Evidence for modulation of GABAergic neurotransmission by nicotine

Nicotinic acetylcholine receptors and epilepsy

Despite recent advances in understanding the causes of epilepsy, especially the genetic, comprehending the biological mechanisms that lead to the epileptic phenotype remains difficult. A paradigmatic case is constituted by the epilepsies caused by altered neuronal nicotinic acetylcholine receptors (nAChRs), which exert complex physiological functions in mature as well as developing brain. The ascending cholinergic projections exert potent control of forebrain excitability, and wide evidence implicates nAChR dysregulation as both cause and effect of epileptiform activity. First, tonic-clonic seizures are triggered by administration of high doses of nicotinic agonists, whereas non-convulsive doses have kindling effects. Second, sleep-related epilepsy can be caused by mutations on genes encoding nAChR subunits widely expressed in the forebrain (CHRNA4, CHRNB2, CHRNA2). Third, in animal models of acquired epilepsy, complex time-dependent alterations in cholinergic innervation are observed following repeated seizures. Heteromeric nAChRs are central players in epileptogenesis. Evidence is wide for autosomal dominant sleep-related hypermotor epilepsy (ADSHE). Studies of ADSHE-linked nAChR subunits in expression systems suggest that the epileptogenic process is promoted by overactive receptors. Investigation in animal models of ADSHE indicates that expression of mutant nAChRs can lead to lifelong hyperexcitability by altering i) the function of GABAergic populations in the mature neocortex and thalamus, ii) synaptic architecture during synaptogenesis. Understanding the balance of the epileptogenic effects in adult and developing networks is essential to plan rational therapy at different ages. Combining this knowledge with a deeper understanding of the functional and pharmacological properties of individual mutations will advance precision and personalized medicine in nAChR-dependent epilepsy.

Central histaminergic transmission modulates the expression of chronic nicotine withdrawal induced anxiety-like and somatic behavior in mice

The present study investigated the plausible modulatory role of central histaminergic transmission on the expression of nicotine withdrawal induced anxiety and somatic behavior in mice. Abrupt cessation of chronic nicotine (2 mg/kg, i.p. × 3/day) treatment for 12 days to mice, expressed increased anxiety in light & dark test and total abstinence (somatic) score at 24 h post nicotine withdrawal time. The somatic signs includes a composite score of all behaviors such as grooming, rearing, jumping, body shakes, forelimb tremors, head shakes, abdominal constrictions, scratching, empty mouth chewing or teeth chattering, genital licking, tail licking. Mice exhibited higher expression to nicotine withdrawal induced anxiety in light & dark test at 24 h post-nicotine withdrawal time on pre-treatment centrally (i.c.v) with histaminergic agents like histamine (0.1, 50 μg/mouse), histamine H₃ receptor inverse agonist, thioperamide (2, 10 μg/mouse), histamine H₁ receptor agonist, FMPH (2, 6.5 μg/mouse) or H₂ receptor agonist amthamine (0.1, 0.5 μg/mouse) or intraperitoneally (i.p.) with histamine precursor, l-histidine (250, 500 mg/kg) as compared to control nicotine withdrawn animals. Furthermore, mice pre-treated with all these histaminergic agents except histamine H₁ receptor agonist, FMPH shows exacerbated expression to post-nicotine withdrawal induced total abstinence (somatic) score in mice. On the other hand, central injection of selective histamine H₁ receptor antagonist, cetirizine (0.1 μg/mouse, i.c.v.) or H₂ receptor antagonist, ranitidine (50 μg/mouse, i.c.v) to mice 10 min before 24 h post-nicotine withdrawal time completely alleviated the expression of nicotine withdrawal induced anxiety and somatic behavior. Thus, it can be contemplated that the blockade of central histamine H₁ or H₂ receptor during the nicotine withdrawal phase could be a novel approach to mitigate the nicotine withdrawal associated anxiety-like manifestations. Contribution of endogenous histamine via H₁ or H₂ receptor stimulation in the nicotine withdrawal induced anxiety and somatic behavior is proposed.

Stability or Plasticity? - A Hierarchical Allostatic Regulation Model of Medial Prefrontal Cortex Function for Social Valuation

The medial prefrontal cortex (mPFC) has long been recognized as the key component of the neurocircuitry involved in various social as well as non-social behaviors, however, little is known regarding the organizing principle of distinctive subregions in the mPFC that integrates a wide range of mPFC functions. The present study proposes a hierarchical model of mPFC functionality, where three functionally dissociable subregions, namely, the ventromedial prefrontal cortex (vmPFC), rostromedial prefrontal cortex (rmPFC), and dorsomedial prefrontal cortex (dmPFC), are differentially involved in computing values of decision-making. According to this model, the mPFC subregions interact with each other in such a way that more dorsal regions utilize additional external sensory information from environment to predict and prevent conflicts occurring in more ventral regions tuned to internal bodily signals, thereby exerting the hierarchically organized allostatic regulatory control over homeostatic reflexes. This model also emphasizes the role of the thalamic reticular nucleus (TRN) in arbitrating the transitions between different thalamo-cortical loops, detecting conflicts between competing options for decision-making, and in shifting flexibly between decision modes. The hierarchical architecture of the mPFC working in conjunction with the TRN may play a key role in adjusting the internal (bodily) needs to suit the constraints of external (environmental) variables better, thus effectively addressing the stability-plasticity dilemma.

