GABA and glycine in synaptic microcircuits associated with physiologically characterized primary afferents of cat trigeminal principal nucleus

Descending projections from the insular cortex to the trigeminal spinal subnucleus caudalis facilitate excitatory outputs to the parabrachial nucleus in rats

ABSTRACT

Nociceptive information from the orofacial area projects to the trigeminal spinal subnucleus caudalis (Sp5C) and is then conveyed to several nuclei, including the parabrachial nucleus (PBN). The insular cortex (IC) receives orofacial nociceptive information and sends corticofugal projections to the Sp5C. The Sp5C consists of glutamatergic and GABAergic/glycinergic interneurons that induce excitatory postsynaptic currents and inhibitory postsynaptic currents, respectively, in projection neurons. Therefore, quantification of glutamatergic IC inputs in combination with identifying postsynaptic neuronal subtypes is critical to elucidate IC roles in the regulation of Sp5C activities. We investigated features of synaptic transmission from the IC to glutamatergic and GABAergic/glycinergic Sp5C neurons of laminae I/II using vesicular GABA transporter-Venus transgenic rats that received an injection of adeno-associated virus-channelrhodopsin-2-mCherry into the IC. Selective stimulation of IC axon terminals in Sp5C slice preparations induced monosynaptic excitatory postsynaptic currents in both excitatory glutamatergic and inhibitory GABAergic/glycinergic Sp5C neurons with a comparable amplitude. Paired whole-cell patch-clamp recordings showed that unitary inhibitory postsynaptic currents from inhibitory neurons influencing excitatory neurons, including neurons projecting to the PBN, exhibited a high failure rate and were suppressed by both bicuculline and strychnine, suggesting that excitatory neurons in the Sp5C receive both GABAergic and glycinergic inhibition with low impact. Moreover, selective stimulation of IC axons increased the firing rate at the threshold responses. Finally, we demonstrated that selective stimulation of IC axons in the Sp5C by a chemogenetic approach decreased the thresholds of both mechanical and thermal nociception. Thus, IC projection to the Sp5C is likely to facilitate rather than suppress excitatory outputs from the Sp5C.

Ultrastructure of Rat Rostral Nucleus of the Solitary Tract Terminals in the Parabrachial Nucleus and Medullary Reticular Formation

Neurons in the rostral nucleus of the solitary tract (rNST) receive taste information from the tongue and relay it mainly to the parabrachial nucleus (PBN) and the medullary reticular formation (RF) through two functionally different neural circuits. To help understand how the information from the rNST neurons is transmitted within these brainstem relay nuclei in the taste pathway, we examined the terminals of the rNST neurons in the PBN and RF by use of anterograde horseradish peroxidase (HRP) labeling, postembedding immunogold staining for glutamate, serial section electron microscopy, and quantitative analysis. Most of the anterogradely labeled, glutamate-immunopositive axon terminals made a synaptic contact with only a single postsynaptic element in PBN and RF, suggesting that the sensory information from rNST neurons, at the individual terminal level, is not passed to multiple target cells. Labeled terminals were usually presynaptic to distal dendritic shafts in both target nuclei. However, the frequency of labeled terminals that contacted dendritic spines was significantly higher in the PBN than in the RF, and the frequency of labeled terminals that contacted somata or proximal dendrites was significantly higher in the RF than in the PBN. Labeled terminals receiving axoaxonic synapses, which are a morphological substrate for presynaptic modulation frequently found in primary sensory afferents, were not observed. These findings suggest that the sensory information from rNST neurons is processed in a relatively simple manner in both PBN and RF, but in a distinctly different manner in the PBN as opposed to the RF.

