Depolarization-induced [3H]arachidonic acid accumulation: effects of external Ca2+ and phospholipase inhibitors

Activity-driven sharpening of the retinotectal projection: the search for retrograde synaptic signaling pathways

Patterned visual activity, acting via NMDA receptors, refines developing retinotectal maps by shaping individual retinal arbors. Because NMDA receptors are postsynaptic but the retinal arbors are presynaptic, there must be retrograde signals generated downstream of Ca(++) entry through NMDA receptors that direct the presynaptic retinal terminals to stabilize and grow or to withdraw. This review defines criteria for retrograde synaptic messengers, and then applies them to the leading candidates: nitric oxide (NO), brain-derived neurotrophic factor (BDNF), and arachidonic acid (AA). NO is not likely to be a general mechanism, as it operates only in selected projections of warm blooded vertebrates to speed up synaptic refinement, but is not essential. BDNF is a neurotrophin with strong growth promoting properties and complex interactions with activity both in its release and receptor signaling, but may modulate rather than mediate the retrograde signaling. AA promotes growth and stabilization of synaptic terminals by tapping into a pre-existing axonal growth-promoting pathway that is utilized by L1, NCAM, N-cadherin, and FGF and acts via PKC, GAP43, and F-actin stabilization, and it shares some overlap with BDNF pathways. The actions of both are consistent with recent demonstrations that activity-driven stabilization includes directed growth of new synaptic contacts. Certain nondiffusible factors (synapse-specific CAMs, ephrins, neurexin/neuroligin, and matrix molecules) may also play a role in activity-driven synapse stabilization. Interactions between these pathways are discussed.

Presynaptic protein kinase C controls maturation and branch dynamics of developing retinotectal arbors: possible role in activity-driven sharpening

Visual activity refines developing retinotectal maps and shapes individual retinal arbors via an NMDA receptor-dependent mechanism. As retinal axons grow into tectum, they slow markedly and emit many transient side branches behind the tip, assuming a "bottlebrush" morphology. Some branches are stabilized and branch further, giving rise to a compact arbor. The dynamic rate of branch addition and deletion is increased twofold when MK801 is used to block NMDA receptors, as if this prevents release of a stabilizing signal such as arachidonic acid (AA) from the postsynaptic neuron. In optic tract, AA mediates NCAM and L1 stimulation of axon growth by activating presynaptic protein kinase C (PKC) to phosphorylate GAP-43 and stabilize F-actin, and, if present in tectum, this growth control pathway could be modulated by postsynaptic activation. To test for the effects on arbor morphology of blocking PKC or AA release, we examined DiO-labeled retinal axons of larval zebrafish with time-lapse videomicroscopy. Bath application of the selective PKC inhibitor bisindolylmaleimide from 2 or 3 days onward doubled the rate at which side branches were added and deleted, as seen with MK801, and also prevented maturation of the arbor so that it retained a "bottlebrush" morphology. In order to selectively block the PKC being transported to retinal terminals, we injected the irreversible inhibitor calphostin C into the eye from which the ganglion cells were labeled, and this produced both effects seen with bath application. In contrast, there were no effects of control injections, which included Ringers into the same eye and the same dose into the opposite eye (actually much closer to the tectum of interest), to rule out the possibility that the inhibitor leaked from the eye to act on tectal cells. For comparison, we examined arbors treated with the NMDA blocker MK801 at half-hour time-lapse intervals, and detected the twofold rise in rates of branch addition and deletion previously reported in Xenopus larvae, but not the structural effect seen with the PKC inhibitors. In addition, we could produce both effects seen with PKC inhibitors by using RHC80267 to block AA release from DAG lipase, indicating that AA is the main drive for PKC activation. Thus, the results show a distinct role of AA and presynaptic PKC in both maturation of arbor structure and in the dynamic control of branching. The effects on branch dynamics were present regardless of the level of maturity of arbor structure. The fact that they mimicked those of MK801 suggests that presynaptic PKC may be involved in the NMDA receptor-driven stabilization of developing retinal arbors.

