Activation of the central cholinergic system mediates the reversal of hypotension by centrally administrated U-46619, a thromboxane A2 analog, in hemorrhaged rats

Central α7 and α4β2 nicotinic acetylcholine receptors offset arterial baroreceptor dysfunction in endotoxic rats

Cardiac autonomic neuropathy is a prominent feature of endotoxemia. Given the defensive role of the cholinergic pathway in inflammation, we assessed the roles of central homomeric α7 and heteromeric α4β2 nAChRs in arterial baroreceptor dysfunction caused by endotoxemia in rats. Endotoxemia was induced by i.v. administration of lipopolysaccharides (LPS, 10 mg/kg), and baroreflex activity was measured by the vasoactive method, which assesses reflex chronotropic responses to increments (phenylephrine, PE) or decrements (sodium nitroprusside, SNP) in blood pressure. Shifts caused by LPS in PE/SNP baroreflex curves and associated decreases in baroreflex sensitivity (BRS) were dose-dependently reversed by nicotine (25-100 μg/kg, i.v.). The nicotine effect disappeared after intracisternal administration of methyllycaconitine (MLA) or dihydro-β-erythroidine (DHβE), selective blockers of α7 and α4β2 receptors, respectively. The advantageous effect of nicotine on BRSPE was replicated in rats treated with PHA-543613 (α7-nAChR agonist) or 5-iodo-A-85380 (5IA, α4β2-nAChRs agonist) in dose-dependent fashions. Conversely, the depressed BRSSNP of endotoxic rats was improved after combined, but not individual, treatments with PHA and 5IA. Central α7 and α4β2 nAChR activation underlies the nicotine counteraction of arterial baroreflex dysfunction induced by endotoxemia. Moreover, the contribution of these receptors depends on the nature of the reflex chronotropic response (bradycardia vs. tachycardia).

Modulation of nesfatin-1-induced cardiovascular effects by the central cholinergic system

Nesfatin-1, a peptide whose receptor is yet to be identified, has been shown to be involved in the modulation of feeding, stress, and metabolic responses. Recently, increasing evidence has supported a modulatory role of nesfatin-1 in cardiovascular activity. We have previously reported that nesfatin-1 causes an increase in blood pressure in normotensive and hypotensive rats by increasing plasma catecholamine, vasopressin, and renin levels. Recent reports suggest that nesfatin-1 may activate the central cholinergic system. However, there is no evidence showing an interaction between central nesfatin-1 and the cholinergic system. Therefore, this study aimed to determine whether the central cholinergic system may have a functional role in the nesfatin-1-induced cardiovascular effect observed in normotensive rats. Intracerebroventricular injection of nesfatin-1 caused short-term increases in mean arterial pressure and heart rate responses including bradycardic/tachycardic phases in normotensive animals. Central injection of nesfatin-1 increased the acetylcholine and choline levels in the posterior hypothalamus, as shown in microdialysis studies. Central pretreatment with the cholinergic muscarinic receptor antagonist atropine and/or nicotinic receptor antagonist mecamylamine blocked nesfatin-1-induced cardiovascular effects. In conclusion, the results show that centrally administered nesfatin-1 produces a pressor effect on blood pressure and heart rate responses including bradycardic/tachycardic phases in normotensive rats. Moreover, according to our findings, the central cholinergic system can modulate nesfatin-1-evoked cardiovascular activity.

Activation of the central histaminergic system mediates arachidonic-acid-induced cardiovascular effects

The aim of this study was to explain the involvement of the central histaminergic system in arachidonic acid (AA)-induced cardiovascular effects in normotensive rats using hemodynamic, immunohistochemistry, and microdialysis studies. Intracerebroventricularly (i.c.v.) administered AA (0.25, 0.5, and 1.0 μmol) induced dose- and time-dependent increases in mean arterial pressure and decreased heart rate in conscious normotensive Sprague–Dawley rats. Central injection of AA (0.5 μmol) also increased posterior hypothalamic extracellular histamine levels and produced strong COX-1 but not COX-2 immunoreactivity in the posterior hypothalamus of rats. Moreover, the cardiovascular effects and COX-1 immunoreactivity in the posterior hypothalamus induced by AA (0.5 μmol; i.c.v.) were almost completely blocked by the H2 receptor antagonist ranitidine (50 and 100 nmol; i.c.v.) and partially blocked by the H1 receptor blocker chlorpheniramine (100 nmol; i.c.v.) and the H3–H4 receptor antagonist thioperamide (50 and 100 nmol; i.c.v.). In conclusion, these results indicate that centrally administered AA induces pressor and bradycardic responses in conscious rats. Moreover, we suggest that AA may activate histaminergic neurons and increase extracellular histamine levels, particularly in the posterior hypothalamus. Acting as a neurotransmitter, histamine is potentially involved in AA-induced cardiovascular effects under normotensive conditions.

