Long-term potentiation of intrinsic excitability in trigeminal motoneurons

Trigeminal motoneurons (TMNs) relay the final output signals generated within the oral-motor pattern-generating circuits to the jaw muscles for execution of various patterns of motor activity. Activity-dependent plasticity, referred to as long-term potentiation (LTP), in the central nervous system has been the subject of many studies. The mechanisms of plasticity in the trigeminal system, an important component of the oral-motor system underlying mastication, swallowing, and other behaviors, remain to be fully elucidated. In the present study, we investigated long-term potentiation of intrinsic excitability (LTP-IE) in TMNs. Experiments were performed using extracellular recording and whole-cell patch-clamp recording to assess the intrinsic excitability of TMNs. Intrinsic response properties were examined using an induction pulse with ionotropic transmission blocked. The output of the trigeminal motor branch exhibited long-lasting potentiation of intrinsic neuronal excitability following induction. Applying brainstem transection techniques to the neonatal rat brainstem in vitro, we found that the activity of the motoneuron population recorded from the motor branch of the trigeminal nerve exhibited LTP-IE. We thus demonstrated the usefulness of this type of preparation for the study of rudimentary oral-motor activity and observed changes in TMN excitability. In addition, on testing with the whole-cell patch-clamp method, TMNs exhibited a significant increase in excitability with a leftward shift in F–I curves generated with depolarizing current injections, whereas resting membrane potential and input resistance exhibited no remarkable changes. These findings indicate that TMNs exhibit LTP of intrinsic excitability.