Synergistic effect of galantamine with risperidone on impairment of social interaction in phencyclidine-treated mice as a schizophrenic animal model

Social withdrawal is the first sign and key component of the negative symptoms of schizophrenia. The efficacy of risperidone, an atypical antipsychotic, on the symptom is practically limited by dose-dependent side effects in clinical trials, therefore there is the need for adjuvant treatments. In the present study, we aimed to investigate the synergistic effect and mechanism of risperidone and galantamine, which is a nicotinic acetylcholine receptor (nAChR)-allosteric modulator and a modest cholinesterase inhibitor, on phencyclidine (PCP)-treated mouse model of social withdrawal. At non-effective doses by themselves, co-administration of galantamine (0.05mg/kg) and risperidone (0.05mg/kg) showed synergistic effects on PCP-induced impairments of social interaction and dopamine release in the medial prefrontal cortex (mPFC). The behavioral synergistic effect was abolished by the administration of a dopamine-D(1) receptor antagonist, SCH 23390 (0.02mg/kg, systemic; or 0.02microg/0.5microL/mouse, intra-mPFC), and a nAChR antagonist, mecamylamine (3mg/kg), but not a muscarinic receptor antagonist, scopolamine (0.1mg/kg). Mecamylamine (3mg/kg) also abolished the synergistic effect on dopamine release in the mPFC. We conclude that galantamine may have synergistic effect with risperidone on the negative symptom of social withdrawal in schizophrenia, which is mediated by dopamine-D(1) receptors in the mPFC through nAChR activation-increased dopamine release.     

Region-specific effects of N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide on nicotine-induced increase in extracellular dopamine in vivo

BACKGROUND AND PURPOSE: Systemic administration of N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), an antagonist of nicotinic acetylcholine receptors (nAChRs) attenuated the nicotine-induced increase in dopamine levels in nucleus accumbens (NAcc).EXPERIMENTAL APPROACH: Using in vivo microdialysis, we investigated the effects of local perfusion of the novel nAChR antagonist bPiDDB into the NAcc or ventral tegmental area (VTA) on increased extracellular dopamine in NAcc, induced by systemic nicotine. We also examined the concentration-dependent effects of bPiDDB on the acetylcholine (ACh)-evoked response of specific recombinant neuronal nAChR subtypes expressed in Xenopus oocytes, using electrophysiological methods.KEY RESULTS: Nicotine (0.4 mg kg(-1), s.c.) increased extracellular dopamine in NAcc, which was attenuated by intra-VTA perfusion of mecamylamine (100 microM). Intra-VTA perfusion of bPiDDB (1 and 10 microM) reduced nicotine-induced increases in extracellular dopamine in NAcc. In contrast, intra-NAcc perfusion of bPiDDB (1 or 10 microM) failed to alter the nicotine-induced increase in dopamine in NAcc. Intra-VTA perfusion of bPiDDB alone did not alter basal dopamine levels, compared to control, nor the increased dopamine in NAcc following amphetamine (0.5 mg kg(-1), s.c.). Using Xenopus oocytes, bPiDDB (0.01-100 microM) inhibited the response to ACh on specific combinations of rat neuronal nAChR subunits, with highest potency at alpha3beta4beta3 and lowest potency at alpha6/3beta2beta3.CONCLUSIONS AND IMPLICATIONS: bPiDDB-Sensitive nAChRs involved in regulating nicotine-induced dopamine release are located in the VTA, rather than in the NAcc. As bPiDDB has properties different from the prototypical nAChR antagonist mecamylamine, further development may lead to novel nAChR antagonists for the treatment of tobacco dependence.     

Adjunctive galantamine, but not donepezil, enhances the antipsychotic-like effect of raclopride in rats

Acetylcholine (ACh) esterase inhibitors like galantamine and donepezil have been tested as adjunct treatment in schizophrenia. Although ACh esterase inhibition might confer some antipsychotic activity, the role of allosteric potentiation of nicotinic ACh receptors (nAChRs), which is an additional mechanism of galantamine, remains elusive. Therefore, the potential antipsychotic-like effects of galantamine and donepezil, respectively, alone, and in combination with the dopamine D2/3 receptor antagonist, raclopride, were tested in the conditioned avoidance response (CAR) test and extrapyramidal side-effect liability was assessed with the catalepsy test. Neither galantamine nor donepezil alone suppressed CAR selectively. Galantamine, but not donepezil, enhanced the raclopride-induced suppression of CAR, predicting augmentation of antipsychotic activity. In contrast to donepezil, galantamine did not increase catalepsy, alone or combined with raclopride. These data suggest that allosteric potentiation of nAChRs may mediate the antipsychotic-like effect of adjunctive galantamine and provide support for the development of alpha7 nAChR-selective allosteric potentiators for schizophrenia.     