Centrally administered CDP-choline induced cardiovascular responses are mediated by activation of the central phospholipase-prostaglandin signaling cascade

The present study was designed to determine the involvement of central prostaglandin synthesis on the pressor and bradycardic effect of cytidine 5'-diphosphocholine (CDP-choline). Intracerebroventricular (i.c.v.) administration of CDP-choline was made and blood pressure and heart rate were recorded in male Sprague Dawley rats throughout this study. Microdialysis and immunohistochemical studies were performed to measure extracellular total prostaglandin concentration and to show cyclooxygenase-1 and -2 (COX-1 and -2) immunoreactivities, respectively, in the posterior hypothalamic area. Moreover, rats were pretreated (i.c.v) with mepacrine [a phospholipase A2 (PLA2) inhibitor], ibuprofen [a nonselective COX inhibitor], neomycine [a phospholipase C (PLC) inhibitor] or furegrelate [a thromboxane A2 (TXA2) synthesis inhibitor] 5 min prior to the injection of CDP-choline to determine the effects of these inhibitors on cardiovascular responses to CDP-choline. Control rats were pretreated (i.c.v) with saline. CDP-choline caused a dose- and time-dependent increase in blood pressure and decrease in heart rate. Immunohistochemical studies showed that CDP-choline increased COX-1 and -2 immunoreactivities in the posterior hypothalamic area. CDP-choline also elevated hypothalamic extracellular total prostaglandin concentration by 62%, as shown in microdialysis studies. Mepacrine or ibuprofen pretreatments almost completely blocked the pressor and bradycardic responses to CDP-choline while neomycine or furegrelate partially attenuated the drug-induced cardiovascular effects. The results suggest that CDP-choline may stimulate prostaglandin synthesis through the activation of PLA2, cyclooxygenases (COX-1 and -2) and prostaglandins and at least TXA2, may mediate the drug׳s cardiovascular effects.

The effect of centrally injected CDP-choline on respiratory system; involvement of phospholipase to thromboxane signaling pathway

CDP-choline is an endogenous metabolite in phosphatidylcholine biosynthesis. Exogenous administration of CDP-choline has been shown to affect brain metabolism and to exhibit cardiovascular, neuroendocrine neuroprotective actions. On the other hand, little is known regarding its respiratory actions and/or central mechanism of its respiratory effect. Therefore the current study was designed to investigate the possible effects of centrally injected CDP-choline on respiratory system and the mediation of the central cholinergic receptors and phospholipase to thromboxane signaling pathway on CDP-choline-induced respiratory effects in anaesthetized rats. Intracerebroventricularly (i.c.v.) administration of CDP-choline induced dose- and time-dependent increased respiratory rates, tidal volume and minute ventilation of male anaesthetized Spraque Dawley rats. İ.c.v. pretreatment with atropine failed to alter the hyperventilation responses to CDP-choline whereas mecamylamine, cholinergic nicotinic receptor antagonist, mepacrine, phospholipase A2 inhibitor, and neomycin phospholipase C inhibitor, blocked completely the hyperventilation induced by CDP-choline. In addition, central pretreatment with furegrelate, thromboxane A2 synthesis inhibitor, also partially blocked CDP-choline-evoked hyperventilation effects. These data show that centrally administered CDP-choline induces hyperventilation which is mediated by activation of central nicotinic receptors and phospholipase to thromboxane signaling pathway.

