Pairing elevation of [cyclic GMP] with inhibition of PKA produces long-term depression of glutamate release from isolated rat hippocampal presynaptic terminals

Data suggest both presynaptic and postsynaptic changes contribute to activity-dependent long-term synaptic plasticity. We have shown that pairing elevation of intracellular [cyclic GMP], using the type V phosphodiesterase inhibitor zaprinast, with inhibition of cyclic AMP-dependent protein kinase (PKA), is sufficient to elicit chemical long-term depression (CLTD) of synaptic transmission at Schaffer collateral-CA1 and mossy fibre-CA3 synapses in rat hippocampus. CLTD does not require synaptic activity, and selective postsynaptic drug injections do not affect it, suggesting it is presynaptically induced and expressed. To directly evaluate this hypothesis, we tested whether CLTD of transmitter release can be expressed in isolated presynaptic nerve terminals. Presynaptic nerve terminals (synaptosomes) were isolated from rat hippocampi by Percoll density gradient centrifugation. Synaptosomes were loaded with [3H]glutamate, and basal and depolarisation-induced release of [3H]glutamate measured in control medium versus medium containing zaprinast (20 microm) plus or minus the PKA inhibitor H-89 (10 microm). Zaprinast produced a significant decrease in basal [3H]glutamate release. However, only combining zaprinast with H-89 significantly depressed K+-evoked [3H]glutamate release. After a 20-min drug washout, basal release returned to normal in all conditions, but K+-evoked [3H]glutamate release was persistently reduced only by the combination of zaprinast plus H-89. Long-term reduction of [3H]glutamate release from synaptosomes was completely prevented by the PKG inhibitor KT5823 (5 microm). These data demonstrate the existence of a presynaptic, cyclic GMP-PKG dependent cascade capable of expressing LTD of glutamate release from isolated hippocampal nerve terminals.     

The effect of heliox treatment in a rat model of focal transient cerebral ischemia

At nerve terminals G protein coupled receptors modulate neurotransmitter release probability. We recently showed that prolonged activation of metabotropic glutamate receptor 7, mGlu7 receptor, potentiates glutamate release. This signalling involves phospholipase C activation via a pertussis toxin insensitive G protein, the hydrolysis of phosphatidylinositol (4,5)-bisphosphate, and the subsequent activation of the non-kinase diacylglycerol binding protein Munc13-1 which primes synaptic vesicle for exocytosis at the active zone. Here we found that inhibitors of diacylglycerol metabolism (diacylglycerol kinase inhibitor II and diacylglycerol lipase inhibitor RHC80267) remarkably reduce the time of mGlu7 receptor stimulation required for glutamate release potentiation in mice cerebrocortical nerve terminals. We conclude that changes in diacylglycerol levels at nerve terminals control the efficiency of the exocytotic release machinery.     

Eating ourselves to death (and despair): The contribution of adiposity and inflammation to depression

Obesity and related metabolic conditions are of epidemic proportions in most of the world, affecting both adults and children. The accumulation of lipids in the body in the form of white adipose tissue in the abdomen is now known to activate innate immune mechanisms. Lipid accumulation causes adipocytes to directly secrete the cytokines interleukin (IL) 6 and tumor necrosis factor α (TNFα), but also monocyte chemoattractant protein 1 (MCP-1), which results in the accumulation of leukocytes in fat tissue. This sets up a chronic inflammatory state which is known to mediate the association between obesity and conditions such as cardiovascular disease, type 2 diabetes, and cancer. There is also a substantial literature linking inflammation with risk for depression. This includes the observations that: (1) people with inflammatory diseases such as multiple sclerosis, cardiovascular disease, and psoriasis have elevated rates of depression; (2) many people administered inflammatory cytokines such as interferon α develop depression that is indistinguishable from depression in non-medically ill populations; (3) a significant proportion of depressed persons show upregulation of inflammatory factors such as IL-6, C-reactive protein, and TNFα; (4) inflammatory cytokines can interact with virtually every pathophysiologic domain relevant to depression, including neurotransmitter metabolism, neuroendocrine function, and synaptic plasticity. While many factors may contribute to the association between inflammatory mediators and depression, we hypothesize that increased adiposity may be one causal pathway. Mediational analysis suggests a bi-directional association between adiposity and depression, with inflammation possibly playing an intermediary role.     

