In vivo comparative study of the seizure- and ischemia-induced synthesis of eicosanoids in the brain of gerbils.

After transient cerebral ischemia induced by bilateral ligation of carotid arteries, followed by 5 min reperfusion, concentrations of prostaglandin D2 and LTC4-like material increased with time in the gerbil brain. At least a 1 min occlusion time was necessary to elevate the eicosanoid concentrations significantly over the basal levels. Spontaneous tonic-clonic seizures of about 20 sec duration induced an increase in prostaglandin D2 and LTC4-like material comparable to the values found after a 2 min occlusion time. Following carotid artery occlusion, the eicosanoid levels were found to be elevated in midbrain, hypothalamus, striatum, hippocampus and cortex, i.e., those brain areas dependent upon the blood supply from the carotid arteries. In contrast, following spontaneous seizures, prostaglandin D2 concentrations were increased in the striatum, hippocampus and cortex only, and the LTC4-like material in the cortex. Hippocampus, striatum and cortex are brain areas which participate in the generation and propagation of seizures. It appears, therefore, unlikely that the seizure-induced eicosanoid synthesis is triggered off by a hypoxic event due to an impaired breathing caused by convulsions. The regional pattern of the eicosanoid synthesis following the seizures may rather depend on the intensity of the neuronal activity than on regional differences in the eicosanoid-synthesizing capacity.

The phorbol ester TPA potentiates cholera toxin- and isoproterenol-stimulated cyclic AMP-synthesis in primary astrocyte cultures.

Cellular responses to changes in the extracellular environment are mediated by intracellular signaling systems. One of the most extensively studied systems is adenylate cyclase which generates the second messenger molecule cAMP. Another one is the phosphatidylinositol (PI) second messenger system giving rise to IP3 and diacylglycerol, the latter stimulating protein kinase C. Recently, a third potential signaling system has attracted increased scientific attention: the phospholipase A2 system which generates arachidonic acid. This substance may be used for eicosanoid synthesis or serve as a second messenger molecule. The present report gives more evidence about mechanisms how these signaling pathways interact in cultured astrocytes. Substances commonly used for stimulation of arachidonic acid release and prostaglandin synthesis in these cultures (A23187, TPA) had no influence on intracellular cAMP levels. Pertussis toxin that had previously been shown to inhibit prostaglandin synthesis, had no influence on cAMP levels either. Cholera toxin, however, raised intracellular cAMP significantly, although much less than the beta-adrenoceptor agonist isoproterenol. Cholera toxin also caused a marked change in astroglial morphology even at reduced concentrations (1-10 ng/ml). A23187 used in combination with Ctx had a moderate stimulatory effect on cAMP synthesis. In contrast, in the presence of Ctx, the PKC-activating phorbol ester TPA synergistically stimulated cAMP production, raising cAMP levels as high as isoproterenol-stimulated levels. The TPA effect was concentration-dependent. It was also dependent on an intact PKC since preincubation of cells with the phorbol ester completely abolished the synergistic effect. The synergistic effect of the phorbol ester was also observed at subthreshold concentrations of isoproterenol. The data reveal that the sole activation of most Gs molecules is a necessary but not sufficient prerequisite to achieve maximal adenylate cyclase activity. The fine-tuning of this activity apparently occurs at the catalytic subunit which is under the (partial) control of phosphorylation by PKC.

Meningocortical lesion increases expression of the cholecystokinin gene in rat cerebral cortex: evidence for the involvement of platelet-activating factor (PAF).

In rat neocortex, interneurons express the gene encoding cholecystokinin. After an injury to the meninges and the underlying cortex the levels of cholecystokinin mRNA are transiently enhanced in the ipsilateral hemisphere. In the present study, we have investigated, whether platelet-activating factor plays a role in this phenomenon. Two antagonists of platelet-activating receptors, i.e. WEB 2086 (1.5 mg/kg) and brotizolam (10 mg/kg), were used. When injected 30 min prior to the injury of the parietal cortex, both agents reduced the rise in the concentration of cholecystokinin mRNA in frontal cortex by approximately 60%. They had no significant effect when given 30 min after the injury. Our finding that antagonists of platelet-activating factor receptors diminish the injury-induced change in the activity of cholecystokinin-interneurons opens the possibility that these agents may also affect other pathophysiological aspects of brain trauma.

Functionally diverse purinergic P2Y-receptors mediate prostanoid synthesis in cultured rat astrocytes: the role of ATP-induced phosphatidyl-inositol breakdown.

