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

The effects of carbachol (0.01-30 μM) and muscarine (10-30 μM) on the excitatory synaptic potentials were studied using conventional intracellular recordings from dopaminergic neurons in rat mesencephalic slices. Both muscarinic agonists reversibly reduced the excitatory synaptic potentials, evoked by local electrical stimulation. The EC₅₀ for carbachol was determined to be 4.5 mM. The maximal degree of the excitatory synaptic potentials suppression caused by carbachol and muscarine was around 40% of control. This suppression was completely blocked by the non-specific muscarinic antagonist atropine (1 μM) and the selective M3 antagonist 4- diphenylacetoxy-N-methylpiperidine methiodide (1 μM). Other antagonists, preferentially acting at M1, M2 and M4 receptors, were not effective. Furthermore, the acetylcholinesterase inhibitor, physostigmine (50 μM), decreased the amplitude of the excitatory synaptic potentials, indicating that ambient acetylcholine can depress this potential. Direct depolarizing responses to glutamate were not changed by muscarine. In addition, muscarine facilitated the second excitatory synaptic potentials during a paired-pulse protocol. Thus, the effect of the muscarinic agonists is attributable to a presynaptic locus of action. The action of muscarine was not mediated by an N-ethylmaleimide-sensitive G-protein since it was not modified by a treatment of the slices with this agent. The calcium channels blockers, ω-conotoxin GIVA, ω-agatoxin IVA and ω-conotoxin MVIIC did not affect the action of muscarine on the 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 excitatory transmission to midbrain dopaminergic neurons via a N-ethylmaleimide- insensitive G-protein which activates mechanisms neither linked to N-, P-, Q- type calcium channels nor to barium- and 4-aminopyridine-sensitive potassium channels.