Evoked prefrontal gamma oscillation by hippocampal train stimulation in anesthetized rats

Inhibition of fatty acid amide hydrolase reverses aberrant prefrontal gamma oscillations in the sub-chronic PCP model for schizophrenia

Hypofunctioning of NMDA receptors, and the resulting shift in the balance between excitation and inhibition, is considered a key process in the pathophysiology of schizophrenia. One important manifestation of this phenomenon is changes in neural oscillations, those above 30 Hz (i.e., gamma-band oscillations), in particular. Although both preclinical and clinical studies observed increased gamma activity following acute administration of NMDA receptor antagonists, the relevance of this phenomenon has been recently questioned given the reduced gamma oscillations typically observed during sensory and cognitive tasks in schizophrenia. However, there is emerging, yet contradictory, evidence for increased spontaneous gamma-band activity (i.e., at rest or under baseline conditions). Here, we use the sub-chronic phencyclidine (PCP) rat model for schizophrenia, which has been argued to model the pathophysiology of schizophrenia more closely than acute NMDA antagonism, to investigate gamma oscillations (30-100 Hz) in the medial prefrontal cortex of anesthetized animals. While baseline gamma oscillations were not affected, oscillations induced by train stimulation of the posterior dorsal CA1 (pdCA1) field of the hippocampus were enhanced in PCP-treated animals (5 mg/kg, twice daily for 7 days, followed by a 7-day washout period). This effect was reversed by pharmacological enhancement of endocannabinoid levels via systemic administration of URB597 (0.3 mg/kg), an inhibitor of the catabolic enzyme of the endocannabinoid anandamide. Intriguingly, the pharmacological blockade of CB1 receptors by AM251 unmasked a reduced gamma oscillatory activity in PCP-treated animals. The findings are consistent with the observed effects of URB597 and AM251 on behavioral deficits reminiscent of the symptoms of schizophrenia and further validate the potential for cannabinoid-based drugs as a treatment for schizophrenia.

Oscillatory activity and phase-amplitude coupling in the human medial frontal cortex during decision making

Electroencephalogram oscillations recorded both within and over the medial frontal cortex have been linked to a range of cognitive functions, including positive and negative feedback processing. Medial frontal oscillatory characteristics during decision making remain largely unknown. Here, we examined oscillatory activity of the human medial frontal cortex recorded while subjects played a competitive decision-making game. Distinct patterns of power and cross-trial phase coherence in multiple frequency bands were observed during different decision-related processes (e.g., feedback anticipation vs. feedback processing). Decision and feedback processing were accompanied by a broadband increase in cross-trial phase coherence at around 220 msec, and dynamic fluctuations in power. Feedback anticipation was accompanied by a shift in the power spectrum from relatively lower (delta and theta) to higher (alpha and beta) power. Power and cross-trial phase coherence were greater following losses compared to wins in theta, alpha, and beta frequency bands, but were greater following wins compared to losses in the delta band. Finally, we found that oscillation power in alpha and beta frequency bands were synchronized with the phase of delta and theta oscillations ("phase-amplitude coupling"). This synchronization differed between losses and wins, suggesting that phase-amplitude coupling might reflect a mechanism of feedback valence coding in the medial frontal cortex. Our findings link medial frontal oscillations to decision making, with relations among activity in different frequency bands suggesting a phase-utilizing coding of feedback valence information.

Rat prefrontal response and prestimulation local field potential power in vivo

To determine whether spontaneous local field potential (LFP) activities in the rat medial prefrontal cortex influence the responses evoked by hippocampal stimulation, we investigated the relationship between the evoked responses and the LFP powers immediately before the stimulation using anesthetized rats. We demonstrated that the degrees of evoked response showed significant inverse correlations with the prefrontal LFP powers in a specific frequency band (including the gamma band) immediately before the stimulation. The results indicate that the specific frequency band activities in the prefrontal LFPs may be involved in prefrontal responsiveness. Spontaneous LFP activities may have a role in information processing in the hippocampus-prefrontal cortex pathway.

Gamma-band power elevation of prefrontal local field potential after posterior dorsal hippocampus-prefrontal long-term potentiation induction in anesthetized rats

Previously, we reported an elevation in the power of the medial prefrontal cortex (PFC) local field potential spontaneous gamma-band (40-100 Hz) after long-term depression (LTD) but not long-term potentiation (LTP) in the rat ventral hippocampus CA1 (vCA1)-PFC pathway. In the present study, we analyzed the PFC local field potential before and after the induction of posterior dorsal hippocampus CA1 (pdCA1)-PFC LTP/LTD in vivo. In contrast with vCA1-PFC, the present study found that gamma-band power elevation was associated with pdCA1-PFC LTP but not LTD, although we observed a correlation between LTD and the gamma-band power change. The differences between vCA1- and pdCA1-PFC pathways might be related to differences in synaptic plasticity and behavioral functions. Since the neural connection of the hippocampus and PFC is believed to be involved in the function of the working memory, sustained gamma-band power elevation in the PFC might be related to this function.

Differences between paired-pulse facilitation and long-term potentiation in the dorsal and ventral hippocampal CA1-prefrontal pathways of rats

We studied the interaction between paired-pulse facilitation (PPF) and long-term potentiation (LTP) in the hippocampo-prefrontal cortex (PFC, prelimbic area) pathway, stimulating the ventral or posterior dorsal CA1 region (vCA1 or pdCA1). In the vCA1-PFC, the group averaged PPF did not change after the LTP induction, and there was a negative correlation between the post-LTP PPF change and LTP magnitude. In contrast, the post-LTP PPF of the pdCA1-PFC appeared to decrease significantly, and the PPF change was independent of the LTP magnitude. We found that there were at least two mechanisms of PPF regulation following LTP induction in the pathway resulting from extensive CA1 projections into the prelimbic area. The results imply that the CA1-PFC pathway regulates the PFC PPF quantitatively in LTP-dependent and independent manners, which depend on the local properties of the CA1 regions.