Imaging receptors by autoradiography: computer-assisted approaches

A method for acetylcholinesterase staining of brain sections previously processed for receptor autoradiography

Receptor autoradiography using selective radiolabeled ligands allows visualization of brain receptor distribution and density on film. The resolution of specific brain regions on the film often can be difficult to discern owing to the general spread of the radioactive label and the lack of neuroanatomical landmarks on film. Receptor binding is a chemically harsh protocol that can render the tissue virtually unstainable by Nissl and other conventional stains used to delineate neuroanatomical boundaries of brain regions. We describe a method for acetylcholinesterase (AChE) staining of slides previously processed for receptor binding. AChE staining is a useful tool for delineating major brain nuclei and tracts. AChE staining on sections that have been processed for receptor autoradiography provides a direct comparison of brain regions for more precise neuroanatomical description. We report a detailed thiocholine protocol that is a modification of the Koelle-Friedenwald method to amplify the AChE signal in brain sections previously processed for autoradiography. We also describe several temporal and experimental factors that can affect the density and clarity of the AChE signal when using this protocol.

Radio-imaging for quantitative autoradiography in biology

We present here an overview of new in vitro and ex vivo radio-imaging systems developed to overcome the limitations of films and emulsions currently used in histological autoradiography experiments. The shortcomings of films for quantitative studies are first introduced. Principles and performances of each family of imagers are discussed and illustrated in various biological contexts. Finally, perspectives of development including nonradioactive labeling techniques are briefly presented.

Three-dimensional visualization and quantification of the benzodiazepine receptor population within a living human brain using PET and MRI

Positron emission tomography (PET) in combination with receptor-selective high-affinity radioligands allows the characterization of neuroreceptor distributions in the living human brain. Thus far, the visualization and quantification of receptors with PET have been limited to series of two-dimensional (2D) image planes of the anatomic receptor distribution. The development of high-resolution PET has increased the number of planes to approximately 50, supplying an excessive amount of image information from a single experiment. The inherent limitations of 2D techniques make them insufficient to apprehend and efficiently analyze this cumbersome amount of data. In the present communication we describe procedures to visualize and quantify in three dimensions (3D) the total image information from the compound set of 47 2D planes of a PET experiment using commercially available software. Three-dimensional computer graphic and volume rendering techniques were used to analyze and display the results. For the experimental application the benzodiazepine (BZ) antagonist [11C]flumazenil was used as radioligand to visualize the BZ receptor (BZR) population in the brain of a healthy human subject. Three-dimensional images of the radioligand binding receptor population were displayed with regard to volume and form in relation to the corresponding anatomic structures in the brain reconstructed from MR images. The volume-rendering technique allowed the inspection of PET signals representing BZR populations in the interior of the hemisphere as viewed from the medial projection. Thresholding and seeding techniques were used to define volumes and quantities. Using the PET data and volume rendering, the total amount of cerebral BZRs (NCerebrum) and the apparent volume they take into account (V(BZR, Cerebrum)app) could he calculated for the first time using an automated procedure. The cerebrum of the healthy subject contained 17.6 nmol of BZRs in a volume of approximately 1.25 L. The principles and application of the technical development described offer new dimensions to clinical neuroscience and should be practically useful for automated quantitative determination of neuroreceptor number in brain regions of patients with neuropsychiatric disorders and in relation to drug treatment.

Radioimagers as an alternative to film autoradiography for in situ quantitative analysis of 125I-ligand receptor binding and pharmacological studies

Three radioimagers, the mu-imager, the beta-imager and the phosphorimager, were tested as alternatives to quantitative autoradiography on film, for receptor imaging and pharmacological in situ quantitative analysis. Two iodinated ligands 125I-interleukin-1 alpha and 125I-gonadotropin releasing hormone agonist were used for receptor characterization in mouse brain and pituitary sections. Due to the high number of the agonist receptors in rat pituitary gland, this tissue was used to compare measurements obtained from digital autoradiograms with classical gamma detector determination. This permits the evaluation of radioimager efficiency and absolute quantification. Radioimagers represent an improvement in terms of time of image acquisition. All the radioimagers are more sensitive than film for the detection of low levels of radioactivity. The spatial resolution provided by the mu-imager compares favourably with that obtained on film autoradiograms while digital autoradiograms from the phosphorimager and beta-imager did not show precise definition under our experimental conditions. Superimposition of histological structures from the stained sections with radiolabelled areas in the autoradiograms remains, at this time, the unique advantage of film. In conclusion, radioimagers represent an alternative to autoradiography on film or emulsion for in situ quantitative studies on tissue sections. They combine precise imaging for in situ binding studies with easy and direct access to counts in cpm. The improvement in radioimaging technology has, therefore, brought in situ analysis of iodinated ligand binding to the level of accuracy that is obtained with classical detectors of radioactivity.

Mapping of [3H]vasopressin binding sites in the brain of jerboa (Jaculus orientalis) by an high resolution beta-radio imager

The distribution of vasopressin receptors in the brain of the jerboa (Jaculus orientalis) was studied using tritiated arginin vasopressin ([3H]vasopressin). beta-Particles emitted from tritiated ligand bound to brain sections were detected by a newly developed beta-radio imager to generate a light spot which was read by a charge coupled device camera. The number and coordinate of the center of gravity of the light spot were recorded. After summation in pixels of the counts collected during 2-20 h, an image was produced representing the distribution of [3H]vasopressin bound to brain sections. Specific vasopressin binding was detected in various brain regions such as the cerebral cortex, islands of Calleja, pallidum, amygdala and the hippocampus as well as in the pituitary gland. The intensity of the binding was quantified directly from the images obtained and expressed in decays/min/surface unit. The linearity of this method of detection allowed a relevant measurement of non-specific binding, therefore its subtraction from images representing the total binding. Three-dimensional reconstructions of labeled structures were also performed. The presence of numerous vasopressin receptors in the jerboa hippocampus suggests a major role for this neuropeptide in this part of the brain.