Vahide Savci

Restoration of blood pressure by choline treatment in rats made hypotensive by haemorrhage

1 Intracerebroventricular (i.c.v.) injection of choline (25–150 μg) increased blood pressure in rats made acutely hypotensive by haemorrhage. Intraperitoneal administration of choline (60 mg kg−1) also increased blood pressure, but to a lesser extent. Following i.c.v. injection of 25 μg or 50 μg of choline, heart rate did not change, while 100 μg or 150 μg i.c.v. choline produced a slight and short lasting bradycardia. Choline (150 μg) failed to alter the circulating residual volume of blood in haemorrhaged rats 2 The pressor response to i.c.v. choline (50 μg) in haemorrhaged rats was abolished by pretreatment with mecamylamine (50 μg, i.c.v.) but not atropine (10 μg, i.c.v.). The pressor response to choline was blocked by pretreatment with hemicholinium‐3 (20 μg, i.c.v.) 3 The pressor response to i.c.v. choline (150 μg) was associated with a several fold increase in plasma levels of vasopressin and adrenaline but not of noradrenaline and plasma renin 4 The pressor response to i.c.v. choline (150 μg) was not altered by bilateral adrenalectomy, but was attenuated by systemic administration of either phentolamine (10 mg kg−1) or the vasopressin antagonist [β‐mercapto‐β,β‐cyclopenta‐methylenepropionyl1, O‐Me‐Tyr2,Arg8‐vasopressin (10 μg kg−1) 5 It is concluded that the precursor of acetylcholine, choline, can increase and restore blood pressure in acutely haemorrhaged rats by increasing central cholinergic neurotransmission. Nicotinic receptor activation and an increase in plasma vasopressin and adrenaline level appear to be involved in this effect of choline.

EFFECTS OF INTRACEREBROVENTRICULAR INJECTED CHOLINE ON CARDIOVASCULAR FUNCTIONS AND SYMPATHOADRENAL ACTIVITY

Summary: Intracerebroventricular (i.c.v.) injection of choline (50–150 μg) increased blood pressure (SP) and decreased heart rate (HR) in freely moving rats. Intracerebroventricular pretreatment of rats with mecamylamine (50 μg) blocked the reduction in HR and reduced the increase in SP induced by i.c.v. choline (150 μg). Central muscarinic blockade with atropine (10 μg, i.c.v.) reduced the pressor response to i.c.v. choline (150 μg) by about 70%, without influencing the decrease in HR. The decrease in HR induced by i.c.v. choline was prevented by intraarterial (i.a.) treatment of atropine methylnitrate (2 mg/kg). Intracerebroventricular choline (150 μg) produced a fivefold increase in catecholamine concentrations in adrenal venous plasma. Bilateral adrenalectomy reduced, but did not block, cholines effect on SP. Intracerebroventricular choline (150 μg) showed an ability to increase and restore SP in rats subjected to spinal cord transection or pretreatment with hexamethonium (15 mg/kg, i.a.) or with phentolamine (10 mg/kg, i.a.). Intracerebroventricular choline (150 μg) increased plasma vasopressin (VP) levels from 2.2 × 0.4 to 25.6 × 2.5 pg/ml. Pretreatment of rats with a VP antagonist reduced the pressor response to i.c.v. choline. It is concluded that (a) the reduction in HR results from a central nicotinic receptor-mediated increase in vagal tone, (b) the increase in SP appears to be due to activation of both nicotinic and muscarinic central cholinergic receptors, and that (c) the central activation of the adrenal medulla and the increase in plasma levels of VP are involved in the pressor response to i.c.v. choline.

Restoration of Blood Pressure by Centrally Injected U-46619, a Thromboxane A2 Analog, in Hemorrhaged Hypotensive Rats: Investigation of Different Brain Areas

In the present study, we investigated the cardiovascular effects of centrally injected U-46619, a thromboxane A2 (TXA2) analog, and the central and peripheral mechanisms of these effects in hemorrhagic shock conditions. Hemorrhage was performed by withdrawing a total volume of 2.1 ml of blood/100 g body weight over a period of 10 min. Injections were made into the lateral cerebral ventricle (LCV), nucleus tractus solitarius (NTS), rostral ventrolateral medulla (RVLM) and paraventricular nucleus of hypothalamus (PVN). U-46619 (0.1, 1 and 2 µg) increased blood pressure and reversed hypotension in hemorrhagic shock. The pressor effect was dose- and time-dependent in all investigated brain areas. Heart rate changes were not significantly different in all groups. Pretreatment of rats with an injection of SQ-29548 (4 or 8 µg), a TXA2 receptor antagonist, into the LCV, NTS, RVLM and PVN completely blocked the pressor effect of U-46619 (1 µg) injected into respective brain areas. Hemorrhage itself increased plasma adrenaline, noradrenaline, vasopressIN levels and renin activity. U-46619 (1 µg) injected into the LCV, PVN, RVLM and NTS produced additional increases in these hormone levels and in renin activity. Intravenous pretreatments of rats with prazosin (0.5 mg/kg), an α1-adrenoceptor antagonist, [β-mercapto-β,β-cyclopentamethylenepropionyl1, O-Me-Tyr2,Arg8]- vasopressin (10 µg/kg), a vasopressin V1-receptor antagonist, or saralasin (250 µg/kg), an angiotensin II receptor antagonist, in hemorrhaged rats partially blocked the pressor response to U-46619 (1 µg) injected into the LCV, PVN, RVLM and NTS. Results show that centrally administered U-46619, a TXA2 analog, increases blood pressure and reverses hypotension in hemorrhagic shock. Activation of central TXA2 receptors mediates the pressor effect of the drug. Furthermore, the increases in plasma adrenaline, noradrenaline, vasopressin levels and renin activity are involved in these effects.

