The effect of topically applied atropine on resting and evoked cortical acetylcholine release

Muscarinic acetylcholine activity modulates cortical silent period, but not motor evoked potentials, during muscle contractions

This study used transcranial magnetic stimulation (TMS) to determine if muscarinic receptor blockade affects muscle responses during voluntary contractions. Motor evoked potentials (MEPs) were recorded from biceps brachii in 10 subjects (age: 23 ± 2) during 10%, 25%, 50%, 75%, and 100% maximal voluntary contractions (MVCs). Each contraction intensity was examined under non-fatigued and fatigued conditions. All measurements were obtained post-ingestion of 25 mg promethazine or placebo. MEP area and the duration of the TMS-evoked silent period (SP) were calculated for all contractions. No drug-related differences were detected for MEP area during non-fatigued or fatigued contractions. A main effect of drug was detected for the SP (p = 0.019) where promethazine increased SP duration by an average of 0.023 [Formula: see text] 0.015 s. This drug effect was only identified for the unfatigued contractions and not following the sustained fatiguing contractions (p = 0.105). The cholinergic system does not influence corticospinal excitability during voluntary muscle contractions, but instead affects neural circuits associated with the TMS-evoked SP. Given the prevalence of cholinergic properties in prescription and over-the-counter medications, the current study enhances our understanding of mechanisms that may contribute to motor side-effects.

Neuromodulation of Persistent Activity and Working Memory Circuitry in Primate Prefrontal Cortex by Muscarinic Receptors

Neuromodulation by acetylcholine plays a vital role in shaping the physiology and functions of cerebral cortex. Cholinergic neuromodulation influences brain-state transitions, controls the gating of cortical sensory stimulus responses, and has been shown to influence the generation and maintenance of persistent activity in prefrontal cortex. Here we review our current understanding of the role of muscarinic cholinergic receptors in primate prefrontal cortex during its engagement in the performance of working memory tasks. We summarize the localization of muscarinic receptors in prefrontal cortex, review the effects of muscarinic neuromodulation on arousal, working memory and cognitive control tasks, and describe the effects of muscarinic M1 receptor stimulation and blockade on the generation and maintenance of persistent activity of prefrontal neurons encoding working memory representations. Recent studies describing the pharmacological effects of M1 receptors on prefrontal persistent activity demonstrate the heterogeneity of muscarinic actions and delineate unexpected modulatory effects discovered in primate prefrontal cortex when compared with studies in rodents. Understanding the underlying mechanisms by which muscarinic receptors regulate prefrontal cognitive control circuitry will inform the search of muscarinic-based therapeutic targets in the treatment of neuropsychiatric disorders.

Regulation of neural ion channels by muscarinic receptors

The excitable behaviour of neurons is determined by the activity of their endogenous membrane ion channels. Since muscarinic receptors are not themselves ion channels, the acute effects of muscarinic receptor stimulation on neuronal function are governed by the effects of the receptors on these endogenous neuronal ion channels. This review considers some principles and factors determining the interaction between subtypes and classes of muscarinic receptors with neuronal ion channels, and summarizes the effects of muscarinic receptor stimulation on a number of different channels, the mechanisms of receptor - channel transduction and their direct consequences for neuronal activity. Ion channels considered include potassium channels (voltage-gated, inward rectifier and calcium activated), voltage-gated calcium channels, cation channels and chloride channels. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Muscarinic acetylcholine receptors (mAChRs) in the nervous system: some functions and mechanisms

This article summarizes some of the effects of stimulating muscarinic receptors on nerve cell activity as observed by recording from single nerve cells and cholinergic synapses in the peripheral and central nervous sytems. It addresses the nature of the muscarinic receptor(s) involved and the ion channels and subcellular mechanisms responsible for the effects. The article concentrates on three effects: postsynaptic excitation, postsynaptic inhibition, and presynaptic (auto) inhibition. Postsynaptic excitation results primarily from the inhibition of potassium currents by M(1)/M(3)/M(5) receptors, consequent upon activation of phospholipase C by the G protein Gq. Postsynaptic inhibition results from M2-activation of inward rectifier potassium channels, consequent upon activation of Gi. Presynaptic inhibition results from M(2) or M(4) inhibition of voltage-gated calcium channels, consequent upon activation of Go. The segregation receptors, G proteins and ion channels, and the corelease of acetylcholine and glutamate from cholinergic fibres in the brain are also discussed.

Simultaneous release of glutamate and acetylcholine from single magnocellular "cholinergic" basal forebrain neurons

