Effects of the cannabinoid receptor 1 positive allosteric modulator GAT211 and acute MK-801 on visual attention and impulsivity in rats assessed using the five-choice serial reaction time task

https://doi.org/10.1016/j.pnpbp.2020.110235Get rights and content

Highlights

  • Acute MK-801 decreased accuracy and increased premature responding in the five choice serial reaction time task.

  • The cannabinoid 1 receptor positive allosteric modulator GAT211 did not affect task performance.

  • GAT211 did not affect the disruption in task performance cause by MK-801.

  • These findings are discussed in the context of the cognitive deficits observed in schizophrenia.

Abstract

Altered interactions between endocannabinoid and glutamate signaling may be involved in the pathophysiology of schizophrenia and acute psychosis. As cognitive disturbances are involved in schizophrenia, increased understanding of the roles of these neurotransmitter systems in cognition may lead to the development of novel therapeutics for disorder. In the present study, we examined the effects of a recently synthesized cannabinoid receptor 1 (CB1R) positive allosteric modulator GAT211 in a rodent model of acute psychosis induced by systemic treatment with MK-801. To assess cognitive function, we used the Five-Choice Serial Reaction Time (5CSRT) task, conducted in touchscreen-equipped operant conditioning chambers. Our measures of primary interest were accuracy – indicative of visual attentional capacity – and the number of premature responses – indicative of impulsivity. We also measured latencies, omissions, and perseverative responding during all test sessions. Thirteen adult male Long Evans rats were trained on the 5CSRT and were then tested using a repeated measures design with acute MK-801 (0 or 0.15 mg/kg, i.p.) and GAT211 (0, 3, or 10 mg/kg, i.p.) administration. Acute MK-801 severely impaired accuracy, increased omissions, and increased the number of premature responses. MK-801 also significantly increased correct response latencies, without significant effects on incorrect or reward correction latencies. GAT211 had no significant effects when administered alone, or in combination with acute MK-801. These data confirm the dramatic effects of acute MK-801 treatment on behavioral measures of attention and impulsivity. Continued investigation of CB1R positive allosteric modulators as potential treatments for the cognitive symptoms of schizophrenia and related disorders should be pursued in other rodent models.

Introduction

Schizophrenia is a chronic neuropsychiatric disorder affecting 0.4% of the global population (Nicholl et al., 2010). The majority of antipsychotics in use today are dopamine antagonists, though many of these drugs cause serious side effects, including heart disease, diabetes, and extrapyramidal motor symptoms (Li et al., 2016; Goff et al., 2017). In addition, the efficacy of these dopamine antagonists are questionable in certain populations, and they tend to treat only the positive symptoms of schizophrenia while having little or no effect on the negative and cognitive symptoms (Keefe et al., 2007; Stafford et al., 2015). As a result, some researchers suggest that abnormalities in dopamine signaling are likely the “final common pathway” to schizophrenia and result from the dysregulation of other neurotransmitter systems, including at N-methyl-d-aspartate (NMDA) receptors, a subtype of ionotropic glutamate receptors (Coyle, 2012; Cannon, 2015). Indeed, non-competitive NMDA receptor antagonists are routinely used as a drug-induced model for schizophrenia, as they can replicate the aforementioned NMDA receptor hypofunction and induce a state resembling some aspects of acute psychosis.

