Zhu Li

Atypical antipsychotic drugs, quetiapine, iloperidone, and melperone, preferentially increase dopamine and acetylcholine release in rat medial prefrontal cortex: role of 5-HT1A receptor agonism.

Preferential increases in both cortical dopamine (DA) and acetylcholine (ACh) release have been proposed to distinguish the atypical antipsychotic drugs (APDs) clozapine, olanzapine, risperidone and ziprasidone from typical APDs such as haloperidol. Although only clozapine and ziprasidone are directly acting 5-HT(1A) agonists, WAY100635, a selective 5-HT(1A) antagonist, partially attenuates these atypical APD-induced increases in cortical DA release that may be due to combined 5-HT(2A) and D(2) blockade. However, WAY100635 does not attenuate clozapine-induced cortical ACh release. The present study determined whether quetiapine, iloperidone and melperone, 5-HT(2A)/D(2) antagonist atypical APDs, also increase cortical DA and ACh release, and whether these effects are related to 5-HT(1A) agonism. Quetiapine (30 mg/kg), iloperidone (1-10 mg/kg), and melperone (3-10 mg/kg) increased DA and ACh release in the medial prefrontal cortex (mPFC). Iloperidone (10 mg/kg) and melperone (10 mg/kg), but not quetiapine (30 mg/kg), produced an equivalent or a smaller increase in DA release in the nucleus accumbens (NAC), respectively, compared to the mPFC, whereas none of them increased ACh release in the NAC. WAY100635 (0.2 mg/kg), which alone did not affect DA or ACh release, partially attenuated quetiapine (30 mg/kg)-, iloperidone (10 mg/kg)- and melperone (10 mg/kg)-induced DA release in the mPFC. WAY100635 also partially attenuated quetiapine (30 mg/kg)-induced ACh release in the mPFC, but not that induced by iloperidone (10 mg/kg) or melperone (10 mg/kg). These results indicate that quetiapine, iloperidone and melperone preferentially increase DA release in the mPFC, compared to the NAC via a 5-HT(1A)-related mechanism. However, 5-HT(1A) agonism may be important only for quetiapine-induced ACh release.

N-desmethylclozapine, a major metabolite of clozapine, increases cortical acetylcholine and dopamine release in vivo via stimulation of M1 muscarinic receptors

The active moiety of clozapine, the prototypical antipsychotic drug, consists of clozapine and its major metabolite, N-desmethylclozapine (NDMC). Previous studies have suggested that NDMC may be more important than the patent compound itself for the improvement in cognition in patients with schizophrenia treated with clozapine. While the pharmacology of clozapine and NDMC are similar in most respects, NDMC has been shown to be an M1 muscarinic receptor partial agonist whereas clozapine is an M1 antagonist in vitro and in vivo. We hypothesized that NDMC may improve cognition by increasing dopamine (DA) and acetylcholine (ACh) release in medial prefrontal cortex (mPFC) via direct stimulation of M1 receptors, whereas both NDMC and clozapine itself would do so by other mechanisms as well, and that clozapine would inhibit the M1 agonist effect of NDMC. In the present study, using microdialysis in awake, freely moving rats, we found that NDMC at doses of 10 and 20, but not 5 mg/kg, significantly increased DA and ACh release in the mPFC and HIP, but not in the nucleus accumbens (NAC). The M1-preferring antagonist, telenzepine (3 mg/kg), completely blocked NDMC (10 mg/kg)-induced increases in cortical DA and ACh release. Clozapine (1.25 mg/kg), which by itself had no effect on DA or ACh release in the cortex, blocked NDMC (10 mg/kg)-induced ACh, but not DA, release in the mPFC. The 5-HT1A receptor antagonist, WAY100635 (0.2 mg/kg) blocked NDMC (20 mg/kg)-induced cortical DA but not ACh release. These findings suggest that: (1) NDMC is an M1 agonist while clozapine is an M1 antagonist in vivo; (2) M1 agonism of NDMC can contribute to the release of cortical ACh and DA release; (3) NDMC, because of its M1 agonism, may more effectively treat the cognitive impairments observed in schizophrenia than clozapine itself; and (4) M1 receptor agonism may be a valuable target for the development of drugs that can improve cognitive deficit in schizophrenia, and perhaps other neuropsychiatric disorders as well.

