Characterization of nicotinic acetylcholine receptor-mediated [(3)H]-dopamine release from rat cortex and striatum

Dystonia and Parkinson's disease: Do they have a shared biology?

Parkinsonism and dystonia co-occur across many movement disorders and are most encountered in the setting of Parkinson's disease. Here we aim to explore the shared neurobiological underpinnings of dystonia and parkinsonism through the clinical lens of the conditions in which these movement disorders can be seen together. Foregrounding the discussion, we briefly review the circuits of the motor system and the neuroanatomical and neurophysiological aspects of motor control and highlight their relevance to the proposed pathophysiology of parkinsonism and dystonia. Insight into shared biology is then sought from dystonia occurring in PD and other forms of parkinsonism including those disorders in which both can be co-expressed simultaneously. We organize these within a biological schema along with important questions to be addressed in this space.

Striatal cholinergic dysfunction as a unifying theme in the pathophysiology of dystonia

Dystonia is a movement disorder of both genetic and non-genetic causes, which typically results in twisted posturing due to abnormal muscle contraction. Evidence from dystonia patients and animal models of dystonia indicate a crucial role for the striatal cholinergic system in the pathophysiology of dystonia. In this review, we focus on striatal circuitry and the centrality of the acetylcholine system in the function of the basal ganglia in the control of voluntary movement and ultimately clinical manifestation of movement disorders. We consider the impact of cholinergic interneurons (ChIs) on dopamine-acetylcholine interactions and examine new evidence for impairment of ChIs in dysfunction of the motor systems producing dystonic movements, particularly in animal models. We have observed paradoxical excitation of ChIs in the presence of dopamine D2 receptor agonists and impairment of striatal synaptic plasticity in a mouse model of DYT1 dystonia, which are improved by administration of recently developed M1 receptor antagonists. These findings have been confirmed across multiple animal models of DYT1 dystonia and may represent a common endophenotype by which to investigate dystonia induced by other types of genetic and non-genetic causes and to investigate the potential effectiveness of pharmacotherapeutics and other strategies to improve dystonia.

Striatal cholinergic interneuron regulation and circuit effects

The striatum plays a central role in motor control and motor learning. Appropriate responses to environmental stimuli, including pursuit of reward or avoidance of aversive experience all require functional striatal circuits. These pathways integrate synaptic inputs from limbic and cortical regions including sensory, motor and motivational information to ultimately connect intention to action. Although many neurotransmitters participate in striatal circuitry, one critically important player is acetylcholine (ACh). Relative to other brain areas, the striatum contains exceptionally high levels of ACh, the enzymes that catalyze its synthesis and breakdown, as well as both nicotinic and muscarinic receptor types that mediate its postsynaptic effects. The principal source of striatal ACh is the cholinergic interneuron (ChI), which comprises only about 1-2% of all striatal cells yet sends dense arbors of projections throughout the striatum. This review summarizes recent advances in our understanding of the factors affecting the excitability of these neurons through acute effects and long term changes in their synaptic inputs. In addition, we discuss the physiological effects of ACh in the striatum, and how changes in ACh levels may contribute to disease states during striatal dysfunction.

Attention-deficit/hyperactivity disorder: neurophysiology, information processing, arousal and drug development

In this review, we draw on literature from both animal and human neurophysiological studies to consider the neurochemical mechanisms underlying attention-deficit/ hyperactivity disorder (ADHD). Psychophysiological and neuropsychological research is used to propose possible etiological endophenotypes of ADHD. These are conceptualized as patients with distinct cortical-arousal, information-processing or maturational abnormalities, or a combination thereof, and how the endophenotypes can be used to help drug development and optimize treatment and management. To illustrate, the paper focuses on neuro- and psychophysiological evidence that suggests cholinergic mechanisms may underlie specific information-processing abnormalities that occur in ADHD. The clinical implications for a cholinergic hypothesis of ADHD are considered, along with its possible implications for treatment and pharmacological development.

