Schizophrenia Research Zeroes In on Fast-Spiking Interneurons

by Jim Schnabel

May 20, 2015

Schizophrenia’s cause has long been a mystery, and current treatments fall well short of curing the debilitating,usually lifelong illness. But in a new study, researchers reversed schizophrenia-like signs in transgenic mice by restoring the normal activity of fast-spiking interneurons (FSINs) in the prefrontal cortex.

The findings highlight the recent convergence of schizophrenia research on this special type of brain cell, and suggest a possible new schizophrenia treatment strategy—one that could be tested in the clinic soon, using off-the-shelf drugs.

“We’re certainly talking to people about that now,” said Vikaas Sohal of the University of California–San Francisco, whose lab reported the study in Neuron on March 18.

FSINs are faster-working members of the broad class of neurons known as inhibitory interneurons, whose job is generally to quieten and otherwise modulate the main excitatory neurons of the cortex. FSINs in particular seem principally responsible for establishing the relatively fast, “gamma wave” oscillations in neuronal activity—with frequencies of about 25 to 120 cycles per second—that can be recorded on the surface of the human brain by electroencephalography (EEG).

As basic as these neuronal rhythms seem to be, scientists still don’t know precisely why they are there. Many suspect that synchronous activity generally helps align neuronal inputs and outputs, thereby enabling neurons to communicate and form functional networks across the brain more efficiently than they would otherwise. Gamma rhythms, the fastest of the defined brain rhythms, have been proposed as a mechanism of attention, and even as a key mediator of conscious awareness.

Schizophrenia and gamma waves

FSINs and gamma rhythms are relevant to schizophrenia because they don’t work properly in the disease. People with schizophrenia show abnormal gamma rhythms over the prefrontal cortex during certain cognitive tasks—tasks on which they are impaired. Studies of patients’autopsied brains also show molecular abnormalities in prefrontal FSINs.

Further hints that FSIN abnormalities might be a key to schizophrenia include evidence that some of the environmental factors believed to predispose to the disease—including inflammation from maternal infections before birth, early-life stress, and early-life lead exposure—can have particularly strong adverse impacts on interneurons.

Moreover, the recreational drugs ketamine and PCP, which can induce schizophrenia-like psychoses in humans, as well as schizophrenia-like brain and behavior changes in lab animals, apparently do so by reducing the functionality of a receptor found on FSINs, which can in turn impair FSIN activity.

For all these reasons, schizophrenia researchers have begun to focus on disease-related FSIN abnormalities and apparently related cognitive deficits, to understand better how they arise during childhood and adolescence. These cognitive deficits of schizophrenia are less dramatic and less well characterized than the classic symptoms involving auditory hallucinations and paranoid delusions. But many researchers, among them Sohal,suspect that cognitive problems are in fact more central to the disorder, since they are typically seen earlier, are more persistent, and are less amenable to standard schizophrenia drug therapies.

A subtle deficit with big consequences

A major goal of research in this area has been to create mouse models in which FSINs develop abnormally, causing impaired gamma rhythms as well as the same kind of impaired cognition seen in schizophrenia. Ideally such mice would start to show these abnormal signs in adolescence or early adulthood, when schizophrenia typically emerges in humans.

It’s a tricky task, in part because there is no “smoking gun” from gene studies—no gene, known to be closely involved in FSIN development, that is missing or abnormal in a large subset of patients. Moreover, knocking out any gene that governs the formation of interneurons, a very important cell type,is likely to keep mice from developing normally enough to survive until birth.

The transgenic mice used in Sohal’s study were not full-gene knockouts. Developed in the laboratory of UCSF colleague John Rubinstein, the animals were missing just one copy—not both, which would be lethal—of two genes, Dlx5 and Dlx6. These are among several genes known to be required for the normal development of FSINs in a part of the prefrontal cortex where schizophrenia patients show FSIN abnormalities.

“These cells are not absent in schizophrenia, they just have abnormal markers, and so it seemed a reasonable bet that these transgenic mice would have [similar] subtle abnormalities in their cells,” says Sohal.

