Muscarinic M 1 modulation of N and L types of calcium channels is mediated by protein kinase C in neostriatal neurons

Abstract

In some neurons, muscarinic M 1-class receptors control L-type (Ca V1) Ca 2+-channels via protein kinase C (PKC) or calcineurin (phosphatase 2B; PP-2B) signaling pathways. Both PKC and PP-2B pathways start with phospholipase C (PLC) activation. In contrast, P/Q- and N-type (Ca V2.1, 2.2, respectively) Ca 2+-channels are controlled by M 2-class receptors via G proteins that may act, directly, to modulate these channels. The hypothesis of this work is that this description is not enough to explain muscarinic modulation of Ca 2+ channels in rat neostriatal projection neurons. Thus, we took advantage of the specific muscarinic toxin 3 (MT-3) to block M 4-type receptors in neostriatal neurons, and leave in isolation the M 1-type receptors to study them separately. We then asked what Ca 2+ channels are modulated by M 1-type receptors only. We found that M 1-receptors do modulate L, N and P/Q-types Ca 2+ channels. This modulation is blocked by the M 1-class receptor antagonist (muscarinic toxin 7, MT-7) and is voltage-independent. Thereafter, we asked what signaling pathways, activated by M 1-receptors would control these channels. We found that inactivation of PLC abolishes the modulation of all three channel types. PKC activators (phorbol esters) mimic muscarinic actions, whereas reduction of intracellular calcium virtually abolishes all modulation. As expected, PKC inhibitors prevented the muscarinic reduction of the afterhyperpolarizing potential (AHP), an event known to be dependent on Ca 2+ entry via N- and P/Q-type Ca 2+ channels. However, PKC inhibitors (bisindolylmaleimide I and PKC-1936) only block modulation of currents through N and L types Ca 2+ channels; while the modulation of P/Q-type Ca 2+ channels remains unaffected. These results show that different branches of the same signaling cascade can be used to modulate different Ca 2+ channels. Finally, we found no evidence of calcineurin modulating these Ca 2+ channels during M 1-receptor activation, although, in the same cells, we demonstrate functional PP-2B by activating dopaminergic D 2-receptor modulation.