Molecular determinants for competitive inhibition of alpha4beta2 nicotinic acetylcholine receptors

The Erythrina alkaloids erysodine and dihydro-beta-erythroidine (DHbetaE) are potent and selective competitive inhibitors of alpha4beta2 nicotinic acetylcholine receptors (nAChRs), but little is known about the molecular determinants of the sensitivity of this receptor subtype to inhibition by this class of antagonists. We addressed this issue by examining the effects of DHbetaE and a range of aromatic Erythrina alkaloids on [(3)H]cytisine binding and receptor function in conjunction with homology models of the alpha4beta2 nAChR, mutagenesis, and functional assays. The lactone group of DHbetaE and a hydroxyl group at position C-16 in aromatic Erythrina alkaloids were identified as major determinants of potency, which was decreased when the conserved residue Tyr126 in loop A of the alpha4 subunit was substituted by alanine. Sensitivity to inhibition was also decreased by substituting the conserved aromatic residues alpha4Trp182 (loop B), alpha4Tyr230 (loop C), and beta2Trp82 (loop D) and the nonconserved beta2Thr84; however, only alpha4Trp182 was predicted to contact bound antagonist, suggesting alpha4Tyr230, beta2Trp82, and beta2Thr84 contribute allosterically to the closed state elicited by bound antagonist. In addition, homology modeling predicted strong ionic interactions between the ammonium center of the Erythrina alkaloids and beta2Asp196, leading to the uncapping of loop C. Consistent with this, beta2D196A abolished sensitivity to inhibition by DHbetaE or erysodine but not by epierythratidine, which is not predicted to form ionic bonds with beta2Asp196. This residue is not conserved in subunits that comprise nAChRs with low sensitivity to inhibition by DHbetaE or erysodine, which highlights beta2Asp196 as a major determinant of the receptor selectivity of Erythrina alkaloids.

Molecular and cellular mechanisms of action of nicotine in the CNS

Nicotine achieves its psychopharmacological effects by interacting with nicotinic acetylcholine receptors (nAChRs) in the brain. There are numerous subtypes of nAChR that differ in their properties, including their sensitivity to nicotine, permeability to calcium and propensity to desensitise. The nAChRs are differentially localised to different brain regions and are found on presynaptic terminals as well as in somatodendritic regions of neurones. Through their permeability to cations, these ion channel proteins can influence both neuronal excitability and cell signalling mechanisms, and these various responses can contribute to the development or maintenance of dependence. However, many questions and uncertainties remain in our understanding of these events and their relevance to tobacco addiction. In this chapter, we briefly overview the fundamental characteristics of nAChRs that are germane to nicotine's effects and then consider the cellular responses to acute and chronic nicotine, with particular emphasis on dopamine systems because they have been the most widely studied in the context of nicotine dependence. Where appropriate, methodological aspects are critically reviewed.

Alpha-conotoxin Arenatus IB[V11L,V16D] [corrected] is a potent and selective antagonist at rat and human native alpha7 nicotinic acetylcholine receptors

A recently developed alpha-conotoxin, alpha-conotoxin Arenatus IB-[V11L,V16D] (alpha-CtxArIB[V11L,V16D]) [corrected], is a potent and selective competitive antagonist at rat recombinant alpha7 nicotinic acetylcholine receptors (nAChRs), making it an attractive probe for this receptor subtype. alpha7 nAChRs are potential therapeutic targets that are widely expressed in both neuronal and non-neuronal tissues, where they are implicated in a variety of functions. In this study, we evaluate this toxin at rat and human native nAChRs. Functional alpha7 nAChR responses were evoked by choline plus the allosteric potentiator PNU-120596 [1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxazol-3-yl)-urea] in rat PC12 cells and human SH-SY5Y cells loaded with calcium indicators. alpha-CtxArIB[V11L,V16D] specifically inhibited alpha7 nAChR-mediated increases in Ca2+ in PC12 cells. Responses to other stimuli, 5-I-A-85380 [5-iodo-3-(2(S)-azetidinylmethoxy)pyridine dihydrochloride], nicotine, or KCl, that did not activate alpha7 nAChRs were unaffected. Human alpha7 nAChRs were also sensitive to alpha-CtxArIB[V11L, V16D]; acetylcholine-evoked currents in Xenopus laevis oocytes expressing human alpha7 nAChRs were inhibited by alpha-CtxArIB[V11L,V16D] (IC(50), 3.4 nM) in a slowly reversible manner, with full recovery taking 15 min. This is consistent with the time course of recovery from blockade of rat alpha7 nAChRs in PC12 cells. alpha-CtxArIB[V11L,V16D] inhibited human native alpha7 nAChRs in SHSY5Y cells, activated by either choline or AR-R17779 [(2)-spiro[1-azabicyclo[2.2.2]octane-3,59-oxazolidin]-29-one] plus PNU-120596. Rat brain alpha7 nAChRs contribute to dopamine release from striatal minces; alpha-CtxArIB[V11L,V16D] (300 nM) selectively inhibited choline-evoked dopamine release without affecting responses evoked by nicotine that activates heteromeric nAChRs. This study establishes that alpha-CtxArIB[V11L,V16D] selectively inhibits human and rat native alpha7 nAChRs with comparable potency, making this a potentially useful antagonist for investigating alpha7 nAChR functions.

