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

Participation of the Cholinergic System in the Development of Polycystic Ovary Syndrome

In rats with polycystic ovary syndrome (PCOS) induced by injection of estradiol valerate (EV), unilateral or bilateral section of the vagus nerve restores ovulatory function in 75% of animals, suggesting that the vagus nerve participates in the development of PCOS. Since the vagus nerve is a mixed nerve through which mainly cholinergic-type information passes, the objective of the present study was to analyze whether acetylcholine (ACh) is involved in the development of PCOS. Ten-day-old rats were injected with 2.0 mg EV, and at 60 days of age, they were microinjected on the day of diestrus in the bursa of the left or right ovary with 100 or 700 mg/kg of ovarian weight atropine, a blocker of muscarinic receptors, and sacrificed for histopathological examination after the surgery. Animals with PCOS microinjected with 100 mg of atropine showed a lack of ovulation, lower serum concentrations of progesterone and testosterone, and cysts. Histology of the ovaries of animals microinjected with 700 mg of atropine showed corpus luteum and follicles at different stages of development, which was accompanied by a lower concentration of progesterone and testosterone. These results allow us to suggest that in animals with PCOS, ACh, which passes through parasympathetic innervation, is an important component in the persistence and development of the pathophysiology.

PET Imaging Estimates of Regional Acetylcholine Concentration Variation in Living Human Brain

Acetylcholine (ACh) has distinct functional roles in striatum compared with cortex, and imbalance between these systems may contribute to neuropsychiatric disease. Preclinical studies indicate markedly higher ACh concentrations in the striatum. The goal of this work was to leverage positron emission tomography (PET) imaging estimates of drug occupancy at cholinergic receptors to explore ACh variation across the human brain, because these measures can be influenced by competition with endogenous neurotransmitter. PET scans were analyzed from healthy human volunteers (n = 4) and nonhuman primates (n = 2) scanned with the M1-selective radiotracer [11C]LSN3172176 in the presence of muscarinic antagonist scopolamine, and human volunteers (n = 10) scanned with the α4β2* nicotinic ligand (-)-[18F]flubatine during nicotine challenge. In all cases, occupancy estimates within striatal regions were consistently lower (M1/scopolamine human scans, 31 ± 3.4% occupancy in striatum, 43 ± 2.9% in extrastriatal regions, p = 0.0094; nonhuman primate scans, 42 ± 26% vs. 69 ± 28%, p < 0.0001; α4β2*/nicotine scans, 67 ± 15% vs. 74 ± 16%, p = 0.0065), indicating higher striatal ACh concentration. Subject-level measures of these concentration differences were estimated, and whole-brain images of regional ACh concentration gradients were generated. These results constitute the first in vivo estimates of regional variation in ACh concentration in the living brain and offer a novel experimental method to assess potential ACh imbalances in clinical populations.

First-in-Human Assessment of 11C-LSN3172176, an M1 Muscarinic Acetylcholine Receptor PET Radiotracer

This was a first-in-human study of the PET radiotracer ¹¹C-LSN3172176 for the muscarinic acetylcholine receptor subtype M1. The objectives of the study were to determine the appropriate kinetic model to quantify binding of the tracer to M1 receptors, and the reliability of the chosen quantification method. Methods: Six healthy subjects completed the test-retest protocol, and 5 healthy subjects completed the baseline-scopolamine blocking protocol. Multiple modeling methods were applied to calculate total distribution volume (V _T) and nondisplaceable binding potential (BP _ND) in various brain regions. The reference region was selected from the blocking study. The occupancy plot was applied to compute receptor occupancy by scopolamine and nondisplaceable distribution volume. Results: Tracer uptake was highest in the striatum, followed by neocortical regions and white matter, and lowest in the cerebellum. Regional time-activity curves were fitted well by all models. The 2-tissue-compartment (2TC) model fits were good, but the 2TC parameters often could not be reliably estimated. Because V _T correlated well between the 2TC and 1-tissue-compartment (1TC) models after exclusion of unreliable estimates, the 1TC model was chosen as the most appropriate. The cerebellum showed the lowest V _T, consistent with preclinical studies showing little to no specific binding in the region. Further, cerebellar V _T did not change between baseline and blocking scans, indicating that the cerebellum is a suitable reference region. The simplified reference tissue model (SRTM) slightly underestimated 1TC BP _ND, and the simplified reference tissue model 2 (SRTM2) improved BP _ND estimation. An 80-min scan was sufficient to quantify V _T and BP _ND The test-retest study showed excellent absolute test-retest variability for 1TC V _T (≤5%) and BP _ND (≤10%). In the baseline and blocking studies, occupancy values were lower in the striatum than in nonstriatal regions, as may be attributed to differences in regional acetylcholine concentrations. Conclusion: The 1TC and SRTM2 models are appropriate for quantitative analysis of ¹¹C-LSN3172176 imaging data. ¹¹C-LSN3172176 displayed excellent test-retest reproducibility and is a highly promising ligand to quantify M1 receptors in the human brain.

Cocaine, nicotine, and their conditioned contexts enhance consolidation of object memory in rats

To test the hypothesis that drugs of abuse and their conditioned stimuli (CSs) enhance memory consolidation, the effects of post-training exposure to cocaine and nicotine were compared to the effects of post-training exposure to contextual stimuli that were paired with the effects of these drugs. Using the object recognition (OR) task, it was first demonstrated that both 10 and 20 mg/kg cocaine, and 0.2 and 0.4 mg/kg nicotine, enhanced recognition memory when administered immediately after, but not 6 h after the sample phase. To establish the drug CSs, rats were confined for 2 h in a chamber (the CS+) after injections of 20 mg/kg cocaine, or 0.4 mg/kg nicotine, and in another chamber (the CS−) after injections of vehicle. This was repeated over 10 d (5 drug/CS+ and 5 vehicle/CS− pairings in total). At the end of this conditioning period, when tested in a drug-free state, rats displayed conditioned hyperactivity in the CS+ relative to the CS−. More important, immediate, but not delayed, post-sample exposure to the cocaine CS+, or nicotine CS+, enhanced OR memory. Therefore, this study reports for the first time that contextual stimuli paired with cocaine and nicotine, like the drugs themselves, have the ability to enhance memory consolidation.

Choline availability and acetylcholine synthesis in the hippocampus of acetylcholinesterase-deficient mice

Mice deficient for acetylcholinesterase (AChE) have strongly increased extracellular levels of acetylcholine (ACh) in the dorsal hippocampus [Hartmann, J., Kiewert, C., Duysen, E.G., Lockridge, O., Greig, N.H., Klein, J., 2007. Excessive hippocampal acetylcholine levels in acetylcholinesterase-deficient mice are moderated by butyrylcholinesterase activity. J. Neurochem. 100, 1421-1429]. Using microdialysis, we found that increased ACh levels are accompanied by decreased levels of extracellular choline which were 1.60 microM in AChE-deficient mice and 4.36 microM in wild-type mice. Addition of choline (10 microM) to the perfusion fluid, while ineffective in wild-type animals, more than doubled extracellular ACh levels in AChE-deficient mice. High-affinity choline uptake (HACU), as measured ex vivo in corticohippocampal synaptosomes, was more than doubled in AChE-deficient mice. Inhibition of HACU by hemicholinium-3 (HC-3) in vivo reduced extracellular levels of ACh by 60% in wild-type mice but by more than 90% in AChE-deficient mice. Decreased ACh levels caused by infusion of HC-3 or tetrodotoxin (TTX) were accompanied by increased levels of free choline. Infusion of scopolamine (1 microM) caused a fivefold increase of ACh levels in wild-type animals but only a 50% increase in AChE-deficient mice. In conclusion, absence of AChE causes dynamic changes in the ratio of choline to ACh. High levels of extracellular ACh are accompanied by reduced levels of extracellular choline, and ACh release becomes strongly dependent on choline availability. Similar changes may take place in patients chronically exposed to AChE inhibitors.