The mediation of the central histaminergic system in the pressor effect of intracerebroventricularly injected melittin, a phospholipase A2 activator, in normotensive rats

Melittin is a polypeptide component of bee venom that leads to an increase in arachidonic acid release and subsequently in prostaglandin synthesis by activating phospholipase A(2). Recently we demonstrated that centrally or peripherally administrated melittin caused pressor effect and central thromboxane A(2) (TXA(2)) and cholinergic system mediated these effects of melittin. Also centrally injected histamine leads to pressor and bradycardic response by activating central histamine receptors in normotensive rats and central cholinergic system involved the effects of histamine. The present study demonstrates an involvement of the central histaminergic system in melittin-induced cardiovascular effect in normotensive rats. Experiments were carried out in male Sprague Dawley rats. Intracerebroventricularly (i.c.v.) injected melittin (0.5, 1 and 2 nmol) caused dose- and time-dependent increases in mean arterial pressure (MAP) and decrease in heart rate (HR) as we reported previously. Moreover, H(2) receptor antagonist ranitidine (50 nmol; i.c.v.) almost completely and H(3)/H(4) receptor antagonist thioperamide (50 nmol; i.c.v.) partly blocked melittin-evoked cardiovascular effects, whereas H(1) receptor blocker chlorpheniramine (50 nmol; i.c.v.) had no effect. Also centrally injected melittin was accompanied by 28% increase in extracellular histamine concentration in the posterior hypothalamus, as shown in microdialysis studies. In conclusion, results show that centrally administered melittin causes pressor and bradycardic response in conscious rats. Moreover, according to our findings, there is an involvement of the central histaminergic system in melittin-induced cardiovascular effects.

The role of the central arachidonic acid-thromboxane A2 cascade in cardiovascular regulation during hemorrhagic shock in rats

The aim of the current study was to elucidate the underlying central mechanism(s) of the cardiovascular effects evoked by centrally injected melittin and arachidonic acid (AA) in hemorrhaged hypotensive condition, specifically, from central AA release from the cell membrane under the influence of phospholipase A(2) (PLA(2)) to central thromboxane A(2) (TXA(2)) signaling via the cyclooxygenase (COX) pathway. As the main control of the study, melittin (3 μg) or AA (150 μg) was injected intracerebroventricularly (i.c.v.) after the hemorrhage procedure, which was performed by withdrawing a total volume of 2.2 ml of blood/100g body weight over a period of 10 min. Both treatments generated a pressor response and abolished the hypotension-induced hemorrhage. Pretreatment with the PLA(2) inhibitor mepacrine (500 μg; i.c.v.) completely blocked the pressor response to melittin in the hemorrhagic hypotensive state. Pretreatments with the nonselective COX inhibitor indomethacin (200 μg; i.c.v.) or the TXA(2) synthesis inhibitor furegrelate (250 or 500 μg; i.c.v.) were made to test the role of central COX activity and, subsequently, the TXA(2) signaling pathway in the melittin- or AA-mediated reversal of hemorrhagic hypotension. Indomethacin completely prevented the pressor response to melittin and AA in the hemorrhaged, hypotensive state, but furegrelate did so only partially. In conclusion, these findings suggest that central COX activity and, subsequently, the central TXA(2) signaling pathway, are, at least in part, involved in the melittin- or AA-induced reversal effect during hemorrhagic shock.

Central mechanism underlying pressor and bradycardic effect of intracerebroventricularly injected arachidonic acid

The aim of the current study was to determine the central cyclooxygenase (COX) pathway and central thromboxane signaling in the cardiovascular effects evoked by arachidonic acid (AA). As a main control for the study, different doses of AA (75, 150, or 300 µg) were administered intracerebroventricularly (i.c.v.). Centrally injected AA dose- and time-dependently increased mean arterial pressure and decreased heart rate in conscious normotensive Sprague–Dawley rats. The maximal cardiovascular effects of AA were observed at min 10 of the injection and lasted almost 30 min. To investigate the central mechanism of the AA-induced cardiovascular effect in conscious normotensive animals, pretreatment with nonselective COX inhibitor indomethacin (200 µg; i.c.v.), thromboxane A2 (TXA2) synthesis inhibitor furegrelate (250 or 500 µg; i.c.v.), or TXA2 receptor antagonist SQ-29548 (8 or 16 µg; i.c.v.) was carried out 15 min before AA (150 µg; i.c.v.) injection. While indomethacin completely prevented the pressor and bradycardic responses to AA, furegrelate and SQ-29548 attenuated these effects in part in awake normotensive rats. In conclusion, these findings suggest that the pressor and bradycardic cardiovascular effects of centrally injected AA are dependent on COX activity being totally central and the TXA2 signaling pathway being subsequently central, at least in part.