Clozapine increases both acetylcholine and dopamine release in rat ventral hippocampus: role of 5-HT1A receptor agonism.

Atypical antipsychotic drugs (APDs) such as clozapine, but not the typical APD haloperidol, improve some aspects of cognition in schizophrenia. This advantage has been attributed, in part, to the ability of the atypical APDs to markedly increase acetylcholine (ACh) and dopamine (DA) release in rat medial prefrontal cortex (mPFC), while producing a minimal effect in the nucleus accumbens (NAC) or striatum. The atypical APD-induced preferential release of DA, but not ACh, in the mPFC is partially inhibited by the selective 5-HT(1A) antagonist WAY100635. However, little is known about these effects of atypical APDs in the ventral hippocampus (vHIP), another possible site of action of atypical APDs with regard to cognitive enhancement. The present study demonstrates that clozapine (10 mg/kg) comparably increases both ACh and DA release in the vHIP and mPFC. The increases in DA, but not ACh, release in both regions were partially attenuated by WAY100635 (0.2 mg/kg), which had no effect by itself on the release of either neurotransmitter in either region. Tetrodotoxin (TTX; 1 microM), a Na(+) channel blocker, in the perfusion medium, eliminated the clozapine (10 mg/kg)-induced ACh and DA release in the vHIP, indicating their neuronal origin. Haloperidol produced a slight increase in ACh release in the vHIP at 1 mg/kg, and DA release in the mPFC at 0.1 mg/kg. In conclusion, clozapine increases ACh and DA release in the vHIP and mPFC, whereas haloperidol has minimal effects on the release of these two neurotransmitters in either region. These differences may contribute, at least in part, to the superior ability of clozapine, compared to haloperidol, to improve cognition in schizophrenia. 5-HT(1A) agonism is important to the ability of clozapine and perhaps other atypical APDs to increase DA, but not ACh, release in the vHIP, as well as the mPFC. The role of hippocampus in the cognitive effects of atypical APDs warrants more intensive study.

SR46349-B, a 5-HT2A/2C Receptor Antagonist, Potentiates Haloperidol-induced Dopamine Release in Rat Medial Prefrontal Cortex and Nucleus Accumbens

The combination of M100907, a putative antipsychotic drug (APD) and serotonin (5-HT)2A antagonist, and the typical APD haloperidol, can enhance dopamine (DA) release in rat medial prefrontal cortex (mPFC), an effect which has been postulated to be of value to improve cognition and negative symptoms. The present study demonstrated that another putative APD and 5-HT2A/2C antagonist, SR46349-B (10 mg/kg, but not 1–3 mg/kg) alone, but not M100907 (0.1 and 3 mg/kg) alone, increased mPFC DA release, whereas neither drug alone affected nucleus accumbens (NAC) DA release. Neither SR46349-B nor M100907 alone affected nucleus accumbens (NAC) DA release. Neither SR46349-B nor M100907 alone affected nucleus accumbens (NAC) DA release. SR46349-B (3 mg/kg) potentiated haloperidol-induced DA release in both regions, whereas M100907 (0.1 mg/kg) potentiated haloperidol (0.1 mg/kg)-induced mPFC DA release and inhibited it in the NAC. WAY100635 (0.2 mg/kg), a 5-HT1A antagonist, abolished the effects of haloperidol plus M100907 as well as SR46349-B on DA release in the mPFC, but did not do so in the NAC. Thus, 5-HT2A and 5-HT2A/2C antagonism together with haloperidol-induced D2 antagonism may potentiate mPFC DA release via 5-HT1A agonism, whereas the combined effects of these agents on NAC DA release is not dependent upon 5-HT1A receptor stimulation. Interestingly, similar to the effect of SR46349-B, high dose M100907 (3 mg/kg), which might have antagonist activity at 5-HT2C receptors, potentiated 1 mg/kg haloperidol-induced DA release in the mPFC and NAC. These results suggest that 5-HT2A/2C antagonism may be more advantageous than selective 5-HT2A antagonism as an adjunct to D2 antagonists to improve cognition and negative symptoms in schizophrenia.

