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In early March, the FDA approved a nasal spray for depression based on ketamine, a substance once known only as a rave drug. Despite its reputation, ketamine is so promising as an anti-depressant that it will soon be available in licensed clinics throughout the country. A study published in Science on Thursday proposes the new treatment can be made even better.
In their paper, a team of scientists at Weill Cornell’s Medicine’s Feil Family Brain and Mind Research Institute show that ketamine can help the brain reform synapses, crucial connections between neurons, that can alleviate depressive symptoms. Ketamine is already famous for working quickly to relieve depressive symptoms — within days or hours — co-author Conor Liston, Ph.D., tells Inverse, but maintaining those crucial connections is key to extending its effects.
“Our study shows that the formation of new connections (synapses) between brain cells is required for sustaining ketamine’s antidepressant effects in the days after treatment,” says Liston, also professor of neuroscience at Weill Cornell. “Ketamine is an exciting new treatment for depression that differs from drugs like SSRIs in that it relieves symptoms rapidly. However, those effects are not always sustained.”
Growing Back Dendritic Spines
In a mouse model, Liston and his co-authors demonstrated that doses of ketamine helped mouse brains regrow dendritic spines, small protrusions on neurons that help them pick up signals rom other cells that, crucially, degrade during exposure to chronic stress. These dendritic spines are a key part of synapse formation.
The degradation of these spines is not a perfect analog to human depression, but humans have them as well, and Liston points out that some of the most important features of depression in humans are also present in chronically stressed mice.
To create depression-like conditions, the team degraded the dendritic spines in their mice using stress hormones. Then, they gave one group a dose of ketamine, which they expected to have anti-depressive effects.
The dose of ketamine not only changed the mice’s behavior — they tried harder to escape their cages — it also helped reform the dendritic spines in their brains. Interestingly, the ketamine didn’t form random dendritic spines but actually seemed to replace the old ones that had been degraded by constant stress. Of the new spines formed, 47.7 percent grew within two micrometers of where the old ones once were.
Why Dendritic Spines Are Important
The new dendritic spines serve an important purpose in the mouse brains. Within three hours of treatment, previously damaged circuits in the prefrontal cortex were starting to come back online, but this happened before new synapses form. At the end of the experiment, an estimated 20.4 to 31.0 percent of the lost synapses were restored after the mice took ketamine.
The fact that the circuits were restored before the synapses reformed suggests that ketamine jump-starts a two-step process that fights depression. The first step is the rapid anti-depressant effect that is seen in so many studies. The second step — regrowing the spines and restoring synapses — occurs more slowly, which means it’s the one scientists should focus on if they’re looking to make ketamine’s effects on depression last longer, Liston says.
When Liston used blue light to artificially remove the newly grown spines in a follow-up experiment, the mice relapsed into depressive symptoms. It suggested that maintaining these dendritic spines is important in keeping depression at bay.
“Our results suggest that interventions aimed at enhancing the survival of newly formed connections in prefrontal brain circuits could be useful for augmenting ketamine’s antidepressant effects by increasing their durability in the days and weeks after treatments.”
The FDA’s approval of a ketamine-based drug to treat depression was groundbreaking in itself, especially since it works differently than other anti-depressant drugs. But just because it’s been approved doesn’t mean there aren’t ways to improve it. Depression can be alleviated with ketamine, but for now the illness constantly threatens individuals with remission. Preventing the potential for a relapse with the promise of longer-lasting effects is one way to make this already remarkable drug even more helpful.
Depression is an episodic form of mental illness, yet the circuit-level mechanisms driving the induction, remission, and recurrence of depressive episodes over time are not well understood. Ketamine relieves depressive symptoms rapidly, providing an opportunity to study the neurobiological substrates of transitions from depression to remission and to test whether mechanisms that induce antidepressant effects acutely are distinct from those that sustain them.
Contrasting changes in dendritic spine density in prefrontal cortical pyramidal cells have been associated with the emergence of depression-related behaviors in chronic stress models and with ketamine’s antidepressant effects. But whether and how dendritic spine remodeling is causally involved, or whether it is merely correlated with these effects, is unclear. To answer these questions, we used two-photon imaging to study how chronic stress and ketamine affect dendritic spine remodeling and neuronal activity dynamics in the living prefrontal cortex (PFC), as well as a recently developed optogenetic tool to manipulate the survival of newly formed spines after ketamine treatment.
The induction of depression-related behavior in multiple chronic stress models was associated with targeted, branch-specific elimination of postsynaptic dendritic spines and a loss of correlated multicellular ensemble activity in PFC projection neurons. Antidepressant-dose ketamine reversed these effects by selectively rescuing eliminated spines and restoring coordinated activity in multicellular ensembles that predicted motivated escape behavior. Unexpectedly, ketamine’s effects on behavior and ensemble activity preceded its effects on spine formation, indicating that spine formation was not required for inducing these effects acutely. However, individual differences in the restoration of lost spines were correlated with behavior 2 to 7 days after treatment, suggesting that spinogenesis may be important for the long-term maintenance of these effects. To test this, we used a photoactivatable probe to selectively reverse the effects of ketamine on spine formation in the PFC and found that the newly formed spines play a necessary and specific role in sustaining ketamine’s antidepressant effects on motivated escape behavior. By contrast, optically deleting a random subset of spines unrelated to ketamine treatment had no effect on behavior.
Prefrontal cortical spine formation sustains the remission of specific depression-related behaviors after ketamine treatment by restoring lost spines and rescuing coordinated ensemble activity in PFC microcircuits. Pharmacological and neurostimulatory interventions for enhancing and preserving the rescue of lost synapses may therefore be useful for promoting sustained remission.
A better understanding of the mechanisms underlying the action of antidepressants is urgently needed. Moda-Sava et al. explored a possible mode of action for the drug ketamine, which has recently been shown to help patients recover from depression (see the Perspective by Beyeler). Ketamine rescued behavior in mice that was associated with depression-like phenotypes by selectively reversing stress-induced spine loss and restoring coordinated multicellular ensemble activity in prefrontal microcircuits. The initial induction of ketamine’s antidepressant effect on mouse behavior occurred independently of effects on spine formation. Instead, synaptogenesis in the prefrontal region played a critical role in nourishing these effects over time. Interventions aimed at enhancing the survival of restored synapses may thus be useful for sustaining the behavioral effects of fast-acting antidepressants.