Ultrastructural localization and identification of adrenergic and cholinergic nerve terminals in the olfactory mucosa

Pharmacological and ultrastructural methods were used to demonstrate alpha-adrenergic regulation of secretory granule content of acinar cells of Bowman's glands and to localize and identify adrenergic and cholinergic axonal varicosities and terminals in the olfactory mucosa of the tiger salamander. The alpha-adrenergic agonist phenylephrine caused secretory granule depletion from Bowman's glands; the alpha-adrenergic antagonist phentolamine partially blocked this effect. These observations were quantified using light microscopic computer-assisted morphometric techniques. Both drugs caused morphological signs of electrolye/water transport. Adrenergic axonal varicosities were identified by the presence of small granular vesicles (SGVs, 45-60 nm in diameter) containing electron-dense material that was enhanced by 5-hydroxydopamine loading and chromaffin reaction fixation techniques. Throughout the lamina propria, small fascicles with axons containing SGVs as well as varicosities and terminals with SGVs were located adjacent to blood vessels, Bowman's gland acini, and melanocytes. Mean vesicle diameters at these sites were 54 +/- 7 nm, 50 +/- 9 nm, and 56 +/- 8 nm, respectively; varicosities were located approximately 0.1-1.0 microns from their presumed cellular targets. Axonal varicosities containing small agranular vesicles (AGVs, 65 +/- 8 nm in diameter), identified as cholinergic by their size and by the absence of electron-dense material after 5-hydroxydopamine loading and chromaffin reaction fixation, were located between adjacent acinar cells. In addition, adrenergic varicosities containing SGVs (56 +/- 6 nm in diameter) were found within 1 micron of blood vessels associated with Bowman's gland ducts and sustentacular cells near the base of the olfactory epithelium. These results characterize the ultrastructural basis for adrenergic and cholinergic regulation of vasomotor tone and secretion within the olfactory mucosa.     

Cardiac connexins Cx43 and Cx45: formation of diverse gap junction channels with diverse electrical properties

HeLa cells expressing rat connexin43 (Cx43) and/or mouse Cx45 were studied with the dual voltage-clamp technique. Different types of cell pairs were established and their gap junction properties determined, i.e. the dependence of the instantaneous and steady-state conductances (g_j,inst, g_j,ss) on the transjunctional voltage (V_j) and the kinetics of inactivation of the gap junction current (I_j). Pairs of singly transfected cells showed homogeneous behaviour at both V_j polarities. Homotypic Cx43-Cx43 and Cx45-Cx45 cell pairs yielded distinct symmetrical functions g_j,inst=f(V_j) and g_j,ss=f(V_j). Heterotypic Cx43-Cx45 preparations exhibited asymmetric functions g_j,inst=f(V_j) and g_j,ss=f(V_j) suggesting that connexons Cx43 and Cx45 gate with positive and negative V_j, respectively. Preparations containing a singly (Cx43 or Cx45) or doubly (Cx43/45) transfected cell showed quasi-homogeneous behaviour at one V_j polarity and heterogeneous behaviour at the other polarity. The former yielded Boltzmann parameters intermediate between those of Cx43-Cx43, Cx45-Cx45 and Cx43-Cx45 preparations; the latter could not be explained by homotypic and heterotypic combinations of homomeric connexons. Each pair of doubly transfected cells (Cx43/Cx45) yielded unique functions g_j,inst=f(V_j) and g_j,ss=f(V_j). This can not be explained by combinations of homomeric connexons. We conclude that Cx43 and Cx45 form homomeric-homotypic, homomeric-heterotypic channels as well as heteromeric-homotypic and heteromeric-heterotypic channels. This has implications for the impulse propagation in specific areas of the heart.     

Pharmacological characterization of ionotropic glutamate receptors in the zebrafish olfactory bulb

