The alpha2C-adrenoceptor as a neuropsychiatric drug tar-get - PET studies in human subjects

Positron emission tomography imaging has both academic and applied uses in revealing the distribution and density of different molecular targets in the central nervous system. Following the significant progress made with the dopamine D2 receptor, advances have been made in developing PET tracers to allow analysis of receptor occupancy of many other receptor types as well as evaluating changes in endogenous synaptic transmitter concentrations of transmitters e.g. serotonin and noradrenaline. Noradrenergic receptors are divided into α1-, α2- and β-adrenoceptor subfamilies, in humans each of which is composed of three receptor subtypes. The α2-adrenoceptors have an important presynaptic auto-inhibitory function on noradrenaline release but they also have postsynaptic roles in modulating the release of other neurotransmitters, such as serotonin and dopamine. One of the subtypes, the α2C-adrenoceptor, has been detected at distinct locations in the central nervous system, most notably the dorsal striatum. Several serious neurological conditions causing dementia, Alzheimer’s disease and Parkinson’s disease have been linked to disturbed noradrenergic signaling. Furthermore, altered noradrenergic signaling has also been implicated in conditions like ADHD, depression, anxiety and schizophrenia. In order to benefit future research into these central nervous system disorders as well as being useful in the clinical development of drugs affecting brain noradrenergic neurotransmission, validation work of a novel tracer for positron emission tomography studies in humans was performed. Altogether 85 PET imaging experiments were performed during four separate clinical trials. The repeatability of [11C]ORM-13070 binding was tested in healthy individuals, followed by a study to evaluate the dose-dependent displacement of [11C]ORM-13070 from α2C-adrenoceptors by a competing ligand, and the final two studies examined the sensitivity of [11C]ORM-13070 binding to reflect changes in endogenous noradrenaline levels. The repeatability of [11C]ORM-13070 binding was very high. The binding properties of the tracer allowed for a reliable estimation of α2C-AR occupancy by using the reference tissue ratio method with low test-retest variability. [11C]ORM-13070 was dose-dependently displaced from its specific binding sites by the subtype-nonselective α2-adrenoceptor antagonist atipamezole, and thus it proved suitable for use in clinical drug development of novel α2C-adrenoceptor ligands e.g. to determine the best doses and dosing intervals for clinical trials. Convincing experimental evidence was gained to support the suitability of [11C]ORM-13070 for detecting an increase in endogenous synaptic noradrenaline in the human brain. Tracer binding in the thalamus tended to increase in accordance with reduced activity of noradrenergic projections from the locus coeruleus, although statistical significance was not reached. Thus, the investigation was unable to fully validate [11C]ORM-13070 for the detection of pharmacologically evoked reductions in noradrenaline levels.Siirretty Doriast

Dysconnection in schizophrenia: from abnormal synaptic plasticity to failures of self-monitoring

Over the last 2 decades, a large number of neurophysiological and neuroimaging studies of patients with schizophrenia have furnished in vivo evidence for dysconnectivity, ie, abnormal functional integration of brain processes. While the evidence for dysconnectivity in schizophrenia is strong, its etiology, pathophysiological mechanisms, and significance for clinical symptoms are unclear. First, dysconnectivity could result from aberrant wiring of connections during development, from aberrant synaptic plasticity, or from both. Second, it is not clear how schizophrenic symptoms can be understood mechanistically as a consequence of dysconnectivity. Third, if dysconnectivity is the primary pathophysiology, and not just an epiphenomenon, then it should provide a mechanistic explanation for known empirical facts about schizophrenia. This article addresses these 3 issues in the framework of the dysconnection hypothesis. This theory postulates that the core pathology in schizophrenia resides in aberrant N-methyl-D-aspartate receptor (NMDAR)–mediated synaptic plasticity due to abnormal regulation of NMDARs by neuromodulatory transmitters like dopamine, serotonin, or acetylcholine. We argue that this neurobiological mechanism can explain failures of self-monitoring, leading to a mechanistic explanation for first-rank symptoms as pathognomonic features of schizophrenia, and may provide a basis for future diagnostic classifications with physiologically defined patient subgroups. Finally, we test the explanatory power of our theory against a list of empirical facts about schizophrenia

