Central Cholinergic Transmission Research Articles

Effect of the subchronic treatment with the acetylcholinesterase inhibitor heptastigmine on central cholinergic transmission and memory impairment in aged rats

The effect of subchronic administration of the acetylcholinesterase (AChE) inhibitor heptastigmine (HEP 0.6 mg/kg s.c. daily for 15 days) was investigated on cortical extracellular acetylcholine (ACh) levels and on memory function in aged male rats (26 months old at the beginning of the experiments) using microdialysis and behavioural techniques. Twenty-four hours after the last treatment, cortical ACh levels were significantly higher in rats subchronically treated with HEP than in rats treated with saline and AChE activity was still inhibited in cortex, hippocampus and striatum. The injection of a challenge dose of HEP (0.6 mg/kg s.c.) 24 h after the last treatment produced a faster and a more substained increase of ACh in the cortex of subchronically treated rats compared to those repeatedly injected with saline. However, the maximum increase of ACh levels after injection of the challenge was comparable in both groups. In an object recognition test in which the pretest and test phase were spaced by 45 days, HEP prevented the deterioration of spatial memory occurring during this period, but had no effect on non-spatial memory. The present results suggest that moderate inhibition of brain AChE is able to maintain high levels of cortical extracellular ACh in aged rats and that this increase matches facilitatory effect of HEP on spatial memory.

Central muscarinic antinociception induced by ET-142 and SS-20 in rodents

The antinociceptive effects of ET-142 (10–50 mg kg–1 sc; 10–30 μg per mouse icv) and SS-20 (10–50 mg kg–1 sc; 5–30 μg per mouse icv) were examined in mice by using the hot-plate and abdominal constriction tests. A similar antinociceptive profile for both compounds (20–40 mg kg–1 ip) was also observed in rats using the paw pressure test. In the antinociceptive dose-range, ET-142 and SS-20 did not impair mouse gross behavior and motor coordination evaluated, respectively, by the Irwin and rotarod tests. The increase in the pain threshold produced by ET-142 and SS-20 was prevented by atropine, dicyclomine, pirenzepine, and hemicholinium-3, but not by naloxone, atropine methyl bromide, and CGP 35348. In vitro experiments showed that the two investigated compounds amplified electrically evoked guinea pig ileum contractions. On the basis of the above data, it can be postulated that ET-142 and SS-20 exert their antinociceptive effect through a potentiation of central cholinergic transmission. Drug Dev. Res. 42:26–34, 1997. © 1997 Wiley-Liss, Inc.

Central muscarinic antinociception induced by ET‐142 and SS‐20 in rodents

The antinociceptive effects of ET-142 (10–50 mg kg–1 sc; 10–30 μg per mouse icv) and SS-20 (10–50 mg kg–1 sc; 5–30 μg per mouse icv) were examined in mice by using the hot-plate and abdominal constriction tests. A similar antinociceptive profile for both compounds (20–40 mg kg–1 ip) was also observed in rats using the paw pressure test. In the antinociceptive dose-range, ET-142 and SS-20 did not impair mouse gross behavior and motor coordination evaluated, respectively, by the Irwin and rotarod tests. The increase in the pain threshold produced by ET-142 and SS-20 was prevented by atropine, dicyclomine, pirenzepine, and hemicholinium-3, but not by naloxone, atropine methyl bromide, and CGP 35348. In vitro experiments showed that the two investigated compounds amplified electrically evoked guinea pig ileum contractions. On the basis of the above data, it can be postulated that ET-142 and SS-20 exert their antinociceptive effect through a potentiation of central cholinergic transmission. Drug Dev. Res. 42:26–34, 1997. © 1997 Wiley-Liss, Inc.

