The involvement of the sympathetic nervous system in the centrogenic pressor and tachycardiac effects of prostaglandins E2 and F2α in anaesthetised cats

The intracerebroventricular (i.c.v.) administration of prostaglandin E₂ (PGE₂, 1 μg) and prostaglandin F_2α (PGF_2α, 10 μg) produced prolonged pressor and tachycardiac responses in chloralose-anaesthetised cats. Phenoxybenzamine-pretreatment completely prevented the pressor response without altering the tachycardiac response, whereas propranolol intervention completely inhibited the tachycardiac response and also attenuated the pressor response. The pretreatment with pentolinium completely antagonised both the pressor and tachycardiac responses to i.c.v. PGE₂ and PGF_2α. The results suggest that the centrally administered PGE₂ and PGF_2α augment sympathetic outflow to the heart and vascular system and thereby cause excitatory cardiovascular responses in anaesthetised cats.     

Time course of release in vivo of PGE2, PGF2 alpha, 6-keto-PGF1 alpha, and TxB2 into the brain extracellular space after 15 min of complete global ischemia in the presence and absence of cyclooxygenase inhibition

The time-dependent release of prostaglandin E2 (PGE2), prostaglandin F2 alpha (PGF2 alpha), thromboxane (Tx) B2, and 6-keto-PGF1 alpha (6-keto) from brain was measured before, during, and after a 15-min interval of total ischemia (four-vessel occlusion) in halothane-anesthetized cats using the technique of cerebroventricular perfusion. Resting levels of PGE2, PGF2 alpha, 6-keto, and Tx were: 253 +/- 75, 953 +/- 300, 650 +/- 200, and 550 +/- 170 pg/ml, respectively. During the 15-min ischemia, all prostanoids rose significantly, yet the highest levels were not observed until the first 15-60 min of the reflow at which time levels of PGE2, PGF2 alpha, 6-keto, and Tx, as compared with the preischemic baseline, rose approximately 8, 3.4, 3, and 55-fold, respectively. Significantly, although all prostanoids showed increases relative to baseline, the ratios of PGF2 alpha/6-keto and PGE2/6-keto remained stable throughout the experiment in both groups of animals. In contrast, the Tx/6-keto ratio rose from approximately 1 to approximately 30 during the 60 min after reflow in untreated cats. Treatment with zomepirac sodium (5 mg/kg, i.v.), a cyclooxygenase inhibitor, resulted in highly significant reductions in the levels of all prostanoids during the preischemic period. In zomepirac sodium-treated animals, there were also highly significant reductions in the prostanoid response to ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Histamine and prostaglandin interaction in regulation of oxytocin and vasopressin secretion

Prostaglandins and histamine in the hypothalamus are involved in the regulation of oxytocin and vasopressin secretion, and appear to be involved in the mediation of pituitary hormone responses to immunochallenges. Therefore, we investigated in conscious male rats: (i) whether blockade of H1 or H2 receptors affected the oxytocin and vasopressin responses to prostaglandins and (ii) whether blockade of prostaglandin synthesis affected the oxytocin and vasopressin responses to histamine or to Escherichia coli lipopolysaccharide (LPS), in order to determine any interaction between prostaglandins and histamine in the hypothalamus. Oxytocin secretion was dose-dependently stimulated by intracerebroventricular infusion of 1 or 5 microg of PGE1, PGE2 or PGF2alpha, with PGE2 being the most potent of the compounds used. Prior central infusion of the H1 receptor antagonist mepyramine or the H2 receptor antagonist cimetidine significantly inhibited the oxytocin response to all three prostaglandins by approximately 50%. Vasopressin secretion was increased by PGE1 but not by PGE2 or PGF2alpha. The stimulatory effect of PGE1 was almost annihilated by prior administration of mepyramine or cimetidine. Central infusion of histamine or immunochallenge with LPS administered intraperitoneally increased oxytocin and vasopressin secretion four- and two-fold, respectively. Pretreatment with systemic injection of the prostaglandin synthesis inhibitor indomethacin dose-dependently reduced the oxytocin response and prevented the vasopressin response to histamine or LPS. We conclude that histamine and PGE1, PGE2 or PGF2alpha interact in the regulation of oxytocin secretion, whereas histamine and only PGE1 interact in the regulation of vasopressin secretion. Furthermore, histamine as well as LPS may affect oxytocin and vasopressin neurones via activation of prostaglandins, probably in the hypothalamic supraoptic nucleus.     

