L. Beani

The cerebral acetylcholine release in conscious rabbits with semi-permanently implanted epidural cups

Abstract A technique is described for the semi-permanent implantation of epidural cups in the rabbit skull. This method was employed to examine the relationship between behaviour, electrocortical activity and cortical acetylcholine output in conscious, freely moving animals. The drugs tested fall into two groups: (i) those which change behaviour, electrocortical activity and cortical acetylcholine output in a parallel fashion e.g. pentobarbital and amphetamine; (ii) those which “dissociate” the three parameters investigated e.g. atropine, morphine and chlorpromazine. The effect of these drugs on the activity of cholinergic pathways belonging to the reticulocortical ascending system is discussed.

The effect of catecholamines and sympathetic stimulation on the release of acetylcholine from the guinea-pig colon

1 In isolated guinea‐pig terminal colon, the effect of sympathetic stimulation on contraction and acetylcholine release elicited by pelvic and transmural stimulation was investigated. 2 Sympathetic stimulation reduced the nerve‐mediated contractile responses more than those produced by added acetylcholine. 3 Sympathetic stimulation also reduced the acetylcholine released during pelvic and transmural stimulation at low frequency. The inhibitory effect on acetylcholine released from resting colons is concealed by the simultaneous release of acetylcholine in considerable amounts from stimulated periarterial nerves which probably contain parasympathetic fibres. 4 The inhibitory effect of endogenous and exogenous catecholamines prevails when cholinergic neurones fire at low rates. It was confirmed that adrenaline is more active than noradrenaline. 5 The release of acetylcholine from unstimulated colons was for the most part maintained by nerve‐conducted activity, because tetrodotoxin was able to reduce it to about one‐tenth. 6 It is suggested that the sympathetic control of gastrointestinal tone and motility is exerted through two different routes: inhibition of intramural cholinergic plexuses and direct relaxation of smooth muscle cells. 7 The possible site and mechanism of action of catecholamines on intramural cholinergic structures is briefly discussed.

Effect of amantadine on cerebral acetyl-choline release and content in the guinea pig

Abstract The effect of amantadine on cortical acetylcholine release and on cerebral acetyl-choline content in guinea pigs was examined in comparison with amphetamine. Both drugs stimulated gross behaviour and increased cortical acetylcholine release in conscious freely-moving animals. The effects of amantadine were still evident in guinea pigs pretreated with reserpine or α-methyl-p-tyrosine, but were abolished by the combined inhibition of catecholamine synthesis and granular uptake. On the contrary, α-methyl-p-tyrosine pretreatment alone, prevented amphetamine effects both on behaviour and on acetylcholine release. Amantadine increased acetylcholine content in the caudate nucleus, α-methyl-p-tyrosine showed the opposite effect, while amphetamine was ineffective. The discussion is based on the hypothesis that the activation of the cerebral adrenergic neurons is associated with a cholinergic counter-action. The conclusion is reached that the role of dopamine in determining this counter-action is predominant.

Effects of metoclopramide on isolated guinea-pig colon. 2. Interference with ganglionic stimulant drugs.

Abstract Possible effects of metoclopramide on intramural nervous structures of isolated guinea-pig distal colon were investigated to ascertain if stimulation produced by the drug on the gastrointestinal peristalsis depended on actions other than the demonstrated peripheral sensitization to acetylcholine. Nerve-mediated responses were obtained with nicotine and 5-hydroxytryptamine, which stimulated not only cholinergic neurones, but also non-cholinergic nervous structures in the presence of postganglionic cholinergic and adrenergic blockade. Metoclopramide enhanced the cholinergic responses to nicotine, but blocked the others. Conversely, the drug completely prevented the effects of 5-hydroxytryptamine. Since: (i) the stimulatory effect of nicotine on non-adrenergic, non-cholinergic structures was abolished during 5-hydroxytryptamine tachyphylaxis; (ii) 5-hydroxytryptamine maintained its own effects during nicotine tachyphylaxis; (iii) metoclopramide antagonized 5-hydroxytryptamine, it is suggested that the drug blocks tryptaminergic receptors, necessary for activating unknown nervous elements involved in the control of gastrointestinal tone and motility.

Vagal non‐adrenergic inhibition of guinea‐pig stomach

1. The effect of vagal and sympathetic stimulation on the mechanical and electrical activity (intracellular recording) of the body of the guinea‐pig stomach was investigated in vitro.

