Opiates have brought pain relief to humankind for hundreds of years, but they don't come without consequences. Motor impairment and respiratory depression are a couple of potential side effects, but from opium-dependent Chinese of the mid-19th century to the morphine-riddled soldiers of the Vietnam War, the risk of addiction remains the biggest problem. Researchers have now developed a new painkiller they claim to be as strong as morphine, but without much of this unwanted baggage.

Scientists at Tulane University in New Orleans engineered a version of a chemical called endomorphin, which occurs naturally in the body. The peptide-based drug targets the same pain-relieving opioid receptor as morphine, so by measuring their performance side-by-side the researchers aimed to establish how its safety and effectiveness stacked up.

The researchers treated rats with similarly potent dosages of both the new endomorphin drug and morphine. Motor skills were significantly impaired in those receiving morphine, as was their breathing, while the rats receiving endomorphin experienced no substantial respiratory depression or impairment of motor skills. The pain relief offered by the endomorphin was equal to or greater than the morphine.

"These side effects were absent or reduced with the new drug," says leader of the research James Zadina, professor of medicine, pharmacology and neuroscience at Tulane University School of Medicine. "It's unprecedented for a peptide to deliver such powerful pain relief with so few side effects."

The team also carried out experiments to test how addictive the new drug might be. It found that while rats would spend more time in a compartment where they had received morphine, treatment with the endomorphin did not induce this behavior. It also rigged up a system where both groups of rats could press a bar to receive a dose of their drug, but only the morphine-treated group showed increased efforts to secure further helpings. Zadina says these tests are predictive of human drug abuse.

Furthermore, endomorphin did not result in spinal glial cell activation, an established effect of morphine that is known to help build up tolerance of the drug and in turn abuse or risk of overdose. The researchers are hopeful of commencing human clinical trials within the next two years.

The research was published in the journal Neuropharmacology.

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