at 100 Hz (POP-stimulation) or with 36 pulses at 3 Hz in presence of yohimbine). Clonidine, an alpha 2-adrenoceptor agonist, was used to impose a distinct alpha 2-adrenoceptor-mediated inhibition of release elicited by POP-stimulation. Only, however, in the presence of 30 nmol/l clonidine, causing maximum inhibition of approximately 80% of 3H-overflow, but not in the presence of 6 nmol/l clonidine, causing approximately 50% inhibition, a significant diminution of the inhibitory effect of R-PIA was seen. Similarly, the alpha 2-adrenoceptor mechanism was affected only by 10 mumols/l R-PIA causing maximum inhibition of approximately 80%, but remained unchanged in the presence of 30 nmol/l R-PIA diminishing release by 50%.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

Forskolin modulates acetylcholine release in the hippocampus independently of adenylate cyclase activation.

[3H]Acetylcholine release from slices of rabbit hippocampus was elicited by electrical field stimulation (360 pulses/3 Hz). Both forskolin, commonly used as a specific activator of adenylate cyclase, as well as 1,9-dideoxy-forskolin, which fails to activate adenylate cyclase, increased the evoked transmitter release in an almost identical manner. In addition, the phosphodiesterase inhibitor, rolipram, and the membrane-permeable analogue of cAMP, 8-Br-cAMP, did not influence acetylcholine release. These data show that forskolin is not specific to adenylate cyclase and that the increase in acetylcholine release in the rabbit hippocampus occurs through a mechanism other than activation of adenylate cyclase.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1990

Regulatory proteins in presynaptic function.

Activation of alpha 2-adrenoceptors, opioid, A1-adenosine, and PGE receptors inhibited the stimulation-induced [3H]noradrenaline release in brain tissue in a concentration-dependent manner. Under experimental conditions (360 pulses/3 Hz) where the released noradrenaline activated the presynaptic alpha 2-autoreceptors, the effects of the heteroreceptor (k-opioid, A1-adenosine, PGE) agonists were decreased. By avoiding autoinhibition by either blockade of the alpha 2-autoreceptors with yohimbine or stimulating the tissue with four pulses/100 Hz, the heteroreceptor-mediated inhibition of [3H]noradrenaline release was markedly increased. The dependence of the heteroreceptor-mediated inhibition of evoked noradrenaline release on the extent of alpha 2-autoreceptor activation suggests a common postreceptor signal transduction pathway. PTX-catalyzed [32P]ADP ribosylation of synaptosomal membrane proteins revealed three bands of polypeptides with molecular weights corresponding to the alpha subunits of Go (39,000) and the Gi proteins (40,000, 41,000). Pretreatment with NEM reduced the PTX-induced 32P labeling by alkylating the alpha subunits at or near the site that is ADP ribosylated by PTX in a concentration-dependent manner. K(+)-evoked release of [3H]noradrenaline from synaptosomes indicated the presynaptic localization of the PTX-sensitive G proteins coupled to alpha 2-, k-, and A1-receptors of noradrenergic nerve terminals. Electrically evoked [3H]noradrenaline release was only increased by PTX or NEM in a time- and concentration-dependent manner when autoinhibition was present. The alpha 2-, opioid, and A1-adenosine receptor-mediated inhibition of [3H]noradrenaline release was impaired similarly by PTX or NEM treatment. In contrast, the inhibitory effect of PGE2 remained unaffected. These results indicate that presynaptic alpha 2-, opioid, and A1-receptors but not PGE receptors of noradrenergic nerve terminals are linked to PTX-sensitive G proteins. The interaction between the alpha 2-autoreceptors and the PGE receptors therefore does not occur at the level of a common pool of G proteins but at some subsequent step of the signal transduction mechanism.

Animals↗

PubMed1989

3,4-Diaminopyridine-induced noradrenaline release from CNS tissue as a model for action potential-evoked transmitter release: effects of phorbol ester.

