Robert Kerwin

Functional Connectivity and Brain Networks in Schizophrenia

Schizophrenia has often been conceived as a disorder of connectivity between components of large-scale brain networks. We tested this hypothesis by measuring aspects of both functional connectivity and functional network topology derived from resting-state fMRI time series acquired at 72 cerebral regions over 17 min from 15 healthy volunteers (14 male, 1 female) and 12 people diagnosed with schizophrenia (10 male, 2 female). We investigated between-group differences in strength and diversity of functional connectivity in the 0.06–0.125 Hz frequency interval, and some topological properties of undirected graphs constructed from thresholded interregional correlation matrices. In people with schizophrenia, strength of functional connectivity was significantly decreased, whereas diversity of functional connections was increased. Topologically, functional brain networks had reduced clustering and small-worldness, reduced probability of high-degree hubs, and increased robustness in the schizophrenic group. Reduced degree and clustering were locally significant in medial parietal, premotor and cingulate, and right orbitofrontal cortical nodes of functional networks in schizophrenia. Functional connectivity and topological metrics were correlated with each other and with behavioral performance on a verbal fluency task. We conclude that people with schizophrenia tend to have a less strongly integrated, more diverse profile of brain functional connectivity, associated with a less hub-dominated configuration of complex brain functional networks. Alongside these behaviorally disadvantageous differences, however, brain networks in the schizophrenic group also showed a greater robustness to random attack, pointing to a possible benefit of the schizophrenia connectome, if less extremely expressed.

Association between clozapine response and allelic variation in 5-HT2A receptor gene

We report allelic association between a polymorphism (T102C) within the coding region of the 5-HT2A gene (HTR2A, 13q14-21) and response to clozapine in schizophrenic patients. Homozygosity for the C102 allele was more frequent (30/57, 53%) among patients who did not respond to clozapine than in those who responded (23/92, 25%). This finding is evidence that allelic variation of genes which encode neurotransmitter receptors can influence clinical response to antipsychotic drugs.

Pharmacogenetic prediction of clozapine response

We did association studies in multiple candidate genes to find the combination of polymorphisms that give the best predictive value of response to clozapine in schizophrenic patients. A combination of six polymorphisms in neurotransmitter-receptor-related genes resulted in 76.7% success in the prediction of clozapine response (p=0.0001) and a sensitivity of 95% (+/- 0.04) for satisfactory response. These results will form the basis for a simple test to enhance the usefulness of clozapine in psychiatric treatment.

Meta-analysis of studies on genetic variation in 5-HT2A receptors and clozapine response

Serotonin (5-HT) neurotransmitter receptors are targeted by atypical antipsychotic drugs. We hypothesized that genetic variation in these receptors may affect clinical response to the drugs targeting them. This hypothesis has been tested by several studies in which the correlation between polymorphic variants in the 5-HT2A receptor gene and clinical response to the atypical antipsychotic clozapine was investigated. The results of these studies either found association between 5-HT2A genetic variants and clozapine response or found differences in the same direction which did not reach statistical significance. Meta-analysis of these studies including 373 patients who responded to the treatment and 360 non-responders showed association between two 5-HT2A polymorphisms, 102-T/C and His452Tyr, and clozapine response. Statistical analysis of extreme responders showed a clearer association of the 102-T/C with clozapine response. These results reinforce the hypothesis and strengthen the candidacy of these receptors as important therapeutic targets.

Latent inhibition in drug naive schizophrenics: relationship to duration of illness and dopamine D2 binding using SPET

The dual aims of the study were (1) to examine the effect of neuroleptic medication on the expression of latent inhibition (LI) by studying LI in drug naive schizophrenic patients, and (2) to investigate the relationship between LI and dopamine D2 receptor binding in the basal ganglia using single photon emission tomography (SPET). Subjects constituted a sub-set of patients investigated in a major study of in vivo D2 receptor binding in schizophrenia (Pilowsky et al., 1993). Striatal D2 receptor binding was assessed in 15 neuroleptic naive schizophrenic patients and 13 healthy volunteers. The performance of subjects on a within-subject auditory latent inhibition paradigm was also assessed. There was found to be no significant difference in LI between schizophrenic patients and normal controls, both groups showing a strong within-subject LI effect. There was also found to be no association between LI and dopamine D2 receptor binding in either the left or the right basal ganglia. This lack of association indicates that LI is not directly related to post-synaptic D2 receptor levels in the striatum. LI was, however, found to be correlated with duration of illness in the schizophrenic group. Patients with a relatively short duration of illness (< 12 months) tended to show reversed, or absent, LI whereas patients with a longer illness duration (> 12 months) showed intact LI. The effect on LI of duration of illness is consistent with previous findings that LI is disrupted specifically in acute, but not chronic, schizophrenia. Previous studies have assumed that this pattern of results is due to the stabilising effect of long-term neuroleptic medication. The present findings in a sample of neuroleptic naive schizophrenic patients indicate that this is unlikely to be the case. Rather, it appears that the reinstatement of LI in schizophrenic patients over time is due to a factor(s) intrinsic to the evolution of the schizophrenic illness.