Systemic administration of a centrally acting acetylcholinesterase inhibitor improves outcome from hemorrhagic shock during acute alcohol intoxication

Previously, we have demonstrated that acute alcohol intoxication impairs hemodynamic counter-regulation to hemorrhage in unanesthetized rats, and that this phenomenon is associated with an impaired neuroendocrine response to blood loss. Moreover, we demonstrated that central acetylcholinesterase inhibition restores the hemodynamic and neuroendocrine responses to hemorrhage in alcohol-intoxicated rats. We hypothesized that similar responses could be elicited by systemic administration of physostigmine, an acetylcholinesterase inhibitor that penetrates the blood brain barrier. The relevance of this approach was to establish effectiveness of a more clinically applicable route of drug administration than that used previously. Chronically catheterized adult male Sprague-Dawley rats (250-275 g) were administered a bolus of physostigmine (i.v., 100 microg/kg) at rest, and in a separate study, simultaneously with Ringer's lactate solution after an overnight intragastric infusion of 30% alcohol (approximately 7 g/kg for 15 h) or 52% isocaloric dextrose and fixed-pressure hemorrhage. I.v. physostigmine administration immediately increased sympathetic outflow via activation of central nicotinic receptors and improved the pressor response to fluid resuscitation in both dextrose controls and alcohol-intoxicated animals. The improved hemodynamic recovery achieved with physostigmine was also associated with attenuation of the rises in the markers of liver and renal damage alanine aminotransferase and blood urea nitrogen in alcohol-intoxicated animals. Additional studies are warranted to determine the effect of central acetylcholinesterase inhibition on tissue injury and survival after severe blood loss, as well as its effects on long-term metabolic and inflammatory responses.

Cardiovascular effect of peripheral injected melittin in normotensive conscious rats: Mediation of the central cholinergic system

Recently we demonstrated that centrally administrated melittin, a phospholipase A(2) (PLA(2)) activator, caused the pressor effect in normotensive, conscious rats. In the present study, we aimed to determine the cardiovascular effect of peripherally injected melittin and the involvement of the central cholinergic system on these effects in the normotensive conscious rats. For this reason, 250, 500 or 1000microg/kg doses of melittin were injected intraperitoneally to normotensive male Sprague Dawley rats. Melittin produced dose- and time-dependent increases in mean arterial pressure (MAP) and heart rate (HR). Both peripheral (5mg/kg; i.p.) and central (500microg; i.c.v.) pretreatment with indomethacin, nonselective inhibitor of cyclooxygenase (COX) 1 and 2, totally abolished cardiovascular effect of melittin. Intraperitoneal (i.p.) pretreatment with propranolol, a nonselective beta-adrenergic receptor blocker, completely abolished the tachycardic response to melittin. Also, the pressor effect of melittin was partially attenuated in these rats. In order to test the mediation of the central cholinergic system on the pressor and tachycardic effects of melittin, the rats were pretreated with atropine sulfate (10microg; i.c.v.), a cholinergic nonselective muscarinic receptor antagonist, mecamylamine (50microg; i.c.v.), a cholinergic nonselective nicotinic receptor antagonist, methyllycaconitine (10microg; i.c.v.) or alpha-bungarotoxin (10microg; i.c.v.), selective antagonists of alpha7 subtype nicotinic acetylcholine receptors (alpha7nAChRs) 15min prior to melittin (500microg/kg; i.p.) injection. Pretreatment with mecamylamine, methyllycaconitine or alpha-bungarotoxin partially diminished the pressor and tachycardic response to melittin in the normotensive conscious rats whereas pretreatment with atropine sulfate had no effect. In conclusion, our data demonstrate that peripherally administered melittin exerts a clear pressor and tachycardic effect by activating COX pathway. The activation of central cholinergic nicotinic receptors, predominantly alpha7nAChRs, appears to be involved in the pressor and tachycardic effects of the drug.

Central acetylcholinesterase inhibition improves hemodynamic counterregulation to severe blood loss in alcohol-intoxicated rats

