Role of Ca2+ ions in nicotinic facilitation of GABA release in mouse thalamus

Failure of nicotine-dependent enhancement of synaptic efficacy at Schaffer-collateral CA1 synapses of AD11 anti-nerve growth factor transgenic mice

Alzheimer's disease is a neurodegenerative disorder characterized by neuronal loss associated with a progressive impairment of cognitive functions. Early consequences of Alzheimer's disease include deficit of cholinergic signalling in particular regions controlling memory processes, such as the cortex and hippocampus, and accumulation of beta-amyloid (Abeta) peptide in neuritic plaques. The cholinergic system depends for its integrity and function on nerve growth factor. Chronic nerve growth factor deprivation in transgenic mice (AD11) engineered to produce recombinant neutralizing anti-nerve growth factor antibodies leads to progressive age-dependent Alzheimer's-like neurodegenerative pathology similar to that found in patients with Alzheimer's disease, associated with a selective loss of cholinergic neurones in the basal forebrain. Here we show that in the hippocampus of 6-month-old AD11 mice, Abeta aggregates started appearing in the CA1 region. The accumulation of Abeta was associated with a loss of cholinergic function at CA3-CA1 synapses. Whereas in wild-type mice nicotine induced a persistent increase of synaptic efficacy via alpha7 nicotine acetylcholine receptors, in AD11 mice this alkaloid failed to modify synaptic strength. Moreover, nicotine failed to transiently enhance the frequency of spontaneous miniature glutamatergic currents (miniature excitatory postsynaptic currents) recorded from CA1 but not from CA3 pyramidal neurones of AD11 mice. However, in CA3 principal cells of AD11 mice, the potentiating effect of nicotine on miniature excitatory postsynaptic currents was prevented when Abeta peptide 1-42 was added to the extracellular solution. These data suggest that in AD11 mice, Abeta interferes with nicotine acetylcholine receptors at the level of presynaptic glutamatergic terminals, inhibiting their function possibly through calcium signalling via presynaptic alpha7 nicotine acetylcholine receptors.

In vivo neurochemical characterization of Anatoxin-a evoked dopamine release from striatum

Anatoxin-a (AnTx) is a natural neurotoxin, which acts as a potent and stereoselective agonist at the nicotinic acetylcholine receptors. The in vivo actions of the AnTx on dopamine (DA) release are scarcely characterized. The aim of this study was to determine the neurochemical bases for AnTx-induced striatal DA release, using the brain microdialysis technique, in freely moving rats. Local application of AnTx (3.5 mM) through the microdialysis probe produced an increase in striatal DA levels (701 +/- 51% with respect to basal values). The effect of infusion of AnTx in Ca(2+)-free Ringer medium, in Na(+)-free Ringer medium and with TTX in the medium, was inhibited. Also, reserpine pre-treatment blocked the action of AnTx on striatal DA levels. To investigate the involvement of the DA transporter, the effects of AnTx were observed in the presence of nomifensine. The coadministration of AnTx and nomifensine evoked an additive effect on striatal DA levels. The latter results show that the DA release is not mediated by a decreased DA uptake. Taken as a whole, these results suggest that the effects of AnTx are predominantly mediated by an exocytotic mechanism, Ca(2+)-, Na(+)- and TTX-dependent, and not by a mechanism mediated by the DA transporter.

Modulation of GABAergic synaptic transmission by terminal nicotinic acetylcholine receptors in the central autonomic nucleus of the neonatal rat spinal cord

Using patch clamp recordings from an in vitro spinal cord slice preparation of neonatal rats (9-15days old), we characterized the GABAergic synaptic transmission in sympathetic preganglionic neurones (SPN) of the central autonomic nucleus (CA) of lamina X. Local applications of isoguvacine (100microM), a selective agonist at GABA(A) receptors, induced in all cells tested a chloride current which was abolished by bicuculline, a competitive antagonist at GABA(A) receptors. In addition, 25% of the recorded cells displayed spontaneous tetrodotoxin-insensitive and bicuculline-sensitive chloride miniature inhibitory postsynaptic currents (mIPSCs). Acetylcholine (100microM) increased the frequency of GABAergic mIPSCs without affecting their amplitudes or their kinetic properties indicating a presynaptic site of action. The presynaptic effect of ACh was restricted to GABAergic neurones synapsing onto sympathetic preganglionic neurones. The facilitatory effect of ACh was abolished in the absence of external calcium or in the presence of 100microM cadmium added to the bath solution. Choline 10mM, an agonist at alpha7 nicotinic acetylcholine receptors (nAChRs) or muscarine (10microM), a muscarinic receptor agonist, did not reproduce the presynaptic effect of ACh. The presynaptic effect of ACh was blocked by 1microM of dihydro-beta-erythroidine (DHbetaE), an antagonist of non-alpha7 nAChRs but was insensitive to alpha7 nAChRs antagonists (strychnine, alpha-bungarotoxin and methyllycaconitine) or to the muscarinic receptor antagonist atropine (10microM). It was concluded that SPNs of the central autonomic nucleus displayed a functional GABAergic transmission which is facilitated by terminal non alpha7 nAChRs.

Facilitation of glutamate release by nicotine involves the activation of a Ca2+/calmodulin signaling pathway in rat prefrontal cortex nerve terminals

The effect of nicotine on evoked glutamate release from isolated nerve terminals (synaptosomes) from rat prefrontal cortex was examined. We found that nicotine significantly potentiated 4-aminopyridine (4AP)-evoked glutamate release, and this potentiatory effect was mimicked by the selective alpha7 nicotinic receptor agonist choline and was blocked by the selective alpha7 nicotinic receptor antagonist methyllycaconitine, indicating its mediation by alpha7 nicotinic receptors. Examination of the effect of nicotine on cytosolic [Ca(2+)] revealed that the potentiation of glutamate release was associated with an increase in voltage-dependent Ca(2+) influx through N- and P/Q-type Ca(2+) channels. The potentiatory effect of nicotine on Ca(2+) influx seems to be attributed to its increasing synaptosomal excitability because nicotine significantly increased depolarization-evoked increase in the intrasynaptosomal free Na(+) concentration and 4AP-evoked depolarization of the synaptosomal plasma membrane potential. Also, Ca(2+) ionophore ionomycin-induced glutamate release was enhanced by nicotine, and this action was blocked by methyllycaconitine. These results suggest that nicotine exerts its potentiatory effect presynaptically, likely through the activation of alpha7 nicotinic receptors, resulting in Na(+) influx and local depolarization, which subsequently enhances the Ca(K+) entry through voltage-dependent N-and P/Q-type Ca(2+) channels as well as the vesicular release machinery to cause an increase in evoked glutamate release from rat prefrontocortical nerve terminals. Moreover, in this release potentiation may involve an activation of Ca(2+)/calmodulin signaling pathway as nicotine-mediated potentiation of 4AP- and ionomycin-evoked glutamate release were significantly attenuated by KN62, a selective inhibitor of Ca(2+)/calmodulin-dependent kinase II.

Dysregulation of thalamic sensory "transmission" in schizophrenia: neurochemical vulnerability to hallucinations

Cholinergic arousal mechanisms predispose thalamic and cortical neurons to fire action potentials at gamma rhythms, which have a tendency to resonate in thalamocortical networks, thereby forming coherent assemblies under constraints of sensory input to specific thalamic nuclei, on the one hand, and prefrontal and limbic attentional mechanisms, on the other. Perception may be based on sustained assemblies of coherent gamma oscillations in thalamocortical circuits. In schizophrenia, the impact of sensory input on self-organization of thalamocortical activity may be generally reduced. As a result, processes underlying perception can become uncoupled from sensory input, particularly at times of hyperarousal, leading to domination of attentional mechanisms and the emergence of hallucinations. Evidence is reviewed that implicates excessive neuronal noise in specific thalamic nuclei in the generation of hallucinations in schizophrenia. Nicotinic receptor abnormalities, dopaminergic hyperactivity and glutamate-receptor hypofunction are reconciled within a model of psychotic symptom generation that places crucial emphasis on dysfunction of the reticular thalamic nucleus.