Cannabinoid receptor 1 (CB1R) is the most abundant G protein-coupled receptor (GPCR) in the central nervous system and its activation regulates neurotransmitter release in glutamatergic neurons in the cortico-striatal-limbic circuitry involved in schizophrenia (Robson et al., 2014; Pertwee, 2015; Laprairie et al., 2017; Alaverdashvili and Laprairie, 2018). The 2 main endogenous cannabinoids acting on CB1R are 2-arachidonoylglycerol (2-AG, a full CB1R agonist) and anandamide (AEA, a partial CB1R agonist). 2-AG and AEA primarily act via retrograde signaling, meaning they are released by postsynaptic neurons and bind to the CB1Rs expressed on presynaptic neurons to modulate presynaptic neurotransmitter release (Van Winkel and Kuepper, 2014). Recent studies show that patients with schizophrenia have lower CB1R availability in many brain regions when compared to controls (Ranganathan et al., 2016; Volk and Lewis, 2016; Borgan et al., 2019). Therefore, compounds which enhance CB1R activity may be promising therapeutic agents. However, the roles of cannabinoids and CB1R in schizophrenia are complex. For example, the phytocannabinoid cannabidiol (CBD) may improve some symptoms of psychosis via unknown mechanisms, whereas the CB1R partial agonist Δ9-tetrahydrocannabinol (THC) significantly worsens some symptoms of psychosis (Leweke et al., 2012; Manseau and Goff, 2015). These differences may be attributed to the pluri-dimensional signaling of CB1R, such that different ligands may elicit different responses from the same receptor (Hua et al., 2017). They may also be attributed to distinct interactions between exogenous cannabinoids and endogenous cannabinoids. For example, CBD has been found to inhibit the reuptake of AEA, and the concentration of AEA in the cerebrospinal fluid is found to be inversely related to the severity of psychosis symptoms in schizophrenia patients (Manseau and Goff, 2015; Giuffrida et al., 2004). Therefore, there is merit in studying the modulation of CB1R in the context of schizophrenia (Mielnik et al., 2020).

A positive allosteric modulator (PAM) is a ligand that binds to a site on its receptor that is distinct from the main (i.e., orthosteric) site of endogenous binding and in doing so enhances the potency of the orthosteric ligand and/or the efficacy of the receptor's signaling (Laprairie et al., 2017). True PAMs lack intrinsic efficacy and thus only modulate endogenous ligand actions on the receptor (Laprairie et al., 2017). CB1R PAMs may thus help keep the receptor's activity at a homeostatic level and normalize endogenous cannabinoid system (ECS) tone (Laprairie et al., 2017; Mielnik et al., 2020). Existing pre-clinical studies have shown that CB1R allosteric modulators are safer than traditional CB1R agonists in terms of tolerance and abuse liability (Laprairie et al., 2017; Alaverdashvili and Laprairie, 2018; Mielnik et al., 2020). GAT211 is a CB1R allosteric modulator composed of 2 enantiomers: GAT228, an allosteric agonist and PAM; and GAT229, a pure CB1R PAM without agonist actions (Laprairie et al., 2017). Therefore, GAT211 is a mixed agonist-PAM (ago-PAM). Several recent studies have demonstrated the therapeutic potential of GAT211 in various conditions including pathological pain, PTSD, ocular hypertension, and Huntington's disease (Shekhar and Thakur, 2018; Slivicki et al., 2018; Laprairie et al., 2019; Slivicki et al., 2020). However, to the best of our knowledge, no previous studies have assessed the effects of GAT211, or any CB1R PAM or ago-PAM, in rodent models of the cognitive symptoms of schizophrenia. Therefore, in the present experiment, we chose to test the effects of GAT211 on the Five-Choice Serial Reaction Time (5CSRT) task in rats.

The 5CSRT task enables the measurement of visual attention, impulsivity, and perseveration using well established, automated procedures in operant conditioning chambers (Bari et al., 2008). Our apparatus made use of touchscreens for displaying the stimuli and measuring the rats' responses during the task (Mar et al., 2013; Hvoslef-Eide et al., 2015). The effects of GAT211 on the 5CSRT task were assessed alone and in combination with acute administration with the NMDA receptor antagonist MK-801 in an effort to recapitulate some aspects of NMDA receptor hypofunction as observed in schizophrenia.

