Possible existence of two subsets of platelet-activating factor receptor to mediate polyphosphoinositide breakdown and calcium influx in neuroblastoma x glioma hybrid NG 108-15 cells

Neuronal platelet-activating factor receptor signal transduction involves a pertussis toxin-sensitive G-protein

In most nonneural systems, platelet-activating factor (PAF) receptor effects are mediated by G-proteins that are often pertussis toxin-sensitive. The activation of pertussis toxin-sensitive G-proteins linked to PAF receptors results in the mobilization of intracellular calcium, at least in part, through the second messenger inositol triphosphate. We have sought to determine if a pertussis toxin-sensitive G-protein is involved in the PAF receptor-mediated phenomena of growth cone collapse and of synaptic enhancement in primary neuronal culture. Using infrared differential interference contrast microscopy and patch-clamp recording techniques, pertussis toxin, but not the inactive B oligomer of the toxin, was found to block both the growth cone collapse and the enhanced synaptic release of excitatory transmitter induced by a nonhydrolyzable PAF receptor agonist, making it likely that Go, Gq, or Gi is the G-protein transducer of PAF receptors in primary neurons. We believe that PAF acts directly on neuronal receptors, which are linked to pertussis toxin-sensitive G-proteins, on the tips of developing neurites, and on presynaptic nerve terminals, leading to growth cone collapse and enhanced synaptic release of transmitter.

Platelet-activating factor in the CNS

Platelet-activating factor (PAF) is a phospholipid synthesized in a variety of cells throughout the body. Platelet-activating factor has been identified in the CNS and has a number of diverse physiological and pathological functions. It has been shown to be a modulator of many CNS processes, ranging from long-term potentiation (LTP) to neuronal differentiation. Excessive levels of PAF appear to play an important role in neuronal cell injury, such as that resulting from ischaemia, inflammation, human immunodeficiency syndrome (HIV) and meningitis. The beneficial effects of PAF receptor antagonists are many and give rise to possible therapeutic strategies for neurotrauma.

Presence of specific platelet-activating factor binding-sites in neuroblastoma N1E-115 cells

In this study we reported evidence for the existence of specific binding sites for platelet-activating factor (PAF) in neuroblastoma N1E-115 cells. The specific [3H]PAF binding reached a steady state level within 60 min at 25 degrees C. Scatchard analysis of the specific [3H]PAF binding revealed the presence of two apparent populations of binding sites. The high-affinity binding site possessed a Kd1 of 2.5 +/- 0.6 pM and Bmax1 = 57.3 +/- 20.0 fmol/mg protein. The low-affinity binding site possessed a Kd2 = 3.2 +/- 1.0 nM and Bmax2 = 4.4 +/- 2.1 pmol/mg protein. Furthermore, the total [3H]PAF binding was partially displaced by unlabelled PAF, PAF antagonists BN 52021 and BN 50730 in a dose-dependent manner. This study confirms the presence of specific PAF receptors in neuronal cells.

Agonist-induced down-regulation of platelet-activating factor receptor gene expression in U937 cells

Prolonged exposure (8-24 h) of human promonocytic U937 cells to 100 nM 1-O-hexadecyl-2-N-methylcarbamyl-sn-glycero-3-phosphocholine (carbarmyl-PAF), a non-metabolizable analogue of platelet-activating factor (PAF), reduced the numbers of PAF receptors by 50-75%, as determined by the radioligand-binding assay. To clarify whether the down-regulation of receptor numbers is due to decreased expression level of the PAF-receptor gene, the effect of carbamyl-PAF on the steady-state level of PAF-receptor mRNA was examined by a highly sensitive reverse-transcriptase PCR method. A 50% decline in the level of PAF-receptor mRNA was observed in U937 cells pretreated with 100 nM carbamyl-PAF for 24 h. The effect of carbamyl-PAF was dose-dependent, with an EC50 value around 10 nM. PAF-receptor antagonist, SRI-63675, was able to attenuate the effect of carbamyl-PAF. Furthermore lysoPAF, at 1 uM, was unable to induce a significant decrease in PAF-receptor mRNA after incubation for 24 h, indicating that the effect of carbamyl-PAF was specific. The half-life of the PAF-receptor mRNA measured in the presence of actinomycin D was unaffected by carbamyl-PAF treatment. In contrast, nuclear run-off experiments demonstrated that the transcription rate of the PAF-receptor gene in carbamyl-PAF-treated cells was about 65% of that in control cells. These results suggest that the PAF receptor in U937 cells is subject to down-regulation by agonist, at least partly, at the transcriptional level.

Eicosanoid and inositol phosphate response to platelet-activating factor (PAF) and to a PAF antagonist in rat astroglial cells

The effects of different concentrations of exogenous platelet-activating factor (PAF) on the formation of arachidonic acid-cyclooxygenase metabolites and on the production of inositol phosphates have been investigated in a primary culture of rat astroglial cells. The cells were used at confluence and the purity was checked by immunostaining of the culture with specific antibodies against glial fibrillary acidic protein. Incubation of the cells with PAF (range 10(-9) to 10(-6) M) resulted in maximal accumulation of total inositol phosphate (620 +/- 60% increment over basal values, P < 0.001) at the concentration of 10(-8) M, after 1 min of stimulation. Smaller inositol phosphate accumulation occurred at higher concentrations of the agonist and at longer stimulation time. After 1 min of stimulation with PAF, the accumulation of the cyclooxygenase metabolites, thromboxane B2 (630 +/- 58 vs 20 +/- 2 pg/mg protein in non-stimulated samples) and 6-keto-prostaglandin F1 alpha (132 +/- 15 vs 55 +/- 7 pg/mg protein in non-stimulated samples) was also maximal at 10(-8) M concentration of the agonist. When the cultures were stimulated with PAF or Ca(2+)-ionophore after preincubation with equimolar concentration of the PAF inhibitor BN 52021, a significant inhibition in the synthesis of both inositol phosphates and cyclooxygenase metabolites occurred only in the PAF-stimulated cells.

Platelet-activating factor is a messenger in the electroconvulsive shock-induced transcriptional activation of c-fos and zif-268 in hippocampus

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), undetectable in resting neural tissue, accumulates in brain during seizures. A hetrazepine, BN-50730, is shown here to displace [3H]PAF-specific binding from microsomal, but not from synaptosomal membranes, indicating selectivity for a high affinity intracellular binding site. Rats pretreated with BN-50730 by intraperitoneal or intracerebroventricular injection exhibited an inhibition of the electroconvulsive shock (ECS)-induced expression of c-fos and zif-268 in hippocampus. A much more pronounced, dose-dependent inhibition of ECS-induced zif-268 mRNA in hippocampus by intracerebroventricular injection of BN-50730 was observed. It is concluded that, in the hippocampus, PAF is a mediator of the expression of zif-268 and, to a lesser extent, c-fos through an intracellular specific binding site. Thus, PAF may be a messenger in signal regulated zinc-finger transcription factors, and in other immediate-early genes involved in long-term synaptic plasticity changes.