Metabotropic glutamate receptor mGluR5 subcellular distribution and developmental expression in hypothalamus

Age-related changes in the expression of axonal and glial group I metabotropic glutamate receptor in the rat substantia nigra pars reticulata

Neuronal systems undergo many significant changes during the course of brain development. To characterize the developmental changes in the substantia nigra pars reticulata (SNr) associated with the expression of group I metabotropic glutamate receptors (mGluRs), we used the immunoperoxidase and immunogold methods at the electron microscope level to determine whether the subcellular and subsynaptic patterns of distribution of mGluR1a and mGluR5 differ between young (P14-P18) and adult (>2 months) rats. The SNr of young rats contained a significantly higher density of labeled unmyelinated axons for both receptor subtypes. In addition, mGluR5-immunoreactive glial processes were very abundant in young rats but absent in the adults. On the other hand, the relative proportion of immunoreactive dendrites was the same for both age groups. Analysis of immunogold-labeled rat SNr revealed similar proportions of plasma membrane-bound mGluR1a and mGluR5 in adult (59.8 and 19.4%, respectively) and young (60.6 and 18.4%, respectively) rats. The pattern of subsynaptic localization of mGluR1a also remained the same between young and adults. However, the proportion of extrasynaptic mGluR5 decreased, whereas proportions of gold particles associated with symmetric synapses increased in adults. The results of this study demonstrate significant differences in the expression of group I mGluRs in the SNr of young and adult rats. These findings support a role for group I mGluRs during development and emphasize the importance of using brain tissue from age-matched subjects when attempting to correlate functional data from young rat brain slices with immunocytochemical localization of group I mGluRs.

Gene structure of the human metabotropic glutamate receptor 5 and functional analysis of its multiple promoters in neuroblastoma and astroglioma cells

The metabotropic glutamate receptor 5 (mGluR5) has a discrete tissue expression mainly limited to neural cells. Expression of mGluR5 is developmentally regulated and undergoes dramatic changes in association with neuropathological disorders. We report the complete genomic structure of the mGluR5 gene, which is composed of 11 exons and encompasses approximately 563 kbp. Three clusters of multiple transcription initiation sites located on three distinct exons (IA, IB, and II), which undergo alternative splicing, have been identified. The 5'-flanking regions of these exons were isolated and, using a luciferase reporter gene assay, shown to possess active promoter elements in SKN-MC neuroblastoma and U178-MG astroglioma cells. Promoter IA was characterized by a CpG island; promoter IB contained a TATA box, and promoter II possessed three active Oct-1-binding sites. Preferential luciferase activity was observed in SKN-MC concomitant with differential DNA binding activity to several responsive elements, including CREB, Oct-1, C/EBP, and Brn-2. Exposure to growth factors produced enhanced expression of promoters IB and II in astroglioma cells and activation of NF-kappa B. These results suggest that alternative 5'-splicing and usage of multiple promoters may contribute regulatory mechanisms for tissue- and context-specific expression of the mGluR5 gene.

Metabotropic glutamate receptor 5 (mGluR5)-mediated phosphoinositide hydrolysis and NMDA-potentiating effects are blunted in the striatum of aged rats: a possible additional mechanism in striatal senescence

The aim of the present work was to verify whether an impairment of subtype 5 metabotropic glutamate receptor-mediated neurotransmission did occur in the aged striatum. To this end, the ability of the subtype 5 metabotropic glutamate receptor agonist, RS-2-chloro-5-hydroxyphenylglycine, to stimulate phosphoinositide hydrolysis and to potentiate N-methyl-d-aspartate-induced effects in striatal slices from young (3 months) and aged (24 months) rats was compared. The ability of RS-2-chloro-5-hydroxyphenylglycine to induce maximal phosphoinositide turnover and to potentiate N-methyl-d-aspartate effects was significantly reduced in slices from old vs. young rats. These changes were associated with a significant reduction in the expression of subtype 5 metabotropic glutamate receptor protein (-28.8%) and phospholipase C-beta1 (-55.8%) in old striata, while receptor messenger ribonucleic acid expression was unchanged. These results show that the signalling associated with subtype 5 metabotropic glutamate receptors undergoes significant age-related changes and that a reduced expression of subtype 5 metabotropic glutamate receptors and, more importantly, phospholipase C-beta1 may account for the functional decline of subtype 5 metabotropic glutamate receptors.

Role of immediate early gene expression in cortical morphogenesis and plasticity

During the development of the central nervous system, there is a fundamental requirement for synaptic activity in transforming immature neuronal connections into organized functional circuits (Katz 1996). The molecular mechanisms underlying activity-dependent adaptive changes in neurons are believed to involve regulated cascades of gene expression. Immediate early genes (IEGs) comprise the initial cascade of gene expression responsible for initiating the process of stimulus-induced adaptive change, and were identified initially as transcription factors that were regulated in brain by excitatory synaptic activity. More recently, a class of neuronal immediate early genes has been identified that encodes growth factors, signaling molecules, extracellular matrix and adhesion proteins, and cytoskeletal proteins that are rapidly and transiently expressed in response to glutamatergic neurotransmission. This review focuses on the neuronal immediate early gene (nIEG) response, in particular, the class of "effector" immediate early gene proteins that may directly modify neuronal and synaptic function.

Endogenous activation of group-II metabotropic glutamate receptors inhibits the hypothalamic-pituitary-adrenocortical axis

Systemic injection of the mGlu2/3 receptor antagonist, LY341495 (1 mg/kg, i.p.), increased plasma corticosterone in mice to an extent similar to that induced by the despair test. Treatment with the mGlu2/3 receptor agonist, LY379268 (1 mg/kg, i.p.), or the non-competitive mGlu5 receptor antagonist, MPEP (5 mg/kg, i.p.), failed to induce significant changes in corticosterone levels. Searching for a site of action of LY341495, we examined the expression of mGlu receptor subtypes in the various anatomical regions of the mouse hypothalamic-pituitary-adrenal (HPA) axis. Only mGlu5 and -7 receptor mRNAs were detected in the adrenal gland by RT-PCR, whereas mGlu -1, -3, -4, -5, -7 and -8 receptor mRNAs were detected in the anterior pituitary. All transcripts (with the exception of mGlu5 and mGlu6 receptor mRNAs) were detected in the hypothalamus. However, Western blot analysis showed the presence of mGlu2/3 receptor proteins only in the hypothalamus and not in the anterior pituitary. This was consistent with functional data showing that LY341495 (0.1 and 1 microM) failed to affect ACTH secretion from isolated mouse anterior pituitaries. Moving from these observations, we examined whether LY341495 could activate the HPA axis by inhibiting mGlu2/3 receptors at hypothalamic level. We measured the release of corticotropin releasing hormone (CRH) in isolated mouse hypothalami incubated in the presence of subtype-selective mGlu receptor agonists or antagonists. Among all the drugs we have tested, only LY341495 was able to increase CRH secretion. With high concentrations of LY341495 (1 microM) this increase was similar to that induced by 50 mM K(+). The action of LY341495 was prevented by the combined application of the mGlu2/3 receptor agonist, LY379268. We conclude that group-II mGlu receptors tonically regulate the HPA axis by controlling CRH secretion at hypothalamic level.

