Effects of GABA(B) receptor activation on excitability of IB4-positive maxillary trigeminal ganglion neurons: Possible involvement of TREK2 activation

IB4-positive maxillary trigeminal ganglion (TG) neurons are a subtype of afferent neurons involving nociception in orofacial regions, and excitability of these neurons is associated with orofacial nociceptive sensitivity. TREK-2 channel is a member of two-pore domain potassium (K2P) channel family mediating leak K(+) currents. It has been shown previously that TREK-2 channel activity can be enhanced following GABAB receptor activation, leading to a reduction of cortical neuron excitability. In the present study, we have characterized TREK-2 channel expression on maxillary TG neurons and investigated the effect of the GABAB agonist baclofen on electrophysiological properties of small-sized maxillary TG neurons of rats. We show with immunohistochemistry that TREK-2 channels are predominantly expressed in small-sized IB4-positive maxillary TG neurons. Patch-clamp recordings on neurons in ex vivo TG preparations show that baclofen hyperpolarizes resting membrane potentials, increases outward leak currents, and decreases input resistances in IB4-positive maxillary TG neurons. Moreover, baclofen significantly reduces action potential (AP) firing in IB4-positive maxillary TG neurons. In contrast, baclofen shows no significant effect on electrophysiological properties of small-sized nociceptive-like and non-nociceptive-like maxillary trigeminal neurons that are IB4-negatve. Our results suggest that TREK-2 channel activity can be enhanced by baclofen, leading to reduced excitability of IB4-positive maxillary TG neurons. This finding provides new insights into the role of TREK-2 and GABAB receptors in controlling nociceptive sensitivity in orofacial regions, which may have therapeutic implications.