Blockade Of Nicotinic Receptors Research Articles

Amyloid beta from axons and dendrites reduces local spine number and plasticity

Excessive synaptic loss is thought to be one of the earliest events in Alzheimer's disease. Amyloid beta (Abeta), a peptide secreted in an activity-modulated manner by neurons, has been implicated in the pathogenesis of Alzheimer's disease by removing dendritic spines, sites of excitatory synaptic transmission. However, issues regarding the subcellular source of Abeta, as well as the mechanisms of its production and actions that lead to synaptic loss, remain poorly understood. In rat organotypic slices, we found that acute overproduction of either axonal or dendritic Abeta reduced spine density and plasticity at nearby ( approximately 5-10 mum) dendrites. The production of Abeta and its effects on spines were sensitive to blockade of action potentials or nicotinic receptors; the effects of Abeta (but not its production) were sensitive to NMDA receptor blockade. Notably, only 30-60 min blockade of Abeta overproduction permitted induction of plasticity. Our results indicate that continuous overproduction of Abeta at dendrites or axons acts locally to reduce the number and plasticity of synapses.

Selective potentiation of crossed vs. uncrossed inputs from lateral geniculate nucleus to visual cortex by the basal forebrain: Potential facilitation of rodent binocularity

Cholinergic projections originating in the basal forebrain (BF) play important roles in the heterosynaptic facilitation of synaptic strength in various sensory cortices, including the primary visual cortex (V1). Here, using urethane-anesthetized rats, we find that pairing burst stimulation of the BF with single pulse stimulation of the lateral geniculate nucleus (LGN) does not consistently increase field postsynaptic potentials (fPSPs) in V1 elicited by ipsilateral LGN stimulation. However, longer latency fPSPs recorded in V1 in response to stimulation of the contralateral LGN, reflecting crossed, polysynaptic inputs, show significant potentiation when paired with preceding BF stimulation. This synaptic enhancement requires relatively short time intervals between paired BF burst and LGN pulse stimulation (40 ms) and is abolished by systemic or local V1 muscarinic receptor blockade (scopolamine), while systemic nicotinic receptor blockade (mecamylamine) is ineffective. Together, these data provide evidence for a differential capacity for cholinergic/muscarinic-dependent plasticity induction among different signals in V1, with inputs reaching V1 from the contralateral LGN exhibiting potentiation in the face of stable strength in ipsilateral LGN-V1 projections. This preferential readiness for potentiation in crossed fiber systems could serve to amplify binocular responses in V1 elicited by synchronized excitation of ipsi- and contralateral LGN neurons.

Input-selective potentiation and rebalancing of primary sensory cortex afferents by endogenous acetylcholine

Acetylcholine (ACh) plays important roles in the modulation of activity and plasticity of primary sensory cortices, thus influencing sensory detection and integration. We examined this in urethane-anesthetized rats, comparing cholinergic modulation of short latency, large amplitude field postsynaptic potentials (fPSPs) in the visual cortex (V1) evoked by stimulation of the ipsilateral lateral geniculate nucleus (LGN), reflecting direct thalamocortical inputs, with longer latency, smaller amplitude fPSPs elicited by contralateral LGN stimulation, reflecting indirect, polysynaptic inputs. Basal forebrain (BF) stimulation (100 Hz) produced a significant (∼45%), gradually developing potentiation of the smaller, contralateral fPSPs, while ipsilateral fPSPs showed less enhancement (∼15%), shifting the relative strength of dominant/ipsi- and weaker/contralateral inputs to V1. Systemic or local, cortical blockade of muscarinic receptors (scopolamine) reduced potentiation of contralateral fPSP without affecting ipsilateral enhancement, thus preventing the relative amplification of contralateral inputs following BF stimulation. Systemic nicotinic receptor blockade (mecamylamine) resulted in depression of ipsilateral, and reduced enhancement of contralateral fPSPs after BF stimulation. N-methyl-d-aspartate receptor blockade (systemic MK-801) abolished ipsilateral fPSP enhancement without affecting contralateral potentiation. Neither drug reduced the amplification of contralateral relative to ipsilateral signals in V1. In a second experiment in the barrel cortex, BF stimulation enhanced multiunit activity elicited by whisker deflection in a muscarinic-sensitive manner. Similar to the observations in V1, this effect was more pronounced for weaker multiunit activity driven by a surround whisker than activity following principal whisker deflection. These experiments demonstrate that ACh release following BF stimulation exerts surprisingly selective effects to amplify non-dominant inputs to sensory cortices. We suggest that, by diminishing the imbalance between different afferent signals, ACh release during states of behavioral activation acts to induce a long-lasting facilitation of the detection and/or integration of signals in primary sensory fields of the cortical mantle.

