PET quantification of muscarinic cholinergic receptors with [N-11C-methyl]-benztropine and application to studies of propofol-induced unconsciousness in healthy human volunteers

A Molecular Toolbox of Positron Emission Tomography Tracers for General Anesthesia Mechanism Research

With appropriate radiotracers, positron emission tomography (PET) allows direct or indirect monitoring of the spatial and temporal distribution of anesthetics, neurotransmitters, and biomarkers, making it an indispensable tool for studying the general anesthesia mechanism. In this Perspective, PET tracers that have been recruited in general anesthesia research are introduced in the following order: 1) ¹¹C/¹⁸F-labeled anesthetics, i.e., PET tracers made from inhaled and intravenous anesthetics; 2) PET tracers targeting anesthesia-related receptors, e.g., neurotransmitters and voltage-gated ion channels; and 3) PET tracers for studying anesthesia-related neurophysiological effects and neurotoxicity. The radiosynthesis, pharmacodynamics, and pharmacokinetics of the above PET tracers are mainly discussed to provide a practical molecular toolbox for radiochemists, anesthesiologists, and those who are interested in general anesthesia.

Drug Development in Alzheimer's Disease: The Contribution of PET and SPECT

Clinical trials aiming to develop disease-altering drugs for Alzheimer’s disease (AD), a neurodegenerative disorder with devastating consequences, are failing at an alarming rate. Poorly defined inclusion-and outcome criteria, due to a limited amount of objective biomarkers, is one of the major concerns. Non-invasive molecular imaging techniques, positron emission tomography and single photon emission (computed) tomography (PET and SPE(C)T), allow visualization and quantification of a wide variety of (patho)physiological processes and allow early (differential) diagnosis in many disorders. PET and SPECT have the ability to provide biomarkers that permit spatial assessment of pathophysiological molecular changes and therefore objectively evaluate and follow up therapeutic response, especially in the brain. A number of specific PET/SPECT biomarkers used in support of emerging clinical therapies in AD are discussed in this review.

Development of (18)F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

Positron emission tomography (PET) is an in vivo molecular imaging tool which is widely used in nuclear medicine for early diagnosis and treatment follow-up of many brain diseases. PET uses biomolecules as probes which are labeled with radionuclides of short half-lives, synthesized prior to the imaging studies. These probes are called radiotracers. Fluorine-18 is a radionuclide routinely used in the radiolabeling of neuroreceptor ligands for PET because of its favorable half-life of 109.8 min. The delivery of such radiotracers into the brain provides images of transport, metabolic, and neurotransmission processes on the molecular level. After a short introduction into the principles of PET, this review mainly focuses on the strategy of radiotracer development bridging from basic science to biomedical application. Successful radiotracer design as described here provides molecular probes which not only are useful for imaging of human brain diseases, but also allow molecular neuroreceptor imaging studies in various small-animal models of disease, including genetically-engineered animals. Furthermore, they provide a powerful tool for in vivo pharmacology during the process of pre-clinical drug development to identify new drug targets, to investigate pathophysiology, to discover potential drug candidates, and to evaluate the pharmacokinetics and pharmacodynamics of drugs in vivo.

Hipnose, imobilidade tônica e eletroencefalograma

OBJETIVO: Apresentar uma revisão sobre as características da atividade elétrica cerebral que acompanha a hipnose animal, estado induzido em laboratório em mamíferos por manipulações experimentais, bem como sobre as alterações encontradas no EEG durante o estado de hipnose, visando à discussão dos resultados encontrados na busca de evidências dos fundamentos filogenéticos que possam conduzir ao entendimento dos rudimentos neurais da hipnose humana. MÉTODO: Livros e bases eletrônicas de dados foram consultados. Critério de inclusão: artigos originais publicados entre 1966-2012. Critério de exclusão: artigos que se afastavam da visão eletroneurofisiológica da hipnose. RESULTADOS: Foram encontradas 662 referências, tendo sido selecionados os artigos e livros referenciados. Além desses artigos, foi incluído no estudo o artigo de Hoagland, publicado em 1928, que é um clássico na área de imobilidade tônica em vertebrados. CONCLUSÕES: O estado de hipnose humano resulta de processamentos em inúmeros circuitos paralelos distribuídos em uma complexa rede neuronal, envolvendo, dessa forma, uma ampla área do encéfalo. Na trajetória evolutiva, a grande ampliação dos recursos corticais pode ter tornado as respostas de imobilidade tônica passíveis de modulação consciente, respostas essas ainda presentes nos humanos e que se manifestam involuntariamente em situações de grande ameaça. Vários estudos têm evidenciado mecanismos neurofisiológicos capazes de reforçar a visão da hipnose não só como um eficiente recurso para procedimentos médicos e odontológicos, funcionando como auxiliar na analgesia e sedação, mas também como excelente ferramenta psicoterapêutica.

Propofol decreases in vivo binding of 11C-PBR28 to translocator protein (18 kDa) in the human brain

The PET radioligand (11)C-PBR28 targets translocator protein (18 kDa) (TSPO) and is a potential marker of neuroimmune activation in vivo. Although several patient populations have been studied using (11)C-PBR28, no investigators have studied cognitively impaired patients who would require anesthesia for the PET procedure, nor have any reports investigated the effects that anesthesia may have on radioligand uptake. The purpose of this study was to determine whether the anesthetic propofol alters brain uptake of (11)C-PBR28 in healthy subjects.

