In vitro effects of extracellular calcium concentrations on hippocampal pyramidal cell responses.

This study examined the effects of varying the extracellular Ca2+ concentration [( Ca2+]o) on membrane stabilization, response to paired stimuli, and long-term potentiation (LTP) in the CA1 pyramidal cells of the rat hippocampal slice. To approximate the in vivo state, 1.5 mM [Ca2+]o was used as the control condition. Raising [Ca2+]o caused a leftward shift of the input-output curve (stimulus intensity vs population spike amplitude) and reduced the tendency toward multiple spiking. Lowering [Ca2+]o shifted the input-output curve down and to the right. When [Ca2+]o was lowered to 0.75 mM an epileptiform pattern of extracellular field responses was produced. Paired-pulse phenomena were studied at an interstimulus interval of 20 ms using both input-output curves and action potential thresholds for single units. Only facilitation was seen using single unit thresholds. Using input-output curves, inhibition was seen in 2.5 and 3.5 mM [Ca2+]o at high stimulus intensities but not at all in 1.5 mM [Ca2+]. LTP was shown to be Ca2+-dependent, maximal at 1.5 mM [Ca2+]o and absent at 1.0 mM [Ca2+]o. LTP was present but less prominent in [Ca2+]o greater than 1.5 mM.