Nicotine modulation of fear memories and anxiety: Implications for learning and anxiety disorders

Anxiety disorders are a group of crippling mental diseases affecting millions of Americans with a 30% lifetime prevalence and costs associated with healthcare of $42.3 billion. While anxiety disorders show high levels of co-morbidity with smoking (45.3% vs. 22.5% in healthy individuals), they are also more common among the smoking population (22% vs. 11.1% in the non-smoking population). Moreover, there is clear evidence that smoking modulates symptom severity in patients with anxiety disorders. In order to better understand this relationship, several animal paradigms are used to model several key symptoms of anxiety disorders; these include fear conditioning and measures of anxiety. Studies clearly demonstrate that nicotine mediates acquisition and extinction of fear as well as anxiety through the modulation of specific subtypes of nicotinic acetylcholine receptors (nAChRs) in brain regions involved in emotion processing such as the hippocampus. However, the direction of nicotine's effects on these behaviors is determined by several factors that include the length of administration, hippocampus-dependency of the fear learning task, and source of anxiety (novelty-driven vs. social anxiety). Overall, the studies reviewed here suggest that nicotine alters behaviors related to fear and anxiety and that nicotine contributes to the development, maintenance, and reoccurrence of anxiety disorders.

Nicotinic receptors, memory, and hippocampus

Nicotinic acetylcholine receptors (nAChRs) modulate the neurobiological processes underlying hippocampal learning and memory. In addition, nicotine's ability to desensitize and upregulate certain nAChRs may alter hippocampus-dependent memory processes. Numerous studies have examined the effects of nicotine on hippocampus-dependent learning, as well as the roles of low- and high-affinity nAChRs in mediating nicotine's effects on hippocampus-dependent learning and memory. These studies suggested that while acute nicotine generally acts as a cognitive enhancer for hippocampus-dependent learning, withdrawal from chronic nicotine results in deficits in hippocampus-dependent memory. Furthermore, these studies demonstrated that low- and high-affinity nAChRs functionally differ in their involvement in nicotine's effects on hippocampus-dependent learning. In the present chapter, we reviewed studies using systemic or local injections of acute or chronic nicotine, nAChR subunit agonists or antagonists; genetically modified mice; and molecular biological techniques to characterize the effects of nicotine on hippocampus-dependent learning.

Acetylcholine as a neuromodulator: cholinergic signaling shapes nervous system function and behavior

Acetylcholine in the brain alters neuronal excitability, influences synaptic transmission, induces synaptic plasticity, and coordinates firing of groups of neurons. As a result, it changes the state of neuronal networks throughout the brain and modifies their response to internal and external inputs: the classical role of a neuromodulator. Here, we identify actions of cholinergic signaling on cellular and synaptic properties of neurons in several brain areas and discuss consequences of this signaling on behaviors related to drug abuse, attention, food intake, and affect. The diverse effects of acetylcholine depend on site of release, receptor subtypes, and target neuronal population; however, a common theme is that acetylcholine potentiates behaviors that are adaptive to environmental stimuli and decreases responses to ongoing stimuli that do not require immediate action. The ability of acetylcholine to coordinate the response of neuronal networks in many brain areas makes cholinergic modulation an essential mechanism underlying complex behaviors.

Effects of nicotine on cerebral metabolism

Interest in identifying brain areas mediating the behavioural effects of nicotine led to autoradiographic studies on the distribution of cerebral metabolic responses to nicotine. The 2-deoxy-D-[1-14C] glucose method was used to map and quantitate nicotine's effects in the rat brain. The method allows simultaneous measurement of the regional cerebral metabolic rate(s) for glucose (rCMRglc), an index of functional activity, throughout the central nervous system. It provides information about sites of initial drug interactions, and of secondary effects propagated via afferents to remote areas. In rats given acute systemic (-)nicotine, stimulation occurs in brain areas which contain specific binding sites for [3H] nicotine, indicating that the sites are true receptors, linked to functional activity. Doses of nicotine that are discriminated by rats and that produce behavioural and physiological effects stimulate rCMRglc. The stimulation is transient and is antagonized by mecamylamine. Affected areas include limbic structures, components of the visual system, brainstem nuclei important in cardiovascular reflexes, and areas involved in motor function. The distribution of nicotine's in vivo effects on rCMRglc implicates various brain regions in the behavioural and physiological effects of nicotine. Future studies employing positron emission tomography will assess relations between nicotine's effects on mood and rCMRglc in man.

Interaction of the Nicotinic Cholinergic System with Ethanol Withdrawal

The observation that alcohol and nicotine are commonly abused together suggests that the two drugs have common sites of action. In vitro studies indicate that nicotinic acetylcholine receptor (nAChR) function is enhanced by ethanol. Furthermore, some ethanol-related behaviors are associated with a region of mouse chromosome 2 that contains the gene encoding the alpha4 subunit of the nAChR (Chrna4). We have identified a polymorphism in Chrna4 that results in an alanine (A) or threonine (T) residue at position 529 in the second intracellular loop of the protein. Nicotinic receptors expressing the A variant have greater responses to nicotine and ethanol than receptors with the T variant when measured in vitro, but the possible effects of the polymorphism on the severity of ethanol withdrawal have not been assessed. The handling-induced convulsion (HIC) assay is an established method for studying drug withdrawal in vivo. We monitored the HIC responses of mice for 8 h after an injection of ethanol (4 g/kg). A survey of 16 mouse strains, as well as previously published data, indicated an association of the A/T polymorphism with ethanol withdrawal. This association was also found in wild-type animals from an F2 intercross of the A/J (A529-genotype) strain with C57BL/6J (T529-genotype) mice that also lack expression of the beta2 nAChR subunit. Beta2 -/- animals, which do not express alpha4beta2 nAChRs in the brain, exhibited significantly lower HIC responses and no effect of the polymorphism. These results suggest that the nicotinic cholinergic system and the A/T polymorphism modulate ethanol withdrawal.

