Ionic mechanisms of muscarinic depolarization in entorhinal cortex layer II neurons.
The mechanisms underlying direct muscarinic depolarizing responses in the stellate cells (SCs) and non-SCs of medial entorhinal cortex layer II were investigated in tissue slices by intracellular recording and pressure-pulse applications of carbachol (CCh). Subthreshold CCh depolarizations were largely potentiated in amplitude and duration when paired with a short DC depolarization that triggered cell firing. During Na+ conductance block, CCh depolarizations were also potentiated by a brief DC depolarization that allowed Ca2+ influx and the potentiation was more robust in non-SCs than in SCs. Also, in non-SCs, CCh depolarizations could be accompanied by spikelike voltage oscillations at a slow frequency. In both SCs and non-SCs, the voltage-current (V-I) relations were similarly affected by CCh, which caused a shift to the left of the steady-state V-I relations over the entire voltage range and an increase in apparent slope input resistance at potentials positive to about -70 mV. CCh responses potentiated by Ca2+ influx demonstrated a selective increase in slope input resistance at potentials positive to about -75 mV in relation to the nonpotentiated responses. K+ conductance block with intracellular injection of Cs+ (3 M) and extracellular Ba2+ (1 mM) neither abolished CCh depolarizations nor resulted in any qualitatively distinct effect of CCh on the V-I relations. CCh depolarizations were also undiminished by block of the time-dependent inward rectifier Ih, with extracellular Cs . However, CCh depolarizations were abolished during Ca2+ conductance block with low-Ca2+ (0.5 mM) solutions containing Cd2+, Co2+, or Mn2+, as well as by intracellular Ca2+ chelation with bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid. Inhibition of the Na+-K+ ATPase with strophanthidin resulted in larger CCh depolarizations. On the other hand, when NaCl was replaced by N-methyl-D-glucamine, CCh depolarizations were largely diminished. CCh responses were blocked by 0.8 microM pirenzepine, whereas hexahydro-sila-difenidolhydrochloride,p-fluoroanalog (p-F-HHSiD) and himbacine were only effective antagonists at 5- to 10-fold larger concentrations. Our data are consistent with CCh depolarizations being mediated in both SCs and non-SCs by m1 receptor activation of a Ca2+-dependent cationic conductance largely permeable to Na+. Activation of this conductance is potentiated in a voltage-dependent manner by activity triggering Ca2+ influx. This property implements a Hebbian-like mechanism whereby muscarinic receptor activation may only be translated into substantial membrane depolarization if coupled to postsynaptic cell activity. Such a mechanism could be highly significant in light of the role of the entorhinal cortex in learning and memory as well as in pathologies such as temporal lobe epilepsy.