Health risk assessment of long term exposure to chemicals: application to cadmium and manganese

In vivo trans-synaptic tract tracing from the murine striatum and amygdala utilizing manganese enhanced MRI (MEMRI)

Small focal injections of manganese ion (Mn(2+)) deep within the mouse central nervous system combined with in vivo high-resolution MRI delineate neuronal tracts originating from the site of injection. Previous work has shown that Mn(2+) can be taken up through voltage-gated Ca(2+) channels, transported along axons, and across synapses. Moreover, Mn(2+) is a paramagnetic MRI contrast agent, causing positive contrast enhancement in tissues where it has accumulated. These combined properties allow for its use as an effective MRI detectable neuronal tract tracer. Injections of low concentrations of MnCl(2) into either the striatum or amygdala produced significant contrast enhancement along the known neuronal circuitry. The observed enhancement pattern is different at each injection site and enhancement of the homotopic areas was observed in both cases. Ten days postinjection, the Mn(2+) had washed out, as evidenced by the absence of positive contrast enhancement within the brain. This methodology allows imaging of neuronal tracts long after the injection of the ion because Mn(2+) concentrates in active neurons and resides for extended periods of time. With appropriate controls, differentiation of subsets of neuronal pathways associated with behavioral and pharmacological paradigms should be feasible.

Regional distribution of manganese found in the brain after injection of a single dose of manganese-based contrast agents

Manganese (Mn) complexes are unstable and dissociate in vivo. Because of the release of this metal, there exists some concern about the potential long-term neurotoxicity associated with the use of Mn-based contrast agents. This latter problem arises because manganese is known to accumulate in specific regions of the brain of people intoxicated by this metal. It was previously demonstrated that Mn can accumulate in the mice brain after administration of 5 micromol/kg of MnCl2, Mn-diethylenetriaminepentaacetate (Mn-DTPA), or Mn-dipyridoxal diphosphate (Mn-DPDP). In order to better characterize the behavior of Mn complexes after administration, this study assesses the regional distribution of Mn in the brain after i.v. injection of a single dose of MnCl2 or Mn-DTPA. Male Wistar rats received an i.v. injection of 5 micromol/kg of 54Mn as MnCl2 or Mn-DTPA. The rats were killed at one and two weeks post exposure. The distribution of the radioactivity in the slices was monitored by autoradiography. For both MnCl2 or Mn-DTPA, we observed that the radioactivity was dispersed in the entire brain, but the radioactivity was higher in several regions. No difference was observed between MnCl2 or Mn-DTPA in the regional distribution of Mn, and no difference was observed between the two times of exposure (1 week or 2 weeks). The uptake of Mn was minimal in corpus callosum. Maximal Mn concentration was observed in the hippocampal region, thalamus, colliculi, amygdala, olfactory nuclei, and cerebellum.

Evidence for the dissociation of the hepatobiliary MRI contrast agent Mn-DPDP

These experiments assessed and quantitated the release of free manganese Mn++ from the hepatobiliary contrast agent Mn-DPDP (manganese dipyridoxal diphosphate), using several magnetic resonance techniques (EPR spectroscopy, 31P-NMR spectroscopy, and relaxometry) to differentiate between free Mn++ and Mn++ in complexes in various preparations. The presence of calcium and magnesium in physiological concentrations in aqueous solutions induced the release of Mn++ from the complex, as did incubation of the complex in liver homogenates. After intravenous injection of 15 mumol/kg of Mn-DPDP, both EPR and 31P-NMR spectroscopy demonstrated that Mn-DPDP is partly dissociated (approximately 25%) in the liver. By comparing in vitro and ex vivo data from the liver, we concluded that the dissociation of Mn-DPDP occurs primarily in the liver, whereas a minor portion of the dissociated. Mn found in the liver comes from dissociation of the complex in the blood. Most of the dissociated Mn in liver becomes bound to macromolecules and is responsible for the enhancement of relaxivity observed with this agent.