Sridhar Natesan

Converging lines of evidence implicate synaptic loss in cognitive impairment associated with schizophrenia. However, it remains unknown whether synaptic terminal density and premorbid intellectual functioning are related in vivo, or whether there are age-related changes in them in schizophrenia. To address this, we investigated whole brain grey matter synaptic vesicle glycoprotein 2A (SV2A) levels and examined their relationship with intellectual functioning and age, in forty-three patients with schizophrenia (SCZ) and 26 healthy volunteers (HV), using [ ¹¹ C]UCB-J positron emission tomography (PET). Whole brain grey matter [ ¹¹ C]UCB-J distribution volume ratio (DVR) was significantly lower in the SCZ than the HV group (Cohen’s d = 0.64, p = 0.01), and negatively correlated with age in both groups (Spearman’s rho = –0.46 to –0.55), with no significant group difference in magnitude of DVR-age correlations ( z = 0.44, p = 0.66). Current (Cohen’s d = 0.73) and premorbid IQ (Cliff’s delta = 0.37) were significantly lower in the SCZ than the HV group, though DVR was not significantly associated with current or premorbid IQ in either group (including in chronic medicated and early-course unmedicated SCZ subgroups). The group differences in DVR are consistent with a global deficit in synaptic terminal density in schizophrenia, with similar age-related changes in people with schizophrenia and healthy volunteers. The lack of significant relationships between DVR and premorbid or current cognitive measures are not consistent with the hypothesis that lower levels of synaptic terminal density observed in schizophrenia underlie lower levels of intellectual functioning in the disorder.

Loss of glutamatergic terminals is hypothesised to contribute to excitation-inhibition imbalance in schizophrenia, supported by evidence that the normal positive association between glutamate concentrations and synaptic terminal density is not found in patients with chronic schizophrenia. However, it is unknown whether the relationship between synaptic terminal density and glutamate levels is altered early in the course of illness. To address this, we investigated [¹¹C]UCB-J distribution volume ratio (DVR) and glutamatergic markers in healthy volunteers (HV) and in antipsychotic-naïve/free patients with schizophrenia (SCZ) recruited from first-episode psychosis services. Forty volunteers (HV n = 19, SCZ n = 21) underwent [¹¹C]UCB-J positron emission tomography and proton magnetic resonance spectroscopy (¹H-MRS) imaging in the anterior cingulate cortex (ACC) and left hippocampus to index [¹¹C]UCB-J DVR and creatine-scaled glutamate (Glu/Cr) and glutamate in combination with glutamine (Glx/Cr). In the HV but not SCZ group, [¹¹C]UCB-J DVR was significantly positively associated with Glu/Cr (Spearman’s rho = 0.55, p = 0.02) and Glx/Cr (Spearman’s rho = 0.73, p = 0.0004) in the ACC, and with Glu/Cr in the left hippocampus (Spearman’s rho = 0.77, p = 0.0001). DVR was significantly lower in the ACC in the SCZ group compared to the HV group (Kolmogorov-Smirnov Z = 1.44, p = 0.03). Together, these findings indicate that the normal relationship between levels of a synaptic terminal density marker and levels of glutamate is disrupted early in the course of schizophrenia. This is consistent with the hypothesis that there is loss of glutamatergic terminals at illness onset.

Purpose: Elevated dopamine synthesis capacity is part of the pathophysiology of schizophrenia thought to underlie psychosis. Drugs that reduce this phenomenon could thus be potential treatments for these disorders. In this study, we evaluated the ability of the trace amine-associated receptor 1 (TAAR1) partial agonist ralmitaront to reduce presynaptic dopamine synthesis capacity. Procedures: Ralmitaront (3 mg/kg, i.p.), a TAAR1 partial agonist, was evaluated using [¹⁸F]DOPA PET for its ability to modulate presynaptic dopamine synthesis capacity in naïve mice as well as mice in an induced hyperdopaminergic state following acute cocaine administration (20 mg/kg, i.p.). Results: Cocaine treatment on its own did not induce elevated dopamine synthesis capacity when compared to the control group. Pretreatment with ralmitaront significantly reduced dopamine synthesis capacity when given either alone (44%) or in combination with the psychostimulant cocaine (50%) when compared to the control group. Conclusions: The TAAR1 agonist ralmitaront reduces striatal dopamine synthesis capacity, indexed as Ki^Mod, both in naïve animals and when given prior to acute cocaine. This indicates the potential of TAAR1 agonism to address disorders characterized by striatal hyperdopaminergia.

