A quantitative estimate of the role of striatal D-2 receptor proliferation in dopaminergic behavioral supersensitivity: the contribution of mesolimbic dopamine to the magnitude of 6-OHDA lesion-induced agonist sensitivity in the rat

Meta-analysis and systematic review of vesicular monoamine transporter (VMAT-2) inhibitors in schizophrenia and psychosis

RATIONALE

Dopamine antagonists induce dopamine receptor supersensitivity. This may manifest in late-appearing movement disorders (tardive dyskinesia (TD). VMAT-2 inhibitors reduce dopaminergic transmission but have limited activity at postsynaptic receptors and so may have antipsychotic activity with lower risk of tardive dyskinesia.

METHODS

We conducted a systematic database search from inception to September 2022 for articles describing the use of VMAT-2 inhibitors in psychosis. Inclusion criteria were as follows: Population: adults diagnosed with psychosis or schizophrenia; Intervention: treatment with tetrabenazine, deutetrabenazine or valbenazine; Comparison: comparison with placebo or/and antipsychotic drug; Outcomes: with efficacy outcomes (e.g. Brief Psychiatric Rating Scale (BPRS) change or clinician assessment) and adverse effects ratings (e.g. rating scale or clinician assessment or dropouts); and Studies: in randomised controlled trials and non-randomised studies.

RESULTS

We identified 4892 records relating to VMAT-2 inhibitor use of which 5 (173 participants) met our a priori meta-analysis inclusion criteria. VMAT-2 inhibitors were more effective than placebo for the outcome 'slight improvement' (risk ratio (RR) = 1.77 (95% CI 1.03, 3.04)) but not for 'moderate improvement' (RR 2.81 (95% CI 0.27, 29.17). VMAT-2 inhibitors were as effective as active comparators on both measures for-'slight improvement' (RR 1.05 (95% CI 0.6, 1.81)) and 'moderate improvement' (RR 1.11 (95% CI 0.51, 2.42). Antipsychotic efficacy was also suggested by a narrative review of 37 studies excluded from the meta-analysis.

CONCLUSIONS

VMAT-2 inhibitors may have antipsychotic activity and may offer promise for treatment of psychosis with the potential for a reduced risk of TD.

Dopamine Dynamics and Neurobiology of Non-Response to Antipsychotics, Relevance for Treatment Resistant Schizophrenia: A Systematic Review and Critical Appraisal

Treatment resistant schizophrenia (TRS) is characterized by a lack of, or suboptimal response to, antipsychotic agents. The biological underpinnings of this clinical condition are still scarcely understood. Since all antipsychotics block dopamine D2 receptors (D2R), dopamine-related mechanisms should be considered the main candidates in the neurobiology of antipsychotic non-response, although other neurotransmitter systems play a role. The aims of this review are: (i) to recapitulate and critically appraise the relevant literature on dopamine-related mechanisms of TRS; (ii) to discuss the methodological limitations of the studies so far conducted and delineate a theoretical framework on dopamine mechanisms of TRS; and (iii) to highlight future perspectives of research and unmet needs. Dopamine-related neurobiological mechanisms of TRS may be multiple and putatively subdivided into three biological points: (1) D2R-related, including increased D2R levels; increased density of D2Rs in the high-affinity state; aberrant D2R dimer or heteromer formation; imbalance between D2R short and long variants; extrastriatal D2Rs; (2) presynaptic dopamine, including low or normal dopamine synthesis and/or release compared to responder patients; and (3) exaggerated postsynaptic D2R-mediated neurotransmission. Future points to be addressed are: (i) a more neurobiologically-oriented phenotypic categorization of TRS; (ii) implementation of neurobiological studies by directly comparing treatment resistant vs. treatment responder patients; (iii) development of a reliable animal model of non-response to antipsychotics.

