Cross reference: Receptors in General
A ligand "is a signal triggering molecule, binding to a site on a target protein." "Ligand binding to a receptor alters the chemical conformation, that is the three dimensional shape of the receptor protein. The conformational state of a receptor protein determines the functional state of a receptor. Ligands include substrates, inhibitors, activators, and neurotransmitters. "
Note that hormones, neuropeptides and neurotransmitters interact with specific receptors. The difference between them is that hormones circulate in the blood, neuropeptides are released by neurons in the general vicinity of other neurons, and neurotransmitters are only released into synapses.
Serotonergic modulation of behaviour ... (PubMed)
Also excerpted at: Neurotransmitter Evolution
"The orchestration of complex behaviour by the nervous system relies on a surprisingly small number of neurotransmitters. Each transmitter, however, may act in multiple ways: it may bind to a variety of receptors to open or close ion channels or to activate any of several different second messenger systems. Furthermore, a single transmitter may, in different systems, function in any of three basic modes: as a classical neurotransmitter, as a neuromodulator, or as a neurohormone. A classical neurotransmitter exerts its effects directly on a postsynaptic cell. A neuromodulator does not directly alter the activity of a postsynaptic cell, but rather alters either presynaptic release of, or postsynaptic response to, another compound which acts as the primary neurotransmitter at a synapse. Neuromodulators may be released in a diffuse manner into a region of the nervous system rather than onto discrete postsynaptic sites. A neurohormone is released into the general circulation rather than onto a specific postsynaptic cell and can therefore exert modulatory effects throughout an entire
For more specific information on the various kinds of ligands, see:
Hormones in General
Neuropeptides in General
Neurotransmitters in General