Expression of P2X3 receptor in the trigeminal sensory nuclei of the rat

Trigeminal primary afferents expressing P2X₃ receptor are involved in the transmission of orofacial nociceptive information. However, little is known about their central projection pattern and ultrastructural features within the trigeminal brainstem sensory nuclei (TBSN). Here we use multiple immunofluorescence and electron microscopy to characterize the P2X ₃-immunopositive (+) neurons in the trigeminal ganglion and describe the distribution and synaptic organization of their central terminals within the rat TBSN, including nuclei principalis (Vp), oralis (Vo), interpolaris (Vi), and caudalis (Vc). In the trigeminal ganglion, P2X₃ immunoreactivity was mainly in small and medium-sized somata, but also frequently in large somata. Although most P2X₃⁺ somata costained for the nonpeptidergic marker IB4, few costained for the peptidergic marker substance P. Most P2X₃⁺ fibers in the sensory root of trigeminal ganglion (92.9%) were unmyelinated, whereas the rest were small myelinated. In the TBSN, P2X₃ immunoreactivity was dispersed in the rostral TBSN but was dense in the superficial laminae of Vc, especially in the inner lamina II. The P2X₃⁺ terminals contained numerous clear, round vesicles and sparse large, dense-core vesicles. Typically, they were presynaptic to one or two dendritic shafts and also frequently postsynaptic to axonal endings, containing pleomorphic vesicles. Such P2X₃⁺ terminals, showing glomerular shape and complex synaptic relationships, and those exhibiting axoaxonic contacts, were more frequently seen in Vp than in any other TBSN. These results suggest that orofacial nociceptive information may be transmitted via P2X₃⁺ afferents to all TBSN and that it may be processed differently in different TBSN.