Abstract:

Repeated drug consumption may progress to problematic use by triggering neuroplastic adaptations that attenuate sensitivity to natural rewards while increasing reactivity to craving and drug cues. Converging evidence suggests a single sub-anesthetic dose of the N-methyl-D-aspartate receptor antagonist ketamine may work to correct these neuroadaptations and restore motivation for non-drug rewards. Using an established laboratory model aimed at evaluating behavioral shifts in the salience of cocaine now vs money later, we found that ketamine, as compared to the control, significantly decreased cocaine self-administration by 67% relative to baseline at greater than 24 h post-infusion, the most robust reduction observed to date in human cocaine users and the first to involve mechanisms other than stimulant or dopamine agonist effects. These findings signal new directions in medication development for substance use disorders.

Neural plasticity is defined as the cellular and structural reorganisation
of the brain. Synaptogenesis is a crucial mechanism for
plasticity, since for change to happen within brain circuitry new
synapses between neurons must be formed. Surface expression of
AMPARs and upregulation of other synaptic proteins are involved in
the process of synaptogenesis. Diminished glutamatergic synaptic
transmission and reduced plasticity are thought to be associated
with addiction. Existing models suggest that ketamine’s blockade of NMDA receptors
increases synaptogenesis by stimulating protein synthesis
and the insertion of AMPA receptors. Hence, ketamine’s
effects help to reverse the glutamatergic changes associated
with depression and addiction. 

Animal models of addiction, depression and other psychiatric disorders
have been linked to a reduction in adult neurogenesis . It has been suggested that in addiction
the loss of neurogenesis, especially in cortical and hippocampal
regions, may contribute to levels of self-administration and the
vulnerability of relapsing. The reduction of neurogenesis in addiction is supported in
humans by the reduction in BDNF serum levels. In a study, 37
subjects with diagnosis of alcohol dependence showed significantly
reduced BDNF serum levels compared to healthy individuals
. Similarly, cocaine- and heroin-dependentpatients have significantly lower serum BDNF levels and these
seem to recover during withdrawal. Rapid and transient up-regulation of the neuroplasticity marker
BDNF is implicated as a critical component of the antidepressant
mechanism of ketamine . BDNF knock-out mice do not show anti-depressant response to
ketamine in animal models of depression.

Recent research has
demonstrated that ketamine increases peripheral plasma BDNF in
depressed people who respond to treatment but not in treatment
non-responders or patients receiving an active placebo. These BDNF increases in depressed people given ketamine
are robustly correlated with the drug’s antidepressant effects.

It has been found there is a dispersion in normal brain connectivity and the disruption of the usual pattern of communication  in depression and addictions. . The integrity of functional networks decreased, being the
change maximal in functional hubs such as the thalamus, putamen
and high-level association cortices. In particular, connectivity
within the Default Mode Network was reduced between the posterior
cingulate cortex and the mPFC .
The connectivity between the parahippocampal and the retrosplenial
cortex also decreased as well as the segregation between
other major functional networks such as the salience, attention and
different visual networks Infusions of ketamine have shown to decrease connectivity
between and within resting-state consciousness networks.
Connectivity between the mPFC and the rest of the Default
Mode Network (via the posterior cingulate cortex) has been found
to be reduced, along with the integrity and activity of the salience
and visual networks are also affected. Since it is known
that connectivity with the mPFC is elevated in depression , the reduction of connectivity in the Default Mode
Network observed during the psychedelic experience might be a
mechanism that helps treat depressive states, which are very
common in addicts and predictive of relapse.

Given addiction is highly co-morbid with depression   and ketamine’s role within psychiatry changed
dramatically when it was discovered to be an anti-depressant, we
now briefly describe the research concerning ketamine and
depression. In 2000, the first clinical trial hinted at the potential of
ketamine as a treatment for depression. Four subjects diagnosed
with depression were intravenously administered 0.5 mg/kg of
ketamine in a randomised, double-blind design. The results were
compared to the injection of saline solutions in 3 subjects with an
equivalent diagnosis. Comparison on the Hamilton Rating Scale for
Depression (HAM-D) showed moderate evidence for a greater
reduction in scores after ketamine infusion compared to saline
(Berman et al., 2000). The reduction was rapid and outlasted the
subjective effects of ketamine, lasting for 3 days after infusion.
Despite the small sample size and the limited follow-up, this result
and anti-depressant effects observed in animal models of depression
encouraged researchers in the field to perform more studies in humans . Since then, over 30 studies have
examined the antidepressants effects of ketamine in patients with
treatment-resistant major depressive and bipolar disorders.

