Differential distribution of vesicular glutamate transporters in the rat cerebellar cortex

The transmembrane semaphorin Sema6A controls cerebellar granule cell migration

The transmembrane semaphorin protein Sema6A is broadly expressed in the developing nervous system. Sema6A repels several classes of developing axons in vitro and contributes to thalamocortical axon guidance in vivo. Here we show that during cerebellum development, Sema6A is selectively expressed by postmitotic granule cells during their tangential migration in the deep external granule cell layer, but not during their radial migration. In Sema6A-deficient mice, many granule cells remain ectopic in the molecular layer where they differentiate and are contacted by mossy fibers. The analysis of ectopic granule cell morphology in Sema6a-/- mice, and of granule cell migration and neurite outgrowth in cerebellar explants, suggests that Sema6A controls the initiation of granule cell radial migration, probably through a modulation of nuclear and/or soma translocation. Finally, the analysis of mouse chimeras suggests that this function of Sema6A is primarily non-cell-autonomous.

Vesicular glutamate transporter-1 colocalizes with endogenous opioid peptides in axon terminals of the rat locus coeruleus

We have previously shown that a subset of axon terminals in the locus coeruleus (LC) containing methionine(5)-enkephalin (ENK) forms type I (asymmetric-type) synaptic specializations that are characteristic of excitatory-type transmitters. In addition, we previously provided ultrastructural evidence showing that ENK is colocalized with glutamate using a combination of pre- and postembedding immunohistochemistry. To examine cellular substrates for interactions between glutamate and other endogenous opioid peptides in the LC, we examined the localization of the vesicular glutamate transporter 1 (VGLUT1), a transporter protein involved in the accumulation of the transmitter glutamate into synaptic vesicles, with either ENK or preprodynorphin (ppDYN). Dual-immunofluorescence and electron microscopy showed prominent coexistence of VGLUT1 and ENK in varicose processes of the LC, confirming our previous report using postembedding immunolabeling for glutamate. Likewise, VGLUT1 and ppDYN were identified in common varicose processes in the LC using confocal fluorescence microscopy. Immunoelectron microscopy using gold-silver labeling for VGLUT1 and peroxidase labeling for ppDYN established that this endogenous opioid peptide also colocalizes with glutamate transporters. The majority of these formed asymmetric-type synapses. Taken together, these results demonstrate that excitatory LC afferents are enriched with endogenous opioid peptides and are positioned to modulate LC neuronal activity dually.

Frequent coexpression of the vesicular glutamate transporter 1 and 2 genes, as well as coexpression with genes for choline acetyltransferase or glutamic acid decarboxylase in neurons of rat brain

It is widely believed that expression of the vesicular glutamate transporter genes VGLUT1 and VGLUT2 is restricted to glutamatergic neurons and that the two transporters segregate in different sets of neurons. Using single-cell multiplex RT-PCR (sc-RT-mPCR), we show that VGLUT1 and VGLUT2 mRNAs were coexpressed in most of the sampled neurons from the rat hippocampus, cortex, and cerebellum at postnatal Day (P)14 but not P60. In accordance, changes in VGLUT1 and VGLUT2 mRNA concentrations were found to occur in these and other brain areas between P14 and P60, as revealed by semiquantitative RT-PCR and quantitated by ribonuclease protection assay. VGLUT1 and -2 coexpression in the hippocampal formation is supported further by in situ hybridization data showing that virtually all cells in the CA1-CA3 pyramidal and granule cell layers were highly positive for both transcripts until P14. It was revealed using sc-RT-mPCR that transcripts for VGLUT1 and VGLUT2 were also present in neurons of the cerebellum, striatum, and septum that expressed markers for gamma-aminobutyric acid (GABA)ergic or cholinergic phenotypes, as well as in hippocampal cells containing transcripts for the glial fibrillary acidic protein. Our study suggests that VGLUT1 and VGLUT2 proteins may often transport glutamate into vesicles within the same neuron, especially during early postnatal development, and that they are expressed widely in presumed glutamatergic, GABAergic, and cholinergic neurons, as well as in astrocytes. Furthermore, our study shows that such coexpressing neurons remain in the adult brain and identifies several areas that contain them in both young and adult rats.

