A nigro-rubro-bulbar pathway to the parvicellular reticular formation in the rat.

A possible pathway from the substantia nigra pars reticulata (SNr) to the parvicellular reticular formation (RFp) via the red nucleus (RN) was examined light and electron microscopically by combining anterograde and retrograde tracing techniques. After contralateral injections of biotinylated dextranamine (BDA) into the dorsolateral part of the SNr and cholera toxin B subunit (CTb) into the RFp, many CTb-labelled neurones were distributed contralaterally in the dorsal part of the RN, where numerous BDA-labelled axon terminals originating from the ipsilateral SNr were found. After contralateral injections of BDA into the dorsolateral part of the SNr and wheat germ agglutinin-horseradish peroxidase (WGA-HRP) into the RFp, ipsilateral axons labelled with BDA were found to make synaptic contacts with the somata and dendrites of contralateral neurones labelled with WGA-HRP in the dorsal part of the RN.