The effect of nicotine and cytisine on 3H-acetylcholine release from cortical slices of guinea-pig brain

Cytisine: State of the art in pharmacological activities and pharmacokinetics

Cytisine is a naturally occurring bioactive compound, an alkaloid mainly isolated from legume plants. In recent years, various biological activities of cytisine have been explored, showing certain effects in smoking cessation, reducing drinking behavior, anti-tumor, cardiovascular protection, blood sugar regulation, neuroprotection, osteoporosis prevention and treatment, etc. At the same time, cytisine has the advantages of high efficiency, safety, and low cost, has broad development prospects, and is a drug of great application value. However, a summary of cytisine's biological activities is currently lacking. Therefore, this paper summarizes the pharmacological action, mechanism, and pharmacokinetics of cytisine by referring to numerous databases, and analyzes the new and core targets of cytisine with the help of computer simulation technology, to provide reference for doctors.

Cytisine Exerts an Anti-Epileptic Effect via α7nAChRs in a Rat Model of Temporal Lobe Epilepsy

Background and Purpose: Temporal lobe epilepsy (TLE) is a common chronic neurological disease that is often invulnerable to anti-epileptic drugs. Increasing data have demonstrated that acetylcholine (ACh) and cholinergic neurotransmission are involved in the pathophysiology of epilepsy. Cytisine, a full agonist of α7 nicotinic acetylcholine receptors (α7nAChRs) and a partial agonist of α4β2nAChRs, has been widely applied for smoking cessation and has shown neuroprotection in neurological diseases. However, whether cytisine plays a role in treating TLE has not yet been determined.

Experimental Approach: In this study, cytisine was injected intraperitoneally into pilocarpine-induced epileptic rats for three weeks. Alpha-bungarotoxin (α-bgt), a specific α7nAChR antagonist, was used to evaluate the mechanism of action of cytisine. Rats were assayed for the occurrence of seizures and cognitive function by video surveillance and Morris water maze. Hippocampal injuries and synaptic structure were assessed by Nissl staining and Golgi staining. Furthermore, levels of glutamate, γ-aminobutyric acid (GABA), ACh, and α7nAChRs were measured.

Results: Cytisine significantly reduced seizures and hippocampal damage while improving cognition and inhibiting synaptic remodeling in TLE rats. Additionally, cytisine decreased glutamate levels without altering GABA levels, and increased ACh levels and α7nAChR expression in the hippocampi of TLE rats. α-bgt antagonized the above-mentioned effects of cytisine treatment.

Conclusion and Implications: Taken together, these findings indicate that cytisine exerted an anti-epileptic and neuroprotective effect in TLE rats via activation of α7nAChRs, which was associated with a decrease in glutamate levels, inhibition of synaptic remodeling, and improvement of cholinergic transmission in the hippocampus. Hence, our findings not only suggest that cytisine represents a promising anti-epileptic drug, but provides evidence of α7nAChRs as a novel therapeutic target for TLE.

Analgesic and antineuropathic drugs acting through central cholinergic mechanisms

The role of muscarinic and nicotinic cholinergic receptors in analgesia and neuropathic pain relief is relatively unknown. This review describes how such drugs induce analgesia or alleviate neuropathic pain by acting on the central cholinergic system. Several pharmacological strategies are discussed which increase synthesis and release of acetylcholine (ACh) from cholinergic neurons. The effects of their acute and chronic administration are described. The pharmacological strategies which facilitate the physiological functions of the cholinergic system without altering the normal modulation of cholinergic signals are highlighted. It is proposed that full agonists of muscarinic or nicotinic receptors should be avoided. Their activation is too intense and un-physiological because neuronal signals are distorted when these receptors are constantly activated. Good results can be achieved by using agents that are able to a) increase ACh synthesis, b) partially inhibit cholinesterase activity c) selectively block the autoreceptor or heteroreceptor feedback mechanisms. Activation of M(1) subtype muscarinic receptors induces analgesia. Chronic stimulation of nicotinic (N(1)) receptors has neuronal protective effects. Recent experimental results indicate a relationship between repeated cholinergic stimulation and neurotrophic activation of the glial derived neurotrophic factor (GDNF) family. At least 9 patents covering novel chemicals for cholinergic system modulation and pain control are discussed.

Co-expression of alpha7 and beta2 nicotinic acetylcholine receptor subunit mRNAs within rat brain cholinergic neurons

Nicotine enhances cognitive and attentional processes through stimulation of the basal forebrain cholinergic system. Although muscarinic cholinergic autoreceptors have been well characterized, pharmacological characterization of nicotinic autoreceptors has proven more difficult. The present study used double-labeling in situ hybridization to determine expression of nicotinic acetylcholine receptor (nAChR) subunit mRNAs within basal forebrain cholinergic neurons in order to gain information about possible nAChR autoreceptor properties. Cholinergic cells of the mesopontine tegmentum and striatal interneurons were also examined, as were septohippocampal GABAergic neurons that interact with cholinergic neurons to regulate hippocampal activity. alpha7 and beta2 nAChR mRNAs were found to be co-expressed in almost all cholinergic cells and in the majority of GABAergic neurons examined. alpha4 nAChR mRNA expression was restricted to cholinergic cells of the nucleus basalis magnocellularis, and to non-cholinergic cells of the medial septum and mesopontine tegmentum. These data suggest possible regional differences in the pharmacological properties of nicotinic autoreceptors on cholinergic cells. Whereas most cholinergic cells express rapidly desensitizing alpha7 homomers or alpha7beta2 heteromers, cortical projection neurons may also express a pharmacologically distinct alpha4beta2 nAChR subtype. There may also be differential nAChR regulation of cholinergic and non-cholinergic cells within the mesopontine tegmentum that are implicated in acquisition of nicotine self-administration.

Brain-selective stimulation of nicotinic receptors by TC-1734 enhances ACh transmission from frontoparietal cortex and memory in rodents

The authors have described the effect of TC-1734, a brain-selective nicotinic acetylcholine receptor (nAChR) agonist, on acetylcholine (ACh) release in the frontoparietal cortex of rats and on cognitive function in mice. Oral administration of TC-1734 (5, 10 and 20 mg/kg) stimulated ACh release in a dose-dependent manner, as measured by transversal microdialysis. The maximal effect on the amplitude of ACh release was observed at a dose of 10 mg/kg (about 70% above baseline), whereas the maximal effect on the duration of ACh release was observed at the dose of 20 mg/kg. By contrast, oral administration of nicotine (1, 2.5 and 5 mg/kg) did not stimulate ACh release in a dose-dependent manner but produced the same maximal effect on the amplitude of ACh release (about 50% above baseline) at all the doses tested. The ability of both TC-1734 (10 mg/kg) and nicotine (1 mg/kg) to increase ACh levels was antagonized by mecamylamine (1 mg/kg s.c.), suggesting a specific nicotine receptor-mediated effect of both agonists. No tolerance to TC-1734- and nicotine-stimulated ACh release was observed after repeated treatment with TC-1734 (10 mg/kg) or nicotine (1 mg/kg) for 4 days. TC-1734 (1 mg/kg p.o.) improved memory in the object recognition test in mice, and this effect was antagonized by mecamylamine (2.5 mg/kg i.p.). Taken together, these results show that TC-1734 stimulates nAChR in the brain to induce an increase of ACh release in the cortex of rats and enhance memory in mice.