Cultured rat astrocytes possess purinergic P2Y-receptors. Stimulation of these receptors with ATP (10(-3) M) results in increased phosphatidylinositol biphosphate (PIP2)-breakdown and prostanoid formation. We have investigated the relevance of the PIP2-pathway in prostanoid synthesis. The intracellular Ca(2+)-mobilizing agent thapsigargin (TG) (10(-6) M) and the diacylglycerol (DAG)-mimetic tetradecaoylphorbol acetate (TPA) (10(-8)-10(-6) M) both stimulate prostaglandin D2 production. ATP-induced prostanoid formation can be mimicked by combined addition of TG and TPA, suggesting the importance of the second messengers IP3 and DAG, generated during P2Y-receptor mediated PIP2-breakdown. Inhibition of ATP-induced PIP2-hydrolysis by TPA (IC50 about 5 x 10(-8) M) or by 10(-4) M neomycine, however, does not affect astroglial prostanoid synthesis, showing that P2Y-receptor mediated prostanoid formation may occur also in the absence of PIP2-hydrolysis. These findings suggest that additional postreceptor mechanisms exist in the signal transduction chain of ATP-induced astroglial prostanoid synthesis. A possible involvement of phospholipase A2 and/or of Ca(2+)-channels, directly coupled to P2Y-receptors is proposed.

Characterization of cysteinyl-leukotriene formation in primary astroglial cell cultures.

The formation and composition of cysteinyl-leukotrienes (LT) in primary astroglial cell cultures prepared from newborn rat brain has been studied. Small amounts of cysteinyl-LT determined in terms of LTC4-like material in the supernatants of the cultures, became detectable after stimulation of the cells with 10(-5) M ionophore A23187. Cysteinyl-LT formation increased with time, reaching about 600 pg (mg protein)-1 after 60 min incubation. In contrast, considerable thromboxane (TX) B2 synthesis was found at 5 min following A23187-stimulation (about 30 ng TXB2 (mg protein)-1). The synthesis of cysteinyl-LT was abolished by 5 x 10(-5) M nordihydroguaiaretic acid (NDGA). Irrespective of the duration of incubation, blockage of prostanoid synthesis by 10(-6) M indomethacin did not result in increased cysteinyl-LT production. Reversed phase HPLC combined with radioimmunological detection showed that, after 60 min incubation in the presence of A23187, LTC4 and LTD4 accounted for practically all the LTC4-like immunoreactive material in the supernatants of cell cultures. No significant amounts of LTE4 could be detected. The results show that astrocytes may contribute to brain LTC4 and LTD4 synthesis. However, the cellular site of cerebral LTE4 formation seems to be other than the astroglia.

Differential prostaglandin formation induced by convulsions in the brain of mice susceptible (DBA/2J) and resistant (CFLP) to acoustic stimulation.

Endogenous cerebral prostanoids possess anticonvulsant properties. This study investigates possible age-dependent anomalies of prostanoid synthesis in the brain of seizure-prone DBA/2J (DBA2) mice as compared to sound stimulus-resistant CFLP mice. Irrespective of the age of the animals, a large increase of prostaglandin (PG) D2 and E2 in the brain of CFLP mice was observed in response to pentylenetetrazol (PTZ)-, or electroshock (ES)-induced seizures. Significantly less PGD2 and PGE2 was formed in the brain of DBA2 mice at day 21 after birth when subjected to PTZ or ES convulsions. At 42 days of age, however, this deficit of cerebral PGD2 synthesis in DBA2 mice disappeared concomitantly with the age-related decrease in audiogenic seizure (AS) susceptibility, whereas the deficit of PGE2 formation persisted. These results suggest that a deficiency of cerebral PGD2 synthesis may be one of the factors responsible for the AS susceptibility of the DBA2 mice. In contrast to PTZ or ES convulsions, acoustically induced seizures of the DBA2 mice were not accompanied by cerebral prostanoid synthesis. This supports the view that the pathways involved in AS are different from those involved in PTZ or ES models of epilepsy.

Formation and function of eicosanoids in the central nervous system.