Attenuation of Phencyclidine-Induced Object Recognition Deficits by the Combination of Atypical Antipsychotic Drugs and Pimavanserin (ACP 103), a 5-Hydroxytryptamine2A Receptor Inverse Agonist

Subchronic administration of the N-methyl-d-aspartate receptor antagonist, phencyclidine (PCP), in rodents has been shown to produce impairment in novel object recognition (NOR), a model of visual learning and memory. We tested the hypothesis that the selective 5-HT2A inverse agonists, pimavanserin and (R)-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl]-4-piperidinemethanol (M100907), would potentiate subeffective doses of atypical antipsychotic drugs (APDs) to reverse the NOR deficits. Female rats received vehicle or PCP (2 mg/kg b.i.d.) for 7 days, followed by a 7-day washout. Pimavanserin (3 mg/kg) or M100907 (1 mg/kg) alone, or four atypicial APDs, risperidone (0.05–0.1 mg/kg), melperone (1–3 mg/kg), olanzapine (1–2 mg/kg), or N-desmethylclozapine (1–2 mg/kg), and the typical APD, haloperidol (0.05–0.1 mg/kg), were administered alone, or in combination with pimavanserin or M100907, before NOR testing. The exploration times of objects during 3-min acquisition and retention trials, separated by a 1-min interval, were compared by analysis of variance. Vehicle-, but not PCP-treated, animals, explored the novel object significantly more than the familiar in the retention trial (p < 0.05–0.01). Pretreatment with the higher doses of the atypical APDs, but not pimavanserin, M100907, or haloperidol alone, reversed the effects of PCP. The effect of risperidone was blocked by haloperidol pretreatment. Coadministration of pimavanserin or M100907, with ineffective doses of the atypical APDs, but not haloperidol, also reversed the PCP-induced deficit in NOR. These results support the importance of 5-hydroxytryptamine2A receptor blockade relative to D2 receptor blockade in the ability of atypicals to ameliorate the effect of subchronic PCP, a putative measure of cognitive dysfunction in schizophrenia.

Cholinergic modulation of basal and amphetamine-induced dopamine release in rat medial prefrontal cortex and nucleus accumbens

Behavioral evidence suggests that muscarinic/cholinergic inhibition of brain dopaminergic activity may be a useful principle for developing novel antipsychotic drugs (APDs). Thus, oxotremorine, a muscarinic agonist, attenuates amphetamine-induced locomotor activity in rodents, an effect also produced by a wide variety of proven APDs, whereas scopolamine, a muscarinic antagonist, has the opposite effect. Since atypical APDs such as clozapine, olanzapine, risperidone, ziprasidone and quetiapine, increase brain acetylcholine as well as dopamine (DA) release in a region-specific manner, their effects on cholinergic and dopaminergic neurotransmission may also contribute to various actions of these drugs. Oxotremorine (0.5-1.5 mg/kg) dose-dependently and preferentially increased DA release in rat medial prefrontal cortex (mPFC), compared to the nucleus accumbens (NAC). However, S-(-)-scopolamine (0.5-1.5 mg/kg) produced similar increases in DA release in the mPFC, but the effect was much less than that of oxotremorine. Whereas a dose of S-(-)-scopolamine of 0.5 mg/kg comparably increased DA release in the mPFC and NAC, 1.5 mg/kg had no effect on DA release in the NAC. Oxotremorine-M (0.5 mg/kg), a M(1/4)-preferring agonist, also increased DA release in the mPFC, but not the NAC, an effect completely abolished by telenzepine (3 mg/kg), a M(1/4)-preferring antagonist, which by itself had no effect on DA release in either region. Oxotremorine (0.5, but not 1.5, mg/kg) attenuated amphetamine (1 mg/kg)-induced DA release in the NAC, whereas S-(-)-scopolamine did not. Oxotremorine (1.5 mg/kg) and S-(-)-scopolamine (0.5 mg/kg) modestly but significantly potentiated amphetamine (1 mg/kg)-induced DA release in the mPFC. These results suggest that stimulation of muscarinic receptors, in particular M(1/4), as indicated by the effect of oxotremorine-M and telenzepine, may preferentially increase cortical DA release and inhibit amphetamine-induced DA release in the NAC.

