Motivation-enhancing drug | |
---|---|
Drug class | |
Class identifiers | |
Synonyms | Motivation-enhancing agent; Motivation-enhancing medication; Pro-motivational drug; [1] Pro-motivational agent; Pro-motivational medication |
Use | To increase motivation and treat disorders of diminished motivation |
Legal status | |
In Wikidata |
A motivation-enhancing drug, [2] [3] also known as a pro-motivational drug, [1] is a drug which increases motivation. [4] [1] Drugs enhancing motivation can be used in the treatment of motivational deficits, for instance in depression, schizophrenia, and attention deficit hyperactivity disorder (ADHD). [5] [4] They can also be used in the treatment of disorders of diminished motivation (DDMs), including apathy, abulia, and akinetic mutism, disorders that can be caused by conditions like stroke, traumatic brain injury (TBI), and neurodegenerative diseases. [6] [7] Motivation-enhancing drugs are used non-medically by healthy people to increase motivation and productivity as well, for instance in educational contexts. [8] [1] [9] [10]
There are limited clinical data on medications in treating motivational deficits and disorders. [11] [12] In any case, drugs used for pro-motivational purposes are generally dopaminergic agents, for instance dopamine reuptake inhibitors (DRIs) like methylphenidate and modafinil, dopamine releasing agents (DRAs) like amphetamine, and other dopaminergic medications. [4] [1] [13] Adenosine receptor antagonists, like caffeine and istradefylline, can also produce pro-motivational effects. [13] [14] [15] [16] Acetylcholinesterase inhibitors, like donepezil, have been used as well. [17] [18] [6] [11]
Some drugs do not appear to increase motivation and can actually have anti-motivational effects. [4] [13] [19] Examples of these drugs include selective serotonin reuptake inhibitors (SSRIs), [19] [20] [21] selective norepinephrine reuptake inhibitors (NRIs), [19] and antipsychotics (which are dopamine receptor antagonists or partial agonists). [22] [23] [24] [25] Cannabinoids, for instance those found in cannabis, have also been associated with motivational deficits. [26] [27] [28] [4] [29]
Dopaminergic agents that have been found to produce pro-motivational effects in animals and/or humans include the following: [4] [13]
Dopamine D2-like receptor agonists, including pramipexole, ropinirole, rotigotine, piribedil, bromocriptine, cabergoline, pergolide, and lisuride, have also been used to treat disorders of diminished motivation in humans. [18] [6] [7] [12] [40] [41] [42] The clinical data on these agents for this use is very limited, but therapeutic successes have been reported. [12] [41] D2-like receptor agonists are known to have sedative-like and non-rewarding effects in humans. [43] [44] [45] In any case, dopamine D2-like receptor antagonists, like haloperidol and other antipsychotics, are known to produce anti-motivational effects in animals [4] [13] [12] [1] and humans. [22] [23] [46] [47] [48] [49] Bromocriptine has been reported to improve anergia and motivation in humans in very limited clinical reports. [40] [50] [51] On the other hand, pergolide failed to show pro-motivational effects in animals. [52]
Other dopaminergic drugs that have been used or suggested in the treatment of disorders of diminished motivation include rasagiline (a selective monoamine oxidase B (MAO-B) inhibitor; but see more below), tolcapone (a centrally-acting catechol-O-methyltransferase (COMT) inhibitor), and amantadine (an indirectly acting dopaminergic agent that acts via unknown mechanisms). [12] [18] [53] [17] [54] Tolcapone, the only marketed COMT inhibitor that is centrally acting (as opposed to peripherally selective), shows antidepressant- and anti-anhedonia-like effects, stimulates exploratory behavior, and enhances the locomotor hyperactivity induced by psychostimulants like amphetamine and nomifensine in animals. [55] [56] [57] Amantadine is widely used to treat multiple sclerosis-related fatigue, among other fatigue- and motivation-related disorders, and is recommended by the United Kingdom National Institute for Health and Care Excellence (NICE) guidelines for this use, although clinical data are limited. [54] [58] [59] [60] [61]
Dopamine levels and signaling in the nucleus accumbens, part of the ventral striatum and the mesolimbic reward pathway, are thought to play a key role in mediating behavioral activation and motivation. [4] [19] [13] [12] Dopamine releasing agents like dextroamphetamine are able to rapidly increase striatal dopamine levels by 700 to 1,500% of baseline in rodents. [62] These drugs show greater magnitudes of impact on dopamine levels than do dopamine reuptake inhibitors like methylphenidate. [62] [63] In addition, whereas dopamine reuptake inhibitors show a clear dose–effect ceiling in their effects on dopamine levels, dopamine releasing agents do not and have been found to maximally increase dopamine levels by more than 5,000%. [62] [64] Atypical dopamine reuptake inhibitors like modafinil can also increase dopamine levels in the striatum and nucleus accumbens in animals, but have further reduced impacts on dopamine levels compared to psychostimulants like amphetamine and methylphenidate. [65]
