Characteristics of serotonin receptors in the rat brain

Two biochemical methods are currently available for studying 5-HT receptors in the central nervous system. The first consists of measuring the specific high affinity binding of 3H-5-HT to synaptic membranes. The other derives from the discovery of an adenylate cyclase which can be activated by 5-HT in brain homogenates. Whereas the specific 3H-5-HT binding is measurable in young as well as in adult rats, the 5-HT-sensitive adenylate cyclase can be quantitatively estimated only during the first three weeks following birth. Later on, the increment of adenylate cyclase activity produced by 5-HT is too low to permit valid measurements, notably in tissues from adult rats. Studies on the effects of various agonists and antagonists demonstrated that the specific binding site characterized by a high affinity for 3H-5-HT (Kd = 1.5 nM) exhibited the expected properties of a 5-HT receptor in brain. Performing chemical lesions on serotoninergic neurons by an intracerebral injection of 5, 7-dihydroxytryptamine or the blockade of central 5-HT receptors by the peripheral administration of methiothepin resulted in a subsequent increase in the number of specific binding sites for 3H-5-HT particularly in the hippocampus (+30 to +45%). In contrast, preliminary attempts to detect any supersensitivity of the 5-HT-sensitive adenylate cyclase after selective raphe lesions were unsuccessful. Indeed, several observations strongly suggested that the high affinity binding site for 3H-5-HT did not correspond to the 5-HT receptor coupled to adenylate cyclase in synaptic membranes: 1) the apparent affinity of the 5-HT-sensitive adenylate cyclase for 5-HT was about 300 times lower (Kd = 0.5 microM) than that of the specific 3H-5-HT binding site; 2)the ontogenic evolutions of 3H-5-HT binding and 5-HT-sensitive adenylate cyclase were not parallel, notably in the hippocampus; 3) they were differently affected by several drugs. For instance, quipazine, a putative 5-HT agonist, effectively displaced 3H-5-HT from its specific binding site (Ki = 0.23 microM) whereas it did not affect 5-HT-sensitive adenylate cyclase. In conclusion, it is likely that the high affinity binding site for 3H-5-HT and the 5-HT-sensitive adenylate cyclase belong to two different postsynaptic 5-HT receptors in the rat brain.

Chitinase 3-like proteins as diagnostic and prognostic biomarkers of multiple sclerosis

Background:

Despite sensitivity of MRI to diagnose multiple sclerosis (MS), prognostic biomarkers are still needed for optimized treatment.

Objective:

The objective of this paper is to identify cerebrospinal fluid (CSF) diagnostic biomarkers of MS using quantitative proteomics and to analyze their expression at different disease stages.

Methods:

We conducted differential analysis of the CSF proteome from control and relapsing–remitting MS (RRMS) patients followed by verification by ELISA of candidate biomarkers in CSF and serum in control, clinically isolated syndrome (CIS), RRMS and progressive MS (PMS) patients.

Results:

Twenty-two of the 527 quantified proteins exhibited different abundances in control and RRMS CSF. These include chitinase 3-like protein 1 (CHI3L1) and 2 (CHI3L2), which showed a strong expression in brain of MS patients, especially in astrocytes and microglial cells from white matter plaques. CSF and serum CHI3L1 levels increased with the disease stage and CIS patients with high CSF (>189 ng/ml) and serum (>33 ng/ml) CHI3L1 converted more rapidly to RRMS (log rank test, p < 0.05 and p < 0.001, respectively). In contrast, CSF CHI3L2 levels were lower in PMS than in RRMS patients. Accordingly, CSF CHI3L1/CHI3L2 ratio accurately discriminated PMS from RRMS.

Conclusions:

CSF CHI3L1 and CHI3L2 and serum CHI3L1 might help to define MS disease stage and have a prognostic value in CIS.

The nucleus accumbens 5-HTR₄-CART pathway ties anorexia to hyperactivity

In mental diseases, the brain does not systematically adjust motor activity to feeding. Probably, the most outlined example is the association between hyperactivity and anorexia in Anorexia nervosa. The neural underpinnings of this 'paradox', however, are poorly elucidated. Although anorexia and hyperactivity prevail over self-preservation, both symptoms rarely exist independently, suggesting commonalities in neural pathways, most likely in the reward system. We previously discovered an addictive molecular facet of anorexia, involving production, in the nucleus accumbens (NAc), of the same transcripts stimulated in response to cocaine and amphetamine (CART) upon stimulation of the 5-HT(4) receptors (5-HTR(4)) or MDMA (ecstasy). Here, we tested whether this pathway predisposes not only to anorexia but also to hyperactivity. Following food restriction, mice are expected to overeat. However, selecting hyperactive and addiction-related animal models, we observed that mice lacking 5-HTR(1B) self-imposed food restriction after deprivation and still displayed anorexia and hyperactivity after ecstasy. Decryption of the mechanisms showed a gain-of-function of 5-HTR(4) in the absence of 5-HTR(1B), associated with CART surplus in the NAc and not in other brain areas. NAc-5-HTR(4) overexpression upregulated NAc-CART, provoked anorexia and hyperactivity. NAc-5-HTR(4) knockdown or blockade reduced ecstasy-induced hyperactivity. Finally, NAc-CART knockdown suppressed hyperactivity upon stimulation of the NAc-5-HTR(4). Additionally, inactivating NAc-5-HTR(4) suppressed ecstasy's preference, strengthening the rewarding facet of anorexia. In conclusion, the NAc-5-HTR(4)/CART pathway establishes a 'tight-junction' between anorexia and hyperactivity, suggesting the existence of a primary functional unit susceptible to limit overeating associated with resting following homeostasis rules.

Metabotropic glutamate receptors: from the workbench to the bedside

Metabotropic glutamate (mGlu) receptors were discovered in the mid 1980s and originally described as glutamate receptors coupled to polyphosphoinositide hydrolysis. Almost 6500 articles have been published since then, and subtype-selective mGlu receptor ligands are now under clinical development for the treatment of a variety of disorders such as Fragile-X syndrome, schizophrenia, Parkinson's disease and L-DOPA-induced dyskinesias, generalized anxiety disorder, chronic pain, and gastroesophageal reflux disorder. Prof. Erminio Costa was linked to the early times of the mGlu receptor history, when a few research groups challenged the general belief that glutamate could only activate ionotropic receptors and all metabolic responses to glutamate were secondary to calcium entry. This review moves from those nostalgic times to the most recent advances in the physiology and pharmacology of mGlu receptors, and highlights the role of individual mGlu receptor subtypes in the pathophysiology of human disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Physical interaction between the serotonin transporter and neuronal nitric oxide synthase underlies reciprocal modulation of their activity

The spatiotemporal regulation of neurotransmitter transporters involves proteins that interact with their intracellular domains. Using a proteomic approach, we identified several proteins that interact with the C terminus of the serotonin transporter (SERT). These included neuronal nitric oxide synthase (nNOS), a PSD-95/Disc large/ZO-1 (PDZ) domain-containing protein recruited by the atypical PDZ binding motif of SERT. Coexpression of nNOS with SERT in HEK293 cells decreased SERT cell surface localization and 5-hydroxytryptamine (5-HT) uptake. These effects were absent in cells transfected with SERT mutated in its PDZ motif to prevent physical association with nNOS, and 5-HT uptake was unaffected by activation or inhibition of nNOS enzymatic activity. 5-HT uptake into brain synaptosomes was increased in both nNOS-deficient and wild-type mice i.v. injected with a membrane-permeant peptidyl mimetic of SERT C terminus, which disrupted interaction between SERT and nNOS, suggesting that nNOS reduces SERT activity in vivo. Furthermore, treating cultured mesencephalic neurons with the mimetic peptide similarly increased 5-HT uptake. Reciprocally, indicating that 5-HT uptake stimulates nNOS activity, NO production was enhanced on exposure of cells cotransfected with nNOS and SERT to 5-HT. This effect was abolished by 5-HT uptake inhibitors and absent in cells expressing SERT mutated in its PDZ motif. In conclusion, physical association between nNOS and SERT provides a molecular substrate for their reciprocal functional modulation. In addition to showing that nNOS controls cell surface localization of SERT, these findings provide evidence for regulation of cellular signaling (NO production) by a substrate-carrying transporter.