5-HT6 receptor antagonist SB-399885 potentiates haloperidol and risperidone-induced dopamine efflux in the medial prefrontal cortex or hippocampus

Many studies suggest that the 5-HT6 receptors are involved, along with other 5-HT receptors, in the pathophysiology and pharmacotherapy of schizophrenia. It is a putative therapeutic target of atypical antipsychotic drugs, notably clozapine, as well as some other psychotropic agents. Preferential potentiation of dopamine (DA) efflux in the medial prefrontal cortex (mPFC) and hippocampus (HIP) has been suggested to contribute to the ability of atypical antipsychotic drugs (APDs), e.g. clozapine, risperidone, olanzapine and ziprasidone, to improve cognitive function in schizophrenia. The present study demonstrated that SB-399885, a selective 5-HT6 receptor antagonist, at doses of 3 and 10 mg/kg, had no effect on cortical DA release in freely moving rats. However, both doses of SB-399885 slightly but significantly increased DA release in the HIP. Of particular interest, SB-399885, 3 mg/kg, significantly potentiated the ability of a typical antipsychotic drug haloperidol, a D2 receptor antagonist, at a dose of 0.1 mg/kg, to increase DA release in the HIP but not the mPFC. The atypical antipsychotic drug risperidone, a multireceptor antagonist, which lacks 5-HT6 receptor antagonist properties, at doses of 0.1, 0.3 and 1.0 mg/kg, produced a bell-shaped dose response effect on DA efflux in the mPFC and HIP. SB-399885 potentiated risperidone (1.0 mg/kg)-induced DA efflux in both regions. The increase in the HIP, but not the mPFC, DA efflux by 0.3 mg/kg risperidone was also potentiated by SB-399885, 3 mg/kg. These results suggest that the combined blockade of 5-HT6 and D2 receptors may contribute to the potentiation of haloperidol- and risperidone-induced DA efflux in the mPFC or HIP. The present data provides additional evidence in support of a possible therapeutic role for 5-HT6 receptor antagonism, as an addition on therapy, to enhance cognitive function in schizophrenia.

SLV313 (1-(2,3-dihydro-benzo[1,4]dioxin-5-yl)-4- [5-(4-fluoro-phenyl)-pyridin-3-ylmethyl]-piperazine monohydrochloride): a novel dopamine D2 receptor antagonist and 5-HT1A receptor agonist potential antipsychotic drug.

Combined dopamine D2 receptor antagonism and serotonin (5-HT)1A receptor agonism may improve efficacy and alleviate some side effects associated with classical antipsychotics. The present study describes the in vitro and in vivo characterization of 1-(2,3-dihydro-benzo[1,4]dioxin-5-yl)-4-[5-(4-fluoro-phenyl)-pyridin-3-ylmethyl]-piperazine monohydrochloride (SLV313), a D2/3 antagonist and 5-HT1A agonist. SLV313 possessed high affinity at human recombinant D2, D3, D4, 5-HT2B, and 5-HT1A receptors, moderate affinity at 5-HT7 and weak affinity at 5-HT2A receptors, with little-no affinity at 5-HT4, 5-HT6, α1, and α2 (rat), H1 (guinea pig), M1, M4, 5-HT3 receptors, and the 5-HT transporter. SLV313 had full agonist activity at cloned h5-HT1A receptors (pEC50=9.0) and full antagonist activity at hD2 (pA2=9.3) and hD3 (pA2=8.9) receptors. In vivo, SLV313 antagonized apomorphine-induced climbing and induced 5-HT1A syndrome behaviors and hypothermia, the latter behaviors being antagonized by the 5-HT1A antagonist WAY100635. In a drug discrimination procedure SLV313 induced full generalization to the training drug flesinoxan and was also antagonized by WAY100635. In the nucleus accumbens SLV313 reduced extracellular 5-HT and increased dopamine levels in the same dose range. Acetylcholine and dopamine were elevated in the hippocampus and mPFCx, the latter antagonized by WAY100635, suggesting possible 5-HT1A-dependent efficacy for the treatment of cognitive and attentional processes. SLV313 did not possess cataleptogenic potential (up to 60 mg/kg p.o.). The number of spontaneously active dopamine cells in the ventral tegmental area was reduced by SLV313 and clozapine, while no such changes were seen in the substantia nigra zona compacta following chronic administration. These results suggest that SLV313 is a full 5-HT1A receptor agonist and full D2/3 receptor antagonist possessing characteristics of an atypical antipsychotic, representing a potential novel treatment for schizophrenia.