Summary: Valproate is widely used in psychiatry and neurology, including off-label use. Here we consider its potential benefits and risks, particularly for women of childbearing potential, and the evidence that clinical guidelines are adhered to. Finally, we consider the implications for clinical practice and research into its efficacy in off-label indications.

Positron emission tomography (PET) using the radiotracer [¹⁸F]-FDOPA provides a tool for studying brain dopamine synthesis capacity in animals and humans. We have previously standardised a micro-PET methodology in mice by intravenously administering [¹⁸F]-FDOPA via jugular vein cannulation and assessment of striatal dopamine synthesis capacity, indexed as the influx rate constant [ ] of [¹⁸F]-FDOPA, using an extended graphical Patlak analysis with the cerebellum as a reference region. This enables a direct comparison between preclinical and clinical output values. However, chronic intravenous catheters are technically difficult to maintain for longitudinal studies. Hence, in this study, intraperitoneal administration of [¹⁸F]-FDOPA was evaluated as a less-invasive alternative that facilitates longitudinal imaging. Our experiments comprised the following assessments: (i) comparison of [¹⁸F]-FDOPA uptake between intravenous and intraperitoneal radiotracer administration and optimisation of the time window used for extended Patlak analysis, (ii) comparison of [ ] in a within-subject design of both administration routes, (iii) test-retest evaluation of [ ] in a within-subject design of intraperitoneal radiotracer administration, and (iv) validation of [ ] estimates by comparing the two administration routes in a mouse model of hyperdopaminergia induced by subchronic ketamine. Our results demonstrate that intraperitoneal [¹⁸F]-FDOPA administration resulted in good brain uptake, with no significant effect of administration route on [ ] estimates (intraperitoneal: [ ], intravenous: [ ], [ ]) and similar coefficient of variation (intraperitoneal: 19.6%; intravenous: 18.4%). The technique had a moderate test-retest validity ([ ], [ ]) and thus supports longitudinal studies. Following subchronic ketamine administration, elevated [ ] as compared to control condition was measured with a large effect size for both methods (intraperitoneal: Cohen’s [ ]; intravenous: Cohen’s [ ]), providing further evidence that ketamine has lasting effects on the dopamine system, which could contribute to its therapeutic actions and/or abuse liability.

Glutamatergic excitotoxicity is hypothesised to underlie synaptic loss in schizophrenia pathogenesis, but it is unknown whether synaptic markers are related to glutamatergic function in vivo. Additionally, it has been proposed that N-acetyl aspartate (NAA) levels reflect neuronal integrity. Here, we investigated whether synaptic vesicle glycoprotein 2 A (SV2A) levels are related to glutamatergic markers and NAA in healthy volunteers (HV) and schizophrenia patients (SCZ). Forty volunteers (SCZ n = 18, HV n = 22) underwent [¹¹C]UCB-J positron emission tomography and proton magnetic resonance spectroscopy (¹H-MRS) imaging in the left hippocampus and anterior cingulate cortex (ACC) to index [¹¹C]UCB-J distribution volume ratio (DVR), and creatine-scaled glutamate (Glu/Cr), glutamate and glutamine (Glx/Cr) and NAA (NAA/Cr). In healthy volunteers, but not patients, [¹¹C]UCB-J DVR was significantly positively correlated with Glu/Cr, in both the hippocampus and ACC. Furthermore, in healthy volunteers, but not patients, [¹¹C]UCB-J DVR was significantly positively correlated with Glx/Cr, in both the hippocampus and ACC. There were no significant relationships between [¹¹C]UCB-J DVR and NAA/Cr in the hippocampus or ACC in healthy volunteers or patients. Therefore, an appreciable proportion of the brain ¹H-MRS glutamatergic signal is related to synaptic density in healthy volunteers. This relationship is not seen in schizophrenia, which, taken with lower synaptic marker levels, is consistent with lower levels of glutamatergic terminals and/or a lower proportion of glutamatergic relative to GABAergic terminals in the ACC in schizophrenia.

