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Keywords:

  • Gβγ subunit;
  • muscarinic receptor;
  • patch clamp technique;
  • rat SCG neuron

Abstract

One postsynaptic action of the transmitter acetylcholine in sympathetic ganglia is to inhibit somatic N-type Ca2+ currents: this reduces Ca2+-activated K+ currents and facilitates high-frequency spiking. Previous experiments on rat superior cervical ganglion neurons have revealed two distinct pathways for this inhibitory action: a rapid, voltage-dependent inhibition through activation of M4 muscarinic acetylcholine receptors (mAChRs), and a slower, voltage-independent inhibition via M1 mAChRs (Hille (1994) Trends in Neurosci., 17, 531–536]. We have analysed the mechanistic basis for this divergence at the level of the individual G-proteins and their α and βγ subunits, using a combination of site-directed antibody injection, plasmid-driven antisense RNA expression, over-expression of selected constitutively active subunits, and antagonism of endogenously liberated βγ subunits by over-expression of βγ-binding β-adrenergic receptor kinase 1 (βARK1) peptide. The results indicate that: (i) M4 mAChR-induced inhibition is mediated by GoA; (ii) α and βγ subunits released from the activated GoA heterotrimer produce separate voltage-insensitive and voltage-sensitive components of inhibition, respectively; and (iii) voltage-insensitive M1 mAChR-induced inhibition is likely to be mediated by the α subunit of Gq. Hence, Ca2+ current inhibition results from the concerted, but independent actions of three different G-protein subunits.