Calcium sensitivity of a sodium-activated nonselective cation channel in lobster olfactory receptor neurons

Phosphatidylinositol 4,5-bisphosphate-dependent regulation of the output in lobster olfactory receptor neurons

Transient receptor potential (TRP) channels often play a role in sensory transduction, including chemosensory transduction. TRP channels, a common downstream target of phosphoinositide (PI) signaling, can be modulated by exogenous phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2], phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] and/or diacylglycerol (DAG). Lobster olfactory receptor neurons (ORNs) express a TRP-related, non-selective, calcium/magnesium-permeable, sodium/calcium-gated cation (SGC) channel. Here we report that PIs regulate the function of the calcium-activated form of the lobster channel. Sequestering of endogenous PI(4,5)P2, either with an anti-PI(4,5)P2 antibody or by electrostatic screening with polyvalent cations, blocks the channel. Exogenous PI(3,4,5)P3 activates the channel independently of intracellular sodium and/or calcium. Exogenous non-hydrolysable DAG analogs fail to change the gating parameters of the channel, suggesting the channel is insensitive to DAG. Electrophysiological recording from lobster ORNs in situ using a panel of pharmacological tools targeting the key components of both PI and DAG metabolism (phospholipase C, phosphoinositide 4-kinase and DAG kinase) extend these findings to the intact ORN. PI(4,5)P2 depletion suppresses both the odorant-evoked discharge and whole-cell current of the cells, and does so possibly independently of DAG production. Collectively, our results argue that PIs can regulate output in lobster ORNs, at least in part through their action on the lobster SGC channel.

The Na+/Ca2+ exchanger inhibitor, KB-R7943, blocks a nonselective cation channel implicated in chemosensory transduction

The mechanism(s) of olfactory transduction in invertebrates remains to be fully understood. In lobster olfactory receptor neurons (ORNs), a nonselective sodium-gated cation (SGC) channel, a presumptive transient receptor potential (TRP)C channel homolog, plays a crucial role in olfactory transduction, at least in part by amplifying the primary transduction current. To better determine the functional role of the channel, it is important to selectively block the channel independently of other elements of the transduction cascade, causing us to search for specific pharmacological blockers of the SGC channel. Given evidence that the Na(+)/Ca(2+) exchange inhibitor, KB-R7943, blocks mammalian TRPC channels, we studied this probe as a potential blocker of the lobster SGC channel. KB-R7943 reversibly blocked the SGC current in both inside- and outside-out patch recordings in a dose- and voltage-dependent manner. KB-R7943 decreased the channel open probability without changing single channel amplitude. KB-R7943 also reversibly and in a dose-dependent manner inhibited both the odorant-evoked discharge of lobster ORNs and the odorant-evoked whole cell current. Our findings strongly imply that KB-R7943 potently blocks the lobster SGC channel and likely does so directly and not through its ability to block the Na(+)/Ca(2+) exchanger.

Acidity enhances the effectiveness of active chemical defensive secretions of sea hares, Aplysia californica, against spiny lobsters, Panulirus interruptus

Sea hares such as Aplysia californica, gastropod molluscs lacking a protective shell, can release a purple cloud of chemicals when vigorously attacked by predators. This active chemical defense is composed of two glandular secretions, ink and opaline, both of which contain an array of compounds. This secretion defends sea hares against predators such as California spiny lobsters Panulirus interruptus via multiple mechanisms, one of which is phagomimicry, in which secretions containing feeding chemicals attract and distract predators toward the secretion and away from the sea hare. We show here that ink and opaline are highly acidic, both having a pH of approximately 5. We examined if the acidity of ink and opaline affects their phagomimetic properties. We tested behavioral and electrophysiological responses of chemoreceptor neurons in the olfactory and gustatory organs of P. interruptus, to ink and opaline of A. californica within their natural range of pH values, from approximately 5 to 8. Both behavioral and electrophysiological responses to ink and opaline were enhanced at low pH, and low pH alone accounted for most of this effect. Our data suggest that acidity enhances the phagomimetic chemical defense of sea hares.

Ca2+-dependent regulation of a non-selective cation channel from Aplysia bag cell neurones

Ca2+-activated, non-selective cation channels feature prominently in the regulation of neuronal excitability, yet the mechanism of their Ca2+ activation is poorly defined. In the bag cell neurones of Aplysia californica, opening of a voltage-gated, non-selective cation channel initiates a long-lasting afterdischarge that induces egg-laying behaviour. The present study used single-channel recording to investigate Ca2+ activation in this cation channel. Perfusion of Ca2+ onto the cytoplasmic face of channels in excised, inside-out patches yielded a Ca2+ activation EC50 of 10 microm with a Hill coefficient of 0.66. Increasing Ca2+ from 100 nm to 10 microm caused an apparent hyperpolarizing shift in the open probability (Po) versus voltage curve. Beyond 10 microm Ca2+, additional changes in voltage dependence were not evident. Perfusion of Ba2+ onto the cytoplasmic face did not alter Po; moreover, in outside-out recordings, Po was decreased by replacing external Ca2+ with Ba2+ as a charge carrier, suggesting Ca2+ influx through the channel may provide positive feedback. The lack of Ba2+ sensitivity implicated calmodulin in Ca2+ activation. Consistent with this, the application to the cytoplasmic face of calmodulin antagonists, calmidazolium and calmodulin-binding domain, reduced Po, whereas exogenous calmodulin increased Po. Overall, the data indicated that the cation channel is activated by Ca2+ through closely associated calmodulin. Bag cell neurone intracellular Ca2+ rises markedly at the onset of the afterdischarge, which would enhance channel opening and promote bursting to elicit reproduction. Cation channels are essential to nervous system function in many organisms, and closely associated calmodulin may represent a widespread mechanism for their Ca2+ sensitivity.

Pharmacological properties and functional role of a TRP-related ion channel in lobster olfactory receptor neurons

Odors activate lobster olfactory receptor neurons (ORNs) through phosphoinositide signaling that appears to target a Na(+)-gated nonselective cation channel. The Na(+)-gated channel is a potential member of the growing family of transient receptor potential (TRP) channels. Here, we test the effect of potential antagonists on the channel in cell-free patches from cultured lobster ORNs. We show that the channel is antagonized by H+ and the TRP channel blockers 2-aminoethoxydiphenyl borate, SKF96365, ruthenium red, Al3+, Gd3+, and La3+. We then use this enhanced antagonist profile together with the agonists Na+ and Ca2+ to implicate the channel in signal amplification in the cells.