While awaiting new cholinergic agonists, we can begin to address the cholinergic deficit in schizophrenia. First is to “do no harm” by avoiding the use of highly anticholinergic regimens that can exacerbate cognitive deficits. For example, if the use of anticholinergics to treat extrapyramidal syndrome (EPS) appears to be exacerbating cognitive symptoms, consider amantadine, which treats EPS but is not an anticholinergic. In a double–blind, cross–over study, Silver and Geraisy17 showed that biperiden (an anticholinergic), but not amantadine, interferes with memory and, in particular, visual memory.

It is extremely important to help those with schizophrenia to stop smoking; bear in mind, however, that they may smoke because nicotine improves their cognitive symptoms. While the smoking itself should cease, nicotine replacement therapy may need to be continued indefinitely to prevent a worsening of cognition.

D1 dopamine–mediated processes

A link has been shown between prefrontal dysfunction and the cognitive deficits observed in schizophrenia.18,19 Goldman–Rakic20 has suggested that disruption of D1 dopamine receptor activity can contribute to the cognitive symptoms of schizophrenia, while stimulation of the D1 dopamine receptor improves cognition.21

Modafinil has been found to improve short–term verbal memory span, visual memory, and spatial planning in patients with chronic schizophrenia. It is reasonable to hypothesize that it does this, at least in part, by stimulating D1 dopamine receptors.22

Hypofunction of the NMDAglutamate system

In the 1980s, phencyclidine (PCP), “angel dust,” was a widely used recreational drug of abuse. Some people were brought to psychiatric emergency departments with schizophrenia–like symptoms, including positive, negative, and cognitive symptoms. The hypothesis that schizophrenia may be a result of hypofunction of the NMDA glutamate system emerged when it was found that PCP blocked calcium efflux through channels controlled by NMDA glutamate receptors.

In the NMDA glutamate system, glutamate binding to a subset of receptors leads to the opening of the calcium channel, but only if a second site is simultaneously occupied by either glycine or D–serine, both of which are released into the synapse by astrocytes. Glycine’s action is terminated when it binds to a glycine transporter protein and is brought back to the astrocytes where it is oxidized. High doses of dietary glycine added to antipsychotic regimens can lead to clinical improvement, but in clinical practice, glycine–induced nausea limits its utility.23

Recently, another promising strategy has emerged. Glycine levels in the synapses can be raised by glycine transport inhibitors that prevent glycine from entering the surroundingastrocytes. Consequently, more glycine remains in the synapse.

Several glycine transport inhibitors are presently in or are entering clinical trials. One promising candidate is N–methylglycine, or sarcosine.24 Preliminary studies indicate that added to antipsychotics, 1 to 2 g of sarcosine per day can lead to significant improvement in positive, negative, and cognitive symptoms.

Conclusion

The next decade should bring new drugs that will treat cognitive symptoms in schizophrenia. While practicing clinicians wait for these new agents, this article suggests pharmacological agents and strategies that may improve cognitive symptoms in persons with schizophrenia.
Pages: 1  2