Section snippets

Subjects

Adult male Long Evans rats (Charles River Laboratories, Kingston, NY, USA) were used for this experiment. Upon arrival at the vivarium at the University of Saskatchewan animal facility, the rats were given food (Purina Rat Chow) and water ad libitum, pair-housed in plastic ventilated home cages, and left undisturbed for 1 week before handling. The vivarium was temperature- and humidity-controlled, and provided a 12:12 h light:dark cycle, with the light-on period beginning at 0700 daily. All

Results

MK-801 (0.15 mg/kg) significantly disrupted task performance on several parameters. GAT211 failed to have effects on its own or affect the MK-801-induced disruptions. Response accuracy, derived from the number of correct responses divided by the total number of responses, was significantly reduced by MK-801 (Fig. 2a; main effect of MK-801: F(1,12) = 8.71; p = 0.012). GAT211 failed to impact accuracy levels alone (main effect of GAT211: F(2,24) = 0.47; p = 0.51) and did not significantly reverse

Discussion

The present study assessed the effects of acute MK-801 and GAT211 treatments on performance of the 5CSRT task in male rats. Our results support a potent effect of subanesthetic doses of acute MK-801 in impairing accuracy, increasing omissions, and increasing premature responding in the 5CSRT task. Administration of GAT211, a CB1R ago-PAM, did not reverse these impairments or have effects on its own.

Previous studies have explored the effects of MK-801 on the 5CSRT task. Acute treatment with low

Conclusion

This experiment demonstrated that acute MK-801 treatment potently alters accuracy and premature responding in the 5CSRT task. While CB1R ago-PAMs, such as GAT211, are promising new compounds for increasing CB1R activity, GAT211 treatment did not significantly affect the effects of MK-801 on the 5CSRT task. Further characterization of the effects of altering CB1R activity on behavioral models of schizophrenia, including NMDA receptor hypofunction models, may facilitate the development of novel

Ethical statement (Onofrychuk et al. 2020)

Ethical guidelines related to the ARRIVE guidelines for the use of animals in research have been followed. This is indicated in the Methods section of the submitted paper.

Declaration of Competing Interest

None.

Acknowledgements

Funding for the project was provided by a Saskatchewan Health Research Foundation (SHRF) Establishment Grant and a Canadian Institutes of Health Research (CIHR) Partnership Grant with GlaxoSmithKline to RBL; and funds from the College of Medicine, University of Saskatchewan, and a CIHR Project Grant to JGH. This work was also supported by grant from National Institutes of Health to GAT (EY024717). TJO, SC, DLM, and AJR received scholarship support from Natural Sciences and Engineering Research

References (43)

  • A. Bari et al.

    The application of the 5-choice serial reaction time task for the assessment of visual attentional processes and impulse control in rats

    Nat. Protoc.

    (2008)
  • S.A. Barnes et al.

    Impaired limbic cortico-striatal structure and sustained visual attention in a rodent model of schizophrenia

    Int. J. Neuropsychopharmacol.

    (2015)
  • A. Benn et al.

    Investigating glutamatergic mechanism in attention and impulse control using rats in a modified 5-choice serial reaction time task

    PLoS One

    (2014)
  • F. Borgan et al.

    METSY group. In vivo availability of cannabinoid 1 receptor levels in patients with first-episode psychosis

    JAMA Psychiatry

    (2019)
  • E.A. Cairns et al.

    The in vivo effects of the CB1-positive allosteric modulator GAT229 on intraocular pressure in ocular normotensive and hypertensive mice

    J. Ocul. Pharmacol. Ther.

    (2017)
  • Z.A. Cope et al.

    Premature responses in the five-choice serial reaction time task reflect rodents’ temporal strategies: evidence from no-light and pharmacological challenges

    Psychopharmacology

    (2016)
  • J.T. Coyle

    NMDA receptor and schizophrenia: a brief history

    Schizophr. Bull.

    (2012)
  • S. Garai et al.

    Application of fluorine- and nitrogen-walk approaches: defining the structural and functional diversity of 2-phenylindole class of cannabinoid 1 receptor positive allosteric modulators

    J. Med. Chem.

    (2020)
  • C. García et al.