Patients with schizophrenia show increased striatal dopamine synthesis capacity in imaging studies. The mechanism underlying this is unclear but may be due to N-methyl-D-aspartate receptor (NMDAR) hypofunction and parvalbumin (PV) neuronal dysfunction leading to disinhibition of mesostriatal dopamine neurons. Here, we develop a translational mouse model of the dopamine pathophysiology seen in schizophrenia and test approaches to reverse the dopamine changes. Mice were treated with sub-chronic ketamine (30 mg/kg) or saline and then received in vivo positron emission tomography of striatal dopamine synthesis capacity, analogous to measures used in patients. Locomotor activity was measured using the open-field test. In vivo cell-type-specific chemogenetic approaches and pharmacological interventions were used to manipulate neuronal excitability. Immunohistochemistry and RNA sequencing were used to investigate molecular mechanisms. Sub-chronic ketamine increased striatal dopamine synthesis capacity (Cohen’s d = 2.5) and locomotor activity. These effects were countered by inhibition of midbrain dopamine neurons, and by activation of PV interneurons in pre-limbic cortex and ventral subiculum of the hippocampus. Sub-chronic ketamine reduced PV expression in these cortical and hippocampal regions. Pharmacological intervention with SEP-363856, a novel psychotropic agent with agonism at trace amine receptor 1 (TAAR1) and 5-HT_1A receptors but no appreciable action at dopamine D₂ receptors, significantly reduced the ketamine-induced increase in dopamine synthesis capacity. These results show that sub-chronic ketamine treatment in mice mimics the dopaminergic alterations in patients with psychosis, that this requires activation of midbrain dopamine neurons, and can be ameliorated by activating PV interneurons and by a TAAR1/5-HT_1A agonist. This identifies novel therapeutic approaches for targeting presynaptic dopamine dysfunction in patients with schizophrenia and effects of ketamine relevant to its therapeutic use for treating major depression.

The pathophysiology of the majority of neuropsychiatric disorders, including schizophrenia and mood disorders, involves synaptic dysfunction and/or loss, manifesting as lower levels of several presynaptic and postsynaptic marker proteins. Whether chronic exposure to antipsychotic drugs may contribute to this pattern of synaptic loss remains controversial. In contrast, the mood stabiliser lithium has shown to exhibit neurotrophic actions and is thought to enhance synapse formation. Whilst these data are not unequivocal, they suggest that antipsychotic drugs and lithium have contrasting effects on synapse density. We therefore investigated the effect of chronic exposure to lithium and to two different antipsychotics, haloperidol and olanzapine, on presynaptic Synaptic Vesicle glycoprotein 2A (SV2A) and postsynaptic Neuroligin (NLGN) clusters in the rat frontal cortex. Chronic exposure (28 days) to haloperidol (0.5 mg/kg/d) or olanzapine (7.5 mg/kg/d) had no effect on either SV2A or NLGN clusters and no overall effect on synaptic clusters. In contrast, chronic lithium exposure (2 mmol/L eq./d) significantly increased NLGN cluster density as compared to vehicle, but did not affect either SV2A or total synaptic clusters. These data are consistent with and extend our prior work, confirming no effect of either antipsychotics or lithium on SV2A clustering, but suggest contrasting effects of these drugs on the post-synapse. Although caution needs to be exerted when extrapolating results from animals to patients, these data provide clarity with regard to the effect of antipsychotics and lithium on synaptic markers, thus facilitating discrimination of drug from illness effects in human studies of synaptic pathology in psychiatric disorders.

Synaptic dysfunction is hypothesised to play a key role in schizophrenia pathogenesis, but this has not been tested directly in vivo. Here, we investigated synaptic vesicle glycoprotein 2A (SV2A) levels and their relationship to symptoms and structural brain measures using [¹¹C]UCB-J positron emission tomography in 18 patients with schizophrenia and 18 controls. We found significant group and group-by-region interaction effects on volume of distribution (V_T). [¹¹C]UCB-JV_Twas significantly lower in the frontal and anterior cingulate cortices in schizophrenia with large effect sizes (Cohen’sd = 0.8-0.9), but there was no significant difference in the hippocampus. We also investigated the effects of antipsychotic drug administration on SV2A levels in Sprague-Dawley rats using western blotting, [³H]UCB-J autoradiography and immunostaining with confocal microscopy, finding no significant effects on any measure. These findings indicate that there are lower synaptic terminal protein levels in schizophrenia in vivo and that antipsychotic drug exposure is unlikely to account for them.