Basal forebrain (BF) neurons provide the principal cholinergic drive to the hippocampus and cortex. Their degeneration is associated with the cognitive defects of Alzheimer's disease. Immunohistochemical studies suggest that some of these neurons contain glutamate, so might also release it. To test this, we made microisland cultures of single BF neurons from 12- to 14-d-old rats. Over 1-8 weeks in culture, neuronal processes made autaptic connections onto the neuron. In 34 of 36 cells tested, a somatically generated action potential was followed by a short-latency EPSC that was blocked by 1 mM kynurenic acid, showing that they released glutamate. To test whether the same neuron also released acetylcholine, we placed a voltage-clamped rat myoball expressing nicotinic receptors in contact with a neurite. In six of six neurons tested, the glutamatergic EPSC was accompanied by a nicotinic (hexamethonium-sensitive) myoball current. Stimulation of the M2-muscarinic presynaptic receptors (characterized using tripitramine and pirenzepine) produced a parallel inhibition of autaptic glutamatergic and myoball nicotinic responses; metabotropic glutamate receptor stimulation produced similar but less consistent and weaker effects. Atropine enhanced the glutamatergic EPSCs during repetitive stimulation by 25 +/- 6%; the anti-cholinesterase neostigmine reduced the train EPSCs by 37 +/- 6%. Hence, synaptically released acetylcholine exerted a negative-feedback inhibition of coreleased glutamate. We conclude that most cholinergic basal forebrain neurons are capable of releasing glutamate as a cotransmitter and that the release of both transmitters is subject to simultaneous feedback inhibition by synaptically released acetylcholine. This has implications for BF neuron function and for the use of cholinesterase inhibitors in Alzheimer's disease.

Modality- and region-specific acetylcholine release in the rat neocortex

The basal forebrain is the major source of acetylcholine in the neocortex, and this projection has been variously described as either diffuse or highly specific. We used in vivo microdialysis to examine this discrepancy by collecting acetylcholine release simultaneously from visual, somatosensory and prefrontal cortical areas. Urethane-anesthetized rats were presented with visual and somatosensory stimulation in counter-balanced order and acetylcholine was measured using HPLC. Evoked acetylcholine release was modality-specific, i.e. visual stimulation evoked a large (75%) increase from visual cortex and little (24%) change from the somatosensory area whereas skin stimulation had the opposite effect. No increase was apparent in prefrontal cortex with either stimulation protocol. This experiment extends early studies using cortical cups to collect acetylcholine, and is consistent with the concept of functional specificity within the cholinergic basal forebrain with respect to both its sensory inputs and projections to the neocortex. This functional specificity within the cholinergic basal forebrain might be utilized in the modulation of different cortical regions during selective attention and plasticity.

Activation of cholinergic and adrenergic receptors increases the concentration of extracellular adenosine in the cerebral cortex of unanesthetized rat

Adenosine is an inhibitory neuromodulator in the CNS. For extracellular adenosine to play a physiological role in the brain, it must be present at effective concentrations. Acetylcholine and noradrenaline are known to play an important role in modulating the activity of cortical neurons, and they might have a role also in the release of adenosine in the cerebral cortex in vivo. We examined whether activation of cholinergic and adrenergic receptors affects extracellular adenosine levels in the cerebral cortex of unanesthetized rats using in vivo microdialysis. All drugs were administered locally within the cortex by reverse dialysis. Both acetylcholine and the acetylcholinesterase inhibitor neostigmine increased extracellular adenosine levels, and the effect of neostigmine was blocked by the nicotinic receptor antagonist mecamylamine. Both nicotine and the nicotinic receptor agonist epibatidine increased the concentration of extracellular adenosine. Activation of muscarinic receptors using the nonselective agonist oxotremorine and a selective M1 receptor agonist also increased extracellular adenosine levels. Noradrenaline and the noradrenergic reuptake inhibitor desipramine increased extracellular adenosine levels. The alpha(1)-adrenergic receptor agonist phenylephrine and the beta-adrenergic agonist isoproterenol increased extracellular adenosine levels, whereas the alpha(2)-adrenergic receptor agonist clonidine did not have an effect. These findings indicate that activation of specific cholinergic and adrenergic receptors can increase extracellular levels of adenosine in the cortex, and suggest that cholinergic and adrenergic receptor-mediated regulation of adenosine levels may represent a mechanism for controlling the excitability of cortical neurons.

Inhibition of synaptically evoked cortical acetylcholine release by adenosine: an in vivo microdialysis study in the rat

The release of cortical acetylcholine from the intracortical axonal terminals of cholinergic basal forebrain neurons is closely associated with electroencephalographic activity. One factor which may act to reduce cortical acetylcholine release and promote sleep is adenosine. Using in vivo microdialysis, we examined the effect of adenosine and selective adenosine receptor agonists and antagonists on cortical acetylcholine release evoked by electrical stimulation of the pedunculopontine tegmental nucleus in urethane anesthetized rats. All drugs were administered locally within the cortex by reverse dialysis. None of the drugs tested altered basal release of acetylcholine in the cortex. Adenosine significantly reduced evoked cortical acetylcholine efflux in a concentration-dependent manner. This was mimicked by the adenosine A(1) receptor selective agonist N(6)-cyclopentyladenosine and blocked by the selective A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). The A(2A) receptor agonist 2-[p-(2-carboxyethyl)-phenethylamino]-5'-N-ethylcarboxamidoadenosi ne hydrochloride (CGS 21680) did not alter evoked cortical acetylcholine release even in the presence of DPCPX. Administered alone, neither DPCPX nor the non-selective adenosine receptor antagonist caffeine affected evoked cortical acetylcholine efflux. Simultaneous delivery of the adenosine uptake inhibitors dipyridamole and S-(4-nitrobenzyl)-6-thioinosine significantly reduced evoked cortical acetylcholine release, and this effect was blocked by the simultaneous administration of caffeine. These data indicate that activation of the A(1) adenosine receptor inhibits acetylcholine release in the cortex in vivo while the A(2A) receptor does not influence acetylcholine efflux. Such inhibition of cortical acetylcholine release by adenosine may contribute to an increased propensity to sleep during prolonged wakefulness.