The distribution of N-methyl-D-aspartate- (NMDA) and kainic acid- (KA) sensitive ionotropic glutamate receptors (iGluR) in the zebrafish olfactory bulb was assessed using an activity-dependent labeling method. Olfactory bulbs were incubated with an ion channel permeant probe, agmatine (AGB), and iGluR agonists in vitro, and the labeled neurons containing AGB were visualized immunocytochemically. Preparations exposed to 250 microM KA in the presence of a NMDA receptor antagonist (D-2-amino-5-phosphono-valeric acid) and an alpha-amino-3-hydroxyl-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist (sym 2206), revealed KA receptor-mediated labeling of approximately 60-70% of mitral cells, juxtaglomerular cells, tyrosine hydroxylase-positive cells and granule cells. A higher proportion of ventral olfactory bulb neurons were KA-sensitive. Application of 333 microM NMDA in the presence of an AMPA/KA receptor antagonist (6-cyano-7-nitroquinoxaline-2,3-dione) resulted in NMDA receptor-mediated labeling of almost all neurons. The concentrations eliciting 50% of the maximal response (effective concentration: EC(50)s) for NMDA-stimulated labeling of different cell types were not significantly different and ranged from 148 microM to 162 microM. These results suggest that while NMDA receptors with similar binding affinities are widely distributed in the neurons of the zebrafish olfactory bulb, KA receptors are heterogeneously expressed among these cells and may serve unique roles in different regions of the olfactory bulb.     

Ultrastructural localization of telencephalin, a telencephalon-specific membrane glycoprotein, in rabbit olfactory bulb.

Localization of a telencephalon-specific glycoprotein, telencephalin (TCLN), in the olfactory bulb of the rabbit was studied with an electron microscope. Anti-TCLN antisera appeared to stain plasma membrane, Golgi apparatus and multivesicular bodies of granule cells which are local circuit interneurons in the bulb. Principal neurons, mitral and tufted cells, were not immunoreactive. No glial cells showed immunoreactivity. Thus, expression of telencephalin is specific not only to the telencephalic segment of the brain, but also to the neuronal types.     

Cellular localization of metabotropic glutamate receptors mGluR1, 2/3, 5 and 7 in the main and accessory olfactory bulb of the rat.

The cellular localization of metabotropic glutamate receptors (mGluRs) (mGluR1alpha, 2/3, 5a and 7) in the main and accessory olfactory bulb (MOB and AOB) of adult rats was compared by using affinity purified polyclonal antibodies directed to their C-termini. mGluR1alpha and mGluR5a immunoreactivities were located in comparable structures of the MOB and AOB with different levels of intensity. mGluR5a reactivity was high in the AOB. mGluR2/3 showed a different pattern of expression in the MOB compared to that observed in the AOB; the periglomerular region of the MOB was strongly stained, but in the AOB it was the mitral/tufted cell layer that was intense. The mitral cell bodies in the MOB were strongly immunoreactive for mGluR7. These differences in the distribution of mGluRs in the MOB and AOB may reflect differences in synaptic transmission and sensitivity to neuromodulation in the two systems.     

Heterogeneous expression of endothelial connexin (Cx) 37, Cx40, and Cx43 in rat large veins

Gap junctions are clusters of transmembrane protein channels for intercellular communication and are composed of connexin (Cx). The vascular endothelial cells express Cx37, Cx40, and Cx43. We herein examined the spatial distribution of the endothelial connexins Cx37, Cx40, and Cx43 in rat large veins including the cranial vena cava, thoracic section of the caudal vena cava, and abdominal section of the caudal vena cava. We also examined the mean size of the endothelial cells and quantified the protein expression levels of the endothelial connexins. We found that the large veins heterogeneously expressed Cx37, Cx40, and Cx43 as follows: Cx40 > Cx37 > > Cx43 in the cranial vena cava, Cx37 > Cx43 > > Cx40 in the thoracic section of the caudal vena cava, and Cx40 > Cx43 > > Cx37 in the abdominal section of the caudal vena cava. Double immunostaining of two of the endothelial connexins revealed that the gap-junction plaques were composed of various combinations of endothelial connexins. The mean size of the endothelial cells was large, moderate, or small in the cranial vena cava, the abdominal section of the caudal vena cava, or the thoracic section of the caudal vena cava, respectively. The heterogeneity of the endothelial cells of the rat large veins in terms of the connexin expression suggests that the endothelial cells are differently coupled in the large veins. The present data are useful for investigating, for example, disease-related alterations in expression of endothelial connexins in large veins.     

