Thalamic processing of vibrissal information in the rat. I. Afferent input to the medial ventral posterior and posterior nuclei

Retrograde tracing with true blue (TB) and diamidino yellow (DY) and anterograde tracing with either wheatgerm agglutinin–conjugated horseradish peroxidase (WGA‐HRP) or Phaseolus vulgaris leucoagglutinin (PHA‐L) were employed to investigate the projections from trigeminal nucleus principalis (PrV) and trigeminal subnucleus interpolaris (SpI) to their targets in the medial ventral posterior (VPM) and posterior (POm) nuclei of the thalamus. Many more cells in both PrV and SpI were labeled by tracer injections into VPM than into POm. Only a very small number of double‐labeled neurons were observed in either PrV or SpI. However, a significantly higher percentage of SpI cells projected to POm or to both POm and VPM than was the case for PrV. Anterograde tracing with WGA‐HRP showed that the projections from both PrV and SpI to VPM were much denser than those from the same nuclei to POm. Small injections of PHA‐L into either PrV or SpI produced a focus of fairly dense labeling in VPM and much more diffuse terminal labeling in POm. These anatomical data provide evidence for two separate trigeminothalamic pathways, one originating from PrV and the second originating from SpI. Both of these pathways converge and diverge at the thalamic level. That is, information from the PrV pathway and from the SpI pathway are both provided to VPM in a morphologically restricted fashion and to POm in a morphologically widespread fashion.