The role of basal forebrain neurons in tonic and phasic activation of the cerebral cortex

The basal forebrain and in particular its cholinergic projections to the cerebral cortex have long been implicated in the maintenance of cortical activation. This review summarizes evidence supporting a close link between basal forebrain neuronal activity and the cortical electroencephalogram (EEG). The anatomy of basal forebrain projections and effects of acetylcholine on cortical and thalamic neurons are discussed along with the modulatory inputs to basal forebrain neurons. As both cholinergic and GABAergic basal forebrain neurons project to the cortex, identification of the transmitter specificity of basal forebrain neurons is critical for correlating their activity with the activity of cortical neurons and the EEG. Characteristics of the different basal forebrain neurons from in vitro and in vivo studies are summarized which might make it possible to identify different neuronal types. Recent evidence suggests that basal forebrain neurons activate the cortex not only tonically, as previously shown, but also phasically. Data on basal forebrain neuronal activity are presented, clearly showing that there are strong tonic and phasic correlations between the firing of individual basal forebrain cells and the cortical activity. Close analysis of temporal correlation indicates that changes in basal forebrain neuronal activity precede those in the cortex. While correlational, these data, together with the anatomical and pharmacological findings, suggest that the basal forebrain has an important role in regulating both the tonic and the phasic functioning of the cortex.

Detection and modulation of acetylcholine release from neurites of rat basal forebrain cells in culture

1. Nicotinic acetylcholine (ACh) receptor-rich patches prepared from rat myotubes were used as focal ACh detectors to record the release of ACh from magnocellular basal forebrain (MBF) neurones from 11- to 14-day-old postnatal rats maintained in dissociated cell culture. 2. An action potential generated by intracellularly stimulating the MBF cell soma through a patch electrode induced a brief (mean tau(decay), 6.3 ms) short latency (1.35-5.1 ms; median 3.1 ms) burst of nicotinic channel openings in the detector patch when the latter was positioned at discrete loci along the MBF neurites. Detected ACh concentrations ranged from approximately 480 nM to > 50 microM. Concentrations increased markedly during the first 14 days in vitro and were inversely related to response latency. 3. Sites of release were generally confined to the more proximal neurites within 100 microm of the cell body and were invariably associated with the presence of small (2-3 microm diameter) phase-dark puncta located at discrete intervals along the length of the neurites or at points where short collaterals branched from the main process. Release was never detected from the cell soma except under extreme non-physiological conditions but could occasionally be elicited from growth cones at the ends of the shorter thicker neurites in the absence of a target cell. 4. Evoked release was abolished by tetrodotoxin (0.5 microM) and by superfusing with low Ca(2+)-high Mg(2+)-containing solutions (0.25 mM Ca(2+), 5 mM Mg(2+)). Myotube patch responses were antagonized by d-tubocurarine (3 microM). 5. Muscarine (10 microM) inhibited release by 70 +/- 3% (n = 12 cells). This effect was antagonized by 100 nM methoctramine but not by 100 nM pirenzepine, indicating that it was mediated by M(2) muscarinic ACh receptors. 6. These results indicate that ACh release from the processes of magnocellular cholinergic basal forebrain neurones arises from highly specialized and discrete sites, and that it can be inhibited through activation of muscarinic receptors. It is suggested that the latter results from inhibition of presynaptic Ca(2+) channels and that it might be responsible for feedback autoinhibition of ACh release from cortical afferents of nucleus basalis neurones in vivo.

The effect of muscarinic and nicotinic ACh antagonist on the facilitation of rat visual cortical responses

Combined blockade of both muscarinic and nicotinic receptors fully eliminated the effect of reticular facilitation in rat visual cortex. However, this effect lasted only 2 h, what can suggest that there may be other excitatory input to the cholinergic neurones in rat visual cortex which is activated after blockade of cholinergic transmission.

Modification of neocortical acetylcholine release and electroencephalogram desynchronization due to brainstem stimulation by drugs applied to the basal forebrain

Acetylcholine released from the cerebral cortex was collected using microdialysis while stimulating the region of the pedunculopontine tegmentum in urethane-anesthetized rats. Electrical stimulation in the form of short trains of pulses delivered once per minute produced a 350% increase in acetylcholine release and a desynchronization of the electroencephalogram, as measured by relative power in the 20-45 Hz range (low-voltage fast activity). Perfusion of the region of cholinergic neurons believed to be responsible for the cortical release of acetylcholine, the nucleus basalis magnocellularis, was carried out using a second microdialysis probe. Exposure of the nucleus basalis magnocellularis to blockers of neural activity (tetrodotoxin or procaine) or to blockers of synaptic transmission (calcium-free solution plus magnesium or cobalt) produced a substantial decrease in the release of acetylcholine and desynchronization evoked by brainstem stimulation. Exposure of the nucleus basalis magnocellularis to the glutamate antagonist, kynurenate, resulted in a decrease in evoked acetylcholine release and electroencephalogram desynchronization similar in magnitude to that produced by nonspecific blockers, whereas application of muscarinic or nicotinic cholinergic blockers to the nucleus basalis magnocellularis did not reduce acetylcholine release or electroencephalogram desynchronization. Application of tetrodotoxin to the collection site in the cortex abolished the stimulation-evoked acetylcholine release, but not the low baseline release indicating that cholinergic nucleus basalis magnocellularis neurons have a low spontaneous firing rate in urethane-anesthetized animals. The results of this study suggest that the major excitatory input to the cholinergic neurons of the nucleus basalis magnocellularis from the pedunculopontine tegmentum is via glutamatergic and not cholinergic synapses.