Acute alcohol intoxication results in impaired hemodynamic counterregulation to blood loss and is associated with an attenuated hemorrhage-induced release of catecholamines and AVP. We speculated that restoration of the neuroendocrine response to hemorrhage would improve mean arterial blood pressure (MABP) recovery during acute alcohol intoxication. Previously, we demonstrated that intracerebroventricular (i.c.v.) choline, a precursor of acetylcholine, transiently increases sympathetic nervous system (SNS) outflow but is not capable of improving neuroendocrine and hemodynamic compensation to hemorrhage in alcohol-treated rats. We hypothesized that prolongation of the observed effect via i.c.v. neostigmine, an acetylcholinesterase inhibitor, would enhance SNS outflow, restore the neuroendocrine response, and in turn improve hemodynamic responses to hemorrhage during acute alcohol intoxication. I.c.v. neostigmine (1 microg) increased MABP, catecholamines, and AVP within 5 min and reversed hypotension due to 40% hemorrhage and intragastric alcohol (30% wt/vol, 2.5 g/kg) administration in chronically catheterized male Sprague-Dawley rats (225-250 g body wt). Acute alcohol intoxication before 50% hemorrhage decreased basal MABP, accentuated hypotension midhemorrhage, suppressed the hemorrhage-induced release of norepinephrine and AVP, and prevented restoration of MABP to basal levels after fluid resuscitation with lactated Ringer solution. I.c.v. neostigmine (0.5 microg) produced a sustained increase in MABP beginning at 30 min of hemorrhage that persisted throughout fluid resuscitation in control and alcohol-treated animals. I.c.v. neostigmine enhanced epinephrine responses and restored the hemorrhage-induced release of norepinephrine and AVP in alcohol-treated rats. These results demonstrate that inhibition of acetylcholinesterase in the central nervous system enhances SNS outflow, restores the neuroendocrine response to severe blood loss, and thereby improves hemodynamic counterregulation during acute alcohol intoxication. This study provides evidence for a central (and not peripheral) role of alcohol in impairing hemodynamic stability during hemorrhagic shock.

Mechanism of the prostanoid TP receptor agonist U46619 for inducing emesis in the ferret

U46619 is a potent thromboxane A(2) mimetic with emesis-inducing actions that are mediated via prostanoid TP receptors. We investigated its emetic mechanism of action in more detail using the ferret as model animal. The emesis induced by U46619 (30 microg/kg, intraperitoneal) was antagonized significantly by (+)-(2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine hydrochloride (CP-99,994; 1 and 10 mg/kg; P < 0.05) and metoclopramide (0.3 and 3 mg/kg), but not by domperidone (3 mg/kg), sulpiride (0.1 mg/kg), ondansetron (0.1 and 1 mg/kg) alone or combined with droperidol (3 mg/kg), GR125487 (1 mg/kg), promethazine (3 mg/kg), or scopolamine (3 mg/kg); GR 125487 (1 mg/kg) prevented the anti-emetic action of metoclopramide (3 mg/kg). U46619 0.3 microg administered into the fourth ventricle rapidly induced emesis. However, bilateral abdominal vagotomy was ineffective in reducing the emetic response (P > 0.05). Our data suggests that U46619 induces emesis via an extra-abdominal mechanism, probably within the brain. Metoclopramide probably has a mechanism of action to prevent U46619-induced emesis via 5-HT(4) receptor activation and NK(1) tachykinin receptor antagonists could be useful to prevent emesis induced by TP receptor activation in man.

Peripheral mechanisms involved in the pressor and bradycardic effects of centrally administered arachidonic acid

In the current study, we aimed to determine the cardiovascular effects of arachidonic acid and peripheral mechanisms mediated these effects in normotensive conscious rats. Studies were performed in male Sprague Dawley rats. Arachidonic acid was injected intracerebroventricularly (i.c.v.) at the doses of 75, 150 or 300 microg and it caused dose- and time-dependent increase in mean arterial pressure and decrease in heart rate in normal conditions. Maximal effects were observed 10 min after 150 and 300 microg dose of arachidonic acid and lasted within 30 min. In order to evaluate the role of main peripheral hormonal mechanisms in those cardiovascular effects, plasma adrenaline, noradrenaline, vasopressin levels and renin activity were measured after arachidonic acid (150 microg; i.c.v.) injection. Centrally injected arachidonic acid increased plasma levels of all these hormones and renin activity. Intravenous pretreatments with prazosin (0.5 mg/kg), an alpha1 adrenoceptor antagonist, [beta-mercapto-beta,beta-cyclopentamethylenepropionyl1, O-Me-Tyr2-Arg8]-vasopressin (10 microg/kg), a vasopressin V1 receptor antagonist, or saralasin (250 microg/kg), an angiotensin II receptor antagonist, partially blocked the pressor response to arachidonic acid (150 microg; i.c.v.) while combined administration of these three antagonists completely abolished the effect. Moreover, both individual and combined antagonist pretreatments fully blocked the bradycardic effect of arachidonic acid. In conclusion, our findings show that centrally administered arachidonic acid increases mean arterial pressure and decreases heart rate in normotensive conscious rats and the increases in plasma adrenaline, noradrenaline, vasopressin levels and renin activity appear to mediate the cardiovascular effects of the drug.

