Local and downstream effects of excitotoxic lesions in the rat medial prefrontal cortex on In vivo 1H-MRS signals

The rat medial prefrontal cortex (mPFC) regulates subcortical dopamine transmission via projections to the striatum and ventral tegmental area. We used in vivo proton magnetic resonance spectroscopy (1H-MRS) at 4.7 T to determine whether excitotoxic lesions of the mPFC result in alterations of N-acetylaspartate (NAA), a marker of neuronal integrity, both locally and downstream in the striatum. Lesioned rats exhibited persistent reductions of NAA and other metabolites within the prefrontal cortex; selective reductions of NAA were seen in the striatum, but not in the parietal cortex. Consistent with earlier reports, lesioned rats exhibited a transient enhancement in amphetamine-induced hyperlocomotion. Prefrontal NAA losses correlated with lesion extent. In the striatum, while there was no change in tissue volume, expression of striatal glutamic acid decarboxylase-67 mRNA was significantly reduced. In vivo NAA levels thus appear sensitive to both local and downstream alterations in neuronal integrity, and may signal meaningful effects at cellular and behavioral levels.

Vascular protection by chloroquine during brain tumor therapy with Tf-CRM107

Tf-CRM107 is a conjugate of transferrin and a point mutant of diphtheria toxin that selectively kills cells expressing high levels of the transferrin receptor. Tf-CRM107 has been infused intratumorally into patients with malignant brain tumors. Although approximately half of the patients exhibit tumor responses, patients receiving higher doses of Tf-CRM107 may develop magnetic resonance image (MRI) evidence of toxicity indicative of small vessel thrombosis or petechial hemorrhage. Consistent with these clinical results we found that intracerebral injection of Tf-CRM107 into rats at total doses > or =0.025 microg causes brain damage detectable by MRI and histology. To widen the therapeutic window of Tf-CRM107, we explored ways to prevent this damage to the vasculature. We reasoned that the vasculature may be protected to a greater extent than tumor from Tf-CRM107 infused into brain parenchyma by i.v. injection of reagents with low blood-brain barrier permeability that block the toxicity of Tf-CRM107. Chloroquine, a well-characterized antimalarial drug, blocks the toxicity of diphtheria toxin and Tf-CRM107. Systemic administration of chloroquine blocked the toxicity of Tf-CRM107 infused intracerebrally in rats and changed the maximum tolerated dose of Tf-CRM107 from 0.2 to 0.3 microg. Moreover, chloroquine treatment completely blocked the brain damage detected by MRI caused by intracerebral infusion of 0.05 microg of Tf-CRM107. In nude mice bearing s.c. U251 gliomas, chloroquine treatment had little effect on the antitumor efficacy of Tf-CRM107. Thus, chloroquine treatment may be useful to reduce the toxicity of Tf-CRM107 for normal brain without inhibiting antitumor efficacy and increase the therapeutic window of Tf-CRM107 for brain tumor therapy.

On the feasibility of MRI-guided focused ultrasound for local induction of gene expression

Gene therapy is a promising approach to the treatment of many forms of disease, including cancer. Of critical concern in its implementation is the ability to control the location, duration, and level of expression of the therapeutic gene. Here, we propose the use of local heat in combination with a heat-sensitive promoter to help accomplish this. Certain members of the family of heat shock protein (hsp) promoters display a regulation that depends strongly on temperature. We present a study of natural hsp70 induction in rat leg by MRI-guided focused ultrasound to investigate the hsp70 promoter as a possible candidate for use in control of gene expression with local heat. A temperature increase of 5-8 degrees C in the focal region for 45 minutes led to a differential expression of the hsp70 mRNA between the focal region and the surrounding tissue ranging from a factor of 3 to 67.

Fast echo-shifted gradient-recalled MRI: combining a short repetition time with variable T2* weighting

The principles of a fast T2*-sensitized MR imaging method (Moonen et al., Magn. Reson. Med. 26, 184 (1992)) are extended to further increase T2* sensitivity. It is shown that the period of T2*-weighting can be lengthened by n TR-periods by appropriate gradient schemes without RF refocusing resulting in progressively delayed gradient-recalled echoes. This extension of the echo-shifting concept thus introduces large flexibility in the choice of T2*-weighting without changing total imaging time. The coherence pathway formalism is used to evaluate and describe the selection of the desired echo and the attenuation of unwanted coherences. The new techniques are demonstrated for tracking a bolus of susceptibility contrast agent in cat brain. Relative blood-volume maps are derived with expected contrast between white and gray matter.