What is the chemical nature of thought? What is memory?  Why, over 2,000 years since Aristotle first asked such questions, are we still searching for answers? Animal nervous systems range from the simple nerve nets found in jellies to the complexity of the human brain. While the spatial architecture of the human brain is unmatched, perhaps surprisingly, our brains are more similar than people think. Many of the important neurotransmitters and other brain chemicals are conserved across the animal kingdom.
Neurotransmitters affect behavior. While we tend to think of serotonin as related to depression and dopamine as related to addiction, these neurotransmitters are used by almost every animal studied and impact a wide range of behaviors. For example, because animals use the same neurotransmitters, studying learning and memory in a sea snail has let to insights into human memory formation.

A range of measurements are described to measure brain chemistry.  A number of expected and unusual brain molecules have been discovered in a wide variety of animals.  While we still do not fully understand the chemical nature of thought, University of Illinois research lends interesting insights.