Axo-axonic synapses: Diversity in neural circuit function

The chemical synapse is the principal form of contact between neurons of the central nervous system. These synapses are typically configured as presynaptic axon terminations onto postsynaptic dendrites or somata, giving rise to axo-dendritic and axo-somatic synapses, respectively. Beyond these common synapse configurations are less-studied, non-canonical synapse types that are prevalent throughout the brain and significantly contribute to neural circuit function. Among these are the axo-axonic synapses, which consist of an axon terminating on another axon or axon terminal. Here, we review evidence for axo-axonic synapse contributions to neural signaling in the mammalian nervous system and survey functional neural circuit motifs enabled by these synapses. We also detail how recent advances in microscopy, transgenics, and biological sensors may be used to identify and functionally assay axo-axonic synapses.

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.

Synaptic ultrastructure changes in trigeminocervical complex posttrigeminal nerve injury

Trigeminal nerves collecting sensory information from the orofacial area synapse on second-order neurons in the dorsal horn of subnucleus caudalis and cervical C1/C2 spinal cord (Vc/C2, or trigeminocervical complex), which is critical for sensory information processing. Injury to the trigeminal nerves may cause maladaptive changes in synaptic connectivity that plays an important role in chronic pain development. Here we examined whether injury to the infraorbital nerve, a branch of the trigeminal nerves, led to synaptic ultrastructural changes when the injured animals have developed neuropathic pain states. Transmission electron microscopy was used to examine synaptic profiles in Vc/C2 at 3 weeks postinjury, corresponding to the time of peak behavioral hypersensitivity following chronic constriction injury to the infraorbital nerve (CCI-ION). Using established criteria, synaptic profiles were classified as associated with excitatory (R-), inhibitory (F-), and primary afferent (C-) terminals. Each type was counted within the superficial dorsal horn of the Vc/C2 and the means from each rat were compared between sham and injured animals; synaptic contact length was also measured. The overall analysis indicates that rats with orofacial pain states had increased numbers and decreased mean synaptic length of R-profiles within the Vc/C2 superficial dorsal horn (lamina I) 3 weeks post-CCI-ION. Increases in the number of excitatory synapses in the superficial dorsal horn of Vc/C2 could lead to enhanced activation of nociceptive pathways, contributing to the development of orofacial pain states.

Distribution of gastrin-releasing peptide in the rat trigeminal and spinal somatosensory systems

Gastrin-releasing peptide (GRP) has recently been identified as an itch-specific neuropeptide in the spinal sensory system in mice, but there are no reports of the expression and distribution of GRP in the trigeminal sensory system in mammals. We characterized and compared GRP-immunoreactive (ir) neurons in the trigeminal ganglion (TG) with those in the rat spinal dorsal root ganglion (DRG). GRP immunoreactivity was expressed in 12% of TG and 6% of DRG neurons and was restricted to the small- and medium-sized type cells. In both the TG and DRG, many GRP-ir neurons also expressed substance P and calcitonin gene-related peptide, but not isolectin B4 . The different proportions of GRP and transient receptor potential vanilloid 1 double-positive neurons in the TG and DRG imply that itch sensations via the TG and DRG pathways are transmitted through distinct mechanisms. The distribution of the axon terminals of GRP-ir primary afferents and their synaptic connectivity with the rat trigeminal sensory nuclei and spinal dorsal horn were investigated by using light and electron microscopic histochemistry. Although GRP-ir fibers were rarely observed in the trigeminal sensory nucleus principalis, oralis, and interpolaris, they were predominant in the superficial layers of the trigeminal sensory nucleus caudalis (Vc), similar to the spinal dorsal horn. Ultrastructural analysis revealed that GRP-ir terminals contained clear microvesicles and large dense-cored vesicles, and formed asymmetric synaptic contacts with a few dendrites in the Vc and spinal dorsal horn. These results suggest that GRP-dependent orofacial and spinal pruriceptive inputs are processed mainly in the superficial laminae of the Vc and spinal dorsal horn.

