IDENTIFICATION OF SPINAL INTERNEURONS ANTECEDENT TO ADRENAL SYMPATHETIC PREGANGLIONIC NEURONS USING TRANSSYNAPTIC TRANSPORT OF HERPES-SIMPLEX VIRUS TYPE-1

Control of sympathetic preganglionic neurons appears to be mediated, i n part, through polysynaptic pathways using spinal interneurons. To id entify spinal interneurons antecedent to adrenal sympathetic pregangli onic neurons, we injected herpes simplex virus type 1 into the adrenal gland of hamsters as this virus is an effective trans-synaptic tracer of neural pathways. After a three day survival period, immunocytochem istry was used to visualize virus-infected spinal cord cells. Infected sympathetic preganglionic neurons with somata that were either kite-s haped, elliptical or fusiform and that had extensive dendrite arbors w ere identified as well as a group of smaller round cells with finer pr ocesses. For comparison, in additional hamsters, labelling with the re trograde tracer Fluoro-Gold and histochemical reactions for the enzyme nicotinamide adenine dinucleotide phosphate-diaphorase were used to i dentify sympathetic preganglionic neurons. Sympathetic preganglionic n eurons identified with Fluoro-Gold or herpes virus were present mostly in the nucleus intermediolateralis, pars intermediolateralis and nucl eus intermediolateralis, pars funicularis of the spinal cord. The smal ler herpes virus-infected cells were found mostly medial to the pregan glionic neurons in lamina VII and also dorsally in lamina V of the spi nal cord. Assessing immunoreactivity for glial fibrillary acidic prote in demonstrated that the smaller herpes virus-infected cells were not reactive astrocytes. Furthermore, these cells were immunoreactive for two neuronal markers, neuron-specific enolase and for microtubule-asso ciated protein 2. These findings suggest that these smaller round cell s with finer processes are distinct from sympathetic preganglionic neu rons and astrocytes and may be interneurons antecedent to the sympathe tic preganglionic neurons.