Recent Advances in the Pharmacology of Tardive Dyskinesia

Tardive dyskinesia (TD) is a syndrome of abnormal involuntary movements that follows treatment with dopamine D2-receptor antagonists. Recent approval of vesicular monoamine transporter-2 (VMAT2) inhibitors offers hope for reducing the impact of TD. Although these drugs represent a significant advance in patient care and a practical step forward in providing relief for patients with TD, understanding of the pharmacology of TD that could inform future research to prevent and reverse TD remains incomplete. This review surveys evidence for the effectiveness of VMAT2 inhibitors and other agents in the context of theories of pathogenesis of TD. In patients for whom VMAT2 inhibitors are ineffective or intolerable, as well as for extending therapeutic options and insights regarding underlying mechanisms, a review of clinical trial results examined as experimental tests of etiologic hypotheses is worthwhile. There are still compelling reasons for further investigations of the pharmacology of TD, which could generate alternative preventive and potentially curative treatments. Finally, benefits from novel drugs are best realized within an overall treatment strategy addressing the condition and needs of individual patients.

Overcoming barriers to effective management of tardive dyskinesia

Tardive dyskinesia (TD) is a heterogeneous syndrome of involuntary hyperkinetic movements that is often persistent and occurs belatedly during treatment with antipsychotics. Recent approval of two dopamine-depleting analogs of tetrabenazine based on randomized controlled trials offers an evidence-based therapeutic approach to TD for the first time. These agents are optimally used within the context of a comprehensive approach to patient management that includes a practical screening and monitoring program, sensitive and specific criteria for the diagnosis of TD, awareness of the severity and impact of the disorder, informed discussions with patients and caregivers, and a rational basis for prescribing decisions about continued antipsychotic and adjunctive agents. Areas of limited or inconclusive data, bias and misunderstandings about key aspects, and neglect of training about TD in recent years contribute to barriers in providing effective care and promoting patient safety.

Dopamine in the Brain: Hypothesizing Surfeit or Deficit Links to Reward and Addiction

Recently there has been debate concerning the role of brain dopamine in reward and addiction. David Nutt and associates eloquently proposed that dopamine (DA) may be central to psycho stimulant dependence and some what important for alcohol, but not important for opiates, nicotine or even cannabis. Others have also argued that surfeit theories can explain for example cocaine seeking behavior as well as non-substance-related addictive behaviors. It seems prudent to distinguish between what constitutes “surfeit” compared to” deficit” in terms of short-term (acute) and long-term (chronic) brain reward circuitry responsivity. In an attempt to resolve controversy regarding the contributions of mesolimbic DA systems to reward, we review the three main competing explanatory categories: “liking”, “learning”, and “wanting”. They are (a) the hedonic impact -liking reward, (b) the ability to predict rewarding effects-learning and (c) the incentive salience of reward-related stimuli -wanting. In terms of acute effects, most of the evidence seems to favor the “surfeit theory”. Due to preferential dopamine release at mesolimbic-VTA-caudate-accumbens loci most drugs of abuse and Reward Deficiency Syndrome (RDS) behaviors have been linked to heightened feelings of well-being and hyperdopaminergic states.The “dopamine hypotheses” originally thought to be simple, is now believed to be quite complex and involves encoding the set point of hedonic tone, encoding attention, reward expectancy, and incentive motivation. Importantly, Willuhn et al. shows that in a self-administration paradigm, (chronic) excessive use of cocaine is caused by decreased phasic dopamine signaling in the striatum. In terms of chronic addictions, others have shown a blunted responsivity at brain reward sites with food, nicotine, and even gambling behavior. Finally, we are cognizant of the differences in dopaminergic function as addiction progresses and argue that relapse may be tied to dopamine deficiency. Vulnerability to addiction and relapse may be the result of the cumulative effects of dopaminergic and other neurotransmitter genetic variants and elevated stress levels. We therefore propose that dopamine homeostasis may be a preferred goal to combat relapse.

