Why are schizophrenics three to four times more likely to smoke cigarettes than the general population? Over at Brainblogger, Dirk Hanson has a fascinating summary of a few recent papers that tried to understand this saga of self-medication. Nicotine, it turns out, can significantly reduce the sensory symptoms associated with the mental illness. And given that we’re unlikely to untangle the genetic causes of schizophrenia anytime soon – the most recent genetic analyses demonstrated just how stunningly complex the illness is – the palliative effects of cigarettes are an important window into the anatomy of the disease:
The review of studies through 1999, undertaken by Lyon and published in Psychiatric Services, shows unequivocally that schizophrenic smokers are self-medicating to improve processing of auditory stimuli and to reduce many of the cognitive symptoms of the disease. “Neurobiological factors provide the strongest explanation for the link between smoking and schizophrenia,” Lyons writes, “because a direct neurochemical interaction can be demonstrated.”
Of particular interest is the interaction between nicotine and dopamine in the nucleus accumbens and prefrontal cortex. Several of the symptoms of schizophrenia appear to be associated with dopamine release in these brain areas. A 2005 German study concluded that nicotine improved cognitive functions related to attention and memory. “There is substantial evidence that nicotine could be used by patients with schizophrenia as a ‘self-medication’ to improve deficits in attention, cognition, and information processing and to reduce side effects of antipsychotic medication,” the German researchers concluded.
In addition, the process known as “sensory gating,” which lowers response levels to repeated auditory stimuli, so that a schizophrenic’s response to a second stimulus is greater than a normal person’s, is also impacted by cigarettes. Sensory gating may be involved in the auditory hallucinations common to schizophrenics. Receptors for nicotine are involved in sensory gating, and several studies have shown that sensory gating among schizophrenics is markedly improved after smoking.
I’d be curious to see what effect, if any, cigarettes have on working memory deficits in schizophrenics. There’s some initial evidence that nicotine improves visual working memory in people with the illness, which is particularly interesting since it doesn’t seem to improve working memory in control groups. (So don’t start smoking just to make yourself smarter. It won’t work.)
On a completely unrelated note, I’m still fascinated by this 2007 paper, which found that smokers with damaged insulas – the brain area was usually damaged by a stroke – were 136 times more likely to quit cigarettes than smokers with damage in other parts of their brains. Why? According to the scientists, the insula is largely responsible for detecting the pleasant bodily rituals and signals associated with smoking – the escalated pulse, the slow inhalation, the slight nicotine rush – which then get integrated into our mental state. When we crave a cigarette what we are actually craving are these somatic changes: they are the emotional core of addiction. It’s also likely that having an offline insula makes it easier to deal with the negative feelings of nicotine withdrawal. The insula, after all, has long been seen as a crucial component of the pain pathway.