How can I interface cAMP signal or membrane potential from slime mold to an electrode?
For cAMP there is only one electrophysiological technique I've found.  You can use cyclic nucleotide-gated channels in the cell as a secondary indicator of cAMP activity.  When cAMP increases in the cell, these channel will fast gate which you should be able to pick up.

The issue is standard CNGC's are not overly sensitive to cAMP and are typically more sensitive to cGMP.  But there have been mutants created that have a high affinity for cAMP.  You can transplant these in and have an easier time with the detection.

This paper goes into a a lot more detail on it.