Ventroposterior medialis parvocellularis (VPMpc) of thalamus, the thalamic relay nucleus for gustatory sensation, receives primary input from parabrachial nucleus, and projects to insular cortex. To reveal unique properties of gustatory thalamus in comparison to archetypical sensory relay nuclei, this study examines the morphology of synaptic circuitry in VPMpc, focusing on parabrachiothalamic driver input and corticothalamic feedback. Anterogradely visualized parabrachiothalamic fibers in VPMpc bear large swellings. At electron microscope resolution, parabrachiothalamic axons are myelinated and make large boutons, forming multiple asymmetric, adherent and perforated synapses onto large caliber dendrites and dendrite initial segments. Labeled boutons contain dense-core vesicles, and they resemble a population of calcitonin gene-related peptide containing terminals within VPMpc. As typical of primary inputs to other thalamic nuclei, parabrachiothalamic terminals are over 5 times larger than other inputs, while constituting only 2% of all synapses. Glomeruli and triadic arrangements, characteristic features of other sensory thalamic nuclei, are not encountered. As revealed by anterograde tracer injections into insular cortex, corticothalamic projections in VPMpc form a dense network of fine fibers bearing small boutons. Corticothalamic terminals within VPMpc were also observed to synapse on cells that were retrogradely filled from the same injections. The results constitute an initial survey in describing unique anatomical properties of rodent gustatory thalamus.
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