Dopamine Hypothesis Research Articles

Dysfunction of the NMDA Receptor in the Pathophysiology of Schizophrenia and/or the Pathomechanisms of Treatment-Resistant Schizophrenia.

For several decades, the dopamine hypothesis contributed to the discovery of numerous typical and atypical antipsychotics and was the sole hypothesis for the pathophysiology of schizophrenia. However, neither typical nor atypical antipsychotics, other than clozapine, have been effective in addressing negative symptoms and cognitive impairments, which are indices for the prognostic and disability outcomes of schizophrenia. Following the development of atypical antipsychotics, the therapeutic targets for antipsychotics expanded beyond the blockade of dopamine D2 and serotonin 5-HT2A receptors to explore the partial agonism of the D2 receptor and the modulation of new targets, such as D3, 5-HT1A, 5-HT7, and metabotropic glutamate receptors. Despite these efforts, to date, psychiatry has not successfully developed antipsychotics with antipsychotic properties proven to be superior to those of clozapine. The glutamate hypothesis, another hypothesis regarding the pathophysiology/pathomechanism of schizophrenia, was proposed based on clinical findings that N-methyl-D-aspartate glutamate receptor (NMDAR) antagonists, such as phencyclidine and ketamine, induce schizophrenia-like psychotic episodes. Large-scale genome-wide association studies (GWASs) revealed that approximately 30% of the risk genes for schizophrenia (the total number was over one hundred) encode proteins associated with glutamatergic transmission. These findings supported the validation of the glutamate hypothesis, which was inspired by the clinical findings regarding NMDAR antagonists. Additionally, these clinical and genetic findings suggest that schizophrenia is possibly a syndrome with complicated pathomechanisms that are affected by multiple biological and genetic vulnerabilities. The glutamate hypothesis has been the most extensively investigated pathophysiology/pathomechanism hypothesis, other than the dopamine hypothesis. Studies have revealed the possibility that functional abnormalities of the NMDAR play important roles in the pathophysiology/pathomechanism of schizophrenia. However, no antipsychotics derived from the glutamatergic hypothesis have yet been approved for the treatment of schizophrenia or treatment-resistant schizophrenia. Considering the increasing evidence supporting the potential pro-cognitive effects of glutamatergic agents and the lack of sufficient medications to treat the cognitive impairments associated with schizophrenia, these previous setbacks cannot preclude research into potential novel glutamate modulators. Given this background, to emphasize the importance of the dysfunction of the NMDAR in the pathomechanism and/or pathophysiology of schizophrenia, this review introduces the increasing findings on the functional abnormalities in glutamatergic transmission associated with the NMDAR.

Current treatments in schizophrenia

The understanding of schizophrenia has remarkably expanded over the past few decades, moving beyond the dopamine hypothesis to encompass neurotransmitter interplays, structural brain changes and genetic and environmental determinants. This has led to innovations in therapeutic interventions that offer improved outcomes. This paper explores past and current treatments available for schizophrenia, the latest pharmacological advances and the significance of psychotherapeutic approaches.

The Underlying Neurobiological Mechanisms of Psychosis: Focus on Neurotransmission Dysregulation, Neuroinflammation, Oxidative Stress, and Mitochondrial Dysfunction.

Psychosis, defined as a set of symptoms that results in a distorted sense of reality, is observed in several psychiatric disorders in addition to schizophrenia. This paper reviews the literature relevant to the underlying neurobiology of psychosis. The dopamine hypothesis has been a major influence in the study of the neurochemistry of psychosis and in development of antipsychotic drugs. However, it became clear early on that other factors must be involved in the dysfunction involved in psychosis. In the current review, it is reported how several of these factors, namely dysregulation of neurotransmitters [dopamine, serotonin, glutamate, and γ-aminobutyric acid (GABA)], neuroinflammation, glia (microglia, astrocytes, and oligodendrocytes), the hypothalamic-pituitary-adrenal axis, the gut microbiome, oxidative stress, and mitochondrial dysfunction contribute to psychosis and interact with one another. Research on psychosis has increased knowledge of the complexity of psychotic disorders. Potential new pharmacotherapies, including combinations of drugs (with pre- and probiotics in some cases) affecting several of the factors mentioned above, have been suggested. Similarly, several putative biomarkers, particularly those related to the immune system, have been proposed. Future research on both pharmacotherapy and biomarkers will require better-designed studies conducted on an all stages of psychotic disorders and must consider confounders such as sex differences and comorbidity.