Vglut2 afferents to the medial prefrontal and primary somatosensory cortices: a combined retrograde tracing in situ hybridization study [corrected]

Glutamate transmission is critical for controlling cortical activity, but the specific contribution of the different isoforms of vesicular glutamate transporters in subcortical pathways to the neocortex is largely unknown. To determine the distribution and neocortical projections of vesicular glutamate transporter2 (Vglut2)-containing neurons, we used in situ hybridization and injections of the retrograde tracer Fluoro-Gold into the medial prefrontal and primary somatosensory cortices. The thalamus contains the majority of Vglut2 cells projecting to the neocortex (approximately 90% for the medial prefrontal cortex and 96% for the primary somatosensory cortex) followed by the hypothalamus and basal forebrain, the claustrum, and the brainstem. There are significantly more Vglut2 neurons projecting to the medial prefrontal cortex than to the primary somatosensory cortex. The medial prefrontal cortex also receives a higher percentage of Vglut2 projection from the hypothalamus than the primary somatosensory cortex. About 50% of thalamic Vglut2 projection to the medial prefrontal cortex and as much as 80% of the thalamic projection to primary somatosensory cortex originate in various relay thalamic nuclei. The remainder arise from different midline and intralaminar nuclei traditionally thought to provide nonspecific or diffuse projection to the cortex. The extrathalamic Vglut2 corticopetal projections, together with the thalamic intralaminar-midline Vglut2 corticopetal projections, may participate in diffuse activation of the neocortex.

Colocalization of vesicular glutamate transporters in the rat superior olivary complex

Vesicular glutamate transporters (VGLUTs) are responsible for the accumulation of the excitatory neurotransmitter glutamate into synaptic vesicles. It is currently controversial whether the two isoforms found in glutamatergic neurons, VGLUT1 and VGLUT2, are present at the same synapse or have entirely complementary patterns of distribution. Using fluorescent immunohistochemistry, this study examines the colocalization of these two transporters in the rat superior olivary complex (SOC) between postnatal day (P) 5 and 29. The medial and lateral superior olives (MSO; LSO) stain for both VGLUT1 and VGLUT2 at all ages studied, with VGLUT1 levels doubling over this developmental period and VGLUT2 levels remaining unchanged. The ventral nucleus of the trapezoid body (VNTB) strongly labels only for VGLUT2, despite the fact that glutamatergic synapses are present that are formed from collaterals of axons that go on to form synapses containing both VGLUT1 and VGLUT2. Principal neurons of the medial nucleus of the trapezoid body (MNTB) are surrounded by the calyx of Held presynaptic terminal, which is large enough to allow examination of VGLUT localization within a synapse. Throughout its postnatal developmental period a single calyx synapse contains both VGLUT1 and VGLUT2. Whereas VGLUT1 levels are greatly up-regulated from P5 to P29, VGLUT2 levels remain high. As the abundance of VGLUT determines the quantal size, this up-regulation will increase excitatory postsynaptic currents (EPSCs) and have influences on synaptic physiology.

Correlation between multiple climbing fibre regression and parallel fibre response development in the postnatal mouse cerebellum

At the neuromuscular junction elimination of supernumerary synaptic connections during development is owing to a competitive process between single neuronal populations, whereas in the central nervous system the interaction of different types of input could affect the process. In the cerebellum, the regression from multiple- to mono-innervation of the Purkinje cells by climbing fibres is virtually completed during the first two weeks of postnatal development. While it is clear that parallel fibres are important in the control of the regression, there are conflicting results in relation to whether an early phase of regression is independent of parallel fibre effects. We studied the precise timing of climbing fibre synapse development and decline and the relationship with the functional maturation of parallel fibres. Until postnatal day (P) 6 or 7, the synaptic currents generated by different climbing fibres become progressively more uniform in amplitude. However, between P7 and 14, the amplitudes of the currents increasingly diverge until only one fibre remains connected. These data are taken as evidence that, in multiply innervated Purkinje cells, competition between different climbing fibres appears at P7 and continues during the second postnatal week. Morphological and electrophysiological data demonstrate that parallel fibres synapses appear at P7 and their development is significantly correlated with the time course of the climbing fibre regression. These results provide no evidence for climbing fibre regression independent of parallel fibres before P7 and also suggest a dominant role of the parallel fibre input in the later phase.