We used rabbit hippocampus slices preincubated with [3H]noradrenaline (NA) and applied short pulses of 3,4-diaminopyridine (3,4-DAP) during superfusion to investigate the mechanism underlying the 3H overflow evoked by 3,4-DAP and the effects of the protein kinase C (PKC) activator, 4 beta-phorbol 12,13-dibutyrate (PDB), in this model. The 3H overflow evoked by 200 microM 3,4-DAP (about 4-5% of tissue-tritium) was largely Ca2+-dependent, tetrodotoxin-sensitive and markedly reduced by clonidine, but it was enhanced by yohimbine. We also demonstrated that the response could be inhibited via presynaptic adenosine (A1-) and opioid (kappa-) receptors. PDB (1 microM) markedly increased the 3,4-DAP-evoked 3H overflow, its effect being almost unchanged following activation of presynaptic alpha 2-, A1- or kappa-receptors. Inhibitors of PKC (polymyxin B, staurosporine) almost abolished the 3,4-DAP-evoked 3H overflow and antagonized the effects of PDB. It is concluded that application of 3,4-DAP (200 microM for 2 min) to brain slices leads to depolarization of the neuronal membrane, Na+ current-carried action potentials, Ca2+ influx and the exocytotic release of NA, which in many aspects resembles the release evoked by electrical field stimulation. The findings with phorbol ester further support the involvement of PKC in transmitter release. Activation of PKC apparently does not directly interfere with signal transduction mechanisms of presynaptic inhibitory receptors on noradrenergic nerve terminals.

4-Aminopyridine↗

PubMed1989

Effects of botulinum A toxin on presynaptic modulation of evoked transmitter release.

A possible influence of botulinum A toxin on the modulation of evoked neurotransmitter release was investigated in hippocampus tissue. Rabbit hippocampal slices prelabelled with [3H]noradrenaline ([3H]NA), [3H]5-hydroxytryptamine ([3H]5-HT) or [3H]choline were superfused with physiological medium and were stimulated electrically during superfusion. The evoked release of [3H]NA, [3H]5-HT and [3H]acetylcholine [( 3H]ACh) was inhibited by botulinum A toxin in a concentration- and time-dependent manner. Neither the inhibition of release of [3H]NA and [3H]5-HT by the alpha 2-adrenoceptor agonist clonidine nor facilitation of release in the presence of alpha 2-antagonists were influenced by pretreatment of the tissue with botulinum toxin. The toxin caused no [32P]ADP ribosylation of synaptosomal proteins of hippocampus. The facilitation of the stimulation-induced [3H]NA and [3H]5-HT release by the specific protein kinase C (PKC) activator 4 beta-phorbol-12,13-dibutyrate (PDB) was significantly diminished by botulinum A toxin. These results show that the evoked transmitter release is inhibited by botulinum A toxin by a mechanism which does not involve ADP ribosylation or an interaction with the alpha 2-adrenoceptor mechanism.

Acetylcholine↗

PubMed1989

Endogenous noradrenaline impairs the prostaglandin-induced inhibition of noradrenaline release.

The effects of prostaglandin E2 (PGE2) on electrically evoked noradrenaline release in rat brain cortex were studied under conditions under which autoinhibition of release was avoided. When stimulation was carried out with 36 pulses at 3 Hz, 1 mumol/l PGE2, produced about 50% inhibition of release. In the presence of the alpha 2-adrenoceptor antagonist yohimbine (1 mumol/l) the effect of PGE2 was markedly increased. When release was elicited by 3 pulses/100 Hz the period of stimulation was too short to permit development of autoinhibition by released noradrenaline. Then the concentration-response-curve for PGE2 was very similar to that obtained under the above conditions (36 pulses/3 Hz, in the presence of yohimbine). These data suggest that both the alpha 2-adrenoceptor and the PGE2-receptor are linked to a common pathway. Since indometacin (10 mumol/l) did not enhance evoked transmitter release, an influence of endogenous PG's on in vitro release of noradrenaline from rat brain cortex slices can be excluded.