Involvement of dopamine D2 receptors in the induction of long-term potentiation in the basolateral amygdala-dentate gyrus pathway of anesthetized rats

We have previously found that synaptic pathway from the basolateral amygdala (BLA) to the dentate gyrus (DG) displays N-methyl-D-aspartate (NMDA) receptor-independent form of long-term potentiation (LTP), which should be a valuable model for elucidating neural mechanisms linking emotion and memory. To explore its cellular mechanisms, we investigated possible involvement of the beta-adrenergic, muscarinic cholinergic and dopaminergic systems on LTP in this pathway of anesthetized rats. The induction of BLA-DG LTP was not affected by administration of the beta-adrenoceptor antagonist propranolol (50-150nmol, i.c.v.), the muscarinic receptor antagonist scopolamine (2-6mg/kg, i.p.), the cholinesterase inhibitor physostigmine (50 nmol, i.c.v.) or the dopamine D(1) receptor antagonist SCH23390 (100nmol, i.c.v.), but significantly inhibited by the dopamine D2 receptor antagonists, chlorpromazine (15nmol, i.c.v.) and haloperidol (0.15-0.5mg/kg, i.p.), and significantly promoted by the dopamine D2 receptor agonist quinpirole (78nmol, i.c.v.). Furthermore, lesioning with 6-hydroxydopamine of the ventral tegmental area (VTA), the origin of mesolimbic dopaminergic neurons, resulted in attenuated BLA-DG LTP. These results suggest that the D2-dopaminergic system, but not the beta-adrenergic, muscarinic or D1-dopaminergic system, is involved in the induction of BLA-DG LTP. In addition, inhibition of BLA-DG LTP by haloperidol or VTA lesion was abolished by blockade of GABAergic inhibition with picrotoxin. It is probable that the D2-dopaminergic system promotes the induction of BLA-DG LTP by suppressing GABAergic inhibition.     

Synergistic effect of combined treatment with risperidone and galantamine on phencyclidine-induced impairment of latent visuospatial learning and memory: Role of nAChR activation-dependent increase of dopamine D1 receptor-mediated neurotransmission

The clinically achievable efficacy of the atypical antipsychotics on cognitive symptoms of schizophrenia is practically limited by their dose-dependent side effects. Thus, there is the need for adjuvant treatments or strategies for the cognitive impairments. Further, human autopsy and genetic data in schizophrenia have indicated the existence of the abnormality of nicotinic acetylcholine receptors (nAChR). In the present study, we aimed to investigate the synergistic effect and mechanisms of a combined treatment with an atypical antipsychotic risperidone and galantamine, which is a nAChR-allosteric modulator and a modest cholinesterase inhibitor, on the impairment of latent visuospatial learning and memory in mice resembling the cognitive impairment of schizophrenia. Repeated treatment with phencyclidine (PCP, 10 mg/kg, 14 days)-induced cognitive impairment in mice in a one trial water-finding test was used as a model of the cognitive impairment of schizophrenia. In vivo microdialysis was used to investigate the extracellular concentration of dopamine in the medial prefrontal cortex (mPFC). Combined treatment with galantamine and risperidone, at low, ineffective doses (both at 0.05 mg/kg) showed a synergistic effect to reverse cognitive impairment and increase extracellular concentration of dopamine in the mPFC. The synergistic behavioral effect was abolished by a dopamine-D1 receptor antagonist, SCH 23390, and a nAChR antagonist, mecamylamine, but not a muscarinic AChR (mAChR) antagonist, scopolamine. Mecamylamine also blocked the synergistic effect on dopamine release in the mPFC of PCP-treated mice. The study indicates that galantamine and risperidone may have synergistic effect on the cognitive impairments in schizophrenia patients by synergistically promoting the nAChR activation-dependent increase of dopamine D1 receptor-mediated neurotransmission.     

