Epilepsy & Behavior 29 (2013) 599–601

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Epilepsy & Behavior

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Letter to the Editor

The optimal segment for spinal cordstimulation in intractable epilepsy: A virallymediated transsynaptic tracing study inspinally transected transgenic mice

To the Editor

Several lines of evidence have indicated the success of spinal cordstimulation (SCS) for the treatment of movement disorders and pain[1–5]. Because advantages of reversibility and adjustability exist, SCSis expected to have therapeutic effects for patients with intractableepilepsy [6].

Studies in rodents have highlighted that SCS, consisting of electricalstimulation of the dorsal spinal cord using epidural electrodes, has beenshown to achieve seizure remission [6,7]. However, the exact locationof the optimal spinal segment for stimulation is not fully understood.It is well known that two main segments for stimulation are used:the high cervical and the upper thoracic epidural space. Nevertheless,it is still unclear whether the high cervical or the upper thoracic seg-ment is tightly linked to the benefits of spinal stimulation in intractableepilepsy.

The sympathetic or motor regulation of the spinal cord is known tobe involved in the neuroplasticity and synaptic connectivity mecha-nisms of stimulation for several different movement disorders, suchas neuropsychiatric disorders, intractable pain, and Parkinson's disease[8–10]. A number of studies have verified that the pseudorabiesvirus (PRV) technique provides a highly specific method of tracingpolysynaptic pathways, and it has been used to map the sympa-thetic and motor pathways [11–16]. We explored the hypothesisthat the functional integrity of synapses in the upper thoracic seg-ment is essential to both sympathetic and motor projections, where-as that in the high cervical segment is not essential to sympatheticprojection.

We had characterized projections from the left gastrocnemius mus-cle to the intermediolateral cell column (IML) of the upper thoracic spi-nal cord in spinally transected [16] transgenic mice by using retrogradetracing techniques of transgenic recombinants of an attenuated PRVstrain, PRV-614, expressing a novel monomeric red fluorescent protein(mRFP1) under the control of the cytomegalovirus immediate-earlypromoter, for direct visualization under a fluorescence microscope[17–19].

We found that injection of PRV-614 into the gastrocnemius mus-cle resulted in retrograde infection of neurons in the IML of the upperthoracic spinal cord (Fig. 1D), a finding which was in agreementwith that of a previous immunohistochemical study investigatingthe IML of the thoracic cord that controlled the adrenal glandthrough the transneuronal tracer PRV-Bartha [15], suggesting that di-rect sympathetic neuronal circuits exist in the upper thoracic spinalcord. Consistent with previously published reports [17,20–22], sym-pathetic preganglionic neurons connected to the stomach, kidney,liver, and adrenal glands via transneuronal tracing of multisynaptic

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pathways were also scattered throughout the IML, from the T2 toL1 spinal levels, with the greatest numbers of PRV detected at theIML of thoracic spinal levels. Otherwise, by using a melanocortin-4receptor (MC4R)-green fluorescent protein (GFP) transgenic mousemodel, our group demonstrated that neurons expressing MC4R-GFP were distributed in the IML of the thoracic cord (Fig. 1C), afinding which was in line with that of a previous immunohisto-chemical study showing that the IML of the thoracic cord exhibitedmoderate levels of GFP immunoreactivity using a mouse line inwhich GFP is expressed under the control of the MC4R gene pro-moter [23], suggesting thatMC4R signaling in the IMLmay be involvedin melanocortinergic regulation. Because the central regulation ofsympathetic activity is a major component of melanocortinergic ac-tion [23–25], we proposed the existence of direct melanocortin-sympathetic neuronal circuits in the upper thoracic spinal cord.Our results and previously published data [26] showed that directsympathetic neuronal circuits did not exist in the high cervical seg-ment of the spinal cord on because of a lack of IML in the cervicalspinal cord.

Many studies have shown that rapid eye movement (REM) sleepsuppressed seizure occurrence [27–30]. Simultaneously, REM sleep ischaracterized by postural muscle atonia that accompanies sympatheticnervous system hypoactivity [15], suggesting that seizure occurrenceis tightly linked to the activity of the sympathetic nervous system.Based on the direct sympathetic neuronal circuits existing in theupper thoracic spinal cord and not in the high cervical segment of thespinal cord, it is postulated that the optimal segment for spinal cordstimulation in intractable epilepsy may be the upper thoracic segmentand not the high cervical segment.

Conflict of interest statement

The work is original, and there is no conflict of interest to disclose.

Funding

This work was supported by grants from the National NaturalScience Foundation of PR China (No. 81271766 to H.X., No. 30901395to X.T., and No. 31000417 to W.M.), Research Fund for the DoctoralProgram of Higher Education of China (No. 20090142120012 to X.T.),Special Fund of Fundamental Scientific Research Business Expense forHigher School of Central Government (2012 TS060 to H.X.), and the2010 Clinical Key Disciplines Construction Grant from the Ministry ofHealth of PR China.

Acknowledgments

The authors gratefully acknowledge Dr. Lynn Enquist for kindlyproviding PRV-614 and Dr. Joel Elmquist (UT Southwestern MedicalCenter) for providing the MC4R-GFP transgenic mice.

Fig. 1. Diagrams of cross sections of the spinal cord at the cervical (C) and thoracic (T) levels. (A) In contrast to the upper thoracic segment of the spinal cord, there is no IML in the highcervical segment. Anatomical plates are taken from Brichta A.M. and Grant G. (1985) [26]. (B) Overlap of C andD in the T2 spinal cord; (C) neurons expressingMC4R-GFPwere distributedin the T2 spinal cord; (D) sections immunoreacted for PRV-614 (red). Injections of PRV-614 into the gastrocnemiusmuscle resulted in retrograde infection of neurons in the T2 spinal cord.IML, intermediolateral cell column; PRV-614, pseudorabies virus-614. Scale bar: 100 μm.

600 Letter to the Editor

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601Letter to the Editor

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Da-Wei Ye1

Cancer Center, Tongji Hospital of Tongji Medical College,Huazhong University of Science and Technology, Wuhan,

Hubei 430030, PR China

De-Fang Ding1

Department of Pain Management, Wuhan Pu'ai Hospital,Tongji Medical College, Huazhong University of Science and Technology,

Wuhan 430034, PR China

Tao-Tao LiuXue-Bi TianCheng Liu

Department of Anesthesiology and Pain Medicine,Tongji Hospital of Tongji Medical College,

Huazhong University of Science and Technology,Wuhan, Hubei 430030, PR China

1 These authors contributed equally to this work.

Rong-Chun LiDepartment of Pain Management, Wuhan Pu'ai Hospital,

Tongji Medical College, Huazhong University of Science and Technology,Wuhan 430034, PR China

Hong-Bing XiangDepartment of Anesthesiology and Pain Medicine,

Tongji Hospital of Tongji Medical College,Huazhong University of Science and Technology,

Wuhan, Hubei 430030, PR ChinaCorresponding author. Fax: +86 27 83662853.

E-mail address: xianghongbing0204@163.com (H.-B. Xiang).

Chi-Wai CheungDepartment of Anaesthesiology,

The University of Hong Kong, Queen Mary Hospital,Hong Kong, PR China

⁎Correspondence author. Fax: +852 22553384.E-mail address: cheucw@hku.hk (C.W. Cheung).

10 September 2013