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

Hypothyroidism alters the expression of prostaglandin D2 synthase/beta trace in specific areas of the developing rat brain

Lipocalin-type prostaglandin D2 synthase is the enzyme responsible for the synthesis of prostaglandin D2, a major prostaglandin in the central nervous system. We analysed the effects of thyroid hormone deprivation on prostaglandin D2 synthase gene expression in the developing rat brain. By in situ hybridization, the strongest prostaglandin D2 synthase mRNA signal was detected in the leptomeninges and choroid plexus. The signal was greatly reduced in the cerebellar interlaminar meninges of hypothyroid rats aged 15 and 25 days. Immunohistochemical studies defined changes in the location of the prostaglandin D2 synthase protein. In control but not in hypothyroid animals, Cajal-Retzius neurons of cortical layer I, and pyramidal cortical plate neurons were intensely stained on postnatal day 5. Conversely, prostaglandin D2 synthase protein levels were higher in neurons of the CA1 and CA3 regions and the dentate gyrus of the hippocampus of hypothyroid animals on postnatal days 5, 15 and 25, and also in subplate neurons on postnatal days 15 and 25. In agreement with the in situ hybridization and northern blotting data, the major difference was found in the cerebellar interlaminar meninges of hypothyroid animals, where the protein was clearly down-regulated on postnatal days 15 and 25. These results show that hypothyroidism causes both age- and region-specific alterations in the expression and location of the prostaglandin D2 synthase during postnatal brain development, probably reflecting a cell-specific regulatory effect of thyroid hormone on the prostaglandin D2 synthase.

Prostaglandins F2 alpha synthesis in the hippocampal mossy fiber synaptosomal preparation: II. Effects of receptor activation

Mossy fiber nerve endings were isolated from rat hippocampi and used to determine the effects of receptor activation on the production of prostaglandin F2 alpha (PGF2 alpha). Glutamate and its agonists had no effect on PGF2 alpha synthesis. Similarly, acetylcholine, gamma-aminobutyric acid, histamine and purinergic receptor agonists did not affect PGF2 alpha accumulation in this preparation. However, norepinephrine, serotonin and dopamine exerted receptor-mediated stimulations of PGF2 alpha production. The agonist-evoked increases in PGF2 alpha production were attenuated by phospholipase A2 inhibitors, L-type voltage-sensitive Ca2+ blockers and a K+ channel activator, but they were insensitive to tetrodotoxin. In addition, a kappa opioid agonist decreased PGF2 alpha synthesis in unstimulated and depolarized synaptosomes. It appeared, therefore, that certain receptor agonists were able to modulate PGF2 alpha synthesis in the hippocampal mossy fiber synaptosomal preparation.

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.

Reciprocal regulation of fatty acid release in the brain by GABA and glutamate

Several model systems have been used to test the hypothesis that the release of FFA in the brain is regulated by depolarization of neurons. This FFA release is likely the result of the activation of phospholipase A2. The increased neuronal activity that occurs due to synchronous depolarization during seizures causes activation of phospholipase A2. Decreasing neuronal activity by administering the anxiolytic, diazepam, appears to decrease the activity of phospholipase A2. The GABA antagonist, bicuculline, which causes depolarization by negating the hyperpolarizing tone imposed on neurons by GABA, causes FFA release in synaptosomes and in neurons in tissue culture. Likewise, the glutamate agonist, kainic acid, which depolarizes neurons by opening sodium channels, increases the activity of phospholipase A2. PC-specific phospholipase C, another enzyme important in the generation of the second messenger, DG, is also activated by depolarization. Several important questions remain to be answered. The site of FFA release, in terms of the pre-vs. postsynaptic membrane, is not clear, although the experiments with synaptosomes support the hypothesis that activation of phospholipase A2 may be an important regulator of presynaptic events. This idea has also been suggested by studies on the phenomenon of long-term potentiation, where free 20:4 or its metabolites may be involved in presynaptic facilitation of neurotransmitter release (Freeman et al., 1990; Massicotte et al., 1990; Williams et al., 1989; also see Dorman, this volume). The activation of the PI cycle and subsequent stimulation of protein kinase C may be a postsynaptic event important in the integration of inputs at the dendrite and soma or a presynaptic event involved in the modulation of neurotransmitter release (Taniyama et al., 1990; El-Fakahany et al., 1990; also see Nishizuka, this volume). Therefore the stimulation of a PC-specific phospholipase C, which is capable of generating large amounts of DG over a prolonged period of time (Exton, 1990; Martinson et al., 1990; Diaz-Laviada et al., 1990), could occur at either site. Another important question is the role of FFA and DG in affecting cell-cell signaling events, particularly with regard to ion fluxes. Modulation of an acetylcholine-linked K+ channel in the heart by FFA and their oxygenation products has been reported (Kim and Clapham, 1989). The cardiac muscarinic receptor is linked to a hyperpolarizing K+ channel via a G protein.(ABSTRACT TRUNCATED AT 400 WORDS)