The LDAEP as a potential biomarker for central serotonergic activity: challenges to overcome

Biological indicators for neurotransmitter activity are of great interest for a better understanding of the pathophysiology of psychiatric disorders. The monoaminergic neurotransmitter serotonin, in particular, plays an important role in the aetiology of many mental disorders. Serotonin has homeostatic effects on brain functioning by regulating the general excitability of neurons, and thus plays a role in modulating perception and behaviour. The loudness dependence of auditory evoked potentials (LDAEP) is a measure of the excitability of neurons in the auditory cortex during the processing of tones of different intensities. Due to a strong serotonergic innervation of the primary auditory cortex it is assumed that serotonin has modulatory effects on these brain areas. A strong LDAEP is thought to reflect a weak serotonergic activity and vice versa. The LDAEP has been successfully applied in several fields of research. One of its most promising applications is the prediction of treatment responses. However, the LDAEPs validity has been challenged. Particularly, research on some clinical diagnoses is constrained by an inherent heterogeneous symptom constellation within clinical diagnoses based on ICD-10 or DSM-V. It is therefore necessary to assess the symptoms with dimensional measures in order to give evidence to the underlying biological abnormalities. On the other hand, no standardized protocols exist for the application and analysis of the LDAEP. Basically, in order to measure the electrophysiological response of the auditory cortex, dipole source analysis, distributed imaging methods (LORETA) and single electrode estimation have been applied. However, results obtained with different approaches show major inconsistencies. The aim of the present thesis was to progress in the search of biomarkers suitable for a daily clinical use. So far, findings in schizophrenia research using LDAEP are inconsistent and lack comparability due to the above-mentioned methodological issues. Study I conducted within this thesis investigated patients with schizophrenia by means of LDAEP taking dimensional measures of the symptoms into account. Patients showed higher LDAEP values compared to healthy controls, indicating a lower serotonergic activity. Moreover, predominant negative symptoms were associated with the LDAEP. These findings are in line with other studies, which showed a dysfunctional serotonergic neurotransmission in the genesis of negative symptoms. Study II examined the underlying neural generators during loudness processing using magnetoencephalography (MEG). Magnetic field tomography analysis revealed additional activation of brain regions outside of the auditory cortex. Time course analysis further specified that tones with high intensities were processed within hundreds of milliseconds from the primary auditory cortex as well as the primary somatosensory cortex via the posterior cingulate cortex into the premotor cortex. These findings have strong implications on the comparability of the different analysis approaches. In summary, in search of clinical biomarkers it is important to shed light on possible influences of methodological factors in order to improve their reliability and validity. Their application in daily clinical practice would lead to more precise diagnoses and improve treatment strategies by enabling a better prediction of therapeutic outcome. In der psychiatrischen Forschung sind biologische Indikatoren für die Neurotransmitter-Aktivität im Gehirn von grossem Interesse. Insbesondere Serotonin, ein monoaminer Neurotransmitter, spielt in der Ätiologie vieler psychischer Erkrankungen eine wichtige Rolle. Serotonin wird eine regulierende Funktion im Gehirn zugeschrieben, indem es das Erregungsniveau von Nervenzellen mit der Wahrnehmung und dem Verhalten des Organismus abstimmt. Die Lautstärkeabhängigkeit Akustisch Evozierter Potentiale (LAAEP) ist ein Mass für die Reagibilität der Neurone im auditorischen Kortex bei der Verarbeitung unterschiedlich lauter Töne. Angesichts einer starken serotonergen Innervation des primären auditorischen Kortex wird vermutet, dass Serotonin einen modulierenden Einfluss auf die Aktivität dieses Kortexareals hat. Es wird angenommen, dass eine starke LAAEP eine schwache serotonerge Aktivität wiederspiegelt und vice versa. Trotz eines beachtlichen Erfolges der LAAEP in verschiedenen Bereichen der neurowissenschaftli- chen Forschung in der Psychiatrie, ist dieser Indikator noch nicht genügend validiert, um im klinischen Alltag verwendet werden zu können. Ausserdem sind derartige Biomarker bei Untersuchungen an psychiatrischen Stichproben mit heterogenen Symptomkonstellationen nicht uneingeschränkt anwendbar. Symptome, die unter derselben klinischen Diagnose subsumiert sind, unterliegen möglicherweise unterschiedlichen biologischen Ursachen. Im Kontext solcher Forschung ist es daher erforderlich, dimensionale Beschreibungen der Symptome zu berücksichtigen und mit der LAAEP zu assoziieren. Zum anderen existieren bis anhin keine standardisierten Protokolle für die Applikati- on und Auswertung der LAAEP. Um Potentialveränderungen im auditorischen Kortex zu erfassen, wurden einerseits Dipolquellenmodelle sowie verteilte Quellenmodelle (LORETA) und andererseits Ableitungen an einzelnen Elektroden vorgenommen. Untersuchungen zeigen, dass die Ergebnisse zwischen diesen Auswertungsansätzen inkohärent sind. Ziel der vorliegenden Arbeit war es, diesen Limitationen Rechnung zu tragen um die Etablierung der LAAEP voranzubringen. Erkenntnisse zur LAAEP bei schizophrenen Patienten sind uneinheitlich und aus methodischen Gründen schwer miteinander zu vergleichen. In der Studie I wurden Patienten mit Schizophrenie mittels LAAEP und unter Berücksichtigung des Schweregrads unterschiedlicher Symptomcluster, namentlich Negativ- und Positivsymptome, untersucht. Patienten zeigten eine signifikant höhere LAAEP im Vergleich zur gesunden Kontrollgruppe, beziehungsweise eine erniedrigte serotonerge Aktivität. Darüberhinaus war die Ausprägung der Negativsymptomatik stark mit der LAAEP assoziiert. Dies ist in Übereinstimmung mit früherer Literatur, die eine Dysfunktion des serotonergen Systems in der Genese von Negativsymptomen hypothetisierte. In der Studie II wurden die der LAAEP zugrundeliegenden Generatoren im Gehirn näher untersucht. Mithilfe von Magnetoencephalographie (MEG) und eines verteilten Imaging Verfahrens wurde ermittelt, dass nebst dem auditorischen Kortex auch andere Hirnregionen involviert sind. Eine Analyse der Zeitverläufe ergab eine sequenzielle Verarbeitung von den sensorischen Arealen über den posterioren cingulären Cortex zum Prämotor-Cortex im Millisekundenbereich. Diese zusätzlichen Aktivierungen dürften eine Auswirkung auf die Vergleichbarkeit der unterschiedlichen Auswertungsmethoden haben. Zusammenfassend ist es von grosser Bedeutung, den Einfluss möglicher methodologischer Faktoren auf die LAAEP zu untersuchen um die Reliabilität und Validität dieses klinischen Biomarkers zu verbessern. Der Einsatz im klinischen Alltag würde eine präzisere Diagnostik und eine bessere Therapieprädiktion begünstigen und so die Behandlung von Patienten massgeblich verbessern. i

