The supratrigeminal region of the rat sends GABA/glycine-cocontaining axon terminals to the motor trigeminal nucleus on the contralateral side

Top-down descending facilitation of spinal sensory excitatory transmission from the anterior cingulate cortex

Spinal sensory transmission is under descending biphasic modulation, and descending facilitation is believed to contribute to chronic pain. Descending modulation from the brainstem rostral ventromedial medulla (RVM) has been the most studied, whereas little is known about direct corticospinal modulation. Here, we found that stimulation in the anterior cingulate cortex (ACC) potentiated spinal excitatory synaptic transmission and this modulation is independent of the RVM. Peripheral nerve injury enhanced the spinal synaptic transmission and occluded the ACC-spinal cord facilitation. Inhibition of ACC reduced the enhanced spinal synaptic transmission caused by nerve injury. Finally, using optogenetics, we showed that selective activation of ACC-spinal cord projecting neurons caused behavioral pain sensitization, while inhibiting the projection induced analgesic effects. Our results provide strong evidence that ACC stimulation facilitates spinal sensory excitatory transmission by a RVM-independent manner, and that such top-down facilitation may contribute to the process of chronic neuropathic pain.

It is known that descending facilitation of spinal responses may contribute to chronic pain, however many studies have focussed on brainstem mechanisms. Here the authors show that stimulation of the anterior cingulate cortex increases excitatory transmission in the dorsal horn, and that this may be via a direct pathway independent of the brainstem.

Supratrigeminal Bilaterally Projecting Neurons Maintain Basal Tone and Enable Bilateral Phasic Activation of Jaw-Closing Muscles

Anatomical studies have identified brainstem neurons that project bilaterally to left and right oromotor pools, which could potentially mediate bilateral muscle coordination. We use retrograde lentiviruses combined with a split-intein-mediated split-Cre-recombinase system in mice to isolate, characterize, and manipulate a population of neurons projecting to both the left and right jaw-closing trigeminal motoneurons. We find that these bilaterally projecting premotor neurons (BPNs) reside primarily in the supratrigeminal nucleus (SupV) and the parvicellular and intermediate reticular regions dorsal to the facial motor nucleus. These BPNs also project to multiple midbrain and brainstem targets implicated in orofacial sensorimotor control, and consist of a mix of glutamatergic, GABAergic, and glycinergic neurons, which can drive both excitatory and inhibitory inputs to trigeminal motoneurons when optogenetically activated in slice. Silencing BPNs with tetanus toxin light chain (TeNT) increases bilateral masseter activation during chewing, an effect driven by the expression of TeNT in SupV BPNs. Acute unilateral optogenetic inhibition of SupV BPNs identifies a group of tonically active neurons that function to lower masseter muscle tone, whereas unilateral optogenetic activation of SupV BPNs is sufficient to induce bilateral masseter activation both during resting state and during chewing. These results provide evidence for SupV BPNs in tonically modulating jaw-closing muscle tone and in mediating bilateral jaw closing.

SIGNIFICANCE STATEMENT

We developed a method that combines retrograde lentiviruses with the split-intein-split-Cre system in mice to isolate, characterize, and manipulate neurons that project to both left and right jaw-closing motoneurons. We show that these bilaterally projecting premotor neurons (BPNs) reside primarily in the supratrigeminal nucleus and the rostral parvicellular and intermediate reticular nuclei. BPNs consist of both excitatory and inhibitory populations, and also project to multiple brainstem nuclei implicated in orofacial sensorimotor control. Manipulation of the supratrigeminal BPNs during natural jaw-closing behavior reveals a dual role for these neurons in eliciting phasic muscle activation and in maintaining basal muscle tone. The retrograde lentivirus carrying the split-intein-split-Cre system can be applied to study any neurons with bifurcating axons innervating two brain regions.