Prostaglandin F2 alpha synthesis in the hippocampal mossy fiber synaptosomal preparation: I. Dependence in arachidonic acid, phospholipase A2, calcium availability and membrane depolarization

Isolated hippocampal mossy fiber synaptosomes were used to characterize control mechanisms of prostaglandin F2 alpha (PGF2 alpha) synthesis at a central mammalian synapse. Exogenous arachidonic acid stimulated the dose-dependent synthesis of PGF2 alpha, as did the addition of phospholipase A2 or the activation of endogenous phospholipase A2. Phospholipase A2 inhibitors attenuated prostaglandin synthesis, but phospholipase C inhibitors had no effect. However, a diglyceride kinase inhibitor reduced PGF2 alpha accumulation. The cyclooxygenase inhibitor ibuprofen eliminated PGF2 alpha production, while the lipoxygenase inhibitors baicalein and NDGA reduced PGF2 alpha accumulation. The CA(2+)-ionophore-dependent stimulation of PGF2 alpha synthesis was abolished by Cd2+ or Ni2+. Further more, PGF2 alpha production appeared to be dependent on Ca2+ influx via L-type, but not N- or T-type, voltage-sensitive Ca2+ channels. Membrane depolarization with KC1, veratridine or 4-aminopyridine stimulated the synthesis of PGF2 alpha. This depolarization-dependent stimulation of PGF2 alpha synthesis was attenuated by L-type voltage-sensitive Ca2+ channel blockers, phospholipase A2 inhibitors, a K+ channel activator and a Na+ channel blocker. The activation of protein kinase C also led to a reduction of PGF2 alpha accumulation in depolarized nerve endings. These results may be used to suggest that PGF2 alpha production by hippocampal mossy fiber synaptosomes was controlled by the Ca(2+)- and phospholipase A2-dependent accumulation of unesterified arachidonic acid and was modulated by membrane depolarization and the activity of protein kinase C.

Characterization of a triacylglycerol lipase that liberates arachidonic acid from bovine chromaffin cells during secretion

Primary cultures of chromaffin cells from bovine adrenal medullae were used as a model to study lipolytic events during stimulus-secretion coupling. It has been shown that chromaffin cells liberate arachidonic acid in addition to their main secretion product, the catecholamines. To understand more about the mechanism of arachidonic acid liberation, chromaffin cells were labeled with radioactive arachidonic acid, stimulated, and then analyzed for changes in lipid composition. After stimulation with 10(-4) M acetylcholine, the radioactivity of triacylglycerols decreased to the same extent that the free arachidonic acid level rose. This finding suggests that in bovine chromaffin cells a stimulation-dependent triacylglycerol lipase (triacylglycerol hydrolase; EC 3.1.1.3) is involved in arachidonic acid liberation. Further work was performed on detection, characterization, and isolation of this enzyme. Triacylglycerol lipase activity was found in whole cell homogenates and in plasma membrane fractions isolated from adrenal medullary tissue. The plasma membrane lipase showed a pH optimum of 4.3. The apparent Michaelis constant was determined as 3.3 x 10(-4) mol/L. Ca2+ did not influence the enzymatic activity. To differentiate the plasma membrane triacylglycerol lipase from the previously described plasma membrane diacylglycerol lipase of chromaffin cells, the influence of RG 80267, a specific diacylglycerol lipase inhibitor, was examined. RG 80267 (50 microM) inhibited the triacylglycerol lipase by only 24%, although diacylglycerol lipase was totally inhibited with only 20 microM RG 80267. The pH optimum of homogenate lipase was broad, lying between 4 and 7. Starting from the soluble fraction of whole cell homogenates, the triacylglycerol lipase was partially purified by ultracentrifugation and size-exclusion chromatography.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of Glutamate Release From Hippocampal Mossy Fiber Nerve Endings By Arachidonic Acid And Eicosanoids

Arachidonic acid has been implicated in normal synaptic transmission processes, including those related to the development of hippocampal long-term synaptic potentiation. Hippocampal mossy fiber (MF) synaptosomes were used to investigate the role of arachidonate in the evoked accumulation of presynaptic Ca2+ and the release of endogenous glutamate, since these nerve terminals express long-term potentiation and selectively release glutamate as the excitatory transmitter. It was demonstrated that membrane depolarization evoked the accumulation of Ca2+, the release of glutamate, and the production of unesterified arachidonic acid. These events may be functionally related, since exogenous arachidonate and phospholipase A2 activation mimicked the effects of depolarization on Ca2+ availability and glutamate release, while secretion processes were attenuated in the presence of phospholipase A2 inhibitors. In addition, pretreatment of the nerve terminals with arachidonate or melittin allowed for the facilitated release of glutamate in response to a subsequent depolarizing stimulus. Inhibition of cyclooxygenase or lipoxygenase activities also potentiated presynaptic responses to membrane depolarization. In contrast, 12-lipoxygenase products attenuated the depolarization-evoked accumulation of intraterminal free Ca2+ and glutamate release. It is suggested that arachidonic acid acts as a positive modulator of mossy fiber secretion processes, including those involved in the increased glutamate release required for the induction of long-term potentiation, while 12-lipoxygenase metabolites provide negative feedback signals designed to limit neurotransmitter secretion.