Whisker-related circuitry in the trigeminal nucleus principalis: ultrastructure

Trigeminal (V) nucleus principalis (PrV) is the requisite brainstem nucleus in the whisker-to-barrel cortex model system that is widely used to reveal mechanisms of map formation and information processing. Yet, little is known of the actual PrV circuitry. In the ventral "barrelette" portion of the adult mouse PrV, relationships between V primary afferent terminals, thalamic-projecting PrV neurons, and gamma-aminobutyric acid (GABA)-ergic terminals were analyzed in the electron microscope. Primary afferents, thalamic-projecting cells, and GABAergic terminals were labeled, respectively, by Neurobiotin injections in the V ganglion, horseradish peroxidase injections in the thalamus, and postembedding immunogold histochemistry. Primary afferent terminals (Neurobiotin- and glutamate-immunoreactive) display asymmetric and multiple synapses predominantly upon the distal dendrites and spines of PrV cells that project to the thalamus. Primary afferents also synapse upon GABAergic terminals. GABAergic terminals display symmetric synapses onto primary afferent terminals, the somata and dendrites (distal, mostly) of thalamic-projecting neurons, and GABAergic dendrites. Thus, primary afferent inputs through the PrV are subject to pre- and postsynaptic GABAergic influences. As such, circuitry exists in PrV "barrelettes" for primary afferents to directly activate thalamic-projecting and inhibitory local circuit cells. The latter are synaptically associated with themselves, the primary afferents, and with the thalamic-projecting neurons. Thus, whisker-related primary afferent inputs through PrV projection neurons are pre- and postsynaptically modulated by local circuits.

Inhibitory control of nociceptive responses of trigeminal spinal nucleus cells by somatosensory corticofugal projection in rat

The caudal division of the trigeminal spinal nucleus (Sp5C) is an important brainstem relay station of orofacial pain transmission. The aim of the present study was to examine the effect of cortical electrical stimulation on nociceptive responses in Sp5C neurons. Extracellular recordings were performed in the Sp5C nucleus by tungsten microelectrodes in urethane-anesthetized Sprague-Dawley rats. Nociceptive stimulation was produced by application of capsaicin cream on the whisker pad or by constriction of the infraorbital nerve. Capsaicin application evoked a long-lasting increase in the spontaneous firing rate from 1.4±0.2 to 3.4±0.6 spikes/s. Non-noxious tactile responses from stimuli delivered to the receptive field (RF) center decreased 5 min. after capsaicin application (from 2.3±0.1 to 1.6±0.1 spikes/stimulus) while responses from the whisker located at the RF periphery increased (from 1.3±0.2 to 2.0±0.1 spikes/stimulus under capsaicin). Electrical train stimulation of the primary (S1) or secondary (S2) somatosensory cortical areas reduced the increase in the firing rate evoked by capsaicin. Also, S1, but not S2, cortical stimulation reduced the increase in non-noxious tactile responses from the RF periphery. Inhibitory cortical effects were mediated by the activation of GABAergic and glycinergic neurons because they were blocked by bicuculline or strychnine. The S1 and S2 cortical stimulation also inhibited Sp5C neurons in animals with constriction of the infraorbital nerve. Consequently, the corticofugal projection from S1 and S2 cortical areas modulates nociceptive responses of Sp5C neurons and may control the transmission of nociceptive sensory stimulus.

Ultrastructural analysis of the synaptic connectivity of TRPV1-expressing primary afferent terminals in the rat trigeminal caudal nucleus