Inhibition of prandial and waterspray-induced rat grooming by 8-OH-DPAT

The effects of 8-OH-DPAT treatment on rat grooming behaviour, elicited either prandially or in response to spraying with water were investigated. Dose (≤0.1 mg/kg s.c.) response studies employed momentary time sampling over 30 or 60 min with behaviour being scored in one of 6 or 7 (depending on food availability) mutually exclusive categories (feeding, active, scratching, face-grooming, body grooming, genital-grooming and resting) at 15 s intervals. In non-deprived rats, tested with wet mash available, feeding and activity frequencies were increased, but resting and total grooming were inhibited by 8-OH-DPAT. Face-, body- and genital-grooming occurred at higher levels than scratching, but all categories were reduced with reductions in scratching occurring at a lower dose (0.01 mg/kg). Misting rats with a fine water spray selectively increased body grooming and decreased activity without altering feeding, while 8-OH-DPAT increased feeding and reduced face-, body- and genital-grooming, without affecting already low levels of scratching. In misted rats, tested without food, 8-OH-DPAT reduced face-, body- and genital-grooming and increased resting. These results confirm i) that the water spray technique is a useful method for increasing grooming and ii) that 8-OH-DPAT has a suppressant effect on grooming independent of response competition from enhanced feeding

Probing brain function with pharmacological MRI

Lo sviluppo di tecniche di risonanza magnetica funzionale (fMRI) ha rivoluzionato le ricerca neuroscientifica clinica, determinando la possibilit\ue0 di investigare le dinamiche spazio-temporali dell\u2019attivit\ue0 cerebrale in maniera non invasiva e con grande accuratezza. Sebbene la tecnica sia stata originariamente sviluppata in ambito clinico, essa ha il potenziale di poter essere utilizzata in ambito preclinico come efficace strumento investigativo e traslazionale. Tuttavia, l\u2019implementazione preclinica di questi metodi \ue8 complicata da una serie di costrizioni sperimentali, in primis l\u2019utilizzo di anestetici, che minano fortemente il potenziale traslazionale di queste tecniche. Il recente sviluppo di tecniche di "MRI farmacologico" (phMRI) offre la possibilit\ue0 di superare alcune delle limitazioni sperimentali correlate all\u2019implementazione di approcci fMRI classici in animali da laboratorio. La tecnica si basa sull'utilizzo di metodi fMRI per mappare alterazioni di attivit\ue0 cerebrale prodotte dalla somministrazione di sostanze psicoattive. Studi preliminari hanno evidenziato la capacit\ue0 di generare robusti e specifici segnali phMRI anche in condizioni di anestesia, ed ha dimostrato la possibilit\ue0 di stimolare selettivamente diversi sistemi di neurotrasmettitori. Sfruttando la conservazione di circuiti cerebrali tra specie, tecniche phMRI offrono quindi l\u2019opportunit\ue0 di ampliare in maniera significativa il repertorio di stimolazione neuronale a disposizione in ambito preclinico, consentendo di indagare selettivamente specifici aspetti della funzione cerebrale in diversi stati di precondizionamento neuronale. In tale contesto, le attivit\ue0 di ricerca di questa tesi sono state finalizzate ad ampliare il campo di applicazione di metodi phMRI preclinici in due diversi ambiti sperimentali: a) come modalit\ue0 di indagine traslazionale, qualora applicata a modelli di malattia clinicamente rilevanti, b) pi\uf9 in generale come piattaforma investigativa per l'indagine della funzione cerebrale e della sua topologia funzionale in contesti sperimentali diversi. In un primo gruppo di studi, tecniche phMRI sono state impiegate per mappare i circuiti neuronali attivati da antagonisti del recettore del glutammato NMDA nel cervello del ratto (Sezione 4.1). Tali composti, grazie alle loro propriet\ue0 psicotogeniche, sono ampiamente sfruttati come modelli sperimentali di schizofrenia in animali ed in volontari allo scopo di valutare e validare nuovi trattamenti per la malattia. I risultati di questa ricerca hanno evidenziato uno specifico circuito corticolimbo- talamico che risulta essere attivato da antagonisti NMDAR sia nell'uomo che in Riassunto XII specie precliniche, e che \ue8 risultato essere modulabile da meccanismi antipsicotici diversi (Sezione 4.2). Il potenziale traslazionale dei metodi phMRI \ue8 stato ulteriormente avvalorato da un secondo gruppo di studi, in cui un approccio multi-parametrico \u201cphMRI-based\u201d \ue8 stato impiegato per indagare molteplici aspetti della funzione cerebrale in un modello murino di dipendenza da cocaina. Questa linea di investigazione ha evidenziato multiple alterazioni della funzione cerebrale basale e reattiva nel cervello di roditori esposti alla cocaina strettamente connesse a quelle osservate in analoghi studi di imaging su pazienti cocaina-dipendenti (Sezione 4.2). In una terza linea d\u2019 investigazione, l'uso combinato di avanzate strategie di targeting neuro-genetico (pharmaco-genetic silencing) e phMRI si \ue8 dimostrato efficace nello stabilire correlazioni dirette tra cellule, circuito e comportamento in linee di topo geneticamente modificate. Questi studi hanno portato all\u2019identificazione di una nuova e