Centrally injected CDP-choline increases plasma vasopressin levels by central cholinergic activation.

In the present study, both the effects of intracerebroventricular (i.c.v.) injection of cytidine‐5′‐diphosphate choline (CDP‐choline) on plasma vasopressin levels and the choline involvement of these effects were investigated. I.c.v. administration of CDP‐choline (0.5, 1.0 and 2.0 μmol) increased plasma vasopressin levels dose‐ and time‐dependently. I.c.v. injection of equimolar dose of choline (1 μmol) produced similar vasopressin response. However equimolar dose of cytidine (1 μmol; i.c.v.), the other hydrolysis product of CDP‐choline, did not affect plasma vasopressin levels. Pretreatment of rats with hemicholinium‐3, neuronal high affinity choline uptake inhibitor (20 μg; i.c.v.) blocked the vasopressin response to i.c.v. CDP‐choline (1 μmol). Pretreatment of rats with mecamylamine (50 μg; i.c.v.), a nonselective nicotinic receptor antagonist, abolished the increase in plasma vasopressin induced by CDP‐choline while atropine (10 μg; i.c.v.), nonselective muscarinic receptor antagonist, failed to change the response. In conclusion, intracerebroventricularly injected CDP‐choline can increase plasma vasopressin levels by activating central nicotinic cholinergic receptors through the activation of presynaptic cholinergic mechanisms.

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

In the present study, we investigated the role of the central cholinergic system in mediating the pressor effect of intracerebroventricularly administrated U-46619, a thromboxane A2 (TxA2) analog, in hemorrhaged hypotensive rats. Hemorrhage was performed by withdrawing a total volume of 2.1 ml of blood per 100 g body weight over a period of 10 min. Intracerebroventricular (i.c.v.) injection of U-46619 (0.5, 1, 2 micro g) produced a dose- and time-dependent increase in arterial pressure and reversed the hypotension of this condition. Hemorrhage caused small increases in extracellular hypothalamic acetylcholine and choline levels. Intracerebroventricular administration of U-46619 (1 micro g) further increased the levels of extracellular acetylcholine and choline by 57% and 41%, respectively. Pretreatment with SQ-29548 (8 mug; i.c.v.), a selective TxA2 receptor antagonist, completely abrogated the effects of subsequent injection of U-46619 (1 mug; i.c.v.) on arterial pressure and extracellular acetylcholine and choline levels. Pretreatment with mecamylamine (50 micro g; i.c.v.), a cholinergic nonselective nicotinic receptor antagonist, attenuated the pressor effect of U-46619 (1 micro g, i.c.v.) in hemorrhaged rats whereas pretreatment with atropine (10 micro g; i.c.v.), a cholinergic nonselective muscarinic receptor antagonist, had no effect. Interestingly, pretreatment of rats with methyllycaconitine (10 micro g; i.c.v.) or alpha-bungarotoxin (10 micro g; i.c.v.), selective antagonists of alpha-7 subtype nicotinic acetylcholine receptors (alpha7nAChRs), partially abolished the pressor effect of U-46619 (1 micro g; i.c.v.) in the hypotensive condition. Pretreatment with a combination of mecamylamine plus methyllycaconitine or mecamylamine plus alpha-bungarotoxin attenuated the reversal effect of U-46619, but only to the same extent as pretreatment with either antagonist alone. In conclusion, i.c.v. administration of U-46619 restores arterial pressure and increases posterior hypothalamic acetylcholine and choline levels by activating central TxA2 receptors in hemorrhaged hypotensive 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 U-46619 under these conditions.

Involvement of brain thromboxane A2 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.

CDP‐choline increases plasma ACTH and potentiates the stimulated release of GH, TSH and LH: the cholinergic involvement

