How Fish Got Hooked on Hydrocodone

By Pat Anson, Editor

We hear it all the time from PNN readers. They don’t trust academic research about opioids and addiction, and feel much of it is biased or just plain fishy.

You can certainly say the latter about a new study by researchers at the University of Utah.

They devised a system that allows zebrafish, a small tropical fish popular in home aquariums, to self-administer doses of the painkiller hydrocodone. In less than a week, researchers say the fish were hooked on hydrocodone and showed signs of drug-seeking behavior and withdrawal.

"We didn't know if zebrafish would be a relevant model for opioid addiction, much less self-administer the drug," said Randall Peterson, PhD, a professor of Pharmacology and Toxicology, and senior author of the study published in the journal Behavioral Brain Research.

"What is exciting about this work is that we see many of the hallmarks of addiction in zebrafish. This could be a useful and powerful model."

How is this useful and how does it relate to people?

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Zebrafish have more in common with people than you might think. They have 70 percent of the genes that humans have, including similar biological pathways that can lead to addiction. Like people, zebrafish have a μ-opioid receptor and two neurotransmitters, dopamine and glutamate, that trigger the natural reward system in the brain.

"Drugs of abuse target the pathways of the pleasure centers very effectively," said first author Gabriel Bossé, PhD. "These pathways are conserved in zebrafish, and the fish can experience some of the same signs of addiction and withdrawal as people."

Bossé and Peterson tested their system in a tank with a food dispenser equipped with a motion detector that the fish could trigger by swimming nearby. It didn’t take long for the zebrafish to learn how to get food.

Then the researchers removed the food dispenser and replaced it with one that injected small doses of hydrocodone into the water when a fish swam nearby. A continuous flow of water flushed the tank, which forced the fish to trigger the dispenser to receive another dose of hydrocodone.

Over the course of five days, the fish learned how to self-administer the drug. You can watch a demonstration below:

"The fish needed to perform an action to get the drug rather than receiving it passively," said Bossé. "Drug-seeking has been modeled before in rodents and primates, but having a model to study this in zebrafish could move the [study of addiction] forward."

The drug-seeking behavior increased when the zebrafish were forced to receive the opioid in progressively shallower water, a stressful environment that unconditioned fish would normally avoid.

"This was important, because we forced the fish to do more work to receive the drug, and they were more than willing to do more work," said Peterson.

The researchers took their experiment a step further by exposing the conditioned fish to naloxone, a drug used to treat overdoses that blocks opioid receptors. Sure enough, naloxone appeared to reduce the fish’s drug-seeking behavior.

The researchers believe their zebrafish model can lead to new drug therapies, because it can be used to rapidly test thousands of different chemical compounds. They also believe the genetic make-up of zebrafish can be altered to explore the specific biological pathways associated with addiction.

Zebrafish do have other qualities humans can learn from. Researchers at Duke University are studying proteins that enable a zebrafish to completely heal its spine -- even after it was severed. They hope this knowledge will someday lead to new therapies to repair damaged spinal cords in humans.

Zebrafish Could Help Repair Human Spinal Cord Injuries

By Pat Anson, Editor

A freshwater fish popular in many home aquariums might hold the key to repairing damaged spinal cords in humans.

In a study published in the journal Science, Researchers at Duke University say the zebrafish is able to completely heal its spine -- even after it was severed -- because of a protein that helps to rebuild damaged spinal nerves.

"This is one of nature's most remarkable feats of regeneration," said the study's senior investigator Kenneth Poss, a professor of cell biology and director of the Regeneration Next initiative at Duke. "Given the limited number of successful therapies available today for repairing lost tissues, we need to look to animals like zebrafish for new clues about how to stimulate regeneration."

Poss and his colleagues say when the zebrafish's spinal cord is severed, dozens of genes are activated by the injury. Within days, they produce new molecules and proteins to bridge the gap between the severed spine. One of the proteins, called connective tissue growth factor (CTGF), appears to play a key role in regenerating glia nerve cells.

Within 8 weeks, new nerve tissue has filled the gap in the spinal cord and the zebrafish has fully reversed its paralysis.

"The fish go from paralyzed to swimming in the tank. The effect of the protein is striking," said lead author Mayssa Mokalled, a postdoctoral fellow at Duke. “We thought that these glial cells and this gene must be important.”

The zebrafish belongs to the minnow family and is native India, Pakistan and the Himalayan region. It is widely used in scientific research because if it’s regenerative abilities, and was the first vertebra to be cloned. A 2012 study published in The Journal of Neuroscience also documented the fish’s ability to bridge the gap between spinal nerve cells.

Humans and zebrafish share many genes, and the human CTGF protein is nearly 90% similar in its amino acids to the zebrafish’s.

Remarkably, when Duke researchers added the human version of CTGF to the severed spinal cords in zebrafish, it boosted regeneration and the fish began swimming two weeks after the injury.

Healing damaged spinal cords is more complex in mammals, in part because scar tissue forms around the injury. Researchers say CTGF is probably not sufficient on its own for people to regenerate their spinal cords, but further animal studies are needed.

"Mouse experiments could be key," Mokalled says. "When do they express CTGF, and in what cell types?"

The Duke research team also plans to follow up with other proteins that were secreted after injury, which may provide further hints into the zebrafish's ability to regenerate nerve cells.

"I don't think CTGF is the complete answer, but it's a great thing to have in hand to inform new ways to think about the real challenge of trying to improve regeneration," Poss said.

This research was supported by the National Institutes of Health, the Max Planck Society, and Duke University School of Medicine. Duke is seeking patent applications related to the research.