Properties and interconnections of trigeminal interneurons of the lateral pontine reticular formation in the rat

Numerous evidence suggests that interneurons located in the lateral tegment um at the level of the trigeminal motor nucleus contribute importantly to t he circuitry involved in mastication. However, the question of whether thes e neurons participate actively to genesis of the rhythmic motor pattern or simply relay it to trigeminal motoneurons remains open. To answer this ques tion, intracellular recordings were performed in an in vitro slice preparat ion comprising interneurons of the peritrigeminal area (PeriV) surrounding the trigeminal motor nucleus (NVmt) and the parvocellular reticular formati on ventral and caudal to it (PCRt). Intracellular and extracellular injecti ons of anterograde tracers were also used to examine the local connections established by these neurons. In 97% of recordings, electrical stimulation of adjacent areas evoked a postsynaptic potential (PSP). These PSPs were pr imarily excitatory, but inhibitory and biphasic responses were also induced . Most occurred at latencies longer than those required for monosynaptic tr ansmission and were considered to involve oligosynaptic pathways. Both the anatomical and physiological findings show that all divisions of PeriV and PCRt are extensively interconnected. Most responses followed high-frequency stimulation (50 Hz) and showed little variability in latency indicating th at the network reliably distributes inputs across all areas. In all neurons but one, excitatory postsynaptic potentials (EPSPs) or inhibitory postsyna ptic potentials (IPSPs) were also elicited by stimulation of NVmt, suggesti ng the existence of excitatory and inhibitory interneurons within the motor nucleus. In a number of cases, these PSPs were reproduced by local injecti on of glutamate in lieu of the electrical stimulation. All EPSPs induced by stimulation of PeriV, PCRt, or NVmt were sensitive to ionotropic glutamate receptor antagonists 6-cyano-7-dinitroquinoxaline and D,L-2-amino-5-phosph onovaleric acid, while IPSPs were blocked by bicuculline and strychnine, an tagonists of GABAA and glycine receptors. Examination of PeriV and PCRt int rinsic properties indicate that they form a fairly uniform network. Three t ypes of neurons were identified on the basis of their firing adaptation pro perties. These types were not associated with particular regions. Only 5% o f all neurons showed bursting behavior. Our results do not support the hypo thesis that neurons of PeriV and PCRt participate actively to rhythm genera tion, but suggest instead that they are driven by rhythmical synaptic input s. The organization of the network allows for rapid distribution of this rh ythmic input across premotoneuron groups.