Carbachol induces inward current in neostriatal neurons through M1-like muscarinic receptors.
The effects of carbachol on rat neostriatal neurons were examined in the slice and the freshly dissociated neuron preparations using intracellular and whole-cell voltage-clamp recording methods. Superfusion of carbachol (30 microM) produced a depolarization concomitant with an increase in the rate of spontaneous action potentials. This depolarization was associated with an increase in the input resistance. The carbachol-induced membrane depolarization was blocked by pirenzepine (1 microM), a selective M1 muscarinic receptor antagonist. In other experiments, we observed that carbachol induced a transient inward current on the freshly dissociated neostriatal neuron at a holding potential of -60 mV in a concentration-dependent manner underlying the whole-cell voltage-clamp mode. The inward current caused by carbachol was not reduced by tetrodotoxin (1 microM), calcium-free recording solution or Cd2+ (100 microM). However, it was blocked by Ba2+ (100 microM). In addition, the carbachol-induced inward current reversed polarity at about the potassium equilibrium potential. The whole-cell membrane inward current in response to voltage-clamp step from -90 to -140 mV was reduced by 30 microM carbachol. With stronger hyperpolarization beyond the potassium equilibrium potential, carbachol produced a progressively greater reduction in membrane current. This inhibitory effect was also abolished by Ba2+ (100 microM). A concentration of 30 microM carbachol-induced inward current could be reversibly antagonized by the M1 muscarinic receptor antagonist pirenzepine (0.1-1 microM), with an estimated IC50 of 0.3 microM. However, other muscarinic receptor subtype (M2 or M3) antagonists could also block the carbachol-induced inward current. The rank order of antagonist potency was: pirenzepine (M1 antagonist) > 4-diphenylacetoxy-N,N-methyl-piperidine methiodide (M3/M1 antagonist) > gallamine (M2 antagonist). Based on these pharmacological data, we concluded that carbachol can act at M1-like muscarinic receptors to reduce the membrane K+ conductances and excite the neostriatal neurons.
Hsu
K S
KS
Department of Pharmacology, College of Medicine, National Cheng-Kung University, Tainan, Taiwan.
Yang
C H
CH
Huang
C C
CC
Gean
P W
PW
D000818:Animals; D002217:Carbachol / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects*; D017072:Neostriatum / Q000187:drug effects*; D018408:Patch-Clamp Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley; D011976:Receptors, Muscarinic / Q000187:drug effects*
D011976:Receptors, Muscarinic; D002217:Carbachol