Low Doses of Psilocybin and Ketamine Enhance Motivation and Attention in Poor Performing Rats: Evidence for an Antidepressant Property. Guy A. Higgins. Front. Pharmacol., February 26 2021. https://doi.org/10.3389/fphar.2021.640241
Abstract: Long term benefits following short-term administration of high psychedelic doses of serotonergic and dissociative hallucinogens, typified by psilocybin and ketamine respectively, support their potential as treatments for psychiatric conditions such as major depressive disorder. The high psychedelic doses induce perceptual experiences which are associated with therapeutic benefit. There have also been anecdotal reports of these drugs being used at what are colloquially referred to as “micro” doses to improve mood and cognitive function, although currently there are recognized limitations to their clinical and preclinical investigation. In the present studies we have defined a low dose and plasma exposure range in rats for both ketamine (0.3–3 mg/kg [10–73 ng/ml]) and psilocybin/psilocin (0.05–0.1 mg/kg [7–12 ng/ml]), based on studies which identified these as sub-threshold for the induction of behavioral stereotypies. Tests of efficacy were focused on depression-related endophenotypes of anhedonia, amotivation and cognitive dysfunction using low performing male Long Evans rats trained in two food motivated tasks: a progressive ratio (PR) and serial 5-choice (5-CSRT) task. Both acute doses of ketamine (1–3 mg/kg IP) and psilocybin (0.05–0.1 mg/kg SC) pretreatment increased break point for food (PR task), and improved attentional accuracy and a measure of impulsive action (5-CSRT task). In each case, effect size was modest and largely restricted to test subjects characterized as “low performing”. Furthermore, both drugs showed a similar pattern of effect across both tests. The present studies provide a framework for the future study of ketamine and psilocybin at low doses and plasma exposures, and help to establish the use of these lower concentrations of serotonergic and dissociative hallucinogens both as a valid scientific construct, and as having a therapeutic utility.
Discussion
The present series of experiments were designed to evaluate the behavioral properties of low doses and plasma concentrations of ketamine and psilocybin in the rat, with a view to identifying behavioral effects that might be relevant to the antidepressant and other therapeutic potential of both drugs. One of the first challenges to this line of research is defining a low dose range of ketamine and psilocybin. The approach taken in this study was to establish doses and plasma exposures of each drug for stereotyped behaviors characteristic of each drug and its distinct pharmacological class. Since behavioral stereotypies are often considered as the preclinical proxy for their psychomimetic property (Hanks and Gonzalez-Maeso, 2013; Halberstadt and Geyer, 2018), we focused on doses just below threshold for their induction. Based on this criterion we identified ketamine and psilocybin doses (and plasma exposures) of 0.3–3 mg/kg (10–70 ng/ml) and 0.05–0.1 mg/kg (7–12 ng/ml [psilocin]) respectively for investigation.
Preclinical studies explicitly examining low (“micro”) doses of ketamine and psilocybin are beginning to appear in the literature (Horsley et al., 2018; Meinhardt et al., 2020), albeit without any demonstration of potential beneficial effects. One of the limitations to these studies is that antidepressant potential has been typically investigated using tests such as forced swim and elevated plus maze, which lack human equivalence. These tests also overlook the trend to deconstruct complex clinical disorders into endophenotypes that may be more amenable to preclinical study and translation across the preclinical-clinical spectrum (Day et al., 2008; Markou et al., 2009). A diagnosis of MDD includes symptoms of depressed mood, anhedonia, fatigue/loss of energy (anergia), cognitive deficits including diminished/slowed ability to think or concentrate and feelings of guilt, worthlessness and suicidal ideation (van Loo et al., 2012; American Psychiatric Association, 2013). Therefore endophenotypes related to depression include anhedonia (impaired reward function), amotivation (lack of motivation/purpose) and impaired cognitive function (Hasler et al., 2004; Atique-Ur-Rehman and Neill, 2019; Treadway and Zald, 2011) which we addressed through the progressive ratio and 5-choice tasks.
