CARBACHOL INDUCES INWARD CURRENT IN NEOSTRIATAL NEURONS THROUGH M(1)-LIKE MUSCARINIC RECEPTORS

The effects of carbachol on rat neostriatal neurons were examined in t he slice and the freshly dissociated neuron preparations using intrace llular and whole-cell voltage-clamp recording methods. Superfusion of carbachol (30 mu M) produced a depolarization concomitant with an incr ease in the rate of spontaneous action potentials. This depolarization was associated with an increase in the input resistance. The carbacho l-induced membrane depolarization was blocked by pirenzepine (1 mu M), a selective M(1) 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 volta ge-clamp mode. The inward current caused by carbachol was not reduced by tetrodotoxin (1 mu M), calcium-free recording solution or Cd2+ (100 mu M). However, it was blocked by Ba2+ (100 mu M). In addition, the c arbachol-induced inward current reversed polarity at about the potassi um equilibrium potential. The whole-cell membrane inward current in re sponse to voltage-clamp step from -90 to -140 mV was reduced by 30 mu M carbachol. With stronger hyperpolarization beyond the potassium equi librium potential, carbachol produced a progressively greater reductio n in membrane current. This inhibitory effect was also abolished by Ba 2+ (100 mu M). A concentration of 30 mu M carbachol-induced inward cur rent could be reversibly antagonized by the M(1) muscarinic receptor a ntagonist pirenzepine (0.1-1 mu M), with an estimated IC50 of 0.3 mu M . However, other muscarinic receptor subtype (M(2) or M(3)) antagonist s could also block the carbachol-induced inward current. The rank orde r of antagonist potency was: pirenzepine (M(1) antagonist) > 4-dipheny lacetoxy-N,N-methyl-piperidine methiodide (M(3)/M(1) antagonist) > gal lamine (M(2) antagonist). Based on these pharmacological data, we conc luded that carbachol can act at M(1)-like muscarinic receptors to redu ce the membrane K+ conductances and excite the neostriatal neurons. Co pyright (C) 1996 IBRO.