[The Lemierre syndrome: a complicated oropharyngeal infection]

The Lemierre syndrome or 'necrobacillosis' is a post angina sepsis caused by an acute oropharyngeal infection with a secondary thrombophlebitis of the internal jugular vein. There are often septic emboli in the lungs, although intestinal organs can also be affected. This syndrome is caused by the strictly anaerobic gram-negative pathogen Fusobacterium necrophorum, sometimes in combination with other pathogens. The patient typically presents with high fever, pain in the neck, malaise and dyspnoea one week after the start of an angina. Plain chest radiograph shows bilateral nodular infiltrates, ultrasound reveals a thrombophlebitis of the internal jugular vein. CT scan can be useful to confirm the diagnosis and possible complications. In the beginning there is often a transient hyperbilirubinemia with toxic inflammatory blood results. Under the correct antibiotic regime complete recovery can be obtained.

GPCR-interacting proteins (GIPs): nature and functions

The simplistic idea that seven transmembrane receptors are single monomeric proteins that interact with heterotrimeric G-proteins after agonist binding is definitively out of date. Indeed, GPCRs (G-protein-coupled receptors) are part of multiprotein networks organized around scaffolding proteins. These GIPs (GPCR-interacting proteins) are either transmembrane or cytosolic proteins. Proteomic approaches can be used to get global pictures of these 'receptosomes'. This approach allowed us to identify direct but also indirect binding partners of serotonin receptors. GIPs are involved in a wide range of functions including control of the targeting, trafficking and signalling of GPCRs. One of them, Shank, which is a secondary and tertiary partner of metabotropic and ionotropic glutamate receptors, respectively, can induce the formation of a whole functional glutamate 'receptosome' and the structure to which it is associated, the dendritic spine.

Modulation of memory processes and cellular excitability in the dentate gyrus of freely moving rats by a 5-HT4 receptors partial agonist, and an antagonist

Firstly, olfactory association learning was used to determine the modulating effect of 5-HT4 receptor involvement in learning and long-term memory. Secondly, the effects of systemic injections of a 5-HT4 partial agonist and an antagonist on long-term potentiation (LTP) and depotentiation in the dentate gyrus (DG) were tested in freely moving rats. The modulating role of the 5-HT4 receptors was studied by using a potent, 5-HT4 partial agonist RS 67333 [1-(4-amino-5-chloro-2-methoxyphenyl)-3-(1-n-butyl-4-piperidinyl)-1-propanone] and a selective 5-HT4 receptor antagonist RS 67532 [1-(4-amino-5-chloro-2-(3,5-dimethoxybenzyloxyphenyl)-5-(1-piperidinyl)-1-propanone]. Agonist or antagonist systemic chronic injections prior to five training sessions yielded a facilitatory effect on procedural memory during the first session only with the antagonist. Systemic injection of the antagonist only before the first training session improved procedural memory during the first session and associative memory during the second session. Similar injection with the 5-HT4 partial agonist had an opposite effect. The systemic injection of the 5-HT4 partial agonist prior to the induction of LTP in the dentate gyrus by high-frequency stimulation was followed by a population spike increase, while the systemic injection of the antagonist accelerated the depotentiation 48 h later. The behavioural and physiological results pointed out the involvement of 5-HT4 receptors in processing related to the long-term hippocampal-dependent memory system, and suggest that specific 5-HT4 agonists could be used to treat amnesic patients with a dysfunction in this particular system.

5-HT4 Receptors

Serotonin 4 receptors (5-HT(4)Rs) were discovered 15 years ago. They are coded by a very complex gene (700Kb, 38 exons) which generates eight carboxy-terminal variants (a, b, c, d, e, f, g, n). Their sequences differ after position L(358). Another variant is characterized by a 14 residue insertion within the extracellular loop 2. Highly selective potent 5-HT(4) receptor antagonists and partial agonists which cross the blood-brain barrier have been synthesized, but a specific full agonist for brain studies is still missing. Based on physiological and behavioral experiments, 5-HT(4)Rs may be targets to treat cognitive deficits, abdominal pain and feeding disorders. One 5-HT(4)R-directed drug (SL65.0155) is already in phase II to treat patients suffering from memory deficits or dementia.

Neuroprotective activity of antazoline against neuronal damage induced by limbic status epilepticus

Imidazoline drugs exert neuroprotective effects in cerebral ischaemia models. They also have effects against mouse cerebellar and striatal neuronal death induced by N-methyl-D-aspartate (NMDA) through the blockade of NMDA currents. Here, we investigated the effects of antazoline on NMDA toxicity and current in rat hippocampal neuronal cultures, and on an in vivo model of status epilepticus. In hippocampal cultures, antazoline (30 microM) decreased NMDA-mediated neurotoxicity and also blocked the NMDA current with voltage-dependent and fast-reversible action (inhibition by 85+/-3% at -60 mV). Status epilepticus was induced by injecting pilocarpine (200 nmol) directly into the right pyriform cortex of male adult rats. The rats then received immediately three consecutive i.p. injections at 30-min intervals of either PBS (control group) or antazoline at 10 mg/kg (low-dose group) or at 45 mg/kg (high-dose group). During the 6-h recording, status epilepticus lasted more than 200 min in all groups. In the high-dose group only, seizures completely ceased 1 h after the third injection of antazoline, then started again 1 h later. Rats were killed 1 week later, and Cresyl Violet-stained sections of their brain were analysed for damage quantification. On the ipsilateral side to the pilocarpine injection, pyriform cortex and hippocampal CA1 and CA3 areas were significantly protected in both antazoline-treated groups, whilst prepyriform and entorhinal cortices were only in the high-dose group. On the contralateral side to the pilocarpine injection, only the hippocampal CA3 area was significantly protected in the low-dose group, but all investigated structures were in the high-dose group. In conclusion, antazoline is a potent neuroprotective drug in different models of neuronal primary culture, as previously shown in striatal and cerebellar granule neurons [Neuropharmacology 39 (2000) 2244], and here in hippocampal neurons. Antazoline is also neuroprotective in vivo in the intra-pyriform pilocarpine-induced status epilepticus model.