Nicotine enhances contextual fear conditioning in C57BL/6J mice at 1 and 7 days post-training

Nicotine has been demonstrated to enhance learning processes. The present experiments extend these results to examine the effects of nicotine on acquisition and consolidation of contextual and cued fear conditioning, and the duration of nicotine's enhancement of conditioned fear. C57BL/6 mice were trained with two pairings of an auditory CS and a foot shock US. Multiple doses of nicotine were given before or immediately after training and on testing day (0.0, 0.050, 0.125, 0.250, and 0.375 mg/kg, i.p). Freezing to both the context and auditory CS was measured 24h after training and again 1 week after training. Mice did not receive nicotine for the 1-week retest. Nicotine (0.125 and 0.250 mg/kg) given on both training and testing days enhanced freezing to the context at 24h. In addition, elevated freezing to the context was seen 1 week post-training in mice previously treated with 0.125 and 0.250 mg/kg nicotine. Thus, nicotine-treated mice did show elevated levels of freezing when retested 1 week later, even though no nicotine was administered at the 1-week retest. Mice that received nicotine on training day or testing day only and mice that received nicotine with mecamylamine, a nicotinic receptor antagonist, were not different from saline-treated mice. In addition, post-training administration of nicotine did not enhance fear conditioning. The present results indicate that nicotine enhancement of contextual fear conditioning depends on administration of nicotine on training and test days but results in a long-lasting enhancement of memories of contextual fear conditioning that remains in the absence of nicotine.

GABAergic systems modulate nicotinic receptor-mediated seizures in mice

The pharmacology of nicotinic receptor-mediated seizures was investigated in C3H mice. Eleven nicotinic agonists and six antagonists were administered centrally (i.c.v.). Epibatidine and epiboxidine were the most potent agonists tested, whereas acetylcholine and the alpha7*-selective compounds 3-(2,4-dimethoxybenzylidene)-anabaseine (GTS-21) and anabasine, were the least potent. Nicotine-induced seizures were blocked by cotreatment with either the nonselective antagonist mecamylamine or the alpha7*-selective antagonist methyllycaconitine. The alpha4beta2*-selective antagonist dihydro-beta-erythroidine was ineffective at blocking seizures. However, high doses of all six antagonists tested were fully efficacious in producing seizures, with d-tubocurarine being the most potent and mecamylamine the least potent. Potential relationships between nicotinic receptor-mediated seizures and drug effects on GABA function were also investigated. No correlation was seen between potencies of the agonists in producing seizures and stimulating [3H]GABA release or between potencies of the antagonists in producing seizures and antagonist inhibition of nicotine-stimulated [3H]GABA release. However, a robust correlation was detected between potencies of the agonists in producing seizures and the IC50 values for inhibition of nicotine-stimulated [3H]GABA release produced by agonist-induced receptor desensitization. We also compared inbred mouse strain sensitivity to nicotine, picrotoxin, bicuculline, and kainate-induced seizures. Robust positive correlations were revealed for nicotine-induced seizures and seizures induced by either picrotoxin or bicuculline, both GABAA receptor antagonists. No correlation was found between nicotine-induced seizures and those induced by the excitatory amino acid receptor agonist kainate. Based on these findings, we present a model for nicotinic receptor-mediated seizures mediated through GABAergic systems.

Nicotine produces a within-subject enhancement of contextual fear conditioning in C57BL/6 mice independent of sex

Nicotine enhances learning including contextual fear conditioning. The present study extends previous work on nicotine and conditioned fear to examine the nature of nicotine's enhancement of contextual fear conditioning and sex differences in contextual fear conditioning in C57BL/6 mice using a within-subjects design. Mice were trained by pairing of an auditory stimulus of 80 dB, 6 cps train of broad-band clicks conditioned stimulus (CS) with a 2 sec., 0.35 mA shock unconditioned stimulus (US). Twenty-four hours later mice were tested for freezing in the original context, and one hour later mice were retested in the same context. A 0.5 mg/kg dose of nicotine was given either for three conditions: (1) before training, testing, and retesting; (2) before training and retesting; and (3) before retesting only. The use of a within-subjects design allowed for testing if nicotine would produce state-dependent deficits in contextual fear conditioning. Nicotine did enhance contextual fear conditioning in the groups that received nicotine for both training and testing. Nicotine, however, did not alter freezing when given on training but not testing or testing but not training. No sex differences, however, existed for conditioning or for nicotine's effects on conditioning. These results suggest that nicotine enhanced acquisition and retrieval processes but did not produce state-dependent deficits when administered just for training or just for testing.