Dual effects of nicotine on dopamine neurons mediated by different nicotinic receptor subtypes

Burst firing of dopaminergic neurons has been found to represent a particularly effective means of increasing dopamine release in terminal areas as well as activating immediate early genes in dopaminoceptive cells. Spontaneous burst firing is largely controlled by the level of activation of NMDA receptors in the ventral tegmental area (VTA) as a consequence of glutamate released from afferents arising mainly in the prefrontal cortex. Nicotine has been found to effectively increase burst firing of dopaminergic cells. This effect of nicotine may be due to an alpha 7 nicotinic receptor-mediated presynaptic facilitation of glutamate release in the VTA. By the use of in-vivo single-cell recordings and immunohistochemistry we here evaluated the role of alpha 7 nicotinic receptors in nicotine-induced burst firing of dopamine cells in the VTA and the subsequent activation of immediate early genes in dopaminoceptive target areas. Nicotine (0.5 mg/kg s.c.) was found to increase firing rate and burst firing of dopaminergic neurons. In the presence of methyllycaconitine (MLA, 6.0 mg/kg i.p.) nicotine only increased firing rate. Moreover, in the presence of dihydro-beta-erythroidine (DH beta E, 1.0 mg/kg i.p.), an antagonist at non-alpha 7 nicotinic receptors, nicotine produced an increase in burst firing without increasing the firing rate. Nicotine also increased Fos-like immunoreactivity in dopamine target areas, an effect that was antagonized with MLA but not with DH beta E. Our data suggest that nicotine's augmenting effect on burst firing is, indeed, due to stimulation of alpha 7 nicotinic receptors whereas other nicotinic receptors seem to induce an increase in firing frequency.     

Induction of Fos-immunostaining by nicotine and nicotinic receptor antagonists in rat brain.

Using Fos protein immunohistochemistry, we have studied the effects of acute nicotine (0.5 mg/kg s.c.) and nicotinic acetylcholine receptor (nAChR) antagonists in eleven rat brain areas. Acute nicotine elevated Fos-like immunostaining (Fos IS) significantly in all studied areas except the medial prefrontal cortex. Nicotine increased the Fos IS in cortical, limbic and hypothalamic areas by 2-10-fold, and in the interpeduncular nucleus as well as in the visual areas the increases were 15-150-fold. When given alone, the nAChR antagonists mecamylamine (1.0 or 5.0 mg/kg i.p.) and dihydro-beta-erythroidine (DHE; 1.4 or 2.8 mg/kg i.p.) increased Fos IS in most brain areas maximally by 2-10-fold, but methyllycaconitine (MLA; 4.0 mg/kg i.p.) only in three areas and maximally by 4-fold. The efficacy of nAChR antagonists in blocking nicotine's effects on Fos IS varied noticeably with respect to region and antagonist, and the combined effect of nicotine+antagonist did not exceed that of either treatment alone. Mecamylamine and DHE significantly reduced nicotine-induced Fos IS in most of the studied areas, and MLA only in two areas. Thus, nAChRs seem to mediate the effects of nicotine on Fos IS, and the differences in the effects of the antagonists studied suggest that more than one subtype of nAChRs are involved. The present experiments also provide evidence that nAChR blockade itself may result in increased Fos protein expression in the brain. This could be due to blockade of presynaptic nAChRs modulating transmitter release or interruption of complex polysynaptic feedback pathways.     

Nicotine attenuates place aversion induced by naloxone in single-dose,morphine-treated rats

Rationale Acute physical dependence refers to the withdrawal syndrome precipitated by an opioid antagonist administered several hours after either a single dose or a short-term infusion of an opioid agonist.Objectives We examined the mechanism of nicotine-induced attenuation of naloxone-precipitated withdrawal syndrome when used to produce an aversive motivational state in a place-conditioning paradigm.Methods The effect of nicotine was investigated through place aversion induced by naloxone in morphine-pretreated rats. Additionally, the mechanism of nicotine action in this model was explored specifically in relation to the dopaminergic system through the use of dopamine receptor antagonist and agonist.Results Place avoidance behavior was potently elicited by naloxone (0.5 mg/kg s.c.) 24 h after a single exposure to morphine (10 mg/kg s.c.). Avoidance behavior was attenuated by pretreatment with a 0.2-mg/kg dose of nicotine 15 min prior to naloxone administration. The effect of nicotine was completely blocked by mecamylamine, but not hexamethonium. The dopamine receptor antagonists haloperidol (0.05, 0.1 mg/kg, s.c.), SCH23390 (0.1 mg/kg, s.c.), raclopride (1.0 mg/kg, s.c.) and eticlopride (0.1 mg/kg, s.c.) showed effects similar to mecamylamine. Additionally, the dopamine receptor agonist apomorphine (0.03, 0.1, 0.3 mg/kg, s.c.) inhibited naloxone-induced place aversion in morphine-treated rats.Conclusion The inhibitory effect of nicotine on place aversion induced by naloxone-precipitated morphine withdrawal may involve a dopaminergic portion of the central nervous system.     