Role of central tachykinin peptides in cardiovascular regulation in rats.

The mechanisms of action of tachykinin peptides thought to be involved in central cardiovascular regulation were examined. Intracerebroventricular injections (i.c.v.) of tachykinin peptides caused dose-dependent increases in blood pressure and heart rate. The pressor responses to substance P (SP) (10 micrograms, i.c.v.) and neurokinin A (NKA) (10 micrograms, i.c.v.) were blocked by peripheral administration of pentolinium or phentolamine, and partially attenuated by adrenalectomy. In contrast, the only initial pressor response to the neurokinin B (NKB) analogue senktide (10 micrograms, i.c.v.) was blocked by pentolinium or phentolamine. The pressor response to senktide was inhibited by pretreatment with a vasopressin V1 receptor antagonist (d(CH2)5OMe(Tyr)AVP) (10 micrograms/kg, i.v.), and senktide (10 micrograms, i.c.v.) caused an increase in plasma vasopressin level. However, the vasopressin antagonist did not influence the SP- and NKA-induced pressor responses. These results suggest that central SP and NKA increase the blood pressure and heart rate via sympathetic nerve activity, whereas central NKB increases the blood pressure mainly via release of vasopressin from the hypothalamus.     

Contribution of N-methyl-d-aspartate receptors in the anteroventral third ventricular region to vasopressin secretion,but not to cardiovascular responses provoked by hyperosmolality and prostaglandin E2 in conscious rats

The aim of this study is to pursue roles of N-methyl-d-aspartate (NMDA) receptors in the anteroventral third ventricular region (AV3V; a pivotal area for autonomic functions) in controlling vasopressin (AVP) release and cardiovascular system. In conscious rats, we examined effects of AV3V infusion of MK-801 (a selective antagonist for NMDA receptor) on plasma AVP, osmolality, electrolytes, arterial pressure and heart rate, in the absence or presence of NMDA, hyperosmotic or prostaglandin (PG) E2 stimulus. The AV3V infusion of NMDA caused significant increases in plasma AVP, osmolality and sodium, hematocrit, arterial pressure and heart rate after 5 or 15min. When NMDA was administered into the cerebral ventricle, relatively smaller elevations were observed only in plasma AVP and arterial pressure. The effects of AV3V infusion of NMDA were nearly completely prevented by MK-801 applied to the same region before 15min. The application of MK-801 was also potent to block rises of plasma AVP elicited by AV3V injection of PGE2 or i.v. infusion of hypertonic saline. However, it inhibited neither increases of arterial pressure and heart rate due to the PGE2 treatment nor those of arterial pressure, plasma osmolality and sodium in response to the osmotic load. Histological analysis on the AV3V infusion sites of NMDA, MK-801 and PGE2 indicated that they had been located in the structures such as the median and medial preoptic nuclei, periventricular nucleus and medial preoptic area. These results suggest that stimulation of AV3V NMDA receptors in the basal state may facilitate AVP secretion and cause pressor and tachycardiac actions, and that these receptors may be involved in both the hyperosmolality- and PGE2-induced hormone release, but not in the cardiovascular responses to these stimuli.     

Vagal afferent-mediated inhibition of a nociceptive reflex by i.v. serotonin in the rat. II. Role of 5-HT receptor subtypes