Animals↗

PubMed1989

A role for protein kinase C in the electrically evoked release of [3H] gamma-aminobutyric acid in rabbit caudate nucleus.

A possible participation of protein kinase C (PKC) in depolarization-induced release of gamma-aminobutyric acid (GABA) in rabbit caudate nucleus was examined by means of phorbol esters and staurosporine. Slices of caudate nucleus were loaded with [3H]GABA, then superfused and stimulated electrically (3 ms, 5 Hz, 24 mA, 5 V/cm) for 2 min. Aminooxyacetic acid and the uptake inhibitor nipecotic acid were present throughout. The PKC activator 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB) markedly enhanced the evoked [3H]GABA release. In contrast, its biologically inactive isomer, 4 alpha-PDB, did not facilitate transmitter release. Staurosporine, an inhibitor of PKC, diminished [3H]GABA release and counteracted the effects caused by 4 beta-PDB. The above results suggest a participation of PKC in depolarization-induced GABA release in rabbit caudate nucleus. The mechanism underlying the modulation of GABA release by PKC seems to be independent of presynaptic GABA, dopamine and 5-hydroxytryptamine receptors.

Alkaloids↗

PubMed1989

Presynaptic kappa-opioid receptors on noradrenergic nerve terminals couple to G proteins and interact with the alpha 2-adrenoceptors.

Stimulation-induced noradrenaline (NA) release in rabbit hippocampus is inhibited by activation of presynaptic alpha 2-adrenoceptors and kappa-opioid receptors. The purpose of the present study was to investigate (a) an interference between the alpha 2- and kappa-mechanisms, and (b) a coupling of the opioid receptors to pertussis toxin (PT)-sensitive guanine nucleotide-binding proteins (G proteins), as has been previously shown for the alpha 2-receptors. [3H]NA release from hippocampal slices was evoked by electrical field stimulation (360 pulses/3 Hz). Inhibition of stimulation-evoked NA release by the preferential kappa-receptor agonist ethylketocyclazocine (EKC) was increased in the presence of the alpha 2-adrenoceptor antagonist yohimbine (0.1 or 1.0 microM). When autoinhibition was completely removed, EKC (1 microM) almost abolished transmitter release. Pretreatment of hippocampal tissue with either PT (8 micrograms/ml; 18 h) or N-ethylmaleimide (NEM) (30 microM; 30 min), which has been shown to alkylate PT substrates, diminished the EKC-produced inhibition of NA release. The kappa-mechanism was still impaired by these compounds when the alpha 2-receptors were blocked with yohimbine. An effect of NEM on the active site of the kappa-receptor seems to be unlikely, because NEM diminished the EKC-induced inhibition of release irrespective of whether or not the opioid receptor was occupied by EKC during exposure to NEM. The present results suggest an interference of both alpha 2- and kappa-opioid receptor-coupled signal transduction possibly through competition for a common pool of G proteins.

Animals↗

PubMed1988

Phorbol ester-mediated enhancement of hippocampal noradrenaline release: which ion channels are involved?

Enhancement of neurotransmitter release following phorbol ester-induced activation of protein kinase C (PKC) may be mediated by changes in ion conductance through the presynaptic membrane. This question was studied with rabbit hippocampal slices preincubated with [3H]noradrenaline ([3H]NA). NA release was evoked by pulses of either high K+ or Ca2+ (in the presence of high K+), or by electrical field stimulation. 4 beta-Phorbol 12,13-dibutyrate (PDB) increased and polymyxin B (PMB) reduced the K+-evoked NA release independent of the K+ concentration used for depolarization. The effects of PDB and PMB were not reduced by tetrodotoxin. PDB still enhanced the NA release triggered by short Ca2+ pulses in depolarized, axon terminal membranes (30 mM K+ and no Ca2+). The electrically evoked NA release was markedly enhanced by PDB even in the absence of Cl- in the medium or in the presence of the K+ channel blockers, tetraethylammonium, 4-amino- and 3,4-diaminopyridine. The inhibitory effect of the Ca2+ channel blocker, Cd2+, remained almost unchanged in the presence of PDB. It is concluded that PKC activation facilitates NA release in the hippocampus but not via presynaptic changes in Na+, K+ or Cl- currents. Whether phorbol ester mediates an increased intracellular Ca2+ availability, or whether a triggering 'normal' Ca2+ influx simply initiates, and synergistically supports, the PKC-mediated reactions leading to enhanced exocytosis, cannot be decided from the results of the present experiments.