Localization and developmental expression of GABA(B) receptors in the rat olfactory bulb

In this study, we investigated the distribution and developmental expression of the GABA(B) receptor subunits, GABA(B1) and GABA(B2), in the main and accessory olfactory bulbs of the rat. Antibodies raised against these subunits strongly labelled the glomerular layer, suggesting that olfactory and vomeronasal nerve fibers express functional GABA(B) receptors. Using postembedding immunogold cytochemistry, we found that GABA(B) receptors can be present at both extrasynaptic and presynaptic sites of olfactory nerve terminals, and in the latter case they are preferentially associated with the peripheral part of the synaptic specialization. Olfactory nerve fibers expressed GABA(B1) and GABA(B2) at early developmental stages, suggesting that GABA(B) receptors may play a role in olfactory development. Output and local neurons of the main and accessory olfactory bulbs were also labelled for GABA(B1) and GABA(B2), although the subcellular distribution patterns of the two subunits were not completely overlapping. These results indicate that presynaptically located GABA(B) receptors modulate neurotransmitter release from olfactory and vomeronasal nerve fibers and that, in addition to this presynaptic role, GABA(B) receptors may regulate neuronal excitability in infraglomerular circuits.     

Ultrastructural characterisation of the olfactory mucosa of the armadillo Dasypus hybridus (Dasypodidae, Xenarthra)

The ultrastructure of the olfactory mucosa of the armadillo Dasypus hybridus was studied. A comparison with the olfactory mucosa of another armadillo ( Chaetophractus villosus ) was made. The olfactory mucosa of D. hybridus shows many features which are similar to those of other mammals. Interestingly, it differs from the olfactory mucosa of the armadillo C. villosus . A suggestion is made that these differences may be due to differences in the digging habits of these species. In Dasypus , the supporting cells (SCs) showed dense vacuoles, multivesicular bodies and lysosome-like bodies probably related with the endocytotic system. The SCs show a dense network of SER presumably associated with xenobiotic mechanisms. The olfactory receptor neurons exhibit lysosome-like bodies and multivesicular bodies in their perikarya. These organelles suggest the presence of an endocytotic system. Duct cells of Bowman's glands exhibit secretory activities. Bowman's glands are compound-branched tubulo-acinar mixed glands with merocrine secretory mechanisms.     

Ouabain-induced changes in extracellular aspartate, glutamate and GABA levels in the rabbit olfactory bulb in vivo

The effect of ouabain on extracellular amino acid levels was investigated in the rabbit olfactory bulb using brain dialysis. Extracellular field potentials, elicited by stimulation of the lateral olfactory tract (LOT), were recorded simultaneously. Ouabain (100 microM) induced a rapid increase in extracellular aspartate, glutamate and gamma-aminobutyric acid. LOT-evoked potentials changed concomitantly, suggesting a neuronal depolarization.     

Identification and localization of dopamine receptor subtypes in rat olfactory mucosa and bulb: a combined in situ hybridization and ligand binding radioautographic approach

Olfactory bulb (OB) of mammals contains a large population of dopaminergic interneurons within the glomerular layer. Dopamine has been shown in vivo to modulate several aspects of olfactory information processing. The dopamine receptors of olfactory bulb and mucosa are assessed here at the levels of mRNAs and radioligand binding sites with presently available tools. D1A mRNA was found in OB glomerular-, plexiform-, mitral-cell and granular layers, but not in olfactory mucosa. D1B mRNA was absent in olfactory bulb and mucosa. D1-like binding sites were detected with two distinct radioligands, in glomerular-, plexiform-, mitral cell- and granular layers of OB but not in olfactory mucosa. We thus demonstrate the previously doubtful presence of D1-like receptors in OB. D2 mRNAs were localized in the glomerular and granular layers of OB and in olfactory mucosa; lesser amounts of D3 mRNAs were found in OB glomerular and granular layer, but not in olfactory mucosa. No D4 mRNA was detected in either structure. High densities of D2-like, [¹²⁵I]Iodosulpride-labelled binding sites, were revealed within lamina propria of olfactory mucosa, and confirmed in the olfactory nerve- and glomerular layers of OB. A faint but significant density of [³H]7-hydroxy-dipropyl-aminotetralin (OH-DPAT) labelled, D3 binding sites was detected in olfactory nerve- and glomerular layers of OB, but not in olfactory mucosa. Competition of [¹²⁵I]Iodosulpride specific binding by three D2/D3 selective drugs yielded kinetics typical of the D2 receptor subtype in olfactory bulb and mucosa. Olfactory nerve- and glomerular layers of OB are proved thus to contain a predominant contingent of D2 receptors and a minor population of D3 receptors, while olfactory mucosa expresses only D2 receptors.     

