Carbachol-induced changes in excitability and [Ca2+](i) signalling in projection cells of medial entorhinal cortex layers II and III

The entorhinal cortex (EC) is a major gateway for sensory information into the hippocampus and receives a cholinergic input from the forebrain. Theref ore, we studied muscarinic effects on excitability and intracellular CA(2+) signalling in layer II stellate and layer III pyramidal projection neurons of the EC. In both classes of neurons, local pressure-pulse application of carbachol (1 mM) caused small, atropine-sensitive membrane depolarizatians that were not accompanied by any detectable changes in [Ca2+](i). At a hig her concentration (10 mM), carbachol induced a larger membrane depolarizati on associated with synaptic oscillations and epileptiform activity in both classes of neurons. In contrast to the intrinsic theta rhythm in stellate c ell with one dominant peak frequency at similar to 7 Hz, the synaptically m ediated oscillation induced by carbachol showed three characteristic peaks in the theta and gamma frequency range at similar to 11, 23 and 40 Hz. Alth ough carbachol-induced epileptiform activity was associated with increases in intracellular free Ca2+ in both layer II and III cells, the observed [Ca 2+](i) accumulation was significantly larger in layer III than in layer II cells. Responses to intracellular current injection showed differences in C a2+ accumulation in layer II and III cells at the same membrane potentials, suggesting a dominant expression of low- and high-voltage-activated Ca2+ c hannels i these layer II and III cells, respectively. In conclusion, we pre sent evidence for significant differences in the [Ca2+](i) regulation betwe en layer II stellate and layer III pyramidal cells of the medial EC.