Pharmacological evidence that protein kinase C modulates monosynaptic excitations in the olfactory cortex

Summary

The possible occurrence and role of protein kinase C at the lateral olfactory tract (LOT)-pyramidal cell synapse of the rat olfactory cortex slice has been investigated by determining the effects of both activators (4-β-phorbol-12,13,diacetate [PDAc] and 1,2-dioctanoyl-sn-glycerol) and inhibitors (5-isoquinolinylsulphonyl)-2-methylpiperazine [H-7], sangivamycin and polymyxin B of the enzyme on the surface field potential known as the N-wave. PDAc (0.3 to 20 μmol/l) and 1,2-dioctanoyl-sn-glycerol (25 to 250 μmol/l) increased the area and amplitude of the potential. In control slices in which a population spike was recorded, PDAc also triggered the appearance of multiple spikes. In a series of input-output experiments, PDAc (2.5 or 5 μmol/l) increased the area and amplitude of the N-wave relative to that of the action potential but did not significantly affect pyramidal cell excitability. The effects of PDAc on the N-wave were antagonised by all three protein kinase C inhibitors but not by the calmodulin antagonist calmidazolium and were greater in slices perfused with solution containing 10 rather than 1 mmol/l Mg²⁺ or 1.25 rather than 5 mmol/l Ca²⁺. The effect of PDAc on the amplitude but not area of the N-wave was blocked by the potassium channel blocker tetraethylammonium (10 mmol/l) but not by 4-aminopyridine (0.25 mmol/l). In a series of conditioning experiments, PDAc (1 to 5 μmol/l) reduced the amplitude of the N-wave evoked by a second stimulus compared to that evoked by the first conditioning pulse. PDAc also caused a selective, concentration-dependent reduction in the response evoked by Nmethyl-D-aspartate (NMDA), an action blocked by H-7. None of these effects of PDAc was reproduced by 4-\-bphorbol and 4-α-phorbol (10 μmol/l), two analogues of phorbol which do not affect protein kinase C activity. It is concluded that: (i) the effects of PDAc are mediated by activation of protein kinase C; (ii) activation of protein kinase C at the LOT-pyramidal cell synapse potentiates transmission by increasing transmitter release, although an effect of pyramidal cell excitability cannot be ruled out; (iii) protein kinase C may regulate the activity of NMDA receptors.