He and his colleagues, including postdoctoral researcher and lead author Kathleen Cho, found that the affected FSINs did appear in normal numbers in the mice, as they do in human schizophrenia. But just after the mice passed through adolescence—corresponding to the time when humans with schizophrenia tend to exhibit the first clear signs of the disorder—the mouse FSINs began to show pronounced abnormalities.

Experiments revealed that the affected FSINs had unusual, relatively sluggish responses to input signals. At the same time, the post-adolescent transgenic mice performed poorly on a behavioral test of cognitive flexibility—a cognitive domain in which schizophrenia patients also perform poorly. The mice showed more anxiety than normal mice—yet another sign of schizophrenia—but only after adolescence.

For the cognitive flexibility tests, and during social interactions with other mice, EEG recordings indicated little or no increase in the intensity of prefrontal gamma oscillations, in contrast to the increases in gamma power seen in normal mice in these situations. Yet the steady-state, or “baseline,” gamma power was greater in the transgenic mice. Both features—higher baseline gamma power and lower responsiveness during cognitive flexibility tasks—have been noted in studies of schizophrenia patients, and point to a basic dysfunction of rhythm-generating FSINs.

Remarkably, Cho and Sohal and their colleagues were able to reproduce many of these effects in normal mice, by using optogenetics techniques to directly inhibit the firing of prefrontal interneurons. Conversely, they were able to reverse the gamma oscillation and cognitive deficits in the transgenic mice by optogenetically forcing prefrontal FSINs to fire at the gamma range frequency of 40 cycles per second. The effect on improved cognition persisted for at least a week after this optogenetics treatment, hinting at a learning effect—which Sohal notes is also seen in cognitive remediation therapies for schizophrenia.

Finally, the scientists showed that they could also reverse the cognitive deficit and gamma deficiency in the transgenic mice using clonazepam, a common benzodiazepine drug.

Clonazepam is normally prescribed to prevent seizures or anxiety attacks, but researchers from the University of Washington–Seattle reported in 2012 that a low dose—so low that it wouldn’t cause drowsiness—successfully treated autism-like behaviors and cognitive deficits in transgenic mice. Clonazepam apparently works by enhancing the activity of the GABA receptor on excitatory neurons through which interneurons exert their restraining influence. (A recent clinical trial of another GABA-activity-enhancing compound failed to show a benefit for schizophrenia patients, though it is unclear that low-dose clonazepam would work in the same way.)

Aside from trying to set up a pilot trial of low-dose clonazepam in schizophrenia patients, Sohal and his colleagues are now trying to understand better how FSINs and gamma rhythms are important for cognition, and why seemingly subtle impairments in these cells can lead to such a profound disorder as schizophrenia. “We’re focusing on the idea that these [prefrontal FSINs] might enhance the signal-to-noise ratio in brain signaling and thereby allow signals to be propagated through groups of neurons more effectively to downstream structures,” he says.

In principle, he adds, FSIN impairments could explain not only the cognitive deficits of schizophrenia but also the classic symptoms such as auditory hallucinations. For example, an FSIN-related failure of communications across brain regions might leave the auditory cortex unaware that a stimulus had originated internally, as the person’s own thought—so it would mistakenly treat the thought as something spoken by someone in the outside world.

Still some big questions

Kazutoshi Nakazawa, a scientist at the University of Alabama–Birmingham who developed one of the first mouse models relating to schizophrenia and interneurons, and co-authored a recent review of the subject, calls the Sohal study very interesting, and says he is particularly keen “to know the mechanisms downstream of the 40Hz oscillation increase that cause the rescue of cognitive deficits.”

But he emphasizes that important questions remain unanswered. One is that “if interneurons are important for schizophrenia, in which brain areas are they important?” Many researchers focus on cortical and prefrontal interneurons, but he notes that there is some evidence for the involvement of interneurons in the thalamoreticular nucleus, a brain region that is densely wired to the cortex and is suspected of having a role in attention.

Moreover, he says, researchers still need a better understanding of the specific cognitive deficits of schizophrenia and how those might relate to the dysfunctions of interneurons.

The good news is that this is now a very active field. “Lots of papers have been coming out regarding the function of [FSINs] in many brain areas, including cortex,” Nakazawa says.