In the present study, we investigated the effect of intracerebroventricular (i.c.v.) administration of cytidine‐5′‐diphosphate (CDP) choline on plasma adrenocorticotropin (ACTH), serum growth hormone (GH), thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH) and luteinizing hormone (LH) levels in conscious rats. The involvement of cholinergic mechanisms in these effects was also determined. In basal conditions, CDP‐choline (0.5, 1.0 and 2.0 μmol, i.c.v.) increased plasma ACTH levels dose‐ and time‐dependently, but it did not affect the TSH, GH, FSH and LH levels. In stimulated conditions, i.c.v. administration of CDP‐choline (1 μmol, i.c.v.) produced an increase in clonidine‐stimulated GH, thyrotyropin‐releasing hormone (TRH)‐stimulated TSH, LH‐releasing hormone (LHRH)‐stimulated LH, but not FSH levels. Injection of equimolar dose of choline (1 μmol, i.c.v.) produced similar effects on hormone levels, but cytidine (1 μmol, i.c.v.) failed to alter plasma levels of these hormones. Pretreatment with hemicholinium‐3, a neuronal high affinity choline uptake inhibitor, (20 μg, i.c.v.) completely blocked the observed hormone responses to CDP‐choline. The increase in plasma ACTH levels induced by CDP‐choline (1 μmol, i.c.v.) was abolished by pretreatment with mecamylamine, a nicotinic receptor antagonist, (50 μg, i.c.v.) but not atropine, a muscarinic receptor antagonist, (10 μg, i.c.v.). The increase in stimulated levels of serum TSH by CDP‐choline (1 μmol, i.c.v.) was blocked by atropine but not by mecamylamine pretreatment. However, CDP‐choline induced increases in serum GH and LH levels were greatly attenuated by both atropine and mecamylamine pretreatments. The results show that CDP‐choline can increase plasma ACTH and produce additional increases in serum levels of TSH, GH and LH stimulated by TRH, clonidine and LHRH, respectively. The activation of central cholinergic system, mainly through the presynaptic mechanisms, was involved in these effects. Central nicotinic receptors solely mediated the increase in plasma ACTH levels while the activation of central muscarinic receptors was involved in the increase in TSH levels. Both muscarinic and nicotinic receptor activations, separately, mediated the increases in serum GH and LH levels after CDP‐choline.

Centrally administered choline increases plasma prolactin levels in conscious rats

Intracerebroventricular (i.c.v.) administration of choline, a precursor of acetylcholine (ACh) increased plasma prolactin levels in a time and dose-dependent manner in conscious rats. Pretreatment of rats with the cholinergic muscarinic antagonist, atropine (10 microg, i.c.v.), blocked the increase in plasma prolactin level. The increase was not influenced by pretreatment with the cholinergic nicotinic antagonist, mecamylamine (50 microg, i.c.v.). Pretreatment with hemicholinium-3 (HC-3; 20 microg, i.c.v.), a high affinity choline uptake inhibitor, attenuated the choline-induced increase of plasma prolactin levels. These results show that choline increases plasma prolactin levels by activating muscarinic receptors via presynaptic mechanisms.

Effect of intracerebroventricularly injected choline on plasma ACTH and β-endorphin levels in conscious rats

In the present study, we examined the effect of intracerebroventricularly injected choline on plasma ACTH (adrenocorticotrophin) and beta-endorphin levels in conscious rats. The intracerebroventricularly injection of choline (50-150 micrograms) elevated plasma ACTH levels in a dose-dependent manner. Plasma beta-endorphin levels were also significantly increased. Pretreatment of rats with mecamylamine (50 micrograms; intracerebroventricularly), the nicotinic receptor antagonist, completely inhibited the ACTH and beta-endorphin response to choline (150 micrograms; intracerebroventricularly). An antagonist of the muscarinic receptor, atropine (10 micrograms; intracerebroventricularly), failed to alter these effects. Pretreatment of rats with hemicholinium-3 (20 micrograms; intracerebroventricularly), a drug which inhibits the uptake of choline into cholinergic neurons, abolished the choline-induced increases in both plasma ACTH and beta-endorphin levels. These results indicate that choline can increase plasma concentrations of ACTH and beta-endorphin through the activation of central nicotinic acetylcholine receptors.

Intracerebroventricular injection of choline increases plasma oxytocin levels in conscious rats

In the present study, we examined the effect of intracerebroventricularly (i.c.v.) injected choline on both basal and stimulated oxytocin release in conscious rats. I.c.v. injection of choline (50-150 micrograms) caused time- and dose-dependent increases in plasma oxytocin levels under normal conditions. The increase in plasma oxytocin levels in response to i.c.v. choline (150 micrograms) was greatly attenuated by the pretreatment of rats with atropine (10 micrograms; i.c.v.), muscarinic receptor antagonist. Mecamylamine (50 micrograms; i.c.v.), a nicotinic receptor antagonist, failed to suppress the effect of 150 micrograms choline on oxytocin levels. Pretreatment of rats with 20 micrograms of hemicholinium-3 (HC-3), a specific inhibitor of choline uptake into nerve terminals, greatly attenuated the increase in plasma oxytocin levels in response to i.c.v. choline injection. Osmotic stimuli induced by either oral administration of 1 ml hypertonic saline (3 M) following 24-h dehydration of rats (type 1) or an i.c.v. injection of hypertonic saline (1 M) (type 2) increased plasma oxytocin levels significantly, but hemorrhage did not alter basal oxytocin concentrations. The i.c.v. injection of choline (50, 150 micrograms) under these conditions caused an additional and significant increase in plasma oxytocin concentrations beyond that produced by choline in normal conditions. These data show that choline can increase plasma oxytocin concentrations through the stimulation of central cholinergic muscarinic receptors by presynaptic mechanisms and enhance the stimulated oxytocin release.