Usualy brain effects are studied when we are looking for new antiepileptic drug. All new potential neurological drugs are tested in slice of brain, on patch-clamping on isolate neurons, in next time on MES, in audiogenic animals, and so.
New drugs had to have the ability to open and close ionic channels of neuronal membrane, to decraese excitabilty and to increase inhibitory system.
But, when they are oral adiministred they have to reach the brain in the right concentration to develop the same effect observed in vitro.
Since '70s antidepressant SSRIs are suggested to treat depression, the mechanism proposed is to increase the time of action of serotonin in synaptic cleft, but today (this year) Costa, Guidotti and Pinna found that at the low doses that these drugs reach the brain they are not able to inhibit.serotonin re-uptake. At these low doses they are able to stimulate neursteroid synthesis.
An house of cards seem falling down, but in the intestinal brain these drugs, orally adiministred, are in higher concentrations than in the brain, then in intestinal brain they are able to inhibit serotonin reuptake. Neuroendocrine system response is to increase brain synthesis of neuropeptides.