The 40 Hz click train-elicited gamma oscillations show neurophysiologic and pharmacologic differentiation at the prefrontal vs temporal cortical sites of the female SD rat.

Auditory sensory neural networks in the brain naturally entrain to rhythmic stimuli. Such synchronization is an accessible index of local and long distance network function as captured by the technique of EEG. Across species, click trains delivered ~ 40 Hz show strong EEG entrainment with auditory cortex being a primary source. Imaging studies have revealed other cortical sources as well but it is unclear if these are functionally differentiated. In female SD rats, we recorded 40 Hz click train-elicited gamma oscillations using epidural screw electrodes situated at two distant sites; one above the secondary motor cortex M2, considered a part of the prefrontal cortex (PFC) and the other above the primary auditory cortex (Actx), in the temporal lobe, after dosing with saline (1 ml/kg, sc) or MK801 (0.025-0.1 mpk), using a randomized crossover design. Post-saline, both regions showed strong 40 Hz auditory steady state response (ASSR). The latency for the P1-N1-P2 registration responses were ~ 16 ms (Actx) and ~ 34 ms (PFC). Narrow band pass (38-42 Hz) filtering revealed further differences. Gamma oscillations appeared rapidly (<40 ms) after train onset at Actx but took ~ 200 ms at PFC. Relative to saline, MK801 disinhibited gamma oscillations at Actx while showing a dose-dependent inhibition at the PFC. Event-related gamma (but not beta) coherence across sites, an index of long-distance connectivity, dose-dependently decreased with MK801. We conclude that 40 Hz ASSR and its pharmacology differs substantially in association cortex relative to primary sensory cortex and MK801 strongly and selectively disrupted event-related gamma coherence.