A long-form alpha-neurotoxin from cobra venom produces potent opioid-independent analgesia

AIM

In light of the antinociceptive activity of the short-chain neurotoxin, cobrotoxin, and other acetylcholine antagonists, the antinociceptive activity and mechanisms of cobratoxin (CTX), a long-chain postsynaptic alpha-neurotoxin, was investigated in rodent pain models.

METHODS

CTX was administered intraperitoneally (30, 45, 68 microg/kg), intra-cerebral ventricularly (4.5 microg/kg) or microinjected into periaqueductal gray (PAG; 4.5 microg/kg). The antinociceptive action was tested using the hot-plate and acetic acid writhing tests in mice and rats. The involvement of the cholinergic system and opioid system in CTX-induced analgesia was examined by pretreatment of animals with atropine (0.5 mg/kg, im; or 10 mg/kg, ip) or naloxone (1 and 5 mg/kg, ip). The effect of CTX on motor activity was tested using the Animex test.

RESULTS

CTX exhibited a dose-dependent analgesic action in mice as determined by both the hot-plate and acetic acid writhing tests. The peak effect of analgesia was seen 3 h after administration. In the mouse acetic acid writhing test, the intra-cerebral ventricular administration of CTX at 4.5 microg/kg (1/12th of a systemic dose) produced marked analgesic effects. Microinjection of CTX (4.5 microg/kg) into the PAG region did not elicit an analgesic action in rats in the hot-plate test. Atropine at 0.5 mg/kg (im) and naloxone at 1 and 5 mg/kg (ip) both failed to block the analgesic effects of CTX, but atropine at 10 mg/kg (ip) did antagonize the analgesia mediated by CTX in the mouse acetic acid writhing test. Acetylsalicylic acid (300 mg/kg) did not enhance the analgesic effects of CTX. At the highest effective dose of 68 microg/kg the neurotoxin did not change the spontaneous mobility of mice.

CONCLUSION

CTX has analgesic effects, which are mediated in the central nervous system though not through the PAG. The central cholinergic system but not opioid system appears to be involved in the antinociceptive action of CTX.

Nicotinic modulation of neuronal networks: from receptors to cognition

RATIONALE

Nicotine affects many aspects of human cognition, including attention and memory. Activation of nicotinic acetylcholine receptors (nAChRs) in neuronal networks modulates activity and information processing during cognitive tasks, which can be observed in electroencephalograms (EEGs) and functional magnetic resonance imaging studies.

OBJECTIVES

In this review, we will address aspects of nAChR functioning as well as synaptic and cellular modulation important for nicotinic impact on neuronal networks that ultimately underlie its effects on cognition. Although we will focus on general mechanisms, an emphasis will be put on attention behavior and nicotinic modulation of prefrontal cortex. In addition, we will discuss how nicotinic effects at the neuronal level could be related to its effects on the cognitive level through the study of electrical oscillations as observed in EEGs and brain slices.

RESULTS/CONCLUSIONS

Very little is known about mechanisms of how nAChR activation leads to a modification of electrical oscillation frequencies in EEGs. The results of studies using pharmacological interventions and transgenic animals implicate some nAChR types in aspects of cognition, but neuronal mechanisms are only poorly understood. We are only beginning to understand how nAChR distribution in neuronal networks impacts network functioning. Unveiling receptor and neuronal mechanisms important for nicotinic modulation of cognition will be instrumental for treatments of human disorders in which cholinergic signaling have been implicated, such as schizophrenia, attention deficit/hyperactivity disorder, and addiction.

Nicotinic Acetylcholine Receptor Subtypes Involved in Facilitation of GABAergic Inhibition in Mouse Superficial Superior Colliculus

The superficial superior colliculus (sSC) is a key station in the sensory processing related to visual salience. The sSC receives cholinergic projections from the parabigeminal nucleus, and previous studies have revealed the presence of several different nicotinic acetylcholine receptor (nAChR) subunits in the sSC. In this study, to clarify the role of the cholinergic inputs to the sSC, we examined current responses induced by ACh in GABAergic and non-GABAergic sSC neurons using in vitro slice preparations obtained from glutamate decarboxylase 67-green fluorescent protein (GFP) knock-in mice in which GFP is specifically expressed in GABAergic neurons. Brief air pressure application of acetylcholine (ACh) elicited nicotinic inward current responses in both GABAergic and non-GABAergic neurons. The inward current responses in the GABAergic neurons were highly sensitive to a selective antagonist for α3β2- and α6β2-containing receptors, α-conotoxin MII (αCtxMII). A subset of these neurons exhibited a faster α-bungarotoxin-sensitive inward current component, indicating the expression of α7-containing nAChRs. We also found that the activation of presynaptic nAChRs induced release of GABA, which elicited a burst of miniature inhibitory postsynaptic currents mediated by GABAA receptors in non-GABAergic neurons. This ACh-induced GABA release was mediated mainly by αCtxMII-sensitive nAChRs and resulted from the activation of voltage-dependent calcium channels. Morphological analysis revealed that recorded GFP-positive neurons are interneurons and GFP-negative neurons include projection neurons. These findings suggest that nAChRs are involved in the regulation of GABAergic inhibition and modulate visual processing in the sSC.

Performance of alpha7 nicotinic receptor null mutants is impaired in appetitive learning measured in a signaled nose poke task

Wild-type and mutant mice lacking expression of alpha5, alpha7, beta2, beta3, or beta4 neuronal nicotinic cholinergic receptors (nAChRs) were compared on a signaled nose poke task, a multi-phased task used to measure appetitive learning and impulsivity. In the early phases of training, mutants of all nicotinic lines did not differ compared to wild types in the days to reach criterion when mice were required to nose poke for a sucrose reward on FR1 or FR3 schedules, or in their ability to respond to an auditory clicker to receive a sucrose reward. However, mutants lacking alpha7 nAChRs, but not lines lacking other nAChRs, showed impairments when task difficulty was increased such that an auditory stimulus was presented on a variable schedule and mice were required to withhold their responses until the presentation of the auditory cue to obtain a reward. alpha7 mutants were impaired compared to wild types in appetitive learning as measured by the percentage of conditioned responses but overcame their deficits with extensive training for 10 days. However, when efficiency ratios were used to measure impulsivity, alpha7 mutants exhibited lower efficiency ratios even after 10 days of training. These results support a role of the alpha7 nicotinic receptor in mediating appetitive learning and suggest a potential role for the alpha7 nAChRs in the regulation of behavioral disinhibition.

Excitability of motor cortex inhibitory circuits in Tourette syndrome before and after single dose nicotine

The pathophysiology underlying the involuntary tics of Gilles de la Tourette syndrome (GTS) remains unknown. Here we used transcranial magnetic stimulation (TMS) to examine the excitability of two different inhibitory systems in the human motor cortex: short interval intracortical inhibition (SICI) and short interval afferent inhibition (SAI) in 10 healthy non-smoking controls and eight untreated non-smoking patients with GTS. Compared with the healthy control group, both SICI (measured at a range of conditioning intensities) and SAI were reduced in patients. This is consistent with the suggestion that reduced excitability of cortical inhibition is one factor that contributes to the difficulty that patients have in suppressing involuntary tics. In addition, the reduced SAI indicates that impaired intracortical inhibition may not be limited to the motor cortex but also involves circuits linking sensory input and motor output. A single dose of nicotine reduced tic severity as assessed by blind video scoring in the majority of patients. In addition, it abolished the difference between patients and controls in SICI and SAI. There was no effect of nicotine, and no difference between controls and patients in measures of motor or SICI threshold. This indicates that cholinergic input can modulate the efficiency of SICI and SAI differently in GTS and healthy controls.