A further consideration in the design of these experiments was an expectation that any effect of ketamine and psilocybin at low plasma concentrations was likely to be subtle, and potentially variable across a sample study population (see Horsley et al., 2018; Cameron et al., 2019; Meinhardt et al., 2020). We therefore exploited the heterogeneous nature of the performance level of rat populations across tasks such as PR and 5-CSRTT. Rats may be categorized based on performance differences in progressive ratio breakpoint, and thus serve as models of high vs. low motivation (Randall et al., 2012; Randall et al., 2015). Similarly rats may be categorized according to attentional accuracy or impulsive action under specific challenge conditions, thus providing models of high vs. low attention or impulsivity (Blondeau and Dellu-Hagedorn, 2007; Jupp et al., 2013; Hayward et al., 2016; Higgins et al., 2020a; Higgins et al., 2020b). Consequently, rats showing low motivation and/or attention may represent models of specific depression-relevant endophenotypes (Hasler et al., 2004; Treadway and Zald, 2011; Atique-Ur-Rehman and Neill, 2019). We identified three important considerations to this approach of subgrouping. Firstly, a requirement to identify an enduring nature to any performance subgroup classification. Secondly to establish “poor” performance is not a consequence of factors such as ill health, and thirdly a requirement for large sample sizes to ensure that subgroups were adequately separated and powered (Button et al., 2013). To address the former challenge, high/low performance subgroups were allotted based on 5–10 days baseline performance. Control experiments were conducted on the PR and 5-choice study cohorts which confirmed “low performance” was not associated with ill health or sensorimotor deficit. To address the third challenge, and to ensure at least some separation between subgroups but having due consideration to the principal of the 3R’s (replacement, refinement, reduction), we adopted the extreme tertile groups.
Considered as a whole, i.e. without subgrouping, despite group sizes of N = 24–72, we failed to identify any positive effect of ketamine or psilocybin on motivation or attention over the tested dose range. The most robust finding was a trend for a decline in performance following the 6 mg/kg dose of ketamine, which indicated the early phase of the descending limb of a biphasic dose response. This was confirmed by parallel experiments identifying even greater performance decline at 10 mg/kg (data not shown, but see Gastambide et al., 2013; Benn and Robinson, 2014; Nikiforuk and Popik, 2014).
Subgrouping rats based on break point and number of lever presses for food made available under a PR schedule of reinforcement identified rats that consistently ceased responding early (“low” responders), leading to low break points. Interestingly these rats had similar body weights, free feeding measures and open field activity compared to their high responder counterparts, suggesting any differences were unrelated to general health status, neurological function or appetite. In these low performers, both psilocybin (0.05–0.1 mg/kg) and ketamine (1–3 mg/kg) increased break point suggesting an increase in task motivation. These findings suggest that low doses of ketamine may relieve certain clinical signs related to depression (Xu et al., 2016), and further suggest that the doses and plasma concentrations of ketamine and psilocybin as described in the present study may have utility in treating subtypes of mental illnesses characterized by amotivation and anhedonia in particular.
In the 5-CSRTT, the effects of ketamine and psilocybin were evaluated in two separate task schedules. In the first, rats were tested under standard conditions of 0.75 s SD, 5 s ITI. Segregation of rats into high and low performers based on accuracy (% correct), revealed a trend for both psilocybin and ketamine to increase accuracy at equivalent doses to those effective in the PR task. In the case of psilocybin, the more robust measure of efficacy was the % hit measure, which also accounts for errors of omission as well as commission (incorrect response). Speed of responding was also marginally increased further supporting a performance improvement.
The second 5-CSRTT experiment utilized conditions of extended ITI (5 s vs. 10 s) and reduced stimulus duration (0.75 s vs. 0.3 s). The principal challenge is to response control, lengthening the ITI from 5 s to 10 s produces a significant increase in both PREM and PSV responses, a consistent and widely reported finding (Robbins, 2002; Jupp et al., 2013; Barlow et al., 2018; Higgins et al., 2020a,b). Subgrouping rats, based on the level of PREM responses under the 10 s ITI schedule, into “Low” and “High” impulsives (LI vs. HI) highlights a wide range of responders typically seen under this schedule (Jupp et al., 2013; Fink et al., 2015; Barlow et al., 2018; Higgins et al., 2020a). Importantly there is a reasonable consistency of performance on this measure over repeated tests as demonstrated by the HI rats having higher PREM scores under the 5 s ITI, albeit at markedly lower levels. PSV responses are also higher in the HI cohort, consistent with the HI rats demonstrating a deficit in inhibitory response control.
