Abstract
Under physiological conditions, the voltage-gated sodium channel Nav1.8 is expressed almost exclusively in primary sensory neurons. The mechanism restricting Nav1.8 expression is not entirely clear, but we have previously described a 3.7 kb fragment of the Scn10a promoter capable of recapitulating the tissue-specific expression of Nav1.8 in transfected neurons and cell lines (Puhl and Ikeda, 2008). To validate these studies in vivo, a transgenic mouse encoding EGFP under the control of this putative sensory neuron specific promoter was generated and characterized in this study. Approximately 45% of dorsal root ganglion neurons of transgenic mice were EGFP-positive (mean diameter = 26.5 μm). The majority of EGFP-positive neurons bound isolectin B4, although a small percentage (∼10%) colabeled with markers of A-fiber neurons. EGFP expression correlated well with the presence of Nav1.8 transcript (95%), Nav1.8-immunoreactivity (70%), and TTX-R INa (100%), although not all Nav1.8-expressing neurons expressed EGFP. Several cranial sensory ganglia originating from neurogenic placodes, such as the nodose ganglion, failed to express EGFP, suggesting that additional regulatory elements dictate Scn10a expression in placodal-derived sensory neurons. EGFP was also detected in discrete brain regions of transgenic mice. Quantitative PCR and Nav1.8-immunoreactivity confirmed Nav1.8 expression in the amygdala, brainstem, globus pallidus, lateral and paraventricular hypothalamus, and olfactory tubercle. TTX-R INa recorded from EGFP-positive hypothalamic neurons demonstrate the usefulness of this transgenic line to study novel roles of Nav1.8 beyond sensory neurons. Overall, Scn10a-EGFP transgenic mice recapitulate the majority of the Nav1.8 expression pattern in neural crest-derived sensory neurons.