Ultrastructural immunocytochemical localization of B-50/GAP43, a protein kinase C substrate, in isolated presynaptic nerve terminals and neuronal growth cones

Summary Accumulating evidence indicates that the neuron-specific B-50/GAP43, a substrate for protein kinase C, plays a role in neuronal differentiation and neuritogenesis during nervous tissue development and axonal regeneration. An ultrastructural immunocytochemical study on the localization of B-50 in presynaptic terminals (synaptosomes) and neuronal growth cones was carried out by means of cryoultramicrotomy with affinity-purified B-50 antibodies. Detection was accomplished with colloidal gold, conjugated either to protein-A or goat anti-rabbit immunoglobulins. In synaptosomes, isolated from the frontal cortex of 6-week-old rats, and in neuronal growth cones, isolated from forebrains of 5-day-old rats, the majority of B-50 is detected at the surrounding neuronal plasma membrane. In both neuronal growth cones and synaptosomes, a relatively small fraction of B-50 in the cytoplasm was not evidently associated with internal membranes. Our results indicate that B-50 is mainly located at the cytoplasmic face of the synaptosomal and neuronal growth cone plasma membrane. The similar B-50 localization in neuronal growth cones and synaptosomes suggests that, both in extending axons and mature synaptic terminals, B-50 may exert identical functions as a protein kinase C substrate at the plasma membrane.     

Immunolocalization of B-50 (GAP-43) in the mouse olfactory bulb: Predominant presence in preterminal axons

Summary Because the growth-associated protein B-50 (GAP-43) has been implicated in neurite outgrowth as well as in synaptic plasticity, we studied its light and electron microscopical distribution in the mouse olfactory bulb, an area of the nervous system which exhibits a high degree of synaptic plasticity. Immunofluorescent staining with monospecific affinity-purified anti-B-50 antibodies revealed that B-50 is most abundantly expressed in the olfactory nerve fibre layer and the granule cell layer neuropil, while little staining was observed in the external plexiform layer and in cell bodies. B-50 is absent from dendrites and myelinated axons as indicated by double labelling with monoclonal antibodies against microtubule-associated protein 2 and the large neurofilament protein, respectively. Using post-embedding immunogold labelling on ultrathin Lowicryl sections, B-50 was found to be highly concentrated in presumed growth cones in the olfactory nerve fibre layer and in thin unmyelinated axons and presynaptic terminals in the granule cell layer neuropil. Near background immunolabelling was seen in perikarya, dendrites and myelinated axons. In view of the implication of B-50 in plasticity-related phenomena, its abundance in the thin unmyelinated preterminal axons suggests that these are potential sites of extrasynaptic plasticity.     

Calcium permeable AMPA receptors and autoreceptors in external tufted cells of rat olfactory bulb

Glomeruli are functional units of the olfactory bulb responsible for early processing of odor information encoded by single olfactory receptor genes. Glomerular neural circuitry includes numerous external tufted (ET) cells whose rhythmic burst firing may mediate synchronization of bulbar activity with the inhalation cycle. Bursting is entrained by glutamatergic input from olfactory nerve terminals, so specific properties of ionotropic glutamate receptors on ET cells are likely to be important determinants of olfactory processing. Particularly intriguing is recent evidence that AMPA receptors of juxta-glomerular neurons may permeate calcium. This could provide a novel pathway for regulating ET cell signaling. We tested the hypothesis that ET cells express functional calcium-permeable AMPA receptors. In rat olfactory bulb slices, excitatory postsynaptic currents (EPSCs) in ET cells were evoked by olfactory nerve shock, and by uncaging glutamate. We found attenuation of AMPA/kainate EPSCs by 1-naphthyl acetyl-spermine (NAS), an open-channel blocker specific for calcium permeable AMPA receptors. Cyclothiazide strongly potentiated EPSCs, indicating a major contribution from AMPA receptors. The current-voltage (I-V) relation of uncaging EPSCs showed weak inward rectification which was lost after > approximately 10 min of whole-cell dialysis, and was absent in NAS. In kainate-stimulated slices, Co(2+) ions permeated cells of the glomerular layer. Large AMPA EPSCs were accompanied by fluorescence signals in fluo-4 loaded cells, suggesting calcium permeation. Depolarizing pulses evoked slow tail currents with pharmacology consistent with involvement of calcium permeable AMPA autoreceptors. Tail currents were abolished by Cd(2+) and (+/-)-4-(4-aminophenyl)-2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX), and were sensitive to NAS block. Glutamate autoreceptors were confirmed by uncaging intracellular calcium to evoke a large inward current. Our results provide evidence that calcium permeable AMPA receptors reside on ET cells, and are divided into at least two functionally distinct pools: postsynaptic receptors at olfactory nerve synaptic terminals, and autoreceptors sensitive to glutamate released from dendrodendritic synapses.     

