Stimulation of prostaglandin synthesis in rat cerebral cortex via a beta-adrenoceptor

Stress mediators regulate brain prostaglandin synthesis and peroxisome proliferator-activated receptor-gamma activation after stress in rats

Stress exposure leads to oxidative/nitrosative and neuroinflammatory changes that have been shown to be regulated by antiinflammatory pathways in the brain. In particular, acute restraint stress is followed by cyclooxygenase (COX)-2 up-regulation and subsequent proinflammatory prostaglandin (PG) E2 release in rat brain cortex. Concomitantly, the synthesis of the antiinflammatory prostaglandin 15d-PGJ(2) and the activation of its nuclear target the peroxisome proliferator-activated receptor (PPAR)-gamma are also produced. This study aimed to determine the possible role of the main stress mediators: catecholamines, glucocorticoids, and excitatory amino acids (glutamate) in the above-mentioned stress-related effects. By using specific pharmacological tools, our results show that the main mediators of the stress response are implicated in the regulation of prostaglandin synthesis and PPARgamma activation in rat brain cortex described after acute restraint stress exposure. Pharmacological inhibition (predominantly through beta-adrenergic receptor) of the stress-released catecholamines in the central nervous system regulates 15d-PGJ(2) and PGE(2) synthesis, by reducing COX-2 overexpression, and reduces PPARgamma activation. Stress-produced glucocorticoids carry out their effects on prostaglandin synthesis through their interaction with mineralocorticoid and glucocorticoid receptors to a very similar degree. However, in the case of PPARgamma regulation, only the actions through the glucocorticoid receptor seem to be relevant. Finally, the selective blockade of the N-methyl-d-aspartate type of glutamate receptor after stress also negatively regulates 15d-PGJ(2) and PGE(2) production by COX-2 down-regulation and decrease in PPARgamma transcriptional activity and expression. In conclusion, we show here that the main stress mediators, catecholamines, GCs, and glutamate, concomitantly regulate the activation of proinflammatory and antiinflammatory pathways in a possible coregulatory mechanism of the inflammatory process induced in rat brain cortex by acute restraint stress exposure.

Pathophysiology of inner ear fluid imbalance

Maintenance of homeostasis of inner ear fluids and biochemical integrity of inner ear tissue are essential for proper functioning of the auditory and vestibular end organs. Although various regulatory mechanisms exist in a different portion of the labyrinth, the inner ear is known to respond to systemic challenges. The association of Meniere's syndrome with an imbalance of inner ear fluid homeostasis has been hypothesized for the past century. Among many factors, the effects of hormonal imbalance on inner ear fluid composition and inner ear function have however scarcely been studied. The purpose of this study was to explore the relationship between the autonomic nervous system and inner ear function and possible mechanisms of functional disturbances in an experimental condition. An infusion of supraphysiologic amounts of epinephrine, a stress related hormone, resulted in an elevation of osmolality in serum and perilymph. Furthermore, the infusion of epinephrine resulted in elevation of threshold, prolongation of latency, and depression of amplitude in the compound action potential of the auditory nerve. These findings were most marked at high frequencies. We hypothesized that the epinephrine-induced hearing loss was brought about by an increase in perilymphatic osmolality, as well as by the ionic imbalance caused by the osmotic gradient. Since emotional stress has been implicated as a mechanism of inducing a Meniere's attack, evaluation of the relationship between the autonomic system and cochlear function may contribute to the understanding of possible mechanisms of inner ear dysfunction caused by hormonal imbalances.

Microdialysis: a system for localized drug delivery into the brain

To determine why intrahypothalamic microinjections of pyrogen-free saline (PFS) often induce core temperature (Tco) rises, guide cannulas were implanted bilaterally into the preoptic-anterior hypothalamus (POA) of guinea pigs; 1 week later, injectors were inserted to 1 mm beyond the guides and either no injection or 1 microliter PFS was administered. Injector insertion without injection evoked a 0.5 degrees C Tco rise within 40 min, culminating in 3.7 hr. PFS microinjection elicited a 0.9 degrees C Tco rise within 10 min, culminating in 3.8 hr. PFS injected 4 hr later caused a further Tco rise. Indomethacin (10 mg/kg, IM), given 30 min before, prevented these effects. To determine whether microdialysis obviates them, a guide cannula was implanted unilaterally into the POA; 1 week later, a dialysis probe (nominal cutoff, 10kD) was inserted to 1 mm beyond the guide. PFS or prostaglandin E2 (PGE2, 1 microgram/microliter) was perfused 2 days later (2 microliter/min for 3 hr). Tco was unchanged during PFS perfusion but increased during PGE2 perfusion to 1.5 degrees C in 1.6 hr, and plateaued until 2 hr after dialysis. These results indicate the Tco rise induced by PFS microinjection is mediated by prostaglandins, probably released due to tissue puncture by the injectors and injury by the PFS droplet. Microdialysis prevents these effects. It should, therefore, be preferred over microinjection for intracerebral drug administration.