Supersensitive presynaptic dopamine D2 receptor inhibition of the striatopallidal projection in nigrostriatal dopamine-deficient mice

The dopamine (DA) D2 receptor (D2R)-expressing medium spiny neurons (D2-MSNs) in the striatum project to and inhibit the GABAergic neurons in the globus pallidus (GP), forming an important link in the indirect pathway of the basal ganglia movement control circuit. These striatopallidal axon terminals express presynaptic D2Rs that inhibit GABA release and thus regulate basal ganglion function. Here we show that in transcription factor Pitx3 gene mutant mice with a severe DA loss in the dorsal striatum mimicking the DA denervation in Parkinson's disease (PD), the striatopallidal GABAergic synaptic transmission displayed a heightened sensitivity to presynaptic D2R-mediated inhibition with the dose-response curve shifted to the left, although the maximal inhibition was not changed. Functionally, low concentrations of DA were able to more efficaciously reduce the striatopallidal inhibition-induced pauses of GP neuron activity in DA-deficient Pitx3 mutant mice than in wild-type mice. These results demonstrate that presynaptic D2R inhibition of the striatopallidal synapse becomes supersensitized after DA loss. These supersensitive D2Rs may compensate for the lost DA in PD and also induce a strong disinhibition of GP neuron activity that may contribute to the motor-stimulating effects of dopaminergic treatments in PD.

U, R.L.W. C. 2-Hydroxyestradiol enhances binge onset in female rats and reduces prefrontal cortical dopamine in male rats

Women are more likely to suffer from a bingeing-related eating disorder, which is surprising, since estradiol reduces meal size and is associated with reduced binge frequency. This apparent contradiction may involve the estradiol metabolite, 2-hydroxyestradiol. We previously reported that female rats had faster escalations in shortening intake during the development of bingeing than did males, but acute administration of 2-hydroxyestradiol increased the intake of vegetable shortening to a greater extent in male rats once bingeing was established. Here, we report two separate studies that follow up these previous findings. In the first, we hypothesized that chronic exposure to 2-hydroxyestradiol would promote escalation of bingeing during binge development in ovariectomized female rats. In the second, we hypothesized that acute exposure to 2-hydroxyestradiol would enhance dopamine signaling in the prefrontal cortex after bingeing was established in male rats. In study 1, non-food-deprived female rats were separated into 3 groups: ovariectomized (OVX) with chronic 2-hydroxyestradiol supplementation (E), OVX with vehicle supplementation (O), and intact with vehicle (I). Each group was given access to an optional source of dietary fat (shortening) on Mon, Wed, and Fri for 4 weeks. 2-hydroxyestradiol supplementation prevented OVX-induced weight gain and enhanced escalation of shortening intake over the four-week period (ps<0.05). Additionally, in week 4, rats in the E group ate significantly more shortening than I controls, less chow than either the O or I group, and had a higher shortening to chow ratio than O or I (ps<0.05). Study 2 indicated that acute injection of 2-hydroxyestradiol abolished shortening-evoked dopamine efflux in the prefrontal cortex of bingeing male rats (p<0.05). Together, these studies indicate that 2-hydroxyestradiol can exacerbate bingeing as it develops and can suppress dopamine signaling in the prefrontal cortex once bingeing is established.

Neurogenetics of dopaminergic receptor supersensitivity in activation of brain reward circuitry and relapse: proposing "deprivation-amplification relapse therapy" (DART)

BACKGROUND AND HYPOTHESIS

It is well known that after prolonged abstinence, individuals who use their drug of choice experience a powerful euphoria that often precipitates relapse. While a biological explanation for this conundrum has remained elusive, we hypothesize that this clinically observed "supersensitivity" might be tied to genetic dopaminergic polymorphisms. Another therapeutic conundrum relates to the paradoxical finding that the dopaminergic agonist bromocriptine induces stronger activation of brain reward circuitry in individuals who carry the DRD2 A1 allele compared with DRD2 A2 allele carriers. Because carriers of the A1 allele relative to the A2 allele of the DRD2 gene have significantly lower D2 receptor density, a reduced sensitivity to dopamine agonist activity would be expected in the former. Thus, it is perplexing that with low D2 density there is an increase in reward sensitivity with the dopamine D2 agonist bromocriptine. Moreover, under chronic or long-term therapy with D2 agonists, such as bromocriptine, it has been shown in vitro that there is a proliferation of D2 receptors. One explanation for this relates to the demonstration that the A1 allele of the DRD2 gene is associated with increased striatal activity of L-amino acid decarboxylase, the final step in the biosynthesis of dopamine. This appears to be a protective mechanism against low receptor density and would favor the utilization of an amino acid neurotransmitter precursor like L-tyrosine for preferential synthesis of dopamine. This seems to lead to receptor proliferation to normal levels and results in significantly better treatment compliance only in A1 carriers.