Impaired somatodendritic responses to pituitary adenylate cyclase-activating polypeptide (PACAP) of supraoptic neurones in PACAP type I -receptor deficient mice

The role of pituitary adenylate cyclase-activating polypeptide (PACAP) type I receptor (PAC1 receptor) in regulating hypothalamic supraoptic neurones was investigated using PAC1 receptor-deficient male mice (PAC1-/-). The effects of PACAP on [Ca2+]i were investigated in freshly dissociated supraoptic neurones and on the somatodendritic release of vasopressin and oxytocin, examined on intact supraoptic nuclei. In supraoptic neurones from wild-type mice (PAC1+/+), 100 nm PACAP induced an increase in [Ca2+]i and release of vasopressin and oxytocin, whereas in heterozygous (PAC1+/-) and null-mutant mice (PAC1-/-), PACAP was much less effective. PACAP had no effect on these two parameters when applied to isolated neurohypophysial nerve terminals of PAC1+/+ and PAC1-/- mice, and rats. In conclusion, the PAC1 receptor is solely responsible for the PACAP-induced [Ca2+]i signalling and secretion of vasopressin and oxytocin in the somatodendritic region of supraoptic neurones.

Activation of a Large-conductance Ca2+-Dependent K+ Channel by Stimulation of Glutamate Phosphoinositide-coupled Receptors in Cultured Cerebellar Granule Cells

Trans-1-amino-cyclopentyl-1,3-dicarboxylic acid (trans-ACPD), a specific agonist of the glutamate phosphoinositide-coupled receptor (Qp receptor), increased the amplitude of the outward K+ current recorded in the whole-cell configuration of the patch-clamp technique in mouse cultured cerebellar granule cells. This effect was abolished by buffering internal Ca2+ with BAPTA [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid]. Activation of a large-conductance K+ channel was observed when trans-ACPD or quisqualic acid (QA), another Qp receptor agonist, was applied outside the cell-attached patch pipettes. No activation was observed with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a specific agonist of ionotropic non-N-methyl-d-aspartate (non-NMDA) receptors. The effects of trans-ACPD or QA were potentiated in the presence of external Ca2+. The channel was also directly activated by both micromolar concentrations of internal Ca2+ and membrane depolarization. Its unitary conductance was 100 - 115 pS under asymmetrical K+ and 195 - 235 pS under high symmetrical K+ conditions. In the absence of agonist, the channel was blocked by 1 mM external tetraethylammonium. This is the first description of a large conductance Ca2+-activated K+ channel in cultured cerebellar granule cells. It possesses properties similar to those of the so-called 'big K+ channel' described in other preparations. Our cell-attached experiments demonstrated an indirect coupling between Qp receptors and this channel. The most likely hypothesis is that the second messenger system inositol 1,4,5-triphosphate (IP3)-Ca2+ was involved in the coupling process. This hypothesis was further strengthened by our whole-cell experiments. On the basis of the voltage- and Ca2+-sensitivities of the studied channel, we estimated an increase of 350 to 570 nM in internal Ca2+ concentration when Qp receptors were stimulated by 100 microM trans-ACPD. Under physiological conditions, stimulation of Qp receptors by the endogenous neurotransmitter should lead to similar K+ channel activation and therefore would tend to reduce the efficacy of ionotropic glutamate synaptic receptor stimulation responsible for cell excitation.

A Nitric Oxide Synthase Activity Selectively Stimulated by NMDA Receptors Depends on Protein Kinase C Activation in Mouse Striatal Neurons

In mouse striatal neurons in primary culture, the maximal increase in intracellular cyclic guanosine monophosphate level evoked by N-methyl-d-aspartic acid (NMDA) receptor activation was twice that induced by kainate, KCl and ionomycin. Quisqualate was almost inactive. All responses were mediated by nitric oxide (NO) production since they were blocked by haemoglobin (a NO scavenger) and by l-NG-monomethylarginine and l-NG-nitroarginine, the effects of both arginine analogues being reversed by an excess of l-arginine. Several results indicate that NMDA receptors stimulate a specific NO synthase activity. This specifically NMDA-activated NO synthase was blocked by nanomolar concentrations of l-NG-nitroarginine, whereas the responses evoked by other agents, including kainate, KCl and ionomycin, were only blocked by micromolar concentrations of this NO synthase inhibitor. The NMDA response could not be totally reproduced by an increase in cytosolic calcium (Ca2+) alone. In contrast, in the presence of staurosporine, an inhibitor of protein kinases C (PKC), as well as after desensitization of PKC induced by long-term treatment with the phorbol ester, phorbol-12, 13-dibutyrate, NMDA-stimulated NO production was selectively reduced, reaching the level evoked by kainate or Ca2+ increase. In conclusion, our results suggest that in striatal neurons, NMDA selectively stimulates a NO synthase activity which is inhibited by low concentrations of l-NG-nitro-arginine, through a mechanism involving PKC.

PICK1 is required for the control of synaptic transmission by the metabotropic glutamate receptor 7

Both postsynaptic density and presynaptic active zone are structural matrix containing scaffolding proteins that are involved in the organization of the synapse. Little is known about the functional role of these proteins in the signaling of presynaptic receptors. Here we show that the interaction of the presynaptic metabotropic glutamate (mGlu) receptor subtype, mGlu7a, with the postsynaptic density-95 disc-large zona occludens 1 (PDZ) domain-containing protein, PICK1, is required for specific inhibition of P/Q-type Ca(2+) channels, in cultured cerebellar granule neurons. Furthermore, we show that activation of the presynaptic mGlu7a receptor inhibits synaptic transmission and this effect also requires the presence of PICK1. These results indicate that the scaffolding protein, PICK1, plays an essential role in the control of synaptic transmission by the mGlu7a receptor complex.

Group 2 metabotropic glutamate receptors induced long term depression in mouse striatal slices

We studied the roles of mGlu2/3 receptors (mGlu2/3) in glutamatergic transmission at corticostriatal synapses in mice brain slices. Perfusion of the selective mGlu2/3 agonists LY354740 and L-CCG1 caused the long term depression (LTD) of evoked synaptic responses. Photonic and electronic microscopy showed mGlu2/3 on axonal fibers and glial processes but not on striatal dendrites. mGlu2/3-LTD was independent of synaptic activity and insensitive to specific antagonists of dopamine D1, D2, GABA(B), N-methyl-D-aspartate or adenosine A1 receptors. Manipulation of the cAMP/protein kinase A cascade had no effect on the mGlu2/3-LTD. In contrast, MEK1-2 inhibitors reduced both mGlu2/3 initial depression and LTD suggesting the involvement of the mitogen activated kinase pathway in mGlu2/3-LTD.

The C terminus of the metabotropic glutamate receptor subtypes 2 and 7 specifies the receptor signaling pathways

There is accumulating evidence that the specificity of the transduction cascades activated by G protein-coupled receptors cannot solely depend on the nature of the coupled G protein. To identify additional structural determinants, we studied two metabotropic glutamate (mGlu) receptors, the mGlu2 and mGlu7 receptors, that are both coupled to G(o) proteins but are known to affect different effectors in neurons. Thus, the mGlu2 receptor selectively blocks N- and L-type Ca(2+) channels via a protein kinase C-independent pathway, whereas the mGlu7 receptor selectively blocks P/Q-type Ca(2+) channels via a protein kinase C-dependent pathway, and both effects are pertussis toxin-sensitive. We examined the role of the C-terminal domain of these receptors in this coupling. Chimeras were constructed by exchanging the C terminus of these receptors and transfected into neurons. Different chimeric receptors bearing the C terminus of mGlu7 receptor blocked selectively P/Q-type Ca(2+) channels, whereas chimeras bearing the C terminus of mGlu2 receptor selectively blocked N- and L-type Ca(2+) channels. These results show that the C terminus of mGlu2 and mGlu7 receptors is a key structural determinant that allows these receptors to select a specific signaling pathway in neurons.