Adenosine A2A receptors in ventral striatum, hypothalamus and nociceptive circuitry implications for drug addiction, sleep and pain

Adenosine A2A receptors localized in the dorsal striatum are considered as a new target for the development of antiparkinsonian drugs. Co-administration of A2A receptor antagonists has shown a significant improvement of the effects of l-DOPA. The present review emphasizes the possible application of A2A receptor antagonists in pathological conditions other than parkinsonism, including drug addiction, sleep disorders and pain. In addition to the dorsal striatum, the ventral striatum (nucleus accumbens) contains a high density of A2A receptors, which presynaptically and postsynaptically regulate glutamatergic transmission in the cortical glutamatergic projections to the nucleus accumbens. It is currently believed that molecular adaptations of the cortico-accumbens glutamatergic synapses are involved in compulsive drug seeking and relapse. Here we review recent experimental evidence suggesting that A2A antagonists could become new therapeutic agents for drug addiction. Morphological and functional studies have identified lower levels of A2A receptors in brain areas other than the striatum, such as the ventrolateral preoptic area of the hypothalamus, where adenosine plays an important role in sleep regulation. Although initially believed to be mostly dependent on A1 receptors, here we review recent studies that demonstrate that the somnogenic effects of adenosine are largely mediated by hypothalamic A2A receptors. A2A)receptor antagonists could therefore be considered as a possible treatment for narcolepsy and other sleep-related disorders. Finally, nociception is another adenosine-regulated neural function previously thought to mostly involve A1 receptors. Although there is some conflicting literature on the effects of agonists and antagonists, which may partly be due to the lack of selectivity of available drugs, the studies in A2A receptor knockout mice suggest that A2A receptor antagonists might have some therapeutic potential in pain states, in particular where high intensity stimuli are prevalent.     

The dopamine receptor D2 agonist bromocriptine inhibits glucose-stimulated insulin secretion by direct activation of the α2-adrenergic receptors in beta cells

Treatment with the dopamine receptor D2 (DRD2) agonist bromocriptine improves metabolic features in obese patients with type 2 diabetes by a still unknown mechanism. In the present study, we investigated the acute effect of bromocriptine and its underlying mechanism(s) on insulin secretion both in vivo and in vitro. For this purpose, C57Bl6/J mice were subjected to an intraperitoneal glucose tolerance test (ipGTT) and a hyperglycemic (HG) clamp 60 min after a single injection of bromocriptine or placebo. The effects of bromocriptine on glucose-stimulated insulin secretion (GSIS), cell membrane potential and intracellular cAMP levels were also determined in INS-1E beta cells. We report here that bromocriptine increased glucose levels during ipGTT in vivo, an effect associated with a dose-dependent decrease in GSIS. During the HG clamp, bromocriptine reduced both first-phase and second-phase insulin response. This inhibitory effect was also observed in INS-1E beta cells, in which therapeutic concentrations of bromocriptine (0.5-50 nM) decreased GSIS. Mechanistically, neither cellular energy state nor cell membrane depolarization was affected by bromocriptine whereas intracellular cAMP levels were significantly reduced, suggesting involvement of G-protein-coupled receptors. Surprisingly, the DRD2 antagonist domperidone did not counteract the effect of bromocriptine on GSIS, whereas yohimbine, an antagonist of the α2-adrenergic receptors, completely abolished bromocriptine-induced inhibition of GSIS. In conclusion, acute administration of bromocriptine inhibits GSIS by a DRD2-independent mechanism involving direct activation of the pancreatic α2-adrenergic receptors. We suggest that treatment with bromocriptine promotes beta cells rest, thereby preventing long-lasting hypersecretion of insulin and subsequent beta cell failure.     

1,3-dichloro-2-propanol induced lipid accumulation in HepG2 cells through cAMP/protein kinase A and AMP-activated protein kinase pathways via Gi/o-coupled receptors

1,3-dichloro-2-propanol (1,3-DCP) is a food born hepatoxic chloropropanol contaminant that has been detected in a wide range of foods. In the present study, we investigated the effects and mechanisms of 1,3-DCP on lipid accumulation in HepG2 cells. The data showed 1,3-DCP significantly increased intracellular content of triglyceride (TG) and total cholesterol (TC) at 0.5–2 μg/mL. Further results showed that 1,3-DCP greatly decreased cyclic AMP (cAMP) level. In addition, 1,3-DCP inhibited PKA and AMPK signaling pathway, but had no influence on intracellular calcium and regulated proteins. Moreover, Gi/o protein inhibitor PTX significantly inhibited 1,3-DCP induced decrease of cAMP, p-PKA and p-AMPK expression. Furthermore, 1,3-DCP significantly decreased GPR41 and GPR43 expression, but had no effect on GPR109B.Thus, we concluded that 1,3-DCP induced lipid accumulation in HepG2 cells through cAMP/PKA and AMPK signaling pathways via Gi/o-coupled receptor.     

Hypothalamic insulin and glucagon-like peptide-1 levels in an animal model of depression and their effect on corticotropin-releasing hormone promoter gene activity in a hypothalamic cell line

Background

In depression, excessive glucocorticoid action may cause maladaptive brain changes, including in the pathways controlling energy metabolism. Insulin and glucagon-like peptide-1 (GLP-1), besides regulation of glucose homeostasis, also possess neurotrophic properties. Current study was aimed at investigating the influence of prenatal stress (PS) on insulin, GLP-1 and their receptor (IR and GLP-1R) levels in the hypothalamus. GLP-1 and GLP-1R were assayed also in the hippocampus and frontal cortex – brain regions mainly affected in depression. The second objective was to determine the influence of exendin-4 and insulin on CRH promoter gene activity in in vitro conditions.