Metabotropic glutamate receptor 5 antagonist-induced stimulation of hypothalamic-pituitary-adrenal axis activity: interaction with serotonergic systems

The mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine (MPEP) produces anxiolytic or antidepressant effects in several rodent models through incompletely described mechanisms. Anxiolytics and antidepressants share several neuroendocrine features, including acute activation of the hypothalamic-pituitary-adrenal (HPA)-axis, desensitization of neuroendocrine responses with repeated dosing, and desensitization of the HPA axis to 5-HT1A agonist stimulation. We characterized these neuroendocrine parameters in rats treated systemically with MPEP and compared them to those induced by the anxiolytic buspirone. Acutely, MPEP dose-dependently (0.1-10 mg/kg i.p.) increased plasma corticosterone concentrations. These responses were blocked by 50% with the 5-HT1A antagonist WAY100635. The corticosterone responses to both 3 mg/kg MPEP and buspirone were decreased by 80% after 5 days of twice-daily injections. Repeated injection with MPEP decreased HPA-axis sensitivity to buspirone challenge by 75%. This desensitization was not associated with changes in mGluR5 or 5-HT1A receptor binding properties, expression of G-protein subunits coupled to these receptors, or in 5-HT-stimulated binding of [(3)H]-GTPgammaS to membranes. We conclude that MPEP acutely disinhibits the HPA axis, in part through uncharacterized changes in serotonergic signaling. Desensitization of 5-HT1A responses after repeated MPEP administration may indicate that, like other anxiolytics and antidepressants, plasticity in 5-HT signal transduction pathways has occurred.

Neuron-to-glia signaling mediated by excitatory amino acid receptors regulates ErbB receptor function in astroglial cells of the neuroendocrine brain

Hypothalamic astroglial erbB tyrosine kinase receptors are required for the timely initiation of mammalian puberty. Ligand-dependent activation of these receptors sets in motion a glia-to-neuron signaling pathway that prompts the secretion of luteinizing hormone-releasing hormone (LHRH), the neuropeptide controlling sexual development, from hypothalamic neuroendocrine neurons. The neuronal systems that may regulate this growth factor-mediated back signaling to neuroendocrine neurons have not been identified. Here we demonstrate that hypothalamic astrocytes contain metabotropic receptors of the metabotropic glutamate receptor 5 subtype and the AMPA receptor subunits glutamate receptor 2 (GluR2) and GluR3. As in excitatory synapses, these receptors are in physical association with their respective interacting/clustering proteins Homer and PICK1. In addition, they are associated with erbB-1 and erbB-4 receptors. Concomitant activation of astroglial metabotropic and AMPA receptors results in the recruitment of erbB tyrosine kinase receptors and their respective ligands to the glial cell membrane, transactivation of erbB receptors via a mechanism requiring metalloproteinase activity, and increased erbB receptor gene expression. By facilitating erbB-dependent signaling and promoting erbB receptor gene expression in astrocytes, a neuron-to-glia glutamatergic pathway may represent a basic cell-cell communication mechanism used by the neuroendocrine brain to coordinate the facilitatory transsynaptic and astroglial input to LHRH neurons during sexual development.

Modulation of stress-induced and stimulated hyperprolactinemia with the group II metabotropic glutamate receptor selective agonist, LY379268

It is well recognized that glutamate is an integral excitatory neurotransmitter in the neuroendocrine control of several hormonal factors. While the ability of pharmacological agents acting at ionotropic glutamate receptors to modulate the levels of serum prolactin levels has been investigated, there have been few reports of the effects mediated by the G-protein coupled, metabotropic glutamate (mGlu) receptors. The present work was undertaken to investigate the role of the Group II mGlu receptors, mGlu2 and mGlu3 in the regulation of serum polactin levels. LY379268, a Group II selective agonist, did not alter basal levels of circulating prolactin in young (36-40 day old) male rats. However, when an immobilization stress-induced hyperprolactinemia was examined, 10 mg/kg s.c. of LY379268 significantly lowered serum prolactin levels. Similarly, pretreatment with LY379268 was able to reverse the hyperprolactinemia induced with the catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine (aMPT). This inhibition of hyperprolactinemia could be prevented by pretreatment with LY341495, a Group II mGlu receptor antagonist. The Group II antagonist alone had no effect on either basal nor stimulated prolactin levels. The agonist LY379268 was able to prevent the transient hyperprolactinemia associated with stimulation of serotonin 5-HT2A receptors by 2,5-dimethoxy-4-iodoamphetamine (DOI), but did not alter the high levels of circulating prolactin induced with the D2 antagonist, haloperidol. When treatment with LY379268 was delayed until 1 h after aMPT, a time demonstrated to show a full effect of aMPT on serum prolactin levels, the Group II agonist was similarly able to reverse hyperprolactinemia, suggesting LY379268 did not act by preventing the partial catecholamine depletion by aMPT. Similarly, high doses of amphetamine, a dopamine (DA) releaser, were able to reverse the aMPT-induced hyperprolactinemia, consistent with sufficient levels of dopamine remaining after aMPT treatment to modulate prolactin levels. LY379268 did not alter the hyperprolactinemia seen in estrogen-primed, ovariectomized female rats. Taken together the results indicate that stimulation of mGlu2/3 has an indirect inhibitory action on pituitary prolactin release. It is speculated that disinhibition of tubero-infundibular DA release by presynaptic Group II mGlu receptors located on inhibitory inputs to the arcuate hypothalamic nucleus is a possible explanation for the findings.

The activation of metabotropic glutamate receptors differentially affects GABA and alpha-melanocyte stimulating hormone release from the hypothalamus and the posterior pituitary of male rats

The aim of the present study was to investigate the effect of metabotropic glutamate receptor (mGluR) activation on gamma-aminobutyric acid (GABA) and alpha-melanocyte stimulating hormone (alpha-MSH) release from hypothalamic fragments and posterior pituitaries. The actions of a number of subtype-selective mGluR agonists were monitored. A group I mGluR agonist, (S)-3-hydroxyphenylglycine (3-HPG; 0.5 mM), decreased K+-induced hypothalamic GABA release. (RS)-1-Aminoindan-1,5-dicarboxylic acid (AIDA), a specific group I mGluR antagonist (0.2 mM), blocked the effect of 3-HPG. (2S, 1'S, 2'S)-2-(Carboxycyclopropyl) glycine (L-CCG-I) and L-serine-O-phosphate (L-SOP; 0.01-1 mM), agonists of group II and III mGluRs, respectively, did not modify hypothalamic evoked GABA release. Group I mGluR activation decreased, whereas group III increased and group II induced no changes in GABA release from the posterior pituitary. 3-HPG (1 mM) and L-CCG-I (0.1 mM) decreased, whereas L-SOP (0.01-0.1 mM) did not change alpha-MSH release from hypothalamic fragments. No agonists of the three mGluR groups modified alpha-MSH release from the posterior pituitary. These results indicate that activation of mGluRs differentially affects GABA and alpha-MSH release from the hypothalamus and the posterior pituitary.

Expression and plasticity of glutamate receptors in the supraoptic nucleus of the hypothalamus

Magnocellular neuroendocrine cells (MNCs) of the supraoptic nucleus of the hypothalamus (SON) produce and release the hormones vasopressin (VP) and oxytocin (OT) in response to a variety of stimuli to regulate body water and salt, parturition and lactation. Hormone release is influenced by the pattern of neuronal firing of these MNCs, which, in turn, is governed by intrinsic conductances and synaptic inputs, including those mediated by the neurotransmitter glutamate. Functional and molecular evidence has confirmed the expression of AMPA-, NMDA-, and metabotropic-type glutamate receptors in the SON, that together may orchestrate the effects of glutamatergic transmission on neuroendocrine function. However, the specific roles of the different subtypes of glutamate receptors is not yet clear. As with other central neurons, the subunit composition of glutamate receptors on MNCs will likely determine their properties and may potentially help define the differential properties of VP- and OT-producing MNCs. Possible functions of glutamate receptors on SON MNCs include altering excitatory synaptic transmission of osmotic information, neuronal firing, hormone production and release, and calcium signaling. Of interest are the anatomical, molecular, and functional changes at glutamatergic synapses in the SON that occur in response to pertinent physiological stimuli or development. These types of plasticity may include changes in glutamatergic synaptic density, glutamate receptor levels, or glutamate receptor subunit expression, all of which can affect the efficiency of synaptic transmission.

