Too little and too much: hypoactivation and disinhibition of medial prefrontal cortex cause attentional deficits

Abstract

Attentional deficits are core symptoms of schizophrenia, contributing strongly to disability. Prefrontal dysfunction has emerged as a candidate mechanism, with clinical evidence for prefrontal hypoactivation and disinhibition (reduced GABAergic inhibition), possibly reflecting different patient subpopulations. Here, we tested in rats whether imbalanced prefrontal neural activity impairs attention. To induce prefrontal hypoactivation or disinhibition, we microinfused the GABA-A receptor agonist muscimol (C4H6N2O2; 62.5, 125, 250 ng/side) or antagonist picrotoxin (C30H34O13; 75, 150, 300 ng/side), respectively, into the medial prefrontal cortex. Using the five-choice serial reaction time (5CSRT) test, we showed that both muscimol and picrotoxin impaired attention (reduced accuracy, increased omissions). Muscimol also impaired response control (increased premature responses). In addition, muscimol dose dependently reduced open-field locomotor activity, whereas 300 ng of picrotoxin caused locomotor hyperactivity; sensorimotor gating (startle prepulse inhibition) was unaffected. Therefore, infusion effects on the 5CSRT test can be dissociated from sensorimotor effects. Combining microinfusions with in vivo electrophysiology, we showed that muscimol inhibited prefrontal firing, whereas picrotoxin increased firing, mainly within bursts. Muscimol reduced and picrotoxin enhanced bursting and both drugs changed the temporal pattern of bursting. Picrotoxin also markedly enhanced prefrontal LFP power. Therefore, prefrontal hypoactivation and disinhibition both cause attentional deficits. Considering the electrophysiological findings, this suggests that attention requires appropriately tuned prefrontal activity. Apart from attentional deficits, prefrontal disinhibition caused additional neurobehavioral changes that may be relevant to schizophrenia pathophysiology, including enhanced prefrontal bursting and locomotor hyperactivity, which have been linked to psychosis-related dopamine hyperfunction.

Keywords: attention; cognitive deficits; disinhibition; hypoactivation; in vivo electrophysiology; prefrontal cortex.

Figure 1.

Infusion cannula placements in behavioral studies. A, Cresyl-violet-stained section showing exemplary prefrontal infusion sites. B, Approximate locations of infusion cannula tips (black dots) in prefrontal cortex depicted separately for the different experiments testing the effects of muscimol or picrotoxin infusion on the 5CSRT test or the sensorimotor tests (locomotor activity and startle/PPI). Locations are shown on coronal plates adapted from the atlas by Paxinos and Watson (1998), with numbers indicating distance from bregma in millimeters as shown in the atlas.

Figure 2.

Prefrontal muscimol infusions impair attention and response control on the 5CSRT test. Key performance measures are shown after infusion of saline, 62.5, 125, or 250 ng/side muscimol (n = 12). Data are presented as mean ± SEM. Asterisks indicate a significant difference compared with the saline condition and the plus sign indicates a significant difference compared with the lower doses.

Figure 3.

Prefrontal picrotoxin infusions cause attentional deficits on the 5CSRT test. Key performance measures are shown after infusion of saline or 75, 150, or 300 ng/side picrotoxin (n = 12). Data are presented as mean ± SEM. Asterisks indicate a significant difference compared with the saline condition and the plus sign indicates a significant difference compared with the lower dose(s).

Figure 4.

Prefrontal muscimol infusions dose dependently reduce, whereas picrotoxin infusions dose dependently increase locomotor activity. A, Muscimol infusions. After muscimol infusion, locomotor activity was dose dependently reduced during the complete 90 min open-field session. The inset shows average activity per 10 min block for the four infusion groups that had received saline or 62.5, 125, or 250 ng/side picrotoxin. Asterisks indicate a significant difference compared with saline. B, Picrotoxin infusions. Activity did not differ between groups during the 30 min before infusion, whereas activity was significantly increased during the second and third 10 min block after infusion in rats that had received 300 ng/side picrotoxin compared with all other groups (indicated by asterisks). Data are presented as mean ± SEM.

