Huntington’s disease (HD) is an inherited autosomal dominant neuropathological disease caused by a mutation in the huntingtin gene (htt). Preclinical animal models are necessary for the study of HD and the development of therapeutic interventions for the disease. Ideally, an animal model should demonstrate all the behavioral and neuropathological characteristics observed in HD patients. Numerous genetic mutant models of HD have been created, including transgenic rodents harboring the HD human mutation in specific neural regions. While these models provide a valuable tool to study the molecular pathways affected by the polyQ huntingtin mutation, they fail to present some primary neuropathological conditions of the disease. Prominently, they do not show the overt striatal cell loss that occurs in Huntington’s disease. Therefore, the excitotoxic lesion models, such as Quinolinic acid lesioned models, are employed when researchers need to understand the functional impact of cell loss or to test the potential therapeutics aimed at reducing the neural degeneration.
Quinolinic acid (QUIN) has an excitatory effect on the nervous system. It is a selective agonist at the N-methyl-D-aspartate (NMDA) receptors containing the NR2A and NR2B subunits with low receptor affinity. Increased levels of Quinolinic acid can cause excitotoxic cell death in the hippocampus and striatum. QUIN directly interacts with the free iron ions to form toxic complexes that aggravate the formation of reactive oxygen species (ROS), oxidative stress, and excitotoxicity. An intrastriatal injection of quinolinic acid produces axon-sparing neuronal lesion and mimics the pathological and neurobehavioral features of Huntington’s disease (HD). Besides NMDA receptor agonism, it also induces lipid peroxidation, increases inducible nitric oxide synthase (iNOS) expression, produces ROS, decreases superoxide dismutase (SOD) activity, and causes mitochondrial dysfunction (Beal., Ferrante., Swartz., & Kowall., 1991).
Unilateral and bilateral lesions
Neurotoxins could be infused in specific subregions of the striatum to generate specified motor or cognitive impairments, and the lesions could be created either unilaterally or bilaterally. Unilateral lesions are generated when a model of motor impairments is required, although cognitive or visuospatial changes could also be observed in this model by performing lateralized behavioral tasks. The unilateral lesion models assure good animal welfare since they maintain an intact hemisphere, ensuring minimal impact on feeding and motivational behaviors. Bilateral lesion models are required when more complex cognitive or behavioral functions are being investigated. In bilateral lesions, the chosen neural system is completely disrupted to examine the role of this circuitry in supporting behaviors or for the assessment and development of treatments for more complex non-motor symptoms. Bilateral lesions of quinolinic acid in the striatum induce hyperactivity and increase exploratory and rearing behaviors in rats. Bilateral lesions in the dorsolateral neostriatum could also harm habit formation, disrupt response learning, and impair Pavlovian stimulus–outcome learning. Bilateral damage in the medial striatal impacts upon behavioral flexibility and attentional sets formation, and serial spatial reversal learning. Finally, the ventral striatum damage impairs the motivated and rewarded behaviors, such as responses to the food-reinforced progressive ratio task (Lelos. & Dunnett., 2018).
Excitotoxicity produced by QUIN
Quinolinic acid selectively acts at NMDA receptors, specifically with the receptors containing the NR2A and NR2B subunits, with massive calcium entry into astrocytes and neurons. Therefore, it significantly exerts damage to neurons where these receptor subtypes are present. Hippocampus and striatum are most sensitive to QUIN neurotoxicity because of the wide distribution of NMDA receptors. Striatal spiny neurons having 𝛾-aminobutyric acid (GABA) and substance P are also vulnerable to quinolinic acid toxicity. Besides cell damage, QUIN can increase glutamate release and inhibit its reuptake in astrocytes, thereby increasing its concentration in the microenvironments, causing further neurotoxicity. It also limits glutamate to glutamine recycling in astrocytes by decreasing the glutamine synthetase activity. Additionally, chronic exposure of QUIN causes significant structural changes such as microtubular disruption, dendritic beading, and organelle damage. Acute intrastriatal administration of quinolinic acid damages the phosphorylating system of neural striatal cells, causing intermediate hyperphosphorylation of the cytoskeleton; this effect is catalyzed by Ca+2 influx through NMDA channels and oxidative stress (Huitrón et al., 2013).