Beta-amyloid activates the mitogen-activated protein kinase cascade via hippocampal alpha7 nicotinic acetylcholine receptors: In vitro and in vivo mechanisms related to Alzheimer's disease

Alzheimer's Disease (AD) is the most common of the senile dementias, the prevalence of which is increasing rapidly, with a projected 14 million affected worldwide by 2025. The signal transduction mechanisms that underlie the learning and memory derangements in AD are poorly understood. beta-Amyloid (Abeta) peptides are elevated in brain tissue of AD patients and are the principal component of amyloid plaques, a major criterion for postmortem diagnosis of the disease. Using acute and organotypic hippocampal slice preparations, we demonstrate that Abeta peptide 1-42 (Abeta42) couples to the mitogen-activated protein kinase (MAPK) cascade via alpha7 nicotinic acetylcholine receptors (nAChRs). In vivo elevation of Abeta, such as that exhibited in an animal model for AD, leads to the upregulation of alpha7 nAChR protein. alpha7 nAChR upregulation occurs concomitantly with the downregulation of the 42 kDa isoform of extracellular signal-regulated kinase (ERK2) MAPK in hippocampi of aged animals. The phosphorylation state of a transcriptional mediator of long-term potentiation and a downstream target of the ERK MAPK cascade, the cAMP-regulatory element binding (CREB) protein, were affected also. These findings support the model that derangement of hippocampus signal transduction cascades in AD arises as a consequence of increased Abeta burden and chronic activation of the ERK MAPK cascade in an alpha7 nAChR-dependent manner that eventually leads to the downregulation of ERK2 MAPK and decreased phosphorylation of CREB protein.

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.

Nicotine reverses GABAergic inhibition of long-term potentiation induction in the hippocampal CA1 region

Nicotine is known to enhance cognitive function but the mechanism is unknown. The present study examined the modulatory effect of nicotine on the induction of long-term potentiation (LTP), a synaptic model of learning and memory. A weak tetanic stimulation consisting of 20 pulses at 100 Hz induced stable LTP in the hippocampal CA1. The induction of LTP was completely blocked if the tetanus was delivered in the presence of muscimol (2.5 microM), a gamma-aminobutyric acid (GABA) receptor agonist. This inhibition was sensitive to, and reversed by, not only nicotinic acetylcholine receptor (nAChR) agonists (nicotine and epibatidine), but also the alpha7 nAChR-selective antagonist methyllycaconitine (MLA). Furthermore, ACh-puff activation of alpha7 nAChRs on feedforward interneurons induced inhibitory postsynaptic currents in pyramidal cells that were blocked by nicotine or MLA. In addition, nicotine reduced field monosynaptic inhibitory postsynaptic potentials in the presence of MLA. These results suggest not only two pathways of nicotine-induced disinhibition of pyramidal cells, one involving desensitization of alpha7 nAChRs and the other involving non-alpha7 nAChRs, but also two potential mechanisms underlying the modulatory effect of nicotine on LTP induction, both reducing GABAergic inhibition, thereby indirectly increasing the excitability of pyramidal cells.

Impairments of long-term potentiation in hippocampal slices of beta-amyloid-infused rats

In this study, we investigated the neuronal activity of hippocampal slices from the beta-amyloid protein-infused (300 pmol/day for 10-11 days) rats using the extracellular recording technique. Perfusion of nicotine (50 microM) reduced the amplitude of electrically evoked population spikes in the CA1 pyramidal cells of the vehicle control rats, but not in those of the beta-amyloid protein-infused rats, suggesting the impairment of nicotinic signaling in the beta-amyloid protein-infused rats. Long-term potentiation induced by tetanic stimulations in CA1 pyramidal cells, which was readily observed in the vehicle control rats, was also impaired in the beta-amyloid protein-infused rats. Nicotinic blockade by adding hexamethonium into the perfused solution inhibited long-term potentiation induction. Taken together, our previous and present results suggest that beta-amyloid protein infusion impairs the signal transduction mechanisms via nicotinic acetylcholine receptors. This dysfunction may be responsible, at least in part, for the impairment of long-term potentiation induction and may lead to learning deficits.

Functional nicotinic ACh receptors on interneurones in the rat hippocampus

1. Neuronal nicotinic ACh receptors (nAChRs) were studied in the rat hippocampal slice preparation using whole-cell patch-clamp recording techniques. 2. Responses to ACh (100 microM) were detected on inhibitory interneurones in the Ca1 field of the hippocampus proper and in the dentate gyrus, but not on principal excitatory neurones in either region. The different neuronal types were identified based on their morphology and location. 3. ACh excited interneurones in the hippocampus and dentate gyrus in current-clamp recordings. In voltage-clamp recordings, ACh-activated inward currents were recorded from interneurones in the presence of blockers of synaptic transmission and the muscarinic ACh receptor antagonist atropine. The zero current potential for this response to ACh was near 0 mV. 4. The effect of ACh was mimicked by the nAChR-selective agonists nicotine (100 microM) and 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP, 100 microM). The response to ACh was reversibly antagonized by the neuronal nAChR antagonist mecamylamine (10 microM). The nAChR alpha 7 subunit-selective antagonists alpha-bungarotoxin (100 nM) and methyllycaconitine (10 nM) also inhibited the response to ACh. 5. These observations demonstrate the presence of functional nAChRs on inhibitory interneurones in the rat hippocampus. Thus, a novel mechanism by which ACh can regulate neuronal activity in the hippocampus is revealed.

Cholinergic deafferentation enhances rat hippocampal pyramidal neuron responsiveness to locally applied nicotine

We tested whether cholinergic denervation of the hippocampus of young rats would result in an enhancement of CA1 pyramidal cell responsiveness to nicotine. Electrolytic lesions of the medial septal area were performed in young male Fisher 344 rats. One month later the rats were anesthetized with pentobarbital and nicotine was locally applied to CA1 pyramidal neurons using pressure microejection. The dose of nicotine required to excite the pyramidal neurons was significantly lower for cells recorded from rats with septal lesions. However, no changes in hippocampal cytisine or alpha-bungarotoxin binding were found.