Animals↗

PubMed1988

Participation of protein kinase C and regulatory G proteins in modulation of the evoked noradrenaline release in brain.

1. In the present paper two questions are discussed: (A) Does protein kinase C (PKC) participate in the modulation of evoked noradrenaline release in brain tissue? and (B) Is there any link between presynaptic alpha 2-adrenoceptors and regulatory G proteins? 2. Slices of the middle part of the rabbit hippocampus, labeled with 3H-noradrenaline, were superfused with medium containing the reuptake inhibitor cocaine. During superfusion the tissue was stimulated twice electrically for 2 min each. 3. The PKC activators 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB) and 12-O-tetradecanoyl phorbol 13-acetate (TPA) increased the stimulation-evoked transmitter release in a concentration-dependent manner. 4 alpha-PDB and 4-O-methyl-TPA, which do not activate PKC, were without effect on transmitter release. Polymyxin B, an inhibitor of PKC, diminished the stimulus-evoked overflow and counteracted the effects of the phorbol esters. The increases in release caused by phorbol esters and the alpha 2-adrenoceptor antagonist yohimbine were additive. 4. Treatment of hippocampal tissue with islet-activating protein (IAP) or N-ethylmaleimide (NEM), both known to inactivate the regulatory G proteins Gi and Go by chemical modification, led to a marked increase in evoked noradrenaline release. In addition, the effects of both the alpha 2-adrenoceptor agonist clonidine and the alpha 2-adrenoceptor antagonist yohimbine were inhibited. 5. The facilitatory effects of IAP and NEM on transmitter release were not additive. In synaptosomes prepared from rabbit hippocampus two polypeptides with molecular weights corresponding to those of alpha i and alpha o were 32P-ADP-ribosylated with IAP. Pretreatment of synaptosomes with NEM reduced the subsequent ADP ribosylation by IAP concentration dependently. 6. The above results suggest that PKC is involved in the modulation of noradrenaline release in the rabbit hippocampus. The presynaptic alpha 2-autoreceptors modulate transmitter release by a mechanism which is not directly affected by PKC. The alpha 2-autoreceptor-mediated signals seem to be transduced across the plasma membrane via regulatory G proteins.

Adenylyl Cyclases↗

PubMed1988

Multiple pertussis toxin substrates as candidates for regulatory G proteins of adenylate cyclase coupled to the somatostatin receptor in primary rat astrocytes.

The involvement of G proteins in receptor mediated astroglial cAMP formation was studied. Isoproterenol or prostaglandin E2 stimulated adenylate cyclase of primary astroglial cells was inhibited by somatostatin. Preincubation of cells with increasing concentrations of islet activating protein (IAP) diminished somatostatin inhibition of adenylate cyclase. At an IAP concentration of 50 ng/ml somatostatin inhibition was completely abolished. Studies on IAP catalyzed 32P-ADP-ribosylation of astroglial cell particulate material revealed an incorporation of radiolabel into three polypeptides in the molecular weight range of 41,000-39,000 Dalton. Pretreatment of intact cells with IAP reduced radiolabeling of this molecular species in a concentration dependent manner. No further radiolabeling above background level was detectable after pretreatment of cultures with 10 ng IAP/ml or more. At present, the occurrence of at least three IAP substrates (G proteins) does not permit an identification of the somatostatin receptor coupled G protein. Rather, the finding reveals that astrocytes are endowed with multiple variants of GTP binding proteins likely to be coupled to different receptors.