Controls of Tonic and Phasic Dopamine Transmission in the Dorsal and Ventral Striatum

Dopamine (DA) release varies within subregions and local environments of the striatum, suggesting that controls intrinsic and extrinsic to the DA fibers and terminals regulate release. While applying fast-scan cyclic voltammetry and using tonic and phasic stimulus trains, we investigated the regulation of DA release in the dorsolateral to ventral striatum. The ratio of phasic-to-tonic-evoked DA signals varied with the average ongoing firing frequency, and the ratio was generally higher in the nucleus accumbens (NAc) compared with the dorsolateral striatum. At the normal average firing frequency, burst stimulation produces a larger increase in the DA response in the NAc than the dorsolateral striatum. This finding was comparable whether the DA measurements were made using in vitro brain slices or were recorded in vivo from freely moving rodents. Blockade of the dopamine transporters and dopamine D(2) receptors particularly enhanced the tonic DA signals. Conversely, blockade of nicotinic acetylcholine receptors (nAChRs) containing the beta(2) subunit (beta(2)(*)) predominantly suppressed tonic DA signals. The suppression of tonic DA release increased the contrast between phasic and tonic DA signals, and that made the frequency-dependent DA dynamics between the dorsolateral striatum and NAc more similar. The results indicate that intrinsic differences in the DA fibers that innervate specific regions of the striatum combine with (at least) DA transporters, DA receptors, and nAChRs to regulate the frequency dependence of DA release. A combination of mechanisms provides specific local control of DA release that underlies pathway-specific information associated with motor and reward-related functions.

Cytisine-Based Nicotinic Partial Agonists as Novel Antidepressant Compounds

Nicotine and other nicotinic agents are thought to regulate mood in human subjects and have antidepressant-like properties in animal models. Recent studies have demonstrated that blockade of nicotinic acetylcholine receptors (nAChRs) including those containing the beta2 subunit (beta2(*)), results in antidepressant-like effects. Previous studies have shown that cytisine, a partial agonist at alpha4/beta2(*) nAChRs, and a full agonist at alpha3/beta4(*) and alpha7 nAChRs, has antidepressant-like properties in several rodent models of antidepressant efficacy; however, it is not clear whether more selective partial agonists will also be effective in these models. We tested cytisine and two derivatives, 5-bromo-cytisine (5-Br-Cyt) and 3-(pyridin-3'-yl)-cytisine (3-pyr-Cyt) for their ability to act as a partial agonist of different nAChR subtypes and to show antidepressant-like activity in C57/BL6 mice in the tail suspension, the forced-swim, and the novelty-suppressed feeding tests. 3-pyr-Cyt was a partial agonist with very low efficacy at alpha4/beta2(*) nAChRS but had no agonist effects at other nAChRs normally targeted by cytisine, and it was effective in mouse models of antidepressant efficacy. Animals showed dose-dependent antidepressant-like effects in all three behavioral paradigms. 5-Br-Cyt was not effective in behavioral tests when administered peripherally, probably because of its inability to penetrate the blood-brain barrier, because it efficiently reduced immobility in the tail suspension test when administered intraventricularly. These results suggest that novel nicotinic partial agonists may provide new possibilities for development of drugs to treat mood disorders.