Epibatidine induces long-term potentiation (LTP) via activation of alpha4beta2 nicotinic acetylcholine receptors (nAChRs) in vivo in the intact mouse dentate gyrus: both alpha7 and alpha4beta2 nAChRs essential to nicotinic LTP

Activation of nicotinic acetylcholine receptors (nAChRs) induces nocotinic long-term potentiation (LTPn) in vivo in the mouse dentate gyrus. We have found that alpha4beta2 nAChRs activated by epibatidine induce LTPn, the full size of which requires the involvement of alpha4beta2 and alpha7 nAChRs, in the intact mouse dentate gyrus using extracellular recording techniques. Intraperitoneal application of epibatidine, a potent alpha4beta2 nAChR agonist, at 0.3 - 3.0 mug/kg induced a long-lasting increase similar to LTPn induced by choline, a selective alpha7 nAChR agonist, and at 10 mug/kg caused a transient increase followed by a depression. The LTPn induced by epibatidine at 3.0 mug/kg or choline at 30 mg/kg was significantly suppressed by pre-treatment but not post-treatment with mecamylamine (0.5 mg/kg, i.p.), a non-selective neuronal nicotinic antagonist. Post-application of nicotine at 3.0 mg/kg enhanced epibatidine-induced LTPn to the same level of nicotine-induced LTPn, but post-application of epibatidine had no effect on nicotine-induced LTPn. Epibatidine-induced LTPn was additionally increased by post-application of choline, and vice versa, reaching the same level of nicotine-induced LTPn. The present study revealed that epibatidine induced the LTPn via alpha4beta2 nAChRs and that both alpha7 and alpha4beta2 nAChRs were essential for full-sized LTPn, suggesting that both nAChRs play an important role in synaptic plasticity.     

Nicotinic acetylcholine receptors in the ventral tegmental area mediate the dopamine activating and reinforcing properties of ethanol cues

Rationale Cues associated with alcohol can elicit craving, support drug-seeking and precipitate relapse. Objectives We investigated the possible involvement of nicotinic acetylcholine receptors (nAChRs) in the ventral tegmental area (VTA) in the conditioned reinforcing properties of ethanol-associated stimuli in the rat. Materials and methods First, using in vivo microdialysis, we analyzed the effect of VTA perfusion of the nonselective nAChR antagonist mecamylamine (MEC) or the selective α4β2* nAChR antagonist dihydro-β-erythroidine (DHβE) on the nucleus accumbens (nAc) dopaminergic response to the presentation of an ethanol-associated conditioned stimulus (CS). Second, rats were trained to associate a tone + light CS with the presentation of 10% ethanol and were subsequently tested on the acquisition of a new instrumental response with conditioned reinforcement (CR) after local VTA infusion of MEC, DHβE, or α-Conotoxin MII (α-CtxMII, a selective α3β2* and α6* nAChR antagonist). Results The ethanol-associated CS elevated nAc dopamine, an effect that was blocked by VTA perfusion of MEC but not DHβE. Systemic administration of MEC or local VTA infusion of MEC or α-CtxMII selectively blocked ethanol-associated CR, whereas systemic DHβE had no effect. Conclusions We hypothesize a novel mechanism by which alcohol-associated cues promote drug-seeking behavior via activation of dopamine-stimulating α-CtxMII-sensitive nAChRs in the VTA. Pharmacological manipulations of selective nAChRs may thus be possible treatment strategies to prevent cue-induced relapse.     