Chronic nicotine alters nicotinic receptor-induced presynaptic Ca2+ responses in isolated nerve terminals

Brain nicotinic receptors display pronounced permeability for Ca2+ and localize to presynaptic nerve terminals, in addition to postsynaptic sites. Chronic exposure to nicotine has been shown to alter brain nicotinic receptor expression, but the functional consequences for presynaptic Ca2+ have not been directly examined. Here, we used confocal imaging to assess Ca2+ responses in individual nerve terminals from cortices of mice treated up to 14 days with nicotine as compared to vehicle-treated controls. Chronic nicotine treatment led to substantially enhanced amplitudes of presynaptic Ca2+ responses to acute application of nicotine at concentrations of 50 nM (2-fold) and 500 nM (1.7-fold), but not 50 microM. In addition, increased expression of high-affinity nicotinic receptors on isolated terminals was observed following chronic treatment, as determined immunocytochemically and pharmacologically. These findings suggest that chronic exposure to nicotine may lead to enhanced sensitivity to nicotine at select presynaptic sites in brain via up-regulation of high-affinity nicotinic receptors.

The serotonin 2C receptor agonist Ro-60-0175 attenuates effects of nicotine in the five-choice serial reaction time task and in drug discrimination

RATIONALE

There is evidence that serotonin(2C) (5-HT(2C)) receptors can modulate some behavioural effects of nicotine, but the generality of this action is not known.

OBJECTIVE

To analyse the influence of the 5-HT(2C) agonist Ro-60-0175 on responses to nicotine in the five-choice serial reaction time task (5-CSRTT) and on its discriminative stimulus effect; these procedures constitute models for attention-enhancing and subjective effects of nicotine, respectively.

MATERIALS AND METHODS

In the 5-CSRTT, rats were trained to obtain food reinforcers by detecting light stimuli and then challenged with Ro-60-0175 (0.3-0.8 mg/kg) and nicotine (0.2 mg/kg). For drug discrimination studies, rats were trained to discriminate nicotine (0.2 mg/kg) from saline in a two-lever procedure using a tandem schedule of food reinforcement.

RESULTS

In the 5-CSRTT, nicotine positively influenced most response indices, confirming previous results. Ro-60-0175 increased response latencies and omission errors and reduced anticipatory responding but had little effect on response accuracy; importantly, it counteracted the effects of nicotine on response speed and omission errors. Pentobarbitone (10-14 mg/kg) also slowed performance of the 5-CSRTT but did not weaken the nicotine-induced enhancement of performance. In the drug discrimination procedure, Ro-60-0175 was not generalised with nicotine but shifted the nicotine dose-response curve to the right in a dose-related manner.

CONCLUSIONS

The data suggest that selective occupancy of 5-HT(2C) receptors can attenuate some effects of nicotine in the 5-CSRTT and weaken the nicotine discriminative stimulus; these effects cannot be explained by a sedative action of Ro-60-0175.

Structure−Activity Studies and Analgesic Efficacy of N-(3-Pyridinyl)-Bridged Bicyclic Diamines, Exceptionally Potent Agonists at Nicotinic Acetylcholine Receptors

A series of exceptionally potent agonists at neuronal nicotinic acetylcholine receptors (nAChRs) has been investigated. Several N-(3-pyridinyl) derivatives of bridged bicyclic diamines exhibit double-digit-picomolar binding affinities for the alpha 4 beta 2 subtype, placing them with epibatidine among the most potent nAChR ligands described to date. Structure-activity studies have revealed that substitutions, particularly hydrophilic groups in the pyridine 5-position, differentially modulate the agonist activity at ganglionic vs central nAChR subtypes, so that improved subtype selectivity can be demonstrated in vitro. Analgesic efficacy has been achieved across a broad range of pain states, including rodent models of acute thermal nociception, persistent pain, and neuropathic allodynia. Unfortunately, the hydrophilic pyridine substituents that were shown to enhance agonist selectivity for central nAChRs in vitro tend to limit CNS penetration in vivo, so that analgesic efficacy with an improved therapeutic window was not realized with those compounds.

