Responses to magnetic stimuli recorded in peripheral nerves in the marine nudibranch mollusk Tritonia diomedea.

Prior behavioral and neurophysiological studies provide evidence that the nudibranch mollusk Tritonia orients to the earth's magnetic field. Earlier studies of electrophysiological responses in certain neurons of the brain to changing ambient magnetic fields suggest that although certain identified brain cells fire impulses when the ambient field is changed, these neuron somata and their central dentritic and axonal processes are themselves not primary magnetic receptors. Here, using semi-intact animal preparations from which the brain was removed, we recorded from peripheral nerve trunks. Using techniques to count spikes in individual nerves and separately also to identify, then count individual axonal spikes in extracellular records, we found both excitatory and inhibitory axonal responses elicited by changes in the direction of ambient earth strength magnetic fields. We found responses in nerves from many locations throughout the body and in axons innervating the body wall and rhinophores. Our results indicate that primary receptors for geomagnetism in Tritonia are not focally concentrated in any particular organ, but appear to be widely dispersed in the peripheral body tissues.

Localization of serotonin and its possible role in early embryos of Tritonia diomedea(Mollusca: Nudibranchia).

A classical neurotransmitter serotonin (5-HT) was detected immunochemically using laser scanning microscopy at the early stages of Tritonia diomedea development. At the one- to eight-cell stages, immunolabeling suggested the presence of 5-HT in the cytoplasm close to the animal pole. At the morula and blastula stages, a group of micromeres at the animal pole showed immunoreactivity. At the gastrula stage no immunoreactive cells were detected, but they arose again at the early veliger stage. Antagonists of 5-HT(2) receptors, ritanserin and cyproheptadine, as well as lipophilic derivatives of dopamine blocked cleavage divisions or distorted their normal pattern. These effects were prevented by 5-HT and its highly lipophilic derivates, serotoninamides of polyenoic fatty acids, but not by the hydrophilic (quaternary) analog of 5-HT, 5-HTQ. The results confirm our earlier suggestion that endogenous 5-HT in pre-nervous embryos acts as a regulator of cleavage divisions in nudibranch molluscs.