Approaches to regulate neuronal network activity of hypoglossal motoneurons: nAChR agonists and riluzole

Mammalian brainstem nuclei control vital functions of the body via the III-XII cranial nerves. The nucleus hypoglossus (XII nucleus) i1mervates the tongue musculature and, therefore, regulates several motor behaviors, including mastication, swallowing, sucking, and speech and is involved in respiration. These motor commands are generated by hypoglossal motoneuron (HM) network activity, which employs mainly glutamatergic nemotransmission. Neuronal network outputs are modulated by various factors, such as nicotinic acetylcholine receptors (nAChRs), persistent inward currents (PICs) and ambient levels of glutamate. To study the mechanism responsible for neuronal network changes induced by different drugs, I used an in vitro brain slice preparation from neonatal rat. I employed whole cell voltage- and current clamp recordings, nerve recording and immunohistochemical staining technique. I found that nAChRs of XII nucleus are composed of a4, a7 and f32 subunits, which were abundantly expressed on HM somata and fibers. Presynaptic nAChRs upregulated GABAergic neurotransmission. The effect was calcium dependent. However, GABA responses of the motoneuron membrane were unchanged by nAChR activation. The nAChR agonist nicotine enhanced glutamatergic release at premotoneuronal network level. In 40 % of HMs, enhanced glutamatergic neurotransmission produced theta frequency oscillations. Nicotine-induced oscillations persisted for at least eight minutes and required sustained nAChR activation. They relied on network nAChRs, as motoneuron receptors desensitized quickly and were not involved in the oscillatory behaviour. Oscillations required both chemical and electrical transmission, since they were blocked by AMPA, mGluRl and gap junction blockers. Synaptic inhibition and PICs were not essential for oscillatory behaviour. Oscillations made neuronal firing regular, since spikes appeared at the peak of each oscillation. Furthermore, XII nerve recording revealed that oscillations were not a local phenomenon, but were rhythmically transmitted from the XII nucleus to the effector muscles. The neuroprotective drug riluzole depressed excitatory synaptic transmission in a frequencydependent fashion. Riluzole also blocked the persistent sodium current ONaP ), but the effect was not specific, since it also inhibited the persistent calcium current OcaP ). The fast sodium current was unaffected by riluzole. Riluzole inhibited glutamatergic neurotransmission in the presence of TTX by interfering with the facilitatory role of presynaptic NMDA receptors. We propose that riluzole blocked PKC activity necessary to support presynaptic NMDA receptors. These results outline various targets for pharmacological approaches to conditions like sleep apnea or dysphagia characterized by dysfunction of the nucleus hypoglossus.