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.

Staurosporine counteracts the phorbol ester-induced enhancement of neurotransmitter release in hippocampus

The effects of staurosporine, introduced as a very potent inhibitor of protein kinase C (PKC), on evoked neurotransmitter release were investigated and compared with those of the other PKC inhibitors: 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7) and polymyxin B (PMB). Slices of rabbit hippocampus, prelabelled with either [3H]noradrenaline, [3H]5-hydroxytryptamine or [3H]choline were superfused with physiological medium. During superfusion the slices were stimulated either electrically (3 Hz, 5 V/cm, 24 mA, 2 msec) or by high K+ (30 mM) for 2 min, respectively. Both the electrically and potassium evoked overflow were increased by the PKC activator 4 beta-phorbol 12,13-dibutyrate (PDB). The degree of the enhancement by PDB was dependent on the transmitter and the stimulation conditions used. These results may be explained by differences in the extent of activation of PKC during electrically or potassium evoked release. The PDB-induced enhancement of electrically or potassium evoked release of the 3 transmitters was counteracted by staurosporine (1 microM) in concentrations much lower than those required for H7 (100 microM) and PMB (100 microM). PMB, which has been shown to decrease electrically evoked transmitter release, similarly diminished K+-evoked release. In contrast, only the potassium evoked [3H]acetylcholine release was significantly diminished by staurosporine (1 microM) and H7 (100 microM). In conclusion, these results show again that facilitation of neurotransmitter release by phorbol esters is due to activation of PKC.

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.