LITERATURE REVIEWS

ABSTRACT l. Pain Distribution: Growing Evidence for Its Importance. —Wigers, S.H., Skrondal, A., Finset, A., and Gotestam, K.G.: Measuring change in fibromyalgic pain: the relevance of pain distribution. J. Musculoskel. Pain, 5(2):29-41, 1997. Reviewed by CBMT Staff. 2. Review of Myofascial Pain Syndrome —Han, S.C. and Harrison, P.: Myofascial pain syndrome and trigger-point management. Regional Anesthesia, 22(1):89-101, 1997. Reviewed by CBMT Staff. 3. Acupressure Found Effective in Preventing Nausea During Elective Cesarean Section in the Awake Patient. —Stein, D.J., Bimbach, D.J., Danzer, B.I., Kuroda, M.M., Grune-baum, A., and Thys, D.M.: Acupressure versus intravenous meto-clopramide to prevent nausea and vomiting during spinal anesthesia for cesarean section. Anesthesia & Analgesia, 84(2):342-345, 1997. Reviewed by CBMT Staff. 4. Organophosphate Insecticides May Be Instrumental in the Etiology of Central Nervous System Effects of Chronic Fatigue Syndrome. —Chaudhuri, A., Majeed, T., Dinan, T., and Behan, P.O.: Chronic fatigue syndrome: a disorder of central cholinergic transmission. J. Chron. Fatigue Synd., 3(1):3-16, 1997. Reviewed by CBMT Staff.

ANTINOCICEPTION INDUCED BY SM 32 DEPENDS ON A CENTRAL CHOLINERGIC MECHANISM

The antinociceptive effect of SM 32 was examined in mice by using the hot-plate (10–40mgkg−1i.p; 3–30gmg per mouse i.c.v.) and abdominal constriction (10–30mgkg−1i.p) tests. In the antinociceptive dose-range, SM 32 did not impair mouse spontaneous motility and motor coordination evaluated respectively by the Animex and rota-rod tests. The increase in the pain threshold produced by SM 32 was prevented by dicyclomine, pirenzepine and hemicholinium-3 but not by naloxone and CGP 35348.In vitroexperiments showed that the SM 32 amplified electrically- and nicotine-evoked guinea-pig ileum contractions. On the basis of the above data, it can be postulated that SM 32 exerts its antinociceptive effect through a potentiation of central cholinergic transmission.

Quantal release at a neuronal nicotinic synapse from rat adrenal gland.

The neuronal nicotinic synapse in tissue slices of the adrenal medulla was studied with whole-cell patch-clamp. Excitatory postsynaptic currents (EPSCs) were evoked by local field stimulation or occurred spontaneously especially when external [K+] was increased. EPSCs were carried by channels sharing biophysical and pharmacological properties of neuronal-type nicotinic receptors (nAChRs). A single-channel conductance (gamma) of 43-45 pS was found from nonstationary variance analysis of EPSCs. Spontaneous EPSCs were tetrodotoxin-insensitive and Ca(2+)-dependent and occurred in burst-like clusters. Quantal analysis of spontaneous EPSCs gave a quantal size of 20 pA and amplitude histograms were well described by binomial models with low values of quantal content, consistent with a small number of spontaneously active release sites. However, rare large amplitude EPSCs suggest that the total number of sites is higher and that extrajunctional receptors are involved. Our estimates of quantal content and size at the chromaffin cell neuronal nicotinic synapse may be useful in characterizing central neuronal-type nicotinic receptor-mediated cholinergic synaptic transmission.

Effects of nicotinamide on central cholinergic transmission and on spatial learning in rats

High-dose nicotinamide (1000 mg/kg) leads to a minor increase of plasma choline but to a major increase of the choline concentrations in the intra- and extracellular spaces of the brain. In the hippocampus, the nicotinamide-induced increase in choline was associated with an increase in the release of acetylcholine under stimulated conditions. In young rats, nicotinamide in doses between 10 and 1000 mg/kg did not influence spatial learning, as tested in the Morris water maze. In old rats, low doses of nicotinamide were ineffective whereas the high dose of 1000 mg/kg even impaired spatial learning. The combined administration of choline and nicotinamide had a synergistic effect on brain choline levels but had similar effects as nicotinamide given alone in the behavioural experiments. Additional tests for spontaneous behaviour and locomotion revealed procholinergic and sedative effects of the compound. We conclude that the ineffectiveness of the putative cognition enhancer nicotinamide in the learning task may be due to the observed sedative effect. Therefore, the development of nonsedative nicotinamide derivatives is recommended.

