Do developmental glitches in adolescent brains lead to the psychological disorder?
by Aparna Vidyasagar
illustration by Grace Molteni
Schizophrenia is a disorder of the brain, which profoundly affects a person’s thoughts and behavior. Symptoms range from hallucinations and delusions —perceiving sights, smells, and sounds that do not exist; having false beliefs that remain unshaken even in the face of proof — to having odd or garbled speech patterns, or not wanting to talk much at all. Symptoms of schizophrenia come to light during the late teens to early thirties. This coincides with a time when “synaptic pruning,” a process of change within the brain is coming to an end. Pruning is the process of rewiring of a child’s brain and shaping it into that of an adult. Scientists are examining the phenomenon as a potential mechanism by which schizophrenia occurs.
We take in a lot when we emerge from the unvarying confines of the womb into a world filled with plenty of stimulation. However, not all information learned in those early years is essential. Synaptic pruning is a means of house cleaning initiated within the brain. It begins around the age of two or three and continues at least into the late teens. Some recent evidence suggests it can continue into one’s thirties.
The junction between two neurons, or the cells of the brain, is called a synapse. It is the point at which information is transmitted from one cell to another. As babies, we learn and glean new information from our surroundings by using our senses of touch, sight, sound, smell and taste. With each piece of sensory information, our brain creates connections between neurons . For example, the early experiences of perceiving color and movement create synapses in a baby’s visual cortex. In a newborn, one neuron forms 2,500 synapses. By age two or three, this number increases to 15,000 synapses.
In fact, this is the peak density of synapses during childhood. Once synaptic pruning starts, underused synapses are dismantled and superfluous neurons are dusted away into oblivion. Similarly, synaptic connections that are used regularly, such as those controlling language and reasoning, are further nurtured and strengthened over time. By the end of pruning, the number of synapses will have declined by at least half. The brain also loses a certain amount of gray matter through the elimination of unused neurons.
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Besides the fact that the onset of schizophrenia coincides with the near end of synaptic pruning, there are other indications that the process may be involved in schizophrenia. People with the disorder can have unusually decreased amounts of gray matter, suggesting an unnecessary loss of neurons. A 2010 study led by scientists from Johns Hopkins looked into how synapses are lost or weakened in schizophrenia at the molecular level. They showed that low levels of a protein, appropriately named “Disrupted-in-Schizophrenia” (DISC-1), led to fewer and smaller-sized structures called “dendritic spines” in rats. These spiky structures sit at the synapse and help in the efficiency of information transfer. So, fewer dendritic spines may indicate poorly functioning or nonfunctional synapses. The scientists built upon this data in 2014 by developing a drug that countered the effects of low levels of DISC-1. They were able to maintain a healthy number of dendritic spines in mice. Though studying mice is a good first step in understanding the human brain, there are some differences in proteins and molecular pathways between the two. The team is yet to confirm whether the molecular pathways work in exactly the same way in humans as well.
While a body of evidence is growing, synaptic pruning is yet to be definitively established as an event that causes schizophrenia. More experiments that specifically target the various steps and molecules involved in pruning are sure to yield answers in the near future.
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Aparna Vidyasagar is a freelance science writer who writes about life-sciences research and health. She lives in Portland, Oregon. Follow her on Twitter.
Grace Molteni is a Midwest born and raised designer, illustrator, and self-proclaimed bibliophile, currently calling Chicago home. For more musings, work, or just to say hey check her out on Instagram or at her personal website.
