Cholinergic modulation of primary afferent glutamatergic transmission in rat medullary dorsal horn neurons

Although muscarinic acetylcholine (mACh) receptors are expressed in trigeminal ganglia, it is still unknown whether mACh receptors modulate glutamatergic transmission from primary afferents onto medullary dorsal horn neurons. In this study, we have addressed the cholinergic modulation of primary afferent glutamatergic transmission using a conventional whole cell patch clamp technique. Glutamatergic excitatory postsynaptic currents (EPSCs) were evoked from primary afferents by electrical stimulation of trigeminal tract and monosynaptic EPSCs were recorded from medullary dorsal horn neurons of rat horizontal brain stem slices. Muscarine and ACh reversibly and concentration-dependently decreased the amplitude of glutamatergic EPSCs and increased the paired-pulse ratio. In addition, muscarine reduced the frequency of miniature EPSCs without affecting the current amplitude, suggesting that muscarine acts presynaptically to decrease the probability of glutamate release onto medullary dorsal horn neurons. The muscarine-induced decrease of glutamatergic EPSCs was significantly occluded by methoctramine or AF-DX116, M₂ receptor antagonists, but not pirenzepine, J104129 and MT-3, selective M₁, M₃ and M₄ receptor antagonists. The muscarine-induced decrease of glutamatergic EPSCs was highly dependent on the extracellular Ca²⁺ concentration. Physostigmine and clinically available acetylcholinesterase inhibitors, such as rivastigmine and donepezil, significantly shifted the concentration-inhibition relationship of ACh for glutamatergic EPSCs. These results suggest that muscarine acts on presynaptic M₂ receptors to inhibit glutamatergic transmission by reducing the Ca²⁺ influx into primary afferent terminals, and that M₂ receptor agonists and acetylcholinesterase inhibitors could be, at least, potential targets to reduce nociceptive transmission from orofacial tissues.