PROPOSAL AND CONCLUSION

We propose that low D2 receptor density and polymorphisms of the D2 gene are associated with risk for relapse of substance abuse, including alcohol dependence, heroin craving, cocaine dependence, methamphetamine abuse, nicotine sensitization, and glucose craving. With this in mind, we suggest a putative physiological mechanism that may help to explain the enhanced sensitivity following intense acute dopaminergic D2 receptor activation: "denervation supersensitivity." Rats with unilateral depletions of neostriatal dopamine display increased sensitivity to dopamine agonists estimated to be 30 to 100 x in the 6-hydroxydopamine (6-OHDA) rotational model. Given that mild striatal dopamine D2 receptor proliferation occurs (20%-40%), it is difficult to explain the extent of behavioral supersensitivity by a simple increase in receptor density. Thus, the administration of dopamine D2 agonists would target D2 sensitization and attenuate relapse, especially in D2 receptor A1 allele carriers. This hypothesized mechanism is supported by clinical trials utilizing amino acid neurotransmitter precursors, enkephalinase, and catechol-O-methyltransferase (COMT) enzyme inhibition, which have resulted in attenuated relapse rates in reward deficiency syndrome (RDS) probands. If future translational research reveals that dopamine agonist therapy reduces relapse in RDS, it would support the proposed concept, which we term "deprivation-amplification relapse therapy" (DART). This term couples the mechanism for relapse, which is "deprivation-amplification," especially in DRD2 A1 allele carriers with natural D2 agonist therapy utilizing amino acid precursors and COMT and enkepalinase inhibition therapy.

Psychosis pathways converge via D2high dopamine receptors

The objective of this review is to identify a target or biomarker of altered neurochemical sensitivity that is common to the many animal models of human psychoses associated with street drugs, brain injury, steroid use, birth injury, and gene alterations. Psychosis in humans can be caused by amphetamine, phencyclidine, steroids, ethanol, and brain lesions such as hippocampal, cortical, and entorhinal lesions. Strikingly, all of these drugs and lesions in rats lead to dopamine supersensitivity and increase the high-affinity states of dopamine D2 receptors, or D2High, by 200-400% in striata. Similar supersensitivity and D2High elevations occur in rats born by Caesarian section and in rats treated with corticosterone or antipsychotics such as reserpine, risperidone, haloperidol, olanzapine, quetiapine, and clozapine, with the latter two inducing elevated D2High states less than that caused by haloperidol or olanzapine. Mice born with gene knockouts of some possible schizophrenia susceptibility genes are dopamine supersensitive, and their striata reveal markedly elevated D2High states; suchgenes include dopamine-beta-hydroxylase, dopamine D4 receptors, G protein receptor kinase 6, tyrosine hydroxylase, catechol-O-methyltransferase, the trace amine-1 receptor, regulator of G protein signaling RGS9, and the RIIbeta form of cAMP-dependent protein kinase (PKA). Striata from mice that are not dopamine supersensitive did not reveal elevated D2High states; these include mice with knockouts of adenosine A2A receptors, glycogen synthase kinase GSK3beta, metabotropic glutamate receptor 5, dopamine D1 or D3 receptors, histamine H1, H2, or H3 receptors, and rats treated with ketanserin or aD1 antagonist. The evidence suggests that there are multiple pathways that convergetoelevate the D2High state in brain regions and that this elevation may elicit psychosis. This proposition is supported by the dopamine supersensitivity that is a common feature of schizophrenia and that also occurs in many types of genetically altered, drug-altered, and lesion-altered animals. Dopamine supersensitivity, in turn, correlates with D2High states. The finding that all antipsychotics, traditional and recent ones, act on D2High dopamine receptors further supports the proposition.

