Role of ATP in fast excitatory synaptic potentials in locus coeruleus neurones of the rat

1. Intracellular recordings were made in a pontine slice preparation of the rat brain containing the nucleus locus coeruleus (LC). The pressure 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-dependently inhibited the alpha,beta-meATP-induced depolarization, although with 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) slightly depressed it. 2. Pressure-applied (S)-alpha-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 (CNQX; 50 mu M) abolished the response to AMPA. Suramin (100 mu M) potentiated the AMPA effect. 3. Pressure-applied noradrenaline hyperpolarized LC neurones. Suramin (100 mu M) did not after the effect of noradrenaline. 4. Focal electrical stimulation evoked biphasic synaptic potentials consisting of a fast depolarization (p.s.p.) followed by a slow hyperpolarization (i.p.s.p.). A mixture of D(-)-2-amino-5-phosphonopentanoic 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 suramin (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), picrotoxin (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) almost abolished the p.s.p. without changing the i.p.s.p. 5. In the presence of kynurenic acid (500 mu M) and picrotoxin (100 mu M), PPADS (30 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 further inhibition. Neither PPADS (30 mu M) nor suramin (100 mu M) altered the i.p.s.p. 6. It was concluded that the cell somata of LC. neurones are endowed with excitatory P2-purinoceptors. ATP may be released either as the sole transmitter from purinergic neurones terminating at the LC or as a co-transmitter of noradrenaline from recurrent axon collaterals or dendrites of the LC neurones themselves.