Differential mechanisms involved in the effect of nicotinic agonists DMPP and lobeline to release [3H]5-HT from rat hippocampal slices

In the present study we investigated the effect of different nicotinic agonists (dimethylphenyl-piperazinium-iodide (DMPP), (-)nicotine, cytisine, (-)-lobeline, and (-)epibatidine) and antagonists (mecamylamine and dihydro-beta-erythroidine) on the release of [3H]5-HT from hippocampal slices. The nicotinic agonists DMPP and lobeline and electrical field stimulation, released [3H]5-HT from the hippocampus; other nicotinic agonists, such as (-)-nicotine, cytisine, and (-)-epibatidine had no effect. Unlike lobeline-induced release of [3H]5-HT, the effect of DMPP (10 and 40 microM) was antagonized by mecamylamine (20 and 10 microM). The effect of DMPP was [Ca2+]o-independent. In experiments carried out at 7 degrees C, i.e. the membrane carrier proteins are inhibited and the release by lobeline was abolished while the DMPP-induced release of 5-HT was rather potentiated. It is proposed that the effect of DMPP and lobeline, to enhance the release of [3H]5-HT from the hippocampus, was mediated by two different mechanisms. While DMPP-induced 5-HT release can be linked to a non-classical nAChR activation ([Ca2+]o-independence), the effect of lobeline was likely mediated by uptake carriers.

Mechanisms of the formation of reactions of cat motor cortex neurons associated with the triggering of the conditioned placing reflex: a hypothesis

The possible physiological mechanisms of the generation of reactions by neurons of the cat motor cortex during the triggering of a conditioned placing reflex are examined. It is hypothesized that neurons of the motor cortex function as a neuronal generator similar to the spinal generator of locomotion, which can be triggered through ion channels that are controlled by NMDA-type glutamate receptors. The conditions necessary for the opening of these channels under the influence of glutamate are achieved by the conditioned "arousal reaction", the neurochemical equivalent of which is an intensification of the cholinergic and noradrenergic and/or serotoninergic afferentation to the cortex.

Cytisine-induced behavioral activation: delineation of neuroanatomical locus of action

Systemic injections of nicotine increase locomotor activity. The present study was designed to determine whether there is a circumscribed region in the ventral tegmentum that mediates the locomotor-activating effects of nicotine. The mapping technique was used to delineate this region: bilateral injections of cytisine (1 nmol/0.5 microliters per side) were made into sites in and around the ventral tegmentum and the amount of locomotion associated with each site was quantified. The distribution of injection sites spanned between 0.8 and 4.6 mm posterior to bregma. The amount of locomotion varied with relation to the area into which cytisine was injected and a region was identified within which injections of cytisine preferentially increased locomotion. This region appears to have reasonably well-defined anterior and posterior boundaries. Since a portion of the delineated area overlaps the dopamine-containing cell-body region, the possibility remains that the activation of the mesolimbic dopamine system may contribute to the behavioral activation associated with ventral tegmental injections of cytisine. This same system may contribute to the locomotor-activating effects associated with systemic injections of nicotine.

Benzodiazepine receptor mediation of the anxiolytic-like effect of (-)-nicotine in mice

The anxiolytic-like effect of (-)-nicotine (1.9 mumol/kg, IP) on the elevated plus-maze in CD1 mice was blocked by the benzodiazepine receptor antagonist flumazenil (1 and 10 mumol/kg, IP). On the other hand, the cholinergic nicotinic channel blocker mecamylamine (1 to 15 mumol/kg, IP), did not affect the anxiolytic-like properties of diazepam in the same test. These data suggest that the reduction in anxiety induced by (-)-nicotine occurs indirectly via the release of endogenous substances that can activate the benzodiazepine receptor.

Augmenting action of nicotine on population spikes in the dentate gyrus of the guinea pig

Effects of nicotinic cholinergic agents on field potentials recorded from the dentate gyrus were studied in thin transverse sections of the hippocampus of the guinea pig. Nicotine augmented the population spike elicited by the second stimulus of a paired stimulation to the molecular layer. The threshold concentration of nicotine to cause this effect was 5-10 microM. The augmentation of the spike was not accompanied by an increase in the rising slope of population excitatory postsynaptic potentials, and was not observed in the presence of bicuculline. Carbamylcholine had a weak and inconsistent effect. D-tubocurarine and mecamylamine also augmented the population spike. The action of nicotine was blocked by hexamethonium. These results suggest that nicotine facilitates the generation of action potentials in granule cells by depressing inhibitory processes, and that properties of nicotinic cholinergic receptors are different in different subfields of the hippocampal formation, presumably reflecting the diversity of the receptors.

Chronic nicotine reverses age-associated increases in tail-flick latency and anxiety in rats

The chronic consumption of low doses of nicotine in drinking water for two years consistently increased the sensitivity of rats to a nociceptive thermal stimulus (tail-flick test), but reduced aversiveness in the elevated plus-maze test, relative to the responses of age-matched controls in these tests. The responses of aged nicotine-consuming rats were indistinguishable from those of young adult rats that did not receive nicotine. To determine whether these effects were due to a nicotine-induced retardation of age-related changes, young adult rats were similarly treated with nicotine for three months and similar changes in the tail-flick latency and performance in the plus-maze test were observed during nicotine consumption. These changes were reversed following withdrawal from nicotine. It is concluded that the maintenance of circulating low levels of nicotine (and/or its metabolites) increased the nociceptive sensitivity of the rats and reduced their aversions in the plus-maze test regardless of their age.

