Detailed knowledge of the inhibitory input to trigeminal motoneurons is needed to understand better the central mechanisms of jaw movements. Here a quantitative analysis of terminals contacting somata of jaw-closing (JC) and jaw-opening (JO) alpha-motoneurons, and of JC gamma-motoneurons, was performed by use of serial sectioning and postembedding immunogold cytochemistry. For each type of motoneuron, the synaptic boutons were classified into four groups, i.e., immunonegative boutons or boutons immunoreactive to glycine only, to gamma-aminobutyric acid (GABA) only, or to both glycine and GABA. The density of immunolabeled boutons was much higher for the alpha- than for the gamma-motoneurons. In the alpha-motoneuron populations, the immunolabeled boutons were subdivided into one large group of boutons containing glycine-like immunoreactivity only, one group of intermediate size harboring both glycine- and GABA-like immunoreactivity, and a small group of boutons containing GABA-like immunoreactivity only. The percentage of immunolabeled boutons was higher for JC than JO alpha-motoneurons, the most pronounced difference being observed for glycine-like immunoreactivity. In contrast, on the somatic membrane of gamma-motoneurons, the three types of immunoreactive bouton occurred at similar frequencies. These results indicate that trigeminal motoneurons are strongly and differentially controlled by premotoneurons containing glycine and/or GABA and suggest that these neurons play an important role for the generation of masticatory patterns.

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A previous study revealed that rostrodorsomedial oralis (Vo.r) neurons synapsing on trigeminal motoneurons use GABA and/or glycine as neurotransmitters. To determine the number and spatial distribution of contacts, injections of biotinamide and horseradish peroxidase were made into a Vo.r neuron and an alpha-motoneuron in the jaw-closing (JC) and jaw-opening (JO) motor nucleus, respectively, in 39 cats. All Vo.r neurons responded to low-threshold mechanical stimulation of the oral tissues. Single Vo.r neurons terminating in the JC nucleus (Vo.r-dl neurons; n = 5) issued, on average, 10 times more boutons than Vo.r neurons terminating in the JO nucleus (Vo.r-vm neurons; n = 5; 4437 vs 445). The Vo.r-dl neuron-JC alpha-motoneuron pairs (n = 4) made contacts on either the soma-dendritic compartment or dendrites, and the Vo.r-vm neuron-JO motoneuron pairs (n = 2) made contacts on dendrites, with a range of two to seven contacts. In five of the six pairs, individual or groups of two to three terminals contacted different dendritic branches of a postsynaptic cell. The Vo.r-dl neurons innervated a greater number of counter-stained motoneuronal somata than did the Vo.r-vm neurons (216 vs 26). Total number of contacts per Vo.r neuron was higher for the Vo.r-dl than Vo.r-vm neurons (786 vs 72). The present study demonstrates that axonal branches of Vo.r neurons are divided into two types with different innervation domains on the postsynaptic neuron and that they are highly divergent. The overall effect exerted by these neurons is predicted to be much greater within the JC than JO motoneuron pool.

Little is known about the ultrastructure of synaptic boutons contacting trigeminal motoneurons. To address this issue, physiologically identified premotor neurons (n = 5) in the rostrodorsomedial part of the oral nucleus (Vo.r) were labeled by intracellular injections of horseradish peroxidase (HRP) in cats. The ultrastructure of 182 serially sectioned axon terminals from the five neurons was both qualitatively and quantitatively analyzed. In addition, the effects of the glycine antagonist strychnine, GABA(A) antagonist bicuculline, NMDA antagonist 2-amino-5-phosphonovalerate (APV), and non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on Vo.r-induced postsynaptic potentials in trigeminal motoneurons (n = 11) were examined to evaluate potential signaling substances of the premotor neurons. Labeled boutons made synaptic contacts with either jaw-closing or -opening motoneurons. All the boutons contained pleomorphic vesicles, and most formed a single symmetric synapse either on the somata or on primary dendrites. Morphometric analyses indicated that bouton volume, bouton surface area, apposed surface area, total active zone area, and mitochondrial volume were not different between boutons on jaw-closing and -opening motoneurons. Vesicle number and density, however, were higher for boutons on jaw-closing motoneurons. The five morphological parameters were positively correlated with bouton volume. Vesicle density was the exception, which tending to be negatively correlated. Intravenous infusion of strychnine or bicuculline suppressed Vo.r-induced inhibitory postsynaptic potentials (IPSPs) in jaw-closing motoneurons. Abolition of Vo. r-induced excitatory postsynaptic potentials in jaw-opening motoneurons with APV and CNQX unmasked IPSPs. The present results suggest that premotor neurons in the Vo.r are inhibitory and that positive correlations between the ultrastructural parameters associated with synaptic release and bouton size are applicable to the interneurons, as they are in primary afferents.

phological properties of parasympathetic neurons controling the paratid and von Ebner salivary glands in rats. J Neurophysiol 93: 678 -686, 2005; doi:10.1152/jn.00277.2004. The inferior salivatory nucleus (ISN) contains parasympathetic neurons controlling the parotid and von Ebner salivary glands. To characterize the neurophysiological and morphological properties of these neurons, intracellular recordings were made from anatomically identified ISN neurons in rat brain slices. Neurons were also filled with Lucifer yellow and morphometrically analyzed. Based on responses to membrane hyperpolarization followed by depolarization, three types of repetitive discharge patterns were defined for neurons innervating the parotid gland. The regular, repetitive discharge response to membrane depolarization was changed by hyperpolarization resulting either in a delay in the occurrence of the first spike or to an increase in the length of the first interspike interval in the action potential train. Membrane hyperpolarization had little effect on the discharge pattern of some neurons. Similar response discharge patterns were found for neurons innervating the von Ebner salivary gland, which also included a further group of neurons that responded with a short burst of action potentials. Neurons innervating the parotid salivary glands differed morphologically from the von Ebner salivary glands having significantly larger soma and more and longer dendrites than von Ebner gland neurons. In addition, the mean membrane input resistance, time constant, and spike half-width of parotid gland neurons was significantly lower than in von Ebner gland neurons. These differences in intrinsic membrane properties and morphology may relate to the functions of the von Ebner and parotid glands. von Ebner glands are involved in taste stimulus delivery and removal from posterior tongue papillae while the parotid glands contribute saliva to the entire mouth. I N T R O D U C T I O NParasympathetic secretomotor neurons with efferent fibers that project to the periphery in the glossopharyngeal nerve (IX) are grouped in the inferior salivatory nucleus (ISN). Preganglionic axons originating in the ISN travel in the glossopharyngeal nerve to the otic ganglion from where postganglionic fibers supply the parotid salivary gland (Chibuzo and Cummings 1980;Satomi et al. 1979). Parasympathetic fibers in the glossopharyngeal nerve also travel to the intralingual ganglia (Remak's ganglia) in the posterior tongue from which postganglionic fibers innervate the von Ebner glands that drain into the clefts of the circumvallate and foliate papillae (Bradley et al. 1985).The ISN lies principally along the medial edge of the nucleus of the solitary tract (NST) before extending into the ventral reticular formation at its rostral extent (Contreras et al. 1980). This configuration suggests potential interactions between the ISN and NST. However, although the general location of the ISN has been described including brief morphological descriptions of the neurons (Contreras et...