Brainstem reticular nuclei that project to the thalamus in rats: a retrograde tracer study

The precise nuclear origins of projections from the brainstem reticular formation to the thalamus were identified in rats using two retrograde tracing substances: wheat germ agglutinin-horseradish peroxidase conjugate, and Fluoro-Gold. Injections of these tracers were made into a variety of thalamic nuclei, including the intralaminar nuclei (most of these also involved the lateral part of the mediodorsal nucleus), the central part of the mediodorsal nucleus, the ventrolateral/ventromedial nuclei, and the ventral posterolateral/ventral posteromedial nuclei. Counts of retrogradely labeled cells were done on a large sample of select cases. The data generated by these cell counts indicate that brainstem reticular projections to the intralaminar/lateral mediodorsal complex are fairly strong, as are those to the ventrolateral/ventromedial nuclear complex. Ascending reticular projections to the mediodorsal nucleus per se are somewhat weaker, while those to the ventrobasal complex (or at least the ventral posterolateral nucleus) are weaker still. As a whole, reticular neurons projecting to the thalamus are by far most numerous in the midbrain, and then decline gradually at successively caudal levels through the pons and medulla. Midbrain reticular groups evincing very strong ascending projections include nucleus reticularis (n.r.) pedunculopontinus (particularly its pars compactus), n.r. cuneiformis and n.r. subcuneiformis (together known as the deep mesencephalic nucleus). Strong thalamic projections arise from the medial part of n.r. pontis oralis, the medial (beta) part of n.r. pontis caudalis, and the mid-pontine dorsomedial tegmental area. Within the medullary reticular formation, a 'trans-nuclear field' of neurons encompassing n.r. paragigantocellularis dorsalis and dorsal parts of n.r. gigantocellularis and n.r. parvocellularis was consistently labeled contralateral to the injection site. In general, ascending reticulothalamic projections are largely ipsilateral from midbrain reticular groups, bilateral from pontine reticular groups, and contralateral from medullary reticular groups. Within individual reticular nuclei, the morphology of labeled neurons is identical to that reported previously by this laboratory subsequent to spinal, cortical, or cerebellar tracer injections, thus strengthening our hypothesis that the various brainstem reticular nuclei can be distinguished on the basis of neuronal morphology. As a whole, thalamic-projecting reticular neurons are mostly small or medium-sized cells.