The Journal of Neurophysiology Vol. 81 No. 1 January 1999, pp. 72-84
Copyright ©1999 The American Physiological Society

Muscarine Modulates Ca²⁺ Channel Currents in Rat Sensorimotor Pyramidal Cells Via Two Distinct Pathways

Ansalan E. Stewart, Zhen Yan, D. James Surmeier, and Robert C. Foehring

Department of Anatomy and Neurobiology, University of Tennessee at Memphis, Memphis, Tennessee 38163

Stewart, Ansalan E., Zhen Yan, D. James Surmeier, and Robert C. Foehring. Muscarine modulates Ca²⁺ channel currents in rat sensorimotor pyramidal cells via two distinct pathways. J. Neurophysiol. 81: 72-84, 1999. We used the whole cell patch-clamp technique and single-cell reverse transcription-polymerase chain reaction (RT-PCR) to study the muscarinic receptor-mediated modulation of calcium channel currents in both acutely isolated and cultured pyramidal neurons from rat sensorimotor cortex. Single-cell RT-PCR profiling for muscarinic receptor mRNAs revealed the expression of m1, m2, m3, and m4 subtypes in these cells. Muscarine reversibly reduced Ca²⁺ currents in a dose-dependent manner. The modulation was blocked by the muscarinic antagonist atropine. When the internal recording solution included 10 mM ethylene glycol-bis(-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) or 10 mM bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA), the modulation was rapid (_onset ~ 1.2 s). Under conditions where intracellular calcium levels were less controlled (0.0-0.1 mM BAPTA), a slowly developing component of the modulation also was observed (_onset ~17 s). Both fast and slow components also were observed in recordings with 10 mM EGTA or 20 mM BAPTA when Ca²⁺ was added to elevate internal [Ca²⁺] (~150 nM). The fast component was due to a reduction in both N- and P-type calcium currents, whereas the slow component involved L-type current. N-ethylmaleimide blocked the fast component but not the slow component of the modulation. Preincubation of cultured neurons with pertussis toxin (PTX) also greatly reduced the fast portion of the modulation. These results suggest a role for both PTX-sensitive G proteins as well as PTX-insensitive G proteins in the muscarinic modulation. The fast component of the modulation was reversed by strong depolarization, whereas the slow component was not. Reblock of the calcium channels by G proteins (at 90 mV) occurred with a median  of 68 ms. We conclude that activation of muscarinic receptors results in modulation of N- and P-type channels by a rapid, voltage-dependent pathway and of L-type current by a slow, voltage-independent pathway.

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