Immunoreactive thromboxane synthase is measurable in ovine fetal hypothalamus as early as 86 days' gestation

Involvement of brain thromboxane A in hypotension induced by haemorrhage in rats

1. In the present study, we aimed to determine the involvement of brain thromboxane A2 (TXA2) in blood pressure decreases evoked by acute and/or graded haemorrhage in rats. 2. Sprague-Dawley rats were used throughout the study. Acute haemorrhage was achieved by withdrawing a total volume of 2.1 and 2.5 mL blood/100 g bodyweight over a period of 10 min. A microdialysis study was performed in a hypothalamic area to measure extracellular TXA2 levels. Graded haemorrhage was conducted successively by withdrawing carotid arterial blood (0.55 mL/100 g bodyweight) over a 10 s period four times (S1-S4) at 5 min intervals. Furegrelate (125, 250 and 500 microg), a TXA2 synthase inhibitor, was injected intracerebroventricularly (i.c.v.) 60 min before acute or graded haemorrhage was initiated. U-46619 (0.5, 1 and 2 microg, i.c.v.), a synthetic TXA2 analogue, was administered 5 min before acute haemorrhage (2.1 mL/100 g bodyweight). 3. Acute haemorrhage produced a severe and long-lasting decrease in blood pressure and had a tendency to increase heart rate. Both haemorrhage protocols (2.1 or 2.5 mL/100 g) generated similar approximate twofold increases in extracellular hypothalamic TXA2 levels. Intracerebroventricular furegrelate (250 microg) pretreatment completely blocked the TXA2 increases induced by acute haemorrhage. Furegrelate administration (100, 250 and 500 microg, i.c.v.) attenuated the fall in arterial pressure evoked by acute haemorrhage and caused significant increases in heart rate at all doses injected. 4. Graded haemorrhage progressively lowered arterial pressure and increased plasma vasopressin and adrenaline levels in the last period. Furegrelate-injected rats were greatly resistant to the hypotensive effect of haemorrhage for all degrees of blood removed. Plasma adrenaline and vasopressin levels were significantly elevated in furegrelate-pretreated rats compared with the saline-treated group during S2-S3 and S4, respectively. U-46619 administration caused small but statistically significant decreases in arterial pressure induced by haemorrhage. 4. The results show that acute hypotensive haemorrhage increases extracellular hypothalamic TXA2 levels. The increase in brain endogenous TXA2 levels involves a decrease in blood pressure evoked by haemorrhage because the blockade of TXA2 synthesis by furegrelate pretreatment attenuated the haemorrhagic hypotension. Increases in plasma adrenaline and vasopressin levels may mediate this effect.

Cellular localization of thromboxane synthase in ovine spinal cord and hindbrain

We and others have demonstrated that endogenously-produced prostanoids modify the function of the hypothalamus-pituitary-adrenal (HPA) axis. We have demonstrated that exogenously-administered thromboxane mimetic stimulates ACTH secretion in fetal sheep, and that the endogenous production of thromboxane modifies the HPA response to cardiovascular stress. The purpose of this study was to identify the structures within the fetal and adult ovine medulla and hindbrain which express immunoreactive thromboxane synthase. Using immunohistochemical techniques, we demonstrated thromboxane synthase immunostaining in regions important for cardiovascular afferent signaling (nucleus tractus solitarius, ventrolateral medulla) in both cell bodies and axons. Thromboxane synthase was also apparent in neuroanatomical locations which are consistent with afferent and efferent projections from the cerebellum. We observed staining in the superior cerebellar peduncle in the rostal pons, in the corticopontocerebellar fibers, and in Purkinje cells. The enzyme was found in motor regions, including the dorsal motor nucleus of the vagus nerve, and in the motor neurons of the dorsal column of the spinal cord. In addition to the apparent neuronal staining, there was positive staining in the ventricular ependymal cells. We conclude that, consistent with physiological evidence, thromboxane synthase is present in brain regions which are important for afferent and efferent cardiovascular signaling.

Roles of brain prostaglandin E2 and thromboxane A2 in the activation of the central sympatho-adrenomedullary outflow in rats

We examined the effects of centrally administered active metabolites of the arachidonic acid cascade on activation of the central sympatho-adrenomedullary outflow using urethane-anaesthetized rats. Intracerebroventricularly (i.c.v.) administered prostaglandin E(2) (0.3 nmol/animal) significantly elevated plasma levels of noradrenaline while levels of adrenaline were not affected. Prostaglandin D(2), prostaglandin F(2 alpha) and prostaglandin I(2) at the same dose (0.3 nmol/animal, i.c.v.) had no effect on plasma levels of either catecholamine. Thromboxane A(2) mimetic, 7-[3-[3-hydroxy-4-(4-iodophenoxy)-1-butenyl]-7-oxabicyclo [2.2.1]hept-2-yl], [1S-[1 alpha,2 alpha(Z),3beta(1E,3S),4 alpha]]-5-heptenoic acid (I-BOP) (5 and 10 pmol/animal) microinjected into the paraventricular nucleus of the hypothalamus significantly elevated plasma levels of adrenaline, but had little effect on plasma levels of noradrenaline. The I-BOP-induced (10 pmol/animal) elevation of plasma adrenaline levels was abolished by (+)-(1R,2R,3S,4S)-(5Z)-7-(3-[4-3H]-phenylsulphonyl-aminobicyclo[2.2.1]hept-2-yl)hept-5-enoic acid sodium salt [(+)-S-145] (a blocker of thromboxane A(2) receptors) [625 nmol (250 micro g)/animal, i.c.v.]. These results suggest that brain prostaglandin E(2) and thromboxane A(2) are respectively involved in the activation of the central sympathetic and adrenomedullary outflow in rats.

Perfusion of the hypothalamic paraventricular nucleus with N-methyl-D-aspartate produces thromboxane A2 and centrally activates adrenomedullary outflow in rats

We applied a microdialysis technique for the measurement of hypothalamic thromboxane B2, a stable metabolite of thromboxane A2, in urethane-anesthetized rats. Perfusion with N-methyl-D-aspartate (1.5 and 2.5mM) of the paraventricular nucleus by microdialysis probe concentration-dependently elevated the levels of thromboxane B2 in this region and plasma levels of catecholamines. The elevation of adrenaline was much more marked than that of noradrenaline. Pretreatment with dizocilpine maleate (0.1 mM), a non-competitive antagonist of N-methyl-D-aspartate receptors, of the paraventricular nucleus by microdialysis probe attenuated the N-methyl-D-aspartate (1.5 mM)-induced elevations of both thromboxane B2 and plasma catecholamines. Intracerebroventricular administration of furegrelate (250 microg/animal), a thromboxane A2 synthase inhibitor, also abolished the responses evoked by N-methyl-D-aspartate. These results indicate that N-methyl-D-aspartate applied into the paraventricular nucleus produces thromboxane A2 in this region and elevates plasma levels of catecholamines, especially adrenaline. Thromboxane A2 produced in this hypothalamic nucleus is probably involved in the N-methyl-D-aspartate-induced central adrenomedullary outflow.