Development of γ-aminobutyric acid-, glycine-, and glutamate-immunopositive boutons on rat jaw-opening motoneurons

Increase of glutamate in satellite glial cells of the trigeminal ganglion in a rat model of craniofacial neuropathic pain

Introduction

Satellite glial cells (SGCs) that envelop the cell bodies of neurons in sensory ganglia have been shown to both release glutamate, and be activated by glutamate in the context of nociceptive signaling. However, little is known about the subpopulations of SGCs that are activated following nerve injury and whether glutamate mechanisms in the SGCs are involved in the pathologic pain.

Methods

To address this issue, we used light and electron microscopic immunohistochemistry to examine the change in the glutamate levels in the SGCs and the structural relationship between neighboring neurons in the trigeminal ganglion (TG) in a rat model of craniofacial neuropathic pain, CCI-ION.

Results

Administration of ionomycin, ATP and Bz-ATP induced an increase of extracellular glutamate concentration in cultured trigeminal SGCs, indicating a release of glutamate from SGCs. The level of glutamate immunostaining in the SGCs that envelop neurons of all sizes in the TG was significantly higher in rats with CCI-ION than in control rats, suggesting that SGCs enveloping nociceptive as well as non-nociceptive mechanosensitive neurons are activated following nerve injury, and that the glutamate release from SGCs increases in pathologic pain state. Close appositions between substance-P (SP)-immunopositive (+) or calcitonin gene-related peptide (CGRP)+, likely nociceptive neurons, between Piezo1+, likely non-nociceptive, mechanosensitive neurons and SP+ or CGRP+ neurons, and between SGCs of neighboring neurons were frequently observed.

Discussion

These findings suggest that glutamate in the trigeminal SGCs that envelop all types of neurons may play a role in the mechanisms of neuropathic pain, possibly via paracrine signaling.

Long-term changes in oral feeding behaviors of growing rats

Oral feeding is critical for survival in both humans and animals. However, few studies have reported quantitative behavioral measures associated with the development of oral feeding behaviors. Therefore, the present study investigated developmental changes in the oral feeding behaviors of rats by quantitatively assessing pasta eating and licking behaviors. In the pasta eating test, the time to finish pasta sticks of three different thicknesses (Φ = 0.9, 1.4, and 1.9 mm, 4 cm long) was recorded between postnatal day 29 (P29) and P49, because all rats were able to finish eating these pasta sticks on P29. A developmental decrease in the time to finish pasta sticks of all thicknesses was observed during the initial period of recordings and plateaued before P35. The extent of this decrease was dependent on the thickness of pasta sticks. In the licking test, the number of licks per 10 s and the total intake volume during the test were recorded between P19 and P49, because all rats were able to access and lick the solution on P19. The time courses of developmental increases in the number of licks and the total intake volume were similar to the results obtained in the pasta eating test. Collectively, these results suggest that developmental changes in pasta eating and licking behaviors markedly differed between the weanling and periadolescent periods. The present study also demonstrated the applicability of the pasta eating and licking tests to the quantification of developmental changes in the oral feeding behaviors of rats.

Postnatal Maturation of Glutamatergic Inputs onto Rat Jaw-closing and Jaw-opening Motoneurons

Motoneurons that innervate the jaw-closing and jaw-opening muscles play a critical role in oro-facial behaviors, including mastication, suckling, and swallowing. These motoneurons can alter their physiological properties through the postnatal period during which feeding behavior shifts from suckling to mastication; however, the functional synaptic properties of developmental changes in these neurons remain unknown. Thus, we explored the postnatal changes in glutamatergic synaptic transmission onto the motoneurons that innervate the jaw-closing and jaw-opening musculatures during early postnatal development in rats. We measured miniature excitatory postsynaptic currents (mEPSCs) mediated by non-NMDA receptors (non-NMDA mEPSCs) and NMDA receptors in the masseter and digastric motoneurons. The amplitude, frequency, and rise time of non-NMDA mEPSCs remained unchanged among postnatal day (P)2-5, P9-12, and P14-17 age groups in masseter motoneurons, whereas the decay time dramatically decreased with age. The properties of the NMDA mEPSCs were more predominant at P2-5 masseter motoneurons, followed by reduction as neurons matured. The decay time of NMDA mEPSCs of masseter motoneurons also shortened remarkably across development. Furthermore, the proportion of NMDA/non-NMDA EPSCs induced in response to the electrical stimulation of the supratrigeminal region was quite high in P2-5 masseter motoneurons, and then decreased toward P14-17. In contrast to masseter motoneurons, digastric motoneurons showed unchanged properties in non-NMDA and NMDA EPSCs throughout postnatal development. Our results suggest that the developmental patterns of non-NMDA and NMDA receptor-mediated inputs vary among jaw-closing and jaw-opening motoneurons, possibly related to distinct roles of respective motoneurons in postnatal development of feeding behavior.