Chronic pre-treatment with nicotine enhances nicotine-evoked striatal dopamine release and alpha6 and beta3 nicotinic acetylcholine receptor subunit mRNA in the substantia nigra pars compacta of the rat

Whilst local intrastriatal infusion of nicotine consistently elicits striatal dopamine release, systemic administration often fails to do so. Since chronic nicotine administration is known to result in desensitisation-induced upregulation of nicotinic acetylcholine receptors (nAChRs), the present study investigated whether chronic pre-treatment could enhance the response to systemic nicotine and, if so, whether increases in specific nAChR subunit mRNA levels in the substantia nigra pars compacta (SNc) may underlie this effect. In vivo microdialysis studies in male Sprague-Dawley rats revealed that following 4 days pre-treatment with nicotine (0.8 mg kg(-1)s.c.), local intrastriatal nicotine infusion (3 mM) elicited significantly higher dopamine efflux compared to vehicle pre-treated controls (peak release: 1273 +/- 199% basal versus 731 +/- 113% basal), whereas systemic nicotine challenge (0.8 mg kg(-1)s.c.) elicited no response. In contrast, following 8 days pre-treatment with nicotine (0.8 mg kg(-1)s.c.), systemic nicotine challenge (0.8 mg kg(-1)s.c.) now produced significantly higher dopamine efflux than that of vehicle pre-treated controls (147 +/- 30% basal versus 91 +/- 5% basal). Eight days pre-treatment with nicotine also significantly elevated the levels of alpha6 (approximately 55%) and beta3 (approximately 43%) nAChR subunit mRNA in the SNc, suggesting that up-regulation of these nAChR subunit genes in the nigrostriatal tract may contribute to the enhanced nicotine-evoked striatal dopamine release.     

Galantamine improves apomorphine-induced deficits in prepulse inhibition via muscarinic ACh receptors in mice

Background and purpose

Galantamine, a weak acetylcholine esterase (AChE) inhibitor and allosteric potentiator of nicotinic ACh receptors (nAChRs), improves apomorphine-induced deficits in prepulse inhibition (PPI), sensory information-processing deficits, via a nAChR-independent mechanism. The present study examined the role of muscarinic ACh receptors (mAChRs) in the effect of galantamine, and studied the mechanism of galantamine-induced increases in prefrontal ACh levels in mice.

Experimental approach

Apomorphine (1 mg kg⁻¹) was administered to male ddY mice (9–10 weeks old) to create a PPI deficit model. Extracellular ACh concentrations in the prefrontal cortex were measured by in vivo microdialysis.

Key results

Galantamine- and donepezil-mediated improvements in apomorphine-induced PPI deficits were blocked by the preferential M₁ mAChR antagonist telenzepine. The mAChR agonist oxotremorine also improved apomorphine-induced PPI deficits. Galantamine, like donepezil, increased extracellular ACh concentrations in the prefrontal cortex. Galantamine-induced increases in prefrontal ACh levels were partially blocked by the dopamine D₁ receptor antagonist {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390, but not by antagonists of mAChRs (telenzepine) and nAChRs (mecamylamine). Galantamine increased dopamine, but not 5-HT, release in the prefrontal cortex.

Conclusions and implications

Galantamine improves apomorphine-induced PPI deficits by stimulating mAChRs through increasing brain ACh levels via a dopamine D₁ receptor-dependent mechanism and AChE inhibition.     

Nicotinic acetylcholine receptors containing the α6 subunit contribute to ethanol activation of ventral tegmental area dopaminergic neurons

Nicotine and alcohol are often co-abused suggesting a common mechanism of action may underlie their reinforcing properties. Both drugs acutely increase activity of ventral tegmental area (VTA) dopaminergic (DAergic) neurons, a phenomenon associated with reward behavior. Recent evidence indicates that nicotinic acetylcholine receptors (nAChRs), ligand-gated cation channels activated by ACh and nicotine, may contribute to ethanol-mediated activation of VTA DAergic neurons although the nAChR subtype(s) involved has not been fully elucidated. Here we show that expression and activation of nAChRs containing the α6 subunit contribute to ethanol-induced activation of VTA DAergic neurons. In wild-type (WT) mouse midbrain sections that contain the VTA, ethanol (50 or 100 mM) significantly increased firing frequency of DAergic neurons. In contrast, ethanol did not significantly increase activity of VTA DAergic neurons in mice that do not express CHRNA6, the gene encoding the α6 nAChR subunit (α6 knock-out (KO) mice). Ethanol-induced activity in WT slices was also reduced by pre-application of the α6 subtype-selective nAChR antagonist, α-conotoxin MII[E11A]. When co-applied, ethanol potentiated the response to ACh in WT DAergic neurons; whereas co-application of ACh and ethanol failed to significantly increase activity of DAergic neurons in α6 KO slices. Finally, pre-application of α-conotoxin MII[E11A] in WT slices reduced ethanol potentiation of ACh responses. Together our data indicate that α6-subunit containing nAChRs may contribute to ethanol activation of VTA DAergic neurons. These receptors are predominantly expressed in DAergic neurons and known to be critical for nicotine reinforcement, providing a potential common therapeutic molecular target to reduce nicotine and alcohol co-abuse     

