Cultured smooth muscle targets lack survival activity for ciliary ganglion neurons

Cultured neurons require specific trophic agents in order to survive. This dependence is thought to resemble the neuron-target interdependence that develops in vivo during synaptogenesis and neuronal cell death. The notion that neurons in general derive trophic support from their synaptic targets is based primarily on studies of peripheral neurons and motor neurons. To assess the general applicability of this nerve-target relationship, we tested the ability of vascular smooth muscle (VSM) to support dissociated neurons from the chick ciliary ganglion. The ciliary ganglion contains 2 distinct neuronal populations, one of which innervates striated muscle, the other VSM. Striated muscle cocultures are known to support all of the neurons in the ganglion for extended periods. Dissociated neurons were therefore cocultured in microwells containing VSM derived from the rat or chick aorta and from the choroid coat of the chick eye. Surviving neurons were counted after 1, 2, 5, and 7 d. Striated muscle is able to support full neuronal survival in the same assay. However, in no case was VSM capable of contributing to neuronal survival in vitro. The neurons in the VSM cocultures were able to form neurites and make contacts with their putative targets, as confirmed by scanning electron and light microscopy. The presence of viable and differentiated smooth muscle cells was demonstrated in the cultures by transmission electron microscopy and analysis of smooth muscle alpha-actin. The failure of VSM and even the choroid target tissue to support the survival of their innervating neurons suggests that novel mechanisms may operate to provide trophic support for neurons innervating VSM targets.