The neuropeptide proctolin potentiates contractions and reduces cGMP concentration via a PKC-dependent pathway

Activation mechanism of a neuromodulator-gated pacemaker ionic current

The neuromodulator-gated current (I_MI) found in the crab stomatogastric ganglion is activated by neuromodulators that are essential to induce the rhythmic activity of the pyloric network in this system. One of these neuromodulators is also known to control the correlated expression of voltage-gated ionic currents in pyloric neurons, as well as synaptic plasticity and strength. Thus understanding the mechanism by which neuromodulator receptors activate I_MI should provide insights not only into how oscillations are initiated but also into how other processes, and currents not directly activated by them, are regulated. To determine what specific signaling molecules are implicated in this process, we used a battery of agonists and antagonists of common signal transduction pathways. We found that the G protein inhibitor GDPβS and the G protein activator GTPγS significantly affect I_MI amplitude, suggesting that its activation is mediated by G proteins. Interestingly, when using the more specific G protein blocker pertussis toxin, we observed the expected inhibition of I_MI amplitude but, unexpectedly, in a calcium-dependent fashion. We also found that antagonists of calcium- and calmodulin-associated signaling significantly reduce I_MI amplitude. In contrast, we found little evidence for the role of cyclic nucleotide signaling, phospholipase C (PLC), or kinases and phosphatases, except two calmodulin-dependent kinases. In sum, these results suggest that proctolin-induced I_MI is mediated by a G protein whose pertussis toxin sensitivity is altered by external calcium concentration and appears to depend on intracellular calcium, calmodulin, and calmodulin-activated kinases. In contrast, we found no support for I_MI being mediated by PLC signaling or cyclic nucleotides.NEW & NOTEWORTHY Neuronal rhythmic activity is generated by either network-based or cell-autonomous mechanisms. In the pyloric network of decapod crustaceans, the activation of a neuromodulator-gated pacemaker current is crucial for the generation of rhythmic activity. This current is activated by several neuromodulators, including peptides and acetylcholine, presumably via metabotropic receptors. We have previously demonstrated a novel extracellular calcium-sensitive voltage-dependence mechanism of this current. We presently report that the activation mechanism depends on intracellular and extracellular calcium-sensitive components.

Cardioactive properties of Solanaceae plant extracts and pure glycoalkaloids on Zophobas atratus

Glycoalkaloids, the biologically active secondary metabolites produced by Solanaceae plants, are natural defenses against animals, insects and fungi. In this paper, the effects of glycoalkaloids present in extracts of Solanaceae plants (potato, tomato and black nightshade) or pure commercial glycoalkaloids on the coleopteran Zophobas atratus F. were evaluated by in vitro and in vivo bioassays using heart experimental models. Each tested extract induced a dose-dependent cardioinhibitory effect. The perfusion of Zophobas atratus semi-isolated heart using the highest potato and tomato extract concentration (1 mmol/L) caused irreversible cardiac arrests, while extract from black nightshade produced fast but reversible arrests. Pure commercial glycoalkaloids caused similar but less evident effects compared with extracts. Our results showed that the bioactivity of tested compounds depended on their structure and suggested the existence of synergistic interactions when combinations of the main glycoalkaloids of potato and black nightshade were used for trials. Surprisingly, injection of tomato and potato extracts in 1-day-old pupae of Zophobas atratus induced reversible positive chronotropic effects and decreased the duration of the both phases (anterograde and retrograde) of the heart contractile activity. Furthermore, these extracts affected the amplitude of the heart contractions.

Cell specific dopamine modulation of the transient potassium current in the pyloric network by the canonical D1 receptor signal transduction cascade

Dopamine (DA) modifies the motor pattern generated by the pyloric network in the stomatogastric ganglion (STG) of the spiny lobster, Panulirus interruptus, by directly acting on each of the circuit neurons. The 14 pyloric neurons fall into six cell types, and DA actions are cell type specific. The transient potassium current mediated by shal channels (I(A)) is a common target of DA modulation in most cell types. DA shifts the voltage dependence of I(A) in opposing directions in pyloric dilator (PD) versus lateral pyloric (LP) neurons. The mechanism(s) underpinning cell-type specific DA modulation of I(A) is unknown. DA receptors (DARs) can be classified as type 1 (D1R) or type 2 (D2R). D1Rs and D2Rs are known to increase and decrease intracellular cAMP concentrations, respectively. We hypothesized that the opposing DA effects on PD and LP I(A) were due to differences in DAR expression patterns. In the present study, we found that LP expressed somatodendritic D1Rs that were concentrated near synapses but did not express D2Rs. Consistently, DA modulation of LP I(A) was mediated by a Gs-adenylyl cyclase-cAMP-protein kinase A pathway. Additionally, we defined antagonists for lobster D1Rs (flupenthixol) and D2Rs (metoclopramide) in a heterologous expression system and showed that DA modulation of LP I(A) was blocked by flupenthixol but not by metoclopramide. We previously showed that PD neurons express D2Rs, but not D1Rs, thus supporting the idea that cell specific effects of DA on I(A) are due to differences in receptor expression.