Conditioning of the responsiveness of the central nervous system in the life cycle of the lungfish ( Protopterus annectens)

Abstract

Protopterus annectens lives under a tropical climate which is characterized by three major factors that are responsible for the presence of two different phases: an active aquatic phase and a resting aerial phase during which the fish is surrounded by a hardened mucous cocoon. These factors are (1) presence of water vs. absence of water; (2) relatively high temperature vs. relatively low temperature; (3) abundant nourishment vs. scanty nourishment. The regular alternation of those conditions of life induces certain physiological modifications responsible for the preadaptation of the fish. We have studied the mentioned phenomena utilizing the “olfactive awakening reaction” as a criterion. This reaction is characterized by a series of rhythmic puffs observable during the EEG recording. Those rhythmic puffs are detectable at the level of certain archaic structures of the telencephalon. The mentioned reaction can be produced only during the aquatic phase, when the activity of the fish is maximal. It depends on the synthesis of the thyroid hormones as well as that of the cholinesterasic enzymes. The synthesis of these substances depends itself on the mentioned climatic factors. If the rate of synthesis of one or the other of these substances is not sufficient, the awakening reaction can never be obtained.