AMPA receptor activation induces association of G-beta protein with the alpha subunit of the sodium channel in neurons

Glutamatergic transmission is mediated by ionotropic receptors that directly gate cationic channels and metabotropic receptors that are coupled to second messenger generating systems and to ionic channels via heterotrimeric guanine-nucleotide binding- (G) proteins. This distinction cannot be made for the ionotropic receptor subclass activated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), which has been shown to be physically associated with the alpha-subunit of Gi1 protein and activates this G-protein. Here, we report that, in addition to a Ca2+ influx, AMPA induces the mobilization of Ca2+ from the mitochondrial pool by reversing the mitochondrial Na+/Ca2+ exchanger in mouse neurons in primary culture. Both processes required the activation of tetrodotoxin-sensitive Na+ channels. AMPA receptor activation modified the gating properties of the Na+ channel, independently of the AMPA current, suggesting a G-protein-mediated process. Indeed, co-immunoprecipitation experiments indicated that AMPA receptor activation induced the association of Gbeta with the alpha-subunit of the Na+ channel. These results suggest that, in addition to its ionic channel function, the AMPA receptor is coupled to Na+ channels through G-proteins and that this novel metabotropic function is involved in the control of neuronal excitability.

Molecular events involved in neuronal death induced in the mouse hippocampus by in-vivo injection of kainic acid

Apoptosis results from the activation of a programmed cellular cascade involving several mechanisms. In the present study, we have investigated the implication of three molecules of this cascade, p53, Bax and caspase-3, in neuronal death induced by kainic acid (KA) administration in mouse hippocampus. Using immunocytochemistry, western blot and quantification of enzyme activity, we observed in p53+/+ and p53-/- animals that KA induced neuronal death by both p53-dependent and independent pathways. Moreover, apoptosis (labeled by TUNEL) and the increase of bax and caspase-3 protein expression after the neurotoxic insult appeared to clearly depend on p53 expression.

Distribution of metabotropic glutamate receptor DmGlu-A in Drosophila melanogaster central nervous system

L-glutamate is the excitatory neurotransmitter at neuromuscular junctions in insects. It may also be involved in neurotransmission within the central nervous system (CNS), but its function therein remains elusive. The roles of glutamatergic synapses in the Drosophila melanogaster CNS were investigated, with focus on the study of DmGluRA, a G-protein-coupled glutamate receptor. In a first attempt to determine the function of this receptor, we describe its distribution in the larval and adult Drosophila CNS, using a polyclonal antibody raised against the C-terminal sequence of the protein. DmGluRA is expressed in a reproducible pattern both in the larva and in the adult. In particular, DmGluRA can be found in the antennal lobes, the optic lobes, the central complex, and the median bundle in the adult CNS. However, DmGluRA-containing neurons represented only a small fraction of all CNS neurons. DmGluRA immunoreactivity was not detected at the larval neuromuscular junction nor in the body wall muscles. The correlations between DmGluRA distribution and previously described glutamate-like immunoreactivity patterns, as well as the implications of these observations concerning the possible functions of DmGluRA in the Drosophila CNS, are discussed.

Agonist-independent activation of metabotropic glutamate receptors by the intracellular protein Homer

G-protein-coupled receptors (GPCRs) transduce signals from extracellular transmitters to the inside of the cell by activating G proteins. Mutation and overexpression of these receptors have revealed that they can reach their active state even in the absence of agonist, as a result of a natural shift in the equilibrium between their inactive and active conformations. Such agonist-independent (constitutive) activity has been observed for the glutamate GPCRs (the metabotropic glutamate receptors mGluR1a and mGluR5) when they are overexpressed in heterologous cells. Here we show that in neurons, the constitutive activity of these receptors is controlled by Homer proteins, which bind directly to the receptors' carboxy-terminal intracellular domains. Disruption of this interaction by mutagenesis or antisense strategies, or expression of endogenous Homer1a (H1a), induces constitutive activity in mGluR1a or mGluR5. Our results show that these glutamate GPCRs can be directly activated by intracellular proteins as well as by agonists.

Characterization of the methylation-sensitive promoter of the imprinted ZAC gene supports its role in transient neonatal diabetes mellitus

ZAC is a recently isolated zinc finger protein that induces apoptosis and cell cycle arrest. The corresponding gene is imprinted maternally through an unknown mechanism and maps to 6q24-q25, within the minimal interval harboring the gene responsible for transient neonatal diabetes mellitus (TNDM) and a tumor suppressor gene involved in breast cancer. Because of its functional properties, imprinting status, and expression pattern in mammary cell lines and tumors, ZAC is the best candidate so far for both disease conditions. In the present work, we delineated ZAC genomic organization and mapped its transcriptional start site. It is noteworthy that the ZAC promoter localized to the CpG island harboring the methylation imprint associated with TNDM and methylation of this promoter silenced its activity. These data indicate that the methylation mark may have a direct effect on the silencing of the ZAC imprinted allele. Our findings further strengthen the hypothesis that ZAC is the gene responsible for TNDM and suggest a novel mechanism for ZAC inactivation in breast tumors.

Interaction of serotonin 5-hydroxytryptamine type 2C receptors with PDZ10 of the multi-PDZ domain protein MUPP1

By using the yeast two-hybrid system, we previously isolated a cDNA clone encoding a novel member of the multivalent PDZ protein family called MUPP1 containing 13 PDZ domains. Here we report that the C terminus of the 5-hydroxytryptamine type 2C (5-HT(2C)) receptor selectively interacts with the 10th PDZ domain of MUPP1. Mutations in the extreme C-terminal SSV sequence of the 5-HT(2C) receptor confirmed that the SXV motif is critical for the interaction. Co-immunoprecipitations of MUPP1 and 5-HT(2C) receptors from transfected COS-7 cells and from rat choroid plexus verified this interaction in vivo. Immunocytochemistry revealed an SXV motif-dependent co-clustering of both proteins in transfected COS-7 cells as well as a colocalization in rat choroid plexus. A 5-HT(2C) receptor-dependent unmasking of a C-terminal vesicular stomatitis virus epitope of MUPP1 suggests that the interaction triggers a conformational change within the MUPP1 protein. Moreover, 5-HT(2A) and 5-HT(2B), sharing the C-terminal EX(V/I)SXV sequence with 5-HT(2C) receptors, also bind MUPP1 PDZ domains in vitro. The highest MUPP1 mRNA levels were found in all cerebral cortical layers, the hippocampus, the granular layer of the dentate gyrus, as well as the choroid plexus, where 5-HT(2C) receptors are highly enriched. We propose that MUPP1 may serve as a multivalent scaffold protein that selectively assembles and targets signaling complexes.

Alternative splicing of the imprinted candidate tumor suppressor gene ZAC regulates its antiproliferative and DNA binding activities

ZAC encodes a zinc finger protein with antiproliferative activity, is maternally imprinted and is a candidate for the tumor suppressor gene on 6q24. ZAC expression is frequently lost in breast and ovary tumor-derived cell lines and down-regulated in breast primary tumors. In this report, we describe ZACDelta2, an alternatively spliced variant of ZAC lacking the sequence encoding the two N-terminal zinc fingers. Messenger RNAs encoding ZAC or ZACDelta2 were equally abundant and both proteins were nuclear. ZACDelta2 displayed an improved transactivation activity and an enhanced affinity for a ZAC binding site, suggesting that the two N-terminal zinc fingers negatively regulated ZAC binding to its target DNA sequences. Both proteins were equally efficient in preventing colony formation, indicating similar overall antiproliferative activities. However, these activities resulted from a differential regulation of apoptosis vs cell cycle progression since ZACDelta2 was more efficient at induction of cell cycle arrest than ZAC, whereas it was the reverse for apoptosis induction. Hence, these data further underline that ZAC gene is critically controlled, both at the transcriptional level through imprinting and at the functional level through alternative splicing.

