Supraspinal contribution to splanchnic sympathetic activity in neonatal mouse and rat brainstem-spinal cord in vitro

Under optimal in vitro conditions, isolated spinal cords of neonatal Sprague-Dawley (SD) rats spontaneously generate sympathetic nerve discharge (SND). To aid future studies involving genetically modified mice, we established a preparation to assess SND generation within the mouse spinal cord. Brainstem-spinal cord-splanchnic nerve preparations of neonatal 129S6/SvEvTac (129S6) mice, C57BL/6 (B6) mice, Long-Evan (LE) rats, and SD rats were used. The contributions of the brainstem to splanchnic SND (sSND) were not significantly affected by cervical cord transections in LE and SD rats. However, the transections caused approximately a 70% reduction in sSND in both mice species. Power spectral analyses characterized distinct features of sSND oscillations. With intact brainstem-spinal cord, comparable maximal power peaks at approximately 1-2Hz were observed in mouse and rat spectra, although the spectral peak widths were broader in mice. Cervical cord transections reduced the maximal peak powers and the peak widths in mice but not in rats. For comparisons across animal groups, the amounts of sSND were normalized to ambient current noise and expressed in signal/noise units. Similar amounts of normalized sSND were recorded from mice and rats with intact brainstem-spinal cords. However, the level of mouse sSND was reduced following cervical cord transection, whereas rat sSND was not. Our results demonstrate a species related difference in sSND recorded from neonatal rat and mouse spinal cord preparations in vitro. The current experimental model is applicable to evaluate the SND strength in neonatal rodents of various genetic backgrounds.