Most peptide-containing sensory neurons lack proteins for exocytotic release and vesicular transport of glutamate

Glutamatergic functions of primary afferent neurons with special emphasis on vagal afferents

Glutamate has been identified as the main transmitter of primary afferent neurons. This was established based on biochemical, electrophysiological, and immunohistochemical data from studies on glutamatergic receptors and their agonists/antagonists. The availability of specific antibodies directed against glutamate and, more recently, vesicular glutamate transporters corroborated this and led to significant new discoveries. In particular, peripheral endings of various classes of afferents contain vesicular glutamate transporters, suggesting vesicular storage in and exocytotic release of glutamate from peripheral afferent endings. This suggests that autocrine mechanisms regulate sensory transduction processes. However, glutamate release from peripheral sensory terminals could also enable afferent neurons to influence various cells associated with them. This may be particularly relevant for vagal intraganglionic laminar endings, which could represent glutamatergic sensor-effector components of intramural reflex arcs in the gastrointestinal tract. Thus, morphological analysis of the relationships of putative glutamatergic primary afferents with associated tissues may direct forthcoming studies on their functions.

Galanin receptor 1 is expressed in a subpopulation of glutamatergic interneurons in the dorsal horn of the rat spinal cord

The 29/30 amino acid neuropeptide galanin has been implicated in pain processing at the spinal level and local dorsal horn neurons expressing the Gal(1) receptor may play a critical role. In order to determine the transmitter identity of these neurons, we used immunohistochemistry and antibodies against the Gal(1) receptor and the three vesicular glutamate transporters (VGLUTs), as well as in situ hybridization, to explore a possible glutamatergic phenotype. Gal(1) protein, which could not be demonstrated in Gal(1) knockout mice, colocalized with VGLUT2 protein, but not with glutamate decarboxylase, in many nerve endings in lamina II. Moreover, Gal(1) and VGLUT2 transcripts were often found in the same cell bodies in laminae I-IV. Gal(1)-protein and galanin-peptide showed an overlapping distribution but were not colocalized. Gal(1) staining did not appear to be affected by dorsal rhizotomy. Taken together, these findings provide strong evidence that Gal(1) is a heteroreceptor expressed on excitatory glutamatergic dorsal horn interneurons. Activation of such Gal(1) receptors may thus decrease the inhibitory tone in the superficial dorsal horn, and possibly cause antinociception.

Two types of neurotransmitter release patterns in isolectin B4-positive and negative trigeminal ganglion neurons

Mammalian nociceptors have been classified into subclasses based on differential neurotrophin sensitivity and binding of the plant isolectin B4 (IB4). Most of the nerve growth factor-responsive IB4-negative (IB4 (-)) nociceptors contain neuropeptides such as substance P and calcitonin gene-related peptide, whereas the glial-derived neurotrophic factor-responsive IB4-positive (IB4 (+)) neurons predominantly lack such neuropeptides. We hypothesized that the differences in neuropeptide content between IB4 (+) and (-) neurons might be reflected in differences in stimulated exocytosis and/or endocytosis. To address this, we monitored the secretory activity of acutely dissociated neurons from adult rat trigeminal ganglia (TRG) using cell membrane capacitance (Cm) measurements and the fluorescent membrane-uptake marker N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino)phenyl)hexatrienyl)pyridinium dibromide (FM4-64). Cm measurements were performed under whole-cell voltage clamp and neurons were depolarized from -75 mV to +10 mV to elicit exocytosis. Both types of TRG neurons showed similarly-sized, calcium-dependent increases in Cm, demonstrating that both IB4 (+) and (-) TRG neurons are capable of stimulated exocytosis. However, the peak Cm of IB4 (+) neurons decayed faster toward baseline than that of IB4 (-) neurons. Also, IB4 (+) neurons had stable Cm responses to repeated stimuli whereas IB4 (-) neurons loss their secretory response during repeated stimulation. These data suggested that the IB4 (+) neurons possess a faster rate of endocytosis and vesicle replenishment than IB4 (-) neurons. To test this, we measured vesicle trafficking with the fluorescent membrane dye FM4-64. FM4-64 staining showed that IB4 (-) neurons exhibit a larger pool of endocytosed vesicles than IB4 (+) neurons because the peak fluorescence increases in IB4 (-) neurons were larger but slower than in IB4 (+) neurons. However, the recycled vesicles were released faster in IB4 (+) compared with IB4 (-) neurons. Taken together these data suggest that the IB4 (+) TRG neurons have faster exocytosis and endocytosis than the IB4 (-) neurons.