A limitation of certain dopaminergic medications used to improve motivation, like psychostimulants, is development of tolerance to their effects. [66] [67] Rapid acute tolerance to amphetamines is believed to be responsible for the dissociation between their relatively short durations of action (~4 hours for main desired effects) and their much longer elimination half-lives (~10 hours) and durations in the body (~2 days). [67] [68] [69] [70] [71] [72] [73] It appears that continually increasing or ascending concentration–time curves are beneficial for prolonging effects, which has resulted in administration multiple times per day and development of delayed- and extended-release formulations. [67] [69] [70] Drug holidays and breaks can be helpful in resetting tolerance. [66]
Another possible limitation of amphetamine specifically is dopaminergic neurotoxicity, which might occur even at therapeutic doses. [74] [75] [76] [77] [78] [79]
A limitation of bupropion as a dopaminergic agent is that it achieves very limited clinical occupancy of the dopamine transporter (DAT). [80] [81] [82] [83]
Adenosine receptor antagonists, including caffeine, istradefylline (KW-6002), Lu AA47070, MSX-3, MSX-4, preladenant (SCH-420814), and theophylline, have shown pro-motivational effects in animals and humans. [13] [14] [15] [84] [16] [85] Caffeine and theophylline act as non-selective antagonists of the adenosine receptors (including A1, A2A, A2B, and A3). [13] [86] [87] [88] Conversely, agents like istradefylline and preladenant are selective adenosine A2A receptor antagonists. [13] Adenosine A2A receptor antagonists, including the non-selective antagonists like caffeine, show pro-motivational effects in animals, whereas selective adenosine A1 receptor antagonists, like DPCPX and CPX, do not. [13] [89] Adenosine A2A receptor antagonists appear to exert their pro-motivational effects in the nucleus accumbens core and can reverse the anti-motivational effects of dopamine D2 receptor antagonists like haloperidol in animals. [13] [14] [15] [90] [91] Istradefylline is approved in the treatment of Parkinson's disease and has been found to improve symptoms of apathy, anhedonia, and depression in people with the condition. [16] [85]
Acetylcholinesterase inhibitors, like donepezil, rivastigmine, and galantamine, have been used in the treatment of disorders of diminished motivation. [17] [18] [6] [11] These drugs inhibit acetylcholinesterase, which metabolizes the neurotransmitter acetylcholine, thereby increasing acetylcholine levels in the brain and augmenting activation of the muscarinic acetylcholine and nicotinic acetylcholine receptors. [92] They are approved and used in the treatment of Alzheimer's disease and provide modest cognitive improvements in people with the disease. [92] [93] [94] Although acetylcholinesterase inhibitors have been used to treat disorders of diminished motivation, the muscarinic acetylcholine receptor agonist pilocarpine has actually shown anti-motivational effects in animals that can be reversed by the muscarinic acetylcholine receptor antagonist scopolamine. [90] In addition, xanomeline, a muscarinic acetylcholine M1 and M4 receptor agonist, shows indirect antidopaminergic effects in the mesolimbic pathway in animals and, in combination with trospium, is approved as an antipsychotic in the treatment of schizophrenia. [95] [96] [97] Furthermore, scopolamine has been found to reverse the anti-motivational effects of the dopamine D2 receptor antagonist haloperidol in animals. [90] In any case, in spite of the preceding findings, acetylcholinesterase inhibitors have been found to be clinically effective, albeit modestly, for apathy in dementia and Parkinson's disease. [98] [99] [100]
Serotonin 5-HT2C receptor antagonists, such as SB-242084, show pro-motivational effects in animals. [101] [102] [103] [104] [105] In addition, SB-242084 has shown modest stimulant-like and reinforcing effects in monkeys. [106] In accordance with the preceding, serotonin 5-HT2C receptor antagonists like SB-242084 and SB-206553 have been found to increase dopamine levels in the nucleus accumbens. [107] [108]
Agomelatine is a potent melatonin MT1 and MT2 receptor agonist and weak serotonin 5-HT2B and 5-HT2C receptor antagonist that is marketed as an antidepressant. [109] It has been found to increase norepinephrine and dopamine levels in the frontal cortex in rodents, though notably not in the striatum or nucleus accumbens (in contrast to other serotonin 5-HT2C receptor antagonists), and for this reason has sometimes been described as a "norepinephrine–dopamine disinhibitor" ("NDDI"). [109] [110] Due to its indirect dopaminergic effects, the drug has been suggested as a possible treatment for disorders of diminished motivation like anhedonia and abulia. [111] It has been found to be effective in the treatment of apathy in people with dementia [112] [98] [113] [114] and was reported to reverse escitalopram-associated apathy in a case report. [111] [115]
The GPR139 agonist zelatriazin (TAK-041; NBI-1065846) has shown pro-motivational effects in animals. [116] [117] On the basis of these findings, it has been speculated that the drug might be useful in the treatment of apathy in humans. [116] [117] Zelatriazin was under development for the treatment of anhedonia in major depressive disorder and the negative symptoms of schizophrenia and reached phase 3 clinical trials. [118] [119] [120] However, its development was discontinued due to lack of clinical effectiveness. [118] [121]