Selective blockade of dopamine D(3) versus D(2) receptors enhances frontocortical cholinergic transmission and social memory in rats: a parallel neurochemical and behavioural analysis

Though dopaminergic mechanisms modulate cholinergic transmission and cognitive function, the significance of specific receptor subtypes remains uncertain. Here, we examined the roles of dopamine D(3) versus D(2) receptors. By analogy with tacrine (0.16-2.5 mg/kg, s.c.), the selective D(3) receptor antagonists, S33084 (0.01-0.63) and SB277,011 (0.63-40.0), elicited dose-dependent, pronounced and sustained elevations in dialysis levels of acetylcholine (ACh) in the frontal cortex, but not the hippocampus, of freely-moving rats. The actions of these antagonists were stereospecifically mimicked by (+)S14297 (1.25), whereas its inactive distomer, (-)S17777, was ineffective. The preferential D(2) receptor antagonist, L741,626 (10.0), failed to modify levels of ACh. S33084 (0.01-0.63) and SB277,011 (0.16-2.5) also mimicked tacrine (0.04-0.63) by dose-dependently attenuating the deleterious influence of scopolamine (1.25) upon social memory (recognition by an adult rat of a juvenile conspecific). Further, (+)S14297 (1.25) versus (-)S17777 stereospecifically blocked the action of scopolamine. Using an intersession interval of 120 min (spontaneous loss of recognition), S33084 (0.04-0.63), SB277,011 (0.16-10.0) and (+)S14297 (0.63-10.0) likewise mimicked tacrine (0.16-2.5) in enhancing social memory. In contrast, L741,626 (0.16-10.0) displayed amnesic properties. In conclusion, selective blockade of D(3) receptors facilitates frontocortical cholinergic transmission and improves social memory in rats. These data support the pertinence of D(3) receptors as a target for treatment of disorders in which cognitive function is compromised.

Role of presynaptic nicotinic acetylcholine receptors in the regulation of gastrointestinal motility

Presynaptic nicotinic acetylcholine receptors (nAChRs) located on cholinergic terminals facilitate the release of acetylcholine (ACh), thereby constituting a fail-safe mechanism at strategic locations, such as the neuromuscular junction, where reliable transmission is vital. Accumulating data indicate that myenteric neurons in the enteric nervous system possess not only somatodendritic nAChRs, which mediate cholinergic transmission between neurons, but also presynaptic nAChRs. Functional evidence shows that these receptors mediate a positive feedback with respect to ACh release from myenteric motoneurons, and might therefore play an important role in the regulation of gastrointestinal motility. These presynaptic nAChRs were found to be more sensitive to nicotinic ligands than somatodendritic nAChRs and could therefore be primary targets of exogenous compounds, such as nicotine. This interaction might provide a neurochemical basis for the effect of smoking on gastrointestinal motility. Another important human pharmacological implication is based on our recent observation that monoamine uptake inhibitor-type antidepressant drugs are able to inhibit presynaptic nAChRs in the enteric nervous system. The disruption of the nAChR-mediated positive feedback modulation by antidepressants might explain the frequent occurrence of constipation, a common side effect, attributed to these drugs. Clarification of the role of presynaptic nAChRs in feedback mechanisms in the enteric nervous system might be instrumental in the development of new drugs affecting gastrointestinal motility.

Inhibition of acetaldehyde metabolism decreases acetylcholine release in medial frontal cortex of freely moving rats

The effect of high acetaldehyde (ACe) on acetylcholine (ACh) release was studied in vivo in the medial frontal cortex (mfc) of freely moving rats using brain microdialysis coupled with high performance liquid chromatography and an electrochemical detector. Ethanol (EtOH) and ACe concentrations were quantified simultaneously in the mfc of awake rats by in vivo microdialysis followed by head-space gas chromatography. Rats were treated intraperitoneally with saline, EtOH (1 and 2 g/kg) or cyanamide (CY, 50 mg/kg, a potent aldehyde dehydrogenase inhibitor) plus EtOH (1 and 2 g/kg). No significant effect on ACh levels was observed in saline groups, as compared to baseline value. The basal level of ACh in the dialysate was about 0.30 +/- 0.04 pmol/20 microl, and this value was reduced significantly in the EtOH (1 and 2 g/kg) and CY + EtOH (1 and 2 g/kg) groups for 240 min after EtOH administration. The time courses of ACh release continued to decrease significantly after EtOH administration in the CY + EtOH (1 and 2 g/kg) groups compared to the values in the saline and EtOH (1 and 2 g/kg) groups. A significant decrease in ACh release was observed from 140 to 240 min after EtOH dosing in the EtOH (1 and 2 g/kg) groups, as compared to saline groups. EtOH and ACe concentrations in the mfc were first determined at 15 min after a dose of EtOH, reached a peak at 30 min and then gradually decreased in the CY + EtOH (1 and 2 g/kg) groups. The present study suggests that both EtOH and ACe concentration in the brain can decrease in vivo ACh release in the mfc of free-moving rats, and the ACe-induced decrease in ACh levels was significantly higher than EtOH.

Nicotine-induced changes in neurotransmitter levels in brain areas associated with cognitive function

Nicotine, one of the most widespread drugs of abuse, has long been shown to impact areas of the brain involved in addiction and reward. Recent research, however, has begun to explore the positive effects that nicotine may have on learning and memory. The mechanisms by which nicotine interacts with areas of cognitive function are relatively unknown. Therefore, this paper is part of an ongoing study to evaluate regional effects of nicotine enhancement of cognitive function. Nicotine-induced changes in the levels of three neurotransmitters, dopamine (DA), serotonin (5-HT), norepinepherine (NE), their metabolites, homovanillic acid (HVA), dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindoleacetic acid (5-HIAA), and their precursor, L-DOPA, were evaluated in the ventral and dorsal hippocampus (VH and DH), prefrontal and medial temporal cortex (PFC and MTC), and the ventral tegmental area (VTA) using in vivo microdialysis in awake, freely moving, male Sprague-Dawley rats. The animals were treated with acute nicotine (0.5 mg/kg, s.c.) halfway through the 300-min experimental period. The reuptake blockers, desipramine (100 microM) and fluoxetine (30 microM), were given to increase the levels of NE and 5-HT so that they could be detected. Overall, a nicotine-induced DA increase was found in some areas, and this increase was potentiated by desipramine and fluoxetine. The two DA metabolites, HVA and DOPAC, increased in all the areas throughout the experiments, both with and without the inhibitors, indicating a rapid metabolism of the released DA. The increase in these metabolites was greater than the increase in DA. 5-HT was increased in the DH, MTC, and VTA in the presence of fluoxetine; its metabolite, 5-HIAA, was increased in the presence and absence of fluoxetine. Except in the VTA, NE levels increased to a similar extent with desipramine and fluoxetine. Overall, nicotine appeared to increase the release and turnover of these three neurotransmitters, which was indicated by significant increases in their metabolites. Furthermore, DA, and especially HVA and DOPAC, increased for the 150 min following nicotine administration; 5-HT and NE changes were shorter in duration. As gas chromatography experiments showed that nicotine levels in the brain decreased by 75% after 150 min, this may indicate that DA is more susceptible to lower levels of nicotine than 5-HT or NE. In conclusion, acute nicotine administration caused alterations in the levels of DA, 5-HT, and NE, and in the metabolism of DA and 5-HT, in brain areas that are involved in cognitive processes.