A unifying hypothesis of Alzheimer's disease. IV. Causation and sequence of events

Contrary to common concepts, the brain in Alzheimer's disease (AD) does not follow a suicide but a rescue program. Widely shared features of metabolism in starvation, hibernation and various conditions of energy deprivation, e.g. ischemia, allow the definition of a deprivation syndrome which is a phylogenetically conserved adaptive response to energetic stress. It is characterized by hypometabolism, oxidative stress and adjustments of the glucose-fatty acid cycle. Cumulative evidence suggests that the brain in aging and AD actively adapts to the progressive fuel deprivation. The counterregulatory mechanisms aim to preserve glucose for anabolic needs and promote the oxidative utilization of ketone bodies. The agent mediating the metabolic switch is soluble Abeta which inhibits glucose utilization and stimulates ketone body utilization at various levels. These processes, which are initiated during normal aging, include inhibition of pro-glycolytic neurohormones, cholinergic transmission, and pyruvate dehydrogenase, the key transmitter and effector systems regulating glucose metabolism. Hormonal and effector systems which promote ketone body utilization, such as glucocorticosteroid and galanin activity, GABAergic transmission, nitric oxide, lipid transport, Ca2+ elevation, and ketone body metabolizing enzymes, are enhanced. A multitude of risk factors feed into this pathophysiological cascade at a variety of levels. Taking into account its pleiotropic regulatory actions in the deprivation response, a new name for Abeta is suggested: deprivin. On the other hand, cumulative evidence, taken together compelling, suggests that senile plaques are the dump rather than the driving force of AD. Moreover, the neurotoxic action of fibrillar Abeta is a likely in vitro artifact but does not contribute significantly to the in vivo pathophysiological events. This archaic program, conserved from bacteria to man, aims to ensure the survival of a deprived organism and controls such divergent processes as sporulation, hibernation, aging and aging-related diseases. In contrast to the immature brain, ketone body utilization of the aged brain is no longer sufficient to meet the energetic demands and is later supplemented by lactate, thus recapitulating in reverse order the sequential fuel utilization of the immature brain. The transduction pathways which operate to switch metabolism also convey the programming and balancing of the de-/redifferentiation/apoptosis cell cycle decisions. This encompasses the reiteration of developmental processes such as transcription factor activation, tau hyperphosphorylation, and establishment of growth factor independence by means of Ca2+ set point shift. Thus, the increasing energetic insufficiency results in the progressive centralization of metabolic activity to the neuronal soma, leading to pruning of the axonal/dendritic trees, loss of neuronal polarity, downregulation of neuronal plasticity and, eventually, depending on the Ca2+ -energy-redox homeostasis, degeneration of vulnerable neurons. Finally, it is outlined that genetic (e.g. Down's syndrome, APP and presenilin mutations and apoE4) and environmental risk factors represent progeroid factors which accelerate the aging process and precipitate the manifestation of AD as a progeroid systemic disease. Aging and AD are related to each other by threshold phenomena, corresponding to stage 2, the stage of resistance, and stage 3, exhaustion, of a metabolic stress response.

No involvement of nicotinic receptors in the facilitation of acetylcholine outflow in mouse cortex in the presence of neostigmine and atropine

The role of nicotinic and muscarinic receptors in the modulation of acetylcholine release was studied using field stimulated mouse cortex slices incubated with [(3)H]-choline. Both acetylcholine (100 microM) and the cholinesterase inhibitor neostigmine (100 microM) inhibited the stimulation-induced (S-I) outflow of radioactivity but in the presence of atropine (0.3 microM) an enhancement was seen, which may be indicative of facilitatory nicotinic receptors. Mecamylamine (100 microM) was unable to antagonize the enhancement seen in the presence of acetylcholine and atropine. The nicotinic agonist dimethylphenylpiperazinium (30 microM) did not facilitate S-I outflow of radioactivity. A range of nicotinic blockers had no effect on the enhancement seen in the presence of neostigmine and atropine, nor did indomethacin, the 5HT(3) antagonist MDL 7222 nor the NMDA antagonist MK-801. The inability to block this effect suggests that nicotinic receptors are not involved. We postulate, at least for neostigmine, that the facilitation is an artefact because of the use of [(3)H]-choline as a radiotracer whereby the efflux of radioactivity is enhanced because the radiolabelled acetylcholine is not metabolized to choline and therefore flows out of the tissue more readily.

Neuronal nicotinic receptors in the human brain

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of ligand gated ion channels which are widely distributed in the human brain. Multiple subtypes of these receptors exist, each with individual pharmacological and functional profiles. They mediate the effects of nicotine, a widely used drug of abuse, are involved in a number of physiological and behavioural processes and are additionally implicated in a number of pathological conditions such as Alzheimer's disease, Parkinson's disease and schizophrenia. The nAChRs have a pentameric structure composed of five membrane spanning subunits, of which nine different types have thus far been identified and cloned. The multiple subunits identified provide the basis for the heterogeneity of structure and function observed in the nAChR subtypes and are responsible for the individual characteristics of each. A substantial amount of information on human nAChR structure and function has come from studies on neuroblastoma cell lines which naturally express nAChRs and from recombinant nAChRs expressed in Xenopus oocytes. In vitro brain nAChR distribution can be mapped with a number of appropriate agonist and antagonist radioligands and subunit distribution may be mapped by in situ hybridization using subunit specific mRNA probes. Receptor distribution in the living human brain can be studied with noninvasive imaging techniques such as PET and SPECT, with a significant reduction in nAChRs in the brains of Alzheimer's patients having been identified with [11C] nicotine in PET studies. Despite the significant body of knowledge now accumulated about nAChRs, much remains to be elucidated. This review will attempt to describe the current knowledge on the nAChR subtypes in the human brain, their functional roles and neuropathological involvement.