Effect of muscarinic receptor agonists xanomeline and sabcomeline on acetylcholine and dopamine efflux in the rat brain; comparison with effects of 4-[3-(4-butylpiperidin-1-yl)-propyl]-7-fluoro-4H-benzo[1,4]oxazin-3-one (AC260584) and N-desmethylclozapine

We have demonstrated that the main metabolite of clozapine, N-desmethylclozapine, has a significant role in the ability of clozapine to improve some aspects of cognition in schizophrenia. Furthermore, there is also evidence to suggest that it is the muscarinic M(1) receptor agonist effect of N-desmethylclozapine that underlies its cognitive effects. In the present study we examined the efficacy of two muscarinic receptor agonists xanomeline and sabcomeline to increase the efflux of acetylcholine and dopamine in rat medial prefrontal cortex and nucleus accumbens. Microdialysis in awake, freely moving rats was used to demonstrate that xanomeline at 10, but not 1 or 3 mg/kg (s.c.), significantly increased acetylcholine efflux in both the medial prefrontal cortex and nucleus accumbens. Sabcomeline, at 1 but not 0.1 or 0.5 mg/kg (s.c.), significantly increased acetylcholine efflux in the medial prefrontal cortex but not the nucleus accumbens. Both xanomeline and sabcomeline dose-dependently increased dopamine efflux in the medial prefrontal cortex but only high dose of xanomeline (10 mg/kg, s.c.) and sabcomeline (1 mg/kg, s.c.) increased that in the nucleus accumbens. The acetylcholine and dopamine efflux induced by xamomeline (10 mg/kg, s.c.) and sabcomeline (1 mg/kg, s.c.) were significantly blocked by the preferential muscarinic M(1) receptor antagonist telenzepine (3 mg/kg, s.c.), but significantly potentiated by the atypical antipsychotic drug risperidone (0.1 mg/kg, s.c.), which does not have much affinity for muscarinic receptor(s). According to the analysis of net-AUC (area under the curve) values of acetylcholine and dopamine levels, the rank order of ability of these drugs to increase acetylcholine or dopamine levels is sabcomeline>xanomeline approximately AC260584>N-desmethylclozapine. The present study suggests that the binding potency of muscarinic M(1) receptors is greatly related to their ability to increase cortical acetylcholine and dopamine efflux, and that this may have some relevance for treatment of the cognitive deficit of schizophrenia.     

The allosteric potentiation of nicotinic acetylcholine receptors by galantamine is transduced into cellular responses in neurons: Ca2+ signals and neurotransmitter release

Neuronal nicotinic acetylcholine receptors (nAChR) modulate a variety of cellular responses, including Ca2+ signals and neurotransmitter release, which can influence neuronal processes such as synaptic efficacy and neuroprotection. In addition to receptor activation through the agonist binding site, an allosteric modulation of nAChR has also been described for a novel class of allosteric ligands. Of these, the acetylcholinesterase inhibitor and Alzheimer drug galantamine represents the prototypical allosteric ligand, based on its potentiation of nAChR-evoked single-channel and whole-cell currents. The aim of this study was to establish whether the allosteric potentiation of nAChR currents is transduced in downstream cellular responses to nAChR activation, namely increases in intracellular Ca2+ and [3H]noradrenaline release. In SH-SY5Y cells, galantamine potentiated nicotine-evoked increases in intracellular Ca2+ and [3H]noradrenaline release with a bell-shaped concentration-response profile; maximum enhancement of nicotine-evoked responses occurred at 1 muM galantamine. This potentiation was blocked by mecamylamine, whereas galantamine had no effect on these measures in the absence of nicotine. Galantamine did not compete for radioligand binding to the agonist binding sites of several nAChR subtypes, consistent with an allosteric mode of action. Unlike galantamine, the acetylcholinesterase inhibitors rivastigmine and donepezil did not potentiate nAChR-mediated responses, whereas donepezil was a reasonably potent inhibitor of nicotine- and KCl-evoked increases in Ca2+. nAChR-mediated [3H]noradrenaline release from hippocampal slices was also potentiated by galantamine, with an additional component attributable to acetylcholinesterase inhibition and subsequent increase in acetylcholine. These results indicate that the allosteric regulation of nAChR results in the potentiation of receptor-dependent cellular processes relevant to many of the physiological consequences of nAChR activation.     