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.

C3-halogenation of cytisine generates potent and efficacious nicotinic receptor agonists

Neuronal nicotinic acetylcholine receptors subserve predominantly modulatory roles in the brain, making them attractive therapeutic targets. Natural products provide key leads in the quest for nicotinic receptor subtype-selective compounds. Cytisine, found in Leguminosae spp., binds with high affinity to alpha4beta2* nicotinic receptors. We have compared the effect of C3 and C5 halogenation of cytisine and methylcytisine (MCy) on their interaction with native rat nicotinic receptors. 3-Bromocytisine (3-BrCy) and 3-iodocytisine (3-ICy) exhibited increased binding affinity (especially at alpha7 nicotinic receptors; Ki approximately 0.1 microM) and functional potency, whereas C5-halogenation was detrimental. 3-BrCy and 3-ICy were more potent than cytisine at evoking [3H]dopamine release from striatal slices (EC50 approximately 11 nM), [3H]noradrenaline release from hippocampal slices (EC50 approximately 250 nM), increases in intracellular Ca2+ in PC12 cells and inward currents in Xenopus oocytes expressing human alpha3beta4 nicotinic receptor (EC50 approximately 2 microM). These compounds were also more efficacious than cytisine. C3-halogenation of cytisine is proposed to stabilize the open conformation of the nicotinic receptor but does not enhance subtype selectivity.

Desensitized nicotinic receptors in brain

Desensitization is an intriguing characteristic of ligand-gated channels, whereby a decrease or loss of biological response occurs following prolonged or repetitive stimulation. Nicotinic acetylcholine receptors (nAChRs), as a member of transmitter gated ion channels family, also can be desensitized by continuous or repeated exposure to agonist. Desensitization of nicotinic receptors can occur as a result of extended nicotine exposure during smoking or prolonged acetylcholine when treatment of Alzheimer's disease (AD) with cholinesterase inhibitors, or anticholinesterase agent poisoning. Studies from our lab have shown that nAChRs desensitization is not a nonfunctional state and we proposed that desensitized nAChRs could increase sensitivity of brain muscarinic receptor to its agonists. Here, we will review the regulation of nicotinic receptor desensitization and discuss the important biological function of desensitized nicotinic receptors in light of our previous studies. These studies provide the critical information for understanding the importance of nicotinic receptors desensitization in both normal physiological processing and in various disease states.

In vitro and in vivo effects of an alpha3 neuronal nicotinic acetylcholine receptor antisense oligonucleotide

In the mammalian central nervous system (CNS), a family of alpha and beta subunits (alpha2-7, beta2-4) assemble to form both hetero- and homopentameric neuronal nicotinic acetylcholine receptors (nAChRs). In contrast to alpha4beta2 and alpha7, the predominant brain subtypes, far less is known regarding the functional expression and significance of alpha3-containing nAChRs in the CNS. In trying to better understand the role alpha3 in the CNS, an antisense knockdown strategy was utilized in the present studies. Specifically, Isis 106567 was identified out of 80 antisense oligonucleotides (aONs) designed and screened for their ability to reduce alpha3 mRNA expression in PC-12 cells. In addition to reducing alpha3 mRNA by greater than 75%, Isis 106567 attenuated nicotine-induced calcium influx in alpha3-expressing F11 cells. In vivo studies revealed significant reduction of alpha3 mRNA levels in both thalamus and medial habenula, regions known to express alpha3, following continuous (7 days) intracerebroventricular (i.c.v.) infusion of Isis 106567 in rats. Consistent with functional alpha3 knockdown, epibatidine-induced c-Fos expression in the medial habenula was attenuated in aON-treated rats. Known physiological responses elicited by epibatidine, such as hypothermia and micturition, were not affected by alpha3 aON treatment. However, the incidence of epibatidine-induced seizures was reduced in alpha3-antisense aON-treated rats, suggesting that alpha3 may be involved in mediating seizures produced by the nAChR agonist. Results of our studies suggest that Isis 106567 may be a useful in vivo tool for characterizing the functional significance of alpha3 expression in the CNS.