Neuronal firing during experimental convulsions triggered a large increase in brain eicosanoid synthesis. Mature astrocytes are an important source of cerebral prostanoids. Endogenously formed prostaglandins possess anticonvulsive properties of biological relevance. These conclusions suggest new ideas that might explain the formation and functions of prostanoids in the brain. First, as augmented neuronal discharge is a prerequisite for enhanced prostanoid synthesis during seizures, a functional coupling between firing neurons and prostanoid-forming astrocytes may be expected. Second, the anticonvulsive effects of endogenous prostanoids suggest that astroglia-derived substances might regulate neuronal activity. The phenomenon of convulsion-induced prostanoid synthesis may, therefore, represent a new example of neuron-glia interaction. Neither K+-induced membrane depolarization nor receptor activation by drugs with affinity to alpha or beta adrenoceptors, dopamine, serotonin, muscarine, histamine, GABA, glutamate, aspartate, adenosine, and opioid receptors evoked eicosanoid synthesis in astrocytes. The only physiologically relevant ligand that induced prostanoid synthesis concentration dependently in astrocytes was ATP and related nucleotide triphosphates, as well as nucleotide disphosphates. In peripheral nerves ATP serves as a cotransmitter. The effect of the P2 agonists was reduced by pertussis toxin. The mechanism by which eicosanoids regulate neuronal activity remains to be elucidated.

Multiple pertussis toxin substrates as candidates for regulatory G proteins of adenylate cyclase coupled to the somatostatin receptor in primary rat astrocytes.

The involvement of G proteins in receptor mediated astroglial cAMP formation was studied. Isoproterenol or prostaglandin E2 stimulated adenylate cyclase of primary astroglial cells was inhibited by somatostatin. Preincubation of cells with increasing concentrations of islet activating protein (IAP) diminished somatostatin inhibition of adenylate cyclase. At an IAP concentration of 50 ng/ml somatostatin inhibition was completely abolished. Studies on IAP catalyzed 32P-ADP-ribosylation of astroglial cell particulate material revealed an incorporation of radiolabel into three polypeptides in the molecular weight range of 41,000-39,000 Dalton. Pretreatment of intact cells with IAP reduced radiolabeling of this molecular species in a concentration dependent manner. No further radiolabeling above background level was detectable after pretreatment of cultures with 10 ng IAP/ml or more. At present, the occurrence of at least three IAP substrates (G proteins) does not permit an identification of the somatostatin receptor coupled G protein. Rather, the finding reveals that astrocytes are endowed with multiple variants of GTP binding proteins likely to be coupled to different receptors.

Characterization of seizure-induced cysteinyl-leukotriene formation in brain tissue of convulsion-prone gerbils.

Tonic-clonic seizures elicited in convulsion-prone gerbils resulted in a large increase in immunoreactive prostaglandin (PG) F2 alpha and in a smaller increase in immunoreactive leukotriene (LT) C4-like material in brain tissue. Brain tissue contents of both eicosanoids were found to reach a maximum at 6 min after the onset of seizures and were still elevated at 54 min after the beginning of convulsions. By reversed phase HPLC the immunoreactive LTC4-like material was identified as LTC4 and LTD4 at 6 min after the onset of convulsions, whereas at 54 min after the onset, transformation of LTD4 to LTE4 could be detected as well. In gerbils showing only weak seizure activity a small increase in PGF2 alpha but no increase in immunoreactive LTC4-like material could be detected at 6 min after the onset of convulsions. Pretreatment with indomethacin abolished the formation of PGF2 alpha but significantly enhanced the biosynthesis of immunoreactive LTC4-like material at 18 min after the beginning of seizures. The results demonstrate formation of cysteinyl-LT following tonic-clonic convulsions in spontaneously convulsing gerbils which could be enhanced by inhibition of the cyclooxygenase pathway of arachidonic acid metabolism. Since cysteinyl-LT have potent biological actions in various organs this finding warrants further investigations on the potential role of cysteinyl-LT in the CNS.

The stable prostacyclin-analogue, iloprost, unlike prostanoids and leukotrienes, potently stimulates cyclic adenosine monophosphate synthesis of primary astroglial cell cultures.

The effect of different eicosanoids on adenosine-3', 5'-cyclic-monophosphate (cAMP) accumulation in primary astroglial cell cultures prepared from newborn rat brain was studied. The stable prostacyclin-analogue, iloprost, effectively stimulated cAMP synthesis in a concentration-dependent, saturable manner, the EC50 being about 3 x 10(-8) M. Prostaglandin (PG) E2 was less potent, without reaching plateau even at 10(-5) M. Prostaglandins D2 and F2 alpha, and the stable thromboxane A2-analogue, U 46619, as well as leukotrienes (LT) B4, C4, D4 and E4 were not effective and did not attenuate basal or isoprenaline (10(-8) M)-stimulated astroglial cAMP formation. This is the first indication for the existence of a prostacyclin receptor coupled positively to the adenylate cyclase in astrocytes. Other eicosanoids are unlikely to be involved in receptor-mediated regulation of astroglial cAMP levels.