In the rat, intravenous (i.v.) serotonin (5-HT) is a noxious stimulus which produces distinct vagal afferent-mediated pseudoaffective responses, a passive avoidance behavior, a vagal afferent-mediated inhibition of the nociceptive tail-flick (TF) reflex and a complex triad of cardiovascular responses. In the present study, we have used a variety of 5-HT receptor antagonists to characterize the receptor subtype(s) in the rat that mediate (1) inhibition of the TF reflex and (2) the cardiovascular responses produced by i.v. 5-HT. 5-HT produced a dose-dependent (3-72 micrograms/kg, i.v.) inhibition of the TF reflex (ED50 = 15.3 +/- 0.7 micrograms/kg). Following administration of the 5-HT2 receptor-selective antagonists ketanserin (50-250 micrograms/kg, i.v.) or xylamidine (10-100 micrograms/kg, i.v.), or the 5-HT3 receptor-selective antagonists ICS 205-930 (50-250 micrograms/kg, i.v.) or MDL 72222 (25-250 micrograms/kg, i.v.), there appeared to be a parallel shift of the 5-HT dose-response curve to the right. Following co-administration of xylamidine (50 micrograms/kg, i.v.) with ICS 205-930 (100 micrograms/kg, i.v.), the 5-HT-induced inhibition of the TF reflex was completely abolished at all doses of 5-HT tested (3-288 micrograms/kg, i.v.). In contrast, administration of the centrally acting 5-HT2 receptor-selective antagonist LY 53857 (10-100 micrograms/kg, i.v.) or the non-specific receptor antagonist methysergide (25-500 micrograms/kg, i.v.) resulted in a dose-dependent, but not parallel shift of the 5-HT dose-response curve to the right. The maximal doses of LY 53857 and methysergide tested (250 micrograms/kg and 500 micrograms/kg, respectively) completely abolished the effects of 5-HT (3-288 micrograms/kg, i.v.). Administration of the alpha 1-adrenoceptor antagonist prazosin (25-100 micrograms/kg, i.v.) failed to alter the 5-HT dose-response curve, indicating that the effects of ketanserin were due to blockage of 5-HT2 receptors rather than alpha 1 receptors. Administration of each of the antagonists also produced marked, but selective effects on components of the complex cardiovascular response to i.v. 5-HT. Each of the 5-HT3 receptor selective antagonists (ICS 205-930 or MDL 72222) produced a dose-dependent attenuation of the Bezold-Jarisch reflex-mediated hypotension and bradycardia, and each of the 5-HT2 receptor selective antagonists (xylamidine, ketanserin or LY 53857) produced a dose-dependent attenuation of the pressor response. The non-specific 5-HT receptor antagonist methysergide produced a dose-dependent attenuation of the 5-HT-induced pressor response.(ABSTRACT TRUNCATED AT 400 WORDS)     

Prostaglandin E2-induced sensitization of bradykinin-evoked responses in rat dorsal root ganglion neurons is mediated by cAMP-dependent protein kinase A

Primary cultures of neonatal rat dorsal root ganglion (DRG) neurons were used to examine the mechanisms underlying both the direct activation and the sensitization of sensory neurons by prostanoids. Prostaglandin E2 (PGE2) elevated cytosolic calcium concentration ([Ca2+]i) in a subpopulation of small (< 19 microm) diameter, capsaicin-sensitive DRG neurons. PGE2 also stimulated substance P (SP) release from DRG cultures. In contrast to bradykinin, PGE2 did not stimulate phosphoinositidase C (PIC) and the PGE2-evoked increase in [Ca2+]i was dependent on extracellular calcium. Pre-treatment with PGE2 potentiated bradykinin-evoked increases in [Ca2+]i in small diameter neurons and increased the number of cells that responded to low concentrations of bradykinin. A similar effect was seen with prostaglandin I2 (PGI2) but not prostaglandin F2alpha (PGF2alpha). PGE2 pretreatment also potentiated bradykinin-evoked release of SP, inducing a leftward shift in the bradykinin concentration-response curve and an increase in the maximum response. PGE2 stimulated adenylyl cyclase activity in DRG cultures, at concentrations and times consistent with those required to observe both the direct and sensitizing effects of the prostanoid on [Ca2+]i responses. Furthermore, the direct and sensitizing effects of PGE2, on both [Ca2+]i responses and SP release, were mimicked by the membrane permeant cAMP analogue dibutyryl cAMP and inhibited by H89, an inhibitor of cAMP-dependent protein kinase A (PKA). These observations are consistent with the hypothesis that both direct activation and sensitization of sensory neurons by prostanoids, such as PGE2, are mediated by PKA-dependent phosphorylation mechanisms.     