Effects of chronic ethanol on cholinergic actions in rat hippocampus: electrophysiological studies and quantification of m1-m5 muscarinic receptor subtypes

The effects of chronic ethanol treatment (CET) on cholinergic modulation of CA1 evoked field potentials and recurrent inhibition were investigated in rat hippocampal slices. Densities of muscarinic receptor subtypes were quantified in remaining hippocampal tissue by immunoprecipitation. Iontophoretic application of ACh in stratum pyramidale results in facilitation of single evoked population spikes; application in stratum radiatum results in depression of field EPSPs. CET decreased cholinergic facilitation of population spikes, while cholinergic inhibition of field EPSPs remained unaffected. Integrity of feedback (recurrent) inhibitory circuitry was evaluated by paired-pulse stimulation. As previously demonstrated, recurrent inhibition was significantly reduced after CET; cholinergic disinhibition was also significantly reduced. Thus, CET appears to disrupt a subset of cholinergic effector systems within hippocampal neurons. The reductions in cholinergic function produced by CET does not appear to be due to receptor loss, since muscarinic receptor subtype densities were not found to be significantly altered in this tissue. These results support the hypothesis that muscarinic receptor function is impaired in CA1 pyramidal cells through a disruption of intracellular signal transduction mechanisms. While it is unclear whether cholinergic function is reduced in interneurons directly, these results suggest that modulation of neuronal firing in the hippocampus is markedly altered following CET due to impairment of both cholinergic and GABAergic systems.

P and NP rats respond differently to the discriminative stimulus effects of nicotine

Rats selectively bred for ethanol preference, that is, alcohol-preferring (P) and -nonpreferring (NP) rats, were trained to discriminate the interoceptive stimuli produced by IP-administered 1,000 mg/kg ethanol (10% v/v) in a two-lever, food-motivated, operant task. Once criterion performance was attained, dose-response experiments indicated an ED50 value for P rats = 354.1 mg/kg, whereas NP rats generated an ED50 value of 495.1 mg/kg, not significantly different from each other. In contrast to these similar sensitivities to ethanol, administration of doses of nicotine (0.4-1.2 mg/kg, SC) indicated that P rats were significantly more sensitive to the ethanol-like effects of nicotine than were NP rats. The results provide additional evidence for a possible hereditary co-occurrence of alcohol and nicotine sensitivity.

Differential effects of nicotine but not cathinone on motor activity of P and NP rats

The locomotor stimulatory effects of nicotine (0.4 and 0.8 mg/kg) and cathinone (0.5 and 1.0 mg/kg) were assessed in alcohol-preferring (P) and -nonpreferring (NP) rats. Whereas P rats demonstrated enhanced (0.8 mg/kg) or no change (0.4 mg/kg) in spontaneous locomotor activity (SMA) to nicotine, NP animals showed no change (0.4 mg/kg) or depression of activity (0.8 mg/kg). However, following cathinone administration both P and NP rats exhibited an increase in SMA. The above results are discussed in light of the genotypic variations between P/NP rats and the potential mediation of differential neurotransmitter effects in the two lines.

Enhancement of optokinetic and vestibuloocular responses in the rabbit by cholinergic stimulation of the flocculus

Bilateral microinjections into the cerebellar flocculus of the rabbit of carbachol, a general cholinergic agonist, profoundly affect vestibuloocular (VOR) and optokinetic (OKR) reflexes. For sinusoidal stimuli (0.15 Hz, 5 deg peak to peak), the gain of the OKR was strongly increased, while the gain of the VOR was moderately increased. These effects were partially mimicked by floccular injection of the acetylcholinesterase inhibitor eserine. Floccular injection of the muscarinic blocker atropine significantly lowered the gain of the OKR. The effects of the nicotinic blocker mecamylamine were not significant. Optokinetic nystagmus (OKN) in response to constant stimulus velocities (1-30 deg/second) showed a markedly accelerated buildup and a shortened optokinetic after-nystagmus (OKAN) after floccular injections of carbachol. The steady-state gain of OKN remained unaffected. None of the described effects occurred after floccular injection of the solvent, saline. It is postulated that cholinergic cerebellar afferents, one probable source of which are the vestibular nuclei, enhance the optokinetic and vestibular modulation of floccular Purkinje cells.

Nicotinic systems and cognitive function

Nicotinic acetylcholine receptors have been found to be important for maintaining optimal performance on a variety of cognitive tasks. In humans, nicotine-induced improvement of rapid information processing is particularly well documented. In experimental animals nicotine has been found to improve learning and memory on a variety of tasks, while the nicotinic antagonist mecamylamine has been found to impair memory performance. Nicotine has been found to be effective in attenuating memory deficits resulting from lesions of the septohippocampal pathway or aging in experimental animals. Nicotinic receptors are decreased in the cortex of patients with Alzheimer's disease. Preliminary studies have found that some aspects of the cognitive deficit in Alzheimer's disease can be attenuated by nicotine. Nicotine may prove to be useful therapeutic treatment for this and other types of dementia.

