In entorhinal cortex layer-II neurons, muscarinic receptor stimulation promotes depolarization via activation of a non-specific cation current (INCM). Under muscarinic influence, these neurons also develop changes in excitability that result in activity-dependent induction of delayed firing and bursting activity. In order to identify the membrane processes underlying these phenomena, we examined whether INCM may undergo activity-dependent regulation. Our voltage-clamp experiments revealed that appropriate depolarizing protocols increased the basal level of inward current activated during muscarinic stimulation, and suggested that this effect was due to INCM up-regulation. In the presence of low buffering for intracellular Ca2+, this up-regulation was transient and its decay could be followed by a phase of INCM down-regulation. Both up- and down-regulation were elicited by depolarizing stimuli able to activate voltage-gated Ca2+ channels (VGCC); both were sensitive to increasing concentrations of intracellular Ca2+-chelating agents, with down-regulation being abolished at lower Ca2+-buffering capacities; both were reduced or suppressed by VGCC block or in the absence of extracellular Ca2+. These data indicate that relatively small increases in [Ca2+]i driven by firing activity can induce up-regulation of a basal muscarinic depolarizing-current level, whereas more pronounced [Ca2+]i elevations can result in INCM down-regulation. We propose that the interaction of activity-dependent positive and negative feedback mechanisms on INCM allows entorhinal cortex layer-II neurons to exhibit emergent properties, such as delayed firing and enhanced or suppressed responses to repeated stimuli, that may be of importance in the memory functions of the temporal lobe and in the pathophysiology of epilepsy.
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