Connections between EM2-containing terminals and GABA/μ-opioid receptor co-expressing neurons in the rat spinal trigeminal caudal nucleus

Endomorphin-2 (EM2) demonstrates a potent antinociceptive effect via the μ-opioid receptor (MOR). To provide morphological evidence for the pain control effect of EM2, the synaptic connections between EM2-immunoreactive (IR) axonal terminals and γ-amino butyric acid (GABA)/MOR co-expressing neurons in lamina II of the spinal trigeminal caudal nucleus (Vc) were investigated in the rat. Dense EM2-, MOR- and GABA-IR fibers and terminals were mainly observed in lamina II of the Vc. Within lamina II, GABA- and MOR-neuronal cell bodies were also encountered. The results of immunofluorescent histochemical triple-staining showed that approximately 14.2 or 18.9% of GABA-IR or MOR-IR neurons also showed MOR- or GABA-immunopositive staining in lamina II; approximately 45.2 and 36.1% of the GABA-IR and MOR-IR neurons, respectively, expressed FOS protein in their nuclei induced by injecting formalin into the left lower lip of the mouth. Most of the GABA/MOR, GABA/FOS, and MOR/FOS double-labeled neurons made close contacts with EM2-IR fibers and terminals. Immuno-electron microscopy confirmed that the EM2-IR terminals formed synapses with GABA-IR or MOR-IR dendritic processes and neuronal cell bodies in lamina II of the Vc. These results suggest that EM2 might participate in pain transmission and modulation by binding to MOR-IR and GABAergic inhibitory interneuron in lamina II of the Vc to exert inhibitory effect on the excitatory interneuron in lamina II and projection neurons in laminae I and III.

Morphological evidence for a neurotensinergic periaqueductal gray-rostral ventromedial medulla-spinal dorsal horn descending pathway in rat

Neurotensin (NT) is an endogenous neuropeptide that exerts potent opioid-independent analgesic effects, most likely via the type 2 NT receptor (NTR2). Previous morphological and electrophysiological studies suggested that the NT-NTR2 system is primarily localized in structures that constitute the descending pain control pathway, such as the periaqueductal gray (PAG), the rostral ventromedial medulla (RVM), and the spinal dorsal horn (SDH). However, relevant morphological evidence for this neurotensinergic (NTergic) circuit is lacking. Thus, the aim of the present study was to morphologically elucidate the potential sites and connections in the NT-NTR2 system that are involved in the descending pain control pathway. Based on light and electron microscopy combined with anterograde and retrograde tracing, we found evidence that NTR2-immunoreactive (IR) neurons in the RVM receive NT-IR projections originating from the PAG; express NT, serotonin (5-HT), or both; and send projections that terminate in laminae I and II of the SDH. These results suggest that NTR2 may contribute to pain control by binding to NT in the PAG-RVM-SDH pathway. In conclusion, our data provide morphological evidence for an NTergic PAG-RVM-SDH pathway, implicating novel mechanisms of NT-induced analgesia.

Electrophysiological and morphological properties of rat supratrigeminal premotor neurons targeting the trigeminal motor nucleus

The electrophysiological and morphological characteristics of premotor neurons in the supratrigeminal region (SupV) targeting the trigeminal motor nucleus (MoV) were examined in neonatal rat brain stem slice preparations with Ca(2+) imaging, whole cell recordings, and intracellular biocytin labeling. First, we screened SupV neurons that showed a rapid rise in intracellular free Ca(2+) concentration ([Ca(2+)]i) after single-pulse electrical stimulation of the ipsilateral MoV. Subsequent whole cell recordings were generated from the screened SupV neurons, and their antidromic responses to MoV stimulation were confirmed. We divided the antidromically activated premotor neurons into two groups according to their discharge patterns during the steady state in response to 1-s depolarizing current pulses: those firing at a frequency higher (HF neurons, n = 19) or lower (LF neurons, n = 17) than 33 Hz. In addition, HF neurons had a narrower action potential and a larger afterhyperpolarization than LF neurons. Intracellular labeling revealed that the axons of all HF neurons (6/6) and half of the LF neurons (4/9) entered the MoV from its dorsomedial aspect, whereas the axons of the remaining LF neurons (5/9) entered the MoV from its dorsolateral aspect. Furthermore, the dendrites of three HF neurons penetrated into the principal sensory trigeminal nucleus (Vp), whereas the dendrites of all LF neurons were confined within the SupV. These results suggest that the types of SupV premotor neurons targeting the MoV with different firing properties have different dendritic and axonal morphologies, and these SupV neuron classes may play unique roles in diverse oral motor behaviors, such as suckling and mastication.