Bulbar Acetylcholine Enhances Neural and Perceptual Odor Discrimination

Experimental and modeling data suggest that the circuitry of the main olfactory bulb (OB) plays a critical role in olfactory discrimination. Processing of such information arises from the interaction between OB output neurons local interneurons, as well as interactions between the OB network and centrifugal inputs. Cholinergic input to the OB in particular has been hypothesized to regulate mitral cell odorants receptive fields (ORFs) and behavioral discrimination of similar odorants. We recorded from individual mitral cells in the OB in anesthetized rats to determine the degree of overlap in ORFs of individual mitral cells after exposure to odorant stimuli. Increasing the efficacy of the cholinergic neurotransmission in the OB by addition of the anticholinesterase drug neostigmine (20 mM) sharpened the ORF responses of mitral cells. Furthermore, coaddition of either the nicotinic antagonist methyllycaconitine citrate hydrate (MLA) (20 mM) or muscarinic antagonist scopolamine (40 mM) together with neostigmine (20 mM) attenuated the neostigmine-dependent sharpening of ORFs. These electrophysiological findings are predictive of accompanying behavioral experiments in which cholinergic modulation was manipulated by direct infusion of neostigmine, MLA, and scopolamine into the OB during olfactory behavioral tasks. Increasing the efficacy of cholinergic action in the OB increased perceptual discrimination of odorants in these experiments, whereas blockade of nicotinic or muscarinic receptors decreased perceptual discrimination. These experiments show that behavioral discrimination is modulated in a manner predicted by the changes in mitral cell ORFs by cholinergic drugs. These results together present a first direct comparison between neural and perceptual effects of a bulbar neuromodulator.

P01-82 New potential molecular targets for cannabis addition

Cannabis is considerer the most widely abused illicit drug in the world. The recent rising prevalence of cannabis use by young adults and increasing evidence of adverse health effects makes the search for new pharmacotherapy to reduce cannabis abuse extremely important. To date no medication has been approved for the treatment of cannabis addition.This study reviews recent results with potential interest for future pharmacological treatment of Cannabis dependence. Most of the relevant data obtained for treatment of cannabis dependence target the endocannabinoid or the central cholinergic systems, both involved and interact in brain systems implicated drug reinforcement. In laboratory animals blockade of cannabinoids CB1 receptors reverses central effects of cannabinoids. Rimonabant is a selective, orally active, cannabinoid CB1 receptor antagonist (inverse agonist) that has been shown in animals to modulate cannabinoid signaling in brain reward circuit. In humans, it has been shown that rimonabant, single or repeated oral doses blocked psychological and physiological effects of smoked marijuana (1). Although psychiatric adverse side affects like depression were reported with rimonabant, this compound was already been approved for treatment of obesity and metabolic syndrome. Very recently, blockade of α7 nicotinic receptors was shown to reverse abuse-related behavioral and neurochemical effects of cannabinoids in rats (2).In conclusion, besides cannabinoid CB1 receptor, the homomeric α7 nicotinic receptors are novel molecular targets in the development of new drugs for treatment of cannabis addition.

Chronic underactivity of medial frontal cortical β2-containing nicotinic receptors increases clozapine-induced working memory impairment in female rats

Nicotinic receptor decreases in the frontal cortex and hippocampus are important mediators of cognitive impairment in both schizophrenia and Alzheimer's disease. Drug treatments for these diseases should take into account the impacts of compromised brain function on drug response. This study investigated the impact of compromised nicotinic receptor activity in the frontal cortex in rats on memory function. Since both Alzheimer's disease and schizophrenia can involve psychosis, antipsychotic drugs are often given. The impacts of antipsychotic drugs on cognitive function have been found to be quite variable. It is the hypothesis of this and previous studies that the cognitive effects of antispychotic drugs on cognitive function depend on the integrity of brain systems involved in cognition. Previously in studies of the hippocampus, we found that chronic inhibition of β2-containing nicotinic receptors with dihydro-β-erythrodine (DHβE) impaired working memory and that this effect was attenuated by the antipsychotic drug clozapine. In contrast, chronic hippocampal α7 nicotinic receptor blockade with methyllycaconitine (MLA) potentiated the clozapine-induced memory impairment which is seen in rats without compromised nicotinic receptor activity. The current study determined medial frontal cortical α7 and β2-containing nicotinic receptor involvement in memory and the interactions with antipsychotic drug therapy with clozapine. Chronic DHβE and MLA infusion effects and interactions with systemic clozapine were assessed in female rats tested for memory on the radial-arm maze. Antipsychotic drug interactions with chronic systemic nicotine were investigated because nicotinic procognitive treatment has been proposed. The same local infusion DHβE dose that impaired memory with hippocampal infusion did not impair memory when infused in the medial frontal cortex. Frontal DHβE infusion potentiated clozapine-induced memory impairment, whereas previously the memory impairment caused by hippocampal DHβE infusion was attenuated by clozapine. Frontal cortical MLA infusions at a dose that previously was found to potentiate the clozapine-induced memory impairment with hippocampal infusion had no significant effect when infused into the medial frontal cortex. The location and subtype of nicotinic receptor underactivity are critical determinates for clozapine effects on memory. Patients with hippocampal β2-containing nicotinic receptor loss may be well treated with clozapine therapy, while those with frontal cortical β2-containing receptor loss may have a potentiated memory impairment caused by clozapine.

