大鼠A7核區正腎上腺素神經元GABAB受器媒介之持續性抑制

Abstract

大鼠橋腦的A7核區由正腎上腺素神經元所構成，投射至脊髓背角，一般認為跟痛覺的調控有關。然而這些A7正腎上腺素神經元的自發性活動在一般生理情況下並不高，過去研究者猜測A7正腎上腺素神經元可能受到週遭GABAergic中間神經元的持續性抑制。另外，A7核區也受到其他核區的投射，例如週邊導水管灰質區(periaquiductal gray, PAG)；以及腹內側延腦（ventromedial mudlla, RVM）。這些投射中有一部分為抑制性的神經傳導物質，例如腦啡肽(enkephalin)。當來自PAG及RVM的抑制性投射抑制了A7核區的中間神經元後，正腎上腺素神經元受到的持續性抑制就被減弱，造成正腎上腺素神經元活性上升，而達到止痛的效果。本篇研究試圖證實是否A7正腎上腺素神經元在一般生理情況下，確實受到來自GABAergic中間神經元GABAB受器媒介的持續性抑制。我們在生理溫度下，以全細胞實驗方式記錄出生7-9天新生大鼠的A7核區神經元，並在實驗過程後進行免疫組織螢光染色，確認細胞為正腎上腺素神經元。我們證明GABAB受器不僅存在於正腎上腺素神經元上，也存在於投射至正腎上腺素的神經末梢。我們從三個面向來探討GABAB受器在調控正腎上腺素神經元所扮演的角色：意即對突觸後正腎上腺素神經元的影響，以及突觸前興奮性或抑制性訊息傳導的影響。並發現GABAB受器能夠降低正腎上腺素神經元的自發性放電活動，也能夠減弱投射至正腎上腺素神經元的神經訊息傳導。然而，並非所有的GABAB受器都能夠對正腎上腺素神經元產生持續性抑制，在本篇中，我們的結果顯示只有在正腎上腺素神經元上GABAB受器，才能造成顯著的持續性抑制。

The A7 catecholamine cell group consists of noradrenergic (NAergic) neurons that project axonal terminals to the dorsal spinal cord, and are believed to play important role in modulating nociceptic input. It has been suggested that NAergic neurons receive tonic inhibition from local GABAergic interneurons which is controlled by enkephalin released from neurons in the periaquiductal gray (PAG) and ventraomedial medulla (RVM). Activation of these nuclei would relieve the tonic inhibition of A7 NAergic neurons, which in turn, would increase the release of NE in the dorsal spinal cord and result in modulation of pain. In this study, we provided electrophysiological evidences of the existence tonic inhibition mediated by GABAB receptors on A7 NAergic neurons. Whole-cell patch recording were made from A7 NAergic neurons, confirmed by post hoc immunohistochemistry using antibody against dopamine-β-hydroxylase, in sagittal brainstem slices taken from rats aged 7-9 days. We investigated the role of GABAB receptors in three dimensions: intrinsic properties of NAergic neurons; excitatory synaptc inputs to NAergic neurons; and inhibitory synaptic inputs to NAergic neurons. The present data suggested that GABAB receptors are located not only on the postsynaptic NAergic neurons, but also on the presynaptic excitatory and inhibitory axon terminals projecting to them. And these GABAB receptors could either directly reduced the spontaneous firing frequency of NAergic neurons or decreased the synaptic transmission (excitatory or inhibitory). However, not all of these GABAB receptors are tonically activated, and our data proposed that only the postsynaptic GABAB receptors mediated tonic inhibition on A7 NAergic neurons.