Table 2.

Chemical/hormonal terminology (adapted from Nelson (2005) and Norris (2007)).

agents	description
hormone	an organic chemical messenger released from endocrine cells that travels through the blood system to interact with cells some distance away and cause a bio-response. Secreted by specialized non-neural cells into the blood
neurohormone	substance secreted by neurons into the blood that may be stored in neurohemal organ prior to release
neuropeptide	a peptide hormone produced by a neuron
neurosteroid	a steroid hormone produced by a neuron
neuromodulator	substances that do not directly activate post-synaptic receptors but that enhance the excitatory or inhibitory responses of these receptors
neurotransmitter	chemical messenger that acts across the synaptic space
chemical messenger	substance produced by a cell that will in turn affect the function of another cell