Regulation of brain-derived neurotrophic factor transcripts by neuronal activation in rat hypothalamic neurons

Brain-derived neurotrophic factor (BDNF) belongs to the neurotrophin family and regulates the survival, differentiation, and maintenance of function in different neuronal populations. BDNF is strongly expressed in hypothalamic neurons, where it exerts long- or short-lasting actions. Because glutamate has been associated with regulations of hypothalamic hormones, we examined the regulation of the four promoters of the BDNF gene by glutamate in fetal hypothalamic neurons. The expression levels of BDNF transcripts were investigated using semiquantitative RT-PCR. BDNF protein was determined by enzyme immunoassay, and BDNF and Trk B (BDNF receptor) gene variations were determined by RNAse protection assay. By RT-PCR, we showed that, under basal conditions, BDNF transcripts from exons I, II, and III but not from IV were expressed in the hypothalamic neurons. Glutamate increased expression of both the protein and the four transcripts via N-methyl-D-aspartate receptors, with maximal stimulations after 3 hr of application for exon I and II mRNAs and after 1 hr for exon III and IV mRNAs. Actinomycin D blocked the increase of all transcripts, whereas cycloheximide treatment inhibited stimulation only of exon I and II mRNAs. Trk B mRNA was rapidly and transiently reduced after glutamate application. Our results demonstrate that glutamate 1) regulates BDNF mRNA expression at an early developmental stage in hypothalamic neurons and 2) exerts a differential regulation of BDNF transcripts.

Characterization of a metabotropic glutamate receptor type 5-green fluorescent protein chimera (mGluR5-GFP): pharmacology, surface expression, and differential effects of Homer-1a and Homer-1c

Metabotropic glutamate receptor 5 (mGluR5) can modulate synaptic transmission by increasing intracellular Ca2+ and it plays a role in several forms of synaptic plasticity. We have constructed a fusion of human mGluR5 and green fluorescent protein (mGluR5-GFP). Expression of mGluR5-GFP in clonal cell lines yielded a functional fluorescent receptor with pharmacological profiles similar to wild-type mGluR5. mGluR5-GFP coimmunoprecipitated with Homer-1c, indicating that addition of GFP to the C-terminal did not prevent Homer binding. Coexpression of wild-type mGluR5 or mGluR5-GFP with Homer 1c, but not Homer-1a, resulted in reduced receptor surface localization and the formation of intracellular clusters. Neither Homer-1a nor Homer-1c had any effect on mGluR1 or mGluR1-GFP distribution. mGluR5-GFP expressed alone or in combination with Homer-1a formed dimers in HEK cells. Coexpression with Homer-1c, however, prevented mGluR5-GFP dimerization. Neither Homer altered the agonist profiles of mGluR5 or mGluR5-GFP. These data indicate that the functional expression of mGluR5 is regulated by Homer-1c and demonstrate that mGluR5-GFP provides a useful tool to study the molecular pharmacology and cell biology of mGluRs in real-time.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Characterisation of Neuropeptide Y Receptor Subtypes by Synthetic NPY Analogues and by Anti-receptor Antibodies

Neuropeptide Y (NPY), a 36-mer neuromodulator, binds to the receptors Y₁, Y₂, Y₄ and Y₅ with nanomolar affinity. They all belong to the rhodopsin-like G-protein coupled, seven transmembrane helix spanning receptors. In this study, Ala-substituted and centrally truncated NPY analogues were compared with respect to affinity to the Y-receptors. Furthermore, antibodies against the second (E2) and the third (E3) extracellular loop of NPY Y₁-, Y₂- and Y₅-receptor subtypes were raised and affinity to intact cells was tested by immunofluorescence assays. Both methods were applied in order to receive subtype selective tools and to characterise ligand binding. The analogues [A¹³]-pNPY and [A²⁷]-pNPY showed subtype selectivity for the Y₂-receptor. Sera against the E2 loop of the Y₁-receptor and against the E2 loop of the Y₂-receptor were subtype selective. Two antibodies against the Y₅ E2 and E3 loop recognised the Y₅- and Y₂-receptor subtypes. In combination, these sera are able to distinguish between the Y₁-, Y₂-, and Y₅-receptor subtypes. The analogues and antibodies represent valuable tools to distinguish NPY receptors on membranes and intact cells.

Maturation of vulnerability to excitotoxicity: intracellular mechanisms in cultured postnatal hippocampal neurons

Neuronal vulnerability to excitotoxicity changes dramatically during postnatal maturation. To study the intracellular mechanisms by which maturation alters vulnerability in single neurons, we developed techniques to maintain hippocampal neurons from postnatal rats in vitro. After establishing their neuronal phenotype with immunohistochemistry and electrophysiology, we determined that these neurons exhibit developmentally regulated vulnerability to excitotoxicity. At 5 days in vitro, NMDA-induced cell death at 24 h increased from 3.6% in 3-day-old rats to >90% in rats older than 21 days. Time-lapse imaging of neuronal morphology following NMDA demonstrated increasingly prevalent and severe injury as a function of postnatal age. Neither high- nor low-affinity calcium dyes demonstrated differences in peak NMDA-induced [Ca(2+)](i) increases between neurons from younger and older animals. However, neurons from older animals were uniformly distinguished from those from younger animals by their subsequent loss of [Ca(2+)](i) homeostasis. Because of the role of mitochondrial Ca(2+) buffering in [Ca(2+)](i) homeostasis, we measured NMDA-induced changes in mitochondrial membrane potential (DeltaPsi) as a function of postnatal age. NMDA markedly dissipated DeltaPsi in neurons from mature rats, but minimally in those from younger rats. These data demonstrate that, in cultures of postnatal hippocampal neurons, (a) vulnerability to excitotoxicity increases as a function of the postnatal age of the animal from which they were harvested, and (b) developmental regulation of vulnerability to NMDA occurs at the level of the mitochondrion.

Differential distribution of metabotropic glutamate receptors 1a, 1b, and 5 in the rat spinal cord

Metabotropic glutamate receptors (mGluRs) modulate somatosensory, autonomic, and motor functions at spinal levels. mGluR postsynaptic actions over spinal neurons display the pharmacologic characteristics of type I mGluRs; however, the spinal distribution of type I mGluR isoforms remains poorly defined. In this study, the authors describe a differential distribution of immunoreactivity to various type I mGluR isoforms (mGluR1a, mGluR5a,b, and mGluR1b) that suggests a correlation between specific isoforms and particular aspects of spinal cord function. Two different antisera raised against mGluR5a,b detected intense immunoreactivity within nociceptive afferent terminal fields (laminae I and II) and also in autonomic regions (parasympathetic and sympathetic). In contrast, two of three anti-mGluR1a antibodies did not immunostain lamina I or II. Laminae I and II immunostaining by a third anti-mGluR1a antibody was competed by a peptide sequence obtained from a homologous region in mGluR5, suggesting possible cross reactivity in fixed tissue. Autonomic neurons did not express mGluR1a immunoreactivity. All anti-mGluR1a antibodies strongly and specifically immunolabeled dendritic and somatic membranes of neurons in the deep dorsal horn (lamina III-V) and the ventral horn (lamina VI-IX). Somatic motoneurons expressed mGluR1a immunoreactivity but little or no mGluR5 immunoreactivity. Phrenic and pudendal motoneurons expressed the highest level of mGluR1a immunoreactivity in the spinal cord. Intense mGluR1b immunoreactivity was restricted to a few scattered neurons and a prominent group of neurons in lamina X. Lamina II neurons expressed low levels of mGluR1b immunoreactivity. Ultrastructurally, type I mGluR immunoreactivity was found mostly at extrasynaptic sites on the plasma membrane, but it was also found perisynaptically, in the body of the postsynaptic regions or in relation to intracytoplasmic structures.