Excitotoxic lesions of the ventral anteroventral third ventricle and pressor responses to central sodium, ouabain and angiotensin II

To clarify the role of neurones in the anteroventral third ventricle (AV3V) area in cardiovascular responses to CSF sodium, ouabain and angiotensin II (ANG II), we employed excitotoxic lesions of the ventral AV3V (vAV3V). In conscious lesioned Wistar rats with systemic vasopressin blockade, pressor and tachycardiac responses to intracerebroventricular (i.c.v.) artificial CSF containing 0.3 M NaCl or ouabain were significantly attenuated by 26–32% whereas responses to ANG II were not affected. Thus, in rats with systemic blockade of vasopressin mechanisms, the vAV3V region partially mediates acute pressor responses to i.c.v. sodium and ouabain but not to ANG II.     

Prostaglandin profiles in relation to local circulatory changes following focal cerebral ischemia in cats

We explored the temporal and topographic relations between local cerebral blood flow and regional brain prostaglandin profile following prolonged or transient occlusion of the middle cerebral artery in cats. Each experimental group was subjected to a sham operation, prolonged ischemia, or recirculation. Local cerebral blood flow was measured by the hydrogen clearance method. Following in situ freezing, cortical samples were obtained from each gyrus for determination of prostaglandin (PG) F2 alpha, PGE2, 6-keto-PGF1 alpha, and thromboxane (TX) B2 concentrations by radioimmunoassay. During prolonged ischemia, the concentrations of PGF2 alpha and PGE2 within the middle cerebral artery territory were significantly increased. Immediately after recirculation, there was a prominent but transient increase in PGF2 alpha and PGE2 in gyri that had been exposed to moderate ischemia (perifocal area). By contrast, the increases in these prostaglandins were slow and less prominent in gyri that had been exposed to severe ischemia (the focal area). The concentration of 6-keto-PGF1 alpha did not change during prolonged ischemia but transiently increased following recirculation in both the focal and perifocal areas. The TXB2 concentration did not change in any experimental group. Our study revealed a homogeneous increase in the regional brain content of PGE2 or PGF2 alpha in spite of the heterogeneous reduction of local cerebral blood flow during prolonged ischemia. Following recirculation, the focal and perifocal areas exhibited different patterns of prostanoid content. No correlation was found between local cerebral blood flow and the regional concentration of any prostaglandin examined.     

Role of prostaglandin,endothelin and sympathetic nervous system on the L-NAME-induced pressor responses in spontaneously hypertensive rats

We tested the hypothesis that in spontaneously hypertensive rat (SHR) NO produced centrally influences the resting arterial blood pressure by attenuating mechanisms involving prostaglandins, angiotensin II, endothelin and sympathetic nervous system. L-NAME (200 micro g/5 micro l), an inhibitor of NO synthase, administered intracerebroventricularly (i.c.v.) to awake and freely moving rats increased mean arterial blood pressure (MABP) in a biphasic pattern: an early transient increase within 1 min and a late prolonged response starting at 45 min and persisting for the duration of experiment (180 min). The two pressor responses involve different neurochemical mechanisms and, based on their latencies, they appear to reflect different anatomical sites of action of L-NAME. The late, but not the early pressor response, was prevented by pretreatment with chlorisondamine (2.5 mg/kg, i.v.), a ganglionic blocker, indicating its dependence on the sympathetic nervous system. Both pressor responses were abolished by i.c.v. pretreatment with indomethacin (200 micro g/5 micro l, i.c.v.), an inhibitor of cyclo-oxygenase, showing that they are mediated by prostaglandin(s). In contrast, losartan (25 micro g/5 micro l), an angiotensin II AT(1) receptor antagonist, had no effect. The initial pressor response was also attenuated by pretreatment with the endothelin ET(A)/ET(B) receptor antagonist, PD 145065 (48 micro g/2 micro l, i.c.v.). Intravenous pretreatment with another ET(A)/ET(B) receptor antagonist, L-754,142 (15 mg/kg as a bolus+15 mg/kg/h for 180 min), however, attenuated both responses to L-NAME. It is possible that L-754,142 crossed the blood-brain barrier and blocked, in addition, central ET(A)/ET(B) receptors. These studies show that NO synthesized in the brain attenuates pressor mechanisms involving prostaglandin, endothelin and sympathetic nervous system, but not angiotensin II, to modulate resting arterial blood pressure.     

