Determination of a chemical compound in a non-homogeneous sample
You should not actually homogenize samples because histamine works locally. If you know which region is releasing histamine then you can just study those regions. You can actually create a transgenic fish with GFP that is a target of histamine receptor signaling. 

Using mathematical models or statistical correlations you can obtain the level of histamine based on fluorescence. 

Activated histamine receptor H1 causes production of inositol triphosphate (IP3) which in turn causes release of calcium form ER. You can use Cameleon-GFP to sense the change in calcium levels. However, there are many events that may result in calcium release; you should carefully design your experiment.

This is another possibility (just a guess)- a FRET based study using GFP-YFP FRET Pairs (or other variants). Since histamine receptors are G-protein coupled receptors you may fuse the GFP to alpha subunit and YFP to beta subunit. When upon signaling, the subunits dissociate, you would see a decrease in FRET.