The tumor necrosis factor α (TNF-α) monoclonal antibody infliximab has been found to increase motivation in people with depression with high inflammation (as measured by high C-reactive protein levels). [122] [123] It has also been found to reduce symptoms of depression and anhedonia, for instance in people with high inflammation. [124] [125] [122]
Selective serotonin reuptake inhibitors (SSRIs) like escitalopram and norepinephrine reuptake inhibitors (NRIs) like atomoxetine have been used and recommended in the treatment of disorders of diminished motivation. [7] [17] [126] However, SSRIs like fluoxetine and citalopram, NRIs like desipramine and atomoxetine, and MAO-A-inhibiting monoamine oxidase inhibitors (MAOIs) like moclobemide and pargyline, have all not shown pro-motivational effects in animals. [4] [13] [30] [127] [39] In fact, these drugs can produce further motivational deficits in animals. [19] [127] [128] [39] [103] Serotonergic antidepressants like SSRIs and serotonin–norepinephrine reuptake inhibitors (SNRIs) have also been implicated in inducing apathy and emotional blunting in humans. [20] [21] [129] Activation of the serotonin 5-HT2C receptor by serotonergic agents like SSRIs may contribute to or mediate their anti-motivational effects, whereas co-administration of serotonin 5-HT2C receptor antagonists may reverse the effects. [102] [103]
In contrast to selegiline, selective MAO-B inhibitors without concomitant catecholaminergic activity enhancer (CAE) actions, like rasagiline, SU-11739, and lazabemide, are poorly effective in reversing behavioral deficits induced by the dopamine depleting agent tetrabenazine in animals. [130] [131]
Antipsychotics, which classically act as dopamine receptor antagonists (mostly of the D2-like receptors), are well-known as having robust and dose-dependent anti-motivational effects. [4] [13] [22] [23] [46] [47] [49] In fact, these effects may play a key role in their effectiveness against the positive and psychotic symptoms of schizophrenia by blunting the emotions underlying delusions. [22] [23] [46] [47] [49]
A novel class of antipsychotics, sometimes referred to as third-generation antipsychotics, act as dopamine D2-like receptor partial agonists instead of as pure antagonists, and hence have mixed agonistic and antagonistic effects. [132] [133] These drugs include aripiprazole, brexpiprazole, and cariprazine. [133] Aripiprazole has been suggested, at low doses, as a possible treatment for disorders of diminished motivation. [53] However, aripiprazole and cariprazine showed anti-motivational effects in animals and failed to reverse the motivational deficits induced by the dopamine depleting agent tetrabenazine. [25] [24] Accordingly, aripiprazole reduced activation of the mesolimbic motivational pathway in humans similarly to but less robustly than haloperidol. [134] [135] On the other hand, another study found that aripiprazole reversed stress-induced motivational anhedonia in animals, an antidepressant-like effect. [136] [137] Different dopamine receptor partial agonists that are used in the treatment of schizophrenia are known to vary in their intrinsic activities at the dopamine receptors, so each drug may be expected to have a different profile of effects. [138]
Some atypical DRIs, like JJC8-091, in contrast to other DRIs, are not effective in producing pro-motivational effects in animals. [139] This has been attributed to binding to an occluded conformation of the dopamine transporter (DAT) that results in a diminished increase in dopamine levels. [139]
Aripiprazole, sold under the brand names Abilify and Aristada, among others, is an atypical antipsychotic primarily used in the treatment of schizophrenia, bipolar disorder, and irritability associated with autism spectrum disorder; other uses include as an add-on treatment for major depressive disorder and tic disorders. Aripiprazole is taken by mouth or via injection into a muscle. A Cochrane review found low-quality evidence of effectiveness in treating schizophrenia.
Dopamine receptors are a class of G protein-coupled receptors that are prominent in the vertebrate central nervous system (CNS). Dopamine receptors activate different effectors through not only G-protein coupling, but also signaling through different protein interactions. The neurotransmitter dopamine is the primary endogenous ligand for dopamine receptors.
Tetrabenazine is a drug for the symptomatic treatment of hyperkinetic movement disorders. It is sold under the brand names Nitoman and Xenazine among others. On August 15, 2008, the U.S. Food and Drug Administration approved the use of tetrabenazine to treat chorea associated with Huntington's disease. Although other drugs had been used "off label," tetrabenazine was the first approved treatment for Huntington's disease in the U.S. The compound has been known since the 1950s.
Dopaminergic means "related to dopamine", a common neurotransmitter. Dopaminergic substances or actions increase dopamine-related activity in the brain.
Agomelatine, sold under the brand names Valdoxan and Thymanax, among others, is an atypical antidepressant most commonly used to treat major depressive disorder and generalized anxiety disorder. One review found that it is as effective as other antidepressants with similar discontinuation rates overall but fewer discontinuations due to side effects. Another review also found it was similarly effective to many other antidepressants.
Trace amine-associated receptor 1 (TAAR1) is a trace amine-associated receptor (TAAR) protein that in humans is encoded by the TAAR1 gene.