The vulnerability of striatal projection neurons and interneurons to excitotoxicity is differentially regulated by dopamine during development

The maturation of striatal projection neurons and interneurons is influenced by the development and integrity of their connectivity. In the present work, we have analyzed the modulation of striatum vulnerability to quinolinate (QUIN)-induced excitotoxicity in different neuronal populations by the nigrostriatal dopaminergic pathway during postnatal development. A single striatal lesion with 6-hydroxydopamine (6-OHDA) at the second postnatal day (P) 2 or QUIN at P7 induced a reduction in the striatal volume at P30, whereas an additive effect was observed when these two lesions were performed in the same animal. The analysis of different striatal neuronal populations showed that the excitotoxic lesion induced by QUIN over projection neurons stained with calbindin was partially reverted by the previous injection of 6-OHDA at P2. However, cholinergic interneurons were affected neither by the lack of dopamine innervation nor by QUIN treatment. This neuronal population also remained intact after the double lesion. In contrast, the number of other type of striatal interneurons, parvalbumin-positive neurons, were reduced by the dopaminergic ablation and also by the QUIN-induced excitotoxicity and this effect was additive after the double lesion when it was measured at P30. On the other hand, we studied the effect on the striatal outputs measuring the density of substance P-positive fibers in the substantia nigra and enkephalin-positive fibers in the globus pallidus. A reduction in substance P-positive fibers was observed in 6-OHDA injected animals, while the density of enkephalin-positive fibers was only decreased after QUIN treatment. The double lesion did not modify the effects of the single lesions. In conclusion, our results show that dopamine modulates the vulnerability to excitotoxicity during striatal postnatal development, and this effect is specific for projection neurons. Furthermore, striatonigral and striatopallidal pathways are differentially regulated by the activation of dopamine or glutamate receptors.

Iron alters glutamate secretion by regulating cytosolic aconitase activity

Glutamate has many important physiological functions, including its role as a neurotransmitter in the retina and the central nervous system. We have made the novel observations that retinal pigment epithelial cells underlying and intimately interacting with the retina secrete glutamate and that this secretion is significantly affected by iron. In addition, iron increased secretion of glutamate in cultured lens and neuronal cells, indicating that this may be a common mechanism for the regulation of glutamate production in many cell types. The activity of the iron-dependent enzyme cytosolic aconitase (c-aconitase) is increased by iron. The conversion of citrate to isocitrate by c-aconitase is the first step in a three-step process leading to glutamate formation. In the present study, iron increased c-aconitase activity, and this increase was associated with an increase in glutamate secretion. Inhibition of c-aconitase by oxalomalate decreased glutamate secretion and completely inhibited the iron-induced increase in glutamate secretion. Derangements in both glutamate secretion and iron metabolism have been noted in neurological diseases and retinal degeneration. Our results are the first to provide a functional link between these two physiologically important substances by demonstrating a significant role for iron in the regulation of glutamate production and secretion in mammalian cells resulting from iron regulation of aconitase activity. Glutamatergic systems are found in many nonneuronal tissues. We provide the first evidence that, in addition to secreting glutamate, retinal pigment epithelial cells express the vesicular glutamate transporter VGLUT1 and that regulated vesicular release of glutamate from these cells can be inhibited by riluzole.

