Postnatal changes of local neuronal circuits involved in activation of jaw-closing muscles

Monosynaptic premotor circuit tracing reveals neural substrates for oro-motor coordination

Feeding behaviors require intricately coordinated activation among the muscles of the jaw, tongue, and face, but the neural anatomical substrates underlying such coordination remain unclear. In this study, we investigate whether the premotor circuitry of jaw and tongue motoneurons contain elements for coordination. Using a modified monosynaptic rabies virus-based transsynaptic tracing strategy, we systematically mapped premotor neurons for the jaw-closing masseter muscle and the tongue-protruding genioglossus muscle. The maps revealed that the two groups of premotor neurons are distributed in regions implicated in rhythmogenesis, descending motor control, and sensory feedback. Importantly, we discovered several premotor connection configurations that are ideally suited for coordinating bilaterally symmetric jaw movements, and for enabling co-activation of specific jaw, tongue, and facial muscles. Our findings suggest that shared premotor neurons that form specific multi-target connections with selected motoneurons are a simple and general solution to the problem of orofacial coordination.DOI: http://dx.doi.org/10.7554/eLife.02511.001.

Inhibition of excitatory synaptic transmission in the trigeminal motor nucleus by the nitric oxide-cyclic GMP signaling pathway

Nitric oxide (NO) and cyclic GMP (cGMP) suppressed glutamatergic synaptic transmission to trigeminal motoneurons in brain stem slices of neonatal rats. Histological studies showed guanylate cyclase (GC) containing fibers in the trigeminal motor pool. Glutamatergic excitatory postsynaptic currents (EPSCs) were recorded from neonatal trigeminal motoneurons in response to stimulation of the supratrigeminal nucleus (SuV). The NO donors DETA/NONOate (DETA/NO), at a concentration which released 275.1 nM of NO, and Spermine/NONOate (Sper/NO) reduced the amplitude of the EPSC to 52.7±0.6% and 60.1±10.8% of control values, respectively. These actions were not blocked by the GC inhibitors, ODQ or NS-2028. However, in the presence of YC-1 or BAY41-2272, modulators of GC that act as NO sensitizers, lower and otherwise ineffective concentrations of DETA/NO induced a reduction of the EPSC to 60.6±5.2%. Moreover, NO effects were mimicked by 8BrcGMP and by Zaprinast, an inhibitor of Phosphodiesterase 5. Glutamatergic currents evoked by exogenous glutamate were not reduced by DETA/NO nor 8BrcGMP. Paired-pulse facilitation was increased by NO donors. Under "minimal stimulation" conditions NO donors and cGMP increased the failure rate of evoked EPSCs. Protein kinase inhibitors antagonized cGMP effects. The results suggest that NO, through the synthesis of cGMP, presynaptically inhibits glutamatergic synaptic transmission on trigeminal motoneurons. We propose that NO has complex actions on motor pools; specific studies are needed to elucidate their physiological significance in the behaving animal.