Novel Insights into Psychosis and Antipsychotic Interventions: From Managing Symptoms to Improving Outcomes.

For the past 70 years, the dopamine hypothesis has been the key working model in schizophrenia. This has contributed to the development of numerous inhibitors of dopaminergic signaling and antipsychotic drugs, which led to rapid symptom resolution but only marginal outcome improvement. Over the past decades, there has been limited research on the quantifiable pathological changes in schizophrenia, including premature cellular/neuronal senescence, brain volume loss, the attenuation of gamma oscillations in electroencephalograms, and the oxidation of lipids in the plasma and mitochondrial membranes. We surmise that the aberrant activation of the aryl hydrocarbon receptor by toxins derived from gut microbes or the environment drives premature cellular and neuronal senescence, a hallmark of schizophrenia. Early brain aging promotes secondary changes, including the impairment and loss of mitochondria, gray matter depletion, decreased gamma oscillations, and a compensatory metabolic shift to lactate and lactylation. The aim of this narrative review is twofold: (1) to summarize what is known about premature cellular/neuronal senescence in schizophrenia or schizophrenia-like disorders, and (2) to discuss novel strategies for improving long-term outcomes in severe mental illness with natural senotherapeutics, membrane lipid replacement, mitochondrial transplantation, microbial phenazines, novel antioxidant phenothiazines, inhibitors of glycogen synthase kinase-3 beta, and aryl hydrocarbon receptor antagonists.

The Dopamine Hypothesis as a Causal Explanation of Schizophrenia

Schizophrenia, a severe psychiatric disorder, profoundly impacts individuals, families, and society, characterized by symptoms such as delusions, hallucinations, disorganized thinking, speech, and motor behavior, as outlined in the DSM-5 criteria. Coined by Eugen Bleuler in 1911 to emphasize fragmented cognition, the term "schizophrenia" replaced Emile Kraepelin's "dementia praecox"(1908) reflecting its chronic nature, with an estimated prevalence of 1% in the general population and a significant heritability rate of around 79%. The dopamine hypothesis, central to schizophrenia research, suggests heightened dopaminergic transmission as a primary factor in its development, supported by the efficacy of antipsychotic drugs targeting dopamine receptors. However, recent studies have revealed complexities beyond dopamine dysfunction, including the glutamate hypothesis, which proposes deficits in glutamate activity as an alternative explanation. This essay critically evaluates the dopamine hypothesis within the broader biopsychosocial framework, emphasizing the interaction of biological, psychological, and social factors in schizophrenia etiology. Despite its foundational role, the dopamine hypothesis has limitations in fully clarifying the multifaceted nature of schizophrenia, highlighting the need for comprehensive approaches integrating diverse perspectives and methodologies to enhance understanding of this complex disorder. Keywords: Dopamine Hypothesis, Schizophrenia, Mental Illness, Psychotic Symptoms, GENE x ENVIROMENT Interaction

The New Horizon of Antipsychotics beyond the Classic Dopaminergic Hypothesis-The Case of the Xanomeline-Trospium Combination: A Systematic Review.

Although the dopamine hypothesis of schizophrenia explains the effects of all the available antipsychotics in clinical use, there is an increasing need for developing new drugs for the treatment of the positive, negative, and cognitive symptoms of chronic psychoses. Xanomeline-trospium (KarXT) is a drug combination that is based on the essential role played by acetylcholine in the regulation of cognitive processes and the interactions between this neurotransmitter and other signaling pathways in the central nervous system, with a potential role in the onset of schizophrenia, Alzheimer's disease, and substance use disorders. A systematic literature review that included four electronic databases (PubMed, Cochrane, Clarivate/Web of Science, and Google Scholar) and the US National Library of Medicine database for clinical trials detected twenty-one sources referring to fourteen studies focused on KarXT, out of which only four have available results. Based on the results of these trials, the short-term efficacy and tolerability of xanomeline-trospium are good, but more data are needed before this drug combination may be recommended for clinical use. However, on a theoretical level, the exploration of KarXT is useful for increasing the interest of researchers in finding new, non-dopaminergic, antipsychotics that could be used either as monotherapy or as add-on drugs.

Current Perspectives on Attention-deficit Hyperactivity Disorder.