Effects of metoclopramide on isolated guinea-pig colon. 1)Peripheral sensitization to acetylcholine

Abstract An investigation was carried out on isolated guinea-pig distal colon to contribute to an understanding of the mechanism of action of metoclopramide, a compound known to enhance gastrointestinal motility. The drug at low concentrations: (i) did not change acetylcholine release, or directly affect electrical activity of the smooth muscle cells; (ii) selectively increased the response of the colon both to exogenous acetylcholine and to pelvic nerve stimulation. Another prominent effect was increased intestinal tone; this effect was ascribed to enhanced cholinergic drive on the muscle because it was prevented by atropine and tetrodotoxin. Interestingly, morphine, but not hexamethonium, counteracted effects of metoclopramide suggesting interference of the drug on non-adrenergic, non-cholinergic intramural nervous structures, in addition to the demonstrated peripheral sensitization of muscarinic receptors.

The effect of tubocurarine on acetylcholine release from motor nerve terminals

The mechanism of Wedensky inhibition is still under discussion: some authors (Thesleff, 1959) suggest that it is due to refractoriness of the postjunctional membrane towards ACh; others (Brooks & Thies, 1962) call attention to the concomitant reduction of ACh release. The failure of the neuromuscular junction to transmit impulses at high frequencies probably has several causes, which may not be the same when the muscle is treated with tubocurarine (TC), or witb anticholinesterase drugs. End-plate refractoriness might explain the inhibition seen with anticholinesterases, since excess ACh could desensitize the end-plate. On the other hand, presynaptic failure might be a contributory factor in Wedensky inhibition in the curarized preparation. In this work, briefly reported elsewhere (Beani & Bianchi, 1961a, b), experiments are described which show that TC is able, under certain conditions of stimulation, to reduce the ACh release from the motor nerve endings.

Further evidence for the presence of non-adrenergic inhibitory structures in the guinea-pig colon

Abstract A pharmacological analysis was carried out of the responses of guinea-pig isolated terminal colon to transmural, parasyinpathetic and sympathetic electrical stimulation. The contraction produced by stimulating the pelvic nerves was abolished by atropine and the inhibition produced by stimulating the periarterial nerves was suppressed by dibenamine plus propranolol and by adrenergic blocking agents. After complete pharmacological blockade of extrinsic sympathetic and parasympathetic supply, transmural stimulation produced a pure inhibitory response particularly when low frequencies (1–2/sec) were used. This response was abolished by tetrodotoxin. Some physiologically occurring substances were tested with the aim of mimicking such inhibition, but this was unsuccessful. 5-Hydroxy-tryptamine displayed complex effects, mediated through the intramural nervous structures; moreover it evoked a non-adrenergic inhibitory response which was abolished by tetrodotoxin. The presence of non-adrenergic inhibitory structures was confirmed in this part of the gastrointestinal tract. The possible nature of the contractile response which follows the primary inhibition due to transmural stimulation is discussed. It is suggested that the possible release of excitatory and inhibitory substances, different from the so-called “neurotransmitters”, is a consequence of physical and chemical stimulation of the intestinal wall.

Correlation of brain catecholamines with cortical acetylcholine outflow, behaviour and electrocorticogram

Abstract The effects of changes in the brain DA/NA ratio were correlated with cortical acetylcholine outflow, behaviour and E.Co.G. in unanaesthetized, unrestrained guinea pigs. The prevailing reduction in DA content determined by α-methyl-p-tyrosine and the increase in NA levels caused by d,1-dihydroxyphenylserine, were associated with reduced acetylcholine outflow, sedation and E.Co.G. synchronization. Conversely, the predominance of DA over NA, obtained with L-dopa and Fla-63, was associated with increased acetylcholine outflow and desynchronized E.Co.G., although sedation occurred after Fla-63. The results are in agreement with the concept that DA enhances and NA restrains the functional level of the cholinergic telecenphalic structures.

Effects of some anti-epileptic drugs on brain acetylcholine

The effects exerted by some anti-epileptic drugs on gross behaviour, electrocorticogram, brain acetylcholine content and cortical acetylcholine release have been investigated in normal and epileptic guinea-pigs. Focal epilepsy was provoked by heating the eserinized solution contained in an epidural cup placed on the parietal cortex. In normal guinea-pigs di-n-propylacetic acid, di-n-propylacetamide and diazepam caused sedation, synchronized the ECoG, increased the acetylcholine content in the forebrain and decreased its cortical outflow. In contrast, trimethadione and diphenylhydantoin (DPH) were ineffective in normal animals but in epileptic guinea-pigs they suppressed the convulsive symptoms and reduced the abnormally high cortical acetylcholine outflow. This finding supports the idea that specific anti-epileptic drugs, like trimethadione and DPH, influence the physiological disposition of acetylcholine only indirectly, by restraining the paroxysmal neuronal activity.