Williams F G, Mullet M A, Beitz A J
Department of Veterinary Biology, College of Veterinary Medicine, University of Minnesota, St. Paul 55108, USA.
Brain Res. 1995 Sep 4;690(2):207-16. doi: 10.1016/0006-8993(95)00554-4.
The periaqueductal gray (PAG) contains neural circuits that participate in descending antinociception. Anatomical and electrophysiological evidence suggests that these circuits might employ opioid peptides and GABA in series to remove a tonic inhibition of descending PAG output neurons. The present studies examined the release of the antinociceptive peptides Met-enkephalin and neurotensin in the ventrolateral PAG, and investigated the interaction between GABA and Met-enkephalin release. In awake and freely moving rats the ventrolateral PAG was dialysed using 25 ga. concentric probes. Basal release of peptide in 12 min or 40 min fractions was determined using radioimmunoassays. To establish how the ventrolateral PAG responds to nociception, dialysis was performed following unilateral hindpaw inflammation using Complete Freund's Adjuvant. Twenty-four hours after inflammation was induced, neurotensin release was increased 133% and Met-enkephalin release was increased 353% compared to control animals. Seven days after inflammation was induced, neurotensin release declined precipitously, while basal Met-enkephalin release remained elevated 313% above controls. Thus, unlike enkephalin, increased basal neurotensin release is not sustained with persistent tonic nociception. In addition, we confirmed in normal animals that the ventrolateral PAG is induced to release Met-enkephalin by systemic morphine. A 43% increase in basal Met-enkephalin release was observed immediately following a 12 mg/kg i.p. morphine injection. Morphine should have the opposite effect (inhibit peptide release) if it acts directly on the enkephalinergic neurons. Thus, we examined the hypothesis that GABAergic interneurons in the PAG mediated morphine-stimulated enkephalin release. When the GABAantagonist bicuculline (0.25 microM to 25 microM) was co-infused with the dialysis medium, Met-enkephalin release increased in a dose-dependent fashion and peaked 68% above pre-infusion levels. These data elucidate the reciprocal inhibitory relationship between GABA and enkephalin in the ventrolateral PAG. We hypothesize that, when nociception induces Met-enkephalin release within this region, the tonic GABAergic inhibition is overcome, resulting in greater sensitivity of PAG enkephalinergic neurons. Ultimately, this enhanced enkephalin release should result in greater excitability of the descending PAG output neurons that are responsible for antinociception.
导水管周围灰质（PAG）包含参与下行性抗伤害感受的神经回路。解剖学和电生理学证据表明，这些回路可能串联使用阿片肽和GABA来消除对下行PAG输出神经元的紧张性抑制。本研究检测了腹外侧PAG中抗伤害感受肽甲硫氨酸脑啡肽和神经降压素的释放，并研究了GABA与甲硫氨酸脑啡肽释放之间的相互作用。在清醒且自由活动的大鼠中，使用25号同心探针透析腹外侧PAG。使用放射免疫分析法测定12分钟或40分钟组分中肽的基础释放量。为了确定腹外侧PAG对伤害感受的反应方式，在使用完全弗氏佐剂诱导单侧后爪炎症后进行透析。诱导炎症24小时后，与对照动物相比，神经降压素释放增加了133%，甲硫氨酸脑啡肽释放增加了353%。诱导炎症7天后，神经降压素释放急剧下降，而基础甲硫氨酸脑啡肽释放仍比对照升高313%。因此，与脑啡肽不同，持续的紧张性伤害感受不会使基础神经降压素释放持续增加。此外，我们在正常动物中证实，全身注射吗啡可诱导腹外侧PAG释放甲硫氨酸脑啡肽。腹腔注射12mg/kg吗啡后，立即观察到基础甲硫氨酸脑啡肽释放增加了43%。如果吗啡直接作用于脑啡肽能神经元，应该会产生相反的效果（抑制肽释放）。因此，我们检验了PAG中的GABA能中间神经元介导吗啡刺激的脑啡肽释放这一假设。当GABA拮抗剂荷包牡丹碱（0.25μM至25μM）与透析介质共同注入时，甲硫氨酸脑啡肽释放呈剂量依赖性增加，峰值比注入前水平高68%。这些数据阐明了腹外侧PAG中GABA与脑啡肽之间的相互抑制关系。我们假设，当伤害感受诱导该区域内甲硫氨酸脑啡肽释放时，紧张性GABA能抑制被克服，导致PAG脑啡肽能神经元的敏感性增加。最终，这种增强的脑啡肽释放应导致负责抗伤害感受的下行PAG输出神经元的兴奋性增加。
