Cutaneous Radiation Injuries: Models, Assessment and Treatments.

Many cases of human exposures to high-dose radiation have been documented, including individuals exposed during the detonation of atomic bombs in Hiroshima and Nagasaki, nuclear power plant disasters (e.g., Chernobyl), as well as industrial and medical accidents. For many of these exposures, injuries to the skin have been present and have played a significant role in the progression of the injuries and survivability from the radiation exposure. There are also instances of radiation-induced skin complications in routine clinical radiotherapy and radiation diagnostic imaging procedures. In response to the threat of a radiological or nuclear mass casualty incident, the U.S. Department of Health and Human Services tasked the National Institute of Allergy and Infectious Diseases (NIAID) with identifying and funding early- to mid-stage medical countermeasure (MCM) development to treat radiation-induced injuries, including those to the skin. To appropriately assess the severity of radiation-induced skin injuries and determine efficacy of different approaches to mitigate/treat them, it is necessary to develop animal models that appropriately simulate what is seen in humans who have been exposed. In addition, it is important to understand the techniques that are used in other clinical indications (e.g., thermal burns, diabetic ulcers, etc.) to accurately assess the extent of skin injury and progression of healing. For these reasons, the NIAID partnered with two other U.S. Government funding and regulatory agencies, the Biomedical Advanced Research and Development Authority (BARDA) and the Food and Drug Administration (FDA), to identify state-of-the-art methods in assessment of skin injuries, explore animal models to better understand radiation-induced cutaneous damage and investigate treatment approaches. A two-day workshop was convened in May 2019 highlighting talks from 28 subject matter experts across five scientific sessions. This report provides an overview of information that was presented and the subsequent guided discussions.

Basal ganglia dopamine loss due to defect in purine recycling.

Several rare inherited disorders have provided valuable experiments of nature highlighting specific biological processes of particular importance to the survival or function of midbrain dopamine neurons. In both humans and mice, deficiency of hypoxanthine-guanine phosphoribosyl transferase (HPRT) is associated with profound loss of striatal dopamine, with relative preservation of other neurotransmitters. In the current studies of knockout mice, no morphological signs of abnormal development or degeneration were found in an exhaustive battery that included stereological and morphometric measures of midbrain dopamine neurons, electron microscopic studies of striatal axons and terminals, and stains for degeneration or gliosis. A novel culture model involving HPRT-deficient dopaminergic neurons also exhibited significant loss of dopamine without a morphological correlate. These results suggest that dopamine loss in HPRT deficiency has a biochemical rather than anatomical basis and imply that purine recycling to be a biochemical process of particular importance to the function of dopaminergic neurons.

The role of dopamine receptors in the neurobehavioral syndrome provoked by activation of L-type calcium channels in rodents.

In rodents, activation of L-type calcium channels with +/-BayK 8644 causes an unusual behavioral syndrome that includes dystonia and self-biting. Prior studies have linked both of these behaviors to dysfunction of dopaminergic transmission in the striatum. The current studies were designed to further elucidate the relationship between +/-BayK 8644 and dopaminergic transmission in the expression of the behavioral syndrome. The drug does not appear to release presynaptic dopamine stores, since microdialysis of the striatum revealed dopamine release was unaltered by +/-BayK 8644. In addition, the behaviors were preserved or even exaggerated in mice or rats with virtually complete dopamine depletion. On the other hand, pretreatment of mice with D(3) or D(1/5) dopamine receptor antagonists attenuated the behavioral effects of +/-BayK 8644, while pretreatment with D(2) or D(4) antagonists had no effect. In D(3) receptor knockout mice, +/-BayK 8644 elicited both dystonia and self-biting, but these behaviors were less severe than in matched controls. In D(1) receptor knockout mice, behavioral responses to +/-BayK 8644 appeared exaggerated. These results argue that the behavioral effects of +/-BayK 8644 are not mediated by a presynaptic influence. Instead, the behaviors appear to result from a postsynaptic activation of the drug, which does not require but can be modified by D(3) or D(1/5) receptors.