The Relationship Between Suicide Ideation and Parasuicide: An Electrophysiological Investigation Using the Loudness Dependence of Auditory Evoked Potential

The loudness dependence of the auditory evoked potential (LDAEP) has been proposed as a promising valid and a non-invasive indicator of behaviorally relevant central 5-HT functioning. There is limited research on the utility of the LDAEP in discriminating individuals who engage in various degrees of suicidal behavior. The primary purpose of the present study was to examine if the LDAEP, as a measure of central serotonergic functioning, can be useful in distinguishing groups of individuals who: (a) solely experience suicidal ideation (SI group); (b) experience suicidal ideation and have engaged in deliberate self-harm acts (SH group); and (c) individuals with no history of suicidal ideation or deliberate self-harm behavior (control group). I was also interested in observing whether the nature of individuals’ suicidal behavior (i.e., cognitive versus cognitive and behavioral) would differentiate individuals’ performance on Self-Aggression Paradigm (behavioral measure of self-aggression; SAP) and Suicide-Implicit Association Test (reaction-time based measure of implicit cognitive associations with death/suicide; S-IAT). Forty-eight participants consisting of college students and community members were recruited for this study. I predicted that (1) The SH group would exhibit the largest LDAEP slope, followed by the SI group, and finally the Control group; (2) The SH group would obtain the largest mean shock score and would self-select the highest number of “20” shock, followed by the SI group, and finally the Control group; (3) SI and SH groups would obtain a more negative S-IAT index, indicating pro-suicide tendencies, than the Control group; (4) the LDAEP slope would be positively related to the SAP indexes and negatively related to S-IAT index; and (5) the LDAEP would be positively related to the self-report measures of self-harm behavior and aggression, and negatively related to self-report measure of reasons for living. Contrary to expectations, most of our predictions were not supported. Clinical implications and future research directions are discussed