Olfactory nerve stimulation-evoked mGluR1 slow potentials, oscillations, and calcium signaling in mouse olfactory bulb mitral cells

Fast synaptic transmission between olfactory receptor neurons and mitral cells (MCs) is mediated through AMPA and NMDA ionotropic glutamate receptors. MCs also express high levels of metabotropic glutamate receptor 1 (mGluR1) whose functional significance is less understood. Here we characterized a slow mGluR1-mediated potential that was evoked by high-frequency (100-Hz) olfactory nerve (ON) stimulation in the presence of NBQX and D-APV, blockers of ionotropic glutamate receptors, and that was associated with a local Ca2+ transient in the MC dendritic tuft. High-frequency ON stimulation in the presence of NBQX and D-APV also evoked a slow, nearly 2-Hz oscillation of MC membrane potential that was abolished by the mGluR1 antagonist LY367385 (50 microM). Both mGluR slow potential and slow oscillation persisted in the presence of gabazine (10 microM), a GABA(A) receptor antagonist, and intracellular QX-314 (10 mM), a Na+ channel blocker. In contrast to a slow mGluR1 potential in cerebellar Purkinje neurons, the MC mGluR1 potential was not depressed by SKF96365 (< or =250 microM) and thus is likely not mediated by TRPC1 cation channels, nor was it potentiated by an elevation of intracellular Ca2+ level. Imaging with the Na+ indicator SBFI revealed a Na+ transient in the MC dendrite accompanying the mGluR1 slow potential. We conclude that the MC mGluR1 potential triggered by glutamate released from the ON supports oscillations and synchronizations of MCs associated within one glomerulus.     

Ultrastructural immunohistochemical localization of gastrin,somatostatin and serotonin in endocrine cells of human antral gastric mucosa

Five types of endocrine cells are found in the human antral gastric mucosa: gastrin (G) cells, somatostatin (D) cells, enterochromaffin (EC) cells and cells with an unknown secretory product (D1 cells and P cells). The content of secretory granules, gastrin, somatostatin and serotonin, was evaluated using electron microscopic immunohistochemistry and was compared with the granular content in G cells, D cells and EC cells as determined by routine electron microscopy. Semi-quantitative scoring of the granular content was performed on a scale 1-4 (empty-full). The content of gastrin (2.5 +/- 0.2) and somatostatin (3.3 +/- 0.2) in the granules was not different from the granular content in G cells (2.5 +/- 0.3; p > 0.05) and D cells (3.5 +/- 0.2; p > 0.05). Gastrin was also found in G cells in a nongranular form. The content of serotonin in granules (2.8 +/- 0.3) was smaller than the granular content in EC cells (3.7 +/- 0.2; p < 0.05). In intermediate-full and intermediate-empty granules, serotonin was localized in the periphery of granules whereas the granular content in EC cells was localized in an eccentric or central pattern. The granular content of D1 cells and P cells was 3.8 +/- 0.2, and 3.4 +/- 0.2, respectively. It is concluded that gastrin and somatostatin immunostaining in granules of G cells and D cells reflects the granular content in G cells and D cells, respectively, whereas serotonin immunostaining does not agree with the granular content of EC cells.     

Distribution and synaptic localisation of the metabotropic glutamate receptor 4 (mGluR4) in the rodent CNS

