Effects of muscarinic acetylcholine receptor activation on membrane currents and intracellular messengers in medium spiny neurones of the rat striatum

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Abstract
Acetylcholine, acting through muscarinic receptors, modulates the excitability of striatal medium spiny neurones. However, the underlying membrane conductances and intracellular signalling pathways have not been fully determined. Our aim was to characterize excitatory effects mediated by M1 muscarinic acetylcholine receptors in these neurones using whole‐cell patch‐clamp recordings in brain slices of postnatal rats. Under voltage‐clamp, muscarine evoked an inward current associated with an increase in cell membrane resistance. The current, which reversed at −85 mV, was sensitive to the M1 receptor antagonist pirenzepine. Blocking the potassium conductance attenuated the response and the residual current was further reduced by ruthenium red (50 µm) and reversed at +15 mV. Simultaneous recordings from cholinergic interneurones and medium spiny neurones in conjunction with spike‐triggered averaging revealed small unitary excitatory postsynaptic currents in four of 39 cell pairs tested. The muscarine‐induced inward current was attenuated by a phospholipase C (PLC) inhibitor, U73122, but not by a protein kinase C inhibitor, chelerythrine, or by the intracellular calcium chelator 1,2‐bis(2‐aminophenoxy) ethane‐N,N,N′,N′‐tetra‐acetic acid, suggesting that the current was associated with PLC in a protein kinase C‐ and Ca2+‐independent manner. The phosphatidylinositol 4‐kinase inhibitor wortmannin (10 µm) reduced the recovery of the inward current, indicating that the recovery process was dependent on the removal of diacylglycerol and/or inositol 1,4,5 triphosphate or resynthesis of phospholipid phosphatidylinositol 4,5‐bisphophate. Ratiometric measurement of intracellular calcium after cell loading with fura‐2 demonstrated a muscarine‐induced increase in calcium signal that originated mainly from intracellular stores. Thus, the cholinergic excitatory effect in striatal medium spiny neurones, which is important in motor disorders associated with altered cholinergic transmission in the striatum such as Parkinson's disease, is mediated through M1 receptors and the PLC‐dependent pathway.