Stress-induced increase of cortical dopamine metabolism: attenuation by a tachykinin NK1 receptor antagonist

The present study examined the potential role of tachykinin NK1 receptors in modulating immobilisation stress-induced increase of dopamine metabolism in rat medial prefrontal cortex. In agreement with previous studies, 20 min immobilisation stress significantly increased medial prefrontal cortex dopamine metabolism as reflected by the concentration of the dopamine metabolite dihydroxyphenylacetic acid (DOPAC). Pretreatment with the high affinity, selective, tachykinin NK1 receptor antagonist (3(S)-(2-methoxy-5-(5-trifluoromethyltetrazol-1-yl)-phenylmethyl amino)-2(S)-phenylpiperidine) ((S)-GR205171, 10 mg/kg, s.c.), a dose that in ex vivo binding studies extensively occupied rat brain tachykinin NK1 receptors for approximately 60 min, significantly attenuated the stress-induced increase of mesocortical DOPAC concentration without affecting cortical DOPAC levels per se. In contrast, pretreatment of animals with the less active enantiomer (R)-GR205171 (10 mg/kg, s.c.), which demonstrated negligible tachykinin NK1 receptor occupancy ex vivo, failed to affect either basal or stress-induced DOPAC concentration in medial prefrontal cortex. Furthermore, pretreatment of animals with the benzodiazepine/GABAA receptor antagonist, flumazenil (15 mg/kg, i.p.), did not affect the ability of (S)-GR205171 to attenuate the increase of medial prefrontal cortex DOPAC concentration by acute stress. Results demonstrate that the selective tachykinin NK1 receptor antagonist, (S)-GR205171, attenuated the stress-induced activation of mesocortical dopamine neurones by a mechanism independent of the benzodiazepine modulatory site of the GABAA receptor.     

The alpha-7 nicotinic receptor partial agonist/5-HT3 antagonist RG3487 enhances cortical and hippocampal dopamine and acetylcholine release

Rationale

Alpha-7 nicotinic acetylcholine receptor (nAChR) agonists may ameliorate cognitive deficits in schizophrenia, in part, because of their ability to enhance dopaminergic and cholinergic neurotransmission.

Objectives

In the current study, the effects of partial nAChR agonist and 5-HT₃ receptor antagonist RG3487 (previously R3487/MEM3454) on dopamine (DA) and acetylcholine (ACh) effluxes in rat prefrontal cortex (mPFC) and hippocampus (HIP) were investigated in awake, freely moving rats.

Results

R3487/MEM3454, at doses of 0.1–10 mg/kg, s.c., enhanced DA and ACh effluxes in rat mPFC and (HIP), with a peak effect at 0.3- to 0.6-mg/kg doses, producing a bell-shaped dose–response curve. Pretreatment with the selective nAChR antagonist, methyllycaconitine (1.0 mg/kg), completely blocked RG3487-induced (0.45 mg/kg) DA but not ACh efflux, while the selective 5-HT₃ receptor agonist 1-(m-chlorophenyl)-biguanide (1.0 mg/kg) partially inhibited cortical ACh but not DA efflux. RG3487 (0.45 mg/kg) combined with atypical antipsychotic drug (APD) risperidone (0.1 mg/kg), but not typical APD haloperidol (0.1 mg/kg), induced a significantly greater increase in HIP ACh efflux. Their combined effect on DA efflux was additive. RG3487, combined with other atypical APDs, namely aripiprazole (0.3 mg/kg), olanzapine (1.0 mg/kg), and quetiapine (30 mg/kg), also produced additive effects on DA efflux.