Protocol (Lelos. & Dunnett., 2018)
Stock aliquots preparation
- Add 125 mg QUIN in 750 μl of sterile phosphate-buffered saline (PBS).
- Add 50 μl of 10 M sodium hydroxide (NaOH) in the solution.
- Dissolve the solution by sonication for 15 minutes.
- Add 3200 μl of PBS.
- Calculate the pH of the solution. Add 50 μl of 10 M sodium hydroxide to the solution to get a pH of 7.4. Adjust the pH with sodium hydroxide or concentrated hydrochloric acid (HCl), drop-by-drop, as needed.
- Add 2200 μl of PBS to make it a total of 0.12 M.
- Aliquot 50 μl into 200 μl Eppendorf tubes and store the solution at -20 °C until use.
- On the day of surgery, make the final concentration to 0.09 M for striatal lesions, and thaw one 50 μl aliquot and add 16.7 μl of PBS. Store it on the ice during surgery.
- Place the animal in the induction box, and anesthetize it with 4-5% isoflurane.
- Inject Metacam subcutaneously to relieve pain.
- Shave the animal’s head.
- Set the tooth bar to the appropriate coordinates.
- Place the animal in a stereotaxic frame, and ensure that the skull is flat and the ear bars are evenly spaced. Clamp the nose firmly onto the nose bar without causing injury. Apply the anesthetic mask.
- Maintain the anesthesia level at 1.5-2.5% isoflurane, in carrier gases 0.8 L/min oxygen and 0.4 L/min nitrous oxide. Check the breathing regularly. Also, monitor the level of liquid isoflurane throughout the surgical session.
- Cover the animal with a sterile drape to reduce the chances of contamination during incision.
- Clean the incision site with iodine and apply ointment to the eyes.
- Make an incision in the skin above the skull and wipe it to clear blood and tissues using sterile cotton buds.
- Identify bregma under the surgical microscope. Place the stereotaxic arm with mounted drill onto the stereotaxic frame and position the drill over bregma.
- Adjust the drill placement for the lesion as per the specified coordinates.
- Drill at the desired site for the lesion.
- Exchange the stereotaxic arm harboring the drill with the arm carrying the syringe. Place the injection needle over the drill hole in the skull.
- Flush the syringe with toxin by first flushing with distilled water, then expelling to 0.5 μl, and draw in ~0.2 μl air, then position in the toxin solution and fill to 10 µl by drawing back the plunger.
Note: A small air bubble is seen in the syringe. The free movement of the air bubble without compression confirms the delivery of the toxin without needle blockage. The syringe drive is set at the speed of ~0.25 μl of toxin/minute.
- Lower down the syringe into the burr hole. As the needle touches the dura, a subtle dip could be observed in the fluid surrounding the brain. Stop advancing the needle, retract by 0.1 mm, and use this site as 0 for dorsal/ventral coordinates.
- Raise the syringe and mark the dura with a fine surgical needle to allow smooth needle passage into the brain without resistance.
- Lower the syringe in the brain and confirm the depth coordinates. Start the syringe drive. Confirm correct toxin delivery by observing the movement of the bubble.
- After complete infusion, leave the syringe in place for 3 minutes to diffuse the toxin completely into the surrounding parenchyma and to reduce the expulsion of the fluid.
- Retract the syringe. Run the syringe drive between the injection sites again to ensure the smooth toxin flow.
- After the toxin infusions are completed, gently use sterile cotton buds to reposition the skin over the skull and suture the wounds.
- For hydration, inject 5 ml glucose saline subcutaneously.
- Inject Diazepam intramuscularly or subcutaneously.
- Place the animal in a warm recovery chamber for 2 hours.