Presynaptic localization of an AMPA-type glutamate receptor in corticostriatal and thalamostriatal axon terminals

The neostriatum is known to receive glutamatergic projections from the cerebral cortex and thalamic nuclei. Vesicular glutamate transporters 1 and 2 (VGluT1 and VGluT2) are located on axon terminals of corticostriatal and thalamostriatal afferents, respectively, whereas VGluT3 is found in axon terminals of cholinergic interneurons in the neostriatum. In the present study, the postsynaptic localization of ionotropic glutamate receptors was examined in rat neostriatum by the postembedding immunogold method for double labelling of VGluT and glutamate receptors. Immunoreactive gold particles for AMPA receptor subunits GluR1 and GluR2/3 were frequently found not only on postsynaptic but also on presynaptic profiles immunopositive for VGluT1 and VGluT2 in the neostriatum, and GluR4-immunoreactive particles were observed on postsynaptic and presynaptic profiles positive for VGluT1. Quantitative analysis revealed that 27-45% of GluR1-, GluR2-, GluR2/3- and GluR4-immunopositive particles found in VGluT1- or VGluT2-positive synaptic structures in the neostriatum were associated with the presynaptic profiles of VGluT-positive axons. In contrast, VGluT-positive presynaptic profiles in the neostriatum showed almost no immunoreactivity for NMDA receptor subunits NR1 or NR2A/B. Furthermore, almost no GluR2/3-immunopositive particles were observed in presynaptic profiles of VGluT3-positive (cholinergic) terminals that made asymmetric synapses in the neostriatum, or in those of VGluT1- or VGluT2-positive terminals in the neocortex. The present results indicate that AMPA receptor subunits but not NMDA receptor subunits are located on axon terminals of corticostriatal and thalamostriatal afferents, and suggest that glutamate released from these axon terminals controls the activity of the terminals through the presynaptic AMPA autoreceptors.

Corticosterone controls the developmental emergence of fear and amygdala function to predator odors in infant rat pups

In many altricial species, fear responses such as freezing do not emerge until sometime later in development. In infant rats, fear to natural predator odors emerges around postnatal day (PN) 10 when infant rats begin walking. The behavioral emergence of fear is correlated with two physiological events: functional emergence of the amygdala and increasing corticosterone (CORT) levels. Here, we hypothesize that increasing corticosterone levels influence amygdala activity to permit the emergence of fear expression. We assessed the relationship between fear expression (immobility similar to freezing), amygdala function (c-fos) and the level of corticosterone in pups in response to presentation of novel male odor (predator), littermate odor and no odor. CORT levels were increased in PN8 pups (no fear, normally low CORT) by exogenous CORT (3 mg/kg) and decreased in PN12 pups (express fear, CORT levels higher) through adrenalectomy and CORT replacement. Results showed that PN8 expression of fear to a predator odor and basolateral/lateral amygdala activity could be prematurely evoked with exogenous CORT, while adrenalectomy in PN12 pups prevented both fear expression and amygdala activation. These results suggest that low neonatal CORT level serves to protect pups from responding to fear inducing stimuli and attenuate amygdala activation. This suggests that alteration of the neonatal CORT system by environmental insults such as alcohol, stress and illegal drugs, may also alter the neonatal fear system and its underlying neural control.

A quantitative analysis of the laminar distribution of synaptic boutons in field CA3 of the rat hippocampus

We analyzed the laminar distribution of synaptic boutons in field CA3 of the rat hippocampus using a large montage electron micrograph. The size of boutons and synaptic vesicles was measured using a computer-assisted digitizing system. In all, 3353 synaptic boutons were observed in a 15 microm x 100 microm strip. Of these, 86.3% contained spherical vesicles (S-boutons), 12% contained flat vesicles (F-boutons), and 1.7% were mossy terminals (M-boutons). S-boutons were distributed widely in the strata moleculare (st. Mol), radiatum (st. Rad), and oriens (st. Ori), but there were only a few in the strata lucidum (st. Luc) and pyramidale (st. Pyr). The upper portions of both the st. Rad and Ori contained slightly fewer boutons. In terms of the location of synaptic contacts, 83% of all S-boutons were found on the dendritic spines and the rest were on the dendritic shafts. S-boutons on the dendritic shafts were observed more frequently in the st. Mol than in the other strata. According to the morphometry of the size of synaptic vesicles, S-boutons with small vesicles (mean vesicle area <1109 nm(2)) were located exclusively in the st. Mol, S-boutons with medium-sized vesicles (mean vesicle area 1109-1482 nm(2)) were observed in all strata, and S-boutons with large vesicles (mean vesicle area >1482 nm(2)) were distributed in the st. Luc and Ori, but not in the st. Mol. F-boutons were predominantly distributed in the upper half of the st. Mol and in the area around the st. Pyr, although they were observed in all strata. In the st. Mol, all the F-boutons were in contact with dendritic shafts, while near the st. Pyr, F-boutons were found exclusively on somata, the proximal parts of the dendritic shafts, and the initial segments of axons. The average F-bouton was smaller in the st. Mol (0.23 microm(2)) than near the st. Pyr (0.39 microm(2)). In this synapto-architectural study of the hippocampal CA3 region using large montage electron micrographs, we observed (1) an intimate relationship between synapse distribution and the dendritic structure of pyramidal neurons, (2) the distribution of different types of boutons containing vesicles of various size, and (3) two different plausible foci of postsynaptic inhibition where F-boutons were distributed densely, and (4) estimated the input ratios of pyramidal neurons.