Asenapine Increases Dopamine, Norepinephrine, and Acetylcholine Efflux in the Rat Medial Prefrontal Cortex and Hippocampus

Atypical antipsychotic drugs, which are more potent direct acting antagonists of brain serotonin (5-HT)2A than dopamine (DA) D2 receptors, preferentially enhance DA and acetylcholine (ACh) efflux in the rat medial prefrontal cortex (mPFC) and hippocampus (HIP), compared with the nucleus accumbens (NAc). These effects may contribute to their ability, albeit limited, to improve cognitive function and negative symptoms in patients with schizophrenia. Asenapine (ASE), a new multireceptor antagonist currently in development for the treatment of schizophrenia and bipolar disorder, has complex serotonergic properties based upon relatively high affinity for multiple serotonin (5-HT) receptors, particularly 5-HT2A and 5-HT2C receptors. In the current study, the effects of ASE on DA, norepinephrine (NE), 5-HT, ACh, glutamate, and γ-aminobutyric acid (GABA) efflux in rat mPFC, HIP, and NAc were investigated with microdialysis in awake, freely moving rats. ASE at 0.05, 0.1, and 0.5 mg/kg (s.c.), but not 0.01 mg/kg, significantly increased DA efflux in the mPFC and HIP. Only the 0.5 mg/kg dose enhanced DA efflux in the NAc. ASE, at 0.1 and 0.5 mg/kg, significantly increased ACh efflux in the mPFC, but only the 0.5 mg/kg dose of ASE increased HIP ACh efflux. ASE did not increase ACh efflux in the NAc at any of the doses tested. The effect of ASE (0.1 mg/kg) on DA and ACh efflux was blocked by pretreatment with WAY100635, a 5-HT1A antagonist/D4 agonist, suggesting involvement of indirect 5-HT1A agonism in both the actions. ASE, at 0.1 mg/kg, increased NE, but not 5-HT, efflux in the mPFC and HIP. ASE, at 0.1 mg/kg (s.c.), had no effect on glutamate and GABA efflux in either the mPFC or NAc. These findings indicate that ASE is similar to clozapine and other atypical antipsychotic drugs in preferentially increasing the efflux of DA, NE, and ACh in the mPFC and HIP compared with the NAC, and suggests that, like these agents, it may also improve cognitive function and negative symptoms in patients with schizophrenia.

Effect of repeated administration of phencyclidine on spatial performance in an eight-arm radial maze with delay in rats and mice

Phencyclidine (PCP) is an N-methyl-D-aspartate (NMDA) glutamate receptor channel noncompetitive antagonist that produces some of the symptoms of schizophrenia, including delusions, hallucinations, and negative symptoms as well as cognitive impairment. Thus, administration of PCP to rodents and nonhuman primates has been suggested to provide a potential animal model for schizophrenia. There have been some reports that 7-14 days of PCP administration can bring about enduring impairments in working memory in rodents but not all studies have been consistent in this regard. The present study determined whether repeated PCP administration impaired spatial performance in rats or mice trained to make minimal errors in an eight-arm radial maze task with a delay. Male Sprague-Dawley rats and C57BL/6J mice received 14 daily injection of vehicle or PCP (10 mg/kg, s.c.) followed by a withdrawal period of 1 week. The number of arm reentry errors and the distance traveled to complete the task were not significantly different between PCP-treated and vehicle-treated rats on 2, 8, and 14 days of PCP administration or 8 days following withdrawal of PCP. Mice treated with PCP for up to 2 weeks also had no significant differences in the number of arm reentry errors, travel distances, the numbers of visits to different arms during the first eight choices, or latencies to take all eight pellets compared to the vehicle-treated group. Thus, the present study failed to demonstrate that repeated administration of PCP to rats or mice produces enduring memory impairment. Factors potentially contributing to the discrepancies between various studies are discussed.