Early and Late Stage Mechanisms for Vocalization Processing in the Human Auditory System

The human auditory system is able to rapidly process incoming acoustic information, actively filtering, categorizing, or suppressing different elements of the incoming acoustic stream. Vocalizations produced by other humans (conspecifics) likely represent the most ethologically-relevant sounds encountered by hearing individuals. Subtle acoustic characteristics of these vocalizations aid in determining the identity, emotional state, health, intent, etc. of the producer. The ability to assess vocalizations is likely subserved by a specialized network of structures and functional connections that are optimized for this stimulus class. Early elements of this network would show sensitivity to the most basic acoustic features of these sounds; later elements may show categorically-selective response patterns that represent high-level semantic organization of different classes of vocalizations. A combination of functional magnetic resonance imaging and electrophysiological studies were performed to investigate and describe some of the earlier and later stage mechanisms of conspecific vocalization processing in human auditory cortices. Using fMRI, cortical representations of harmonic signal content were found along the middle superior temporal gyri between primary auditory cortices along Heschl\u27s gyri and the superior temporal sulci, higher-order auditory regions. Additionally, electrophysiological findings also demonstrated a parametric response profile to harmonic signal content. Utilizing a novel class of vocalizations, human-mimicked versions of animal vocalizations, we demonstrated the presence of a left-lateralized cortical vocalization processing hierarchy to conspecific vocalizations, contrary to previous findings describing similar bilateral networks. This hierarchy originated near primary auditory cortices and was further supported by auditory evoked potential data that suggests differential temporal processing dynamics of conspecific human vocalizations versus those produced by other species. Taken together, these results suggest that there are auditory cortical networks that are highly optimized for processing utterances produced by the human vocal tract. Understanding the function and structure of these networks will be critical for advancing the development of novel communicative therapies and the design of future assistive hearing devices

Inherited Obsession: The Role of Genetics and Serotonin in the Etiology of Obsessive-Compulsive Disorder

We still do not understand why some individuals are more likely to develop OCD than others. Research has implicated the serotonin system specifically the serotonin transporter and the 5-HT2A receptor as potential neurochemical underpinnings of OCD. Innovations in genetics have allowed research to hone in on the specific genes which code for the neurochemical dysfunction implicated in OCD. In this literature review, I gathered data in the form of research which addresses the neurochemical and genetic underpinnings of OCD in order to gain a better understanding of the etiology of the disorder. The findings presented represent my analysis of current research in the field in the hopes of drawing conclusions about the etiology of OCD. My conclusions implicate the specific genes which code for the serotonin transporter and the 5-HT2A receptor as the potential neurochemical and genetic underpinnings of OCD

Cortical mechanisms for tinnitus in humans /

PhD ThesisThis work sought to characterise neurochemical and neurophysiological processes underlying tinnitus in humans. The first study involved invasive brain recordings from a neurosurgical patient, along with experimental manipulation of his tinnitus, to map the cortical system underlying his tinnitus. Widespread tinnitus-linked changes in low- and high-frequency oscillations were observed, along with inter-regional and cross-frequency patterns of communication. The second and third studies compared tinnitus patients to controls matched for age, sex and hearing loss, measuring auditory cortex spontaneous oscillations (with magnetoencephalography) and neurochemical concentrations (with magnetic resonance spectroscopy) respectively. Unlike in previous studies not controlled for hearing loss, there were no group differences in oscillatory activity attributable to tinnitus. However, there was a significant correlation between gamma oscillations (>30Hz) and hearing loss in the tinnitus group, and between delta oscillations (1-4Hz) and perceived tinnitus loudness. In the neurochemical study, tinnitus patients had significantly reduced GABA concentrations compared to matched controls, and within this group there was a positive correlation between choline concentration (potentially linked to acetylcholine and/or neuronal plasticity) and both hearing loss, and subjective tinnitus intensity and distress. In light of present and previous findings, tinnitus may be best explained by a predictive coding model of perception, which was tested in the final experiment. This directly controlled the three main quantities comprising predictive coding models, and found that delta/theta/alpha oscillations (1-12Hz) encoded the precision of predictions, beta oscillations (12-30Hz) encoded changes to predictions, and gamma oscillations represented surprise (unexpectedness of stimuli based on predictions). The work concludes with a predictive coding model of tinnitus that builds upon the present findings and settles unresolved paradoxes in the literature. In this, precursor processes (in varying combinations) synergise to increase the precision associated with spontaneous activity in the auditory pathway to the point where it overrides higher predictions of ‘silence’.Medical Research Council Wellcome Trust and the National Institutes of Healt