Calbindin-D28k immunoreactivity is a marker for a subdivision of the sexually dimorphic nucleus of the preoptic area of the rat: developmental profile and gonadal steroid modulation

Calbindin-D28k (calbindin) is a 28 kilodalton calcium binding protein which potentially plays a role in neuroprotection. We report here the normal development and gonadal steroid modulation of a sexually dimorphic group of calbindin immunoreactive cells within the sexually dimorphic nucleus of the preoptic area (SDN) which we call the calbindin-immunoreactive SDN or CALB-SDN. Beginning on PN2, a faintly immunoreactive CALB-SDN is present, however, the volume is not sexually dimorphic. On PN4, the staining of the CALB-SDN appears more robust but the volume is still not sexually dimorphic. By PN8 and extending through PN12 and PN26, the latest age analysed, the volume of the CALB-SDN is larger in males by two- to fourfold. Cresyl violet counterstain reveals a similar developmental profile of the SDN as well as clusters of darkly staining calbindin immunonegative cells which lie around the CALB-SDN. Castration of males on PN0 decreases the volume of the CALB-SDN by PN12 and administration on the day of birth and PN1 of either testosterone propionate or oestradiol benzoate, but not dihydrotestosterone propionate to females increases the volume of the CALB-SDN by PN12. By demonstrating the sexual dimorphism and gonadal steroid modulation of the CALB-SDN, we hereby establish that calbindin is a specific marker of a subdivision of the SDN and can be used as such in future studies.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Estrogen selectively regulates spine density within the dendritic arbor of rat ventromedial hypothalamic neurons

Estrogen acts in the hypothalamic ventromedial nucleus (VMH) to promote female sexual behavior. One potential mechanism through which estrogen may facilitate this behavior is by reconfiguring synaptic connections within the VMH. Estrogen treatment increases the number of synapses and dendritic spines in the VMH, but how this remodeling occurs within the context of the local, behaviorally relevant microcircuitry is unknown. The goal of this study was to localize estrogen-induced changes in spine density within the VMH and relate these to dendritic morphology and the presence of nuclear estrogen receptor. The hypothalami from ovariectomized rats, treated with either vehicle or estradiol, were lightly fixed, and VMH neurons were iontophoretically filled with Lucifer yellow. Confocal microscopy was used to examine neuronal morphology. Estrogen treatment increased dendritic spine density by 48% in the ventrolateral VMH but had no effect on spine density in the dorsal VMH. The primary dendrites of VMH neurons were differentially affected by estrogen. Estrogen treatment increased spine density twofold on the short primary dendrites but did not affect spine density on long primary dendrites. Immunocytochemical staining showed that none of the filled neurons expressed estrogen receptor-alpha. Thus, although the effect of estrogen on spine density is localized to a VMH subdivision where estrogen receptor is expressed, estrogen treatment induces spines on neurons that lack estrogen receptor. Taken together, our results suggest that the effect of estrogen on ventrolateral VMH spines is selective within the dendritic arbor of a neuron and may be mediated by an indirect, possibly transynaptic, mechanism.

Metabotropic glutamate receptors in acutely isolated hippocampal astrocytes: developmental changes of mGluR5 mRNA and functional expression

We previously found that 82% of glial fibrillary acidic protein (GFAP)-positive hippocampal astrocytes acutely isolated from P1-10 rats responded to glutamate (Glu) with transient intracellular calcium increases via activation of a Group I metabotropic glutamate receptor (mGluR). Fewer cells responded to ATP and none to serotonin (5-HT). In this study we asked the question whether hippocampal astrocytes in older animals retain this relative pattern of expression. We have found that 77% of GFAP (+) cells from P11-20 rats responded to 50 microM Glu, 43% to ATP, and none to 5-HT. Thirty-three percent of GFAP (+) cells from P25-35 rats responded to Glu, 12% to ATP and 3% to 5-HT. In the case of the responses to Glu, pharmacological characterization and single-cell RT-PCR data confirmed that these responses were mediated by the mGluR5 subtype of group I mGluRs. Also, fewer (36%) GFAP mRNA (+) cells from P25-35 rats expressed detectable mGluR5 mRNA than those from P11-20 rats (77%). This number essentially corresponds to the number of GFAP(+) showing a Ca(2+) response to Glu. Both mGluR5a and b were expressed with equal frequency in cells from P11-20 rats, but the b form predominated in cells from older animals. Overall, our studies show that expression of mGluR5 in hippocampal astrocytes decreases with increasing age and the "a" splice variant declines to a greater extent than the "b" splice variant, corresponding to the developmental changes shown in total tissue for mGluR5.

mGluR3 and mGluR5 are the predominant metabotropic glutamate receptor mRNAs expressed in hippocampal astrocytes acutely isolated from young rats

The metabotropic glutamate receptors (mGluRs) that are expressed and not expressed on astrocytes in the brain have not been defined. While immunohistochemistry and in situ mRNA hybridization have been used on a limited basis to address this question, they do not readily enable the proportion of astrocytes expressing a particular mRNA or protein to be determined. Also, for many receptors, expression by cultured astrocytes does not reflect in situ expression. In this study, therefore, we examined expression of mRNA for all the mGluRs except mGluR6 by single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) in freshly isolated hippocampal astrocytes from postnatal day P1-10 rats, as an additional approach to address the question of which mGluRs are expressed on astrocytes in situ. The astrocytic nature of the cells was supported by simultaneously measuring mRNA for the astrocytic marker glial fibrillary acidic protein (GFAP) from the same cells. In these studies, the percentage of cells showing GFAP mRNA expression was the same as the percentage of cells showing immunocytochemical staining for GFAP. We found that only mGluR3 and mGluR5 mRNAs were significantly present in GFAP mRNA(+) cells. The mGluR5 PCR products were primarily of the "a" splice variant. mGluR1, 2, 4, 7, and 8 were very rarely or never detected. mGluR6 mRNA level was too low in whole rat brain and hippocampus to warrant examination. These results show that interpretation of effects involving mGluR3 or 5 activation in the hippocampus of young rats needs to also consider effects due to astrocytes.

Interstitial glial cells of the gerbil pineal gland display immunoreactivity for the metabotropic glutamate receptors mGluR2/3 and mGluR5

Recent studies have strengthened the hypothesis that neuroactive amino acids such as L-glutamate play an important role in the physiology of the mammalian pineal gland. In particular, there is now considerable evidence that L-glutamate is liberated from electron-lucent microvesicles of pinealocytes for a paracrine modulation of melatonin synthesis and release which may at least partially be mediated by the metabotropic glutamate receptor mGluR3. In order to expand our incomplete knowledge of possible pineal target cells and signal transduction mechanisms which are involved in glutamate-dependent intercellular communication, we have performed an immunohistochemical study of the gerbil pineal gland with antibodies directed against the metabotropic glutamate receptors mGluR2/3 and mGluR5. Using microwave irradiation of cryostat sections prior to immunostaining, strong immunoreactivity for both receptor subtypes was constantly observed in a subpopulation of pineal cells. Interestingly, these mGluR-positive cells could be identified as interstitial glial cells since they were labeled by antibodies against the intermediate filament protein vimentin in double immunofluorescence histochemistry. This indicates that interstitial glial cells in the gerbil possess the capacity to express at least two metabotropic glutamate receptors coupled to different intracellular signal transduction pathways. Therefore, it can be concluded that the glutamatergic communication system of the pineal gland may not only enable paracrine crosstalk among pinealocytes but probably also relies on interactions between pinealocytes and interstitial cells analogous to neuronal-glial signaling.