Evidence for association between polymorphisms in the promoter and coding regions of the 5-HT2A receptor gene and response to clozapine

Clozapine is a potent atypical antipsychotic which binds to a variety of neurotransmitter receptors including serotonin (5-HT) receptors. However, the precise neurochemical site of clozapines therapeutic action is unknown. We hypothesize that genetic variation in the neurotransmitter receptors to which the drug binds may influence clozapine response. To test this hypothesis we genotyped a novel −1438-G/A polymorphism detected in the promoter region, and a His452Tyr polymorphism described in the coding region of the 5-HT2A receptor gene in two independent samples of clozapine-treated patients including responders and non-responders. Although the strong association between these polymorphisms and clozapine response observed in the first sample (sample I) was not statistically significant in the second sample (sample II), the results in both samples were in the same direction. Homozygosity for the allele G-1438 was higher among non-responders (56% in sample I, 43% in sample II) than in responders (28% in sample I and 32% in sample II) in both samples. Similarly, the frequency of allele Tyr452 was higher in non-responders (11% in sample I, 16% in sample II) than in responders (6% in sample I and 10% in sample II). A combined analysis of both samples showed association between both polymorphisms and clozapine response. These results provide further evidence suggesting that genetic variation at 5-HT2A receptors may influence clozapine response and strengthen the candidacy of these receptors as important therapeutic targets.

Do antidepressants regulate how cortisol affects the brain

Although the effects of antidepressants on glucocorticoid hormones and their receptors are relevant for the therapeutic action of these drugs, the molecular mechanisms underlying these effects are unclear. Studies in depressed patients, animals and cellular models have demonstrated that antidepressants increase glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) expression and function; this, in turn, is associated with enhanced negative feedback by endogenous glucocorticoids, and thus with reduced resting and stimulated hypothalamic-pituitary-adrenal (HPA) axis activity. In a series of studies conducted over the last few years, we have shown that antidepressants modulate GR function in vitro by inhibiting membrane steroid transporters that regulate the intracellular concentration of glucocorticoids. In this paper, we will review the effects of membrane steroid transporters and antidepressants on corticosteroid receptors. We will then present our unpublished data on GR live microscopy in vitro, showing that ligand-induced translocation of the GR starts within 30 seconds and is completed within minutes. Furthermore, we will present our new data using an in situ brain perfusion model in anaesthetised guinea-pigs, showing that entry of cortisol to the brain of these animals is limited at the blood-brain barrier (BBB). Finally, we will present a comprehensive discussion of our published findings on the effects of chemically unrelated antidepressants on membrane steroid transporters, in mouse fibroblasts and rat cortical neurones. We propose that antidepressants in humans could inhibit steroid transporters localised on the BBB and in neurones, like the multidrug resistance p-glycoprotein, and thus increase the access of cortisol to the brain and the glucocorticoid-mediated negative feedback on the HPA axis. Enhanced cortisol action in the brain might prove to be a successful approach to maximise therapeutic antidepressant effects.

Clozapine, single photon emission tomography, and the D2 dopamine receptor blockade hypothesis of schizophrenia

According to the dopamine hypothesis of schizophrenia, D2 receptor blockade is essential for a drug to have antipsychotic potency, and antipsychotic potency and D2 blockade are linearly related in vitro. To test this assumption in vivo, we have compared clinical response with central D2 dopamine receptor availability measured by 123I-iodobenzamide single photon emission tomography in two groups of schizophrenic patients. 6 patients were on typical antipsychotic drugs and 10 were on the atypical antipsychotic clozapine, including 2 patients from the first group. The patients on typical antipsychotics showed poor therapeutic response despite D2 receptor blockade. Significant clinical improvement occurred in all patients on clozapine, but at a lower level of D2 blockade by the drug. These findings suggest a more complex relation between D2 blockade and clinical efficacy than was previously thought.

Association between clozapine response and allelic variation in the 5-HT2C receptor gene

A cysteine to serine substitution at amino acid 23 in the 5-HT2C receptor gene alters the pharmacological properties of the protein. We investigated this polymorphism in subjects with schizophrenia resistant to conventional neuroleptic drugs, and analysed our data for allelic association between the disease state or clinical response to the atypical antipsychotic drug, clozapine. Ninety percent of subjects who had one or more 5-HT2Cser alleles (19/21) were classified as clozapine responders compared with 59% (84/141) without this allele (χ2 = 7.7, p = 0.005), suggesting that this mutation is a predictor of good response to clozapine. There was no association between schizophrenia and the 5-HT2Cser allele, but our results indicate that the 5-HT2C receptor may contain the major site of action through which clozapine mediates its antipsychotic effects.

Limbic selectivity of clozapine

6motile sperm cells per mL, the inclusion criterion for the use of ICSI in the Netherlands. Both oligozoospermic (n=69) and azoospermic (n=11) men were included. All patients were karyotyped, and a consecutive subgroup (n=58) was also tested for Y-chromosomal deletions and CFTR mutations. We found an abnormal karyotype in peripheral blood lymphocytes in seven (8·8%) of 80 patients, ten-fold more than the overall population incidence of 0·85% 2 (p<0·001, 2 test; table). We analysed Y-chromosomal deletions with a multiplex PCR system with markers for the AZF-a, AZF-b, and AZF-c regions. 3 With markers sY254 and sY255, three (5%) of 58 patients were found to have an AZF-c deletion associated with the deleted-in-azoospermia (DAZ) gene. No AZF deletions were detected with this assay in a study of 100 fertile male controls. 3