Effect of acute and subchronic nicotine treatment on cortical efflux of [3H]-D-aspartate and endogenous GABA in freely moving guinea-pigs

1. The [3H]-D-aspartate preloading of the parietal cortex of freely moving guinea-pigs equipped with epidural cups makes it possible to investigate drug effects on the efflux of this radiolabel, assumed as a marker of the glutamatergic structures underlying the cup. In the same model, the efflux of [3H]-gamma-aminobutyric acid ([3H]-GABA) and endogenous GABA can be measured. 2. Nicotine, 0.9-3.6 mg kg-1, s.c., or 3-5 micrograms, i.c.v., increased the efflux of [3H]-D-aspartate but reduced that of GABA. 3. These effects were mediated through mecamylamine-sensitive receptors but the ganglionic blocking agent was devoid of any primary activity. 4. The inhibition of GABA efflux induced by nicotine 3.6 mg kg-1, s.c., was abolished by methysergide 2 mg kg-1, i.p. and was reduced by naloxone 3 mg kg-1, i.p. pretreatment, suggesting the involvement of tryptaminergic and opioid systems. In contrast, muscarinic and catecholamine antagonists were ineffective. 5. Chronic treatment with nicotine (3.6 mg kg-1, twice daily for 16 days) reduced the facilitatory effect of [3H]-D-aspartate and abolished the inhibition of endogenous GABA efflux. 6. A slight increase in the number of nicotinic binding sites (by use of [3H]-nicotine as ligand) was found in the neocortex of chronically treated guinea-pigs. 7. The higher degree of tolerance to chronic nicotine treatment shown by GABA as compared with [3H]-D-aspartate efflux suggests that adaptative changes of the inhibitory neuronal pools prevail. This may contribute to the reinforcing and addictive properties of nicotine.

Cholinergic modulation of optokinetic and vestibulo-ocular responses: a study with microinjections in the flocculus of the rabbit

In spite of a large body of histochemical evidence for a cholinergic system in the cerebellum, particularly in lobules IX and X, the physiological role of such a system has remained obscure. In view of the important role of these same lobules in the control of the vestibulo-ocular (VOR) and optokinetic (OKR) responses, we tested the effect of microinjections of cholinergic (ant)agonists in the flocculus of the rabbit on these reflexes. Very marked effects were found. Bilateral floccular injection of the aspecific cholinergic agonist carbachol raised the gain of the OKR by about 0.46 above the baseline values, while the gain of the VOR in darkness was raised by about 0.14. These effects were statistically significant and persisted for several hours. Similar, but smaller effects were obtained after injection of eserine, an inhibitor of acetylcholinesterase. Thus, the effects could be produced by increasing the naturally present amount of acetylcholine. Microinjections of the nicotinic blocker mecamylamine reduced the gain of the VOR and OKR, although these effects did not reach statistical significance. The muscarinic blocker atropine significantly reduced the gain of the OKR, but not of the VOR. The present results argue strongly for an important physiological role of the cholinergic system in the cerebellum. Specifically, acetylcholine appears to be involved in the modulation of oculomotor reflexes through the flocculus.

Nicotine exerts differential effects on different CA1 hippocampal cell types

The effects of (-) nicotine hydrogen tartrate (NHT) were examined on several cell types in the CA1 region of rat hippocampus. The results indicate that nicotine may have a preferential net inhibitory effect on basket cells and an excitatory effect on oriens/alveus interneurons. The resultant effects of nicotine on pyramidal cells may thus be a product of complex local circuit interactions.

Nicotine interferes with GABA-mediated inhibitory processes in mouse hippocampus

Previous studies indicated that the excitatory effects of nicotine may be mediated via interference with GABAergic transmission. Here, several variants of the paired-pulse paradigm were employed to ascertain whether nicotine interferes with endogenous inhibitory circuits in the hippocampus. Nicotine attenuated the inhibition evoked by antidromic (alvear) stimulation in the CA1 region in a concentration-dependent manner (EC50 = 60-75 microM). This same phenomenon was also observed for the GABAA receptor antagonist bicuculline (0.1 microM). Orthodromic-orthodromic paired-pulse paradigms were found to be unsuitable for investigating the effects of epileptogenic agents such as nicotine and bicuculline on endogenous inhibition.

5 alpha-Pregnan-3 alpha-ol-20-one blocks nicotine-induced seizures and enhances paired-pulse inhibition

5 alpha-Pregnan-3 alpha-ol-20-one (3 alpha-OH-DHP) blocked seizures induced by nicotine (4 mg/kg, i.p.) in C3H male mice with an ID50 of 2.37 +/- 0.66 mg/kg (average +/- 95% confidence limit). This steroid (1 microM) also increased paired-pulse inhibition in the hippocampus approximately 40% after 50 min exposure; nicotine (200 microM) partially reversed this effect. Since nicotine and 3 alpha-OH-DHP may have opposite effects on endogenous inhibitory systems, it is proposed that nicotine-induced seizures may involve a disinhibitory mechanism and that 3 alpha-OH-DHP protects against seizures by preventing disinhibition.