Trigeminal primary afferents that express the transient receptor potential vanilloid 1 (TRPV1) are important for the transmission of orofacial nociception. However, little is known about how the TRPV1-mediated nociceptive information is processed at the first relay nucleus in the central nervous system (CNS). To address this issue, we studied the synaptic connectivity of TRPV1-positive (+) terminals in the rat trigeminal caudal nucleus (Vc) by using electron microscopic immunohistochemistry and analysis of serial thin sections. Whereas the large majority of TRPV1+ terminals made synaptic contacts of an asymmetric type with one or two postsynaptic dendrites, a considerable fraction also participated in complex glomerular synaptic arrangements. A few TRPV1+ terminals received axoaxonic contacts from synaptic endings that contained pleomorphic synaptic vesicles and were immunolabeled for glutamic acid decarboxylase, the synthesizing enzyme for the inhibitory neurotransmitter γ-aminobutyric acid (GABA). We classified the TRPV1+ terminals into an S-type, containing less than five dense-core vesicles (DCVs), and a DCV-type, containing five or more DCVs. The number of postsynaptic dendrites was similar between the two types of terminals; however, whereas axoaxonic contacts were frequent on the S-type, the DCV-type did not receive axoaxonic contacts. In the sensory root of the trigeminal ganglion, TRPV1+ axons were mostly unmyelinated, and a small fraction was small myelinated. These results suggest that the TRPV1-mediated nociceptive information from the orofacial region is processed in a specific manner by two distinct types of synaptic arrangements in the Vc, and that the central input of a few TRPV1+ afferents is presynaptically modulated via a GABA-mediated mechanism.

Neuronal disinhibition in the trigeminal nucleus caudalis in a model of chronic neuropathic pain

The mechanisms underlying neuropathic facial pain syndromes are incompletely understood. We used a unilateral chronic constriction injury of the rat infraorbital nerve (CCI-IoN) as a facial neuropathic model. Pain-related behavior of the CCI-IoN animals was tested at 8, 15 and 26 days after surgery (dps). The response threshold to mechanical stimulation with von Frey hairs on the injured side was reduced at 15 and 26 dps, indicating the presence of allodynia. We performed unitary recordings in the caudalis division of the spinal trigeminal nucleus (Sp5C) at 8 or 26 dps, and examined spontaneous activity and responses to mechanical and thermal stimulation of the vibrissal pad. Neurons were identified as wide dynamic range (WDR) or low-threshold mechanoreceptive (LTM) according to their response to tactile and/or noxious stimulation. Following CCI-IoN, WDR neurons, but not LTM neurons, increased their spontaneous activity at 8 and 26 dps, and both types of Sp5C neurons increased their responses to tactile stimuli. In addition, the on-off tactile response in neurons recorded after CCI-IoN was followed by afterdischarges that were not observed in control cases. Compared with controls, the response inhibition observed during paired-pulse stimulation was reduced after CCI-IoN. Immunohistochemical studies showed an overall decrease in GAD65 immunoreactivity in Sp5C at 26 dps, most marked in laminae I and II, suggesting that following CCI-IoN the inhibitory circuits in the sensory trigeminal nuclei are depressed. Consequently, our results strongly suggest that disinhibition of Sp5C neurons plays a relevant role in the appearance of allodynia after CCI-IoN.

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.

GABA- and glycine-like immunoreactivity in axonal endings presynaptic to the vibrissa afferents in the cat trigeminal interpolar nucleus

The goal of this study was to analyze the synaptic interaction of primary afferents with GABA- and/or glycine-immunopositive presynaptic endings in the cat trigeminal interpolar nucleus (Vi). Fast adapting vibrissa afferents were labeled by intra-axonal injections of horseradish peroxidase. Postembedding immunogold labeling on serially cut ultrathin sections and quantitative ultrastructural analysis of the labeled boutons and their presynaptic endings (p-endings) in the Vi were performed. The majority of p-endings presynaptic to labeled boutons (83%) were immunopositive for both GABA and glycine and 8% were immunopositive for glycine alone. A small fraction of p-endings were immunopositive for GABA alone (4%) or immunonegative for both GABA and glycine (4%). Ultrastructural parameters related to synaptic release, i.e. bouton volume, mitochondrial volume, and active zone area, were significantly larger in the labeled boutons of primary afferents than in the p-endings. The volume of labeled boutons was positively correlated with the number of the postsynaptic dendrites and p-endings. In addition, fairly large-sized labeled boutons and p-endings were frequently observed in the Vi. These results reveal that large majority of vibrissa afferents in the Vi are presynaptically modulated by interneurons immunopositive for both GABA and glycine, and suggest that the Vi plays a distinct role in the processing of orofacial sensory information, different from that of other trigeminal sensory nuclei.