Potentiation of mouse vagal afferent mechanosensitivity by ionotropic and metabotropic glutamate receptors

Glutamate acts at central synapses via ionotropic (iGluR--NMDA, AMPA and kainate) and metabotropic glutamate receptors (mGluRs). Group I mGluRs are excitatory whilst group II and III are inhibitory. Inhibitory mGluRs also modulate peripherally the mechanosensitivity of gastro-oesophageal vagal afferents. Here we determined the potential of excitatory GluRs to play an opposing role in modulating vagal afferent mechanosensitivity, and investigated expression of receptor subunit mRNA within the nodose ganglion. The responses of mouse gastro-oesophageal vagal afferents to graded mechanical stimuli were investigated before and during application of selective GluR ligands to their peripheral endings. Two types of vagal afferents were tested: tension receptors, which respond to circumferential tension, and mucosal receptors, which respond only to mucosal stroking. The selective iGluR agonists NMDA and AMPA concentration-dependently potentiated afferent responses. Their corresponding antagonists AP-5 and NBQX alone attenuated mechanosensory responses as did the non-selective antagonist kynurenate. The kainate selective agonist SYM-2081 had minor effects on mechanosensitivity, and the antagonist UBP 302 was ineffective. The mGluR5 antagonist MTEP concentration-dependently inhibited mechanosensitivity. Efficacy of agonists and antagonists differed on mucosal and tension receptors. We conclude that excitatory modulation of afferent mechanosensitivity occurs mainly via NMDA, AMPA and mGlu5 receptors, and the role of each differs according to afferent subtypes. PCR data indicated that all NMDA, kainate and AMPA receptor subunits plus mGluR5 are expressed, and are therefore candidates for the neuromodulation we observed.

Structure of peripheral synapses: autonomic ganglia

Final motor neurons in sympathetic and parasympathetic ganglia receive synaptic inputs from preganglionic neurons. Quantitative ultrastructural analyses have shown that the spatial distribution of these synapses is mostly sparse and random. Typically, only about 1%-2% of the neuronal surface is covered with synapses, with the rest of the neuronal surface being closely enclosed by Schwann cell processes. The number of synaptic inputs is correlated with the dendritic complexity of the target neuron, and the total number of synaptic contacts is related to the surface area of the post-synaptic neuron. Overall, most neurons receive fewer than 150 synaptic contacts, with individual preganglionic inputs providing between 10 and 50 synaptic contacts. This variation is probably one determinant of synaptic strength in autonomic ganglia. Many neurons in prevertebral sympathetic ganglia receive additional convergent synaptic inputs from intestinofugal neurons located in the enteric plexuses. The neurons support these additional inputs via larger dendritic arborisations together with a higher overall synaptic density. There is considerable neurochemical heterogeneity in presynaptic boutons. Some synapses apparently lack most of the proteins normally required for fast transmitter release and probably do not take part in conventional ganglionic transmission. Furthermore, most preganglionic boutons in the ganglionic neuropil do not form direct synaptic contacts with any neurons. Nevertheless, these boutons may well contribute to slow transmission processes that need not require conventional synaptic structures.

Differential expression of synaptoporin and synaptophysin in primary sensory neurons and up-regulation of synaptoporin after peripheral nerve injury

Synaptoporin and synaptophysin are integral membrane components of synaptic vesicles. The distribution of synaptoporin and its relationship with synaptophysin in sensory afferent fibers remain unclear. In the present study, we showed that in the rat dorsal root ganglia synaptoporin was expressed in subsets of small neurons that contain either calcitonin gene-related peptide or isolectin B4, and was distributed in their afferent terminals in laminae I-II of the spinal cord. Synaptophysin was expressed in 57% of synaptoporin-containing small dorsal root ganglion neurons and in large dorsal root ganglion neurons. In the spinal dorsal horn, synaptophysin-immunolabeling was weak in the afferent fibers in lamina I, outer lamina II and the dorsal part of inner lamina II, but strong in the afferent fibers in laminae III-IV. However, a subpopulation of isolectin B4-positive small dorsal root ganglion neurons expressed both synaptoporin and synaptophysin, and their afferent fibers were mainly distributed in the ventral part of inner lamina II. After peripheral nerve injury, synaptoporin expression was up-regulated in small dorsal root ganglion neurons, and synaptoporin level was increased in their afferent terminals. Thus, synaptoporin and synaptophysin have topographically distinct distributions in afferent fibers. Synaptoporin is a major synaptic vesicle protein in Adelta- and C-fibers in both physiological and neuropathic pain states.