Vagus nerve mediates the protective effects of melanocortins against cerebral and systemic damage after ischemic stroke.

A vagus nerve-mediated, efferent cholinergic protective pathway activated by melanocortins is operative in circulatory shock and myocardial ischemia. Moreover, melanocortins have neuroprotective effects against brain damage after ischemic stroke. Here we investigated cerebral and systemic pathophysiologic reactions to focal cerebral ischemia in rats induced by intrastriatal microinjection of endothelin-1, and the possible protective role of the melanocortin-activated vagal cholinergic pathway. In the striatum and liver of saline-treated control rats, the activation of extracellular signal-regulated kinases, c-jun N-terminal kinases, and caspase-3, the increase in tumor necrosis factor-alpha (TNF-alpha) concentration and DNA fragmentation, as well as the increase in TNF-alpha plasma levels, occurred 10 and 20 h after the ischemic insult suggesting an activation of inflammatory and apoptotic responses. Treatment with [Nle(4), D-Phe(7)]alpha-melanocyte-stimulating hormone (NDP-alpha-MSH; 3 or 9 h after stroke) suppressed the inflammatory and apoptotic cascades at central and peripheral level. Bilateral vagotomy and pharmacologic blockade of peripheral nicotinic acetylcholine receptors blunted the protective effect of NDP-alpha-MSH. The present results show that focal brain ischemia in rats causes significant effects not only in the brain, but also in the liver. Moreover, our data support the hypothesis that a protective, melanocortin-activated, vagal cholinergic pathway is likely operative in conditions of ischemic stroke.

M5Muscarinic Receptor Knockout Mice Show Reduced Morphine-Induced Locomotion but Increased Locomotion after Cholinergic Antagonism in the Ventral Tegmental Area

M(5) muscarinic receptors are the only muscarinic receptor subtype expressed by mesencephalic dopamine neurons and provide an important excitatory input to mesolimbic and nigrostriatal dopamine systems. Here, we studied locomotion induced by systemic morphine (3, 10, and 30 mg/kg i.p.) in M(5) knockout mice of the C57BL/6 (B6) and CD1 x 129SvJ background strains. M(5) knockout mice of both strains showed reduced locomotion in response to 30 mg/kg morphine. B6 M(5) knockout mice were less sensitive to naltrexone in either the antagonism of morphine-induced locomotion or in the reduction of locomotion by naltrexone alone. This suggests that M(5) knockout mice are less sensitive to the effects of either exogenous or endogenous opiates on locomotion and that spontaneous locomotion in B6 mice is sustained by endogenous opiates. In B6 wild-type mice, ventral tegmental area (VTA) pretreatment with the muscarinic receptor antagonist atropine (3 microg bilateral), but not the nicotinic receptor antagonist mecamylamine (5 microg bilateral), reduced locomotion in response to 30 mg/kg morphine to a similar extent as systemic M(5) knockout, suggesting that reduced morphine-induced locomotion in M(5) knockout mice is due to the loss of M(5) receptors on VTA dopamine neurons. In contrast, in M(5) knockout mice, but not in wild-type mice, either intra-VTA atropine or mecamylamine alone increased locomotion by almost 3 times relative to saline and potentiated morphine-induced locomotion. Therefore, in M(5) knockout mice, blockade of either VTA muscarinic or nicotinic receptors increased locomotion, suggesting that in the absence of VTA M(5) receptors, VTA cholinergic inputs inhibit locomotion.

Brain regions mediating α3β4 nicotinic antagonist effects of 18-MC on methamphetamine and sucrose self-administration