Characterization of intestinal receptors for VIP and PACAP in rat and in PAC1 receptor knockout mouse

The receptors for VIP and PACAP were characterized in vitro on rat ileal and colonic longitudinal smooth muscle with adherent myenteric ganglia. Colon strips from PAC1 receptor knockout and wildtype mice were also examined. VIP, PACAP-38 and PACAP-27 all caused concentration dependent relaxations. In rat ileum three different types of smooth muscle VIP/PACAP receptors were defined: (1) a PACAP-27 preferring receptor coupled to apamin sensitive Ca(2+)-dependent K+ channels, (2) a PAC1 receptor activated by both PACAP-27 and PACAP-38, and (3) a VIP specific receptor regulated by NPY. The receptors identified in rat colon were: (1) a PAC1 receptor localized on NO synthesizing neurones. Activation leads to increased NO production. (2) A smooth muscle PAC1 receptor. The responses elicited by both receptors were abolished by apamin. (3) A smooth muscle VIP specific receptor. PAC1 receptor knockout mice did not respond to PACAP-27 or PACAP-38, whereas VIP induced a relaxatory response indicating the presence of a VIP specific receptor. In wildtype mice all three peptides elicited relaxatory responses. Pharmacological characterization of intestinal VIP/PACAP receptors indicates the existence of receptors, such as a PACAP-27 preferring receptor and a VIP specific receptor, distinct from those that have been cloned (VPAC1, VPAC2, and PAC1).

PAC1 null females display decreased fertility

We report here that PAC1 KO females display decreased fertility, whereas male fertility was normal. ICC on pituitary section showed that FSH, LH, and prolactin synthesis were not affected in KO mice. Moreover, the pituitary-gonadal axis responded properly to an acute fasting test in KO mice. Hence, the phenotype of PAC1 null mice provides clear evidence for the role of PAC1 receptor in reproduction process.

Nerve growth factor upregulates the PAC1 promoter by activating the MAP kinase pathway in rat PC12 cells

We report that: (1) An increase in the transcription activity is a mechanism by which trophic peptides may regulate the expression of PAC1. (2) An activation of the PAC1 promoter does not necessarily correlate with the neurotrophin-promoted neuritogenesis. (3) Activation of the PAC1 promoter is probably an early event since the EGF response is rather weak and transient in PC12 cells. (4) MAP kinase pathway activation is necessary for the NGF effect. The mechanism of the antagonism between PACAP and NGF observed on the PAC1 promoter activity and already described in regulating chromaffin cell proliferation, remains to be explained.

A novel site on the Galpha -protein that recognizes heptahelical receptors

Specific domains of the G-protein alpha subunit have been shown to control coupling to heptahelical receptors. The extreme N and C termini and a region between alpha4 and alpha5 helices of the G-protein alpha subunit are known to determine selective interaction with the receptors. The metabotropic glutamate receptor 2 activated both mouse Galpha(15) and its human homologue Galpha(16), whereas metabotropic glutamate receptor 8 activated Galpha(15) only. The extreme C-terminal 20 amino acid residues are identical between the Galpha(15) and Galpha(16) and are therefore unlikely to be involved in coupling selectivity. Our data reveal two regions on Galpha(16) that inhibit its coupling to metabotropic glutamate receptor 8. On a three-dimensional model, both regions are found in a close proximity to the extreme C terminus of Galpha(16). One module comprises alpha4 helix, alpha4-beta6 loop (L9 Loop), beta6 sheet, and alpha5 helix. The other, not described previously, is located within the loop that links the N-terminal alpha helix to the beta1 strand of the Ras-like domain of the alpha subunit. Coupling of Galpha(16) protein to the metabotropic glutamate receptor 8 is partially modulated by each module alone, whereas both modules are needed to eliminate the coupling fully.

Cannabinoids inhibit GABAergic synaptic transmission in mice nucleus accumbens

In mice nucleus accumbens slices, whole-cell patch clamp recording of medium-spiny neurons revealed that cannabimimetics ((R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphtalenylmethanone) (WIN-2) and ((-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)-phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol) inhibit stimulus-evoked gamma-aminobutyric acid mediated inhibitory post-synaptic currents (IPSC). The actions of WIN-2 were reversed by the selective cannabinoid CB(1) receptor antagonist [N-piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboximide hydrochloride] (SR141716A). WIN-2 modified paired-pulse ratio of evoked IPSCs and decreased miniature IPSC frequency indicating a presynaptic localization of cannabinoid CB(1) receptors.

Constitutively active mutants of 5-HT4 receptors are they in unique active states?

Somatic mutations leading to constitutively active G-protein coupled receptors (GPCRs) are responsible for certain human diseases. A consistent structural description of the molecular change underlying the conversion of GPCRs from an inactive R state to an active R* state is lacking. Here, we show that a series of constitutively active 5-HT4 receptors (mutated or truncated in the C-terminal and the third intracellular loop) were characterized by an increase in their denaturation rate at 55 degrees C. The thermal denaturation kinetics were monophasic, suggesting that we were measuring mainly the denaturation rate of R*. Analysis of these kinetics revealed that constitutively active C-terminal domain mutants, were due to a change in the J constant governing the R/R* equilibrium. However, the constitutive activity of the receptor mutated within the third intracellular loop was the result of both a change in the allosteric J constant and a change in the R* conformation.

Localization and mechanisms of action of cannabinoid receptors at the glutamatergic synapses of the mouse nucleus accumbens

Despite the role of excitatory transmission to the nucleus accumbens (NAc) in the actions of most drugs of abuse, the presence and functions of cannabinoid receptors (CB1) on the glutamatergic cortical afferents to the NAc have never been explored. Here, immunohistochemistry has been used to show the localization of CB1 receptors on axonal terminals making contacts with the NAc GABAergic neurons. Electrophysiological techniques in the NAc slice preparation revealed that cannabimimetics [WIN 55,212,2 (WIN-2) and CP55940] strongly inhibit stimulus-evoked glutamate-mediated transmission. The inhibitory actions of WIN-2 were dose-dependent (EC(50) of 293 +/- 13 nm) and reversed by the selective CB1 antagonist SR 141716A. In agreement with a presynaptic localization of CB1 receptors, WIN-2 increased paired-pulse facilitation, decreased miniature EPSC (mEPSC) frequency, and had no effect on the mEPSCs amplitude. Perfusion with the adenylate cyclase activator forskolin enhanced glutamatergic transmission but did not alter presynaptic CB1 actions, suggesting that cannabinoids inhibit glutamate release independently from the cAMP-PKA cascade. CB1 did not reduce evoked transmitter release by inhibiting presynaptic voltage-dependent Ca(2+) currents through N-, L-, or P/Q-type Ca(2+) channels, because CB1 inhibition persisted in the presence of omega-Conotoxin-GVIA, nimodipine, or omega-Agatoxin-IVA. The K(+) channel blockers 4-aminopyridine (100 micrometer) and BaCl(2) (300 micrometer) each reduced by 40-50% the inhibitory actions of WIN-2, and their effects were additive. These data suggest that CB1 receptors are located on the cortical afferents to the nucleus and can reduce glutamate synaptic transmission within the NAc by modulating K(+) channels activity.

Mild deficits in mice lacking pituitary adenylate cyclase-activating polypeptide receptor type 1 (PAC1) performing on memory tasks

Pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptor subtype 1 (PAC1) have been suggested to play a role in the modulation of learning and memory. However, behavioral evidence for altered mnemonic function due to altered PAC1 activity is missing. Therefore, the role of PAC1 in learning and memory was studied in mouse mutants lacking this receptor (PAC1 knock-out mice), tested in water maze two-choice spatial discrimination, one-trial contextual and cued fear conditioning, and multiple-session contextual discrimination. Water maze spatial discrimination was unaffected in PAC1 mutants, while a mild deficit was observed in multiple session contextual discrimination in PAC1 knock-out mice. Furthermore, PAC1 knock-out mice were able to learn the association between context and shock in one-trial contextual conditioning, but showed faster return to baseline than wild-type mice. Thus, the effects of PAC1 knock-out on modulating performance in these tasks were subtle and suggest that PAC1 only plays a limited role in learning and memory.

Dendritic and axonal targeting of type 5 metabotropic glutamate receptor is regulated by homer1 proteins and neuronal excitation

The physiological actions of neurotransmitter receptors are intimately linked to their proper neuronal compartment localization. Here we studied the effect of the metabotropic glutamate receptor (mGluR)-interacting proteins, Homer1a, b, and c, in the targeting of mGluR5 in neurons. We found that mGluR5 was exclusively localized in cell bodies when transfected alone in cultured cerebellar granule cells. In contrast, mGluR5 was found also in dendrites when coexpressed with Homer1b or Homer1c, and in both dendrites and axons when cotransfected with Homer1a. In dendrites, cotransfected mGluR5 and Homer1b/c formed clusters that colocalized with the synaptic marker synaptophysin. Interestingly when transfected alone, the Homer proteins were also translocated to neurites but did not form such clusters. Depolarization of the neurons with a mixture of ionotropic glutamate receptor agonists, NMDA and kainate, or potassium channel blockers, tetraethylammonium and 4-aminopyridine, induced transient expression of endogenous Homer1a and persistent neuritic localization of transfected mGluR5 even long after degradation of Homer1a. These results suggest that Homer1a/b/c proteins are involved in the targeting of mGluR5 to dendritic synaptic sites and/or axons and that this effect can be regulated by neuronal activity. Because the activity-dependent effect of endogenous Homer1a was also long-lasting, the axonal targeting of mGluR5 by this protein is likely to play an important role in synaptic plasticity.

Cytokine signals propagate through the brain

Interleukin-1 (IL-1) and tumor necrosis factor alpha (TNFalpha) are proinflammatory cytokines that are constitutively expressed in healthy, adult brain where they mediate normal neural functions such as sleep. They are neuromodulators expressed by and acting on neurons and glia. IL-1 and TNFalpha expression is upregulated in several important diseases/disorders. Upregulation of IL-1 and/or TNFalpha expression, elicited centrally or systemically, propagates through brain parenchyma following specific spatio-temporal patterns. We propose that cytokine signals propagate along neuronal projections and extracellular diffusion pathways by molecular cascades that need to be further elucidated. This elucidation is a prerequisite for better understanding of reciprocal interactions between nervous, endocrine and immune systems.

Selective blockade of P/Q-type calcium channels by the metabotropic glutamate receptor type 7 involves a phospholipase C pathway in neurons

Although presynaptic localization of mGluR7 is well established, the mechanism by which the receptor may control Ca(2+) channels in neurons is still unknown. We show here that cultured cerebellar granule cells express native metabotropic glutamate receptor type 7 (mGluR7) in neuritic processes, whereas transfected mGluR7 was also expressed in cell bodies. This allowed us to study the effect of the transfected receptor on somatic Ca(2+) channels. In transfected neurons, mGuR7 selectively inhibited P/Q-type Ca(2+) channels. The effect was mimicked by GTPgammaS and blocked by pertussis toxin (PTX) or a selective antibody raised against the G-protein alphao subunit, indicating the involvement of a G(o)-like protein. The mGuR7 effect did not display the characteristics of a direct interaction between G-protein betagamma subunits and the alpha1A Ca(2+) channel subunit, but was abolished by quenching betagamma subunits with specific intracellular peptides. Intracellular dialysis of G-protein betagamma subunits did not mimic the action of mGluR7, suggesting that both G-protein betagamma and alphao subunits were required to mediate the effect. Inhibition of phospholipase C (PLC) blocked the inhibitory action of mGluR7, suggesting that a coincident activation of PLC by the G-protein betagamma with alphao subunits was required. The Ca(2+) chelator BAPTA, as well as inhibition of either the inositol trisphosphate (IP(3)) receptor or protein kinase C (PKC) abolished the mGluR7 effect. Moreover, activation of native mGluR7 induced a PTX-dependent IP(3) formation. These results indicated that IP(3)-mediated intracellular Ca(2+) release was required for PKC-dependent inhibition of the Ca(2+) channels. Possible control of synaptic transmission by the present mechanisms is discussed.

p53 and Bax implication in NMDA induced-apoptosis in mouse hippocampus

Seven days after in vivo intrahippocampal administration of NMDA, 3'-OH DNA fragmentations and Bax protein expression were detected in hippocampal neurons of p53+/+ but not p53-/- transgenic mice. Interestingly, neurons showing pycnosis, an early apoptotic phenomena, were present in all genotypes. These results confirm that apoptotic 3'OH DNA fragmentations and Bax protein induction during NMDA-induced apoptosis in adult hippocampal neurons are p53 dependent.

Sequential expression of surface antigens and transcription factor NFkappaB by hippocampal cells in excitotoxicity and experimental epilepsy

Neurodegeneration and gliosis have been extensively described after long-lasting seizures; evidence for cytokine involvement in neuron-glia interactions does exist. We have therefore studied the hippocampal expression of molecules responsible for immune and inflammatory reactions, at different time-points following either experimental status epilepticus (SE) or direct excitotoxic damage. Experiments consisting of immunohistochemical labeling of glial markers, major histocompatibility complex (MHC) and nuclear factor kappaB (NFkappaB), were performed. NFkappaB nuclear translocation was controlled and measured using the electrophoretic mobility shift assay. One day after SE, neurodegeneration was obvious in CA3 pyramidal layers; NFkappaB staining in neurons and its translocation to the nucleus enhanced. From day 4 to at least day 8 post-SE, MHC-positive microglia, NFkappaB over-expression in thickened astrocytes, and increased levels of its activated form could be observed. The excitotoxic model caused more severe lesions, but NFkappaB and MHC expression were similar in both models. These results suggest that during long-lasting seizures: (i) neuronal firing activates NFkappaB expression and translocation; (ii) microglia expresses MHC; (iii) astrocytes, probably stimulated by microglial cytokines, over-express NFkappaB, the activation of which induces a cascade of reactions, particularly the transcription of cytokines and or neuroprotective molecules. Further clarification of the toxic or protective consequences of delayed inflammatory responses may be interesting in therapy of epilepsy.

