SELECTIVE COUPLING OF DIFFERENT MUSCARINIC ACETYLCHOLINE-RECEPTORS TO NEURONAL CALCIUM CURRENTS IN DNA-TRANSFECTED CELLS

Acetylcholine (ACh) can inhibit calcium currents (I(Ca)) in nerve cells by activating muscarinic ACh receptors (mAChR). There are several different genetic subtypes of mAChR. It is not known which subtype(s) are responsible for I(Ca) inhibition. To resolve this issue, we measured I(Ca) inhibition by ACh with patch-clamp recording, by using Ba2+ as charge carrier, in clones of NG108-15 neuroblastoma x glioma hybrid cells transfected with DNA for mAChRI, II, III and IV. Control (non-transfected) cells showed a mean maximum inhibition of peak I(Ca) of 12.8 ± 1.8% (n = 36) at 1 mM ACh. No consistent increase in inhibition was detected in vector-transfected cells, or in cells transformed to express mAChRI or mAChRIII. In contrast, inhibition was significantly increased in clones transformed to express mAChRII or mAChRIV. Inhibition was not correlated with the number of muscarinic receptors as determined by 3H-quinuclidinyl benzilate binding. Inhibition in both control and transfected cells was prevented by pretreatment with pertussis toxin (PTx). Inhibition persisted in the presence of extracellular or intracellular dibutyryl cyclic AMP, and hence is not because of inhibition of adenylate cyclase. We conclude that the inhibition of neuronal I(Ca) is mediated preferentially by mAChRII and mAChRIV, via a PTx-sensitive GTP-binding protein.