The novel iboga alkaloid congener 18-methoxycoronaridine (18-MC) is a putative anti-addictive agent that has been shown, in rats, to decrease the self-administration of several drugs of abuse. Previous work has established that 18-MC is a potent antagonist at α3β4 nicotinic receptors. Because high densities of α3β4 nicotinic receptors occur in the medial habenula and the interpeduncular nucleus and moderate densities occur in the dorsolateral tegmentum, ventral tegmental area, and basolateral amygdala, the present study was conducted to determine if 18-MC could act in these brain areas to modulate methamphetamine self-administration in rats. Local administration of 18-MC into either the medial habenula, the interpeduncular area or the basolateral amygdala decreased methamphetamine self-administration. Similar results were produced by local administration into the same brain areas of two other α3β4 nicotinic antagonists, mecamylamine and α-conotoxin AuIB. Local administration of 18-MC, or the other antagonists, into the dorsolateral tegmentum or the ventral tegmental area had no effect on methamphetamine self-administration. In contrast, local administration of 18-MC and the other antagonists decreased sucrose self-administration when administered into the dorsolateral tegmentum or basolateral amygdala but had no effect when infused into the medial habenula, interpeduncular nucleus, or ventral tegmental area. These data are consistent with the hypothesis that 18-MC decreases methamphetamine self-administration by indirectly modulating the dopaminergic mesolimbic pathway via blockade of α3β4 nicotinic receptors in the habenulo-interpeduncular pathway and the basolateral amygdala. The data also suggest that the basolateral amygdala along with a different pathway involving α3β4 receptors in the dorsolateral tegmentum mediate the effect of 18-MC on sucrose self-administration.

Persistent behavioral alterations in rats neonatally exposed to low doses of the organophosphate pesticide, parathion

Although developmental exposures of rats to low levels of the organophosphate pesticides (OPs), chlorpyrifos (CPF) or diazinon (DZN), both cause persistent neurobehavioral effects, there are important differences in their neurotoxicity. The current study extended investigation to parathion (PTN), an OP that has higher systemic toxicity than either CPF or DZN. We gave PTN on postnatal days (PND) 1–4 at doses spanning the threshold for systemic toxicity (0, 0.1 or 0.2 mg/kg/day, s.c.) and performed a battery of emotional and cognitive behavioral tests in adolescence through adulthood. The higher PTN dose increased time spent on the open arms and the number of center crossings in the plus maze, indicating greater risk-taking and overall activity. This group also showed a decrease in tactile startle response without altering prepulse inhibition, indicating a blunted acute sensorimotor reaction without alteration in sensorimotor plasticity. T-maze spontaneous alternation, novelty-suppressed feeding, preference for sweetened chocolate milk, and locomotor activity were not significantly affected by neonatal PTN exposure. During radial-arm maze acquisition, rats given the lower PTN dose committed fewer errors compared to controls and displayed lower sensitivity to the amnestic effects of the NMDA receptor blocker, dizocilpine. No PTN effects were observed with regard to the sensitivity to blockade of muscarinic and nicotinic cholinergic receptors, or serotonin 5HT 2 receptors. This study shows that neonatal PTN exposure evokes long-term changes in behavior, but the effects are less severe, and in some incidences opposite in nature, to those seen earlier for CPF or DZN, findings consistent with our neurochemical studies showing different patterns of effects and less neurotoxic damage with PTN. Our results reinforce the conclusion that low dose exposure to different OPs can have quite different neurotoxic effects, obviously unconnected to their shared property as cholinesterase inhibitors.

Original nootropic drug Noopept prevents memory deficit in rats with muscarinic and nicotinic receptor blockade

Antiamnesic activity of Noopept was studied on the original three-way model of conditioned passive avoidance response, which allows studying spatial component of memory. Cholinoceptor antagonists of both types (scopolamine and mecamylamine) decreased entry latency and reduced the probability for selection of the safe compartment. Noopept abolished the antiamnesic effect of cholinoceptor antagonists and improved spatial preference.

Peripheral and central routes of administration of quaternary ammonium compound IEM-1460 are equally potent in reducing the severity of nicotine-induced seizures in mice

Peripheral administration of nicotinic receptor antagonists with a quaternary ammonium group (hexamethonium and chlorisondamine) did not prevent the development of seizures induced by systemic treatment with nicotine in the toxic dose. The Me3N+ group with stable positive charge inhibits transport of these compounds into the brain through the blood-brain barrier. Intracerebral and peripheral (intraperitoneal) administration of compound IEM-1460 with the Me3N+ group was equally potent in reducing the severity of nicotine-induced seizures in mice. This phenomenon is related to the fact that IEM-1460 acts as a nicotinic receptor antagonist and polyamine agonist, which increases blood-brain barrier permeability for polar compounds. These features contribute to IEM-1460 transport into the brain. High anticonvulsant activity of IEM-1460 on the model of nicotine-induced seizures is associated with combined blockade of nicotinic receptors (alpha3beta4 receptors) and glutamate receptors (GluR1 AMPA receptors).

