Torsten Reese

A stereotaxic MRI template set for the rat brain with tissue class distribution maps and co-registered anatomical atlas: application to pharmacological MRI.

We describe a stereotaxic rat brain MRI template set with a co-registered digital anatomical atlas and illustrate its application to the analysis of a pharmacological MRI (phMRI) study of apomorphine. The template set includes anatomical images and tissue class probability maps for brain parenchyma and cerebrospinal fluid (CSF). These facilitate the use of standard fMRI software for spatial normalisation and tissue segmentation of rat brain data. A volumetric reconstruction of the Paxinos and Watson rat brain atlas is also co-localised with the template, enabling the atlas structure and stereotaxic coordinates corresponding to a feature within a statistical map to be interactively reported, facilitating the localisation of functional effects. Moreover, voxels falling within selected brain structures can be combined to define anatomically based 3D volumes of interest (VOIs), free of operator bias. As many atlas structures are small relative to the typical resolution of phMRI studies, a mechanism for defining composite structures as agglomerations of individual atlas structures is also described. This provides a simple and robust means of interrogating structures that are otherwise difficult to delineate and an objective framework for comparing and classifying compounds based on an anatomical profile of their activity. These developments allow a closer alignment of pre-clinical and clinical analysis techniques.

In vivo mapping of functional connectivity in neurotransmitter systems using pharmacological MRI.

Pharmacological MRI (phMRI) methods map the hemodynamic response to drug challenge as a surrogate for changes in neuronal activity. However, the central effects of drugs can be complex and include activity at the primary site of action, downstream effects in other brain regions and direct effects on vasculature and neurovascular coupling. Univariate analysis, normally applied to phMRI data, does not discriminate between these effects, and can result in anatomically non-specific activation patterns. We analysed inter-subject correlations in the amplitude of the slow phMRI response to map functionally connected brain regions recruited in response to pharmacological challenge. Application of D-amphetamine and fluoxetine revealed well-defined functional structure underlying the widespread signal changes detected via standard methods. Correlated responses were found to delineate key neurotransmitter pathways selectively targeted by these drugs, corroborating a tight correspondence between the phMRI response and changes in neurotransmitter systems specific to the pharmacological action. In vivo mapping of correlated responses in this way greatly extends the range of information available from phMRI studies and provides a new window into the function of neurotransmitter systems in the active state. This approach may provide new important insights regarding the central systems underlying pharmacological action.

Region-specific effects of nicotine on brain activity: a pharmacological MRI study in the drug-naïve rat.

We have applied pharmacological magnetic resonance imaging (phMRI) methods to map the functional response to nicotine in drug-naïve rats. Nicotine (0.35 mg/kg intravenous (i.v.)) increased relative cerebral blood volume (rCBV) in cortical (including medial prefrontal, cingulate orbitofrontal, insular) and subcortical (including amygdala and dorsomedial hippocampus) structures. The pharmacological specificity of the effect was demonstrated by acute pretreatment with the nicotinic acetylcholine receptor (nAChR) ion-channel-blocking agent mecamylamine, which suppressed the rCBV response to nicotine. Control experiments with norepinephrine, a potent non-brain-penetrant vasopressor, at a dose that mimics the cardiovascular response induced by nicotine were performed to assess the potential confounding effects of peripheral blood pressure changes induced by nicotine. In an attempt to highlight the relative contribution of different nAChR subtypes to the observed activation pattern of nicotine, we also investigated the central phMRI response to an acute challenge with (R)-N-(1-azabicyclo[2.2.2]oct-3-yl)(5-(2-pyridyl)thiophene-2-carboxamide) (cpdA, at 5, 10, 20, and 30 mg/kg i.v.) and 5-iodo-A-85380 (5IA, 5 mg/kg i.v.). CpdA is a selective agonist at homomeric α7 nAChRs, while 5IA features high in vivo affinity for the α4β2* and other less-abundant β2-containing nicotinic receptors. CpdA did not produce significant rCBV changes at any of the doses tested, whereas 5IA induced a pattern of activation very similar to that induced by nicotine. The lack of phMRI response to cpdA together with the high spatial overlap between the activation profile of nicotine and 5IA, suggest that the acute functional response to nicotine in drug-naïve rats is mediated by β2-containing nAChR isoforms, presumably belonging to the α4β2* subtype.

Functional Connectivity in the Pharmacologically Activated Brain: Resolving Networks of Correlated Responses to d-Amphetamine

We investigated the functional connectivity structure underlying the widespread relative cerebral blood volume (rCBV) response to d‐amphetamine in the rat brain by systematically analyzing the intersubject correlations between the response amplitudes in 48 atlas‐defined brain structures. A cluster analysis resolved three distinct networks of brain regions that exhibited closely coupled responses: one corresponding to primary dopamine projections from the midbrain to the striatum, a second consisting predominantly of forebrain cortical and basal ganglia regions that share a widespread correlation pattern resembling the univariate group response, and a third including structures in the periventricular dopamine system. These results suggest that different functional networks underlie the brains response to d‐amphetamine. This approach may provide important new insights regarding the central systems that underlie pharmacological action. Magn Reson Med 57:704–713, 2007.

