Laminae-specific distribution of alpha-subunits of voltage-gated sodium channels in the adult rat spinal cord

Abstract

While the voltage-gated sodium channels (VGSCs) are the key molecules for neuronal activities, the precise distribution of them in spinal cord is not clear in previous studies. We examined the expression of mRNAs for alpha-subunits of VGSC (Navs) in adult rat spinal cord before and 7 days after L5 spinal nerve ligation (SPNL) or complete Freund's adjuvant (CFA)-induced paw inflammation by in situ hybridization histochemistry, reverse transcription-polymerase chain reaction, and immunohistochemistry. Nav1.1 and Nav1.6 mRNAs were present in all laminae, except for lamina II, including the spinothalamic tract neurons in lamina I identified by retrograde tracing of Fluoro-gold. Nav1.2 mRNA was predominantly observed in the superficial layers (laminae I, II), and Nav1.3 mRNA was more restricted to these layers. All these transcripts were expressed by the neurons characterized by immunostaining for neuron-specific nuclear protein. Nav1.7 mRNA was selectively expressed by a half of motoneurons in lamina IX. No signals for Nav1.8 or Nav1.9 mRNAs were detected. Immunohistochemistry for Nav1.1, Nav1.2, Nav1.6, and Nav1.7 proteins verified some of these neuronal distributions. L5 SPNL decreased Nav1.1 and Nav1.6 mRNAs, and increased Nav1.3 and Nav1.7 mRNAs in the axotomized spinal motoneurons, without any changes in other laminae of L4-6 spinal segments. Intradermal injection of CFA did not cause any transcriptional change. Our findings demonstrate that spinal neurons have different compositions of VGSCs according to their location in laminae. Pathophysiological changes of spinal neuronal activity may due to post-transcriptional changes of VGSCs. Comparison with our previous data concerning the subpopulation-specific distribution of Nav transcripts in primary afferent neurons provides potentially specific targets for local analgesics at the peripheral nerve and spinal levels.