Attention-deficit hyperactivity disorder (ADHD) is a neurobiological and neurodevelopmental disorder with an idiosyncratic genetic base. ADHD presents various characteristics, such as inattention, hyperactivity, and impulsivity. Over the period, ADHD leads to noticeable functional disability. A five- to ten-fold progressed risk of disorder development is observed in the populations with familial history of ADHD. The abnormal structure of the brain in ADHD results in altered neural mechanisms, such as cognition, attention, and memorial function. The mesolimbic, nigrostriatal, and mesocortical pathways in the brain get affected by the deterioration of the levels of dopamine. The hypothesis of dopamine in ADHD and its etiopathology suggests that detained attention and impaired arousal functions are due to reduced levels of dopamine. The quickest way to improve strategical treatment is by clarifying the etiological aspects of ADHD and identifying the underlying mechanisms of pathophysiology, which will assist in exploring the biomarkers for better diagnosis. The implementation of life course theory is a very important research principle announced by Grand Challenges in Global Health Initiative (GCMHI). Long-term research is needed to define the progression of ADHD. Interdisciplinary collaborations promise a great future for research innovations in ADHD.

Understanding the Impact of the Dopamine Hypothesis on Schizophrenia

Understanding the Impact of the Dopamine Hypothesis on Schizophrenia

Dynamic aberrances of substantia nigra-relevant coactivation patterns in first-episode treatment-naïve patients with schizophrenia.

Although dopaminergic disturbances are well-known in schizophrenia, the understanding of dopamine-related brain dynamics remains limited. This study investigates the dynamic coactivation patterns (CAPs) associated with the substantia nigra (SN), a key dopaminergic nucleus, in first-episode treatment-naïve patients with schizophrenia (FES). Resting-state fMRI data were collected from 84 FES and 94 healthy controls (HCs). Frame-wise clustering was implemented to generate CAPs related to SN activation or deactivation. Connectome features of each CAP were derived using an edge-centric method. The occurrence for each CAP and the balance ratio for antagonistic CAPs were calculated and compared between two groups, and correlations between temporal dynamic metrics and symptom burdens were explored. Functional reconfigurations in CAPs exhibited significant differences between the activation and deactivation states of SN. During SN activation, FES more frequently recruited a CAP characterized by activated default network, language network, control network, and the caudate, compared to HCs (F = 8.54, FDR-p = 0.030). Moreover, FES displayed a tilted balance towards a CAP featuring SN-coactivation with the control network, caudate, and thalamus, as opposed to its antagonistic CAP (F = 7.48, FDR-p = 0.030). During SN deactivation, FES exhibited increased recruitment of a CAP with activated visual and dorsal attention networks but decreased recruitment of its opposing CAP (F = 6.58, FDR-p = 0.034). Our results suggest that neuroregulatory dysfunction in dopaminergic pathways involving SN potentially mediates aberrant time-varying functional reorganizations in schizophrenia. This finding enriches the dopamine hypothesis of schizophrenia from the perspective of brain dynamics.

New Therapeutic Targets and Drugs for Schizophrenia Beyond Dopamine D2 Receptor Antagonists.

Schizophrenia is a disease with a complex pathological mechanism that is influenced by multiple genes. The study of its pathogenesis is dominated by the dopamine hypothesis, as well as other hypotheses such as the 5-hydroxytryptamine hypothesis, glutamate hypothesis, immune-inflammatory hypothesis, gene expression abnormality hypothesis, and neurodevelopmental abnormality hypothesis. The first generation of antipsychotics was developed based on dopaminergic receptor antagonism, which blocks dopamine D2 receptors in the brain to exert antipsychotic effects. The second generation of antipsychotics acts by dual blockade of 5-hydroxytryptamine and dopamine receptors. From the third generation of antipsychotics onwards, the therapeutic targets for antipsychotic schizophrenia expanded beyond D2 receptor blockade to explore D2 receptor partial agonism and the antipsychotic effects of new targets such as D3, 5-HT1A, 5-HT7, and mGlu2/3 receptors. The main advantages of the second and third generation antipsychotics over first-generation antipsychotics are the reduction of side effects and the improvement of negative symptoms, and even though third-generation antipsychotics do not directly block D2 receptors, the modulation of the dopamine transmitter system is still an important part of their antipsychotic process. According to recent research, several receptors, including 5-hydroxytryptamine, glutamate, γ-aminobutyric acid, acetylcholine receptors and norepinephrine, play a role in the development of schizophrenia. Therefore, the focus of developing new antipsychotic drugs has shifted towards agonism or inhibition of these receptors. Specifically, the development of NMDARs stimulants, GABA receptor agonists, mGlu receptor modulators, cholinergic receptor modulators, 5-HT2C receptor agonists and alpha-2 receptor modulators has become the main direction. Animal experiments have confirmed the antipsychotic effects of these drugs, but their pharmacokinetics and clinical applicability still require further exploration. Research on alternative targets for antipsychotic drugs, beyond the dopamine D2 receptor, has expanded the potential treatment options for schizophrenia and gives an important way to address the challenge of refractory schizophrenia. This article aims to provide a comprehensive overview of the research on therapeutic targets and medications for schizophrenia, offering valuable insights for both treatment and further research in this field.