Different nicotinic acetylcholine receptor subtypes mediating striatal and prefrontal cortical [3H]dopamine release

Different nicotinic acetylcholine receptor subtypes appear to modulate dopamine release from the striatum and prefrontal cortex. In this study a combination of subtype-selective antagonists and agonists were used to extensively characterize the nAChRs involved in dopamine release from slice preparations of these two brain regions. alpha-conotoxin-MII inhibited nicotine-evoked [3H]dopamine (DA) release from striatum by 45%, but did not affect cortical dopamine release. Neither methyllycaconitine, alpha-bungarotoxin, nor alpha-conotoxin-ImI affected nicotine-evoked [3H]DA release from either striatum or prefrontal cortex. MG 624, a novel selective nAChR antagonist, inhibited cortical [3H]DA by 53%, but had no effect on striatal release. Compared to nicotine, (+/-)-UB-165 showed less efficacy with respect to dopamine release from striatum, and had no effect on cortical dopamine release. (+/-)-UB-165-evoked striatal dopamine release was completely blocked by mecamylamine, partially blocked (up to 55%) by alpha-conotoxin-MII, and unaffected by methyllycaconitine or alpha-conotoxin-ImI. alpha4beta2* and alpha6beta2beta3* nAChRs appear to play a role in striatal dopamine release, whereas alpha4beta2* nAChRs modulate release from prefrontal cortex. alpha7* nAChRs do not appear to play a role in nAChR-mediated dopamine release from either brain region.

Modulation of dopamine release by the nicotinic agonist epibatidine in the frontal cortex and the nucleus accumbens of naive and chronic nicotine treated rats

The effect of the nicotinic acetylcholine receptors (nAChRs) agonist (+/-)epibatidine on the modulation of dopamine (DA) release was investigated by microdialysis in vivo in the frontal cortex and the nucleus accumbens of naive and chronic nicotine-treated awake rats. (+/-)Epibatidine (2.5 microg/kg, s.c.), contrary to (-)nicotine (0.5 mg/kg, s.c.), decreased the extracellular concentrations of DA in the brain of naive rats. Subchronic nicotine treatment (0.45 mg/kg, s.c., twice daily for 7 days) attenuated the (+/-)epibatidine induced decrease in the DA level. The extracellular concentrations of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were elevated by (+/-)epibatidine administration in both naïve and subchronic treated rats. The findings suggest that the decrease in DA extracellular concentrations induced by the high affinity nAChRs agonist (+/-)epibatidine might be due to inactivation of nAChRs, which can be overcome by subchronic treatment with nicotine. Different mechanisms in modulation of DA release appears to be involved in the rat brain by (+/-)epibatidine compare to (-)nicotine.

Nicotinic Acetylcholine Receptor-Mediated [3H]Dopamine Release from Hippocampus

The mechanism of nicotinic acetylcholine receptor (nAChR)-induced hippocampal dopamine (DA) release was investigated using rat hippocampal slices. nAChRs involved in hippocampal DA and norepinephrine (NE) release were investigated using prototypical agonists and antagonists and several relatively novel compounds: ABT-594 [(R)-5-(2-azetidinylmethoxy)-2-chloropyridine], (+/-)-UB-165 [(2-chloro-5-pyridyl)-9-azabicyclo [4.2.1]non2-ene], and MG 624 [N,N,N-triethyl-2-[4-(2 phenylethenyl)phenoxy]-ethanaminium iodine]. (+/-)-Epibatidine, (+/-)-UB-165, anatoxin-a, ABT-594, (-)-nicotine, 1,1-dimethyl-4-phenyl-piperazinium iodide, and (-)-cytisine (in decreasing order of potency) evoked [(3)H]DA release in a mecamylamine-sensitive manner. Aside from (+/-)-UB-165, all the agonists displayed full efficacy relative to 100 microM (-)-nicotine in [(3)H]DA release. In contrast, (+/-)-UB-165 was a partial agonist, evoking 58% of 100 microM (-)-nicotine response. Mecamylamine, MG 624, hexamethonium, d-tubocurare, and dihydro-beta-erythroidine (in decreasing order of potency), but not alpha-conotoxin-MII, methyllycaconitine, alpha-conotoxin-ImI, or alpha-bungarotoxin, attenuated 100 microM (-)-nicotine-evoked [(3)H]DA release in a concentration-dependent manner. (+/-)-UB-165, ABT-594, and MG 624 exhibited different pharmacologic profiles in the [(3)H]NE release assay when compared with their effect on [(3)H]DA release. ABT-594 was 4.5-fold more potent, and (+/-)-UB-165 was a full agonist in contrast to its partial agonism in [(3)H]DA release. MG 624 potently and completely blocked NE release evoked by 100 microM (-)-nicotine and 10 microM (+/-)-UB-165, whereas it only partially inhibited (-)-nicotine-evoked [(3)H]DA release. In conclusion, we provide evidence that [(3)H]DA can be evoked from the hippocampus and that the pharmacologic profile for nAChR-evoked hippocampal [(3)H]DA release suggests the involvement of alpha3beta4(*) and at least one other nAChR subtype, thus distinguishing it from that of nAChR-evoked hippocampal [(3)H]NE release.