    Fernández-Ruiz J. Cannabinoid–dopamine interactions in the physiology and physiopathology of the basal ganglia. Br

    J. Pharmacol.

    (2016)
  • A. Giuffrida et al.

    Cerebrospinal anandamide levels are elevated in acute schizophrenia and are inversely correlated with psychotic symptoms

    Neuropsychopharmacol.

    (2004)
  • D.C. Goff et al.

    The long-term effects of antipsychotic medication on clinical course in schizophrenia

    Am. J. Psychiatry

    (2017)
  • Cited by (8)

    • The effects of acute Cannabis smoke or Δ<sup>9</sup>-THC injections on the trial-unique, nonmatching-to-location and five-choice serial reaction time tasks in male Long-Evans rats

      2022, Neurobiology of Learning and Memory
      Citation Excerpt :

      The 5-CSRTT was used to evaluate attentional processes and impulsivity in separate rats. The 5-CSRTT is flexible, as it can be modified to adjust attentional load and manipulate impulse responding, requiring rats to activate both sustained and divided attention to select correct stimuli (Asinof & Paine, 2014; Onofrychuk et al., 2021; Robbins, 2002). To enable direct comparisons with previous studies, we also evaluated performance on these tasks following i.p. injections of pure THC distillate (3 mg/kg).

    • Moving from cells to animals: Challenges of studying allosteric modulators in vivo

      2022, Allosteric Modulation of G Protein-Coupled Receptors
    • Design, synthesis, and pharmacological profiling of cannabinoid 1 receptor allosteric modulators: Preclinical efficacy of C2-group GAT211 congeners for reducing intraocular pressure

      2021, Bioorganic and Medicinal Chemistry
      Citation Excerpt :

      We have designed and extensively profiled, in vitro and in vivo, a prototypic 2-phenylindole-based CB1R agonist-positive allosteric modulator (ago-PAM), GAT211 (1), and demonstrated that each of its two enantiomers interacts stereospecifically with the receptor: one GAT211 enantiomer, GAT229, behaves as a pure CB1R PAM, whereas its other enantiomer, GAT228, acts as a CB1R allosteric agonist.16–17,19,20 GAT211 exhibited promising preclinical activity in rodent models of neuropathic pain, Huntington’s disease, morphine-induced analgesia enhancement, epilepsy, glaucoma, and inflammation, albeit with some suboptimal drug-like properties (e.g., aqueous solubility, allosteric potency, metabolic stability).17,20,21–26 Using GAT211 as a scaffold, we recently applied a fluorine-, nitrogen-walk approach to expand our focused library of 2-phenylindole-based CB1R PAMs, an effort that yielded two metabolically improved CB1R ago-PAMs, GAT591 (2) and GAT593 (3), that demonstrated efficacy in models of neuropathic pain (Fig. 1).27

    • Cannabinoids: Emerging developments in neuropsychopharmacology and biological psychiatry

      2021, Progress in Neuro-Psychopharmacology and Biological Psychiatry
    • Positive allosteric modulation of type 1 cannabinoid receptors reduces spike-and-wave discharges in Genetic Absence Epilepsy Rats from Strasbourg

      2021, Neuropharmacology
      Citation Excerpt :

      Both GAT211 and GAT229 have shown promise as treatments in rodent models of neuropathic pain and Huntington's Disease (Shekhar and Thakur, 2018; Slivicki et al., 2018; Trexler et al., 2019; Laprairie et al., 2019). Furthermore, we have recently demonstrated that GAT211 modulates some aspects of rat behaviour in studies using pharmacological models of schizophrenia (Onofrychuk et al., 2020; McElroy et al., 2021). Notably, these behavioural results differed from those of the orthosteric CB1R agonist THC (McElroy et al., 2021).

    View all citing articles on Scopus
    View full text