Control of breathing in newborn mice lacking the beta-2 nAChR subunit

AIM

To study the ventilatory and arousal/defence responses to hypoxia in newborn mutant mice lacking the beta2 subunit of the nicotinic acetylcholine receptors.

METHODS

Breathing variables were measured non-invasively in mutant (n = 31) and wild-type age-matched mice (n = 57) at 2 and 8 days of age using flow barometric whole-body plethysmography. The arousal/defence response to hypoxia was determined using behavioural criteria.

RESULTS

On day 2, mutant pups had significantly greater baseline ventilation (16%) than wild-type pups (P < 0.02). Mutant pups had a decreased hypoxic ventilatory declines. Arousal latency was significantly shorter in mutant than in wild-type pups (133 +/- 40 vs. 146 +/- 20 s, respectively, P < 0.026). However, the duration of movement elicited by hypoxia was shorter in mutant than in wild-type pups (14.7 +/- 5.9 vs. 23.0 +/- 10.7 s, respectively, P < 0.0005). Most differences disappeared on P8, suggesting a high degree of functional plasticity.

CONCLUSION

The blunted hypoxic ventilatory decline and the shorter arousal latency on day 2 suggested that disruption of the beta2 nicotinic acetylcholine receptors impaired inhibitory processes affecting both the ventilatory and the arousal response to hypoxia during postnatal development.

Persistent decrease in synaptic efficacy induced by nicotine at Schaffer collateral-CA1 synapses in the immature rat hippocampus

Neuronal nicotinic acetylcholine receptors (nAChRs) are widely distributed within the brain where they contribute to the regulation of higher cognitive functions. The loss of the cholinergic function in Alzheimer's disease patients, along with the well-known memory enhancing effect of nicotine, emphasizes the role of cholinergic signalling in memory functions. The hippocampus, a key structure in learning and memory, is endowed with nAChRs localized at pre- and postsynaptic levels. In previous work on the immature hippocampus we have shown that, at low probability (P) synapses, activation of alpha7 nAChRs by nicotine or by endogenously released acetylcholine persistently enhanced glutamate release and converted 'presynaptically silent' synapses into functional ones. Here we show that in the same preparation, at high P synapses, nicotine induces long-term depression of AMPA- and NMDA-mediated synaptic currents. This effect was mediated by presynaptic alpha7- and beta2-containing receptors and was associated with an increase in the paired pulse ratio and in the coefficient of variation. High P synapses could be converted into low P and vice versa by changing the extracellular Ca2+/Mg2+ ratio. In these conditions nicotine was able to persistently potentiate or depress synaptic responses depending on the initial P-values. A bi-directional control of synaptic plasticity by nicotine would considerably enhance the computational properties of the network during a critical period of postnatal development thus contributing to sculpt the neuronal circuit.

Nicotine inhibition of pulsatile GnRH secretion is mediated by GABAA receptor system in the cultured rat embryonic olfactory placode

In past work, we suggested that nicotine inhibition of in vivo pulsatile LH release is not mediated by opiate receptors known to be involved in the inhibition of LH release. In the present study, we examined whether nicotine inhibits the pulsatile gonadotropin-releasing hormone (GnRH) release, and whether this inhibition of GnRH release by nicotine is mediated by the GABA receptor system, by checking in vitro pulsatile GnRH release from cultured GnRH neurons obtained from olfactory placodes of rat embryos at E13.5. The mean interpulse interval of pulsatile GnRH release into the medium was 34.2+/-2.0 min in the control period and increased to 95.3+/-19.0 min (n=6) in the period of nicotine treatment at a concentration of 500 nM, showing an inhibitory effect of nicotine on pulsatile GnRH release. The GABA(A) receptor antagonist bicuculline used alone at a concentration of 20 microM caused no significant changes in the pulsatile GnRH release, but when used in combination with 500 nM of nicotine, bicuculline blocked the nicotine inhibition of GnRH release. In a separate experiment, nicotine treatment at a concentration of 500 nM significantly increased GABA release. These results suggest that, in the cultured embryonic olfactory placode, nicotine stimulates GABA release, which then inhibits GnRH release through GABA(A) receptor system.

Nicotinic acetylcholine receptors and the regulation of neuronal signalling

Neuronal nicotinic acetylcholine (nACh) receptors in the brain are more commonly associated with modulatory events than mediation of synaptic transmission. nACh receptors have a high permeability for Ca(2+), and Ca(2+) signals are pivotal in shaping nACh receptor-mediated neuromodulatory effects. In this review, we consider the mechanisms through which nACh receptors convert rapid ionic signals into sustained, wide-ranging phenomena. The complex Ca(2+) responses that are generated after activation of nACh receptors can transmit information beyond the initial domain and facilitate the interface with many intracellular processes. These mechanisms underlie the diverse repertoire of neuronal activities of nicotine in the brain, from the enhancement of learning and memory, to addiction and neuroprotection.

Long-lasting nicotinic modulation of GABAergic synaptic transmission in the rat nucleus accumbens associated with behavioural sensitization to amphetamine

A robust increase in dopaminergic transmission in the nucleus accumbens (NAc) shell has been reported to be consistently associated with the long-term expression of behavioural sensitization to drugs of abuse. However, little is known about how this affects the neuronal network of the NAc. We made cellular recordings in NAc slices of saline- and amphetamine-pretreated adult rats and found that expression of behavioural sensitization was associated with long-lasting changes in the basal firing pattern of cholinergic interneurons up to 3 weeks after the last drug injection. Consequently, upon amphetamine sensitization, an inhibiting effect of the nicotinic receptor blocker mecamylamine on the amplitudes of spontaneous GABAergic synaptic currents as well as on the failure rate of electrically evoked GABAergic currents was found that was not present under control conditions. Thus, behavioural sensitization to amphetamine is associated with an up-regulation of the endogenous activation of nicotinic receptors that, in turn, stimulate the GABAergic synaptic transmission within the NAc shell. This is a new mechanism by which drugs of abuse may induce alterations in the processing and integration of NAc inputs involved in psychomotor sensitization.

Nicotine alters flash-evoked potentials in Long–Evans rats

This experiment examined the effects of nicotine on flash-evoked potentials (FEPs) recorded from both the visual cortex (VC) and the superior colliculus (SC) of chronically implanted male Long-Evans rats. FEPs were recorded at 5, 20, 40, and 60 min following subcutaneous injections of saline, and of 0.4, 0.7, and 1.0 mg/kg nicotine on separate days. In the VC, the amplitude of components N(39), N(53), N(67), and P(88) increased, while the amplitude of components N(30) and P(235) decreased following nicotine administration. P(22), P(47), and N(153) were unchanged. In the SC, components P(27), N(48), and N(53) were reduced in amplitude, while P(37) and N(57) were unaffected by nicotine. Many peak latencies in the VC and SC were increased by nicotine, often at all three doses. However, effects of nicotine on FEPs were both dose- and time-dependent. When body temperature was recorded 65 min after drug administration, significant hypothermia was found with both the 0.7- and 1.0-mg/kg nicotine doses. The 1.0-mg/kg dose of nicotine resulted in a significant increase in movement during the recording sessions, but not in subsequent open-field observations. The results demonstrate that nicotinic acetylcholine receptors (nAChRs) play a differential role in the production/modulation of the various components of FEPs.