Functional magnetic resonance mapping of intracerebroventricular infusion of a neuroactive peptide in the anaesthetised rat

Pharmacological magnetic resonance imaging (phMRI) methods map the cerebral haemodynamic response to challenge with psychotropic agents as a surrogate for drug-induced changes in brain activity. However, many neuroactive compounds present low blood-brain barrier penetration and thus systemic administration may result in insufficient brain concentration. Intracerebroventricular (ICV) administration has been long used as an effective way of bypassing the blood-brain barrier in studies with non-brain-penetrant compounds, such as neuropeptides. In order to extend the range of pharmacological substances accessible to phMRI, we have developed methods to map relative cerebral blood volume (rCBV) changes induced by in situ ICV administration of neuroactive agents in the anaesthetised rat. We have applied this method to study for the first time the phMRI response to central administration of a neuropeptide, the metabolically stable and potent NK1 receptor agonist GR-73632. ICV administration of 4.2 pmol of GR-73632 produced a rapid onset and sustained rCBV increase in several brain structures, such as the amygdala, the caudate putamen and the cortex. These results demonstrate the feasibility of phMRI as a tool to study the functional correlates of brain activity induced by central administration of neuroactive agents.

A general protocol of ultra-high resolution MR angiography to image the cerebro-vasculature in 6 different rats strains at high field

BACKGROUND Differences in the cerebro-vasculature among strains as well as individual animals might explain variability in animal models and thus, a non-invasive method tailored to image cerebral vessel of interest with high signal to noise ratio is required. NEW METHOD Experimentally, we describe a new general protocol of three-dimensional time-of-flight magnetic resonance angiography to visualize non-invasively the cerebral vasculature in 6 different rat strains. Flow compensated angiograms of Sprague Dawley, Wistar Kyoto, Lister Hooded, Long Evans, Fisher 344 and Spontaneous Hypertensive Rat strains were obtained without the use of contrast agents. At 11.7T using a repetition time of 60ms, an isotropic resolution of up to 62μm was achieved; total imaging time was 98min for a 3D data set. RESULTS The visualization of the cerebral arteries was improved by removing extra-cranial vessels prior to the calculation of maximum intensity projection to obtain the angiograms. Ultimately, we demonstrate that the newly implemented method is also suitable to obtain angiograms following middle cerebral artery occlusion, despite the presence of intense vasogenic edema 24h after reperfusion. COMPARISON WITH EXISTING METHODS The careful selection of the excitation profile and repetition time at a higher static magnetic field allowed an increase in spatial resolution to reliably detect of the hypothalamic artery, the anterior choroidal artery as well as arterial branches of the peri-amygdoidal complex and the optical nerve in six different rat strains. CONCLUSIONS MR angiography without contrast agent can be utilized to study cerebro-vascular abnormalities in various animal models.

A longitudinal MRI study on lymph nodes histiocytosis of a xenograft cancer model

Background Efforts are continuously made to detect and investigate the pivotal processes and interplay between the response of sentinel lymph node and malignant cells from a primary tumor. Conversely, some frequently used tumor animal models, such as human cancer xenografts, rarely feature metastasis. Therefore, lymph node alterations are seldom assessed. We consider that studying lymph node response could contribute to the understanding of host reaction to cancer. In the present study, we explored the presence of regional lymph node alterations in parallel with tumor growth using a pancreatic tumor xenograft model which does not develop metastasis. Methods and findings We established an animal cancer model by the subcutaneous inoculation of PANC-1 (a metastatic human pancreatic cancer cell line) in the left upper flank of athymic nude mice. Tumor animals, along with controls (n = 7 / group) were subjected to Magnetic Resonance Imaging (MRI) in order to follow tumor growth and brachial and axillary lymph nodes alterations over several weeks. Further histological analyses were performed at the end of the study. The individual average of the different lymph nodes sizes was 15–40% larger in the tumor animals compared to control animals at week 8 to week 20. The tumor size and lymph node size were not correlated. Histological analysis of the lymph nodes showed paracortical histiocytosis. No metastasis to lymph nodes could be detected by histology. In tumor bearing animals, histiocytosis was associated with isolated apoptotic bodies and migration of human tumoral cells was confirmed by specific immunostaining of human origin markers. Conclusions The lack of metastasis as well as the pathological manifestation of the lymph node alteration in this pre-clinical model established here parallels findings in patients with sinus histiocytosis that is correlated with improved survival.

New Molecular and Functional Imaging Techniques

The ultimate goal of nanomedicine relies on monitoring the efficient targeted drug delivery in a noninvasive manner. Novel imaging biomarkers are currently explored in murine cancer models to correctly diagnose cancer and to guide therapy. Hereby, relevant biomolecules like peptides and aptamers as well as sophisticated functionalized nanoparticles can be successfully radiolabeled for nuclear imaging such as positron emission tomography (PET) and single photon emission computed tomography (SPECT). Magnetic resonance offers novel diagnostic tools with the use of hyperpolarized [1-13C] pyruvate for prostate cancer as well as with potential applications of smart contrast agents as functional biomarkers. Multimodular imaging probes (e.g., radiolabeled quantum dots for optical imaging and PET) are designed to use complementary imaging modalities to assess biodistribution and gain new insights in disease mechanism, and they can be used for cross-validation. Thus, the described new molecular and functional imaging probes may ultimately help to monitor drug delivery and individual patients’ response to new therapies.