Involvement of primary afferent C-fibres in touch-evoked pain (allodynia) induced by prostaglandin E2.

Nociceptive primary afferents have the capacity to induce a state of increased excitability in dorsal horn neurons of the spinal cord or central sensitization causing thermal hyperalgesia and touch-evoked pain (allodynia). It is believed that primary afferent C-fibres become hypersensitive and induce hyperalgesia and that low-threshold Abeta-fibres are responsible for induction of allodynia, the mechanisms of which remain elusive. We previously showed that intrathecal administration of prostaglandin E2 (PGE2) and prostaglandin F2alpha (PGF2alpha) induce allodynia in conscious mice. Here we demonstrated that selective elimination of C-fibres by neonatal capsaicin treatment resulted in the disappearance of allodynia induced by PGE2, but not that by PGF2alpha. PGE2-induced allodynia was not observed in N-methyl-D-aspartate (NMDA) receptor epsilon1 subunit knockout mice and was sensitive to morphine. In contrast, PGF2alpha-induced allodynia was not observed in NMDA epsilon4 subunit knockout mice and was insensitive to morphine. Furthermore, while PGF2alpha showed a capsaicin-insensitive feeble facilitatory action on evoked excitatory postsynaptic currents in dorsal horn neurons, PGE2 induced a long-lasting facilitation of evoked excitatory postsynaptic currents in a capsaicin-sensitive manner. Taken together, the present study demonstrates that there are two pathways for induction of allodynia and that capsaicin-sensitive C-fibres may participate in PGE2-induced allodynia.     

Prostaglandin E2 levels in cerebrospinal fluid of normal and narcoleptic dogs

It has been shown that endogenous prostaglandin D2 and prostaglandin E2 (PGE2) are involved in sleep-wake regulation. Our recent experimental result that exogenously administered PGE2 significantly reduces canine cataplexy (a pathological equivalent of rapid-eye-movement sleep atonia and a symptom of narcolepsy) suggests that PGE2 is involved in the pathophysiology of canine narcolepsy. In order to further investigate the role of prostaglandins (PGs) in this disorder, PG levels in cerebrospinal fluid (CSF) of genetically homozygous narcoleptic, heterozygous (unaffected), and control Doberman pinschers were studied. PGE2 levels were measured by direct radioimmunoassay (RIA) and after high-grade purification using PG affinity columns and high-performance liquid chromatography. PGD2 and PGF2 alpha levels were measured by RIA after high-grade purification. There was no significant difference in PGE2 levels between homozygous narcoleptic and heterozygous or controls dogs, and PGD2 and PGF2 alpha levels were undetectable in most cases. Our results do not favor the hypothesis that central PGE2 levels are modified in canine narcolepsy, assuming that PGE2 levels in cisternal CSF properly reflect PGE2 production in the brain.     

Contribution of the central interaction between calcium and sodium to hemodynamic regulation in spontaneously hypertensive rats.

The contribution of the central interaction between calcium and sodium to hemodynamic regulation was assessed in spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) rats. The effect of a high calcium solution (Ca2+, 130 mg/dl, 10 microliters) infused into the cerebral ventricle (i.c.v.) on hemodynamic responses induced by a high sodium solution (Na+, 1,000 mEq/1, 10 microliters) i.c.v. and the mechanism by which high Ca2+ affects the hemodynamic responses induced by high Na+ i.c.v. were studied. High Na+ i.c.v. induced a pressor response with tachycardia in the SHRs, but induced a pressor response with reflex bradycardia in the WKYs. Prior treatment with high Ca2+ i.c.v. attenuated the pressor response induced by high Na+ i.c.v. (+55.6 +/- 4.4 to +33.1 +/- 3.2 mmHg, P < 0.01) and restored reflex bradycardia (+86.4 +/- 7.7 to -26.7 +/- 7.6 bpm, P < 0.01) in SHRs. Whereas prior treatment with high Ca2+ i.c.v. attenuated the pressor response (+35.7 +/- 2.0 to +22.2 +/- 4.0 mmHg, P < 0.05), it did not alter the degree of reflex bradycardia (-81.7 +/- 7.1 to -69.2 +/- 120 bpm, n.s.) in WKYs. Ganglionic blockade attenuated the pressor response (+56.9 +/- 3.5 to +42.9 +/- 2.3 mmHg, P < 0.05) and restored reflex bradycardia (+82.1 +/- 10.3 to -65.9 +/- 11.0 bpm, P < 0.01) in SHRs, whereas, inhibition of arginine vasopressin attenuated the pressor response without modification of the tachycardic response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of N‐methyl‐D‐aspartate and non‐N‐methyl‐D‐aspartate receptors in the cardiovascular effects of L‐glutamate microinjection into the hypothalamic paraventricular nucleus of unanesthetized rats