Evaluating the neuroprotective effects of pyruvate (Ryu., Kim., & McLarnon., 2003)
The quinolinic acid rat model of Huntington’s disease was employed to explore the neuroprotective effects of pyruvate, the end metabolite of glycolysis. Intrastriatal injection of QUIN caused widespread damage to rat striatum, as demonstrated by cresyl violet staining and immunohistochemical analysis. A significant reduction in striatal lesions was observed following the intraperitoneal administration of pyruvate (500-1000 mg/kg). It was found that pyruvate concentration of 250 mg/kg is not protective. The protective effects of pyruvate were also assessed over a range of time, from application to 1 h post-administration; however, no such results were seen if pyruvate was applied 30 minutes before QUIN injection or 3 hours post-administration. It was also found that pyruvate protects different types of striatal neurons against quinolinic acid toxicity, including cholinergic interneurons, GABAnergic projection neurons, and NADPH-diaphorase interneurons. These results suggested that pyruvate is effective in reducing neuronal damage in Huntington’s disease.
Analysis of receptor changes and microglia activation (Moresco et al., 2008)
Quinolinic acid rat model is a well-studied pharmacological model of Huntington’s disease to produce many of the distinctive features of this neurodegenerative disorder. In this study, this model was used to assess the temporal behavior (from 1 to 60 days from the excitotoxicity) of neuronal cell density and receptor availability (adenosine A2A and dopamine D2 receptors) along with microglia activation. It was observed that the reduction in adenosine A2A and dopamine D2 receptors was paralleled by increased microglial activation. This model represented a more quantitative approach to compare the neuroprotective effects of treatments in slowing down or reversing Huntington’s disease in rodent models with potential applications in clinical practices.
Investigating amino acid neurotransmitter abnormalities (Ellison et al., 1987)
Quinolinic acid lesioned rat models could also be used to measure the concentrations of neurotransmitters. In this study, the levels of gamma-aminobutyric acid (GABA), aspartate, glutamate, and taurine were measured in postmortem brain tissue from the HD patients and in the quinolinic acid (QA) lesioned rat striatum. Significant reductions in both glutamate and GABA were found in HD striatum. These reductions were greater in the severely affected grades of HD, and there was a differential decrease in amino acid level across the striatal nuclei, which was consistent with the known pattern of disease pathology. No significant changes were observed in taurine and aspartate concentrations. These findings suggest that the QA model could be efficiently applied to assess and reproduce the neurochemical features of Huntington’s disease.
Strengths and limitations
- Quinolinic acid is an N-methyl-d-aspartate (NMDA) receptor agonist having high in vivo excitotoxicity, and it closely represents the striatal loss and neuronal loss as observed in HD.
- Quinolinic acid lesioned rat models are designed to target the striatal projection neurons specifically and are employed to study the functional outcomes of striatal atrophy.
- Bilateral and unilateral QUIN lesions could create the required damage to the striatal neurons for the region-specific investigation of Huntington’s disease.
- These models are powerful tools to investigate potential therapeutic interventions that target neuronal degeneration.
- Quinolinic acid could not target the cholinergic, calretinin-positive, and parvalbumin immunoreactive interneurons.
- Beal., F. M., Ferrante., J. R., Swartz., J. K., & Kowall., W. N. (1991). Chronic quinolinic acid lesions in rats closely resemble Huntington’s disease. J Neurosci, 11(6), 1649-59.
- J. M. Lelos., & Dunnett., B. S. (2018). Generating Excitotoxic Lesion Models of Huntington’s Disease. Methods Mol Biol, 1780, 209-220.
- K. J. Ryu., U. S. Kim., & McLarnon., G. J. (2003). Neuroprotective effects of pyruvate in the quinolinic acid rat model of Huntington’s disease. Exp Neurol, 183(2), 700-4.
- M. R. Moresco., T. Lavazza., S. Belloli., M. Lecchi., A. Pezzola., S. Todde., Fazio., F. (2008). Quinolinic acid induced neurodegeneration in the striatum: a combined in vivo and in vitro analysis of receptor changes and microglia activation. Eur J Nucl Med Mol Imaging, 35(4), 704-715.
- R. L. Huitrón., P. U. Muñiz., B. Pineda., J. P. Chaverrí., C. Ríos., & Cruz, V. P. (2013). Quinolinic Acid: An Endogenous Neurotoxin with Multiple Targets. Oxid Med Cell Longev.
- W. D. Ellison., F. M. Beal., F. M. Mazurek., R. J. Malloy., D. E. Bird., & Martin., B. J. (1987). Amino acid neurotransmitter abnormalities in Huntington’s disease and the quinolinic acid animal model of Huntington’s disease. Brain, 110, 1657-73.