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.
Allgaier
C
C
Pharmakologisches Institut der Universität, Freiburg i. Br., Federal Republic of Germany.
Hertting
G
G
Huang
H Y
HY
Jackisch
R
R
D000818:Animals; D002273:Carcinogens / Q000494:pharmacology; D003000:Clonidine / Q000494:pharmacology; D004558:Electric Stimulation; D004789:Enzyme Activation / Q000187:drug effects; D005033:Ethylmaleimide / Q000494:pharmacology; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D009411:Nerve Endings / Q000187:drug effects / Q000378:metabolism; D009638:Norepinephrine / Q000378:metabolism*; D037342:Pertussis Toxin; D015240:Phorbol 12,13-Dibutyrate; D010703:Phorbol Esters / Q000494:pharmacology; D011493:Protein Kinase C / Q000378:metabolism*; D011817:Rabbits; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D002273:Carcinogens; D010703:Phorbol Esters; D011942:Receptors, Adrenergic, alpha; D010566:Virulence Factors, Bordetella; D015240:Phorbol 12,13-Dibutyrate; D037342:Pertussis Toxin; D011493:Protein Kinase C; D003000:Clonidine; D005033:Ethylmaleimide; D009638:Norepinephrine