Department of Neurobiology and Anatomy, Drexel University College of Medicine, Philadelphia, PA 19129, USA.
Exp Neurol. 2010 Jan;221(1):68-78. doi: 10.1016/j.expneurol.2009.10.003. Epub 2009 Oct 17.
Serotonergic (5-HT) receptors are upregulated following spinal cord transection. Stimulation by administration of serotonergic receptor agonists has been successful in improving hindlimb function. We tested whether this strategy would be successful in incomplete injury models (moderate or severe thoracic contusion) where descending projections are partially spared which should produce less denervation-induced receptor upregulation. Adult rats received midthoracic moderate (MOD: 25 mm drop) or severe (SEV: 50 mm drop) contusion injuries. Distribution of 5-HT and its transporter and expression of 5-HT(2C) receptors were evaluated in lumbar spinal cord and motor response to 5-HT receptor activation was assessed using open field locomotion (BBB) score, percent weight supported treadmill stepping (%WS) and evaluation of hindlimb muscle activation (tremor and serotonin syndrome). 5-HT immunostaining 3 months post-contusion revealed few 5-HT fibers caudal to the severe contusion, and more spared caudal to the moderate contusion. The distribution of 5-HT transporter paralleled 5-HT staining, but was more greatly reduced. Thus serotonin reuptake may be less efficient in the injured spinal cord. Immunostaining for the 5-HT(2C) receptor in the dorsal and ventral horns at L5 showed significant upregulation in SEV, compared to sham or MOD rats. Neither 5-HT(2C) nor 5-HT(1A) receptor agonists, alone or in combination, nor the serotonin transporter inhibitor d-fenfluramine modified BBB scores or %WS in either group. Despite the increased sensitivity of post-synaptic targets, agonist treatment did not improve function in SEV rats. We conclude that selective 5-HT(2C) or 5-HT(1A) receptor activation was not effective in improving hindlimb function after incomplete lesions. In contrast, the 5-HT precursor 5-hydroxytryptophan (L-5-HTP), which leads to activation of all classes of 5-HT receptors, increased both %WS and hindlimb activity in the MOD group. While no side effects were observed in normal or MOD rats, SEV rats displayed hindlimb tremors and 33% mortality, indicating hypersensitivity to the precursor.
脊髓横断后，5-羟色胺能（5-HT）受体上调。给予 5-HT 受体激动剂的刺激已成功改善后肢功能。我们测试了这种策略在不完全损伤模型（中度或严重胸段挫伤）中是否成功，其中下行投射部分保留，这应该会产生较少的去神经诱导受体上调。成年大鼠接受中胸段中度（MOD：25mm 下降）或严重（SEV：50mm 下降）挫伤损伤。评估了腰椎脊髓中 5-HT 及其转运体的分布，以及 5-HT（2C）受体的表达，并通过开放场运动（BBB）评分、后肢支撑跑步机行走的百分比（%WS）和后肢肌肉激活（震颤和 5-羟色胺综合征）评估 5-HT 受体激活的反应。挫伤后 3 个月的 5-HT 免疫染色显示，严重挫伤后 5-HT 纤维较少，而中度挫伤后则更多。5-HT 转运体的分布与 5-HT 染色平行，但减少更为明显。因此，损伤脊髓中的 5-HT 再摄取可能效率较低。L5 背角和腹角的 5-HT（2C）受体免疫染色显示，SEV 大鼠与假手术或 MOD 大鼠相比，显著上调。5-HT（2C）或 5-HT（1A）受体激动剂单独或联合使用，以及 5-HT 转运体抑制剂 d-氟苯丙胺，均未改变两组的 BBB 评分或%WS。尽管突触后靶标敏感性增加，但激动剂治疗并未改善 SEV 大鼠的功能。我们得出结论，选择性 5-HT（2C）或 5-HT（1A）受体激活在不完全损伤后改善后肢功能无效。相比之下，5-HT 前体 5-羟色氨酸（L-5-HTP）可激活所有 5-HT 受体类型，增加 MOD 组的%WS 和后肢活动。虽然在正常或 MOD 大鼠中未观察到副作用，但 SEV 大鼠出现后肢震颤和 33%的死亡率，表明对前体药物敏感。