Ketamine has shown a 65-70% response rate in treating
depression within 24 h, which contrasts with the ~47% response
rate of conventional monoaminergic antidepressants after weeks
or months . Furthermore,
ketamine’s antidepressant actions are almost immediate and last
for approximately a week ,
whereas conventional antidepressive medications take weeks to
have an effect, are given daily and most of them fail to exert long lasting
effects . Furthermore, studies
have consistently shown that after a ketamine infusion there is a
significant reduction in suicidal ideation which also lasts for several
days.Depression and addiction’s co-expression is almost ubiquitous
People with alcohol, opioids, cannabis and
cocaine use disorders show notably higher rates of depression than
the average of the general population. Furthermore, high levels of depression and anxiety
may predispose relapse to: heroin, alcohol, cannabis and cocaine.

Memories and their creation and alteration is felt to be at the heart of cues and triggers and relapse in addiction. Once consolidated, memories are thought to be stored in a
stabilised state after initial acquisition. Shortly after reactivation
(i.e. remembered) of consolidated memories, these are rendered
transiently unstable and labile, before they then re-stabilise. This
process has been named reconsolidation . After reconsolidation,
the memories are stored again, but they may have been slightly
altered or updated. Each time memories are reactivated the latest
version is retrieved and they are again susceptible to change. During reconsolidation memories may be vulnerable to
manipulation and disruption. This was first demonstrated in animals
using fear conditioning. Rodents were trained to associate a
neutral stimulus with a shock such that the neutral stimulus elicited
a fear response. Researchers eliminated this fear response by
pharmacologically disrupting the reconsolidation process . Reward memories can also be disrupted such that a
neutral stimulus that once elicited appetitive behaviour no longer
does so. Therefore, non-pharmacological and drug therapies that
aim at weakening drug-cue memories via manipulation of reconsolidation
are of interest. Preclinical studies have shown that ketamine affects reconsolidation
of drug memories. . A recent review has suggested that ketamine (along with other psychedelics)
may be able to disrupt maladaptive appetitive memories
(Fattore et al., 2017).  Psychedelics and reconsolidation of traumatic and appetitive maladaptive memories: focus on cannabinoids and ketamine

Article ABSTRACT:

Rationale

Clinical data with 3,4-methylenedioxymethamphetamine (MDMA) in post-traumatic stress disorder (PTSD) patients recently stimulated interest on the potential therapeutic use of psychedelics in disorders characterized by maladaptive memories, including substance use disorders (SUD). The rationale for the use of MDMA in PTSD and SUD is being extended to a broader beneficial “psychedelic effect,” which is supporting further clinical investigations, in spite of the lack of mechanistic hypothesis. Considering that the retrieval of emotional memories reactivates specific brain mechanisms vulnerable to inhibition, interference, or strengthening (i.e., the reconsolidation process), it was proposed that the ability to retrieve and change these maladaptive memories might be a novel intervention for PTSD and SUD. The mechanisms underlying MDMA effects indicate memory reconsolidation modulation as a hypothetical process underlying its efficacy.

Objective

Mechanistic and clinical studies with other two classes of psychedelic substances, namely cannabinoids and ketamine, are providing data in support of a potential use in PTSD and SUD based on the modulation of traumatic and appetitive memory reconsolidation, respectively. Here, we review preclinical and clinical data on cannabinoids and ketamine effects on biobehavioral processes related to the reconsolidation of maladaptive memories.

Results

We report the findings supporting (or not) the working hypothesis linking the potential therapeutic effect of these substances to the underlying reconsolidation process. We also proposed possible approaches for testing the use of these two classes of drugs within the current paradigm of reconsolidation memory inhibition.

Furthermore, a meta-analysis of pre-clinical
studies found evidence suggesting that NMDAR antagonists can
be used to target reward memory reconsolidation, and more successfully
than adrenergic antagonists such as propranolol (Das
et al., 2013)  Das, R.K., Freeman, T.P., Kamboj, S.K., 2013. The effects of N-methyl d-aspartate and B-adrenergic receptor antagonists on the reconsolidation of reward memory: a meta-analysis. Neurosci. Biobehav. Rev. 37, 240-255.:

Abstract

Pharmacological memory reconsolidation blockade provides a potential mechanism for ameliorating the maladaptive reward memories underlying relapse in addiction. Two of the most promising classes of drug that interfere with reconsolidation and have translational potential for human use are N-methyl-d-aspartate receptor (NMDAR) and B-Adrenergic receptor (B-AR) antagonists. We used meta-analysis and meta-regression to assess the effects of these drugs on the reconsolidation of reward memory in preclinical models of addiction. Pharmacokinetic, mnemonic and methodological factors were assessed for their moderating impact on effect sizes. An analysis of 52 independent effect sizes (NMDAR = 30, B-AR = 22) found robust effects of both classes of drug on memory reconsolidation, but a far greater overall effect of NMDAR antagonism than B-AR antagonism. Significant moderating effects of drug dose, relapse process and primary reinforcer were found. The findings suggest that reward memory reconsolidation can be robustly targeted by NMDAR antagonists and to a lesser extent, by B-AR antagonists. Implications for future clinical work are discussed.

Highlights

► Meta-analysis of NMDAR and B-adrenergic antagonists in preclinical reward reconsolidation. ► Larger effects of NMDAR (r = .613) than B-adrenergic (r = .24) antagonists were found. ► ‘Relapse process’, trace type, reinforcer and drug dose moderated effect sizes. ► NMDAR antagonists particularly might be of clinical use in treating addiction.

 

.

                                                           Mystical experiences and psychedelic effects

Mystical experiences and psychedelic effects provoked by
classic psychedelic drugs have been shown to be psychologically
beneficial in long-term studies.They have not only been linked with positive
outcomes in various treatments, but also to ‘life-changing’,
‘spiritually meaningful’ and ‘eye opening’ events.In the ketamine studies described
above, anecdotal and qualitative reports suggest that the subjective
psychedelic experience seemed to help patients. For example, to
help them: undergo a cathartic process, improve relationships with
the world and other people, maintain positive psychological
changes and enhance self-awareness and personal growth.During KPT, patients reported a feeling of ‘resolution’ and
‘catharsis’ of some psychological problems, mainly those related to
alcohol. Furthermore, the degree of mystical experience was also
linked to the insight and impact of KPT reported by patients
. Interestingly, the intensity of the negative experiences (experiences associated
with negative emotions, fear and horror) during the
ketamine session was associated with longer remission. This was
blindly and quantitatively assessed by analysing patient’s selfreports.
Moreover, spirituality, self-concept, emotional attitudes
to other people and positive changes in life values and purposes
were improved after the ketamine experience.

Notably, ketamine’s mystical experiences, but not dissociative
effects, were found to mediate ketamine’s increase motivation to
quit 24 h after the infusion in cocaine addicts .
Moreover, consistent with previous studies, it was also observed
that mystical experiences were positively dose-dependent. This
study therefore provides evidence that the mystical experience
induced by ketamine is important in its therapeutic mechanism
. Speculatively, mystical experiences may help
to rapidly shift patients’ mindsets towards the integration and
acceptance of a sober lifestyle.

The acute disruptions of the functional networks, especially the
alterations to the default mode network, are related to the psychedelic
experience. In fact, the degree of network dissolution in
LSD and psilocybin is correlated with the intensity of the psychedelic
experience . The disruption to the default mode network may engender a reduction
in rumination and maladaptive repetitive thoughts. Psychological
therapies for addiction often aim to help the patient consider
different ways of life, especially those without the drug, and a
pharmacological agent such as ketamine which expedites that
process may be useful in treating addiction.

Speculatively, ketamine can
provide a unique mental state during and after acute drug effects
that facilitates and enriches therapeutic experiences, which in turn
may improve efficacy and lengthen treatment effects. Furthermore, synaptogenesis
and neurogenesis are putatively critical in learning new
information . The uptake of psychological therapy may
therefore be facilitated after ketamine infusions due increases in
synaptogenesis and neurogenesis, and thus improved learning of
relapse-reducing strategies, such as those used in relapseprevention
based cognitive behavioural therapy (CBT). In fact, the
idea that neurogenesis and synaptogenesis work synergistically
with psychological therapies is becoming recognised as a new
approach in the treatment of mental disorders . Theoretically, the administration of ketamine (which can
produce a ‘psychedelic’ experience) may open people’s minds so
they are more able to embrace what is presented during therapy as
well as enhancing the uptake of new therapeutic content.

The promise of ketamine in the treatment of addiction is supported
by research with large treatment effect sizes, especially in
comparison to existing treatments. In recently detoxified alcoholics,
ketamine treatment increased one-year abstinence rates in
alcoholics from 24% in the control to 66% in the ketamine group
(Krupitsky and Grinenko, 1997) and reduced cocaine self administration
by 67% relative to baseline in non-treatment
seeking cocaine users (Dakwar et al., 2016). These results clearly
demonstrate profound effects of ketamine administration (with
and without therapy) on drug and alcohol use, of an order of
magnitude which is 2 or 3 times more effective than existing
pharmacotherapies.

Ketamine for the treatment of addiction Evidence and potential mechanisms