Vesicular glutamate transporter immunoreactivity in the central and peripheral endings of muscle-spindle afferents

Expression of vesicular glutamate transporters (VGLUTs: VGLUT1, VGLUT2 and VGLUT3) in muscle spindle afferents was examined in rats. VGLUT1 immunoreactivity was detected in the sensory endings on the equatorial and juxta-equatarial regions of intrafusal fibers as well as in many axon terminals within lamina IX of the spinal cord. VGLUT1 might be expressed not only in the central axon terminals but also in the peripheral sensory endings of muscle-spindle afferents.

An essential role for vesicular glutamate transporter 1 (VGLUT1) in postnatal development and control of quantal size

Quantal neurotransmitter release at excitatory synapses depends on glutamate import into synaptic vesicles by vesicular glutamate transporters (VGLUTs). Of the three known transporters, VGLUT1 and VGLUT2 are expressed prominently in the adult brain, but during the first two weeks of postnatal development, VGLUT2 expression predominates. Targeted deletion of VGLUT1 in mice causes lethality in the third postnatal week. Glutamatergic neurotransmission is drastically reduced in neurons from VGLUT1-deficient mice, with a specific reduction in quantal size. The remaining activity correlates with the expression of VGLUT2. This reduction in glutamatergic neurotransmission can be rescued and enhanced with overexpression of VGLUT1. These results show that the expression level of VGLUTs determines the amount of glutamate that is loaded into vesicles and released and thereby regulates the efficacy of neurotransmission.

Localization of VGLUT3, the vesicular glutamate transporter type 3, in the rat brain

We have recently identified a third subtype of glutamate vesicular transporter (VGLUT) named VGLUT3. In the present study, we provide a detailed account of the regional and cellular distributions of VGLUT3 in the rat brain, using specific nucleotide probes and antisera. The distribution of VGLUT3 protein was compared with that of the other vesicular transporters (VGLUT1 and VGLUT2). All the areas expressing VGLUT3 also contain high levels of VGLUT1 and -2 proteins, but, at a finer level of analysis, the distribution of the three subtypes differs. Unlike VGLUT1 and -2, VGLUT3 expression is limited to discrete cell populations. Neurons containing VGLUT3 transcript are essentially observed in the caudate-putamen, the olfactory tubercle, the nucleus accumbens, the hippocampus, the interpeduncular nucleus and the dorsal and medial raphe nuclei. More scattered populations of VGLUT3 expressing neurons are found in the cerebral cortex. The distribution of the VGLUT3 protein, as determined with specific antisera, overlaps with that of the transcript in the caudate-putamen, olfactory tubercles, hippocampus, cortex, interpeduncular nucleus, and raphe nuclei, suggesting that VGLUT3 is essentially present in local projection neurons in these regions. Microscopic examination reveals staining of terminals and perikarya. Furthermore, co-localization studies indicate that VGLUT3 is present in GABAergic interneurons in the hippocampus, as well as in the interpeduncular nucleus. However, other regions, such as the substantia nigra (pars compacta), the ventral tegmental area, and the parabigeminal nucleus, receive a dense VGLUT3 terminal labeling although they do not contain VGLUT3 expressing neurons. In these regions, VGLUT3 immunoreactivity may be present in terminals of long projecting neurons. This subclass of glutamatergic afferents differs from other "classical" excitatory terminals that express VGLUT1 or VGLUT2. The distribution of VGLUT3 in the rat brain suggests an unsuspected function of vesicular glutamate transport in subsets of interneurons and in neuromodulatory neurons.

