Does the reticular thalamic nucleus project to the midbrain?

In this study, we investigated whether the reticular thalamic nucleus has a projection to major centres of the midbrain in rats, rabbits and cats. Various tracers (biotinylated dextran, cholera toxin B subunit, fluorescent latex beads) were injected either into the midbrain tectum (deep layers of the superior colliculus) or tegmentum (midbrain reticular and pedunculopontine nuclei). In other experiments, different coloured latex beads (red and green) were injected into the deep layers of the superior colliculus and into the midbrain reticular nucleus of the same animal (rabbits). Our major finding is that in rats, rabbits and cats, there are no retrogradely labelled cells in the reticular thalamic nucleus after tracer injections into the above mentioned midbrain centres. In rabbits and cats, however, there are retrogradely labelled cells lying close to the ventromedial edge of the reticular thalamic nucleus after such injections. We show, by means of immunocytochemical double-labelling, that these retrogradely labelled cells do not lie in the reticular thalamic nucleus as suggested by previous studies, but in the inner small-celled region, a group of small cells that forms part of the zona incerta. Although these appears to be no clear topography of projection of the inner small-celled region, our tracer double-labelling experiments show that separate cells in the inner small-celled region project to individual centres of the midbrain (i.e., there are very few double-labelled cells after double injections). In rats, unlike in rabbits and cats, there is no clearly defined inner small-celled region and there are no retrogradely labelled cells seen along the ventromedial edge of the reticular thalamic nucleus. Our results suggest that in rats, rabbits and cats, there is no projection of the reticular thalamic nucleus to major centres of the midbrain, suggesting that the nucleus may not have a very strong influence on midbrain function, as it does on dorsal thalamic function.