Baroreceptive and somatosensory convergent thalamic neurons project to theposterior insular cortex in the rat

Connectivity between the rat posterior insula and the ventrobasal thalamus has been demonstrated anatomically. Neurons convergent for baroreceptor and nociceptive input have also been identified in the homologous anterior ins ula of the primate. Whether similar convergent cells exist in the ventrobas al thalamus was investigated in 30 urethane anesthetized male Sprague-Dawle y rats. Six classes of cells were identified in the right ventrobasal thala mus: (a) 83/159 (52%) baroreceptive and nociceptive convergent units; (b) 2 /159 (1%) convergent cells responding to baroreceptor activation and light touch; (c) 44/159 (28%) purely nociceptive units; (d)10/159 (6%) purely bar oreceptive units; (e) 1/159 (0.6%) cells responding to brush alone and (f) 19/159 (12%) unresponsive units. Of the viscerosomatic convergent cells, 66 /85 (78%) were situated in the ventroposterolateral nucleus (VPL), 6/85 (7% ) in the ventroposterolateral parvicellular nucleus (VPLpc), and 13/85 (15% ) in the ventroposteromedial nucleus (VPM). Fifteen right ventrobasal thala mic units were antidromically activated and 34 units orthodromically activa ted by right posterior insular microstimulation. Cobalt injection into the right ventrobasal thalamus blocked the right insular response to barorecept or activation by > 70%. These data indicate: (a) baroreceptive and somatose nsory nociceptive convergent units exist in the ventrobasal thalamus; (b) t halamic convergent neurons project directly to the ipsilateral posterior in sula and receive reciprocal insulothalamic projections; and (c) a significa nt proportion of baroreceptor input relays to the posterior insula through the ipsilateral ventrobasal thalamus. (C) 2000 Elsevier Science B.V. All ri ghts reserved.