The hearing hippocampus

The hippocampus has a well-established role in spatial and episodic memory but a broader function has been proposed including aspects of perception and relational processing. Neural bases of sound analysis have been described in the pathway to auditory cortex, but wider networks supporting auditory cognition are still being established. We review what is known about the role of the hippocampus in processing auditory information, and how the hippocampus itself is shaped by sound. In examining imaging, recording, and lesion studies in species from rodents to humans, we uncover a hierarchy of hippocampal responses to sound including during passive exposure, active listening, and the learning of associations between sounds and other stimuli. We describe how the hippocampus' connectivity and computational architecture allow it to track and manipulate auditory information – whether in the form of speech, music, or environmental, emotional, or phantom sounds. Functional and structural correlates of auditory experience are also identified. The extent of auditory-hippocampal interactions is consistent with the view that the hippocampus makes broad contributions to perception and cognition, beyond spatial and episodic memory. More deeply understanding these interactions may unlock applications including entraining hippocampal rhythms to support cognition, and intervening in links between hearing loss and dementia

Tinnitus, biomarkers and quality of life in an older population

Tinnitus is a symptom involving the perception of sound in the ears or head, without a corresponding external acoustic stimulus. It is related to many different conditions and has a major impact on quality of life of the affected person. Currently, its diagnosis and monitoring are based on subjective audiometric and psychometric measures. There are no objective methods for tinnitus identification. In addition, the pathophysiological mechanisms underlying tinnitus remains unknown. The purpose of this thesis was to study the mechanisms underlying tinnitus and their relationship to hearing loss, being that hearing loss is the comorbidity most frequently associated with tinnitus. It also aimed to evaluate the contribution of genetic, audiological and immunological factors to the etiology of tinnitus. For this purpose, systematic reviews (SR) were performed, in order to account the state of art, the perspectives of the patient and their relatives, and previous clinical trials of tinnitus treatments. SRs contributed to the identification of a pool of tinnitus-related complaint domains used by COMIT’ID (Core outcome measures in tinnitus international Delphi) in a 3-round internet-based Delphi survey to identifying core outcome sets (COS), i.e., which complaints related to tinnitus are essential for evaluation in clinical trials. These recommendations are specific to the three main therapeutic modalities: sound, psychological, and pharmacological. In order to contribute to the standardization of tinnitus clinical evaluation and treatment, TINNET, a European network for scientific tinnitus research, was created. Among the different activities carried out in were a systematic review of existing national clinical practice guidelines for the diagnosis and treatment of tinnitus. This review contributed to the development of a multidisciplinary European guideline for tinnitus: diagnosis, evaluation and treatment. This guideline was presented at TINNET final meeting and it is being disseminated widely. Another aim of the present thesis was to review work on somatosensory tinnitus (pathophysiology, diagnosis, treatment and the participation in an international Delphi consensus group on the diagnosis of this subtype of tinnitus), to contribute to a better understanding of this subtype of tinnitus. In order to achieve the objectives of this PhD study, 114 participants aged 55 to 75 years were recruited from the Portuguese population. Participants were divided into four groups according to the presence/absence of tinnitus and hearing loss. The completion of the study protocol gave rise to four original research articles, including a demographic characterization, relevant psychological and quality of life aspects comparing the studied population and the published literature, audiologic markers of tinnitus, and immunological profile of population and biomarkers of presbycusis and tinnitus. The results point to hearing loss as a risk factor for the development of