The Ca(2+) channel β subunit determines whether stimulation of G(q)-coupled receptors enhances or inhibits N current

In superior cervical ganglion (SCG) neurons, stimulation of M(1) receptors (M(1)Rs) produces a distinct pattern of modulation of N-type calcium (N-) channel activity, enhancing currents elicited with negative test potentials and inhibiting currents elicited with positive test potentials. Exogenously applied arachidonic acid (AA) reproduces this profile of modulation, suggesting AA functions as a downstream messenger of M(1)Rs. In addition, techniques that diminish AA's concentration during M(1)R stimulation minimize N-current modulation. However, other studies suggest depletion of phosphatidylinositol-4,5-bisphosphate during M(1)R stimulation suffices to elicit modulation. In this study, we used an expression system to examine the physiological mechanisms regulating modulation. We found the β subunit (Ca(V)β) acts as a molecular switch regulating whether modulation results in enhancement or inhibition. In human embryonic kidney 293 cells, stimulation of M(1)Rs or neurokinin-1 receptors (NK-1Rs) inhibited activity of N channels formed by Ca(V)2.2 and coexpressed with Ca(V)β1b, Ca(V)β3, or Ca(V)β4 but enhanced activity of N channels containing Ca(V)β2a. Exogenously applied AA produced the same pattern of modulation. Coexpression of Ca(V)β2a, Ca(V)β3, and Ca(V)β4 recapitulated the modulatory response previously seen in SCG neurons, implying heterogeneous association of Ca(V)β with Ca(V)2.2. Further experiments with mutated, chimeric Ca(V)β subunits and free palmitic acid revealed that palmitoylation of Ca(V)β2a is essential for loss of inhibition. The data presented here fit a model in which Ca(V)β2a blocks inhibition, thus unmasking enhancement. Our discovery that the presence or absence of palmitoylated Ca(V)β2a toggles M(1)R- or NK-1R–mediated modulation of N current between enhancement and inhibition identifies a novel role for palmitoylation. Moreover, these findings predict that at synapses, modulation of N-channel activity by M(1)Rs or NK-1Rs will fluctuate between enhancement and inhibition based on the presence of palmitoylated Ca(V)β2a.