The organization of lateral ventromedial thalamic connections in the rat: A link for the distribution of nociceptive signals to widespread cortical regions

We have used several anatomical tracing techniques to study the organizatio n of the lateral ventromedial thalamic nucleus in the rat, a region that is selectively activated by cutaneous nociceptive inputs from any part of the body. The lateral ventromedial thalamic projections are organized as a wid espread dense band covering mainly layer I of the dorsolateral anterior-mos t aspect of the cortex. This band diminishes progressively as one moves cau dally, disappearing completely at 1 mm caudal to bregma level. These widesp read projections contrast with the circumscribed projections to the deep la yers of the primary somatosensory and insular cortices from the adjacent ve ntral posteromedial and ventroposterior parvicellular thalamic regions, res pectively. Injections into the lateral part of the ventromedial thalamic nu cleus of an anterograde/retrograde tracer showed that the cortical layer I areas showing the densest projections from this thalamic region also contai n the greatest number of retrogradely labeled cells in cortical layers V an d VI. The same injections retrogradely labeled numerous cells which were co nfined to the dorsal subnucleus reticularis dorsalis in an area that contai ns a concentration of neurons with widespread nociceptive convergence. Fina lly, the lateral part of the ventromedial thalamic nucleus was also differe ntially labeled following a topical application of tetramethylrhodamine-lab eled dextran on the dorsolateral anterior cortex. These findings suggest that lateral ventromedial thalamic neurons could be part of a spino-reticulo-thalamo-cortical network that allows signals of pa in from any part of the body surface to spread across widespread cortical a reas. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved .