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

Cloning and characterisation of NMDA receptors in the Pacific oyster, Crassostrea gigas (Thunberg, 1793) in relation to metamorphosis and catecholamine synthesis

Bivalve metamorphosis is a developmental transition from a free-living larva to a benthic juvenile (spat), regulated by a complex interaction of neurotransmitters and neurohormones such as L-DOPA and epinephrine (catecholamine). We recently suggested an N-Methyl-D-aspartate (NMDA) receptor pathway as an additional and previously unknown regulator of bivalve metamorphosis. To explore this theory further, we successfully induced metamorphosis in the Pacific oyster, Crassostrea gigas, by exposing competent larvae to L-DOPA, epinephrine, MK-801 and ifenprodil. Subsequently, we cloned three NMDA receptor subunits CgNR1, CgNR2A and CgNR2B, with sequence analysis suggesting successful assembly of functional NMDA receptor complexes and binding to natural occurring agonists and the channel blocker MK-801. NMDA receptor subunits are expressed in competent larvae, during metamorphosis and in spat, but this expression is neither self-regulated nor regulated by catecholamines. In-situ hybridisation of CgNR1 in competent larvae identified NMDA receptor presence in the apical organ/cerebral ganglia area with a potential sensory function, and in the nervous network of the foot indicating an additional putative muscle regulatory function. Furthermore, phylogenetic analyses identified molluscan-specific gene expansions of key enzymes involved in catecholamine biosynthesis. However, exposure to MK-801 did not alter the expression of selected key enzymes, suggesting that NMDA receptors do not regulate the biosynthesis of catecholamines via gene expression.

D-Amino acids in mammalian endocrine tissues

D-Aspartate, D-serine and D-alanine are a regular occurrence in mammalian endocrine tissues, though in amounts varying with the type of gland. The pituitary gland, pineal gland, thyroid, adrenal glands and testis contain relatively large amounts of D-aspartate in all species examined. D-alanine is relatively abundant in the pituitary gland and pancreas. High levels of D-serine characterize the hypothalamus. D-leucine, D-proline and D-glutamate are generally low. The current knowledge of physiological roles of D-amino acids in endocrine tissues is far from exhaustive, yet the topic is attracting increasing interest because of its potential in pharmacological application. D-aspartate is known to act at all levels of the hypothalamus-pituitary-testis axis, playing a key role in reproductive biology in several vertebrate classes. An involvement of D-amino acids in the endocrine function of the pancreas is emerging. D-Aspartate has been immunolocalized in insulin-containing secretory granules in INS-1 E clonal β cells and is co-secreted with insulin by exocytosis. Specific immunolocalization of D-alanine in pituitary ACTH-secreting cells and pancreatic β-cells suggests that this amino acid participates in blood glucose regulation in mammals. By modulating insulin secretion, D-serine probably participates in the control of systemic glucose metabolism by modulating insulin secretion. We anticipate that future investigation will significantly increase the functional repertoire of D-amino acids in homeostatic control.

Phenotypic characterization of Grm1crv4 mice reveals a functional role for the type 1 metabotropic glutamate receptor in bone mineralization

Recent increasing evidence supports a role for neuronal type signaling in bone. Specifically glutamate receptors have been found in cells responsible for bone remodeling, namely the osteoblasts and the osteoclasts. While most studies have focused on ionotropic glutamate receptors, the relevance of the metabotropic glutamate signaling in bone is poorly understood. Specifically type 1 metabotropic glutamate (mGlu1) receptors are expressed in bone, but the effect of its ablation on skeletal development has never been investigated. Here we report that Grm1^crv4/crv4 mice, homozygous for an inactivating mutation of the mGlu1 receptor, and mainly characterized by ataxia and renal dysfunction, exhibit decreased body weight, bone length and bone mineral density compared to wild type (WT) animals. Blood analyses of the affected mice demonstrate the absence of changes in circulating factors, such as vitamin D and PTH, suggesting renal damage is not the main culprit of the skeletal phenotype. Cultures of osteoblasts lacking functional mGlu1 receptors exhibit less homogeneous collagen deposition than WT cells, and present increased expression of osteocalcin, a marker of osteoblast maturation. These data suggest that the skeletal damage is directly linked to the absence of the receptor, which in turn leads to osteoblasts dysfunction and earlier maturation. Accordingly, skeletal histomorphology suggests that Grm1^crv4/crv4 mice exhibit enhanced bone maturation, resulting in premature fusion of the growth plate and shortened long bones, and further slowdown of bone apposition rate compared to the WT animals. In summary, this work reveals novel functions of mGlu1 receptors in the bone and indicates that in osteoblasts mGlu1 receptors are necessary for production of normal bone matrix, longitudinal bone growth, and normal skeletal development.

Epigenomic and metabolic responses of hypothalamic POMC neurons to gestational nicotine exposure in adult offspring

Background

Epidemiological and animal studies have reported that prenatal nicotine exposure (PNE) leads to obesity and type-2 diabetes in offspring. Central leptin-melanocortin signaling via hypothalamic arcuate proopiomelanocortin (POMC) neurons is crucial for the regulation of energy and glucose balance. Furthermore, hypothalamic POMC neurons were recently found to mediate the anorectic effects of nicotine through activation of acetylcholine receptors. Here, we hypothesized that PNE impairs leptin-melanocortinergic regulation of energy balance in first-generation offspring by altering expression of long non-coding RNAs (lncRNAs) putatively regulating development and/or function of hypothalamic POMC neurons.

Methods

C57BL/6J females were exposed ad libitum to nicotine through drinking water and crossed with C57BL/6J males. Nicotine exposure was sustained during pregnancy and discontinued at parturition. Offspring development was monitored from birth into adulthood. From the age of 8 weeks, central leptin-melanocortin signaling, diabetes, and obesity susceptibility were assessed in male offspring fed a low-fat or high-fat diet for 16 weeks. Nicotine-exposed and non-exposed C57BL/6J females were also crossed with C57BL/6J males expressing the enhanced green fluorescent protein specifically in POMC neurons. Transgenic male offspring were subjected to laser microdissections and RNA sequencing (RNA-seq) analysis of POMC neurons for determination of nicotine-induced gene expression changes and regulatory lncRNA/protein-coding gene interactions.

Results

Contrary to expectation based on previous studies, PNE did not impair but rather enhanced leptin-melanocortinergic regulation of energy and glucose balance via POMC neurons in offspring. RNA-seq of laser microdissected POMC neurons revealed only one consistent change, upregulation of Gm15851, a lncRNA of yet unidentified function, in nicotine-exposed offspring. RNA-seq further suggested 82 cis-regulatory lncRNA/protein-coding gene interactions, 19 of which involved coding genes regulating neural development and/or function, and revealed expression of several previously unidentified metabolic, neuroendocrine, and neurodevelopment pathways in POMC neurons.

Conclusions

PNE does not result in obesity and type 2 diabetes but instead enhances leptin-melanocortinergic feeding and body weight regulation via POMC neurons in adult offspring. PNE leads to selective upregulation of Gm15851, a lncRNA, in adult offspring POMC neurons. POMC neurons express several lncRNAs and pathways possibly regulating POMC neuronal development and/or function.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-016-0348-2) contains supplementary material, which is available to authorized users.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Role of metabotropic glutamate receptors in the control of neuroendocrine function

Glutamate exerts its effects through binding and activation of two classes of specific receptors: ionotropic (iGluRs) and metabotropic (mGluRs). Group I mGluR includes mGluR1 and mGluR5 subtypes, group II includes mGluR2 and mGluR3 subtypes and group III includes the subtypes mGluR 4, 6, 7 and 8. Glutamate and its receptors are found in all key hypothalamic areas critically involved in reproduction and neuroendocrine function. To date, considerable data support an important role for iGluRs in the control of neuroendocrine function; however, the role of mGluRs as regulators of hypothalamic-pituitary function has not been clearly elucidated. mGluRs could be exerting a fine tune on the release of hypothalamic factors that regulate hormone release such as Substance P, GABA, alpha-MSH and CRH. Group II mGluR exert a direct inhibitory effect on anterior pituitary prolactin and GH secretion. Moreover, some group II mGluR agonists, like LY 354,740 and LY 379,268, can modulate PRL secretion from the anterior pituitary through their actions as dopamine receptor agonists. Evidence suggests a role for group III mGluR subtypes in stress-related behavioral disorders. Several reports indicate that selective ligands for mGluR subtypes have potential for the treatment of a wide variety of neurological and psychiatric disorders, including depression, anxiety disorders, schizophrenia, epilepsy and Alzheimer's disease among others. Since converging lines of evidence suggest a role for mGluRs subtypes in neuroendocrine regulation of hormone secretion, mGluRs neuroendocrine actions must be taken in consideration to insure proper treatment of these diseases. Moreover, discovery of selective agonists provides an opportunity to investigate the physiological role of mGluR subtypes and to directly test the neuroendocrine actions of mGluRs. Finally, mGluRs selective agonists may have an impact in the treatment of conditions involving chronic stress, such as depression and anxiety disorders, since they regulate neuroendocrine stress circuits involving the HPA axis and stress-sensitive hormones such as oxytocin and prolactin. This review aims to provide a survey of our current understanding of the effects of mGluR activation on neuroendocrine function.

Glutamate suppresses GABA release via presynaptic metabotropic glutamate receptors at baroreceptor neurones in rats

The nucleus tractus solitarii (NTS) is essential for coordinating arterial baroreflex control of blood pressure. The primary baroreceptor afferent fibres make their first excitatory synaptic contact at second-order NTS neurones with glutamate as the major neurotransmitter. Glutamate regulates its own release by activating presynaptic metabotropic glutamate autoreceptors (mGluRs) on the baroreceptor central terminals to suppress its further release in frequency-dependent manner. Gamma-aminobutyric acid (GABA) interneurones provide the major inhibitory synaptic input. It is the integration of excitatory and inhibitory inputs that shapes the NTS output of baroreceptor signals. We hypothesized that glutamate released from the primary central afferent terminals can spill over to presynaptic mGluRs on GABA interneurones to suppress GABA release at the second-order baroreceptor neurones. We assessed GABA transmission in second-order baroreceptor neurones identified by attached aortic depressor nerve (ADN) boutons. The medial NTS was stimulated to evoke GABA inhibitory postsynaptic currents (eIPSCs). Glutamate spillover, generated by brief 2 s, 25 Hz trains of stimuli applied to the tractus solitarius (TS), induced a small (10%) but significant reduction in the eIPSC amplitudes. The depression was enhanced to a 25% decrease by increasing glutamate in the cleft with a glutamate-uptake inhibitor (M-trans-pyrrolidine-2,4-dicarboxylic acid, 1 mum), blocked by a Group II mGluR antagonist (LY341495, 200 nm) and mimicked by a Group II agonist ((2S,3S,4S)-CCG/(2S,1'S,2'S)-2-carboxycyclopropyl; L-CCG-I). A presynaptic mGluR locus was established by the mGluR agonist-mediated increase in the paired-pulse ratio of two consecutive eIPSCs in conjunction with the decrease in the first eIPSC, and a decrease in the frequency (39-46% reduction at EC(50) concentration), but not amplitude, of spontaneous and miniature GABA IPSCs. The data indicate that endogenous glutamate activation of Group II presynaptic mGluRs can decrease GABA release at the first central synapses, suggesting a heterosynaptic role for the Group II mGluRs in shaping baroreceptor signal transmission.

Plasma membrane and vesicular glutamate transporter mRNAs/proteins in hypothalamic neurons that regulate body weight

After synaptic release, glutamate is taken up by the nerve terminal via a plasma membrane-associated protein termed excitatory amino acid transporter 3 (EAAT3). Following entry into the nerve terminal, glutamate is pumped into synaptic vesicles by a vesicular transport system. Three different vesicular glutamate transporter proteins (VGLUT1-3) representing unique markers for glutamatergic neurons were recently characterized. The presence of EAAT3, glutaminase and VGLUT1-3 was examined in mouse, rat and rabbit species at mRNA and protein levels in hypothalamic neurons which are involved in the regulation of body weight using in situ hybridization and immunohistochemistry. EAAT3 and glutaminase mRNAs were demonstrated in all parts of the arcuate nucleus in the dorsomedial and ventromedial hypothalamic nuclei and lateral hypothalamic area. VGLUT1 mRNA was present in the magnocellular lateral hypothalamic nucleus. VGLUT2 mRNA was demonstrated in a subpopulation of neurons in the arcuate nucleus and in the ventromedial and dorsomedial hypothalamic nuclei and lateral hypothalamic area. Few VGLUT3 mRNA expressing neurons were scattered throughout the medial and lateral hypothalamus. EAAT3-like immunoreactivity (-li) was demonstrated in glutamate, neuropeptide Y (NPY), agouti-related peptide (AGRP), pro-opiomelanocortin (POMC), cocaine and amphetamine-regulated transcript (CART), melanin-concentrating hormone and orexin-immunoreactive (-ir) neurons. VGLUT2-li could only be demonstrated in POMC- and CART-ir neurons of the ventrolateral arcuate nucleus. The results show that key neurons involved in regulation of energy balance are glutamatergic and/or densely innervated by glutamatergic nerve terminals. Whereas orexigenic NPY/AGRP neurons situated in the ventromedial part of the arcuate nucleus are mainly GABAergic, it is shown that several anorexigenic POMC/CART neurons of the ventromedial arcuate nucleus are most likely glutamatergic [corrected].

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.