We report on the cardiovascular effects of L‐glutamate (L‐glu) microinjection into the hypothalamic paraventricular nucleus (PVN) as well as the mechanisms involved in their mediation. L‐glu microinjection into the PVN caused dose‐related pressor and tachycardiac responses in unanesthetized rats. These responses were blocked by intravenous (i.v.) pretreatment with the ganglion blocker pentolinium (PE; 5 mg/kg), suggesting sympathetic mediation. Responses to L‐glu were not affected by local microinjection of the selective non‐NMDA receptor antagonist NBQX (2 nmol) or by local microinjection of the selective NMDA receptor antagonist LY235959 (LY; 2 nmol). However, the tachycardiac response was changed to a bradycardiac response after treatment with LY235959, suggesting that NMDA receptors are involved in the L‐glu heart rate response. Local pretreatment with LY235959 associated with systemic PE or dTyr(CH₂)₅(Me)AVP (50 μg/kg) respectively potentiated or blocked the response to L‐glu, suggesting that L‐glu responses observed after LY235959 are vasopressin mediated. The increased pressor and bradycardiac responses observed after LY + PE was blocked by subsequent i.v. treatment with the V₁‐vasopressin receptor antagonist dTyr(CH₂)₅(Me)AVP, suggesting vasopressin mediation. The pressor and bradycardiac response to L‐glu microinjection into the PVN observed in animals pretreated with LY + PE was progressively inhibited and even blocked by additional pretreatment with increasing doses of NBQX (2, 10, and 20 nmol) microinjected into the PVN, suggesting its mediation by local non‐NMDA receptors. In conclusion, results suggest the existence of two glutamatergic pressor pathways in the PVN: one sympathetic pathway that is mediated by NMDA receptors and a vasopressinergic pathway that is mediated by non‐NMDA receptors. © 2009 Wiley‐Liss, Inc.     

Difference in sensitivity of cardiovascular and respiratory control neurons in the subretrofacial nucleus to glutamate receptor subtype agonists in SHR, WKY and cats.

Using spontaneously hypertensive rats (SHR), Wistar-Kyoto rats (WKY) and cats, either glutamate or a glutamate receptor subtype agonist was injected into the subretrofacial nucleus (SRF) in the rostral ventrolateral medulla at the site where the pressor response had been evoked by electrical stimulation. The sensitivity of SRF neurons to the electrical stimulation or glutamate receptor agonist was estimated by the threshold current or dose required to evoke the pressor response. The threshold of SRF neurons to electrical stimulation was similar in the three animal groups, while that to the glutamate receptor agonist was different. The significance of the difference in threshold between WKY and SHR was calculated as was that between WKY and cats. The threshold for kainate stimulation was ten times lower for SHR (0.016 pmol, P less than 0.001) and five times higher for cats (0.78 pmol, P less than 0.05); that for quisqualate stimulation was fifty times lower for SHR (0.016 pmol, P less than 0.001) but similar for cats; that for NMDA stimulation was twelve times lower for SHR (0.13 pmol, P less than 0.001) but seven times higher for cats (11 pmol, P less than 0.01); that for glutamate stimulation was ten times lower for SHR (4.2 pmol, P less than 0.001) but similar for cats. The heart rate and respiratory responses associated with the pressor response were tachycardiac and hypopneic in SHR and WKY, but bradycardiac and hyperpneic in cats. These responses were less dominant than the pressor response. We suggest that the pathogenesis of hypertension in SHR may be partly due to abnormal properties of glutamate receptor subtypes acting on vasomotor control neurons in the SRF.     