Pharmacological characterization of nicotine-induced acetylcholine release in the rat hippocampus in vivo: evidence for a permissive dopamine synapse

In this study, the mechanism of nicotine-induced hippocampal acetylcholine (ACh) release in awake, freely moving rats was examined using in vivo microdialysis. Systemic administration of nicotine (0.4 mg kg(-1), s.c.) increased the levels of ACh in hippocampal dialysates. The nicotine-induced hippocampal ACh release was sensitive to the pretreatment of neuronal nicotinic acetylcholine receptor (nAChR) antagonists mecamylamine (3.0 mg kg(-1), s.c.) and dihydro-beta-erythrodine (DHbetaE; 4.0 mg kg(-1), s.c.) as well as systemic administration of the dopamine (DA) D1 receptor antagonist SCH-23390 (R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-benzaz epine; 0.3 mg kg(-1), s.c.). Local perfusion of mecamylamine (100 microM), DHbetaE (100 microM) or SCH-23390 (10 microM) through microdialysis probe did not increase basal hippocampal ACh release. Hippocampal ACh release elicited by systemic administration of nicotine (0.4 mg kg(-1), s.c.) was antagonized by local perfusion of SCH-23390 (10 microM), but not by MEC (100 microM) or DHbetaE (100 microM). Direct perfusion of nicotine (1 mM, but not 0.1 mM) increased hippocampal ACh levels; however, this effect was relatively insensitive to blockade by co-perfusion of either mecamylamine (100 microM) or SCH-23390 (10 microM). These results suggest that nicotine-induced hippocampal ACh release occurs by two distinct mechanisms: (1) activation of nAChRs outside the hippocampus leading to DA release and subsequent ACh release involving a permissive DA synapse, and (2) direct action of nicotine within the hippocampus leading to ACh release via non-DA-ergic mechanism.

The pharmacology of (-)-nicotine and novel cholinergic channel modulators

Advances in the understanding of the molecular biology and pharmacology of nAChRs may provide targets for the development of novel and selective modulators of nAChRs in the brain. This contention is supported by the dissimilar behavioral effects observed following systemic administration of currently available nicotinic ligands. The concept of multiple subtypes of nAChRs is not unique, as evidenced by the pharmacology of other ligand-gated ion channels, such as GABA-A receptor, which also exist in multiple subtypes. At present, with respect to the nAChRs, relatively few of the subtypes identified have been cloned from human tissue and pharmacologically evaluated, but several groups are focusing their research efforts in this direction. With a thorough understanding of the pharmacological and functional characteristics of more of the putative human nAChR subtypes, this knowledge will facilitate the discovery of more efficacious and less toxic ChCMs that may provide potential novel therapeutic agents for a variety of CNS conditions.

Pharmacology of nicotine and its therapeutic use in smoking cessation and neurodegenerative disorders

During the last decade, nicotine has been used increasingly as an aid to smoking cessation and has been found to be a safe and efficacious treatment for the symptoms of nicotine withdrawal. This period has also seen significant advances in our understanding of the mechanisms underlying the psychopharmacological responses to nicotine, including, particularly, those that have been implicated in nicotine addiction. This paper reviews this decade of progress in the specific context of the therapeutic application of nicotine to the treatment of smoking cessation. Other putative future applications, particularly in the treatment of neurodegenerative disorders, are also reviewed.

5-Hydroxytryptamine-mediated effects of nicotine on endogenous GABA efflux from guinea-pig cortical slices

1. The effect of nicotine on endogenous basal GABA outflow was studied in guinea-pig cerebral cortex slices. 2. Nicotine 1.86-18.6 mumol l-1 significantly decreased the basal, tetrodotoxin-sensitive GABA efflux, whereas at higher concentrations (186-620 mumol l-1) nicotine increased it. The inhibition was prevented by mecamylamine while the facilitation was blocked by mecamylamine, (+)-tubocurarine and tetrodotoxin. 3. The effect of nicotine was due to an indirect 5-hydroxytryptaminergic action. In fact, MDL 72222 (1 mumol l-1) completely prevented the alkaloid inhibition and methysergide (1 mumol l-1) reversed the facilitation into inhibition; concomitant treatment with methysergide and MDL 72222 antagonized the effect of nicotine at 186 mumol l-1 4. Lower concentrations of 5-HT (3-10 mumol l-1) decreased, whereas higher concentrations (30-100 mumol l-1) increased, spontaneous GABA outflow. The inhibition of GABA efflux was prevented by MDL 72222 whereas the facilitation was reversed by methysergide (1 mumol l-1) into inhibition, and prevented by MDL 72222 1 mumol l-11. 5. These results suggest that, by activating nicotinic receptors present on 5-hydroxytryptaminergic terminals, nicotine releases 5-HT which, in turn, inhibits or increases the secretory activity of cortical GABA interneurones via 5-HT3 and methysergide-sensitive receptors, respectively.

Improvement in performance of a delayed matching-to-sample task by monkeys following ABT-418: a novel cholinergic channel activator for memory enhancement

ABT-418, a newly characterized centrally acting cholinergic channel activator (ChCA), was evaluated for its ability to improve performance in a delayed matching-to-sample (DMTS) task by mature macaques well trained in the task. Previous studies in rodents have indicated that ABT-418 shares the memory/cognitive enhancing actions of nicotine, but without many of nicotine's dose-limiting side effects. As DMTS provides a measure both of general cognitive function (the matching concept) and of recent memory, it was hypothesized that some doses of ABT-418 would enhance the monkeys' ability to correctly perform the DMTS task. Intramuscular administration of ABT-418 significantly enhanced DMTS performance at low (2-32.4 nmol/kg) doses. In fact, the drug was slightly more potent that nicotine in this regard, and all eight animals tested in this study exhibited enhanced performance at one or more doses. ABT-418 produced the greatest improvement in DMTS performance at the longest delay interval. In animals repeatedly tested with their individualized "Best Dose", DMTS performance increased on average by 10.1 +/- 3.5 percentage points correct, which was equivalent to an increase of 16.2% over baseline performance. ABT-418 did not significantly affect response times, i.e., latencies to make a choice between stimuli, or latencies to initiate new trials. Whereas nicotine enhanced DMTS performance both on the day of administration and on the following day (in the absence of drug), ABT-418-induced enhanced performance was detected only on the day of administration. Finally, single daily administration of the individualized best dose in three monkeys over a period of 8 days generally maintained enhancement of DMTS performance. Thus, the data were not consistent with the development of significant tolerance to the drug's mnemonic actions. In contrast to nicotine, no overt toxicity or side effects to acute or repeated administration of the drug were noted. Thus, ABT-418 represents a prototype of a new class of nicotinic agonists designed for the potential treatment of human dementias having a low profile of toxicity.

