A New Transgenic Rat Model to Study the Role of μ-Opioid Receptors in Addiction-Related Behaviors
Jennifer M. Bossert, Carlos A. Mejias-Aponte, Thomas Saunders, Lindsay Altidor, Michael Emery et al.
(see pages 1692–1713)
The μ-opioid receptor (MOR) is the target of both endogenous and exogenous opioids, such as heroin, morphine, and fentanyl. MORs are found throughout the brain, and their activation mediates the rewarding and analgesic effects of opioid drugs. But their role in circuits controlling complex behaviors in animal models of opioid addiction has remained obscured. Transgenic mice have been used as a model organism to study cell-specific effects of MOR activation, but their small size and limited capacity for sophisticated learning behaviors has restricted what researchers could glean from them.
Rats provide a better model to study more complex addiction-related behaviors, but the genetic tools to probe them have not been developed. Now, Bossert et al. have used Cre line technology to create a CRISPR-mediated knock-in rat (Oprm1-Cre) that coexpresses an improved Cre recombinase and the MOR protein. Validation of the model showed that the transgenic rats did not differ from wild-type rats in expression of Oprm1 mRNA in nucleus accumbens (NAc), dorsal striatum, or dorsal hippocampus, nor did it differ in receptor activity. MOR protein density was slightly elevated in dorsal striatum in transgenic compared with wild-type rats.
Food self-administration, heroin self-administration, and heroin relapse-related behaviors were also similar regardless of genotype. Pain-related responses to aversive mechanical and thermal stimuli also did not differ between transgenic and wild-type rats, and rats of either genotype showed morphine dose-dependent effects, tolerance, and reversal of analgesia by naloxone, as well as intact operant learning. Bossert et al. next tested the effect of lesioning MOR cells from the NAc using a Cre-dependent adenovirus strategy. The lesion had no effect on feeding behaviors but decreased acquisition of heroin self-administration behavior in male but not female rats. In contrast, drug administration was decreased in females but not males when the rats had extended access to heroin. In males and females, lesioned rats showed decreased radioactive binding of an MOR agonist.
The study reveals potential sex differences in the role of NAc MOR in heroin self-administration and serves as a proof of concept that this transgenic rat model provides a novel way to manipulate MOR-expressing neurons in the brain. This will allow further elucidation of the mechanisms of complex opioid addiction-related behaviors and other MOR-mediated behaviors like opioid analgesia.
Colocalization (yellow) of Oprm1+ (red) neurons and Cre+ (green) neurons in the nucleus accumbens of a transgenic rat.
Drug Improves Pathology, Behavior in Batten Disease Mouse Model
Malabendu Jana, Debashis Dutta, Jit Poddar, and Kalipada Pahan
(see pages 1814–1829)
Batten disease, or juvenile neuronal ceroid lipofuscinosis (JNCL), is a rare but devastating disease that strikes in childhood and is always fatal, with life expectancy only in the teens or twenties. Children first present with vision loss or loss of motor coordination and progress to seizures, cognitive decline, and often end up bedbound and unable to communicate. This week, Malabendu et al. find astonishing results in a mouse model of JNCL with a Food and Drug Administration-approved lipid-lowering drug.
JNCL arises from an inability to clear the metabolic product lipofuscin due to a mutation in the Cln3 gene, which encodes a protein involved in lysosomal function. Pathology of JNCL is marked by accumulation of autofluorescent material including lipoproteins and glycoproteins in neurons as well as peripheral non-neuronal cells. Astroglial and microglial activation are rampant in areas of neuronal loss in autopsy samples, providing a clue to the pathophysiology. The drug gemfibrozil is used to lower triglycerides and reduce the risk of hyperlipidemia, but recent studies have shown that it also has anti-inflammatory properties and has provided benefits in animal models of multiple sclerosis, Alzheimer's disease, and Parkinson's disease. In a transgenic mouse model of JNCL (Cln3ΔJNCL), which recapitulates the most common mutation seen in patients, oral treatment with gemfibrozil reduced the widespread glial activation normally seen in the somatosensory barrel field (SBF) cortex. Proinflammatory molecules inducible nitric oxide synthase, pro-IL-1β, IL-1β, and TNFα were elevated in the SBF cortex of Cln3ΔJNCL mice but were reduced by gemfibrozil treatment. Transcription factor-EB (TFEB) regulates lysosomal biogenesis and clearance of cellular material and is markedly decreased in SBF cortex of 6-month-old Cln3ΔJNCL mice, but TFEB was normalized by gemfibrozil treatment. Downstream proteins controlled by TFEB were also increased with treatment, as was autophagy. That led to a decrease in the disease-associated accumulation of autofluorescent storage material in the cortex. Importantly, treatment also improved functional motor performance on open-field, pole, and rotarod tests. The authors traced the neuroprotective effects of gemfibrozil to its increased activation of peroxisome proliferator-activated receptor α (PPARα). The exciting findings suggest that oral treatment with gemfibrozil has the potential to improve outcomes for people with JNCL.
Footnotes
This Week in The Journal was written by Stephani Sutherland