Distribution of vesicular glutamate transporters 1 and 2 in the rat spinal cord, with a note on the spinocervical tract

To evaluate whether the organization of glutamatergic fibers systems in the lumbar cord is also evident at other spinal levels, we examined the immunocytochemical distribution of vesicle glutamate transporters 1 and 2 (VGLUT1, VGLUT2) at several different levels of the rat spinal cord. We also examined the expression of VGLUTs in an ascending sensory pathway, the spinocervical tract, and colocalization of VGLUT1 and VGLUT2. Mainly small VGLUT2-immunoreactive varicosities occurred at relatively high densities in most areas, with the highest density in laminae I-II. VGLUT1 immunolabeling, including small and medium-sized to large varicosities, was more differentiated, with the highest density in the deep dorsal horn and in certain nuclei such as the internal basilar nucleus, the central cervical nucleus, and the column of Clarke. Lamina I and IIo displayed a moderate density of small VGLUT1 varicosities at all spinal levels, although in the spinal enlargements a uniform density of such varicosities was evident throughout laminae I-II in the medial half of the dorsal horn. Corticospinal tract axons displayed VGLUT1, indicating that the corticospinal tract is an important source of small VGLUT1 varicosities. VGLUT1 and VGLUT2 were cocontained in small numbers of varicosities in laminae III-IV and IX. Anterogradely labeled spinocervical tract terminals in the lateral cervical nucleus were VGLUT2 immunoreactive. In conclusion, the principal distribution patterns of VGLUT1 and VGLUT2 are essentially similar throughout the rostrocaudal extension of the spinal cord. The mediolateral differences in VGLUT1 distribution in laminae I-II suggest dual origins of VGLUT1-immunoreactive varicosities in this region.

Nicotinamide adenine dinucleotide is released from sympathetic nerve terminals via a botulinum neurotoxin A-mediated mechanism in canine mesenteric artery

Using high-performance liquid chromatography techniques with fluorescence and electrochemical detection, we found that beta-nicotinamide adenine dinucleotide (beta-NAD) is released in response to electrical field stimulation (4-16 Hz, 0.3 ms, 15 V, 120 s) along with ATP and norepinephrine (NE) in the canine isolated mesenteric arteries. The release of beta-NAD increases with number of pulses/stimulation frequencies. Immunohistochemistry analysis showed dense distribution of tyrosine hydroxylase-like immunoreactivity (TH-LI) and sparse distribution of TH-LI-negative nerve processes, suggesting that these blood vessels are primarily under sympathetic nervous system control with some contribution of other (e.g., sensory) neurons. Exogenous NE (3 micromol/l), alpha,beta-methylene ATP (1 micromol/l), neuropeptide Y (NPY, 0.1 micromol/l), CGRP (0.1 micromol/l), vasoactive intestinal peptide (VIP, 0.1 micromol/l), and substance P (SP, 0.1 micromol/l) had no effect on the basal release of beta-NAD, suggesting that the overflow of beta-NAD is evoked by neither the sympathetic neurotransmitters NE, ATP, and NPY, nor the neuropeptides CGRP, VIP, and SP. Botulinum neurotoxin A (BoNTA, 0.1 micromol/l) abolished the evoked release of NE, ATP, and beta-NAD at 4 Hz, suggesting that at low levels of neural activity, release of these neurotransmitters results from N-ethylmaleimide-sensitive factor attachment protein receptor/synaptosomal-associated protein of 25 kDa-mediated exocytosis. At 16 Hz, however, the evoked release of NE, ATP, and beta-NAD was reduced by BoNTA by approximately 90, 60, and 80%, respectively, suggesting that at higher levels of neural activity, beta-NAD is likely to be released from different populations of synaptic vesicles or different populations of nerve terminals (i.e., sympathetic and sensory terminals).