Istradefylline, sold under the brand name Nourianz, is a medication used as an add-on treatment to levodopa/carbidopa in adults with Parkinson's disease (PD) experiencing "off" episodes. Istradefylline reduces "off" periods resulting from long-term treatment with the antiparkinson drug levodopa. An "off" episode is a time when a patient's medications are not working well, causing an increase in PD symptoms, such as tremor and difficulty walking.
SB-242084 is a selective antagonist of the serotonin 5-HT2C receptor which is used in scientific research.
Preladenant is a drug that was developed by Schering-Plough which acted as a potent and selective antagonist of the adenosine A2A receptor. It was being researched as a potential treatment for Parkinson's disease. Positive results were reported in Phase II clinical trials in humans, but it did not prove itself to be more effective than a placebo during Phase III trials, and so was discontinued in May 2013.
A serotonin releasing agent (SRA) is a type of drug that induces the release of serotonin into the neuronal synaptic cleft. A selective serotonin releasing agent (SSRA) is an SRA with less significant or no efficacy in producing neurotransmitter efflux at other types of monoamine neurons, including dopamine and norepinephrine neurons.
SB-206553 is a drug which acts as a mixed antagonist for the 5-HT2B and 5-HT2C serotonin receptors.
EPPTB, also known as RO5212773 or RO-5212773, is a drug developed by Hoffmann-La Roche which acts as a potent and selective antagonist or inverse agonist of the trace amine-associated receptor 1 (TAAR1). The drug was the first selective antagonist developed for the TAAR1. It is a potent agonist of the mouse and rat TAAR1, but is dramatically less potent as an agonist of the human TAAR1. EPPTB has been used in scientific research to demonstrate an important role for TAAR1 in regulation of dopaminergic signaling in the limbic system.
Peripherally selective drugs have their primary mechanism of action outside of the central nervous system (CNS), usually because they are excluded from the CNS by the blood–brain barrier. By being excluded from the CNS, drugs may act on the rest of the body without producing side-effects related to their effects on the brain or spinal cord. For example, most opioids cause sedation when given at a sufficiently high dose, but peripherally selective opioids can act on the rest of the body without entering the brain and are less likely to cause sedation. These peripherally selective opioids can be used as antidiarrheals, for instance loperamide (Imodium).
Locomotor activity is a measure of animal behavior which is employed in scientific research.
Disorders of diminished motivation (DDM) are a group of disorders involving diminished motivation and associated emotions. Many different terms have been used to refer to diminished motivation. Often however, a spectrum is defined encompassing apathy, abulia, and akinetic mutism, with apathy the least severe and akinetic mutism the most extreme.
CT-005404, or CT-5404, is an atypical dopamine reuptake inhibitor (DRI) that was derived from modafinil. It shows pro-motivational effects in animals and reverses motivational deficits induced by tetrabenazine and interleukin-1β. CT-005404 is described as being orally active in animals and having a long duration of action. It is under development by Chronos Therapeutics for treatment of motivational disorders. The drug was first described by 2018.
CE-158 is an atypical dopamine reuptake inhibitor (DRI) that was derived from modafinil. It is often but not always referred to as the enantiopure enantiomer (S,S)-CE-158 instead.
The conditioned avoidance response (CAR) test, also known as the active avoidance test, is an animal test used to identify drugs with antipsychotic-like effects. It is most commonly employed as a two-way active avoidance test with rodents. The test assesses the conditioned ability of an animal to avoid an unpleasant stimulus. Drugs that selectively suppress conditioned avoidance responses without affecting escape behavior are considered to have antipsychotic-like activity. Variations of the test, like testing for enhancement of avoidance and escape responses, have also been used to assess other drug effects, like pro-motivational and antidepressant-like effects.
ENX-104, also known as deuterated nemonapride enantiomer, is a selective dopamine D2 and D3 receptor antagonist which is under development for the treatment of major depressive disorder. It is specifically under development for the treatment of major depressive disorder characterized by anhedonia. The drug is being developed for use at low doses to preferentially block presynaptic dopamine D2 and D3 autoreceptors and hence to enhance rather than inhibit dopaminergic neurotransmission. It is taken by mouth.
The ethical considerations of pharmacological enhancement of cognition in the healthy population have been debated elsewhere (Farah et al. 2004; Porsdam Mann & Sahakian 2015). It is likely that putative pro-motivational drugs deserve a similar level of scrutiny.
The ethical threat posed by Adderall and other drugs that improve motivation [...] If it isn't justified – that is, if her options are limited purely due to unjust socio-political forces – then motivation enhancing drugs start to look more like political complacence pills. [...] It's the sort of spectre that permeates dystopian visions of the future, and it's one that is very much raised by the prospect of motivation enhancing drugs.
Adenosine A2A receptor antagonists have been studied for their potential antiparkinsonian effects (Ferré 1997; Morelli and Pinna 2002; Correa et al. 2004), and istradefylline (Nourianz) has been approved for use in several countries. Particularly relevant for the present review, drugs that act on adenosine A2A receptors induce substantial effects on instrumental behavior and effort-related choice. [...] Caffeine, theophylline, and several adenosine A2A receptor antagonists (MSX-3, MSX-4, Lu AA47070, istradefylline) can reverse the low-effort bias induced by systemically administered DA D2 antagonists (Farrar et al. 2007; Worden et al. 2009; Mott et al. 2009; Collins et al. 2012; Nunes et al. 2010; Santerre et al. 2012; Randall et al. 2012; Pardo et al. 2020), and MSX-3 and preladenant reverse the effects of TBZ (Nunes et al. 2013; Randall et al. 2014; Yohn et al. 2015a; Salamone et al. 2018). [...] Furthermore, A2A receptor knockout mice are resistant to the effort-related effects of haloperidol (Pardo et al. 2012). [...] Along with adenosine A2A antagonists such as istradefylline and preladenant (Nunes et al. 2013; Randall et al. 2014; Yohn et al. 2015a; Salamone et al. 2018), and D1 agonists (Yohn et al. 2015b), atypical DAT inhibitors offer promise as potential treatments for effort-related motivational symptoms.
Similarly, augmenting DA using a D1 agonist (PF-06412562; 6mg, 15mg, 30mg) increased willingness to exert physical effort for reward (Soutschek et al., 2020).
Dopamine agonists have been reported to have positive effects in treating the neuropsychiatric sequalae of brain injury. One case series reported 19 out of 30 patients with severe head injury and aggression to respond to amantadine over the course of a year (15). Other case series have also shown positive response of cognitive function, attention and motivation in persons with head injury in the rehabilitative setting (16–18). Open label trials have shown improved neuropsychiatric outcomes in braininjured patients with bromocriptine and amantadine (19,20). Case studies have also reported improvement with the use of dopaminergic therapy in patients with neuropsychiatric sequalae of stroke. A combination of carbidopa/levodopa and pergolide has been reported to substantially improve the outcome of post-infarct akinetic mutism (21). Ropinirole has been reported to have had a dramatic effect on post-stroke apathy (22). However most of the reported associations to date have been limited by considerable methodological shortcomings. Case studies are anecdotal evidence, whereas larger case series may report improvement but are uncontrolled. This is critical in studies of neurological injury, where a degree of improvement may be expected by neuronal recovery over time. Similarly trials to date which have reported positive results have been open-label and consequently susceptible to placebo effect. Thus a systematic review of rigorous double-blind randomised controlled trials (RCTs) is needed.
{{cite book}}
: CS1 maint: DOI inactive as of November 2024 (link)It also enhances locomotor hyperactivity induced by amphetamine and nomifensine and stereotypy induced by amphetamine, and stimulates exploratory activity in the open field test in rats and mice.14 Tolcapone potentiates levodopa antagonism of haloperidol-induced catalepsy in MPP+-lesioned mice (murine model of Parkinson's disease) and potentiates and prolongs levodopa-induced circling behavior in rats with 6-hydroxydopamine-induced nigrostriatal pathway lesions (another animal model of Parkinson's disease).23, 24 [...] The effect of tolcapone on animal models of depression was evaluated in two studies. In rats with chronic mild stress-induced anhedonia, tolcapone 10 or 30 mg/kg twice/day by intraperitoneal injection prevented the stress-induced anhedonic state compared with vehicle-treated controls.28 Another rat study using the forced swimming test and learned helplessness paradigm, found no significant antidepressant activity of the agent.29 The relevance of these findings to the management of depression in humans with both parkinsonian and nonparkinsonian disease is unknown.
Tolcapone administered 6 h before amphetamine challenge was found to significantly increase locomotor activity in rats treated with 0.5 and 2.0 mg kg-1 amphetamine. In rats given 4.0 mg kg-1 amphetamine, tolcapone produced a marked decrease in locomotor activity and increased two-fold the duration of the stereotyped behaviour.
Currently, different pharmacological agents are used for the treatment for fatigue in patients with MS, including amantadine, modafinil and pemoline.99,100 Of these, the most commonly used is amantadine. Its main mechanism of action is not yet fully understood, although its effects on fatigue seem to be related to its dopaminergic effects, supporting the dopamine imbalance theory for MS-related fatigue.101 In general, all trials that compared amantadine with placebo showed a significant effect of amantadine on fatigue. However, the results of these trials need to be interpreted with caution because of the low number of participants included in the trials and the short duration of the interventions.8 The daily dose of amantadine used in all published studies was 200 mg, which is the standard amount administered today. Amantadine is the only oral treatment that is currently recommended by the National Institute for Health and Care Excellence (NICE) for the treatment of MS-related fatigue.102
Intraperitoneal administration of dl-threo-MPH 10 mg/kg to spontaneously hypertensive rats elicits a rapid 3–4-fold increase in extracellular concentrations of noradrenaline in the prefrontal cortex and dopamine in the striatum, peaking within 45 min of dosing, and remaining above control levels for at least 3 h [48]. [...] Intraperitoneal administration of d-AMF 1 mg/kg to spontaneously hypertensive rats elicits a 15-fold increase in striatal dopamine concentrations 30 min post-dose that return to control levels within 90 min, and a fourfold increase in noradrenaline concentrations in the prefrontal cortex within 45 min of dosing that remain above control levels for at least 3 h.
Both d- and l-[amphetamine (AMP)] evoked rapid increases in extraneuronal concentrations of [noradrenaline (NA)] and [dopamine (DA)] that reached a maximum 30 or 60 min after administration. However, the [spontaneously hypertensive rats (SHRs)] were much more responsive to AMP's enantiomers than the [Sprague-Dawleys (SDs)]. Thus, 3 mg/kg d-AMP produced a peak increase in [prefrontal cortex (PFC)] NA of 649 ± 87% (p<0.001) in SHRs compared with 198 ± 39% (p<0.05) in SDs; the corresponding figures for [striatal (STR)] DA were 4898 ± 1912% (p<0.001) versus 1606 ± 391% (p<0.001). At 9 mg/kg, l-AMP maximally increased NA efflux by 1069 ± 105% (p<0.001) in SHRs compared with 157 ± 24% (p<0.01) in SDs; the DA figures were 3294 ± 691% (p<0.001) versus 459 ± 107% (p<0.001).
MOD binding to DAT differs from that of other typical, cocaine-like, DAT blockers (Schmitt and Reith, 2011). In contrast to cocaine, MOD prefers to bind to, or stabilize the DAT protein in a more inward-facing occluded conformation (Schmitt and Reith, 2011; Loland et al., 2012) that still inhibits uptake and results in increases in extracellular DA in the NAcc (Ferraro et al., 1996c; Zolkowska et al., 2009), the NAcc shell (NAS) (Loland et al., 2012; Mereu et al., 2020), and the striatum (Rowley et al., 2014). MOD also increases electrically evoked DA in the DS and VS (Bobak et al., 2016) (summarized in Table 2) like abused psychostimulants (Nisell et al., 1994; Pontieri et al., 1996; Munzar et al., 2004; Kohut et al., 2014). However, while acute administration of MOD (Mereu et al., 2017, 2020) or its enantiomers (Loland et al., 2012; Keighron et al., 2019a, b) increases extracellular NAcc DA levels in rodents, these effects, even at very high doses, elicited a limited stimulation of DA in striatal areas compared to the stimulation elicited by abused psychostimulants (Loland et al., 2012; Mereu et al., 2017, 2020). This limited efficacy of MOD to increase DA levels, as compared to abused psychostimulants, also predicts a limited potential for abuse.
Metabolism does not appear to be altered by chronic exposure, thus dose escalation appears to arise from pharmacodynamic rather than pharmacokinetic tolerance [24]. [...] The terminal plasma half-life of methamphetamine of approximately 10 hours is similar across administration routes, but with substantial inter-individual variability. Acute effects persist for up to 8 hours following a single moderate dose of 30 mg [30]. [...] peak plasma methamphetamine concentration occurs after 4 hours [35]. Nevertheless, peak cardiovascular and subjective effects occur rapidly (within 5–15 minutes). The dissociation between peak plasma concentration and clinical effects indicates acute tolerance, which may reflect rapid molecular processes such as redistribution of vesicular monoamines and internalization of monoamine receptors and transporters [6,36]. Acute subjective effects diminish over 4 hours, while cardiovascular effects tend to remain elevated. This is important, as the marked acute tachyphylaxis to subjective effects may drive repeated use within intervals of 4 hours, while cardiovascular risks may increase [11,35].
For several decades, clinical benefits of amphetamines have been limited by the pharmacologic half-life of around 4 hours. Although higher doses can produce higher maximum concentrations, they do not affect the half-life of the dose. Therefore, to achieve longer durations of effect, stimulants had to be dosed at least twice daily. Further, these immediate-release doses were found to have their greatest effect shortly after administration, with a rapid decline in effect after reaching peak blood concentrations. The clinical correlation of this was found in comparing math problems attempted and solved between a mixed amphetamine salts preparation (MAS) 10 mg once at 8 am vs 8 am followed by 12 pm [14]. The study also demonstrated the phenomenon of acute tolerance, where even if blood concentrations were maintained over the course of the day, clinical efficacy in the form of math problems attempted and solved would diminish over the course of the day. These findings eventually led to the development of a once daily preparation (MAS XR) [15], which is a composition of 50% immediate-release beads and 50% delayed release beads intended to mimic this twice-daily dosing with only a single administration.
It has been suggested that the association between PD and ADHD may be explained, in part, by toxic effects of these drugs on DA neurons.241 [...] An important question is whether amphetamines, as they are used clinically to treat ADHD, are toxic to DA neurons. In most of the animal and human studies cited above, stimulant exposure levels are high relative to clinical doses, and dosing regimens (as stimulants) rarely mimic the manner in which these drugs are used clinically. The study by Ricaurte and colleagues248 is an exception. In that study, baboons orally self-administered a racemic (3:1 d/l) amphetamine mixture twice daily in increasing doses ranging from 2.5 to 20 mg/day for four weeks. Plasma amphetamine concentrations, measured at one-week intervals, were comparable to those observed in children taking amphetamine for ADHD. Two to four weeks after cessation of amphetamine treatment, multiple markers of striatal DA function were decreased, including DA and DAT. In another group of animals (squirrel monkeys), d/l amphetamine blood concentration was titrated to clinically comparable levels for four weeks by administering varying doses of amphetamine by orogastric gavage. These animals also had decreased markers of striatal DA function assessed two weeks after cessation of amphetamine.
Recently, however, new data from Ricaurte et al. (2005) indicate that primates may be much more susceptible than rats to AMPH-induced neurotoxicity. They examined the effect of the drug in adult baboons and squirrel monkeys, as clinically used to treat ADHD. In the first two studies, baboons were trained to orally selfadminister a mixture of AMPH salts (a 3:1 ratio of dextro [S(+)] and levo [R(-)] AMPH, which simulated a common formulation for ADHD treatment). AMPH was administered twice daily for approximately 4 weeks at escalating doses of 2.5 to 20 mg (0.67 to 1.00 mg/kg). During the second study, plasma AMPH concentrations were determined at the end of each week. In the third study, AMPH was administered by orogastric gavage to squirrel monkeys and doses were adjusted (to 0.58-0.68 mg/kg) so that for approximately the last 3 weeks plasma drug concentrations were comparable to those reported in clinical populations of children receiving chronic AMPH treatment—100 to 150 ng/ml (McGough et al., 2003). Measurements in all three investigations were taken 2 to 4 weeks after drug treatment. Results from the first two studies showed significant reductions in striatal dopamine concentration, dopamine transporter density, and vesicular monoamine transporter sites. Plasma AMPH concentration at the end of the 4 week treatment period was 168 ± 25 ng/ml. In squirrel monkeys, brain dopamine concentrations and vesicular transporter sites were also significantly reduced although dopamine transporter decreases were not statistically significant. These results raise obvious concerns about clinical drug treatment of ADHD, although extrapolation to human populations may be premature until possible species differences in mechanism of action, developmental variables, or metabolism are determined.
Amphetamine treatment similar to that used for ADHD has been demonstrated to produce brain dopaminergic neurotoxicity in primates, causing the damage of dopaminergic nerve endings in the striatum that may also occur in other disorders with long-term amphetamine treatment (57).
Repeated exposure to moderate to high levels of methamphetamine has been related to neurotoxic effects on the dopaminergic and serotonergic systems, leading to potentially irreversible loss of nerve terminals and/or neuron cell bodies (Cho and Melega, 2002). Preclinical evidence suggests that d-amphetamine, even when administered at commonly prescribed therapeutic doses, also results in toxicity to brain dopaminergic axon terminals (Ricaurte et al., 2005).
Though the paradigm used by Ricaurte et al. 53 arguably still incorporates amphetamine exposure at a level above much clinical use,14,55 it raises important unanswered questions. Is there a threshold of amphetamine exposure above which persistent changes in the dopamine system are induced? [...]
Recently, PD patients have been treated with istradefylline, an adenosine A2A receptor antagonist used for treatment of motor symptoms. The drug was given to 14 PD patients for 12 weeks, measuring anhedonia, apathy and depression using the SHAPS, Apathy Scale and BDI. On istradefylline, SHAPS, Apathy Scale and BDI scores significantly reduced from baseline scores at 4-, 8- and 12-weeks, with mean SHAPS scores at week 12 about 50% reduced from baseline scores, indicating that istradefylline reduces anhedonia (Nagayama et al. 2019). As apathy and depression rates dropped as well as anhedonia, this trial also provided evidence for the overlapping relationship between the three symptoms. [...] Taken together, there is some evidence that dopamine agonists such as pramipexole and piribedil, or the adenosine A2A receptor antagonist istradefylline can improve anhedonia and apathy in PD.
Serotonin 2C receptor antagonism: Enhancement of dopamine release and motivation [...]
[...] 5-HT2C receptors are linked to some of the adverse motivational effects corresponding with dimensions of approach/avoidance behaviours in both aversive and appetitive paradigms. 5-HT2CR antagonism reduces the inhibitory effects of citalopram and SERT-KO mice on reward-seeking after conditioned reinforcers (Brown & Fletcher, 2016), paralleling other experiments showing that 5-HT2CR antagonism exacerbates impulsivity (Winstanley et al, 2004b; Robinson et al, 2008b). 5-HT2CR antagonists increase motivation alongside extracellular DA in the dorsomedial striatum (Bailey et al, 2018), and others have suggested that 5-HT2CR antagonists may be a valuable adjunctive treatment to SSRIs to attenuate motivational side effects such as apathy, decreased libido, and general emotional blunting (Demireva et al, 2018). 5-HT2C receptors, including its negative interactions with DA, appear to increase the latency of active responses to rewards and constrain motivation, consistent with its inhibitory effects on locomotion also seen in uncertain and potentially aversive contexts. [...] 5-HT2C antagonists could prove to be a useful adjunctive treatment to attenuate unwanted anxiogenic and motivational deficits associated with SSRIs.
Additionally, recent animal studies showed interests in using different pharmacologic targets such as antagonist of muscarinic acetylcholine receptors (Hailwood et al., 2019) or selective 5-HT2C receptor ligand (Bailey et al., 2018) to enhance amotivation and goal-oriented behaviors.
[...] the selective 5-HT2C receptor antagonist SB-242084 was shown to enhance DA levels in the nucleus accumbens, an effect attributed to the disinhibition of DA firing via 5-HT2C receptors expressed on GABAergic interneurons in the VTA[17]. In agreement with this, SB-242084 dose dependently increased the firing rate and bursting activity of DA neurons in the VTA[28]. Behaviorally, SB-242084 was found to potentiate dexamphetamine-induced locomotor hyperactivity in rats[21].
Binding studies show that [agomelatine] has a high affinity for human melatonin MT1- and MT2-receptors (Ki: 0.1nM; 0.12nM respectively) and acts as an agonist at these receptors [7]. It has little affinity (Ki > 10μM) for most other receptors, [...] [Agomelatine] binds to the 5-HT2C receptor (Ki = 631nM) as well as cloned, human 5-HT2B receptors (Ki = 660nM), but has negligible affinity at 5-HT2A receptors [7]. At 5-HT2B and 5-HT2C receptors agomelatine acts as an antagonist. The interaction with 5-HT2C receptors may be more nuanced than simple antagonism since this receptor is subject to RNA editing, which can generate multiple isoforms of the receptor with various properties (e.g., affinity, coupling and constitutive activity) [9]. Blockade of the 5-HT2C receptor is believed to be responsible for the dose dependent increase in the extracellular concentrations of both noradrenaline and dopamine observed in the prefrontal cortex following acute drug administration [7]. By contrast dopamine concentrations in the nucleus accumbens or the striatum were not affected by agomelatine [7]. Furthermore, there was no change in extracellular concentrations of serotonin.
In 2021, Reichard et al., (2021) developed the GPR139 agonist TAK041, also known as NBI-1065846. TAK-041 has good physical and chemical properties, can cross the blood–brain barrier, and shows potential in preclinical studies to treat schizophrenia symptoms. Several clinical trials indicate that TAK-041 is safe and metabolically stable (Kamel et al., 2021; Reichard et al., 2021; Yin et al., 2022). Apathy is a condition characterised by a lack of motivation, emotion, or interest and is a common symptom of many psychiatric and neurological disorders. Munster et al. (2022) provided preclinical evidence supporting GPR139 agonism (using TAK-041) as a molecular mechanism for treating apathy. The research and development of TAK-041 have effectively promoted the process of de-orphaning GPR139 and its clinical application value.
The investigational NBI-1065846, as part of the collaboration with Takeda Pharmaceutical Company Limited (Takeda), did not meet its primary endpoint in the Phase 2 TERPSIS™ study evaluating its efficacy compared to placebo in patients with anhedonia in major depressive disorder. No further development with NBI-1065846 is planned at this time.
Aripiprazole, brexpiprazole, and cariprazine are representative of a new class of APs that act as "dopamine stabilizers", namely partial agonists at D2R/D3R [16]. Partial agonists may act as functional agonists or antagonists, depending on the surrounding levels of endogenous ligand. According to this view, D2R partial agonists may act as functional antagonists within the mesolimbic system, where a hyperdopaminergic state may contribute to positive symptoms; on the other hand, they act as functional agonists in the mesocortical pathway, where extracellular dopamine levels are low, thus mitigating, or at least not worsening, negative and cognitive symptoms [17], [18]. [...]
Accordingly, the task-related BOLD-fMRI response in the mesolimbic motivational system was diminished in the haloperidol group compared to the placebo group, particularly in the ventral striatum, whereas the aripiprazole group showed task-related activations intermediate of the placebo and haloperidol groups.
In self-administration protocols, the schedule used to assess the motivation to work for a natural (or a drug) reward is commonly the progressive ratio (PR) schedule (Hodos, 1961) where increasing effort is required to obtain the reward as the ratio requirement progressively increases, and the last ratio completed is the breaking point. The breaking point measures the effort the animal is willing to exert to obtain the reinforcing stimulus and is then considered an index of motivation, or of the perceived reinforcing value of the stimulus. Thus, a decrease in breaking point may be regarded as a core symptom in animal models of anhedonia, although this decrease is not reliably observed in all the models. Reductions in breakpoints for sucrose have been reported in a genetic animal model of depression, the congenital learned helpless rat (Vollmayr et al., 2004), in a chronic unavoidable stress protocol in rats (Marchese et al., 2013; Scheggi et al., 2016), and in rats and mice exposed to chronic social defeat (Bergamini et al., 2016; Spierling et al., 2017). This index of reduced motivation for a natural reward can be restored to control values by treatments endowed with antidepressant and/or promotivational activity, for example, lithium, clozapine, aripiprazole, and lamotrigine (Marchese et al., 2013; Scheggi et al., 2015, 2017b; Scheggi, Pelliccia, De Montis and Gambarana, unpublished data). Conversely, exposure to the CMS model does not usually affect sucrose breaking point.