tinnitus and psychological complaints as a risk factor for more severe tinnitus and consequently less quality of life in patients with this symptom. In characterizing audiological markers, the presence of previous noise exposure and the hearing loss increased the probability of developing tinnitus. Also, participants with an abrupt onset of tinnitus and who had a negative effect or rebound on residual inhibition were more likely to develop severe or catastrophic tinnitus. For the population with tinnitus, a reduction in amplitude of auditory evoked potentials wave I and a higher values in the 'Ratio of Waves V/I for both ears' were associated with a greater probability of developing severe or catastrophic tinnitus. The inflammatory profile of the study population showed significant differences in IL10 levels between the group with and without tinnitus. IL1α was significantly higher in patients with tonal tinnitus, while IL2 was higher in participants who reported negative or rebound effect on residual inhibition of tinnitus. A negative correlation was also found between IL10 and tinnitus duration, and between HSP70 and tinnitus intensity. Biomarkers were explored in this thesis. A systematic review was performed to synthesize evidence for the existence and clinical usefulness of biomarkers. GRM7 and NAT2 were evaluated in the thesis population. The results indicate a higher prevalence of the T allele in the GRM7 gene (60.3% T/T and 33.3% A/T). Participants with a T/T genotype appeared to be at a higher risk for ARHL development, and 33% have a lower risk of developing tinnitus compared to participants with A/A and A/T genotype. Regarding the NAT2 phenotype, the slow acetylator (53%) was most common, followed by the intermediate acetylator (35.9%). These results suggest that the AT allele of GRM7 and the slow acetylating phenotype of Nat2 are potential biomarkers of tinnitus severity. The results in this thesis are very interesting and original, showing us the need for future research in larger samples, and employing rigorous methodological design in order to control for confounding variables. On the other hand, translational studies may be the key to clarifying the pathophysiologic dilemmas of tinnitus.O acufeno é um sintoma referente à perceção de um som nos ouvidos ou na cabeça, sem que exista um estímulo acústico externo correspondente. Está presente em diferentes patologias (otológicas ou não) e tem um impacto importante na qualidade de vida da pessoa afetada. Atualmente, o seu diagnóstico e monitorização são baseados em medidas subjetivas audiométricas e psicométricas, sendo que não existem métodos objetivos para a identificação do acufeno. Além disso, os mecanismos fisiopatológicos subjacentes ao acufeno subjectivo permanecem desconhecidos. O objetivo da presente tese é estudar os mecanismos subjacentes ao acufeno subjectivo e a sua relação com a surdez, visto que a surdez é a co-morbilidade mais frequentemente associada ao acufeno. Pretende-se também avaliar a contribuição dos fatores genéticos, audiológicos e imunológicos na etiologia do acufeno. Para isso, foram realizadas revisões sistemáticas (RS) sobre esta temática de forma a conhecer o estado de arte, primeiramente em relação à forma como os pacientes e os familiares percecionam o acufeno e também sobre os ensaios clínicos existentes acerca da eficácia do tratamento do acufeno. Ambas as RS contribuíram para a identificação de um conjunto de domínios relacionados com o acufeno, usado pelo COMIT’ID (Core outcome measures in tinnitus international Delphi), num método de consensos Delphi, baseado na Internet, com o objetivo de identificar um ‘Core Outcome Set’ (ou seja definir quais as queixas relacionadas com o acufeno que são imprescindíveis para a sua avaliação) recomendado para ensaios clínicos de eficácia terapêutica para o acufeno assim como para o seu diagnóstico. Estas recomendações são específicas para as três modalidades terapêuticas principais: sonora, psicológica e farmacológica uma vez que cada modalidade tem fundamentos específicos e por isso visam avaliar diferentes aspetos do acufeno. Com o objetivo de contribuir para a padronização da avaliação e do tratamento clínico do acufeno, foi constituída a TINNET, uma rede europeia para a investigação científica do acufeno. Considerando o objetivo do presente estudo e a hipótese de integrar esta rede europeia, foram desenvolvidas um conjunto de atividades que em muito contribuíram para o conhecimento sobre o acufenos. Entre