Molecular characterization of the human neuropeptide Y Y2-receptor

Five neuropeptide Y receptors, the Y1-, Y2-, Y4-, Y5- and y6-subtypes, have been cloned, which belong to the rhodopsin-like G-protein-coupled, 7-transmembrane helix-spanning receptors and bind the 36-mer neuromodulator NPY (neuropeptide Y) with nanomolar affinity. In this study, the Y2-receptor subtype expressed in a human neuroblastoma cell line (SMS-KAN) and in transfected Chinese hamster ovary cells (CHO-hY2) was characterized on the protein level by using photoaffinity labeling and antireceptor antibodies. Two photoactivatable analogues of NPY were synthesized, in which a Tyr residue was substituted by the photoreactive amino acid 4-(3-trifluoromethyl)-3H-diazirin-3-ylphenylalanine ((Tmd)Phe), [Nalpha-biotinyl-Ahx2,(Tmd)Phe36]NPY (Tmd36), and the Y2-receptor subtype selective [Nalpha-biotinyl-Ahx2,Ahx5-24,(Tmd)Phe27]N PY (Tmd27). Both analogues were labeled with [3H]succinimidyl-propionate at Lys4 and bind to the Y2-receptor with affinity similar to that of the native ligand. A synthetic fragment of the second (E2) extracellular loop was used to generate subtype selective antireceptor antibodies against the Y2-receptor. Photoaffinity labeling of the receptor followed by SDS-PAGE and detection of bound radioactivity and SDS-PAGE of solubilized receptors and subsequent Western blotting revealed the same molecular masses. Two proteins correspondingly have been detected for each cell line with molecular masses of 58 +/- 4 and 50 +/- 4 kDa, respectively.

Group I metabotropic glutamate receptors mediate phospholipase D stimulation in rat cultured astrocytes

We have studied the activation of phospholipase D (PLD) by glutamate in rat cultured astrocytes by measuring the PLD-catalyzed formation of [32P]phosphatidylbutanol in [32P]Pi-prelabeled cells, stimulated in the presence of butanol. Glutamate elicited the activation of PLD in cortical astrocytes but not in cortical neurons, whereas similar glutamate activation of phosphoinositide phospholipase C was found in both astrocytes and neurons. The extent of PLD stimulation by glutamate was similar in astrocytes from brain cortex and hippocampus, but no effect was found in cerebellar astrocytes. In cortical astrocytes, the glutamate response was insensitive to antagonists of ionotropic glutamate receptors and was reproduced by agonists of metabotropic glutamate receptors (mGluRs) with a rank order of agonist potency similar to that reported for group I mGluR-mediated phosphoinositide phospholipase activation [quisqualate > (S)-3,5-dihydroxyphenylglycine > (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid]. The response to (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid was inhibited by the mGluR antagonist (S)-alpha-methyl-4-carboxyphenylglycine and, less potently, by 1-aminoindan-1,5-dicarboxylic acid and 4-carboxyphenylglycine, two antagonists of group I mGluRs that display higher potency on mGluR1 than on mGluR5. The mGluR5-selective agonist (RS)-2-chloro-5-hydroxyphenylglycine also activated PLD in astrocytes. These findings indicate the involvement of group I mGluRs, most likely mGluR5, in the glutamate activation of PLD in cultured rat cortical astrocytes.

Expression and signaling of group I metabotropic glutamate receptors in astrocytes and microglia

Stimulation of astrocytes with the excitatory neurotransmitter glutamate leads to the formation of inositol 1,4,5-trisphosphate and the subsequent increase of intracellular calcium content. Astrocytes express both ionotropic receptors and metabotropic glutamate (mGlu) receptors, of which mGlu5 receptors are probably involved in glutamate-induced calcium signaling. The mGlu5 receptor occurs as two splice variants, mGlu5a and mGlu5b, but it was hitherto unknown which splice variant is responsible for the glutamate-induced effects in astrocytes. We report here that both mRNAs encoding mGlu5 receptor splice variants are expressed by cultured astrocytes. The expression of mGlu5a receptor mRNA is much stronger than that of mGlu5b receptor mRNA in these cells. In situ hybridization experiments reveal neuronal expression of mGlu5b receptor mRNA in adult rat forebrain but a strong neuronal expression of mGlu5a mRNA only in olfactory bulb. Signals for mGlu5a receptor mRNA in the rest of the brain were diffuse and weak but consistently above background. Activation of mGlu5 receptors in astrocytes yields increases in inositol phosphate production and transient calcium responses. It is surprising that the rank order of agonist potency [quisqualate > (2S,1 'S,2'S)-2-(carboxycyclopropyl)glycine = trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (1S,3R-ACPD) > glutamate] differs from that reported for recombinantly expressed mGlu5a receptors. The expression of mGlu5a receptor mRNA and the occurrence of 1S,3R-ACPD-induced calcium signaling were found also in cultured microglia, indicating for the first time expression of mGlu5a receptors in these macrophage-like cells.

Metabotropic glutamate receptor modulation of glutamate responses in the suprachiasmatic nucleus

Glutamate is the primary excitatory transmitter in the suprachiasmatic nucleus (SCN). Ionotropic glutamate receptors (iGluRs) mediate transduction of light information from the retina to the SCN, an important circadian clock phase shifting pathway. Metabotropic glutamate receptors (mGluRs) may play a significant modulatory role. mGluR modulation of SCN responses to glutamate was investigated with fura-2 calcium imaging in SCN explant cultures. SCN neurons showed reproducible calcium responses to glutamate, kainate, and N-methyl-D-aspartate (NMDA). Although the type I/II mGluR agonists L-CCG-I and t-ACPD did not evoke calcium responses, they did inhibit kainate- and NMDA-evoked calcium rises. This interaction was insensitive to pertussis toxin. Protein kinase A (PKA) activation by 8-bromo-cAMP significantly reduced iGluR inhibition by mGluR agonists. The inhibitory effect of mGluRs was enhanced by activating protein kinase C (PKC) and significantly reduced in the presence of the PKC inhibitor H7. Previous reports show that L-type calcium channels can be modulated by PKC and PKA. In SCN cells, about one-half of the calcium rise evoked by kainate or NMDA was blocked by the L-type calcium channel antagonist nimodipine. Calcium rises evoked by K+ were used to test whether mGluR inhibition of iGluR calcium rises involved calcium channel modulation. These calcium rises were primarily attributable to activation of voltage-activated calcium channels. PKC activation inhibited K+-evoked calcium rises, but PKC inhibition did not affect L-CCG-I inhibition of these rises. In contrast, 8Br-cAMP had no effect alone but blocked L-CCG-I inhibition. Taken together, these results suggest that activation of mGluRs, likely type II, modulates glutamate-evoked calcium responses in SCN neurons. mGluR inhibition of iGluR calcium rises can be differentially influenced by PKC or PKA activation. Regulation of glutamate-mediated calcium influx could occur at L-type calcium channels, K+ channels, or at GluRs. It is proposed that mGluRs may be important regulators of glutamate responsivity in the circadian system.