Coordination of NMDA-induced rhythmic activity in the trigeminal and hypoglossal nerves of neonatal mice in vitro

Suckling is a rhythmic jaw movement that is symmetrical on the left and right side and is highly coordinated with tongue movement. Thus, we investigated the neuronal mechanisms of the left/right and jaw/tongue coordinations during N-methyl-d-aspartate (NMDA)-induced fictive suckling using isolated brainstem-spinal cord preparations obtained from neonatal mice. We observed synchronous low-frequency rhythmic activity in the left/right trigeminal motor nerves, which differed from respiration, and high-frequency rhythmic trigeminal activity, which was side-independent. The low-frequency rhythmic trigeminal activity was also synchronized with the hypoglossal nerve activity. After a complete midline separation of the preparation or a partial midline transection of the brainstem from the anterior inferior cerebellar artery to the junction of the vertebral artery, the low-frequency rhythmic trigeminal activity disappeared, whereas the high-frequency rhythmic trigeminal activity and low-frequency rhythmic hypoglossal activity still remained. These results suggest that the neuronal network that generates low-frequency rhythmic activity likely contributes to the synchronized activity of the left/right jaw muscles and to the jaw/tongue muscles, where it sends its command to the trigeminal motoneurons mainly via the commissural pathway that crosses the transected midline region. Such a neuronal network may underlie the coordinated movements of the jaw and tongue during suckling.

Involvement of ryanodine receptors in tetanic sciatic stimulation-induced long-term potentiation of spinal dorsal horn and persistent pain in rats

Tetanic stimulation of the sciatic nerve induces long-term potentiation (LTP) of C-fiber-evoked field potentials in the spinal dorsal horn and persistent pain, suggesting that spinal LTP may be a substrate for central sensitization of the pain pathway. However, its cellular mechanism remains unclear. The present study provides electrophysiological and behavioral evidence for the involvement of ryanodine receptor (RyR) in the induction of spinal LTP and persistent pain in rats. The specific inhibitor of ryanodine receptor, ryanodine and dantrolene, dose dependently blocked the induction, but not maintenance, of spinal LTP and reduced persistent pain behaviors induced by tetanic sciatic stimulation. Both cyclic ADP ribose (cADPR), an endogenous agonist of RyR, and (±)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester (Bay K 8644), an agonist of L-type calcium channel, attenuated ryanodine-induced inhibition. Immunohistochemistry and electron microscopic observation showed that RyR subtypes RyR1 and RyR3 were located in the spinal dorsal horn. The results suggest that RyRs are involved in synaptic plasticity of the spinal pain pathway and may be a novel target for treating pain. © 2012 Wiley Periodicals, Inc.

Ultrastructural analysis of glutamate-, GABA-, and glycine-immunopositive boutons from supratrigeminal premotoneurons in the rat trigeminal motor nucleus

The supratrigeminal region (Vsup) is important for coordination of smooth jaw movement. However, little is known about the synaptic connections of the Vsup premotoneurons with the trigeminal motor neurons. In the present study, we examined axon terminals of Vsup premotoneurons in the contralateral trigeminal motor nucleus (Vmo) by a combination of anterograde tracing with cholera toxin B-horseradish peroxidase (CTB-HRP), postembedding immunohistochemistry for the amino acid transmitters glutamate, GABA, and glycine, and electron microscopy. Tracer injections resulted in anterograde labeling of axon terminals of the Vsup premotoneurons in the motor trigeminal nucleus (Vmo). The labeled boutons in Vmo exhibited immunoreactivity for glutamate, GABA, or glycine: glutamate-immunopositive boutons (69%) were more frequently observed than GABA- or glycine-immunopositive boutons (19% and 12%, respectively). Although most labeled boutons (97%) made synaptic contacts with a single postsynaptic dendrite, a few glutamate-immunopositive boutons (3%) showed synaptic contact with two dendrites. No labeled boutons participated in axoaxonic synaptic contacts. Most labeled boutons (78%) were presynaptic to dendritic shafts, and the remaining 22% were presynaptic to somata or primary dendrites. A large proportion of GABA- or glycine-immunopositive boutons (40%) were presynaptic to somata or primary dendrites, whereas most glutamate-immunopositive boutons (86%) were presynaptic to dendritic shafts. These results indicate that axon terminals of Vsup premotoneurons show simple synaptic connection with Vmo neurons. This may provide the anatomical basis for the neural information processing responsible for jaw movement control.