Adenylate Cyclase Toxin↗

PubMed1988

Protein kinase C and presynaptic modulation of acetylcholine release in rabbit hippocampus.

1. The involvement of protein kinase C in the presynaptic modulation of stimulated acetylcholine release was investigated in rabbit hippocampus. 2. Slices of the rabbit hippocampus, labelled with [3H]-acetylcholine, were superfused with medium and stimulated electrically during superfusion. 3. The protein kinase C activating phorbol ester 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB) enhanced the electrically evoked tritium overflow in a concentration-dependent manner. Its biologically inactive 4 alpha-isomer was without any effect on transmitter release. 4. The protein kinase C inhibitor polymyxin B decreased the stimulation-evoked tritium overflow and counteracted the enhancement of release caused by 4 beta-PDB. 5. The stimulation-evoked tritium overflow was facilitated when the muscarine receptor antagonist atropine was present. The effects of both atropine and 4 beta-PDB, given in combination, were additive. 6. The net inhibition of the evoked tritium overflow caused by the muscarine receptor agonists carbachol and oxotremorine was similar, irrespective of whether 4 beta-PDB was present or not. 7. Similar results to those for muscarine autoreceptor-mediated inhibition, were obtained for inhibition of the stimulated tritium overflow caused by the adenosine receptor agonist (-)-N6-(R-phenylisopropyl)-adenosine ((-)-PIA) and the opioid receptor agonist ethylketocyclazocine (EKC). The net inhibition of both agonists was independent of the presence of the phorbol ester. 8. The above results provide further evidence for participation of a presynaptically located protein kinase C in the modulation of acetylcholine release. However, the modulatory mechanisms which are coupled to presynaptic receptors and mediate inhibition of release seem not to be directly affected by protein kinase C.

Acetylcholine↗

PubMed1987

Possible involvement of protein kinase C (PKC) in the regulation of electrically evoked serotonin (5-HT) release from rabbit hippocampal slices.

Protein kinase C (PKC)-activating phorbol esters enhanced the electrically evoked 5-HT release from rabbit hippocampal slices preincubated with [3H]5-HT. The release was diminished by polymyxin B, an inhibitor of PKC. These results are compatible with a stimulatory effect of PKC on the 5-HT release induced by action potentials. The mutual effects of PKC affecting drugs on 5-HT release suggest a functional but not a competitive interaction. The attenuation or the enhancement of effects of 5-HT autoreceptor ligands at various 5-HT biophase concentrations found after PKC-affecting drugs are in line with the view that autoreceptor-mediated events are not directly influenced by the enzyme PKC.

Action Potentials↗

PubMed1987

The adenosine receptor-mediated inhibition of noradrenaline release possibly involves an N-protein and is increased by alpha 2-autoreceptor blockade.

