NMDA R1 receptor distribution in the cyprid of Balanus amphitrite (=Amphibalanus amphitrite) (Cirripedia, Crustacea)

Cloning and characterisation of NMDA receptors in the Pacific oyster, Crassostrea gigas (Thunberg, 1793) in relation to metamorphosis and catecholamine synthesis

Bivalve metamorphosis is a developmental transition from a free-living larva to a benthic juvenile (spat), regulated by a complex interaction of neurotransmitters and neurohormones such as L-DOPA and epinephrine (catecholamine). We recently suggested an N-Methyl-D-aspartate (NMDA) receptor pathway as an additional and previously unknown regulator of bivalve metamorphosis. To explore this theory further, we successfully induced metamorphosis in the Pacific oyster, Crassostrea gigas, by exposing competent larvae to L-DOPA, epinephrine, MK-801 and ifenprodil. Subsequently, we cloned three NMDA receptor subunits CgNR1, CgNR2A and CgNR2B, with sequence analysis suggesting successful assembly of functional NMDA receptor complexes and binding to natural occurring agonists and the channel blocker MK-801. NMDA receptor subunits are expressed in competent larvae, during metamorphosis and in spat, but this expression is neither self-regulated nor regulated by catecholamines. In-situ hybridisation of CgNR1 in competent larvae identified NMDA receptor presence in the apical organ/cerebral ganglia area with a potential sensory function, and in the nervous network of the foot indicating an additional putative muscle regulatory function. Furthermore, phylogenetic analyses identified molluscan-specific gene expansions of key enzymes involved in catecholamine biosynthesis. However, exposure to MK-801 did not alter the expression of selected key enzymes, suggesting that NMDA receptors do not regulate the biosynthesis of catecholamines via gene expression.

Nitric oxide synthase (NOS) in the cyprid of Amphibalanus amphitrite (Cirripedia, Crustacea)

The Amphibalanus amphitrite barnacle is a sessile marine crustacean and a major constituent of benthic as well as intertidal communities. A. amphitrite is also an important component of biofouling on artificial substrates. The role of nitric oxide (NO) was recently observed in the settlement of this species. In this work, we used immunohistochemical and histoenzymatic methods to investigate, for the first time, the presence and distribution of NO synthetic enzymes (NOS) in the competent-for-settlement cyprid of A. amphitrite. NOS-like immunoreactivity was observed in various regions of the cyprid: gut mucosa, mantel epithelium, thoracic muscle, and abductor muscles. Intense immunoreactivity was also present in the cement gland and oil cells, while widespread immunoreactivity was observed in the compound eye. NADPH-diaphorase method was used to provide further data and understand NOS-distribution. The results show that NOS is likely to be present in structures - such as muscles and cement gland - which are key for settlement.

The regulatory role of the NO/cGMP signal transduction cascade during larval attachment and metamorphosis of the barnacle Balanus (=Amphibalanus) amphitrite

The barnacle Balanus amphitrite is among the most dominant fouling species on intertidal rocky shores in tropical and subtropical areas and is thus a target organism in antifouling research. After being released from adults, the swimming nauplius undertakes six molting cycles and then transforms into a cyprid. Using paired antennules, a competent cyprid actively explores and selects a suitable substratum for attachment and metamorphosis (collectively known as settlement). This selection process involves the reception of exogenous signals and subsequent endogenous signal transduction. To investigate the involvement of nitric oxide (NO) and cyclic GMP (cGMP) during larval settlement of B. amphitrite, we examined the effects of an NO donor and an NO scavenger, two nitric oxide synthase (NOS) inhibitors and a soluble guanylyl cyclase (sGC) inhibitor on settling cyprids. We found that the NO donor sodium nitroprusside (SNP) inhibited larval settlement in a dose-dependent manner. In contrast, both the NO scavenger carboxy-PTIO and the NOS inhibitors aminoguanidine hemisulfate (AGH) and S-methylisothiourea sulfate (SMIS) significantly accelerated larval settlement. Suppression of the downstream guanylyl cyclase (GC) activity using a GC-selective inhibitor ODQ could also significantly accelerate larval settlement. Interestingly, the settlement inhibition effects of SNP could be attenuated by ODQ at all concentrations tested. In the developmental expression profiling of NOS and sGC, the lowest expression of both genes was detected in the cyprid stage, a crucial stage for the larval decision to attach and metamorphose. In summary, we concluded that NO regulates larval settlement via mediating downstream cGMP signaling.

G-protein alpha subunits distribution in the cyprid of Balanus amphitrite (=Amphibalanus amphitrite) (Cirripedia, Crustacea)

The acorn barnacle Balanus amphitrite is a marine crustacean with six nauplius and one cyprid larval stages and a sessile adult, that represent one of the main constituents of sea biofouling. The cyprid is the last larval stage, specialized for settlement, and the study of its biology is interesting also in the frame of antifouling strategies. In this study, a novel approach to the neurobiology of B. amphitrite cyprid has undertaken, studying immunohistochemically the distribution of some G-protein α subunits (Gαs, Gαo Gαi, and Gαq) on B. amphitrite cyprid. Gαs-like immunoreactivity was observed in the intestinal mucosa, oral cone, epithelial cells along the outer face of the mantle and thorax; Gαo into the fibers of the neuropile of the central nervous system; Gαi in oil cells, epithelial cells, and limbs and thorax muscles; Gαq was not detected. The results suggest the involvement of the G-protein α subunits in different tissues and functions that seem to be in agreement with the distribution of the ones from the same class of G-proteins in vertebrates.