circoscritta popolazione neuroni nell'amigdala, in grado di controllare qualitativamente la risposta comportamentale alla paura attraverso il reclutamento di circuiti colinergici corticali (Sezione 4.3) Infine, l'approccio phMRI si \ue8 dimostrato uno strumento potente e versatile per l\u2019implementazione di misure di connettivit\ue0 funzionale nel cervello di roditori. Questo aspetto ha permesso l\u2019esplorazione di nuovi approcci statistici per l\u2019analisi della topologia funzionale del cervello basati sulla rappresentazione di misure di connettivit\ue0 in termini di reti complesse (Sezione 4.4). Complessivamente, i risultati di questo lavoro avvalorano il potenziale traslazionale di metodi phMRI nell\u2019ambito di diverse aree delle neuroscienze e della psicofarmacologia. La combinazione di phMRI e tecniche di manipolazione genetica avanzate definisce una nuova, potente piattaforma tecnologica per lo studio delle basi circuitali del comportamento in animali da laboratorio.The development of functional Magnetic Resonance Imaging (fMRI) has heralded a revolution in neuroscience, providing clinicians with a method to non-invasively investigate the spatio-temporal patterns of neuro-functional activity. Although primarily developed for human investigations, there exists significant scope for the application of fMRI in pre-clinical species as a translational and investigational platform across different areas of neuroscience and psychiatry research. However, the realization of this potential is hampered by a number of experimental constraints which make the application of fMRI methods to animal models less than straightforward. As a result, most fMRI research in laboratory species has been reduced to the employment of basic somato-sensory stimulation paradigms, thus greatly limiting the translational potential of the technique. An interesting approach to overcome some of these limitations has been dubbed \u201cpharmacological MRI\u201d (phMRI) and relies on the use of fMRI to map patterns of brain activity induced by psychoactive drugs. The approach has demonstrated the ability to elicit reliable fMRI signals even under anaesthesia, and to enable selective stimulation of different neurotransmitter systems. Building upon the homology between brain circuits in humans and laboratory animals, phMRI techniques thus offer the opportunity of significantly expanding the stimulation repertoire available to preclinical fMRI research, by allowing to selectively probe specific aspects of brain function under different preconditioning states. Within this framework, the research presented herein was aimed to broaden the scope of application of preclinical phMRI both as a translational technique, when applied to clinically-relevant disease models, and more generally as a versatile platform for the pre-clinical investigation of brain activity and its functional topology as a function of behavioural, pharmacological or genetic preconditioning. In a first group of studies, we developed a phMRI assay to map the circuitry activated by NMDAR antagonists in the rat brain. These psychotogenic compounds are widely exploited to model schizophrenia symptoms and to provide experimental models that may prove useful in the development of novel treatments for the disorder. The results of this research highlighted a conserved cortico-limbo-thalamic circuit that is activated by NMDAR antagonists both in humans and preclinical species, which can be modulated by existing and novel antipsychotic drugs (Section 4.1). The translational potential of phMRI measurements was further corroborated by a second group of studies, where a multi-parametric phMRI-based approach was applied to investigate multiple facets of brain function in a rodent cocaine selfSummary X administration model, a behavioural paradigm of established construct-validity for research of drug addiction. This line of investigation revealed specific basal and reactive functional alterations in the brain of cocaine-exposed rodents closely related to those observed in analogous neuroimaging studies in humans (Section 4.2). In a third line of investigation, the combined use of advanced neuro-genetic targeting strategies (i.e. pharmacogenetic silencing) and phMRI has proven successful in establishing direct correlations between cells, circuit and complex behaviours in genetically engineered mouse lines. These studies (Section 4.3) have led to the identification of a novel cell population in the amygdala that controls the behavioural response to fear through the recruitment of cholinergic circuits. Finally, the phMRI approach has proven a powerful tool to explore functional connectivity in rodents, and to map a variety of different neurotransmitter pathways by performing measures of correlated responses in spatially remote brain areas. This has provided a useful playground to explore novel statistical methods of analysis of functional connectivity represented in terms of complex networks (Section 4.4). Collectively, the results of this work strongly corroborate the translational use of phMRI approaches, and pave the way to the integrated implementation of phMRI and advance genetic manipulation as a novel powerful platform for basic neurobiological research