Presynaptic inhibition by noradrenaline of the EPSC evoked in neonatal rat sympathetic preganglionic neurons

Abstract

Visually identified and electrophysiologically characterized sympathetic preganglionic neurons (SPNs) were recorded using the whole-cell voltage clamp technique in slices of neonatal rat spinal cord. Monosynaptic excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of the nucleus intercalatus in the presence of strychnine (5 μM) and bicuculline (10 μM). These EPSCs were abolished by the antagonist of AMPA-type glutamate receptors, 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX; 10 μM). Bath applied noradrenaline (NA; 0.5–50 μM) dose-dependently and reversibly decreased by up to around 60% the amplitude of the EPSC, without affecting the holding current. The EPSC depression by NA was not accompanied by a change in EPSC reversal potential (around +5 mV), nor were inward currents generated by pressure application of glutamate affected by NA application. A comparable degree of EPSC depression was also seen with the α 2-adrenoceptor agonist clonidine (5 μM), and the α 2A-agonist oxymetazoline (5 μM), while the α 1-agonist phenylephrine (100 μM) caused only a 22% depression. The EPSC depression caused by NA (10 μM) was completely antagonized by either the α-antagonist phentolamine (10 μM) or the α 2-antagonist idazoxan (2 μM). Conversely, the β-adrenoceptor antagonist popranolol (5 μM), and the α 1-, α 2B- and α 2C-antagonist prazosin (2 μM) were without effect. These results indicate that activation of presynaptic α 2A-adrenoceptors on inputs to SPNs decreases glutamate release.