Abstract Our principal was to demonstrate the convergence of the trigeminal ganglion population from the transversal meningeal sinus and the infraorbital nerve. Recent data suggest that some trigeminal ganglion cells simultaneously send neuronal projections to craniofacial structures via the V1 and V2 trigeminal branches; however, no conclusive evidence exists. Hence, immunohistological assays using retrograde neural tracers injected at the V1 (meningeal dura mater at the transverse sinus level; true-blue, (TB) and V2 (infraorbital nerve; fluoro-gold, (FG) branches were performed to test the hypothesis regarding the convergence of different trigeminal branches to one trigeminal ganglion cell. Besides, bifurcated labeled sensory trigeminal nerves were quantified in adult (60 days old) and young rats (30 days old). The correlation between oxytocin receptor (OTR), calcitonin gene-related peptide (CGRP), and IB4 was analyzed. Cells were labeled concomitantly with FG and TB neuro tracers at the trigeminal ganglion. Interestingly, young animals showed more labeled cells than did adult rats. Triple-labeled neurons (i.e., CGRP/FG/TB and OTR/FG/TB) were found in trigeminal ganglion cells, implying that CGRPergic fibers can be modulated by OTR activation in meningeal or infraorbital nerve fibers. These data support the contention that bifurcated trigeminal ganglion cells simultaneously run via two trigeminal branches (V1 and V2). Finally, the significant decrease in labeled ganglionic cells at 60 days compared to rats 30 days old needs to be followed to understand the developmental process of the orofacial system.

Abstract CSF-contacting neurons are involved in detecting changes in the cerebrospinal (CSF) circulation and recent studies report their role in nociception. Using neuronal tracers (Fluoro-Gold™, Cholera toxin subunit B, BDA), immunofluorescence (Anti-FG, Anti-OT-neurophysin, Anti-vasopressin, Anti-GABA, Anti-OTR) and electron microscopy, we describe oxytocinergic and vasopressinergic CSF-contacting neurons within the hypothalamic paraventricular nucleus (PVN) and their projections towards the rostral agranular insular cortex (RAIC). Our results show the presence of CSF-contacting neurons along the PVN that are labelled by oxytocin (OT) or/and vasopressin (AVP) and could secrete these peptides into CSF by dendritic projections. Besides, we report some oxytocinergic CSF-contacting neurons that send projections towards the RAIC. Inside the RAIC, our ultrastructural analysis shows that axons from PVN sustain synaptic connections with cortical GABAergic neurons that express oxytocin receptor (OTR) where we identify OT molecules as well. These findings support the possible role of CSF-contacting neurons in the neuronal modulation by releasing neuromodulators both at CSF and synaptic levels. Also, our results signal the extended means of oxytocinergic transmission, including its release inside RAIC promoting local GABAergic activity and its secretion towards CSF probably modulating many areas close to the ventricles, that can be involved in different conditions as nociception modulation.