Voltage gated P/Q and N-type calcium channels mediate the nicotinic facilitation of GABAergic and glycinergic inputs to cardiac vagal neurons

Previous work has shown endogenous cholinergic activity facilitates both GABAergic and glycinergic neurotransmission to premotor cardiac vagal neurons. Exogenous application of nicotine increases the frequency of glycinergic and GABAergic inhibitory postsynaptic currents (IPSCs) and miniature IPSCs (mIPSCs) to cardiac vagal neurons. In this study we examined whether the nicotine evoked facilitation of GABAergic and glycinergic neurotransmission to cardiac vagal neurons is dependent or independent of activation of voltage dependent calcium channels. Nicotine evoked increases in GABAergic and glycinergic mIPSCs in cardiac vagal neurons which were blocked by the non-specific calcium channel antagonist cadmium (100 microM). Application of the L (Cav 1) type calcium channel antagonist nimodipine (10 microM) had no effect. However, the increase in both GABAergic and glycinergic mIPSCs elicited by nicotine was abolished by the P/Q (Cav 2.1) voltage gated calcium channel antagonist omega-agatoxin IVA (100 nM). Omega-conotoxin GVIA (1 microM), a specific blocker of N (Cav 2.2) type voltage gated calcium currents, inhibited the nicotine elicited augmentation of GABA and abolished the increase in glycine mIPSC frequency. This work demonstrates that the nicotine evoked facilitation of GABAergic and glycinergic neurotransmission to cardiac vagal neurons is dependent upon activation of P/Q (Cav 2.1) and N (Cav 2.2) type calcium channels.

Abnormal functional organization in the dorsal lateral geniculate nucleus of mice lacking the beta 2 subunit of the nicotinic acetylcholine receptor

Spontaneous activity patterns in the developing retina appear important for the functional organization of the visual system. We show here that an absence of early retinal waves in mice lacking the beta2 subunit of the nicotinic acetylcholine receptor (nAChR) is associated with both gain and loss of functional organization in the dorsal lateral geniculate nucleus (dLGN). Anatomical studies show normal gross retinotopy in the beta2(-/-) dLGN but suggest reduced topographic precision in the retinogeniculate projection. Physiological recordings reveal normal topography in the dorsoventral visual axis but a lack of fine-scale mapping in the nasotemporal visual plane. In contrast, unlike wild-type mice, on- and off-center cells in the beta2(-/-) dLGN are spatially segregated. The presence of the beta2 subunit of the nAChR in the CNS is therefore important for normal functional organization in the retinogeniculate projection.

The nicotinic acetylcholine receptor subtypes and their function in the hippocampus and cerebral cortex

Nicotinic acetylcholine receptors (nAChRs) are widely distributed in the central nervous system and have been implicated in multiple behavioral paradigms and pathological conditions. Nicotinic therapeutic interventions require an extensive characterization of native nAChRs including mapping of their distribution and function in different brain regions. Here, we describe the roles played by different nAChRs in affecting neuronal activity in the hippocampus and cerebral cortex. At least three distinct functional nAChR subtypes (alpha 7, alpha 4 beta 2, alpha 3 beta 4) can be detected in the hippocampal region, and in many instances a single neuron type is found to be influenced by all three nAChRs. Further, it became clear that GABAergic and glutamatergic inputs to the hippocampal interneurons are modulated via different subtypes of nAChRs. In the cerebral cortex, GABAergic inhibition to the layer V pyramidal neurons is enhanced predominantly via activation of alpha 4 beta 2 nAChR and to a minor extent via activation of alpha 7 nAChR. Such diversity offers pathways by which nicotinic drugs affect brain function.

Hallucinations: synchronisation of thalamocortical gamma oscillations underconstrained by sensory input

What we perceive is the product of an intrinsic process and not part of external physical reality. This notion is consistent with the philosophical position of transcendental idealism but also agrees with physiological findings on the thalamocortical system. gamma-Frequency rhythms of discharge activity from thalamic and cortical neurons are facilitated by cholinergic arousal and resonate in thalamocortical networks, thereby transiently forming assemblies of coherent gamma oscillations under constraints of sensory input and prefrontal attentional mechanisms. Perception and conscious experience may be based on such assemblies and sensory input to thalamic nuclei plays merely a constraining role in their formation. In schizophrenia, the ability of sensory input to modulate self-organisation of thalamocortical gamma activity may be generally reduced. If during arousal thalamocortical self-organisation is underconstrained by sensory input, then attentional mechanisms alone may determine the content of perception and hallucinations may arise.

Beta-amyloid regulation of presynaptic nicotinic receptors in rat hippocampus and neocortex

Alteration by beta-amyloid (Abeta) of signaling via nicotinic acetylcholine receptors (nAChRs) has been implicated in the early stages of Alzheimer's disease. nAChRs function both post- and presynaptically in the nervous system; however, little is known about the functional consequence of the interaction of Abeta with these receptors, particularly those on presynaptic nerve terminals. In view of the strong correlation between loss of synaptic terminals and dementia, together with the reduction in nAChRs in Alzheimer's disease, the possibility exists that presynaptic nAChRs may be targets for Abeta. To explore this possibility, we assessed the effect of Abeta peptides on nicotine-evoked changes in presynaptic Ca2+ level via confocal imaging of isolated presynaptic nerve endings from rat hippocampus and neocortex. Abeta1-42 appeared to inhibit presynaptic nAChR activation by nicotine. Surprisingly, picomolar Abeta1-42 was found to directly evoke sustained increases in presynaptic Ca2+ via nAChRs, revealing that the apparent inhibitory action of Abeta1-42 was the result of an occlusion of nicotine to further stimulate the receptors. The direct effect of Abeta was found to be sensitive to alpha-bungarotoxin, mecamylamine, and dihydro-beta-erythroidine, indicating involvement of alpha7-containing nAChRs and non-alpha7-containing nAChRs. Prior depolarization strongly attenuated subsequent Abeta-evoked responses in a manner dependent on the amplitude of the initial presynaptic Ca2+ increase, suggesting that nerve activity or Ca2+ channel density may control the impact of Abeta on presynaptic nerve terminal function. Together, these results suggest that the sustained increases in presynaptic Ca2+ evoked by Abeta may underlie disruptions in neuronal signaling via nAChRs in the early stages of Alzheimer's disease.

Conditional expression in corticothalamic efferents reveals a developmental role for nicotinic acetylcholine receptors in modulation of passive avoidance behavior

Prenatal nicotine exposure has been linked to attention deficit hyperactivity disorder and cognitive impairment, but the sites of action for these effects of nicotine are still under investigation. High-affinity nicotinic acetylcholine receptors (nAChRs) contain the beta2 subunit and modulate passive avoidance (PA) learning in mice. Using an inducible, tetracycline-regulated transgenic system, we generated lines of mice with expression of high-affinity nicotinic receptors restored in specific neuronal populations. One line of mice shows functional beta2 subunit-containing nAChRs localized exclusively in corticothalamic efferents. Functional, presynaptic nAChRs are present in the thalamus of these mice as detected by nicotine-elicited rubidium efflux assays from synaptosomes. Knock-out mice lacking high-affinity nAChRs show elevated baseline PA learning, whereas normal baseline PA behavior is restored in mice with corticothalamic expression of these nAChRs. In contrast, nicotine can enhance PA learning in adult wild-type animals but not in corticothalamic-expressing transgenic mice. When these transgenic mice are treated with doxycycline in adulthood to switch off nAChR expression, baseline PA is maintained even after transgene expression is abolished. These data suggest that high-affinity nAChRs expressed on corticothalamic neurons during development are critical for baseline PA performance and provide a potential neuroanatomical substrate for changes induced by prenatal nicotine exposure leading to long-term behavioral and cognitive deficits.