Local circuitry in the IMHV of the domestic chick (Gallus domesticus)

The responses to local stimulation have been recorded from neurons in the intermediate part of the medial hyperstriatum ventrale (IMHV) of the domestic chick, by using an in vitro slice preparation. When the slice is bathed in gassed Krebs' solution, a single stimulus evokes a short-lasting diphasic response. The first phase is negative and lasts some 3 ms, whereas the second, positive phase is often of lower amplitude and usually persists for about 15 ms. The first phase is little altered by perfusion with either Ca2(+)-free Krebs' solution or Krebs' solution containing a high concentration of Mg2+. In contrast, the second phase is abolished by these procedures. The post-synaptic phase is positive when it is recorded anywhere between 0.1-1.25 mm from the stimulated point; however, in the immediate vicinity (0.0-0.1 mm) of the stimulating electrodes, the post-synaptic response is strongly negative. A pair of stimuli has to be separated by at least 10 s to guarantee complete recovery of excitability of the post-synaptic response. The recovery curve for this response shows a refractory period of some 5 ms, a peak of excitability at an interval of about 20 ms, and then a sharp trough of relative inexcitability at about 200 ms. The post-synaptic response is considerably reduced in magnitude and duration by the addition of AP-5 to the perfusion fluid; the remaining post-synaptic response is completely abolished by kynurenic acid. The addition of bicuculline methiodide in concentrations of at least 1 x 10(-6) M increases both the magnitude and duration of the second, positive phase of the response to single stimuli. This extended positive response (which may last from 500-800 ms) is abolished by perfusion with bicuculline dissolved in Ca2(+)-free Krebs' solution. For the entire duration of the extended post-synaptic positive response produced by bicuculline, the irregular discharge of single neurons can be recorded. Like the post-synaptic positive response in Krebs' solution, the much larger response produced by bicuculline shows a very localized negativity beneath the stimulating electrodes and displays an almost identical time-course for the recovery of excitability following a single stimulus. The bicuculline induced positive response is also considerably reduced by the presence of AP-5; the addition of kynurenic acid abolishes the remaining post-synaptic response completely. A post-synaptic response, similar to that produced under bicuculline, can be produced by the addition of a maximally effective dose of d-tubocurarine.(ABSTRACT TRUNCATED AT 400 WORDS)

The epileptogenic action of the taurine analogue guanidinoethane sulfonate may be caused by a blockade of GABA receptors

The aim of this paper is to clarify the mechanism through which the taurine analogue guanidinoethane sulfonate (GES) produces its epileptogenic effects. Experiments were performed in the rat hippocampus in vivo, using a brain dialysis probe also containing a recording electrode. Perfusion of 10 mM GES induced an enhancement of extracellular taurine levels probably as a result of forced efflux through the taurine uptake systems in a heteroexchange process. This taurine increase was highly reversible. GES also induced an increase of neuronal excitability and an impairment of recurrent inhibition as judged by the neuronal pattern discharge of evoked potentials. These results indicate the possible implication of GABA receptors in the epileptogenic effect of GES. Specific binding of [3H]-GABA to P2 fractions was inhibited by both bicuculline methiodide (BMI) and GES with the same potency. Similar results were obtained using cerebral sections. Autoradiographic experiments confirm the binding results. GES and BMI completely displaced [3H]-GABA binding. All these results suggest that the epileptogenic GES action is due to a direct antagonism on GABAA receptors.

Nicotine effects in mouse hippocampus are blocked by mecamylamine, but not other nicotinic antagonists

Previous data indicated that bath-application of nicotine to mouse hippocampal slices resulted in a concentration-dependent increase in the amplitude of the orthodromic population spike and the appearance of multiple population spikes in the CA1 pyramidal cell layer. d-Tubocurarine (4-100 microM), alpha-bungarotoxin (10-160 microM), and atropine (40-200 microM) had similar effects, although for alpha-bungarotoxin these excitatory effects were transient. Mecamylamine (1.6-3.2 mM) inhibited the population spike, while hexamethonium (3.2 mM) had no effect. These cholinergic antagonists were tested for their ability to block excitatory effects of nicotine (800 microM) at antagonist concentrations which were at or near threshold for intrinsic effects. Of the 5 antagonists tested, only mecamylamine (400 microM) effectively inhibited the nicotine-induced increase of the population spike amplitude and the appearance of multiple population spikes. These results suggest that nicotine exerts electrophysiological effects via a subclass of nicotinic cholinergic receptors that is neither neuromuscular nor ganglionic in the classical sense; these brain nicotinic receptors are sensitive to mecamylamine, but not to hexamethonium, alpha-bungarotoxin, or D-tubocurarine.

A direct nicotinic receptor-mediated inhibition recorded intracellularly in vitro

Acetylcholine activates both nicotinic and muscarinic receptors in the central nervous system. Although the action of acetylcholine at muscarinic receptor has been well characterized, relatively little is known at the cellular level concerning nicotinic receptor stimulation in brain. Central nicotinic receptors have been implicated in Alzheimer's disease, seizure activity, the generation of slow-wave theta rhythm in the hippocampus and the potential abuse liability of nicotine. At the neuronal level, nicotinic agonists have been most often associated with postsynaptically mediated excitation and membrane depolarization at various sites, including Renshaw spinal motoneurons, locus coeruleus and the medial habenular nucleus. Nicotine acting presynaptically can produce either excitation or inhibition indirectly through the release of endogeneous transmitters or modulators. Whereas a direct inhibitory effect of nicotine has been suggested by one in vivo extracellular recording study in rat cerebellar Purkinje neurons, the mechanism(s) underlying this action is not yet known. We now report our findings obtained using in vitro intracellular methods in a submerged brain slice preparation in which application of nicotinic agonists to rat dorsolateral septal neurons reveal a direct membrane hyperpolarization mediated by an increase in potassium conductance.