Smoking: A Deadly Pleasant Habit—Is There a Way Out?

It was not until 1958 that the first major epidemiologic study demonstrated a strong correlation between smoking and cardiovascular disease. Although not providing definitive evidence that tobacco smoke was responsible for the increased coronary risk, it prompted the first anti-smoking measures by the US Surgeon General in his 1964 report. Smoking is a highly addictive (biological and psychological) habit. The severity of withdrawal symptoms that patients find distressing can be reduced by nicotine replacement therapy. Bupropion SR was the first nonnicotine medication shown to be effective for smoking cessation. Its possible mechanisms of action include blockade of neuronal reuptake of dopamine and norepinephrine and blockade of nicotinic acetylcholinergic receptors. Varenicline, a nicotine receptor partial agonist, is the most recently developed nonnicotine preparation. The odds of smoking cessation after 12 weeks of 1 mg twice-daily varenicline treatment was approximately twice that achieved with 150 mg twice-daily bupropion and nearly 4-fold greater than with placebo.

Presynaptic opioid and nicotinic receptor modulation of dopamine overflow in the nucleus accumbens.

Behaviorally relevant stimuli prompt midbrain dopamine (DA) neurons to switch from tonic to burst firing patterns. Similar shifts to burst activity are thought to contribute to the addictive effects of opiates and nicotine. The nucleus accumbens DA overflow produced by these drugs is a key element in their pathological effects. Using electrochemical techniques in brain slices, we explored the effects of opioids on single-spike and burst stimuli-evoked DA overflow in the dorsal and ventral striatum. In specific subregions of the nucleus accumbens, mu-opioids inhibit DA overflow elicited with single-spike stimuli while leaving that produced by burst stimuli unaffected. This is similar to published effects of nicotinic receptor blockade or desensitization, and is mediated by opioid receptor-induced inhibition of cholinergic interneurons. Whereas delta-opioids have similar effects, kappa-opioids inhibit evoked DA overflow throughout the striatum in a manner that is not overcome with high-frequency stimuli. These observations reveal remarkable mechanistic overlap between the effects of nicotine and opiates within the dopamine reward pathway.

Temporal resolution and temporal transfer properties: Gabaergic and cholinergic mechanisms

Temporal resolution is a basic property of the visual system and critically depends upon retinal temporal coding properties which are also of importance for directional coding. Whether the temporal coding properties for directional coding derive form inherent properties or critically depend upon the temporal coding mechanisms is unclear. Here, the influence of acetylcholine and GABA upon photopic temporal coding was investigated in goldfish, using flicker stimuli, in a behavioral and an electrophysiological (ERG) approach. The goldfish temporal resolution ability decreased from more than 90% correct choices at 20 Hz flicker frequency to about 65% at 45 Hz flicker frequency with a flicker fusion frequency of approximately 39 Hz. Blockade of GABAa-receptors reduced the flicker fusion frequency to about 23 Hz, not affecting temporal resolution below 20 Hz flicker frequency. Partial blockade of nicotinic acetylcholine receptors reduced the flicker fusion frequency slightly and lowered the temporal resolution ability in the 25-30 Hz range. Blockade of muscarinic acetylcholine receptors had a smaller effect than the partial blockade of nicotinic acetylcholine receptors. In ERG-recordings, blocking GABAa-receptors increased the a- and b-wave amplitude, induced a delay, an increase and a slow fall-off of the d-wave. Blocking GABAc-receptors had little effect. Blocking GABAa- or GABAa/c-receptors changed the temporal resolution, when expressed as a linear filter, from a 3rd degree filter with resonance to a low order low-pass filter with a low upper limit frequency. The temporal transfer properties were barely changed by blocking either nicotinic or muscarinic acetylcholine receptors, although ERG-components increased in amplitude to varying degrees. The behavioral and electrophysiological data indicate the important role of GABA for temporal processing but little involvement of the cholinergic system. It is proposed that the interaction of the GABAergic amacrine cell network and bipolar cells determines the gain of the retinal temporal coding in the upper frequency range.