Central blockade of muscarinic cholinergic receptors disrupts affective and attentional set-shifting

Impairments in multiple aspects of attentional and executive function follow damage to cholinergic neurons in the central nervous system. Affective and attentional set-shifting represent two aspects of executive function controlled by different sectors of the prefrontal cortex. The involvement of cholinergic neural mechanisms in these aspects of executive function has not been specified. To determine whether central muscarinic cholinergic receptors were involved in affective and/or attentional set-shifting, we tested rats on a series of discrimination learning problems, which included affective (reversal learning) and attentional set (extradimensional shift)-shifting components, under the systemic influence of scopolamine, a muscarinic antagonist. Scopolamine impaired both reversal learning and extradimensional shifting, but was without effect on learning new discrimination problems that did not require an affective or attentional shift. Systemic administration of methylscopolamine, which does not cross the blood-brain barrier, did not impair affective or attentional set-shifting, indicating that the scopolamine effects were centrally mediated. These data implicate muscarinic receptors in the central nervous system in the control of executive function. Taken together with other recent data, they may also suggest an important role for cholinergic receptors outside of the neocortex in regulating these aspects of attention and executive function.

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.

Piribedil enhances frontocortical and hippocampal release of acetylcholine in freely moving rats by blockade of alpha 2A-adrenoceptors: a dialysis comparison to talipexole and quinelorane in the absence of acetylcholinesterase inhibitors

In a dialysis procedure not requiring perfusate addition of acetylcholinesterase inhibitors to "boost" basal levels of acetylcholine (ACh), the influence of the antiparkinson agent piribedil upon levels of ACh in frontal cortex and dorsal hippocampus of freely moving rats was compared with those of other antiparkinson drugs and selective ligands at alpha(2)-adrenoceptors (ARs). Suggesting a tonic, inhibitory influence of alpha(2A)-ARs upon cholinergic transmission, the alpha(2)-AR agonist 5-bromo-6-[2-imidazolin-2-yl-amino]-quinoxaline tartrate (UK14,304), and the preferential alpha(2A)-AR agonist guanabenz reduced levels of ACh. They were elevated by the antagonists 2(2-methoxy-1,4 benzodioxan-2-yl)-2-imidazoline HCl (RX821002) and atipamezole and by the preferential alpha(2A)-AR antagonist 2-(2H-(1-methyl-1,3-dihydroisoindole)methyl)-4,5-dihydroimidazole (BRL44008). In contrast, trans-2,3,9,13b-tetrahydro-1,2-dimethyl-1H-dibenz[c,f]imidazo[1,5-a]azepine (BRL41992) and prazosin, preferential alpha(2B/2C)-AR antagonists, were inactive. The dopaminergic agonist and antiparkinson agent piribedil, which behaves as an antagonist at alpha(2)-ARs, dose dependently increased extracellular levels of ACh. This action was absent upon pretreatment with a maximally effective dose of RX821002. On the other hand, a further dopaminergic agonist and antiparkinson agent, talipexole, which possesses agonist properties at alpha(2)-ARs, dose dependently reduced levels of ACh. This action was also blocked by RX821002. In contrast to piribedil and talipexole, quinelorane, which interacts with dopaminergic receptors but not alpha(2)-ARs, failed to affect ACh levels. Finally, in analogy to the frontal cortex, piribedil likewise elicited a dose-dependent increase in extracellular levels of ACh in the dorsal hippocampus. In conclusion, in distinction to talipexole and quinelorane, and reflecting its antagonist properties at alpha(2A)-ARs, piribedil reinforces cholinergic transmission in the frontal cortex and dorsal hippocampus of freely moving rats. These actions may be related to its facilitatory influence upon cognitive function.

Selective 5-HT1A antagonists WAY 100635 and NAD-299 attenuate the impairment of passive avoidance caused by scopolamine in the rat

Systemic administration of the muscarinic-receptor antagonists atropine and scopolamine produces cognitive deficits in humans, nonhuman primates and rodents. In humans, these deficits resemble symptoms of dementia seen in Alzheimer's disease. The passive avoidance (PA) task has been one of the most frequently used animal models for studying cholinergic mechanisms in learning and memory. The present study examined the ability of two selective 5-HT(1A) receptor antagonists WAY 100635 and NAD-299 (robalzotan) and two acetylcholinesterase (AChE) inhibitors tacrine and donepezil to attenuate the impairment of PA retention caused by the nonselective muscarinic receptor antagonist scopolamine in the rat. Although demonstrating differences in their temporal kinetics, both WAY 100635 and NAD-299 attenuated the impairment of PA caused by scopolamine (0.3 mg/kg s.c.). Donepezil did not block the PA deficit caused by the 0.3 mg/kg dose of scopolamine, but it prevented the inhibitory effects of the 0.2 mg/kg dose of scopolamine. In contrast, tacrine was effective vs both the 0.2 and 0.3 mg/kg doses of scopolamine. These results indicate that (1). a functional 5-HT(1A) receptor antagonism can attenuate the anterograde amnesia produced by muscarinic-receptor blockade, and (2). the AChE inhibitors tacrine and donepezil differ in their ability to modify muscarinic-receptor-mediated function in vivo. These results suggest that 5-HT(1A) receptor antagonists may have a potential in the treatment of cognitive symptoms in psychopathologies characterized by reduced ACh transmission such as Alzheimer's disease.

Neurobiology of the nicotine withdrawal syndrome

The aversive aspects of withdrawal from chronic nicotine exposure are thought to be an important motivational factor contributing to the maintenance of the tobacco habit in human smokers. Much emphasis has been placed on delineating the underlying neurobiological mechanisms mediating different components of the nicotine withdrawal syndrome. Recent studies have shown that both central and peripheral populations of nicotinic acetylcholine receptors (nAChRs) are involved in mediating somatic signs of nicotine withdrawal as measured by the rodent nicotine abstinence scale. However, only central populations of nAChRs are involved in mediating affective aspects of nicotine withdrawal, as measured by elevations in brain-stimulation reward thresholds and conditioned place aversion. Nicotine interacts with several neurotransmitter systems, including acetylcholine, dopamine, opioid peptides, serotonin, and glutamate systems. Evidence so far suggests that these neurotransmitters play a role in nicotine dependence and withdrawal processes. The available evidence also suggests that different underlying neurochemical deficits mediate somatic and affective components of nicotine withdrawal. The aim of the present review is to discuss preclinical findings concerning the neuroanatomical and neurochemical substrates involved in these different aspects of nicotine withdrawal.

Effect of chronic stress on cholinergic transmission in rat hippocampus

We previously demonstrated that chronic stress impaired prefrontal cortex-sensitive working memory, but not reference memory. Since the hippocampal cholinergic system is also involved in these memories, we examined the effects of chronic stress on cholinergic transmission in the rat hippocampus. A microdialysis study revealed that the stress did not affect the basal acetylcholine release, but enhanced the KCl-evoked response. These results suggest that cholinergic transmission in the chronically stressed hippocampus does not contribute to working memory impairment, but it may be involved in maintenance of reference memory.

The effect of nitric oxide on acetylcholine release in the rabbit bladder

We evaluated the effects of nitric oxide (NO) on acetylcholine release and the contractile response induced by electrical field stimulation in rabbit bladder smooth muscles using a muscle bath and high performance liquid chromatography coupled with microdialysis. Electrical field stimulation (supramaximum voltage, pulse duration 0.5 ms, frequency 5 and 20 Hz) was applied to a smooth muscle strip isolated from rabbit bladder. With low-frequency (5 Hz) stimulation, pretreatment with Nomega-nitro-L-arginine (L-NNA) (100 microM) significantly increased electrical field stimulation-induced acetylcholine release and contractile response, which were reduced by the addition of L-arginine. Pretreatment with sodium nitroprusside in the absence or presence of L-NNA significantly decreased electrical field stimulation-induced acetylcholine release and contractile response. In contrast, with high frequency (20 Hz) stimulation, pretreatment with L-NNA and sodium nitroprusside had no significant effect on either contractile response or acetylcholine release. Pretreatment with sodium nitroprusside caused no significant changes in carbachol and ATP-induced contractile responses. Sodium nitroprusside and L-NNA had no significant effects on the atropine-resistant part of the contraction induced by electrical field stimulation in rabbit bladder smooth muscles. The results suggest that there is a NO-mediated mechanism inhibiting acetylcholine release from cholinergic nerve endings in rabbit bladder, which may contribute to bladder function.