Morphological and immunohistochemical characterization of interneurons within the rat trigeminal motor nucleus

Three series of experiments were carried out to characterize interneurons located within the trigeminal motor nucleus of young rats aged 5-24 days. Cholera toxin injections were made bilaterally into the masseter and, sometimes, digastric muscles to label motoneurons. In the first set of experiments, thick slices were taken from the pontine brainstem and cholera toxin-positive and cholera toxin-negative neurons located inside the trigeminal motor nucleus were filled with biocytin through whole-cell recording patch electrodes. Positively identified motoneurons (cholera toxin+) of various shapes and sizes always had a thick, unbranched axon that entered the motor root following a tight zigzag course. Many cholera toxin-negative neurons were also classified as motoneurons after biocytin filling based on this particularity of their axon. These are probably either fusimotor motoneurons or motoneurons supplying other jaw muscles. The cholera toxin-negative neurons classified as interneurons differed markedly from motoneurons in that they had thin, usually branched axons that supplied the ipsilateral reticular region surrounding the trigeminal motor nucleus (peritrigeminal area), the main trigeminal sensory nucleus, the trigeminal mesencephalic nucleus, the medial reticular formation of both sides, and the contralateral medial peritrigeminal area. Most often, their dendrites were arranged in bipolar arbors that extended beyond the borders of the trigeminal motor nucleus into the peritrigeminal area. Immunohistochemistry against glutamate, GABA and glycine was used to further document the nature and distribution of putative interneurons. Immunoreactive neurons were uniformly distributed throughout the rostro-caudal extent of the trigeminal motor nucleus. Their concentration seemed greater toward the edges of the nucleus and they were scarce in the digastric motoneuron pool. Glutamate- outnumbered GABA- and glycine-immunoreactive neurons. There was no clear segregation between the three populations. In the final experiment, 1,1'-dioctadecyl-3,3,3',3'-tetra-methylindocarbocyanine perchlorate crystals were inserted into one trigeminal motor nucleus in thick slices and allowed to diffuse for several weeks. This procedure marked commissural fibers and interneurons in the contralateral trigeminal motor nucleus. Together these results conclusively support the existence of interneurons in the trigeminal motor nucleus.

Neuron numbers in the sensory trigeminal nuclei of the rat: A GABA- and glycine-immunocytochemical and stereological analysis

The volume, total neuron number, and number of GABA- and glycine-expressing neurons in the sensory trigeminal nuclei of the adult rat were estimated by stereological methods. The mean volume is 1.38+/-0.13 mm3 (mean+/-SD) for the principal nucleus (Vp), 1.59+/-0.06 for the n. oralis (Vo), 2.63+/-0.34 for the n. interpolaris (Vip), and 3.73+/-0.11 for the n. caudalis (Vc). The total neuron numbers are 31,900+/-2,200 (Vp), 21,100+/-3,300 (Vo), 61,600+/-8,300 (Vip), and 159,100+/-25,300 (Vc). Immunoreactive (-ir) neurons were classified as strongly stained or weakly stained, depending on qualitative criteria, cross-checked by a densitometric analysis. GABA-ir cells are most abundant in Vc, in an increasing rostrocaudal gradient within the nucleus. Lower densities are found in Vip and Vp. The mean total number of strongly labeled GABA-ir neurons ranges between 1,800 in Vp to 7,800 in Vip and 22,900 in Vc, and varies notably between subjects. Glycine-ir neurons are more numerous and display more homogeneous densities in all nuclei. Strongly labeled Gly-ir cells predominate in all nuclei, their total number ranging between 9,400 in Vp to 24,300 in Vip and 34,200 in Vc. A substantial fraction of immunolabeled neurons in all nuclei coexpress GABA and glycine. In general, all neurons strongly immunoreactive for GABA are small, while weakly GABA-ir cells which coexpress Gly are larger. In Vc, one-third of all neurons are immunoreactive: 16.6% of them are single-labeled for GABA and 31.6% are single-labeled for glycine. The remaining 51.8% express GABA and glycine in different combinations, with those showing strong double labeling accounting for 22.6%.