METHODS

Ten healthy subjects (5 men; 5 women) each underwent 2 dynamic brain PET scans on the same day, first at baseline and then with intravenous propofol anesthesia. The subjects were injected with 680 ± 14 MBq (mean ± SD) of (11)C-PBR28 for each PET scan. Brain uptake was measured as total distribution volume (V(T)) using the Logan plot and metabolite-corrected arterial input function.

RESULTS

Propofol decreased V(T), which corrects for any alteration of metabolism of the radioligand, by about 26% (P = 0.011). In line with the decrease in V(T), brain time-activity curves showed decreases of about 20% despite a 13% increase in plasma area under the curve with propofol. Reduction of V(T) with propofol was observed across all brain regions, with no significant region X condition interaction (P = 0.40).

CONCLUSION

Propofol anesthesia reduces the V(T) of (11)C-PBR28 by about 26% in the brains of healthy human subjects. Given this finding, future studies will measure neuroimmune activation in the brains of autistic volunteers and their age and sex-matched healthy controls using propofol anesthesia. We recommend that future PET studies using (11)C-PBR28 and concomitant propofol anesthesia, as would be required in impaired populations, include a control arm to account for the effects of propofol on brain measurements of TSPO.

Localisation of pre- and postsynaptic cholinergic markers in the human brain

The cholinergic neurotransmission in the central nervous system plays an important role in modulating cognitive processes such as learning, memory, arousal and sleep as well as in modulating locomotor activity. Dysfunction of the central cholinergic system is involved in numerous neuropsychiatric diseases. This review will provide a synopsis on the regional localisation of cholinergic and cholinoceptive structures within the adult human brain. On the cholinergic site data based on the distribution of choline acetyltransferase-immunoreactive structures are in the focus, complemented by data from acetylcholinesterase and vesicular acetylcholine transporter studies. On the cholinoceptive site, the distribution and localisation of receptors that transduce the acetylcholine message, i.e. the muscarinic and the nicotinic acetylcholine receptors is summarized. In addition to these data obtained on post mortem brain an overview of markers which allow for the in vivo monitoring of the cholinergic system in the brain is given. The detailed knowledge on the distribution and localisation of cholinergic markers in human brain will provide further information on the cholinergic circuits of neurotransmission - a prerequisite for the interpretation of in vivo imaging data and the development of selective diagnostic and therapeutic compounds.

Imaging the effects of propofol on human cerebral glucose metabolism using positron emission tomography

The effects of propofol on glucose metabolism in different cerebral regions were observed, using positron emission tomography (PET) technology, to determine a possible cerebral target region. Seven healthy volunteers were injected with (18)F-fluorodeoxyglucose developing agent for PET scanning whilst awake (control group T1), during sedation (induced by 1.5 microg/ml propofol administered by target controlled injection [TCI], group T2) and when unconsciousness (induced by 2.5 microg/ml propofol administered by TCI, group T3). Whole brain glucose metabolism was reduced during propofol anaesthesia; this was initially observed in the cortical areas at the lower dose of propofol (group T2) but extended to the subcortical regions, especially the thalamus and hippocampus, at the higher dose (group T3). This suggests that these regions of the brain might be important targets that are susceptible to propofol.

Neuroimaging and genetic associations of attentional and hypnotic processes

In the aftermath of the human genome project, genotyping is fast becoming an affordable and technologically viable complement to phenotyping. Whereas attempts to characterize hypnotic responsiveness have been largely phenomenological, data emanating from exploratory genetic data may offer supplementary insights into the genetic bases of hypnotizability. We outline our genetic and neuroimaging findings and discuss potential implications to top-down control systems. These results may explain individual differences in hypnotizability and propose new ideas for studying the influence of suggestion on neural systems.

Brain imaging in research on anesthetic mechanisms: studies with propofol

Brain imaging helps to refine our understanding of the anesthetic effect and is providing novel information that result in the formulation of hypotheses. They have shown that anesthetics act on specific structures that have been known to be important for consciousness at large. They have also helped to show that anesthetics act on specific structures regionally, rather than being non-specific, general depressant of the central nervous system (CNS). A constant finding is that the drugs that we use seem to exert their action on specific sites within the CNS. This is true for a wide variety of drugs like midazolam, anesthetic vapors and opiates. The thalamus has consistently shown marked deactivation coincident with the anesthesia-induced loss of consciousness, appearing to be a very important target of anesthetic effect. Additionally, when vibro-tactile or pain stimulation is given, anesthetics significantly effect cortical structures even before loss of consciousness while loss of transmission at the thalamic level seems to coincide with loss of consciousness. Finally, the use of radioligands allow in vivo characterization of anesthetic effects on neurotransmitter systems.

Attention and hypnosis: neural substrates and genetic associations of two converging processes

Although attention is a central theme in psychological science, hypnosis researchers rarely incorporate attentional findings into their work. As with other biological systems, attention has a distinct anatomy that carries out basic psychological functions. Specific brain injuries, states, and drugs can all influence attentional networks. Investigation into these networks using modern neuroimaging techniques has revealed important mechanisms involved in attention. In this age of genomics, genetic approaches can supplement these neuroimaging techniques. As genotyping becomes an affordable and technologically viable complement to phenotyping, exploratory genetic assays offer insights into the genetic bases of both attention and hypnotizability. This paper discusses relevant aspects of attentional mechanisms and their underlying neuroanatomy as they relate to hypnosis. Underlining data from attentional networks, neuroimaging, and genetics, these findings should help to explain individual differences in hypnotizability and the neural systems subserving hypnosis.