Acetylcholine release in the hippocampus of the urethane anaesthetised rat positively correlates with both peak theta frequency and relative power in the theta band

The need to achieve a clearer understanding of relations between hippocampal theta characteristics and cholinergic septohippocampal neuron activity, prompted us to re-examine, in the urethane-anaesthetised rat, the statistical relationships between the electrophysiological and neurochemical variables using a procedure which is believed to enhance significantly the degree of confidence with which parameters of theta recorded with classic macroelectrodes can be related to concomitant acetylcholine output measured by high-performance liquid chromatography with electrochemical detection. Firstly, the theta rhythm and the acetylcholine content were derived from the same hippocampus. Secondly, the hippocampal electroencephalogram was quantified using spectral analysis which permits the more objective quantitative evaluation of selected electroencephalogram samples. Thirdly, a larger number of rats than in our previous study was used here, thus enhancing the validity of statistical results. This procedure yielded, in our time-course determination, two main findings. The first finding is that acetylcholine release was positively correlated with frequency at the peak power of the theta band which reflects the frequency of the theta signal. This finding had not been reported yet. The second finding is that hippocampal acetylcholine outflow also covaried with relative power of the theta band which reflects the amplitude of the theta signal. This finding is consistent with our previous study in which EEG was quantified by means of a traditional method. These findings suggest that the cholinergic component of the septohippocampal system, which is the main source of hippocampal acetylcholine, and neurophysiological mechanisms involved in the modulation of both the amplitude and the frequency of theta are functionally related. The possibility that, at least in the urethane-anaesthetised rat, hippocampal acetylcholine is involved in these modulator mechanisms is discussed.

Evidence for a complex influence of nicotinic acetylcholine receptors on hippocampal serotonin release

The effects of nicotine on 5-hydroxytryptamine (5-HT) release from serotonergic nerve endings in rat dorsal hippocampal slices were studied. Nicotine (50-500 microM:) caused a concentration-dependent increase in 5-HT release. This effect was antagonised by mecamylamine (0.5 microM:), indicating an action at nicotinic receptors. Nicotine-evoked 5-HT release was not affected by tetrodotoxin (3 microM:), cadmium chloride (0.1 mM:), or the absence of Ca(2+) or Na(+) in the superfusion medium. Unexpectedly, higher concentrations of mecamylamine alone (1-50 microM:) increased 5-HT release. This suggested the presence of inhibitory input to 5-HT neurones and that these inhibitory neurones possess tonically active nicotinic receptors. The effect of mecamylamine (50 microM:) on 5-HT release was reduced by the muscarinic M(1) receptor agonist, McN-A-343 (100 microM:), but pirenzepine (0.005-1 microM:), which blocks M(1) receptors, alone increased 5-HT release. Hippocampal serotonergic neurones are known to possess both excitatory nicotinic receptors and inhibitory M(1) receptors. Although there may be several explanations for our results, one possible explanation is that nicotine stimulates 5-HT release by activating nicotinic heteroreceptors on 5-HT terminals. Mecamylamine (0.5 microM:) antagonises this effect, but higher concentrations increase 5-HT release indirectly by blocking the action of endogenous acetylcholine on nicotinic receptors situated on cholinergic neurones that provide muscarinic inhibitory input to 5-HT neurones.

The scopolamine-induced impairment of spatial cognition parallels the acetylcholine release in the ventral hippocampus in rats

We investigated the relationship between the induction of spatial cognition impairment in the 8-arm radial maze task and regional changes (ventral hippocampus (VH), dorsal hippocampus, frontal cortex, and basolateral amygdala nucleus) in brain acetylcholine (ACh) release using microdialysis in rats treated with muscarinic (M) receptor antagonists. In a behavioral study, two M1 antagonists, scopolamine (0.5 mg/kg, i.p. and 20 microg, i.c.v.) and pirenzepine (80 microg, i.c.v.), but not an M2 antagonist, AF-DX116 (40-80 microg, i.c.v.), disrupted spatial cognition in the 8-arm radial maze task. In brain microdialysis with Ringer's solution containing 0.1 mM eserine sulfate, scopolamine and AF-DX116, but not pirenzepine, increased ACh release in the VH. Moreover, in the bilateral injection of scopolamine (2 microg/side), the VH and dorsomedial thalamus nucleus were important regions for scopolamine-induced impairment of spatial cognition. A simultaneous determination of the behavioral changes revealed that scopolamine (0.5 mg/kg, i.p.) markedly decreased the ACh contents and also increased the ACh release in all regions tested. Especially, the changes in the ACh release of the VH closely paralleled the induction of the scopolamine-induced impairment of spatial cognition. These results suggest that the blocking balance between M1 and M2 muscarinic receptor in the VH therefore plays a major role in the spatial cognition impairment induced by scopolamine in the 8-arm radial maze task.

Prejunctional muscarinic receptors modulating acetylcholine release in rabbit detrusor smooth muscles

We investigated the presence and subtypes of functionally prejunctional receptors in cholinergic nerve endings of rabbit detrusor smooth muscle strips using high-performance liquid chromatography coupled with a microdialysis procedure. The effects of pretreatment with various drugs on acetylcholine (ACh) release and contractile responses induced by electrical field stimulation were evaluated. Although atropine (a muscarinic nonselective antagonist) and 4-DAMP (a muscarinic M3 antagonist) did not influence the ACh release, they markedly reduced the contractile responses. Pirenzepine (M1 antagonist) decreased ACh release and contractile responses. Methoctramine (M2 antagonist) increased the ACh release, but did not influence to the contractile responses. These results suggest that the muscarinic receptors in the rabbit detrusor smooth muscle are heterogeneous, prejunctional facilitatory (M1 receptors), and inhibitory (M2 receptors) for ACh release and postjunctional M3 receptors mediating contractile responses.

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.

Contribution of cholinergic and gabaergic functions to memory processes in BALB/cANnCrlBR mice

Several lines of evidence indicate that glucose influences on memory depend on interactions between glucose, glucoregulation and hippocampal cholinergic function. We previously demonstrated that glucose and scopolamine differentially affected memory consolidation for an operant bar pressing task in two closely-related BALB/c mouse strains. Whereas glucose normally improves memory in several animal strains, memory consolidation was not effected by systemic glucose injections in BALB/cANnCrlBR mice. Moreover, these mice were relatively insensitive to the normally observed amnestic effects of scopolamine. We therefore sought to determine whether cholinergic mechanisms in the dorsal hippocampus were involved in such atypical drug effects on memory processing in that strain of mice. In Experiment 1, we examined whether post-training oxotremorine would also atypically influence memory consolidation for an appetitively reinforced operant bar pressing task following microinjection in the dorsal hippocampus. In Experiment 2, we examined the effects of intrahippocampal GABAA drugs on memory consolidation. The non-selective muscarinic agonist, oxotremorine, dose-dependently impaired memory and the GABAA antagonist, bicuculline, improved retention in BALB/cANnCrlBR mice. It was concluded that GABA-mediated influences on hippocampal pyramidal output in BALB/cANnCrlBR mice and other strains are similar; but the amnestic effects of oxotremorine from the dorsal hippocampus were opposite to facilitating effects normally observed in other animal strains. Results are discussed relative to possible altered septo-hippocampal cholinergic neurotransmission in BALB/cANnCrlBR mice.

