Non-invasive characterization of beta-amyloid(1-40) vasoactivity by functional magnetic resonance imaging in mice.

Neurovascular regulation, which is critical to the efficient functioning of the brain, is impaired in Alzheimer's disease and in transgenic mice overexpressing Abeta. Although senile plaques and neurofibrillary tangles represent neuropathological hallmarks of Alzheimer's disease, deposition of Abeta in cerebral blood vessels also likely plays a significant role in this debilitating and fatal disease. Further, soluble Abeta, which shows greater correlation with disease progression and severity than deposited plaques or tangles, displays strong vasoactive properties. The aim of this study was to develop a non-invasive model of cerebral vasoactivity that would ultimately be translatable to Alzheimer's disease as a marker for disease-modifying efficacy of novel small molecule and biologics drugs. Relative changes in cerebral blood volume following relevant doses of soluble Abeta(1-40) (0.01 or 0.1 mg/mouse), PBS, or the reverse peptide, Abeta(40-1) (0.01 or 0.1 mg/mouse), were monitored non-invasively by contrast-enhanced functional magnetic resonance imaging in anesthetized C57BL/6 mice. Experiments were performed on a 7T horizontal bore scanner using gradient echo echo-planar imaging. As expected, PBS and Abeta(40-1) did not induce any significant change in vascular response. In contrast, Abeta(1-40) significantly decreased CBV in a quantifiable, dose-related and region-specific manner. These data demonstrate for the first time the feasibility of characterizing pathogenic Abeta(1-40)-induced vascular dysfunction in vivo using a non-invasive approach. Further, this technique can be readily applied to preclinical screening in a longitudinal manner for novel drugs or antibodies targeting disease modification.

Differential effects of cannabinoid receptor agonists on regional brain activity using pharmacological MRI.

BACKGROUND AND PURPOSE: Activation of cannabinoid CB1 and/or CB2 receptors mediates analgesic effects across a broad spectrum of preclinical pain models. Selective activation of CB2 receptors may produce analgesia without the undesirable psychotropic side effects associated with modulation of CB1 receptors. To address selectivity in vivo, we describe non-invasive, non-ionizing, functional data that distinguish CB1 from CB2 receptor neural activity using pharmacological MRI (phMRI) in awake rats. EXPERIMENTAL APPROACH: Using a high field (7 T) MRI scanner, we examined and quantified the effects of non-selective CB1/CB2 (A-834735) and selective CB2 (AM1241) agonists on neural activity in awake rats. Pharmacological specificity was determined using selective CB1 (rimonabant) or CB2 (AM630) antagonists. Behavioural studies, plasma and brain exposures were used as benchmarks for activity in vivo. KEY RESULTS: The non-selective CB1/CB2 agonist produced a dose-related, region-specific activation of brain structures that agrees well with published autoradiographic CB1 receptor density binding maps. Pretreatment with a CB1 antagonist but not with a CB2 antagonist, abolished these activation patterns, suggesting an effect mediated by CB1 receptors alone. In contrast, no significant changes in brain activity were found with relevant doses of the CB2 selective agonist. CONCLUSION AND IMPLICATIONS: These results provide the first clear evidence for quantifying in vivo functional selectivity between CB1 and CB2 receptors using phMRI. Further, as the presence of CB2 receptors in the brain remains controversial, our data suggest that if CB2 receptors are expressed, they are not functional under normal physiological conditions.

An improved method of evaluation of drug-evoked changes in gastric emptying in mice.

INTRODUCTION: The increased availability of transgenic mice prompts a need for the adaptation to mice of whole-animal assays traditionally performed in larger laboratory animals. Gastric emptying studies are frequently conducted in dogs and rats. Mouse-based gastric emptying models currently available often use inert, nonnutrient liquid meals containing nonabsorbable markers or radionuclides. We have developed a mouse gastric emptying assay that features a favorable throughput and the use of a semisolid, high-calorie meal. METHODS: A carbohydrate- and protein-rich semisolid test meal was prepared from common laboratory reagents. Gastric emptying was determined by subtracting the mass of test meal remaining in the stomach from the mass of test meal administered. A time-course study of basal emptying of a semisolid, paste-like test meal high in carbohydrate and protein from the stomachs of overnight-fasted mice was conducted. Agents known to either inhibit (propantheline, 0.3-10 mg/kg sc; corticotropin-releasing factor [CRF], 3-100 nmol/kg ip) or accelerate (metoclopramide, 1-10 mg/kg ip; bethanechol, 1-30 mg/kg ip) gastric emptying were tested. A single time-point variation of the assay can be used for quickly screening compounds for effects on gastric emptying. RESULTS: In time-course studies, the test meal emptied from the stomach with a half-emptying time of 30.6 min (95% CI: 27.3-34.7). The gastric emptying data were successfully modeled by a two-parameter exponential decay function. No lag phase was observed, indicating that the meal empties from the stomach as a liquid. The anticholinergic agent propantheline increased gastric half-emptying time (t(1/2)) approximately threefold, while metoclopramide decreased gastric half-emptying time approximately twofold compared to basal emptying. Single time-point screening studies correctly detected the gastrokinetic activity of bethanechol and the inhibitory effect of CRF. DISCUSSION: The mouse gastric emptying assay reported here is simple, inexpensive, and not labor-intensive. It is capable of detecting either stimulation or inhibition of gastric motor activity. This assay should prove useful for identifying drug-evoked changes in gastric emptying as well as for assessing the gastric motility effects of altered gene expression in genetically modified mice.

Effect of novel motilide ABT-229 versus erythromycin and cisapride on gastric emptying in dogs.

ABT-229 (8,9-anhydro-4"-deoxy-3'-N-desmethyl-3'-N-ethylerythromycin B-6,9-hemiacetal), a synthetic derivative of erythromycin (ERY) with no antibiotic activity, has been shown to bind to motilin receptors and stimulate contractile activity of the antrum and small intestine. The objective of this study was to determine the effect of ABT-229 on canine gastric emptying (GE) and contractile activity of the antrum and duodenum in response to a solid meal. Six beagles were used to determine GE of a solid meal and contractile activity in response to either vehicle, ABT-229 (0.17, 0.83, 2.5, or 5.0 microg/kg/min), ERY (33.3 microg/kg/min), or cisapride (CIS) (10 microg/kg/min). Lag (t(lag)), half-emptying (t(1/2)), and complete emptying (t(full)) times were determined. Contractile data were analyzed for motility index and gastroduodenal coordination. Compared with vehicle, ABT-229 dose dependently accelerated GE, t(lag) was decreased at the two highest doses, t(1/2) was decreased compared with vehicle at the three highest doses, and t(full) was decreased at all doses compared with vehicle. ERY also decreased t(1/2) and t(full), whereas CIS decreased all GE parameters. The slopes of the linear phase of GE curves for all drugs and doses were greater than those for vehicle. ABT-229 dose dependently increased the motility index as well as gastroduodenal coordination. ABT-229 (two highest doses) and CIS accelerated GE of a solid meal by decreasing the lag phase and increasing the rate of GE, whereas ERY only increased the rate of GE. The data suggest that ABT-229 is 7- to 40-fold more potent than ERY in accelerating GE.