Investigations of the cholinergic modulation of GABA release in rat thalamus slices

The thalamus receives a dense cholinergic projection from the pedunculopontine tegmentum. A number of physiological studies have demonstrated that this projection causes a dramatic change in thalamic activity during the transition from sleep to wakefulness. Previous anatomical investigations have found that muscarinic type 2 receptors are densely distributed on the dendritic terminals of GABAergic interneurons, as well as the somata and proximal dendrites of GABAergic cells in the thalamic reticular nucleus. Since these structures are the synaptic targets of cholinergic terminals in the thalamus, it appears likely that thalamic pedunculopontine tegmentum terminals can activate muscarinic type 2 receptors on GABAergic cells. To test whether activation of muscarinic type 2 receptors affects the release of GABA in the thalamus, we have begun pharmacological studies using slices prepared from the rat thalamus. We have found that the application of the nonspecific muscarinic agonist, methacholine, and the muscarinic type 2-selective agonist, oxotremorine.sesquifumarate, diminished both the baseline, and K(+) triggered release of [(3)H]GABA from thalamic slices. This effect was calcium dependent, and blocked by the nonselective muscarinic antagonist atropine, the muscarinic type 2-selective antagonist, methoctramine, but not the muscarinic type 1 antagonist, pirenzepine. Thus, it appears that one function of the pedunculopontine tegmentum projection is to decrease the release of GABA through activation of muscarinic type 2 receptors. This decrease in inhibition may play an important role in regulating thalamic activity during changes in states of arousal.

Nucleus isthmi enhances calcium influx into optic nerve fiber terminals in Rana pipiens

We examined the role of nucleus isthmi in enhancing intracellular calcium concentrations in retinotectal fibers in the frog optic tectum in vitro. The intracellular calcium levels were measured using the fluorescent calcium-sensitive dye, Calcium Green-1 3000 mw dextran conjugate (CG-1), which was injected into one optic nerve. Electrical stimulation of the labeled optic nerve alone increased tectal CG-1 fluorescence whereas electrical stimulation of nucleus isthmi alone had no effect on CG-1 fluorescence. Electrical stimulation of the nucleus isthmi ipsilateral to the labeled tectum, followed by electrical stimulation to the optic nerve can enhance calcium uptake more than a double pulse stimulation of the optic nerve alone. Maximum enhancement of the calcium signal by nucleus isthmi occurs when optic nerve stimulation follows the ipsilateral nucleus isthmi stimulation by 10 ms. These results suggest that nucleus isthmi input can facilitate retinotectal neurotransmission, and the mechanism could be used to allow the frog to attend to a single prey stimulus in an environment of several prey stimuli.

Nicotinic receptors mediate increased GABA release in brain through a tetrodotoxin-insensitive mechanism during prolonged exposure to nicotine

The effects of nicotine on the spontaneous release of GABA from nerve terminals in the chick lateral spiriform nucleus were examined using whole cell patch-clamp recording in brain slices. Exposure to 1 microM nicotine produced an early immediate increase in the frequency of spontaneous postsynaptic GABAergic currents. This effect was blocked in the presence of 0.5 microM tetrodotoxin. However, a prolonged application of 0.1-1 microM nicotine (>3 min) caused a tetrodotoxin-insensitive increase in the frequency of spontaneous GABAergic currents. This late tetrodotoxin-insensitive effect was blocked by the nicotinic antagonists dihydro-beta-erythroidine (30 microM) and mecamylamine (10 microM), but not by methyllycaconitine (50-100 nM), indicating that activation of high affinity nicotine receptors was mainly responsible for this effect. This enhancement was inhibited by the high threshold Ca(2+) channel blocker Cd(2+) (100 microM), but not by dantrolene or ryanodine. The tetrodotoxin-insensitive enhancement of the frequency of GABA currents by nicotine was reduced by inhibition of cAMP-dependent protein kinase with HA1004 (30 microM), but not by inhibition of protein kinase C with staurosporine (1 microM), and was facilitated by forskolin (10 microM) or bromo-cAMP (50 microM). The results indicate that nicotine-enhanced GABA release can operate through both tetrodotoxin-sensitive and -insensitive mechanisms in a single brain region and that a second messenger cascade may be involved in the tetrodotoxin-insensitive enhancement by nicotine.

Nicotine-induced changes in cerebrocortical neuroactive steroids and plasma corticosterone concentrations in the rat

Nicotine, one of the most widely used psychotropic substances, is able to induce both anxiolytic and anxiogenic effects. The effect of this drug on the brain and plasma concentrations of neuroactive steroids was examined in the rat. Anxiolytic doses of nicotine (0.03-0.3 mg/kg) had no significant effect, whereas administration of anxiogenic doses (0.5 to 2 mg/kg) produced a dose- and time-dependent increase in the cerebrocortical concentrations of pregnenolone, progesterone, and allopregnanolone, with the greatest observed effects (+180%, +223%, and +124%, respectively) apparent at the dose of 2 mg/kg. In contrast, nicotine (1-2 mg/kg) decrease by 31% and 38%, respectively, the concentration of 3alpha,21-dihydroxy-5alpha-pregnan-20-one (allotetrahydrodeoxycorticosterone, or THDOC) in the cerebral cortex. Nicotine also increased the plasma concentrations of pregnenolone and progesterone, whereas failed to affect significantly those of allopregnanolone or THDOC. Nicotine induced a dose- and time-dependent increase in the plasma concentration of corticosterone, indicating that this drug activates the hypothalamic-pituitary-adrenal (HPA) axis. These results suggest that the changes in emotional behavior elicited by nicotine, similar to those induced by stressful stimuli or other anxiogenic drugs, are associated with an increase in neuroactive steroids content of the brain.

Nicotine induces glutamate release from thalamocortical terminals in prefrontal cortex

It has been proposed that activation of nicotinic acetylcholine receptors (nAChRs) can activate the prefrontal cortex, enhancing attention and cognition. Nicotine can stimulate the release of several different neurotransmitters in many brain regions. In the present study, we found that stimulation of nAChRs by nicotine or the endogenous agonist, acetylcholine (ACh), induces a large spontaneous increase in glutamate release onto layer V pyramidal neurons of the prefrontal cortex. This release of glutamate, measured by spontaneous excitatory postsynaptic currents (sEPSCs) in the prefrontal cortical slice, depends on intact thalamocortical terminals. It can be suppressed by mu-opioids or eliminated by blocking action potentials. The increase in sEPSCs is sensitive to low concentrations of nicotine, suggesting the involvement of high-affinity (eg alpha(4)beta(2)) nAChRs. Recent work has shown alterations in prefrontal alpha(4)beta(2) nAChRs in autism and schizophrenia, two conditions that are distinguished by abnormal prefrontal cortical activation as well as difficulty in certain aspects of cognition and integrating social and emotional cues. We show that mice lacking the beta(2) nAChR subunit do not show increased sEPSCs with either nicotine or ACh, again implicating high-affinity nicotinic receptors. These findings give new insight into the mechanism by which nicotine affects excitatory neurotransmission to the output neurons of the cerebral cortex in a pathway that is critical for cognitive function and reward expectation.

