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.
Klink
R
R
Department of Neurology and Neurosurgery, Montreal Neurological Institute and McGill University, Quebec, Canada.
Alonso
A
A
D000818:Animals; D002118:Calcium / Q000502:physiology*; D002217:Carbachol / Q000494:pharmacology*; D002463:Cell Membrane Permeability / Q000187:drug effects; D004553:Electric Conductivity; D018728:Entorhinal Cortex / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018721:Muscarinic Agonists / Q000494:pharmacology*; D026902:Potassium Channel Blockers; D051381:Rats; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000493:pharmacokinetics; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013779:Tetrodotoxin / Q000494:pharmacology
D018721:Muscarinic Agonists; D026902:Potassium Channel Blockers; D011976:Receptors, Muscarinic; D013779:Tetrodotoxin; D002217:Carbachol; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium