Is there some "standar" way for calculate the amplitude of action potentials? If there are differents ways, wich can be better? For example, to compare if they change under different culture conditons...
You can optically measure action potentials by loading your cell cultures with voltage sensitive dyes and measuring the changes in membrane potential using the intensity of the fluorescence.
see this paper in the Biological Bulletin
Imaging membrane potential with voltage-sensitive dyes
M Zochowski, M Wachowiak, CX Falk, LB Cohen, YW Lam, S Antic and D Zecevic
The Biological Bulletin, (2000) Vol 198, Issue 1 1-21
Membrane potential can be measured optically using a variety of molecular probes. These measurements can be useful in studying function at the level of an individual cell, for determining how groups of neurons generate a behavior, and for studying the correlated behavior of populations of neurons. Examples of the three kinds of measurements are presented. The signals obtained from these measurements are generally small. Methodological considerations necessary to optimize the resulting signal-to-noise ratio are discussed.
Also Here is a site with a list of different voltage sensitive dyeshttp://square.umin.ac.jp/optical/optical/dye.html
Alternatively you can patch clamp you cells in current clamp mode an directly measure the changes in membrane potential by electrophysiology