Mapping of presynaptic nicotinic acetylcholine receptors using fluorescence imaging of neuritic calcium

Neuronal nicotinic receptors (nAChRs) appear to function at both pre- and postsynaptic sites, to modulate the release of neurotransmitter, and to mediate synaptic transmission, respectively. Localization of functional nAChRs at presynaptic structures has only been possible under the best of circumstances where the presynaptic structure is very large allowing direct nAChR channel recording. We report here a novel and simple method that allows the visualization of stimulus-evoked changes in Fura-2 fluorescence in the presynaptic structures of essentially any neuron type in vitro. Following 'loading' of all neurons by incubation with the calcium-sensitive dye, Fura-2-AM, we selectively reduced the fluorescent signal in the postsynaptic neuron by injecting the Fura-2 quenching agent, Mn(2+), into the postsynaptic neuron. After quenching, nicotine treatment elicits calcium transients that can be observed in spatially distinct regions of neurite bundles contacting the Mn(2+)-infused neuron. Thus, the approach described allows one to readily map the distribution of activated nAChRs on presynaptic inputs in vitro.

Effects of halothane and propofol on excitatory and inhibitory synaptic transmission in rat cortical neurons

General anesthetics are thought to act on both excitatory and inhibitory neuronal pathways at both post- and presynaptic sites. However, the literature in these regards is somewhat controversial. The aim of the present study was to reassess the relative importance of the various anesthetic actions using a common preparation. Rat cortical neurons in primary culture were used to record spontaneous miniature postsynaptic currents by the whole-cell patch-clamp technique. Halothane at clinically relevant concentrations prolonged the decay phase of spontaneous miniature inhibitory postsynaptic currents (mIPSCs) recorded in the presence of tetrodotoxin and at higher concentrations decreased the frequency of mIPSCs. The mIPSC amplitudes underwent little change. Spontaneous action potential-dependent IPSCs recorded in the absence of tetrodotoxin were similarly affected by halothane. Halothane also decreased the frequency of spontaneous miniature non-N-methyl-D-aspartate (NMDA) excitatory postsynaptic currents (mEPSCs) as well as spontaneous action potential-dependent NMDA EPSCs and non-NMDA EPSCs without affecting their decay phase. The halothane effect on mIPSC and mEPSC frequency was dependent on the external calcium concentration. In contrast to halothane, the only effect of propofol was the prolongation of the decay phase of mIPSCs and IPSCs. The prolongation of mIPSCs and IPSCs by halothane and propofol coupled with the ineffectiveness on mEPSCs and EPSCs suggests a selective postsynaptic modulation of GABA(A) receptors. The additional calcium-dependent inhibition of mIPSC and mEPSC frequency by halothane (but not propofol) suggests a more general mechanism by this anesthetic on presynaptic transmitter release.

Modulation of inhibitory synaptic activity by a non-alpha4beta2, non-alpha7 subtype of nicotinic receptors in the substantia gelatinosa of adult rat spinal cord

The GABA/glycine-mediated inhibitory activity in the substantia gelatinosa (SG) of the spinal cord is critical in the control of nociceptive transmission. We examined whether and how SG inhibitory activity might be regulated by neuronal nicotinic receptors (nAChRs). Patch-clamp recordings were performed in SG neurons of spinal slice preparations from adult rats. We provided electrophysiological evidence that inhibitory presynaptic terminals in the SG expressed nAChRs and their activation resulted in large increases in the frequency of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) in over 90% SG neurons tested. The enhancement of inhibitory activity was mediated by increases in the release of GABA/glycine, and direct Ca(2+) entry through SG presynaptic nAChRs appeared to be involved. Miniature IPSC frequency could be enhanced by the nAChR agonists nicotine or cytisine. Nicotine could still elicit large increases in mIPSC frequency in the presence of the alpha4beta2 nAChR antagonist dihydro-beta-erythroidine (5 microM) and the alpha7 nAChR-selective antagonist methyllycaconitine (40 nM). However, nicotine did not produce a significant enhancement of mIPSC frequency in the presence of the broad spectrum nAChR antagonist mecamylamine (5 microM). Nicotinic agonist-evoked whole-cell currents from SG neurons and the antagonist profiles also indicated the presence of a subtype of nAChRs, which were different from the major central nervous system nAChR subtypes, i.e. alpha4beta2* or alpha7 nAChRs. Together, our results suggest that a subtype of nAChR, possibly alpha3beta4* nAChR or a new nAChR type, is highly expressed at the inhibitory presynaptic terminals in SG of adult rats and play a role in the control of inhibitory activity in SG.

Nicotinic receptors and Alzheimer's disease

Nicotinic receptors (NRs) belong to the group of polymeric receptors of the cell membrane and are key elements of cholinergic transmission. Numerous subtypes of NRs exist with the alpha 4 beta 2 and alpha 7 types being encountered most frequently. Deficiencies in NRs seem to play a role in Alzheimer's disease, which is characterised by accumulation of senile plaques, mainly composed of beta-amyloid peptide (beta A). Although the aetiology of this disease is unknown, different pathogenesis hypotheses implicating alpha 7 NRs have been proposed, with the receptors exerting a direct or indirect action on the mechanism of beta A toxicity. Allosteric modulators of NRs, such as the cholinesterase inhibitor galantamine, that facilitate the action of acetylcholine on these receptors may provide therapeutic benefits in the areas of cognition, attention and antineurodegenerative activity.

Nicotinic receptors in circuit excitability and epilepsy

Neuronal nicotinic acetylcholine receptors belong to the family of excitatory ligand-gated channels and result from the assembly of five subunits. Functional heteromeric nictonic receptors are present in the hippocampus and neocortex, thalamus, mesolimbic dopamine system and brainstem motor nuclei, where they may play a role, respectively, in memory, sensory processing, addiction and motor control. Some forms of autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) have been found to be associated with mutations in the genes coding for the alpha 4 or beta2 subunits of the nicotinic receptor. Mutant receptors display an increased acetylcholine sensitivity with respect to normal receptors. Since the thalamus and the cortex are strongly innervated by cholinergic neurons projecting from the brainstem and basal forebrain, an unbalance between excitation and inhibition, brought about by the presence of mutant receptors, could generate seizures by facilitating and synchronizing spontaneous oscillations in thalamo-cortical circuits.

Cellular and synaptic mechanisms of nicotine addiction

The tragic health effects of nicotine addiction highlight the importance of investigating the cellular mechanisms of this complex behavioral phenomenon. The chain of cause and effect of nicotine addiction starts with the interaction of this tobacco alkaloid with nicotinic acetylcholine receptors (nAChRs). This interaction leads to activation of reward centers in the CNS, including the mesoaccumbens DA system, which ultimately leads to behavioral reinforcement and addiction. Recent findings from a number of laboratories have provided new insights into the biologic processes that contribute to nicotine self-administration. Examination of the nAChR subtypes expressed within the reward centers has identified potential roles for these receptors in normal physiology, as well as the effects of nicotine exposure. The high nicotine sensitivity of some nAChR subtypes leads to rapid activation followed in many cases by rapid desensitization. Assessing the relative importance of these molecular phenomena in the behavioral effects of nicotine presents an exciting challenge for future research efforts.