Interaction of arachidonic acid metabolites and adrenergic nervous system

Arachidonic acid, which is esterified mainly at the Sn-2 position of phospholipids, is deacylated in response to a wide variety of neurohormonal stimuli, including sympathetic nerve stimulation and catecholamines. In most cardiovascular tissues, arachidonic acid is metabolized via the cyclooxygenase pathway into prostaglandins. The kind and quantity of prostaglandins formed in response to adrenergic stimuli vary in different tissues. For example, PGE2 is the major product of arachidonic acid formed in the spleen and kidney and PGI2 in the heart in response to sympathetic nerve stimulation and norepinephrine. The sequence of events, including the type of adrenergic receptor involved in the action of catecholamines on PG synthesis, varies from tissue to tissue. For example, PG synthesis elicited by adrenergic stimulation in the heart is mediated via activation of beta-1 adrenergic receptors, whereas in the kidney it is linked to stimulation of alpha-1 adrenergic receptors. Moreover, prostaglandin synthesis elicited by alpha-1 adrenergic receptor activation in the kidney requires only partially extra- but mainly intracellular Ca++ and calmodulin. However, beta-1 adrenergic receptor stimulated PG synthesis in the heart is absolutely dependent upon extracellular Ca++ but not on calmodulin. Furthermore, alpha-1 but not beta-1 adrenergic receptor stimulated PG synthesis is inhibited by mepacrine, a phospholipase A2 and C inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine dramatically increases prostanoid synthesis in piglet parietal cortex

We investigated effects of exogenous acetylcholine on prostanoid synthesis by parietal cortex in neonatal pigs. Cerebrospinal fluid (CSF) with no drug, and CSF containing acetylcholine at 10(-6) to 10(-3) M was injected under a 'closed' cranial window, and after 5 min the CSF was collected and analyzed by radioimmunoassay for prostaglandin (PG) E2, PGF2 alpha, PGD2, 6-keto-PGF1 alpha (the hydrolysis product of prostacyclin), and thromboxane (TX) B2 (the hydrolysis product of TXA2). PGE2 and PGF2 alpha were the predominant prostanoids in CSF under control conditions. Levels of all CSF prostanoids increased after topical application of acetylcholine, with the largest increases being for PGE2 and PGF2 alpha. During control conditions, levels were 1294 +/- 170 (mean +/- S.E.M.) pg/ml for PGE2 (n = 16), 1032 +/- 143 pg/ml for PGF2 alpha (n = 3), 659 +/- 92 pg/ml for 6-keto-PGF1 alpha (n = 15), 141 +/- 44 pg/ml for TXB2 (n = 12), and were below detectable levels for PGD2. Following application of 10(-3) M acetylcholine, levels were 34,535 +/- 5438 pg/ml for PGE2, 15,539 +/- 2772 pg/ml for PGF2 alpha, 2967 +/- 547 pg/ml for 6-keto-PGF1 alpha, 580 +/- 105 pg/ml for TXB2, and 556 +/- 221 pg/ml for PGD2. These results suggest that prostanoids could play a role in mediating effects of acetylcholine in the brain, or in modulating acetylcholine release via a negative feedback mechanism.

Effect of adrenergic agonists on eicosanoid output from isolated rabbit choroid plexus and iris-ciliary body

Prostanoid production by rabbit choroid plexus (CP) and iris-ciliary body (ICB), and the effects of adrenergic agonists thereon, were studied in vitro. Immunoreactive prostaglandin (PG) E2 was the major prostanoid released by both tissues; the output from ICB was some two orders of magnitude greater than from CP. Immunoreactive 6-keto PGF1 alpha and thromboxane (TX) B2, the dehydration products of prostacyclin and TXA2, respectively, were detected in smaller quantities. Epinephrine stimulated the outputs of PGE2 and 6-keto PGF1 alpha, but not of TXB2, from both tissues. ICB responded to epinephrine concentrations of 10(-4) and 10(-5), while only 10(-4) was effective in stimulating prostanoid synthesis in the CP. Phenylephrine, an adrenergic agonist, stimulated prostanoid output from the ICB, but not from the CP. It is concluded that adrenergic mechanisms stimulate the biosynthesis of prostanoids in the rabbit CP and ICB. The implications of such interactions to aqueous humor and cerebrospinal fluid dynamics, or to other processes in brain and ocular physiology, are discussed.

