The characterization of the firing patterns and their cholinergic modulation represents a fundamental step to understand the integrative and transfer properties of the perirhinal cortex (PRC).

Methods: 

In this study we combined ISI and CV2 analysis to whole-cell patch-clamp recordings performed on GAD67-GFP mouse brain slices to define the firing patterns of pyramidal and GABAergic neurons in the superficial and deep layers of the PRC.

Results: 

Among the pyramidal neurons (n=63) we found that 37% of them were regular firing, 42% adapting, 12% bursting, 7% late spiking and 2% single spike cells. Bursting cells have only been recorded in the deep layers. GABAergic interneurons were recorded in layer I, II, III and V. They were regular spiking (49%), adapting (32%), irregular firing (12%) and late spiking (7%). Such heterogeneity in the generation of action potentials confers to the PRC a powerful computational potentiality. Since cholinergic modulation strongly influence the functional state of the PRC we have characterized whether or not muscarinic modulation affects the firing discharge and the excitability of both pyramidal and GABAergic neurons. In most cases muscarinic activation induced a depolarization of the membrane potential of both pyramidal (D[asymp] 12 mV) and GABAergic (D[asymp] 8 mV) neurons. In two pyramidal neurons carbachol induced an inhibitory potential whereas in three cells no effect has been observed. To dissect the direct from possible indirect effects elicited by charbachol application the recordings were repeated by adding, to the bath solution, CPP and NBQX as glutamatergic blockers and/or bicuculline as GABAA blockers.

Conclusion: 

Altogether these data suggest that muscarinic cholinergic activation could either increase or decrease the excitability of single pyramidal neurons in the PRC and, hence, affects the integrative and transfer processes in the PRC at cellular level.