Cholinergic modulation of purinergic and GABAergic co-transmission at in vitro hypothalamic synapses

The lateral hypothalamus (LH) is an important center for the integration of autonomic and limbic information and is implicated in the modulation of visceral motor and sensory pathways, including those underlying feeding and arousal behaviors. LH neurons in vitro release both ATP and GABA. The control of ATP and GABA co-transmission in LH may underlie the participation of LH in basic aspects of arousal and reinforcement. LH neurons receive cholinergic input from the pedunculopontine and laterodorsal tegmental nuclei as well as from cholinergic interneurons within the LH per se. This study presents evidence for nicotinic acetylcholine receptor (nAChR)-mediated enhancement of GABAergic, but not of purinergic, transmission despite the co-transmission of ATP and GABA at LH synapses in vitro. Facilitation of GABAergic transmission by nicotine is inhibited by antagonists of (alphabeta)*-containing nAChRs, but is unaffected by an alpha7-selective antagonist, consistent with a nAChR-mediated enhancement of GABA release mediated by non-alpha7-containing nAChRs. Activation of muscarinic ACh receptors enhances the release of ATP while concomitantly depressing GABAergic transmission. The independent modulation of ATP/GABAergic transmission may provide a new level of synaptic flexibility in which individual neurons utilize more than one neurotransmitter but retain independent control over their synaptic activity.

Nicotinic receptors regulate the release of glycine onto lamina X neurones of the rat spinal cord

Whole-cell patch clamp recordings were performed on neurones in the lamina X of rat spinal cord slices in order to characterize glycinergic synaptic currents and their modulation by nicotinic acetylcholine receptors. In the presence of TTX, bicuculline and kynurenic acid, glycine-induced currents and miniature glycinergic postsynaptic currents (mIPSCs) were recorded. These currents reversed near the chloride ion equilibrium potential and were blocked by strychnine (1 microM). A selective nicotinic acetylcholine receptor (nAChR) agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP), increased the frequency of glycinergic mIPSCs without altering significantly their amplitude distributions or their kinetic properties. The effects of DMPP were mimicked by different nAChRs agonists with the following apparent order of potency: ACh > DMPP > nicotine > cytisine. The effect of DMPP on mIPSCs was blocked by both d-tubocurarine and hexamethonium, and was reduced by dihydro-beta-erythroidine and methyllycaconitine (MLA), antagonists of non alpha7- and alpha7-containing nAChRs, respectively. In the absence of TTX, strychnine-sensitive glycinergic electrically evoked postsynaptic currents (eIPSCs) could be recorded. DMPP blocked the appearance of electrically evoked IPSCs while still inducing the appearance of spontaneous glycine IPSCs. These data demonstrate that neurones surrounding the central canal of the spinal cord present a glycinergic synaptic transmission which is modulated by terminal nAChRs.

beta 2 nicotinic acetylcholine receptor subunit modulates protective responses to stress: A receptor basis for sleep-disordered breathing after nicotine exposure

Nicotine exposure diminishes the protective breathing and arousal responses to stress (hypoxia). By exacerbating sleep-disordered breathing, this disturbance could underpin the well established association between smoking and the increased risk of sudden infant death syndrome. We show here that the protective responses to stress during sleep are partially regulated by particular nicotinic acetylcholine receptors (nAChRs). We compared responses of sleeping wild-type and mutant mice lacking the beta2 subunit of the nAChR to episodic hypoxia. Arousal from sleep was diminished, and breathing drives accentuated in mutant mice indicating that these protective responses are partially regulated by beta2-containing nAChRs. Brief exposure to nicotine significantly reduced breathing drives in sleeping wild-type mice, but had no effect in mutants. We propose that nicotine impairs breathing (and possibly arousal) responses to stress by disrupting functions normally regulated by beta2-containing, high-affinity nAChRs.

Prolonged stimulation of presynaptic nicotinic acetylcholine receptors in the rat interpeduncular nucleus has differential effects on transmitter release

Alterations in nicotinic acetylcholine (nAChR) receptor number can be induced by chronic exposure to nicotine possibly by stabilization of the desensitized state(s) of the receptor. Since within the central nervous system (CNS), many nAChRs are localized presynaptically, we have investigated the physiological consequences of prolonged nicotine applications on spontaneous transmitter release. In the presence of glutamate receptor antagonists, bicuculline-sensitive spontaneous GABA inhibitory synaptic currents (IPSCs) could be readily resolved in whole-cell recordings from neurons in the interpeduncular nucleus (IPN) maintained as brain slices. Nicotine (300nM) caused a marked enhancement in the frequency of spontaneous events. During a 15min exposure to nicotine, the time course of changes in IPSC frequency could be divided into two groups. In most neurons, there was a fast increase in event frequency followed by a decline to a lower steady-state level that remained above baseline. In the remaining neurons, the effect of nicotine was more slowly developing and outlasted the application. Interestingly, the rapid effect was associated with a shift to higher amplitude events, whereas, no change in the IPSC amplitude histogram was observed during the slow onset effect. These data show that prolonged stimulation of presynaptic nicotinic receptors can have different outcomes that could potentially contribute to the diverse effects of nicotine on central information processing.

Use of knock-out mice to determine the molecular basis for the actions of nicotine

Recombinant DNA techniques have been used to identify the family of molecules that mediate nicotine's effects on the brain. Nicotine binds and activates nicotinic acetylcholine receptors (nAChRs) which are made up of combinations of individual nicotinic subunits. It is important to determine which of the many possible subunit combinations are responsible for the physiological and behavioral effects of nicotine that lead to addiction. Molecular genetic tools such as antisense strategies have been useful in elucidating the electrophysiological properties of nAChRs in different tissues. Use of knock-out mice lacking individual nAChR subunits has also begun to elucidate how nicotine exerts its actions from the molecular level to the behavioral level. Experiments using mice lacking the beta2 subunit of the nAChR have shown that binding of nicotine to receptors containing this subunit is the first step in a pathway leading to increased dopamine levels in the mesolimbic dopamine system, and ultimately to the behavioral effects of nicotine in a test of nicotine reinforcement. Mice deficient in various alpha subunits of the nAChR will identify the partners of beta2 mediating the addictive properties of nicotine. In addition, more data needs to be gathered on the electrophysiological properties of different subunit combinations, the effects of nicotine on different neurotransmitter systems and the links between the molecular biology of nicotine receptors, their physiology and the ultimate role of individual receptor subtypes in complex behaviors. Multidisciplinary approaches to nAChR function will be essential to answering these questions.

Excitatory and inhibitory responses of dopamine neurons in the ventral tegmental area to nicotine

In the present electrophysiological study the mechanisms by which nicotine activates dopamine neurons in the ventral tegmental area in anesthetized Sprague-Dawley rats were analyzed. Intravenous administration of nicotine caused a dose-dependent increase in firing rate and percentage of spikes fired in bursts of ventral tegmental area dopamine neurons. However, this activation was preceded by an instantaneous but short-lasting inhibition of the firing rate. The excitation of dopamine neurons by nicotine (1.5-400 microg/kg i.v.) was antagonized and even reversed into an inhibitory response by elevated levels (four-fold) of the endogenous glutamate receptor antagonist kynurenic acid, as induced by a potent inhibitor of kynurenine 3-hydroxylase (PNU 156561A, 40 mg/kg, i.v., 5-9 h). The antagonistic action induced by PNU 156561A pretreatment was prevented by administration of D-cycloserine (128 mg/kg, i.v., 5 min). Administration of the GABA(B)-receptor antagonist CGP 35348 (200 mg/kg, i.v., 3 min) facilitated the nicotine-induced increase in burst firing activity of dopamine neurons and antagonized the short-lasting decrease in firing rate by nicotine. The results of the present study show that nicotine produces both inhibition and excitation of ventral tegmental area dopamine neurons, actions that appear to be related to the release of GABA and glutamate, respectively. Whereas the excitatory action of nicotine may be associated with motivational processes underlying learning and cognitive behavior, the inhibitory action of the drug may play a more prominent role in the situation of a profound dysregulation of the mesocorticolimbic dopamine system and may help to explain the high prevalence of tobacco-smoking in schizophrenics.