Independent inputs by VGLUT2- and VGLUT3-positive glutamatergic terminals onto rat sympathetic preganglionic neurons

To characterize glutamatergic axon terminals onto sympathetic preganglionic neurons (SPNs), we visualized immunohistochemically three vesicular glutamate transporters (VGLUTs) in the intermediolateral cell column (IML) of rat thoracic spinal cord. VGLUT2 and VGLUT3 immunoreactivities but not VGLUT1 immunoreactivity were distributed in the IML and found in terminals making asymmetric synapses and apposed to dendrites immunopositive for choline acetyltransferase, an SPN marker. VGLUT2 and VGLUT3 immunoreactivities were not co-localized with each other. A population of VGLUT2-immunoreactive but not VGLUT3-immunoreactive terminals were adrenergic or noradrenergic. Some of VGLUT3-immunoreactive but not VGLUT2-immunoreactive terminals contained serotonin. These results indicate at least two independent glutamatergic terminal populations, which include a distinct monoaminergic subpopulation, making excitatory inputs onto SPNs.

Vesicular glutamate transporters VGLUT1 and VGLUT2 define two subsets of unipolar brush cells in organotypic cultures of mouse vestibulocerebellum

Different isoforms of a vesicular glutamate transporter (VGLUT) mediate glutamate uptake into synaptic vesicles of excitatory neurons. There is agreement that the VGLUTs are differentially expressed in brain, and that two isoforms, VGLUT1 and VGLUT2, are localized to excitatory axon terminals in the cerebellar cortex. While granule cells express solely VGLUT1, there is no report about the VGLUT(s) of the unipolar brush cell (UBC), the second type of glutamatergic interneuron residing in the cerebellar granular layer. In the mouse, UBCs are particularly numerous in the uvula (lobule IX) and nodulus (lobule X). These folia contain two distinct subsets of UBCs: one kind expresses the calcium-binding protein calretinin (CR), and the other kind expresses the metabotropic glutamate receptor (mGluR) 1alpha. UBCs give rise to an extensive system of intrinsic mossy fibers (MF), whose terminals innervate granule cells and other UBCs, altogether similar to those formed by the extrinsic MFs. The presence of both extrinsic and intrinsic MFs in the vestibulocerebellum makes it difficult to determine which type of VGLUT is contained in MFs formed by the UBC axons. Hence, the nodulus was isolated from sagittal cerebellar slices from postnatal day 10 mice, and cultured for 15-20 days in vitro. Double immunofluorescence and confocal microscopy showed that mossy terminals of CR-positive (CR(+)) UBCs were immunoreactive for VGLUT1 and VGLUT2, while mossy terminals of mGluR1alpha-positive (mGluR1alpha(+)) UBCs were provided with VGLUT1 only. Moreover, CR(+) dendritic brushes were contacted by mossy terminals provided with both transporters, while mGluR1alpha(+) dendritic brushes were contacted by mossy terminals immunopositive for VGLUT1 and immunonegative for VGLUT2. These data indicate that the two UBC subsets use different modalities of vesicular glutamate storage and form separate networks. We consider it possible that expressions of CR with VGLUT1/VGLUT2 and mGluR1alpha(+) with VGLUT1 in the two subsets of vestibulocerebellar UBCs are determined by specific vestibular inputs, carried by groups of primary and/or secondary vestibular afferents.