Vesicular glutamate transporter 1 immunoreactivity in extrinsic and intrinsic innervation of the rat esophagus

Encouraged by the recent finding of vesicular glutamate transporter 2 (VGLUT2) immunoreactivity (-ir) in intraganglionic laminar endings (IGLEs) of the rat esophagus, we investigated also the distribution and co-localization patterns of VGLUT1. Confocal imaging revealed substantial co-localization of VGLUT1-ir with selective markers of IGLEs, i.e., calretinin and VGLUT2, indicating that IGLEs contain both VGLUT1 and VGLUT2 within their synaptic vesicles. Besides IGLEs, we found VGLUT1-ir in both cholinergic and nitrergic myenteric neuronal cell bodies, in fibers of the muscularis mucosae, and in esophageal motor endplates. Skeletal neuromuscular junctions, in contrast, showed no VGLUT1-ir. We also tested for probable co-localization of VGLUT1-ir with markers of extrinsic and intrinsic esophageal innervation and glia. Within the myenteric neuropil we found, besides co-localization of VGLUT1 and substance P, no further co-localization of VGLUT1-ir with any of these markers. In the muscularis mucosae some VGLUT1-ir fibers were shown to contain neuronal nitric oxide synthase (nNOS)-ir. VGLUT1-ir in esophageal motor endplates was partly co-localized with vesicular acetylcholine transporter (VAChT)/choline acetyltransferase (ChAT)-ir, but VGLUT1-ir was also demonstrated in separately terminating fibers at motor endplates co-localized neither with ChAT/VAChT-ir nor with nNOS-ir, suggesting a hitherto unknown glutamatergic enteric co-innervation. Thus, VGLUT1-ir was found in extrinsic as well as intrinsic innervation of the rat esophagus.

Post-stimulus potentiation of transmission in pelvic ganglia enhances sympathetic dilatation of guinea-pig uterine artery in vitro

Vasodilatation produced by stimulation of preganglionic neurones in lumbar and sacral pathways to pelvic ganglia was studied using an in vitro preparation of guinea-pig uterine artery and associated nerves in a partitioned bath allowing selective drug application to the ganglia or artery. Arterial diameter was monitored using real time video imaging. Vasodilatations produced by hypogastric nerve stimulation (HN; 300 pulses, 10 Hz) were significantly larger and longer in duration than with pelvic nerve stimulation (N = 18). Stimulation of ipsilateral lumbar splanchnic nerves or ipsilateral third lumbar ventral roots also produced prolonged vasodilatations. Blockade of ganglionic nicotinic receptors (0.1-1 mM hexamethonium) delayed the onset and sometimes reduced the peak amplitude of dilatations, but slow dilatations persisted in 16 of 18 preparations. These dilatations were not reduced further by 3 microM capsaicin applied to the artery and ganglia, or ganglionic application of 1 microM hyoscine, 30-100 microM suramin or 10 microM CNQX. Dilatations were reduced slightly by ganglionic application of NK1 and NK3 receptor antagonists (SR140333, SR142801; 1 microM), but were reduced significantly by bathing the ganglia in 0.5 mM Ca2+ and 10 mM Mg2+. Intracellular recordings of paracervical ganglion neurones revealed fast excitatory postsynaptic potentials (EPSPs) in all neurones on HN stimulation (300 pulses, 10 Hz), and slow EPSPs (3-12 mV amplitude) in 25 of 37 neurones. Post-stimulus action potential discharge associated with slow EPSPs occurred in 16 of 37 neurones (firing rate 9.4 +/- 1.5 Hz). Hexamethonium (0.1-1 mM) abolished fast EPSPs. Hexamethonium and hyoscine (1 microM) did not reduce slow EPSPs and associated post-stimulus firing in identified vasodilator neurones (with VIP immunoreactivity) or non-vasodilator paracervical neurones. These results demonstrate a predominantly sympathetic origin of autonomic pathways producing pelvic vasodilatation in females. Non-cholinergic mediators of slow transmission in pelvic ganglia produce prolonged firing of postganglionic neurones and long-lasting dilatations of the uterine artery. This mechanism would facilitate maintenance of pelvic vasodilatation on stimulation of preganglionic neurones during sexual activity.