Synaptic mechanisms underlie nicotine-induced excitability of brain reward areas

A single nicotine exposure increases dopamine levels in the mesolimbic reward system for hours, but nicotine concentrations experienced by smokers desensitize nAChRs on dopamine neurons in seconds to minutes. Here, we show that persistent modulation of both GABAergic and glutamatergic synaptic transmission by nicotine can contribute to the sustained increase in dopamine neuron excitability. Nicotine enhances GABAergic transmission transiently, which is followed by a persistent depression of these inhibitory inputs due to nAChR desensitization. Simultaneously, nicotine enhances glutamatergic transmission through nAChRs that desensitize less than those on GABA neurons. The net effect is a shift toward excitation of the dopamine reward system. These results suggest that spatial and temporal differences in nicotinic receptor activity on both excitatory and inhibitory neurons in reward areas coordinate to reinforce nicotine self-administration.

Nicotine modulates the spontaneous synaptic activity in cultured embryonic rat spinal cord interneurons

The nicotine-induced modulation of the synaptic activity was studied in cultured spinal cord neurons from embryonic rats, using the patch-clamp technique, alone or in combination with Ca(2+) imaging. Morphologically, neurons could be divided into two populations: multipolar nerve cells and bipolar, spindle-shaped neurons. Neurons were predominantly GABAergic, with approximately 70% of bipolar cells and 60% of multipolar cells positive for GABA immunostaining. Nicotine (Nic) did not affect the activity of the spontaneous postsynaptic current (sPSC) in multipolar neurons, whereas bipolar cells responded to Nic applications with an enhancement of both inhibitory and excitatory synaptic activity (threefold for 100 microM Nic). No change in the mean event amplitude was observed. The increase of sPSC frequency was detectable at 1-10 microM Nic, and was prevented by dihydro-beta-erythroidine (DHbetaE) but not by alpha-bungarotoxin. Choline, a selective alpha7-nAChR agonist, did not mimic the Nic action. Simultaneous treatment with inhibitors of ionotropic glutamate receptors, CNQX (20 microM) and AP5 (20 microM), completely blocked the excitatory sPSC activity but did not prevent the Nic-induced enhancement of inhibitory sPSC activity. Tetrodotoxin (1 microM) reduced the basal spontaneous activity but did not block the Nic-induced effects on bipolar neurons. In a subset of bipolar neurons (12%) exposed to AP5 and CNQX, Nic activated DHbetaE-sensitive inward currents, associated with an elevation of cytosolic Ca(2+) ([Ca(2+)](i)). Our results provide the first evidence of modulation of both excitatory and inhibitory neurotransmitter release in embryonic spinal cord interneurons by non-alpha7-containing nicotinic receptors.

Absence of alpha7-containing neuronal nicotinic acetylcholine receptors does not prevent nicotine-induced seizures

Nicotine is the primary addictive component in tobacco, and at relatively low doses it affects cardiovascular responses, locomotor activity, thermoregulation, learning, memory, and attention. At higher doses nicotine produces seizures. The mechanisms underlying the convulsive effects of nicotine are not known, but studies conducted on a number of inbred strains of mice have indicated a positive correlation between the number of alpha-bungarotoxin (alpha-BTX) binding sites in the hippocampus and the sensitivity to nicotine-induced seizures. Because alpha7-containing neuronal nicotinic acetylcholine receptors (nAChRs) represent the major binding site for alpha-BTX, mice lacking the alpha7 nAChR subunit were predicted to be less sensitive to the convulsive effects of nicotine. To test this hypothesis, we injected nicotine intraperitoneally in alpha7 mutant mice and found that the dose-response curve for nicotine-induced seizures was similar in the alpha7 +/+, alpha7 +/- and alpha7 -/- mice. The retained sensitivity to the convulsant effects of nicotine could not be explained by the presence of cholinergic compensatory mechanisms such as increases in mRNA levels for other nAChR subunits, or changes in binding levels or affinity for nicotinic ligands such as epibatidine and nicotine. These findings indicate that alpha7 may not be necessary for the mechanisms underlying nicotine-induced seizures.

Evidence for nicotinic acetylcholine receptor activation in rat cerebellar slices

Neuronal nicotinic ACh receptor (nAChR) activation is known to enhance glutamate and GABA release in different brain areas. Moreover, nAChRs play an important role in neuronal differentiation. By using the patch-clamp technique, we have investigated the presence of nAChRs in cerebellar granule cells in slices from P5-P14 rats. Application of ACh (1 mM) could elicit a variety of effects. Some cells did not respond at all. In other cells, a somatic current was activated. In a proportion of cells, postsynaptic currents (PSCs), with or without somatic current, were elicited. Somatic nAChRs are likely to be of the alpha(4)beta(2) subtype, but the presence of other subunit combinations (alpha(7)- or beta(4)-containing receptors) cannot be ruled out. The ACh-induced PSCs were glutamatergic in nature. Thus, in a reasonable proportion of cells, nicotinic receptors are present presynaptically. They are likely to be alpha(7) receptors whose activation elicits Glu release via a TTX-sensitive mechanism. Our experiments are the first electrophysiological evidence showing, in a native cerebellar preparation, the presence of nicotinic receptors at the mossy fibre-granule cell synapse at early developmental stages.

Neuronal nicotinic acetylcholine receptor subunit knockout mice: physiological and behavioral phenotypes and possible clinical implications

Nicotinic acetylcholine receptors (nAChRs) in the muscle, autonomic ganglia, and brain are targets for pharmacologically administered nicotine. Several of the subunits that combine to form neuronal nicotinic receptors have been deleted by knockout or mutated by knockin in mice using homologous recombination. We will review the biochemical, pharmacological, anatomical, physiological, and behavioral phenotypes of mice with genetically altered neuronal nAChR subunits. Clinically relevant mutations in nAChR genes will also be discussed. In addition, some of the signal transduction pathways activated through nAChRs will be described in order to delineate the longer-term changes that might result from persistent activation or inactivation of nAChRs. Genetically manipulated mice have greatly increased our understanding of the subunit composition and physiological properties of nAChRs in vivo. In addition, these mice have provided a model system to determine the molecular basis for many of the pharmacological actions of nicotine on neurotransmitter release and behavior. Genetic manipulations in mice have also elucidated the role of nAChR subunits in various disease states, and suggest several avenues for drug development.

Frequent occurrence of nicotinic acetylcholine receptors in GABAergic neurons of the chick visual system

Double-labeling immunohistochemical methods were used to investigate the occurrence of the alpha8 and alpha5 nicotinic receptor subunits in presumptive GABAergic neurons of the chick nervous system. Nicotinic receptor immunoreactivity was often found in cells exhibiting GABA-like immunoreactivity, especially in the visual system. The alpha8 subunit appeared to be present in presumptive GABAergic cells of the ventral lateral geniculate nucleus, nucleus of the basal optic root of the accessory optic system, and the optic tectum, among several other structures. The alpha5 subunit was also found in GABA-positive neurons, as observed in the lentiform nucleus of the mesencephalon and other pretectal nuclei. The numbers of alpha8- and alpha5-positive neurons that were also GABA-positive represented high percentages of the total number of neurons containing nicotinic receptor labeling in several brain areas, which indicates that most of the alpha8 and alpha5 nicotinic receptor subunits are present in GABAergic cells. Taken together with data from other studies, our results indicate an important role of the nicotinic acetylcholine receptors in the functional organization of GABAergic circuits in the visual system.