cGMP/cGMP-dependent protein kinase pathway modulates nicotine-induced currents through the activation of α-bungarotoxin-insensitive nicotinic acetylcholine receptors from insect neurosecretory cells

The calcium-calmodulin-dependent protein kinase kinase inhibitor, STO-609, inhibits nicotine-induced currents and intracellular calcium increase in insect neurosecretory cells

Insect neuronal nicotinic acetylcholine receptors (nAChRs) are transmembrane receptors that play a key role in the development and synaptic plasticity of both vertebrates and invertebrates and are considered to be major targets of neonicotinoid insecticides. We used dorsal unpaired median (DUM) neurons, which are insect neurosecretory cells, in order to explore the intracellular mechanisms leading to the regulation of insect neuronal nAChRs in more detail. Using whole-cell patch-clamp and fura-2AM calcium imaging techniques, we found that a novel CaMKK/AMPK pathway could be involved in the intracellular regulation of DUM neuron nAChRs. The CaMKK selective inhibitor, STO, reduced nicotinic current amplitudes, and strongly when co-applied with α-Bgt. Interestingly, intracellular application of the AMPK activator, A-76, prevented the reduction in nicotine-induced currents observed in the presence of the AMPK inhibitor, dorsomorphin. STO prevented the increase in intracellular calcium induced by nicotine, which was not dependent on α-Bgt. Currents induced by 1 mM LMA, a selective activator of nAChR2, were reduced under bath application of STO, and mecamylamine, which blocked nAChR2 subtype, inhibited the increase in intracellular calcium induced by LMA. These findings provide insight into potential complex mechanisms linked to the modulation of the DUM neuron nAChRs and CaMKK pathway.

Isolation and electrophysiological recording of Ixodes ricinus synganglion neurons

The central nervous system of hard ticks (Ixodidae) consists of a concentrated merged nerve mass known as the synganglion. Although knowledge of tick neurobiology has dramatically improved over the last two decades, this is the first time that isolation and electrophysiological recordings have been carried out on tick neurons from the synganglion. Method: We developed a simple protocol for synganglion neuron isolation and used a whole-cell patch clamp to measure ionic currents induced by acetylcholine, nicotine and muscarine. Relatively large neurons (∼ 25 μm and ∼ 35 μm) were isolated and 1 mM acetylcholine was used to induce strong inward currents of -0.38 ± 0.1 nA and - 1.04 ± 0.1 nA, respectively, with the corresponding cell capacitances being at around 142 pF and 188 pF. In addition, successive application of 1 mM acetylcholine through ∼25 μm and ∼ 35 μm cells for increasing amounts of time resulted in a rapid reduction in current amplitudes. We also found that acetylcholine-evoked currents were associated with a reversible increase in intracellular calcium levels for each neuronal type. In contrast, 1 mM muscarine and nicotine induced a strong and non-reversible increase in intracellular calcium levels. This study serves as a proof of concept for the mechanical isolation of tick synganglion neurons followed by their electrophysiological recording. This approach will aid investigations into the pharmacological properties of tick neurons and provides the tools needed for the identification of drug-targeted sites and effective tick control measures.

Neonicotinoids can cause arrested pupal ecdysis in Lepidoptera

Recently, we reported a novel mode of action in monarch butterfly (Danaus plexippus) larvae exposed to neonicotinoid insecticides: arrest in pupal ecdysis following successful larval ecdysis. In this paper, we explore arrested pupal ecdysis in greater detail and propose adverse outcome pathways to explain how neonicotinoids cause this effect. Using imidacloprid as a model compound, we determined that final-instar monarchs, corn earworms (Helicoverpa zea), and wax moths (Galleria mellonella) showed high susceptibility to arrested pupal ecdysis while painted ladies (Vanessa cardui) and red admirals (Vanessa atalanta) showed low susceptibility. Fall armyworms (Spodoptera frugiperda) and European corn borers (Ostrinia nubilalis) were recalcitrant. All larvae with arrested ecdysis developed pupal cuticle, but with incomplete shedding of larval cuticle and unexpanded pupal appendages; corn earworm larvae successfully developed into adults with unexpanded appendages. Delayed initiation of pupal ecdysis was also observed with treated larvae. Imidacloprid exposure was required at least 26 h prior to pupal ecdysis to disrupt the molt. These observations suggest neonicotinoids may disrupt the function of crustacean cardioactive peptide (CCAP) neurons, either by directly acting on their nicotinic acetylcholine receptors or by acting on receptors of inhibitory neurons that regulate CCAP activity.

Effects of the DAG analogue 1,2-dioctanoyl-sn-glycerol (DiC8) on nicotine- and clothianidin-evoked currents through α-bungarotoxin-insensitive nicotinic acetylcholine receptors expressed on cockroach neurosecretory cells

We previously demonstrated that the cockroach α-bungarotoxin-sensitive nicotinic acetylcholine receptors, nAChR1 and nAChR2 subtypes, are differently sensitive to intracellular calcium pathways. Here, using whole cell patch-clamp recordings, we studied the effects of the diacylglycerol (DAG) analogue 1,2-dioctanoyl-sn-glycerol (DiC8) on nicotine- and clothianidin-evoked currents under an α-bungarotoxin treatment. Our results demonstrated that DiC8 reduced nicotine and clothianidin evoked currents. 10 μM DiC8 suppressed the increase in nicotine-induced currents which was brought about by application of 5 mM caffeine or 9 mM Ca²⁺, whereas DiC8 did not affect the decrease in nicotine-induced currents induced by BAPTA. Similarly, bath application of caffeine or 9 mM Ca²⁺ did not change the clothianidin effects, and the amplitude of clothianidin-induced currents was not affected. However, co-application of both 10 μM DiC8 with 9 mM Ca²⁺, caffeine or BAPTA reduced clothianidin current amplitudes. We conclude that nicotine and clothianidin differently modulate nAChR1 and nAChR2 subtypes under DiC8 treatment, and that nicotine activates nAChR1, whereas clothianidin activates both nAChR1 and nAChR2 subtypes.

A review of the actions of Nitric Oxide in development and neuronal function in major invertebrate model systems

Ever since the late-eighties when endothelium-derived relaxing factor was found to be the gas nitric oxide, endogenous nitric oxide production has been observed in virtually all animal groups tested and additionally in plants, diatoms, slime molds and bacteria. The fact that this new messenger was actually a gas and therefore didn't obey the established rules of neurotransmission made it even more intriguing. In just 30 years there is now too much information for useful comprehensive reviews even if limited to animals alone. Therefore this review attempts to survey the actions of nitric oxide on development and neuronal function in selected major invertebrate models only so allowing some detailed discussion but still covering most of the primary references. Invertebrate model systems have some very useful advantages over more expensive and demanding animal models such as large, easily identifiable neurons and simple circuits in tissues that are typically far easier to keep viable. A table summarizing this information along with the major relevant references has been included for convenience.