Eicosanoid synthesis elicited by norepinephrine in piglet parietal cortex

We investigated effects of exogenous norepinephrine and isoproterenol on pial arterial diameter and cerebral eicosanoid synthesis in anesthetized newborn pigs. Norepinephrine in artificial cerebrospinal fluid (CSF) constricted pial arteries from 203 +/- 27 micron (X +/- S.E.M.) to 164 +/- 18 micron (20 +/- 2%) (n = 21 vessels from 16 animals) at 10(-4) M. In the same animals, norepinephrine caused the concentration in CSF of 6-keto-prostaglandin F1 alpha to increase from 768 +/- 91 to 1544 +/- 151 pg/ml, thromboxane B2 to increase from 188 +/- 37 to 269 +/- 38 pg/ml, and prostaglandin E2 to increase from 2067 +/- 448 to 6575 +/- 751 pg/ml. Topical application of prostaglandin E2 in CSF to the cortical surface demonstrated that concentrations as low as 10,000 pg/ml were able to dilate pial arteries substantially. Blockade of cyclo-oxygenase activity by indomethacin (5-10 mg/kg, i.v.) potentiated pial arterial constriction to norepinephrine. Topical isoproterenol dilated pial arteries, but isoproterenol did not affect levels of measured eicosanoids in CSF. We conclude that norepinephrine elicits release of prostanoids from the cortical surface, and that these substances limit cerebrovascular constriction to norepinephrine.

Central nervous system prostaglandins and ethanol

There is growing interest in the effect of ethanol on cellular membranes, generally, and neuronal membrane systems, in particular. Perturbations of membranes have led to numerous enzymatically mediated processes, one of which is prostaglandin production. This paper reviews the nature and role of prostaglandins in the central nervous system, and what is known about the effect of ethanol on prostaglandin production in brain. Areas of central nervous system physiological function in which prostaglandins may mediate the actions of ethanol are discussed. Methodological considerations and future directions for research in the area of ethanol and prostaglandins are highlighted.

Brain arachidonic acid metabolites. Functions and interactions with ethanol

The formation and potential function of arachidonic acid metabolites in brain are reviewed briefly. Cyclooxygenase metabolites of arachidonic acid are formed in brain, but the lipoxygenase derivatives have not been demonstrated. Evidence clearly indicates that the cyclooxygenase products act as neuromodulators. Possible effects of leukotrienes on brain function remain in doubt, although several studies have suggested a role for the leukotrienes and it appears that these agents have a long-lasting effect. Potential interactions between ethanol and brain arachidonic acid metabolites have received minimal attention. Several studies indicate that pretreatment with prostaglandin synthetase inhibitors produces antagonism of ethanol's behavioral effects. These observations may be related to preliminary findings that ethanol increases brain prostaglandin content.

Beta-adrenoceptor mediated inhibition of behavioral action of desipramine and of central noradrenergic activity in forced swimming rats

The immobility-reducing action of desipramine (DMI) in forced swimming rats was attenuated by intracerebroventricular (i.c.v.) injection of isoproterenol (ISO) and potentiated by i.c.v. atenolol (ATE), a beta 1-adrenoceptor antagonist. The effect of ISO was blocked by ATE. When administered i.c.v. in normal rats, ISO reduced the contents of 3-methoxy-4-hydroxyphenylethyleneglycol sulfate (MHPG-SO4), a major metabolite of noradrenaline, in the septal area, thalamus and hypothalamus while ATE had no effect in most of the brain regions. However, in forced swimming rats treated with DMI, ISO reduced MHPG-SO4 in 6 out of 8 brain regions tested and conversely, ATE increased the levels in the amygdala, septal area and hypothalamus. Similar to the behavioral effect, the effect of ISO was antagonized by ATE. These results support the hypothesis that central beta 1-adrenergic mechanisms inhibit the immobility-reducing action of DMI by reducing the activity of noradrenergic neurons in the brain.