The stimulation-evoked overflow of [3H]-noradrenaline from slices of the rabbit hippocampus is inhibited by alpha 2-autoreceptors as well as by adenosine (A1)-receptors. Slices of rabbit hippocampus were labelled with [3H]-noradrenaline, superfused continuously and stimulated twice electrically (rectangular pulses; 2 ms, 3 Hz, 24 mA, 5 V cm-1). Treatment of hippocampal slices with N-ethylmaleimide (NEM, 30 microM; 30 min), which functionally disturbs certain N-proteins, decreased the inhibitory action of adenosine receptor agonists like (-)-N6-(R-phenylisopropyl)-adenosine ((-)-PIA) and adenosine on noradrenaline release. Release inhibition caused by (-)-PIA (0.03-1 microM) was antagonized by NEM in a non-competitive manner in the absence and in the presence of the alpha 2-adrenoceptor antagonist yohimbine. The adenosine receptor antagonist 8-phenyltheophylline significantly increased the evoked noradrenaline release by about 15% in control slices by diminishing the inhibitory action of endogenous adenosine. In NEM-treated slices this effect of 8-phenyltheophylline was not seen. In the presence of (-)-PIA (0.1 microM), i.e. under conditions of an increased inhibitory tone, release facilitation by 8-phenyltheophylline was decreased by NEM compared to that in the respective controls. Occupation of the A1-receptor with (-)-PIA prior to and during the NEM treatment did not protect the A1-receptor-coupled signal transduction system from being affected by NEM. In the presence of the alpha 2-adrenoceptor antagonist yohimbine, the inhibitory action of (-)-PIA was strongly increased. The above results suggest the involvement of a regulatory N-protein in the A1-receptor-mediated inhibition of noradrenaline release and an interaction between the alpha 2-autoreceptor and the A1-receptor-coupled signal transduction system, possibly at the level of a N-protein.

Adenosine↗

PubMed1987

Protein kinase C activation and alpha 2-autoreceptor-modulated release of noradrenaline.

1 Effects of phorbol esters on the evoked noradrenaline release were studied in slices of the rabbit hippocampus, labelled with [3H]-noradrenaline, superfused continuously with a medium containing the reuptake inhibitor cocaine and stimulated electrically for 2 min (stimulation parameters: 2 ms, 24 mA, 5 V cm-1, 3 or 0.3 Hz). 2 The electrically-evoked overflow of [3H]-noradrenaline in the slices was increased in a concentration-dependent manner by the protein kinase C (PKC) activators 12-O-tetradecanoylphorbol 13-acetate (TPA) and 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB). Phorbol esters, which do not activate PKC, 4-O-methyl-TPA and 4 alpha-PDB, showed no effect on neurotransmitter release. The effect of 4 beta-PDB was abolished in the presence of tetrodotoxin and in the absence of calcium. The PKC inhibitor polymyxin B inhibited the evoked noradrenaline release. 3 In the presence of 4 beta-PDB the inhibitory effects of the alpha 2-adrenoceptor agonist clonidine or the facilitatory effects of the alpha 2-adrenoceptor antagonist yohimbine seemed to be modified only by changes in the concentration of noradrenaline in the synaptic region. At a stimulation frequency of 3 Hz the inhibitory action of clonidine was reduced whereas the facilitatory effect of the yohimbine was even slightly enhanced by the phorbol ester. At 0.3 Hz and in the presence of 4 beta-PDB the effect of clonidine remained and that of yohimbine was strongly enhanced. 4 Pretreatment of the slices with islet-activating protein or N-ethylmaleimide significantly reduced the enhancement of noradrenaline release caused by 4 beta-PDB. It is possible that a regulatory N-protein is involved in steps following PKC activation. 5 These results suggest that PKC participates in the mechanism of action-potential-induced noradrenaline release from noradrenergic nerve terminals of the rabbit hippocampus and that effects on the autoinhibitory feedback system were not responsible for the 4 beta-PDB-induced increase of neurotransmitter release.

Animals↗

PubMed1986

Enhancement of noradrenaline release by 12-O-tetradecanoyl phorbol-13-acetate, an activator of protein kinase C.

12-O-Tetradecanoyl phorbol-13-acetate (TPA), an activator of protein kinase C (PKC), enhanced the electrically evoked overflow of [3H]noradrenaline in a concentration-dependent manner in rabbit hippocampal slices. 4-O-Methyl-TPA, which lacks the ability to activate PKC had no effect on the evoked tritium overflow. The enhancement of noradrenaline release by TPA was affected by neither the alpha 2-adrenoceptor antagonist yohimbine nor the alpha 2-adrenoceptor agonist clonidine. It is concluded from these results that PKC is involved in the mechanism of stimulus-secretion coupling in noradrenergic nerve terminals of the rabbit hippocampus.

Animals↗