Group III metabotropic glutamate receptors (mGluRs) are selectively activated by L-2-amino-4-phosphonobutyrate (L-AP4), which produces depression of synaptic transmission. The relative contribution of different group III mGluRs to the effects of L-AP4 remains to be clarified. Here, we assessed the distribution of mGluR4 in the rat and mouse brain using affinity-purified antibodies raised against its entire C-terminal domain. The antibodies reacted specifically with mGluR4 and not with other mGluRs in transfected COS 7 cells. No immunoreactivity was detected in brains of mice with gene-targeted deletion of mGluR4. Pre-embedding immunocytochemistry for light and electron microscopy showed the most intense labelling in the cerebellar cortex, basal ganglia, the sensory relay nuclei of the thalamus, and some hippocampal areas. Immunolabelling was most intense in presynaptic active zones. In the basal ganglia, both the direct and indirect striatal output pathways showed immunolabelled terminals forming mostly type II synapses on dendritic shafts. The localisation of mGluR4 on GABAergic terminals of striatal projection neurones suggests a role as a presynaptic heteroreceptor. In the cerebellar cortex and hippocampus, mGluR4 was also localised in terminals establishing type I synapses, where it probably operates as an autoreceptor. In the hippocampus, mGluR4 labelling was prominent in the dentate molecular layer and CA1-3 strata lacunosum moleculare and oriens. Somatodendritic profiles of some stratum oriens/alveus interneurones were richly decorated with mGluR4-labelled axon terminals making either type I or II synapses. This differential localisation suggests a regulation of synaptic transmission via a target cell-dependent synaptic segregation of mGluR4.Our results demonstrate that, like other group III mGluRs, presynaptic mGluR4 is highly enriched in the active zone of boutons innervating specific classes of neurones. In addition, the question of alternatively spliced mGluR4 isoforms is discussed.     

Ultrastructural localization of enkephalin and mu-opioid receptors in the rat ventral tegmental area

Enkephalins are endogenous ligands for opioid receptors whose activation potently modulates the output of mesocorticolimbic dopaminergic neurons within the ventral tegmental area. Many of the reinforcing effects of enkephalins in the mesocorticolimbic system are mediated by mu-opioid receptors. To determine the sites for Leu(5)-enkephalin activation of mu-opioid receptors in the ventral tegmental area, we examined the dual electron microscopic immunocytochemical localization of their respective antigens in this region of rat brain. Enkephalin immunoperoxidase reaction product and mu-opioid receptor immunogold-silver labeling showed similar cellular and subcellular distribution in both the paranigral and parabrachial subdivisions of the ventral tegmental area. Enkephalin immunoreactivity was mainly localized in small unmyelinated axons (50.4%) and in axon terminals (40.4%). The majority of these terminals formed symmetric, inhibitory-type synapses, many of which were on dendrites expressing plasmalemmal mu-opioid receptors. Appositional contacts were also often seen between axons or terminals that were differentially labeled for the two antigens. In addition, some of the enkephalin-labeled terminals and a few somatodendritic profiles showed a plasmalemmal or vesicular localization of mu-opioid receptors.Our results indicate that dendritic targets of inhibitory terminals, as well as nearby axon terminals, are potential sites for enkephalin activation of mu-opioid receptors throughout the ventral tegmental area. Moreover, co-localization of enkephalin and mu-opioid receptors in selective neuronal profiles may indicate an autoregulatory role for these receptors or their internalization along with the bound ligand in this brain region.     

Merkel cells, corpuscular nerve endings and free nerve endings in the mouse palatine mucosa express three subtypes of vesicular glutamate transporters

The hard palate of rodents is a mucous membrane covered by a keratinized epithelium that typically contains Merkel cell (MC)-neurite complexes. MCs have engendered considerable research activity because of their involvement in mechanoreception and possibly also Merkel cell carcinomas. MCs derive from the neural crest, differentiate under control of peripheral nerve factors, are enriched in large dense core vesicles, and secrete neuropeptides and other neuroactive molecules. Upon stimulation, MC-neurite complexes produce slowly adapting type I responses. Here we emphasize that the murine hard palate is a highly differentiated sensory region, as shown by intravital staining with a styryl dye and immunocytochemistry with antibodies to vesicular glutamate transporters (VGLUTs). The entire palate contained densities of sensory endings and MC-neurite complexes, that nearly paralleled in abundance the vibrissal pads. MCs were differentially distributed in the murine palate; clusters of MCs were most abundant in the antemolar and intermolar rugae, while individual MCs were particularly enriched in the rugae at the mid-portion of the palate and in the postrugal field. VGLUT1, VGLUT2 and VGLUT3 were expressed in MCs throughout, although immunostained MCs were most frequently encountered in intermolar than antemolar rugae. The same transporters were also present in corpuscular endings at the summit of the rugae and in intraepithelial free nerve endings throughout the palate. VGLUTs presumably load glutamate into large dense core vesicles in MCs and into small clear vesicles in corpuscular and free nerve endings. The data suggest that glutamate release, or co-release, is likely to represent an important functional aspect of palatine Merkel cells and neighboring corpuscular and free nerve endings.