Human nicotinic receptors--their role in aging and dementia

Multiple nicotinic receptors seem to exist in brain as revealed by neurophysiological, neurochemical, molecular and immunological studies. The mechanisms for their involvement in higher functions including learning and memory are still relatively unknown. The nicotinic receptor subtypes in human brain undergo changes during aging. Deficits of brain nicotinic receptors have been traced in neurodegenerative disorders as Alzheimer's disease and Parkinson's disease. Brain imaging studies in patients and neurochemical studies in autopsy brain tissue from Alzheimer patients reveal significant losses of the nicotinic receptors. New therapeutic compounds tried in Alzheimer's disease, aiming to increase cholinergic activity in the brain, act via the nicotinic receptors in brain. Augmentation of nicotinic receptor function in brain might be of importance for alleviating some of the cognitive impairments in Alzheimer's disease.

Muscarinic and nicotinic modulation of cortical acetylcholine release monitored by in vivo microdialysis in freely moving adult rats

The aim of the present study was to investigate, using in vivo dialysis, the existence of muscarinic and nicotinic receptors controlling acetylcholine release in the cortex of freely behaving rats. Various muscarinic receptor antagonists, including the nonselective blocker atropine, and a variety of M2 drugs (AF-DX116, AF-DX384, AQ-RA 741) potently stimulated, in a concentration-dependent manner, the in vivo release of acetylcholine in the rat cortex. The effects of all these antagonists were long lasting. The nature of these putative muscarinic autoreceptors is likely of the pharmacologically defined M2 subtype on the basis of the high potency of the antagonists of the AF-DX series and the variability and shorter duration of action of the effects of the prototypic M1 blocker, pirenzepine. 4-DAMP, a purported M3 blocker, also potently stimulated in vivo cortical acetylcholine release, but this likely relates to its now established, rather limited selectivity for any given muscarinic receptor subtypes. Peripheral and central injections of nicotine also induced the in vivo release of acetylcholine in the rat cortex, albeit with a lower potency and shorter duration of action than muscarinic antagonists. Interestingly, the combination of a muscarinic antagonist, such as atropine, AF-DX 116, or AF-DX384, in the presence of nicotine, induced tremendous releases of cortical acetylcholine up to 8- to 10-fold over basal values. This is clearly more than a simply additive effect, and it reveals the great capacity of cortical cholinergic nerve terminals to synthesize and release acetylcholine. Optimal pharmacological manipulations of these putative muscarinic and nicotinic autoreceptors could thus be useful in disorders in which the activity of cholinergic inputs is decreased, such as in Alzheimer's disease.

Scopolamine reversal of nicotine enhanced delayed matching-to-sample performance in monkeys

The basis for the memory enhancing action of nicotine was evaluated in five adult monkeys (Macaca fascicularis) well trained in the performance of a delayed matching-to-sample (DMTS) paradigm. Nicotine (1.25-20 micrograms/kg, IM) produced a dose-dependent improvement in performance of the task. The optimal dose of nicotine for each monkey also improved performance when the animals were tested 24 h later in the no-drug situation. In the same animals, low doses of scopolamine produced a dose-dependent decrement in DMTS performance. A subthreshold dose (defined by DMTS performance decrement) of scopolamine was administered 20 min prior to the optimal dose of nicotine. Scopolamine pretreatment completely blocked the enhanced performance observed earlier with nicotine. The results of this study are consistent with the hypothesis that the enhanced cognitive performance associated with nicotine is due to central acetylcholine release and subsequent muscarinic receptor stimulation.

Mechanism of the hypertensive response to central injection of nicotine in conscious rats

The purpose of this study was to examine the effects of nicotine administered directly into the CNS on mean arterial pressure (MAP) and heart rate to avoid the direct peripheral action of the drug. Also, because nicotine has been reported to enhance the release of endogenous brain acetylcholine, we sought to determine the role of this mechanism in mediating the cardiovascular response. Normotensive Wistar rats were previously implanted with indwelling intracerebroventricular (ICV) cannula guides and an arterial line (iliac artery) for central injection of drugs and measurement of MAP and heart rate, respectively. Rats received a series of increasing doses of nicotine (or saline vehicle) from 2-100 micrograms (in a 10 microliter volume) with each dose separated by at least 1 day. MAP increased immediately following all doses of nicotine; however, the maximal response was obtained following the 50 micrograms dose (higher doses actually produced lower responses). In general, the hypertensive response began immediately after injection, peaked within 2-3 min and returned to baseline within about 20 min. Heart rate changes were often not dramatic and highly variable. In order to examine the dependence of the pressor response to nicotine on brain acetylcholine, rats were pretreated with 20 micrograms (ICV) of hemicholinium-3 (HC-3) 1 h prior to nicotine to deplete endogenous acetylcholine. HC-3 pretreatment resulted in a significant reduction in the magnitude and duration of the pressor response to nicotine. Likewise, pretreatment with atropine inhibited the pressor response to subsequent injection of nicotine. Nicotine enhanced the release of [3H]acetylcholine from brain slices in vitro at concentrations likely achieved in the in vivo studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in hippocampal cholinergic receptors and hippocampal behaviors after early exposure to nicotine

Mice were exposed to nicotine prenatally by injecting the mother with 1.5 mg/kg nicotine SC twice daily on gestation days 9-18 (PreN mice) or neonatally by daily SC injections of 1.5 mg/kg nicotine on postnatal days 2-21 (NeoN mice). At age 50 days, hippocampal muscarinic receptors Bmax of PreN and NeoN mice were 58% and 79% above control, respectively (p less than 0.01); Kd was unaffected by early nicotine exposure. Eight-arm maze performance of nicotine-exposed animals fell behind control level. Both PreN and NeoN made approximately 10% less correct responses in the first eight trials than controls throughout the test period (p less than 0.01). By the last day of testing, PreN needed 23% and NeoN 31% more trials than controls to enter all arms (p less than 0.001). In addition, PreN needed 35 and NeoN 42% more days than controls to reach criterion (p less than 0.05). Similarly, while 61% of controls reached criterion by day 6 only 17% of PreN and 25% of NeoN reached criterion (p less than 0.01). In the Morris maze, PreN needed from 43-119% more time to reach the platform (p less than 0.001). In the spatial probe test, PreN animals made 35% fewer crosses over the area of the missing platform (p less than 0.001). The study suggests that nicotine administered to the fetus or neonate alters septohippocampal chemistry and induces deficits in hippocampus-related behaviors. The possible reversal of the behavioral changes by manipulating the cholinergic innervations should be the subject of future investigations.

