Cholinergic Modulation in A7 Noradrenergic Neurons in Rats

乙醯膽鹼（acetylcholine, ACh）對於痛覺的調控有重要的功能。文獻指出乙醯膽鹼能引起鎮痛效果（analgesia），且在脊髓中，乙醯膽鹼或其他膽鹼性促效劑（cholinergic agonists）參與了由α2正腎上腺素受體（α2-adrenergic receptor）調控之止痛功能。行為實驗也提出正腎上腺素之下行性痛覺調控路徑（NAergic descending pain modulation pathway）與蕈毒膽鹼性系統（muscarinic cholinergic system）在腦內有交互作用的可能性。然而，目前並無直接的證據支持此項論述。本研究利用碳醯膽鹼素（carbachol, CCh）這類不易被乙醯膽鹼水解酵素（cholinesterase）分解的膽鹼性促效劑，檢視其對於腦幹中分泌正腎上腺素之A7兒茶酚胺細胞群（catecholamine cell group）的影響。文獻已證實位於腦幹的A7兒茶酚胺細胞群會投射其軸突至脊髓背角（dorsal horn），分泌正腎上腺素並進行痛覺訊息傳遞的調控。在此實驗中，我們對幼鼠腦幹薄片中的A7細胞進行全細胞記錄（whole-cell recording），當膜電位被箝定在-70 mV時，碳醯膽鹼素可於A7細胞上引發一內流電流，且可被蕈毒膽鹼性受器拮抗劑─阿托品（atropine）抑制。利用喜八辛（himbacine）在不同濃度下可抑制不同亞型之蕈毒膽鹼性受器的特性，發現此電流應是活化類M1亞型受器而引發，且G-protein以及磷脂酶C（phospholipase C）的活化並非參與此電流的主要分子機制。另外，此電流的反轉電位（reversal potential）約為-12.6 mV，並在NMDG取代胞外鈉離子時被減弱，顯示可能開啟了非專一性陽離子通道（nonselective cation channel），如Transient Receptor Potential通道（TRP channels）。實驗結果顯示在三種TRP通道抑制劑（2APB, SKF96365, ruthenium red）下，碳醯膽鹼素的反應皆會被顯著抑制。 藉由電刺激腳橋被蓋核（pedunculopontine tegmental nucleus, PPTg），可於A7正腎上腺素細胞中引發一可被阿托品部分抑制的突觸後興奮性電流（excitatory postsynaptic currents），顯示腳橋被蓋核的軸突有投射至A7核區，並可釋放乙醯膽鹼以增強A7細胞的興奮性。在此突觸連結上，也發現有自主回饋抑制（auto-inhibition）的調控現象。 綜合以上結果，此篇研究證實了蕈毒膽鹼性受器可透過開啟TRP通道調控A7正腎上腺素分泌細胞的活性，但並非主要經由活化G蛋白與磷脂酶Ｃ的胞內分子訊息傳遞路徑，且腳橋被蓋核可釋放內生性乙醯膽鹼至A7核區，影響A7細胞的活性。此外，更為脊髓以上蕈毒膽鹼性系統與正腎上腺素之下行性痛覺調控路徑的交互作用提供了有利的證據。Acetylcholine (ACh) is one of principal neurotransmitters involved in pain modulation. Many behavioral studies have shown that central or peripheral ACh administrations can evoke analgesia, and have proved that cholinergic agonists can serve as a synergistic role of α2 adrenergic receptors-mediated antinociception in the spinal cord. Moreover, recent behavioral researches also indicate that there might be supraspinal interactions between muscarinic cholinergic system and noradrenergic (NAergic) pain descending pathway. Nevertheless, there is currently no direct evidence to support this argument. In this study, we investigated the effect of carbachol (CCh), a cholinergic agonist, on NAergic neurons of A7 catecholamine cell group, which projects NAergic fibers to the dorsal horn of the spinal cord to modulate nociceptive signaling. Whole-cell recordings were made from A7 neurons in voltage-clamp mode with membrane voltage clamped at -70 mV in brainstem slices taken from rat pups. Bath application of 25 μM CCh evoked inward currents, which were blocked by 1.5 μM atropine, a muscarinic acetylcholine receptor (mAChR) antagonist, suggesting that carbachol-induced currents (ICCh) were mediated through mAChR. Furthermore, ICCh were significantly attenuated with the existence of high concentration of himbacine, a dose-selective antagonist of mAChRs, showing that mAChRs on NAergic A7 neurons activated by CCh were M1-like mAChRs. Surprisingly, the ICCh were not blocked with internal administration of GDP-β-S, a non-catalytic analogue of GDP, suggesting that the ICCh were G-protein-independent. Bath application of U73122, a phospholipase C inhibitor, slightly but significantly blocked the ICCh, showing that phospholipase C was not the major participant in ICCh. The ICCh were reversed at about -12.6 mV and blocked by extracellular application of NMDG substituted for Na+, showing that ICCh were caused through opening a nonselective cation channel, presumably by transient receptor potential (TRP) channels. Indeed, ICCh were significantly attenuated by several antagonists of TRP channels, including 2APB, SKF96365 and ruthenium red. Besides, high frequency stimulation at pedunculopontine tegmental nucleus (PPTg) evoked an inward current partially blocked by atropine, suggesting PPTg projected their axons to NAergic A7 neurons. There was an auto-inhibition in PPTg-A7 synaptic transmission. These results indicate that mAChR modulate the NAergic A7 neurons via activating TRP channels without the requirement of G-protein and phospholipase C, and there is endogenous ACh released from PPTg onto NAergic A7 neurons. The above results provide an evidence of supraspinal interaction between muscarinic cholinergic system and NAergic descending pain pathway