PGF2 alpha synthesis in isolated cerebellar glomeruli: effects of membrane depolarization, calcium availability and phospholipase activity

The controlling factors for PGF2 alpha production were assessed in isolated cerebellar glomeruli, since this prostaglandin has been shown to stimulate the release of neurotransmitters from the mossy fiber terminals associated with this synaptic preparation. The metabolism of PGE2 was also examined, in order to determine the specificity of any treatment effects. It was observed that K(+)-dependent membrane depolarization or the activation of voltage-sensitive Na+ channels with veratradine stimulated the production of PGF2 alpha. The syntheses of both prostanoids were dependent on available calcium and were blocked by cyclooxygenase inhibitors. The lipoxygenase inhibitor NDGA also reduced the accumulation of PGE2 and PGF2 alpha. In addition, PGF2 alpha synthesis was stimulated by the phospholipase A2 activator melittin and was reduced due to phospholipase inhibition with dibucaine. These results are consistent with a role for PGF2 alpha in the evoked release of neurotransmitter from cerebellar mossy fiber terminals.

Presynaptic facilitation of glutamate release from isolated hippocampal mossy fiber nerve endings by arachidonic acid

Hippocampal mossy fiber synaptosomes were used to investigate the role of arachidonic acid in the release of endogenous glutamate and the long-lasting facilitation of glutamate release associated with long-term potentiation. Exogenous arachidonate induced a dose-dependent efflux of glutamate from the hippocampal mossy fiber synaptosomes and this effect was mimicked by melittin. Neither treatment induced the release of occluded lactate dehydrogenase at the concentrations used in these experiments. In each case, removal of the biochemical stimulus allowed for glutamate efflux to return to spontaneous levels. However, there was a persistent effect of exposure to either arachidonate or melittin, since these compounds facilitated the glutamate release induced by the subsequent addition of 35 mM KCl. This facilitation of glutamate release resulted from an enhancement of both the magnitude and duration of the response to depolarization. Although exogenous prostanoids were also able to stimulate the release of glutamate, they appeared to play no direct role in secretion processes, since inhibition of eicosanoid synthesis potentiated the glutamate efflux in response to membrane depolarization or exogenous arachidonic acid. We suggest that the calcium-dependent accumulation of arachidonic acid in presynaptic membranes plays a central role in the release of endogenous glutamate and that the persistent effects of arachidonic acid may be related to the maintenance of long-term potentiation in the hippocampal mossy fiber-CA3 synapse.

Modulation by neuropeptides of bradykinin-stimulated second messenger release in dorsal root ganglion neurons

Fetal rat dorsal root ganglion neurons (7-8 days in culture) were labeled with [3H]arachidonic acid for 24 h. Stimulation with 10 microM bradykinin (BK) for 30 s resulted in nearly 2-fold increases in levels of radioactive diglyceride and arachidonic acid. A similar result was obtained in the absence of receptor stimulation using the Ca2+ channel agonist BAY K 8644 (10 microM, in the presence of 100 mM potassium chloride) or the Ca2+ ionophore, ionomycin (2.5 microM). If Ca2+ influx was inhibited by adding 3 mM Co2+, a blocker of voltage-sensitive calcium channels, or 2.5 mM EDTA, then BK-stimulated accumulation of both arachidonate and diglyceride was inhibited. These data suggest Ca2+ influx is required for ligand-stimulated accumulation of both arachidonate (a product of diglyceride-lipase or phospholipase A2) and diglyceride (a product of phospholipase C). Two distinct populations of channels may be involved in these reactions since pretreatment with 10 microM nifedipine or 50 microM verapamil (agents which block a subset of voltage-sensitive Ca2+ channels) inhibited BK-stimulated accumulation of arachidonic acid, but did not inhibit diglyceride accumulation. Such functional discrimination appears to have physiological importance; the inhibitory effect of nifedipine and verapamil on BK-stimulated arachidonate release was mimicked by pretreatment with peptides which decrease Ca2+ channel conductance in dorsal root ganglion neurons. The three peptides used were 1 microM neuropeptide Y, 10 microM somatostatin, and 10 microM [N-MePhe3,D-Pro4]-morphiceptin. The effect of neuropeptide Y was blocked by pretreatment with pertussis toxin.(ABSTRACT TRUNCATED AT 250 WORDS)