Neurochemistry and pharmacology of the major hippocampal transmitter systems: synaptic and nonsynaptic interactions

Hippocampus plays a crucial role in important brain functions (e.g. memory, learning) thus in the past two decades this brain region became a major objective of neuroscience research. During this period large number of anatomical, neurochemical and electrophysiological data have been accumulated. While excellent reviews have been published on the anatomy and electrophysiology of hippocampal formation, the neurochemistry of this area has not been thoroughly surveyed. Therefore the aim of this review is to summarize the neurochemical and pharmacological data on the release of the major neurotransmitters found in the hippocampal region: glutamate (GLU), gamma-amino butyric acid (GABA), acetylcholine (ACh), noradrenaline (NA) and serotonin (5-HT). In addition, this review analyzes the synaptic and nonsynaptic interactions between hippocampal neuronal elements and overviews how auto- and heteroreceptors are involved in the presynaptic modulation of transmitter release. The presented data clearly show that transmitters released from axon terminals without synaptic contact play an important role in the fine tuning of communication between neurons within a neuronal circuit.

Measurement of acetylcholine released from rabbit detrusor smooth muscle using HPLC with electro-chemical detection coupled with microdialysis procedure

We measured the amount of acetylcholine (ACh) released from rabbit detrusor smooth muscles induced by electrical field stimulation (EFS) using microdialysis procedure. The dialysis probe was inserted through the detrusor muscle strip and was continuously perfused with a Ringer solution containing physostigmine sulfate, at a rate of 2 microl/min. The strip was suspended in an organ bath filled with the modified Krebs-Henseleit solution and then EFS was delivered. The isometric force was recorded and monitored in each muscle preparation. The dialysates were collected every 10 min. ACh was determined by a high performance liquid chromatography with electro-chemical detection. The contraction of the muscle strip and ACh release induced by EFS were increased in a frequency and duration dependent manner. There were some differences between frequency response curves of contraction and frequency dependent ACh release. In the contractile response, the maximum contractions were observed at lower frequencies, while ACh releases reached the maximum at higher frequencies. There was a significant, but not simple correlation between EFS-induced contraction and ACh release. The results suggest that this new method is useful to investigate the ACh release from rabbit detrusor smooth muscles, and that other neurotransmitters than ACh possibly contribute to EFS-induced contraction.

Brain choline has a typical precursor profile

Choline is product and precursor to both acetylcholine and membrane phospholipids, and, in the brain, is ultimately provided by the circulation. The brain is protected from excess choline and choline deprivation by a refined system of homeostatic mechanisms that maintain a level of extracellular choline that, for its role as precursor, meets saturation criteria under normal conditions. The kinetic and activity profiles of choline are typical for a biosynthetic precursor.

Nefiracetam elevates extracellular acetylcholine level in the frontal cortex of rats with cerebral cholinergic dysfunctions: an in vivo microdialysis study

We determined the effect of nefiracetam, a novel cognitive enhancer, on the extracellular acetylcholine (ACh) level in the frontal cortex of freely moving rats, using a microdialysis technique without an acetylcholinesterase inhibitor in the perfusate. Treatment with nefiracetam (10 mg/kg, p.o.) produced a significant increase in the level of ACh in the brain dialysate, compared with the vehicle-treated group. This enhancing effect was also observed when the ACh level was elevated by administration of scopolamine (1 mg/kg, i.p.) at 45 min after the treatment with nefiracetam. In addition, perfusion of nefiracetam at the concentration of 10 microM significantly increased the extracellular ACh level in the frontal cortex of basal forebrain (BF)-lesioned rats, in which a marked decrease of the basal ACh level was observed in this region. These results suggest that enhancement of cortical ACh release by nefiracetam may contribute to an anti-amnesic effect on the learning deficits induced by treatment of scopolamine or BF-lesion in rats.

Effects of ventral hippocampal galanin on spatial learning and on in vivo acetylcholine release in the rat

The neuropeptide galanin coexists in the medial septum and diagonal band of Broca with a population of acetylcholine neurons which project mainly to the ventral hippocampus. The present studies investigated the role of ventral hippocampal galanin in spatial learning in the male rat using a spatial learning task. In addition, the effects of galanin on cholinergic function were monitored by in vivo microdialysis and high-performance liquid chromatography. Bilateral microinjections of galanin (3 nmol/ rat) via chronic cannulae placed into the ventral hippocampus (i.v.h.) produced a slight but significant impairment of acquisition of the spatial task, while the 1 nmol dose of galanin facilitated acquisition. The 6 nmol dose of galanin failed to affect performance. A trend for an impairment of long-term memory retention (examined seven days after the last training session) was observed after 3 nmol of galanin, while the 1 nmol dose facilitated retention performance. Scopolamine (0.1 mg/kg, s.c.) caused a marked impairment of acquisition. Galanin (3 nmol/rat) given i.v.h. failed to modify the acquisition impairment caused by scopolamine (0.1 mg/kg, s.c.). These results suggest that galanin given i.v.h. produces a biphasic dose-dependent effects on spatial learning. In freely moving rats, galanin (3 nmol/10 microliters) given into the lateral ventricle (i.c.v.) did not affect basal acetylcholine release. In contrast, perfusion (100 min) with galanin (0.1 or 0.3 nmol/1.25 microliters/min) through the ventral hippocampal probe resulted in a reduction of basal acetycholine release which was dose-dependent and reversible. Galanin given i.c.v. (3 nmol/10 microliters) or through the probe (0.3 nmol/1.25 microliters/min) attenuated the increase in acetylcholine release evoked by the muscarinic antagonist scopolamine (0.1 mg/kg, s.c.; 0.001 nmol/1.25 microliters/min through the probe). The galanin plus scopolamine combinations produced a 50% lower increase in the extracellular acetylcholine concentrations than scopolamine alone. This suggests that the mechanism(s) behind scopolamine- and galanin-induced stimulation of acetylcholine differ. These results indicate that ventral hippocampal galanin plays a role in cognition and that it has a powerful and modulatory effect on cholinergic transmission. However, the effects of exogenous galanin on spatial learning cannot be directly related to changes in in vivo cholinergic transmission in the ventral hippocampus. These discrepancies may relate to effects on subtypes of galanin receptors with different functional coupling. In addition, other hippocampal neurotransmitter systems (e.g. noradrenergic neurons) important for cognitive functions may also be modulated by ventral hippocampal galanin.

Acetylcholine release from frontal cortex in the waking rat measured by microdialysis without acetylcholinesterase inhibitors: effects of diisopropylfluorophosphate

Acetylcholine (ACh) release was measured in frontal cortex of awake quietly resting rats by microdialysis without using cholinesterase blockers in the perfusate. Resting release was 16.61 +/- 2.05 fmol/h (+/- S.E.M., n = 18). Injection of sublethal doses of the acetylcholinesterase blocker, diisopropylfluorophosphate produced dose-dependent increases in ACh release, reaching 79.9 fmol/h with a dose of 0.7-times the LD50. Although this irreversible inactivation of acetylcholinesterase increased ACh recovery to more than 700% of control values, levels of ACh in the perfusate never reached those seen in physostigmine-treated animals. The relationship between the amount of acetylcholinesterase inactivation and the quantity of ACh in the perfusate suggests that the extracellular ACh concentrations are controlled by simple enzyme kinetics. Within 2 h after enzyme inactivation, extracellular choline levels fell significantly, suggesting that ACh degradation by acetylcholinesterase plays an important role in regulating the amount of choline in the extracellular space.

Potassium, but not atropine-stimulated cortical acetylcholine efflux, is reduced in aged rats

Using in vivo microdialysis, cortical acetylcholine (ACh) efflux was measured in freely moving Brown Norway/Fischer 344 F1 rats, aged 4 or 22 months. The effects of local, intracortical perfusion of atropine (1.0 or 100.0 microM) via the dialysis probe were compared to local K+ (100.0 mM) stimulation in the presence of elevated extracellular Ca2+ (2.5 mM). Basal cortical ACh efflux in aged rats was similar to that of young animals. Administration of atropine (1.0 or 100.0 microM) via the cortical dialysis probe substantially increased cortical ACh efflux, but did not differentially stimulate ACh efflux in young and aged rats. In contrast, ACh efflux stimulated locally with K+ and Ca2+ was significantly reduced in aged rats relative to young adults. The implications of the dissociable effects of K(+)-depolarization and muscarinic blockade for local regulation of cortical ACh efflux in aged animals are discussed.