Muscarinic modulation of acetylcholine release from slices of guinea pig nucleus basalis magnocellularis

Spontaneous and electrically evoked endogenous acetylcholine release and [3H]-choline efflux from slices of guinea pig nucleus basalis magnocellularis (nbM) were studied. Tetrodotoxin reduced the spontaneous endogenous release by 55%, while the Ca(2+)-free medium reduced it by about 30%. Evoked [3H]-choline efflux was Na+ and Ca2+ dependent and frequency related. Physostigmine, 30 microM, nearly halved the stimulation-evoked efflux; atropine, 0.15 microM, not only antagonized, but even reversed this effect into facilitation. Pirenzepine, 1 microM, and AFDX 116, 1 microM, were less effective than atropine, and reversed the inhibitory effect of physostigmine only when applied together. 4-DAMP, 0.01 microM, was ineffective. These findings indicate that acetylcholine release in guinea pig nbM slices is inhibited by the cooperation of muscarinic autoreceptors, possibly belonging to the M1 and M2 subclasses.

Neonatal nicotine exposure induces permanent changes in brain nicotinic receptors and behaviour in adult mice

The effects of neonatal nicotine exposure on spontaneous and nicotine-induced behaviour in 4-month-old mice and on the development of brain nicotinic receptors were studied. The behaviour study showed that mice treated with nicotine 66 micrograms (-)nicotine base/kg body weight (bw) s.c. twice daily between 10 and 16 days postnatally displayed a hypoactive condition, whereas mice treated with saline displayed a hyperactive condition. When the nicotinic receptors in the brain cortex were analyzed, the displacement curves for [3H]nicotine(-)nicotine revealed an almost equal proportion of high- and low-affinity binding sites in 17-day-old mice, while the high-affinity sites predominated in 4-month-old mice, with affinity constants for both high- and low-affinity binding sites 10 times higher in 4-month-old mice than in 17-day-old-mice. A decrease in the number of nicotinic receptors was observed from day 17 to 4 months, mainly of the low-affinity nicotinic type. Interestingly, the displacement curves in neonatally nicotine-treated mice showed only one population of high-affinity binding sites in 17-day- and 4-month-old mice though the total binding sites in 4-month-old mice were the same for the neonatally nicotine-treated and saline-treated mice. These results indicate that neonatal nicotine treatment prevents the development of low-affinity nicotinic sites in the brain and this earlier exposure to nicotine induces a different behaviour response in adult animals to a test dose of nicotine. Days 10-16 postnatally appear to be a critical period for the effects of nicotine on the brain.

Effect of acute administration of nicotine on in vivo release of noradrenaline in the hippocampus of freely moving rats: a dose-response and antagonist study

The effect of systemic administration of (-)-nicotine on release of noradrenaline in the hippocampus was studied by in vivo microdialysis in freely moving rats, using dialysate containing nomifensine (5 microM). (-)-Nicotine, at both 0.4 and 0.8 mg/kg but not 0.2 mg/kg, rapidly and significantly increased extracellular levels of noradrenaline. Extracellular levels of dopamine were also increased, but this was only significant after the larger dose. Both 0.4 and 0.8 mg/kg also produced a significant increase in extracellular levels of the metabolites of dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid. Extracellular levels of the metabolite of 5-hydroxytryptamine, 5-hydroxyindoleacetic acid, increased after 0.8 mg/kg but this effect was only apparent much later. Injection of a second 0.8 mg/kg challenge of (-)-nicotine, 150 min after the first, produced similar increases in extracellular levels of noradrenaline, dopamine, 3-4-dihydroxyphenylacetic acid and homovanillic acid. Over the experimental period, there was no further increase in extracellular levels of 5-hydroxyindoleacetic acid. Increases in extracellular levels of noradrenaline, dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid, in response to 0.8 mg/kg (-)-nicotine, were prevented by the systemic administration of mecamylamine, but not hexamethonium (both at 5 mg/kg). Mecamylamine also inhibited the delayed increase in extracellular levels of 5-hydroxyindoleacetic acid, produced by the first injection of (-)-nicotine. These results suggest that (-)-nicotine, dose-dependently stimulated the release and metabolism of amine transmitters by an action at central nicotinic receptors. However, the precise site of action, i.e. at nerve terminals, cell bodies or both, requires further elucidation.

Beneficial effects of nicotine administered prior to a delayed matching-to-sample task in young and aged monkeys

Our earlier studies have demonstrated that administration of low micrograms/kg doses of nicotine to young adult monkeys prior to a delayed matching-to-sample (DMTS) task resulted in a centrally mediated improvement in performance of the task, particularly when delay intervals which most greatly challenged the animal's capabilities were involved. The present study confirmed these findings using a completely computer driven and automated procedure. In addition, performance on the DMTS was observed to be enhanced when animals were again tested 24 h after the dose of nicotine. Further analysis of the data indicated that the majority of enhancement to nicotine could be accounted for by a greatly increased performance at the least preferred stimulus color. Position preference (left vs. right stimulus) was not a factor in nicotine-induced enhancement. Two aged monkeys (34 years old Macaca mulatta) were significantly more difficult to train in the DMTS task and their longest delay capabilities were significantly shorter than the young animals (Macaca fascicularis). Nevertheless, the aged animals were essentially similar in most respects in their responses to nicotine administration. These data are consistent with a role for central nicotinic systems in memory performance and with the ability of nicotine to produce enhancement of selective features of mnemonic strategy in young and old monkeys. Furthermore, it is possible that either model, the aged animal, or the young animal stressed to his mnemonic capability may provide a good model for learning and memory disorders in humans.

Effect of nicotine and tacrine on acetylcholine release from rat cerebral cortical slices

The effect of nicotine (1-10 microM) and tacrine (9-amino-1,2,3,4-tetrahydroacridine; THA) on stimulation evoked release of [3H]acetylcholine from the rat brain slice preparation preincubated with [3H]choline was investigated. In these preparations, nicotine enhanced while tacrine inhibited evoked [3H]acetylcholine release. These effects were blocked by (+)tubocurarine (1 microM) and atropine (0.1 microM) respectively. In the presence of idazoxan (0.3 microM) plus atropine (0.1 microM), nicotine (3 microM) continued to enhance evoked [3H]acetylcholine release while the inhibitory effect of tacrine (1 microM) on evoked [3H]acetylcholine release was reversed to an enhancement. Under these circumstances the effects of both nicotine and tacrine were blocked by (+)tubocurarine (1 microM). These findings demonstrate that tacrine can both inhibit or enhance [3H]acetylcholine release, most likely through its activity as a cholinesterase inhibitor. Under normal circumstances following tacrine the predominant effect of the elevated levels of acetylcholine will be activation of inhibitory presynaptic muscarine receptors on cholinergic nerves and an inhibition of evoked [3H]acetylcholine release. Under conditions where both presynaptic inhibitory muscarine and alpha 2-adrenoceptors are blocked, the elevated levels of acetylcholine produced by tacrine will lead to the activation of facilitatory presynaptic nicotine cholinoceptors on cholinergic nerves and an enhancement of evoked [3H]acetylcholine release.