Projections of the second cervical dorsal root ganglion to the cochlear nucleus in rats

Physiological, anatomical, and clinical data have demonstrated interactions between somatosensory and auditory brainstem structures. Spinal nerve projections influence auditory responses, although the nature of the pathway(s) is not known. To address this issue, we injected biotinylated dextran amine into the cochlear nucleus or dorsal root ganglion (DRG) at the second cervical segment (C2). Cochlear nucleus injections retrogradely labeled small ganglion cells in C2 DRG. C2 DRG injections produced anterograde labeling in the external cuneate nucleus, cuneate nucleus, nucleus X, central cervical nucleus, dorsal horn of upper cervical spinal segments, and cochlear nucleus. The terminal field in the cochlear nucleus was concentrated in the subpeduncular corner and lamina of the granule cell domain, where endings of various size and shapes appeared. Examination under an electron microscope revealed that the C2 DRG terminals contained numerous round synaptic vesicles and formed asymmetric synapses, implying depolarizing influences on the target cell. Labeled endings synapsed with the stalk of the primary dendrite of unipolar brush cells, distal dendrites of presumptive granule cells, and endings containing pleomorphic synaptic vesicles. These primary somatosensory projections contribute to circuits that are hypothesized to mediate integrative functions of hearing.

The synaptic microcircuitry associated with primary afferent terminals in the interpolaris and caudalis of trigeminal sensory nuclear complex

Previous ultrastructural studies indicating a higher number of axoaxonic contacts on individual low-threshold mechanoreceptive afferents in the principalis (Vp) than in the oralis (Vo) of cat trigeminal sensory nuclear complex (TSNC) suggest that the synaptic microcircuitry associated with primary afferents manifests unique differences across the sensory nuclei of TSNC. To address this issue, we analyzed synaptic microcircuits associated with fast adapting vibrissa afferent terminals in the interpolaris (Vi) and caudalis (Vc, laminae III/IV) by using intraaxonal injections of horseradish peroxidase (HRP) in cats. Forty-two and 65 HRP-labeled boutons were analyzed in the Vi and Vc, respectively. The labeled boutons contained clear, spherical vesicles. They most frequently formed asymmetric axodendritic synapses and were commonly postsynaptic to unlabeled axon terminals containing pleomorphic vesicles (p-endings) with symmetric junctions. The examination of synaptic contacts over the entire surface of individual boutons indicated that the afferent boutons made contacts with an average of two postsynaptic targets in the Vi and Vc. In contrast, axoaxonic contacts, and labeled boutons participating in synaptic triads, where p-endings contacted both the boutons and their postsynaptic targets, were, on average, higher in the Vi than in the Vc. These results suggest that the output of sensory information conveyed through low-threshold mechanoreceptive afferents is more strongly controlled at the level of the first synapse by presynaptic and postsynaptic mechanisms in the Vi responsible for sensory discriminative functions than in the Vc for sensorimotor reflexive functions.

Neural mechanisms controlling jaw-jerk reflex in the cat

Signal substances of axon terminals presynaptic to jaw spindle Ia afferents and their ultrastructural features were examined using a combination of intra-axonal horseradish peroxidase injection and postembedding immunogold-labeling techniques in cats. A total of 35 axon terminals presynaptic to 22 horseradish peroxidase-labeled Ia boutons were examined. Of the 35 presynaptic axon terminals, 14 (40%) were immunoreactive for both gamma-aminobutyric acid and glycine, 9 (26%) for gamma-aminobutyric acid alone and 9 (26%) for glycine alone. The bouton volume, mitochondrial volume, active zone area, and apposed surface area were larger for Ia boutons than for presynaptic axon terminals, while each of the values is similar among the three types of presynaptic axon terminals. These results suggest that gamma-aminobutyric acid and glycine play an important role for modulating the jaw-jerk reflex presynaptically and that the smaller size of presynaptic axon terminals is important to prevent action potential generation from Ia afferents.