Regional differences of the effect of sigma receptor ligands on the acetylcholine release in the rat brain

We found that sigma receptor ligands differentially regulated the acetylcholine (ACh) neurotransmission in the rat brain. Acute administration of (+)-N-allylnormetazocine [(+)-SKF-10,047], a prototype sigma 1 receptor ligand, and 1,3-di(2-tolyl)guanidine (DTG), a non-specific sigma receptor ligand, increased the extracellular ACh level in the rat hippocampus. This increase of hippocampal extracellular ACh level elicited by (+)-SKF-10,047 was more potent than that elicited by DTG. On the other hand, the striatal extracellular ACh level was slightly affected by (+)-SKF-10,047. In addition, DTG did not affect the striatal extracellular ACh level. Our previous studies have shown that both (+)-SKF-10,047 and DTG increased the extracellular ACh level in the rat frontal cortex. Taking all these data into consideration, the regulation of ACh neurotransmission by sigma receptor ligands are different depending upon the brain region.

Age-related changes in diurnal acetylcholine release in the prefrontal cortex of male rats as measured by microdialysis

Extracellular levels of acetylcholine in the prefrontal cortex were measured using the micro-dialysis method in freely moving young (three to four months old) and old (23 to 24 months old) male rats over a period of 24 h to examine the effect of aging on prefrontal acetylcholine release. Prefrontal acetylcholine release during a 24 h period exhibited a diurnal variation with higher levels during the dark cycle than during the light cycle in young rats but not in old rats. In addition, prefrontal acetylcholine release was closely associated with spontaneous activity in young rats but not in old rats. The present study suggests that aging reduces diurnal changes in the prefrontal acetylcholine release and that there is a cross-correlation between the prefrontal acetylcholine release and spontaneous locomotor activity in male rats.

Changes in cortical acetylcholine release in the rat during day and night: differences between motor and sensory areas

By sampling simultaneously from two microdialysis probes placed in the left and right hindlimb somatosensory cortex, or in the somatosensory and visual or in the somatosensory and motor cortices, we compared the release of acetylcholine in functionally different regions. Samples were taken hourly from freely moving, adult male Sprague-Dawley rats for periods of 10-24h. A generalized increase in acetylcholine release occurred in all cortical regions with the transition to the night-time period of wakefulness and activity; however, the change was significantly greater in the two sensory regions (56%) than in the motor cortex (20%). Decrements in release during the active period seldom decreased the amount released below the values observed during sleep. During the active period, the amount of acetylcholine released in the somatosensory cortex was strongly correlated with the amount released in the contralateral somatosensory region and was only slightly less well correlated with the amount released in either the visual or motor cortex. The correlation between release in the somatosensory and motor cortex was not present during the day, when rats habitually sleep. These data confirm that a global change in the level of acetylcholine release occurs with a transition in behavioural state; however, because the change is not equal in all areas and, because the correlation between motor and sensory cortex can be uncoupled, it seems likely that there are additional mechanisms available for independent control of acetylcholine release within specific cortical regions.

Cholinergic modulation of a decrement in social investigation following repeated contacts between mice

Social recognition has been inferred from a decline in olfactory investigation of conspecific intruders during repeated or protracted confrontation with a resident rat. A stimulus-response relationship defined by lack of response remains somewhat ambiguous. Since it is likely that behavior continues to be emitted by the resident animal, how behavior reorganizes as the resident becomes familiar with an intruder represents an important issue in the characterization of recognition. We examined the decline in olfactory investigation of ovariectomized females by adult male mice. The duration and frequency of olfactory investigation was measured during four 1 minute confrontations with 10-min intertrial intervals (Training trials). If the same female was presented in each trial, investigation declined to less than 50% of initial levels. Aggressive behavior gradually increased with repeated trials. No decline in investigation or increased aggression was measured when females were changed in each trial. Administration of doses of scopolamine (0.16-1.0 mg/kg, IP) blocked decrements in olfactory investigation in repeated confrontations and significantly reduced aggression. Co-administration of heptylphysostigmine (0.32-5.0 mg/kg, IP) reversed scopolamine's effects on olfactory investigation but not aggression. Acetylcholinesterase inhibitors heptylphysostigmine, galanthamine (0.63-2.5 mg/kg, IP) and tacrine (0.63-10.0 mg/kg, IP) all enhanced the rate of decrement of olfactory investigation when administered alone, but had differential effects on aggression. The decline in investigation corresponds to criteria for habituation. Increased responsivity expressed as aggression indicates recognition may also be characterized as a change in behavioral strategy dependent on the sexual and social status of the stimulus animal. Pharmacological data support a role for acetylcholine release in the development of social recognition as an olfactory memory, or through modulation of olfactory perception.

Glucose enhancement of scopolamine-induced increase of hippocampal high-affinity choline uptake in mice: relation to plasma glucose levels

The administration of glucose has been shown to improve memory for various learning tasks in rodents. In humans, glucose also increases declarative memory performance in elderly people and in some patients with mild Alzheimer's disease. One of the possible physiological bases for the effect of glucose on memory processes is a facilitation of cholinergic function through increased synthesis. In support of this hypothesis, glucose was shown to attenuate the amnesia induced by scopolamine and, in similar conditions, glucose increased extracellular levels of acetylcholine following a scopolamine injection. To further examine the interaction between glucose and cholinergic function, the present experiment measured the effects of combined injections of glucose and scopolamine on hippocampal sodium-dependent high-affinity choline uptake, an indirect index of cholinergic activity. Results showed that the injection of 3 g/kg glucose enhanced the increase in high affinity choline uptake in hippocampal synaptosomes produced by scopolamine. A regression analysis revealed the existence of a positive correlation between plasma blood glucose level and hippocampal choline uptake particularly in the animals receiving a combined injection of scopolamine and glucose. These data further support the hypothesis that glucose administration can facilitate acetylcholine synthesis under certain conditions and that this action could explain how glucose attenuates scopolamine-induced amnesia.

Effects of local and repeated systemic administration of (-)nicotine on extracellular levels of acetylcholine, norepinephrine, dopamine, and serotonin in rat cortex

Systemically administered (-)nicotine (0.2-1.2 mg/kg, s.c.) significantly increased the release of acetylcholine (ACh), norepinephrine (NE) and dopamine (DA) in rat cortex. The lowest dose of (-)nicotine examined (0.2 mg/kg, s.c.) also significantly elevated extracellular serotonin (5-HT) levels, and the maximal increases of extracellular ACh (122% at 90 min post injection) and DA levels (249% at 120 min post-injection) were observed following this dose. In contrast, the maximal increase of NE release (157% at 30 min post-injection) was observed following the highest dose of (-)nicotine injected (1.2 mg/kg, s.c.). This higher dose consistently produced generalized seizures. Repeating the (-)nicotine (0.58 mg/kg, s.c.) injection four hours after the first administration significantly elevated extracellular NE levels and also appeared to increase DA and ACh release. In addition, extracellular ACh and DA levels increased significantly in the dialysate after (-)nicotine was administered directly to the neocortex through the microdialysis probe membrane. Norepinephrine levels appeared to be elevated in the cortex following local administration as well.