Figure 5.

Prefrontal muscimol or picroxin infusions do not substantially affect startle and PPI. A, Muscimol infusions. On the infusion day, all groups showed comparable startle (left) and PPI (right) measures, even though rats infused with 62.5 ng muscimol tended to show reduced startle responses during pulse-alone trials and the rats receiving 250 ng tended to show lower PPI values at lower prepulse intensities (where all groups showed very little or no PPI). Importantly, the prospective infusion groups already presented with similar differences during baseline testing on the day before the infusion (insets), indicating that the differences are unrelated to the infusion, but instead reflect interindividual variability. B, Picrotoxin infusions. After infusion of saline or picrotoxin, all groups showed very similar startle (left) and PPI (right) measures. Data are presented as mean ± SEM.

Figure 6.

Electrode placements. A, Photograph showing the assembly of infusion cannula and eight-microwire-electrode array used to measure the effects of drug microinfusions on prefrontal multiunit and LFP activity. The array was arranged parallel to the midline of the brain, with the infusion cannula located just lateral to the center of the array. B, Photographs of coronal sections through the prefrontal cortex showing markings for the anterior (top) and posterior (bottom) electrodes of the array. C, Approximate locations of markings for the tips of the anterior (black dots) and posterior (gray dots) electrodes in prefrontal cortex depicted separately for the different experimental groups that received infusions of 300 or 150 ng picrotoxin, saline, or 250 ng muscimol. Locations are shown on coronal plates adapted from the atlas by Paxinos and Watson (1998), with numbers indicating distance from bregma in millimeters as shown in the atlas.

Figure 7.

LFP spike-wave discharges and intensified multiunit burst-firing pattern within prefrontal cortex after picrotoxin infusion. Multiunit recording traces (top, electrode 8) and LFP traces (bottom, electrodes 1, 3, 6, and 8) from one exemplary experiment involving infusion of 300 ng picrotoxin. The two stippled horizontal lines in the multiunit traces indicate the zero-voltage line and the predefined spike-detection threshold (−240 μV). A, Baseline recording ∼5 min before infusion. B, Recordings ∼15 min after picrotoxin infusion. Note the compressed, intensified burst periods in the multiunit recordings and the marked spike-wave discharges in the LFP. For the multiunit traces, 50 ms segments during a burst period are shown with expanded time line, illustrating intensified spiking during burst periods after picrotoxin infusion.

Figure 8.

Muscimol inhibits and picrotoxin disinhibits prefrontal neuron firing within bursts and picrotoxin increases the proportion of spikes fired in bursts. A, B, Time courses of within-burst firing rates (A) and percentage of spikes fired in bursts (B) during baseline recordings and after infusions of muscimol, picrotoxin, or saline. Values are normalized to the average of the 6 baseline 5 min blocks and are presented as mean ± SEM. The stippled horizontal line indicates baseline.

Figure 9.

Additional multiunit firing parameters and effects of muscimol and picrotoxin. Shown are the time courses of overall firing rate (A), number of bursts (B), burst duration (C), and interburst intervals (D) during baseline recordings and after infusion of muscimol, picrotoxin, or saline (for 1 rat that received a muscimol infusion, there was not 1 electrode that recorded more than 1 burst in all 5 min blocks; therefore, this rat had to be excluded from the analysis of interburst intervals, so that only 5 rats remained in the muscimol group for this analysis). Values are normalized to the average of the 6 baseline 5 min blocks and are presented as mean ± SEM. The stippled horizontal line indicates baseline.

Figure 10.

Picrotoxin increases prefrontal LFP power. Time courses of overall LFP power, measured as AUC of PSD, during baseline recordings and after infusion of muscimol, picrotoxin, or saline. Values are normalized to the average of the 6 baseline 5 min blocks and are presented as mean ± SEM. The stippled horizontal line indicates baseline.