Effects of cholinergic drugs on extracellular levels of acetylcholine and choline in rat cortex, hippocampus and striatum studied by brain dialysis

A brain dialysis technique was used to investigate the effects of cholinergic drugs on acetylcholine (ACh) release and on the extracellular choline levels. Scopolamine (0.5 mg/kg s.c.) markedly increased ACh release in frontal cortex, hippocampus and corpus striatum. Conversely, it significantly decreased choline levels in extracellular spaces of these three regions. Oxotremorine (0.5 mg/kg i.p.) induced no significant ACh release in these three regions, probably due to the presence of highly concentrated physostigmine in the perfusate, while it induced an increase of the choline levels in both frontal cortex and hippocampus but not in corpus striatum. Nicotine (0.5 mg/kg s.c.) significantly increased the ACh release in these three regions with no change in choline levels. Nicotine had a biphasic effect on ACh release in frontal cortex and hippocampus but not in corpus striatum. It should be noted that all such cholinergic drugs, in our time course determinations, yielded certain differences among these brain regions for both the magnitude and the response pattern of ACh and choline levels. The present investigation appears to indicate that the in vivo brain dialysis technique can be useful for probing cholinergic neurotransmission of cholinergic drugs via presynaptic terminals.

Effect of acute and subchronic nicotine treatment on cortical acetylcholine release and on nicotinic receptors in rats and guinea-pigs

1. The effect of acute and chronic (16 days) administration of nicotine on cortical acetylcholine (ACh) release, gross behaviour and brain nicotinic binding sites was investigated in rats and guinea-pigs. 2. The drug, injected either subcutaneously (0.45-0.90 mg kg-1) or intracerebroventricularly (1, 3 and 5 micrograms) increased the cortical ACh release, in a dose-dependent manner, through mecamylamine-sensitive receptors for 1-2 h in both species. 3. Chronic treatment significantly increased basal ACh release in the rat and slightly lowered it in the guinea-pig, but the response to a challenging dose of nicotine was proportionally maintained in both species. 4. The number of nicotinic receptors was four times higher in the rat than in the guinea-pig and was not dependent on the radioligand used ([3H]-nicotine or [3H]-ACh, in the presence of atropine) to determine this. The nicotinic binding sites showed an apparent increase in chronically treated rats but no change in guinea-pigs. 5. Tolerance to the inhibitory effect of the drug, assessed with the T maze test, was found in the rat. No apparent change in gross behaviour was detected in the guinea-pig. 6. It is concluded that chronic nicotine treatment causes evident tolerance to its inhibitory effect on behaviour in the rat, but no adaptation to its excitatory properties on the cholinergic brain structures in rats and guinea-pigs.

Cholinomimetic drugs modify betamethasone suppression in the rat

This study was performed to investigate the effect of different cholinomimetic drugs (CD) on ACTH levels in betamethasone (BET) pretreated rats (40 mg/kg in the drinking water for 24 h). Animals were injected with atropine methylbromide (1 mg/kg, i.p.) and, after 10 min, with physostigmine (25, 50, 100, 200 micrograms/kg), arecoline (AR) or oxotremorine (OX) (150, 300 micrograms/kg) intravenously. They were killed 10 and 20 min after CD injection, to measure ACTH plasma levels. The lowest dose of AR and both doses of OX were able to induce an escape from BET suppression. The hypothesis that a stimulation of central muscarinic (and, probably, not nicotinic) receptors is involved in the CD-induced escape from glucocorticoid suppression is discussed.

Factors influencing the calculation of results from studies of the release of tritiated neurotransmitters from superfused slices of guinea pig striata

Slices of guinea pig striata were incubated with tritiated choline, dopamine, or serotonin and the release of radioactive transmitter was studied in a superfusion system. Some experiments were also done on the release of [3H]acetylcholine from rat striatal slices. For analysis of the results, various parameters of the system were determined in order to establish the most reliable method of assessing drug effects on transmitter release. Peaks of radioactivity (S1 and S2) above basal release of radioactivity (B1 and B2) were observed after two depolarizations of the nerve endings with high K+ buffer. Each stimulation was for 2 min, and S2 occurred 20 min after S1. There was a highly significant correlation between S1 and the protein content of the slices for acetylcholine release from guinea pig striata. Basal release of radioactivity, and the ratio S2/S1, were not sensitive to minor changes in the experimental conditions. It was concluded that S2/S1, rather than S1 or S1/B1, should be used as a measure of drug effects on release. The experiments also demonstrated that re-uptake of released neurotransmitter is operative in the superfusion system for dopamine, but not for serotonin. Differences were observed between the rat and the guinea pig with respect to the release of [3H]acetylcholine.

Control of transmitter release from the motor nerve by presynaptic nicotinic and muscarinic autoreceptors

Until recently, release studies have failed to indicate the existence of autoreceptors on motor nerves. Ignaz Wessler now reports on a refinement of the technique - the measurement of newly synthesized [3H]acetylcholine released from the phrenic nerve - which provides clear evidence in support of release-modulating autoreceptors. Presynaptic nicotinic receptors mediate a positive feedback mechanism, can rapidly be desensitized and appear to differ in their pharmacological profile from the postsynaptic receptors. In addition, inhibitory and facilitatory muscarinic receptors appear to be involved in the presynaptic control of transmitter release from the phrenic nerve.

Effects of nicotinic agonists and antagonists on N-methyl-D-aspartate-induced 3H-norepinephrine release and 3H-(1-[1-(2-thienyl)cyclohexyl]-piperidine) binding in rat hippocampus

The nicotinic agonists dimethylphenylpiperazinium iodide (DMPP) and carbachol (CARB) as well as (-)nicotine [-)NIC) were tested alone and in combination with N-methyl-D-aspartate (NMDA) for their abilities to enhance the efflux of 3H-norepinephrine (NE) from slices of rat hippocampus. CARB and (-)NIC produced small, transient increases in NE efflux, while DMPP produced larger, longlasting increases. Inasmuch as the nicotinic antagonists mecamylamine (MECA) and hexamethonium (C6) did not consistently inhibit the increases in NE efflux produced by these agonists, the role of a nicotinic receptor in mediating these responses is uncertain. CARB and DMPP enhanced the ability of NMDA to stimulate NE release, while (-)NIC did not. MECA, but not C6, was found to selectively antagonize NMDA-stimulated NE release that did not appear to involve a nicotinic receptor. Binding studies indicated that MECA and the related nicotinic antagonist pempidine produced an inhibition of NMDA-stimulated NE release by an action at the PCP receptor that is known to be linked to the NMDA receptor-ionophore complex. These data suggest that the actions of these ganglionic blocking agents on excitatory responses in the hippocampus involve inhibition of excitatory amino acid as well as nicotinic receptors.

