Cholinergic Modulation of Fast Inhibitory and Excitatory Transmission to 1 Pedunculopontine Thalamic Projecting Neurons

21 The pedunculopontine nucleus (PPN) is part of the cholinergic arm of the reticular 22 activating system, which is mostly active during waking and REM sleep. The PPN 23 projects to the thalamus, and receives cholinergic inputs from the laterodorsal tegmental 24 nucleus and contralateral PPN. We employed retrograde labeling and whole-cell 25 recordings to determine the modulation of GABAergic, glycinergic and glutamatergic 26 transmission to PPN thalamic projecting neurons, and their post-synaptic responses to the 27 nonspecific cholinergic agonist carbachol. M2 and M4 muscarinic receptor-modulated 28 inhibitory post-synaptic responses were observed in 73% of PPN output neurons; in 29 12.9%, M1 and nicotinic receptor-mediated excitation was detected; and muscarinic and 30 nicotinic-modulated fast inhibitory followed by slow excitatory biphasic responses were 31 evident in 6.7% of cells. A significant increase in the frequency of spontaneous 32 excitatory post-synaptic currents (EPSCs) and inhibitory post-synaptic currents (IPSCs) 33 during carbachol application was observed in 66.2% and 65.2% of efferent neurons, 34 respectively. This effect was blocked by a M1 antagonist or non-selective muscarinic 35 blocker, indicating that glutamatergic, GABAergic and/or glycinergic neurons projecting 36 to PPN output neurons are excited through muscarinic receptors. Decreases in the 37 frequency of miniature EPSCs, and amplitude of electrical stimulation evoked EPSCs, 38 were blocked by a M2 antagonist, suggesting the presence of M2Rs at terminals of pre39 synaptic glutamatergic neurons. CAR induced multiple types of post-synaptic responses, 40 enhancing both inhibitory and excitatory fast transmission to PPN thalamic projecting 41 neurons through muscarinic receptors. These results provide possible implications for the 42