Similar findings for both ketamine and psilocybin were noted in this test schedule. While neither drug affected accuracy (measured as % correct), either in all, or HI/LI classified rats; both increased PREM and PSV responses in the LI cohort, supporting an increase in impulsive action. It should be noted that the magnitude of change produced by both ketamine and psilocybin was relatively small (∼2-fold) and confined to the LI subgroup. Certainly, the magnitude of change contrasted sharply with the 4-fold increase noted in rats pretreated with dizocilpine under the same 10 s ITI schedule (see also Higgins et al., 2005; 2016; Benn and Robinson, 2014). Previous studies have also described increased PREM responses following pretreatment with the phenethylamine 5-HT2A agonist DOI (Koskinen et al., 2000; Koskinen and Sirvio, 2001; Blokland et al., 2005; Wischhof and Koch, 2012; Fink et al., 2015), typically at doses lower than those which induce signs of WDS/BMC (Fink et al., 2015; Halberstadt and Geyer, 2018).
Impulsivity is a construct that may be viewed in two forms: functional and dysfunctional (Dickman, 1990). Dysfunctional impulsivity is associated with psychiatric conditions such as substance abuse and OCD and thus carries a negative context. For example, associations between high impulsive trait and drug seeking behaviors have been reported both preclinically and clinically (Grant and Chamberlain, 2004; Jupp et al., 2013). Functional impulsivity has been described as a tendency to make quick decisions when beneficial to do so, and may be related to traits such as enthusiasm, adventurousness, activity, extraversion and narcissism. Individuals with a high functional impulsivity are also reported to have enhanced executive functioning overall (Dickman, 1990; Zadravec et al., 2005; Burnett Heyes et al., 2012). Viewed in this more positive context, the feature of psilocybin and ketamine to promote impulsive behavior selectively in a LI cohort may be relevant in supporting a potential to treat depression and other mental disorders.
One advantage of being able to study pharmacological effects at low doses in an experimental setting, is the ability to probe for an underlying neurobiological mechanism, which would serve to establish this pattern of use within a scientific framework. Presumably these doses result in a low level of target site occupancy, which in the case of psilocybin is the serotonin 5-HT2A receptor (Vollenweider et al., 1998; Tylš et al., 2014; Nichols, 2016; Kyzar et al., 2017). At higher doses and plasma exposure, and consequently higher levels of target occupancy, psychomimetic effects begin to emerge. In this respect, the recent study of Madsen et al., (2019) is of interest. These workers reported a correlation between the psychedelic effects of psilocybin (40–100% Likert scale maximum) and CNS 5-HT2A receptor occupancy (43–72%) and plasma psilocin levels (2–15 ng/ml). Increases in subjective intensity was correlated with both increases in 5-HT2A receptor occupancy and psilocin exposure. Based on these data, it is estimated that at 5-HT2A receptor occupancies up to ∼15%, no perceptual effects occur (Madsen and Knudsen, 2020).
5-HT2A receptors are widely distributed within cortical zones, notably layer II-V (Santana et al., 2004; Mengod et al., 2015), and also in subcortical regions such as the DA nigrostriatal and mesocorticolimbic pathways where they appear to positively regulate tone, at least under certain physiological conditions (Doherty and Pickel, 2000; Nocjar et al., 2002; Bortolozzi et al., 2005; Alex and Pehek, 2007; Howell and Cunningham, 2015; De Deurwaerdère, and Di Giovanni, 2017). One plausible hypothesis is that at low nanomolar plasma concentrations, psilocybin (or LSD, mescaline etc.) may preferentially target a subset of 5-HT2A receptors, possibly those localized to subcortical DA systems where activation has been reported to increase firing and tonicity of these pathways (Alex and Pehek, 2007; Howell and Cunningham, 2015; De Deurwaerdère, and Di Giovanni, 2017 for reviews). In turn this might be expected to promote behaviors related to motivation, attention and impulse control as noted in the PR and 5-choice experiments. Activation of cortical 5-HT2A receptors may account for the subjective/perceptual effects once a critical (higher) drug [plasma] threshold has been reached (Nichols, 2016; Kyzar et al., 2017; Madsen et al., 2019; Vollenweider and Preller, 2020).
In the case of ketamine, the relevant target is most likely the NMDA subtype glutamate receptor (Lodge and Mercier, 2015; Mathews et al., 2012; Corriger and Pickering, 2019; although note; Zanos et al., 2018), which is comprised of a tetrameric receptor complex composed of NR1 subunits, combined with NR2A-D subunits and, in some cases, NR3A-B subunits. The NR2A-D subunits exist in an anatomically distinct manner, with the NR2A and NR2B subunits predominant in forebrain; the NR1 subunit having a broader distribution being a constituent of all NMDA channels (Kew and Kemp, 2005; Traynelis et al., 2010). Potentially at low ketamine doses, there may be a preferential interaction between ketamine and specific NMDA channel subtypes (see Lodge and Mercier, 2015), and/or regional subpopulations which underlies the pharmacological effects of these doses of ketamine in preclinical and clinical contexts. We and others have reported on apparently pro-cognitive effects of non-competitive NMDA antagonists, typically dizocilpine, when tested at low doses (Mondadori et al., 1989; Jackson et al., 2004; Higgins et al., 2003; 2016; Guidi et al., 2015). A better understanding of the neurobiological mechanisms that underlie these effects may provide useful insight toward understanding the clinical benefit of low doses of ketamine in humans.
An interesting feature to emerge from this work was the similar profile of ketamine and psilocybin across the PR and 5-choice experiments. Both drugs increased break point in low performers, improved attention in low performer subgroups, and increased PREM/PSV responses in LI rats. Horsley et al., (2018) also reported a similar pattern of both drugs across various elevated plus maze measures, although the effects were suggestive of a mild anxiogenic profile. Despite their differing pharmacology, there is accumulating evidence from a variety of sources that the NMDA and 5-HT2A receptors are functionally intertwined. Vollenweider has highlighted the overlapping psychotic syndromes produced by serotonergic hallucinogens and psychotomimetic anesthetics associated with a marked activation of the prefrontal cortex and other overlapping changes in temporoparietal, striatal, and thalamic regions (Vollenweider, 2001; Vollenweider and Kometer, 2010) suggesting that both classes of drugs may act upon a common final pathway. Secondly, 5-HT2A receptor antagonists attenuate a variety of putative psychosis-related behaviors induced by NMDA channel block, including behavioral stereotypy and disrupted PPI (Varty and Higgins, 1995; Varty et al., 1999; Higgins et al., 2003), a property that likely contributes to the antipsychotic efficacy of atypical neuroleptics such as clozapine, risperidone (Meltzer, 1999; Remington, 2003). Furthermore, a cellular coexpression of 5-HT2A and NMDA receptors has been described in multiple brain regions, including VTA, striatum and cortex (Wang and Liang, 1998; Rodriguez et al., 1999; Rodriguez et al., 2000). Therefore, studying these drugs at the low dose range may also provide further insights into how these receptor systems may interact.
In conclusion, the present studies have characterized for the first time, a positive effect of ketamine (0.3–3 mg/kg [plasma] 10–70 ng/ml) and psilocybin (0.05–0.1 mg/kg [psilocin plasma] 7–12 ng/ml) on behaviors related to endophenotypes of amotivation and anhedonia. The overall effect sizes are modest, which might be expected at the doses and concentrations studied, where the degree of target occupancy is likely to be low and subject to individual differences in drug pharmacodynamics and pharmacokinetics. Each of these factors will impact on treatment response across a study population (Levy, 1998; Dorne, 2004). Limitations to the present study include a restriction to male test subjects, and on single acute doses. Future studies should extend to both male and female subjects, and alternative dosing schedules. Nonetheless, the studies are important in that they define a potentially efficacious dose and plasma exposure range and provide a framework for early safety studies and further scientific investigation into the neurobiology of these drugs in the low dose range.
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