Chronic nicotine causes functional upregulation of ionotropic glutamate receptors mediating hippocampal noradrenaline and striatal dopamine release

It has been proposed that (-)-nicotine can activate release-stimulating presynaptic nicotinic acetylcholine receptors (nAChRs) on glutamatergic nerve terminals to release glutamate, which in turn stimulates the release of noradrenaline (NA) and dopamine (DA) via presynaptic ionotropic glutamate receptors on catecholaminergic terminals. The objective of this study was to compare the function of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazide-4-propionic acid (AMPA) glutamate receptors in synaptosomes of rat hippocampus and striatum following acute and chronic (-)-nicotine administration. In hippocampal synaptosomes, prelabeled with [3H]NA, both the NMDA- and AMPA-evoked releases were higher in (-)-nicotine-treated (10 days) than in (-)-nicotine-treated (1 day) or vehicle-treated (1 or 10 days) rats. In striatal synaptosomes prelabeled with [3H]DA, the NMDA-evoked, but not the AMPA-evoked, release of [3H]DA was higher in (-)-nicotine-treated (10 days) than in nicotine-treated (1 day) or vehicle-treated (1 or 10 days) animals. Chronic (-)-nicotine did not affect catecholamine uptake, basal release and release evoked by high-K+ depolarization. Thus, chronic exposure to nicotine enhances the function of ionotropic glutamate receptors mediating noradrenaline release in the hippocampus and dopamine release in the striatum.

Pharmacological characterization of dopamine, norepinephrine and serotonin release in the rat prefrontal cortex by neuronal nicotinic acetylcholine receptor agonists

Neuronal nicotinic acetylcholine receptors (nAChRs) modulate synaptic transmission by regulating neurotransmitter release, an action that involves multiple nAChRs. The effects of four nAChR agonists, nicotine (NIC), 1,1-dimethyl-4-phenylpiperzinium iodide (DMPP), cytisine (CYT) and epibatidine (EPI) were investigated on [3H]-norepinephrine (NE), [3H]-dopamine (DA) and [3H]-serotonin (5-HT) release from rat prefrontal cortical (PFC) slices. All four agonists evoked [3H]-DA release to a similar magnitude but with a differing rank order of potency of EPI>>DMPP approximately NIC approximately CYT. Similarly, all four agonists also increased [3H]-NE release, but with a differing rank order of potency of EPI>>CYT approximately DMPP>NIC. NIC-induced [3H]-NE and [3H]-DA release responses were both calcium-dependent and attenuated by the sodium channel antagonist, tetrodotoxin (TTX) and by the nAChR antagonists mecamylamine (MEC) and dihydro-beta-erythroidine (DHbetaE), but not by D-tubocurare (D-TC). The modulation of [3H]-5-HT release by nAChR agonists was distinct from that seen for catecholamines. DMPP produced robust increases with minimal release observed with other agonists. DMPP-induced [3H]-5-HT release was neither sensitive to known nAChR antagonists nor dependent on external calcium. The differences between nicotinic agonist induced catecholamine and serotonin release suggest involvement of distinct nAChRs.