A modulatory role of central cholinergic transmission in control of the 10-Hz rhythm in sympathetic nerve discharge

In 43 urethane-anesthetized or decerebrate, baroreceptor-denervated cats, spectral analysis showed that most of the power in sympathetic nerve discharge (SND) was at frequencies < 6 Hz. In 18 of these cats, physostigmine (100 μg/kg i.v.) induced a 10-Hz rhythm in inferior cardiac SND that was eliminated by atropine sulfate (0.25 mg/kg i.v.; n = 6). In contrast, the naturally occurring 10-Hz rhythm that appeared in SND in other experiments was atropine-insensitive ( n = 6). The data indicate that central muscarinic cholinergic transmission is not essential for the naturally occurring 10-Hz rhythm. Nonetheless, facilitation of cholinergic transmission can induce a 10-Hz rhythm.

Comparative effects of chronic administration of some psychotropic drugs on rat brain cortex acetylcholinesterase activity

1. The response of central cholinergic neurotransmission to the chronic administration of some psychotropic drugs to rats was investigated using brain acetylcholinesterase activity as a neurochemical marker for cholinergic neurons. 2. Rats were divided into four groups. Three experimental groups were given chlorpromazine, amitriptyline, or diazepam respectively, for a period of 30 days; and the control group received physiological saline only. 3. Long-term treatment of chlorpromazine and amitriptyline resulted in significant increases in rat brain cortex enzyme activity, whereas only a slight increase was observed in the diazepam-treated group. 4. These results indicate that the chronic treatment with some psychotropic drugs causes changes in central cholinergic transmission.

Presynaptic cholinergic modulators as potent cognition enhancers and analgesic drugs. 1. Tropic and 2-phenylpropionic acid esters.

Previous studies have shown that (R)-(+)-hyoscyamine has analgesic activity as a consequence of increased ACh release following antagonism of central muscarinic autoreceptors. Since the enhancement of central cholinergic transmission could be beneficial for cognitive disorders, we manipulated (R)-(+)-hyoscyamine, synthesizing several derivatives of tropic and 2-phenylpropionic acids, with the aim of obtaining drugs which are able to increase ACh release and consequently to show analgesic and nootropic activities. The results showed that several new compounds are indeed potent analgesics (with an analgesic efficacy comparable to that of morphine) and that the most potent one ((+/-)-19, PG9) also has remarkable cognition-enhancing properties. Our study confirmed that the mechanism of action involves ACh release even if it is still unclear whether only muscarinic autoreceptors or, also, heteroreceptors are involved.

Effect of NIK-247 on basal concentrations of extracellular acetylcholine in the cerebral cortex of conscious, freely moving rats.

We studied the effect of orally administered NIK-247 (9-amino-2,3,5,6,7,8-hexahydro-1H-cyclopenta[b]quinoline monohydrochloride monohydrate) on basal extracellular acetylcholine (ACh) concentrations in the rat cerebral cortex using microdialysis without the addition of cholinesterase inhibitor to the perfusion fluid and radioimmunoassay for ACh. In addition, the effect of oral administration of NIK-247 on acetylcholinesterase (AChE) activity in rat cerebral cortex was determined. The mean basal ACh content in the perfusate from the cerebral cortex of freely moving rats was 123.2 +/- 21.8 fmol/30 min (n = 7). NIK-247 (2.5-10.0 mg/kg, p.o.) increased the ACh content of the perfusate in a dose-dependent manner. NIK-247 at 10 mg/kg significantly increased the ACh content in the perfusate from 0.5 to 2.5 hr after administration, and the maximum increase was attained at 1 hr after administration. 9-Amino-1,2,3,4-tetrahydroacridine (5 mg/kg, p.o.) and physostigmine (0.5 mg/kg, i.p.) significantly increased the ACh content in the perfusate from 1 to 2 hr and from 0.5 to 1.5 hr after administration, respectively. AChE activities in the cerebral cortex were about 32% and 12% below the control value at 1 hr and 3 hr after administration of NIK-247 at 10 mg/kg, respectively. These findings demonstrate that NIK-247 increases extracellular ACh concentration and inhibits AChE activity in the cerebral cortex after oral administration, and they suggest that NIK-247 facilitates central cholinergic transmission.