Time-dependent recovery from the effects of 6-hydroxydopamine lesions of the rat nucleus accumbens on cocaine self-administration and the levels of dopamine in microdialysates

RATIONALE

Neurotoxin induced lesions of dopamine-releasing neurons that innervate the nucleus accumbens (NAcc) alter cocaine self-administration. In addition, elevated extracellular levels of NAcc dopamine (DA) are thought to be central to the biological mechanisms that underlie this behavior.

OBJECTIVES

This study assessed the long-term effects of 6-hydroxydopamine (6-OHDA) induced lesions of the NAcc on cocaine self-administration and the dialysate levels of dopamine ([DA](d)) in this structure to determine if recovery of drug intake was correlated with the DA response.

METHODS

Rats implanted with jugular catheters and bilateral cannulas were trained to self-administer cocaine and subsequently received bilateral intracranial micro-injections of 6-OHDA or vehicle into the NAcc. The levels of DA and cocaine were determined in microdialysates of the NAcc collected during experimental sessions 6-7, 14-16, 29-30, and 44-46 days post-treatment.

RESULTS

The 6-OHDA induced lesions significantly reduced cocaine self-administration for 3 weeks while vehicle treatment had a moderate effect for the first several days. Cocaine-induced increases in NAcc [DA](d) did not return to sham/vehicle treated control levels for 6 weeks in the lesioned group and DA content in the NAcc was 46% of control at 44 days post-lesion.

CONCLUSIONS

Although dopaminergic lesions of the NAcc produced profound effects on cocaine self-administration, responding recovered to control levels before cocaine-induced increases in NAcc [DA](d) while content of DA in the NAcc did not recover. These data suggest that the plasticity of neuronal systems in the NAcc related to cocaine self-administration and their response following 6-OHDA lesions is more complex than restoration of DAergic tone.

Dopamine receptor gene transfer into rat striatum using a recombinant adenoviral vector: rotational behaviour

To study the role of dopamine (DA) receptor expression on dopamine-mediated rotational behaviour, adenovirus expressing the lacZ reporter gene (AdCMVLacZ) or D2R expressing adenoviral vector (AdRSVD2) viruses, mediating expression of beta-galactosidase and DAD2 receptors, respectively, were microinjected stereotactically into Sprague-Dawley rat striatum. Apomorphine stimulated rotational behaviour was measured in rats unilaterally injected with either AdCMVLacZ or AdRSVD2. No significant difference in rotational direction was observed until day 14 post-injection, when animals showed a tendency to rotate away from the injected side. Our data indicate that unilateral changes in receptor density mediated by a non-cell type selective adenoviral vector results in minor changes in rotational behavior. This suggests that supersensitivity in dopamine receptor signaling, rather than receptor levels per se, are the major factor in determining rotational response with dopamine agonist stimulation in unilateral striatal dopamine depleted animals.

N-type calcium channel blockers - tools for modulation of cerebral functional units?

According to in vitro and in vivo studies, the direct application of N-type calcium channel blockers as for instance omega-conotoxin GVIA (omega-ctx) potently inhibits the release of neurotransmitters like dopamine. To find out whether this effect could be used for modulation of neurological functions, omega-ctx was used for continuous infusion into the functionally well characterized rat striatum. Over the 2-week time course of intrastriatal application, rats developed a decrease in spontaneous motor activity, spontaneous rotational asymmetry towards the side of application, and behavioral supersensitivity to apomorphine. After the end of infusion period, all functional deficits showed reversibility. The pattern of spontaneous neurological deficits - in particular supersensitivity to apomorphine - points to a substantial unilateral alteration of dopaminergic transmission due to omega-ctx, which is suggested also by an increase in dopamine receptor protein expression within the ipsilateral striatum. Time course and reversibility of neurological deficits caused by omega-ctx, as well as a lack of dopamine depletion contrast findings after selective destruction of dopaminergic neurons and support a functional modulation of dopaminergic transmission. The present study suggests that omega-ctx is an effective potent tool for the unilateral and reversible intracerebral modulation of neuronal circuits. Intracerebral application of omega-ctx could possibly open the way to therapeutic interventions.

