Rapid desensitization of the metabotropic glutamate receptor that facilitates glutamate release in rat cerebrocortical nerve terminals

The metabotropic autoreceptor of glutamatergic nerve terminals from the cerebral cortex of adult rats has been characterized. Receptor activation involves a rapid and transient increase in diacylglycerol, which is sensitive to L-2-amino-3-phosphonopropionate (L-AP3) and L-2-amino-4-phosphonobutanoic acid (L-AP4) and is partially blocked by pertussis toxin. Protein kinase C (PKC) has a negative feedback control in this transduction pathway because the activation of the kinase, either by phorbol esters or by the endogenous diacylglycerol produced by the receptor, results in a reversible receptor desensitization, with loss of the ability to further facilitate glutamate release. It is concluded that the facilitatory metabotropic receptor located at the glutamatergic nerve endings belongs to the subclass coupled to phosphoinositide hydrolysis and that the rapid and use-dependent desensitization of the facilitatory pathway may underlie a mechanism to prevent its permanent activation and thereby to avoid neurotoxicity.

Partial splenic embolization for the treatment of hypersplenism in cirrhosis

Hypersplenism is of great relevance in the management of cirrhosis because of the widespread use of myelodepressant drugs such as interferon or antineoplastic agents. Because no standard therapy exists for this complication, we have evaluated the efficacy and risks of splenic embolization in the treatment of hypersplenism in cirrhosis. Partial splenic embolization was performed in 40 consecutive patients with the following indications: 25 patients with active viral cirrhosis before interferon therapy, 8 patients with unresectable hepatocellular carcinoma before anti-neoplastic chemotherapy and 7 patients with thrombocytopenia associated with spontaneous bleeding events, with high risk of central nervous system hemorrhage or facing major surgery. After selective catheterization of the splenic artery, partial splenic embolization was performed by means of repeated injections of gelatin sponge until a 40% to 60% reduction in the splenic blood flow was achieved. After partial splenic embolization a significant and sustained increase in platelet and white blood cell count was observed during the follow-up period (mean = 13.9 +/- 2.2 mo; range = 1 to 36 mo). The goal of partial splenic embolization was achieved in all but two patients in whom a dose reduction of interferon was needed. Hypersplenism relapsed in only seven patients, and all of them exhibited an embolization of less than 50% of the splenic mass. Postembolization syndrome was the main side effect, but no life-threatening complications were detected. In conclusion, partial splenic embolization is a safe and effective therapy of hypersplenism in cirrhosis.

Positive feedback of glutamate exocytosis by metabotropic presynaptic receptor stimulation

Glutamate is important in several forms of synaptic plasticity such as long-term potentiation, and in neuronal cell degeneration. Glutamate activates several types of receptors, including a metabotropic receptor that is sensitive to trans-1-amino-cyclopenthyl-1,3-dicarboxylate, coupled to G protein(s) and linked to inositol phospholipid metabolism. The activation of the metabotropic receptor in neurons generates inositol 1,4,5-trisphosphate, which causes the release of Ca2+ from intracellular stores and diacylglycerol, which activates protein kinase C. In nerve terminals, the activation of presynaptic protein kinase C with phorbol esters enhances glutamate release. But the presynaptic receptor involved in this protein kinase C-mediated increase in the release of glutamate has not yet been identified. Here we demonstrate the presence of a presynaptic glutamate receptor of the metabotropic type that mediates an enhancement of glutamate exocytosis in cerebrocortical nerve terminals. Interestingly, this potentiation of glutamate release is observed only in the presence of arachidonic acid, which may reflect that this positive feedback control of glutamate exocytosis operates in concert with other pre- or post-synaptic events of the glutamatergic neurotransmission that generate arachidonic acid. This presynaptic glutamate receptor may have a physiological role in the maintenance of long-term potentiation where there is an increase in glutamate release mediated by postsynaptically generated arachidonic acid.

Activation of Protein Kinase C by Phorbol Esters and Arachidonic Acid Required for the Optimal Potentiation of Glutamate Exocytosis

The effects of arachidonic acid and phorbol esters in the Ca(2+)-dependent release of glutamate evoked by 4-aminopyridine (4-AP) in rat cerebrocortical synaptosomes were studied. In the absence of arachidonic acid, high concentrations (500 nM) of 4 beta-phorbol dibutyrate (4 beta-PDBu) were required to enhance the release of glutamate. However, in the presence of arachidonic acid, low concentrations of 4 beta-PDBu (1-50 nM) were effective in potentiating glutamate exocytosis. This potentiation of glutamate release by phorbol esters was not observed with the methyl ester of arachidonic acid, which does not activate protein kinase C. Moreover, pretreatment of synaptosomes with the protein kinase inhibitor staurosporine also prevented the stimulatory effect by arachidonic acid and phorbol esters. These results suggest that the activation of protein kinase C by both arachidonic acid and phorbol esters may play a role in the potentiation of glutamate exocytosis.

Inhibition of glutamate release by arachidonic acid in rat cerebrocortical synaptosomes

The action of arachidonic acid on glutamate release in rat cerebrocortical synaptosomes was investigated. The Ca(2+)-dependent release of glutamate evoked by 4-aminopyridine (4-AP) was inhibited by arachidonic acid (0.5-10 microM), but the KCl-evoked release was not modified. The Ca(2+)-independent release of glutamate was insensitive to low concentrations of arachidonic acid, but higher concentrations of this free fatty acid (30 microM) induced a slow efflux of cytoplasmic glutamate. The decrease in the Ca(2+)-dependent release of glutamate by arachidonic acid was consistent with a reduction in both the depolarization and the subsequent rise in the cytoplasmic free Ca2+ concentration induced by 4-AP in the nerve terminal. The inhibitory action by arachidonic acid observed in glutamate release was reversed in the presence of the K(+)-channel blocker tetraethylammonium.

Glutamate exocytosis evoked by 4-aminopyridine is inhibited by free fatty acids released from rat cerebrocortical synaptosomes

The Ca(2+)-dependent release of glutamate induced by 4-aminopyridine (4-AP) in rat cerebral cortical synaptosomes was reduced by removal of bovine serum albumin (BSA) from the incubation medium. The decrease in the glutamate release in the absence of BSA was consistent with a reduction in the rise in cytosolic free [Ca2+] after depolarization with 4-AP. Contrarily, neither the glutamate release nor the elevation in cytosolic free [Ca2+] after depolarization with 30 mM KCl was altered by the removal of BSA. The inhibitory action of the free fatty acids released during the incubation of synaptosomes was also observed when exogenous free fatty acids were added to the medium in the presence of BSA. The highest inhibition of 4-AP-induced release of glutamate was observed in the presence of arachidonic acid. The results strongly suggest an inhibitory action of free fatty acids by decreasing Ca2+ entry and glutamate release in rat cerebrocortical synaptosomes.