Glutamate inhibits GABA excitatory activity in developing neurons

In contrast to the mature brain, in which GABA is the major inhibitory neurotransmitter, in the developing brain GABA can be excitatory, leading to depolarization, increased cytoplasmic calcium, and action potentials. We find in developing hypothalamic neurons that glutamate can inhibit the excitatory actions of GABA, as revealed with fura-2 digital imaging and whole-cell recording in cultures and brain slices. Several mechanisms for the inhibitory role of glutamate were identified. Glutamate reduced the amplitude of the cytoplasmic calcium rise evoked by GABA, in part by activation of group II metabotropic glutamate receptors (mGluRs). Presynaptically, activation of the group III mGluRs caused a striking inhibition of GABA release in early stages of synapse formation. Similar inhibitory actions of the group III mGluR agonist L-AP4 on depolarizing GABA activity were found in developing hypothalamic, cortical, and spinal cord neurons in vitro, suggesting this may be a widespread mechanism of inhibition in neurons throughout the developing brain. Antagonists of group III mGluRs increased GABA activity, suggesting an ongoing spontaneous glutamate-mediated inhibition of excitatory GABA actions in developing neurons. Northern blots revealed that many mGluRs were expressed early in brain development, including times of synaptogenesis. Together these data suggest that in developing neurons glutamate can inhibit the excitatory actions of GABA at both presynaptic and postsynaptic sites, and this may be one set of mechanisms whereby the actions of two excitatory transmitters, GABA and glutamate, do not lead to runaway excitation in the developing brain. In addition to its independent excitatory role that has been the subject of much attention, our data suggest that glutamate may also play an inhibitory role in modulating the calcium-elevating actions of GABA that may affect neuronal migration, synapse formation, neurite outgrowth, and growth cone guidance during early brain development.

Pituitary adenylate cyclase-activating peptide (PACAP) in the retinohypothalamic tract: a potential daytime regulator of the biological clock

The retinohypothalamic tract (RHT) relays photic information from the eyes to the suprachiasmatic nucleus (SCN). Activation of this pathway by light plays a role in adjusting circadian timing via a glutamatergic pathway at night. Here we report a new signaling pathway by which the RHT may regulate circadian timing in the daytime as well. We used dual immunocytochemistry for pituitary adenylate cyclase-activating peptide (PACAP) and the in vivo tracer cholera toxin subunit B and observed intense PACAP-immunoreactivity (PACAP-IR) in retinal afferents in the rat SCN as well as in the intergeniculate leaflet (IGL) of the thalamus. This PACAP-IR in the SCN as well as in the IGL was nearly lost after bilateral eye enucleation. PACAP afferents originated from small ganglion cells distributed throughout the retina. The phase of circadian rhythm measured as SCN neuronal activity in vitro was significantly advanced (3.5 +/- 0.4 hr) by application of 1 x 10(-6) M PACAP-38 during the subjective day [circadian time (CT)-6] but not at night (CT14 and CT19). The phase-shifting effect is channeled to the clock via a PACAP-R1 receptor, because mRNA from this receptor was demonstrated in the ventral SCN by in situ hybridization. Furthermore, vasoactive intestinal peptide was nearly 1000-fold less potent in stimulating a phase advance at CT6. The signaling mechanism was through a cAMP-dependent pathway, which could be blocked by a specific cAMP antagonist, Rp-cAMPS. Thus, in addition to its role in nocturnal regulation by glutamatergic neurotransmission, the RHT may adjust the biological clock by a PACAP/cAMP-dependent mechanism during the daytime.

Comparison of antibodies directed against receptor segments of NPY-receptors

The Y1-, Y2-, Y4- and Y5-receptor, which belong to the rhodopsin-like G-protein coupled, 7 transmembrane helix spanning receptors, bind the 36-mer neuromodulator NPY (neuropeptide Y) with nanomolar affinity. Synthetic fragments of the second (E2) and third (E3) extracellular loop were used to generate subtype selective anti-receptor antibodies against the Y-receptors. As investigated on intact receptors by ELISA and on solubilized receptors by SDS-PAGE and subsequent Western blotting, subtype selectivity was only partly achieved. Nevertheless, selectivity can be obtained by using several antisera in combination. These antibodies represent tools for molecular mass determination, receptor purification by affinity chromatography with antibody-columns and receptor localization studies.

Coexpression of multiple metabotropic glutamate receptors in axon terminals of single suprachiasmatic nucleus neurons

Glutamate is the primary excitatory transmitter in axons innervating the hypothalamic suprachiasmatic nucleus (SCN) and is responsible for light-induced phase shifts of circadian rhythms generated by the SCN. By using self-innervating single neuron cultures and patch-clamp electrophysiology, we studied metabotropic glutamate receptors (mGluRs) expressed by SCN neurons. The selective agonists for group I (3,5-dihydroxy-phenylglycine), group II ((S)-4-carboxy-3-hydroxyphenylglycine), and group III ((+)-2-amino-4-phosphonobutyric acid) mGluRs all depressed the evoked IPSC in a subset (33%) of single autaptic neurons, suggesting a coexpression of all three groups of mGluRs in the same axon terminals of a single neuron. Other neurons showed a variety of combinations of mGluRs, including an expression of only one group of mGluR (18%) or coexpression of two groups of mGluRs (27%). Some neurons had no response to any of the three agonists (22%). The three mGluR agonists had no effect on postsynaptic gamma-aminobutyric acid (GABA) receptor responses, indicating a presynaptic modulation of GABA release by mGluRs. We conclude that multiple mGluRs that act through different second messenger pathways are coexpressed in single axon terminals of SCN neurons where they modulate the release of GABA presynaptically, usually inhibiting release.

Pharmacological characterisation of metabotropic glutamatergic and purinergic receptors linked to Ca2+ signalling in hippocampal astrocytes

Intracellular Ca2+ ([Ca2+]i) signals induced by metabotropic glutamate receptor (mGluR) agonists and by purinergic agonists in cultured hippocampal astrocytes were investigated using [Ca2+]-sensitive fluorophores. The mGluR agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) and (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced [Ca2+]i responses in 76 and 93% of the cells, respectively. The broad-spectrum mGluR antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) and the mGluR1 antagonists (S)-4-carboxy-3-hydroxyphenylglycine (4C3HPG) and (S)-4-carboxyphenylglycine (4CPG) suppressed the agonist-evoked [Ca2+]i response in about 25% of the cells completely and in about 60% partially, depending on the agonist concentration employed. Together with immunohistochemical receptor localisations these results suggest the presence of at least two subpopulations of class I mGluRs recruited from the truncated splice variants of mGluR1 (mGluR 1b, 1c, 1d) and/or hitherto unknown glial-specific class I mGluRs. Of the hippocampal astrocytes 88, 92 or 83% of the cells responded with a [Ca2+]i elevation (mostly oscillations) to application of ATP, ADP, or 2-methylthio-ATP (2-MeS-ATP), respectively, whereas only 14 and 5% responded to AMP and adenosine, respectively, indicating the predominance of P2 receptors. The ATP-induced [Ca2+]i signal was suppressed by suramin. Release of Ca2+ from intracellular stores was involved in the response to ATP because the cells also exhibited [Ca2+]i elevations in Ca2+-free medium. Cells did not respond to 10 microM UTP. We conclude that the P2Y subtype represents the main [Ca2+]i-linked purinoceptor in hippocampal astrocytes. Sequential application of ATP and DHPG in Ca-free medium showed that metabotropic glutamate and purinergic receptors initiate release of Ca2+ from subsets of cyclopiazonic acid-sensitive Ca2+ stores which are partly independent.