Self-biting induced by activation of L-type calcium channels in mice: dopaminergic influences.

The L-type calcium channel activator +/-Bay K 8644 induces repetitive self-biting and self-injurious behavior in young mice. Since dopaminergic systems have been implicated in prior studies of these behaviors in both humans and animals, the present experiments were designed to test whether drugs influencing the dopaminergic systems could modify the behavioral responses to +/-Bay K 8644. The ability of +/-Bay K 8644 to provoke self-biting and self-injurious behavior was increased by amphetamine and GBR 12909, drugs that augment synaptic dopaminergic concentrations by blocking the reuptake and/or stimulating the release of dopamine. Conversely, self-biting and self-injurious behavior were decreased by tetrabenazine or reserpine, two drugs that deplete vesicular stores of dopamine. These results suggest that dopaminergic systems may play a role in the ability of +/-Bay K 8644 to provoke self-biting and self-injurious behavior.

Expression of c-FOS in the brain after activation of L-type calcium channels.

In rodents, administration of the L-type calcium channel activators, +/-Bay K 8644 and FPL 64176, causes an unusual neurobehavioral syndrome that includes dystonia and self-injurious biting. To determine the regional influence of these drugs in the brain, the induction of c-FOS was mapped after administration of these drugs to mice. In situ hybridization with an antisense riboprobe directed to c-FOS mRNA revealed widespread induction, with the highest levels in the striatum, cortex, hippocampus, locus coeruleus, and cerebellum. The induction of c-FOS mRNA was dose dependent, reached maximal expression approximately 60 min after drug treatment, and could be blocked by pretreatment with the L-type calcium channel antagonist, nifedipine. Immunohistochemical stains with an antibody directed to c-FOS protein revealed a pattern of induction similar to that obtained with in situ hybridization in most brain regions. These results demonstrate a very heterogeneous influence of L-type calcium channel activation in different brain regions, despite the nearly universal expression of these channels implied by more classical anatomical methods.

Self-biting induced by activation of L-type calcium channels in mice: serotonergic influences.

The L-type calcium channel activator +/-Bay K 8644 has recently been shown to provoke self-injurious biting in young mice. Since the serotonergic systems have been implicated in the expression of self-injurious behavior in both humans and animals, the present studies tested whether drugs influencing serotonin systems could modify the ability of +/-Bay K 8644 to cause this behavior. The ability of +/-Bay K 8644 to provoke self-biting behavior was increased by the serotonin uptake inhibitor fluoxetine or the monoamine oxidase inhibitor clorgyline. On the other hand, the ability of +/-Bay K 8644 to provoke self-biting was decreased by depletion of serotonin with p-chlorophenylalanine or 5,7-dihyroxytryptamine. These results suggest that the ability of +/-Bay K 8644 to provoke self-injurious behaviors may be mediated by serotonergic influences.

Pharmacologic thresholds for self-injurious behavior in a genetic mouse model of Lesch-Nyhan disease.

Congenital deficiency of hypoxanthine-guanine phosphoribosyl transferase (HPRT) causes Lesch-Nyhan disease in humans, which is associated with severe and recurrent self-injurious behavior (SIB). The HPRT-deficient knockout mouse model, however, does not display this unusual behavior. The present studies tested whether these mice might be more vulnerable to pharmacologic agents known to cause SIB in normal rodents, including clonidine, Bay K 8644, GBR 12909, methamphetamine, pemoline and caffeine. The results provided three conclusions. First, normal mice did not display SIB using some drugs known to provoke the behavior in rats (GBR 12909, caffeine), indicating important species differences in the expression of the behavior. Second, the C57BL/6J mice did not display SIB using drugs effective for other strains of mice (methamphetamine, pemoline), indicating important strain differences in expression of the behavior. Finally, there was no evidence that the HPRT-deficient mice were more susceptible to SIB when it occurred (clonidine, Bay K 8644).