Possible mechanisms of the effect of physostigmine on the facilitation of acetylcholine release in the guinea pig myenteric plexus

The automodulation of acetylcholine (ACh) release in the guinea pig myenteric plexus-longitudinal muscle preparation was investigated by studying the electric stimulation-evoked release of radiolabeled ACh. When the release associated with neuronal activity was challenged by the muscarinic antagonist atropine, the release was not significantly enhanced. When the acetylcholinesterase (AChE) blocker physostigmine was present, the well-established muscarinic receptor-mediated autoinhibition was operative, i.e., the release was significantly reduced. However, when both drugs were added together, the release was much higher than under control conditions. Therefore, it seems likely that there is also a facilitatory system. We made an effort to clear up the mechanism of this facilitation by blocking possible nicotinic presynaptic receptors, by excluding the alpha 2-adrenoceptor-mediated masking effect of noradrenergic heteromodulation, by preventing a possible ATP-mediated mechanism, and by attempting to prevent the direct effect of physostigmine. None of these manipulations resulted in a decrease of the surplus release. It is concluded, that when the negative feedback modulation of ACh is inhibited and AChE activity is reduced, high levels of ACh facilitates additional release of ACh from the nerve terminals, possibly through a not yet verified class of receptors.

Muscarinic receptors mediate attenuation of extracellular acetylcholine levels in rat cerebral cortex after cholinesterase inhibition

Muscarinic autoregulation of extracellular acetylcholine levels was investigated by microdialysis in the cerebral cortex of freely moving rats under basal conditions as well as following systemic administration of a reversible cholinesterase inhibitor. Atropine (2.2 mg/kg s.c. or 0.2 microM via the dialysis probe) did not affect basal extracellular acetylcholine levels in the cerebral cortex. However, it did potentiate the elevation of extracellular acetylcholine levels produced by a dose of systemic heptylphysostigmine which inhibited 25% of cortical and 40% of plasma cholinesterase activity. These observations suggest that the extracellular concentration of acetylcholine following moderate acetylcholinesterase inhibition is regulated through muscarinic receptors.

Frequency-dependent increase in cortical acetylcholine release evoked by stimulation of the nucleus basalis magnocellularis in the rat

Acetylcholine was collected from the somatosensory cortex of anesthetized rats, using the microdialysis technique. Electrical stimulation of the nucleus basalis magnocellularis (NBM) with trains of 10 pulses at 100 Hz delivered every second produced a 3-4-fold increase in acetylcholine release. Stimulation with an intratrain frequency of 10, 50, 100 or 200 Hz demonstrated that 100 Hz trains produced the greatest increase, while the other frequencies were about half as effective. The cortical release of acetylcholine in this paradigm supports the hypothesis that the previously demonstrated enhancement by NBM stimulation of cortical sensory inputs is due to cholinergic activation.

Participation of cholinergic mechanisms in the origination of the dendritic potential of the cerebral cortex

The influence of a series of muscarinic (M) and nicotinic (N) substances on the dendritic potentials of the cerebral cortex was studied in acute experiments on adult cats under deep nembutal anesthesia. M-Antagonists in large doses and M-agonists, N-agonists and acetylcholine elicited a decrease in the amplitude of dendritic potentials. M-Antagonists in small doses and N-antagonists elicited an increase in the amplitude of the dendritic potentials. The application of an anticholinesterase substance, proserine, to the cortex elicited an increase in the amplitude and duration of the dendritic potentials. It is hypothesized that there are postsynaptic M-cholinoreceptors of the M1 type, presynaptic M-cholinoreceptors of the M2 type, and presynaptic N-cholinoreceptors in the neuropil of layer I of the cortex.

Insulin-like growth factor 1 stimulates the release of acetylcholine from rat cortical slices

The effect of somatomedin, or insulin-like growth factors (IGF-1 and IGF-2), on the basal and potassium induced release of [3H]acetylcholine ([3H]Ach) from rat cortical slices, previously preincubated with [3H]choline ([3H]Ch), was studied in vitro. IGF-1 (1.4 x 10(-9) to 1.4 x 10(-8) M) had no effect on the basal release of [3H]ACh, while IGF-1 (1.4 x 10(-9) to 4.3 x 10(-8) M) increased the potassium induced release of [3H]ACh from rat brain slices in a concentration-dependent manner. However IGF-2 (1.4 x 10(-8) M) had no effect. Insulin (1.8 x 10(-8) to 5.3 x 10(-8) M), similarly, did not have any influence on the release of [3H]ACh, demonstrating that the facilitatory effect of IGF-1 on [3H]ACh release is not mediated via insulin receptors. This report demonstrates for the first time that IGF-1 has an effect on neurotransmission in the adult brain.