Muscarinic and nicotinic presynaptic modulation of EPSCs in the nucleus accumbens during postnatal development

We have studied the modulatory effects of cholinergic agonists on excitatory postsynaptic currents (EPSCs) in nucleus accumbens (nAcb) neurons during postnatal development. Recordings were obtained in slices from postnatal day 1 (P1) to P27 rats using the whole cell patch-clamp technique. EPSCs were evoked by local electrical stimulation, and all experiments were conducted in the presence of bicuculline methchloride in the bathing medium and with QX-314 in the recording pipette. Under these conditions, postsynaptic currents consisted of glutamatergic EPSCs typically consisting of two components mediated by AMPA/kainate (KA) and N-methyl-D-aspartate (NMDA) receptors. The addition of acetylcholine (ACh) or carbachol (CCh) to the superfusing medium resulted in a decrease of 30-60% of both AMPA/KA- and NMDA-mediated EPSCs. In contrast, ACh produced an increase ( approximately 35%) in both AMPA/KA and NMDA receptor-mediated EPSCs when administered in the presence of the muscarinic antagonist atropine. These excitatory effects were mimicked by the nicotinic receptor agonist 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) and blocked by the nicotinic receptor antagonist mecamylamine, showing the presence of a cholinergic modulation mediated by nicotinic receptors in the nAcb. The antagonistic effects of atropine were mimicked by pirenzepine, suggesting that the muscarinic depression of the EPSCs was mediated by M(1)/M(4) receptors. In addition, the inhibitory effects of ACh on NMDA but not on AMPA/KA receptor-mediated EPSC significantly increased during the first two postnatal weeks. We found that, under our experimental conditions, cholinergic agonists produced no changes on membrane holding currents, on the decay time of the AMPA/KA EPSC, or on responses evoked by exogenous application of glutamate in the presence of tetrodotoxin, but they produced significant changes in paired pulse ratio, suggesting that their action was mediated by presynaptic mechanisms. In contrast, CCh produced consistent changes in the membrane and firing properties of medium spiny (MS) neurons when QX-314 was omitted from the recording pipette solution, suggesting that this substance actually blocked postsynaptic cholinergic modulation. Together, these results suggest that ACh can decrease or increase glutamatergic neurotransmission in the nAcb by, respectively, acting on muscarinic and nicotinic receptors located on excitatory terminals. The cholinergic modulation of AMPA/KA and NMDA receptor-mediated neurotransmission in the nAcb during postnatal development could play an important role in activity-dependent developmental processes in refining the excitatory drive on MS neurons by gating specific inputs.

Differential regulation of nicotinic receptor-mediated neurotransmitter release following chronic (-)-nicotine administration

The objective of this study was to compare nAChR-mediated neurotransmitter release from slices of rat striatum, frontal cortex and hippocampus following chronic (-)-nicotine (Nic) administration (tartrate salt, 2 mg/kg twice daily for 10 days). Binding studies were also conducted to measure changes in receptor density. Relative to saline-treated animals, the number of nAChRs measured by [(3)H]-cytisine (CYT) binding was significantly increased in all brain regions examined by 15% to 25% following chronic Nic administration. Using a relatively high throughput method to measure neurotransmitter release, we found that Nic, CYT, and (+/-)-epibatidine (EB) evoked similar concentration-dependent striatal [(3)H]-dopamine (DA) and hippocampal [(3)H]-norepinephrine (NE) release from both saline (rank order of potency for [(3)H]-DA: EB>CYT>Nic; pEC(50) values, EB (9 +/- 0.1), CYT (8 +/- 0.13), Nic (7.3 +/- 0.19); rank order potency for [(3)H]-NE: EB>Nic=CYT; pEC(50) values, EB (8 +/- 0.18), Nic (5.5 +/- 0.09), CYT (5.12 +/- 0.1)) -and Nic-treated animals (pEC(50) values [(3)H]-DA, EB (9.5 +/- 0.15), Nic (8 +/- 0.16, CYT (6.6 +/- 0.52); [(3)H]-NE, EB (8.4 +/- 0.23), Nic (5.19 +/- 0.1), CYT (5.18 +/- 0.29)). Although no change in potency was detected between the two treatment groups, the agonist efficacies in both tissues were significantly reduced by approximately 17-54% following chronic Nic administration. In contrast to striatum, treatment with Nic did not affect the maximal [(3)H]-DA response (efficacy) in the frontal cortex. However, as observed in the striatum, no change in agonist potency was observed in the frontal cortex following chronic Nic administration (pEC(50) values, saline; EB (9.2 +/- 0.2), >CYT (6.95 +/- 0.75) = Nic (6.9 +/- 0.16); Nic-treated, EB (9 +/- 0.42)>CYT (6.88 +/- 0.27) = Nic (7.1 +/- 0.17)). Chronic Nic treatment did not significantly affect KCl-evoked [(3)H]-NE release from hippocampus or [(3)H]-DA release from frontal cortex or striatum. Since previous work has demonstrated that different nAChR subtypes display various sensitivities to chronic Nic exposure, we suggest that the subtypes of nAChRs involved in regulating [(3)H]-DA release may be different in the striatum and frontal cortex. These results support findings from earlier studies comparing the pharmacology of nAChR-evoked striatal versus cortical [(3)H]-DA release.

