Cellular and synaptic properties of trigeminal neurons and their modulation by endogenous neuromessengers

I studied cellular and synaptic properties that contribute to the generation and expression of rhythmic oral-motor behaviors. Specifically, I recorded trigeminal motoneurons, which directly innervate jaw muscles, and trigeminal interneurons, which have projections to the motoneurons, in rodent brain stem slice preparations, using intracellular recording techniques. Bifurcation theory and the testable predictions of mathematical models were employed to empirically identify a novel mechanism for neuronal bursting behavior in trigeminal interneurons. This unique mechanism of intrinsic rhythmicity in trigeminal interneurons may be involved in the central generation of rhythmic oral-motor behaviors. I also show that trigeminal motoneurons, in the presence of the behaviorally relevant neuromessenger serotonin, exhibit bistability (plateau potentials) and bursting. These behaviors are produced by a traditional mechanism that utilizes a region of negative slope resistance in the steady-state current-voltage relationship. Pharmacology and ionic substitution were used to determine the ionic basis of bistability and bursting in trigeminal motoneurons: I characterize both the inactivation-resistant inward currents that are needed to initiate and sustain plateau potentials and bursts, and the slowly activating outward currents that terminate the plateaus and bursts. The bistable and bursting behaviors exhibited by trigeminal motoneurons may be important for motoneuronal expression of rhythmic oral-motor behaviors. Lastly, I show that the endogenous neurotransmitter glutamate, acting at metabotropic glutamate receptors, can modulate the synaptic and intrinsic properties of the trigeminal motoneurons. This activity-dependent form of neuromodulation, mediated by the ubiquitous excitatory transmitter glutamate, probably influences the expression of rhythmic jaw movements by enhancing the signal-to-noise ratio of centrally generated, rhythmic oral-motor patterns.