Age-related alterations in pre-synaptic and receptor-mediated cholinergic functions in rat brain.

Fractional [3H]acetylcholine (ACh) release and regulation of release process by muscarinic receptors were studied in corpus striatum of young and aged rat brains. [3H] Quinuclidinyl benzilate (QNB) binding and carbachol stimulated phosphoinositide turnover, on the other hand, were compared in striatal, hippocampal and cortical tissues. High potassium (10 mM)-induced fractional [3H]ACh release from striatal slices was reduced by aging. Although inhibition of acetylcholinesterase with eserine (20 microM) significantly decreased stimulation-induced fractional [3H]ACh release in two groups of rats, this inhibition slightly lessened with aging. Incubation of striatal slices with muscarinic antagonists reversed eserine-induced inhibition in fractional [3H]ACh release with a similar order of potency (atropine = 4-DAMP > AF-DX 116 > pirenzepine) in young and aged rat striatum, but age-induced difference in stimulated ACh release was not abolish by muscarinic antagonists. These results suggested that fractional [3H]ACh release from striatum of both age groups is modulated mainly by M3 muscarinic receptor subtype. Although both muscarinic receptor density and labeling of inositol lipids with [myo-3H]inositol decreased with aging, carbachol-stimulated [3H]myo inositol-1-fosfat (IP1) accumulation was found similar in striatal, cortical and hippocampal slices.

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 beta-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.

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

1. Intracerebroventricular (i.c.v.) injection of choline (25-150 micrograms) 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 micrograms or 50 micrograms of choline, heart rate did not change, while 100 micrograms or 150 micrograms i.c.v. choline produced a slight and short lasting bradycardia. Choline (150 micrograms) failed to alter the circulating residual volume of blood in haemorrhaged rats. 2. The pressor response to i.c.v. choline (50 micrograms) in haemorrhaged rats was abolished by pretreatment with mecamylamine (50 micrograms, i.c.v.) but not atropine (10 micrograms, i.c.v.). The pressor response to choline was blocked by pretreatment with hemicholinium-3 (20 micrograms, i.c.v.). 3. The pressor response to i.c.v. choline (150 micrograms) 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 micrograms) was not altered by bilateral adrenalectomy, but was attenuated by systemic administration of either phentolamine (10 mg kg-1) or the vasopressin antagonist [beta-mercapto-beta,beta-cyclopenta-methylenepropionyl1, O-Me-Tyr2,Arg8]-vasopressin (10 micrograms 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.

3,4-Diaminopyridine and choline increase in vivo acetylcholine release in rat striatum.

We investigated the effects of choline, 3,4-diaminopyridine and their combination on acetylcholine release from the corpus striatum of freely moving rats which were treated or not with atropine. Intraperitoneal administration of choline or intrastriatal administration of 3,4-diaminopyridine increased acetylcholine levels in striatal dialysates in a dose-dependent manner. When 3,4-diaminopyridine treatment was combined with choline, the observed effect was considerably greater than the sum of the increases produced by choline or 3,4-diaminopyridine alone. Administration of atropine (1 microM) in the dialysing medium was also found to be effective to stimulate striatal acetylcholine levels. 3,4-Diaminopyridine did not affect acetylcholine levels under these conditions. Whereas the choline-induced increase in acetylcholine release was significantly potentiated by atropine, co-administration of 3,4-diaminopyridine with choline failed to produce a further significant increase in the presence of atropine. These results suggest that a highly effective means for increasing acetylcholine release involves two concurrent treatments that increase neuronal choline levels and inhibition of the negative feedback modulation of acetylcholine release.

Effects of intracerebroventricular injected choline on cardiovascular functions and sympathoadrenal activity.

Intracerebroventricular (i.c.v.) injection of choline (50-150 micrograms) increased blood pressure (SP) and decreased heart rate (HR) in freely moving rats. Intracerebroventricular pretreatment of rats with mecamylamine (50 micrograms) blocked the reduction in HR and reduced the increase in SP induced by i.c.v. choline (150 micrograms). Central muscarinic blockade with atropine (10 micrograms, i.c.v.) reduced the pressor response to i.c.v. choline (150 micrograms) 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 micrograms) produced a fivefold increase in catecholamine concentrations in adrenal venous plasma. Bilateral adrenalectomy reduced, but did not block, choline's effect on SP. Intracerebroventricular choline (150 micrograms) 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 micrograms) 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.

Choline as an agonist: determination of its agonistic potency on cholinergic receptors.

These experiments examined the potency of choline as a cholinergic agonist at both muscarinic and nicotinic receptors in rat brain and peripheral tissues. Choline stimulated the contraction of isolated smooth muscle preparations of the stomach fundus, urinary bladder and trachea and reduced the frequency of spontaneous contractions of the right atrium at high micromolar and low millimolar concentrations. The potency of choline to elicit a biological response varied markedly among these tissues; EC50 values ranged between 0.41 mM in the fundus to 14.45 mM in the atrium. Choline also displaced [3H]quinuclidinyl benzilate binding in a concentration-dependent manner although, again, its potency varied among different brain regions (Ki = 1.2 to 3.5 mM) and peripheral tissues (Ki = 0.28 to 3.00 mM). Choline exhibited a comparable affinity for nicotinic receptors. It stimulated catecholamine release from the vascularly perfused adrenal gland (EC50 = 1.3 mM) and displaced L-[3H]nicotine binding to membrane preparations of brain and peripheral tissues (Ki = 0.38 to 1.17 mM). However, the concentration of choline required to bind to cholinergic receptors in most tissues was considerably higher than serum levels either in controls (8-13 microM) or following the administration of choline chloride (200 microM). These results clearly demonstrate that choline is a weak cholinergic agonist. Its potency is too low to account for the central nervous system effects produced by choline administration, although the direct activation of cholinergic receptors in several peripheral tissues may explain some of its side effects.

Involvement of central dopamine in the hyperthermia in rats produced by d-amphetamine.

The effect of amphetamine on the body temperature was studied in 6-hydroxydopamine and pimozide pretreated rats. Amphetamine alone (1, 2.5 and 5 mg/kg) produced a fairly dose-dependent increase in body temperature. The effect was almost totally antagonized by pimozide, and also reduced after pretreatment with 6-hydroxydopamine. These results are discussed with regard to the possible role of central dopamine in mediating amphetamine-induced hyperthermia in rats.