Department of Physiology and Pharmacology, Oregon Health and Science University, Portland, Oregon, USA.
J Neurophysiol. 2011 Oct;106(4):1833-40. doi: 10.1152/jn.00476.2011. Epub 2011 Jul 6.
In the caudal portions of the solitary tract (ST) nucleus, primary sensory afferents fall into two broad classes based on the expression of transient receptor potential vanilloid type 1 (TRPV1) receptors. Both afferent classes (TRPV1+/-) have indistinguishable glutamate release mechanisms for ST-evoked excitatory postsynaptic currents (EPSCs). However, TRPV1+ terminals release additional glutamate from a unique, TRPV1-operated vesicle pool that is temperature sensitive and facilitated by ST activity to generate asynchronous EPSCs. This study tested whether presynaptic γ-aminobutyric acid (GABA)(B) receptors inhibit both the evoked and TRPV1-operated release mechanisms on second-order ST nucleus neurons. In horizontal slices, shocks activated single ST axons and evoked the time-invariant (latency jitter <200 μs), glutamatergic EPSCs, which identified second-order neurons. Gabazine eliminated GABA(A) responses in all recordings. The GABA(B) agonist baclofen inhibited the amplitude of ST-EPSCs from both TRPV1+ and TRPV1- afferents with a similar EC(50) (∼1.2 μM). In TTX, GABA(B) activation decreased miniature EPSC (mEPSC) rates but not amplitudes, suggesting presynaptic actions downstream from terminal excitability. With calcium entry through voltage-activated calcium channels blocked by cadmium, baclofen reduced mEPSC frequency, indicating that GABA(B) reduced vesicle release by TRPV1-dependent calcium entry. GABA(B) activation also reduced temperature-evoked increases in mEPSC frequency, which relies on TRPV1. Our studies indicate that GABA(B) G protein-coupled receptors are uniformly distributed across all ST primary afferent terminals and act at multiple stages of the excitation-release cascades to suppress both action potential-triggered and TRPV1-coupled glutamate transmission pathways. Moreover, the segregated release cascades within TRPV1+ ST primary afferents represent independent, potential targets for differential modulation.
在孤束核（ solitary tract nucleus，ST ）的尾部区域，初级感觉传入纤维根据瞬时受体电位香草酸型 1（ transient receptor potential vanilloid type 1，TRPV1）受体的表达分为两类。两类传入纤维（TRPV1+/-）均具有相同的谷氨酸释放机制，可引发 ST 诱发的兴奋性突触后电流（ excitatory postsynaptic currents，EPSCs）。然而，TRPV1+末端从独特的 TRPV1 操纵的囊泡池中释放额外的谷氨酸，这种囊泡池对温度敏感，并且受到 ST 活动的促进，从而产生非同步 EPSC。本研究测试了 ST 核二级神经元上的 GABA（B）受体是否抑制了谷氨酸释放的这两种诱发和 TRPV1 操纵的释放机制。在水平切片中，刺激可激活单个 ST 轴突，并引发时间不变的（潜伏期抖动<200μs）、谷氨酸能 EPSC，这些 EPSC 可识别二级神经元。gabazine 在所有记录中消除了 GABA（A）反应。GABA（B）激动剂baclofen 以相似的 EC（50）（约 1.2μM）抑制 TRPV1+和 TRPV1-传入纤维的 ST-EPSC 幅度。在 TTX 中，GABA（B）激活降低了微小 EPSC（ miniature EPSC，mEPSC）的速率，但不影响幅度，这表明其作用发生在末端兴奋性之后的突触前。通过用 cadmium 阻断电压激活的钙通道进入钙，baclofen 降低了 mEPSC 频率，表明 GABA（B）通过 TRPV1 依赖的钙内流减少了囊泡释放。GABA（B）激活还降低了温度诱发的 mEPSC 频率增加，这依赖于 TRPV1。我们的研究表明，GABA（B）G 蛋白偶联受体均匀分布在所有 ST 初级传入末端，并在兴奋-释放级联的多个阶段起作用，以抑制动作电位触发和 TRPV1 偶联的谷氨酸传递途径。此外，TRPV1+ST 初级传入纤维内的分离释放级联代表了独立的、潜在的差异调节靶点。
