Protein kinase C involvement in maintenance and modulation of noradrenaline release in the mouse brain cortex

1. The role of protein kinase C in the modulation of noradrenaline release was investigated in mouse cortical slices which were pre-incubated with [3H]-noradrenaline. The aim was to investigate the hypothesis that protein kinase C is activated during high levels of transmitter release to maintain transmitter output. 2. The protein kinase C activators, phorbol myristate acetate (0.01-0.3 microM) and to a greater extent 4 beta-phorbol 12,13-dibutyrate (0.01-0.3 microM) significantly enhanced stimulation-induced noradrenaline release whereas 4 alpha-phorbol 12,13-dibutyrate (0.1 microM) which does not activate protein kinase C was without effect. The effect of the protein kinase C activator, phorbol myristate acetate, on noradrenaline release was attenuated by the protein kinase C inhibitor, polymyxin B (21 microM) which by itself inhibited stimulation-induced noradrenaline release. 3. Protein kinase C was down-regulated by 10 h exposure of the cortical slices to 4 beta-phorbol 12,13-dibutyrate (1 microM). In this case the facilitatory effect of 4 beta-phorbol 12,13-dibutyrate (0.1 microM) on noradrenaline release was abolished as was the inhibitory effect produced by polymyxin B. This indicates that polymyxin B was acting selectively at protein kinase C. 4. The inhibitory effect of polymyxin B on noradrenaline release, when expressed as a percentage of the appropriate frequency control, was constant at 1, 5 and 10 Hz. Furthermore, the ratio of release at 5 Hz to that at 10 Hz was not altered by protein kinase C down-regulation, indicating that there is no additional effect of protein kinase C at higher stimulation frequencies. 5. When transmitter release was elevated by blocking alpha 2-adrenoceptor auto-inhibition with idazoxan (0.1 microM) or K+ channels with tetraethylammonium (300 microM), the elevation in transmitter release was significantly attenuated by protein kinase C down-regulation, suggesting an involvement of protein kinase C. 6. We conclude that protein kinase C is involved in the modulation of noradrenaline release over a wide range of stimulation frequencies, in addition to a role when noradrenaline release is elevated by presynaptic mechanisms.