Morphological evidence for GABA/glycine-cocontaining terminals in synaptic contact with neurokinin-1 receptor-expressing neurons in the sacral dorsal commissural nucleus of the rat

Previous studies have shown that neurons in the sacral dorsal commissural nucleus (SDCN) express neurokinin-1 receptor (NK1R) and can be modulated by the co-release of GABA and glycine (Gly) from single presynaptic terminal. These results raise the possibility that GABA/Gly-cocontaining terminals might make synaptic contacts with NK1R-expressing neurons in the SDCN. In order to provide morphological evidence for this hypothesis, the triple-immunohistochemical studies were performed in the SDCN. Triple-immunofluorescence histochemical study showed that some axon terminals in close association with NK1R-immunopositive (NK1R-ip) neurons in the SDCN were immunopositive for both glutamic acid decarboxylase (GAD) and glycine transporter 2 (GlyT2). In electron microscopic dual- and triple-immunohistochemistry for GAD/GlyT2, GAD/NK1R, GlyT2/NK1R, or GAD/GlyT2/NK1R also revealed dually labeled (GAD/GlyT2-ip) synaptic terminals upon SDCN neurons, as well as GAD- and/or GlyT2-ip axon terminals in synaptic contact with NK1R-ip SDCN neurons. These results suggested that some synaptic terminals upon NK1R-expressing SDCN neurons co-released both GABA and Gly.

Efferent and afferent connections of GABAergic neurons in the supratrigeminal and the intertrigeminal regions

It has been reported in the cat and rat that inhibitory premotor neurons, which send their axons to motoneurons of the trigeminal motor nucleus (Vm) are distributed in the reticular regions around the Vm, especially in the supratrigeminal region (Vsup) and the intertrigeminal region (Vint). In the present study, we examined neuronal connections of GABAergic neurons in the Vsup and Vint in the mouse by utilizing the adult heterozygous GAD67-GFP knock-in mouse, in which green fluorescence protein (GFP) is expressed in GABAergic neurons under the control of the endogenous GAD (GAD67) gene promoter [Yanagawa, Y., Kaneko, K., Kanbara, N., Totsuka, M., Yagi, T., Obata, K., 2001. Development of mouse expressing GFP in GABAergic neurons. Neurosci. Res. Suppl. 25, S77; Tamamaki, N., Yanagawa, Y., Tomioka, R., Miyazaki, J.-I., Obata, K., Kaneko, T., 2003. Green fluorescent protein expression and colocalization with calretinin, parvalbumin and somatostatin in the GAD67-GFP knock-in mouse. J. Comp. Neurol. 467, 60-79]. The connections were examined light- and electron-microscopically by combining the anterograde or the retrograde tract-tracing method with the immunohistochemical method for GFP. The data indicated that the Vsup and Vint of the mouse contained GABAergic neurons, which received projection fibers from the marginal layer of the nucleus of the spinal tract of the trigeminal nerve (Vc) on the ipsilateral side and sent their axons to the Vm on the contralateral side. Some of these GABAergic neurons may represent Vm-premotor neurons that receive nociceptive input from the Vc to elicit jaw-opening reflex by inhibiting jaw-closing Vm-motoneurons.

Overall distribution of GLYT2 mRNA-containing versus GAD67 mRNA-containing neurons and colocalization of both mRNAs in midbrain, pons, and cerebellum in rats

We aimed to clarify the overall distribution of glycinergic neurons in the midbrain, pons, and cerebellum in rats, using in situ hybridization for mRNA encoding glycine transporter 2 (GLYT2), which reliably detects glycinergic cell bodies. We combined this method with in situ hybridization for mRNA encoding glutamic acid decarboxylase isoform 67 (GAD67), and have presented for the first time global and detailed views of the distribution of glycinergic neurons in relation to GABAergic neurons. In addition to this single-detection study, we performed double-detection of GLYT2 mRNA and GAD67 mRNA to determine the distribution of neurons co-expressing these mRNAs. We have shown that many areas of the brainstem and cerebellum, not only areas where previous immunohistochemical studies have specified, involve double-labeled neurons with GLYT2 and GAD67 mRNAs. In particular, when lightly labeled GLYT2 mRNA-positive neurons were distributed within the area of GAD67 mRNA-positive neurons, almost all such GLYT2 mRNA-positive neurons were GAD67 mRNA-positive. Areas or neuron groups expressing exclusively GLYT2 mRNA or GAD67 mRNA were rather limited, such as the superior colliculus, nucleus of the trapezoid body, and Purkinje cells. The present study suggests that the corelease of glycine and GABA from single neurons is more widespread than has been reported.