The development of dorsal root potential and responsiveness to GABA in the isolated spinal cord of rat fetus

Abstract

The sensitivity of dorsal root potential (DRP) to bicuculline and the depolarizing response of primary afferent fibers to GABA were investigated to study the development of GABAnergic neurons using the spinal cord isolated from rat fetuses (ED13.5-21.5). The potential changes generated in primary afferent fibers were recorded extracellularly from L3 dorsal root. When the cord surface close to the dorsal root was stimulated at ED13.5, the primary afferent fibers caused a spike potential which was not affected by lowering Ca concentration, but was blocked by tetrodotoxin (50ng/ml). At ED14.5, in addition to the spike potential, a slow potential change was recorded in the response to stimulation of the cord surface close to the dorsal root and was abolished by lowering Ca concentration. DRP elicited by stimulation of the dorsal root (DR-DRP) was first observed at ED15.5. The half-decay time of DRP was about 100msec at ED15.5, 600msec at ED17.5, 400msec at ED19.5, 270msec at ED21.5. Bicuculline (5μg/ml) began to depress DR-DRP at ED17.5. Stimulation of thoracic segment (T7) of spinal cord induced a spike potential followed by a slow potential change with almost the same time course as that of DR-DRP from ED16.5. This slow potential change was also inhibited by bicuculline (5μg/ml) after ZD17.5. GABA (10uM-lmM) caused the dose-dependent depolarization of the dorsal root from ED13.5. The amplitude of depolarization by GABA gradually increased and attained a maximum at ED17.5. If bicuculline-sensitive DRP reflects GABAnergic activity, it is suggested that GABAnergic activity may develop at ED17.5,and responsiveness to GABA of primary afferent fibers precedes the onset of GABAnergic input.