Developmental changes in GABAergic and glycinergic synaptic transmission to rat motoneurons innervating jaw-closing and jaw-opening muscles

Trigeminal motoneurons (MNs) innervating the jaw-closing and jaw-opening muscles receive numerous inhibitory synaptic inputs from GABAergic and glycinergic neurons, which are essential for oromotor functions, such as the orofacial reflex, suckling, and mastication. The properties of the GABAergic and glycinergic inputs of these MNs undergo developmental alterations during the period in which their feeding behavior proceeds from suckling to mastication; however, the detailed characteristics of the developmental patterns of GABAergic and glycinergic transmission in these neurons remain to be elucidated. This study was conducted to investigate developmental changes in miniature inhibitory postsynaptic currents (mIPSCs) in masseter (jaw-closing) and digastric (jaw-opening) MNs using brainstem slice preparations obtained from Wistar rats on postnatal day (P)2-5, P9-12, and P14-17. The frequency and amplitude of glycinergic mIPSCs substantially increased with age in both the masseter and digastric MNs. The rise time and decay time of glycinergic mIPSCs in both MNs decreased during development. In contrast, the frequency of GABAergic components in masseter MNs was higher at P2-5 than at P14-17, whereas that in the digastric MNs remained unchanged throughout the postnatal period. The proportion of currents mediated by GABA-glycine co-transmission was higher at P2-5, and then it decreased with age in both MNs. These results suggest that characteristics related to the development of inhibitory synaptic inputs differ between jaw-closing and jaw-opening MNs and between GABAergic and glycinergic currents. These distinct developmental characteristics may contribute to the development of feeding behaviors.

Development of γ-aminobutyric acid-, glycine-, and glutamate-immunopositive boutons on the rat genioglossal motoneurons

Detailed information about the development of excitatory and inhibitory synapses on the genioglossal (GG) motoneuron may help to understand the mechanism of fine control of GG motoneuron firing and the coordinated tongue movement during postnatal development. For this, we investigated the development of γ-aminobutyric acid (GABA)-immunopositive (GABA +), glycine + (Gly +), and glutamate + (Glut +) axon terminals (boutons) on the somata of rat GG motoneurons at a postnatal day 2 (P2), P6 and P18 by retrograde labeling of GG motoneurons with horseradish peroxidase, electron microscopic postembedding immunogold staining with GABA, Gly, and Glut antisera, and quantitative analysis. The number of boutons per GG motoneuron somata and the mean length of bouton apposition, measures of bouton size and synaptic covering percentage, were significantly increased from P2/P6 to P18. The number and fraction of GABA + only boutons of all boutons decreased significantly, whereas those of Gly + only boutons increased significantly from P2/P6 to P18, suggesting developmental switch from GABAergic to glycinergic synaptic transmission. The fraction of mixed GABA +/Gly + boutons of all boutons was the highest among inhibitory bouton types throughout the postnatal development. The fractions of excitatory and inhibitory boutons of all boutons remained unchanged during postnatal development. These findings reveal a distinct developmental pattern of inhibitory synapses on the GG motoneurons different from that on spinal or trigeminal motoneurons, which may have an important role in the regulation of the precise and coordinated movements of the tongue during the maturation of the oral motor system.