Imidazoline-induced neuroprotective effects result from blockade of NMDA receptor channels in neuronal cultures

Imidazolines have been shown to be neuroprotective in focal and global ischemia in the rat. However, their mechanism of action is still unclear. We have studied the neuroprotective effects of imidazolines against NMDA-induced neuronal death and hypoxic insult in cerebellar and striatal neuronal cultures. All of the imidazolines tested decreased the NMDA-mediated neurotoxicity in a non-competitive manner. Antazoline was the most effective (IC(50) of 5 microM, maximal neuroprotection reaching 90% at 100 microM). The neuroprotective effects were still present when the imidazolines were applied during the post-insult period. Antazoline, idazoxan and guanabenz also showed neuroprotective effects against hypoxia-induced neuronal death (neuroprotection reaching 95% for antazoline at 100 microM). Antazoline was still active if applied during the reoxygenation period (15% neuroprotection). To determine the mechanism of the neuroprotective effects, the possible interaction of imidazolines with NMDA receptors was studied. Imidazolines dose-dependently and non-competitively inhibited NMDA currents. As found for the neuroprotective effects, antazoline was the most effective imidazoline, with an IC(50) of 4 microM and a maximal inhibition of 90% at 100 microM. This blockade was rapid, reversible and voltage-dependent. We compared these effects to those of the classical non-competitive antagonist of NMDA channels, MK-801. In contrast to imidazolines, blockade of the NMDA current by MK-801 was voltage-independent and reversible only at positive potentials. When co-applied with MK-801, antazoline prevented the long lasting blockade of the NMDA current by MK-801. These results are consistent with the existence of overlapping binding sites for these drugs on the NMDA receptor channel. They indicate that imidazolines exert a strong neuroprotective effect against excitotoxicity and hypoxia in cerebellar and striatal primary neuronal cultures by inhibiting NMDA receptors. Since these effects were non-competitive, imidazolines appear to be interesting new drugs with therapeutic potential.

Differential modulation of the 5-HT(4) receptor agonists and antagonist on rat learning and memory

Recent data suggest that activation of 5-HT(4) receptors may modulate cognitive processes such as learning and memory. In the present study, the effects of two potent and selective 5-HT(4) agonists, RS 17017 [1-(4-amino-5-chloro-2-methoxyphenyl)-5- (piperidin-1-yl)-1-pentanone hydrochloride] and RS 67333 [1(4-amino-5-chloro-2-methoxyphenyl)-3- (1-n-butyl-4-piperidinyl)-1-propanone], were studied in an olfactory associative discrimination task. The implication of 5-HT(4) receptors in the associative discriminative task was suggested by the following observation. Injection of a selective 5-HT(4) receptor antagonist RS 67532 [1-(4-amino-5-chloro-2-(3, 5-dimethoxybenzyloxyphenyl)-5-(1-piperidinyl)-1-pentanone; 1 mg/kg: i.p.] before the third training session induced a consistent deficit in associative memory during the following training sessions. This deficit was absent when the antagonist was injected together with either a specific hydrophilic 5-HT(4) (RS 17017, 1 mg/kg) or a specific hydrophobic (RS 67333, 1 mg/kg) 5-HT(4) receptor agonist. RS 67333 was more potent than RS 17017. This difference in potency certainly reflects a difference in their capacity to enter into the brain. This is also likely to be the reason why, injected alone, the hydrophobic 5-HT(4) agonist (RS 67333) but not the hydrophilic 5-HT(4) agonist (RS 17017) improved learning and memory performance.

Pharmacological properties of 5-Hydroxytryptamine(4) receptor antagonists on constitutively active wild-type and mutated receptors

We studied the pharmacological properties of twenty-four 5-hydroxytryptamine (5-HT)(4) receptor ligands known to act as antagonists on 5-HT(4) receptors positively coupled to adenylyl cyclase endogenously expressed in mouse colliculi neurons. In COS-7 cells expressing human or mouse 5-HT(4(a)) receptors (100-8000 fmol/mg of protein), we found neutral antagonists, partial agonists, and inverse agonists. The majority of neutral antagonists belong to the benzodioxanyl ketone class, whereas partial agonists belong to different chemical classes. We found only two inverse agonists, GR 125487 and SB 207266, which are both indoles. Analysis of pharmacological characteristics of the constitutively active wild-type and constitutively active mutated receptors revealed that 1) the ratio between the efficiencies of the full agonist 5-HT and the partial agonist RS 23597 was invariable when the receptor density increased, but was dependent on receptor structure; 2) similarly, the efficacy of the inverse agonist SB 207266 was not dependent on receptor density but was dependent on receptor structure; 3) when the receptor concentration increased, the EC(50) values of the full agonist 5-HT were not modified and the increase in basal constitutive activity, as well as its stimulation by 5-HT, followed a parallel evolution; and 4) the stimulation of basal constitutive activity by 5-HT was not modified by the overexpression of Galphas. All these results indicate that in COS-7 cells, the coupling of the 5-HT(4) receptor to adenylyl cyclase was linear with no indication of spare receptors even at high receptor density (8 pmol/mg). These results are also in accordance with a precoupling between the activated receptor (f(R*)) and adenylyl cyclase. Such observations allowed us to use the two-state model to calculate the constant J, i.e., the equilibrium allosteric constant denoting the ratio of the receptor in the inactive versus active state (J = [R]/[R*]). We found that J was a receptor structural characteristic, independent of receptor density.

PAC1 receptor-deficient mice display impaired insulinotropic response to glucose and reduced glucose tolerance

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a ubiquitous neuropeptide of the vasoactive intestinal peptide (VIP) family that potentiates glucose-stimulated insulin secretion. Pancreatic beta cells express two PACAP receptor subtypes, a PACAP-preferring (PAC1) and a VIP-shared (VPAC2) receptor. We have applied a gene targeting approach to create a mouse lacking the PAC1 receptor (PAC1(-/-)). These mice were viable and normoglycemic, but exhibited a slight feeding hyperinsulinemia. In vitro, in the isolated perfused pancreas, the insulin secretory response to PACAP was reduced by 50% in PAC1(-/-) mice, whereas the response to VIP was unaffected. In vivo, the insulinotropic action of PACAP was also acutely reduced, and the peptide induced impairment of glucose tolerance after an intravenous glucose injection. This demonstrates that PAC1 receptor is involved in the insulinotropic action of the peptide. Moreover, PAC1(-/-) mice exhibited reduced glucose-stimulated insulin secretion in vitro and in vivo, showing that the PAC1 receptor is required to maintain normal insulin secretory responsiveness to glucose. The defective insulinotropic action of glucose was associated with marked glucose intolerance after both intravenous and gastric glucose administration. Thus, these results are consistent with a physiological role for the PAC1 receptor in glucose homeostasis, notably during food intake.

Conservation of the ligand recognition site of metabotropic glutamate receptors during evolution

Mammalian metabotropic glutamate receptors (mGluRs) are classified into 3 groups based on their sequence similarity and ligand recognition selectivity. Recently, we identified a Drosophila mGluR (DmGlu(A)R) which is about equidistant, phylogenetically, from the 3 mGluR groups. However, both the G-protein coupling selectivity and the pharmacological profile of DmGlu(A)R, as analysed with mutated G-proteins and a few compounds, look similar to those of mammalian group-II mGluRs. In the present study we carefully examined the pharmacological profile of DmGlu(A)R, and compared it to those of the rat mGlu(1a), mGlu(2) and mGlu(4a) receptors, representative of group-I, II and III respectively. The pharmacological profile of DmGlu(A)R was found to be similar to that of mGlu(2)R, and only very small differences could be identified at the level of their pharmacophore models. These data strongly suggest that the binding sites of these two receptors are similar. To further document this idea, a 3D model of the mGlu(2) binding domain was constructed based on the low sequence similarity with periplasmic amino acid binding proteins, and was used to identify the residues that possibly constitute the ligand recognition pocket. Interestingly, this putative binding pocket was found to be very well conserved between DmGlu(A)R and the mammalian group-II receptors. These data indicate that there has been a strong selective pressure during evolution to maintain the ligand recognition selectivity of mGluRs.