Developmental plasticity of NR1 and NR2B subunit expression in the supraoptic nucleus of the rat hypothalamus

Vasopressin and oxytocin neuroendocrine cells within the supraoptic nucleus of the adult hypothalamus (SON) display mRNA expression for the NMDA receptor subunits, NR1 and NR2B, NR2C and NR2D. The NR2B subunit confers slow decay kinetics (relative to NR1/NR2A receptors) and high magnesium sensitivity to NMDA receptor responses--properties which may contribute to the NMDA receptor-mediated bursting manifested by these cells. Therefore, we examined NR2B protein expression and its developmental profile in the SON and compared it to that in the cortex and cerebellum--areas which have been studied previously. We performed Western blot analysis on SON homogenates from embryonic, postnatal (PN7, 14, 21), and adult rats using an NR2B-specific antibody. Adult NR2B levels in the SON and PVN were similar but low relative to those of cortex. SON NR2B protein levels rose in the first postnatal week, remained high through PN21, and later declined to significantly lower levels in the adult. A similar profile was observed in cerebellum, where NR2B expression displayed a sharp peak at PN14 and later declined to minimal or undetectable levels in the adult. In contrast, NR2B continued to be overexpressed through adulthood in the cortex. The ontogenic pattern for NR1 expression, which included unregulation during early postnatal life and adulthood, was similar in the SON and cortex. A different pattern was observed for the cerebellum, where NR1 levels increased gradually after ED17 to reach significantly greater adult levels. Of all three areas studied, the SON displayed the earliest developmental rise in NR1 levels. SON explant cultures proved to be a useful preparation, since they contained neurons which synthesized NR1 and NR2B subunits in quantities similar to those of ED17 SON. Our findings suggest that NMDA receptors on SON neuroendocrine cells are assembled using NR1 and NR2B subunits, and that their plastic expression in early postnatal life may play a role during development.

Changes in subcellular localization of metabotropic glutamate receptor subtypes during postnatal development of mouse thalamus

High resolution immunoelectron microscopy was used to study subcellular localization patterns of three metabotropic glutamate receptor subtypes (mGluR1alpha, mGluR5, and mGluR2/3) during postnatal development of mouse ventral posterior (VP) thalamic nucleus. Immunoreactivity for all three mGluRs was detected from birth (postnatal day 0, P0), but mGluR1alpha showed dramatic changes in localization with age. In the first postnatal week, mGluR1alpha immunoreactivity was mainly found in proximal dendrites and somata and not usually associated with synaptic contacts. From the second postnatal week, it became concentrated in distal dendrites and preferentially associated with corticothalamic (RS) synapses. mGluR5 immunoreactivity was weaker than mGluR1alpha immunoreactivity at all postnatal ages and showed a similar change in subcellular distribution to that of mGluR1alpha. It was also localized in astrocytic processes. mGluR2/3 immunoreactivity was mainly localized in astrocytic processes surrounding neuronal somata and synapses and this pattern was consistently maintained through all postnatal ages. A small number of presynaptic axon terminals were labeled for mGluR2/3 immunoreactivity and formed asymmetrical synapses. This study demonstrates that Group I mGluR proteins (mGluR1alpha and mGluR5) become redistributed in association with the development of corticothalamic function as demonstrated physiologically, whereas Group II mGluR proteins (mGluR2/3) are mainly associated with neuroglia.

Ionotropic and metabotropic glutamate receptor agonist-induced [Ca2+]i increase in isolated rat supraoptic neurons

In the present study, the effects of glutamate and of agonists for ionotropic and metabotropic glutamate receptors on intracellular Ca2+ concentration ([Ca2+]i) were investigated in neurons of the rat supraoptic nucleus (SON). We used the intracellular Ca2+ imaging technique with fura-2, in single magnocellular neurons dissociated from the SON of rats. Glutamate (10(-6)-10(-4) M) evoked a dose-dependent increase in [Ca2+]i. The glutamate agonists exerted similar effects, although with some differences in the characteristics of their responses. The [Ca2+]i response to NMDA was smaller than those of glutamate or the non-NMDA receptor agonists, AMPA and kainate, but was significantly enhanced by the removal of extracellular Mg2+. Glutamate, as well as quisqualate, an agonist for both ionotropic and metabotropic glutamate receptors, evoked a [Ca2+]i increase in a Ca2+-free condition, suggesting Ca2+ release from intracellular Ca2+ stores. This was further evidenced by [Ca2+]i increases in response to a more selective metabotropic glutamate receptor agonist, t-ACPD, in the absence of extracellular Ca2+. Furthermore, the quisqualate-induced Ca2+ release was abolished by the selective metabotropic glutamate receptor antagonist, (S)-4-carboxyphenylglycine. The results suggest that metabotropic glutamate receptors as well as non-NMDA and NMDA receptors are present in the SON neurons, and that activation of the first leads to Ca2+ release from intracellular Ca2+ stores and the activation of the latter two types induces Ca2+ entry. These dual mechanisms of Ca2+ signalling may play a role in the regulation of SON neurosecretory cells by glutamate.

Diversity of structure and function at mammalian central synapses

Our appreciation of the relationship between synaptic structure and function, and in particular our understanding of quantal synaptic transmission, is derived from classical studies on the neuromuscular junction. However, physiological studies of quantal transmission at mammalian CNS synapses have produced a variety of results, and thus no consensus of opinion has emerged. This variability could be due, in part, to experimental and analytical limitations or to differences in the structural and functional features of central synapses, or both. Some of the experimental limitations have recently been overcome by the use of novel preparations that permit direct measurement of quantal synaptic events in the CNS. Although these studies reveal similarities between the synaptic mechanisms of the neuromuscular junction and CNS synapses, important differences and specializations are also evident. The purpose of this review is to highlight the structural and functional diversity of synapses in the mammalian CNS, and to discuss the potential relevance of structural features to synaptic function.

Immunocytochemical localization of the mGluR1b metabotropic glutamate receptor in the rat hypothalamus

The mGluR1 metabotropic glutamate receptor is a G-protein-coupled receptor that exists as different C-terminal splice variants. When expressed in mammalian cells, the mGluR1 splice variants exhibit diverse transduction mechanisms and also slightly differ in their apparent agonist affinities. In the present study, we used an affinity-purified antiserum, specifically reactive to the mGluRlb splice variant, in combination with a highly sensitive preembedding immunocytochemical method for light microscopy to investigate the distribution of this receptor in the rat hypothalamus. An intense immunoreactivity for mGluRlb was observed in distinct hypothalamic nuclei. Thus, neuronal cell bodies and dendrites were stained in the preoptic area, suprachiasmatic nucleus, dorsal hypothalamus, lateral hypothalamus, dorsomedial nucleus, tuberomammilary nucleus, and lateral mammilary body. The ventromedial nucleus exhibited neuropil immunostaining but neuronal cell bodies were not labeled. Strong mGluRlb immunoreactivity was observed in magnocellular neurons of the neuroendocrine supraoptic, paraventricular, and arcuate nuclei. Also, neuronal cell bodies were heavily labeled in the retrochiasmatic nucleus, anterior commissural nucleus, and periventricular nucleus. These immunocytochemical observations, together with previous studies, suggest that mGluRlb is coexpressed with other class I mGluRs in some nuclei throughout the hypothalamus. However, mGluRlb is so far the only receptor of this class strongly expressed in the supraoptic, paraventricular, and arcuate nuclei, which might have relevant implications in the physiological control of the neuroendocrine hypothalamic-pituitary system.

Glial calcium: homeostasis and signaling function

Glial cells respond to various electrical, mechanical, and chemical stimuli, including neurotransmitters, neuromodulators, and hormones, with an increase in intracellular Ca2+ concentration ([Ca2+]i). The increases exhibit a variety of temporal and spatial patterns. These [Ca2+]i responses result from the coordinated activity of a number of molecular cascades responsible for Ca2+ movement into or out of the cytoplasm either by way of the extracellular space or intracellular stores. Transplasmalemmal Ca2+ movements may be controlled by several types of voltage- and ligand-gated Ca(2+)-permeable channels as well as Ca2+ pumps and a Na+/Ca2+ exchanger. In addition, glial cells express various metabotropic receptors coupled to intracellular Ca2+ stores through the intracellular messenger inositol 1,4,5-triphosphate. The interplay of different molecular cascades enables the development of agonist-specific patterns of Ca2+ responses. Such agonist specificity may provide a means for intracellular and intercellular information coding. Calcium signals can traverse gap junctions between glial cells without decrement. These waves can serve as a substrate for integration of glial activity. By controlling gap junction conductance, Ca2+ waves may define the limits of functional glial networks. Neuronal activity can trigger [Ca2+]i signals in apposed glial cells, and moreover, there is some evidence that glial [Ca2+]i waves can affect neurons. Glial Ca2+ signaling can be regarded as a form of glial excitability.

