Lao L, Marvizón J C G
Center for Neurovisceral Sciences and Women's Health, CURE: Digestive Diseases Research Center, Division of Digestive Diseases, Department of Medicine, David Geffen School of Medicine at University of California Los Angeles, Los Angeles, CA, USA.
Neuroscience. 2005;130(4):1013-27. doi: 10.1016/j.neuroscience.2004.10.019.
Our goal was to test the following hypotheses: 1) GABA(A) receptors facilitate neurokinin release from primary afferent terminals; 2) they do this by suppressing an inhibitory effect of GABA(B) receptors; 3) the activation of these two receptors is controlled by the firing frequency of primary afferents. We evoked neurokinin release by stimulating the dorsal root attached to spinal cord slices, and measured it using neurokinin 1 receptor (NK1R) internalization. Internalization evoked by root stimulation at 1 Hz (but not at 100 Hz) was increased by the GABA(A) receptor agonists muscimol (effective concentration of drug for 50% of the increase [EC50] 3 microM) and isoguvacine (EC50 4.5 microM). Internalization evoked by root stimulation at 100 Hz was inhibited by the GABA(A) receptor antagonists bicuculline (effective concentration of drug for 50% of the inhibition [IC50] 2 microM) and picrotoxin (IC50 243 nM). Internalization evoked by incubating the root with capsaicin (to selectively recruit nociceptive fibers) was increased by isoguvacine and abolished by picrotoxin. Therefore, GABA(A) receptors facilitate neurokinin release. Isoguvacine-facilitated neurokinin release was inhibited by picrotoxin, low Cl-, low Ca2+, Ca2+ channel blockers and N-methyl-D-aspartate receptor antagonists. Bumetanide, an inhibitor of the Na(+)-K(+)-2Cl- cotransporter, inhibited isoguvacine-facilitated neurokinin release, but this could be attributed to a direct inhibition of GABA(A) receptors. The GABA(B) agonist baclofen inhibited NK1R internalization evoked by 100 Hz root stimulation (IC50 1.5 microM), whereas the GABA(B) receptor antagonist (2S)-3-[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl phosphinic acid (CGP-55845) increased NK1R internalization evoked by 1 Hz root stimulation (EC50 21 nM). Importantly, baclofen inhibited isoguvacine-facilitated neurokinin release, and CGP-55845 reversed the inhibition of neurokinin release by bicuculline. In conclusion, 1) GABA(B) receptors located presynaptically in primary afferent terminals inhibit neurokinin release; 2) GABA(A) receptors located in GABAergic interneurons facilitate neurokinin release by suppressing GABA release onto these GABA(B) receptors; 3) high frequency firing of C-fibers stimulates neurokinin release by activating GABA(A) receptors and inhibiting GABA(B) receptors, whereas low frequency firing inhibits neurokinin release by the converse mechanisms.
1）GABA(A)受体促进初级传入终末释放神经激肽；2）它们通过抑制GABA(B)受体的抑制作用来实现这一点；3）这两种受体的激活受初级传入神经的放电频率控制。我们通过刺激连接脊髓切片的背根来诱发神经激肽释放，并使用神经激肽1受体（NK1R）内化来进行测量。GABA(A)受体激动剂蝇蕈醇（使增加50%的药物有效浓度[EC50]为3 microM）和异鹅肌肽（EC50为4.5 microM）可增加1 Hz（而非100 Hz）背根刺激诱发的内化。GABA(A)受体拮抗剂荷包牡丹碱（使抑制50%的药物有效浓度[IC50]为2 microM）和苦味毒（IC50为243 nM）可抑制100 Hz背根刺激诱发的内化。用辣椒素孵育背根（以选择性募集伤害性纤维）诱发的内化可被异鹅肌肽增加，并被苦味毒消除。因此，GABA(A)受体促进神经激肽释放。异鹅肌肽促进的神经激肽释放可被苦味毒、低Cl-、低Ca2+、Ca2+通道阻滞剂和N-甲基-D-天冬氨酸受体拮抗剂抑制。布美他尼，一种Na(+)-K(+)-2Cl-共转运体抑制剂，可抑制异鹅肌肽促进的神经激肽释放，但这可能归因于对GABA(A)受体的直接抑制。GABA(B)激动剂巴氯芬可抑制100 Hz背根刺激诱发的NK1R内化（IC50为1.5 microM），而GABA(B)受体拮抗剂(2S)-3-[(1S)-1-(3,4-二氯苯基)乙基]氨基-2-羟丙基次膦酸（CGP-55845）可增加1 Hz背根刺激诱发的NK1R内化（EC50为21 nM）。重要的是，巴氯芬可抑制异鹅肌肽促进的神经激肽释放，而CGP-55845可逆转荷包牡丹碱对神经激肽释放的抑制作用。总之，1）位于初级传入终末突触前的GABA(B)受体抑制神经激肽释放；2）位于GABA能中间神经元的GABA(A)受体通过抑制GABA释放到这些GABA(B)受体上促进神经激肽释放；3）C纤维的高频放电通过激活GABA(A)受体和抑制GABA(B)受体刺激神经激肽释放，而低频放电则通过相反机制抑制神经激肽释放。
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