Richard Lerner, MD, and Scott Hansen, PhD, of Scripps Research may have figured out how anesthesia works, which has otherwise been a mystery throughout its 175 years of use.
Clusters of lipids in the cell membrane serve as a missing go-between in a two-part mechanism, the researchers report. Temporary exposure to anesthesia causes the lipid clusters to move from an ordered state to a disordered one, and then back again, leading to multiple subsequent effects that ultimately cause changes in consciousness.
The research settles the debate on whether anesthetics act directly on cell-membrane gates called ion channels or if they somehow act on the membrane to signal cell changes in a new and unexpected way. The researchers said that it’s a two-step process that begins in the membrane. The anesthetics perturb ordered lipid clusters within the cell membrane known as “lipid rafts” to initiate the signal.
“We think there is little doubt that this novel pathway is being used for other brain functions beyond consciousness, enabling us to now chip away at additional mysteries of the brain,” said Lerner, a member of the National Academy of Sciences, a former president of Scripps Research, and founder of the Scripps Research campus in Jupiter, Florida.
Ether’s ability to induce loss of consciousness was first demonstrated on a tumor patient at Massachusetts General Hospital in Boston in 1846. By 1899, German pharmacologist Hans Horst Meyer and then in 1901 British biologist Charles Ernest Overton concluded that lipid solubility dictated the potency of such anesthetics.
Hansen was investigating how lipid solubility impacted anesthetic potency, thinking that he couldn’t be the only one convinced of the role of membrane lipid rafts in the process, when he found research by Lerner proposing such a mechanism.
“I contacted him, and I said, ‘You are never going to believe this. Your 1997 figure was intuitively describing what I am seeing in our data right now,” said Hansen, who is an associate professor at the Jupiter campus. “It was brilliant.”
“This is the granddaddy of medical mysteries,” said Lerner. “When I was in medical school at Stanford, this was the one problem I wanted to solve. Anesthesia was of such practical importance I couldn’t believe we didn’t know how all of these anesthetics could cause people to lose consciousness.”
Previous researchers have been unable to answer these questions because they lacked key elements, Hansen, said, such as microscopes able to visualize biological complexes smaller than the diffraction limits of light and recent insights into the nature of cell membranes and the complex organization and function of the rich variety of lipid complexes that compose them.
“They had been looking in a whole sea of lipids, and the signal got washed out. They just didn’t see it, in large part, for a lack of technology,” Hansen said.
Using direct stochastical optical reconstruction microscopy (dSTORM), a microscopic technology that won the Nobel Prize, a postdoctoral researcher in Hansen’s lab bathed cells in chloroform and watched something like the opening break shot of a game of billiards, the researchers said.
Exposing the cells to chloroform strongly increased the diameter and area of cell membrane lipid clusters called GM1, Hansen said. It was a shift in the GM1 cluster’s organization from a tightly packed ball to a disrupted mess, he continued. As it grew disordered, GM1 spilled its contents, including an enzyme called phospholipase D2 (PLD2).
Tagging PLD2 with a fluorescent chemical, Hansen watched as PLD2 moved like a billiard ball away from its GM1 home and over to a different, less preferred lipid cluster called PIP2. This activated key molecules within PIP2 clusters, including TREK1 potassium ion channels and their lipid activator, phosphatidic acid (PA). The activation of TREK1 basically freezes neurons’ ability to fire, leading to loss of consciousness, Hansen said.
“The TREK1 potassium channels release potassium, and that hyper-polarizes the nerve. It makes it more difficult to fire and just shuts it down,” Hansen said.
Lerner insisted they validate the findings in a living animal model. The common fruit fly, drosophila melanogaster, provided that data. Deleting PLD expression in the flies rendered them resistant to the effects of sedation. In fact, they required double the exposure to the anesthetic to demonstrate the same response.
All of the fruit flies lost consciousness eventually, suggesting that PLD helps set a threshold, but it isn’t the only pathway that controls anesthetic sensitivity, the researchers said. Also, these discoveries raise tantalizing new possibilities that may explain other mysteries of the brain, including the molecular events that lead us to fall asleep, the researchers said.
Lerner’s original 1997 hypothesis of the role of “lipid matrices” in signaling arose from his inquiries into the biochemistry of sleep and his discovery of a soporific lipid he called oleamide. Hansen’s and Lerner’s collaboration in this arena continues.
“We think this is fundamental and foundational, but there is a lot more work that needs to be done, and it needs to be done by a lot of people,” Hansen said.
“People will begin to study this for everything you can imagine: sleep, consciousness, all those related disorders,” Lerner said. “Ether was a gift that helps us understand the problem of consciousness. It has shined a light on a heretofore unrecognized pathway that the brain has clearly evolved to control higher-order functions.”