ROLE OF ATP IN FAST EXCITATORY SYNAPTIC POTENTIALS IN LOCUS-COERULEUSNEURONS OF THE RAT

1. Intracellular recordings were made in a pontine slice preparation o f the rat brain containing the nucleus locus coeruleus (LC). The press ure application of alpha,beta-methylene ATP (alpha,beta-meATP) caused reproducible depolarizations which were depressed by suramin (30 mu M) and abolished by suramin (100 mu M). Pyridoxal-phosphate-6-azophenyl- 2',4'-disulphonic acid (PPADS; 10, 30 mu M) also concentration-depende ntly inhibited the alpha,beta-meATP-induced depolarization, although w ith a much slower timecourse than suramin. Almost complete inhibition developed with 30 mu M PPADS. Reactive blue 2 (30 mu M) did not alter the effect of alpha,beta-meATP, while reactive blue 2 (100 mu M) sligh tly depressed it. 2. Pressure-applied lpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid (AMPA) also depolarized LC neurones. Kynurenic acid (500 mu M) depressed and 6-cyano-7-nitroquinoxaline-2,3-dione (C NQX; 50 mu M) abolished the response to AMPA. Suramin (100 mu M) poten tiated the AMPA effect. 3. Pressure-applied noradrenaline hyperpolariz ed LC neurones. Suramin (100 mu M) did not after the effect of noradre naline. 4. Focal electrical stimulation evoked biphasic synaptic poten tials consisting of a fast depolarization (p.s.p.) followed by a slow hyperpolarization (i.p.s.p.). A mixture of D(-)-2-amino-5-phosphonopen tanoic acid (AP-5; 50 mu M), CNQX (50 mu M) and picrotoxin (100 mu M) depressed both the p.s.p. and the i.p.s.p. Under these conditions sura min (100 mu M) markedly inhibited the p.s.p., but did not alter the i. p.s.p. In the combined presence of AP-5 (50 mu M), CNQX (50 mu M), pic rotoxin (100 mu M), strychnine (0.1 mu M), tropisetron (0.5 mu M) and hexamethonium (100 mu M), a high concentration of suramin (300 mu M) a lmost abolished the p.s.p. without changing the i.p.s.p. 5. In the pre sence of kynurenic acid (500 mu M) and picrotoxin (100 mu M), PPADS (3 0 mu M) depressed the p.s.p. Moreover, the application of suramin (100 mu M) to the PPADS (30 mu M)-containing medium failed to cause any fu rther inhibition. Neither PPADS (30 mu M) nor suramin (100 mu M) alter ed the i.p.s.p. 6. It was concluded that the cell somata of LC. neuron es are endowed with excitatory P2-purinoceptors. ATP may be released e ither as the sole transmitter from purinergic neurones terminating at the LC or as a co-transmitter of noradrenaline from recurrent axon col laterals or dendrites of the LC neurones themselves.