Characterization of nicotinic agonist-induced [(3)H]dopamine release from synaptosomes prepared from four mouse brain regions

The inhibition of uptake of [(3)H]dopamine into synaptosomes prepared from four mouse brain regions was investigated. The inhibition curves demonstrated that in three regions, striatum, nucleus accumbens, and olfactory tubercle, [(3)H]dopamine was taken up exclusively by dopaminergic terminals. In frontal cortex, however, only a portion of the uptake was into dopaminergic terminals, with a larger amount taken up by noradrenergic terminals, and another small portion by serotonergic terminals. Release studies in frontal cortex indicated that in this region only dopaminergic and noradrenergic terminals are capable of packaging [(3)H]dopamine in a form allowing vesicle-mediated release; additionally, only the dopaminergic terminals have functional presynaptic nAChRs that, when stimulated by nicotinic agonists, evoke [(3)H]dopamine release. Agonist-stimulated [(3)H]dopamine release was characterized from synaptosomes prepared from four mouse brain regions. alpha-Conotoxin MII was a partial inhibitor of dopamine release in all of the brain regions, which suggests that a minimum of two nicotinic cholinergic receptors (nAChRs) are expressed in the nerve terminals of all four brain regions. No nicotine-induced [(3)H]dopamine release was detected in any brain region when the synaptosomes were prepared from beta2 null mutant mice, which indicates that the beta2 subunit is required for all nAChRs mediating this release. Dose-response curves were constructed for seven agonists in each of the brain regions. The pharmacological properties of synaptosomal [(3)H]dopamine release appear similar across the four brain regions. The results suggest that all four regions express the same nAChRs, although subtle regional differences may exist.

Assessment of nicotinic acetylcholine receptor-mediated release of [(3)H]-norepinephrine from rat brain slices using a new 96-well format assay

The study of the modulatory effects of nicotinic acetylcholine receptor (nAChR) agonists on neurotransmitter release from tissue slices has been hampered by laborious and limiting superfusion techniques. A new methodology was developed utilizing 96-well filter plates. This new method produced comparable results to previously published data, yet expanded throughput to permit more complete pharmacological characterization. Rat brain slices, preloaded with [(3)H]-norepinephrine ([(3)H]-NE), were distributed onto 96-well filter plates. Following a 5 min preincubation, the slices were incubated for 5 min with nicotinic agonists or antagonists. (-)-Nicotine (NIC) and 1,1-dimethyl-4-phenylpiperazine (DMPP) evoked release of [(3)H]-NE from a number of brain regions and spinal cord, with the highest response seen in the hippocampus. Concentration-response curves revealed a rank order of potency of (+/-)-epibatidine>>anatoxin-a>A-85380>DMPP=NIC=(-)-cytisine in the hippocampus, thalamus, and frontal cortex. EC(50) values were approximately 0.005, 0.2, 1, 5, 5 and 5 microM, respectively. Concentration-inhibition curves of nicotine evoked [(3)H]-NE release from hippocampal and thalamic slices resulted in a rank order of potency of mecamylamine>hexamethonium>d-tubocurare>DHbetaE. Schild analysis revealed apparent noncompetitive antagonism of [(3)H]-NE release from hippocampus by mecamylamine, hexamethonium, and DHbetaE. In contrast, DHbetaE antagonism of [(3)H]-dopamine release from striatal slices using a similar methodology was competitive.