Hypothalamic CSF-contacting neurons project to the rostral agranular insular cortex: an immunofluorescence and ultrastructural analysis in the rat.

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.