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

Abstract

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 microM) and abolished by suramin (100 microM). Pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 10, 30 microM) also concentration-dependently inhibited the alpha,beta-meATP-induced depolarization, although with a much slower time-course than suramin. Almost complete inhibition developed with 30 microM PPADS. Reactive blue 2 (30 microM) did not alter the effect of alpha,beta-meATP, while reactive blue 2 (100 microM) slightly depressed it. 2. Pressure-applied (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) also depolarized LC neurones. Kynurenic acid (500 microM) depressed and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 50 microM) abolished the response to AMPA. Suramin (100 microM) potentiated the AMPA effect. 3. Pressure-applied noradrenaline hyperpolarized LC neurones. Suramin (100 microM) did not alter 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 microM), CNQX (50 microM) and picrotoxin (100 microM) depressed both the p.s.p. and the i.p.s.p. Under these conditions suramin (100 microM) markedly inhibited the p.s.p., but did not alter the i.p.s.p. In the combined presence of AP-5 (50 microM), CNQX (50 microM), picrotoxin (100 microM), strychnine (0.1 microM), tropisetron (0.5 microM) and hexamethonium (100 microM), a high concentration of suramin (300 microM) almost abolished the p.s.p. without changing the i.p.s.p. 5. In the presence of kynurenic acid (500 microM) and picrotoxin (100 microM), PPADS (30 microM) depressed the p.s.p. Moreover, the application of suramin (100 microM) to the PPADS (30 microM)-containing medium failed to cause any further inhibition. Neither PPADS (30 microM) nor suramin (100 microM) 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.