Ketamine flips a ‘switch’ in brain circuits of mice: study

IAnd the 1950s, scientists on a mission to create better anesthesia drugs have synthesized phencyclidine, commonly known as PCP. Although PCP has worked well in keeping most people unconscious during surgeries, some have experienced what the authors of a 1959 trial described as “a delirium and hallucinations which, although generally of a very pleasant nature, are at times rather terrifying to patients”. This so-called dissociated state – when what the brain experiences is disconnected from reality – lasted for up to 12 hours.

In search of a shorter-acting agent, researchers in the 1960s created a compound structurally related to PCP called ketamine. Ketamine remains a common anesthetic today, says Joe Cichon, a neuroscientist and anesthesiologist at the Perelman School of Medicine at the University of Pennsylvania. At lower doses than would be used for anesthesia, people remain conscious while experiencing a dissociated state similar to PCP, but for much less time. In the 2000s, researchers discovered that these lower, so-called sub-hypnotic doses of ketamine had a long-lasting antidepressant effect. Several weeks, long after the body has metabolized the drug, Cichon says. “And even to this day, after 60 years of human use . . . we still don’t quite understand how it exerts all these different effects over a wide range of doses – that’s the mystery of ketamine.

Now, Cichon and his colleagues have uncovered a new clue: Using calcium photon imaging, they found that the drug flipped a “switch” in the brains of mice, turning off neurons that fired while awake while they were awake. by activating a distinct group of dormant neurons. The conclusions, published on 24 November in Natural neurosciencesuggest that this activity switching might result from suppressing neurotransmission through one set of receptors in some cells while promoting neurotransmission through a different set of receptors in others.

“[This study] allows us to get a view of what’s going on in the brain during this dissociated state, which we didn’t have a very good view of under the microscope,” explains Alex Kwan, a neuroscientist at Cornell University’s Meinig School of Biomedical Engineering, who was not involved in the research. Kwan adds that the researchers studied the switching effect in “many areas of the brain and many layers to confirm their findings, and it seems to be quite widespread.”

Activate and deactivate neurons

To probe how ketamine alters neuronal activity, the researchers gave the drug to transgenic mice and measured its impact on cells in several layers of their neocortex. This region is the six-layered outer surface of the mammalian brain that is responsible for higher functions such as remembering, thinking, learning and reasoning, says study co-author and neuroscientist and anesthesiologist UPenn. Alex Proekt. The mice were engineered so that specific excitatory neurons released calcium upon firing, which the researchers recorded with their imaging technique.

See “CRACK method reveals new type of neuron in mouse brain

“The mouse is a perfect specimen to do this type of study because the skull and the brain itself [are] very flat,” unlike the twisting nooks and crannies of the human brain, Cichon says. Before injecting mice with ketamine, the calcium signal from some neurons fluctuated, indicating that they fired during the wakeful state, while the signal from dormant neurons was essentially flat. About 10 to 20 minutes after injecting sub-hypnotic doses of ketamine into the mice’s abdomens, some neurons seemed to shut down while others suddenly came to life.

graph representing the activity of three groups of neurons.  The red and green lines represent a decrease and an increase in activity respectively.

The activity of six murine cortical neurons recorded during normal wakefulness (left) compared to the same six cells in the presence of subhypnotic doses of ketamine (right), which shows increased activity in three cells (green), reduced activity in two (red), and no change in one (black).


“Ketamine, in a way not previously known, induces a complete change in active circuitry,” Cichon explains. “So you’re exchanging an active circuit for a new circuit that’s basically emerging from the dark.”

The researchers repeated the experiment, but this time applying the drug directly to the neocortex of each mouse. Because they saw a similar switching effect, the researchers concluded that whatever mechanism was at play was happening at the neural level, not somewhere upstream, as it might have been during the injection of the drug into the abdomen of mice. Additionally, Cichon and his colleagues repeated the experiments in different regions as well as deeper layers of the cortex, which again showed a similar exchange of cellular activity.

“I don’t think I know of any other drug that would do such a thing,” Kwan says. “So to observe that, from this study of several [neocortex] areas across multiple layers of neurons – this is just a very surprising finding,” and which he says he intends to research in his own data.