Characterization of N-[3H]methylcarbamylcholine binding sites and effect of N-methylcarbamylcholine on acetylcholine release in rat brain

The present experiments show that N-[3H]-methylcarbamylcholine ([3H]MCC) binds specifically and with high affinity to rat hippocampus, frontal cortex, and striatum. The highest maximal density of binding sites was apparent in frontal cortex and the lowest in hippocampus. [3H]MCC binding was potently inhibited by nicotinic, but not muscarinic, agonists and by the nicotinic antagonist dihydro-beta-erythroidine in all three brain regions studied. The effect of unlabeled MCC on acetylcholine (ACh) release from slices of rat brain was tested. The drug significantly enhanced spontaneous ACh release from slices of hippocampus and frontal cortex, but not from striatal slices. This effect of MCC to increase ACh release from rat hippocampus and frontal cortex was antagonized by the nicotinic antagonists dihydro-beta-erythroidine and d-tubocurarine, but not by alpha-bungarotoxin or by the muscarinic antagonist atropine. The MCC-induced increase in spontaneous ACh release from hippocampal and frontal cortical slices was not affected by tetrodotoxin. The results suggest that MCC might alter cholinergic transmission in rat brain by a direct activation of presynaptic nicotinic receptors on the cholinergic terminals. That this alteration of ACh release is apparent in hippocampus and frontal cortex, but not in striatum, suggests that there may be a regional specificity in the regulation of ACh by nicotinic receptors in rat brain.

Scopolamine induces up-regulation of nicotinic receptors in intact brain but not in nucleus basalis lesioned rats

The effect of chronic scopolamine treatment on muscarinic and nicotinic receptors in frontoparietal cortex in rats was investigated. Administration of the muscarinic antagonist, scopolamine (10 mg/kg i.p./day) for 21 days, produced a significant increase in the density of both muscarinic and nicotinic receptors by 27.7% and 12.1% respectively as measured by the specific binding of (-)-[3H]quinuclidinylbenzilate and (-)-[3H]-nicotine. There was no modification in the affinities for these ligands. Rats, bilaterally lesioned with ibotenic acid at the level of nucleus basalis of Meynert, which innervates the frontoparietal cortex, showed no up-regulation of cortical nicotinic receptors after chronic scopolamine treatment, suggesting the importance of the synaptic integrity in the regulation mechanism.

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.

Change in nicotinic receptor subtypes in temporal cortex of Alzheimer brains

Competition experiments using (-)-[3H]nicotine and unlabelled nicotine revealed both high and low affinity nicotinic binding sites in temporal cortex of control and Alzheimer (AD/SDAT) brains. A significant reduction in the proportion of high affinity nicotinic binding sites (-20%) and a parallel increase in the proportion of low affinity nicotinic binding sites was obtained in AD/SDAT brain cortex compared to control brain. Moreover, a marked decrease was observed in the affinity of the low affinity nicotinic binding sites in AD/SDAT.

Effects of nucleus basalis lesions on the muscarinic and nicotinic modulation of [3H]acetylcholine release in the rat cerebral cortex

Presynaptic muscarinic and nicotinic receptors in the cerebral cortex reportedly inhibit and increase acetylcholine (ACh) release, respectively. In this study, we investigated whether these receptors reside on cholinergic nerve terminals projecting to the cerebral cortex from the nucleus basalis magnocellularis (nbm). Adult male rats received unilateral infusions of ibotenic acid (5 micrograms/1 microliter) in the nbm. Two weeks later, cerebral cortical cholinergic markers (choline acetyltransferase activity, high-affinity choline uptake, and coupled ACh synthesis) were significantly reduced in synaptosomes prepared from the lesioned hemispheres compared to contralateral controls. The depolarization-induced release of [3H]ACh from these synaptosomes was also reduced in the lesioned hemispheres, reflecting the reduced synthesis of transmitter. However, the nbm lesions had no effect on the inhibition of release induced by 100 microM oxotremorine. Synaptosomal [3H]ACh release was not altered by nicotine or the nicotinic agonists anabaseine and 2-(3-pyridyl)-1,4,5,6-tetrahydropyrimidine. Nicotine (10-100 microM) did increase [3H]ACh release in control and lesioned hemispheres in cortical minces, but to a similar extent. These results suggest that neither muscarinic nor nicotinic receptors modulating ACh release reside on nbm-cholinergic terminals.

Influence of N-allyl-normetazocine on acetylcholine release from brain slices: involvement of muscarinic receptors

The effects of (+/-)N-allyl-normetazocine on the release of acetylcholine from different areas of guinea-pig and rat brain were investigated. 1. The drug did not modify the electrically (2 Hz) evoked tritium efflux from guinea-pig cerebral cortex, thalamus and caudate nucleus slices, preloaded with 3H-choline 0.1 mumol/l and superfused with Krebs solution containing hemicholinium-3 10 mumol/l. 2. (+/-)N-allyl-normetazocine 10 mumol/l enhanced the evoked 3H efflux from guinea-pig brain slices superfused with Krebs solution containing physostigmine 30 mumol/l or oxotremorine 0.3-1 mumol/l; the effect was naloxone-insensitive and was abolished by atropine 0.15 mumol/l, but not by pirenzepine 1 mumol/l. 3. (+/-)N-allyl-normetazocine 5 mumol/l enhanced the electrically evoked release of endogenous acetylcholine as well, in a naloxone-insensitive way. 4. Both (+/-) and (+)N-allyl-normetazocine were without effect on 3H efflux from rat caudate nucleus slices electrically stimulated at 0.2 Hz frequency, after preloading with 3H-choline and during superfusion with hemicholinium-3. 5. The results are discussed in view of the antimuscarinic properties of the drug.

Do tetrahydroaminoacridine (THA) and physostigmine restore acetylcholine release in Alzheimer brains via nicotinic receptors?

