Can't say for sure, but I always thought that the fresh Amanita muscaria's contained chiefly ibotenic acid (an amino acid), which is rapidly decomposed (decarboxylised) when drying of the mushroom, into muscimol. Perhaps that's why it decreases in potency?
Ibotenic acid is a potentiator of narcosis, it exhibits potent neuroexcitatory activity:
Johnston et al, Nature 248, 804 (1974).
Use as experimental neurotoxic agent: A. Contestabile et al., Experientia 40, 524 (1984).
Patent BE665249 (http://l2.espacenet.com/dips/viewer?PN=BE665249&CY=gb&LG=en&DB=EPD)
Patent US3459862 (http://l2.espacenet.com/dips/viewer?PN=US3459862&CY=gb&LG=en&DB=EPD)
Muscimol is known as a potent CNS depressant and GABA agonist,
Theobald et al., Arzneimittel-Forsch. 18, 311 (1968)
F. V. DeFeudis, Neurochem. Res. 5, 1047-1068 (1980)
Patent US3242190 (http://l2.espacenet.com/dips/viewer?PN=US3242190&CY=gb&LG=en&DB=EPD)
Patent US3397209 (http://l2.espacenet.com/dips/viewer?PN=US3397209&CY=gb&LG=en&DB=EPD)
Ibotenic acid converts to muscimole, true, but this should increase the potency of the sample. (as muscimole is more active)
https://www.thevespiary.org/rhodium/Rhodium/chemistry/muscimole.html (https://www.thevespiary.org/rhodium/Rhodium/chemistry/muscimole.html)
https://www.thevespiary.org/rhodium/Rhodium/chemistry/psychedelicchemistry/chapter6.html (https://www.thevespiary.org/rhodium/Rhodium/chemistry/psychedelicchemistry/chapter6.html)
The degradation of the muscimole at this point is what is probably causing the problems with decrease in potency. Of course, there is the possibility that potency is decreased b/c in the orginal sample the two compounds potentiated each other and that is now not a possibility b/c there is *only muscimole. *most of the ibotenic acid has decomposed to muscimole.