Stress, Dopamine and the Development of Schizophrenia

Dopaminergic dysregulation and dysfunction have long been seen as the key factors of the onset of psychosis. The dopamine hypothesis postulates hyperactivity of subcortical dopamine release and hypoactivity of cortical dopamine transmission in schizophrenia, indicating the relationship between irregular dopamine activity and the development of positive and negative symptoms of schizophrenia. Meanwhile, psychological stressors affect the brain in multiple ways, including changing the structure of neurons and causing malfunction of the dopamine system. These changes are also critical in causing psychotic experiences. Despite there are abundant studies and findings investigating either the relationship between dopamine and schizophrenia, or the relationship between schizophrenia and stress, few of them have put these three concepts together in discussion. This paper aims at investigating how abnormal dopamine functions, stress and schizophrenia are related to each other, which helps generate a deeper and clearer understanding of the disease from a broader view. The relationship between inappropriate dopamine release and the pathophysiology of schizophrenia is discussed, particularly talks about the dopamine transmission in striatal and cortical regions of the brain, it proceeds into an investigation of the association between stress and schizophrenia from both physiological and sociological scopes, and then summarizes the current ways of treating schizophrenia provided along with the future prospect in developing treatments for this disorder.

“D-Cell Hypothesis of Schizophrenia” Leading to Develop Non-D2 Receptor-Binding Antipsychotics

The latest psychopharmacological study showed effectiveness of a novel non-D2-receptor-binding drug, SEP-363856, for treatment of schizophrenia. The compound is trace amine-associated receptor 1 (TAAR1) full agonist and also 5-hydroxytryptamin 1A (5-HT 1A) receptor partial agonist. I found the TAAR1 ligand neuron, D-neuron, in the striatum and nucleus accumbens (Acc), a neuroleptic acting site, of human brains, though they were scarcely found in the monkey, and D-neuron reduction in these regions by using post-mortem brains of patients with schizophrenia. The reduction of D-neurons was significant (t-test, p<0.05) in Acc of schizophrenia. I proposed “D-cell hypothesis of schizophrenia”, a link of dopamine (DA) hypothesis and neural stem cell (NSC) dysfunction hypothesis, showing NSC dysfunction-based D-neuron reduction being cellular basis of mesolimbic DA hyperactivity of schizophrenia, and predicted prospectiveness of TAAR1 ligands for treatment of schizophrenia. This hypothesis has been verified by effectiveness of a TAAR1 agonist in reducing symptoms of schizophrenia.

Fronto-striato-thalamic circuit connectivity and neuromelanin in schizophrenia: an fMRI and neuromelanin-MRI study

Changes in dopamine and fronto-striato-thalamic (FST) circuit functional connectivity are prominent in schizophrenia. Dopamine is thought to underlie connectivity changes, but experimental evidence for this hypothesis is lacking. Previous studies examined the association in some of the connections using positron emission tomography (PET) and functional MRI (fMRI); however, PET has disadvantages in scanning patients, such as invasiveness. Excessive dopamine induces neuromelanin (NM) accumulation, and NM-MRI is suggested as a noninvasive proxy measure of dopamine function. We aimed to investigate the association between NM and FST circuit connectivity at the network level in patients with schizophrenia. We analysed substantia nigra NM-MRI and resting-state fMRI data from 29 schizophrenia patients and 63 age- and sex-matched healthy controls (HCs). We identified the FST subnetwork with abnormal connectivity found in schizophrenia patients compared to that of HCs and investigated the relationship between constituting connectivity and NM-MRI signal. We found a higher NM signal (t = −2.12, p = 0.037) and a hypoconnected FST subnetwork (FWER-corrected p = 0.014) in schizophrenia patients than in HCs. In the hypoconnected subnetwork of schizophrenia patients, lower left supplementary motor area-left caudate connectivity was associated with a higher NM signal (β = −0.38, p = 0.042). We demonstrated the association between NM and FST circuit connectivity. Considering that the NM-MRI signal reflects dopamine function, our results suggest that dopamine underlies changes in FST circuit connectivity, which supports the dopamine hypothesis. In addition, this study reveals implications for the future use of NM-MRI in investigations of the dopamine system.