Enteric GABA-containing nerves projecting to the guinea-pig inferior mesenteric ganglion modulate acetylcholine release.

1. The effect of GABA and GABA receptor-modulating drugs on release of [3H]acetylcholine was studied in the guinea-pig inferior mesenteric ganglion. 2. GABA caused a dose-dependent increase in [3H]acetylcholine release during stimulation of the lumbar colonic nerves. Muscimol (10 microM) and diazepam (5 microM) also increased [3H]acetylcholine release during stimulation of the lumbar colonic nerves whereas baclofen (10 microM) had no effect. 3. Bicuculline (20-100 microM) and picrotoxin (50 microM) alone reduced [3H]acetylcholine release during electrical stimulation of the lumbar colonic nerves whereas phaclofen (300 microM) had no effect. 4. Bicuculline (100 microM) significantly decreased whereas diazepam (5 microM) significantly increased distension-induced [3H]acetylcholine release. 5. Colonic distension significantly increased [3H]GABA release in the inferior mesenteric ganglion compared to basal periods when the colon was not distended. Distension-induced release of [3H]GABA resulted from active neuronal transmission from the colon to the inferior mesenteric ganglion, since perfusion of the inferior mesenteric ganglion with tetrodotoxin (1 microM) reduced basal release of [3H]GABA and abolished distension-evoked increases in the release of [3H]GABA. 6. In contrast to its excitatory effects on peripheral colonic afferent cholinergic nerves, exogenous GABA caused a dose-dependent decrease in [3H]acetylcholine release during electrical stimulation of the central lumbar splanchnic nerves. Baclofen (10 microM) also inhibited [3H]acetylcholine release whereas muscimol (10 microM) or diazepam (5 microM) had no effect. Phaclofen (300 microM) antagonized the inhibitory effects of exogenous GABA (10 microM) and of baclofen (10 microM). Bicuculline (100 microM), picrotoxin (50 microM) and phaclofen (300 microM) alone had no effect on [3H]acetylcholine release during splanchnic nerve stimulation. 7. Phaclofen (300 microM) increased [3H]acetylcholine release during simultaneous electrical stimulation of the lumbar colonic nerves and splanchnic nerves and when GABAA receptors were blocked by bicuculline (20 microM). 8. The data suggest that GABAA receptors facilitate release of acetylcholine from peripheral cholinergic mechanosensory nerves projecting from the colon to the inferior mesenteric ganglion and that GABAB receptors inhibit release of acetylcholine from central cholinergic nerves. Enteric GABA-containing nerves projecting to the inferior mesenteric ganglion are mechanosensory. Endogenous release of GABA may act on GABAA receptors to facilitate peripheral cholinergic mechanosensory transmission and/or on GABAB receptors to inhibit central cholinergic transmission.

Brief presentation of the story and present status of studies of the vertebrate cholinergic system.

This year marks the seventieth anniversary of Otto Loewi's demonstration of chemical transmission generally and autonomic cholinergic transmission specifically and the fortieth anniversary of John Eccles's proof of the existence of central cholinergic transmission. Following these epochal findings, the subsequent studies of the cholinergic system led to discoveries of similarly important phenomena. This review concerns these phenomena, including chemical structure and molecular biology of cholinergic receptors; electrophysiologic and ionic aspects of pre- and postsynaptic cholinergic events; the quantal expression of cholinergic postsynaptic events and activities of their subunits, the elementary events; second messengers and G proteins; synthesis, storage and release of acetylcholine; cholinesterases, anticholinesterases, and war gases; central cholinergic pathways; central cholinergic functions, behaviors, cholinergic EEG and REM sleep; cholinergic ontogeny and teratology; trophic phenomena; and the clinical aspects of the cholinergic system. This review refers to the history as well as the present status of each of these phenomena; furthermore, it describes briefly the nineteenth-century work with calabar bean, pilocarpine, muscarine, and nicotine, that is, the work performed before the promulgation of the cholinergic era.