Effect of thiopental sodium and barbitone sodium on the total acetylcholine content and acetylcholinesterase activity in the brain tissue of Arvicanthis niloticus

The total ACh content and AChE activity were determined 1 hr after the i.p. injection of different doses of thiopental sodium (5, 10 and 20 mg/ml/100 g body wt) and barbitone sodium (20, 40 and 80 mg/ml/100 g body wt). The effect of different time intervals (1 min, 10 min, 30 min, 1 hr, 2.5 hr, 5 hr, 8 hr, 12 hr, 24 hr and 48 hr) on the total ACh content and AChE activity was investigated after i.p. injection of 10 mg thiopental sodium and 40 mg barbitone sodium/ml/100 g body wt. Both thiopental sodium and barbitone sodium increased the total ACh content in the brain tissue of Arvicanthis niloticus. Both drugs inhibited the brain AChE activity. It is thought that the increase in the total ACh content in the brain tissue of Arvicanthis niloticus may be due to a decrease in the release of ACh from the neuronal tissue and a decrease in AChE activity.

Cortical acetylcholine efflux with hypercapnia and nociceptive stimulation

In rabbits anesthetized with 70% N2O-30% O2, the rate of efflux of acetylcholine (ACh) from the cerebral cortex doubled during hypercapnia (increase of end-tidal CO2 from 4 to 8%), and during mild nociceptive stimulation of the tail. Under 0.7% halothane anesthesia, the control rate of ACh efflux was lower than that under N2O; the rate rose 2-fold during hypercapnia and 4-fold during tail stimulation. In the absence of systemic atropinization, increase in ACh efflux was correlated with a shift in EEG from high- to low-voltage ('activated'); after systemic atropinization EEG remained in the high-voltage state, but the changes in ACh efflux with hypercapnia and stimulation were not affected. Following transection of the midbrain, ACh efflux was markedly depressed and did not change during hypercapnia. Taken in context with the previously known facts that the cerebral hyperemia of hypercapnia is potentiated by cholinesterase inhibition and attenuated by atropine or decerebration, the present results support the concept of a cholinergic regulation of the cerebral vasculature.

Acetylcholine release in the cat caudate nucleus measured with the choline oxidase method

A chemiluminescent assay for the estimation of acetylcholine (ACh) was used to measure ACh release in caudate nuclei (CN) of halothane-anaesthetized cats implanted with push-pull cannulae. The validity of the entire experimental approach used was shown by the fact that ACh release was calcium-dependent and was increased by depolarizing agents (potassium ions, veratridine) as well as by atropine. The effects of GABA (10(-5) M, 30 min) unilateral application into the ventralis medialis and ventralis lateralis thalamic nuclei on ACh release in both CN were then examined. This treatment, known to increase DA release bilaterally, decreased ACh release in both CN. These data further reveal the role of thalamic nuclei in the bilateral regulation of the activity of neurons identified within the basal ganglia and are discussed in the light of the well-known inhibitory influence of nigrostriatal DA neurons on striatal cholinergic neurons.

Evidence for a cholinergic inhibitory feed-back mechanism in the rabbit retina

The effects of muscarine, atropine and nicotinic antagonists on the light-evoked release of radioactivity from rabbit retinas previously exposed to [3H]choline (Ch) was studied. On the basis of previous experiments, this light-evoked release of total radioactivity was taken as a measure of the light-evoked release of [3H]acetylcholine (ACh) from the cholinergic amacrine cells. Atropine (1 microM) in the presence, but not the absence, of eserine more than doubled the light-evoked release of [3H]ACh. Eserine (30 microM) itself had no significant effect on either the spontaneous resting release or the light-evoked release of [3H]ACh. Muscarine (10 microM) in the presence or absence of eserine reduced the light-evoked release of [3H]ACh from the retina by 50%. This effect of muscarine was blocked by atropine used in the absence of eserine. The nicotinic antagonists pempidine, hexamethonium and gallamine had no significant effect on retinal [3H]ACh release. Strychnine (20 microM), which alone had no effect on retinal [3H]ACh release, abolished the effects of both muscarine and atropine on the light-evoked release of [3H]ACh. Bicuculline (5 microM) did not affect the actions of muscarine or atropine on the light-evoked release of [3H]ACh. Previous experiments had shown that glycine and gamma-aminobutyric acid (GABA) reduce the light-evoked release of [3H]ACh from the retina and that these inhibitory effects are selectively blocked by strychnine (20 microM) and bicuculline (5 microM) respectively. These results suggest the presence in the retina of a cholinergic inhibitory feed-back mechanism which involves a neuronal loop, rather than presynaptic or post-synaptic inhibitory muscarinic receptors on the cholinergic amacrine cells themselves. Our experiments do not provide evidence on the nature of the proposed inhibitory loop, except that it apparently includes a glycinergic or taurinergic (amacrine) cell.