Information highways versus feeder roads

Cichon and his colleagues hypothesized that ketamine turned off certain neurons by blocking their N-methyl-D-aspartate (NMDA) receptor, something the drug was already known to do. To probe the mystery of why different neurons fired, the researchers turned to other known effects of ketamine: in particular, the drug’s ability to suppress cells that inhibit excitatory neurons called interneurons, and to block the channels activated by the hyperpolarization of cyclic nucleotides (HCN). —transmembrane proteins that regulate the excitability of neurons.

When the researchers injected the mice with ketamine along with compounds that dampen the drug’s normal suppression of two types of interneurons – essentially forcing these cells to remain active – the change in activity did not occur, indicating that the switching effect depends on the ability of ketamine to subdue them. In another experiment, researchers injected different compounds that blocked HCN channels, mimicking the effect of ketamine without actually injecting it. This induced a similar switching effect, suggesting that these channels also play a role.

So it’s not that ketamine itself treats depression. This is [that] ketamine produces a certain state of the brain which then leads to changes in the brain.

—Alex Proekt, University of Pennsylvania

Given the combined results, a possible explanation for the contribution of NMDA receptors, interneurons, and HCN channels to neural switching emerged from a neuroscience adage: “Neurons that fire together, wire together,” Proekt says. . Neurons that fire synchronously tend to rely on NMDA neurotransmission, and the synaptic connections between these cells grow stronger the more they are used, he says. Like a muscle that bulges with repeated dumbbells, these pathways can become so strong that they end up functioning as information highways, carrying most of the traffic from the cortex. On the other hand, neurons that fire asynchronously may have atrophied synaptic connections connecting them, Proekt says, that may function as front roads that are barely or never traveled.

With ketamine barricading NMDA highways by impeding the ability of interneurons to inhibit arousal and impeding the ability of HCN channels to regulate arousal, the drug could create a scenario where highway traffic is diverted to highways. less frequented secondary ones, stimulating these neurons into action. Although the NMDA receptors on these newly awakened neurons would remain suppressed, the cells could still transmit signals via AMPA receptors.

Kwan calls the team’s calcium imaging technique “state of the art,” but a relatively slow method of measuring brain activity – on the order of seconds. “I think a very good next step would be to use electrophysiology to record the electrical activity of the brain, which will give you millisecond-scale resolution,” he says, adding that this approach could reveal finer details of how the change in activity occurs. Kwan also notes that the researchers’ method is indirect. Because NMDA receptors alter the flow of calcium into the dendrites of neurons, the team’s imaging technique may have, at least in part, captured the action of ketamine on the receptors rather than neuronal activity. Electrophysiology would measure voltage directly, but he admits it would also be more difficult to measure individual cells, as the researchers did in this study.

Proekt points out that the researchers’ mechanistic theory remains speculation, but their findings bring scientists one step closer to understanding the unusual ketamine-induced brain state that helps treat depression, and possibly other conditions such as addiction.

Opening new avenues to treat depression

At subhypnotic levels, ketamine leaves a person’s system in about one to two hours, but the antidepressant benefits last for several weeks, Proekt says. “So it’s not that ketamine itself treats depression,” he argues. “His [that] ketamine produces a certain state of the brain which then leads to changes in the brain. He and his colleagues plan to further explore how the newly discovered change in activity plays into this neuroplasticity, Proekt says, adding that the research could lead to new treatments for depression.

James Murrough, a psychiatrist who was not involved in the work but who studies the antidepressant effects of ketamine at Mount Sinai’s Icahn School of Medicine, points out that using mice in this analysis has a significant limitation when considering is about gleaning information about humans. “The problem we often have in [psychiatric disorders]which are fundamentally disorders of feeling and thought, [is that] these cannot be measured in an animal,” he says. Instead, the researchers rely on behavioral surrogates, such as a characteristic head movement that indicates the dissociative state in mice. In reality, “we don’t know for sure what these mice are going through or if they are dissociated.”

Still, he says, these researchers are “really finding new evidence — at the cellular level — of what ketamine is doing in the brain.” The neural switching effect shown in this article represents an important piece of the puzzle for treating clinical depression, which he compares to a state of obstructed thought patterns that cause persistent negative thinking. Ketamine seems to work like a reboot for the mind, he says, the same way rebooting a frozen computer can unlock it.

“We need the fingerprint of this [the drug] done in the brain for the fundamental discovery of new drugs for depression,” says Murrough. “I think that [research] could bring us a little closer in that direction.

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