as diferentes atividades realizadas com o apoio da TINNET destaca-se a realização de uma revisão sistemática sobre as ‘guidelines’ clínicas existentes para o diagnóstico e tratamento do acufeno. Esta revisão foi uma das bases que conduziu ao desenvolvimento das ‘guidelines’ europeias multidisciplinares para o acufeno: diagnóstico, avaliação e tratamento. Estas ‘guidelines’ foram apresentadas na conferência final do TINNET e estão atualmente em fase de disseminação. Outro foco de interesse da presente tese foi a realização de trabalhos de revisão sobre o acufeno somatosensorial (nomeadamente sobre a fisiopatologia, diagnóstico e tratamento), bem como a participação num grupo de consenso internacional sobre o diagnóstico deste subtipo do acufeno, de forma a contribuir para uma melhor compreensão deste subtipo do acufeno. Também estas atividades contribuíram para o desenvolvimento de competências cientificas essenciais ao desenvolvimento do presente estudo, dado que permitiram uma melhor compreensão deste subtipo do acufeno, demonstrando-se a heterogeneidade e diversidade do acufeno. De forma a alcançar os objetivos deste estudo de doutoramento, recrutaram-se 114 voluntários da população portuguesa com idade dos 55 aos 75 anos. Os indivíduos desta amostra permitiam a realização de diferentes estudos nomeadamente os laboratoriais, tendo a analise dos resultados envolvido a amostra dividida em quatro grupos consoante a presença/ausência do acufeno e de surdez. Dos resultados desta tese fazem parte quatro artigos originais que e incluem uma caracterização demográfica, aspetos relevantes a nível psicológico e de qualidade de vida, marcadores audiológicos do acufeno, perfil imunológico da população e biomarcadores da presbiacusia e do acufeno. Os resultados obtidos sugerem a perda auditiva como fator de risco para o desenvolvimento do acufeno e as queixas a nível psicológico como fator de risco para o acufeno mais grave e consequentemente associado a menor qualidade de vida nos pacientes com este sintoma. A nível da caracterização dos marcadores audiológicos, verificou-se que a presença de antecedentes de exposição ao ruído e a perda auditiva aumentam a probabilidade de desenvolver acufeno. Também, os participantes com um início abrupto do acufeno e que apresentam um efeito negativo ou ‘rebound’ na inibição residual têm maior probabilidade de desenvolver acufeno grave ou catastrófico. Encontrou-se nos Potenciais Evocados Auditivos, uma redução da amplitude na onda I em pacientes com acufeno, bem como valores maiores no ‘Ratio de amplitude das ondas V e I de ambos ouvidos’ estando associados a maiores probabilidades de desenvolver acufeno severo ou catastrófico. O perfil inflamatório da nossa população mostra diferenças significativas entre o grupo com e sem acufeno quando comparados para a IL10. Quanto à relação entre os parâmetros imunológicos e a acufenometria, verificou-se uma correlação entre o aumento da IL1α e acufeno tonal, bem como entre o aumento da IL2 e a inibição residual do acufeno. Foi também encontrada uma correlação negativa para a IL10 e a duração do acufeno e para o HSP70 e a intensidade do acufeno. Estes resultados são muito originais e suscitam a necessidade de estudos futuros que permitam esclarecer os mecanismos subjacentes às correlações encontradas. Em relação aos biomarcadores, foi efetuada uma revisão sistemática com a finalidade de sintetizar evidências para a existência e utilidade clínica dos biomarcadores para o desenvolvimento ou gravidade do acufeno. Foi também realizado um estudo acerca do papel do GRM7 e do NAT2 na nossa amostra. Os resultados apontam para uma maior prevalência do alelo T no gene GRM7 (60,3% T/T e 33,3% A/T). Os participantes com um genótipo T/T parecem ter um maior risco para o desenvolvimento de ARHL e 33% apresentam menor risco para o desenvolvimento do acufeno, em comparação com indivíduos com A/A e genótipo A/T. Em relação ao fenótipo NAT2, o acetilador lento (53%) foi o mais comum seguido pelo intermediário acetilador (35,9%). Os nossos resultados sugerem que o genótipo A/T de GRM7 e o fenótipo acetilador lento de NAT2 como potenciais biomarcadores da severidade do acufeno. Os resultados obtidos são originais e no seu conjunto são muito interessantes, apontando para a necessidade de estudos futuros em larga escala de forma a aprofundar as conclusões aqui obtidas. Por outro lado, os estudos translacionais poderão ser a chave para esclarecer os dilemas da fisiopatologia do acufeno