Presynaptic depression of excitatory synaptic inputs to rat hypoglossal motoneurons by muscarinic M2 receptors

1. Whole cell recordings of glutamatergic excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation in the reticular formation were made from visualized hypoglossal motoneurons (HMs) in rat brain stem slices. 2. Carbachol, muscarine, or physostigmine reduced EPSC amplitude to 50 +/- 3%, 37 +/- 3%, and 54 +/- 7% (mean +/- SE) of control, respectively; effects of carbachol and physostigmine were antagonized by atropine (1-2 mu M). EPSC depression was most effectively antagonized by methoctramine, an M2 muscarinic acetylcholine receptor (mAChR) antagonist with a high affinity constant (pK(B)) of 8.07 for the receptor mediating this response, whereas pirenzepine, an M1 mAChR antagonist, had a pK(B) of <7.0, showing that EPSC depression was mediated by the M2 mAChR. 3. Postsynaptic properties of HMs (holding current and input resistance), EPSCs (reversal potential, rise time, half-width, and decay time constant), and postsynaptic glutamate-gated currents (amplitude and waveform) were not altered by carbachol or muscarine. 4. Muscarine did not decrease presynaptic neuron excitability, because the frequency of spontaneous EPSCs in HMs in the absence of tetrodotoxin (TTX) was either unchanged or increased. Leak and action currents of reticular formation neurons were not significantly altered by muscarine. In contrast, with TTX present, the frequency of spontaneous miniature glutamatergic EPSCs in HMs was decreased by both carbachol (mean change = 203 +/- 46%) and muscarine (mean change = 185 I 26%),with no change in miniature EPSC amplitude distribution. 5. Muscarinic depression of excitatory transmission to HMs thus occurs at the presynaptic terminal, most probably affecting release mechanisms downstream from calcium entry, and is likely to be significant during rapid eye movement sleep, possibly underlying the loss of tongue tone and inspiratory activity during this state.