Manganese toxicity is targeting an early step in the dopamine signal transduction pathway that controls lateral cilia activity in the bivalve mollusc Crassostrea virginica.

Manganese is a neurotoxin causing manganism, a Parkinson-like clinical disorder. Manganese has been shown to interfere with dopaminergic neurotransmission, but the neurotoxic mechanism involved is not fully resolved. In the bivalve mollusc Crassostrea virginica also known as the eastern oyster, beating rates of lateral cilia of the gill are controlled by dopaminergic-serotonergic innervation originating from their cerebral and visceral ganglia. Terminal release of dopamine activates D2-like receptors on these gill cells inhibiting adenylyl cyclase and slowing cilia beating rates. In C. virginica, manganese treatment disrupts this dopaminergic innervation of the gill, preventing the normal cilio-inhibitory response of lateral cells to dopamine. In this study an adenylyl cyclase activator (forskolin) and two different inhibitors (MDL-12,330A and SQ 22,536) were used to determine if manganese had any effects on the adenylyl cyclase step of the dopamine D2 receptor signal transduction pathway. The results showed that neither the adenylyl cyclase activator nor the inhibitors were affected by manganese in the control of lateral ciliary activity. This suggests that in C. virginica the mechanism of manganese toxicity on the dopaminergic control of lateral ciliary activity is targeting an early step in the D2R signal transduction pathway, which may involve interference with D2 receptor activation or alternatively some other downstream signaling activity that does not affect adenylyl cyclase.

Innervation of Gill Lateral Cells in the Bivalve Mollusc Crassostrea virginica Affects Cellular Membrane Potential and Cilia Activity.

Gill lateral cells of Crassostrea virginica are innervated by the branchial nerve, which contains serotonergic and dopaminergic fibers that regulate cilia beating rate. Terminal release of serotonin or dopamine results in an increase or decrease, respectively, of cilia beating rate in lateral gill cells. In this study we used the voltage sensitive fluorescent probe DiBAC4(3) to quantify changes in gill lateral cell membrane potential in response to electrical stimulation of the branchial nerve or to applications of serotonin and dopamine, and correlate these changes to cilia beating rates. Application of serotonin to gill lateral cells caused prolonged membrane depolarization, similar to plateau potentials, while increasing cilia beating rate. Application of dopamine hyperpolarized the resting membrane while decreasing cilia beating rate. Low frequency (5 Hz) electrical stimulations of the branchial nerve, which cause terminal release of endogenous serotonin, or high frequency (20 Hz) stimulations, which cause terminal release of endogenous dopamine, had the same effects on gill lateral cell membrane potentials and cilia beating rate as the respective applications of serotonin or dopamine. The study shows that innervation of gill lateral cells by the branchial nerve affects membrane potential as well as cilia beating rate, and demonstrates a strong correlation between changes in membrane potential and regulation of cilia beating rate. The study furthers the understanding of serotonin and dopamine signaling in the innervation and regulation of gill cilia in bivalves. The study also shows that voltage sensitive fluorescent probes like DiBAC 4(3) can be successfully used as an alternative to microelectrodes to measure changes in membrane potential of ciliated gill cells and other small cells with fast moving cilia.