Presynaptic muscarinic (M3) receptors reduce excitatory transmission in dopamine neurons of the rat mesencephalon

The effects of carbachol (0.01-30 mu M) and muscarine (10-30 mu M) on the e xcitatory synaptic potentials were studied using conventional intracellular recordings from dopaminergic neurons in rat mesencephalic slices. Both mus carinic agonists reversibly reduced the excitatory synaptic potentials, evo ked by local electrical stimulation. The EC50 for carbachol was determined to be 4.5 mu M. The maximal degree of the excitatory synaptic potentials su ppression caused by carbachol and muscarine was around 40% of control. This suppression was completely blocked by the non-specific muscarinic antagoni st atropine (1 mu M) and the selective M3 antagonist 4-diphenylacewxy-N-met hylpiperidine methiodide (1 mu M) Other antagonists, preferentially acting at M1, M2 and M4 receptors, were not effective. Furthermore, the acetylchol inesterase inhibitor, physostigmine (50 mu M), decreased the amplitude of t he excitatory synaptic potentials, indicating that ambient acetylcholine ca n depress this potential. Direct depolarizing responses to glutamate were n ot changed by muscarine. In addition, muscarine facilitated the second exci tatory synaptic potentials during a paired-pulse protocol. Thus, the effect of the muscarinic agonists is attributable to a presynaptic locus of actio n. The action of muscarine was not mediated by an N-ethylmaleimide-sensitiv e G-prorein since it was not modified by a treatment of the slices with thi s agent. The calcium channels blockers, omega-conotoxin GIVA, omega-agatoxi n IVA and omega-conotoxin MVIIC did not affect the action of muscarine on t he excitatory synaptic potentials. When the potassium currents were reduced by extracellular barium and 4-aminopyridine, the muscarinic agonists still depressed the excitatory synaptic potentials. Our data indicate that presynaptically located M3 receptors modulate the ex citatory transmission to midbrain dopaminergic neurons via a N-ethylmaleimi de-insensitive G-protein which activates mechanisms neither linked to N-, P -, Q-type calcium channels nor to barium- and 4-aminopyridine-sensitive pot assium channels. (C) 1999 IBRO, Published by Elsevier Science Ltd.