Ultrastructural immunocytochemical evidence for peptidergic neurotransmission in the pond snail Lymnaea stagnalis

The action of RFamide neuropeptides on molluscs, with special reference to the gastropods Buccinum undatum and Busycon canaliculatum

The ever-growing RFamide neuropeptide superfamily has members in all animal phyla. Their effects in molluscs, on both smooth and cardiac muscle as well as on neurons, has been studied in detail. These neuropeptides exert a variety of functions: excitatory, inhibitory or even biphasic. Firstly, the literature on the excitatory effect of the RFamide neuropeptides on molluscan muscle and neurons has been reviewed, with greater emphasis and examples from the gastropods Buccinum undatum and Busycon canaliculatum. The peptides seem to be potent activators of contraction, sometimes generating slow tonic force and other times twitch activity. Secondly, the literature on the inhibitory effect of the superfamily has been reviewed. These peptides can exert an inhibitory effect, hyperpolarizing the cells rather than depolarizing them. Thirdly, the neuropeptides may play a variety of other roles, such as contributing to the regulation or maturation process of the animals. There have been cases recorded of RFamide neuropeptides acting as potent venoms in members of the Conus sp. The pathway of action of these multiple roles, their interaction with the parent neurotransmitters acetylcholine and serotonin, as well as the calcium dependency of the RFamide neuropeptides has been discussed, again with special reference to the above mentioned gastropods. A better understanding of the mode of action, the effects, and the importance of the RFamide neuropeptides on molluscan physiology and pharmacology has been attempted by reviewing the existing literature, recognizing the importance of the RFamide neuropeptide actions on molluscs.

Isolation of FMRFa-like peptides from the DB-containing connective tissue of Helix aspersa

The endocrine dorsal bodies (DB) of Helix aspersa are innervated by axons from the central nervous system, which establish synapse-like structures (SLS) with the DB cells. Previous immunocytochemical studies suggested the presence of FMRFa-like substances in nerves of the DB area and in SLS. This paper reports on biochemical attempts undertaken in order to investigate the nature of these substances: the use of HPLC and RIA confirms the presence of three FMRFa-like peptides in the DB-containing connective tissue among which one is probably the FMRFa itself.

In vitro inhibition of methionine incorporation in the dorsal bodies of Helix aspersa by synthetic FMRFamide

1. In in vitro conditions, synthetic FMRFamide was shown to inhibit the uptake of labelled methionine by the dorsal bodies (DB)-containing connective tissue of Helix aspersa. 2. This effect occurred at physiological concentrations and in a dose-dependent manner. 3. Among the different cell types of the explants, the DB cells preferentially incorporated the radioactive precursor. 4. These results suggest that FMRFamide plays a biological role in controlling the DB activity (protein synthesis) of Helix aspersa.

Dual inhibitory action of FMRFamide on neurosecretory cells controlling egg laying behavior in the pond snail

We describe here the electrophysiological characterization of a dual inhibitory action of FMRFamide (FMRFa, Phe-Met-Arg-Phe-NH2) on the caudodorsal cells (CDCs) of the pond snail Lymnaea stagnalis: (i) a transient hyperpolarizing response (H-response) and (ii) a suppression of the excitability of the cells, which lasted as long as the peptide was present. Both effects of FMRFa occurred in silent, excitable cells as well as discharging cells. The effects were reversible and dose-dependent in the range of 10(-9) to 10(-5) M. The H-response was not blocked by any of the antagonists to classical neurotransmitters that were tested. The reversal potential of the H-response was dependent on the [K+]o, which suggests that K+ is the major charge carrier in this response. 4-Aminopyridine (4-AP) blocked the H-response but did not affect the suppression of the excitability by FMRFa. This indicates that the effects of the peptide on these cells are independent. Experiments on the mechanism of the inhibition of the excitability indicated that FMRFa blocks the cAMP-dependent activation of the pacemaking mechanism of the CDCs. In experiments with isolated cells it was demonstrated that the actions of FMRFa are mediated directly through receptors on CDCs (H-response: ED50 = 10(-8) M). Finally, anti-FMRFa-positive varicosities and axons close to the somata, the axons and the neurohaemal endings of the CDCs were demonstrated immunocytochemically. The duality of the action of FMRFa on the neural activity of CDCs indicates its role of high priority in the regulation of egg laying behavior.

Immunocytochemistry of peptidergic systems in the pond snail Lymnaea stagnalis

Evidence suggests that there exists in the animal kingdom a family of biologically active peptides whose members are related to the molluscan cardio-active tetrapeptide FMRFamide (Phe-Met-Arg-Phe-NH2). Immunocytochemical and ultrastructural studies indicate that several family-members occur in the pond snail Lymnaea stagnalis. Monoclonal antibodies were raised to whole brain homogenates of the pond snail. Selection of antibody producing hybridomas was carried out by staining sections of the central nervous system of the snail with the supernatants of the hybridomas. Certain antibodies stain selectively known (neuro)endocrine centres of the snail, others are directed against particular groups of neurons. It is argued that these antibodies were raised against biologically active peptides and/or their precursors. The antibodies may be used for the isolation of these peptides.