Influence of growth medium, age in vitro and spontaneous bioelectric activity on the distribution of sensory ganglion-evoked activity in spinal cord explants.

Abstract

The role of serum added to the culture medium and of spontaneous bioelectric activity in the development of sensory afferent connections was studied, employing fetal mouse spinal cord explants with attached dorsal root ganglia (DRG) as an in vitro model system. Afferent DRG terminals in the cord explants were localized on the basis of 'fixed-latency' DRG-evoked action potentials, which were anatomically verified in several experiments using horseradish peroxidase histology. In serum-supplemented medium (HSSM), but not in chemically defined medium (CDM), those DRG fibers which grew into the dorsal side of the cord terminated predominantly within the dorsal cord region, and remained there throughout the experimental period (18-33 days in vitro). In contrast, ventrally entering fibers terminated equally in both the dorsal and the ventral cord regions in young cultures (18-24 days in vitro) but were no longer observed after 27 days in vitro. Cultures grown in HSSM with the addition of xylocaine, in order to chronically suppress spontaneous bioelectric activity, essentially corresponded (at 25-32 days in vitro) to the picture seen in the control series at the same age. On the basis of polysynaptic DRG-evoked responses in the cord, developmental changes in local neuronal networks were inferred which resulted in less spread of DRG-evoked activity with age in HSSM, and more spread with age in CDM-grown cultures. It is concluded that for the formation of selective DRG connections in the spinal cord: (i) a serum-borne factor plays a role: and (ii) functional activity is not required.