In the presence of 9-amino-1,2,3,4-tetrahydroacridine (THA) 10(-4) M or physostigmine 10(-4) M, the in vitro 3H-Acetylcholine (3H-ACh) release from control cortical slices was significantly reduced. In contrast, THA 10(-4) M and physostigmine 10(-4) M significantly increased the release of 3H-ACh in AD/SDAT brain tissue. This facilitating effect on 3H-ACh release was partially blocked (50%) in the presence of the nicotinic antagonist d-tubocurarine 10(-6) M indicating a possible interaction via nicotinic receptors. The muscarinic antagonist atropine 10(-5) M significantly increased the 3H-ACh release both in control and AD/SDAT brains, thus indicating preservation of muscarinic autoreceptors in the AD/SDAT cortical tissue. In receptor competition studies with 3H-nicotine, 3H-ACh and 3H-quinuclidinyl benzilate (3H-QNB) as receptor ligands, THA interfered with both nicotinic and muscarinic receptor ligand binding, while physostigmine had much less effect.

In vitro binding of 3H-acetylcholine to nicotinic receptors in rodent and human brain

The binding of 3H-acetylcholine (3H-ACh) to nicotinic receptors in rodent and human brain was measured in the presence of atropine to prevent binding to muscarinic binding sites. 3H-ACh binds specifically and saturably to rodent brain. From saturation binding Kd was 30 nM in rat cerebral cortex, which is close to that calculated from kinetic experiments. The binding was temperature-dependent, being highest at low temperatures and decreasing at higher temperatures. The regional distribution of binding in mouse brain was not uniform. The binding was highest in the midbrain, intermediate in the cerebral cortex and striatum, and lowest in the cerebellum, hippocampus, hypothalamus and medulla oblongata. No significant correlation was found between the regional 3H-ACh binding and the regional binding of 3H-alpha-bungarotoxin (3H-BTX), 3H-nicotine (3H-NIC), 3H-tubocurarine and the endogenous acetylcholine content, although the correlation value for 3H-ACh/3H-NIC was at the limit for significance. 3H-ACh also bound specifically to human cerebral cortical tissue and this binding was approximately three times lower than in rodent brain, when a low 3H-ACh concentration was used. In contrast to rat brain there appears to exist multiple binding sites for 3H-ACh in human cerebral cortex as suggested by the curvelinear nature of the Scatchard plot. It was calculated that 3H-ACh bound with Kd 4 nM and Bmax 8 pmol/g protein and Kd 112 nM and Bmax 67 pmol/g protein. The Hill number of 1.5 for the binding of low concentration and 2.5 for high concentration of 3H-ACh also suggest that the 3H-ACh-binding sites interaction exhibit positive cooperativity.

Reduced number of [3H]nicotine and [3H]acetylcholine binding sites in the frontal cortex of Alzheimer brains

Nicotinic cholinergic receptors were measured in human frontal cortex using [3H]nicotine and [3H]acetylcholine (in the presence of atropine) as receptor ligands. A parallel marked reduction in number of [3H]nicotine (52%; P less than 0.01) and [3H]acetylcholine (-55%; P less than 0.05) binding was found in the frontal cortex of Alzheimer brains (AD/SDAT) when compared to age-matched control brains. As a comparison the number of muscarinic receptors was quantified using [3H]quinuclidinyl benzilate and found to be significantly increased (+23%; less than 0.01) in AD/SDAT compared to controls.

Presynaptic nicotine receptors mediating a positive feed-back on transmitter release from the rat phrenic nerve

The effects of 1,1-dimethyl-4-phenylpiperazinium (DMPP) and of nicotine receptor antagonists on [3H]acetylcholine release from the rat phrenic nerve preincubated with [3H]choline were investigated in the absence and presence of cholinesterase inhibitors (presynaptic effects). Additionally, the effects of hexamethonium and tubocurarine on the muscle contraction of the indirectly stimulated diaphragm were examined (postsynaptic effects). DMPP (1-30 microM) increased (76-92%), whereas hexamethonium (0.001-1 mM) and tubocurarine (1-10 microM) decreased (52-60%) the release of [3H]acetylcholine following a train of 100 pulses at 5 Hz. The release caused by a longer train (750 pulses at 5 Hz) was only slightly affected by DMPP and tubocurarine. In the presence of neostigmine (10 microM) neither tubocurarine nor DMPP significantly modulated the evoked [3H]acetylcholine release. High DMPP concentrations (10 and 30 microM) enhanced the evoked release only when the pretreatment interval was reduced from 15 min to 20 s. Tubocurarine and hexamethonium concentration-dependently inhibited the end-organ response. Hexamethonium was 250-fold more potent on presynaptic than on postsynaptic nicotine receptors. It is concluded that the motor nerve terminals are endowed with presynaptic nicotine receptors. These autoreceptors mediate a positive feed-back mechanism that can be triggered by previously released endogenous acetylcholine. Receptor desensitization can be produced by high agonist concentrations (endogenous or exogenous agonists) and is probably one mechanism to limit the autofacilitatory process. The presynaptic receptors appear to differ in their pharmacological properties from the postsynaptic receptors.

Physostigmine restores 3H-acetylcholine efflux from Alzheimer brain slices to normal level

A technique was developed, which made it possible to study the in vitro release of 3H-acetylcholine (3H-ACh) from human post mortem brain tissue, collected with short post-mortem delay (2,5-22 hours), both from controls and patients with Alzheimer's disease (AD/SDAT). The tritium (3H) release was investigated during potassium stimulation, and AD/SDAT cortical slices were found to release a decreased amount of 3H compared to control brain slices. Physostigmine, 10(-5) and 10(-4) M, has no significant effect on the release of 3H from control slices, while physostigmine 10(-4) M increased the evoked release from AD/SDAT brain slices over threefold, nearly to the control level.

Subchronic treatment of rats with nicotine: effects on tolerance and on [3H]acetylcholine and [3H]nicotine binding in the brain

The effects of subchronic nicotine treatment on the development of tolerance and on two nicotinic ligand binding sites were investigated. After subcutaneous injection of nicotine (0.45 mg nicotine base/kg) or saline twice a day for 14 days the body weight of rats was significantly lower than that of control animals. A significant tolerance to the acute effects of nicotine on locomotor activity and body temperature was observed after the treatment period. Nicotine treatment also resulted in a significant increase in [3H]acetylcholine (3H-ACh) binding in the midbrain (48.3% increase in comparison with controls) and hippocampus (38.3% increase), whereas the binding of [3H]nicotine (3H-NIC) was unaffected in all brain areas investigated. These results indicate that subchronic s.c. injections of nicotine can differentially affect the binding of two different nicotinic ligands in the brain. It is also concluded that the development of tolerance to the acute effects of nicotine on locomotor activity and temperature is not directly dependent upon changes in binding of [3H]nicotine to the brain.