Antisense strategies in dopamine receptor pharmacology

Recent advances in molecular biology have provided pharmacologists the opportunity of developing an entirely new type of agent for studying and treating a variety of biological disorders. These agents, termed antisense oligodeoxynucleotides, have as their target the messenger RNAs encoding specific proteins. They act by binding to selected portions of these mRNAs through complimentary interactions and thereby prevent the synthesis of these proteins. These novel pharmacological tools have the promise of being easier to design and being more selective and predictable in their actions. In addition, insofar as agents targeted to receptors for neurotransmitters are concerned, unlike the classical pharmacological agents, these new compounds may not lead to the upregulation of the very receptors the drugs are designed to inhibit. The present review summarizes briefly studies on the effect of oligodeoxynucleotides antisense to the mRNAs encoding the various subtypes of the dopamine receptor. The studies show that oligodeoxynucleotides antisense to the D2 dopamine receptor when intracerebroventricularly into brains of rodents are rapidly taken up into the brain tissue, distributed to brain cells, and produce effects characteristic of highly selective D2 dopamine antagonists. The compounds also produced specific reductions in the levels of D2 dopamine receptor mRNA and D2 dopamine receptors. Similarly, injecting an antisense oligodeoxynucleotide targeted to the D1 dopamine receptor mRNA produces effects characteristic of D1 dopamine receptor antagonists. Other studies using these agents has produced evidence that there is a small pool of receptors that turn over very rapidly and which constitute the functional pool of these receptors. The evidence suggests further that antisense oligodeoxynucleotides inhibit the synthesis of this small functional pool of dopamine receptors, thereby providing an explanation of why there is often a discordance between changes in dopaminergic function and changes in the levels of dopamine receptors. Studies of antisense oligodeoxynucleotides targeted to the other subtypes of dopamine receptor may help reveal the biological roles that these and other newly discovered subtypes of neurotransmitter receptors have. They may also provide an entirely new and potentially more selective therapeutic regimen for altering the functions of these receptors.

Dopamine autoreceptor sensitivity is unchanged in rat nucleus accumbens after chronic haloperidol treatment: an in vivo and in vitro voltammetric study

Fast cyclic voltammetry was used to assess the effects of chronic oral haloperidol treatment (0.7 mg/kg/day for 21 days) on the sensitivity of dopamine autoreceptors in the rat nucleus accumbens both in vivo and in vitro. Evoked dopamine overflow was significantly reduced after chronic haloperidol treatment, but the sensitivity of dopamine overflow to sulpiride, an antagonist at release-inhibiting dopamine autoreceptors, and quinpirole, an agonist at these receptors, was unchanged. The estimated EC50 values for quinpirole and sulpiride (52 and 60 nM respectively) obtained in vitro and the receptor distribution profiles published in the literature suggest that the autoreceptors involved in this modulation are mainly of the D3 subtype. The finding that the reduced dopamine overflow in the nucleus accumbens observed after chronic treatment with a classical neuroleptic is not due to dopamine autoreceptor supersensitivity may therefore be the first functional evidence for unchanged autoreceptor activity in the nucleus accumbens, supporting biochemical findings of a lack of D3 autoreceptor up-regulation after chronic haloperidol treatment. It lends further support to the assumption that the long-term changes occurring during chronic neuroleptic treatment may not lie at the level of presynaptic dopamine receptor regulation.

Amphetamine sensitivity in open-field activity vs. the prepulse inhibition paradigm

Amphetamine-induced mesolimbic dopamine release has been reported to reduce prepulse inhibition of the acoustic startle response. In addition, it is well known that mesolimbic dopamine stimulation leads to hyperactivity. The present study was undertaken to explore the possibility that one or the other measure may be a more sensitive in vivo indicator of dopamine release in the nucleus accumbens by determining if the amphetamine dose-response curves for these two behavioral measures were different. The data indicate that the dose-response curves obtained for the different behavioral measures are identical. These data are consistent with the idea that the same dopamine terminal field supports both prepulse inhibiton of the acoustic startle response and dopamine-stimulated hyperactivity.