Muscarinic M1 acetylcholine receptors regulate the non‐quantal release of acetylcholine in the rat neuromuscular junction via NO‐dependent mechanism

Nitric oxide (NO), previously demonstrated to participate in the regulation of the resting membrane potential in skeletal muscles via muscarinic receptors, also regulates non-quantal acetylcholine (ACh) secretion from rat motor nerve endings. Non-quantal ACh release was estimated by the amplitude of endplate hyperpolarization (H-effect) following a blockade of skeletal muscle post-synaptic nicotinic receptors by (+)-tubocurarine. The muscarinic agonists oxotremorine and muscarine lowered the H-effect and the M1 antagonist pirenzepine prevented this effect occurring at all. Another muscarinic agonist arecaidine but-2-ynyl ester tosylate (ABET), which is more selective for M2 receptors than for M1 receptors and 1,1-dimethyl-4-diphenylacetoxypiperidinium (DAMP), a specific antagonist of M3 cholinergic receptors had no significant effect on the H-effect. The oxotremorine-induced decrease in the H-effect was calcium and calmodulin-dependent. The decrease was negated when either NO synthase was inhibited by N(G)-nitro-L-arginine methyl ester or soluble guanylyl cyclase was inhibited by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. The target of muscle-derived NO is apparently nerve terminal guanylyl cyclase, because exogenous hemoglobin, acting as an NO scavenger, prevented the oxotremorine-induced drop in the H-effect. These results suggest that oxotremorine (and probably also non-quantal ACh) selectively inhibit the non-quantal secretion of ACh from motor nerve terminals acting on post-synaptic M1 receptors coupled to Ca(2+) channels in the sarcolemma to induce sarcoplasmic Ca(2+)-dependent synthesis and the release of NO. It seems that a substantial part of the H-effect can be physiologically regulated by this negative feedback loop, i.e., by NO from muscle fiber; there is apparently also Ca(2+)- and calmodulin-dependent regulation of ACh non-quantal release in the nerve terminal itself, as calmidazolium inhibition of the calmodulin led to a doubling of the resting H-effect.

Combined blockade of α3β4 nicotinic acetylcholine receptors and GluR1 AMPA receptors in rats prevents kainate-induced tonic-clonic seizures

Intramuscular injection of IEM-1754, a blocker of cerebral GluR1 AMPA receptors, in doses of 0.5-3.0 mg/kg decreased the incidence of kainate-induced tonic-clonic seizures and mortality rate by 2.7-4 times. IEM-1678, an alpha3beta4 nicotinic acetylcholine receptor antagonist administered intramuscularly in a maximum dose of 3 mg/kg decreased the incidence of kainate-induced tonic-clonic seizures and mortality rate by 2.3-2.7. IEM-1460 blocking both GluR1 AMPA receptors and alpha3beta4 nicotinic acetylcholine receptors, injected intramuscularly in doses of 0.5-3.0 mg/kg produced the maximum anticonvulsant activity and 8-fold decreased the incidence of kainate-induced tonic-clonic seizures and mortality rate.

Comparison of the effects of bupropion on nicotinic receptor-evoked [3H]dopamine release from rat striatal synaptosomes and slices

Tobacco smoking is a nicotine addiction, mediated in part by the ability of nicotine to elicit dopamine release, as a result of the stimulation of nicotinic acetylcholine receptors associated with dopaminergic pathways. The smoking cessation agent bupropion is an inhibitor of the dopamine transporter, but has also been shown to inhibit nicotinic acetylcholine receptors. To assess the relative impact of its actions at these two targets, we have examined the effects of bupropion on nicotine-evoked [(3)H]dopamine release from rat striatal synaptosomes and slices, in the absence of any other transporter inhibitor. Bupropion (10 microM) significantly decreased nicotine-evoked [(3)H]dopamine release by approximately 50% in both preparations, consistent with the blockade of nicotinic receptors. In support of this interpretation, bupropion also selectively inhibited nicotine-evoked Ca(2+) increases in SH-SY5Y cells. In striatal slices (but not in synaptosomes) the concentration-response profile for bupropion has an inverted 'u' shape, with a decrease in nicotine-evoked [(3)H]dopamine release also observed in the presence of 0.1 microM bupropion. This effect of 0.1 microM bupropion (but not 10 microM bupropion) was reversed by the dopamine D(2) receptor antagonist raclopride. We propose that modest blockade of the dopamine transporter by low concentrations of bupropion results in feedback inhibition via dopamine D(2) autoreceptors. This is overcome at higher concentrations of bupropion, before inhibition of nicotinic receptors occurs. Therefore bupropion's inhibition of the dopamine transporter and nicotinic receptors appears to be separated with respect to concentration.