Exogenous choline enhances the synthesis of acetylcholine only under conditions of increased cholinergic neuronal activity

The effect choline (60 mg/kg, i.p.) on fluphenazine- and pentylenetetrazol-induced alterations in the concentration of acetylcholine (ACh) and/or the rate of sodium-dependent high-affinity choline uptake (HACU) in rat striatum and hippocampus was studied. Systemic administration of the dopamine receptor blocking agent fluphenazine hydrochloride (.05 mg/kg, i.p.) decreased the concentration of ACh in the striatum; this effect was prevented by the prior administration of choline. The central nervous system stimulant pentylenetetrazol (30 mg/kg, i.p.) reduced the concentration ACh in both striatum and hippocampus and increased the velocity of HACU in the hippocampus. Pretreatment with choline totally prevented the depletion of ACh induced by pentylenetetrazol in the striatum. In the hippocampus, prior administration of choline prevented the pentylenetetrazol-induced increase in the rate of HACU and attenuated the effect of pentylenetetrazol on the levels of ACh. Results indicate that the acute administration of choline antagonizes pharmacologically induced alterations in cholinergic activity as assessed by the rate of HACU and the steady-state concentration of ACh. Furthermore, data support the hypothesis that the administration of choline increases the ability of central cholinergic neurons to synthesize ACh under conditions of increased neuronal activity.

Presynaptic muscarinic receptors inhibiting active acetylcholine release in the bullfrog sympathetic ganglion

1 The effects of bethanechol and atropine on the release of acetylcholine (ACh) from bullfrog sympathetic preganglionic nerve terminals were examined electrophysiologically. 2 Bethanechol (1 mM) caused no depolarization of sympathetic preganglionic nerve terminals, whereas carbachol or ACh in the same concentration induced marked depolarizations of these terminals. 3 Bethanechol (10 microM) depressed the amplitude of fast excitatory postsynaptic potentials (e.p.s.ps) recorded in Ca2+-high Mg2+ solution, without depolarizing ganglion cells. The quantal content measured from these fast e.p.s.ps by the variance method showed a significant reduction. 4 Amplitudes of both miniature e.p.s.ps and ACh-potentials induced by iontophoresis of ACh were not affected by addition of bethanechol (10 microM). 5 The depressant effect of bethanechol (10 microM) on fast e.ps.ps disappeared in the presence of atropine (3 microM). 6 Atropine (3 microM) increased the quantal content measured from fast e.p.s.ps recorded in low Ca2+-high Mg2+ solution. 7 The depressant effect of bethanechol (10 microM) on fast e.p.s.ps was unaffected by alpha-adrenoceptor blocking agents (phenoxybenzamine (10 microM) or phentolamine (10 microM). 8 These results suggest that presynaptic nerve terminals in bullfrog sympathetic ganglia possess a muscarinic receptor which inhibits active release of ACh.

Effect of stimulation on the incorporation of 14C from glial and neuronal specific substrates into brain proteins in vivo and in vitro

The incorporation of amino acids into brain proteins following brachial plexus stimulation (BPS) was studied in anaesthetised Sprague-Dawley rats following injection of radioactive precursors of both neuronal and glial compartments. Following intraperitoneal injection of [14C]glucose, which is the major neuronal pool precursor, BPS resulted in a significant increase of 37% (P < 0.001) in the incorporation of carbon from [14C]glucose into TCA-insoluble proteins in the contralateral sensorimotor cortex as compared with the ipsilateral area of the same animal. This increase was abolished totally when tetrodotoxin (10 micrograms ml-1) was applied topically to the surface of the stimulated area. Following intraperitoneal injection of [14C]acetate, which is considered to by mainly a glial cell precursor, the same afferent electrical stimuli caused a significant decrease of 21% in the incorporation of amino acids into proteins in the stimulated versus unstimulated sensorimotor cortex. With [4-(3)H]phenylalanine or [1-(14)C]leucine as precursors a significant decrease (12%) or no change was recorded, respectively. A similar decrease in protein synthesis in the stimulated sensorimotor cortex was achieved using different routes of injection. No significant changes were observed in the ratio of the specific radioactivities of the total amino acids of the two hemispheres using either precursor. In vitro, synaptosomes showed a large increase in incorporation into proteins after treatment with electrical pulses, both with [14C]glucose and with [U-14C]acetate as precursors.

Muscarinic autoreceptor regulates acetylcholine release in rat hippocampus: in vitro evidence

Release of 3H-ACh from isolated nerve endings of rat hippocampus was evoked by incubation in Krebs-Ringer's buffer containing 25 or 35 mM potassium. The release was Ca2+-dependent and could be inhibited by Mg2+ (20 mM). The muscarinic antagonist, atropine (10(-10)-10(-6) M), enhanced 3H-ACh-release. The muscarinic agonist, carbachol (10(-5)-10(-3) M) inhibited 3H-ACh release via interaction with muscarinic receptors: this effect could be blocked by atropine (10(-6) M). The presence of the feed-back regulation of 3H-ACh release in a cell-free preparation provides further evidence that the presynaptic regulation is exerted by muscarinic autoreceptors localized on the cholinergic nerve ending itself. The feed back inhibition of the 3H-ACh release does not require the presence of intact neurons or neuronal loops as tetrodotoxin (2.5 x 10(-6) M) does not affect the above results.