Changes of immunocytochemical localization of vesicular glutamate transporters in the rat visual system after the retinofugal denervation

To clarify which vesicular glutamate transporter (VGluT) is used by excitatory axon terminals of the retinofugal system, we examined immunoreactivities and mRNA signals for VGluT1 and VGluT2 in the rat retina and compared immunoreactivities for VGluT1 and VGluT2 in the retinorecipient regions using double immunofluorescence method, anterograde tracing, and immunoelectron microscopy. Furthermore, the changes of VGluT1 and VGluT2 immunoreactivities were studied after eyeball enucleation. Intense immunoreactivity and mRNA signal for VGluT2, but not for VGluT1 immunoreactivity, were observed in most perikarya of ganglion cells in the retina. Immunoelectron microscopy revealed that VGluT1- and VGluT2-immunolabeled terminals made asymmetrical synapses, suggesting that they were excitatory synapses, and that VGluT1-immunolabeled terminals were smaller than VGluT2-labeled ones in many retinorecipient regions, such as the dorsal lateral geniculate nucleus (LGd) and superior colliculus (SC). Double immunofluorescence study further revealed that almost no VGluT2 immunoreactivity was colocalized with VGluT1 in the retinorecipient regions. After wheat germ agglutinin (WGA) injection into the eyeballs, WGA immunoreactivity was colocalized in the single axon terminals of LGd and SC with VGluT2 but not VGluT1 immunoreactivity. After unilateral enucleation, VGluT2 immunoreactivity in the LGd, SC, nucleus of the optic tract, and nuclei of the accessory optic tract in the contralateral side of the enucleated eye was clearly decreased. Although only a small change of VGluT2 immunoreactivity was observed in the contra- and ipsilateral suprachiasmatic nuclei, olivary pretectal nucleus, anterior pretectal nucleus, and posterior pretectal nucleus, moderate reduction of VGluT2 was found in these regions after bilateral enucleation. On the other hand, almost no change in VGluT1 immunoreactivity was found in the structures examined in the present enucleation study. Thus, the present results support the notion that the retinofugal pathways are glutamatergic, and indicate that VGluT2, but not VGluT1, is employed for accumulating glutamate into synaptic vesicles of retinofugal axons.

Vesicular glutamate transporters, VGluT1 and VGluT2, in the trigeminal ganglion neurons of the rat, with special reference to coexpression

Vesicular glutamate transporters are responsible for glutamate transport into synaptic vesicles. In the present study, we examined immunohistochemically the expression of vesicular glutamate transporters, VGluT1 and VGluT2, in trigeminal ganglion neurons of the rat. Immunohistochemistry for VGluT1 and VGluT2 indicated that more than 80% of trigeminal ganglion neurons express VGluT1 and/or VGluT2 in their cell bodies. It also indicated that large and small trigeminal ganglion neurons express VGluT2 more frequently than VGluT1. Dual immunofluorescence histochemistry for VGluT1 and VGluT2 indicated that trigeminal ganglion neurons express VGluT2 more frequently than VGluT1 and that more than 80% of VGluT-expressing trigeminal ganglion neurons express VGluT1 and VGluT2. Many axon terminals in the superficial layers of the medullary dorsal horn also showed VGluT1 and VGluT2 immunoreactivities. Some of these axon terminals were confirmed to form the central core of the synaptic glomerulus. These results indicated that VGluT1 and VGluT2 are coexpressed in the cell bodies and axon terminals in most trigeminal ganglion neurons.

Classical conditioning of responses to an artificial nipple in the rat fetus: mu and kappa opioid systems

The rat fetus exhibits oral grasp responses when presented with an artificial nipple in utero. Preexposure to milk produces a transient decrease in oral grasp responses. The effect of milk on oral grasping is mediated by endogenous activity in brain opioid systems. Specifically, milk triggers mu activity in rostral brain regions and kappa opioid activity in caudal brain regions to decrease oral grasping of the artificial nipple. Reexposure to the artificial nipple after it has been paired with milk during a classical conditioning trial results in a conditioned reduction in oral grasping that is evident when fetuses are reexposed to the nipple. The classically conditioned decrease in oral grasping is mediated by mu opioid activity in rostral brain regions and kappa opioid activity in caudal brain regions. Endogenous opioid systems, activated by exposure to milk and the nipple, may regulate the processing of sensory information during suckling in the rat.