Effect of JTP-2942, a novel thyrotropin-releasing hormone analogue, on pentobarbital-induced anesthesia in rats

The effects of a novel thyrotropin-releasing hormone (TRH) analogue, N alpha-((1S,2R)-2-methyl-4-oxocyclopentylcarbonyl)-L-histidyl-L-pro linamide monohydrate (JTP-2942), on pentobarbital-induced anesthesia in rats were investigated and compared with those of TRH. Intravenous administration of both JTP-2942 and TRH caused a dose-dependent decrease in the recovery time from pentobarbital-induced anesthesia. The minimum effective doses of JTP-2942 and TRH were respectively 0.03 and 1 mg/kg. The effect of JTP-2942 was antagonized by intraperitoneal scopolamine (0.5 mg/kg). Intraperitoneal JTP-2942 (1 mg/kg) caused an increase of acetylcholine release and a decrease of choline release in the frontal cortex and hippocampus of pentobarbital-treated rats. In addition, JTP-2942 ameliorated the decrease of hemicholinium-3-sensitive high-affinity choline uptake and the increase of acetylcholine in these brain regions. However, JTP-2942 had no effect on choline acetyltransferase activity or the choline content, which were also not changed by pentobarbital. Our results indicate that the effect of JTP-2942 on pentobarbital-induced anesthesia was about 30 times more potent than that of TRH, and suggest that JTP-2942 may act by accelerating acetylcholine turnover.

Choline enhances scopolamine-induced acetylcholine release in dorsal hippocampus of conscious, freely-moving rats

We examined the effects of exogenous choline (30, 60, 120 mg/kg, i.p.) on basal and scopolamine-evoked acetylcholine (ACh) release in awake animals, using in vivo microdialysis. After collection of 3-4 baseline dialysate samples (15 min each), rats received either saline or choline chloride and 4 additional samples were collected. All animals then received scopolamine hydrochloride (0.5 mg/kg, i.p.) and 6 additional samples were collected. Basal ACh release in animals receiving choline did not differ from that in rats given saline, nor from ACh release prior to choline administration. Scopolamine alone increased average ACh levels in dialysates from 1.22 +/- 0.54 to 11.18 +/- 3.07 pmol/15 min (mean +/- SD; p = 0.001); administration of 60 mg/kg or 120 mg/kg of choline chloride significantly enhanced maximal scopolamine responses by about 55%. These results suggest that supplemental choline enhances evoked ACh release in hippocampus of freely-moving rats.

Effect of physostigmine on relative acetylcholine output induced by systemic treatment with scopolamine in in vivo microdialysis of rat frontal cortex

By means of an in vivo brain microdialysis, the effect of different concentrations of physostigmine on the acetylcholine level in the dialysate of rat frontal cortex was studied. Perfusion of the various degrees of physostigmine (eserine) concentration (10 nM-10 microM) into the cortex through the dialysis membrane increased the basal acetylcholine level in a dose-dependent manner. In the presence of 10 nM, 0.1 microM and 10 microM physostigmine in the perfusate, systemic treatment with scopolamine (0.5 mg/kg, i.p.) increased 200, 270 and 510%, respectively, the relative acetylcholine level in the dialysates in comparison with the corresponding basal levels, while in the absence of physostigmine the treatment increased it only 40%. From these results, it appears that perfusion of physostigmine at a variety of concentrations, changes not only the basal level of acetylcholine induced by the inhibition of acetylcholinesterase but also the relative acetylcholine output induced by systemic treatment with scopolamine.

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.

Effects of single or repeated administration of a carbamate, propoxur, and an organophosphate, DDVP, on jejunal cholinergic activities and contractile responses in rats

Wistar rats were injected once or repeatedly for 10 days with dichlorvos (DDVP, 5 mg kg-1), propoxur (10 mg kg-1), oxotremorine (0.1 mg kg-1) or atropine (5 mg kg-1). Animals were killed 20 min or 24 h after single or consecutive injections, respectively, for determinations of cholinergic activities and contractile responses to acetylcholine (ACh) of the jejunum. Single treatments: while DDVP and propoxur decreased acetylcholinesterase (AChE) activity, oxotremorine and atropine did not. Although DDVP, propoxur and oxotremorine increased levels of ACh, atropine decreased them. Contractile responses to ACh were enhanced by DDVP and reduced by oxotremorine and atropine. The Bmax value of binding of [3H]quinuclidinyl benzylate (QNB) to muscarinic ACh receptors was decreased by atropine. Consecutive treatments: DDVP and oxotremorine decreased AChE activity markedly and slightly, respectively. Although DDVP and oxotremorine increased levels of ACh, propoxur decreased them. Without affecting the contractile responses, DDVP caused a reduction and propoxur and atropine caused an increase in the Bmax value for binding of [3H]QNB. Both the contractile responses and the value of Bmax for binding of [3H]-QNB were decreased by oxotremorine. In summary, propoxur and DDVP showed similar effects mainly through their anticholinesterase properties in the case of single injection, but DDVP had similar effects to those of oxotremorine and propoxur had similar effects to those of atropine in the case of repeated injection.

Time course of decline of radiolabeled acetylcholine formed following intracerebroventricular administration of tritiated choline: effects of oxotremorine and scopolamine

Rats were injected intracerebroventricularly with 5 microCi of [methyl-3H]choline. The time course of decline of the radiolabeled acetylcholine (ACh) formed was estimated in the ipsilateral cerebral cortex and striatum. The [3H]ACh levels declined biphasically from the cerebral tissue. The initial decline proceeded rapidly, after which labeled ACh declined more slowly. Scopolamine (1 mg/kg, i.v.) caused a significant increase in the rate of [3H]ACh disappearance, which can be interpreted as an enhancement of ACh release. By contrast, oxotremorine (0.8 mg/kg, i.v.) markedly reduced the [3H]ACh disappearance. The results show that drug-induced changes in cholinergic neuronal activities can be estimated from the disappearance of radioactive ACh after labeling the endogenous transmitter through intracerebroventricular administration of labeled choline.

Different effects of scopolamine on extracellular acetylcholine levels in neostriatum and nucleus accumbens measured in vivo: possible interaction with aversive stimulation

The in vivo microdialysis technique was used to measure extracellular concentrations of acetylcholine (ACh) in the neostriatum (NS) and nucleus accumbens (NAc) of freely moving rats after intraperitoneal administration of the muscarinic receptor antagonist scopolamine (0.5 mg/kg) or vehicle. Simultaneously, behavior was monitored. The administration of scopolamine induced an increase in extracellular ACh levels in the NS, which reached a maximum of about 185% within one hour after injection and returned to baseline values about three hours after injection. In the NAc, an increase of similar time-course was observed; however, this increase reached a maximum of 250%, which was significantly higher than the one observed in NS. These changes in ACh levels were accompanied by enhanced locomotion, rearing and grooming; however, the behavioral changes were of shorter time-course than those of extracellular ACh. The injection of vehicle did not affect ACh levels in NS, but induced a significant increase (60%) in the NAc. The levels of behavioral activity after vehicle injection did not differ from pre-injection levels. These results suggest, that the cholinergic systems in the NAc and NS are differently affected by peripheral administration of both scopolamine and vehicle. The differential effects of scopolamine in NS and NAc could reflect pharmacodynamic differences between these two striatal brain areas, perhaps due to a higher density of cholinergic interneurons or muscarinic autoreceptors in the NAc in comparison to the NS. However, the increase of extracellular ACh observed after vehicle injection suggests that factors such as aversive stimulation through the injection procedure can increase ACh release in the NAc and that such a mechanism can interact within the action of scopolamine. Thus, the stronger action of scopolamine on extracellular ACh in the NAc might be an additive effect of the drug with that of the injection procedure.