Effects of cerebral ischemia and reperfusion on prostanoid accumulation in unanesthetized and pentobarbital-treated gerbils

Cerebral ischemia was induced in unanesthetized gerbils using bilateral carotid artery ligations. The effects of 20 min of global ischemia on the concentrations of prostaglandin F2 alpha (PGF2 alpha), PGE2, 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha), and thromboxane B2 were determined after 0-24 h of reperfusion. Ischemia had little effect on eicosanoid production, but significant increases were observed by 5 min of reperfusion, with maximal levels reached by 15 min of reperfusion. PGF2 alpha was the most concentrated prostaglandin in postischemic brain, whereas PGE2 was most concentrated in control cerebra. Pretreatment with anesthetic doses of pentobarbital supported increased accumulation of PGF2 alpha in postischemic cerebra, increased accumulation of 6-keto-PGF1 alpha during the ischemic episode, and decreased accumulation of PGE2 at 120 min of reperfusion. It appears that the protective effects of barbiturate anesthesia are not expressed by the reduced accumulation of the above eicosanoids.     

The involvement of the central cholinergic system in the pressor and bradycardic effects of centrally administrated melittin in normotensive conscious rats

Recently we demonstrated that centrally administrated melittin, a phospholipase A(2) (PLA(2)) activator, caused pressor and bradycardic effect in the normotensive conscious rats. In the current study we aimed to determine the mediation of central cholinergic system in the pressor and bradycardic effect of centrally administrated melittin. Studies were performed in normotensive male Sprague-Dawley rats. 1.5, 3.0 or 6.0microg/5.0microl doses of melittin were injected intracerebroventricularly (i.c.v.). Melittin caused dose- and time-dependent increases in mean arterial pressure (MAP) and decrease in heart rate (HR). In order to test the mediation of central cholinergic system on the pressor and bradycardic effect of melittin, the rats were pretreated with mecamylamine (50microg; i.c.v.), cholinergic nonselective nicotinic receptor antagonist, atropine sulfate (10microg; i.c.v.), a cholinergic nonselective muscarinic receptor antagonist, hemicholinium-3 (20microg; i.c.v.), a high affinity neuronal choline uptake inhibitor, methyllycaconitine (10 and 25microg; i.c.v.) or alpha-bungarotoxin (10 and 25microg; i.c.v.), selective antagonists of alpha-7 subtype nicotinic acetylcholine receptors (alpha7nAChRs), 15min prior to melittin (3.0microg) injection. Pretreatment with mecamylamine, hemicholinium-3, methyllycaconitine or alpha-bungarotoxin partially attenuated the pressor and bradicardia effect of elicited by melittin in the normotensive conscious rats whereas pretreatment with atropine had no effect. In conclusion, i.c.v. administration of melittin increases MAP and decreases HR in conscious rats. The activation of central nicotinic cholinergic receptors, predominantly alpha7nAChRs, partially acts as a mediator in the pressor responses to i.c.v. injection of melittin in the normotensive conscious rats. Moreover, decreased uptake of choline to the cholinergic terminals may consider that melittin activates central choline and acetylcholine release, as well.     

Effects of intracerebroventricular losartan on angiotensin II-mediated pressor responses and c-fos expression in near-term ovine fetus

The renin-angiotensin system plays an important role in cardiovascular control. Intracerebroventricular (i.c.v.) angiotensin (ANG) II causes a reliable pressor response in the fetus at 90% gestation. To determine the roles of brain AT1 and AT2 receptors in this response, the effects of the central AT1 and AT2 receptor antagonists losartan and PD123319 were investigated in chronically prepared near-term ovine fetuses. Losartan at 0.5 mg/kg (i.c.v.) abolished central ANG II-induced pressor responses. High-dose losartan (5 mg/kg, i.c.v.) showed a potentiation of the pressor response to i.c.v. ANG II, accompanied by bradycardia. Associated with the pressor responses, c-fos expression in the cardiovascular controlling areas was significantly different between the low and high doses of losartan. These areas included the subfornical organ, median preoptic nucleus, organum vasculosum of the lamina terminalis, and paraventricular nuclei in the forebrain, and the tractus solitarius nuclei, lateral parabrachial nuclei in the hindbrain. Low-dose losartan markedly reduced c-fos in these areas after i.c.v. ANG II, while the high-dose losartan together with ANG II elicited a much stronger FOS-immunoreactivity in these areas than that induced by i.c.v. ANG II alone. This is a novel finding, that c-fos expression in the brain can be both activated and inhibited under the same condition. Central ANG II-induced fetal pressor responses were not altered by PD123319 (0.8 mg/kg). These results indicate that i.c.v. losartan at a high and a low dose has strikingly different effects on central ANG II-induced pressor responses in fetuses at late gestation, and that the AT1 mechanism plays an important role in fetal cardiovascular regulation.     

