cAMP is a cellular second messenger involved in the intricate process of neurotransmission. It is an essential component in communication between nerve cells. cAMP transmits signals which orginate at the surface (receptor area) of receiving cells, having been sent from any one of multiple 'first messenger' or 'first order' neurons.
In other words, when a signal is sent down a neural pathway and bridges a synaptic intersection, cAMP is the compound affected at the beginning of the next cell (neural circuit); cAMP translates the binding of the signal transmitter into a response (in this case, an impulse which is relayed further down the neural pathway).
Not all drugs and neurotransmitters simply carry signals from point A to point B. Some of these compounds produce an inhibitory neural response by blocking the production or function of this second messenger (cAMP), while other drugs increase a neural cell's excitability by affecting an increase in the production of cAMP (binding of a mu opioid agonist results in the former).