FRI302 Umar Durrani

Abstract Disclosure: U.S. Durrani: None. A. Paracha: None. S. Vasireddy: None. F. Waheed: None. M. Thomure: None. Prolactinomas are tumors of the pituitary gland that induce hyperprolactinemia. Symptoms at presentation can include growth delay, infertility, neurologic deficits and bothersome galactorrhea. Dopamine suppresses prolactin production; therefore, dopamine agonists are first-line treatment for prolactinomas. However, hyperdopaminergia is also associated with psychiatric disorders. The dopamine hypothesis states that cerebral hyperdopaminergia can contribute to symptoms of schizophrenia. Thus, psychiatric disorders are commonly treated with dopamine antagonists. Almost 1 in 8 people in the world have a psychiatric condition. It is almost inevitable that the medical system will have to treat patients with both a psychiatric condition and a prolactinoma; thus, it is essential to identify treatment regimens for these patients. In order to identify potential treatments, we conducted a PubMed literature search focusing on patients with prolactinomas and co-existing psychiatric diagnoses. Our inclusion criteria helped us focus on patients who presented with concurrent prolactinoma and psychiatric condition which were confirmed by brain imaging, serologic prolactin levels, and reports of psychiatric conditions/episodes in their medical history (or chart). Our search yielded 30 studies representing a total of 54 patients with both conditions. We found that four key themes emerged from the literature: 1) discontinuing risperidone in exchange for other atypical antipsychotics such as aripiprazole, olanzapine or ziprasidone, 2) discontinuing thioridazine, thiothixene and remoxipride in exchange for clozapine, 3) dopamine agonist therapy cessation to abate psychiatric symptoms, and 4) surgery and/or radiation after pharmacotherapy. In conclusion, we would recommend focusing on specific antipsychotics (aripiprazole, olanzapine, ziprasidone, or clozapine) while shying away from (risperidone, thioridazine, thiothixene, and remoxipride). We would also recommend ceasing dopamine agonist therapy at least until symptoms improve. If these two methods are not yielding sufficient results, then pursuing surgical options or radiation therapy may be considered. Presentation: Friday, June 16, 2023

Six Decades of Dopamine Hypothesis: Is Aryl Hydrocarbon Receptor the New D2?

In 1957, Arvid Carlsson discovered that dopamine, at the time believed to be nothing more than a norepinephrine precursor, was a brain neurotransmitter in and of itself. By 1963, postsynaptic dopamine blockade had become the cornerstone of psychiatric treatment as it appeared to have deciphered the “chlorpromazine enigma”, a 1950s term, denoting the action mechanism of antipsychotic drugs. The same year, Carlsson and Lindqvist launched the dopamine hypothesis of schizophrenia, ushering in the era of psychopharmacology. At present, six decades later, although watered down by three consecutive revisions, the dopamine model remains in vogue. The latest emendation of this paradigm proposes that “environmental and genetic factors” converge on the dopaminergic pathways, upregulating postsynaptic transmission. Aryl hydrocarbon receptors, expressed by the gut and blood–brain barrier, respond to a variety of endogenous and exogenous ligands, including dopamine, probably participating in interoceptive awareness, a feed-back loop, conveying intestinal barrier status to the insular cortex. The conceptualization of aryl hydrocarbon receptor as a bridge, connecting vagal terminals with the microbiome, may elucidate the aspects of schizophrenia seemingly incongruous with the dopamine hypothesis, such as increased prevalence in urban areas, distance from the equator, autoantibodies, or comorbidity with inflammatory bowel disease and human immunodeficiency 1 virus. In this review article, after a short discussion of schizophrenia outcome studies and insight, we take a closer look at the action mechanism of antipsychotic drugs, attempting to answer the question: do these agents exert their beneficial effects via both dopaminergic and nondopaminergic mechanisms? Finally, we discuss potential new therapies, including transcutaneous vagal stimulation, aryl hydrocarbon receptor ligands, and restoring the homeostasis of the gut barrier.

Taq1A polymorphism in patients with bipolar disorder: A candidate gene study based on the dopamine hypothesis

Taq1A polymorphism in patients with bipolar disorder: A candidate gene study based on the dopamine hypothesis

Xanomeline-Trospium and Muscarinic Involvement in Schizophrenia.

