Widespread dopaminergic projections of the subparafascicular thalamic nucleus in the rat.

The subparafascicular thalamic nucleus (Spf) contains a substantial number of dopaminergic neurons. The present study was designed to investigate in the rat whether or not Spf neurons projecting to a variety of central nervous system structures are dopaminergic. The following eight structures were tested for projection sites of Spf dopamine neurons: the neocortex, olfactory tubercle, nucleus accumbens, striatum, globus pallidus, amygdala, inferior olive, and spinal cord. By using a combination of fluorescent retrograde axonal tracing and immunofluorescence histochemistry for tyrosine hydroxylase, it has been revealed that the Spf provides a dopaminergic innervation, in varying degree, to each of these structures: the neocortex and spinal cord were the largest targets for dopaminergic projections from the Spf. The Spf was also found to contain significant numbers of dopaminergic neurons projecting to the olfactory tubercle and amygdala. In contrast, dopaminergic projections of Spf neurons to the nucleus accumbens, striatum, globus pallidus, and inferior olive were only minor. Furthermore, a series of fluorescent retrograde double-labeling experiments have indicated that individual Spf neurons are poorly collateralized to more than one of the eight terminal fields examined; the Spf neurons descending to the spinal cord relatively more frequently send axon collaterals ascending to the telencephalic structures, including the neocortex, olfactory tubercle, nucleus accumbens, striatum, and amygdala. The present results suggest that the Spf constitutes a major origin of widespread dopaminergic projections arising from the thalamus.