Effect of cyclic nucleotide derivatives on the release of ACh from cortical slices of the rat brain

Atropine (5 nM to 5 muM), but not D-tubocurarine or hexamethonium, enhanced the release of acetylcholine (ACh) from rat brain cortical slices exposed to high K+. The enhancement was dose-dependent, and it could be partially antagonized by oxotremorine (50-500 muM) or by dibutyryl or 8-bromo-cyclic GMP (1 mM), but not by tetrodotoxin (200 nM) or by dibutyryl cyclic AMP (1 mM). The effects of oxotremorine and the cyclic GMP derivatives were not due to diminished ACh synthesis, since these compounds did not influence the reduction of tissue ACh resulting from treatment with K+ and atropine. These results suggest that cyclic GMP may mediate the regulation of ACh release by presynaptic muscarinic receptors.

Mechanism of acetylcholine release: possible involvement of presynaptic muscarinic receptors in regulation of acetylcholine release and protein phosphorylation

Acetylcholine (AcCho) release from purely cholinergic Torpedo synaptosomes was evoked by K+ depolarization in the presence of Ca2+. Activation of muscarinic receptors, present in the synaptosomal fraction, by the agonist oxotremorine resulted in the inhibition of AcCho liberation. This inhibition was abolished by the muscarinic antagonist atropine, which by itself has no effect. These findings suggest that the muscarinic receptor, present in the electric organ of Torpedo is presynaptic and that its physiological function is to regulate AcCho release by negative feedback. The mechanism of presynaptic muscarinic inhibition was investigated by examining the effect of muscarinic ligands on synaptosomal 45Ca2+ uptake and on the level of phosphorylation of specific synaptosomal proteins. Ca2+-dependent K+ depolarization-induced synaptosomal AcCho release was accompanied by 45Ca2+ uptake and by a marked increase in the phosphorylation of a specific synaptosomal protein (band alpha) of approximately 100,000 daltons. Activation of the muscarinic receptor by the agonist oxotremorine had no detectable effect on synaptosomal 45Ca2+ uptake but resulted in the concomitant inhibition of AcCho release and of phosphorylation of band alpha. The muscarinic antagonist atropine abolished the inhibitory effect of oxotremorine both on AcCho liberation and on phosphorylation of band alpha. These findings suggest that phosphorylation of band alpha may be involved in regulation of the presynaptic processes that underly AcCho release and that activation of the muscarinic receptor by agonists may inhibit AcCho release by blocking the phosphorylation of band alpha.

Electrocorticographic effects of topically applied scopolamine

The electrocorticographic effects of topically applied scopolamine were investigated in unanesthetized cats with high cervical transection. After subpial injection of 30 cumu scopolamine in concentrations of 10 to 20%, large amplitude intermittent sharp waves appeared in the electrocorticogram, which developed into long-lasting paroxysmal activity. This effect was antagonized by intravenous eserine, 0.1 mg/kg, when it was produced by topical scopolamine in low dosage. The acoustically evoked cortical response and the generalized epileptiform activity produced by topical succinylcholine disappeared after topical scopolamine in low dosage. It is suggested that the seizure-suppressing effect of scopolamine may be due to its cholinolytic action. The convulsive activity of topical scopolamine in high concentrations may be accounted for by its depolarizing, synchronizing, disinhibiting, and acetylcholine-releasing effects.

Choline Administration: Modification of the Central Actions of Atropine

The anticholinergic agent atropine decreases acetylcholine concentrations and increases high-affinity choline uptake in cortical and hippocampal regions of rat brain. Administration of choline 1 hour before atropine prevents both of these atropine-induced alterations. These findings suggest that alterations in acetylcholine precursor availability may modify the effects of centrally active anticholinergic agents.

The role of the septal nuclei in the release of acetyl-choline from the rabbit cerebral cortex and dorsal hippocampus and the effect of atropine

Acetylcholine (ACh) was collected from the alvear surface of the dorsal hippocampus and cerebral cortex in chloralose-urethane anaesthetized or unanaesthetized rabbits. With anaesthesia, the resting release of ACh from the hippocampus was greater than that from the cortex. Wthout anaesthesia, the resting release from both areas was much higher and very similar. The addition of atropine sulphate (1 microgram/ml) to the collecting fluid or the administration of Artane (2 mg/kg i.v.) increased resting ACh release from both the hippocampus and cortex to similar output levels. Atropine also increased ACh release due to stimulation of the medial septum (MS) or mesencephalic reticular formation (MRF). Removal of the septum abolished the effect of atropine on resting ACh release and on release evoked by MRF stimulation from both the hippocampus and cortex. The data indicate that the septum is an essential pathway for cholinergic fibres ascending to the cerebral cortex and hippocampus. They also demonstrate that the septal cholinergic fibres must be intact and active for atropine to increase ACh release from their terminals.