Thromboxane A2: physiology/pathophysiology, cellular signal transduction and pharmacology

Thromboxane A(2) (TXA(2)), an unstable arachidonic acid metabolite, elicits diverse physiological/pathophysiological actions, including platelet aggregation and smooth muscle contraction. TXA(2) has been shown to be involved in allergies, modulation of acquired immunity, atherogenesis, neovascularization, and metastasis of cancer cells. The TXA(2) receptor (TP) communicates mainly with G(q) and G(13), resulting in phospholipase C activation and RhoGEF activation, respectively. In addition, TP couples with G(11), G(12), G(13), G(14), G(15), G(16), G(i), G(s) and G(h). TP is widely distributed in the body, and is expressed at high levels in thymus and spleen. The second extracellular loop of TP is an important ligand-binding site, and Asp(193) is a key amino acid. There are two alternatively spliced isoforms of TP, TPalpha and TPbeta, which differ only in their C-terminals. TPalpha and TPbeta communicate with different G proteins, and undergo hetero-dimerization, resulting in changes in intracellular traffic and receptor protein conformations. TP cross-talks with receptor tyrosine kinases, such as EGF receptor, to induce cell proliferation and differentiation. TP is glycosylated in the N-terminal region for recruitment to plasma membranes. Furthermore, TP conformation is changed by coupling to G proteins, showing several states of agonist binding. Finally, several drugs modify TP-mediated events; these include cyclooxygenase inhibitors, TXA(2) synthase inhibitors and TP antagonists. Some flavonoids of natural origin also have TP receptor antagonistic activity. Recent advances in TP research have clarified TXA(2)-mediated events in detail, and further study will supply more beneficial information about TXA(2) pathophysiology.

Cardiovascular effects of centrally injected melittin in hemorrhaged hypotensive rats: the investigation of peripheral mechanisms

We have previously shown that centrally injected melittin, a phospholipase A(2) (PLA(2)) activator, increases blood pressure and decreases heart rate in the normotensive conscious rats. In the current study we aimed to determine the cardiovascular effects of melittin in hemorrhaged hypotensive rats and to investigate the mediation of peripheral adrenergic, vasopressinergic and renin angiotensin system in the pressor effect of centrally administrated melittin in both normotensive and hypotensive conditions. Acute hypotensive hemorrhage was performed by withdrawing a total volume of 2.2ml of blood/100g body weight over a period of 10min. Melittin was injected intracerebroventricularly (i.c.v.) at the doses of 1.5microg, 3.0microg or 6.0microg after the stabilization period of hemorrhage procedure. We also repeated previous experiments by injecting melittin (1.5microg, 3.0microg or 6.0microg; i.c.v.) to the normotensive animals. Melittin caused dose- and time-dependent increases in mean arterial pressure (MAP) in normal and hypotensive conditions and decreases in heart rate (HR) in normotensive conscious animals. In hypotensive rats, melittin injected at the dose of 6.0microg completely restored the decrease in blood pressure. Plasma adrenaline, noradrenaline, vasopressin levels and renin activity increased after melittin (3.0microg; i.c.v) administration in normal conditions. Hemorrhage, itself, produced an increase in these plasma hormone levels and melittin (3.0microg; i.c.v.) caused additional increases in plasma adrenaline, noradrenaline, vasopressin levels and renin activity in hypotensive conditions. Intravenous pretreatments of rats with prazosin (0.5mg/kg), an alpha(1) adrenoceptor antagonist, [beta-mercapto-beta,beta-cyclopentamethylenepropionyl(1), O-Me-Tyr(2)-Arg(8)]-vasopressin (10microg/kg), a vasopressin V(1) receptor antagonist, or saralasin (250microg/kg), an angiotensin II receptor antagonist, partially blocked the pressor response to melittin (3.0microg; i.c.v.) in both normotensive and hypotensive conditions. Besides, the combined administration of these three antagonists before melittin completely abolished the pressor responses to drug in both conditions. Results show that centrally administered melittin, a PLA(2) activator, increases blood pressure and reverses hypotension in hemorrhagic shock. The increases in plasma adrenaline, noradrenaline, vasopressin levels and renin activity mediate the pressor responses to melittin in normal and hypotensive conditions.

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.