ACETYLCHOLINE MODULATES AVERAGED SENSORY EVOKED-RESPONSES AND PERFORANT PATH EVOKED FIELD POTENTIALS IN THE RAT DENTATE GYRUS

The effect of localized application of acetylcholine (ACh) on well characterized components of sensory evoked and electrically induced potentials in the dentate gyrus was investigated in rats while performing a tone discrimination task. Local Pressure application of ACh to the granule cell layer of the dentate gyrus through the recording pipette increased the amplitude of perforant path evoked population spikes without changing the amplitude of the field EPSP. When the pipette was relocated to the outer molecular layer of the dentate gyrus (OM), ACh application decreased the amplitude of the perforant path field EPSP. Two major components of the averaged auditory evoked potential (AEP) recorded during criterion performance of the discrimination task were significantly changed by dendritic application of ACh. The N1 component of the OM AEP which has been shown to reflect perforant path synaptic activity decreased in amplitude while the N2 component which represents activity from septal connections, was significantly increased. These effects were not due to the pressure ejection procedure nor drug related changes in behavioral performance of the task. The results suggest that ACh may act to differentially modulate the synaptic excitability of dentate granule cells, allowing them to acquire responses to sensory stimulation during the establishment and maintenance of discrimination learning.