Supraspinal and spinal mediation of naloxone-induced morphine withdrawal in rats

Unanesthetized rats, made physically dependent over 5 days by chronic intra-arterial infusion of increasing concentrations of morphine (35-100 mg/kg/day) underwent withdrawal by naloxone (6 micrograms) injection into either the lateral ventricle (i.c.v.), fourth ventricle (V4), intrathecal subarachnoid space (i.t.), or intra-arterially (i.a.) and were evaluated for cardiovascular and behavioral signs of precipitated abstinence. Naloxone i.c.v. produced a significantly greater increase in the magnitude and duration of withdrawal hypertension than did V4 injection. Naloxone i.t. produced a distinctively different, persistent, pressor response as compared to i.c.v., V4 or i.a. routes of administration, although no quantitative differences in behavioral signs of withdrawal were observed. Morphine-dependent, spinal transected (C1) animals generated an augmented pressor response to i.c.v. or i.t. naloxone. This pressor response was accompanied by a significant reduction in core temperature (0.50-0.79 degrees C). Both the naloxone-induced pressor and hypothermic responses were abolished by ganglionic (hexamethonium, 100 mg/kg, i.a.) or peripheral alpha-adrenergic (phentolamine 4 mg/kg, i.a.) blockade. The hypertensive and hypothermic effects of naloxone also were prevented in transected dependent rats by prior spinal pithing. We conclude that in morphine-dependent rats: supraspinal sites rostral to the V4 mediate a more intense naloxone-induced pressor response than caudal regions; cardiovascular and behavioral signs of withdrawal can be precipitated via the spinal cord of intact animals; and the production of withdrawal hypertension and hypothermia in spinal transected morphine-dependent rats indicates that these abstinence signs can be mediated through neuronal pathways within the spinal cord.     

Type-1 and type-2 astrocytes are distinct targets for prostaglandins D2, E2, and F2 alpha.

Accumulating evidence has revealed that astrocytes are potential targets for various neurotransmitters. Here we investigated the effects of prostaglandins (PGs) on signal transduction in purified primary cultures of rat type-1 and type-2 astrocytes. PGF2 alpha, PGD2, and 9 alpha,11 beta-PGF2, a metabolite of PGD2 and a stereoisomer of PGF2 alpha, evoked a rapid rise in the intracellular Ca2+ concentration ([Ca2+]i) in type-1, but not in type-2, astrocytes. STA2, a stable analogue of thromboxane A2, was less effective, and PGE2 showed little effect. The PG-induced rise in [Ca2+]i was not blocked by an antagonist of either PGD2 receptor or thromboxane A2 receptor. PGF2 alpha and 9 alpha,11 beta-PGF2 stimulated rapid formation of inositol trisphosphate followed by inositol bisphosphate and inositol monophosphate. On the other hand, PGE2 increased the intracellular level of cyclic AMP in type-2 astrocytes, rather than in type-1 astrocytes. The potency of PGs for cyclic AMP formation was in the following order: PGE2 greater than PGE1 greater than or equal to STA2 much greater than iloprost, a stable analogue of PGI2. PGD2 and PGF2 alpha had no effect on cyclic AMP formation. These results demonstrate that type-1 astrocytes preferentially express PGF2 alpha receptors, the activation of which leads to phosphoinositide metabolism and [Ca2+]i elevation, whereas type-2 astrocytes possess PGE receptors that are linked to cyclic AMP formation.