Schizophrenia is a severe mental illness that has its onset in late adolescence or early adulthood and is associated with significant dysfunction across multiple domains. The pathogenesis of schizophrenia remains unknown, but physiologic understanding of the illness has been driven by the dopamine hypothesis. However, acetylcholine (ACh) clearly plays a role with mixed results regarding effect on psychosis. Selective muscarinic M1 and M4 agonists, such as xanomeline, originally developed to aid in cognitive loss with Alzheimer's, showed promise in proof-of-concept study in 20 patients with schizophrenia. Unfortunately, tolerability problems made muscarinic agonists impractical in either condition. However, coadministration of trospium, a lipophobic, non-selective muscarinic antagonist previously used for the treatment of overactive bladder, with xanomeline resulted in a significant reduction of cholinergic adverse effects. A recent randomized, placebo-controlled study of the antipsychotic effects of this combination in 182 patients with acute psychosis revealed improved tolerability with 80% of subjects staying to the end of the 5 weeks study. At the end of the trial, the treatment group saw a -17.4 change in the positive and negative symptom scale (PANSS) score from baseline compared to a -5.9 change in the placebo arm (P < 0.001). Furthermore, the negative symptom subscore, was also superior in the active arm (P < 0.001). These early studies are exciting because they suggest that the cholinergic system may be recruited to treat a severe and disabling disorder with suboptimal treatment options. Xanomeline-trospium combination is currently in phase III studies.

Age Differences in Motivated Cognition: AMeta-Analysis.

The goal of this preregistered study was to synthesize empirical findings on age differences in motivated cognition using a meta-analytic approach, with a focus on the domains of cognitive control and episodic memory. A systematic search of articles published before July 2022 yielded 27 studies of cognitive control (N = 1,908) and 73 studies of memory (N = 5,837). Studies had to include healthy younger and older adults, a within-subjects or between-subjects comparison of motivation (high vs low), and a measure of cognitive control or memory. The Age × Motivation effect size was meta-analyzed using random-effects models, and moderators were examined using meta-regressions and subgroup analyses. Overall, the Age × Motivation interaction was not significant in either cognitive domain, but the effect sizes in both domains were significantly heterogeneous, indicating a possible role of moderating factors in accounting for effect size differences. Moderator analyses revealed significant moderation by incentive type for episodic memory, but not for cognitive control. Older adults' memory was more sensitive to socioemotional rewards, whereas younger adults' memory was more sensitive to financial gains. Findings are discussed with reference to the dopamine hypothesis of cognitive aging and to life-span theories of motivational orientation. None of these theories is fully supported by the meta-analysis findings, highlighting the need for an integration of neurobiological, cognitive process, and life-span-motivational perspectives.

Re: The low dopamine hypothesis: A plausible mechanism underpinning Residual urine, Overactive bladder and Nocturia (RON) syndrome in older patients.

Re: The low dopamine hypothesis: A plausible mechanism underpinning Residual urine, Overactive bladder and Nocturia (RON) syndrome in older patients.

Evidence for upregulation of excitatory synaptic transmission in the substantia nigra in Schizophrenia: a postmortem ultrastructural study.

The dopamine hypothesis of schizophrenia suggests that psychotic symptoms originate from dysregulation of dopaminergic activity, which may be controlled by upstream innervation. We hypothesized that we would find anatomical evidence for the hyperexcitability seen in the SN. We examined and quantified synaptic morphology, which correlates with function, in the postmortem substantia nigra (SN) from 15 schizophrenia and 12 normal subjects. Synapses were counted using stereological techniques and classified based on the morphology of the post-synaptic density (PSD) and the presence or absence of a presynaptic density. The density and proportion of excitatory synapses was higher in the schizophrenia group than in controls, while the proportion (but not density) of inhibitory synapses was lower. We also detected in the schizophrenia group an increase in density of synapses with a PSD of intermediate thickness, which may represent excitatory synapses. The density of synapses with presynaptic densities was similar in both groups. The density of synapses with mixed morphologies was higher in the schizophrenia group than in controls. The human SN contains atypical synaptic morphology. We found an excess amount and proportion of excitatory synapses in the SN in schizophrenia that could result in hyperactivity and drive the psychotic symptoms of schizophrenia. The sources of afferent excitatory inputs to the SN arise from the subthalamic nucleus, the pedunculopontine nucleus, and the ventral tegmental area (VTA), areas that could be the source of excess excitation. Synapses with mixed morphologies may represent inputs from the VTA, which release multiple transmitters.