Conclusions

These results suggest that RG3487 enhances DA efflux by nAChR stimulation, whereas ACh efflux is primarily mediated via 5-HT₃ receptor antagonism, and that RG3487 alone or as augmentation may improve cognitive impairment in schizophrenia.     

Preclinical investigation of the functional effects of memantine and memantine combined with galantamine or donepezil.

Combinations of drugs approved to treat Alzheimer's disease (AD) were tested in older rabbits with delay eyeblink classical conditioning, a form of associative learning severely impaired in AD. In Experiment 1 (n=49 rabbits), low doses (0.1, 0.5, 1.0, and 0.0 (vehicle) mg/kg) of memantine (Namenda) were tested. These three doses neither improved nor impaired acquisition at a statistically significant level. The 0.5 mg/kg dose had the greatest effect numerically and did not cause sensitization or habituation in explicitly unpaired controls. In Experiment 2 (n=56), doses of galantamine (Razadyne; 3.0 mg/kg) and donepezil (Aricept; 0.75 mg/kg) that had comparable magnitudes of cholinesterase inhibition were tested alone and in combination with 0.5 mg/kg memantine. Older rabbits treated with galantamine and with galantamine+memantine learned significantly better than vehicle-treated rabbits, but adding memantine did not improve learning over galantamine alone. Older rabbits treated with donepezil or a combination of memantine and donepezil did not learn significantly better than rabbits treated with vehicle. Galantamine has two mechanisms of action: mild cholinesterase inhibition and allosteric modulation of nicotinic acetylcholine receptors (nAChRs). When equated for cholinesterase inhibition, galantamine had significant efficacy in the eyeblink conditioning model system, but donepezil did not, indicating that modulation of nAChRs may be the mechanism that significantly ameliorates learning deficits in this model. In the absence of AD neuropathology in older rabbits, memantine had no efficacy alone or in combination with the other drugs.     

TC-2559 excites dopaminergic neurones in the ventral tegmental area by stimulating alpha4beta2-like nicotinic acetylcholine receptors in anaesthetised rats

1. The in vivo effects of a selective partial agonist for neuronal nicotinic acetylcholine receptor (nAChRs) alpha4beta2 subtype, TC-2559, characterised recently in in vitro preparations, have been profiled. The brain bioavailability of TC-2559 and its effects on the spontaneous firing and bursting properties of the dopaminergic (DAergic) neurones recorded extracellularly in the ventral tegmental area (VTA) were studied following systemic administration in anaesthetised rats. 2. Cumulative doses of TC-2559 (0.021-1.36 mg kg(-1), i.v.) increased both the firing and bursting activities of VTA DA neurones. The effect of bolus doses of TC-2559 of 0.66 or 1.32 mg kg(-1), i.v., was approximately equivalent to that of 0.0665 mg kg(-1), i.v. nicotine. 3. The excitation evoked by both nicotine and TC-2559 was fully reversed by DHbetaE (0.39-0.77 mg kg(-1), i.v.), an alpha4beta2-subtype-preferring nicotinic antagonist, and application of nicotine after DHbetaE failed to evoke any excitation. MLA (0.23 mg kg(-1), i.v.), an alpha7 selective antagonist, failed to alter TC-2559-evoked excitation and bursting activities, and a novel alpha7 agonist (PSAB-OFP; 0.23 mg kg(-1), i.v.) was also without effect. 4. The present results indicated that TC-2559 fully mimics nicotine by increasing both the excitability and bursting behaviour of VTA DA neurones, effects that are predominantly due to activation of alpha4beta2-like nAChRs. 5. TC-2559 has been demonstrated to be a useful in vivo pharmacological tool for studying the alpha4beta2 subtype of nicotinic receptor.     

The NMDA receptor antagonist MK-801 prevents imipramine-induced supersensitivity to quinpirole.

Chronic treatment with imipramine enhanced the locomotor stimulant response to quinpirole (0.3 mg/kg s.c.), a dopamine D2/D3 receptor agonist. Chronic, but not acute, blockade of the NMDA receptor with the non-competitive antagonist MK-801 (0.3 mg/kg i.p.) prevented the imipramine-induced potentiation of the quinpirole effect. The results suggest that NMDA receptors play a role in the development of supersensitivity to dopamine receptor agonists produced by chronic antidepressant treatment.