Convulsant-anticonvulsant interactions on seizure activity and cortical acetylcholine release

The effects of leptazol and bicuculline on the efflux of endogenous acetylcholine (ACh) from the surface of the cerebral cortex have been related to EEG activity in urethane-anaesthetised rats. During seizure activity there was a calcium dependent increase in ACh efflux which was related to increase EEG activity and clonic muscle movements. ACh release and EEG activity were reduced during convulsive activity by trimethadione but not phenytoin. Phenobarbitone reduced convulsive EEG activity but left ACh release relatively unaffected. Blood pressure changes induced by convulsant and anticonvulsant drugs were not consistently related to EEG activity or ACh release. It is suggested that ACh efflux from the cerebral cortex is closely related to the activity of neurones within the cortex where it is released from nerve endings. Comparison of EEG changes induced by anticonvulsants and urethane during control and convulsant activity showed that only trimethadione produces anticonvulsant activity unaccompanied by general CNS depression.

Effect of atropine and oxotremorine on the release of acetylcholine from the frog spinal cord

The in vitro frog spinal cord has been used to study acetylcholine (ACh) release and spinal root potentials. The preparation bathed in an eserine-containing medium spontaneously released ACh into the bathing fluid. This release was enhanced by atropine in a dose-related and long-lasting manner and transiently by oxotremorine. The release rate of ACh was increased by low frequency ventral root stimulation; this increase was found to be proportionally the same after application of atropine. Oxotremorine did not modify the elctrically-evoked ACh release but blocked or reduced the effect of atropine. It is concluded that the stimulatory action of atropine on ACh output cannot be entirely explained by an interaction of atropine with presynaptic cholinergic receptors and that other indirect mechanisms (via interneurones) may play a role in this effect.

Acetylcholine release from visual and sensorimotor cortices of conditioned rabbits: the effects of sensory cuing and patterns of responding

A technique was devised for the collection of acetylcholine (ACh) released from the cerebral cortex of awake rabbits while they were performing a previously learned operant task. Based on the assumption that ACh release is directly proportional to the activity of cholinergic synapses under the area of collection, two hypotheses of the functional role of cortical cholinergic mechanisms were examined: (1) that activity in cholinergic neurons is related to the inhibition of responding; (2) that cholinergic activity is related to the perception of a 'significant' stimulus. Five groups trained on different behavioral paradigms were used to test these hypotheses. ACh release was collected concurrently from visual and sensorimotor cortices to differentiate diffuse from specific cortical effects. A small (50-100%) increase in ACh release was found in all groups and from both cortical areas. In the case of one group (visually cued, reinforced for low response rates) a significantly greater increase occurred from sensorimotor cortex only. These findings do not support either hypothesis alone, and are interpreted as evidence for two cholinergic systems within, or projecting to the cortex. One is related to generalized behavioral arousal and desynchronization of the electroencephalogram. Activation of the second cholinergic system is dependent on both response inhibition and the presence of a significant stimulus of the visual (but not of the auditory) modality.

The effect of atropine on the turnover of acetylcholine in the mouse brain

The effect of atropine on the acetylcholine (ACh) turnover in the mouse brain has been studied and related to the central effect (motor activity) of the drug. At the threshold dose for maximal increase in motor activity, atropine had no measurable effect in the brain on the initial rate of formation of labelled ACh from labelled choline (Ch) i.v. injected. However, if atropine was injected 3 min after the injection of labelled Ch, when the labelled ACh had reached its peak value in the brain, there was a more rapid exponential decline of labelled ACh. This was assumed to be an indication that atropine increases the turnover of ACh in the brain. The specific radioactivity of ACh was not changed 2-17 min after the atropine injection, which indicates that atropine does not preferentially increase the release of newly synthetized ACh.

Inhibition by oxotremorine of acetylcholine resting release from guinea pig-ileum longitudinal muscle strips

1. Longitudinal muscle strips of the guinea-pig ileum were incubated in Tyrode solution containing either DFP or physostigmine as cholinesterase inhibitor. After a 90 min preincubation period the acetylcholine resting release into the medium was determined. Acetylcholine was estimated by gas chromatography. 2. The resting release was 0.39 nmol/g times min irrespective of the cholinesterase inhibitor used. In the presence of hexamethonium, or after omission of external calcium, the resting release fell by 50 and 55 per cent, respectively. 3. Oxotremorine (10-5 and 10-4M) significantly inhibited the resting release of acetylcholine by 25 and 33 per cent, respectively. The inhibitory effect of oxotremorine was completely reversed by atropine (3 times 10-7 M). Oxotremorine did not reduce the spontaneous release of acetylcholine that occurred either in the presence of hexamethonium or in the absence of external calcium. 4. The acetylcholine content of the muscle strips was approximately doubled during the preincubation with a cholinesterase inhibitor. The subsequent incubation with oxotremorine did not lead to a further increase in the endogenous acetylcholine content. However, incubation of the muscle strips with oxotremorine in the absence of a cholinesterase inhibitor led to a rise in the endogenous acetycholine concentration. In in vivo experiments, oxotremorine also caused an increase in the acetylcholine content of the muscle strips. The possibility is discussed that the rise in the acetylcholine concentration following the administration of oxotremorine is a consequence of the decreased release. 5. It is concluded that oxotremorine inhibits the resting release of acetylcholine by activation of neuronal muscarinic receptors. The inhibitory effect of exotremorine is linked to that fraction of the acetylcholine resting release that is calcium-dependent and that arises from propagated activity in cholinergic neurones. The results are consistent with the hypothesis of a feed-back control of acetylcholine release mediated by inhibitory muscarinic receptors.