Distribution of excitatory and inhibitory axon terminals on the rat hypoglossal motoneurons

Detailed information about the excitatory and inhibitory synapses on the hypoglossal motoneurons may help understand the neural mechanism for control of the hypoglossal motoneuron excitability and hence the precise and coordinated movements of the tongue during chewing, swallowing and licking. For this, we investigated the distribution of GABA-, glycine (Gly)- and glutamate (Glut)-immunopositive (+) axon terminals on the genioglossal (GG) motoneurons by retrograde tracing, electron microscopic immunohistochemistry, and quantitative analysis. Small GG motoneurons (< 400 μm² in cross-sectional area) had fewer primary dendrites, significantly higher nuclear/cytoplasmic ratio, and smaller membrane area covered by synaptic boutons than large GG motoneurons (> 400 μm²). The fraction of inhibitory boutons (GABA + only, Gly + only, and mixed GABA +/Gly + boutons) of all boutons was significantly higher for small GG motoneurons than for large ones, whereas the fraction of Glut + boutons was significantly higher for large GG motoneurons than for small ones. Almost all boutons (> 95%) on both small and large GG motoneurons were GABA + , Gly + or Glut + . The frequency of mixed GABA +/Gly + boutons was the highest among inhibitory boutons types for both small and large GG motoneurons. These findings may elucidate the anatomical substrate for precise regulation of the motoneuron firing required for the fine movements of the tongue, and also suggest that the excitability of small and large GG motoneurons may be regulated differently.

Rat odontoblasts may use glutamate to signal dentin injury

Accumulating evidence indicates that odontoblasts act as sensor cells, capable of triggering action potentials in adjacent pulpal nociceptive axons, suggesting a paracrine signaling via a currently unknown mediator. Since glutamate can mediate signaling by non-neuronal cells, and peripheral axons may express glutamate receptors (GluR), we hypothesized that the expression of high levels of glutamate, and of sensory receptors in odontoblasts, combined with an expression of GluR in adjacent pulpal axons, is the morphological basis for odontoblastic sensory signaling. To test this hypothesis, we investigated the expression of glutamate, the thermo- and mechanosensitive ion channels transient receptor potential vanilloid 1 (TRPV1), transient receptor potential ankyrin 1 (TRPA1), and TWIK-1-related K+channel (TREK-1), and the glutamate receptor mGluR5, in a normal rat dental pulp, and following dentin injury. We also examined the glutamate release from odontoblast in cell culture. Odontoblasts were enriched with glutamate, at the level as high as in adjacent pulpal axons, and showed immunoreactivity for TRPV1, TRPA1, and TREK-1. Pulpal sensory axons adjacent to odontoblasts expressed mGluR5. Both the levels of glutamate in odontoblasts, and the expression of mGluR5 in nearby axons, were upregulated following dentin injury. The extracellular glutamate concentration was increased significantly after treating of odontoblast cell line with calcium permeable ionophore, suggesting glutamate release from odontoblasts. These findings lend morphological support to the hypothesis that odontoblasts contain glutamate as a potential neuroactive substance that may activate adjacent pulpal axons, and thus contribute to dental pain and hypersensitivity.

Expression of glycine receptor alpha 3 in the rat trigeminal neurons and central boutons in the brainstem

Increasing evidence shows that the homomeric glycine receptor is expressed in axon terminals and is involved in the presynaptic modulation of transmitter release. However, little is known about the expression of the glycine receptor, implicated in the presynaptic modulation of sensory transmission in the primary somatosensory neurons and their central boutons. To address this, we investigated the expression of glycine receptor subunit alpha 3 (GlyRα3) in the neurons in the trigeminal ganglion and axon terminals in the 1st relay nucleus of the brainstem by light- and electron-microscopic immunohistochemistry. Trigeminal primary sensory neurons were GlyRα3-immunopositive/gephyrin-immunonegative (indicating homomeric GlyR), whereas GlyRα3/gephyrin immunoreactivity (indicating heteromeric GlyR) was observed in dendrites. GlyRα3 immunoreactivity was also found in the central boutons of primary afferents but far from the presynaptic site and in dendrites at subsynaptic sites. Boutons expressing GlyRα3 contained small round vesicles, formed asymmetric synapses with dendrites and were immunoreactive for glutamate. These findings suggest that trigeminal primary afferent boutons receive presynaptic modulation via homomeric, extrasynaptic GlyRα3, and that different subtypes of GlyR may be involved in pre- and postsynaptic inhibition.