Chemosensory Systems in Lower Organisms: Correlations with Internal Receptor Systems for Neurotransmitters and Hormones

The presence of specific chemical substances in the environment evokes behavioral responses in many lower eukaryotic organisms such as protozoans and invertebrates (for reviews, see Mackie and Grant 1974, Gleeson 1978, Atema 1985, Carr 1987). Beginning with Haldane (1954), investigators have noted that several of the chemical substances evoking these responses in lower life forms include some of the very same compounds that function internally as neurotransmitters, modulators, or hormones (see Table 1). Hence, substances such as acetylcholine and the adenine nucleotides may evoke behavioral responses when present in an external aquatic environment just as they evoke physiological responses when present in a synaptic cleft.