Pharmacological characterization of the rat metabotropic glutamate receptor type 8a revealed strong similarities and slight differences with the type 4a receptor

In the brain, group-III metabotropic glutamate (mGlu) receptors mGlu(4), mGlu(7) and mGlu(8) receptors play a critical role in controlling the release process at many glutamatergic synapses. The pharmacological profile of mGlu(4) receptor has been studied extensively, allowing us to propose a pharmacophore model for this receptor subtype. Surprisingly, the activity of only a few compounds have been reported on mGlu(7) and mGlu(8) receptors. In order to identify new possibilities for the design of selective compounds able to discriminate between the members of the group-III mGlu receptors, we have undertaken a complete pharmacological characterization of mGlu(8) receptor and compared it with that of mGlu(4) receptor, using the same expression system, and the same read out. The activities of 32 different molecules revealed that these two mGlu receptors subtypes share a similar pharmacological profile. Only small differences were noticed in addition to that previously reported with S-carboxyglutamate (S-Gla) being a partial agonist at mGlu(4) receptor and a full antagonist at mGlu(8) receptor. These include: a slightly higher relative potency of the agonists 1S,3R and 1S,3S-aminocyclopentane-1,3-dicarboxylic acid (ACPD), S-4-carboxyphenylglycine (S-4CPG) and S-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG), and a slightly higher potency of the antagonists 2-aminobicyclo[3.1.0]hexane-2, 6-dicarboxylic acid (LY354740) and RS-alpha-methyl-4-phosphonophenylglycine (MPPG) on mGlu(8) receptor. When superimposed on the mGlu(4) receptor pharmacophore model, these molecules revealed three regions that may be different between the ligand binding sites of mGlu(8) and mGlu(4) receptors.

Complex interactions between mGluRs, intracellular Ca2+ stores and ion channels in neurons

Metabotropic glutamate receptors (mGluRs) can increase intracellular Ca2+ concentration via Ins(1,4,5)P3- and ryanodine-sensitive Ca2+ stores in neurons. Both types of store are coupled functionally to Ca2+-permeable channels found in the plasma membrane. The mGluR-mediated increase in intracellular Ca2+ concentration can activate Ca2+-sensitive K+ channels and Ca2+-dependent nonselective cationic channels. These mGluR-mediated effects often result from mobilization of Ca2+ from ryanodine-sensitive, rather than Ins(1,4, 5)P3-sensitive, Ca2+ stores, suggesting that close functional interactions exist between mGluRs, intracellular Ca2+ stores and Ca2+-sensitive ion channels in the membrane.

"Inflammatory" cytokines: neuromodulators in normal brain?

If cytokines are constitutively expressed by and act on neurons in normal adult brain, then we may have to modify our current view that they are predominantly inflammatory mediators. We critically reviewed the literature to determine whether we could find experimental basis for such a modification. We focused on two "proinflammatory" cytokines, interleukin (IL)-1 and tumor necrosis factor-alpha (TNFalpha) because they have been most thoroughly investigated in shaping our current thinking. Evidence, although equivocal, indicates that the genes coding for these cytokines and their accessory proteins are expressed by neurons, in addition to glial cells, in normal brain. Their expression is region- and cell type-specific. Furthermore, bioactive cytokines have been extracted from various regions of normal brain. The cytokines' receptors selectively are present on all neural cell types, rendering them responsive to cytokine signaling. Blocking their action modifies multiple neural "housekeeping" functions. For example, blocking IL-1 or TNFalpha by several independent means alters regulation of sleep. This indicates that these cytokines likely modulate in the brain behavior of a normal organism. In addition, these cytokines are likely involved in synaptic plasticity, neural transmission, and Ca2+ signaling. Thus, the evidence strongly suggests that these cytokines perform neural functions in normal brain. We therefore propose that they should be thought of as neuromodulators in addition to inflammatory mediators.

mGluR7-like receptor and GABA(B) receptor activation enhance neurotoxic effects of N-methyl-D-aspartate in cultured mouse striatal GABAergic neurones

Presynaptic metabotropic glutamate receptors (mGluRs) of group III constitute possible targets for putative neuroprotective drugs acting against glutamate excitotoxic insults. Indeed, in glutamatergic cerebellar granule neurones in culture, high concentrations of L-2-amino-4-phosphonobutyrate (L-AP4, above 0.3 mM, thus activating mGluR7) inhibit NMDA-induced cell death. In contrast, in striatal cultures which are enriched in GABAergic neurones, we show that high concentrations of L-AP4 increased neuronal death in control as well as in NMDA-stimulated cultures. Moreover, similar results were obtained with the GABA(B)R agonist. baclofen. Both the neuroprotective effects in cerebellar granule cells and the neurotoxic effects in striatal neurones were mediated via Gi-Go-coupled mGluRs, suggesting that these effects were probably mediated by mGluR7a or b and GABA(B)R expressed in these neurones. In striatal neurones, we found that L-AP4 and baclofen inhibited both basal and NMDA-stimulated GABA release. These inhibitions of GABA release may be responsible for the increase in basal and NMDA-stimulated neuronal death. Indeed, blockade of GABA(A) receptors with bicuculline increased neuronal death of control and NMDA-treated striatal cultures. Taken together, these results suggest that L-AP4 and baclofen, via mGluR7 and GABA(B)R, reduced the neuroprotective effect of GABA present in striatal cultures acting via GABA(A) receptors. Although caution must be taken when extrapolating from in vitro to in vivo situations, the present experiments and the recent observations that mGluR7 and GABA(B)R are expressed in heterologous synapses, should be taken into consideration when evaluating the neuroprotective action of future mGluR7 specific agonists or GABA(B)R specific antagonists.

Loss of expression of the candidate tumor suppressor gene ZAC in breast cancer cell lines and primary tumors

Loss of chromosome 6q21-qter is the second most frequent loss of chromosomal material in sporadic breast neoplasms suggesting the presence of at least one tumor suppressor gene on 6q. We recently isolated a cDNA encoding a new zinc finger protein which we named ZAC according to its functional properties, namely induction of apoptosis and control of cell cycle progression. ZAC is expressed in normal mammary gland and maps to 6q24-q25, a recognized breast cancer hot spot on 6q. In the present report, we investigated the possible inactivation of ZAC in breast cancer cell lines and primary tumors. We detected no mutation in ZAC coding region in a panel of 45 breast tumors with allelic imbalance of 6q24-q25. However, a survey of eight breast cancer cell lines showed a deeply reduced (three cell lines) or complete loss of (five cell lines) ZAC expression. Treatment of three of these cell lines with the methylation-interfering agent 5-azacytidine induced ZAC re-expression. In addition, Northern blot and RNase protection assay analysis of ZAC expression in 23 unselected primary breast tumors showed a reduced expression in several samples. Together with its functional properties and chromosomal localization, these findings substantiate ZAC as a good candidate for the tumor suppressor gene on 6q24-q25.