13 September 2007. Changes in neuronal activity in the prefrontal cortex (PFC) are thought to give rise to some of the cognitive symptoms of schizophrenia. Successful treatment of these symptoms with antipsychotic medicines predicts better disease management and outcome. Of the drugs available, the atypical antipsychotic clozapine is regarded as the most effective against cognitive symptoms, but how it acts on neurons of the PFC is not understood.
To shed some light on that question, two groups took the tack of recording single neuron activity in living animals treated with NMDA receptor antagonists to induce schizophrenia-like symptoms. The studies, one published by a group led by Pau Celada and Francesco Artigas of the Institut d’Investigacions Biomediques de Barcelona in Spain in PNAS on September 4, and another from Houman Homayoun and Bita Moghaddam of the University of Pittsburgh, Pennsylvania, in Biological Psychiatry last year, come up with similar findings: NMDA antagonists disrupt spontaneous rates and patterns of PFC neuronal spiking, which clozapine restores with a surprising and unique deftness. Compared to the first-generation (typical) medicine haloperidol, clozapine has an ability to fine-tune neuronal activity toward a more normal state, which may help explain its improved clinical efficacy.
In their study, published online October 13, 2006, Homayoun and Moghaddam carried out single neuron recording in awake rats after treatment with clozapine or haloperidol, and the NMDA antagonist MK801, which induces some symptoms of schizophrenia in humans. When they gave clozapine alone, the researchers saw the firing rates of some neurons increase, while other decreased or stayed the same. In all, they saw effects on half the neurons they recorded. Interestingly, clozapine tended to activate neurons with low baseline firing rates, and inhibit those with high baseline rate. This effect was not seen with haloperidol, which caused only transient increases in activity in some neurons, but sustained decreases in others.
MK801 by itself increased activity in 80 percent of recorded neurons, while in animals pretreated with clozapine and then MK801, significantly fewer neurons showed increased activity, and the increases were of lower magnitude. Half of the neurons in the clozapine group showed no change in activity. Haloperidol was much less effective at reducing the number of neurons with high activity, and did not reduce average firing rates at all.
Because the experiments involved awake animals, the researchers could look at behavior, and they found that despite the vastly different effects of the drugs on PFC firing, either antipsychotic reversed the appearance of stereotypical repetitive behaviors induced by MK801. They saw a close correlation between behavioral responses and neuronal firing in individual clozapine-treated animals, but not in those treated with haloperidol. From this, the authors conclude that the behavioral effect of haloperidol is “most probably” mediated at the subcortical level.
“The present data suggest that clozapine modulates PFC neuronal firing on the basis of baseline activity level of different ensembles. This modulatory activity might confer clozapine an ability to fine-tune the PFC function through gating unwanted disturbances in neuronal signal to noise ratio,” Homayoun and Moghaddam write.
The new data from the Spanish group support those results, with the psychotomimetic phencyclidine (PCP) used to induce PFC functional abnormalities in anesthetized rats. In single cell recordings of PFC pyramidal neurons, they found that PCP activated some neurons and inhibited others. First author Lucila Kargieman and colleagues also found a disruption in neuronal synchrony. By measuring local field potentials, they discovered that PCP decreased cortical synchrony in the delta frequency range. In addition, they found that PCP caused induction of c-fos expression in PFC pyramidal neurons, an indicator of increased neuronal activity. Either clozapine or haloperidol treatment reduced the excess neuronal firing and re-established synchrony. Clozapine also inhibited fos induction.
Taken together, the results of both papers suggest that antipsychotic drugs may partly exert their therapeutic effect by normalizing PFC activity, which is required for higher order functions that integrate external information with internal representations to determine behavior. This could occur via blocking dopamine D2 receptors. In the case of clozapine, other receptor targets may help produce its constellation of actions distinct from older drugs like haloperidol. Whatever the cause, the ability of clozapine to modulate neuronal activity both up and down could be one key to its therapeutic actions.—Pat McCaffrey.
Kargieman L, Santana N, Mengod G, Celada P, Artigas F.
Antipsychotic drugs reverse the disruption in prefrontal cortex function produced by NMDA receptor blockade with phencyclidine. Proc Natl Acad Sci U S A. 2007 Sep 4; [Epub ahead of print] Abstract
Homayoun H, Moghaddam B. Fine-tuning of awake prefrontal cortex neurons by clozapine: comparison with haloperidol and N-desmethylclozapine. Biol Psychiatry. 2007 Mar 1;61(5):679-87. Epub 2006 Oct 13. Abstract