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Medullary dorsal horn (MDH), the homolog of spinal dorsal horn, plays essential roles in processing of nociceptive signals from orofacial region toward higher centers, such as the ventral posteromedial thalamic nucleus (VPM) and parafascicular thalamic nucleus (Pf), which belong to the sensory-discriminative and affective aspects of pain transmission systems at the thalamic level, respectively. In the present study, in order to provide morphological evidence for whether neurons in the MDH send collateral projections to the VPM and Pf, a retrograde double tracing method combined with immunofluorescence staining for substance P (SP), SP receptor (SPR) and Fos protein was used. Fluoro-gold (FG) was injected into the VPM and the tetramethylrhodamine-dextran (TMR) was injected into the Pf. The result revealed that both FG- and TMR-labeled projection neurons were observed throughout the entire extent of the MDH, while the FG/TMR double-labeled neurons were mainly located in laminae I and III. It was also found that some of the FG/TMR double-labeled neurons within lamina I expressed SPR and were in close contact with SP-immunoreactive (SP-ir) terminals. After formalin injection into the orofacial region, 41.4% and 34.3% of the FG/TMR double-labeled neurons expressed Fos protein in laminae I and III, respectively. The present results provided morphological evidence for that some SPR-expressing neurons within the MDH send collateral projections to both VPM and Pf and might be involved in sensory-discriminative and affective aspects of acute orofacial nociceptive information transmission., (Copyright © 2019 IBRO. Published by Elsevier Ltd. All rights reserved.)

Several cortico-cortical and limbic-related circuits are enriched in zinc, which is considered as an important modulator of glutamatergic transmission. While heavy metals have been detected in the thalamus, the specific presence of zinc has not been examined in this region. We have used two highly sensitive variations of the Timm method to study the zinc-rich innervation in the rat thalamus, which was compared to the distribution of acetylcholinesterase activity. The origin of some of these zinc-rich projections was also investigated by means of retrograde transport after intracerebral infusions of sodium selenium (Na2SeO3). The overall zinc staining in the thalamus was much lower than in the neocortex, striatum or basal forebrain; however, densely stained terminal fields were observed in the dorsal tip of the reticular thalamic nucleus, the anterodorsal and lateral dorsal thalamic nuclei and the zona incerta. In addition, moderately stained zinc-rich terminal fields were found in the rostral intralaminar nuclei, nucleus reuniens and lateral habenula. Intracerebral infusions of Na2SeO3 in the lateral dorsal nucleus resulted in retrogradely labeled neurons that were located in the postsubiculum, and also in the pre- and parasubiculum. These results are the first to establish the existence of a zinc-rich subicular-thalamic projection. Similar infusions in either the intralaminar nuclei or the zona incerta resulted in labeling of neurons in several brainstem structures related to the reticular formation. Our results provide morphological evidence for zinc modulation of glutamatergic inputs to highly selective thalamic nuclei, arising differentially from either cortical limbic areas or from brainstem ascending activation systems.