Fine-tuning of awake prefrontal cortex neurons by clozapine: comparison with haloperidol and N-desmethylclozapine

Abstract

Background: Mechanisms underlying clozapine's better clinical efficacy in schizophrenia remain poorly understood. The prefrontal cortex (PFC) has been implicated as a primary site for the therapeutic effects of clozapine; however, evidence for how clozapine influences the activity of PFC neurons in behaviorally relevant contexts is lacking.

Methods: Ensemble single unit recording in awake rats was used to measure the activity of PFC neurons in response to clozapine, its main metabolite N-desmethylclozapine (DMClz), and the typical antipsychotic drug haloperidol during baseline conditions and after treatment with the N-methyl-D-aspartate antagonist MK801. Behavioral stereotypy was scored during recording.

Results: Clozapine and DMClz but not haloperidol had an activity-dependent influence on spontaneous firing rate of PFC cells: they increased the activity of neurons with low baseline firing rates and decreased the activity of neurons with higher firing rates. Clozapine and DMClz but not haloperidol also reversed the effect of MK801 on PFC neuronal firing. This reversal was strongly correlated with blockade of MK801-induced behavioral stereotypy.

Conclusions: These findings indicate that clozapine has the capacity to fine-tune spontaneous and disrupted activity of PFC neurons. This effect might contribute, in part, to the therapeutic efficacy of clozapine in schizophrenia.

Figure 1

The predominant types of responses of prefrontal neurons to antipsychotic treatments. (A) Firing responses of all units recorded in experiment 1 were clustered into one of four main response types with a K-means clustering analysis on normalized firing rate histograms with 5-min bins. Each color line shows the average firing rate (± SEM) of all neurons in one of the response clusters (early increase, sustained increase, decrease, or no change). Injections are indicated by arrows. (B) The proportion (% of total) of different response types in each treatment group. Treatments included: vehicle (Veh), clozapine (Clz; 1, 5, or 10 mg/kg), N-desmethylclozapine (DC; 5 mg/kg), or haloperidol (Hal; .1 mg/kg) injected at min 0 followed by saline at min 20. Color codes are as in section A. (C) The average duration of periods of significant increase (left) or decrease (right) in firing rates in all responsive neurons after second injection. *p < .05 compared with vehicle + vehicle group.

Figure 2

State-dependent modulation of firing rate by clozapine and N-desmethylclozapine. The normalized firing rates of individual neurons have been plotted against their baseline firing rates. (A) Vehicle + saline. (B) Clozapine (10 mg/kg) + saline. (C) N-desmethylclozapine (5 mg/kg) + saline. (D) Haloperidol (.1 mg/kg) + saline. Baseline firing rates were calculated for the 30-min period before the first injection, and the normalized responses (y axis) were calculated for the period afterwards. The dashed line indicates normalized value of 1 (no change), and the solid line is the linear regression line.

Figure 3

The predominant response patterns of prefrontal neurons to MK801 after different pre-treatments. (A) Firing response of all neurons in experiment 2 were clustered into one of four response types. Each color line depicts the average firing rate (± SEM) of all neurons with one of identified response types (early transient increase, late robust increase, decrease, or no change). Injections are indicated by arrows. (B) The proportion (% of total) of response types in each treatment group. Treatments included: vehicle (Veh), clozapine (Clz; 1, 5, or 10 mg/kg), N-desmethylclozapine (DC; 5 mg/kg), or haloperidol (Hal; .1 mg/kg) injected at min 0 followed by MK801 (.1 mg/kg) at min 20. Color codes are as in section A. (C–F) Rate histograms of four individual single units displaying different response types in experiment 2. All neurons were treated with a pre-treatment (indicated above each histogram; (C) vehicle; (D) clozapine 10 mg/kg; (E) N-desmethylclozapine; (F) haloperidol) followed by MK801. Arrows indicate the time of injections, and histograms bins are 5 min.

Figure 4

The effects of antipsychotic drugs on the magnitude of MK801-induced excessive firing and stereotypical behavior. (A) The temporal profile of normalized firing rate (mean ± SEM) of all neurons with an increase response after MK801. All groups received MK801 (.1 mg/kg) as second injection, preceded by vehicle (Veh), clozapine (Clz; 1, 5, or 10 mg/kg), N-desmethylclozapine (DC; 5 mg/kg), or haloperidol (Hal; .1 mg/kg). Arrows indicate injections and bins are 5 min. For brevity, data for clozapine 5 mg/kg group is not shown. (B) Average stereotypy score in the same groups. All conventions are as in section A. For clarity, the error bars are not shown. (C) The coefficients of correlation between stereotypy score and neuronal firing response are shown for individual sessions in each treatment group. There was a high correlation between behavior and electrophysiological activity in all groups except for the haloperidol pre-treated group.

Figure 5

Example rate histograms of putative interneurons. Each histogram depicts the response of an individual putative interneuron to treatment with vehicle + MK801 (upper row) or clozapine (10 mg/kg) + MK801 (lower row). Treatments are indicated above each histogram with arrows showing the injection time.