Age-related decline in cholinergic synaptic transmission in hippocampus

Age-dependent changes in central nervous system (CNS) cholinergic synaptic transmission were studied in three age groups of Sprague-Dawley and Fischer 344 rats: 1- to 2-month-old, 8- to 10-month-old, and 18- to 23-month-old. Utilizing intracellular recording techniques and the in vitro hippocampal slice preparation, we report an age-related decline in central cholinergic transmission as a function of age. Slow excitatory postsynaptic potentials (slow EPSPs) were reduced approximately 60% in aged (18- to 23-month-old) compared to younger (1- to 2-month-old) animals. The response of the postsynaptic membrane to the muscarinic agonist, carbachol (0.3 μM), was also reduced with age. These changes were not accompanied by a global decline in muscarinic receptor function since two additional measures of cholinergic function were not changed with age. Both presynaptic inhibition of fast excitatory synaptic transmission and postsynaptic inhibition of the afterhyperpolarization (AHP) following a train of spikes were not changed during aging. Our results suggest that a primary functional decline in central cholinergic mechanisms during aging may be a specific reduction in central cholinergic synaptic transmission.

Blockade of muscarinic receptors potentiates D 1 dependent turning behavior and c-fos expression in 6-hydroxydopamine-lesioned rats but does not influence D 2 mediated responses

In rats with a unilateral 6-hydroxydopamine lesion of the dopaminergic nigrostriatal pathway, blockade of muscarinic receptors by scopolamine potentiates the contralateral turning induced by selective dopaminergic D 1 agonists (SKF 38393, A 68930), but does not influence the contralateral turning induced by the D 2 agonist LY 171555. Studies on the expression of the early gene c-fos as reflected by the immunohistochemical demonstration of the Fos protein, show that administration of scopolamine (5 mg/kg, i.p.) potentiates c-fos expression elicited by SKF 38393 (1.5 mg/kg, s.c.) in the caudate-putamen of the lesioned side. The results indicate that cholinergic transmission is differentially involved in the behavioral expression of D 1 versus D 2 receptor stimulation in a denervated condition and suggest that blockade of central cholinergic transmission might be useful in improving the antiparkinsonian efficacy of D 1 receptor agonists.

Chapter 30: Nerve growth factor affects the cholinergic neurochemistry and behavior of aged rats

The major central cholinergic systems include (1) the projection neurons of the medial septum and diagonal band of Broca (MS/DB) to the hippocampus; (2) the projection neurons of the nucleus basalis magnocellularis (NBM) to the amygdala and cerebral cortex; (3) the inter-neurons of the striatum. Although the basal forebrain projection neurons have been implicated in learning and memory, and the striatal inter-neurons in motor behaviors, it still is not clear how and to what extent the central cholinergic neurons are involved in specific behaviors. Similarly, relatively little is known about the cellular and molecular mechanisms that regulate animal behaviors mediated by the central cholinergic systems. Although acetylcholine (ACh) was the first neurotransmitter discovered in the central nervous system, it remains unclear how the synthesis, storage, and release of ACh are regulated, or how ACh neurochemistry might be altered by environmental stimuli or synaptic experience, both of which would impact on cholinergically mediated behaviors. The central cholinergic systems are particularly dysfunctional in Alzheimer's disease. Thus, our animal research on the regulation of central cholinergic transmission and behavior has focused on the deficits in the systems associated with senescence and the potential amelioration of age-related deficits by the administration of the neurotrophic protein, nerve growth factor (NGF). This chapter describes the current understanding of the regional regulation of ACh synthesis and release in young and old rats, and illustrates several correlations between age-related and NGF-induced alterations in cholinergic neurochemistry, electrophysiology, morphology, and behavior.