Light regulates Homer mRNA expression in the rat suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus (SCN) of the mammal is the circadian pacemaker responsible for generation of circadian rhythms. Several immediate-early genes are expressed in the SCN by light stimuli which induce phase shifts of animal activity rhythms. In the present study, we investigated whether Homer, a PDZ-like protein which is rapidly induced following synaptic activation, mRNA expression is regulated by light in rat SCN. Homer mRNA expression in the SCN of rat killed at 4 h after onset of the light and dark phases was very low. One hour light stimuli during the subjective night dramatically induced Homer mRNA expression in the ventrolateral portion of the SCN, whereas light stimuli during the subjective light phase did not. This finding implies that Homer may be involved in the photic entrainment of the circadian clock.

Action of a metabotropic glutamate receptor agonist in rat lateral septum: induction of a sodium-dependent inward aftercurrent

The mechanism by which (1S,3R)-ACPD, a metabotropic glutamate receptor agonist, induces burst firing in lateral septal neurons of the rat was investigated in coronal brainstem slices. Membrane currents were characterized in voltage clamp using whole-cell recordings. In the presence of (1S,3R)-ACPD, following depolarizing voltage jumps, repolarization towards the holding potential generated an inward aftercurrent. It could have a plateau-like phase and decayed exponentially. This (1S,3R)-ACPD-dependent inward aftercurrent was accompanied by an increase in cell conductance and was reduced following partial replacement of extracellular sodium by N-methyl-D-glucamine. It was unaffected by TEA or barium, and persisted in Cs-loaded neurons or following partial replacement of extracellular chloride by isethionate. This suggests that it was mainly carried by sodium. Loading neurons with the calcium chelator, BAPTA, or blocking transmembrane calcium currents, suppressed the (1S,3R)-ACPD-dependent aftercurrent. By contrast, partial replacement of extracellular sodium by lithium did not affect it. Thus, this current was dependent upon calcium influx but was not due to a sodium/calcium exchanger. It was probably mediated by G protein activation. Indeed, in neurons loaded with GTP-gamma-S, following depolarizing voltage jumps, repolarization towards the holding potential revealed an inward aftercurrent having properties similar to those of the (1S,3R)-ACPD-dependent current. We suggest that (1S,3R)-ACPD induced calcium-activated non-selective channels. In the presence of this agonist, a depolarization-evoked calcium influx could thus evoke a cationic inward current. This current probably promotes the burst firing observed in lateral septal neurons in current clamp.

Differential plasma membrane distribution of metabotropic glutamate receptors mGluR1 alpha, mGluR2 and mGluR5, relative to neurotransmitter release sites

Two group I metabotropic glutamate receptor subtypes, mGluR1 and mGluR5, have been reported to occur in highest concentration in an annulus surrounding the edge of the postsynaptic membrane specialisation. In order to determine whether such a distribution is uniform amongst postsynaptic mGluRs, their distribution was compared quantitatively by a pre-embedding silver-intensified immunogold technique at electron microscopic level in hippocampal pyramidal cells (mGluR5), cerebellar Purkinje cells (mGluR1 alpha) and Golgi cells (mGluR2). The results show that mGluR1 alpha, mGluR5 and mGluR2 each have a distinct distribution in relation to the glutamatergic synaptic junctions. On dendritic spines, mGluR1 alpha and mGluR5 showed the highest receptor density in a perisynaptic annulus (defined as within 60 nm of the edge of the synapse) followed by a decreasing extrasynaptic (60-900 nm) receptor level, but the gradient of decrease and the proportion of the perisynaptic pool (mGluR1 alpha, approximately 50%; vs mGluR5, approximately 25%) were different for the two receptors. The distributions of mGluR1 alpha and mGluR5 also differed significantly from simulated random distributions. In contrast, mGluR2 was not closely associated with glutamatergic synapses in the dendritic plasma membrane of cerebellar Golgi cells and its distribution relative to synapses is not different from simulated random distribution in the membrane. The somatic membrane, the axon and the synaptic boutons of the GABAergic Golgi cells also contained immunoreactive mGluR2 that is not associated with synaptic specialisations. In the hippocampal CA1 area the distribution of immunoparticles for mGluR5 on individual spines was established using serial sections. The results indicate that dendritic spines of pyramidal cells are heterogeneous with respect to the ratio of perisynaptic to extrasynaptic mGluR5 pools and about half of the immunopositive spines lack the perisynaptic pool. The quantitative comparison of receptor distributions demonstrates that mGluR1 alpha and mGluR5, but not mGluR2, are highly compartmentalised in different plasma membrane domains. The unique distribution of each mGluR subtype may reflect requirements for different transduction and effector mechanisms between cell types and different domains of the same cell, and suggests that the precise placement of receptors is a crucial factor contributing to neuronal communication.

Developmentally regulated gene expression of all eight metabotropic glutamate receptors in hypothalamic suprachiasmatic and arcuate nuclei--a PCR analysis

Previous studies have demonstrated the critical role glutamate plays in the hypothalamus, both in the developing and adult brain. The expression of metabotropic glutamate receptor (mGluR) mRNA (mGluR1-8) was studied in the suprachiasmatic (SCN) and arcuate (ARC) nuclei. Using reverse Northern blots and cDNA-PCR, we found that all eight cloned mGluRs were expressed in these brain regions. Most had not previously been detected here. Surprisingly, this included mGluRs that had previously been thought to be restricted to the retina, such as mGluR6. We also detected, cloned, and sequenced a splice variant of mGluR7 (mGluR7b). Developmentally, the age of maximal expression of mGluRs was dependent on the region. For instance, mGluR5 was more strongly expressed in neonatal ARC than in adult, whereas the opposite was true in the SCN. Compared with P10 neonates, mGluR1, R3, R6, R7a, R7b, and R8 showed a greater expression in adult SCN and ARC.

mGluR5 metabotropic glutamate receptor distribution in rat and human spinal cord: a developmental study

By combining biochemical, molecular and immunohistochemical approaches, we have investigated the presence of metabotropic glutamate receptors (mGluRs) belonging to the subtype 5 in rat and human spinal cords and the developmental changes in their expression. A polyclonal antibody raised against the carboxy-terminal portion of mGluR5 was used to study the distribution of the receptor in rat foetal (Et15), neonatal (P8) and adult spinal cords and dorsal root ganglia (DRG). mGluR5 appeared to be predominantly expressed in regions containing the primary sensory afferents. Immunoblotting with anti-mGluR5 antibody revealed lower receptor protein levels in rat adult spinal cord when compared with P8 rat spinal cord. Reverse transcriptase-polymerase chain reaction showed both mGluR5a and mGluR5b mRNAs expression in rat spinal cord. The mGluR5a variant was found more abundant in young animals than in adults. The pattern of mGluR5 immunostaining was also studied in foetal (6-8, 10, 12 and 22 weeks of gestation) and adult human spinal cord. At all stages of human development, a strong mGluR5 immunoreactivity was observed in the dorsal roots and in the dorsal and dorsolateral funiculi with maximum levels of staining at week 12 of gestation. Foetal DRG neurons were heterogeneously labeled. mGluR5 was also diffusely detectable in the mantle layer. In adult human spinal cords, immunoreactivity was confined to laminae I and II of the dorsal horns. These results demonstrate for the first time the presence of mGluR5 in human spinal cord. The distribution of this receptor suggests a role in the development of somatosensory pathways and in the control of nociceptive neurotransmission.