Nefiracetam enhances acetylcholine outflow from the frontal cortex: in vivo microdialysis study in the rat

The effects of nefiracetam [DM-9384; N-(2, 6-dimethylphenyl)-2-(2-oxo-1-pyrrolidinyl)acetamide], a cognitive enhancer, on extracellular acetylcholine and glutamic acid in the frontal cortex were studied using brain microdialysis in freely moving rats. Nefiracetam administration (1 mg/kg, p.o.) doubled the amount of acetylcholine in the dialysate. When tetrodotoxin was added to the perfusion solution, the basal level of acetylcholine decreased and nefiracetam caused no increase in acetylcholine outflow. The amount of glutamic acid in the dialysates was not affected by nefiracetam. These results suggest that nefiracetam increases the amount of acetylcholine within the cholinergic synaptic clefts.

Daily changes in the release of acetylcholine from rat primary somatosensory cortex

Using microdialysis, acetylcholine (ACh) release was measured in the somatosensory cortex of 14 rats over a 24-h period. The release of ACh was 0.195 pmol/min during the day and 0.344 pmol/min at night. The length of exposed dialysis membrane within the cortex was an important source of variability in the absolute amounts of ACh collected. Even after rejecting some cases where the membrane contacted only the superficial cortical layers, this factor accounted for 25% of the variation of absolute amounts collected in different animals. After correcting for the length of exposed membrane, the release of ACh was shown to increase 52% at night during the time when the animals were awake, feeding and grooming. Variability in the measures of ACh release obtained during periods of activity was greater than its variability during periods of inactivity. These data were interpreted in the context of several hypothesized roles for ACh in sensory cortex.

Characterization of BIBN 99: a lipophilic and selective muscarinic M2 receptor antagonist

The present study was designed to characterize the receptor selectivity profile of the novel muscarinic M2 receptor antagonist BIBN 99 (5,11-dihydro-8-chloro-11-[[4-[3-[(2,2-dimethyl-1- oxopentyl)ethylamino]propyl]-1-piperidinyl]acetyl]-6H- pyrido[2,3-b][1,4]benzodiazepin-6-one). In radioligand binding studies BIBN 99 showed high affinity for m2/M2 sites (pKi = 7.52/7.57), intermediate affinity for m4 sites (pKi = 6.76) and low affinity for m1/M1 (pKi = 5.97/6.17), m3/M3 (pKi = 6.11/6.04) and m5 sites (pKi = 5.84). Functional studies in vitro showed BIBN 99 to be a competitive antagonist and to have an 11- to 25-fold higher affinity for M2 receptors than for putative M1 receptors in the rabbit vas deferens or M3 receptors in guinea-pig trachea. In vivo studies revealed that BIBN 99 is able to cross the blood-brain barrier, and although showing an approximately 3-fold higher affinity for M2 binding sites BIBN 99 appeared to be 7- to 18-fold less potent than AF-DX 116 in inhibiting muscarinic agonist or vagally induced bradycardia in rats and guinea-pigs. The results show that BIBN 99 is the first lipophilic muscarinic M2 receptor antagonist to have remarkable M2 versus M1 selectivity (30-fold). In addition, BIBN 99 possesses central nervous system activity and only minor peripheral cardiac effects.

Galanin stimulates striatal acetylcholine release via a mechanism unrelated to cholinergic receptor stimulation

The effect of galanin (GAL) on the basal and the muscarinic agonist/antagonist mediated release of acetylcholine (ACh) in the striatum was investigated in male rats using in vivo microdialysis and HPLC techniques. GAL (300 microM or 3 nmol/10 microliters), perfused through the microdialysis membrane into the striatum, was found to enhance basal ACh release. The GAL evoked ACh release was completely prevented by bupivacaine, a sodium channel blocker (1.5 mM) when coperfused with GAL. This suggests that the effect of GAL depends on intact neuronal activity and thus derives from impulse-dependent release. The muscarinic agonists oxotremorine (0.3 mg/kg, i.p.) or carbachol (100 microM, infusion) reduced ACh release and reduced the stimulation of ACh release by GAL with a magnitude corresponding to that of oxotremorine or carbachol alone. Thus, the resultant effect of GAL on ACh release remained unchanged. When GAL was given at a threshold dose (30 microM), which by itself did not stimulate ACh release, it was unable to attenuate the muscarinic agonist induced inhibition of ACh releases. Furthermore, GAL given in combination with scopolamine (0.25 mg/kg, i.p.) or pirenzepine (1 microM, infusion) added to the stimulating effect by the two muscarinic antagonists. In contrast to the GAL (300 microM) evoked ACh release, the scopolamine (0.25 mg/kg, i.p.) stimulated ACh release was not blocked by M15, a putative GAL antagonist, indicating that the mechanisms behind GAL and scopolamine evoked striatal ACh release differ. These results suggest that the mechanisms behind GAL evoked release do not involve direct interactions with pre- or postsynaptic muscarinic receptor mediated events. It is concluded that the stimulation of the basal ACh release by GAL in the striatum occurs via occupation of GAL receptors located on cholinergic interneurons and that the release process is dependent on intact neuronal activity.

Effects of a novel thyrotropin-releasing hormone analogue, JTP-2942, on extracellular acetylcholine and choline levels in the rat frontal cortex and hippocampus

The effects of a novel thyrotropin-releasing hormone (TRH) analogue, N alpha-[(1S,2R)-2-methyl-4-oxocyclopentanecarbonyl]-L-histidyl-L-pr olinamide (JTP-2942) on acetylcholine (ACh) release and on the extracellular choline level were investigated in rat frontal cortex and hippocampus by microdialysis, and were compared with effects of TRH. JTP-2942 (0.3 mg/kg i.p.) produced a marked (> 300%) and persistent increase of ACh release in both the frontal cortex and hippocampus, while TRH (3 mg/kg i.p.) caused a significant but transient increase of ACh to about 200% in the frontal cortex. Both drugs significantly decreased the choline levels in both brain regions. Investigation of the effects of JTP-2942 (0.001-1 mM) and TRH (1 and 10 mM) on ACh release and choline levels when perfused through the dialysis probe revealed that JTP-2942 had a greater effect than TRH in both the frontal cortex and the hippocampus. The action of JTP-2942 was about 1000-fold more potent than that of TRH in both brain regions. Oral administration of JTP-2942 at a dose of 10 mg/kg markedly and persistently increased the release of ACh and at doses of 1-10 mg/kg decreased the extracellular choline level in the frontal cortex and hippocampus. These results also suggest that JTP-2942 has some selectivity for the hippocampus compared to the frontal cortex after both systemic administration and local injection. The increase of ACh release caused by JTP-2942 was completely antagonized by perfusion with tetrodotoxin (TTX, 1 microM), suggesting that the action of JTP-2942 on cholinergic neurons was mediated via neuronal activity.

Regulation of spontaneous acetylcholine release in the hypothalamic vasopressinergic supraoptic nucleus of a freely moving rat: a study by in vivo microdialysis

We employed a brain microdialysis method to examine the possible regulation of spontaneous acetylcholine (ACh) release in the hypothalamic vasopressinergic supraoptic nucleus (SON) of rats. We monitored the basal ACh release in the SON-microdialysate. The addition of tetrodotoxin (10(-6) M) to the perfusate (saline containing 10(-4) M physostigmine) decreased the basal ACh release. A muscarinic receptor antagonist, atropine (non-selective) or pirenzepine (M1-selective), increased the basal ACh release in a concentration-dependent manner. The maximal increase occurred at 20-40 min after the start of the infusion of antagonists. The ED50 values for the stimulatory effects of atropine and pirenzepine were 9.4 x 10(-8) and approx. 10(-4) M, respectively. The effect of atropine (10(-6) M) was inhibited by simultaneous addition of the muscarinic agonist oxotremorine (10(-5) M). The results showed a negative feedback regulation of the spontaneous ACh release through the activation of muscarinic receptors in the SON. The weak effect of pirenzepine in increasing the ACh release, compared with atropine, suggests that ACh release in the nucleus is mainly regulated by the non-M1-muscarinic receptor subtype.