ATP Mediates Neuron-satellite Glia-microglia Signaling in the Rat Trigeminal Ganglion

Objectives: Extracellular ATP is known as a pain mediator in trigeminal ganglion (TG), however, how extracellular ATP involves in the signal transduction among cells such as neuron, satellite glial cell (SGC), and microglia in TG is still unclear. Recent study indicated that vesicular nucleotide transporter (VNUT) is associated with ATP release, and found in TG neuron. The aim of this study is to reveal ATP mediated signaling in the interaction among neurons, SGCs, and microglia in TG.
Methods: Six-week-old Sprague-Dawley rats were anesthetized and their upper molars were extracted. Expression of VNUT was investigated using immunohistochemistry, and in situ hybridization. Cells in TG were immunostained using anti-VNUT, anti-glutamine synthetase (SGC marker), anti-ATF3 (marker of neuronal damage), anti-Iba1antibody (microglia marker), and anti-P2X receptor (ATP receptor).
Results: After tooth extraction, the expression of VNUT mRNA was strongly detected in TG neurons and SGCs. Three hours after tooth extraction, the number of VNUT-immunoreactive (IR) vesicles increased and strong VNUT-IR vesicles were distributed at the peripheral portion of ATF3-IR damaged neuron. Six hours after tooth extraction, strong VNUT-IR vesicles were found in some SGCs around the ATF3-IR damaged neuron. The localization of VNUT-IR vesicles in SGC was confirmed using an electron microscopy. Furthermore, Iba1-IR microglia were distributed around the neuron-SGC complex. P2X receptors were found in both SGCs and microglia.
Conclusions: These results suggest that ATP released by VNUT-positive vesicles would be involved in neuron-SGC-microglia signaling in TG. These activated microglia in TG would induce the neuropathic pain in trigeminal nerve area.