Effect of Phosphatidylcholine Administration on Choline and Acetylcholine Content in Brain Areas. I.

Although clinical attempts have been made to treat patients with senile dementia using phosphatidylcholine (PC) or acetylcholine (Ach) esterase inhibitors to restore central cholinergic transmission by increasing Ach contents in the brain, definite positive results have yet to be obtained. To investigate further the influence of the systemic administration of PC on brain Ach, we measured choline (Cho) and Ach contents in discrete brain regions of rat by HPLC with an electrochemical detector, following an acute administration of PC. Cho contents increased in the frontal cortex, striatum and substantia innomimata after acute intraperitoneal administration of soybean PC in liposome form. Cho contents were not affected in any region after acute injection of egg PC. Ach contents were not affected significantly in any brain region after acute administration of each PC substance. It thus follows that Cho contents increase by the systemic administration of highly purified PC, and PC may possibly pass the blood brain barrier. However, PC administration is not capable of accelerating Ach synthesis in the brain.

Suppression of in vivo neostriatal acetylcholine release by vesamicol: evidence for a functional role of vesamicol receptors in brain.

Experiments examined the effects of peripheral and central administration of the vesicular acetylcholine transport blocker vesamicol (AH5183) on the content, synthesis, and release of acetylcholine in the rat brain in vivo. In time course studies, a single intraperitoneal dose of DL-vesamicol (5 mg/kg) rapidly and reversibly (within 2 h) doubled the content of acetylcholine in the striatum and hippocampus, without affecting choline levels or the rate of transmitter synthesis. In microdialysis experiments, the same peripheral dose of drug produced a reversible 55% reduction in endogenous striatal acetylcholine release. A similar inhibitory effect was produced by direct intrastriatal perfusion with vesamicol. Moreover, this effect of vesamicol was (a) concentration-dependent and saturable (EC50 = 68 nM), (b) rapidly reversible, (c) stereospecific for the L-isomer, and (d) poorly mimicked by a vesamicol analog with lower plasma membrane permeability. This profile of effects is consistent with an interaction with a specific vesamicol receptor as defined by previous in vitro binding studies. These results support a functional role for vesamicol receptors in modulating central cholinergic transmission in vivo.

Cholinergic neuropharmacology: an update.

The current status of the pharmacology of central cholinergic transmission is reviewed. Particular attention is paid to the compounds that have been or are potential candidates as therapeutic agents for the treatment of mental disorders, particularly senile dementia. Compounds affecting acetylcholine synthesis, storage and release, affecting the enzyme acetylcholinesterase, acting on nicotinic cholinergic receptors, as well as compounds acting on muscarinic cholinergic receptors are reviewed. It is concluded that the most promising approaches for the development of new therapeutic agents might be specific acetylcholinesterase inhibitors and compounds with specific action at only one of the muscarinic cholinergic receptor subtypes.

The effect of the acetylcholine transport blocker vesamicol on central cholinergic pressor neurons

Vesamicol (AH5183) inhibits the uptake of acetylcholine into cholinergic neuronal storage vesicles. Earlier in vitro studies have demonstrated that such inhibition can lead to a failure of transmission, particularly in peripheral cholinergic tissues. The present study was designed to determine whether vesamicol could inhibit central cholinergic transmission in conscious freely moving rats. Central (lateral cerebroventricular) injection of 20 micrograms of vesamicol significantly reduced the hypertensive and bradycardic response to subsequent central injection of physostigmine in spontaneously hypertensive rats. Inhibition of the pressor response was greatest when physostigmine was administered 1 hr after vesamicol. Acetylcholine and choline levels were determined in three brain regions derived from rats treated one hr earlier with either vehicle or vesamicol. Acetylcholine levels were found to be unaltered after vesamicol treatment, although choline levels were significantly higher in two brain regions. These results are consistent with the ability of vesamicol to inhibit the function of central cholinergic cardiovascular regulatory neurons. The mechanism for this inhibition is not related to depletion of total brain acetylcholine content but may be due to depletion of a small critical pool of transmitter.